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Geology and tectonic setting of the Kamloops group, South-central British Columbia Ewing, Thomas Edward 1981

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GEOLOGY AND TECTONIC SETTING OF THE KAMLOOPS GROUP, SOUTH-CENTRAL BRITISH COLUMBIA by THOMAS EDWARD EWING B.A., The Colorado C o l l e g e , 1975 M.S., New Mexico I n s t i t u t e of Mining and Technology, 1977 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l Sciences We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA February 1981 © Thomas Edward Ewing, 1981 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or pu b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of r.pnlnpiVal Sri PTirp.S The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 D a t e February 17, 1981 ABSTRACT The Kamloops Group i s a widespread assemblage of Eocene v o l c a n i c and sedimentary rocks i n s o u t h - c e n t r a l B r i t i s h Columbia. D e t a i l e d mapping of the type area near Kamloops has r e s u l t e d i n i t s s u b d i v i s i o n i n t o two formations and t h i r t e e n formal and i n f o r m a l members. The T r a n q u i l l e Formation, 0-450 metres t h i c k , c o n s i s t s of l a c u s t r i n e sediments which grade upward i n t o p i l l o w e d flows, h y a l o c l a s t i t e b r e c c i a and aquagene t u f f . The o v e r l y i n g Dewdrop F l a t s Formation, with nine members, c o n s i s t s of up to 1000 metres of b a s a l t to a n d e s i t e p h r e a t i c b r e c c i a , flow b r e c c i a and f l a t - l y i n g f lows. In one l a r g e and four minor v o l c a n i c cones, b a s a l p h r e a t i c v o l c a n i c rocks pass upward i n t o s u b a e r i a l flows and b r e c c i a . A f a u l t zone of i n f e r r e d s t r i k e - s l i p displacement to the southwest, and a complex r e v e r s e - f a u l t e d zone to the south of the area, l o c a l i z e d d e p o s i t i o n of the T r a n q u i l l e Formation. L a t e r f a u l t a c t i v i t y c r e a t e d the complex T r a n q u i l l e Canyon graben, i n p a r t f i l l e d with Dewdrop F l a t s Formation v o l c a n i c s . Reconnaissance of the Kamloops Group throughout the Thompson-Okanagan r e g i o n , and d e t a i l e d mapping at McAbee and Savona, have shown that most Kamloops Group s e c t i o n s c o n s i s t of a lower sedimentary and volcanosedimentary u n i t , f o l l o w e d by t h i c k e r , dominantly b a s a l t to a n d e s i t e , flow and b r e c c i a u n i t s . Thick sedimentary accumulations were l o c a l i z e d i n zones of e xtension w i t h i n a network of s t r i k e - s l i p f a u l t s . The Kamloops Group i s a h i g h - a l k a l i c a l c - a l k a l i n e v o l c a n i c s u i t e dominated by a u g i t e - p i g e o n i t e - l a b r a d o r i t e a n d e s i t e and b a s a l t , with u n u s u a l l y high K, Sr and Ba. I n i t i a l s t r o n t i u m or i s o t o p i c r a t i o s d i s t i n g u i s h a boundary between 'ol d ' c r u s t upper mantle to the east and 'young'.'or Rb-depleted m a t e r i a l s to the west. Petrographic and chemical data are c o n s i s t e n t with magma genesis by p a r t i a l m e l t i n g of a l k a l i - e n r i c h e d p e r i d o t i t e between 40 and 75 km depth, with subsequent deep- and shallow-l e v e l 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 producing the observed v o l c a n i c c h e m i s t r y . Compilation of Paleogene geology and geochronometry in the P a c i f i c Northwest shows the Kamloops Group to be p a r t of a robust c a l c - a l k a l i n e v o l c a n i c arc extending from Wyoming to A l a s k a . Superimposed on t h i s arc were d e x t r a l , s t r i k e - s l i p : f a u l t s , sedimentary b a s i n s and r e s e t metamorphic t e r r a n e s . These elements formed a t e c t o n i c , network which accommodated 90 to 450 k i l o m e t r e s of r i g h t - l a t e r a l displacement between c o a s t a l B r i t i s h Columbia and North America. A t e n t a t i v e p l a t e - t e c t o n i c : r e c o n s t r u c t i o n i s based on the c o m p i l a t i o n . A n o r t h e a s t - d i p p i n g subduction zone, a c t i v e along the e n t i r e coast of the Paleocene P a c i f i c Northwest, ceased to be a c t i v e a f t e r 53 Ma. The transform motion between P a c i f i c and North American p l a t e s was d i s t r i b u t e d i n l a n d , d r i v i n g the Eocene t e c t o n i c a c t i v i t y , while the remnant subducted s l a b gave r i s e to the Eocene magmatic a r c . Transform motion l a t e r became l o c a l i z e d along the c o n t i n e n t a l .margin, as the e a s t - d i p p i n g subduction , zone south of 49° l a t i t u d e i n t i a t e d the Cascade v o l c a n i c arc i n the Late Eocene and O l i g o c e n e . i i i TABLE OF CONTENTS I n t r o d u c t i o n 2 I. Geology Of The Kamloops Group Near Kamloops, B r i t i s h Columbia 6 Summary 6 I n t r o d u c t i o n 8 Previou s Work 9 S t r a t i g r a p h y 11 T r i a s s i c : N i c o l a Group 11 N i c o l a V o l c a n i c s (Trn) 11 A r g i l l i t e (a) 11 Iron Mask B a t h o l i t h ( T r i i ) 12 Cherry B l u f f s Pluton ( T r i e ) 12 J u r a s s i c Or Cretaceous 12 Sediments (Mzc) 12 Eocene: Kamloops Group 13 T r a n q u i l l e Formation (Tt) 13 Border F a c i e s 14 Lower Members ( T t l ) 16 Middle Members (Ttm) 18 Middle-upper Member (Ttmu) 20 Upper Members (Ttu) 20 Dewdrop F l a t s Formation (Td) 21 B r e c c i a Of The N i p p l e (Tdn) 21 K i s s i c k B r e c c i a (Tdk) 22 Mara H i l l Member (Tdm) 23 Wheeler Mountain B r e c c i a (Tdw) 23 C a s t l e Butte B r e c c i a (Tdc) 25 Doherty Creek Member (Tdd) 25 Rosseau Mountain B r e c c i a (Tdrm) 27 Red P l a t e a u Member (Tdr) 27 Opax B r e c c i a (Tdo) 29 I n t r u s i v e Rocks ( T i ) 29 B a t t l e B l u f f I n t r u s i v e Complex (Tib) 31 Quaternary (Q) 32 Gravels Of The Midland Surface (Qg) 32 Terrace G r a v e l s (Qtg) 33 G l a c i a l D r i f t (Qd) 33 P r o g l a c i a l Lakebeds (Qlb) 33 P o s t g l a c i a l Raised D e l t a (Qdl) 33 L a n d s l i d e D e b r i s (Qls) 34 A l l u v i a l Fans (Qaf) 34 Fan D e l t a s (Qfd) 34 Al l u v i u m (Qal) 35 S t r u c t u r e 36 P r e - T e r t i a r y S t r u c t u r e 36 S y n d e p o s i t i o n a l S t r u c t u r e 36 Syn- To P o s t - d e p o s i t i o n a l S t r u c t u r e 37 Summary Of Geologic H i s t o r y 43 A l t e r a t i o n And M i n e r a l P o t e n t i a l 46 I I . Regional S t r a t i g r a p h y And S t r u c t u r a l S e t t i n g Of The Kamloops Group, S o u t h - c e n t r a l B r i t i s h Columbia 54 Summary 54 I n t r o d u c t i o n And Previo u s Work 55 New St u d i e s Of Kamloops Group Geology 56 Kamloops-Tranquille Area ( F i g . 12) 57 Mount Savona Area ( F i g . 16 Map And Sect i o n ) ... 63 McAbee Area ( F i g . 17 Map And Sect i o n ) 67 Monte Lake Area ( F i g . 18 S e c t i o n ; F i g . 11 Map) 70 Proposals For S t r a t i g r a p h i c Usage 74 T e c t o n i c S e t t i n g 78 Summary Of Geologic H i s t o r y Of The Kamloops Group 85 I I I . P e t r o l o g y And Geochemistry Of The Kamloops Group V o l c a n i c s , B r i t i s h Columbia 87 Summary 87 I n t r o d u c t i o n 89 Data C o l l e c t i o n And A n a l y s i s 90 Kamloops Group: A H i g h - a l k a l i C a l c - a l k a l i n e S u i t e 91 Major Elements 94 Trace Elements 100 S p a t i a l And Temporal V a r i a t i o n s 104 Pe t r o l o g y And D i f f e r e n t i a t i o n Trends 107 F e l s i c D i f f e r e n t i a t i o n 113 Mafic To Intermediate D i f f e r e n t i a t i o n 115 Magma O r i g i n s 118 Con c l u s i o n 120 IV. Paleogene T e c t o n i c E v o l u t i o n Of The P a c i f i c Northwest .122 Summary . 