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Structure of the western margin of the Queen Charlotte Basin, British Columbia Young, Ian Fairley 1981

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STRUCTURE OF THE WESTERN MARGIN OF THE QUEEN CHAROLETTE BASIN BRITISH COLUMBIA by IAN FAIRLEY YOUNG B. S c , M c G i l l U n i v e r s i t y , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Geology, I n s t i t u t e of Oceanography) We accept t h i s t h e s i s as confirming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1981 t^ cT) Ian F a i r l e y Young, 1981 11 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a gree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f Geology The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V a n c ouver, Canada V6T 1W5 Date May 6, 1981 i i i ABSTRACT The t h e s i s i n v e s t i g a t e s the geology and s t r u c t u r a l develop-ment of the western margin of the Neogene Queen C h a r l o t t e Basin, B r i t i s h Columbia. They study area, s i t u a t e d along the Sandspit F a u l t t r e n d , includes both the onshore p o r t i o n of the f o r e a r c basin on Graham Island (Queen C h a r l o t t e Islands) and o f f s h o r e p o r t i o n i n southwestern Hecate S t r a i t and northwestern Queen C h a r l o t t e Sound. F i e l d i n v e s t i g a t i o n s comprised marine high r e s o l u t i o n seismic and magnetic p r o f i l i n g and sea bottom sampling i n v i c i n i t y of the presumed southern extension of the Sandspit F a u l t and g r a v i t y measurements across the f a u l t zone on Queen C h a r l o t t e I s l a n d s . Samples were c o l l e c t e d f o r geochronometry and a n a l y s i s of magnetic p r o p e r t i e s . A d d i t i o n a l unpublished data, i n c l u d i n g aeromagnetic survey maps and seismic p r o f i l e s , have a l s o been incorporated. A syn t h e s i s of a v a i l a b l e information on the geologic and t e c t o n i c h i s t o r y of the Queen C h a r l o t t e Islands and the Queen C h a r l o t t e Basin and p l a t e motions i n the northeastern P a c i f i c Ocean serve as a basis f o r i n t e r p r e t a t i o n of the f i e l d data. Newly reported r a d i o m e t r i c dates have confirmed or r e v i s e d the ages of Mesozoic-Cenozoic v o l c a n i c and p l u t o n i c rocks of the Queen C h a r l o t t e I s l a n d s . Masset v o l c a n i c episodes occured i n ? Paleocene, Upper Eocene and Lower Miocene time. Coeval "post-t e c t o n i c " plutons were emplaced i n Upper Eocene-Lower Oligocene, Lower-Middle Miocene and l a t e r and appear t o postdate Masset v o l c a n i c episodes. The timing of wrench f a u l t i n g on the Sandspit and Rennel-Louscoone systems i s constrained by the new age data. i v Magnetic s u s c e p t i b i l i t i e s and remnant i n t e n s i t i e s of Queen C h a r l o t t e Islands v o l c a n i c and p l u t o n i c rocks were measured t o a i d i n t e r p r e t a t i o n of airborne and marine magnetic data. High magneti-z a t i o n s and corresponding high amplitude magnetic anomalies are associated with the Masset v o l c a n i c and syn- and p o s t - t e c t o n i c p l utons. High Q r a t i o s and peak demagnetizing f i e l d s i n d i c a t e s Masset NRM i s s t a b l e and comprises the l a r g e r percentage of the magnetic f i e l d . I n t e r p r e t a t i o n of marine magnetics has o u t l i n e d probable seacrop p a t t e r n s on an inner s h e l f between Queen C h a r l o t t e Islands and the basin edge. Anomaly patterns over major basin elements, i n c l u d i n g Hecate and C h a r l o t t e sub-basins and Moresby and Cape S t . James r i d g e s , r e f l e c t depth t o source and nature of pre-Neogene subcrop. Offshore extensions of the Sandspit, Rennel, and Louscoone f a u l t s and a s s o c i a t e d wrench s t r u c t u r e s are suggested by anomaly trends. Seismic p r o f i l e s i l l u s t r a t e s t r u c t u r a l development i n shallow Neogene sediments of the western Queen C h a r l o t t e Basin. In Skidegate I n l e t , the seismic data shows p o s s i b l e evidence of l a t e P l e i s t o c e n e f a u l t i n g . In Hecate sub-basin, continuous p a r a l l e l to en-echelon f o l d s , t h a t e v i d e n t l y experienced some growth i n Neogene time, are draped over Mesozoic wrench f o l d s i n a deformation zone c o l i n e a r with the onshore Sandspit F a u l t . Lack of a throughgoing f a u l t zone i n the sedimentary cover i n d i c a t e s that post-Lower Miocene wrench f a u l t development i s i n an i n c i p i e n t stage of d e v e l -opment. In the northern C h a r l o t t e sub-basin f o l d s are poorly d e v e l -oped and deformation has been l e s s severe. In the southern p o r t i o n of the subbasin f a u l t s that p a r a l l e l the Sandspit trend have been V mapped. Marine a r k o s i c sands dredged from seacrop i n the C h a r l o t t e sub-basin c o n t a i n an Upper Miocene-early Pliocene m i c r o f l o r a l assem-blage. The sands are probably equivalent t o the upper marine member of the Skonun Formation on Graham I s l a n d . The nature and timing of Cenozoic wrench f a u l t i n g and associated volcanism and plutonism on Queen Ch a r l o t t e Islands and i n the western Queen C h a r l o t t e Basin may be r e l a t e d t o changes i n oceanic p l a t e motions and p o s s i b l e p l a t e edge e f f e c t s at the c o n t i n e n t a l margin. v i TABLE OF CONTENTS Page Abstract i i i Table of Contents v i L i s t of Tables * L i s t of Figures x i L i s t of Pl a t e s x v i Acknowledgments x v i i CHAPTER PREFACE 1 I. SUMMARY OF THE GEOLOGY AND TECTONIC HISTORY OF THE 5 QUEEN CHARLOTTE ISLANDS In t r o d u c t i o n 5 Upper Pa l e o z o i c (?) and the Allochtonous 7 In s u l a r Terrane Upper T r i a s s i c Lower J u r a s s i c W r a n g e l l i a 13 Succession Lower-Upper J u r a s s i c Succession 16 Lower-Upper Cretaceous Succession 23 T e r t i a r y Succession 27 I I . HISTORY OF PLATE INTERACTIONS AT QUEEN CHARLOTTE 30 ISLANDS Late Mesozoic-Middle Eocene 31 Middle Eocene-Present 37 Present P l a t e Boundary 42 I I I . RADIOMETRIC AGE DATES OF QUEEN CHARLOTTE ISLANDS 46 VOLCANIC AND PLUTONIC ROCKS Introdu c t i o n 46 Data A n a l y s i s , P r e c i s i o n , and I n t e r p r e t a t i o n 48 Discussion of Age Data 53 Karmutsen Formation 54 Syntectonic I n t r u s i o n s 54 Masset Formation 55 Post-Tectonic I n t r u s i o n s 58 Timing of Movement on Sandspit and Rennel- 62 Louscoone F a u l t s Block R o t a t i o n - T r a n s l a t i o n of Graham Island 63 v i i TABLE OF CONTENTS (cont'd) Page I I I . RADIOMETRIC AGE DATES OF QUEEN CHARLOTTE ISLANDS VOLCANIC AND PLUTONIC ROCKS (cont'd) P o s s i b l e R e l a t i o n s h i p s of Cenozoic Magmatism 65 to " P a c i f i c " P l a t e Motions Masset Volcanics 67 Post-Tectonic Plutons 72 IV. GEOLOGY OF THE QUEEN CHARLOTTE BASIN 75 General S e t t i n g 75 St r a t i g r a p h y 77 Age and C o r r e l a t i o n of Skonun Formation 85 Str u c t u r e 85 V. MAGNETIC PROPERTIES OF QUEEN CHARLOTTE ISLANDS 94 VOLCANIC AND PLUTONIC ROCKS Int r o d u c t i o n 94 Magnetic S u s c e p t i b i l i t y 96 Measurement 96 S t a t i s t i c a l A n a l y s i s 97 Frequency D i s t r i b u t i o n s 101 Remnant Magnetizations 104 Remnant I n t e n s i t i e s 105 A l t e r n a t i n g F i e l d Demagnetization 107 Disc u s s i o n 109 VI. AEROMAGNETIC MAPS OF THE QUEEN CHARLOTTE ISLANDS 111 Int r o d u c t i o n 111 High Level Aeromagnetic Map of B r i t i s h Columbia 114 Northern Moresby Island 116 Southeastern Moresby Island 123 Northeastern Graham Island 125 R e l a t i o n s h i p of Magnetic Anomalies t o Magnetic 126 Rock P r o p e r t i e s V I I . MAGNETIC ANOMALIES OF WESTERN HECATE STRAIT AND 130 NORTHWESTERN QUEEN CHARLOTTE SOUND Int r o d u c t i o n 130 I n t e r p r e t a t i o n Procedure 132 Depth t o Basement Estimates 132 I n t e r p r e t a t i o n of Marine Magnetic Anomalies 134 Inner Shelf 134 Queen C h a r l o t t e Basin 140 Hecate Sub-Basin 141 Moresby Ridge 144 Ch a r l o t t e Sub-Basin 147 v i i i TABLE OF CONTENTS (cont'd) Page V I I . MAGNETIC ANOMALIES OF WESTERN HECATE STRAIT AND NORTHWESTERN QUEEN CHARLOTTE SOUND (cont'd) Oceanic Area Adjacent t o Queen Ch a r l o t t e 149 Sound Tectonic I m p l i c a t i o n of Magnetic Anomalies 153 over Western Queen C h a r l o t t e Sound V I I I . CONTINUOUS SEISMIC REFLECTION PROFILING 161 Introdu c t i o n 161 Previous P u b l i c a t i o n s and Open F i l e Reports 162 Instrumentation and Seismic Record C h a r a c t e r i s t i c s 163 V e l o c i t y of Sound i n Seawater, Sediments, and 168 Bedrock Seawater 169 Quaternay Sediments 171 Bedrock 173 An a l y s i s Procedure of P r o f i l e Data 176 A c o u s t i c - S t r a t i g r a p h i c U n i t s 178 Bedrock S t r u c t u r e of the Western Margin of the 181 Queen C h a r l o t t e Basin Sandspit F a u l t i n Skidegate I n l e t 182 Hecate Sub-Basin 185 Ch a r l o t t e Sub-Basin 198 Morphology and St r u c t u r e of Northwestern 206 Queen C h a r l o t t e Sound Kunghit Channel 207 Moresby Trough 208 Moresby and M i t c h e l l Canyons 211 Continental Slope Off North-Central Queen 213 Ch a r l o t t e Sound IX. BEDROCK SAMPLING AND BOTTOM PHOTOGRAPHY 216 Introdu c t i o n 216 Neogene (Upper Miocene) Bedrock Samples 217 Sample D e s c r i p t i o n s and Microscopy 217 Age of Dredged Samples 222 Bottom Photography 225 X. WRENCH FAULT DEVELOPMENT ON QUEEN CHARLOTTE ISLANDS 229 AND IN WESTERN HECATE STRAIT Int r o d u c t i o n 229 Wrench F a u l t S t r u c t u r a l S t y l e 231 D e f i n i t i o n and Tectonic Habitat 231 Wrench Structures 232 The Rennel-Louscoone Wrench F a u l t System 237 General D e s c r i p t i o n 237 Geologic Expression of F a u l t Zone 237 ix TABLE OF CONTENTS (cont'd) Page X. WRENCH FAULT DEVELOPMENT ON QUEEN CHARLOTTE ISLANDS AND IN WESTERN HECATE STRAIT (cont'd) S t r u c t u r a l Evidence of Wrench F a u l t O r i g i n 239 Folds 240 Synt h e t i c F a u l t s 242 A n t i t h e t i c F a u l t s 243 Main Wrench Fault 244 Displacement 244 Age of F a u l t i n g 246 Sandspit Wrench F a u l t System 248 General D e s c r i p t i o n 248 Geologic Expression of F a u l t Zone 250 Displacement 254 Age of F a u l t i n g 256 Wrench S t y l e Development 260 Queen C h a r l o t t e Islands 260 Western Hecate S t r a i t 261 Cenozoic P l a t e Motions and Wrench F a u l t 263 Development XI. SUMMARY AND CONCLUSIONS 269 REFERENCES 277 APPENDIX I. PROCESSING OF NAVIGATION AND TOTAL 297 MAGNETIC FIELD DATA APPENDIX I I . FORTRAN PROGRAMS 329 APPENDIX I I I . LISTING OF SEISMIC AND MAGNETIC PROFILES 377 APPENDIX IV. DESCRIPTION OF SEABOTTOM SAMPLES 380 X LIST OF TABLES TABLE TITLE PAGE I Table of Formations f o r Queen C h a r l o t t e Islands 6 II Cenozoic P l a t e I n t e r a c t i o n s at Queen C h a r l o t t e 39 Islands I I I Summary of K-Ar Age Determinations: 50 Queen C h a r l o t t e Islands IV K-Ar Age Determinations: S h e l l Canada Wells, 52 Queen C h a r l o t t e Basin V C r i t i c a l S t r a t i g r a p h i c R e l a t i o n s f o r Syn- and 59 Post-Tectonic Intrusions VI Masset and Mean North American D i r e c t i o n s and 65 Poles VII Magnetic S u s c e p t i b i l i t y of Queen C h a r l o t t e 99 Islands and Vancouver Island Rocks VI I I Magnetic Hardness of Queen C h a r l o t t e Islands 108 Core Samples IX Aeromagnetic Survey Data - Queen C h a r l o t t e 113 Islands X R e l a t i o n s h i p of Magnetic Anomalies and Magnetic 128 Rock P r o p e r t i e s to Geologic Units XI Seismic Record C h a r a c t e r i s t i c s 166 XII V e l o c i t y i n T e r t i a r y Masset and Mesozoic 175 Sediments: Queen C h a r l o t t e Islands and Queen C h a r l o t t e Basin X I I I Comparative Mineralogy of Upper Marine Member, 220 Skonun Formation, and Dredge Samples XIV Summary of Major Cenozoic Events at the Queen 265 C h a r l o t t e Islands XV Sample Output from Program MERGE 307 XVI Seismic and Magnetic P r o f i l e s 377 XVII D e s c r i p t i o n of Seabottom Samples 380 x i LIST OF FIGURES FIGURE TITLE PAGE 1 Location of Study Area Showing Ship's Tracks 2 2 D i s t r i b u t i o n of Upper T r i a s s i c Wrangellia 9 Sequences 3 Breakup of P a c i f i c a , Opening of P a c i f i c Ocean, 12 and Emplacement of Allochthonous Terranes 4 Paleogeography of Canadian C o r d i l l e r a i n 18 Lower-Middle J u r r a s i c 5 D i s t r i b u t i o n of Volcanic and P l u t o n i c Rocks on 22 Queen C h a r l o t t e Islands 6 E v o l u t i o n of the Western P a c i f i c Ocean 32 7 Upper Cretaceous-Tertiary P l a t e Reconstruction 34 of Northern P a c i f i c 8 Late Paleogene-Neogene P l a t e Reconstruction 40 of Northeastern P a c i f i c 9 Magnetic L i n e a t i o n s and Tectonic Features of 43 the Gulf of Alaska 10 Sample Locations on Queen C h a r l o t t e Islands 47 f o r K-Ar Dating 11 Histogram of K-Ar and Rb-Sr Dates of T e r t i a r y 66 Masset Volcanics and Post-Tectonic Plutons 12 Neogene Volcanic Trends of Southwestern 68 B r i t i s h Columbia and Offshore P l a t e Boundaries 13 Neogene Displacement of Queen C h a r l o t t e Islands 70 14 T e r t i a r y Plutonism and Ridge M i g r a t i o n 73 15 D i s t r i b u t i o n of Late Mesozoic-Cenozoic Basins 76 of Western North America 16 Well Locations on Queen C h a r l o t t e Islands and 78 i n Queen C h a r l o t t e Basin x i i LIST OF FIGURES (cont'd) FIGURE TITLE PAGE 17 S t r u c t u r a l Cross-Section of Queen C h a r l o t t e 79 Basin 18 Isopach of Queen C h a r l o t t e Basin 82 19 I s o s t a t i c G r a v i t y Anomalies and Models of 87 Queen C h a r l o t t e Basin 20 Total I n t e n s i t y Aeromagnetic Map of Queen 89 Ch a r l o t t e Basin 21 G r a v i t y Map of Queen C h a r l o t t e Basin Region 90 22 Earthquake Epicenters i n V i c i n i t y of Queen 92 Ch a r l o t t e Islands 23 Sample Locations on Queen C h a r l o t t e Islands 95 f o r A n a l y s i s of Magnetic P r o p e r t i e s 24 Histograms of Magnetic S u s c e p t i b i l i t y of 98 Queen C h a r l o t t e Islands and Vancouver Island Samples 25 Comparison of Remnant and Induced Magnetizations 106 of Core Samples from Queen C h a r l o t t e Islands 26 Location of Queen C h a r l o t t e Islands Aeromagnetic 112 Surveys 27 V e r t i c a l Component Magnetic F i e l d Map of South- 115 Western B r i t i s h Columbia 28 Constant A l t i t u d e Aeromagnetic Survey Map i n -poek-et of Northern Moresby Island 29 Constant T e r r a i n Clearance Aeromagnetic in-poeket-Survey Map of Northern Moresby Island 30 Geologic Cross-Section and Magnetic P r o f i l e s 117 of Northern Moresby Island Along A-A' 31 Geologic Cross-Section and Magnetic P r o f i l e s 118 of Northern Moresby Island Along B-B' XI 1 1 LIST OF FIGURES (cont'd) TITLE Aeromagnetic Map of Southern Moresby Island Aeromagnetic Map of Northeastern Graham Island Magnetic Anamaly Map of Southwestern Hecate S t r a i t and Northwestern Queen C h a r l o t t e Sound Magnetic Anomalies Along Ship's Tracks i n Southwestern Hecate S t r a i t Stucture of Moresby Island and Western Hecate S t r a i t Magnetic Anomaly Map of Queen C h a r l o t t e Sound Seismic R e f l e c t i o n and Magnetic Anomaly P r o f i l e Over Cape S t . James Ridge U.B.C. Seismic P r o f i l i n g System I n t e r v a l V e l o c i t i e s of Neogene Sediments from S h e l l Canada Wells as Function of Depth Bathymetry of Southwestern Hecate S t r a i t and Northwestern Queen C h a r l o t t e Sound Geology of Moresby Island and West-Central Queen C h a r l o t t e Basin Bathymetry of Skidegate I n l e t Line Drawings of Seismic P r o f i l e s Across Sandspit F a u l t , Skidegate I n l e t Line Drawings of Seismic P r o f i l e s of Southern Hecate Sub-Basin Seismic P r o f i l e 76-16, Hecate Sub-Basin Seismic P r o f i l e 76-19, Hecate Sub-Basin Seismic P r o f i l e 76-25, Hecate Sub-Basin x i v LIST OF FIGURES (cont'd) FIGURE TITLE PAGE 49 Composite Line Drawing of Seismic P r o f i l e s (s-452), Hecate Sub-Basin 195 50 M u l t i f o l d Seismic P r o f i l e from Hecate Sub-Basin 196 51 Line Drawings of Seismic P r o f i l e s of Northern C h a r l o t t e Sub-Basin 199 52 Seismic P r o f i l e 76-31, C h a r l o t t e Sub-Basin 201 53 M u l t i f o l d Seismic P r o f i l e from C h a r l o t t e Sub-Basin 203 54 Line Drawings of Seismic P r o f i l e s Across Kunghit Channel, C h a r l o t t e Sub-Basin 205 55 Seismic P r o f i l e s Across Moresby Trough, Northern Queen C h a r l o t t e Sound 209 56 Seismic P r o f i l e s Across Moresby and M i t c h e l l Canyons, Northwestern Queen C h a r l o t t e Sound 212 57 Line Drawings of Seismic P r o f i l e s Across Continental Slope o f f Northern Queen Ch a r l o t t e Sound 214 58 Examples of Major F a u l t Systems Characterized by Parallel-Convergent Wrenching 230 59 S t r a i n E l l i p s e and Composite of S t r u c t u r e s f o r Right L a t e r a l Wrench System 233 60 E v o l u t i o n of S t r u c t u r e s Associated with Wrench F a u l t i ng 234 61 Rennel-Louscoone F a u l t on Queen C h a r l o t t e Islands 238 62 Sandspit F a u l t on Queen C h a r l o t t e Islands 249 63 Processing Sequence of Magnetic Data 302 64 D i f f e r e n c e Between IGRF and Third Degree Poly-nomial Reference F i e l d s 311 XV LIST OF FIGURES (cont'd) FIGURE TITLE PAGE 65 Temporal V a r i a t i o n Curves of To t a l magnetic 317 F i e l d 66 Histograms of Crossover Discrepancies 320 67 A l i a s i n g and Smoothing E f f e c t s i n Contour 326 Map Representation of Magnetic Data 68 Flow Diagram of Program MPTAPE 330 69 Flow Diagram of Program NAVF 335 70 Flow Diagram of Program MERGE 341 71 Flow Diagram of Program TRAK 369 x v i LIST OF PLATES PLATE TITLE PAGE I Photomicrographs of Skonun Dredged Samples 219 II Polynomorphs from Skonun Dredge Sample 223 I I I Sea-Bottom Photographs from Western Hecate S t r a i t 226 IV Sandspit F a u l t on Northeastern Moresby Island 251 x v i i ACKNOWLEDGEMENTS The author i s s i n c e r e l y g r a t e f u l t o Dr. R.L. Chase f o r the s e l e c t i o n of the t h e s i s t o p i c , f o r securing funds t o defray f i e l d expenses, f o r constant encouragement during the length of the p r o j e c t , and f o r c r i t i c a l reading of the manuscript. In a d d i t i o n t o the o f f i c e r s and crew of the C.F.A.V. ENDEAVOR, the author extends h i s a p p r e c i a t i o n t o colleagues at the Department of Geologic Sciences, U.B.C. - Dr. R.L. Chase, Mr. R.D. Macdonald (research a s s o c i a t e ) , Mr. D. Reimer (research a s s i s t a n t ) , Mrs. J . Carne (nee Grette) and Ms. D. Runkle (graduate s t u d e n t s ) , and Mr. B. Jensen-Schmidt and Mr. G. M y r f i e l d (under-graduate s t u d e n t s ) , who ably a s s i s t e d i n data a c q u i s i t i o n during I.O.U.B.C. c r u i s e 76-10. The author thanks Dr. R.L. Chase and Mr. R. Maxwell (graduate student) f o r a s s i s t a n c e during the two week-long f i e l d excursions t o the Queen C h a r l o t t e I s l a n d s . Drs. R.M. Clowes and W.C. Barnes, members of the t h e s i s committee, are thanked f o r t h e i r comments and c r i t i q u e of the manuscript. Mr. R.G. C u r r i e (Geological Survey of Canada, P a c i f i c Geoscience Centre) provided considerable advice and as s i s t a n c e i n computer programming and processing of magnetic f i e l d data. He a l s o commented on chapters i n t h i s t h e s i s concerned with magnetics. Dr. R.L. Armstrong and Ms. J . Nelson (Department of Geology, U.B.C.) and Mr. C.J. Zinkan (Esso Resources Canada) c r i t i c a l l y reviewed portions of the t h e s i s . Discussions with Dr. A. Sutherland Brown ( B r i t i s h Columbia Department of Mines) and Mr. D.R. Shouldice ( S h e l l Canada Resources) were of considerable b e n e f i t t o the author. x v i i i Ms. S. M i l l e n (Department of Zoology, U.B.C.) and Dr. G.E. Rouse (Department of Botony, U.B.C.) i d e n t i f i e d epifauna and polynomorphs from bottom photographs and dredge samples respec-t i v e l y . The Geological Survey of Canada (courtesy Dr. D.L. T i f f e n ) and S h e l l Canada Resources L t d . (courtesy Mr. D.W. Smith) k i n d l y released seismic data from the Queen C h a r l o t t e Basin. Aeromagnetic maps of the Queen C h a r l o t t e Islands were provided by Dr. A. Sutherland Brown (B.C. Department of Mines) and Mr. A.T. Avison (Dennison Mines L t d . ) . Magnetic s u s c e p t i b i l i t i e s were measured f o r the author by the D i v i s i o n of Geomagnetism, Earth Physics Branch, Ottawa (courtesy Dr. R.L. Coles) on o r i e n t e d cores c o l l e c t e d by Drs. A. Hicken and E. I r v i n g . Dr. E I r v i n g supplied data on remnant i n t e n s i t i e s . S h e l l Canada Resources and Amoco Canada L t d . are thanked f o r release of data on rad i o m e t r i c age determinations of Queen C h a r l o t t e Islands and w e l l samples. F i n a n c i a l support f o r the p r o j e c t was provided by the Department of Energy, Mines and Resources (E.M.R. Research Agreement 1 135-013-4-25/76), B r i t i s h Columbia Department of Mines and Petroleum Resources; S h e l l Canada Resources L t d . , National Research Council (grants t o Dr. R.L. Chase), and the U n i v e r s i t y of B r i t i s h Columbi a. In a d d i t i o n t o academic and research a s s i s t a n t s h i p s , the author received f i n a n c i a l a s s i s t a n c e i n the form of a Canadian S o c i e t y of E x p l o r a t i o n Geophysicists Trust Fund Scho l a r s h i p (1976-77) and a U n i v e r s i t y of B r i t i s h Columbia Graduate Research Summer Fellowship (1977). x i x Typing a s s i s t a n c e was provided by Ms. L Donders, Ms. L Lee, Mrs. J . Watmore and Ms. D. Gray. The D r a f t i n g and Reproduction Departments at Esso Resources Canada L t d . are thanked f o r help i n preparing some of the i l l u s t r a t i o n s i n the t h e s i s . 1 PREFACE As i n i t i a t e d , the primary purpose of the t h e s i s study was t o i n v e s t i g a t e the geology and s t r u c t u r e of the western margin of the Queen C h a r l o t t e Basin. S p e c i f i c o b j e c t i v e s were: 1) t o map a p o s s i b l e submarine extension of the Sandspit F a u l t and determine, i f p o s s i b l e , t i m i n g , magnitude, and sense of displacement and r e l a t i o n -ship t o complementary wrench f a u l t systems; 2) t o map s t r u c t u r e of shallow Queen C h a r l o t t e Basin Neogene bedrock; 3) t o a s c e r t a i n nature and d i s t r i b u t i o n of subcropping (basement) rocks beneath Neogene s t r a t a . F i e l d i n v e s t i g a t i o n s included high r e s o l u t i o n seismic and magnetic p r o f i l i n g and bottom sampling and photography i n south-western Hecate S t r a i t and northwestern Queen C h a r l o t t e Sound (I.O.U.B.C. Cruise 76-10; June 18-28, 1976, F i g . 1). The marine survey comprises the f i r s t non-proprietary geophysical data ( e x c l u -s i v e of g r a v i t y measurements) t o be acquired f o r the c e n t r a l p o r t i o n of the Queen C h a r l o t t e Basin. Only r e c e n t l y (1979) has the j o i n t program of systematic resource mapping by the Department of Energy, Mines, and Resources ( P a c i f i c Geoscience Centre) and Canadian Hydro-graphic Service been extended over t h i s p o r t i o n of the b a s i n . Two week-long f i e l d excursions t o the Queen C h a r l o t t e Islands were made during May and October, 1976. Work c o n s i s t e d of g r a v i t y and magnetic measurements across the Sandspit F a u l t , l i m i t e d reconnaissance geologic mapping, and sample c o l l e c t i n g f o r r a d i o -metric age determinations and magnetic s u s c e p t i b i l i t y measurements. P r e l i m i n a r y r e s u l t s of both onshore and marine i n v e s t i g a t i o n s were 2 F i g . 1 L o c a t i o n of study area showing ship's t r a c k s . Sandspit F a u l t i s i n d i c a t e d by heavy l i n e . 3 given i n Young and Chase ( 1977a, 1977b, 1977c) and Chase and Young (1977). The f o l l o w i n g chapters describe the r e s u l t s of f i e l d and l a b o r a t o r y s t u d i e s and endeavors t o analyze and i n t e r p r e t the data i n terms of the h i s t o r y of p l a t e motions and t e c t o n i c events recorded on the Queen C h a r l o t t e Islands and western Canadian C o r d i l l e r a . A summary of previous s t u d i e s r e l a t e d t o the geology of the Queen C h a r l o t t e Islands and oceanic 1 P a c i f i c ' - A m e r i c a p l a t e i n t e r a c t i o n s are included (chapters I and I I ) t o introduce the reader t o the r e g i o n a l study area and serve as a basis f o r d i s c u s s i n g the s t r u c t u r a l development of the western margin of the Queen C h a r l o t t e Basin. New and p r e v i o u s l y unpublished r a d i o m e t r i c dates of Queen C h a r l o t t e Islands igneous rocks (chapter I I I ) have r e f i n e d the ages of major v o l c a n i c and p l u t o n i c episodes and c o n s t r a i n e d timing of movement on the Sandspit and Rennel-Louscoone wrench-fault systems. The s t r u c t u r e and s t r a t i g r a p h y of the Neogene Queen Ch a r l o t t e Basin i s summarized i n chapter IV. Magnetic rock proper-t i e s (chapter V) and aeromagnetic patterns (chapter VI) have been used to a i d i n t e r p r e t a t i o n of marine magnetic data. The nature and d i s t r i b u t i o n of pre-Neogene basement seacrop and subcrop, i d e n t i f i e d p r i m a r i l y from magnetics data, i s described i n chapter V I I . Evidence f o r a submarine extension of the Sandspit F a u l t and i t s p o s s i b l e r o l e i n basin margin e v o l u t i o n i s a l s o discussed. The s t r u c t u r e of shallow Neogene bedrock at the western margin of the Queen C h a r l o t t e Basin i s i l l u s t r a t e d by seismic r e f l e c t i o n p r o f i l e s included i n chapter V I I I . Major physiographic 4 f e a t u r e s of the c o n t i n e n t a l s h e l f and slope of northwestern Queen Ch a r l o t t e Sound are a l s o i l l u s t r a t e d . A bedrock sample r a i s e d from the s e a f l o o r of western Hecate S t r a i t and bottom photographs are described i n chapter IX. In Chapter X, a model f o r the g e o l o g i c a l and s t r u c t u r a l development of the western margin of the Queen C h a r l o t t e Basin i s suggested based on observations and conclusions of the study and comparisons with s i m i l a r basins along the P a c i f i c margin. 5 CHAPTER 1 SUMMARY OF THE GEOLOGY AND TECTONIC HISTORY OF THE QUEEN CHARLOTTE ISLANDS INTRODUCTION The Queen C h a r l o t t e Islands form the nothern part of the I n s u l a r Tectonic B e l t of the Canadian C o r d i l l e r a . The Late Paleozoic(?)-Recent g e o l o g i c h i s t o r y of the Islands i s s i m i l a r t o t h a t of Vancouver I s l a n d , although d i f f e r e n c e s i n timing and f a c i e s become more important from the beginning of the Cretaceous. Wrench f a u l t i n g and the i n t e r a c t i o n of oceanic " P a c i f i c " p l a t e s with the western C o r d i l l e r a have played a s i g n i f i c a n t r o l e i n t e c t o n i c d e v e l -opment of the Islands and l i k e l y were the dominant f a c t o r s i n c o n t r o l l i n g the three major periods of volcanism, f o u r main periods of sedimentation and two major periods of plutonism t h a t have been recorded. Sutherland Brown (1968) mapped the Queen C h a r l o t t e Islands on a r e g i o n a l s c a l e (1:125,000) and provided a d e t a i l e d d e s c r i p t i o n of the exposed geologic u n i t s . The f o l l o w i n g summary, which r e l i e s e x t e n s i v e l y on h i s comprehensive study, i s intended as an i n t r o d u c t i o n t o the r e g i o n a l study area. Knowledge of the major rock u n i t s of the Islands (Table I) i s important i n attempting to i n t e r p r e t geophysical data and e s t a b l i s h the approximate d i s t r i b u -t i o n of rock types which may u n d e r l i e Neogene-Recent sediments i n the Queen C h a r l o t t e Basin. Reference i s made t o a l l p u b l i c a t i o n s on s p e c i f i c aspects of the s t r a t i g r a p h y or b i o s t r a t i g r a p h y of the 6 TABLE I : TABLE OF FORMATIONS FOR QUEEN CHARLOTTE ISLANDS1 PERIOD EPOCH/ STAGE GROUP OR FORMATION LITHOLOGY .MAXIMUM HICKNES^ (METERS) [ECTONIC OR DEPOSITIONAL ENVIRONMENT INTRUSIVE ROCKS QUATERNARY RECENT [Alluvium </> PLEISTOCENE Cape Ball Formation frill, sand, s i l t , klay, , LOWER MIOCENE) OLIGOCENE EOCENE PALEOCENE ? Masset Fm. Dana Fades Kootenay Fades Conglomerate Pyroclastic breccias volcanic Ss.norphyja, Rhyollte tuffs*flows dadte, basalt flows 1200+ Basalt Mbr. Basalt flows, pyro-clastlcs. andeslte 1500+ Rhyollte Mbr Rhyollte, ash flows,! basalt flows 2100 TURONIAN . a. P O {j ae U3 CENOMANIAN ALBIAN Mixed Mbr. Skidegate Formation 'Basalt breccias & flows 2000 Is 11ts tone, sandstone) 600+ Honna Formation 'Conglomerate, sand-stone, shale 1200 Halda Fm. Shale Mbr. [Shale, siltstone 330 Sdstone Mbr. Sandstone, slltstonej 820 BARREMIAN HAUTERIVIAN J^LiNGJAN. Longara Formation Slltstone, sandstone) conglomerate 1200+ BAJOCIAN TOARCIAN PLEINSBACHIANI SINEMURIAN HETTANGIAN NORIAN KARNIAN OR PEfWSYLVAWIAN Yakoun Fm. C Mbr. Andesltlc agglome-rates and tuffs 290 B Mbr. A Mbr. Shale, Ss. tuffs lalcareous ami lapl l l i tuffs 30+ 200 Maude Formation Shale, sandstone 225 Kunga Fm. Black Argil-Hte Mbr. Arg1111te.sltstone, shale, Ls., Ss. 580 Black Lime-stone Mbr. Carbonaceous lime-stone, arglllHe 270 Grey Lime-stone Mbr. Limestone 180 Karmutsen Formation Basalt flows & pil-lows, tuffs, minor UsatiSKBJs^, Sicker Group ? Limestone, shale, basalt, diabase 4.300 (not ex posed ?)' Non-mar1ne and near-shore marine Glacial marine _ani noji-marlne Mantle plume ? Rifting ? Divergent wrenching Shallow marine Post Tectonic PIutons Middle Miocene -Upper Eocene Ridge subductlon ? Divergent wrenching ? Near-shore marine or non-marine Near-shore marine Marine Syn tec ton 1c BathoHths Lower Cretaceous (?) Upper Jurassic Collision event Volcanic arc Marine Marine Marine Ocean crust ? Volcanic arc ? Intenirc_bMlnJ> Shallow marine Volcanic arc ? Modified In part from Sutherland Brown (1968). For other data sources, see text. 7 Islands t h a t have appeared i n the l i t e r a t u r e s i n c e 1968; the reader i s r e f e r r e d t o Sutherland Brown (1968) f o r a b i b l i o g r a p h y of e a r l i e r works. Al s o noted are any recent t e c t o n i c syntheses of the C o r d i l l e r a that have described models f o r the development of the In s u l a r B e l t . Regional g e o l o g i c a l c o r r e l a t i o n s and paleomagnetics, i n p a r t i c u l a r , have advanced our understanding of the p o s s i b l e Upper Paleozoic-Mesozoic t e c t o n i c h i s t o r y of the Queen C h a r l o t t e Islands. The s t r u c t u r a l h i s t o r y of the Islan d s , that has involved major wrench f a u l t i n g , i s discussed i n Chapter X. In Chapter I I I , new ra d i o m e t r i c age data that has constrained t i m i n g of the major v o l c a n i c and p l u t o n i c episodes and f a u l t i n g on the Queen C h a r l o t t e I s l a n d s , i s d e s c r i b e d . S i m p l i f i e d geologic maps are i l l u s t r a t e d i n F i g s . 61 and 62. UPPER PALEOZOIC (?) AND THE ALLOCHTHONOUS INSULAR TERRANE P o s i t i v e i d e n t i f i c a t i o n of rocks o l d e r than Upper T r i a s s i c on the Queen C h a r l o t t e Islands has not been made. Sutherland Brown (1968, p.50) however, reported tuffaceous c r i n o i d a l limestones, s h a l e , glomerophorphyritic p i l l o w b a s a l t and diabase s i l l s at the base of Upper T r i a s s i c Karmutsen Formation on eastern Moresby Island ( F i g . 42) which might be Upper P a l e o z o i c . On Vancouver I s l a n d , the Karmutsen unconformably or disconformably o v e r l i e s rocks of the S i c k e r Group (Sutherland Brown, 1966; M u l l e r and Carson, 1969; M u l l e r e t al_, 1974; M u l l e r , 1980) that crop out i n the core of u p l i f t s along the c e n t r a l part of the i s l a n d . I f Upper P a l e o z o i c , some of the Queen C h a r l o t t e Islands rocks might be 8 c o r r e l a t e d with the upper limestone ( B u t t l e Lake Formation) of the S i c k e r Group, considered on the b a s i s of f u s u l i n i d s and m a c r o f o s s i l s to be Pennsylvanian or Lower Permian (Danner, 1977; M u l l e r , 1977). On Vancouver Island B u t t l e Lake limestones c o n t a i n bryozoans t h a t have c l o s e t i e s with those of Timor and A u s t r a l i a ; brachiopods and f u s u l i n i d s , however, are s i m i l a r t o those of the boreal realm and the U r a l s of Russia ( f o r d i s c u s s i o n see Danner, 1977). These faunas represent c o o l e r water c o n d i t i o n s and c o n t r a s t markedly with Tethyan Permian and o l d e r faunas (Danner's T r a f t o n -Cache Creek b e l t ) found f u r t h e r east on San Juan I s l a n d s , northern Cascades, and southern Intermontane B e l t . Though faunal d i f f e r e n c e s could have r e s u l t e d s o l e l y from v a r i a t i o n s i n l o c a l d e p o s i t i o n a l environments, i t i s more l i k e l y t h a t they o r i g i n a t e d by j u x t a p o s i -t i o n of allochthonous "oceanic" t e c t o n i c s l i v e r s representing widely separated biogeographic provinces (Tozer, 1970; Monger and Ross, 1971; Monger et aj_, 1972; Monger, 1977). Scattered outcrop ( i n c l u d i n g the S i c k e r Group), under-l y i n g the Upper T r i a s s i c Wrangellia succession (see f o l l o w i n g d i s c u s s i o n ) has been i n t e r p r e t e d as a t h i c k Upper Paleozoic arc sequence ( R i c h t e r and Jones, 1973; M u l l e r , 1977) o v e r l a i n by Lower Permian a r g i l l i t e and limestone s h e l f sediments. Further work should e s t a b l i s h whether a s i m i l a r sequence l i e s at the base of the Karmutsen Formation on the Queen C h a r l o t t e I s l a n d s . The e a r l y t e c t o n i c h i s t o r y of the Queen C h a r l o t t e Islands must be i n f e r r e d from rocks of an Upper Paleozoic-Lower J u r a s s i c i n s u l a r t e r r a i n t h a t extends along the c o n t i n e n t a l margin 2000 km from southern Alaska t o eastern Oregon ( F i g . 2 ) . U n i f y i n g 9 Wrangellia Location of generalized. Triassic columnar sec-tions (Fig. 2) of typical Wrangellia sequence • Western limit of Triassic rocks related to the North American craton / " 4 . F i g . 2 Map showing d i s t r i b u t i o n of Upper T r i a s s i c Wrangel l ia sequences. Reproduced from Jones et al_ (1977, F i g . 1) 10 c h a r a c t e r i s t i c s of t h i s t e r r ane (termed "Wrangelia" by Jones et a]_, 1977) i n c l u d e g r o s s l y s i m i l a r Upper Paleozoic-Lower J u r a s s i c rock u n i t s , faunas, t e c t o n i c s t y l e and g e o l o g i c h i s t o r y . Although d i r e c t c o n t i n u i t y cannot now be demonstrated between the major outcrop areas (eastern Oregon, Vancouver I s l a n d , Queen C h a r l o t t e I s l a n d s , Baranoff and Chichagof I s l a n d s , Wrangel1-St.Elias Mountains, eastern Alaska mountains) the Upper T r i a s s i c rocks i n p a r t i c u l a r are so s i m i l a r that these now d i s j u n c t areas must be regarded as fomerly c o n s t i t u t i n g one homogeneous, continuous t e r r a n e . Considerable evidence, both geologic and paleomagnetic (see below) suggests that W r a n g e l l i a c o n s t i t u t e s an allochthonous block of subcontinental dimensions that has been c a r r i e d a c o n s i d e r a b l e d i s t a n c e north and p o s s i b l y east of i t s o r i g i n a l p o s i t i o n . W r a n g e l l i a i s juxtaposed t o the e a s t , throughout i t s e n t i r e length, against d i s s i m i l a r terranes a l s o thought to be allochthonous (Monger et a^, 1972; Templeman-Kluit, 1976; Monger, 1977; Danner, 1977; Monger and P r i c e , 1979; Coney et aj_, 1980). In the western Canadian C o r d i l l e r a these i n c l u d e the predominately "oceanic" Paleozoic-Lower Mesozoic Tracy Arm, Taku, Bridge R i v e r and Northern Cascades "suspect" terranes (see Coney et a]_, 1980, F i g . 1). S t r a t i g r a p h i c and s t r u c t u r a l c o n t r o l suggests t h a t Wrangellia and the Alexander terrane of southeastern Alaska (Jones et aj_, 1972; Churkin and E b e r l e i n , 1977) were amalgamted i n Upper Jurassic-Lower Cretaceous before f i n a l c o l l i s i o n and a c c r e t i o n onto the North American margin i n Lower Cretaceous-Early T e r t i a r y (Jones et j H , 1977; Coney et a2, 1980). Concurrent and post a c c r e t i o n a r y convergence caused t r a n s l a t i o n and i n t r a - p l a t e deformation with Ill r e a c t i v a t i o n of o l d sutures t h a t d i s r u p t e d W r a n g e l l i a . Upper Cretaceous- Lower T e r t i a r y northwestward s t r i k e - s l i p displacements of hundreds of kil o m e t e r s on the T i n t i n a , D e n a l i , Yalakom, Rennel-Louscoone, and other Alaskan and Canadian f a u l t s may record f i n a l stages of a c c r e t i o n and l a t e r t e c t o n i c adjustments. In southern A l a s k a , successive a c c r e t i o n of the Upper Mesozoic-Early T e r t i a r y Chugach terrane against W r a n g e l l i a caused subduction systems to step southward i n l a t e Cretaceous time (Coney et a l , 1980). Symons (1971) and I r v i n g and Yole (1972) determined paleomagnetic poles f o r "Wrangellian" Karmutsen b a s a l t s of Vancouver I s l a n d . The poles (though based on r e l a t i v e l y unstable remnant magnetization) l i e 46+15 south and about 15° west of the mean Upper T r i a s s i c pole f o r the s t a b l e North American c r a t o n . In order to r e c o n c i l e Vancouver Island paleomagnetic p o l e s , Vancouver Island would have had t o l i e approximately 3,000 km f u r t h e r south during e x t r u s i o n of Karmutsen l a v a s . (Because of T e r t i a r y ? remagnetization paleomagnetic poles f o r Karmutsen rocks of the Queen C h a r l o t t e Islands (Hicken and I r v i n g , 1977, are not s i g n i f i c a n t l y d i s p l a c e d from mean c r a t o n i c poles.) Paleomagnetic poles f o r Middle-Upper J u r a s s i c , Upper Cretaceous, and Eocene sediments from the Alaska P e n i n s u l a , south-western Alaska (Packer and Stone, 1974; Stone and Packer, 1977; 1979) and Middle-Late T r i a s s i c N i c o l a i b a s a l t s from the Wrangell Mountains ( H i l l h o u s e , 1978) are i n general agreement with r e s u l t s from Vancouver I s l a n d . The data suggests northward t r a n s l a t i o n of southern Alaska 30° si n c e Late T r i a s s i c and 18° si n c e Upper 12 F i g . 3 Schematic model of the breakup of " P a c i f i c a " , opening o f the P a c i f i c Ocean, and the r e s u l t i n g c o l l i s i o n of a l l o c h -thonous t e r r a n e s a t circum P a c i f i c margins. Ages of r e c o n s t r u c t i o n stages are approximate o n l y . Fine l i n e s mark the present-day c o n t i n e n t a l o u t l i n e . Heavy l i n e s mark the former p o s i t i o n of c o n t i n e n t s during g e o l o g i c a l e v o l u t i o n . Reproduced from Nur and Ben-Avraham (1977, F i g . 1 ) . 1'3 J u r a s s i c ( H i l l h o u s e , 1978). Much of t h i s motion may have been delayed u n t i l l a t e s t Cretaceous, accompanied by a la r g e clockwise r o t a t i o n (Stone, 1977; Stone and Packer, 1979). The exact o r i g i n of the Queen C h a r l o t t e Islands and other p o r t i o n s of the allochtonous W r a n g e l l i a terrane remains u n c e r t a i n . The t e r r a n e may have been r a f t e d by large s c a l e s t r i k e -s l i p f a u l t i n g from southwestern North America and Central America (Monger, et aj_, 1972; Jones et al_, 1972; M u l l e r , 1977; Stone, 1977) or may be a fragment of eastern A s i a t h a t c o l l i d e d with North America as North America d r i f t e d northwestward a f t e r opening- of the c e n t r a l A t l a n t i c (Monger et _aj_, 1972; Danner, 1977). Nur and Ben-Avraham (1977) envisage t h a t W rangellia was fo r m e r l y part of a mid-Permian c o n t i n e n t a l mass ( " P a c i f i c a " ) and was c a r r i e d t o i t s present p o s i t i o n embedded i n the ancient Kula and/or F a r a l l o n p l a t e s ( F i g . 3 ) . UPPER TRIASSIC-LOWER JURASSIC WRANGELLIA SUCCESSION On Queen C h a r l o t t e I s l a n d s , rocks of the Wr a n g e l l i a succession i n c l u d e the Karmutsen and Kunga Formations. The Karmutsen c o n s i s t s of t h o l e i i t i c lavas i n a general upward succes-s i o n of c h l o r i t i z e d greenstone, massive amygdaloidal b a s a l t , p i l l o w l a v a , p i l l o w b r e c c i a and aquagene t u f f with very minor amounts of i n t e r c a l a t e d l e n t i c u l a r limestone. The e n t i r e sequence, more than 4,000 meters t h i c k , was deposited mainly or e n t i r e l y subaqueously accompanied by gentle subsidence of the v o l c a n i c p i l e (Sutherland Brown, 1968). Although the age of the Karmutsen Formation on Queen 14 C h a r l o t t e Islands i s p o o r l y d e f i n e d , i t i s probably s i m i l a r t o Vancouver Island where the e n t i r e sequence was produced i n the i n t e r v a l between Upper Ladinian-Upper Karnian (Upper T r i a s s i c ) , a span of p o s s i b l y 10 Ma ( J e l e t z k y , 1970; M u l l e r et _al_, 1974; M u l l e r , 1977). Other than Vancouver I s l a n d , outcrops of Karmutsen p i l l o w b a s a l t s on B o n i l l a I s l a n d , along J e r v i s I n l e t , and i n the Mount Waddington map area suggest c o n t i n u i t y of W r a n g e l l i a i n t o the Coast P l u t o n i c Complex (see Hutchison e_t _al_, 1979). The N i c o l a i greenstone of the Wrangel1-St.El i a s Mountains i s s i m i l a r i n age and major element chemistry t o the Karmutsen but c o n t a i n s a lower percentage of p i l l o w b a s a l t s and aquagene t u f f s ( M u l l e r , 1977; Souther, 1977; Jones e t aj_, 1977). M u l l e r (1977) suggests t h a t r a p i d outpouring of Karmutsen v o l c a n i c s was a d i r e c t r e s u l t of r i f t i n g t h a t separated the southern i n s u l a r t e r r ane from Central America. No unequivocal c r i t e r i a have been e s t a b l i s h e d f o r determining t e c t o n i c environment of the Karmutsen and the rocks have been v a r i o u s l y i n t e r p r e t e d as ocean c r u s t , i . e . ridge or ocean i s l a n d (Sutherland Brown, 1968; J e l e t z k y , 1970; Kuniyoshi, 1972), i s l a n d arc (Monger et al_, 1972; G r i f f i t h s , 1977; Souther, 1977) or m a r g i n a l - i n t e r a r c basin (Mu l l e r et a l , 1974). The Karmutsen Formation i s conformably or paraconform-ably o v e r l a i n by limestone, a r g i l l i t e ( s i l i c e o u s mudstone) and sandstone of the Upper Triassic-Lower J u r a s s i c Kunga Formation (Sutherland Brown, 1968). The formation has been subdivided i n t o three informal members ( o p . c i t . ) : 1) a massive grey limestone member 15 t h a t o v e r l i e s the Karmutsen; 2) a middle thinly-bedded black carbon-aceous member; and 3) an upper t h i n l y bedded black a r g i l l i t e member. The upper two members co n t a i n admixtures of l i t h i c s i l t and sand while the upper member alone contains t h i n beds of s i l i c e o u s t u f f (Sutherland Brown, 1968; Tipper, 1977). Contact between i n d i v -i d u a l members are gr a d a t i o n a l and i n some cases diachronous. The composite t h i c k n e s s of the formation (of type l o c a l i t y s e c t i o n s ) i s at l e a s t 950 meters. The lower member of probable K a m i an age (Sutherland Brown, 1968; Cameron and Tipper, 1981) contains poorly preserved c o r a l s and pelecypods. The middle member contains common p e c t i n a -ceans (Halobi a and Monoti s) and rare ammonites of l a t e Upper Kami an to Upper Norian age. The uppermost two-thirds of the upper member contains Sinemurian a r i e t i t i d ammonites (eg. A r i e t i t e s ) , w h i l e the lowermost i n t e r v a l l a c k s megafossils. However, an abundant assem-blage of p a n t a n e l l i a n r a d i o l a r i a n s from the lower i n t e r v a l i s of probable Hettangian (pre-Sinemurian) age (Pessagno and Blome, 1980). The Quatsino, Parson Bay, and Harbledown Formations of Vancouver Island are 1 i t h o l o g i c a l l y , s t r a t i g r a p h i c a l l y , and paleon-t o l o g i c a l l y s i m i l a r t o the lower, middle, and upper members respec-t i v e l y of the Kunga Formation (Sutherland Brown, 1966, 1968; J e l e t z k y , 1970; M u l l e r , 1977; M u l l e r et a_l_, 1974). Sediments of s i m i l a r age and l i t h o l o g y from other outcrop areas of Wra n g e l l i a have been noted by M u l l e r (1977), Jones et aj_ (1978) and P l a f k e r and Hudson (1980). The Kunga limestone (basal member) was deposited i n open ocean at probable shallow depths, on submerged Karmutsen la v a s . 16 With advancing time the marine basin became barred, the bottom anoxic, and depths may have increased. Fine d e t r i t u s from u p l i f t e d b asic (Karmutsen?) source rocks was introduced, i n i t i a l l y as a slow r a i n and e v e n t u a l l y , w i t h increased r a t e of basin f i l l i n g , by t u r b i d i t y c u r r e n t s (Sutherland Brown, 1968; J e l e t z k y , 1970). Halobia and Monotis bearing sediments (middle member) were deposited i n shallower waters; l a t e r tuffaceous m a t e r i a l , of unknown o r i g i n , was deposited as an a s h f a l l . LOWER-UPPER JURASSIC SUCCESSION Conformably o v e r l y i n g Kunga a r g i l l i t e s on the Queen Ch a r l o t t e Islands i s the Lower-early Middle J u r a s s i c Maude Forma-t i o n , a t h i n (up t o 225 meters) sedimentary u n i t comprising a r g i l -l i t e , s h a l e , calcareous s h a l e , l i t h i c sandstone, and minor t u f f and limestone (lense and nodule) interbeds (Sutherland Brown, 1968). Although considered by e a r l i e r workers t o only i n c l u d e beds of P l e i n s b a c h i a n and Lower Toarcian age, recent d e t a i l e d s t r a t i g r a p h i c and b i o s t r a t i g r a p h i c s t u d i e s has expanded the Maude Formation t o i n c l u d e a l l of P l e i n s b a c h i a n and Toarcian and Lower t o e a r l y Middle Bajocian time (Tipper, 1976; Tipper and Cameron, 1979; Cameron and Tipper, 1981). An abundant megafauna w i t h i n the formation comprises Pleinsbachian-Lower Toarcian ammonites, belemnites, and pelecypods described i n e a r l i e r s t u d i e s (McLearn, 1949; Sutherland Brown, 1968; Freehold, 1967; 1970; Tipper and Frebold, 1970) and Middle Toarcian-e a r l y Middle Bajocian ammonites and belemnites (Cameron and Tipper, 1981). A number of beds have y i e l d e d an abundant and d i s t i n c t i v e 17 microfauna i n c l u d i n g f o r a m i n i f e r s , ostracods and f i s h t eeth (Cameron and Tipper, 1981) and pantanel1inae r a d i o l a r i a n s (Pessagno and Brome, 1980). The occurrence of w e l l preserved Lower Ple i n s b a c h i a n r a d i o l a r i a n s i s r e s t r i c t e d t o black limestone ( c a l c i l u t i t e ) nodules ( o p . c i t . ) . The Maude Formation has been c o r r e l a t e d with the upper part of the Harbledown Formation on northeastern Vancouver Island ( J e l e t z k y , 1970; M u l l e r et al, 1974) and predominately sedimentary rocks i n the Taseko Lakes, Harrison Lake, and Manning Park areas of the southeastern Coast Mountains region (Tipper and Richards, 1976). Presumably these rocks were deposited on the Wrangellia succession i n an extensive northwest trending marine basin ("Vancouver Basin", Tipper and Richards (1976); " t e c t o n i c a l l y quies-cent zone", J e l e t z k y , 1970), bounded t o the east by the Yalakom F a u l t and t o the west on western Vancouver Islan d by c a l c - a l k a l i n e arc v o l c a n c i c s of the Lower J u r a s s i c Bonanza Group ( F i g . 4 ) . (Arc volcanism on Queen C h a r l o t t e I s l a n d s , represented by Yakoun ande-s i t e s , was delayed u n t i l Middle J u r a s s i c ) . Emplacement and u p l i f t of the Coast P l u t o n i c Complex has o b l i t e r a t e d much of the evidence f o r a s i n g l e Lower-Middle J u r a s s i c basin and f o r a northwest extension of the Yalakom f a u l t . According t o Tipper and Richards (1976) the Lower-Middle J u r a s s i c Hazelton Trough of the Intermontane B e l t , a p a r t l y marine and p a r t l y c o n t i n e n t a l v o l c a n i c arc was coextensive with the Vancouver Basin. The present o b l i q u e trend of the Vancouver Basin and i t s j u x t a p o s i t i o n against d i s s i m i l a r rocks of the Hazelton Trough was a r e s u l t of northwestward movement on the Yalakom F a u l t 18 F ig . 4 Paleogeography of the Lower Canadian Co rd i l l e r a showing Basin and Hazel ton Trough. Richards (1976, F i g . 14). and Middle Jurass ic of the locat ion of the Vancouver Reproduced from Tipper and 19 and counter-clockwise r o t a t i o n of the Vancouver Basin. The Bonanza v o l c a n i c s of western Vancouver I s l a n d , p o s s i b l y the Yakoun v o l c a n i c s of Queen C h a r l o t t e I s l a n d s , and the Hazelton v o l c a n i c s , a l l s i m i l a r 1 i t h o l o g i c a l l y and c h e m i c a l l y , would have formed a s i n g l e continuous north-south b e l t . I f t h i s i n t e r p r e t a t i o n i s c o r r e c t , the t e c t o n i c regime of Vancouver and Queen C h a r l o t t e Islands must have changed from r i f t i n g and volcanism i n Upper T r i a s s i c ( M u l l e r , 1977) during i n i t i a l northward t r a n s p o r t of Wrangellia t o subduction and arc volcanism even though t r a n s p o s i t i o n and amalgamation of major c r u s t a l blocks continued i n J u r a s s i c time. In Bajocian time arc volcanism i n the Vancouver Basin s h i f t e d t o the east and northwest, represented by a n d e s i t i c volcan-ism on Queen C h a r l o t t e Islands (Yakoun Formation), and t u f f s and b r e c c i a s at Harrison Lake (Harrison Lake Formation) and i n Manning Park (Ladner Group). A n d e s i t i c lava flows and t u f f s , o v e r l y i n g probable Maude eq u i v a l e n t s i n the B u t t l e Lake and A l b e r n i areas of eastern Vancouver Island ( M u l l e r and Carson, 1969) have been t e n t a -t i v e l y c o r r e l a t e d w i t h the Yakoun ( J e l e t z k y , 1970). The Middle J u r a s s i c Yakoun Formation (McLearn, 1949; Sutherland Brown, 1968) i s a t h i c k (up t o 1800 meters) predominately v o l c a n i c u n i t comprising p y r o c l a s t i c p o r p h y r i t i c a n d e s i t i c agglom-erates and t u f f s but a l s o i n c l u d i n g v o l c a n i c sandstones, s i l t s t o n e , shale, conglomerate and minor c o a l . The formation has been sub-d i v i d e d by Sutherland Brown i n t o f i v e informal members. The lower-most A member c o n s i s t s dominately of c a l c i t e cemented scoriaceous l a p a l i l l i t u f f . The t h i n (30 meter) B member comprises interbedded shales, sandstones and t u f f s , o v e r l a i n by t h i c k s e c t i o n s (greater 20 than 300 meters) of a n d e s i t i c agglomerate (member C), and t u f f , tuffaceous sandstone and conglomerate (member D). The uppermost E member i s a predominately sedimentary u n i t of v o l c a n i c sandstone, shale, calcareous s i l t s t o n e , pebbly conglomerate and c o a l . The Yakoun i s l o c a l l y conformable over the Maude, though r e g i o n a l l y unconformable on a l l o l d e r u n i t s . The sedimentary u n i t s (members B and E) conta i n d i s t i n c -t i v e ammonite index f o s s i l s i n a d d i t i o n t o belemnites, pelecypods, gastropods, c o r a l s , m i c r o f l o r a , and microfauna. The B member contains Middle Bajocian Stephanoceras ammonites (McLearn, 1949; H a l l and Westermann, 1980) while the E member contains Upper Bathonian t o Lower C a l l o v i a n Seymourites (McLearn, 1949; Tipper and Cameron, 1980) and I n i s k i n i t e s ( F r e b o l d , 1979; Tipper and Cameron, 1980) faunas. The E member, though dominated by ammonite genera c h a r a c t e r i s t i c of the Boreal realm ( s i m i l a r t o c o r r e l a t a b l e s t r a t a i n southern Alaska and mainland B r i t i s h Columbia) contains c o l o n i a l c o r a l s , i n d i c a t i n g warmer water c o n d i t i o n s not recognized i n other regions (Tipper and Cameron, 1980). This suggests that s i n c e Middle J u r a s s i c time the Queen C h a r l o t t e Islands have been transported n o r t h e r l y a much greater d i s t a n c e than southern Alaska and mainland B r i t i s h Columbia ( o p . c i t . ) . The Yakoun v o l c a n i c s and r e l a t e d arc rocks ( i n c l u d i n g the Lower J u r a s s i c Bonanza) probably represent evolved products of Karmutsen and o l d e r Upper Pa l e o z o i c arc rocks and sediments derived l a r g e l y from p a r t i a l melting above the broad zone of high heat flow r e l a t e d t o subduction (Sutherland Brown, 1968; Monger et a]_, 1972; G r i f f i t h s , 1977; M u l l e r , 1977). On the east coast of the proto 21 Queen C h a r l o t t e Islands a l i n e of vents b u i l t cones out of the shallow marine Vancouver Basin along a l i n e west and s u b p a r a l l e l t o the l a t e r Sandspit F a u l t (Sutherland Brown, 1966, 1968). Erosive processes d i s t r i b u t e d v o l c a n i c d e t r i t u s east and west of the v o l c a n i c s t r u c t u r e s i n depressions t h a t were a l t e r n a t e l y above and below sea l e v e l . The " t e c t o n i c land" ( J e l e t z k y , 1970) erected by arc volcanism on the Queen C h a r l o t t e Islands extended southeast through the present Queen C h a r l o t t e and Georgia S t r a i t s . Cessation of magmatism along t h i s trend i n the e a r l y Upper J u r a s s i c may mark the time of f i n a l a c c r e t i o n of the southern p o r t i o n of the i n s u l a r terrane (Dickinson, 1976; M u l l e r , 1977). Arc volcanism on Vancouver and Queen Ch a r l o t t e Islands was accompanied or postdated by intense deformation u p l i f t and emplacement of syntectonic-j b a t h o l i t h s and plutons. According to Yorath and Chase (1981), t h e i r tectogenesis was r e l a t e d t o c o l l i s i o n and s u t u r i n g of Wrangellia and Alexander terranes along the Rennel and (?) Yalakom f a u l t s . On Queen C h a r l o t t e Islands the San C h r i s t o v a l and West Kano B a t h o l i t h s , Sandspit and Southern groups, and s i m i l a r plutons, s a t e l l i t i c dikes and s i l l s were intruded ( F i g . 5). The p l u t o n i c rocks, mostly hornblende d i o r i t e to quartz "Syntectonic" and " p o s t - t e c t o n i c " are informal genetic names used by Sutherland Brown (1968, pp. 128-129) t o describe (?) Upper Jurassic-Cretaceous mesozonal and (?) Cretaceous-Lower T e r t i a r y epizonal plutons r e s p e c t i v e l y . Although syn t e c t o n i c plutons were c e r t a i n l y emplaced during a period of major s t r e s s , not a l l p o s t - t e c t o n i c plutons, where dated (Chapter 3 ) , are younger than s i g n i f i c a n t deformation. The informal names, however, have been re t a i n e d i n t h i s t h e s i s because they or other synonyms (eg. " o l d " and "young") have been entrenched i n the 1 i t e r a t u r e . 22 F i g . 5 S i m p l i f i e d g e o l o g i c map of the Queen C h a r l o t t e Islands shewing d i s t r i b u t i o n of v o l c a n i c and p l u t o n i c r o c k s . A f t e r Sutherland Brown (1968, F i g . 5 ) . 23 d i o r i t e of Upper J u r a s s i c t o Lower Cretaceous age (Sutherland Brown, 1968; t h i s study, Chapter 3) were probably generated by a n a t e x i s of p r e - e x i s t i n g c r u s t (Sutherland Brown, 1968; G r i f f i t h s , 1977). A s s o c i a t e d m i g m a t i t i c rocks may have been generated by r e m o b i l i -z a t i o n or p a r t i a l m e l t i n g and r e c r y s t a l l i z a t i o n along major t r a n s v e r s e shears. U n l i k e western Vancouver I s l a n d , where high l e v e l plutons and d i k e s extending from b a t h o l i t h s apparently represent magma chambers and su b v o l c a n i c feeders of Bonanza v o l c a n i c s ( M u l l e r et aY, 1974) the " s y n t e c t o n i c " plutons and Yakoun v o l c a n i c s of Queen C h a r l o t t e I s l a n d s are not s p a t i a l l y r e l a t e d and may not be comag-matic . The Upper J u r a s s i c plutons of Vancouver and Queen C h a r l o t t e Islands are the o l d e s t i n the predominant eastward younging tr e n d which extends across the I n s u l a r B e l t and Coast P l u t o n i c Complex ( J e l e t z k y , 1970; G r i f f i t h s , 1977). S t r u c t u r a l and s t r a t i g r a p h i c evidence suggests t h a t the o r i g i n of the Coast P l u t o n i c Complex may be r e l a t e d t o c o l l i s i o n and a c c r e t i o n of Wrangellia-Alexander t o t e r r a n e s f u r t h e r east i n Mesozoic time (Coney e t aj_, 1980). From Upper J u a r s s i c or Lower Cretaceous onwards, subduction zones were l i k e l y s i t u a t e d west of the I n s u l a r B e l t (Monger et a l , 1972; D i c k i n s o n , 1976; G r i f f i t h s , 1977; M u l l e r , 1977). LOWER-UPPER CRETACEOUS SUCCESSION Since the Lower Cretaceous, sedimentary basins and 24 troughs of Vancouver and Queen C h a r l o t t e Islands appear t o have been l o c a l i z e d i n the I n s u l a r B e l t although some "superjacent" d e p o s i t i o n may be d i s t r i b u t e d across amalgamated t e r r a n e s . Following a period of non-deposition i n the Late J u r a s s i c on Queen C h a r l o t t e Islands and northern Vancouver I s l a n d , marine sediments (Longarm Formation) were deposited i n Valangian-Barremian (E a r l y - M i d d l e Lower Cretaceous) time (Sutherland Brown, 1968). On Queen C h a r l o t t e Islands the Longarm i s at l e a s t 1200 meters t h i c k and r e s t s disconformably t o unconformably on Yakoun and o l d e r u n i t s . The formation i s predominately sedimentary, c h a r a c t e r -i z e d by massive dark calcareous s i l t s t o n e with abundant Inoceramus s h e l l d e b r i s , but a l s o c o n t a i n i n g l i t h i c greywacke, f i n e pebble conglomerates, v o l c a n i c sandstones and minor p y r o c l a s t i c rocks akin t o those of the Yakoun Formation (Sutherland Brown, 1968). Thickest s e c t i o n s of Longarm sediments are found along the Rennel-Louscoone f a u l t l i n k a g e , suggesting i n a d d i t i o n t o marine d e p o s i t i o n , e f f e c t s due t o f a u l t movement, slumping and t u r b i d i t y r e d i s t r i b u t i o n . Facies deposited outside the f a u l t zone (or trough), on eroded margins of Yakoun v o l c a n i c s , c o n t a i n i n a d d i t i o n t o v o l c a n i c d e t r i t u s , c l a s t s of g r a n i t i c and metamorphic rocks. Dates of 142 and 145 Ma (Upper J u r a s s i c ) obtained from g r a n i t i c c l a s t s of the Longarm along the Louscoone F a u l t i n d i c a t e a probably source from the Upper J u r a s s i c (142 Ma) Burnaby Island p l u t o n (Yorath and Chase, 1981). On northern Vancouver Island unnamed rocks of s i m i l a r age and l i t h o l o g y occur i n the Quatsino Sound area ( M u l l e r jer a U 1974; J e l e t z k y , 1976). A succession of a r g i l l i t e s and greywacke 25 exposed on the c o n t i n e n t a l slope west of northern Vancouver Island have al s o been t e n t a t i v e l y c o r r e l a t e d w i t h the Longarm (Yorath et a l , 1977). A marine trough or f o r e a r c basin ( M u l l e r , 1977) was probably continuous from Queen C h a r l o t t e Islands t o northern Vancouver Island during the Lower Cretaceous, bounded t o the east by t e c t o n i c lands encompassing eastern Vancouver Island and Coast Ranges ( J e l e t z k y , 1976) and t o south by the Brooks P e n i s u l a Westcoast C r y s t a l l i n e Complex ( M u l l e r , 1977). I t i s u n l i k e l y , because of Coast Mountain t e c t o n i c b a r r i e r s , that t h i s trough was co-extensive wi t h the Mesozoic Tyaughton-Methow Trough of south-western B r i t i s h Columbia ( l o c a t e d i n the suture zone between Wrangellia and terranes t o the east) as suggested by Dickinson (1976). Unconformably o v e r l y i n g the Longarm Formation and older u n i t s i s a t h i c k succession (maximum 3,000 meters) of uppermost Lower Cretaceous and Upper Cretaceous c l a s t i c sediments of the Queen Ch a r l o t t e Group. The Queen C h a r l o t t e Group i s subdivided i n t o three formations (Sutherland Brown, 1968): the basal Haida Formation of g l a u c o n i t i c sandstones and s h a l e , o v e r l a i n by the Honna Formation of p o l y m i c t i c orthoconglomerates, coarse a r k o s i c sandstone, and minor shale and s i l t s t o n e , o v e r l a i n i n t u r n by the Skidegate Formation of s i l t s t o n e , shale, and sandstone. Contacts between formations are g r a d a t i o n a l (to i n t e r f i n g e r i n g ) , conformable or unconformable. The Haida Formation, c o n t a i n i n g T r i g o n i a , Inoceramus, and other f o s s i l s , i s w e l l dated and i s of Albian-Turonian ( l a t e s t Lower t o e a r l i e s t Upper Cretaceous) age (Sutherland Brown, 1968; McLearn, 1972). The Honna and Skidegate Formations c o n t a i n few f o s s i l s and t h e i r age 26 cannot be defined c l o s e r than Upper Cretaceous. The "lower" sandstones and "upper" shales of the Haida Formation are time e q u i v a l e n t s of the Blumberg Formation and upper shale u n i t (Coal Harbour Group) of northwestern Vancouver Island ( J e l e t z k y , 1976) though only the upper u n i t s are l i t h o l o g i c equiv-a l e n t s . Sediments of the Upper Cretaceous Nanaimo Group of the downfaulted Suquash Basin of northeastern Vancouver Island might be equivalent t o the Honna and Skidegate Formations (Sutherland Brown, 1966; M u l l e r et _al_, 1974) though d i f f e r e n c e s i n t i m i n g and f a c i e s c e r t a i n l y e x i s t . Upper Cretaceous sandstone, s i l t s t o n e , and shale (Skidegate equivalent?) was cored i n the S h e l l Anglo Sockeye E-66, B-10, and S h e l l Anglo Tyee N-39 w e l l s i n western Hecate S t r a i t (Shouldice, 1973). I t i s d i f f i c u l t , based on the l i m i t e d faunal evidence a v a i l a b l e , to determine whether basins were continuous between Vancouver and Queen C h a r l o t t e I s l a n d s . According to Sutherland Brown (1968), the Haida sediments were deposited i n a subsiding marine basin (Skidegate Basin) of considerable topography. D e t r i t u s was supplied mainly from s t i l l emergent Yakoun v o l c a n i c s t r u c t u r e s and s y n t e c t o n i c p l u t o n s ; current d i r e c t i o n s are mainly from the east and northeast. Honna conglomerates were deposited e i t h e r i n the s i n k i n g marine ba s i n , lapping onto i t s margins or were derived e n t i r e l y s u b a e r i a l l y from exposed uplands and reworked by f l u v i a l streams. The Skidegate Formation appears t o have o r i g i n a t e d i n a shallow marine basin. 27 TERTIARY SUCCESSION The T e r t i a r y succession, which comprises Masset volcan-i c s , and o v e r l y i n g sediments of the Skonun Formation (see chapter IV), are the most extensive and t h i c k e s t u n i t s on Queen C h a r l o t t e Islands and i n the Queen C h a r l o t t e Basin. The Masset v o l c a n i c s comprising flows and p y r o c l a s t i c s of d i f f e r i n g aspect have, on the b a s i s of l i m i t e d mapping and p e t r o -genic study, been separated i n t o three d i f f e r e n t f a c i e s (Sutherland Brown, 1968): the Tartu f a c i e s covering most of Graham I s l a n d , and the Kootenay and Dana f a c i e s o c c u r r i n g p r i m a r i l y on the west and east coasts r e s p e c t i v e l y of Moreby Island ( F i g . 5 ) . The formation a l s o i n c l u d e s f e l d s p a r porphyry and gabbroic hypabyssal i n t r u s i v e s , which are associated as p o s s i b l e feeders of the above f a c i e s . E x p l o r a t o r y w e l l s d r i l l e d on northern Graham Island penetrated the Tartu f a c i e s at depth (Sutherland Brown, 1968). Wells d r i l l e d i n Hecate S t r a i t and Queen C h a r l o t t e Sound and bottoming i n Masset v o l c a n i c s (Shouldice, 1973) cored b a s a l t i c flows and p y r o c l a s t i c s of u n c e r t a i n a f f i n i t y . Each of the above f a c i e s o v e r l i e s with angular unconformity a l l o l d e r u n i t s from the Karmutsen Formation t o the Queen C h a r l o t t e Group. The bulk of exposed Masset v o l c a n i c s appear t o have been erupted i n Upper Oligocene t o Lower Miocene (chapter I I I ) . The Tartu f a c i e s i s a plateau sequence with an aggregate t h i c k n e s s of about 5.5 km. I t c o n s i s t s of three members (Sutherland Brown, 1968): a basal mixed member of b a s a l t i c b r e c c i a s , columnar f l o w s , and r h y o l i t i c ash f l o w t u f f s , an o v e r l y i n g r h y o l i t e member of 28 r h y o l i t e ash f l o w t u f f s and minor columnar b a s a l t f l o w s , and a topmost b a s a l t member of columnar b a s a l t flows and minor b a s a l t i c and r h y o l i t i c p y r o c l a s t i c rocks. The Kootenay f a c i e s i s comprised mostly of r h y o l i t e welded t u f f b r e c c i a s and a p h a n i t i c r h y o l i t e s w h i l e the Dana f a c i e s i n c l u d e s mixed c l a s t , f i n e t u f f b r e c c i a s and f e l d s p a r porphyries w i t h minor andesite and d a c i t e . V a r i a t i o n s of the general rock types w i t h i n each f a c i e s are known but are minor. Chemical a n a l y s i s of Masset b a s a l t s have shown most of these rocks t o be h i g h l y s i l i c e o u s , moderately high alumina, a l k a l i b a s a l t . Compared t o Karmutsen b a s a l t s , the Masset i s more alumin-ous, more o x i d i z e d , and more a l k a l i r i c h , and l i k e l y represents a new tap of mantle m a t e r i a l s somewhat contaminated by the o v e r l y i n g c r u s t or s l i g h t l y modified by d i f f e r e n t i a t i o n (Sutherland Brown, 1968). Masset r h y o l i t e s are high i n sodium and may have evolved by f r a c t i o n a l c r y s t a l l i z a t i o n of b a s a l t i c m a t e r i a l (Sutherland Brown, 1968; Souther, 1977). This i n t e r p r e t a t i o n i s supported by i n i t i a l ^ S r / ^ S r r a t i o s of three samples of the Tartu f a c i e s (nos. 6, 9, 10, see Table I I I ) that range from .7039 t o .7044 (R.L. Armstrong, unpublished d a t a ) . The Masset formation, with the exception of the Dana f a c i e s , was erupted s u b a e r i a l l y . Dana mixed b r e c c i a s probably formed as subaqueous p r y r o c l a s t i c flows or subaqueous slumps. Locations of vents from which eruption of Masset b a s a l t s and rhyo-l i t e ash flows occurred are i n d i c a t e d by gabbroic and f e l d s p a r porphyry d i k e s and plugs (Sutherland Brown, 1966, 1968). A second major phase of plutonism on Queen C h a r l o t t e 29 Island i s represented by some seven " p o s t - t e c t o n i c plutons2" that l o c a l l y i n t rude or metamorphose Masset v o l c a n i c s (Sutherland Brown, 1968). The plutons, which range widely i n composition from d i o r i t e t o g r a n i t e (average composition: g r a n o d i o r i t e ) are coeval with the Masset v o l c a n i c s though emplacement appears t o have postdated major v o l c a n i c episodes. Age determinations (chapter I I I ) suggest most of the high l e v e l plutons were emplaced or u p l i f t e d i n Upper Eocene t o Upper Oligocene time. A number of the plutons are l o c a l i z e d along trend of the Rennel-Louscoone wrench f a u l t system. P o s s i b l e r e l a t i o n s h i p s between Cenozoic volcanism and plutonism on the Queen C h a r l o t t e I s l a n d s , " P a c i f i c " p l a t e motions and major t e c t o n i c events are discussed i n chapters I I and X. 2 A number of plutons ( i n c l u d i n g those of the Sandspit and Southern groups), considered by Sutherland Brown " t o represent the " p o s t - t e c t o n i c " phase (Sutherland Brown, 1968) are of Upper J u r a s s i c age (chapter I I I ) and are t h e r e f o r e included with the o l d e r " s y n t e c t o n i c " group. 30 CHAPTER I I HISTORY OF PLATE INTERACTIONS AT QUEEN CHARLOTTE ISLANDS Because the Queen C h a r l o t t e Islands l i e on the western margin of the North America p l a t e , t h e i r t e c t o n i c h i s t o r y must be c l o s e l y l i k e d t o the h i s t o r y of p l a t e i n t e r a c t i o n s recorded on the sea f l o o r of the P a c i f i c Ocean. Oceanic magnetic anomalies, sea-mount paleomagnetism, Deep Sea D r i l l i n g P r o j e c t (DSDP) r e s u l t s and ages of seamounts along hot spot t r a c e s have allowed r e c o n s t r u c t i o n of past p l a t e motions. Cenozoic paleoplate p o s i t i o n s i n the North-eastern P a c i f i c , modelled by various authors, can be compared with the sequence of major t e c t o n i c events on the Queen Ch a r l o t t e I s l a n d s . P r e v i o u s l y , a number of authors (eg. M u l l e r , 1977; Bevier et al_, 1979; Ewing, 1980; Yorath and Chase, 1981) have attempted with varying success t o compare onshore s t u d i e s of the western Canadian C o r d i l l e r a t o h i s t o r y of p l a t e i n t e r a c t i o n s at the c o n t i n -e n t a l margin. Pa l e o p l a t e p o s i t i o n s p r i o r t o Middle Eocene time are somewhat s p e c u l a t i v e as much of the evidence from the ocean f l o o r needed f o r accurate r e c o n s t r u c t i o n s has sin c e been destroyed at subduction zones along the northern and western margins of the North America p l a t e . In a d d i t i o n , Cenozoic t e c t o n i c o v e r p r i n t i n g may have obscured c o n t i n e n t a l expressions of p l a t e i n t e r a c t i o n s . A b r i e f h i s t o r y of the major oceanic p l a t e s t h a t must at one time have l a i n adjacent t o the Queen C h a r l o t t e Islands f o l l o w s , based on recent p u b l i c a t i o n s . P l a t e r e c o n s t r u c t i o n s are not unique, being dependent on the type of data used, modelling technique, and 31 inferences where only sparse data e x i s t s . LATE MESOZOIC - MIDDLE EOCENE H i l d e e t a l (1977) and S h i e l d s (1979) hypothesized t h a t the P a c i f i c p l a t e , which now borders the Queen C h a r l o t t e I s l a n d s , o r i g i n a t e d at the j u n c t i o n of the ancient Kula (Grow and Atwater, 1970) and Phoenix p l a t e s and the F a r a l l o n p l a t e (McKenzie and Morgan, 1969) i n E a r l y J u r a s s i c time. The s i t e of b i r t h i n the southwestern P a c i f i c i s i n f e r r e d from p a l e o l a t i t u d e s determined from P a c i f i c seamounts (Francheteau e t _aj_, 1970) and Mesozoic magnetic l i n e a t i o n s (Larson and Chase, 1972; Larson and Pitman, 1972). The longitude i s un c e r t a i n due t o the ambiguity of paleomagnetic data. A f t e r i t s i n i t i a l f o rmation, the P a c i f i c p l a t e increased i n s i z e with continued spreading along i t s margins ( F i g . 6 ) . By 135 Ma B.P. ( E a r l y Cretaceous) two major t r i p l e j u n c t i o n s had been e s t a b l i s h e d : a northern one between the Kul a , F a r a l l o n , and P a c i f i c p l a t e s , and a southern one between the F a r a l l o n , P a c i f i c and Phoenix p l a t e s ( H i l d e e_t aYt 1977). Increased spreading r a t e s f o r c e d these t r i p l e j u n c t i o n s t o migrate i n opposite d i r e c t i o n s , i n c r e a s i n g the length of the P a c i f i c - F a r a l I o n r i d g e . The Kula and P a c i f i c p l a t e s moved northwestward and the F a r a l l o n p l a t e northeastward t o be subducted along the western margin of North America. North America-Europe was moving northwestward away from A f r i c a at t h i s time ( D i e t z and Holden, 1970). At 80 Ma B.P. (Late Cretaceous) North America changed i t s motion from west t o southwest as a r e s u l t of s i g n i f i c a n t r e o r g a n i z a t i o n i n p l a t e motion, which i n the North 32 F i g . 6 Schematic model of the e v o l u t i o n a r y h i s t o r y of the western P a c i f i c . R e c o n s t r u c t i o n s are shown f o r a) J u r a s s i c -Cretaceous boundary (135 Ma), b) Lower-Upper Cretaceous boundary (100 Ma), c) C r e t a c e o u s - T e r t i a r y boundary (65 Ma), d) Oligocene-Miocene boundary (25 Ma). S o l i d l i n e s i n d i c a t e l o c a t i o n s o f r i d g e s , f r a c t u r e zones and subduction zones. Short dashed l i n e s i n d i c a t e former p o s i t i o n s of r i d g e s and f r a c t u r e zones. Redproduced from Hi 1de e t a l (1977, F i g s . 