122 I n t r o d u c t i o n 123 E a r l y T e r t i a r y Arc Systems 124 Paleocene (65-53 Ma) 124 Eocene (53-42 Ma) 126 Late Eocene And Oligocene (42-30 Ma) 129 Summary 131 E a r l y T e r t i a r y F a u l t i n g , F o l d i n g And Basin Development .132 Paleocene 132 Eocene .133 Late Eocene And Oligocene 139 E a r l y T e r t i a r y Reset Terranes 141 D i s c u s s i o n And Sy n t h e s i s 146 Paleogene Magmatic B e l t s 146 Eocene T e c t o n i c Elements 147 S y n t h e s i s 152 Co n c l u s i o n 157 B i b l i o g r a p h y • 159 Appendices 185 Appendix 1. Notes To Accompany The "Geologic Map Of The Kamloops Lake Area, B r i t i s h Columbia" 185 Appendix 2. Note On The Paleomagnetism Of The Kamloops Group At Kamloops, B r i t i s h Columbia 188 Appendix 3. Note On A V i t r i n i t e Study Of The Kamloops Group 192 Appendix 4. Road Log, Kamloops / T r a n q u i l l e F i e l d T r i p .198 A b s t r a c t 227 Table Of Contents i i i LIST OF FIGURES 2. C o r r e l a t i o n Diagram Of Kamloops Group Members 13 3. Upper T r a n q u i l l e And Lower Dewdrop F l a t s , South Face Mara H i l l . . 18 4. Wheeler Mountain B r e c c i a , Type S e c t i o n 23 5. Wheeler Mountain B r e c c i a , Northwest Of T r a n q u i l l e 25 6. Red P l a t e a u Member, Type S e c t i o n 27 7. Cross S e c t i o n , Afton Mine Area 37 8. Cross S e c t i o n , T r a n q u i l l e Canyon Graben 37 9. Isometric Diagram Of The Map Area 37 10. Paleogeography During Kamloops Group D e p o s i t i o n 43 1. G e o l o g i c Map Of The T r a n q u i l l e Area, Kamloops. In -P-oe-k-e-fc-, Legend On Page 48 11. G e o l o g i c Map Of The Thompson V a l l e y Area 55 l i e . G e o l o g i c Map Of The Thompson-Okanagan Area. In Poe-k-et, See Page 55 12. S t r u c t u r a l S e c t i o n s , T r a n q u i l l e Area 57 13. Lower T r a n q u i l l e L a c u s t r i n e Sediments 57 14. Paleogeography Of The T r a n q u i l l e Basin 61 15. Isometric Diagram Of The T r a n q u i l l e Area 63 16. G e o l o g i c Map And S e c t i o n s , Mount Savona Area 63 17. G e o l o g i c Map And S e c t i o n Of The McAbee Area 67 18. S e c t i o n Through Tuktakamin Mountain 70 19. B a s a l t i c Andesite Flows Near Monte Lake 70 ;20. I d e a l i z e d Sketch Of Major Eocene T e c t o n i c Features .... 81 •21. Displacement Vector Diagram For The Thompson V a l l e y , B r i t i s h 1 C o l u m b i a 81 22. L o c a t i o n Map For Chemical Analyses 90 23. Major-element V a r i a t i o n Diagrams For. Kamloops Group V o l c a n i c s 94 24. S i l i c a O v e r s a t u r a t i o n Vs. D.I 94 25. MgO Vs. Fe203* For Kamloops Group V o l c a n i c s ....... 95 26. Comparison Of A l k a l i s In Kamloops Group And S i m i l a r V o l c a n i c s . .. ...... 98 27. Sr Vs. Ba For Kamloops Group V o l c a n i c s .......100 28. Trace-element V a r i a t i o n With S i l i c a , Kamloops Group V o l c a n i c s .100 29. S p a t i a l V a r i a t i o n Of Parameters 104 30. S t r a t i g r a p h i c V a r i a t i o n ; Of S e l e c t e d Elements At Kamloops :. . • . . . 106 31. Kamloops Group B a s a l t i c Andesite 108 32. B a t t l e B l u f f Diabase 110 33. Composite Mode-norm V a r i a t i o n Diagram, Kamloops ........ 110 34. Acceptable Models From Mixing C a l c u l a t i o n s 113 35. Igneous And T e c t o n i c Elements, Palepgene Of P a c i f i c Northwest .............. 124 36. Eocene Reset Metamorphic Terranes 141 37. R e l a t i o n s h i p Between S t r i k e - s l i p F a u l t i n g And: Extension . ........ ........ . . . i . .... •  . .. . . . .147 38. Map And Displacement Diagram, Eocene F i v e - b l o c k Model .149 39. I n t e r p r e t a t i o n Of E a r l y T e r t i a r y P l a t e T e c t o n i c s .......153 40. G e o l o g i c Map .Of The Kamloops Lake Area, B r i t i s h Columbia. In -P©e-k-e-t, D e s c r i p t i o n . On Page . ... ... ..:..... .185 41. Paleomagnetic F i e l d D i r e c t i o n s , Kamloops Group 188 42. Histograms, V i t r i n i t e R e f l e c t a n c e 192 43. Time-depth Curve For T r a n q u i l l e Formation Samples .....195 44. View Of West Flank, Doherty Creek Cone 198 45. View Of East Flank, Doherty Creek Cone 198 46. View From Bench South Of Mara H i l l 213 47. Route Map With Geology, T r a n q u i l l e F i e l d T r i p 224 M LIST OF TABLES 1. S e l e c t e d Chemical Analyses Of Kamloops Group Igneous I Roc ks 16 4 2. Type L o c a l i t i e s Of Newly-defined U n i t s , Kamloops Group . 57 3. Radiometric Dates From The Kamloops Group 67 4. Chemistry Of Kamloops Group And C o r r e l a t i v e V o l c a n i c Rocks 90 5. 8 , S r / 8 6 S r Data For Kamloops Group V o l c a n i c Rocks 90 6. P r e l i m i n a r y Paleomagnetic R e s u l t s 188 7. V i t r i n i t e R e f l e c t a n c e Of Kamloops Group Sedimentary Successions 192 ACKNOWLEDGEMENTS I must thank my r e s e a r c h s u p e r v i s o r s , Drs. R.L. Armstrong, W.H. Mathews and K.C. McTaggart f o r t h e i r i n v a l u a b l e guidance and support throughout a l l phases of t h i s p r o j e c t . Graham Nixon, Rob Berman and K r i s t a S cott p r o v i d e d much a s s i s t a n c e i n l e a r n i n g to operate the equipment. Gordon White of the B r i t i s h Columbia M i n i s t r y of Energy, Mines and Petroleum Resources pr o v i d e d f i e l d support, while Alan Reed of Afton Mines L t d . , gave i n v a l u a b l e i n f o r m a t i o n and i d e a s . I was a b l y a s s i s t e d i n the f i e l d by John Harrop (1978) and Michael Hauner (1979). B.N. Church of the B r i t i s h Columbia. M i n i s t r y of Energy, Mines and Petroleum Resources c o n t r i b u t e d g r e a t l y to my knowledge of Eocene geology. Cooperative s t u d i e s with Myrl Beck and Peter Schwimmer of Western Washington U n i v e r s i t y , and with R.M. B u s t i n of t h i s Department, l e d to u s e f u l c o n c l u s i o n s . R.L. Armstrong and W.H. Mathews have reviewed many p r e l i m i n a r y d r a f t s . Important ideas have a l s o been generated from d i s c u s s i o n s with J.W.H. Monger, A.Cox, J . M a g i l l , B. Globerman, J . Mortenson, and R. P a r r i s h . T h i s work was funded i n p a r t by a B r i t i s h Columbia M i n i s t r y of Energy, Mines and Petroleum Resources c o n t r a c t with R.L. Armstrong, and N a t i o n a l Research C o u n c i l grants to R.L. Armstrong and W.H. Mathews. INTRODUCTION T h i s work began in January, 1978, as a c o n t r i b u t i o n to the g e o l o g i c a l h i s t o r y of the Eocene v o l c a n i c b e l t of i n t e r i o r B r i t i s h Columbia. I t s f i r s t o b j e c t i v e was the d e s c r i p t i o n of one of that b e l t ' s areas of best exposure — the type area of the Kamloops Group at Kamloops. The p r e v i o u s d e s c r i p t i o n of these rocks was that of G.M. Dawson i n 1895, although they were examined b r i e f l y by W.E. C o c k f i e l d (1948). Despite the e x c e l l e n t exposures in a t e r r a i n of sagebrush desert and open ponderosa pine f o r e s t , up to one k i lometre of r e l i e f and good road access, no s t r a t i g r a p h i c or s t r u c t u r a l i n v e s t i g a t i o n s w i t h i n the Group had been attempted. T h i s p r o j e c t r e c e i v e d funding from the B r i t i s h Columbia M i n i s t r y of Energy, Mines and Petroleum Resources f o r the 1978 f i e l d season, duri n g which the bulk of t h i s d e t a i l e d g e o l o g i c mapping was completed and the s t r a t i g r a p h y e s t a b l i s h e d . A b r i e f r e p o r t of t h i s work has been p u b l i s h e d (Ewing 1979), but i s now superseded by Chapter I, which i n c l u d e s a d d i t i o n a l f i e l d data from the 1979 f i e l d season, as w e l l as some r e s u l t s of chemical and p e t r o l o g i c i n v e s t i g a t i o n s . T h i s chapter has been submitted to the B r i t i s h Columbia M i n i s t r y of Energy, Mines and Petroleum Resources f o r p u b l i c a t i o n . During the 1979 f i e l d season, the p r o j e c t was expanded to i n c l u d e the reconnaissance geology of the Kamloops Group throughout the Thompson-Okanagan area, i n which i t was i n i t i a l l y d e f i n e d (that i s , the area of the Kamloops map-sheet of Dawson 1895). T h i s p r o v i d e d a r e g i o n a l s t r a t i g r a p h i c and s t r u c t u r a l context at a s c a l e of 1:250,000 in which to pl a c e the d e t a i l e d work. The reconnaissance study, i n c l u d i n g the mapping of s e l e c t e d well-exposed l o c a l i t i e s , l e d to the c h a r a c t e r i z a t i o n of. the major t e c t o n i c elements a f f e c t i n g the Eocene rocks. The r e s u l t s of t h i s study are presented i n Chapter I I , which has been submitted to the Canadian J o u r n a l of E a r t h Sciences f o r p u b l i c a t i o n . Samples c o l l e c t e d d u r i n g the 1978 and 1979 f i e l d seasons were examined i n t h i n - s e c t i o n and by X-ray f l u o r e s c e n c e spectrometry i n order to c h a r a c t e r i z e the Kamloops Group magmatism. Strontium i s o t o p i c composition was measured on s e l e c t e d samples dur i n g the s p r i n g and summer of 1980. These s t u d i e s permit c o n s i d e r a t i o n of the magmatic source of the v o l c a n i c rocks and f r a c t i o n a t i o n p a t t e r n s . These r e s u l t s are d e t a i l e d in Chapter I I I , which a l s o has been submitted f o r p u b l i c a t i o n i n the Canadian J o u r n a l of E a r t h Sc i e n c e s . One hope f o r t h i s study was to p l a c e the Kamloops Group volcanism and sedimentation i n a r e g i o n a l t e c t o n i c s e t t i n g . I t was necessary to compile a l l the a v a i l a b l e i n f o r m a t i o n on Paleogene t e c t o n i c , magmatic and sedimentary elements of the P a c i f i c Northwest, from Wyoming and Idaho through Oregon, Washington and B r i t i s h Columbia to A l a s k a . These data, together with g l o b a l and P a c i f i c p l a t e motions, can be combined i n t o a s y n t h e s i s of the Paleogene t e c t o n i c e v o l u t i o n of the P a c i f i c Northwest. P r e l i m i n a r y v e r s i o n s of t h i s s y n t h e s i s have p r e v i o u s l y been o u t l i n e d i n a b s t r a c t form (Ewing 1979, 1980 a,b). The f i n a l v e r s i o n , along with the r e l e v a n t g e o l o g i c and geochronologic data, i s presented i n Chapter IV of t h i s t h e s i s , which has been accepted f o r p u b l i c a t i o n by the J o u r n a l of  Geology. A n c i l l a r y e f f o r t s are repo r t e d i n the Appendices. Appendix 1 presents a 1:50,000 map of the Kamloops Lake area, combining the d e t a i l e d geology of Chapter I with reconnaissance work. The appendix a l s o o u t l i n e s the aeromagnetic c h a r a c t e r i s t i c s of Eocene rock u n i t s i n the area. Appendix 2 r e p o r t s on a paleomagnetic study undertaken in c o l l a b o r a t i o n with Peter Schwimmer and Myrl Beck of Western Washington U n i v e r s i t y , Bellingham, Washington. Appendix 3 r e p o r t s on v i t r i n i t e r e f l e c t a n c e values obtained on s e v e r a l organic samples from the Kamloops Group rocks, i n c o l l a b o r a t i o n with R.M. Bu s t i n of t h i s Department. These r e s u l t s support the e x i s t e n c e of high Eocene thermal g r a d i e n t s . Appendix 4 i s a f i e l d t r i p guide to the Kamloops / T r a n q u i l l e area, emphasizing the Eocene geology. T h i s f i e l d t r i p takes advantage of the e x c e l l e n t road access and good outcrops i n the area, and should serve as a b a s i s f o r f u t u r e g e o l o g i c f i e l d t r i p s p a s s i n g through Kamloops. CHAPTER I. GEOLOGY OF THE KAMLOOPS GROUP NEAR KAMLOOPS, BRITISH COLUMBIA SUMMARY D e t a i l e d mapping of the type area of the Kamloops Group at Kamloops, B r i t i s h Columbia, leads to d e s c r i p t i o n of the T r a n q u i l l e and Dewdrop F l a t s Formations. The lower, T r a n q u i l l e Formation, up to 450 metres t h i c k , can l o c a l l y be d i v i d e d i n t o lower, middle and upper u n i t s . I t i s dominantly l a c u s t r i n e , t u f f a c e o u s and h y a l o c l a s t i c ; b a s a l l a c u s t r i n e sediments pass upwards i n t o t u f f s , p i l l o w e d flows and h y a l o c l a s t i c mudflows and aquagene t u f f s ; a b o r d e r i n g d e l t a i c f a c i e s i s l o c a l l y developed. The o v e r l y i n g Dewdrop F l a t s Formation i s over 1000 metres t h i c k , |t and c o n s i s t of nine members. Basal p h r e a t i c b r e c c i a ( b r e c c i a of. The N ipple) i s o v e r l a i n by the K i s s i c k b r e c c i a s ( f l o w - b r e c c i a and p h r e a t i c b r e c c i a ) , the Mara H i l l member ( t h i n b a s a l t i c -a n d e s i t e f l o w s ) , Wheeler Mountain b r e c c i a ( f l o w - b r e c c i a and some p h r e a t i c b r e c c i a ) , Red P l a t e a u member ( t h i n b a s a l t i c a n d e s i t e and a n d e s i t e flows) and the Opax flow b r e c c i a . To the west a l a r g e composite cone has been mapped. I t s core c o n s i s t s of h y a l o c l a s t i t e and p h r e a t i c b r e c c i a of p l a g i o c l a s e - r i c h a n d e s i t e , c a l l e d the C a s t l e Butte b r e c c i a . I t i s o v e r l a i n g r a d a t i o n a l l y by s u b a e r i a l flows, b r e c c i a and t u f f of the Doherty Creek member, which i s probably the l a t e r a l e q u i v a l e n t of the Red P l a t e a u member. On the western edge of the map area, a d i s t i n c t i v e flow b r e c c i a (Rosseau Mountain b r e c c i a ) o v e r l i e s the Doherty Creek member. Both formations are cut by small dykes. Large i n t r u s i v e complexes, c o n s i s t i n g of s i l l s , dykes, i r r e g u l a r masses and p l u g s , i n t r u d e the T r a n q u i l l e Formation; of these, the most important are the B a t t l e B l u f f i n t r u s i v e s . The most important s t r u c t u r a l elements of the T r a n q u i l l e area are the Afton s t r u c t u r a l zone along the southern boundary of the area, the Cherry Creek F a u l t Zone on the southwest, and the T r a n q u i l l e Canyon complex graben. The f i r s t two elements c o n t r o l l e d T r a n q u i l l e d e p o s i t i o n , while the t h i r d a f f e c t e d Dewdrop F l a t s volcanism. INTRODUCTION The area of t h i s study l i e s immediately west of Kamloops on e i t h e r s i d e of the v a l l e y of the Thompson River and Kamloops Lake. The Trans-Canada Highway and the Afton open-pit copper mine form i t s southern boundary. Mapping at 1:20,000 durin g the 1978 and 1979 f i e l d seasons extended north to the j u n c t i o n of the T r a n q u i l l e River with Watching Creek, and westward to the mouth of Rosseau Creek. T h i s mapping, which con c e n t r a t e d on the Cenozoic geology of the area, forms the b a s i s f o r the g e o l o g i c map presented as F i g . 