4-7). — 33 A t l a n t i c i n i t i a t e d s e p aration of Europe from North America. Rela-t i v e motion of the North America p l a t e s was, however, small compared to t h a t of the ' P a c i f i c ' p l a t e s . For the western Canadian C o r d i l l e r a , the most s i g n i f i -cant aspect of the P a c i f i c p l a t e system during Late Mesozoic-Early Cenozoic was the p o s i t i o n of the northern t r i p l e j u n c t i o n and, i n p a r t i c u l a r , the p o s i t i o n of the K u l a - F a r a l l o n ridge w i t h respect to the c o n t i n e n t a l margin ( F i g . 7). The e x i s t e n c e of the K u l a - F a r a l l o n r i d g e i s postulated because of geometric c o n s t r a i n t s of the magnetic anomaly p a t t e r n known as the Alaskan Magnetic Bight (Hayes and Pitman, 1968; Grow and Atwater, 1970), although the e n t i r e ridge and the anomalies created by i t s spreading were subducted 30-40 Ma B.P. (Atwater, 1970; Cooper et a]_, 1976; Stone, 1977; Coney, 1976, 1978). I t s former p o s i t i o n w i t h respect t o the North America p l a t e remains u n c e r t a i n . The exact geometry of the t r i p l e j u n c t i o n i s unknown a f t e r 50 Ma B.P. (Atwater, 1970). The only i n d i c a t i o n s of e a r l y spreading at the K u l a - F a r a l l o n ridge are northeast trending J u r a s s i c - E a r l y Cretaceous magnetic anomalies ( F i g . 7) trapped i n the Bering Sea 70 Ma B.P. ago when subduction jumped seaward from the eastern Bering Sea Basin t o the A l e u t i a n Trench (Cooper e_t a l , 1976). The assigned ages f o r the anomalies support eastward migra-t i o n of the K u l a - F a r a l l o n r i d g e . Cooper et a l (1976) present two models f o r the t e c t o n i c development of the northeastern P a c i f i c during Late Cretaceous and Middle Eocene, the f i r s t based on paleomagnetic pole p o s i t i o n s from magnetic anomalies (Larson and Chase, 1972; Atwater and Molnar, 1973) and the second ( F i g . 7) based on absolute p l a t e motions 34 140 160 180 160 140 120 - J L 1 1 a—I l i i • • i ; l 140 1S0 180 160 140 120 140 160 180 160 140 120 140 160 180 160 140 120 140 160 180 160 140 120 I 1 6 0 - y^yf^ N - 6 0 / " 4 5 is (i> 4 5 Jt K 7 r /• F 30 M 1 3 ^ 30 M1 15 - 15 0 0 P 15 - - 15 30 • 70 MY i 3 0 140 160 180 160 140 120 Reconstruct ion of Late Cretaceous-Paleogene p l a te l o ca t i on s in the Northeastern P a c i f i c assuming continuous p l a te motion r e -l a t i v e to a s t a t i ona ry hot-spot re ference frame. N = North American p l a t e ; F = Fa ra l l on p l a t e ; K = Kula p l a t e ; P = P a c i f i c p l a t e ; Ml-Ml3 = Mesozoic anomaly sequence 35 derived from the Hawaiian-Emperor hot spot t r a c e (Clague and J a r r a r d , 1973). Both models show that from Late Cretaceous to Middle Eocene, the northern t r i p l e j u n c t i o n moved northeast toward southeastern A l a s k a . S i x t y t o 70 Ma ago, the extension of the K u l a - F a r a l l o n r i d g e (assumed s t r a i g h t with no transforms) was o r i e n t e d perpendicular t o the Gulf of Alaska c o n t i n e n t a l margin, p o s s i b l y i n the v i c i n i t y of Kodiak Island (Marshak and K a r i g , 1977). With constant p l a t e motion, the ridge would have migrated southeast and by 50-35 Ma B.P. would have l a i n adjacent t o the Queen C h a r l o t t e I s l a n d s . D i r e c t l y north of the t r i p l e j u n c t i o n i n south-eastern Alaska o b l i g u e subduction or transform motion would have c h a r a c t e r i z e d the America-Kula p l a t e boundary. The model of Cooper et _al_, 1976, (see a l s o Coney, 1977), i s not unique, being based on r e l o c a t e d p a l e o p l a t e p o s i t i o n s c o n s i s -tent with age and o r i e n t a t i o n of magnetic anomalies i n the Eastern Bering Sea Basin and constant motion of the P a c i f i c p l a t e over the Hawaiian hot spot t o form the Emperor seamount chain. Atwater (1970), Atwater and Molnar (1973) and Coney (1976), i n reconstruc-t i o n s based on r e l a t i v e motion, show the northern t r i p l e j u n c t i o n o f f southern C a l i f o r n i a at 80 Ma ( s i m i l a r t o the model discussed p r e v i o u s l y ) but o r i e n t e d with repect to the North America p l a t e , so t h a t the K u l a - F a r a l l o n r i d g e migrated northwards up the coast t o Vancouver Island 40 Ma B.P. The r i d g e was f i n a l l y subducted beneath northern Queen C h a r l o t t e Islands or southeastern Alaska (Atwater, 1970; Stone, 1977). This model i m p l i e s t h a t the F a r a l l o n p l a t e never extended f u r t h e r north than point of northward migration of the t r i p l e j u n c t i o n . The Kula p l a t e and p o s s i b l y other unknown 36 p l a t e s l a y west of southeast Alaska. The d i f f e r e n c e s i n p o s t u l a t i o n s concerning p o s i t i o n of the K u l a - F a r a l l o n r i d g e , and, since 50 Ma B.P., the p o s i t i o n of the northern t r i p l e j u n c t i o n , are due t o d i f f e r e n c e s between models based on f i x e d hot spots and those based on r e l a t i v e motion of p l a t e s . Though hot spots i n the northern P a c i f i c Ocean appear t o have remained s t a t i o n a r y f o r at l e a s t the past 10 Ma (Minster e_t aj_, 1974), t h e i r v a l i d i t y as an absolute frame of reference f o r g l o b a l r e c o n s t r u c t i o n p r i o r t o 10 Ma i s s t i l l a matter of debate. Analyses of r e l a t i v e motion r e l y on c o n s t r u c t i o n of vector c i r c u i t s f o r various time periods by vector a d d i t i o n of spreading r a t e and d i r e c t i o n across as many as seven p l a t e s (Atwater and Molnar, 1973), extending from F a r a l l o n (and/or Kula p l a t e ) t o the P a c i f i c , A n t a r c t i c , A u s t r a l i a , I n d i a , A f r i c a and f i n a l l y North America p l a t e s . These r e c o n s t r u c t i o n s f o r the period 40-80 Ma B.P. are very d i f f i c u l t i n the Indian Ocean and furthermore motion w i t h i n A n t a r c t i c a i s probable. Both r e l a t i v e motion and hot spot models assume l i t h o s p h e r i c p l a t e s are r i g i d while models based on r e l a t i v e motion assume a l l spreading i s symmetric. Though r e l a t i v e motion and hot spot models, described above, d i f f e r with respect t o p o s i t i o n of the K u l a - F a r a l l o n ridge and hence the p o s i t i o n of the Kula and F a r a l l o n p l a t e s , both models appear t o suggest that the K u l a - F a r a l l o n ridge l a y immediately west of the Queen C h a r l o t t e Islands 30-40 Ma B.P. F u r t h e r , both models support development of a t r i p l e j u n c t i o n of a ridge-trench-transform or r i d g e - t r e n c h - t r e n c h type at the western Norh America p l a t e boundary. A f t e r disappearance of the K u l a - F a r a l l o n ridge a s i m i l a r 37 t r i p l e j u n c t i o n between the F a r a l l o n , P a c i f i c , and North America p l a t e s developed. MIDDLE EOCENE - RECENT A f t e r mid-Eocene time t e c t o n i c s of the northeastern P a c i f i c were c o n t r o l l e d by i n t e r a c t i o n of the F a r a l l o n , P a c i f i c , and North America p l a t e s . A change i n P a c i f i c p l a t e motion from north to northwest, evidenced by the bend i n the Hawaiian-Emperor seamount ch a i n , dated at 42 Ma B.P. (Clague and J a r r a r d , 1977) c o i n c i d e d with a reduced convergence r a t e between the F a r a l l o n and North America p l a t e s . This date c l o s e l y corresponds with a major t e c t o n i c t r a n -s i t i o n i n western North America, which included c e s s a t i o n of arc magmatism i n the Coast P l u t o n i c Complex and f o r e l a n d t h r u s t i n g i n the Rocky Mountain B e l t (Ewing, 1980; Monger and P r i c e , 1979). I n i t i a l subduction of the K u l a - P a c i f i c r i d g e at the A l e u t i a n Trench l i k e l y occurred approximately 30 Ma B.P. (Grow and Atwater, 1970; De Long et a2> 1978) although i n a more recent i n v e s t i g a t i o n Byrne (1979) suggests subduction may have been as e a r l y as 56 Ma ago. P r i o r t o 29 Ma B.P. (Atwater, 1970; Atwater and Molnar, 1973) or 26 Ma B.P. (Handschumacher, 1976) the P a c i f i c - F a r a l I o n ridge was i n t a c t and migrating east r e l a t i v e t o the o v e r r i d i n g North America p l a t e . At 29 or 26 Ma B.P. the ridge c o l l i d e d w i t h the FaralIon-North America trench stopping spreading at point of contact and i n i t i a t i n g transform motion along the San Andreas and r e l a t e d f a u l t s . The F a r a l l o n p l a t e then broke i n t o two separate u n i t s and large s c a l e p l a t e readjustments occurred. The Juan de Fuca complex of rotated 38 and f r a c t u r e d microplates ( S i l v e r , 1971; Barr and Chase, 1974; Chase et a l , 1975; Riddihough, 1976) i s the l a s t v e s t i g e of the F a r a l l o n p l a t e i n the northeastern P a c i f i c . P l a t e r e c o n s t r u c t i o n s f o r the period 40 Ma B.P. t o the present by Atwater and Molnar (1973), Handschumacher (1976) and Stone (1977) support Atwater's (1970) model of constant motion between the North America and P a c i f i c p l a t e s . R e l a t i v e motion a n a l y s i s (Atwater and Molnar, 1973) suggests, however, that some changes i n d i r e c t i o n and r a t e of motion may have occurred during t h i s time p e r i o d . The r e c o n s t r u c t i o n s (Table I I ) though s i m i l a r i n gross p a t t e r n d i f f e r somewhat i n p o s i t i o n of the F a r a l I o n - P a c i f i c ridge with respect to North America. These d i f f e r e n c e s are due t o basis of r e c o n s t r u c t i o n ( r e l a t i v e motion a n a l y s i s or hot s p o t ) , position of North America r e l a t i v e t o the m i d - A t l a n t i c r i d g e system and I c e l a n d i c hot spot (models of Pitman and Talwani, 1972 or P h i l l i p s and F o r s y t h , 1972) and i n f e r r e d r a t e s of motion between North America and P a c i f i c p l a t e s . These d i f f e r e n t models were compared by Stone (1977), who p r e f e r r e d a model based on constant motion along the San Andreas F a u l t , with f i x e d P a c i f i c hot spots combined with the A t l a n t i c model of P h i l i p s and Forsyth (1972) because t h i s i s the o n l y combination th a t does not r e q u i r e the F a r a l l o n - P a c i f i c spreading r i d g e t o be subducted and then reappear. The model, ( F i g . 8) i n agreement with t h a t of Atwater (1970) suggests t h a t transform motion at the Queen Ch a r l o t t e Islands between the North America and P a c i f i c p l a t e s was i n i t i a t e d 15-20 Ma B.P. The t r i p l e j u n c t i o n between the Juan de Fuca 39 TABLE TI CENOZOIC PLATE INERACTI0N5 AT QUEEN CHARLOTTE ISLANDS ATWATER(1°70) ATWATER & M0LNAR(1973) CONEY(1976/ ATWATER(1970) COOPER ET.AL.U976) ST0NE(1977) HANOSCHUMACHER (1976) CONSTANT RELATIVE MOTION PITMAN S TALWANI ATLANTIC MOD& RELATIVE MOTION (0-5 MA) HOT SPOT. SEAMOUNT PALEOMAGNETICS, RE-LATIVE (ATWATER & MOLNAR) PHILIPS 4 FORSYTH ATLANTIC MODEL CONSTANT MOTION PACIFIC HOT SPOT; PHILIPS S FOR-SYTH ATLANTIC MODEL RELATIVE MOTION TRANSFORM .^ CA-PACIFIC) AMERI-TRANSFORM(N. AMER-ICA-PACIFIC SUBOUCTION-FARALLON PLATE TRANSFORM OR OBLIQUE CONVERGENCES. AMERI-CA-KULA) I TRANSFORM OR OBLIQUE CONVERGENCE-N.AMERICA KULA A INTERACTION KULA-FARA-LLON RIDGE-N.AMERICA SU8DUCTION-FARALL0N PLATE TRANSFORM(N.AMER-ICA-PACIFIC) INTERACTION FARA-LLON-PACIFIC RIDGE-N. AMERICA SUBOUCTION FARA-LLON PLATE t INTERACTION-KULA-FARALLON RIDGE-N.AMERICA TRANSFORM OR OBLIQUq CONVERGENCE KULA-N. AMERICA TRANSFORM-PAC-IFIC-N. AMERI-INTERACTION FARALLON-PACIFIC RIDGE-N.AMERICA FARALLON PLATE SUBOUCTION 40 F i g . 8 P r e f e r r e d r e c o n s t r u c t i o n o f the northwestern P a c i f i c f o r l a t e Paleogene-Neogene based on P h i l l i p s and Forsyth (1972) A t l a n t i c model, constant motion on the San Andreas f a u l t and f i x e d Hawaiian-Emperor hot spot. Reproduced from Stone (1977, F i g . 14); a l s o Stone and Packer (1977, F i g . 3 ) . 41 ( F a r a l l o n ) , P a c i f i c , and North America p l a t e s has remained almost s t a t i c f o r the past 10 Ma at a p o s i t i o n west of northern Vancouver Island (Riddihough, 1976). Any coupling between the Juan de Fuca and North America p l a t e s during t h i s time p e r i o d , however, would place the t r i p l e j u n c t i o n f u r t h e r north, probably west of southern Queen C h a r l o t t e Islands. Changes i n d i r e c t i o n and v e l o c i t y of p l a t e motion during Late Cenozoic may have s e v e r e l y a f f e c t e d "oceanic"-North America p l a t e i n t e r a c t i o n s and hence the geology of the Queen C h a r l o t t e Isl a n d and the developing Queen C h a r l o t t e Basin. R o t a t i o n and t r a n s l a t i o n of m i c r o c o n t i n e n t a l blocks i n c l u d i n g northern Queen Ch a r l o t t e Islands (Hicken and I r v i n g , 1977; t h i s s t u d y ) , southern Vancouver Island (Symons, 1973; H e p t o n s t a l l , 1977) and the Oregon Coastal Ranges (Simpson and Cox, 1977) during the Cenozoic may have changed the c o n f i g u r a t i o n of the North America p l a t e boundary and changed the d i r e c t i o n of ' P a c i f i c ' p l a t e convergence. In a d d i t i o n , the i n f l u e n c e of f a c t o r s such as rid g e jumps, asymmetric spreading, and v a r i a b l e d i p of the subducted p l a t e are d i f f i c u l t to d i s c e r n but may have a profound e f f e c t on c o n t i n e n t a l margin t e c t o n i c s . Atwater and Menard (1970) noted the d i f f i c u l t y i n dati n g magnetic anomalies younger than 35 Ma B.P. west of the Queen C h a r l o t t e I s l a n d s : some l i n e a t i o n s are missing or repeated, f a c t u r e zone o f f s e t s disappear and e v o l u t i o n of the present spreading r i d g e system from e a r l i e r ones i s not e n t i r e l y c l e a r . The present magnetic anomaly p a t t e r n west of the Queen C h a r l o t t e I s l a n d s , mapped by Pitman and Hayes (1968), Couch (1969), S r i v a s t a v a et a]_ (1971) and Naugler and Wageman (1973) i s shown i n 42 F i g . 9. The anomalies, generated at the F a r a l I o n - P a c i f i c r i d g e , trend north without apparent o f f s e t from anomalies west of the Juan de Fuca r i d g e system (Raff and Mason, 1961) and terminate at the Queen C h a r l o t t e Trough, west of the Queen Ch a r l o t t e I s l a n d s . Corre-l a t i o n and assigned ages f o r these anomalies ( F i g . 9) are t e n a t i v e because of sparse magnetic data f o r t h i s region. PRESENT PLATE BOUNDARY The present boundary between the North America and P a c i f i c p l a t e s at the Queen C h a r l o t t e Islands i s the Queen C h a r l o t t e transform f a u l t zone (Wilson, 1965; Morgan, 1968; Tobin and Sykes, 1968; Atwater, 1970). The f a u l t zone extends north from the t r i p l e j u n c t i o n between the North America, P a c i f i c and Expl o r e r p l a t e s , located i n the v i c i n i t y of Delwood k n o l l s (Bertrand, 1972; Chase et al_, 1975; Riddihough, 1977; Chase and T i f f i n , 1977 or Tuzo Wilson k n o l l s (Riddihough e_t aj[, 1980) and connects i n onshore and of f s h o r e southeastern Alaska with the Fairweather F a u l t system (Page, 1969; P l a f k e r et aj_, 1978, von Huene et _aj_, 1978). At the A l e u t i a n Trench, the boundary between the America and P a c i f i c p l a t e s changes t o one of convergence. The t r a n s i t i o n area of southeastern Alaska and adjacent areas of B r i t i s h Columbia consequently has a complex t e c t o n i c framework. In the northeastern Gulf of Ala s k a , west of the Fairweather F a u l t , an area of continen-t a l c r u s t (termed Yukutut m i n i p l a t e by P l a f k e r et aj_, 1978) i s probably coupled t o the P a c i f i c p l a t e (Naugler and Wageman, 1973; Taylor and 0 " N e i l , 1974). Because of i t s low d e n s i t y the m i n i p l a t e 43 es* tto# as* go* F i g . 9 Magnetic l i n e a t i o n s and major t e c t o n i c f e a t u r e s of the G u l f of A l a s k a . L i n e a t i o n s shown by heavy s o l i d l i n e s , f r a c t u r e zones by double l i n e s and A l e u t i a n trench by hachured l i n e . Reproduced from Naugler and Wageman (1973, F i g . 1 ) . 44 cannot be s i g n i f i c a n t l y consumed at the A l e u t i a n Trench and the region i s t h e r e f o r e the s i t e of co n s i d e r a b l e deformation, c r u s t a l t h i c k e n i n g , and high s e i s m i c i t y as a r e s u l t of oblique convergence ( P l a f k e r et al_, 1978; Milne e t a U 1978; von Huene et al_, 1978). Published seismic r e f l e c t i o n p r o f i l e s over the Queen Ch a r l o t t e f a u l t zone (Chase and T i f f i n , 1972; Chase et aj_, 1975; S r i v a s t a v a et a2, 1971; S r i v a s t a v a , 1973; von Huene et a]_, 1978) show intense f a u l t i n g and development of compressive f o l d s i n P l i o -P l e i s t o c e n e sediment on the c o n t i n e n t a l slope. Although transform movement may have occurred on s e v e r a l p a r a l l e l or s u b p a r a l l e l s t e e p l y dipping f a u l t s , a recent microearthquake study o f f southern Queen C h a r l o t t e Islands (Hyndman, 1981) i n d i c a t e s f a u l t a c t i v i t y i s c u r r e n t l y beneath the landward of two steep f a u l t scarps on the c o n t i n e n t a l slope. E a r l i e r s t u d i e s suggested that the main f a u l t t r a c e was located at the base of the c o n t i n e n t a l slope (Sutherland Brown, 1968) or along the a x i s of the Queen C h a r l o t t e Trough (Chase and T i f f i n , 1972). The a x i s of a f r e e - a i r g r a v i t y low i s centered over the t e r r a c e that separates the f a u l t scarps (Riddihough, 1981). Linear magnetic anomalies continue t o the base of the c o n t i n e n t a l slope, but cannot be traced beneath i t ( C u r r i e et a l , 1980). F i r s t motion s t u d i e s of shallow earthquakes along the f a u l t zone ( f o r recent summary, see Milne et aj_, 1978) show almost pure s t r i k e - s l i p on a near v e r t i c a l plane. Some element of under-t h r u s t i n g however may be present along the f a u l t zone: ' seismic p r o f i l e s i n d i c a t e a compressional component and model computations of g r a v i t y (Couch, 1969; S r i v a s t a v a , 1973) and seismic r e f r a c t i o n 45 data (Horn et aj_, 1981) suggests shallow d i p (2-6°) of the oceanic c r u s t toward the Queen C h a r l o t t e I s l a n d s . Oblique convergence i s impli e d by the Minster e t a j (1974) a n a l y s i s of instantaneous p l a t e motions which determined an azimuth of d e x t r a l s t r i k e s l i p of N26°W where the topographic expression of the f a u l t i s a c t u a l l y N35°W. Moresby Channel, o f f s e t approximately 50 km from i t s source, Moresby Trough i n C h a r l o t t e Sound ( F i g . 41) i s d i r e c t evidence of d e x t r a l transform movement (Chase ejb a l , 1975). 46 CHAPTER I I I RADIOMETRIC DATES OF QUEEN CHARLOTTE ISLAND VOLCANIC AND PLUTONIC ROCKS INTRODUCTION In t h i s chapter the r e s u l t s of K-Ar and Rb-Sr dating of Mesozoic-Cenozoic v o l c a n i c and p l u t o n i c rocks of the Queen C h a r l o t t e Islands and Queen C h a r l o t t e Basin are reported and an attempt i s made t o r e l a t e timing of major phases of magmatic a c t i v i t y t o changes i n ' P a c i f i c ' p l a t e motion (Chapter I I ) . Dates f o r Masset Tartu f a c i e s v o l c a n i c s c o n s t r a i n timing of major down-to-basin and p o s s i b l e wrench movement on the Sandspit F a u l t . A r e v i s e d model f o r block r o t a t i o n of Graham Island (Hicken and I r v i n g , 1977), based on new age data, i s described. Of the 53 dates discu s s e d , 8 K-Ar dates from mineral separates have been published p r e v i o u s l y : one b i o t i t e date of the Masset v o l c a n i c s (Mathews, 1963; 1964) and seven b i o t i t e - h o r n b l e n d e dates of the syn- and p o s t - t e c t o n i c i n t r u s i o n s (Wanless _et _aj, 1968, p. 19; 1970, p. 11-13; 1972, p. 6-7). A l l of these dates were obtained from samples c o l l e c t e d by A. Sutherland Brown but only the Masset date was a v a i l a b l e when geologic study of the Queen C h a r l o t t e Islands was completed (Sutherland Brown, 1968). In l i g h t of new geochronologic data, some r e v i s i o n i s now required i n Sutherland Brown's proposed ages of the major v o l c a n i c and p l u t o n i c episodes. Three dates of a l a t e Masset p l u t o n i c phase, obtained r e c e n t l y by Richards et _al_, 1979 ( f o r d e t a i l s and d i s c u s s i o n see Champigny and 47 F i g . 10 Sample l o c a t i o n s f o r K-Ar age determinations. 48 S i n c l a i r , i n press) and two dates of 1 s y n t e c t o n i c ' g r a n i t i c c l a s t s of the Longarm Formation (Yorath and Chase, 1981) none of which are t a b u l a t e d here, are discussed elsewhere i n the t h e s i s . Twenty-five K-Ar whole rock and mineral dates and three Rb-Sr mineral dates were released t o the author by S h e l l Canada Resources L t d . These dates were obtained from samples c o l l e c t e d by S h e l l during r e g i o n a l mappping of the Queen C h a r l o t t e Islands i n 1962-63 and from c u t t i n g s and sidewal1-core samples of e x p l o r a t o r y w e l l s d r i l l e d i n the Queen C h a r l o t t e Basin i n 1968-69. In a d d i t i o n , 9 K-Ar whole rock and mineral dates from samples c o l l e c t e d on Graham Island (Steen, 1967) were released by Amoco Canada L t d . (formerly Pan American Petroleum Corp.). Three K-Ar whole rock dates were obtained from samples c o l l e c t e d by the author on Graham Island i n 1976. DATA ANALYSIS, PRECISION, AND INTERPRETATION Samples dated f o r S h e l l , Amoco, and the author were analyzed r e s p e c t i v e l y at S h e l l Development Corporation (Houston, Texas), Geochron Lab o r a t o r i e s of Krueger E n t e r p r i s e s L t d . (Cambridge, Mass.) and the Geochronometry Laboratory, Univ. of B r i t i s h Columbia. (The l a b o r a t o r y techniques used i n d a t i n g by Wanless et al_ at the Geological Survey of Canada are described i n Wanless et aY, 1965, p. 1-7). Potassium content was measured by s t a b l e isotope d i l u t i o n ( S h e l l ) , flame photometry (Geochron) and atomic absorption (U.B.C.). Argon analyses were by s t a b l e isotope d i l u t i o n and conventional mass spectrometric techniques at a l l of 49 the above l a b o r a t o r i e s . Standardized a n a l y t i c a l data are presented i n Tables I I I and IV and sample l o c a t i o n s on Queen C h a r l o t t e Islands are shown i n F i g . 10. Well l o c a t i o n s and a c r o s s - s e c t i o n are included i n Chapter IV ( F i g s . 16 and 17). Reported e r r o r s are at 1 standard d e v i a t i o n (1 6 ) : e r r o r s i n i t i a l l y reported at 36 by S h e l l (G. Edwards, S h e l l Development Corp., w r i t t e n communication, 1977) and at 26 by Wanless et a l have been adjusted a c c o r d i n g l y . The p r e c i s i o n of K-Ar and Rb-Sr analyses and geologic accuracy of i s o t o p i c determinations vary c o n s i d e r a b l y f o r dates reported here. A r e l i a b i l i t y ranking (see Tables I I I and IV) has been assigned t o each date based on known or assumed a n a l y t i c a l p r e c i s i o n , sample q u a l i t y , concordance of dates, argon r e t e n t i v i t y of minerals and geologic accordance. Age determinations made at Geochron and at U.B.C. are based on r e p l i c a t e analyses and are considered t o be the most r e l i a b l e . Because of g e n e r a l l y poor sample q u a l i t y and large atmospheric argon c o r r e c t i o n s , S h e l l dates are considered l e a s t r e l i a b l e . For fou r of the samples analyzed by S h e l l the radiogenic argon content was i n s u f f i c i e n t (< 10"^ ml/g) f o r q u a n t i t a t i v e measurement and maximum ages only are reported. Dates from w e l l c u t t i n g s , because of small sample s i z e and p o s s i b l e mixed l i t h o l o g i e s r e s u l t i n g from caving should be considered estimates o n l y . Dates obtained from s i d e w a l l core samples should be more r e l i a b l e . The m a j o r i t y of dates reported here are from Cenozoic v o l c a n i c and i n t r u s i v e rocks t h a t , with few exceptions, ^have not been a f f e c t e d by s i g n i f i c a n t post-emplacement deformation or thermal 50 TABLE HI: Summary of K-Ar Age Determinations: Queen Charlotte Islands No. Original Sample No. Ut.N/ Long.W Rock Type Material Analyzed K (wt.J) 100.RadJ°Ar Total* 0 Ar Rad^Arj «°K Age or . Mean Age, (Mails) ' Period/ Epochj Relia-b i l i t y , Oata Source,. Formation (Sutherland Brown 1968) Locality/Feature 1 S0-546-N63 54°04.73' 131°47.65' olivine basalt WR 0.48 * <.0003 <5 U. Miocene or younger 5 b Tow H i l l S i l l s Tow H i l l , N shore Graham Is. 2 GS-29-66 54°Q4.4' 131°47.7 olivine basalt WR 0.373, 0.355 56.0,49.9 .00208 35.2*2 L. OUgocene 5 c 3 GS-54-66 54°p4.17' 132°14.25' basalt WR 0.492, 0.453 33.7,33.8 .00169 28.7±2 U. OHgocens 2 c Masset, Tartu Fades Basalt Member South Westacott Pt., W.snore, mouth of Mas set Sound, Graham Is. 4 GS-51-66 54°09.33' 132°39.0' basalt WR 1.97, 1.91 62.1,47.5 .00140 23.8±1.3 L. Miocene 2 c Shag Rock off Klash-wun Pt., N.W.shore Graham Is. 5 SD-256-N63 54003.53, 132°14.33' basalt porphyry WR 0.95 7 .0007 11*3 M. Miocene 3 b Head of Masset Sound, N shore of Graham Is. 6 MR 8 53°24.6" 131°55.5' basalt WR 0.809 0.789 59.6 .001164 19.8t0.7 L. Miocene 2 g Between Lawnpolnt i Lawnhill beach expo-sures shore Graham Is 7 SD-278-N63 S3°25.07' 131°54.78 olivine basalt porphyry WR 0.62 3 .0021 36*3 I. OUgocene 5 b 8 SD-544-N63 53°31.68' 132°21.2' rhyollte ash flow tuff WR 1.44 12 .0012 20*2 L. Miocene 2 b Masset, Tartu Fades Rhyollte Member Mamin River, 10 km SSM of Juskatla, cent-ral Graham Is. 9 MR 9 53030.6' 132D20.0' basalt WR 0.961 0.966 54.8 .001396 23.910.8 L. Miocene 2 9 East of Marmln R., 11 km S of Juskatla, central Graham Is. 10 MR 2 53o30.6' 132°20.0' rhyollte ash flow tuff WR 2.96, 2.93 S5.0 .001404 23.7*0.8 L. Miocene 2 9 11 GS-39-66 53042.08' 132°59.17' basalt WR 0.201 0.180 5.4,12.1 .00125 21.2*4 L. Miocene 2 c Masset, Tartu Fades Mixed Member Solide Is., Port Louis, W. coast Graham Is. 12 AK 378 53°24.2' 132°23.r biotite fleldspar porphyry Bt 2.56 47 .0037 62*3 L. Paleoceni 2 a Masset, Hypabyssa Equivalent 9 km E of Rennel Sound SW Graham Is. 13 GS-49-66 54010.17' 132°58.0' rhyollte ash flox tuff WR 2.00, 1.53 28.1,32.9 .00264 44.6*3 M. Eocene 2 c Fir s t point E of Bruin Bay, Parry Passage on NW t i p of Graham Is. 14 SD-250-N63 53°p6.8' 131°38.23 andesite WR 1.46 20 .'0010 17*1 L. Miocene 3 b • Masset (undivided! Islet E of Point Grey, N.E. Moresby E of Sandspit Fault 15 SO-252-N63 52°41.4' 131°23.27 basalt porphyry WR 0.75 17 .0012 20*1 L. Miocene 2 b Masset, Dana Fades (undivided Skaga Is. E of Lyell Is. 16 SD-253-N63 52°40.4' 131°24.75 basalt porphyry WR 0.92 16 .0016 27*1 U. Oligoceni 2 b Tar Is., E of Lyell Is. 17 G5-50-66 132°58.83 diorite Nb 2.94, 2.83 21.7,20.6 .00136 23.1*1.4 L. Miocene 4 c Post Tectonic (younger) Plutons Langara Pluton: Mc-pherson Point on NW shore Langara Is. 18 SD-257-63 54°Q0.63' 133°00.57 auglte diorite WR 0.44 * <.0003 <5 U. Miocene or younger 5 b Pivot Mountain Plut-o n ^ km SE of Beres-ford Bay, SW Graham Is. 19a GSC 67-16 53°17' 132°26* granodiorite Hb 0.44 6 .0015 26*3 U. OUgocene 1 e Kano Batholith (East phase); S head of Shield Bay, SW Grah-am Island 19b GSC 67-17 Bt 7.52 69 .0017 29*1 U. OUgocene 1 e 20 GSC 70-2 53°13' 132°29' granadioH te Bt 7.08 54 .0018 30*2 U. OUgocene 2,4 f Kano Batholith (Cent-ral phase), head of Dawson Inlet SW Grah-am Island 51 TABLE I I I (cont'd) No. O r i g i n a l Sample No. Lat.N/ long.W Rock Type Ma t e r i a Analyse K (wt ») 100.Rad*°Ar Rad*°Ar Age o r Mean Age, (Ma i l 6 ) ' P e r i o d / Epochj R e l i a -b i l i t y Data Source^ Formation' (Sutherland Brown 1968) L o c a l i t y / F e a t u r e T o t a l , u A r * U k 21 SO-262-63 5^ 036.35• 131 054.43' g r a n i t e Or 10.33 50 .0013 25i2 U. OUgocene 5 b Post T e c t o n i c (Younger) P l u t o n s Pocket B a t h o l i t h (mar-gi n ) S shore, head o f < Pocket I n l e t , M Mores-by I s l a n d 22 S0-263-63 52 036.91' 131°52.08' b l o t l t e granod1or1t< MR 3.10 46 .0018 30±2 U. OUgocene 4 b Pocket B a t h o l i t h (core) H shore, head o f Pocket I n l e t , W Moresby I s l a n d 23a BSC 67-18 52°34' 131°48" g r a n i t e Hb 1.04 33 .0022 3811 U. Eocene 1 e Pocket B a t h o l i t h (core) S shore o f Barry I n -l e t , W Moresby I s . 23b SSC 67-19 Bt 7.30 71 .0023 39±1 U. Eocene 1 e 24 SD-265-63 52°43.67' 131°35.95' q u a r t z monzonite WR 2.04 15 .0014 24±2 U. OUgocene 3,4 b A t H I n l e t P I u ton, N L y e l l I s . SE shore o f Richardson I n l e t 25 GSC 66-14 52°22' 1316]5' g r a n o d i o r i t e Hb 0.41 49 .0086 142H9 U. J u r a s s i c 4 d * Bumaby I s . P l u t o n ; Poole P t , 8urnaby I s . 26 GSC 70-3 53°19' 131°S8' qu a r t z d i o r l t e Hb 0.33 42 .0095 156*10 U. J u r a s s i c 4 f Chfnukundl P l u t o n ; CMnukundl Creek, SE Graham I s . 27 GS-33-66 53°32.17' 132°S7.17' qu a r t z d i o r l t e Hb 0.315, 0.299 57.1, 78.1 .00721 119.416 L. Cretaceous 3.4 c S y n t e c t o n i c ( o l d e r ) Plutons NW p o i n t o f Hlppa I s . , o f f SW coas t o f Grah-am I s . 28 GSC 67-20 S2°54.5' 131°40" q u a r t z d i o r l t e Hb 0.43 54 .0086 14217 U. J u r a s s i c 2,4 e E contact o f San C h r i s t oval B a t h o l i t h ; E Oar-win Sound, western Moresby I s . 29 GSC 70-1 53°17-5' 132°38.5' g n e l s s l c g r a n o d i o r i t e Hb 0.40 47 .0087 14314 U. J u r a s s i c 2.4 f S y n t e c t o n i c p a r t o f Kano B a t h o l i t h (west) SE t i p o f Cadman I s , Kano Bay, SW Graham I s 30 GS 31-66 S3°14.87 132°37.0 b l o t l t e q u a r t z d i o r l t e WR 1.73, 1.67 57.4, 21.5 .00193 32.712.4 L. OUgocene 5 . c Sy n t e c t o n l c p a r t of Kano B a t h o l i t h (west) S shore, mouth o f Van I n l e t , SW Graham I s . 31 SD-261-63 52°40.53 131°59.70 b l o t l t e q u a r t z d i o r l t e Bt 3.80 40 .0019 32l2 L. OUgocene 5 b Near c o n t a c t w i t h Masset, 4 km SE of Mt De l a Touche, W coast Moresby I s . 32 GS 43-6 • 53 u56.93 133°09.58 b a s a l t WR 0.515, 0.539 65.5, 45.1 .0106 173120 M. J u r a s s i c 4 c Karmutsen E s i d e o f F r e d e r i c k Is.west coast Graham I s . ^ M a t e r i a l s analysed: WR » whole r o c k , Bt » blotlte, Hb • hornblende. Or • o r t h o c l a s e 2 C o n s t a n t s used: X £ . 0.585 x I O " 1 0 y r " ' , * 8 -4.72 x 10"'° y r " ' . *°K, ^Bounds o f g e o l o g i c epoch or p e r i o d from time s c a l e s o f Ryan e t al,1974 (Neogene); Berggren e t al.1978 (Paleogene); Van H1nte,1976b (Cretaceous) and Van H1nte,1976a ( J u r a s s i c ) . Reliability ranking o f age measurement based on sample q u a l i t y (degree of a l t e r a t i o n ) , concordance of ages, a n a l y t i c a l p r e c i s i o n , argon r e t e n t l v l t y o f o f m i n e r a l s , and g e o l o g i c accordance: 1) concordant ages, r e l i a b l e ; 2) s i n g l e age, probably reliable; 3) s i n g l e age, r e l i a b i l i t y unconfirmed; 4) minimum age; 5) u n r e l i a b l e age. 'sources: a) Mathews (1963, 1964); b) S h e l l Canada Resources L t d . (unpublished d a t a ) , analyses by S h e l l Development Corp. (Houston, Texas); c) Pan-amerlcan Petroleum Corp. (unpublished data noted 1n Ste i n , 1 9 6 7 ) , analyses by Geochron L a b o r a t o r i e s o f Kreuger E n t e r p r i s e s L t d . (Cambridge, Mass.) d) Wanless e t a l (1968); e) Wanless e t a l (1970); g) Wanless e t a l (1972); f ) t h i s s t u d y , a n a l y s t s : K. S c o t t , J Harakal. *Rad. *°Ar not d e t e c t e d ( l i m i t 1 0 " 7 ml/g) Table IV:K-Ar Age Determloatlona: Shell Canada Wella, Queen Charlotte Basin No. Original Sample No. Well name lac N/LonglT Depth (metes) Prom Seaflmr lock "type Type of Sample K (wt.Z) 100Rad.*°Ar Rad.4 0Ar *g«l Period/ Epoch} lelia-, i l i t y i Comae nt 2 Total*°Ar 40K (Mallo) 33a 4136D Shell Anglo Osprey D-36 S l ^ . l O ' 129°20.79' 2098.9-2101.9 basalt porphyry well cuttings 0.77 3 £017 2914 U. OUgocene 4 Near middle of 303 m. section of basalt and pyroclastlca. Overlain by L.-M. Miocene sediments 33b 4136H " 2411.3 baaalt porphyry sldewdl core 1.77 0 C00013 <10 U Miocene or younger 5 Top of 40(+)m section of basalt, overlain by 55 a of sandstone, minor siltstone, coal 33c 41351 2437.8 baaalt porphyry well cuttings 0.62 15 .0034 5717 Paleocene 4 Base o f above section 34 4143 Shell Anglo Harlequin M6 51°55.06' 129°58.21' 3056.8-3059.9 baaalt porphyry well cuttings 0.94 12 .0025 42t? 11 Eocene 4 Near base of 95(*) • aection of mixed volcanic pyroclastlcs and flows. Over-la i n by L. Miocene sediments 35 413SJ Shell Anglo Auklet G-41 52"20.27" 130o36.55' ' 2166.2 basalt porphyry sldewall core 0.61 5 .0021 3624 L.OUgocene 4 Base of 206(+) • section of basalt and pyroclastlcs. Overlain b/Mlocene(?) . sediments 36 5003* Shell Anglo Murrelet L-15 52024.69• 130°47.63' 2686.8-2708.1 basalt well cuttings 0.32 0.7 <D023 <39 U. OUgocene or younger 5 Near middle of 65(+)m section of basalt with shale lnterbeds. Overlain by Mlocene(?) sediments 37a 413SH Shell Anglo Sockeye B-10 52°49.14' 131<,00.74" 4433.9 basalt porphyry (altered sldewall core 0.36 4 . 11043 7216 U. Cretaceous 3 Top of 99m section of basalt and shale sandstone lnterbeds. Overlain by Mlo-cene(l). or older sediments 37b 4135F 4447.6-44S3.7 basalt well cuttings 0.44 16 j0050 84110 U. Cretaceous 3 =B  38a 413SB2 Shell Anglo Sockeye E-66 52045.41• 130°55.321 2573.4-2576.5 basalt porphyry well cuttings 0.66 19 0071 11817 L. Cretaceoua 4 Near middle of 165(+)a section of basalts, pyroclastlcs. Overlain by lS2n of U. Cret.-Paleocene sediments 38b 413SE 2612.1 basalt (Uteres: sldewall core 1.20 30 J0101 16518 M,Jurassic 3 •• 39a 4124A1 Shell Anglo tyee N-39 53018.91• 131°20.36' 3381.5 mlcro-gabbro well cuttings 0.40 26 J0271 >14tl0 U. Silurian 5 Near base of 97(+)m of predominately Plutonic rock. Overlain by 406a aection of 0. Cret. sediments 39b 4124A2 " •• ,. well cuttings 0.37 20 ,0163 160110 L. Permian 4 •• 39c 4124A3 •• granite? well cuttings 1.81 18 .0105 ^71*9 11 Jurassic 4 39d 4124B 3381.5-3384.5 olcro-gabbro well cuttings 0.325 24 .0167 265111 L. Permian 4 39e 4124C •• 3393.6-3396.7 mlcro-gabbro well cuttings 0.33 15 .0124 201111 11 Triassic 4 •• 39f 4124E 3402.8 micro-gabbro well cuttings 0.44 24 J0153 24517 U. Permian 4 - t* Note: A l l analyses are whole rock. Constants: X e - 0.585 x 10~ 1 0yr~ x, AB- 4.72 x 10~ 1 0yr . K/K - 1.19 x 10"* atom ratio Sample descriptions: B.E.P. Scarborough (Shell Canada Ltd.), Age laboratory: Shell Development Corp., Houston, Texas 1eee footnotes. Table HI,. e l l llthologles and tentative ages for U. Tertiary sediments from Shell Canada Well History Repts. 53 events. S i n g l e K-Ar dates are minimum dates as they may r e l a t e t o t e c t o n i c u p l i f t and c o o l i n g through a c r i t i c a l isotherm f o r Ar r e t e n t i o n ( f o r recent summary, see Harrison et 1978) rat h e r than to time of emplacement and i n i t i a l c r y s t a l l i z a t i o n . Concordant m i n e r a l - p a i r dates-| are i n t e r p r e t e d as evidence of emplacement age (Lanphere and Reed, 1973). Rapid c o o l i n g f o l l o w i n g emplacement of shallow post t e c t o n i c plutons suggests concordant dates provide a c l o s e lower l i m i t t o the time of c r y s t a l l i z a t i o n . Discordant dates i n d i c a t e t h a t the pluton e i t h e r has been reheated or has undergone a long c o o l i n g h i s t o r y and t h a t i t s age i s ol d e r than e i t h e r mineral date. DISCUSSION OF AGE DATA Dates f o r samples from the Queen C h a r l o t t e Islands and Queen C h a r l o t t e Basin are described here. Two K-Ar whole rock dates ( l o c a l i t i e s 1, 2) f o r the P l i o c e n e - P l e i s t o c e n e b a s a l t i c Tow H i l l S i l l s (Sutherland Brown, 1968, p. 127-128) are indeterminate because of t h e i r young age and w i l l not be discussed. Some u n c e r t a i n t y , due to considerable s c a t t e r of K-Ar whole rock dates (see Table IV) e x i s t s i n the age of basement i n t r u s i v e rocks encountered i n the S h e l l Anglo Tyee N-39 w e l l . A mean age of 228 Ma (Lower T r i a s s i c ) from 5 of the 6 samples dated suggests the basement rocks may be equi v a l e n t t o the Upper Paleozoic-Lower Mesozoic arc plutons of the 1 Concordance of mineral p a i r s was determined by using the c r i t i c a l value F t e s t (Dalrymple and Lanphere, 1969, p. 120) at the 95 percent confidence l e v e l . 