1; a d e t a i l e d legend f o r the map f o l l o w s t h i s chapter. A f i e l d t r i p guide to t h i s area has a l s o been prepared (Appendix 4). PREVIOUS WORK Dawson (1895) f i r s t s t u d i e d the T e r t i a r y rocks of the region around Kamloops Lake, d i s t i n g u i s h i n g from the main v o l c a n i c mass a t u f f a c e o u s sedimentary f a c i e s as the " T r a n q u i l l e beds", and a b a s a l nonvolcanic s u c c e s s i o n as the "Coldwater group". Only the f i r s t u n i t i s present i n the Kamloops-Tranquille area. Drysdale (1914) and Rose (1914) proposed the name "Kamloops V o l c a n i c Group" f o r T e r t i a r y v o l c a n i c and sedimentary rocks o v e r l y i n g the Coldwater Group. C o c k f i e l d (1948) i n c l u d e d the Coldwater beds i n the r e d e f i n e d "Kamloops Group" d u r i n g the remapping of the N i c o l a map-area. The d e s i g n a t i o n "Kamloops Group" has s i n c e been extended over a broad area of s o u t h - c e n t r a l B r i t i s h Columbia to r e f e r to the e a r l y T e r t i a r y v o l c a n i c and sedimentary sequence. Mathews (1964) repo r t e d K-Ar dates d i s t i n g u i s h i n g two v o l c a n i c episodes i n s o u t h - c e n t r a l B r i t i s h Columbia: the Kamloops Group proper, of Eocene age (45-52 Ma), and the P l a t e a u l a v a s of Miocene age (10-13 Ma). H i l l s and Baadsgaard (1967) s u p p l i e d a d d i t i o n a l c o n f i r m a t i o n of the lower to middle Eocene age of the Kamloops Group. The Kamloops Group v o l c a n i c s are t i m e - e q u i v a l e n t s to the P r i n c e t o n Group at P r i n c e t o n , B.C. (Rice 1947), the K e t t l e R i v er and Marron Formations at Midway and P e n t i c t o n (Monger 1967; Church 1973), and the S a n p o i l v o l c a n i c s of northern Washington (Pearson and Obradovich, 1977). Notwithstanding the wide d i s t r i b u t i o n and s u b s t a n t i a l t h i c k n e s s of the Kamloops Group, i t s type area and i n t e r n a l s t r a t i g r a p h y have never been s t u d i e d i n d e t a i l . T h i s study d e s c r i b e s t h i s i n t e r n a l s t r a t i g r a p h y , d e f i n i n g two formations and eleven members in the type a r e a . An examination of the r e g i o n a l s t r a t i g r a p h y of the Kamloops Group i s presented i n Chapter I I . STRATIGRAPHY TRIASSIC: NICOLA GROUP N i c o l a v o l c a n i c s (Trn) Outcrops of metavolcanic rocks r e f e r a b l e to the N i c o l a Group occur e x t e n s i v e l y in the r e g i o n . Within the mapped area, these c o n s i s t of p l a g i o c l a s e - p o r p h y r y a n d e s i t e s , with o c c a s i o n a l interbedded t u f f s west of B a t c h e l o r H i l l . N i c o l a v o l c a n i c s have not been mapped i n d e t a i l . In g e n e r a l , they are e a s i l y d i s t i n g u i s h e d from T e r t i a r y rocks by a l b i t i z a t i o n of f e l d s p a r s and patchy epidote and/or hematite a l t e r a t i o n . In a few areas t h i s s e p a r a t i o n i s l e s s c e r t a i n , n o t a b l y south of T r a n q u i l l e and in the D u f f e r i n H i l l a r e a. There, the N i c o l a Group i s d i s t i n g u i s h e d by the appearance i n t h i n s e c t i o n s of stubby a l b i t i z e d and s a u s s u r i t i z e d andesine l a t h s and a l t e r e d hornblende set i n a r e c r y s t a l l i z e d , o f t e n e p i d o t e - b e a r i n g groundmass. A r g i l l i t e (a) Northwest of Kamloops, low h i l l s of grey-black sheared a r g i l l i t e s have been mapped, but not s t u d i e d i n d e t a i l . They may be c o r r e l a t i v e with the a r g i l l i t e s n ortheast of Kamloops, which y i e l d an Upper T r i a s s i c fauna (Smith 1978). Iron Mask B a t h o l i t h ( T r i i ) I n t r u s i v e d i o r i t e and monzonite occur south of the study area east of and i n c l u d i n g the Afton mine:. Northcote (1977) has i n t e r p r e t e d the b a t h o l i t h as the subvolcanic e q u i v a l e n t of the upper N i c o l a v o l c a n i c s . K-Ar dates range from 190 to 205 Ma (Preto et a l . 1979). \ Cherry B l u f f s pluton ( T r i c ) T h i s pluton u n d e r l i e s the l a r g e b l u f f s of Roper H i l l , Cherry B l u f f and B a t t l e B l u f f on either, s i d e of Kamloops Lake. It i s p e t r o l o g i c a l l y s i m i l a r to the Cherry Creek phase of the i r o n Mask B a t h o l i t h . Betmahis '.(1.972) rep o r t e d on the geology of the Roper H i l l area. JURASSIC OR CRETACEOUS : Sediments (Mzc) : Red-brown conglomerate and sandstone of J u r a s s i c or l a t e r age has been r e p o r t e d along the Cherry Creek f a u l t zone west of Af t o n (Carr and Reed, 1976). D r i l l hole Q-245 (shown on map) p e n e t r a t e s these rocks under Hughes- Lake v a l l e y (Reed 1977). S i m i l a r sediments b e a r i n g n a t i v e copper.are found w i t h i n the Afton d e p o s i t (A.J. Reed, p e r s o n a l communication). Conglomerates of s i m i l a r appearance are seen i n a roadc.ut on the Trans-Canada Highway along Cherry Creek, where they are o v e r t h r u s t by N i c o l a Group v o l c a n i c s . Other small patches of these conglomerates along Cherry Creek are mapped with the N i c o l a Group. North of B r o c k l e h u r s t , w e l l - i n d u r a t e d red-brown angular conglomerates form s e v e r a l low h i l l s . The c l a s t s are composed of a r g i l l i t e , greenstone, c h e r t and g r a n i t i c m a t e r i a l . Although C o c k f i e l d (1948) mapped t h i s m a t e r i a l as "Coldwater beds ( ? ) " , i t i s d i s s i m i l a r to the type Coldwater beds at M e r r i t t , and l i t h o l o g i c a l l y s i m i l a r to the m i n e r a l i z e d sediments i n the Afton d e p o s i t . EOCENE: KAMLOOPS GROUP The r e l a t i o n s h i p of the v a r i o u s members of the Kamloops Group are shown on F i g . 2. In the map area, the Kamloops Group i s s u b d i v i d e d i n t o two formations, the T r a n q u i l l e Formation and the Dewdrop F l a t s Formation. Hypabyssal i n t r u s i v e s assigned to the Kamloops Group are a l s o abundant. T r a n q u i l i e Format ion (Tt) The base of the Eocene s u c c e s s i o n c o n s i s t s of sediments, v o l c a n i c s and t u f f s l a r g e l y of l a c u s t r i n e f a c i e s , t o t a l l i n g about 450m. They make up the area north of the Thompson River v a l l e y from B a t t l e B l u f f east past Mara H i l l , which was designated the type area of the " T r a n q u i l l e beds" by Dawson (1895). T h i s s u c c e s s i o n , mappable at s c a l e s to 1:250,000, i s here f o r m a l l y designated the T r a n q u i l l e Formation. I t s type area encompasses i t s e n t i r e outcrop area as d e s c r i b e d i n t h i s r e p o r t , with r e f e r e n c e s e c t i o n s northwest of Afton mine, northeast of Afton mine, and at K i s s i c k s i d i n g south of Mara H i l l . / Tdo? \ ! Tdo Opax RED POINT TRANQUILLE F i g . 2. C o r r e l a t i o n diagram of Kamloops Group members i n the Kamloops-Tranquilie a r e a . Diagram i s roughly arranged west to e a s t ; compare F i g . 8. Tt = T r a n q u i l l e Formation; Td = Dewdrop F l a t s Formation. Border f a c i e s . T r a n q u i l l e sediments not c o r r e l a t a b l e with the t y p i c a l s e c t i o n s are found i n the Afton mine and the Guerin Creek ar e a s . The Guerin Creek sediments c o n s i s t of orange mudstone, a r k o s i c wacke and o r g a n i c - r i c h s h a l e , with a much-weathered c o a l y zone exposed. S i m i l a r c o a l - b e a r i n g sediments form f a u l t s l i c e s w i t h i n the Afton d e p o s i t . A.J. Reed i n t e r p r e t s these sediments as remnants of a nonmarine nearshore f a c i e s l y i n g to the south of the main T r a n q u i l l e d e p o s i t i o n a l b a s i n (personal communication). The north w a l l of the Afton open p i t exposes four T e r t i a r y rock types: a coarse l i t h i c - a r k o s i c wacke, with c l a s t s of p l u t o n i c q u a r t z , " c h e r t " , s a n i d i n e , m i c r o c l i n e , p l a g i o c l a s e , b i o t i t e , p h y l l i t e , s h a l e , meta-andesite and metaplutonic rocks in patchy c a l c i t e cement; interbedded deformed dark shal e , with f l a s e r bedding, c o a l fragments and small growth f a u l t s ; medium-bedded s i l t s t o n e and t u f f ; and a n d e s i t e - d a c i t e b r e c c i a , with o c c a s i o n a l t u f f c l a s t s and two stages of b r e c c i a t i o n and carbonate cementation. Graham and Long (1979) report s i m i l a r rocks, i n c l u d i n g t u f f a c e o u s p r o d e l t a d e p o s i t s and small s o f t -sediment slumps, in d r i l l h o les GG-1 and GG-6 near the mine. Wackes and shales s i m i l a r to those i n the open p i t are a l s o exposed on the lower sl o p e s of the h i l l n o rtheast of the mine s i t e , a s s o c i a t e d with an o l i s t o s t r o m e of N i c o l a Group v o l c a n i c s . The rocks of the border f a c i e s are i n t e r p r e t e d to represent a d e l t a extending northward i n t o the T r a n q u i l l e lake about one k i l o m e t r e , as proposed by Graham and Long (1979). The c l a s t composition i n d i c a t e a v a r i e t y of source t e r r a n e s s i m i l a r to the p r e - T e r t i a r y rocks exposed today i n the surrounding r e g i o n . T h i s d e l t a appears to be e q u i v a l e n t to the middle and upper members of the T r a n q u i l l e Formation i n more t y p i c a l s e c t i o n s to the north (see F i g . 6). Lower members ( T t l ) . The lower member of the T r a n q u i l l e Formation i s best developed south of Kamloops Lake, where i t may be s u b d i v i d e d i n t o three u n i t s . The lower u n i t ( T t l 1 ) c o n s i s t s of 210 metres of both r i d g e - and slope-forming yellow-brown laminated s i l t s t o n e and mudstone, c o n t a i n i n g pumice c l a s t s , mud c l a s t s , and abundant c a l c i c p l a g i o c l a s e l a t h s . L o c a l l y i t s base i s e x t e n s i v e l y i n t r u d e d by s i l l s of the B a t t l e B l u f f I n t r u s i v e Complex. The middle u n i t ( T t l 2 ) c o n s i s t s of 85 metres of c o a r s e -g r a i n e d p o r p h y r i t i c a n d e s i t e (Table 1, TP 21-2), with phenocrysts of a u g i t e , p i g e o n i t e and l a b r a d o r i t e . The rock v a r i e s i n aspect from flow-rock to t u f f , and i s a r i d g e - f o r m i n g marker i n the Afton area. The upper u n i t ( T t l 3 ) c o n s i s t s of slope-forming t u f f a c e o u s mudstone s i m i l a r to the lower u n i t , and i s g r e a t e r than 30 metres t h i c k ; the top i s concealed. North of the Thompson R i v e r , t h i n l y laminated t u f f a c e o u s mudstone i s exposed on the lowest s l o p e s of Mara H i l l ( t h i c k n e s s g r e a t e r than 55 metres). Over 120 metres of l a c u s t r i n e mudstone and s i l t s t o n e i s exposed east of B a t t l e B l u f f u n d e r l y i n g and interbedded with conspicuous s i l l s . T h i s s e c t i o n i s s p a r i n g l y f o s s i l i f e r o u s ( C o c k f i e l d 1948; H i l l s 1965; Rice 1957; Wilson 1977). Ash from t h i s area y i e l d e d K-Ar dates f o r b i o t i t e and f e l d s p a r s of 50 Ma ( H i l l s and Baadsgaard 1967). These rocks may r e s t on N i c o l a Group v o l c a n i c s ; they c o r r e l a t e with the lower u n i t south of Kamloops Lake. West of Red P o i n t , lower T r a n q u i l l e s t r a t a are exposed i n a Table 1 . Selected Chemical Analyses of Kamloops Group Igneous Rocks . T t l 2 6-5 T t l 2 TP 21-2 Tdn - TP 43-Tdk 1 TP 7-4 Tdm TP 4-1 Tdw1 TP 13-2 Si0 2 Ti0 2 48.28 1.23 51.51 1.22 50.26 0.98 51.52 1.23 52.79 1.24 . 56.03 1.14 M 2 ° 3 16.25 15.13 14.55 17.56 16.05 16.53 F e2°3 8.40 8.69 9.58 12.63 8.40 7.63 MnO 0.12 0.14 0.15 0.18 0.07 0.12 MgO 3.71 6.43 10.06 3.18 4.28 4.02 CaO 7.28 7.84 7.40 7.34 7.48 6.95 Na20 2.72 3.07 2.31 3.42 3.79 3.36 K20 2.59 2.22 2.75 2.38 2.98 2.45 P2°5 0.60 0.58 0.56 0.63 0.80 0.59 H20+ 8.24 1.55 1.74 0.03 1.33 0.92 H20- 0.81 2.20 0.51 0.55 1.10 1.29 sum 100.23 100.58 100.85 100.65 100.32 101.03 Tdc 4-2 Tdrm 5-6 Tdr TP 32-3 Tdo TP 39-3 Tib 3 TP 9-1 T i 1 31-5 Si0 2 53.44 50.85 54.44 57.46 49.40 55.19 Ti0 2 1.24 1.09 0.78 0.96 1.12 0.95 M 2 ° 3 19.69 16.19 18.20 15.66 17.23 16.76 F e2°3 6.91 9.29 9.07 . 7.77 10.28 6.60 MnO 0.11 0.17 0.14 0.13 0.17 0.08 MgO 1.85 5.98 2.44 3.67 4.58 2.99 CaO 6.73 8.18 5.80 5.70 6.45 6.07 Na20 3.23 2.79 4.30 3.36 3.44 2.92 K20 P2°5 2.70 2.52 3.28 3.05 2.85 3.01 0.62 0.49 0.59 0.53 0.54 0.45 H20+ 3.10 2.02 0.04 0.79 2.78 2.45 H20- 1.14 0.62 1.08 0.34 1.05 2.67 sum 100.76 100.26 100.16 99.42 99.90 100.14 Analyses by X-ray fluorescence, University of British Columbia. west-dipping homocline. The lower -unit ( T t l 1 ) c o n s i s t s of more than 180 metres of s p a r i n g l y f o s s i l i f e r o u s (Wilson 1977) laminated l a c u s t r i n e sediment ( s i l t s t o n e and mudstone), becoming more t u f f a c e o u s upwards. Minor i n t e r b e d s of s l i d e b r e c c i a with c l a s t s of N i c o l a Group v o l c a n i c s suggest s i g n i f i c a n t , probably f a u l t - i n d u c e d , r e l i e f nearby. O v e r l y i n g t h i s conformably i s a t h i c k b a s a l t flow (Table 1, 6-5) showing colonnade and e n t a b l a t u r e j o i n t p a t t e r n s and a rubbly upper s u r f a c e . T h i s i n d i c a t e s s u b a e r i a l e r u p t i o n of the flow,and suggests that the lower u n i t e s s e n t i a l l y f i l l e d the lake b a s i n . A f u r t h e r 210 metres of t u f f a c e o u s l a c u s t r i n e s t r a t a , however, o v e r l i e s the flow ( T t l 3 ) . . Middle members (Ttm). The v o l c a n i c middle members of the T r a n q u i l l e Formation c o n s i s t s of d i f f e r e n t f a d e s north and south of the Thompson R i v e r . North of the r i v e r , 30 metres of p i l l o w e d a n d e s i t e are exposed i n the lowest s l o p e s of Mara H i l l ( F i g . 3). P i l l o w s range up to one metre in s i z e , and : show dark g l a s s y selvages with i n t e r i o r v e s i c u l a t e d zones. South.of the r i v e r , a n d e s i t e flow-rock with zoned l a b r a d o r i t e and c l i n o p y r o x e n e s forms conspicuous cuestas northwest of Afton mine and southeast of T r a n q u i l l e . Some of these rocks may be i n t r u s i v e , c o r r e l a t i n g with those i d e n t i f i e d . b y Creaney (1979) in d r i l l h o les GG-1 and GG-6. A l t e r n a t i v e l y , these l a t t e r may be • l o c a l i z e d i n the Afton area. A middle member of the T r a n q u i l l e Formation forms the l a r g e cuesta west of Red P o i n t . At i t s base i s . a c o l u m n a r - j o i n t e d b a s a l t i c a n d e s i t e , conformably o v e r l y i n g the lower- t r a n q u i l l e s t r a t a . Above t h i s , a t h i c k sedimentary and v o l c a n i c l a s t i c metres! 2 X. 600-tea 500-l i g h t gray weathering black b a s a l t i c f l o w - b r e c c i a f l o w - b r e c c i a , l e n s i n g bedding; some p h r e a t i c i n f l u e n c e black aphanite f l o w - b r e c c i a h y a l o c l a s t i c b r e c c i a , crude bedding; mudflow appearance, c l a s t s about 20%. h y a l o c l a s t i t e , b r e c c i a , b a s a l t i c : andesite c l a s t s , v e s i c u l a r ; palagonite matrix,: gra n u l a r . o c c a s i o n a l sandstone, shale, ?aquagene t u f f i n t e r l a y e r s . h y a l o c l a s t i t e breccia; p i l l o w e d andesite sheared sediments p i l l o w e d andesite p i l l o w . b r e c c i a , some i n t a c t p i l l o w s . tuffaceous mudstone F i g . 