54 Alexander terrane of southeastern Alaska (Churkin and E b e r l e i n , 1977). Karmutsen Formation: A s i n g l e K-Ar whole rock date of 173 Ma (Middle J u r a s s i c ) was obtained f o r a massive b a s a l t i c lava on Frederick Island (west coast of Graham I s l a n d , l o c a l i t y 32), mapped by Sutherland Brown (1968) as Ladinian-Karnian Karmutsen Formation. A number of p o s s i b l e e f f e c t s might e x p l a i n the anomalously young age: 1) the date r e f l e c t s the age of c a t a c l a s t i c deformation along a small wrench f a u l t t h a t cuts Frederick I s l a n d ; 2) the date r e f l e c t s low-grade metamorphism or heating by nearby Masset d i k e s ( p a r t i a l l o s s of radiogenic argon); 3) the date r e f l e c t s Middle J u r a s s i c Yakoun (predominately a n d e s i t i c ) volcanism. Syntectonic (Old) I n t r u s i o n s : Three K-Ar hornblende dates have been obtained f o r s y n t e c t o n i c i n t r u s i o n s . The two l a r g e s t bodies, the San C h r i s t o v a l and West Kano B a t h o l i t h s ( l o c a l i t i e s 28, 29), have y i e l d e d Upper J u r a s s i c dates of 142 and 143 Ma r e s p e t i v e l y . Both these dates are i n accordance with f i e l d r e l a t i o n s (see Table V) but may i n d i c a t e time of u p l i f t and unroofing r a t h e r than emplacement. A somewhat younger date of 119.4 Ma (Lower Cretaceous) was obtained f o r a small i n t r u s i v e body on Hippa Island ( l o c a l i t y 27). The three K-Ar dates f a l l w i t h i n the range Upper Ju r a s s i c - M i d d l e Cretaceous, suggested by Sutherland/Brown (1968, p. 133) from f i e l d r e l a t i o n s as the age of the s y n t e c t o n i c p lutons, but i n d i c a t e that the major deformation associated w i t h emplacement i s probably of Upper J u r a s s i c age. The i n t r u s i o n s may be comagmatic 55 with Yakoun volcanism ( i f dates r e f l e c t u p l i f t ) or s l i g h t l y post-date volcanism. The Island I n t r u s i o n s on Vancouver I s l a n d , which have y i e l d e d K-Ar dates ranging from 181-142 Ma and a Rb-Sr isochron date of 174 Ma ( M u l l e r , 1977) appear t o be somewhat o l d e r . Masset Formation: The Late Cretaceous?-Tertiary Masset v o l c a n i c succession underlying much of Graham Island and p o r t i o n s of northern Moresby Island comprises t h i n flows of columnar b a s a l t , b a s a l t i c b r e c c i a s , sodic r h y o l i t i c ash flow t u f f s and welded t u f f b r e c c i a s and b r e c c i a s of mixed b a s a l t and r h y o l i t e c l a s t s (see summary, Chapter I ) . Four-teen K-Ar dates (13 whole rock, 1 b i o t i t e mineral separate) f o r the Masset Tartu and Dana f a c i e s (Table I I I ) suggest t h a t much of the v o l c a n i c p i l e exposed on Queen C h a r l o t t e Islands was erupted i n Lower Miocene time. Sutherland Brown (1968, p. 118) considered volcanism t o have spanned Paleocene-Eocene time based on a s i n g l e K-Ar date of 62 Ma from the base of the Tartu f a c i e s , evidence f o r "extensive f a u l t i n g , t i l t i n g , and e r o s i o n p r i o r t o d e p o s i t i o n of Mio-Pliocene Skonun sediments", and occurrence of widespread volcan-ism i n s o u t h - c e n t r a l B r i t i s h Columbia during Eocene time. Ten K-Ar whole rock dates have been obtained from b a s a l t and r h y o l i t e t u f f s of the b a s a l t and r h y o l i t e members of the Tartu f a c i e s . Most of the samples dated (Table I I I ) were c o l l e c t e d from the north shore of Graham Island and near Mamin R i v e r , south of Masset I n l e t ( F i g . 10). Excluding samples no. 5 and 7 (poor sample q u a l i t y ) K-Ar dates range from 17 t o 28.7, w i t h a mean age of 22.3 Ma (Lower Miocene). Though dates f o r the b a s a l t member should be somewhat younger than those f o r the underlying r h y o l i t e member, no 56 systematic age d i f f e r e n c e i s seen. More d e t a i l e d mapping and geochronologic study w i l l be required to r e s o l v e t h i s apparent discrepancy. Tartu f a c i e s ( b a s a l t and r h y o l i t e members) of south-western Graham Island appear t o be somewhat o l d e r than the bulk of Masset v o l c a n i c s exposed f u r t h e r north. These v o l c a n i c s have been metamorphosed by i n t r u s i o n of the c e n t r a l phase of the Kano B a t h o l i t h , dated at 30 Ma (Lower Oligocene). Dates of 62 and 44.6 Ma from i n t r u s i v e hypabbyssal phases on Graham Island ( l o c a l i t i e s 12, 13) are probable minimum ages f o r s t a r t of Tartu f a c i e s volcanism. The i n t r u s i v e phases are thought t o represent c o n s o l i d a t e d vents or upper magma chambers r e l a t e d t o e a r l y Masset flows (Sutherland Brown 1968). Sample no. 12 was obtained from a t a b u l a r b i o t i t i c f e l d s p a r porphyry body, presum-a b l y a s i l l , near the base of the Tartu mixed member on southcentral Graham I s l a n d . Sample no. 13 was obtained from a v o l c a n i c t u f f a s s o c i a t e d w i t h the Cape Knox-Parry Passage b i o t i t i c f e l d s p a r porphyry of northwestern Graham I s l a n d . As suggested by these dates and rock d i s t r i b u t i o n of Tartu members, volcanism may have s h i f t e d northwestward with time, s i m i l a r to a p o s s i b l e progression of Cenozoic Post-Tectonic I n t r u s i o n s . Two K-Ar dates of 20 and 27 Ma have been obtained f o r b a s a l t samples of the Dana f a c i e s from the Tar I s l a n d s , east of L y e l l Island ( l o c a l i t i e s no. 15, 16). Though much of the area u n d e r l a i n by Dana f a c i e s rocks has been metamorphosed by young p o s t - t e c t o n i c i n t r u s i o n s , the sample d e s c r i p t i o n s of the dated samples i n d i c a t e l i t t l e a l t e r a t i o n . Dyke-like connections t o b a s a l t 57 flows on Skaga Island ( l o c a l i t y 15) are thought t o represent a vent f o r Dana v o l c a n i c s (Sutherland Brown, 1968, p. 117). I t appears l i k e l y t h a t the date f o r t h i s l o c a l i t y (20 Ma) must represent a l a t e r e x t r u s i v e phase s i n c e the flow on the a d j o i n i n g Tar I s l a n d , dated at 27 Ma o r i g i n a t e d from the same source. Both dates f a l l w i t h i n the range of dates obtained f o r Tartu b a s a l t s and r h y o l i t i c t u f f s , suggesting synchroneity of these l a t e phase er u p t i o n s . No dates are p r e s e n t l y a v a i l a b l e f o r the Masset Kootenay f a c i e s of western Moresby I s l a n d . A 'Masset-like' dyke i n the Upper J u r a s s i c San C h r i s t o v a l B a t h o l i t h ( l o c a l i t y 30) has y i e l d e d a K-Ar b i o t i t e date of 32.7 Ma and a Rb-Sr b i o t i t e date of 42 Ma. Though located 20 km northwest of the main exposure of Kootenay f a c i e s v o l c a n i c s , the dyke i n t r u s i o n may be r e l a t e d t o an e a r l y Kootenay phase e r u p t i o n . Five K-Ar whole rock dates o f basement' Masset b a s a l t s from C h a r l o t t e subbasin w e l l s (Osprey D-36, Harlequin D-87, Auklet G-41, Murrelet L-15) range from 57 Ma (Lower Paleocene) t o 29 Ma (Upper Oligocene). Two d i s t i n c t v o l c a n i c episodes (57 Ma and 29 Ma) are recorded i n the Osprey D-36 w e l l . Volcanism i n the C h a r l o t t e subbasin must have preceeded late-phase Tartu and Dana eruptions on Queen C h a r l o t t e I s l a n d s . Four K-Ar whole rock dates f o r the Sockeye B-10 and Sockeye E-66 w e l l s record l a t e Mesozoic eruptions i n the Hecate subbasin. Dates of 72 and 84 Ma on b a s a l t i n Sockeye B-10 are i n c l o s e agreement with an Upper Cretaceous p a l y n o l o g i c a l age ( S h e l l Canada L t d . , 1968b) f o r sediments interbedded and underlying the v o l c a n i c s . Volcanism recorded at Sockeye B-10 may be an e a r l y phase 58 Masset e r u p t i o n . Dates of 118 Ma and 165 Ma from flows and pyro-c l a s t i c s underlying Upper Cretaceous sediments i n the Sockeye E-66 w e l l , l i k e l y record Middle J u r a s s i c Yakoun volcanism. Post-Tectonic I n t r u s i o n s Sutherland Brown (1968) mapped some twelve ' h i g h - l e v e l ' plutons on the Queen C h a r l o t t e Islands that range i n composition from d i o r i t e t o g r a n i t e (average composition, g r a n o d i o r i t e ) . In a d d i t i o n t o two large bodies, the Kano and Pocket B a t h o l i t h s , he recognized f i v e major groups of smaller plutons based on c l o s e s p a t i a l r e l a t i o n s h i p s and s i m i l a r m i n e r o l o g i c a l and t e x t u r a l charac-t e r i s t i c s . Local c r o s s - c u t t i n g r e l a t i o n s h i p s suggested t h a t the young i n t r u s t i o n s ranged from E a r l y Cretaceous t o Upper Eocene i n age. Twelve K-Ar dates, i n c l u d i n g two concordant b i o t i t e -hornblende p a i r s , and 3 Rb-Sr mineral p a i r dates are a v a i l a b l e f o r seven of Sutherland Brown's p o s t - t e c t o n i c plutons (Table I I I ) . The r e l i a b i l i t y of i n d i v i d u a l dates and c r i t i c a l s t r a t i g r a p h i c r e l a t i o n s f o r each sample l o c a l i t y i s included i n Table V. The apparent age of the Kano and Pocket B a t h o l i t h s i s now reasonably w e l l estab-l i s h e d . The apparent minimum age f o r emplacement of the c e n t r a l phase of the Kano B a t h o l i t h i s 30 Ma, based on a s i n g l e K-Ar b i o t i t e date ( l o c a l i t y 20). This date i s not s i g n i f i c a n t l y d i f f e r e n t at 95% confidence from concordant mineral p a i r dates of 26 and 29 Ma from the eastern phase of the b a t h o l i t h ( l o c a l i t y 19), though as noted i n Table V the c e n t r a l phase i s judged t o be somewhat o l d e r on geologic grounds. A hybrid K-Ar age of 33 Ma from the o l d e r s y n t e c t o n i c part of the b a t h o l i t h ( l o c a l i t y 30) dates a thermal event r e l a t e d t o TABLE V . CRITICAL STRATI6RAPHIC RELATIOHS FOR 5TH AND POST-TECTONIC INTRUSIONS SAMPLE NO. DATE(S) RELIABILITY CRITICAL STRATIGRAPHIC RELATIOHS. COKNTS 17 23.111.4 (K-Ar, lib) 4 LANGARA PLUTON (LANGARA GROUP). INTRUOES AND METAMORPHOSES TURONIAN HONNA FORMATION. 18 <5 (K-Ar, WR) S PIVOT MOUNTAIN PLUTON (LANGARA GROUP). INTRUDES HASSET TARTU FACIES RIIYOLITES. 19 26tj K-Ar. lib) 29T1 K-Ar, Bt) 1 KANO BATHOLITH (EASTERN PHASE). INTRUDES AHD HETAKORPHOSES CENTRAL PHASE OF BATHOLITH) INTRUDES VALANGIAN-BARREHIAN LONGARM FORMATION; HETAHORPIIOSES MASSET TARTU FACIES MIXED MEMBER BASALTS AND RIIYOLITES. 20 3012 (K-Ar, Bt) 2. 4 KANO BATHOLITH (CENTRAL PHASE). INTRUDES AND HETAHORPIIOSES HASSET TARTU FACIES RIIYOLITES. THIS PHASE JUDGED TO BE SLIGHTLY OLDER THAN EASTERN PHASE BECAUSE OF COARSER 6RAIN SIZE AND MORE ACIDIC CHARACTER (SUTHERLAND BROWN, 1968, p. 139). 30 32.712.4 (K-Ar, WR) S SYNTECTONIC PART OF KANO BATHOLITH (WESTERN PHASE) OF UPPER JURASSIC AGE. RESET AGE DATES THERMAL EVENT RELATED TO INTRUSION OF CENTRAL PHASE. 21 25*2 (K-Ar, Or) 49f2 (Rb-Sr. Or) S POCKET BATHOLITH (MARGINAL ZONE). INTRUDES AHPIIIB0LI2E0 PRE UPPER KARNIAN KARMUTSEN BASALTS OF ITS META-MORPHIC AUREOLE. 22 30»2 (K-Ar. MR) 39*1 (Rb-Sr. Bt) 4 POCKET BATHOLITH (CORE ZONE). INTRUDES UPPER JURASSIC SAN CIIRISTOVAl BATHOLITH AND AHPIIIBOLIZED KARMUTSEN BASALTS 23 3811 (K-Ar. Hb) 39jl (K-Ar, Bt) 1 POCKET BATHOLITH (CORE ZONE). GEOLOGIC RELATIONS SAME AS HO. 22. 24 2412 (K-Ar, WR) 3. 4 ATLI INLET PLUTON (LOUISE GROUP). INTRUDES HASSET DANA FACIES MIXED CLAST VOLCANIC BRECCIAS (K-Ar 21 AND 27 Ma). 25 142M9 (K-Ar. lib) 4 BURHABV ISLAND PLUTON (SOUTHERN GROUP). INTRUDES VALANOIAN-BARREHIAH LONGARM FORMATION. 26 15615 (K-Ar, lib) 4 CHINUKUNDL PLUTON (SANDSPIT GROUP). PLUTON CORED IN ROYAL1IE QUEEN CHARLOTTE NO. 1 WELL AFTER ALBIAN HAIDA AND BAJOCIAN-CAUOVIAN YAKOUN FORMATIONS. UNCERTAIN WHETHER CONTACT WITH YAKOUN METAMORPHIC OR UN-CONFORMABLE. IN UKHETANORPIOSEO CONTACT WITH MIO-PLIOCENE SKONUN FORMATION ALONG SANDSPIT FAULT. SYNTECTONI 27 : PLUTONS: 119.416 (K-Ar, lib) 3. 4 H1PPA ISLAND INTRUSION. OVERLAIN BY HASSET TARTU FACIES MIXED CLAST VOLCANIC BRECCIAS. 28 14217 (K-Ar. lib) 2, 4 SAN CHRISTOVAl BATHOLITH (EASTERN MARGIN). INTRUDES PRE UPPER KARNIAN KARMUTSEN FORMATION AND LOWER PART (KARNIAN-NORIAN) KUNGA FORMATION. 29 I43M (K-Ar. Kb) 2. 4 KANO BATHOLITH (WESTERN PHASE). INTRUDES KARNIAN-SIHEMURIAN KUNGA FORMATION, UNDERLIES HASSET TARTU FACIES MIXED CLAST VOLCANIC BRECCIAS. 60 i n t r u s i o n of the c e n t r a l phase. The minimum age f o r emplacement of the core zone of the Pocket B a t h o l i t h i s 39 Ma based on a concordant K-Ar mineral p a i r ( l o c a l i t y 23) and a Rb-Sr b i o t i t e mineral date ( l o c a l i t y 22). Due to discordance of K-Ar (25 Ma) and Rb-Sr (49 Ma) ort h o c l a s e dates f o r the western phase of the b a t h o l i t h i t i s unce r t a i n whether the marginal zone i s younger or o l d e r than the core. Since o r t h o c l a s e i s not h i g h l y argon r e t e n t i v e the K-Ar date i s probably too young. A l t e r n a t i v e l y , slow c o o l i n g through c r i t i c a l isotherms f o r Ar and Sr r e t e n t i o n or Ar l o s s through reheating may e x p l a i n the discordance. The Langara pluton group of northwestern Graham and Langara Islands i s apparently somewhat younger than both the Kano and Pocket B a t h o l i t h s , though l i k e the Kano B a t h o l i t h , i t s i n t r u s i o n postdates Masset Tartu f a c i e s volcanism. The minimum age f o r emplacement of the Langara Pluton of t h i s group i s 23 Ma based on a s i n g l e K-Ar hornblende date. An apparent age of l e s s than 5 Ma was obtained f o r the P i v o t Mountain Pluton of t h i s same group which intrudes Tartu r h y o l i t e s . The apparent ages of the Langara plutons and Kano and Pocket b a t h o l i t h s suggest a progression of decreasing age of p o s t - t e c t o n i c plutons northward along the west coast of the Queen C h a r l o t t e Islands. With p o s s i b l e exception of the Louise Group plutons and some of the Sandspit Group pl u t o n s , i n d i c a t e d p o s t - t e c t o n i c i n t r u -sions on the east coast of the Queen C h a r l o t t e Islands may be con s i d e r a b l y o l d e r than those along the west coast. An apparent minimum age of 24 Ma was obtained f o r the A t l i I n l e t Pluton 61 ( l o c a l i t y 24) of the Louise group that intrudes Dana f a c i e s v o l c a n i c s . The l o c a t i o n of the p l u t o n , along the Louscoone wrench f a u l t suggests the p o s s i b i l i t y t h a t the K-Ar date may r e f l e c t movement along t h i s t r end. The Southern Group plutons of northeastern Moresby Island and eastern Burnaby Island intrude sediments of the Valangian-Barremian Longarm Formation and older rocks. A s i n g l e K-Ar hornblende date of 142 Ma was obtained f o r the Burnaby Island Pluton of t h i s group. Though somewhat o l d e r than expected t h i s date, i n a d d i t i o n t o f i e l d r e l a t i o n s , suggests Southern Group plutons may be as o l d as s y n t e c t o n i c i n t r u s i o n s . Southern Group plutons are more b a s i c , mafic r i c h and coarser grained than average p o s t - t e c t o n i c i n t r u s i o n s and f a l l w i t h i n compositional l i m i t s of s y n t e c t o n i c i n t r u s i v e s (Sutherland Brown, 1968, p. 137). In a d d i t i o n , they are d i s t i n c t from most l a t e phase i n t r u s i o n s i n t h a t they are not a l i g n e d along a major t r a n s c u r r e n t f a u l t . These plutons may then be consanguinious with the s y n t e c t o n i c i n t r u s i o n s or represent an a d d i t i o n a l i n t r u s i v e phase d i s t i n c t from syn- and p o s t - t e c t o n i c phases. L i k e the Southern plutons, the Sandspit Group plutons are d i s t i n c t l y more b a s i c than average p o s t - t e c t o n i c plutons. A K-Ar hornblende date of 156 Ma (somewhat questionable due t o poor sample q u a l i t y ) was obtained f o r the Chinukundl Pluton of the Sandspit Group. The pluton i n t r u d e s the A l b i a n Haida Formation and p o s s i b l y the B a j o c i a n - C a l l o v i a n Yakoun Formation. More complete dating of the Sandspit Group i s required as t h e i r alignment along the west s i d e of the Sandspit F a u l t zone suggests emplacement may be 62 e l a t e d t o an e a r l y wrench f a u l t or suture zone along t h i s t r e n d . TIMING OF MOVEMENT ON SANDSPIT AND RENNEL-LOUSCOONE FAULTS Dates of Masset Tartu f a c i e s b a s a l t s c o n s t r a i n timing of major down-to-basin and p o s s i b l e wrench movement on the Sandspit F a u l t . Whole rock K-Ar dates of 19.8 and 17.4 Ma were obtained from samples of b a s a l t flows from l o c a l i t i e s (nos. 6 and 14) east of the f a u l t zone. These flows are probably e q u i v a l e n t s of the Tartu b a s a l t member exposed near Masset I n l e t on Graham I s l a n d , west of the f a u l t zone and dated at 20-28 Ma. Dextral wrench displacement of 80km o f f s e t t i n g l o c a l i t y 14 and the southern contact of the b a s a l t member would be required i n order to e x p l a i n the present d i s t r i b u t i o n of the b a s a l t f l o w s . This c o n c l u s i o n must be considered t e n t a t i v e however, as ages of the f l o w are only roughly e q u i v a l e n t s and the flows may not have o r i g i n a t e d from the same source. Sutherland Brown (1968, p. 116) suggests t h a t d i s t r i b u t i o n of Tartu v o l c a n i c s encountered i n A t l a n t i c R i c h f i e l d e_t a]_ w e l l s f i t s a p r o j e c t i o n of 'member contacts' t o eastern Graham I s l a n d . I f c o r r e c t , l i t t l e , i f any, wrench movement on the Sandspit need be invoked. Major down-to - basin movement, l i k e p o s s i b l e wrench movement, must have been i n i t i a t e d a f t e r e x t r u s i o n of T a r t u volcan-i c s , 17.4 Ma ago ( l a t e Lower Miocene), p r i o r t o d e p o s i t i o n of Middle Miocene-Pliocene Skonun sediments. L a t e r displacements accompanied subsidence of t h i s sediment p i l e . U p l i f t of subaqueous p y r o c l a s t i c s and v o l c a n i c b r e c c i a s of the Dana f a c i e s of northeastern Moresby 63 Island may have been associated with movement on the southern Sandspit F a u l t extension. Dates of Masset Dana f a c i e s c o n s t r a i n timing of l a t e wrench displacement on the Louscoone and Beresford f a u l t s (southern strand of the Rennel'-Louscoone f a u l t system). Sutherland Brown (1968, p. 151) suggests that mid p o i n t s of outcrop b e l t s of the Dana f a c i e s may i n i t i a l l y have been opposed across the f a u l t l i n e s , i n d i c a t i n g combined r i g h t l a t e r a l movement of about 20 kilometers across the two f a u l t s . Two K-Ar whole rock dates of 20+1 and 27+1 Ma, obtained from Dana b a s a l t porphries ( l o c a l i t i e s 15, 16) i n d i c a t e t h a t s i g n i f i c a n t displacement occurred a f t e r d e p o s i t i o n i n Upper Oligocene - Lower Miocene. The Louscoone f a u l t l i k e l y c o n t r o l l e d emplacement of the Post Tectonic A t l i I n l e t Pluton (Louise Group), dated at 24+2 May ( l o c a l i t y 24). S i m i l a r l y , emplacement of the shallow East Kano body (K-Ar hornblende and b i o t i t e dates of 26+3 and 29+1 May, l o c a l i t y 19) was l i k e l y c o n t r o l l e d by the Rennell f a u l t (northern strand of the Rennel'-Louscoone f a u l t system). Although movement on the Rennel -Louscoone system may have l a s t e d from before the Late J u r a s s i c or E a r l y Cretaceous t o the present, a major p o r t i o n of t h i s a c t i v i t y appears t o have occurred i n Neogene time. BLOCK ROTATION-TRANSLATION OF GRAHAM ISLAND Hicken and I r v i n g (1977) report a discordant paleomag-n e t i c pole f o r Masset Tartu f a c i e s b a s a l t s and r h y o l i t e s of c e n t r a l Graham I s l a n d . Assuming an age of Paleocene f o r magnetization of the Tartu f a c i e s they show that the Masset paleopole i s d i s p l a c e d 64 53+28 i n a counter-clockwise sense from the mean Paleocene North American c r a t o n i c pole, i n d i c a t i n g that Graham Island must have ro t a t e d at l e a s t 25° clockwise s i n c e t h i s time. Subsequent d a t i n g of the Tartu f a c i e s , reported i n t h i s chapter, has shown th a t these v o l c a n i c s are as young as Upper Miocene i n age. I t i s noteworthy that e i g h t s i t e s p r o v i d i n g coherent magnetization f o r paleomagnetic determinations were a l l c l o s e t o Masset I n l e t , west of the Sandspit F a u l t . K-Ar whole rock dates f o r three samples c o l l e c t e d south of Masset I n l e t ranged from 20-24 Ma. Using a r e v i s e d age of 20 Ma f o r magnetization of Tartu v o l c a n i c s , M.E. Beck J r . ( w r i t t e n communication, A p r i l , 1978) has c a l c u l a t e d the expected d i r e c t i o n s ( d e c l i n a t i o n - i n c l i n a t i o n ) at Graham Island f o r the mean c r a t o n i c pole. S t a t i s t i c s are summarized i n Table VI. Compared t o these d i r e c t i o n s , Masset d i r e c t i o n s show an eastward d e v i a t i o n i n d e c l i n a t i o n of 28 +30 and steepening i n i n c l i n a t i o n of 7.6+6°. Because of the large standard e r r o r i n d e c l i n a t i o n , one cannot s t a t e at the 95 percent confidence l e v e l t h a t the Masset paleopole i s s i g n i f i c a n t l y d i f f e r e n t from the mean c r a t o n i c pole. However, some clockwise r o t a t i o n may be invoked to e x p l a i n the 'apparent' d i f f e r e n c e i n d e c l i n a t i o n . Such r o t a t i o n would be c o n s i s t e n t w i t h r e s u l t s from a large number of paleomag-n e t i c s t u d i e s i n the C o r d i l l e r a . R o t a t i o n of m i c r o c o n t i n e n t a l blocks between en-echelon r i g h t l a t e r a l s t r i k e s l i p f a u l t s has been proposed by Beck (1976) as a mechanism t o e x p l a i n the common discordance of paleomagnetic d i r e c t i o n s . On Graham I s l a n d , such r o t a t i o n could have occurred between the Queen C h a r l o t t e and 65 Sandspit f a u l t s , i n post-Lower Miocene time. Table VI. Masset and Mean North American D i r e c t i o n s and Poles (T = 20 Ma) Masset d i r e c t i o n s 52 Masset poles 52 N.A. d i r e c t i o n s 10 N.A. po l e s * 10 * from I r v i n g (1977) Means D/I or Lat/Long 026, -78 72N, 97W 358, -70.5 88N, 171°W 81 24 175 a°95 6 11 POSSIBLE RELATIONSHIPS OF CENOZOIC MAGMATISM TO 'PACIFIC PLATE MOTIONS Although more d e t a i l e d mapping, p e t r o l o g i c , chemical, and i s o t o p i c data i s s t i l l r e q u i r e d , i t i s tempting t o speculate on p o s s i b l e r e l a t i o n s h i p s between Cenozoic igneous a c t i v i t y on Queen Ch a r l o t t e Islands and ' P a c i f i c ' p l a t e motions. The c l o s e s p a t i a l and temporal r e l a t i o n s h i p between Masset v o l c a n i c s and Cenozoic epizonal plutons i n d i c a t e s they are coe v a l , but as discussed below, may not be cogenetic. Masset erup-t i o n s , i n i t i a t e d i n Late Cretaceous (?) or Paleocene time culminated i n the Lower Miocene ( F i g . 11) with d e p o s i t i o n of a t h i c k plateau l a v a sequence of a l k a l i b a s a l t and l e s s e r sodic r h y o l i t e . Epizonal plutons on the west coast of Queen C h a r l o t t e Islands were emplaced from Upper Eocene t o Upper Oligocene (or l a t e r ? ) . Magmatic a c t i v i t y appears t o have been e p i s o d i c (though not n e c e s s a r i l y p e r i o d i c ) with p o s s i b l e peaks i n Paleocene, Upper Eocene and Lower Miocene times ( F i g . 11). 66 10r 8 h ( 0 z o < K i l l I -U l Q Ul O < u . O <r u i m S 3 B P 0 S T TECTONIC PLUTONS £3 MASSET VOLCANICS, BASALT RHYOLITE fca OTHER 10 20 30 40 SO 60 Ma B.P. P L M I O C E N E O L I G O C E N E E O C E N E P A L E O C E N E F i g . 11 Histogram of K-Ar and Rb-Sr age determinations of Cenozoic Masset v o l c a n i c s and p o s t - t e c t o n i c p l u t o n s . 67 Masset Volcanics The bimodal s u i t e of Masset s i l i c e o u s s u b a l k a l i n e -t r a n s i t i o n a l b a s a l t flows and sodic r h y o l i t e t u f f s and flows are c h a r a c t e r i s t i c of an e x t e n s i o n a l or anorogenic t e c t o n i c environment as defined by Martin and P i w i n s k i i (1972). Close p r o x i m i t y ( l e s s than 100 km) of Masset v o l c a n i c centers t o Cenozoic paleosubduction zone(s) west of the Queen C h a r l o t t e Islands argues against any l i n k of volcanism t o arc r e l a t e d processes. Arc a c t i v i t y during the Paleogene was l a r g e l y confined t o the eastern margin of the Coast P l u t o n i c Complex (see Ewing, 1980). Several t e c t o n i c models have been postulated that might e x p l a i n the o r i g i n of the Masset v o l c a n i c s . At present there i s i n s u f f i c i e n t evidence f o r proof of any of these models, but the one that has gained the widest acceptance t o date r e l a t e s the occurrence and age of the Masset t o passage of the North American p l a t e over a mantle hot spot. Bevier et a]_ (1979) suggest t h a t Upper Miocene Masset v o l c a n i c s may form the western end of a hot spot t r a c e whose propagation eastward i n Upper Miocene-Quaternary produced a l k a l i c and p e r a l k a l i c eruptions along the Anahim v o l c a n i c b e l t i n c e n t r a l B r i t i s h Columbia ( F i g . 12). The east-west trend and propagation r a t e , (estimated at approximately 2.5 cm/year) i s c o n s i s t e n t with the p r e d i c t e d d i r e c t i o n and r a t e of movement of the North American p l a t e r e l a t i v e t o the M i n i s t e r et a l (1974) g l o b a l hot spot frame of reference. I f the Masset v o l c a n i c s form part of t h i s trend t h e i r l o c a t i o n north of the westward extension of the Anahim b e l t would re q u i r e the Queen C h a r l o t t e Islands t o have been d i s p l a c e d northward 68 F i g . 12 Sketch map o f Neogene v o l c a n i c trends o f southwestern B r i t i s h Columbia (modified a f t e r B e v i e r ejt al_ (1979) and present p l a t e boundaries of B r i t i s h Columbia and Washington coasts ( a f t e r Riddihough, 1977) showing Anahim v o l c a n i c b e l t and Masset v o l c a n i c s ( c i r c l e s ) , Pemberton v o l c a n i c b e l t ( s q u a r e s ) , A l e r t Bay v o l c a n i c b e l t ( s t a r s ) , Garabaldi v o l c a n i c b e l t ( t r i a n g l e s ) and K-Ar dates (Ma B.P.). O u t l i n e d dotted area shows extent of Miocene p l a t e a u l a v a s ; s t i p p l e d area shows t r a c e o f former subducted p l a t e edge. Anahim hot spot t r a c e i n d i c a t e d by dashed l i n e s . PP = P a c i f i c p l a t e , EP = E x p l o r e r p l a t e , JdFP - Juan de Fuca p l a t e , NAP = North America p l a t e , Bp = Brooks P e n i n s u l a , PRfz = Paul Revere f r a c t u r e zone, Sfz = Sovance f r a c t u r e zone. Dotted l i n e (Nfz = Nootka f a u l t zone) i s EP-JdFP boundary. 69 i n post Lower Miocene time. Such movement may have occurred along the Sandspit and Rennel-Louscoone f a u l t s (Chase et a l , 1975, see d i s c u s s i o n i n chapter x). Further analyses of Masset v o l c a n i c s are s t i l l r equired however t o determine i f they are s i m i l a r composition-a l l y and i s o t o p i c a l l y t o the a l k a l i c lavas of the Anahim b e l t . Souther (1977) suggests that the b a s a l t - r h y o l i t e a s s o c i a t i o n may be t h o l e i i t i c r a t h e r than a l k a l i n e t o t r a n s i t i o n a l i n t r e n d . A second model r e l a t e s the o r i g i n of the Masset lavas t o an extension and r i f t i n g event i n Queen C h a r l o t t e Sound ( F i g . 13) that f o l l o w e d passage of the Anahim hot spot but preceeded northward d i s l o c a t i o n of the Queen C h a r l o t t e Islands (Yorath and Chase, 1981). Although the bimodal Masset a s s o c i a t i o n supports an exten-s i o n a l o r i g i n , evidence f o r development of northeast trending " c r u s t a l p e n e t r a t i v e l i s t r i c normal f a u l t s " and r i f t basins (Yorath and Chase, 1981) i s i n s u f f i c i e n t at present. S i m i l a r trending major f a u l t s have not been recognized along the Anahim b e l t trend i n c e n t r a l B r i t i s h Columbia, nor are they preserved i n Masset rocks on Graham I s l a n d . Another model postulated f o r the Anahim b e l t (Stacey, 1974, Souther, 1977) and which may be invoked to e x p l a i n the occurrence of Masset rocks, suggests v o l c a n i c a c t i v i t y i s r e l a t e d t o an "edge e f f e c t " of a subducted ocean p l a t e . The r i s e of magmas from deep mantle f r a c t u r e s at the edge of the subducted p l a t e might r e s u l t through f l e x u r e of the l i t h o s p h e r e , i m b r i c a t i o n of the downgoing s l a b , or mantle p e r t u r b a t i o n s causing anomalous accumu-l a t i o n of melt. Evidence against t h i s model i s the apparent coast p a r a l l e l trend of Masset eruption c e n t e r s , and proximity t o the p l a t e margin which may not allow s u f f i c i e n t descent of the subducted 70 F i g . 13 Sketch map showing proposed Neogene displacement o f the Queen C h a r l o t t e I s l a n d s . P r e - d r i f t and d r i f t p o s i t i o n of western Queen C h a r l o t t e I s l a n d s , Anahim hot spot t r a c e , r i f t basins and t h e i r bounding f a u l t s are shown. From Yorath & Chase (1981 71 p l a t e t o permit magma generation. A f o u r t h model, not considered by previous workers, would r e l a t e Masset eruptions t o major wrench f a u l t s and changes i n oceanic p l a t e motions. The Sandspit and Rennel-Louscoone wrench f a u l t s , which developed as deep c r u s t a l shears i n pre-Cenozoic time might have served as conduits f o r p a r t i a l melting and m o b i l i z a t i o n of Masset f l u i d s . Probable Masset eruption centers are located c l o s e t o major f a u l t trends (Sutherland Brown, 1966). A s i g n i f i c a n t divergent component appears t o have c h a r a c t e r i z e d wrench f a u l t development during much of T e r t i a r y time (see Chapter X ) . Major changes i n ' P a c i f i c ' p l a t e motion (Chapter I I ) that occurred i n Upper Oligocene (ca. 26-30 Ha) and Middle Eocene (ca. 40-45 Ma) time r e s u l t e d i n p l a t e readjustments and reduced convergence r a t e s along the North America p l a t e margin. Increased magmatic a c t i v i t y might be expected t o occur at these times as a r e s u l t of increased t e n s i o n a l s t r e s s . Given an approximate time lag of 5 Ma between changes i n p l a t e motion and surface m a n i f e s t a t i o n on Queen Ch a r l o t t e Islands (Snyder et _al_, 1976) apparent peaks of magmatism show a c l o s e time correspondence t o the Middle Eocene and Upper Oligocene events. More accurate p l a t e r e c o n s t r u c t i o n s f o r the Paleogene of the northeastern P a c i f i c and a d d i t i o n a l r a diometric dates f o r the Masset Formation are required t o t e s t t h i s model. Each of the above models documents some supportive evidence but r e q u i r e s a d d i t i o n a l data and f u r t h e r a n a l y s i s t o t e s t i t s v a l i d i t y . Although the nature and p o s s i b l e a s s o c i a t i o n of each of the ? e p i s o d i c T e r t i a r y Masset eruptions has yet t o be determined, only the f o u r t h model suggests a p o s s i b l e mechanism t o support 72 l o n g - l i v e d , plate-edge v o l c a n i c a c t i v i t y . The hot spot or r i f t models might only be used t o e x p l a i n the f i n a l Lower Miocene v o l c a n i c episode. Post-Tectonic I n t r u s i o n s Although c o e v a l , the mantle o r i g i n of Masset v o l c a n i c s c o n t r a s t s w i t h the shallow c r u s t a l o r i g i n of magmas of T e r t i a r y p o s t - t e c t o n i c plutons and suggests p o s s i b l e fundamental d i f f e r e n c e s i n m e l t i n g and t e c t o n i c r e l a t e d processes. One model, described here (see a l s o Chapter X) suggests t h a t the migrating K u l a - F a r a l l o n r i d g e may be r e s p o n s i b l e f o r anomalous near trench plutonism. Marshak and K a r i g (1977) have r e l a t e d high l e v e l Cenozoic plutons of the eastern A l e u t i a n arc t o p a r t i a l melting (anatexis) of the subduction complex by passage of the K u l a - F a r a l l o n r i d g e . Eastward m i g r a t i o n of the r i d g e - t r e n c h - t r i p l e j u n c t i o n along the eastern A l e u t i a n Trench i s c o n s i s t e n t with i s o t o p i c ages f o r the anomalous near trench plutons. The p r o x i m i t y of Miocene s i l i c i c v o l c a n i c s and i n s t r u s i v e s of the Patton Ridge area ( C a l i f o r n i a Continental Borderland) t o a paleotrench, has been explained by southward mi g r a t i o n of the F a r a l I o n - P a c i f i c r i d g e along the c o n t i n -e n t a l margin (Crouch, 1.981; Dickinson and Snyder, 1979). The o r i g i n of the northeast trending A l e r t Bay v o l c a n i c b e l t on northern Vancouver Island appears t o be r e l a t e d t o p a r t i a l melting along the northern edge of the subducted Juan de Fuca ( F a r a l l o n ) p l a t e (Bevier et a l , 1979). The Upper Miocene-Pliocene age of the v o l c a n i c s and e p i z o n a l plutons i s c o n s i s t e n t with a s t a t i o n a r y p o s i t i o n of the F a r a l l o n - P a c i f i c r i d g e at the c o n t i n e n t a l margin during the period 73 ( . A N G A R A P L U T O N ^ K I L O M E T R E S F i g . 14 Sketch map showing pos s ib le r e l a t i o n s h i p between mid-T e r t i a r y 'near t r e n c h ' p lutonism and northward migrat ion o f the Ku l a -Fa ra l l on r i d ge . 74 5-10 Ma ago (Riddihough, 1977). By analogy with the above examples, the o r i g i n of some near trench Upper 01 igocene-Upper Miocene p o s t - t e c t o n i c plutons may be r e l a t e d t o passage of the K u l a - F a r a l l o n r i d g e . According t o p l a t e r e c o n s t r u c t i o n s of Cooper et al_ (1976), Coney (1978), and Stone (1977) t h i s event may have occurred at the Queen C h a r l o t t e Islands 30-40 Ma ago (see d i s c u s s i o n , Chapter I I ) . The trend of decreasing age of the Pocket, Kano and Langara i n t r u s i o n s from 39 t o 23 Ma ago north along the west coast of the Queen C h a r l o t t e I s l a n d s , ( F i g . 14), would i n d i c a t e t h a t the r i d g e migrated north at a r a t e s l i g h t l y greater than 1 cm/year. This i n t e r p r e t a t i o n assumes that the r i d g e was o r i e n t e d roughly perpendicular t o the paleotrench. A l t e r n a t i v e l y , the age progression may be p a r t l y c o i n c i d e n t a l and may be the consequence of transform f a u l t s along the K u l a - F a r a l l o n ridge which would cause s h i f t s i n t r i p l e j u n c t i o n p o s i t i o n as subduction of the r i d g e proceeded. The o r i g i n of smaller epizonal plutons along the Louscoone f a u l t may more r e a d i l y be explained by divergent wrenching (see Chapter X), s i m i l a r t o the model described p r e v i o u s l y f o r the Masset Formation. 