3. S t r a t i g r a p h i c column of the upper T r a n q u i l l e and lower Dewdrop F l a t s Formations on the south face of Mara H i l l ; e l e v a t i o n s i n metres. O Ld s e c t i o n has been f o l d e d i n t o t i g h t recumbent f o l d s on s c a l e s of 10 c e n t i m e t r e s to 20 metres, v e r g i n g g e n e r a l l y n o r t h e a s t . T h i s zone i s i n t e r p r e t e d to represent soft-sediment deformation a s s o c i a t e d with a c t i v i t y along the Cherry Creek f a u l t zone to the southwest. The top of the middle member i s formed by a 10 metre t h i c k bed of graded b r e c c i a - s a n d s t o n e , marking a gr a d a t i o n to the upper member of the formation. The present t h i c k n e s s of the middle member i s about 230 metres. Middle-upper member (Ttmu). South of the Thompson R i v e r , the upper and middle p o r t i o n s of the T r a n q u i l l e Formation form a much-sheared t u f f a c e o u s f a c i e s extending from T r a n q u i l l e s i d i n g southeast to the Trans-Canada Highway. Tuff and r e l a t e d b r e c c i a s form low r i d g e s ; interbedded s i l t s t o n e and mudstone s i m i l a r to the lower T r a n q u i l l e are exposed only i n dry gulches c u t t i n g the d r i f t cover. Dykes are common, and one small patch of b a s a l t i c p h r e a t i c b r e c c i a was found about 1 km north of Polygon Pond. Because of poor outcrop and intense shearing, d e t a i l e d s t r a t i g r a p h y and t h i c k n e s s are not known, but the m a t e r i a l c o r r e l a t e s with the middle and upper members of the T r a n q u i l l e Formation d e f i n e d elsewhere. Upper members ( T t u ) . The upper member of the T r a n q u i l l e Formation north of the Thompson R i v e r ( F i g . 3) c o n s i s t s of over 80 metres of p a l a g o n i t i c a s h - r i c h mudflows with o c c a s i o n a l sandstone i n t e r b e d s l y i n g conformably and g r a d a t i o n a l l y on the p i l l o w e d a n d e s i t e s of the middle member. T h i s m a t e r i a l i s best exposed at K i s s i c k s i d i n g south of Mara H i l l , where c l a s t s of a l t e r e d v e s i c u l a r a n d e s i t e compose about 15% of the rock. To the west, the c r u d e l y bedded flows i n t e r f i n g e r with the b r e c c i a of The N i p p l e , d e s c r i b e d below. The upper s u r f a c e of the member i s w e l l exposed at K i s s i c k , and i s n e a r l y f l a t with no evidence of e r o s i o n p r i o r to d e p o s i t i o n of the Dewdrop F l a t s Formation. The u n i t i s i n t e r p r e t e d to represent the f i l l i n g of the T r a n q u i l l e lake with h y a l o c l a s t i c m a t e r i a l immediately p r i o r to K i s s i c k d e p o s i t i o n . West of Red Point the upper T r a n q u i l l e Formation l i e s g r a d a t i o n a l l y upon the deformed middle member. The lower part of the upper member in t h i s area ( T t u 1 ) c o n s i s t s of 250 metres of volcanogenic mudstones and s i l t s t o n e s with o c c a s i o n a l massive sandstone i n t e r b e d s . The upper part of t h i s u n i t shows some e f f e c t s of soft-sediment deformation. The upper part ( T t u 2 ) begins with a marker h o r i z o n of v o l c a n i c b r e c c i a about 10 metres t h i c k , succeeded by ashy mudflows c o n t a i n i n g l a r g e blocks of b a s a l t i c a n d e s i t e . T h i s u n i t may represent a h y a l o c l a s t i c s u c c e s s i o n e q u i v a l e n t to the upper member at T r a n q u i l l e . Dewdrop F l a t s Format ion (Td) The Dewdrop F l a t s Formation takes i t s name from Dewdrop F l a t s , four to s i x k i l o m e t r e s northwest of T r a n q u i l l e . Imposing b l u f f s north of the F l a t s p r o v i d e t y p i c a l s e c t i o n s of s e v e r a l members of t h i s v o l c a n i c formation. The name r e p l a c e s the i n f o r m a l term "Upper V o l c a n i c s " of Dawson (1895). B r e c c i a of The N i p p l e (Tdn). An area west of T r a n q u i l l e River i s u n d e r l a i n by p h r e a t i c b r e c c i a with c l a s t s of p o r p h y r i t i c b a s a l t (Table 1, TP 43-1). The mafic phenocrysts are p o l y c r y s t a l l i n e intergrowths of a u g i t e , p i g e o n i t e and a l t e r e d o l i v i n e ; l a b r a d o r i t e forms l a r g e s t e l l a t e aggregates. The b r e c c i a matrix i s formed by gr a n u l a t e d p a l a g o n i t i c m a t e r i a l s i m i l a r to the upper T r a n q u i l l e , i n d i c a t i n g involvement of heated groundwater i n the e r u p t i o n and/or t r a n s p o r t of the brecc i a . The b r e c c i a mass d i s p l a y s a moderate primary east d i p , i n t e r f i n g e r i n g eastward with h y a l o c l a s t i t e s of the upper T r a n q u i l l e over a .distance of about 500 m. Thickness of the u n i t ranges from zero to 100 m. K i s s i c k b r e c c i a (Tdk). The K i s s i c k b r e c c i a conformably o v e r l i e s the upper member of the T r a n q u i l l e Formation ( F i g . 3) and (with a l o c a l unconformity) the b r e c c i a of The N i p p l e . I t c o n s i s t s of 160-190 metres of a n d e s i t i c b r e c c i a i n v a r i o u s f a c i e s . The dominant s u b a e r i a l vent f a c i e s i s t y p i c a l l y exposed in the steep c l i f f s n orth of K i s s i c k s i d i n g south of Mara H i l l . There, rudely bedded l e n t i c u l a r flow b r e c c i a and s u b a e r i a l mudflow u n i t s of l i g h t grey weathering a n d e s i t e (Table 1, TP 7¬4) show primary d i p s of 30°, i n d i c a t i n g a vent area to the nor t h . C l o s e r to the i n f e r r e d vent, p a l a g o n i t i c m a t e r i a l predominates, suggesting that the K i s s i c k cone s t r u c t u r e e i t h e r o r i g i n a t e d as a t u f f r i n g , or co n t a i n e d an i n t e r i o r l a k e . The flow b r e c c i a i s t r a c e a b l e northward i n t o p h r e a t i c b r e c c i a s with pronounced development of a p a l a g o n i t i c granule matrix; some exposures show tongues of massive flow b r e c c i a surrounded by p h r e a t i c m a t e r i a l . T h i s i s i n t e r p r e t e d to represent a nearshore f a c i e s of a small lake i n t o which tongues of the s u b a e r i a l b r e c c i a flowed. A K i s s i c k - e q u i v a l e n t p h r e a t i c a n d e s i t e b r e c c i a (Tdk 1) forms the main r i d g e of D u f f e r i n H i l l south of the Thompson R i v e r . I t s source may be the main K i s s i c k vent. Mara H i l l member (Tdm). The Mara H i l l member g r a d a t i o n a l l y o v e r l i e s the K i s s i c k b r e c c i a i n most exposures. I t c o n s i s t s of 150 to 300 metres of red-brown weathering b a s a l t i c a n d e s i t e flows, flow-top b r e c c i a s and r e l a t e d mudflows (Table 1, TP 4-1), forming crude c o u p l e t s of flow: and b r e c c i a . Thin s e c t i o n s d i s c l o s e phenocrysts of plagioclase:, c l i n o p y r o x e n e s and "ghost" hornblende set i n a p l a g i o c l a s e - m a g n e t i t e - q u a r t z h o l o c r y s t a l l i n e groundmass. T h i s sequence i s t y p i c a l l y developed on the western and northern slopes of Mara H i l l ; i t i n t e r t o n g u e s with a l o c a l accumulation of dark grey flow b r e c c i a on the southeast s i d e of Mara H i l l . Wheeler Mountain brecc i a (Tdw). The Wheeler Mountain b r e c c i a conformably o v e r l i e s the Mara H i l l member. I t e x h i b i t s a v a r i e t y of f a c i e s and a range of composition, both v e r t i c a l l y and l a t e r a l l y . The member i s t y p i c a l l y developed on the southwestern c l i f f s of Wheeler Mountain north of Mara H i l l (Fig.: 4), where i t i s 250 metres t h i c k . The lower u n i t (Tdw 1) c o n s i s t s of a n d e s i t e (Table 1; TP 13-2) flow b r e c c i a and mudflows with some p a l a g o n i t i c a l t e r a t i o n grading upwards to p h r e a t i c and s u b a e r i a l slope-forming b r e c c i a . The upper u n i t (Tdw 2) c o n s i s t s of