75 CHAPTER IV GEOLOGY OF THE QUEEN CHARLOTTE BASIN GENERAL SETTING The Queen C h a r l o t t e Basin i s approximately 400 km long i n a northwest-southeast d i r e c t i o n and 100 km wide, with a t o t a l 2 surface area of approximately 53,000 km . The basin u n d e r l i e s eastern Dixon Entrance, northeastern Graham I s l a n d , Hecate S t r a i t , Queen C h a r l o t t e Sound and p o s s i b l y Queen C h a r l o t t e S t r a i t . The basin occupies the Hecate Depression (Holland, 1964), a topographic and s t r u c t u r a l low bounded t o the east by the Coast Mountains and t o the west f o r much of i t s length by I n s u l a r Mountains of Vancouver and Queen C h a r l o t t e I s l a n d s . The depression i n t u r n forms part of an extensive c o a s t a l trough t h a t extends southwards through the Georgia Depression, the Puget and Willamette Lowlands of Washington and Oregon t o the Great V a l l e y of C a l i f o r n i a (Nelson, 1976). The Neogene Queen C h a r l o t t e Basin i s one of a s e r i e s of Late Mesozoic-Cenozoic basins t h a t l i e along the P a c i f i c margin of North America from southwestern Alaska t o the Gulf of C a l i f o r n i a ( F i g . 15). These basins have developed as a r e s u l t of continued i n t e r a t i o n of " P a c i f i c " p l a t e s and the North America p l a t e along a convergent t o transform margin. The Queen C h a r l o t t e Basin, though i n a part a successor f e a t u r e of p r e - e x i s t i n g sedimentary b e l t s (see previous d i s c u s s i o n , Chapter I) i s a r e s i d u a l or composite f o r e a r c basin (Dickinson and Seely, 1979) occupying the gap between the l a t e Mesozoic-Cenozoic Coast Mountain v o l c a n i c - p l u t o n i c arc and an 76 LATE MESOZOIC — CENOZOIC BASINS WESTERN NORTH AMERICA Major sedimentary basins Calc - alkaline plutonic rocks Geosynclinal structural trends Tertiary — Quaternary volcanic cover Spreading center ^ Subduction zone / Transform fault F i g . 15 Geologic sketch map of western North America showing d i s t r i -b u t i o n o f major Late Mesozoic-Cenozoic b a s i n s . Present p l a t e boundaries are i n d i c a t e d . M o d i f i e d , i n p a r t , from Orwig (1976) and Blake e t a i (1978). 77 a n c e s t r a l trench located< west of Vancouver and Queen C h a r l o t t e I s l a n d s . Of the basins shown i n F i g . 15 the Georgia Basin (B.C.), the Cook I n l e t and B r i s t o l Bay basins (Alaska) and San Joaquin-Sacramento Basin ( C a l i f o r n i a ) are most s i m i l a r t o the Queen C h a r l o t t e Basin with respect t o general s i z e and o r i e n t a t i o n , b a s i n a l c o n f i g u r a t i o n and s t r u c t u r a l alignment and r e l a t i o n s h i p of t e c t o n i c framework t o placement of p o s i t i v e source areas. The morphology of the c o n t i n e n t a l s h e l f of Hecate S t r a i t and Queen C h a r l o t t e Sound, c h a r a c t e r i z e d by major banks and channels ( F i g . 16) i s l a r g e l y of g l a c i a l and glaciomarine o r i g i n w i t h minor Holocene i n f i l l i n g (Luternauer, 1972; Luternauer and Murray, i n prep). In general there i s l i t t l e c o r r e l a t i o n between submarine topography and basement s t r u c t u r e mapped on the basis of geophysical data. STRATIGRAPHY Knowledge of the s t r a t i g r a p h y of the Mio-Pliocene Skonun succession of the Queen C h a r l o t t e Basin has been obtained l a r g e l y from e x p l o r -atory w e l l s d r i l l e d on northeastern Graham Island by R i c h f i e l d et a l and i n Hecate S t r a i t and Queen C h a r l o t t e Sound by S h e l l Canada L t d . Scattered outcrops of Skonun rock occur on north-eastern Graham I s l a n d . The w e l l data, described by Sutherland Brown (1968, pp. 118-127) and Shouldice (1973), i s summarized here ( F i g . 17) as a s t r u c t u r a l c r o s s - s e c t i o n showing ages of rocks, b a s i c c o r r e l a t i o n s , l i t h o f a c i e s and d e p o s i t i o n a l environments. Location of these w e l l s and three a d d i t i o n a l w e l l s d r i l l e d o u t s ide the basin on Graham Island i s shown i n F i g . 16. 78 F i g . 16 Well l o c a t i o n s on Queen C h a r l o t t e Islands and i n Queen C h a r l o t t e Basin and morphology o f c o n t i n e n t a l s h e l f . Bathy-m e t r i c contours i n meters. S.L. GRAHAM I S L A N D — * H HECATE STRAIT — Q U E E N CHARLOTTE SOUND SOCK. SOCK. MUR. AUK. HARL. OSP. TOW HILL GOLD CR C. BALL MAS. NADU TLELL COHO TYEE 2-3-4 -Km TT* m 1 V V V I V V V V V V V V V J Pi-* L.P.I U. Mi 228*28 (Ay. of 5) \ Mio-Pliocene Skonun sediments A A A A A A •net 7i I I65t8' I / UK - } 7 2 ± 6 8 4 t l O Non - marine sand- sandstone Non*- marine siltstone r- shale Coal , lignite I Marine sand - sandstone Marine siltstone - shale Conglomerate Upper Cretaceous Queen Charlotte Group* sandstone, siltstone, shale ~--m * S / L . Mi ffW142t2 S.L. hi h2 h3 K m L +++<•, f+++ I H I i I H W W W W W W W W IA A A A JA A A A (A A A A IA A A A i Plio.-Pleistocene Tow Hill Sills* basalt, diabase Lower -Middle Tertiary Masset volcanicsi basalt, rhyolite, breccia Cretaceous volcanics« basalt, pyroclastics Upper Paleozoic intrusivess diorite, gabbro Microflora, Microfauna Unconformity 2914 K - A r age ( M a ± 1 6 ) F i g . 17 S t r u c t u r a l cross s e c t i o n of Queen C h a r l o t t e Basin. Well l o c a t i o n s are shown i n Fig.16 Sutherland Brown (1968, F i g . 20) and Shouldice (1973, F i g s . 8, 9, 11). M o d i f i e d from HO 80 On northeastern Graham Island s i x e x p l o r a t o r y w e l l s penetrated a comparatively t h i n (up t o 1.83 km) succession of e a r l y Middle Miocene-Pliocene Skonun sandstone, s i l t s t o n e , and shale, with minor l i g n i t e , c o a l , marl and c l a y s t o n e (Sutherland Brown, 1968). Conglomerate i s dominant near the base of the Tow H i l l w e l l but i s r a r e i n a l l other w e l l s . A l l w e l l s except f o r Tow H i l l and T l e l l reached base of the Skonun Formation, bottoming i n b a s a l t i c b r e c c i a , r h y o l i t e , or b a s a l t of the Masset Formation (Tartu f a c i e s ) . A basement v o l c a n i c r i d g e extending eastward from the v i c i n i t y of the Masset w e l l appears t o subdivide the Graham Island p o r t i o n of the Queen C h a r l o t t e Basin i n t o two small sub-basins (Sutherland Brown, 1968). The f a c i e s and t e c t o n i c s on e i t h e r side of the r i d g e d i f f e r though s t r a t i g r a p h i c u n i t s i n the southern w e l l s appear t o t h i n and overlap toward the north and e a s t . In the southern sub-basin sediments t h i c k e n eastward, dipping 10-15° NE toward Hecate S t r a i t . In the northern sub-basin steeper dips (25-30°) and f a u l t and f o l d s t r u c t u r e s i n d i c a t e a more a c t i v e t e c t o n i c regime. The western boundary of the Queen C h a r l o t t e Basin on much of Graham Island and northeastern Moresby Island i s the Sandspit F a u l t . The environment of d e p o s i t i o n of Skonun rocks on north-eastern Graham Island (Martin and Rouse, 1966; Sutherland Brown, 1968; A d d i c o t t , 1978; Champigny and S i n c l a i r , i n press) a l t e r n a t e d between marine (shallow water, near shore) and non-marine ( f l u v i a l -d e l t a i c , swamp, lagoon, l a c u s t r i n e ) . The f a c i e s probably i n t e r -f i n g e r t o some degree as a r e s u l t of p u l s a t i n g onlap. The l a s t d e p o s i t i o n i s represented by the upper marine member comprising 81 mollusc-bearing calcareous sandstones, best exposed as outcrop at Skonun Point (Martin and Rouse, 1966; Sutherland Brown, 1968; A d d i c o t t , 1978). Much of the d e t r i t u s f o r Skonun sediments i s v o l c a n i c and was derived from the proto Queen C h a r l o t t e I s l a n d s ; metamorphic and some i n t r u s i v e c l a s t i c c o n s t i t u e n t s must have o r i g i n a t e d from the Coast Mountains and S.E. Alaska. The o f f s h o r e Queen C h a r l o t t e Basin contains a number of small sub-basins ( F i g . 18) but can be d i v i d e d i n t o two major sub-basins separated by an east-west trending basement rid g e (termed here "Moresby Ridge") near the c e n t r a l p o r t i o n of the b a s i n . The r i d g e , extending at l e a s t 55 km east of Burnaby Island (Queen C h a r l o t t e Islands) has been recognized on the b a s i s of c o i n c i d e n t g r a v i t y and magnetic highs (Shouldice, 1973; Stacey, 1975; chapter V I I ) . Sedimentary f a c i e s g e n e r a l l y d i f f e r on e i t h e r side of the r i d g e . In the northern ("Hecate") sub-basin f o u r e x p l o r a t o r y w e l l s penetrated a t h i c k (up to 3 km) succession of Upper Miocene-Lower Plio c e n e non-marine t o t r a n s i t i o n a l sands, s i l t s , shales and c o a l , a t h i n sequence (500 m) of Upper Cretaceous (Queen C h a r l o t t e Group) non-marine e l a s t i c s , bottoming i n Middle J u r a s s i c Yakoun v o l c a n i c s or Upper Paleozoic i n t r u s i v e s ( F i g . 17, Table IV). The major source f o r c o n t i n e n t a l l y derived sediments was the Coast Mountains though a d d i t i o n a l sediment may w e l l have been derived from the Queen C h a r l o t t e Islands and p o s i t i v e basement f e a t u r e s w i t h i n the basin. In the southern (or "Charlotte") sub-basin three out of f o u r e x p l o r -atory w e l l s penetrated a predominately marine Lower Miocene-Upper Pliocene succession of sands, s i l t s , and s h a l e s , bottoming i n Paleogene Masset v o l c a n i c s ( F i g . 17, Table I V ) . The v o l c a n i c s are 82 133° 132° 131° 130° 129° J28° 127° F i g . 18 S i m p l i f i e d isopach map of Neogene Queen C h a r l o t t e Basin showing l o c a t i o n of major basin elements. Contours i n f e e t . M o d i f i e d from Shouldice (1973, F i g . 32). 83 s u b a e r i a l and are interbedded with Paleogene marine sediments i n d i c a t i n g an a l t e r a t i o n of v o l c a n i c episodes with shallow marine t r a n s g r e s s i o n s (Shouldice, 1973). The upper surface of the v o l c a n i c s i s diachronous ( F i g . 17) and the sedimentary h i s t o r y i n d i c a t e d at i n d i v i d u a l w e l l s i s t h e r e f o r e s l i g h t l y d i f f e r e n t . The marine sediments were deposited i n water depths ranging from shallow ( i n n e r n e r i t i c , 0-50 meters) t o deep (upper b a t h y a l , 200-1000 meters). There was a minor re g r e s s i o n near the end of the Miocene i n the area of the Murrelet w e l l where marine Upper Miocene e l a s t i c s are o v e r l a i n by non-marine Upper Miocene and Pliocene sands and shales (Shouldice, 1973). P a l y n o l o g i c a l study of the f o u r C h a r l o t t e sub-basin w e l l s (Hopkins, 1975; 1976) suggests a warm temperate c l i m a t e i n l a t e E a r l y or Middle Miocene, followed by a general c o o l i n g i n t o the P l e i s t o c e n e . The E a r l y Miocene i s considered t o have been somewhat c o o l e r and more damp than the Middle Miocene. Unconsolidated t o c o n s o l i d a t e d sands and sandstones are the dominant l i t h o l o g i e s i n a l l Queen C h a r l o t t e Basin w e l l s and are c h a r a c t e r i s t i c a l l y c h e m i c a l l y and mechanically immature arkoses and l i t h a r k o s e s (Sutherland Brown, 1968, S h e l l Canada Well H i s t o r y Reports, Galloway, 1974). Based on a petrographic study of f o u r of the o f f s h o r e w e l l s Galloway (1974) suggests t h a t basin f i l l i n g must have postdated a c t i v e development of the adjacent Coast Mountains v o l c a n i c - p l u t o n i c arc:sediment d e b r i s appears t o have been derived from unroofing of the plutons r a t h e r than from contemporaneous e x t r u s i v e or shallow i n t r u s i v e igneous masses associated with a c t i v e convergence and arc development. Rapid sedimentation r a t e s and subsequent b u r i a l of the predominately te r r i g e n o u s c l a s t i c sediment 84 has r e s u l t e d i n a systematic, recurrent s e r i e s of d i a g e n e t i c events (Galloway, 1974; 1979). Geothermal gradients i n the Queen C h a r l o t t e Basin, as high as 3.4°C/100 meters i n tu r n have r e s u l t e d i n compari-t i v e l y shallow depths of p o r o s i t y l o s s from cementation and b u r i a l metamophism ( o p . c i t . ) . Some of the d i a g e n e t i c f a b r i c has l i k e l y been a l t e r e d by t e c t o n i c deformation and u p l i f t which followed the extensive basin f i l l and subsidence. Three major unconformities have been mapped on north-eastern Graham Island and i n the offshore Queen C h a r l o t t e Basin (Sutherland Brown, 1968; Shouldice, 1973): 1) Quaternary (Pleistocene-Recent) sediments unconformably to disconformably o v e r l i e Skonun sediments (Late P l i o c e n e - E a r l y P l e i s t o c e n e s u r f a c e ) ; 2) r e g i o n a l seismic evidence (Shouldice, 1973) suggests a hia t u s of E a r l y Pliocene age i n the Skonun succession; 3) Skonun rocks unconformably o v e r l i e Masset v o l c a n i c s and a l l o l d e r rocks (pre Late Miocene s u r f a c e ) . This l a t t e r surface may be r e g i o n a l i n extent as g e o l o g i c a l and geomorphological evidence from the Western C o r d i l l e r a i n d i c a t e a widespread unconformity of post Middle Miocene age (Drummond, 1978). On northeastern Graham I s l a n d , emergence of the b a s i n a l margin, r e s u l t i n g from post d e p o s i t i o n a l u p l i f t and f o l d i n g , has exposed Late Miocene marine sediments. The E a r l y Pliocene and Late Miocene surfaces may be r e l a t e d i n part t o g l o b a l or major i n t e r -r e g i o n a l changes i n sea l e v e l dated at 11 and 4 Ma B.P. ( V a i l et a l , 1977). 85 AGE AND CORRELATION OF SKONUN FORMATION Attempts t o determine p r e c i s e ages f o r Mio-Pliocene rocks of the Queen C h a r l o t t e Basin have not been s u c c e s s f u l . Comparisons of f l o r a and fauna with Neogene sequences i n western North America are of l i t t l e use s i n c e these l a t t e r sequences are themselves inadequately dated (Martin and Rouse, 1966; Hopkins, 1975; 1976). Suggested ages based on e i t h e r m i c r o f l o r a or micro-fauna are commonly • i n disagreement ( S h e l l Canada Well H i s t o r y Reports). B i o s t r a t i g r a p h i c zonations i n d i c a t e d i n F i g . 17 t h e r e f o r e •are t e n t a t i v e and are based on c o r r e l a t i o n s suggested by Shouldice (1973). C o r r e l a t i o n s based on e i t h e r 1 i t h o l o g i e s or mechanical log c h a r a c t e r i s t i c s was not p o s s i b l e ( o p . c i t . ) . F o s s i l assemblages from outcrop of the uppermost marine member on northeastern Graham Island have been studied i n some d e t a i l . M i c r o f l o r a (Martin and Rouse, 1966), microfauna ( R i c h f i e l d O i l Corp., c i t e d i n Sutherland Brown, 1968, p.126) and macrofauna (Sutherland Brown, 1968; A d d i c o t t , 1978) suggest an Upper Micocene or perhaps e a r l y Pliocene age f o r these rocks. The molluscan fauna have been c o r r e l a t e d ( A d d i c o t t , 1978) with fauna of the lower part of the Montesano Formation of western Washington (the Wishkahan stage of the P a c i f i c Northwest molluscan sequence). STRUCTURE The Queen C h a r l o t t e Basin ( F i g . 18) i s an elongate, asymmetric trough or megasyncline t h a t broadens and plunges 86 r e g i o n a l l y t o the southeast. Much of the e a r l y s t r u c t u r a l develop-ment of the basin has been r e l a t e d t o subsidence i n i t i a t e d during u p l i f t of the Coast Ranges i n l a t e Cretaceous-mid T e r t i a r y time and continued during sediment loading i n Neogene time. The f i n a l pulse of u p l i f t , producing the present r e l i e f of the Coast Mountains occurred approximately 10 Ma ago (Harrison et aj_, 1979). According t o B e l l (1967), the Queen C h a r l o t t e Basin probably o r i g i n a t e d as a h a l f graben: maximum i n i t i a l displacement occurred along f a u l t zones along the eastern margin with l e s s severe breakdown and eventual t r a n s g r e s s i v e onlap onto the western margin. Several episodes of u p l i f t and e r o s i o n , a f f e c t i n g l o c a l s t r u c t u r e and s t r a t i g r a p h y with varying degrees of i n t e n s i t y c h a r a c t e r i z e d l a t e r basin h i s t o r y . A s i m p l i f i e d isopach map ( F i g . 18) based on seismic r e f l e c t i o n , r e f r a c t i o n and aeromagnetic data (Shouldice 1973), i l l u s t r a t e s the gross s t r u c t u r a l c o n f i g u r a t i o n of the Queen C h a r l o t t e Basin. The major sub-basins and Moresby Ridge, described p r e v i o u s l y are shown. For comparison, isopach maps prepared from modelling i s o s t a t i c g r a v i t y anomaly data, reproduced from Stacey (1975) ( F i g . 19), show p o s i t i v e basement r i d g e s or u p l i f t s covered with l e s s than 1 km of sediment flanked by subbasins that c o n t a i n up to 5 km of sediment. Though varying c o n s i d e r a b l y i n o v e r a l l c o n f i g u r a t i o n , basins and ridges are g e n e r a l l y elongate i n northeast-southwest and east-west d i r e c t i o n s . The d e t a i l e d isopach map shown i n F i g . 19 ( b ) , though not n e c e s s a r i l y showing true p o s i t i o n or form of a l l subbasins and r i d g e s , probably r e f l e c t s the true complexity of basement s t r u c t u r e . 87 ISOSTATIC ANOMALIES (a) SEDIMENTARY BASIN MODEL (b) I 50 km i Fig. 19(a) Airy-Heiskanen isostafic gravity anomalies for the Queen Charlotte Basin region at 10 mgal contour interval and (b) computed and (c) filtered sedimentary basin models' derived from gravity data at 0.5 km contour interval. Model assumes a density contrast between Neogene sediments and 'basement' of -0.31 gmcm . Reproduced from Stacey (1975, Figs. 4 and 5). 88 The gross s i m i l a r i t y between the s i m p l i f i e d isopach maps ( F i g . 18; F i g . 19 ( c ) ) suggest that g r a v i t y anomalies r e f l e c t sediment t h i c k n e s s or a s i n g l e d e n s i t y c o n t r a s t between sediments and predominantly v o l c a n i c basement rocks. Major d i s c r e p a n c i e s i n these maps have been a t t r i b u t e d by Stacey (1975) t o v a r i a t i o n s i n d e n s i t y of Mesozoic and E a r l y Cenozoic rocks underlying the v o l c a n i c s : negative r e s i d u a l s i n Queen C h a r l o t t e Sound may be due to pre-Masset g r a n i t i c or sedimentary rocks (Queen C h a r l o t t e Group) while p o s i t i v e r e s i d u a l s i n Hecate S t r a i t may be due t o dense v o l c a n i c (Karmutsen) rocks. Like g r a v i t y data, t o t a l i n t e n s i t y aeromagnetics f o r the Queen C h a r l o t t e Basin ( F i g . 20) and magnetic anomaly data f o r Queen C h a r l o t t e Sound ( T i f f i n and C u r r i e 1976) can y i e l d information on v a r i a t i o n s i n th i c k n e s s of T e r t i a r y sediments o v e r l y i n g basement rock. High-amplitude, short wave-length anomalies on the eastern margin of the basin r e f l e c t basement rocks of the Coast P l u t o n i c Complex. Published deep seismic r e f l e c t i o n p r o f i l e s ( F i g s . 50 and 53, Chapter V I I I ) show ge n t l e f o l d s i n b a s i n a l Skonun sediments and onlap onto a basement ( v o l c a n i c ? ) surface of v a r i a b l e topographic r e l i e f . Though not shown i n the above p r o f i l e s , steep d i p s on basement topography may represent f a u l t s (Shouldice, 1973). Considering the importance of major s t r u c t u r a l d i s l o c a t i o n s i n the t e c t o n i c h i s t o r y of the western C o r d i l l e r a (see F i g . 22 f o r l o c a t i o n of major f a u l t s ) one would expect s i g n i f i c a n t deformation to have occurred i n the Queen C h a r l o t t e Basin. F a u l t - b l o c k t e c t o n i c s producing horst and graben s t r u c t u r e s i n basement rocks and Neogene 89 F i g . 20 S i m p l i f i e d t o t a l i n t e n s i t y aeromagnetic map of the Queen C h a r l o t t e Basin. I.G.R.F. values are approximate from a r b i t r a r y reference l e v e l v alues. Redrawn from Shouldice (1973, F i g . 3 ) . 90 F i g . 21 Combined Bouguer (on land) and f r e e - a i r (at sea) g r a v i t y anomaly map of the Queen C h a r l o t t e Basin region a t 10 mgal contour i n t e r v a l . From Earth Physics Branch (1980). 91 cover i s suggested i n i n t e r p r e t a t i o n s of f r e e a i r and Bouguer g r a v i t y anomaly data (Couch, 1969; Stacey and Stephens,1969). Northwest-southeast and east-west lineaments, apparent i n both g r a v i t y and magnetic anomaly maps ( F i g . 20 and 21) subdivide the Queen C h a r l o t t e Basin. The l i n e a r trend and steep gradient of g r a v i t y anomalies ( F i g . 21) marks the approximate p o s i t i o n of the Sandspit F a u l t and a p o s s i b l e southeastern extension of the Clarence S t r a i t F a u l t on the western and eastern margins r e s p e c t i v e l y of the basin (Stacey and Stephens, 1969). Elongate c o i n c i d e n t g r a v i t y and magnetic highs mark the p o s i t i o n of basement r i d g e s . One of these ri d g e s extends d i s c o n t i n u o u s l y between the southern Queen C h a r l o t t e Islands and northern Vancouver Island ( S h o u l d i c , 1973; Stacey and Stephens, 1969). Earthquake e p i c e n t e r s i n the Queen C h a r l o t t e Basin (Milne et j H , 1978) are concentrated i n the western Queen C h a r l o t t e Sound region ( F i g . 22). These events, though normally i n t e r p r e t e d t o be the r e s u l t of f a u l t i n g or l o c a l i s o s t a t i c readjustments, are more l i k e l y due t o secondary e f f e c t s of seismic a c t i v i t y on the Queen C h a r l o t t e F a u l t and the ridge and transform f a u l t system west of Queen C h a r l o t t e Sound. The s c a t t e r i n s e i s m i c i t y may be due i n part t o large ( i n the order of 100 km) systematic l o c a t i o n e r r o r s i n events recorded p r i o r t o 1965 (Milne et. j i l _ , 1978). Seismic r e f r a c t i o n r e s u l t s from Dixon Entrance and northern Graham Island (Shor, 1962; Johnson et aU 1972; Forsyth et a l , 1974) and g r a v i t y anomaly data (Couch, 1969; Stacey and Stephens, 1969) i n d i c a t e s a c r u s t a l t h i c k n e s s of 25-30 km (average c r u s t a l v e l o c i t y of 6.4 km/sec) f o r the Queen C h a r l o t t e Islands and 92 F i g . 22 D i s t r i b u t i o n o f earthquake e p i c e n t e r s of magnitude 2.0 or g r e a t e r i n v i c i n i t y o f Queen C h a r l o t t e I s l a n d s . The c i r c l e diameters are p r o p o r t i o n a l to earthquake magnitude. Earth-quakes o f magnitude l e s s than 3.0 are marked by 'X'. QCF = Queen C h a r l o t t e f a u l t , RLF = Rennel-Louscoone f a u l t , SF - Sandspit f a u l t , CSF = Clarence S t r a i t f a u l t , MF = M a l i s p i n a f a u l t . Double l i n e s are a c t i v e r i d g e s . Redrawn from Mi l n e e t al_ (1978, F i g s . 14 and 15). 93 Queen C h a r l o t t e Basin. G r a v i t y data suggests.„ th a t the Queen C h a r l o t t e Basin i s compensated i s o s t a t i c a l l y . Heat flo w measurements (eg. Hyndman, 1981) i n d i c a t e g e n e r a l l y low values f o r the I n s u l a r B e l t compared t o the oceanic area and Coast P l u t o n i c Complex. Because of the long and complex t e c t o n i c h i s t o r y of the r e g i o n , r e s u l t i n g i n i m p r i n t i n g of major thermal events, paleogeotherms cannot be determined. 94 CHAPTER V MAGNETIC PROPERTIES OF QUEEN CHARLOTTE ISLANDS VOLCANIC AND PLUTONIC ROCKS INTRODUCTION As an aid to interpretat ion of airborne and marine magnetic anomaly maps of the Queen Charlotte Islands and Hecate S t r a i t , the magnetic properties of Queen Charlotte Island volcanic and plutonic rocks have been invest igated. Aeromagnetic maps (see fo l lowing chapter) indicate that volcanic and plutonic rocks have associated magnetic anomalies of s i g n i f i c an t l y d i f fe rent amplitude and pattern which could r e f l e c t changes in magnetic volume suscept-i b i l i t y and remnant magnetization. A study by Coles and Currie (1977) of magnetic properties of Vancouver Island and Coast Plutonic Complex rocks demonstrates the appl icat ion and s u i t a b i l i t y of rock sample analysis when combined with aeromagnetic in terpretat ion. Sampled rock types of the major volcanic and plutonic units of the Queen Charlotte Islands are as fo l lows: 1) Tow H i l l s i l l s (PI io-Pleistocene) : diabase, o l i v i ne basalt . 2) Masset volcanics (01igocene-Miocene): basalt and rhyo l i te mem-bers of the Tartu f ac ie s . 3) Yakoun volcanics (Middle Jurassic) : porphyr i t ic andesite. 95 L A N G A H A WM Q U E E N C H A R L O T T E ISLANDS DISTRIBUTION OF VOLCANIC AND PLUTONIC ROCKS AND LOCATIONS OF MAGNETIC ROCK PROPERTY SAMPLING SITES PALEOCENE-LOWER MIOCENE ] MASSET VOLCANICS MIDOLE JURASSIC h" ••'•!:) YAKOUN VOLCANICS UPPER TRIASSIC E l l KARMUTSEN VOLCANICS LOWER-MIDDLE TERTIARY W2M POST TECTONIC PLUTONS UPPER JURASSIC fcftffi SYNTECTONIC PLUTONS X HAND SPECIMEN ® CORE SITE I F i g . 23 L o c a t i o n o f v o l c a n i c and p l u t o n i c samples analyzed f o r magnetic p r o p e r t i e s . 96 b a s a l t p i l l o w l a v a s , massive b a s a l t flows greenstone. quartz monozonite (Louise and Cumshewa p l u t o n s ) , quartz d i o r i t e (East Kano and Sandspit) g r a n o d i o r i t e (Chinukundl). quartz d i o r i t e and hornblende d i o r i t e (San C h r i s t o v a l B a t h o l i t h ) . The g e n e r a l i z e d d i s t r i b u t i o n of v o l c a n i c and p l u t o n i c rocks and sample l o c a l i t i e s are shown i n F i g . 23. MAGNETIC SUSCEPTIBILITY Measurement The volume magnetic s u s c e p t i b i l i t y of 54 hand specimens c o l l e c t e d by the author from the Tow H i l l , Masset and Yakoun v o l c a n i c s and p o s t - t e c t o n i c plutons on eastern Graham and north-eastern Moresby Islands ( F i g . 23) was determined at the U n i v e r s i t y of B r i t i s h Columbia using a Geophysical S p e c i a l t i e s Company Magnetic S u s c e p t i b i l i t y Bridge (Model Ms-3). The r e p r o d u c i b i l i t y of measure-ment of crushed samples was +2 s c a l e d i v i s i o n s which i s equivalent 4) Karmutsen v o l c a n i c s (Upper T r i a s s i c ) : 5) P o s t - t e c t o n i c plutons (Lower-Middle : T e r t i a r y ) 6) Syntectonic plutons (Upper J u r a s s i c ) : 97 to from +50 t o +1 percent with p r e c i s i o n i n c r e a s i n g with i n c r e a s i n g s u s c e p t i b i 1 i t y . The volume magnetic s u s c e p t i b i l i t y of 214 o r i e n t e d cores c o l l e c t e d i n 1971 by Drs. A. Hicken and E. I r v i n g (Earth Physics Branch) from 31 s i t e s on the Queen C h a r l o t t e Islands was determined f o r the author by the D i v i s i o n of Geomagnetism, Earth Physics Branch (Ottawa). The 1-inch diameter cores analyzed normally exceeded the 3-inch minimum length required f o r standard c a l i b r a t i o n of the MS-3 br i d g e ; exceptions were short 1 inch long specimens f o r which a c o r r e c t i o n f a c t o r was applied (R.L. Coles, w r i t t e n communication, June, 1977). Cores were a v a i l a b l e f o r the Tow H i l l , Masset, and Karmutsen v o l c a n i c s and post and sy n t e c t o n i c plutons ( F i g . 23). Also included i n t h i s study are the r e s u l t s of suscept-i b i l i t y measurements of Vancouver Island samples ( C u r r i e and M u l l e r , 1976; Coles and C u r r i e , 1977; C u r r i e , unpublished d a t a ) , most of which (excluding the Metchosin v o l c a n i c s ) were c o l l e c t e d from the northern p o r t i o n of the I s l a n d . The age, and l i t h o l o g i c a l and chemical s i m i l a r i t y of pre-Cretaceous Queen C h a r l o t t e Islands and Vancouver Island v o l c a n i c and p l u t o n i c rocks has been noted p r e v i o u s l y (see Chapter I ) . The a d d i t i o n of the Vancouver Island measurements has made s t a t i s t i c a l a n a l y s i s more meaningful when only a l i m i t e d number of samples from e q u i v a l e n t u n i t s on the Queen C h a r l o t t e Islands were a v a i l a b l e . S t a t i s t i c a l A n a l y s i s Frequency d i s t r i b u t i o n histograms of measured s u s c e p t i -b i l i t i e s (on l o g a r i t h m i c scale) are presented i n F i g . 24 and r e l a t e d 98 35T 30 25 20+ 15 10 -5-0 EARLY—MIDDLE TERTIARY MASSET (Q.C.I.) N = 44 EARLY TERTIARY METCHOSIN (V.I.) • [ T T T N = 71 TERTIARY VOLCANICS: MASSET (Q.C.I.) AND METCHOSIN (V.I.) N = 115 r35 -30 -25 L20 -10 -5 0 >-u z LU 3 O LU CC 35-r 30-25-20--JURASSIC VOLCANICS YAKOUN (Q.C.I.) AND BONANZA (V.I.) N = 30 TRIASSIC KARMUTSEN (V.I.) N = 136 TRIASSIC KARMUTSEN (V.I. & O.C.I.) rlI_n£xL: N = 152 in-r35 -30 -25 -20 -15 -10 -5 0 35-j 30-25-20-15-10--5 -EARLY—MIDDLE TERTIARY POST TECTONIC PLUTONS (O.C.I.) N = 27 JURASSIC ISLAND INTRUSIONS (V.I.) N = 37 JURASSIC INTRUSIONS (v.i. a Q.C.I.) u EEL N = 43 •35 30 -25 -20 -15 -10 -5 10 100 1000 10000 10 100 1000 SUSCEPTIBILITY K, EMU/cm 3 x 10 10000 10  - m-« 100 1000 10000 F ig . 24 Frequency d i s t r i b u t i o n histograms of magnetic s u s c e p t i b i l i t y of Queen Charlotte Islands and Vancouver Island volcanic and Pluton ic rocks. TABLE VII MAGNETIC SUSCEPTIBILITY OF QUEEN CHARLOTTE ISLANDS AND  VANCOUVER ISLAND ROCKS 1 2 3 4 5 6 7 8 TOW HILL SILLS (Q.C.I.) 3.10 0.12 3.11 1296 321 1260 6 MASSET (Q.C.I.) 3.11 0.32 3.13 1686 1309 1306 44 METCHOSIN (V.I.) 3.35 0.40 3.39 3016 2030 2255 71 TERTIARY VOLCANICS (T) 3.26 0.39 3.30 1863 2166 1830 115 POST TECTONIC PLUTONS (Q.C.I.) 3.08 0.62 3.23 1977 1537 1191 27 YAKOUN (Q.C.I.) 2.95 0.84 3.08 2809 3134 895 12 BONANZA (V.I.) 2.40 0.77 2.50 724 818 249 18 JURASSIC VOLCANICS (T) 2.62 0.83 2.75 1558 2280 416 30 SYNTECTONIC PLUTONS (Q.C.I.) 3.19 0.20 3.25 1708 764 1566 6 ISLAND INTRUSIONS (V.I.) 2.95 0.64 3.20 1675 1277 886 37 JURASSIC INTRUSIONS (T) 2.98 0.60 3.23 1680 1211 959 43 KARMUTSEN (Q.C.I.) 2.73 0.57 2.89 953 848 540 16 KARMUTSEN (V.I.) 2.91 0.71 3.17 1817 1767 816 136 KARMUTSEN (T) 2.89 0.70 3.16 1726 .1712 781 152 KARMUTSEN MODE A 3.24 0.32 3.28 2241 1650 1738 116 KARMUTSEN MODE B 1.76 0.24 1.73 69 46 58 36 1 = Formation 2,3 = Arithmetic mean and standard deviation of the logarithm of susceptibility k = Median of logarithm of susceptibility 5,6 - Arithmetic mean and standard deviation of susceptibility 7 = Antilogarithm of mean susceptibility 8 = Number of samples _g (Susceptibilities in units of 10 emu.) TOO s t a t i s t i c a l parameters i n Table V I I . Histograms have not been constructed f o r the Tow H i l l s s i l l s , s y n t e c t o n i c plutons and Karmutsen v o l c a n i c s because of the l i m i t e d number of sample measure-ments from each of these u n i t s . S i m i l a r l y , because of a small number of measurements the J u r a s s i c Yakoun and Bonanza v o l c a n i c s of the Queen C h a r l o t t e and Vancouver Islands have been combined i n t o a s i n g l e histogram. S u s c e p t i b i l i t i e s of cores from the same s i t e on the Queen C h a r l o t t e Islands were considered to represent d i s c r e t e samples only when measurements d i f f e r e d by more than the group i n t e r v a l ( l o g s u s c e p t i b i l i t y of 0.2) used t o co n s t r u c t the h i s t o -grams. Measurements t h a t d i d not d i f f e r by t h i s a r b i t r a r y amount were averaged, r e s u l t i n g i n 59 " d i s c r e t e " sample measurements from the 214 cores. The s t a t i s t i c a l r epresentativeness and accuracy of s u s c e p t i b i l i t y measurements (Table VII) i s d i f f i c u l t to assess. The geographical d i s t r i b u t i o n of rock samples c o l l e c t e d from Vancouver and Queen C h a r l o t t e Islands i s n e i t h e r systematic nor t r u l y random and i s t h e r e f o r e s t a t i s t i c a l l y u n s a t i s f a c t o r y . In a d d i t i o n , not a l l rock types from each geologic u n i t are represented by s u s c e p t i b i l i t y measurements. Results f o r the Yakoun v o l c a n i c s and s y n t e c t o n i c plutons and, t o a l e s s e r degree, the p o s t - t e c t o n i c plutons, must be considered p r e l i m i n a r y because of small sample populations. R e s u l t s f o r the Masset (N = 44), Metchosin (N = 71), and the Karmutsen (N = 152) are considered however t o be r e p r e s e n t a t i v e and accurate w i t h i n an estimated 5-10%. The tendency of a s u s c e p t i b i l i t y d i s t r i b u t i o n to 101 approximate a log normal (Gaussian) d i s t r i b u t i o n has been noted by a number of workers (eg. T a r l i n g , 1971, p.89; Puranen et a l , 1974). The d i s t r i b u t i o n s of the l a r g e r sample populations shown i n F i g . 24 tend t o confirm t h i s o b s e r v a t i o n . For t h i s reason, the d i s t r i b u -t i o n s are best compared on the basis of the a r i t h m e t i c mean and standard d e v i a t i o n of the logarithm of s u s c e p t i b i l i t y (Table V I I ) . Although i t has been suggested t h a t the median i s a more v a l i d s t a t i s t i c a l measure than the mean ( P l a t o u , 1974) the d i f f e r e n c e i n values i s minimal when the lognormal d i s t r i b u t i o n s are approximately symmetric ( F i g . 24 and Table V I I ) . Frequency Di s t r i but ions The histograms of frequency d i s t r i b u t i o n and a r i t h m e t i c means of log s u s c e p t i b i l i t y i n d i c a t e t h a t , f o r the Queen C h a r l o t t e I s l a n d s , the T e r t i a r y Masset v o l c a n i c s and p o s t - t e c t o n i c plutons and Upper J u r a s s i c syntectonic plutons, have the highest associated magnetic s u s c e p t i b i l i t i e s . Although the d i f f e r e n c e i n mean log s u s c e p t i b i l i t i e s of these u n i t s i s s t a t i s t i c a l l y not s i g n i f i c a n t , the greater log median values and more l e p t o k e r t i c (peaked) d i s t r i -butions f o r the p l u t o n i c rocks suggests a l a r g e r number of high s u s c e p t i b i l i t i e s f o r these u n i t s than f o r the Masset. On northern Vancouver Island the Karmutsen v o l c a n i c s and Island I n t r u s i o n s are the dominant magnetic rocks (Coles and C u r r i e , 1977). The Middle J u r a s s i c Yakoun (Queen Ch a r l o t t e Islands) and Lower J u r a s s i c Bonanza (Vancouver Island) v o l c a n i c s have the lowest magnetic s u s c e p t i b i 1 i t e s and widest range i n values. The small number of sample measurements and high r e l a t i v e d i s p e r s i o n 102 ( i n d i c a t e d by log standard d e v i a t i o n s ) however, make q u a n t i t a t i v e comparison with other u n i t s meaningless. S u s c e p t i b i l i t i e s of the younger Yakoun v o l c a n i c s are somewhat higher than those of the Bonanza v o l c a n i c s . Although t h i s apparent d i f f e r e n c e may s o l e l y be the r e s u l t of sample b i a s , s l i g h t d i f f e r e n c e s i n compostion and g r a i n s i z e may a l s o be a f a c t o r . The Yakoun v o l c a n i c s are predomi-nan t l y p o r p h y r i t i c andesite whereas the Bonanza v o l c a n i c s comprise f i n e r grained andesite, b a s a l t i c andesite and rhyo d a c i t e . Magnetic s u s c e p t i b i l i t i e s of the ol d e r p l u t o n i c rocks ( s y n t e c t o n i c plutons, Island I n t r u s i o n s ) are s i g n i f i c a n t l y greater than coeval v o l c a n i c rocks (Yakoun, Bonanza) and s l i g h t l y greater than the Karmutsen v o l c a n i c s . Syntectonic rocks of hornblende d i o r i t e composition have s u s c e p t i b i 1 i t e s two t o three times greater than those of quartz d i o r i t e composition. The d i s t r i b u t i o n of s u s c e p t i b i l i t i e s of the Island I n t r u s i o n s i s bimodal; the more h i g h l y magnetic samples are of quartz d i o r i t e or g r a n o d i o r i t e composition and the more weakly magnetic samples are of more a c i d i c v a r i e t i e s (eg. quartz monzonite). L i k e the Island I n t r u s i o n s , the frequency d i s t r i b u t i o n histogram of the Karmutsen v o l c a n i c s i s bimodal with a large sub population (A) of high s u s c e p t i b i l i t i e s d i s t i n c t from a small sub population (B) of low s u s c e p t i b i 1 i t e s . This b i m o d a l i t y was noted by C u r r i e and M u l l e r (1976) who a t t r i b u t e d the d i s t r i b u t i o n t o " h o l o c r y s t a l l i n e f i n e grained rocks having higher s u s c e p t i b i l i t i e s than h y p o c r y s t a l l i n e medium grained rocks". The approximate log normal d i s t r i b u t i o n of both A and B modes i s expected s i n c e d i s t r i -bution of s u s c e p t i b i l i t i e s i s oft e n a f u n c t i o n of g r a i n s i z e , 103 which i n any rock type i s lognormally d i s t r i b u t e d . On Queen Ch a r l o t t e I s l a n d s , f o r which only 16 " d i s c r e t e " core sample measure-ments were a v a i l a b l e , high magnetic s u s c e p t i b i l i t y i s associated wit h both p i l l o w and massive b a s a l t i c lavas and low magnetic s u s c e p t i b i l i t y with c h l o r i t i z e d e q u i v a l e n t s ( i . e . greenstones). The mean magnetic s u s c e p t i b i l i t y of the B mode i s lower than any u n i t on Vancouver and Queen C h a r l o t t e Islands r e p o r t e r here; the mean of the A mode i s not s i g n i f i c a n t l y d i f f e r e n t (at 95% confidence) than the more h i g h l y magnetic u n i t s (Masset, post and s y n t e c t o n i c plutons) on the Queen C h a r l o t t e I s l a n d s . Magnetic s u s c e p t i b i l i t i e s of the p o s t - t e c t o n i c plutons (Louise Pluton and Sandspit Group) are r e l a t e d t o composition with more a c i d i c rock types having lower s u s c e p t i b i l i t i e s . The decrease i s g e n e r a l l y g r a d a t i o n a l from the more h i g h l y magnetic quartz d i o r i t e (Sandspit Pluton) to g r a n o d i o r i t e (Chinukunkl) t o quartz monzonite (Louise and Cumshewa). The low magnetic s u s c e p t i b i l i t i e s shown on the histogram ( F i g . 24) are from h i g h l y weathered samples of the East Kano B a t h o l i t h . Excluding these samples increases the mean log s u s c e p t i b i l i t y from 3.08 t o 3.27. S u s c e p t i b i l i t y measurements of the Masset v o l c a n i c s were r e s t r i c t e d t o b a s a l t flows and r h y o l i t e ash flows of the Tartu f a c i e s (Graham I s l a n d ) . R h y o l i t e samples, (see f o l l o w i n g s e c t i o n ) were found t o be s l i g h t l y more magnetic than b a s a l t . Because of s i m i l a r i t y i n age and composition the Kootenay and Dana f a c i e s of Moresby Island l i k e l y have r e l a t i v e l y high magnetic s u s c e p t i b i l i t i e s s i m i l a r t o the Tartu f a c i e s . However, much of the area occupied by p y r o c l a s t i c b r e c c i a s of the Dana f a c i e s has been metamorphosed by 104 post t e c t o n i c i n t r u s i o n s , so r e s u l t i n g c h l o r i t i z a t i o n may have reduced s u s c e p t i b i l i t y of these rocks. The magnetic s u s c e p t i b l i t y of the v o l u m e t r i c a l l y i n s i g n i f i c a n t Tow H i l l s i l l s of north-eastern Graham Island i s s i m i l a r t o t h a t of the Masset v o l c a n i c s . Medium grained diabase samples from s i l l i n t e r i o r s are s l i g h t l y more magnetic than a p h a n i t i c o l i v i n e b a s a l t s from c h i l l margins. S u s c e p t i b i l i t i e s of the Lower T e r i t a r y Metchosin b a s a l t i c t o d a c i t i c lavas of southern Vancouver I s l a n d , included here ( F i g . 24) f o r comparison purposes, are somewhat higher than those of the T e r t i a r y v o l c a n i c s of the Queen C h a r l o t t e Is l a n d s . REMNANT MAGNETIZATIONS Results of measurements of remnant i n t e n s i t i e s of o r i e n t e d cores from the Queen C h a r l o t t e Islands are reported here t o enable the r e l a t i v e magnitudes of remnant and induced magnetizations t o be assessed, and, i n conjunction with r e s u l t s of a l t e r n a t i n g f i e l d demagnetization, t o consider the s t a b i l i t i e s of remnances. The i n i t i a l remnant i n t e n s i t i e s - j were measured, i n conjunction with paleomagnetic study (Hicken and I r v i n g , 1977), on a Schonstedt Spinner Magnetometer SMI. A l t e r n a t i n g f i e l d (AF) demagnetizations were made using apparatus described by Roy et a l (1973). The I n i t i a l remnant i n t e n s i t i e s used t o c a l c u l a t e Q r a t i o s are the means of reverse o r i e n t a t i o n i n t e n s i t y measurements on i n d i v i d u a l cores. 105 r e s u l t s , p r e v i o u s l y unpublished, were supplied by E. I r v i n g ( w r i t t e n communication, Feb. 1978). Remnant I n t e n s i t i e s The r e l a t i o n s h i p between s u s c e p t i b i l i t i e s and n a t u r a l remnant magnetizations (before any e x p l i c i t demagnetization) i s shown i n F i g . 25. The s l o p i n g l i n e s i n d i c a t e the Koenigsberger r a t i o Q between the remnant magnetization R and induced magneti-z a t i o n , defined as Q = R/kH, where k i s the s u s c e p t i b i l i t y and H i s the ambient geomagnetic f i e l d , taken here t o be 0.565 Oe (45.2 A/m). C l u s t e r i n g of data p o i n t s i n F i g . 25 g e n e r a l l y r e f l e c t s core measurements from i n d i v i d u a l s i t e s . The Koenigsberger r a t i o s f o r the post and s y n t e c t o n i c p l u t o n s , both major c o n t r i b u t o r s t o the magnetic anomaly f i e l d because of high s u s c e p t i b i l i t i e s are g e n e r a l l y l e s s than 0.3 and 0.5 r e s p e c t i v e l y . The Q r a t i o s f o r the Karmutsen v o l c a n i c s ( s i n g l e core s i t e only) are l e s s than 1.0. Q r a t i o s f o r the Masset v o l c a n i c s range from 0.3 t o 9.0 w i t h most greater than 1.0, while those f o r the Tow H i l l s i l l s range from 0.7 t o 1.8. R e s u l t s f o r the T e r t i a r y v o l c a n i c s i n d i c a t e a s i g n i f i c a n t NRM component of the t o t a l magnetic anomaly f i e l d . Of eight s i t e s i n the Masset v o l c a n i c s f o r which s t a b l e remnant d i r e c t i o n s were measured, f o u r s i t e s had normal p o l a r i t i e s and f o u r had reversed (Hicken and I r v i n g , 1977). Masset r h y o l i t e s were g e n e r a l l y found t o have higher i n i t i a l remnant i n t e n s i t i e s and Q r a t i o s than Masset b a s a l t s . ( S u s c e p t i b i l i t i e s of r h y o l i t e s were a l s o higher than those of the b a s a l t s . ) Q r a t i o s f o r hornblende d i o r i t e and quartz d i o r i t e from the s y n t e c t o n i c San C h r i s t o v a l B a t h o l i t h are s i m i l a r . As remnant 106 F i g . 25 Comparison of remnant and induced magnetizations f o r core samples from the Queen C h a r l o t t e I s l a n d s . S l o p i n g l i n e s i n d i c a t e the v a r i a t i o n o f Koenigsberger Q r a t i o s . 107 i n t e n s i t i e s were a v a i l a b l e only f o r the Louise p l u t o n , i t could not be determined i f Q r a t i o s v a r i e d w i t h composition of p o s t - t e c t o n i c i n t r u s i v e rocks. A l t e r n a t i n g F i e l d Demagnetization A l l core specimens analyzed f o r NRM (Hicken and I r v i n g , 1977) were subjected t o AF demagnetization, i n order t o remove low r e l a x a t i o n time remnance. Peak demagnetizing f i e l d s up to 1000 Oe (8 x 10 4 A/m) f o r p i l o t specimens were used i n 50 Oe, (4 x 10 3 A/m) steps. F i e l d s t o reduce the NRM i n t e n s i t y by 50% (estimated g r a p h i c a l l y ) are given' i n Table V I I I . In general the 50% f i e l d s v a r i e d from 55 to 120 Oe w i t h f i e l d s f o r Masset v o l c a n i c s samples c o n s i d e r a b l y higher than those of a l l other specimens. In a number of cases (not included i n Table V I I I ) , the remnances were unstable and meaningful measurements were not p o s s i b l e a f t e r AF demagnetization. These included 2 Masset samples (CM2, b a s a l t ; CM9 r h y o l i t e ) , 4 samples of the Louise pluton (CP7, quartz d i o r i t e ; CP8, 10, 11, quartz monzonite) and one sample from the San C h r i s t o v a l B a t h o l i t h (CP16, hornblende quartz d i o r i t e ) . The low-peak 50% f i e l d s and low Koenigsberger r a t i o s f o r the Karmutsen v o l c a n i c s and p l u t o n i c rocks i n d i a t e s t h a t s t a b l e coherent remnant magnetization does not c o n t r i b u t e s i g n i f i c a n t l y to the magnetic anomaly f i e l d . The high magnetic s u s c e p t i b i l i t y of a number of these samples suggests however that low c o e r c i v i t y viscous remnant magnetizations could produce a small coherent c o n t r i b u t i o n . Coles and C u r r i e (1977) found f o r two samples (gabbro, b i o t i t e -hornblende quartz d i o r i t e ) of high magnetic s u s c e p t i b i l i t y 108 TABLE VIII MAGNETIC HARDNESS OF QUEEN CHARLOTTE ISLANDS CORE SAMPLES ROCK TYPE FIELD TO REMOVE KOENIGSBERGER SUSCEPTIBILITY 50% of NRM RATIO Q k (Oersted0 peak AF) emu/cm30 x IO - 6 TOW HILL SILLS CP4-7A Diabase 60 1.17 1480 MASSET VOLCANICS CM1-4C Basalt 85 0.96 1040 CM3-5A Basalt 95 0.29 2580 CM4-7A Basalt 105 2.30 460 CM5-2A Basalt 105 2.80 280 CM6-1A Basalt 115 1.82 870 CM7-1A Basalt 115 2.18 1230 CM8-1A Rhyolite 200 12.08 2560 CM10-3A Basaltic breccia 120 5.71 1760 CM11-1A Rhyolite 120 1.46 2310 CM12-1A Rhyolite 120 0.99 860 CM13-1B Rhyolite 165 0.63 1140 KARMUTSEN VOLCANICS CP12-1A Basalt 70 0.60 2130 POST TECTONIC PLUTONS CP9-3A Quartz Monzonite 85 0.46 910 CP13-1A Quartz Monzonite 65 1.18 500 SYNTECTONIC PLUTONS CP1-1A Hornblende Diorite 90 0.20 1680 CP2-2C Hornblende Diorite 85 0.22 2080 CP3-1A Hornblende Diorite 70 0.27 2730 CP3-4A Hornblende Diorite 80 0.18 3100 CP3-5A Hornblende Diorite 85 0.24 3090 CP14-1A Quartz Diorite 80 0.22 910 CP15-1A Quartz Diorite 55 0.28 830 a Core site and laboratory sample number assigned by Division of Geomagnetism, Earth Physics Branch b 1 Oe = 80 A/m 3 c 1 emu/cm - 12.56 in SI. 109 from the Coast P l u t o n i c Complex, t h a t r a t e s of v i s c o u s buildup were such t h a t a viscous component co u l d add a c o n t r i b u t i o n equal t o about 30% of the induced magnetization t o the anomaly f i e l d . The high peak a l t e r n a t i n g f i e l d s coupled w i t h high Koenigsberger r a t i o s of the Masset v o l c a n i c s i n d i c a t e s t h a t the NRM i s s t a b l e (high c o e r c i v i t y ) and comprises a higher percentage of the anomaly f i e l d than does induced magnetization. DISCUSSION Previous d i s c u s s i o n on s u s c e p t i b i l i t y d i s t r i b u t i o n s and remnant magentization has pointed t o the f o l l o w i n g f e a t u r e s i n magnetic p r o p e r t i e s of v o l c a n i c and p l u t o n i c rocks of the Queen C h a r l o t t e I s l a n d s . A general i n v e r s e c o r r e l a t i o n between s u s c e p t i -b i l i t y and age of v o l c a n i c s i s observed, w i t h younger rocks (Masset, Tow H i l l ) tending t o have higher s u s c e p t i b i l i t i e s than o l d e r rocks (Yakoun, Karmutsen B Mode). The younger v o l c a n i c s a l s o have a p p r e c i a b l y higher remnant i n t e n s i t i e s and Q r a t i o s . The magnetic s u s c e p t i b i l i t y of i n t r u s i v e rocks i s r e l a t e d t o composition, w i t h more b a s i c rocks having higher suscept-i b i l i t i e s . A number of authors (e.g. Puranen e t a l , 1968) have recognized the dependance of s u s c e p t i b i l i t y on the magnetite content of a rock expressed e i t h e r as a volume percent of as Fe2C"2 and FeO percentages. As expected from s u s c e p t i b i l i t y measurements, the model percent of magnetite (the c h i e f ferro-magnetic component) i s higher i n Masset b a s a l t s and r h y o l i t e s than Yakoun p o r p h y r i t i c a n desites or Karmutsen c h l o r i t i z e d b a s a l t i c l avas (Sutherland Brown, 110 1968). Gabbroic and d i a b a s i c dykes associated with both the Karmutsen and Masset v o l c a n i c s , though not sampled f o r magnetic study, have higher modal magnetite contents than the above rock types and would be expected t o have higher s u s c e p t i b i l i t i e s . The FeO and ^^2^3 percentages of A-mode Karmutsen lavas (Sutherland Brown, 1968, M u l l e r e t a l , 1974) exceed those of analyzed Masset flows though Fe202/Fe0 r a t i o s are l e s s . The magnetic s u s c e p t i b i l i t i e s and remnant i n t e n s i t i e s of Masset r h y o l i t e s and b a s a l t s might be considered anomalous si n c e the more h i g h l y s u s c e p t i b l e r h y o l i t e flows have a p p r e c i a b l y lower modal magnetite and FeO and Fe^O^ concentrations (Sutherland Brown, 1968). Haggerty (1979) shows, however, t h a t r h y o l i t e s may be more h i g h l y o x i d i z e d and more magnetic than b a s a l t s . Oxidation s t a t e increases withn i n c r e a s i n g S ^ and although bulk c o n c e n t r a t i o n of magnetic minerals i n a c i d i c s u i t e s i s r e l a t i v e l y small (1-3% by volume) these phases have large m a g n e t i c a l l y s t a b l e components of 3+ Fe i n s o l i d s o l u t i o n . Conversely advanced high temperature o x i d a t i o n , commonly observed i n s u b a e r i a l b a s a l t s , i s a ma g n e t i c a l l y d e s t r u c t i v e mineral transformation process. I l l CHAPTER VI AEROMAGNETIC MAPS OF THE QUEEN CHARLOTTE ISLANDS INTRODUCTION A d i s c u s s i o n of aeromagnetic survey maps covering northern Moreby I s l a n d , southeastern Moresby Island and northeastern Graham I s l a n d i s presented here i n order t o r e l a t e magnetic response t o known geology and magnetic p r o p e r t i e s of rock u n i t s reported i n the previous chapter. D e l i n e a t i o n of magnetic trends and patterns and s i z e and i n t e n s i t y of magnetic f e a t u r e s i s used t o e x t r a p o l a t e g e o l o g i c a l and s t r u c t u r a l data t o offshor e Hecate S t r a i t . Location and p a r t i c u l a r s r e l a t e d t o the three aeromag-n e t i c surveys are given i n F i g . 26 and Table IX. The contoured aeromagnetic data has been reproduced at a scal e of 1:125,000 ( F i g s . 28, 29,32, and 33 i n pocket) superimposed on the r e g i o n a l geologic maps of Sutherland Brown (1968). Volcanic rocks, namely the T r i a s s i c Karmutsen, J u r a s s i c Yakoun and T e r t i a r y Masset Formations have been d i v i d e d i n t o sub-facies t o i n d i c a t e predominant rock types. D i s t r i b u t i o n of non-magnetic T r i a s s i c t o T e r t i a r y sediments are not shown. In order t o s i m p l i f y the i l l u s t r a t i o n s the f l i g h t t r a v e r s e s are not included and the contour i n t e r v a l s have been reduced. The reader i s r e f e r r e d t o the o r i g i n a l maps (see i n d i c a t e d map sources i n Table IX) f o r a d d i t i o n a l d e t a i l . 112 F i g . 26 L o c a t i o n of Queen C h a r l o t t e Islands aeromagnetic maps. TABLE IX AEROMAGNETIC SURVEY DATA - QUEEN CHARLOTTE ISLANDS NORTHERN MORESBY ISLAND SOUTHEASTERN MORESBY ISLAND NORTHEASTERN GRAHAM ISLAND AREA, APPROX. (Mn2) 800 750 1150 AIRCRAFT Cessna 180 Fixed wing aircraft MAGNETOMETER Fluxgate Fluxgate Fluxgate HORIZONTAL CONTROL Airphoto laydown Airphoto laydown Airphoto laydown TERRAIN CLEARANCE 1) 3850 ft (1173 m) constant elevation 2) 500 ft (152 m) mean terrain clearance 500 ft (152 m). mean terrain clearance 1500 ft (457 m) constant elevation FLIGHT LINE SPACING 1320 ft (402 m) 1320 ft (402 m) 5280 ft (1609 m) FLOWN BY Canadian Aero Service Ltd. (now Geoterrex Ltd.) Hunting Survey Corp. Ltd. (now Lockwood Survey Corp.) ORIGINAL MAP SCALE' 1 inch = 2640 ft (>s mile) 1 : 31,680 1 Inch = 1320 ft [h mile) 1 : 15,840 1 Inch = 5280 ft (1 mile) 1 : 63,360 ORIGINAL CONTOUR INTERVAL 25 nT 20 nT 10 nT DATE FLOWN Aug.-Oct., 1959 June-Aug., 1961 BACKGROUND LEVEL (BASE INTENSITY) Arbitrary 3000 nT Arbitrary DATA SOURCE B.C. Dept. Mines Aeromagnetic Maps Series AM 59-1 to AM 59-4 Dennlson Mines Ltd. Airborne Magnetic Survey (four maps), unpublished data Grlnsfelder D., 1960, Report of Air-borne Magnetometer Survey, Graham Island Onshore Area; B.C. (Richfield Oil Corp.), B.C. Dept. Mines and Petroleum Resources, Pet. Res. Br. Open File 861. 114 HIGH LEVEL AEROMAGNETIC MAP OF BRITISH COLUMBIA In 1969, a h i g h - l e v e l , three-component airborne magnet-ometer survey was flown over B r i t i s h Columbia and the adjacent P a c i f i c Ocean (Haines and Hannaford, 1972; Hannaford and Haines, 1974; Coles and C u r r i e , 1977). A contour map of v e r t i c a l f i e l d r e s i d u a l s r e l a t i v e t o the IGRF i s reproduced i n F i g . 27. Compared with the western p o r t i o n of the Coast P l u t o n i c Complex, which i s o u t l i n e d by a high amplitude p o s i t i v e anomaly, the I n s u l a r B e l t i s r e g i o n a l l y m a g n e t i c a l l y q u i e t . Exceptions are two large ( 400 nT) p o s i t i v e anomalies centered over western Graham Island and east-c e n t r a l Moresby I s l a n d . The f i r s t anomaly o v e r l i e s a t h i c k succession of m i d - T e r t i a r y Masset Tartu f a c i e s b a s a l t s and rhyo-l i t e s , w h i l e the second o v e r l i e s , f o r the most p a r t , Masset Dana f a c i e s b a s a l t and r h y o l i t e b r e c c i a s . Proximity of the anomalies to i n d i c a t e d Masset e r u p t i o n centers (Sutherland Brown, 1966) suggests a p o s s i b l e source f o r deep c r u s t a l magnetization. The vents may have been the l o c i i of viscous enhancement, i n c r e a s i n g l y mafic rocks,' or higher oxygen f u g a c i t y t o p r e c i p i t a t e i r o n oxides. Because the thermal p a t t e r n of high heat f l o w associated with Masset v o l c a n i c a c t i v i t y w i l l have remained f o r several m i l l i o n s of year s , deeper more intense magnetizations could only p e r s i s t and be detected i n an area of r e l a t i v e l y q u i e t magnetization a f t e r the c r i t i c a l C u r r i e isotherm at depth had been passed. 115 . 27 Map of v e r t i c a l component magnetic f i e l d r e s i d u a l s f o r southwestern B r i t i s h Columbia and c o n t i n e n t a l s h e l f a t OfjnT contour i n t e r v a l . Bathymetric depths are i n meters, eproduced from Coles and C u r r i e (1977, F i g . 1). 116 NORTHERN MORESBY ISLAND Two airborne surveys of northern Moresby Island ( F i g s . 28 and 29) were flown i n 1959 f o r the B r i t i s h Columbia Department of Mines: one at a constant f l i g h t a l t i t u d e of 1173 m (3,850 f t ) and a second at an average terrane clearance of 152 m (500 f t ) . The aeromagnetic data formed part of the o v e r a l l mapping p r o j e c t of the Queen C h a r l o t t e Islands (Sutherland Brown, 1968), though the r e l a t i o n s h i p of magnetic response t o mapped geology was not discus s e d . In order t o evaluate the magnetic response produced s o l e l y by topographic e f f e c t s , both contoured aeromagnetic maps were compared with National Topographic S e r i e s Maps. Two c r o s s - s e c t i o n s ( F i g s . 30 and 31) were constructed t o i l l u s t r a t e e f f e c t of topo-graphy and geology on draped and constant l e v e l surveys. Northern Moresby Island i s c h a r a c t e r i z e d by mountainous peaks and r i d g e s , formed p r i m a r i l y of p l u t o n i c and v o l c a n i c rocks, r i s i n g t o e l e v a t i o n s as great as 1130 m (3,700 f t ) . The mountainous regions i n c l u d e deep s t r a i t s and i n l e t s t h a t cut the predominate northwest-southeast s t r u c t u r a l t r e n d . The expected e f f e c t of t h i s sharp contast i n topography on the constant e l e v a t i o n survey would be a r e l a t i v e l y stronger response over the peaks and ridges and attenuated or weaker response over v a l l e y s and i n l e t s . In F i g . 31 the constant e l e v a t i o n magnetic curve bears strong resemblance to the r i d g e - v a l l e y topography on both east and west ends of the p r o f i l e suggesting a s i g n i f i c a n t topographic e f f e c t . On c r o s s -s e c t i o n A-A1 ( F i g . 30) however, the constant e l e v a t i o n curve, here SEWELL INLET LOUISE ISLAND 3000-q LOWER-MIDDLE TERTIARY MASSET FORMATION: KOOTENAY FACIES Rhyolitlc lull, mixed volcanics UPPER CRETACEOUS M ™ HONNA FORMATION: Conglomerate UPPER CRETACEOUS HAIDA FORMATION: Shale, sandstone LOWER CRETACEOUS LONGARM FORMATION: Siltstone, sandstone MIDDLE JURASSIC YAKOUN FORMATION: And08ltlc agglomerate, volcanic sandstone UPPER TRIASSIC - LOWER JURASSIC KUNGA FORMATION: Limestone, argllllte UPPER TRIASSIC ^ y y j l KARMUTSEN FORMATION v v v v l Basaltic lava, greenstone LOWER TERTIARY I POST TECTONIC PLUTON: 1 Quartz monzonite E3! Geology modllled after Sutherland Brown (1966, Fig. 6., Cross Section H-H) Vertical exaggeration: x 3.8 Horizontal scale: 5 Kilometres 10 F i g . 30 Geologic c r o s s - s e c t i o n of northern Moresby I s l a n d along A-A' w i t h mean t e r r a i n clearance and constant f l i g h t a l t i t u d e t o t a l magnetic f i e l d p r o f i l e s . B u o z Ul S 3400 u. Ul " 3200 g 3000 I-S 2800 I 2600 P £ 2400-| o U l 2200-| [Z 9 2000 Z 1800 < S 1600 o I-B' (-1800 5 1600 1400 -oc (9 1200 2 I 1000 p 800 1-600 -100 Q - J ui tZ o 5 z e < x _i o T A L U N K W A N IS. SAN CHR ISTOVAL BATHOL ITH Z o P S E A LEVEL < • * - • LOWER—MIDDLE TERTIARY MASSET FORMATION: DANA FACIES Mixed das ! breccias UPPER C R E T A C E O U S HONNA FORMATION: Conglomerates UPPER C R E T A C E O U S I HAIDA FORMATION: Shale, Sandstone UPPER TRIASSIC — LOWER JURASS IC K U N G A FORMATION: Limestone, argllllte UPPER TRIASSIC ~ W V VI V V V 1 K A R M U T S E N FORMATION: Basaltic lava, greenstone, amphlbollte TERTIARY I POST T E C T O N I C PLUTON: I Quartz monzonite • c n • m i E3! Geology modified alter Sutherland Brown (1968, Fig. 6, Cross-Secl lon G - G ) Vertical exaggeration: x 3.8 MIDDLE JURASS IC Y A K O U N FORMATION: Andesitic agglomerate UPPER JURASS IC S Y N T E C T O N I C PLUTON: Hornblende diorite Horizontal scale: 5 10 g. 31 Geologic c ro s s - sec t i on of northern Moresby Island along B-LV with mean t e r r a i n c learance and constant f l i g h t a l t i t u d e to ta l magnetic f i e l d p r o f i l e s . 119 more subdued, bears l i t t l e s i m i l a r i t y t o topography and any t e r r a i n e f f e c t appears small compared to that from c a u s a t i v e rock bodies. On both c r o s s - s e c t i o n s , the smoother, constant e l e v a t i o n curve may be considered a simple f i l t e r e d v e rsion of the mean t e r r a i n clearance magnetic curve because of s i m i l a r i t y i n response. This observation i n i t s e l f suggests that t e r r a i n e f f e c t s are small compared t o g e o l o g i c e f f e c t s . The predominant magnetic trend i n the northern Moresby Island survey area i s o r i e n t e d northwest-southeast, p a r a l l e l to the main s t r u c t u r a l elements. Secondary northeast-southwest and east-west trends c h a r a c t e r i z e the c e n t r a l and southern p o r t i o n s . An arcuate northwest-southeast magnetic trough corresponds with an eastern splay or extension of the Rennel F a u l t on e a s t - c e n t r a l Louise I s l a n d . Continuation of t h i s trough o f f s h o r e , t o the east of Louise Island suggests a submarine extension of the f a u l t . Magnetic expression of the north-northwest trending Louscoone F a u l t i s l i m i t e d , on the constant t e r r a i n clearance magnetic map, to a s m a l l , low r e l i e f , magnetic trough i n the v i c i n i t y of Louise Narrows and c u r v a t i v e or i n f l e c t i o n of magnetic contours near Dana Passage. As s i m i l a r rock types are juxtaposed on e i t h e r s i d e of the f a u l t t r a c e through much of the survey area, magnetic expression may be due i n part t o a topographic e f f e c t . A broad magnetic plateau (on the constant t e r r a i n clearance survey), c h a r a c t e r i z e d by high amplitude magnetic anoma-l i e s , covers the eastern p o r t i o n of Moresby Island west of Dana Passage, eastern Talunkwan I s l a n d , and southern Louise I s l a n d . The m a j o r i t y of t h i s area i s u n d e r l a i n by Masset Dana f a c i e s mixed c l a s t 120 t u f f b r e c c i a s , f e l d s p a r porphyries and r h y o l i t e s . The magnetic plateau corresponds t o the p o s i t i v e magnetic anomaly described p r e v i o u s l y from the high l e v e l v e r t i c a l f i e l d aeromagnetic map ( F i g . 27). The magnetic anomalies c h a r a c t e r i z i n g Masset Kootenay f a c i e s v o l c a n i c s on western Moresby Island ( F i g . 29) are more subdued than those associated with the Dana f a c i e s . Short magnetic ridges and troughs, trending roughly northwest-southeast over the Kootenay v o l c a n i c s , c o n t r a s t with east-west trending low r e l i e f anomalies of f l a n k i n g Karmutsen v o l c a n i c s . A f a u l t t rending east-west and bounding the Kootenay rocks south of Douglas I n l e t i s marked by a steep magnetic gradient. Coarse grained gabbroic Masset i n t r u s i v e s (such as the large l i n e a r body east of B a r r i e r Bay) have associated high magnetic values. The T e r t i a r y p o s t - t e c t o n i c plutons where i n contact with e i t h e r Masset or Karmutsen v o l c a n i c s or metamorphosed equi v a l e n t s have as s o c i a t e d high magnetic r e l i e f . Highest magnetic values associated with the Louise Pluton are s i t u a t e d along i t s western f l a n k where Karmutsen greenstones and Yakoun v o l c a n i c sandstones, t u f f s and h o r n f e l s i c rock have been i n t r u d e d . The core of the Louise Pluton of quartz monzonitic composition, has lower magnetic values, and suggests the p o s s i b i l i t y that i t may be zoned and enveloped by more h i g h l y s u s c e p t i b l e b a s i c i n t r u s i v e rocks. [The quartz monzonite Cumshewa Pluton on northeastern Moresby Island has border phases of more basic rock types (Sutherland Brown, 1968, p.138] A l t e r n a t i v e l y , the high magnetic values may be associated 121 with m i n e r a l i z a t i o n near the i n t r u s i v e contact zone, s i m i l a r t o the synte c t o n i c b a t h o l i t h (see below). Continuation of a magnetic trend across Selwyn I n l e t i n d i c a t e s that the Louise and Talunkwan Plutons are part of the same i n t r u s i v e body. Northward, magnetic highs along the same trend cross a b e l t of metamorphosed ( h o r n f e l s i c ) Yakoun t u f f s and v o l c a n i c sandstones. The high magnetics suggest shallow b u r i a l of a yet unroofed p o r t i o n of the Louise Pluton. The magnetic character of the Lagoon Pluton ( a l s o of quartz monzonite composition) i s s i m i l a r t o th a t of the Louise P l u t o n . Highest magnetic values on the mean t e r r a i n clearance survey map are recorded near contacts with Kootenay f a c i e s r h y o l i t i c t u f f s . The anomaly patte r n on the constant e l e v a t i o n survey map i s represented by a s i n g l e broad magnetic high with three separate magnetic peaks. The broad NE-SW trending magnetic low, south of the Lagoon Pluton, near Sewell I n l e t , covers an area u n d e r l a i n f o r the most part by Upper Cretaceous Queen C h a r l o t t e Group sediments. The Yakoun v o l c a n i c rocks d i s p l a y varying magnetic signature as would be expected by the wide range i n measured magnetic s u s c e p t i b i l i t i e s . Volcanic sandstones and t u f f s east of B a r r i e r Bay and at the head of Selwyn I n l e t d i s p l a y l i t t l e or no magnetic r e l i e f and cannot be d i s t i n g u i s h e d on the magnetic maps from Cretaceous sediments. A magnetic high does however ch a r a c t e r -i z e s i m i l a r Yakoun rocks west of Louise Narrows. Although i t might be expected t h a t Yakoun p o r p h y r i t i c andesites would d i s p l a y higher magnetic values, the exposure on the east side of Louise Island appears as a magnetic low. 122 Magnetic r e l i e f over Karmutsen v o l c a n i c rocks i s gener-a l l y subdued with comparatively low magnetic values, i n c o n t r a s t to northern Vancouver Island where the Karmutsen d i s p l a y s both higher r e l i e f and higher magnetic values above background (Eastwood, 1976). On Moresby I s l a n d , somewhat higher magnetic values are more commonly associated with massive flows than with p i l l o w l a v a s . High r e l i e f and high magnetic values are associated with Karmutsen massive lavas and amphibolites where intruded by the San C h r i s t o v a l B a t h o l i t h (see a l s o c r o s s - s e c t i o n B-B', F i g . 31) i n the southwestern p o r t i o n of the survey area. Kuniyoshi and Liou (1976) have shown th a t Karmutsen v o l c a n i c rocks i n contact metamorphic zones on Vancouver Island commonly conta i n abundant secondary magnetite. Amphibolite d e r i v a t i v e s may thus have high magnetiza-t i o n s as i n d i c a t e d by magnetic values near the San C h r i s t o v a l B a t h o l i t h . Intense l o c a l i z e d magnetic highs (best seen on the constant t e r r a i n clearance survey map) are associated with magnetite iron-copper deposits at Tasu and Garnet mine s i t e s . No appreciable anomaly i s associated w i t h magnetite showings at the Old Townsite Mine. Geology of the Tasu and Garnet ore bodies i s s i m i l a r ; both are located above the top of the Karmutsen, and replace massive Kunga limestones. S k a r n i f i c a t i o n and m i n e r a l i z a t i o n , i n c l u d i n g magnetite replacement, r e s u l t e d from repeated igneous i n t r u s i o n that included emplacement of the San C h r i s t o v a l B a t h o l i t h (Sutherland Brown, 1968). A l o c a l magnetite high i s associated with magnetite i r o n ore at the Iron Duke property on northern Louise I s l a n d . 123 The s y n t e c t o n i c San C h r i s t o v a l B a t h o l i t h has magnetic character s i m i l a r t o that of the Karmutsen v o l c a n i c s . Magnetic lows predominate i n most areas excepting over i n t r u s i v e contacts with the Karmutsen. The o r i e n t a t i o n of magnetic f e a t u r e s r e l a t e d t o the B a t h o l i t h or i t s e n c l o s i n g rocks are o r i e n t e d northwest-southeast, s i m i l a r t o s t r i k e d i r e c t i o n , contact o r i e n t a t i o n and trend of minor f a u l t s . SOUTHEASTERN MORESBY ISLAND An aeromagnetic survey with constant t e r r a i n clearance was flown i n 1961 f o r Denison Mines L t d . along a 50 km s t r i p of southeatern Moresby Island from Burnaby Island south t o Kunghit Island ( F i g . 32). The survey formed part of an i n t e n s i v e e x p l o r -a t i o n program f o r i r o n ore deposits (see Sutherland Brown pp. 194-197, B.C., Ann.Rept., 1963, pp. 18-21). The major f e a t u r e s of the aeromagnetic map are the r e l a t i o n s h i p s of magnetic troughs t o major t r a n s c u r r e n t and minor block f a u l t s and magnetic highs t o Southern Group p o s t - t e c t o n i c plutons and magnetite i r o n ore d e p o s i t s . A north-northwest trending l i n e a r magnetic trough corresponds t o the t r a c e of the Lousconne F a u l t from Kunghit Island south through Luxana Bay. C i r c u l a r t o lobate magnetic lows located over Rose I n l e t suggest a southern extension to a minor f a u l t that trends north from the head of the i n l e t . Elongate east-west to northeast-southwest magnetic depressions, o r i e n t e d roughly perpendicular t o the predominant 124 magnetic trend c h a r a c t e r i z e S k i n k u t t l e I n l e t , and t o a l e s s e r degree Carpenter Bay and Stewart Channel. The s t r u c t u r e of S k i n k u t t l e I n l e t comprises g e n t l y dipping panels of Karmutsen, Kunga and Yakoun rocks, r a r e l y f o l d e d , which trend east to north-east and dip north-ward, cut by a number of steep block f a u l t s (Sutherland Brown, 1968, p. 195). The major magnetic highs correspond i n part t o the magnetite i r o n ore deposits at J i b on southeastern Burnaby Island and Jedway south of H a r r i e t Harbour. The geology of these deposits i s s i m i l a r t o that described p r e v i o u s l y f o r the Tasu and Garnet minesites although the major i n t r u s i v e phase may be younger. The magnetic highs, w i t h r e l i e f greater than 1,000 nT, extend over the quartz monzonitic J i b (south Burnaby) and d i o r i t i c t o quartz d i o r i t i c Jedway st o c k s . Offshore d r i l l i n g of one of the magnetic highs south of J i b has confirmed the presence of magnetite and suggests that the two smaller magnetite peaks east of the main anomaly may a l s o be r e l a t e d t o extensive m i n e r a l i z a t i o n (Sutherland Brown, 1968, p. 194; B.C., Ann.Rept. 1963). Magnetic highs associated with the C o l l i s i o n , Carpenter, and Point Langford plutons are of lower r e l i e f and may r e f l e c t the s l i g h t l y more a c i d i c g r a n i t i c t o quartz monzonitic composition of these plutons compared t o the Jedway and J i b p l u t o n s . The g r a n i t i c Carpenter Pluton has no magnetic expression. The magnetic high located on the northeast corner of Kunghit Island and extending a short d i s t a n c e o f f s h o r e o v e r l i e s a p a r t l y exposed stock t h a t may form part of the Point Langford P l u t o n . The Point Langford and C o l l i s i o n Bay plutons are roughly o u t l i n e d by the 2900 and 3200 nT 125 countour l i n e s . The Upper J u r a s s i c Burnaby Island Pluton of mafic r i c h quartz monzonite, though only p a r t i a l l y covered by the aeromag-n e t i c survey, would be expected t o d i s p l a y high magnetic r e l i e f . Migmatites of mixed hornblende d i o r i t e and amphibolite exposed on Kunghit I s l a n d , west of Luxana Bay, have associated low-moderate magnetic r e l i e f . The magnetic patt e r n of Karmutsen v o l c a n i c s comprises c i r c u l a r t o lobate magnetic ridges and troughs of g e n e r a l l y low magnetic values, s i m i l a r to northern Moresby I s l a n d . Boundaries of Karmutsen lavas or greenstones and Kunga limestones cannot be determined from magnetic data. NORTHEASTERN GRAHAM ISLAND A constant t e r r a i n clearance airborne magnetic survey ( F i g . 