mixed p h r e a t i c b r e c c i a and columnar j o i n t e d a n d e s i t e j ( i n t r u s i v e ? ) , o v e r l a i n by c l i f f - f o r m i n g a n d e s i t e flow b r e c c i a , which in t e r t o n g u e s with the o v e r l y i n g : R e d P l a t e a u member. The rocks show phenocrysts of p l a g i o c l a s e , a u g i t e and p i g e o n i t e , with sparse hornblende and d i s e q u i l i b r i u m quartz in a p l a g i o c l a s e - m a g n e t i t e matrix. B r e c c i a s r e f e r r e d to the Wheeler Mountain b r e c c i a aire w e l l 700 600 \£S. b r e c c i a , flow-breccia?" with some p h r e a t i c i n f l u e n c e massive oflow-breccia, l i t t l e p h r e a t i c i n f l u e n c e c l i f f base; mixed flow.and p h r e a t i c b r e c c i a columnar-jointed aphanites ( s i l l s ? ) i n t e r l a y e r e d with p h r e a t i c b r e c c i a nonvesicular p h r e a t i c b r e c c i a columnar-jointed b a s a l t a n d e s i t i c b r e c c i a , p h r e a t i c b r e c c i a p h r e a t i c b r e c c i a , channeled contact f l o w - b r e c c i a , a p h a n i t i c c l a s t s p h r e a t i c b r e c c i a , some sediments vuggy b a s a l t i c - a n d e s i t e flows, thin-bedded F i g . 4. S t r a t i g r a p h i c column of the Wheeler Mountain B r e c c i a at i t s type l o c a l i t y , southwestern s i d e of Wheeler Mountain, north-northeast of T r a n q u i l l e . Top not exposed. developed north-northwest of T r a n q u i l l e ( F i g . 5), conformably o v e r l y i n g Mara H i l l r o c k s . Here a cone of coarse a n d e s i t e flow b r e c c i a (Tdw 3) t h i n s r a p i d l y eastward and changes f a c i e s i n t o a t h i n p a l a g o n i t i c s u c c e s s i o n ; primary d i p s are gr e a t e r than .20°. O v e r l y i n g the cone i s a su c c e s s i o n of p h r e a t i c b a s a l t b r e c c i a s (Tdw 4). The c o r r e l a t i o n s of t h i s s e c t i o n with u n i t s of the type Wheeler Mountain b r e c c i a are u n c l e a r . C a s t l e Butte b r e c c i a (Tdc). The C a s t l e Butte b r e c c i a forms imposing c l i f f s above Dewdrop F l a t s northwest of B a t t l e B l u f f , and r epresents the lower p a r t of the Doherty Creek s t r a t o c o n e . It c o n s i s t s of up to 550 metres of p h r e a t i c and t r a n s i t i o n a l p h r e a t i c - s u b a e r i a l b r e c c i a s with c l a s t s of c o a r s e l y p l a g i o c l a s e -p h y r i c a n d e s i t e (Table 1, 4-2). The b r e c c i a i s e n t i r e l y p a l a g o n i t i c and f a i r l y w e l l bedded i n the eas t , but coarsens and th i c k e n s westward i n t o massive f l o w - b r e c c i a near the i n f e r r e d vent i n the ea s t e r n draw of Doherty Creek. T h i s vent i s a l s o i n d i c a t e d by quaquaversal d i p s i n the b r e c c i a away from the vent. The lower p a r t of the member i n the v i e i n i ty of the vent i s composed of h y a l o c l a s t i t e with minor sandstone, s i m i l a r to the upper T r a n q u i l l e Formation.; The base of the C a s t l e Butte b r e c c i a i s not exposed.. Doherty Creek member (Tdd)• The Doherty; Creek member forms the . upper part of the Doherty Creek composite cone. I t c o n s i s t s of over .600 metres of b a s a l t i c a n d e s i t e and a n d e s i t e flows, moderately to p o o r l y p l a g i o c l a s e - p h y r i c , with l e s s e r q u a n t i t i e s of intermediate to f e l s i c t u f f s , mudflows and b r e c c i a . Minor i n t r u s i v e s a l s o occur i n t h i s u n i t . The Doherty Creek member l i n e s the top of the e r o s i o n a l b a s i n s developed . over the metres p h r e a t i c b r e c c i a , m i c r o v e s i c u l a r aphanite Tdw^ andesite f l o w - b r e c c i a no pa l a g o n i t e forms massive c l i f f s ' p h r e a t i c b r e c c i a , b a s a l t i c p h r e a t i c b r e c c i a , b a s a l t i c andesite c l a s t s p h r e a t i c b r e c c i a , p a l a g o n i t i c ^ f l o w - b r e c c i a TdW p h r e a t i c b r e c c i a h y a l o c l a s t i t e coarse b r e c c i a t u f f , sediments light-weathering .flow b r e c c i a , some p h r e a t i c i n f l u e n c e mudflow flow + flow-top flow, fused contacts >- -b r e c c i a b a s a l t i c andesite flow andesite flow-flow + flow-top b r e c c i a b r e c c i a j • flow + flow-too lam b r e c c i a h e t e r o l i t h o l o g i c mudflow F i g . 5. S t r a t i g r a p h i c columns of the Wheeler Mountain B r e c c i a northwest of T r a n q u i l l e , showing the r a p i d f a c i e s change of Tdw3 from west ( l e f t ) to east ( r i g h t ) , a d i s t a n c e of about 1 km. e r u p t i v e c e n t r e s . I t dominates the western s i d e of the composite cone, i n t e r f i n g e r i n g eastward with the C a s t l e Butte b r e c c i a . I t may be l a t e r a l l y e q u i v a l e n t to the Red Pl a t e a u member of s i m i l a r l i t h o l o g y ; d i s t i n c t i o n i s based l a r g e l y on the primary d i p s of the Doherty Creek member away from the Doherty Creek vents. The maximum t h i c k n e s s of the Doherty Creek member i s about 1500 metres; the lower boundary i s g r a d a t i o n a l . Rosseau Mountain brecc i a (Tdrm). The Rosseau Mountain b r e c c i a o v e r l i e s the Doherty Creek member on the rid g e n o r t h -northwest of Red P o i n t . I t i s composed of over 100 metres of flow b r e c c i a and mudflows with some (hydrothermal?) p a l a g o n i t i c a l t e r a t i o n . C l a s t s are of g l o s s y black b a s a l t (Table 1, 5-6) with phenocrysts of c l i n o p y r o x e n e , l a b r a d o r i t e and o l i v i n e . Crude bedding suggests a vent area to the north; the u n i t may represent a s a t e l l i t i c vent to the main composite cone. Towards Kamloops Lake the lower p a r t of the member becomes l a r g e l y h y a l o c l a s t i c , forming rough badlands east of the mouth of Rosseau Creek. Red P l a t e a u member ( T d r ) . The Red P l a t e a u member l i e s conformably upon the C a s t l e Butte b r e c c i a i n t y p i c a l exposures on the southeast end of Red P l a t e a u ( F i g . 6); i t i n t e r f i n g e r s with u n d e r l y i n g Wheeler Mountain b r e c c i a on Wheeler Mountain. The member i s composed of 350 to 450 metres of a l t e r n a t i n g b a s a l t i c a n d e s i t e (Table 1, TP 32-3) flows and flow-top b r e c c i a with d e r i v e d mudflows. Within a flow and b r e c c i a c o u p l e t , v e s i c u l a r i t y i n c r e a s e s upward from small (1-3 mm) v e s i c l e s to cavernous elongate v e s i c l e s up to 10 cen t i m e t r e s i n l e n g t h ; these are o v e r l a i n by rounded, h i g h l y v e s i c u l a r a n d e s i t e c l a s t s D_ LU 1x1 1200-1100-10004 900-880 700-tan-weathering flow b r e c c i a v e s i c u l a r red-brown flow flow t u f f i n t e r b e d , i n flow +. flow-top b r e c c i a sequence p l a t y flow flow + flow-top b r e c c i a ired-brown flow, v e s i c u l a r , p l a g i o c l a s e + pyroxene phenocrysts flow+ flow-top b r e c c i a ; p l a g i o c l a s e phenocrysts flow, l a r g e p l a g i o c l a s e phenocrysts flow, l a r g e p l a g i o c l a s e phenocrysts aolamnar-jointed aphanite flow mudflow b r e c c i a , a n d e s i t i c brown flow + flow-top b r e c c i a andesite, abundant p l a g i o c l a s e and pyroxene no flow-top b r e c c i a , some p h r e a t i c b r e c c i a andesite, abundant p l a g i o c l a s e and pyroxene phenocrysts F i g . 