33) was flown over northeastern Graham Island f o r R i c h f i e l d O i l Corporation ( G r i n s f e l d e r , 1960) i n order to map basement s t r u c t u r e of the northwest p o r t i o n of the Queen C h a r l o t t e Basin. Basement rocks penetrated by fo u r R i c h f i e l d et aj_ ex p l o r a t o r y w e l l s included v o l c a n i c b r e c c i a s and conglomerate, r h y o l i t e and massive b a s a l t s of the Masset Tartu f a c i e s (Sutherland Brown, 1968). Magnetic property a n a l y s i s of Tartu v o l c a n i c samples and cores i n d i c a t e d high magnetic s u s c e p t i b i l i t y and s t a b l e remnant magnetization. The magnetic p a t t e r n of northeastern Graham Island comprise s h o r t e r wavelength, higher amplitude anomalies i n the northern p o r t i o n of the survey area and longer wavelength,lower amplitude anomalies i n the southern p o r t i o n . The magnetic 126 'provinces' correspond i n part t o two major sub-basins separated by a 'magnetic' ridge that extends southeastward from v i c i n i t y of the Masset No. 1 w e l l . Geologic data (Sutherland Brown, 1968) and a (?) Pliocene time s t r u c t u r e map ( G r i n s f e l d e r , 1960) that covers the eastern p o r t i o n of Graham Island a l s o i n d i c a t e the presence of a basement r i d g e . In the northern sub-basin, a p a t t e r n of high-amplitude, lobate and b i f u r c a t i n g anomalies i n d i c a t e s shallow depths t o Masset source rocks and p o s s i b l e basement f a u l t i n g . A magnetic d i p o l e p a t t e r n i n the western p o r t i o n of the sub-basin r e f l e c t s p a r t i a l c a n c e l l a t i o n or reinforcement of the t o t a l f i e l d by vector a d d i t i o n of remnant (normal or reverse) and induced magnetizations. A c i r c u l a r magnetic high with r e l i e f of 250 nT corresponds t o Argonaut H i l l , a P l e i s t o c e n e g l a c i a l f e a t u r e that appears to have developed on a l o c a l basement f l e x u r e . Depth t o basement estimates suggests sediment cover as t h i n as 2000' compared t o greater than 6000' at the nearby Tow H i l l w e l l . In the southern sub-basin magnetic anomalies of lower r e l i e f r e f l e c t t h i c k e r sediment i n f i l l over an i r r e g u l a r basement topography that slopes eastward toward the c e n t r a l p o r t i o n of the Queen C h a r l o t t e Basin. RELATIONSHIP OF MAGNETIC ANOMALIES TO MAGNETIC ROCK PROPERTIES The preceeding d i s c u s s i o n on aeromagnetic anomaly patterns and magnetic rock p r o p e r t i e s suggests a number of basic correspondences t o the geologic u n i t s , rock types and major 127 s t r u c t u r e s on the Queen C h a r l o t t e I s l a n d s . Generalized anomaly types and magnetic c h a r a c t e r i s t i c s are summarized i n Table X. As expected from s u s c e p t i b i l i t y and remnant i n t e n s i t y measurements the highest amplitude magnetic anomalies are associated with the T e r t i a r y Masset v o l c a n i c s and p o s t - t e c t o n i c p l u t o n s . Because remnant magnetization comprises the l a r g e r percentage of the Masset anomaly f i e l d , a magnetic d i p o l e p a t t e r n r e f l e c t i n g normal and reverse p o l a r i t i e s i s o f t e n associated w i t h the v o l c a n i c s . The prominently developed aeromagnetic response signature of the p o s t - t e c t o n i c plutons i s a f u n c t i o n of high S iO ^  content and correspondingly high v i s c o s i t y that has constrained the l a t e r a l extent of the epizonal plutons. The r e l a t i v e amplitude of magnetic anomalies associated with the plutons depends on the v a r i e t y of the i n t r u s i v e rock and type of invaded rock. A n a l y s i s of rock proper-t i e s of the p l u t o n i c rocks showed the more basic v a r i e t i e s to have somewhat higher magnetic s u s c e p t i b i l i t i e s . Magnetic anomalies associated w i t h the Karmutsen v o l c a n i c s , excepting those near i n t r u s i v e c o n t a c t s , are of lower r e l i e f and i n t e n s i t y than what might be expected from measured magnetic rock p r o p e r t i e s . In p a r t i c u l a r , the aeromagnetic patterns would appear t o r e f l e c t the weaker 'B' mode magnetic s u s c e p t i b i l -i t y . A l t e r a t i o n and p o s s i b l e demagnetization due t o reheating, i n a d d i t i o n t o sample bia s may e x p l a i n the discrepancy. Because of s i m i l a r i t y i n remnant magnetization d i r e c t i o n s t o Masset v o l c a n i c s , Hicken and I r v i n g (1977) suggested that the Karmutsen v o l c a n i c s may have been weakly remagnetized during T e r t i a r y time. The low amplitude subdued aeromagnetic pattern of the 128 TABLE X RELATIONSHIP OF MAGNETIC ANOMALIES AND MAGNETIC  ROCK PROPERTIES TO GEOLOGIC UNITS QUEEN CHARLOTTE ISLANDS FORMATION/UNIT i MAGNETIC ROCK PROPERTY MAGNETIC ANOMALY CHARACTERISTICS PIio-Pleistocene Tow H i l l S i l l s High S u s c e p t i b i l i t y High Q Not established Lower-Middle Tertiary High S u s c e p t i b i l i t y High amplitude, medium-short wavelengths; Magnetic dipoles Middle Jurassic Yakoun Volcanics Variable S u s c e p t i b i l i t y Generally Low Low amplitude Long wavelength Upper T r i a s s i c High or Low S u s c e p t i b i l i t y Low-medium amplitude, medium wavelength (High amplitudes near intru s i v e contacts) Cretaceous-Tertiary Post Tectonic Plutons High S u s c e p t i b i l i t y Low Q (Variations due to rock type) Medium-high amplitude Medium-short wavelength (Higher amplitudes associated with more basic rock types) Upper Jurassic Syntectonic Plutons Medium-High S u s c e p t i b i l i t y Low Q (Variations due to rock type) Low-medium amplitude Medium wavelengths 129 Yakoun v o l c a n i c s i s expectable from g e n e r a l l y low magnetic s u s c e p t i -b i l i t i e s . The low-medium amplitude response of s y n t e c t o n i c i n t r u -s i v e rocks r e f l e c t s magnetic i n t e n s i t i e s somewhat weaker than i n d i c a t e d by medium-high s u s c e p t i b i l i t i e s . Major wrench f a u l t s are i n d i c a t e d on the aeromagnetic maps by a l i n e a r p a t t e r n , e i t h e r as a steep magnetic gradient or elongate magnetic trough. A number of u n c e r t a i n t i e s a r i s e when attempting to r e l a t e magnetic rock p r o p e r t i e s and aeromagnetic patterns to s p e c i f i c g e o l o g i c a l u n i t s on the Queen C h a r l o t t e I s l a n d s . Geolog-i c a l c o m p l e x i t i e s and s i m i l a r i t y of both magnetic rock p r o p e r t i e s and magnetic patterns ( i . e . Karmutsen with s y n t e c t o n i c plutons and Masset with p o s t - t e c t o n i c plutons) make unique rock type i d e n t i f i -c a t i o n d i f f i c u l t . Combined with other geophysical information however, the magnetic data can c o n s t r a i n the number of p o s s i b l e s o l u t i o n s . 130 CHAPTER VII MAGNETIC ANOMALIES OF WESTERN HECATE STRAIT AND NORTHWESTERN QUEEN CHARLOTTE SOUND INTRODUCTION Total magnetic f i e l d i n t e n s i t y was measured along 1287 km of ships t r a c k during I.O.U.B.C. Cruise No. 76-10. The purpose of the magnetic survey, run i n conjunction w i t h high r e s o l u t i o n seismic p r o f i l i n g (chapter V I I I ) , was t o a i d mapping of basement rocks and s t r u c t u r e along the western margin of the Queen C h a r l o t t e Basin. A c q u i s i t i o n , r e d u c t i o n , c o r r e c t i o n and e r r o r assessment of data are described i n Appendix 1. The contoured r e s i d u a l magnetic f i e l d map (with respect t o I.G.R.F.) i s shown i n F i g . 34 ( i n pocket) and magnetic anomaly p r o f i l e s i n F i g . 35. The data c o l l e c t e d f o r t h i s study has been supplemented by marine magnetic survey data from Queen C h a r l o t t e Sound ( T i f f i n , 1974; T i f f i n and C u r r i e , 1976; see F i g . 34) and the oceanic area adjacent to Queen C h a r l o t t e Sound ( S r i v a s t a v a , 1971; S r i v a s t a v a et a l , 1971; S r i v a s t a v a , 1973). As l a r g e d i s c r e p a n c i e s (80-100 nT) were found when the oceanic survey data was compared t o l a t e r survey data a constant c o r r e c t i o n t o minimize e r r o r was introduced. Relevant p o r t i o n s of a survey conducted i n 1975 i n Hecate S t r a i t and Queen C h a r l o t t e Sound by J.W. Murrary and R.D. Macdonald (I.O.U.B.C. Cruise No. 75-22) have a l s o been included ( f o r l i n e l o c a t i o n , see F i g . 1). The northern and southeastern Moresby Island aeromagnetic 1 3 1 survey data (see previous d i s c u s s i o n ) has been incorporated i n t o the r e s i d u a l magnetic f i e l d map ( F i g . 34), although no attempt has been made to s t r i c t l y r e c o n c i l e the d i f f e r e n t e l e v a t i o n s of marine and airborne data s e t s . For northern Moresby I s l a n d , the constant e l e v a t i o n ( r a t h e r than constant t e r r a i n clearance) survey data was inte g r a t e d with the marine data because of s i m i l a r i t y i n peak to peak f i e l d strength over contiguous areas. Data sets were combined by simple s h i f t to l e v e l of aeromagnetic data (1600 nT contour = 0 nT, r e l a t i v e t o IGRF) t o provide coherency along boundary p o i n t s . The aeromagnetic data f o r southeastern Moresby Island was f i l t e r e d by simple nonweighted smoothing over a 0.5 inch sq. g r i d before being combined with the marine data (2750 nT contour = 0 nT, r e l a t i v e t o IGRF). Upward c o n t i n u a t i o n of the aeromagnetic data was required because the sho r t e r wavelength and greater d e t a i l represented by the draped airborne survey does not r e f l e c t d i f f e r -ences i n geology to the a d j o i n i n g marine area but only d i f f e r e n c e s i n survey type, sample d e n s i t y , and e l e v a t i o n . S t r i c t i n t e g r a t i o n procedure would req u i r e an equivalent source r e p r e s e n t a t i o n of the airborne data before c o n t i n u a t i o n , removal of the IGRF from the aeromagnetic data, and boundary c o r r e c t i o n s (Bhattacharyya e_t _al_, 1978). D i f f i c u l t i e s i n i n t e g r a t i n g the airborne and marine data se t s of southeastern Moresby Island and Hecate S t r a i t can be explained by the simple i n t e r g r a t i o n procedure employed and changes in gradient and curvature of the magnetic f i e l d s i n the two data sets near t h e i r common boundary. 132 INTERPRETATION PROCEDURE The use of marine magnetic data to i n f e r the geology of completely obscured basement rock i n v o l v e s considerable u n c e r t a i n -t i e s , p a r t l y because of the inherent ambiguity of the geophysical method and p a r t l y because of the manyfold correspondences between anomalies and rock types. Magnetic rock property and aeromagnetic data from the Queen C h a r l o t t e Islands does provide considerable information on e m p i r i c a l c o r r e l a t i o n s between gross geologic u n i t s and s t r u c t u r e s and t h e i r associated magnetic f i e l d s . The i n t e r -p r e t a t i o n of the geology of western Hecate S t r a i t and northwestern Queen C h a r l o t t e Sound makes use of such 1ithology-anomaly c o r r e s -pondences but a l s o r e l i e s on extension of known rock d i s t r i b u t i o n s from nearby land. In areas where there i s d i r e c t c o n t i n u a t i o n of we l l defined s t r u c t u r a l trends, the i n t e r p r e t a t i o n i s more c l o s e l y constrained and geologic u n i t s have been mapped with some degree of c e r t a i n t y to the inner s h e l f edge, and beyond. For both the inner s h e l f and basin p o r t i o n s of the survey area, bathymetric, high reso-l u t i o n s e i s m i c , seismic r e f r a c t i o n , g r a v i t y , and w e l l data when combined with magnetic data provide addtional evidence f o r p r e d i c t i o n of gross l i t h o l o g y . Speculation about age and s t r u c t u r e must be considered p r e l i m i n a r y . A d d i t i o n a l data w i l l c e r t a i n l y r e f i n e the present i n t e r p r e t a t i o n . DEPTH TO BASEMENT ESTIMATES Depth t o basement estimates have been c a l c u l a t e d from 133 the t o t a l f i e l d magnetic data t o a i d i n c o m p i l a t i o n of a s i m p l i f i e d s t r u c t u r e map of the western p o r t i o n of the Queen C h a r l o t t e Basin ( F i g . 36 i n pocket). A d d i t i o n a l data sources i n c l u d e a s i m p l i f i e d isopach map of Neogene Skonun sediments (Shouldice, 1973; see F i g . 18), a basin model derived from i s o s t a t i c g r a v i t y anomaly data (Stacey, 1975, see F i g . 19), basement depths from e x p l o r a t o r y w e l l s (Shouldice, 1973; S h e l l Canada Well H i s t o r y Reports) and seismic r e f r a c t i o n p r o f i l e s (Mahannah, 1965). Seismic r e f l e c t i o n p r o f i l e s (Chapter V I I I ) , l i m i t e d because of shallow depth of p e n e t r a t i o n , were used t o d e f i n e the basin edge. Depths t o magnetic basement were c a l c u l a t e d using a number of graphic ( e m p i r i c a l ) estimators and c h a r a c t e r i s t i c curves (Am, 1972). Where the azimuth of magnetic p r o f i l e s deviated from perpendicular t o the s t r i k e of the magnetic body, the depth estimate was m u l t i p l i e d by the cosine of the d e v i a t i o n . I t has been assumed that the d i r e c t i o n of magnetization does not depart s i g n i f i c a n t l y from t h a t of the current earth's f i e l d ( f o r the survey area, i n c l i n -a t i o n : 26°E, d e c l i n a t i o n : 71°N, t o t a l i n t e n s i t y ( F ) : 56,500 nT; Dawson and Newitt, 1977). Comparison of a s u b j e c t i v e average of depth estimates wit h c o n t r o l p o i n t s provided by other data sources suggests that most estimates should be accurate w i t h i n +10-15%. In areas of broad magnetic f e a t u r e s , i n t e r p r e t e d strong supra-basement magnetic e f f e c t s or where pre-Skonun subcrop i s known or assumed t o be non-magnetic (e.g. southern Hecate sub-basin) basement s t r u c t u r e was not mappable from magnetic data. F i n a l i n t e g r a t i o n and recon-c i l i a t i o n of a l l data sources was n e c e s s a r i l y i n t e r p r e t a t i v e . The 134 placement and o r i e n t a t i o n of major depocentres and basement ridges i s considered accurate although small changes i n basement r e l i e f and f a u l t s are not shown on the s t r u c t u r e map. INTERPRETATION OF MARINE MAGNETIC ANOMALIES I n t e r p r e t a t i o n of marine magnetic anomaly patterns f o l l o w s , based on data acquired from: 1) the inner s h e l f region east of southern Queen C h a r l o t t e I s l a n d s , 2) Moresby ridge and Hecate and C h a r l o t t e subbasins of the Queen C h a r l o t t e Basin (western Hecate S t r a i t ) , 3) northwestern Queen C h a r l o t t e Sound, 4) the,oceanic area immediately adjacent to northwestern Queen C h a r l o t t e Sound. Interpreted basement seacrop on the inner s h e l f , subcrop beneath Neogene sediments and basement s t r u c t u r e contours of the Queen C h a r l o t t e Basin are shown i n F i g . 36. Inner Shelf The inner s h e l f region, which exposes Mesozoic-mid Cenozoic sedimentary and c r y s t a l l i n e rocks as seacrop, extends from onshore Queen C h a r l o t t e Islands t o the edge of the Neogene C h a r l o t t e Basin ( F i g . 36). The edge of the basin does not correspond with a change i n submarine topography, except o f f Kunghit Islan d (compare F i g . 36 to F i g . 41) but marks the l i m i t of landward encroachment of Neogene Skonun sediments. From Louise Island south to Kunghit Island the 'basement' (pre-Skonun) seacrop pattern of the inner s h e l f has been 135 mapped ( F i g . 36) from magnetic ( F i g . 34) and bathymetric ( F i g . 41) data and onshore geology ( F i g . 42). Expected seacrop patterns i n channels and i n l e t s of eastern Moresby I s l a n d , outside the marine survey area, are a l s o shown. The f o l l o w i n g general observations concerning the c o a s t a l geology have aided p r e d i c t i o n and mapping of seacrop. 1) Upper J u r a s s i c Yakoun a n d e s i t i c agglomerate, as a more competent rock type, forms small i s l e t s along the east coast of southern Queen C h a r l o t t e Islands and i s l i k e l y exposed on small bathymetric ridges ( p a r t i c u l a r l y i n Laskeek Bay). 2) Ice-deepened channels (e.g. Juan Perez Channel) cut p r o g r e s s i v e l y deeper westward i n t o the ?Upper Paleozoic-Mesozoic s t r a t i g r a p h i c succession exposing o l d e r rocks as seacrop. 3) Upper T r i a s s i c Karmutsen v o l c a n i c s are exposed as the eroded cores of wrench-related a n t i c l i n a l and homoclinal s t r u c t u r e s . Overlying and f l a n k i n g sediment cover may or may not be preserved. 4) S t r u c t u r a l response along the Rennel-Louscoone wrench f a u l t system (with an echelon f o l d s and conjugate s t r i k e - s l i p f a u l t s ) has produced i n some areas a p r e d i c t a b l e rock outcrop p a t t e r n (Chapter X ) . Wrench f a u l t s commonly f o l l o w Kunga limestones because of i t s thin-bedded and carbonaceous nature (Sutherland Brown, 1968, p. 56). From Louise Island south t o L y e l l Island seacrop patterns represent 136 an extension of c o a s t a l geology and s t r u c t u r e east toward the Hecate sub-basin edge. The most c h a r a c t e r i s t i c f e a t u r e of the magnetic anomaly p a t t e r n o f f Louise I s l a n d i s a southeast trending magnetic trough that extends across the inner s h e l f to a p o s s i b l e submarine extension of the Sandspit F a u l t i n Hecate sub-basin ( F i g . 36). The trough (with magnetic lows of -lOOnT) represents an o f f s h o r e exten-s i o n of the Rennel wrench f a u l t system ( F i g . 61) t h a t includes a major through going r i g h t l a t e r a l f a u l t and r e l a t e d p a r a l l e l to s u b p a r a l l e l f a u l t and f o l d s t r u c t u r e s . South of Louise I s l a n d , the magnetic contours trend i n a more sou t h e r l y d i r e c t i o n r e f l e c t i n g the notable d e f l e c t i o n at Louise Island (east of the Louise Pluton) i n the main strand of the Rennel-Louscoone system. The geology of Laskeek Bay (see c r o s s - s e c t i o n B-B 1, F i g . 31) i s c h a r a c t e r i z e d by a s e r i e s of small f a u l t bounded wrench r e l a t e d f o l d s . East of the Louscoone f a u l t zone and extending from southern Louise Island t o northern L y e l l Island an a n t i c l i n a l b e l t of Karmutsen greenstones and Kunga limestones and a r g i l l i t e s are exposed as outcrop and seacrop. Further east i n Laskeek Bay, i n a f a u l t bounded sync!ine, Yakoun p y r o c l a s t i c s and a n d e s i t i c agglom-erates are exposed as seacrop and as outcrop on Reef and Lost Islands. Towards the c e n t r a l p o r t i o n of Laskeek Bay (west of Reef Island) s t r u c t u r e and seacrop patterns are r e l a t e d to the southeast extension of the Rennel F a u l t . A f a u l t bounded, doubly plunging a n t i c l i n e southwest of the main strand, extends from Louise Island southeast toward Reef I s l a n d , exposing Karmutsen v o l c a n i c s and Kunga Limestones. The comparatively low magnetic values i n the Laskeek Bay 137 region p a r t i a l l y r e f l e c t s the weak magnetization (low s u s c e p t i -b i l i t i e s ) a ssociated with Yakoun p y r o c l a s t i c s and Karmutsen greenstones. P a r t i a l demagnetization ( l o s s of primary magnetite) could be associated with the shearing and c a t a c l a s i s of w a l l rocks along the major wrench f a u l t s and s u b s i d i a r y s p l a y s . North of the Rennel F a u l t , Albian-Turonian marine e l a s t i c s of the Haida Formation are exposed i n an almost continuous b e l t that extends from Graham Island southeast to northern Louise Island and Hecate S t r a i t . Most of Cumshewa I n l e t appears t o have been cut i n t o Haida sediments. Seacrop of the marine e l a s t i c s , extending east of northern Louise Island to the Sandspit F a u l t (subbasin edge) i s probably discontinuous. Remnants of o l d Yakoun volcanoes may at one time have been blanketed by a t h i n cover of marine sediments, s i n c e eroded, or the volcanoes may themselves have r e s t r i c t e d marine d e p o s i t i o n w i t h i n the Skidegate Basin (Sutherland Brown, p. 92) to i n t e r v e n i n g lows. On the inner s h e l f , south of the Rennel F a u l t and east of the mapped f o l d b e l t s , Yakoun v o l c a n i c s probably seacrop to the indented s h e l f edge. Reef Island has been i n d i c a t e d as a p o s s i b l e vent of Yakoun a n d e s i t i c agglomerates and t u f f s (Sutherland Brown, 1968, p. 92). The low amplitude of magnetic anomalies over the probable Yakoun rocks i s s i m i l a r to the aeromagnetic response over the v o l c a n i c s on northern Moresby I s l a n d . South of Cumshewa Point high magnetic values ( 900 nT) i n d i c a t e a submarine extension of the Cumshewa Pluton. A s i m i l a r high amplitude anomaly i s p a r t i c u l a r l y n o t i c e a b l e on the s i m p l i f i e d t o t a l magnetic i n t e n s i t y map of the Queen C h a r l o t t e Basin 138 (Shouldice, 1973; see F i g . 20). High magnetic values ( 650 nT) suggest a p o s s i b l e small pluton underlying Laskeek Bay southwest of Reef I s l a n d . Seacrop on the narrow inner s h e l f east of L y e l l and Ramsay Islands comprises v o l c a n i c s of the Masset Dana f a c i e s . (Magnetic coverage does not extend over the inner s h e l f i n t h i s region.) On northeastern L y e l l I s l a n d , the Tar Islands (east of L y e l l Island) and most of Ramsay Island Dana v o l c a n i c b r e c c i a s are exposed. (Whole rock K-Ar dates of 21+1 and 27+2 Ma were obtained from samples from the Tar I s l a n d s ; see d i s c u s s i o n Chapter I I I . ) These v o l c a n i c s dip east to southeast (20-30°) toward the Hecate sub-basin on the Tar Islands and north-northeast (15-25°) on Ramsay Island (Sutherland Brown, 1968, F i g . 5). I n t r u s i v e b i o t i t i c f e l d s p a r porphyries and dykes (Masset hypabyssal e q u i v a l e n t s ) are exposed on southeastern L y e l l I s l a n d . The southern l i m i t of Masset exposure i s i n d i c a t e d i n F i g . 36 as the northern edge of Juan Perez Channel. Here Masset v o l c a n i c s probably r e s t unconformably on Karmutsen v o l c a n i c s . The low magnetic values northeast of L y e l l Island i n d i c a t e that Masset v o l c a n i c s , normally h i g h l y magnetic, do not extend northwards as seacrop i n southern Laskeek Bay. Juan Perez Channel has been cut i n t o predominately southwesterly d i p p i n g panels of Karmutsen v o l c a n i c s and metamorphic eq u i v a l e n t s . Wall rock exposure along the southern p o r t i o n of the channel includes Kunga limestones, Lower Cretaceous Longarm conglom-erates and sandstone and the Upper J u r a s s i c Burnaby Island Pluton. Deposition and t u r b i d i t y r e d i s t r i b u t i o n of Longarm e l a s t i c s i n f a u l t 139 troughs east of the Louscoone f a u l t probably represented extensive basin f i l l t h at o r i g i n a l l y covered much of the outer p o r t i o n of Juan Perez Channel. Magnetic contours east of the Louscoone F a u l t , trend roughly east-southeast, p a r a l l e l to s t r u c t u r a l elements r e l a t e d t o the wrench system. On the wide inner s h e l f , northeast of Burnaby I s l a n d , high amplitude ( 900 nT), short wavelength magnetic anomalies char-a c t e r i z e basement seacrop. The anomaly p a t t e r n continues f u r t h e r east over the northeast trending Moresby r i d g e . (A p o s s i b l e source of these anomalies i s discussed i n the f o l l o w i n g s e c t i o n . ) The l i n e a r trend of steep magnetic gradients that trend north and north-west may i n d i c a t e f a u l t s . Seacrop on the inner s h e l f from Burnaby Island south to Kunghit I s l a n d comprises g e n t l y t i t l e d and f o l d e d panels of Karmutsen and Yakoun v o l c a n i c s and Kunga limestones, intruded and deformed by ?Upper J u r a s s i c Southern Group plutons. The arcuate e a s t e r l y trending homoclinal panels t h a t , as i n d i c a t e d by magnetic anomalies p e r s i s t t o the C h a r l o t t e sub-basin edge and beyond are r e l a t e d t o the Louscoone wrench f a u l t system. Seacrop patterns east of Moresby and Kunghit Islands have been i n f e r r e d mainly from c o a s t a l geology and bathymetric data. Weekly magnetic Karmutsen v o l c a n i c s and nonmagnetic Yakoun v o l c a n i c sandstones and Kunga limestones (Chapter VI) cannot be d i s c r i m i n a t e d on the b a s i s of magnetic anomaly p a t t e r n s . I f the l i n e of Yakoun vents t h a t b u i l t cones out of the middle J u r a s s i c shallow marine basin extends along the east coast to southern Queen C h a r l o t t e Islands, seacrop of Yakoun rocks may be more extensive 140 than i n d i c a t e d i n F i g . 36. Thin bedded limestones of the Kunga Formation are i n f e r r e d t o seacrop over much of the southern inner s h e l f . F a u l t s and t i g h t f o l d s , e a s i l y preserved i n these rocks, may c h a r a c t e r i z e much of the region. South of Kunghit Island magnetic anomalies trend south-e a s t , p a r a l l e l t o trend of c o a s t a l geology. High amplitude magnetic anomalies may be associated with an offshore extension of the i n t r u -s i v e contact of the San C h r i s t o v a l B a t h o l i t h and Karmutsen v o l c a n i c s . The southern extent of the J u r a s s i c Luxana migmatic p l u t o n , c h a r a c t e r i z e d by low-moderate magnetic r e l i e f on Kunghit I s l a n d , i s not known. Sediments of the Lower Cretaceous Longarm Formation and Upper Cretaceous Queen C h a r l o t t e Group are absent or r e s t r i c t e d i n t h e i r occurrence on the southern inner s h e l f . Non-deposition or erosi o n of these sediments i s i n d i c a t e d by l i m i t e d exposure of only Longarm e l a s t i c s on southern Queen C h a r l o t t e Islands and weaker r e s i s t e n c e t o eros i o n compared t o Kunga limestones and c r y s t a l l i n e rocks. During the Cretaceous much of the southern Queen C h a r l o t t e Islands probably formed a t e c t o n i c highland that supplied d e t r i t u s t o r e s t r i c t e d f a u l t bounded troughs along the Rennel-Louscoone f a u l t l i n kage and the Skidegate Basin t o the north, bypassing much of the eastern p o r t i o n of southern Queen C h a r l o t t e I s l a n d s . Queen C h a r l o t t e Basin The p a t t e r n of magnetic anomalies t h a t c h a r a c t e r i z e Moresby Ridge and Hecate and C h a r l o t t e sub-basins i s a product of 141 depth t o magnetic source rock, s t r u c t u r e , and i n t r a and supra-basement changes i n rock magnetizations. Shouldice (1973) noted th a t "the p a t t e r n of broad lob a t e , and b i f u r c a t i n g anomalies over the Queen C h a r l o t t e Basin was the r e s u l t of v a r i a t i o n s i n t h i c k n e s s of T e r t i a r y sediment o v e r l y i n g v o l c a n i c basement". Hecate Sub-basin:The Hecate sub-basin, located north and east of Moresby Ridge ( F i g . 36) comprises a number of major depocenters t h a t c o n t a i n greater than 4 km of Neogene nonmarine sediments, separated by elongate, nose-shaped saddles or low r e l i e f r i d g e s . Steepest basin slopes of p o s s i b l e f a u l t o r i g i n are located east of the indented s h e l f edge o f f L y e l l and Ramsay Isla n d s . The broad, low r e l i e f magnetic patte r n that charac-t e r i z e s the western p o r t i o n of the Queen C h a r l o t t e Basin ( F i g s . 34 and 35) r e f l e c t s a t t e n u a t i o n of the t o t a l magnetic f i e l d due to t h i c k Neogene sediment cover and nonmagnetic or weakly magnetic basement source rocks. A broad magnetic plateau with magnetic values t h a t exceed 550 nT covers the northeastern p o r t i o n of the survey area. A graben-like trough c o n t a i n i n g predominately marine sediments of the Cretaceous Skidegate Basin extends from Louise Island southeast i n t o Hecate S t r a i t . As discussed p r e v i o u s l y , the trend of magnetic anomalies i s c o n s i s t e n t w i t h the i n t e r p r e t a t i o n of an o f f s h o r e extension of the marine basin and f a u l t s and f o l d s associated with the Rennel wrench f a u l t system. From the inner s h e l f edge the basin extends at l e a s t 30 km along trend t o the v i c i n i t y of the Sockeye E-66 and B-10 w e l l s . The Sockeye E-66 w e l l 142 KILOMETRES F i g . 35 Magnetic anomaly p r o f i l e s along s h i p ' s t rack in southwestern Hecate S t r a i t . 143 penetrated 152 meters of Cretaceous nonmarine e l a s t i c s (Skidegate Formation?), bottoming i n Yakoun v o l c a n i c s ( S h e l l Canada L t d . 1968c) while the B-10 w e l l bottomed i n Cretaceous e l a s t i c s (152+ meters; S h e l l Canada L t d . , 1968b). I s o s t a t i c g r a v i t y anomalies (Stacey, 1975; see F i g . 19) i n the v i c i n i t y of the Sockeye w e l l s suggest the presence of l e s s dense Cretaceous rocks. (In the northern p o r t i o n of the Hecate sub-basin the Tyee N-39 w e l l penetrated 101 meters of Cretaceous s e c t i o n ( S h e l l Canada L t d . , 1968a).) Near the c e n t r a l p o r t i o n of southern Hecate subbasin the trend of magnetic anomalies changes from an azimuth of 118°, p a r a l l e l l i n g the Skidegate Basin t o an azimuth of 146°, p a r a l -l e l l i n g the trend of a p o s s i b l e o f f s h o r e extension of the Sandspit F a u l t . Although evidence of l a t e Neogene f a u l t i n g along the Sandspit trend i n Hecate sub-basin has not been recognized on high r e s o l u t i o n seismic p r o f i l e s (chapter V I I I ) , a basement d i s c o n t i n u i t y or f a u l t zone connecting t o the Sandspit F a u l t on northeastern Moresby Island (azimuth 144°) may be present i n t h i s area. I f so, i t would l i k e l y be located along trend of a magnetic trough, 5-10 km wide t h a t extends 65 km from east of Louise Island south t o Moresby Ridge. A s i m i l a r magnetic trough c h a r a c t e r i z e s the Sandspit f a u l t zone where i t crosses Skidegate I n l e t ( F i g . 34). I f r i g h t l a t e r a l wrench displacement has occurred on the Sandspit F a u l t extension, Skidegate Basin rocks east of the presumed f a u l t t r a c e may be d i s p l a c e d southeast of the basin t r e n d . Movement on the Rennel f a u l t and i t s o f f s h o r e extension, which bounds Skidegate Basin to the south, probably predates movement on the Sandspit (Sutherland Brown, 1968). O f f s e t of the Rennel F a u l t i s t h e r e f o r e 144 i n d i c a t e d unless i t terminates at or near the Sandspit extension. Subcrop south of the Rennel f a u l t i n southwestern Hecate sub-basin may l a r g e l y comprise Yakoun v o l c a n i c s and Cretaceous s e d i -ments. The high amplitude magnetic anomaly that c h a r a c t e r i z e s Masset Dana f a c i e s v o l c a n i c s on e a s t - c e n t r a l Moresby and L y e l l Islands does not extend any d i s t a n c e o f f s h o r e (see F i g s . 29 and 34). Basement v e l o c i t i e s from a reversed seismic r e f r a c t i o n p r o f i l e across southwestern Hecate sub-basin are 4.01 and 4.91 km/sec (Mahannah, 1965). These v e l o c i t i e s are c o n s i d e r a b l y l e s s than those obtained from r e f r a c t i o n p r o f i l e s across probable Masset v o l c a n i c s i n C h a r l o t t e sub-basin and t h e r e f o r e may i n d i c a t e Mesozoic s e d i -mentary or v o l c a n i c rocks. Normal step f a u l t s along the steep subbasin edge east of L y e l l and Ramsay Islands have dropped probable Mesozoic rocks t o 4 km depth t o the east and u p l i f t e d subaqueous p y r o c l a s t i c s and v o l c a n i c b r e c c i a s of the Dana f a c i e s t o the west. I s o s t a t i c g r a v i t y anomalies (Stacey, 1975; see F i g . 19) suggests that part of the northern p o r t i o n of the Hecate sub-basin may be u n d e r l a i n by dense v o l c a n i c rocks. High basement v e l o c i t i e s which range from 4.14 to 6.49 (Mahannah, 1965) support t h i s c o n c l u -s i o n . These rocks could be Karmutsen v o l c a n i c s as suggested by Stacey or Masset b a s a l t s . The Tyee N-39 w e l l bottomed i n gabbroic and l e s s dense d i o r i t i c rocks of ?Upper Paleozoic age ( S h e l l Canada L t d . , 1968a). Moresby Ridge: Moresby Ridge, which extends 55 km from Burnaby Island N55°E to c e n t r a l Hecate S t r a i t ( F i g . 36) i s a p o s i t i v e basement f e a t u r e , 20-30 km i n width, that separates the Hecate and 145 C h a r l o t t e sub-basins. Depth t o the top of the r i d g e i s l e s s than 1 km w h i l e r e l i e f exceeds 2.5 km. The ridge had a major i n f l u e n c e on Skonun sedimentation p a t t e r n s , a c t i n g as a p a r t i a l b a r r i e r t o northward t r a n s g r e s s i n g Neogene shallow marine seas that occupied much of southern Hecate S t r a i t ( C h a r l o t t e subbasin) and Queen C h a r l o t t e Sound (Chapter IV). The approximate p o s i t i o n of the ridge was shown by Shouldice (1973; see F i g . 20) and Stacey (1975; see F i g . 19) who a t t r i b u t e d high f r e e - a i r and i s o s t a t i c g r a v i t y anom-a l i e s over the r i d g e t o dense Karmutsen v o l c a n i c s . The magnetic expression of the ridge ( F i g s . 34 and 35) comprises elongate t o subrounded high amplitude anomalies. R e l i e f v a r i e s from greater than 800 nT on the inner s h e l f east of Burnaby Island t o 350-650 nT east of the basin edge. Elongate anomalies trend roughly north-south and may i n d i c a t e f a u l t trends. Upward c o n t i n u a t i o n of the marine magnetic data would l i k e l y produce a broad magnetic high as seen on the t o t a l i n t e n s i t y aeromagnetic map ( F i g . 20). The high magnetic values are a t t r i b u t e d t o rocks of high magnetic s u s c e p t i b i l i t y , i . e . Karmutsen (A mode), Masset v o l c a n i c s , or p o s t - t e c t o n i c i n t r u s i v e rocks. Coincident g r a v i t y and magnetic highs that c h a r a c t e r i z e the ridge would i n d i c a t e that much of the s t r u c t u r e i s comprised of v o l c a n i c rocks. Aeromagnetic maps show that magnetic anomaly patterns associated w i t h Karmutsen v o l c a n i c s are g e n e r a l l y of low t o medium amplitude and r e l i e f . P r o x i m i t y of the ridge t o outcrop of Masset v o l c a n i c s up t o 1.5 t h i c k on L y e l l and Ramsey Islands and Masset "basement" rock i n the Auklet G-41 and Murrelet L-15 w e l l s (northern C h a r l o t t e sub-basin) suggests that much 146 of the r i d g e might, t h e r e f o r e , be composed of or capped by Masset v o l c a n i c s . I f capped by Masset rocks, Moresby Ridge may have a 'core' of s l i g h t l y more dense Karmutsen v o l c a n i c s , as suggested by Stacey (1975). The high amplitude, short wavelength anomalies on the inner s h e l f may r e s u l t from intrabasement magnetic e f f e c t s produced by an o f f s h o r e extension of the Burnaby Island Pluton and p o s s i b l e suprabasement magnetic e f f e c t s produced by m i n e r a l i z a t i o n associated with i r o n ore bodies r e l a t e d t o the i n t r u s i o n . The age and o r i g i n of Moresby Ridge must be considered s p e c u l a t i v e on the b a s i s of a v a i l a b l e geophysical data. I f com-p r i s e d p a r t i a l l y or e n t i r e l y of Masset rocks the r i d g e may be one or more volcanoes from which b a s a l t i c flows and p y r o c l a s t i c s were deposited over northern C h a r l o t t e sub-basin i n Lower Oligocene. (A b a s a l t porphyry, 206 m from the top of a s e c t i o n of Masset b a s a l t s and p y r o c l a s t i c s i n Auklet G-41 gave a K-Ar age of 36+ 4 Ma.) I f the ridge i s comprised l a r g e l y of Upper T r i a s s i c Karmutsen la v a s , i t may have formed a physiographic b a r r i e r t h a t r e s t r i c t e d depostion w i t h i n the Cretacous Skidegate Basin t o the north. Moresby Ridge remains engimatic because i t s trend l i e s o b l i q u e to major s t r u c t u r e s on the Queen C h a r l o t t e Islands and western p o r t i o n of the Queen C h a r l o t t e Basin. (A s i m i l a r major transverse element, the Stockton arch, segments the Great V a l l e y of C a l i f o r n i a ; see Dickinson and Seely, 1979.) Although s t r u c t u r e s associated with the Louscoone wrench f a u l t system on southern Queen C h a r l o t t e Islands trend i s a s i m i l a r d i r e c t i o n they do not extend any great distance east of the main f a u l t t r a c e . I f appears u n l i k e l y t h e r e f o r e t h a t s t r e s s systems r e l a t e d to the major wrenches 147 could have i n f l u e n c e d the development of Moresby Ridge. Because of the importance of major c r u s t a l d i s l o c a t i o n s and block r o t a t i o n s ( r e l a t e d t o l i t h o s p h e r i c p l a t e i n t e r a c t i o n s ) i n the geologic h i s t o r y of the Queen C h a r l o t t e Islands and I n s u l a r Tectonic B e l t , a t e c t o n i c o r i g i n cannot, however, be discounted. C h a r l o t t e Sub-basin: The C h a r l o t t e sub-basin extends south from Moresby r i d g e t o the west c e n t r a l p o r t i o n of Queen C h a r l o t t e Sound. The deepest p o r t i o n s of the sub-basin at greater than 3.5 km l i e immediately south of Moresby r i d g e and northwest of a short r i d g e t h a t trends N40°W near the Harlequin D-86 w e l l . S t r u c t u r e contours ( F i g . 36) i n d i c a t e that the sub-basin f l o o r east of southern Moresby Island and along eastern Hecate S t r a i t slope g e n t l y toward the c e n t r a l p o r t i o n of the sub-basin which l i e s at approxi-mately 2 km depth. The sub-basin edge i s not f a u l t e d except p o s s i b l y against Moresby Ridge. The magnetic p a t t e r n c h a r a c t e r i z i n g the western p o r t i o n of the C h a r l o t t e sub-basin ( F i g . 34) comprises broad arcuate north-east trending magnetic r i d g e s and troughs east of southern Morsby and Kunghit Is l a n d s . Towards the c e n t r a l p o r t i o n of the sub-basin and south of Kunghit Island the trend of magnetic anomalies changesto northwest, p a r a l l e l l i n g the trend of the southern end of the Louscoone and Sandspit f a u l t s . The arcuate magnetic r i d g e s and troughs, with r e l i e f of up t o 250 nT, extend approximately 20 km east of Moresby I s l a n d , 15 km east of the sub-basin edge. They r e f l e c t an o f f s h o r e extension of f o l d s and monoclinal panels on southern Moresby and Kunghit 148 Islands and the inner s h e l f t h a t trend oblique to the Louscoone wrench f a u l t system. High magnetic values ( 500 nT) east of southern Moresby Island may represent an extension of the ?Upper J u r r a s i c Point Langford Pluton. Low magnetic values that f l a n k the r i d g e l i k e l y r e f l e c t f o l d e d panels of Kunga limestones that extend east of the sub-basin edge. Windows may expose o l d e r Karmutsen v o l c a n i c s . Subcrop over the C h a r l o t t e sub-basin as i n d i c a t e d by w e l l r e s u l t s i s l a r g e l y comprised of Masset v o l c a n i c s . At the Murrelet L-15 and Auklet G-41 w e l l s , Upper Eocene or younger b a s a l t s and p y r o c l a s t i c s were penetrated. Further south the Harlequin D-86 w e l l bottomed i n s i m i l a r Upper Eocene (42 Ma) p y r o c l a s t i c s and f l o w s . Basement v e l o c i t i e s from three reversed seismic r e f r a c t i o n p r o f i l e s (Mahannah, 1965) range from 4.85 to 5.55 km/sec, comparable to measured i n t e r v a l v e l o c i t i e s of Masset v o l c a n i c s i n the nearby Harlequin D-86 w e l l . The approximate coincidence of a magnetic high with the short ridge and magnetic lows with a d j o i n i n g sub-basin troughs suggests t h a t a m a g n e t i c a l l y uniform (Masset?) basement source rock extends over the southern p o r t i o n of the sub-basin, the s i g n i f i c a n t ' remnant magnetic e f f e c t associated with the v o l c a n i c s being c o n s i d e r a b l y reduced. Over the southwestern p o r t i o n of the sub-basin large negative i s o s t a t i c g r a v i t y anomalies cannot be accounted f o r s o l e l y by low-density Neogene sediments and suggest t h a t Mesozoic-mid T e r t i a r y g r a n i t i c or sedimentary rocks may u n d e r l i e Skonun rocks (Stacey, 1975; see F i g . 19). In t h i s area Masset v o l c a n i c s may be 149 absent or c o n s t i t u t e o n ly a t h i n cover over the low d e n s i t y rocks. Oceanic Area Adjacent t o Queen C h a r l o t t e Sound Major physiographic f e a t u r e s of the oceanic area at the base of the c o n t i n e n t a l slope o f f n o r t h - c e n t r a l Queen C h a r l o t t e Sound includ e Tuzo Wilson and Delwood k n o l l s , the Queen C h a r l o t t e f a u l t zone, Moresby and Scott deep sea channels and Oshawa Rise (southern p o r t i o n ) . Location and bathymetric expression of these f e a t u r e s i s shown i n F i g . 41. The t o t a l magnetic f i e l d map f o r the region ( F i g . 34) has been compiled^ p r i m a r i l y from magnetic data acquired i n a multiparameter survey of Queen C h a r l o t t e Sound ( T i f f i n , 1974; T i f f i n and C u r r i e , 1976) supplemented by p r o f i l e s obtained during I.O.U.B.C. Cruise 76-10 ( t h i s study) and Hudson '70 ( S r i v a s t a v a , 1971b, S r i v a s t a v a et a U 1971). Sparse magnetic coverage reported by Pitman and Hayes (1968), Naugler and Wageman 1973) and Couch (1969) has not been incorporated. The p a t t e r n of magnetic anomalies over the oceanic area shows.a c l o s e correspondence t o the major physiographic f e a t u r e s . A large p o s i t i v e magnetic plateau that trends roughly W-SW comprises two separate magnetic peaks of greater than 800 and 850 Nt marks the p o s i t i o n of the Tuzo Wilson k n o l l s . Located 60 km south of Cape St. James r i d g e , the k n o l l s r i s e 0.7 km above the ocean f l o o r at 2.1 km Compilation of magnetic f i e l d data was completed before data from surveys southwest of the Queen C h a r l o t t e Islands (Riddihough et a]_ 1980; C u r r i e et j H , 1980) could be incorporated. water depth. Dredge hauls from one of the seamounts recovered f r e s h hawaiite and photographs showed c l e a r l y f r e s h l a v a f l o w s , t e n s i o n a l c o o l i n g f r a c t u r e s and n e g l i g i b l e sediment cover ( T i f f i n , 1974; Chase, 1977). The young age of the seamounts was confirmed by a K/Ar date of l e s s than 0.05 Ma obtained from one of the dredge samples (Chase, personal communication). According t o Chase (1977), geometric, c h r o n o l o g i c a l and p e t r o l o g i c evidence suggests that the seamounts form the a c t i v e end of the northeastern P a c i f i c Kodiak-Bowie seamount ch a i n . More recent chronologic and geochemical evidence however, p r e d i c t s that the present hot spot l o c a t i o n i s 270-360 km northwest of Tuzo Wilson k n o l l s , 40-130 km southeast of the Bowie seamount (Turner et al_, 1980). Riddihough e t a^ (1980) suggest t h a t the Tuzo Wilson k n o l l s are the locus of a c t i v e spreading which i s both complementary and s i m i l a r t o t h a t of the Delwood k n o l l s i n p r o v i d i n g the connection between the Explorer Ridge and Queen C h a r l o t t e F a u l t (see F i g . 12). The 'simple' magnetic p a t t e r n over the k n o l l s i s compar-able t o patterns observed over P a c i f i c midplate seamounts (Francheteau e_t _al_, 1970). S l i g h t o f f s e t of magnetic peaks with respect t o the l o c a l e l e v a t i o n s can be explained by i n c l i n a t i o n of the c urrent earth's f i e l d and non-homogenous magnetic e f f e c t s of the source rock. The strong p o s i t i v e magnetic f i e l d t h a t c h a r a c t e r i z e s the Tuzo Wilson k n o l l s ( F i g . 34) i s the product of the most recent p o l a r i t y epoch (0.7 Ma). Francheteau et ^1_ (1970) i n d i c a t e t h a t seamounts such as the Tuzo Wilson k n o l l s are normally b u i l t over a period of 0.01-0.1 Ma years and w i l l be magnetized predominately i n 151 one d i r e c t i o n , during a s i n g l e p o l a r i t y epoch. The k n o l l s appar-e n t l y l i e on oceanic c r u s t that i s 5-6 Ma o l d ( S r i v a s t a v a et aj[, 1971; Riddibough, 1977). The lack of any i d e n t i f i a b l e symmetry or strong l i n e a t i o n a s s ociated with spreading suggests that the mag-n e t i c s r e l a t e d t o the seamounts has destroyed the remnant magnetic e f f e c t of the ocean f l o o r . An a l t e r n a t e explanation might be that the apparent l i n e of ter m i n a t i o n of the P a c i f i c P l a t e l i n e a t i o n s , which runs northwestwards from the region of the Delwood k n o l l s , might represent a former Queen C h a r l o t t e F a u l t p o s i t i o n . The 'block' t o the east of t h i s f a u l t would have been attached t o the America p l a t e and t h e r e f o r e would not have been the s i t e of ocean c r u s t and magnetic anomaly generation (Riddihough et aj_, 1980). Delwood k n o l l s , which l i e 80 km S-SE of Tuzo Wilson k n o l l s , have been proposed as a spreading centre connecting the Explorer Ridge t o the Juan de Fuca p l a t e systems ( S r i v a s t a v a et cH, 1971; Bertrand, 1972; Chase et a_U 1975; Riddibough et al_, 1980). The patt e r n of magnetic anomalies over the k n o l l s ( F i g . 37) i s p a r t i c u l a r l y complex, c h a r a c t e r i s t i c of seamounts formed at or near the r i d g e c r e s t of the Juan de Fuca - Explorer r i d g e system. Riddihough et a^ (1980) used three-dimensional modelling techniques t o f i t observed magnetic anomalies, mapped on the ba s i s of a d e t a i l e d survey of the k n o l l s , t o a s e r i e s of model anomalies from uniformly magnetized bodies and determined t h a t 1) c a l c u l a t e d magnetizations are comparable t o mean values determined f o r P a c i f i c seamounts by magnetic anomaly f i t t i n g and measured bulk magneti-z a t i o n s of dredge samples from the k n o l l s and the Explorer and Juan de Fuca r i d g e s , and 2) the k n o l l s were almost e n t i r e l y produced by MAGNETIC ANOMALIES - QUEEN CHARLOTTE SOUND » j km Fiq. 37 Magnetic anomaly map of Queen Charlotte Sound and adjacent Pacific Ocean. Dk = Delwood knolls, TWK = fuzo Wilson knolls, QCfz = Queen Charlotte fault zone, CJr - Cape St. James ridge, OR = Oshawa Rise, SC = Scott Channel, CS = Cape Scott. Redrawn from Tif f in and Currie (1976). 153 normally magnetized b a s a l t s during the l a s t p o l a r i t y epoch. A magnetic trough, 10 km wide, azimuth 140°, located between Tuzo Wilson k n o l l s and the Cape St. James magnetic ri d g e ( F i g . 34) marks the probable l o c a t i o n of the a c t i v e Queen Ch a r l o t t e transform f a u l t zone. A short (25 km) magnetic r i d g e , azimuth 65°, north of Tuzo Wilson k n o l l s , o v e r l i e s a t h i c k sedimentary accumulation immediately south of Moresby Channel. To the north-west subdued magnetic r e l i e f (see a l s o Couch, 1969, F i g . 21; S r i v a s t a v a et ^1_; 1971 F i g . 5) c h a r a c t e r i z e s southern Oshawa R i s e , a broad basement s w e l l t h a t connects the k n o l l s t o Bowie Seamount. A southeastern extension of the Tuzo Wilson magnetic p l a t e a u , west of the Queen C h a r l o t t e f a u l t trough ( F i g . 34) suggests a p o s s i b l e elevated basement foundation t h a t connects t o Oshawa R i s e . The trend of low magnetic values north of the Delwood k n o l l s i s approxi-mately c o i n c i d e n t with Scott Channel. Tectonic I m p l i c a t i o n of Magnetic Anomalies Over Western  Queen C h a r l o t t e Sound Magnetic anomalies over the outer c o n t i n e n t a l s h e l f and slope of Queen C h a r l o t t e Sound r e f l e c t major geologic and s t r u c t u r a l elements whose o r i g i n i s c r i t i c a l t o an understanding of the d e v e l -opment of the I n s u l a r Tectonic B e l t . The p a t t e r n of magnetic anomalies ( F i g s . 34 and 37) d i s p l a y s a d i s c o n t i n u o u s , 1 i n e a r trend that j o i n s southern Queen C h a r l o t t e Islands (Cape S t . James) t o northern Vancouver I s l a n d . A subdued magnetic, p a t t e r n o v e r l i e s a major bathymetric re-entrant i n the c e n t r a l p o r t i o n of the Queen C h a r l o t t e Sound s h e l f . 1 5 4 Various g e o l o g i c a l models have been proposed t o e x p l a i n observed geophysical anomalies. S r i v a s t a v a et, aj_ (1971) and S r i v a s t a v a (1973) suggested t h a t a discontinuous p o s i t i v e magnetic anomaly on the outer c o n t i n e n t a l s h e l f i s a " s h e l f edge anomaly" tha t r e f l e c t s a continent-ocean c r u s t a l t r a n s i t i o n and p o s s i b l e convergence along an a c t i v e margin. Shouldice (1973) speculated that a continuous r i d g e may, at one time ( p o s s i b l y e a r l y T e r t i a r y ) , have e x i s t e d between Vancouver and Queen C h a r l o t t e I s l a n d s . On the b a s i s of f r e e - a i r and i s o s t a t i c g r a v i t y anomaly data, Stacey (1975) and Stacey and Stephens (1969) suggested that a c e n t r a l gap i n an otherwise continuous u p l i f t across Queen C h a r l o t t e Sound i s f i l l e d by Cretaceous and T e r t i a r y sediments, o v e r l y i n g e a r l i e r v o l c a n i c rocks. In order t o e x p l a i n the gap i n the s h e l f edge r i d g e , Stacey (1975) suggested that the Queen C h a r l o t t e Islands could have been r a f t e d northwards with respect t o Vancouver I s l a n d , p o s s i b l y i n a manner s i m i l a r t o the separation of southern C a l i f o r n i a from the Mexican mainland. Chase et a]_ (1975) suggested t h a t such separation may have occurred more r e c e n t l y along the l i n e of the Sandspit F a u l t . They recognized t h a t the heads of three submarine canyons at M i t c h e l l and Goose Island Banks i n Queen C h a r l o t t e Sound, c o - l i n e a r (azimuth 140°) with a southern extension of the Sandspit F a u l t , appear t o head b l i n d without apparent source on the c o n t i n e n t a l s h e l f (see F i g . 41). They f u r t h e r speculated that part of the Queen Ch a r l o t t e Islands west of the f a u l t extension may once have l a i n adjacent t o the p l a t f o r m north of Vancouver I s l a n d , c l o s i n g o f f Queen C h a r l o t t e Sound from the P a c i f i c . Subsequent movement along 155 the Sandspit or r e l a t e d f a u l t s would have opened the Sound t o the ocean and moved Queen C h a r l o t t e mountains north t o t h e i r present p o s i t i o n . New ocean f l o o r thus created would be deeply buried by sediments of the outer c o n t i n e n t a l s h e l f and slope, and presumably magnetic anomalies would, i f ever formed, be destroyed by subsequent heating r e l a t e d t o depth of b u r i a l . More r e c e n t l y , Bevier et aj_ (1979) and Yorath and Case (1981) have suggested that northward d i s l o c a t i o n of the western Queen C h a r l o t t e Islands i n Neogene time f o l l o w e d a r i f t i n g event i n Queen C h a r l o t t e Sound associated w i t h Masset volcanism and passage of the Anahim hot spot (see f u r t h e r d i s c u s s i o n , Chapter I I I , F i g s . 12 and 13). Although our a b i l i t y t o detect young ocean f l o o r b a s a l t s i n Queen C h a r l o t t e Sound (or t o d i s t i n g u i s h them from o l d e r v o l c a n i c s ) may be l i m i t e d , the lack of i d e n t i f i a b l e magnetic l i n e -a t i o n s can be explained by magnetic rock p r o p e r t i e s . Carmichael (1970) has shown t h a t the Curie temperature of unaltered submarine b a s a l t i s t y p i c a l l y about 200°C or l e s s . Stacey (1975) showed that as much as 5 km of T e r t i a r y sediment u n d e r l i e s the s h e l f and slope near the c e n t r a l gap i n Queen C h a r l o t t e Sound. Using an average geothermal gradient of 3.0°C/100 m f o r the southern Queen C h a r l o t t e Basin (Galloway, 1974; 1979) a minimum ambient termperature of 150°C would be expected at 5 km. (Higher temperature may be expected because of the p r o x i m i t y of the gap t o a recent spreading centre at Delwood k n o l l s and p o s s i b l e hot spot i n the v i c i n i t y of Tuzo Wilson k n o l l s ) . Total magnetization could thus be reduced by reheating at t h i s depth of b u r i a l . I r v i n g et a}_ (1970) have shown th a t the 156 i n t r u s i o n of magma i n t o sediments r e s u l t s i n slower c o o l i n g and thus a coarser g r a i n s i z e and lower n a t u r a l remnant magnetization. A b a s a l t i c s i l l encountered i n DSDP hole 177 (Winona Basin) has extremely low t o t a l magnetization compared t o average ocean b a s a l t s (Riddihough et a l , 1980). The above evidence i n d i c a t e s t h a t ocean c r u s t could u n d e r l i e the gap i n Queen C h a r l o t t e Sound as suggested by Chase et aj_ 1975). Any model th a t invokes large c r u s t a l d i s l o c a t i o n s or spreading t o open C h a r l o t t e Sound must be constrained by t e c t o n i c events recorded i n i n s u l a r rocks of Vancouver and Queen C h a r l o t t e I s l a n d s . The geol o g i c h i s t o r y of Vancouver Island and the Queen C h a r l o t t e Islands (see Chapter I) show a gradual divergence i n tim i n g and f a c i e s a f t e r the Lower Cretaceous that might be accounted f o r by the present separation of the i s l a n d s . A n d e s i t i c volcanism on Vancouver Island (Bonanza Formation) and Queen C h a r l o t t e Islands (Yakoun Formation) along the western margin of the Vancouver Basin records the f i n a l major t e c t o n i c event that was synchronous on the i s l a n d s , although Lower Cretaceous marine sediments (Longarm Formation) may have been deposited i n f a u l t bounded troughs across Queen C h a r l o t t e Sound. The mountainous spine of Vancouver and Queen Ch a r l o t t e Islands comprises l a r g e l y the o l d e r Upper Paleozoic-Upper T r i a s s i c "Wrangellia" succession t h a t includes Karmutsen v o l c a n i c s , the Kunga Formation (Queen C h a r l o t t e Islands) and Quatsino, Parson Bay, and Harbledown Formations (Vancouver I s l a n d ) . These rocks are e x t e n s i v e l y exposed on southern Queen C h a r l o t t e Islands and northern Vancouver I s l a n d , and l i k e l y form the foundation of the r i d g e at the edge of the s h e l f i n Queen C h a r l o t t e Sound. 157 Major r i g h t l a t e r a l wrenches, i n c l u d i n g Sandspit and Rennel-Louscoone systems (Chapter X), have played a s i g n i f i c a n t r o l e i n the g e o l o g i c a l development of the I n s u l a r Tectonic B e l t (Sutherland Brown, 1966; 1968; J e l e t z k y , 1970). Some of these f a u l t s may have developed over o l d l i n e s of c r u s t a l suture asso-c i a t e d with t r a n s l a t i o n and j u x t a p o s i t o n of the allochtonous W r a n g e l l i a t e r r a n e i n Upper Jurassic-Lower Cretaceous. I f Queen Ch a r l o t t e Islands have been r a f t e d north w i t h respect t o Vancouver I s l a n d , combined movement l i k e l y occurred on a number of these major f a u l t s . Subsidence of the s h e l f edge r i d g e , r e l a t e d t o l o c a l t e c t o n i c readjustments, or opening of Queen C h a r l o t t e Sound as a r e s u l t of t r a n s l a t i o n and p o s s i b l e ocean spreading must have preceeded Neogene shallow t o deep marine c l a s t i c d e p o s i t i o n i n the C h a r l o t t e sub-basin. This would appear t o argue against a spreading ridge i n Queen C h a r l o t t e Sound being r e l a t e d t o the Explorer-Delwood r i d g e segments which formed only i n the l a s t few m i l l i o n years (Riddihough et al_, 1980). The p a t t e r n of magnetic anomalies ( F i g s . 34 and 37) supports the observations of Stacey (1975), Stacy and Stephens (1969) and Shouldice (1973) t h a t a more or l e s s continuous b e l t of predominately Mesozoic rocks forms an arcuate ridge between Vancouver and Queen C h a r l o t t e I s l a n d s . From the Queen C h a r l o t t e Islands (Cape St. James), a magnetic r i d g e , w i t h values that exceed 850 nT, extends 65 km southeast. A narrow elevated s h e l f a l p l a t -form, bounded t o the west by the c o n t i n e n t a l slope and Queen C h a r l o t t e f a u l t zone, and t o the east by Moresby canyon, and 158 c h a r a c t e r i z e d by a c o i n c i d e n t p o s i t i v e f r e e a i r g r a v i t y anomaly ( F i g . 21) extends 45 km from Cape St. James along the same trend as the magnetic r i d g e . Figure 38 shows seismic r e f l e c t i o n and t o t a l magnetic f i e l d p r o f i l e s , perpendicular t o the r i d g e . Acoustic base-ment i s i n d i c a t e d at 1 sec (two way t r a v e l time) or approximately 1 km (assuming sound v e l o c i t y i n sediment of 2 km/sec). Magnetic depth c a l c u l a t i o n s , using a number of depth e s t i m a t o r s , gives a maximum depth t o source of the anomaly of 1.7 to 2.1 km. The discrepancy between magnetic and a c o u s t i c basement depths suggest that the "magnetic r i d g e " i s o v e r l a i n by sediments, probably Mesozoic, below the a c o u s t i c break. The p o s i t i v e g r a v i t y and magnetic anomalies suggest that the ridge must be p r i m a r i l y of v o l c a n i c rocks, p o s s i b l y Karmutsen v o l c a n i c s which outcrop on southern Moreby and Kunghit Is l a n d s . I f we assume magnetic homo-geneity of the r i d g e , r e l i e f of the r i d g e i s approximately 1.5 - 2.0 km. The magnetic trough east of the ridge i n d i c a t e s the p o s i t i o n of a p o s s i b l e o f f s h o r e extension of the Louscoone F a u l t . East of and p a r r a l l e l t o the S t . James r i d g e , a second elongate magnetic r i d g e comprising two separate peaks of 600 and 700 nT extends over the s h e l f edge from the v i c i n i t y of Kunghit Channel, 75 km t o M i t c h e l l canyon. The steep magnetic gradient along the eastern edge of the ridge c o i n c i d e s with a p o s s i b l e f a u l t l i n e that j o i n s the heads of the " b l i n d " submarine canyons i n Queen C h a r l o t t e Sound. O f f s e t of the two magnetic peaks might be explained by r i g h t l a t e r a l displacement along a p a r a l l e l f a u l t . In a d d i t i o n , seismic r e f l e c t i o n p r o f i l e s (Chapter V I I I ) i n d i c a t e s f a u l t i n g along t h i s trend and earthquake e p i c e n t r e s ( F i g . 22), though not p r e c i s e l y 159 38 Magnetic anomaly p r o f i l e and l i n e - d r a w i n g over Cape St. James r i d g e ( l i n e 73-25). of s e i s m i c p r o f i l e 160 located i n t h i s area, may i n d i c a t e recent displacement. Lower amplitude, lobate t o subrounded anomalies form an arcuate band that extends southeast from the magnetic ridges to northern Vancouver I s l a n d . P o s s i b l e f a u l t trends are not c l e a r l y i d e n t i f i e d , but as contacts of major rock u n i t s on Vancouver Island are commonly represented by f a u l t s , extension of major s t u c t u r e s t o the o f f s h o r e region appears l i k e l y . Magnetic anomalies on the t o t a l i n t e n s i t y aeromagnetic map ( F i g . 37) c l e a r l y extend northwest from Vancouver I s l a n d . A l a r g e p o s i t i v e f r e e - a i r g r a v i t y anomaly ( F i g . 21) extends over the platform northwest of Vancouver I s l a n d . Seacrop west of Cape Scott includes Noegene and nearshore Paleogene sediments which r e s t unconformably on a p r e - T e r t i a r y succession of v o l c a n i c , p l u t o n i c , metamorphic and minor sedimentary rocks that i n c l u d e Lower J u r a s s i c Bonanza v o l c a n i c s , Lower Cretaceous west coast c r y s t a l l i n e complex metamorphic rocks (Yorath et aj_, 1977; Yorath, 1980). Magnetic anomalies are s e v e r l y attenuated i n a zone, 40 km wide (NW-SE) that extends over the outer c o n t i n e n t a l s h e l f and slope i n c e n t r a l Queen C h a r l o t t e Sound, r e f l e c t i n g t h i c k sediment f i l l i n the ridge "gap". 161 CHAPTER VIII CONTINUOUS SEISMIC REFLECTION PROFILING INTRODUCTION Seismic r e f l e c t i o n p r o f i l e s were recorded along a t o t a l of 2,040 km of ships track during two c r u i s e s of the C.F.A.V. ENDEAVOR i n southern Hecate S t r a i t and northern Queen C h a r l o t t e Sound; the f i r s t form J u l y 10 to J u l y 20, 1975 (I.O.U.B.C. Cruise Ref. No. 75-22) and the second from June 18 to June 28, 1976 (I.O.U.B.C. Cruise Ref. No. 76-10). Widely spaced i n t e r s e c t i n g track l i n e s were used i n the i n i t i a l survey to i n v e s t i g a t e the broad s t r u c t u r a l framework of the c e n t r a l p o r t i o n of the Queen C h a r l o t t e Basin. Data acquired during the second c r u i s e , f o r the main survey area described i n t h i s t h e s i s , covered the western margin of the basin, east and southeast of Moresby I s l a n d . Lines f o r t h i s survey were run about 5 km apart i n a northeast-southwest d i r e c t i o n , perpendicular to the predominant s t r u c t u r a l trend. F i v e short p r o f i l e s were recorded i n Skidegate I n l e t . The above data base has been augmented by twelve p r e v i o u s l y unpublished seismic p r o f i l e s , t o t a l l i n g 534 km, released by the Geo l o g i c a l Survey of Canada, from southwestern Hecate S t r a i t and northwestern Queen C h a r l o t t e Sound, that were obtained i n 1973 in a j o i n t c r u i s e by the Geo l o g i c a l Survey, Earth Physics Branch, and the Canadian Hydrographic Service ( T i f f i n , 1974). In a d d i t i o n , 407 km of seismic data from nine gas exploder and one shallow 162 sparker l i n e were released by S h e l l r e p r e s e n t a t i v e of some 25,000 km of during t h e i r program of e x p l o r a t i o n 1963-69. Canada Ltd. These p r o f i l e s are seismic l i n e s recorded by S h e l l o f f the Canadian west coast i n PREVIOUS PUBLICATIONS AND OPEN FILE REPORTS Shouldice (1973) described i n general terms s t r u c t u r a l and s t r a t i g r a p h i c r e l a t i o n s w i t h i n the Queen C h a r l o t t e Basin (see chapter IV) and included two multichannel processed seismic records. Though d e t a i l s of much of S h e l l ' s seismic data remain c o n f i d e n t i a l , a 'seismic d ip map' based on sparker and gas exploder data and r e s u l t s of seismic r e f r a c t i o n work were released (Mahannah, 1964). A subsurface contour map (C. S t e i n , i n G r i n s f e l d e r , 1960) and s t r u c t u r e map (A.J. S t i r l i n g , i n G r i n s f e l d e r , 1959), prepared f o r R i c h f i e l d O i l Corporation, were compiled from sparker p r o f i l e s o f f Graham I s l a n d . A report f o r Canadian Husky O i l L t d . (Fowlie, 1962) included two gas exploder p r o f i l e s from Hecate S t r a i t , east of Graham I s l a n d . Luternauer (1972) included i n t e r p r e t a t i o n s of s i x sparker p r o f i l e s from Queen C h a r l o t t e Sound and discussed physio-graphic f e a t u r e s and shallow subbottom s t r u c t u r e and sediments. Line drawing i n t e r p r e t a t i o n s of seismic r e f l e c t i o n p r o f i l e s over the outer c o n t i n e n t a l s h e l f and slope of Queen C h a r l o t t e Sound have been published by S r i v a s t a v a et a l , 1971; S r i v a s t a v a , 1973; Chase and T i f f i n , 1972 and Chase et 1975. Seven r e g i o n a l seismic p r o f i l e s (775 km) across Hecate S t r a i t , recorded i n 1979, 1980 by the 163 G e o l o g i c a l Survey of Canada have not yet been published. INSTRUMENTATION AND SEISMIC RECORD CHARACTERISTIC^ Because of varying survey requirements and equipment design, the C.S.P. records reproduced i n t h i s t h e s i s , d i f f e r sub-s t a n t i a l l y i n t h e i r o v e r a l l q u a l i t y , r e s o l u t i o n and the amount of seismic p e n e t r a t i o n t h a t was achieved. The tow depth, e f f e c t i v e seismic energy of the source, means of s i g n a l enhancement ( a m p l i -f y i n g and f i l t e r i n g ) and recording c h a r a c t e r i s t i c s , each a f f e c t e d the output seismic d i s p l a y . Because seismic data was not recorded d i g i t a l l y on tape f o r reprocessing, a l l p r o f i l e s reproduced i n t h i s t h e s i s are s i n g l e channel, unmigrated analogue records. S t r u c t u r a l i n t e r p r e t a t i o n of these records i s complicated by the r e l a t i v e l y shallow penetration achieved and the presence of p e r s i s t e n t water bottom m u l t i p l e s . The seismic p r o f i l i n g system of the U.B.C. Marine Geology Group ( F i g . 39), used during both the 1975 and 1976 surveys, included a Par Model 600B ( B o l t Associates) airgun f i r e d at an optimum pressure of 10 MPa (2000 p s i ) . The airgun was f i t t e d with a 3 3 656 cm (40 i n ) compression chamber and pulse shaper f o r the e n t i r e 1975 survey and f o r 'deeper water 1 l i n e s i n Queen C h a r l o t t e For d e t a i l e d d i s c u s s i o n on the theory, p r i n c i p a l s , and instrumentation of s i n g l e channel seismic r e f l e c t i o n p r o f i l i n g , the reader i s r e f e r r e d to the f o l l o w i n g p u b l i c a t i o n s : Kramer et a l (1968), Moore (1969); Leenhardt (1969); T i f f i n (1969); Sargent (1970); McQuillan and Ardus (1977) and Van Overeem (1978). 164 U.B.C. CONTINUOUS SEISMIC PROFILING SYSTEM KEY PULSE I EPC MODEL 4100 GRAPHIC RECORDER KENNEDY RAMPED GAIN AMPLIFIED AND FILTER | OSCILLOSCOPE | TELEDYNE D - 52 TRIGGER AIR COMPRESSOR 70 scfm (INGERSOLL RAND) A KENNEDY PREAMPLIFIER BOLT ASSOCIATES AIRGUN U.B.C. MARINE GROUP HYROPHONE ARRAY (25 GEOSPACE MP-17 HYROPHONES AT ONE FOOT SPACING; SENSITIVITY 40juv/u BAR) F i g . 39 Schematic o f U n i v e r s i t y o f se i s m i c p r o f i l i n g system. B r i t i s h Columbia continuous 165 Sound i n 1976; an 82 cm 0 (5 in°) chamber was used f o r survey l i n e s run i n Hecate S t r a i t and Skidegate I n l e t i n 1976. Seismic r e f l e c t i o n s were received by an 8 meter l i n e a r hydrophone array (U.B.C. Marine Geology Group) c o n s i s t i n g of 25 Geospace MP-17 hydro-phones ( s e n s i t i v i t y , 40 jjV/ubar) at 30 cm spacing and a Kennedy p r e a m p l i f i e r . Seismic s i g n a l s were i n t u r n fed i n t o a Kennedy ramped (time varied) gain a m p l i f i e r and f i l t e r system and recorded on an E.P.C. (Model 4100) graphic recorder. The 1973 Geo l o g i c a l Survey of Canada seismic p r o f i l i n g 3 system included a Par 82 cm airgun, 8 meter l i n e a r hydrophone array (G.S.C.), Geospace a m p l i f i e r , Krohn-Hite band pass f i l t e r and an E.P.C. graphic recorder. S h e l l Canada L t d . , employed a GASSP 30 meter l i n e a r gas exploder^ (Teledyne I n d u s t r i e s L t d . , li c e n c e e of S h e l l Development Corporation) of fo u r 6 meter by 0.2 meter modules, and a 15 KJ SSP (Teledyne I n d u s t r i e s Ltd.) 5 t i p sparker. Seismic r e f l e c t i o n s from the gas exploder and sparker systems were received by 62.5 meters (41 element) and 6.7 meters (11 element) l i n e a r hydrophone arrays r e s p e c t i v e l y , a m p l i f i e d and f i l t e r e d , and recorded on Alden 419 (wet paper) P r e c i s i o n Graphic Recorders. Table XI summarizes instrumentation and ac o u s t i c c h a r a c t e r i s t i c s of the seismic p r o f i l i n g systems. The various instrumental and geometric f a c t o r s l i s t e d , i n a d d i t i o n to the type of a c o u s t i c source, modify the recorded seismic data. The 'high Seismic energy f o r the GAASP system i s produced by spark-plug i g n i t i o n of an oxygen-acetelyne gas mixture confined l a r g e l y to the c e n t r a l chamber i n each module. For more complete d e s c r i p t i o n see Kramer et aj_ (1968, p.51-52). \ -TABLE XI SEISMIC RECORD CHARACTERISTICS CRU1SE 1.0.U.B.C. 76-10 ;G.S.C. (1973) 1.0.U.B.C. 75- 22 76- 10 SHELL CANADA (1963-64) SHELL CANADA (1963-64) Type of Source Airgun (82 cm 3) Airgun (82 cm 3) Airgun (656 cm 3) gas exploder sparker F i r i n g Rate of Source 1 per sec. 1 per 2, 4, or 8 sec. 1 per 2 or 4 sec. 1 per 6 sec. 1 per sec. Average Tow Depth (meters) 1 4.5 1 6 6 Range of Acous t i ca l Bandpass F i I t e r s (H 2) 800-5,000 100-500 100-500 19-60 120-150 E f f e c t i v e Seismic Energyj ( jou les ) 3,000 2,000 30,000 60,000 7,000 Recorder Sweep Rate (cm/sec) 100 25, 50, or 100 25 or 50 25 100 Maximum Resolut ion (meters) 0.5 2.5 4 8 3 Maximum Penetrat ion (meters) 200 400 400-800 800 200 1 Estimate of e f f e c t i v e seismic energy ( i n t r i n s i c energy of system and po tent i a l energy of bubble pulse o s c i l l a t i o n ) determined from R a l e i g h - W i l l i s formula (see Kramer et aj^, 1968). 167 r e s o l u t i o n 1 seismic systems (82 cm J airgun and SSP) d i f f e r s u b s t a n t i a l l y i n output c h a r a c t e r i s t i c s from the 'intermediate r e s o l u t i o n seismic systems (656 cm airgun and GAASP). The high r e s o l u t i o n systems use a smaller i n t r i n s i c energy source with more ra p i d f i r i n g r a t e , generating a higher fundamental frequency and shorter pulse length. R e f l e c t o r s at a spacing of 3 meters or l e s s can be resolved by these systems, but because the higher frequency s i g n a l i s more e a s i l y attenuated, seismic events at depths exceeding 200 meters may not be recorded. A l l systems employed a time v a r i e d gain t o increase the a m p l i f i c a t i o n of r e f l e c t i o n s from deeper l a y e r s . The high r e s o l u t i o n records of best q u a l i t y , i n c l u d i n g most reproduced i n t h i s t h e s i s , are the U.B.C. (1976) seismic p r o f i l e s . E x c e l l e n t t h i n bed r e s o l u t i o n of le s s than 0.5 meters i n Holocene sediments (see p r o f i l e 76-25, F i g . 48) was achieved by using a wider band pass f i l t e r to record high harmonic frequencies, and by towing the a c o u s t i c source at or j u s t below the seasurface. The shallow tow depth improved r e s o l u t i o n by reducing the period of o s c i l l a t i o n of the bubble pulse, thereby damping the power of low frequency components (Kramer et £]_, 1968; Leenhardt, 1971; Van Overeem, 1978). Because the length of the outgoing pulse i s shortened, the e f f e c t i v e seismic energy and penetration i s decreased. Such a ' t r a d e o f f i s s u i t a b l e as l i t t l e u s e f u l seismic information can be obtained where the penetration depth c a p a b i l i t y of the high r e s o l u t i o n system exceeds the equivalent water depth. The 6.S.C. small airgun and S h e l l sparker sources were towed at 4.5 t o 6 meters depth r e s u l t i n g i n s l i g h t l y greater pene-t r a t i o n but poorer r e s o l u t i o n . The deeper tow depth r e s u l t e d i n 168 pulse s t r e t c h i n g (lengthening of the outgoing pulse due to surface r e f l e c t i o n s ) and seismic events were recorded as m u l t i t r a c e s instead of d i s c r e t r e f l e c t o r s on seismic p r o f i l e s . The S h e l l sparker records and some of the G.S.C. airgun records s u f f e r from a lower s i g n a l / n o i s e r a t i o , due p a r t i a l l y to high e l e c t r o n i c background noise ( r i n g i n g of f i l t e r s ) , rough sea c o n d i t i o n s , and ship noise due t o lower recording frequencies and narrower passbands. The 'intermediate r e s o l u t i o n ' systems have an expected depth of p e n e t r a t i o n of 600-800 meters, but r e s o l u t i o n i s l i m i t e d to 4-8 meters. In shallow waters of Hecate S t r a i t the amount of us e f u l 3 seismic data obtained with the 656 cm airgun, below eq u i v a l e n t water depth, was l i m i t e d to strong r e f l e c t o r s i n areas of steep d i p . The S h e l l gas exploder however was able to resolve seismic r e f l e c t o r s below the primary water bottom m u l t i p l e to a depth of 0.6 km i n the same region. The high r e s o l u t i o n seismic p r o f i l e s proved most usef u l in determining the nature of Pleistocene-Recent sediments and map-ping s t r u c t u r e of shallow bedrock m a t e r i a l . The intermediate r e s o l u t i o n p r o f i l e s were used t o map deeper r e f l e c t o r s i n shallow water (Hecate S t r a i t ) and i n northwestern Queen C h a r l o t t e Sound. Because the geometry of a c o u s t i c r e f l e c t o r s d i f f e r s u b s t a n t i a l l y depending on the type of recording system, seismic events could not be c a r r i e d with any degree of confidence from p r o f i l e to p r o f i l e . VELOCITY OF SOUND IN SEAWATER, SEDIMENTS, AND BEDROCK V e l o c i t y of sound through the water column, 169 unconsolidated sediments and bedrock m a t e r i a l i s required i n order to convert r e f l e c t i o n times recorded on seismic p r o f i l e s to depths. Determination of t r u e t h i c k n e s s , d i p , and shape of a c o u s t o s t r a t i -graphic u n i t s i s then p o s s i b l e . Seawater The v e l o c i t y of sound i n seawater i s a f u n c t i o n of i n s i t u s a l i n i t y , temperature, and pressure and v a r i e s from 1.40 to 1.56 km/sec. At 0°C, atmospheric pressure, and normal s a l i n i t y (34 parts per thousand by weight) the v e l o c i t y of sound i s 1.44 km/sec (Department of the Navy, 1946). Increase i n e i t h e r temperature, pressure, or s a l i n i t y causes the sound v e l o c i t y to increase: f o r standard seawater (as above) the increase i n temperature i s 4.7 m/sec per degree C e l s i u s , f o r s a l i n i t y the increase i s 1.4 m/sec per one/thousandth part s a l i n i t y , and f o r pressure the increase i s 1.8 m/sec per 100 meters (op c i t ) . In northwestern Queen C h a r l o t t e Sound and southwestern Hecate S t r a i t , water s t r u c t u r e during the summer months (when a l l surveys were c a r r i e d out) i s c o n t r o l l e d p r i m a r i l y by offsho r e t r a n s -port of c o a s t a l near-surface waters and penetration i n t o the Sound of deeper ocean waters (Barber 1957; 1958). Surface s a l i n i t i e s i n the survey area range from 30 to 32 parts per thousand, decreasing s l i g h t l y toward the mainland coast ( J o i n t Committee on Oceanography, 1955a; 1955b). Below a depth of 100 meters and extending to a depth of 175 meters i s a w e l l developed h a l o c l i n e i n which s a l i n i t y increases to 33.8 parts per thousand. Below t h i s depth, i n Queen 170 C h a r l o t t e Sound, s a l i n i t y increases to 34 parts per thousand at a depth of 500 meters. S a l i n i t i e s i n Hecate S t r a i t are s l i g h t l y l e s s than those i n the Sound at corresponding de