6. S t r a t i g r a p h i c column of the Red P l a t e a u member i n i t s type l o c a l i t y , southern s i d e of Red P l a t e a u , northwest of T r a n q u i l l e . i n v a r i a b l y a l t e r e d matrix. Tuffaceous sediments s i m i l a r to the lower T r a n q u i l l e are l o c a l l y developed as i n t e r f l o w : u n i t s . In good exposures, up to f i f t e e n flow and b r e c c i a c o u p l e t s can be r e c o g n i z e d . T h i s s t r a t i g r a p h y , along with s u b s t a n t i a l l a t e r a l c o n t i n u i t y and low primary d i p s , suggest f i s s u r e - f e d s h i e l d e d i f i c e s formed by low v i s c o s i t y b a s a l t i c a n d e s i t e magma. Thin s e c t i o n s show phenocrysts of l a b r a d o r i t e , a u g i t e , p i g e o n i t e , o c c a s i o n a l b i o t i t e and "ghost" hornblende, with o c c a s i o n a l cognate x e n o l i t h s i n a p l a g i o c l a s e - r i c h matrix. Opax b r e c c i a (Tdo). The Opax b r e c c i a o v e r l i e s and i n t e r f i n g e r s with the Red P l a t e a u member, as t y p i c a l l y exposed on the west si d e of Opax H i l l at the northern edge of the study a r e a . I t i s composed of over 300 metres of c l i f f - f o r m i n g grey a n d e s i t e (Table 1, TP 39-3) flow b r e c c i a with very l i t t l e m a t rix. C l a s t s are near-aphanites, with very i n f r e q u e n t resorbed p l a g i o c l a s e , amphibole and c l i n o p y r o x e n e i n a g l a s s y matrix showing s w i r l e d flow-banding. I t s uniform s t r a t i g r a p h y suggests that i s may have been fed as a sheet from the b r e c c i a dykes which are observed i n the area, rather than from d i s c r e t e cone s t r u c t u r e s . I n t r u s i v e rocks ( T i ) The Kamloops Group sediments and v o l c a n i c s are i n t r u d e d by a v a r i e t y of dykes, s i l l s and p l u g s . Composite dykes of c o l u m n a r - j o i n t e d a n d e s i t e and slabby red-brown d a c i t e are abundant and conspicuous at K i s s i c k s i d i n g south of Mara H i l l . Dykes of massive or b r e c c i a t e d a n d e s i t e and d a c i t e are present in most other areas, but can r a r e l y be mapped f o r any d i s t a n c e along t h e i r s t r i k e . A s m a l l , a l t e r e d i n t r u s i v e i s observed i n the vent of the Doherty Creek s t r a t o c o n e . V a r i o u s masses of T e r t i a r y igneous rock l a c k i n g d i a g n o s t i c s t r u c t u r e have been l a b e l l e d i n t r u s i v e s on the map, mostly along the Cherry Creek f a u l t zone. They are a n d e s i t e s and b a s a l t i c a n d e s i t e s forming fault-bounded s l i c e s which cannot be i n c o r p o r a t e d i n t o the v o l c a n i c s t r a t i g r a p h y . These are i n t e r p r e t e d to represent shallow i n t r u s i v e and e x t r u s i v e bodies which rose along an a c t i v e f a u l t zone. Another, p o s s i b l y i n t r u s i v e body i s l o c a t e d near D u f f e r i n H i l l , but i t may be an a t y p i c a l l y f r e s h phase of the N i c o l a Group. In the K i s s i c k area, three conspicuous dome-shaped a n d e s i t e (Table 1, 31-5) i n t r u s i o n s have been mapped, with r e l a t e d dykes and s i l l s ( T i 1 ) . These i n t r u s i o n s c o n t a i n complex j o i n t i n g and c o o l i n g p a t t e r n s . The i n t r u s i o n nearest the road i s covered with shallow concave-out d i s h s t r u c t u r e s roughly 3 metres a c r o s s , which form p e r p e n d i c u l a r to c o o l i n g j o i n t s . These are formed by a l t e r n a t i n g c l a y and carbonate a l t e r a t i o n zones i n the plug which weather d i f f e r e n t i a l l y . C o o l i n g e v i d e n t l y proceeded from a small number of f o c i around the i n t r u s i o n , forming fanning columns, and t h e r e f o r e dish-shaped a l t e r a t i o n zones. Immediately west of t h i s i n t r u s i o n i s a set of t h i n a n d e s i t e s i l l s , with a wide v a r i e t y of baked c o n t a c t s and complex columnar j o i n t i n g . One s i l l appears to steepen eastward and merge with the i n t r u s i o n . : At M i s s i o n F l a t s south of the Thompson R i v e r , hypabyssal i n t r u s i v e rocks are exposed i n r a i l r o a d c u t s ( T i 2 ) . These rocks, showing l i t t l e s t r u c t u r e and no b r e c c i a t i o n , are massive b a s a l t i c a n d e s i t e s , s i m i l a r to most of the Kamloops Group v o l c a n i c s . They occur i n the K i s s i c k - M a r a H i l l s e c t i o n , and may be connected with the e r u p t i o n of these or o v e r l y i n g members. B a t t l e B l u f f i n t r u s i v e complex ( T i b ) . Medium- to co a r s e -g r a i n e d diabase i n t r u s i o n s are found both n o r t h and south of Kamloops Lake at T r a n q u i l l e . R e s i s t a n t : s i l l s form cuestas north of B a t t l e B l u f f and southwest of Saltwort Pond. North of the R i v e r , three s i l l s are d i s t i n g u i s h e d , both i n outcrop and i n d r i l l hole RDH-75-1 ( d e s c r i b e d by Carr 1975). The b a s a l s i l l ( T i b 1 ) , 20 metres t h i c k , f o l l o w s the c o n t a c t between the T r a n q u i l l e formation and basement. r o c k s ; i t outcrops d i s c o n t i n u o u s l y behind B a t t l e B l u f f . The l a r g e r middle and upper s i l l s ( T i b 2 and T i b 3 , 40 metres and over 60 metres t h i c k , r e s p e c t i v e l y ) form concordant u n i t s with conspicuous c h i l l e d zones and h o r n f e l s e d .'• sediments at t h e i r lower c o n t a c t s . The s i l l s are composed of s u b o p h i t i c diabase (Table 1, TP 9-1) c o n t a i n i n g abundant l a b r a d o r i t e with e x t e n s i v e magmatic overgrowths of oligocla.se,-'. p i g e o n i t e , a u g i t e ; with some overgrowths, s k e l e t a l magnetite, b i o t i t e and rare o l i v i n e . Mathews (1964) obtained a b i o t i t e K-Ar date of 49 Ma; from the s i l l s (51 Ma using modern decay c o n s t a n t s ; see Chapter I I ) . Small (one to f i v e c entimetre wide) m i c r o s y e n i t e d y k e l e t s are seen i n s e v e r a l p l a c e s toward the top of the s i l l s . In t h i n s e c t i o n they show b i o t i t e and aegerine-rimmed a u g i t e , : set i n t o an o r t h o c l a s e - a l b i t e - c a l c i t e groundmass. South of Kamloops Lake, three s i l l s are again seen, but these are probably not d i r e c t l y c o r r e l a t i v e with those near B a t t l e B l u f f . The b a s a l s i l l ( T i b 1 , about 70 metres t h i c k ) i n t r u d e s the basal c o n t a c t of the T r a n q u i l l e Formation, f i l l i n g v a l l e y s i n the pre-Eocene e r o s i o n s u r f a c e . The s i l l i s very c o a r s e - g r a i n e d , but of s i m i l a r mineralogy to the other s i l l s . The middle s i l l ( T i b 2 ) c o n s i s t s of 70 metres of f i n e - g r a i n e d e q u i g r a n u l a r diabase, while the upper s i l l ( T i b 3 ) c o n s i s t s of 95 metres of medium-grained diabase. The lower and upper s i l l s form conspicuous cuestas from Saltwort Pond south to A f t o n , and were found i n d r i l l holes N-3, N-4, and N-5 (data i n M i l l e r 1972). A s i m i l a r but higher s i l l was found i n d r i l l hole N-4 ( F i g . 6). Sedimentary ^ i n t e r c a l a t i o n s of r e c e s s i v e lower T r a n q u i l l e lakebeds are i n t e r m i t t e n t l y exposed between the s i l l s . A t h i c k mass of c o a r s e - g r a i n e d diabase i s exposed along the Canadian P a c i f i c t r a c k s e a s t of Cherry B l u f f s . The high c l i f f d i s p l a y s conspicuous r e g u l a r banding at about 2-metre i n t e r v a l s , a p p a r e n t l y a c o o l i n g phenomenon. T h i s p l u t o n i c mass may mark the i n t r u s i v e c e n t r e of the s i l l s n orth and south of Kamloops Lake. S i m i l a r rocks are p o o r l y exposed eastwards on the Canadian P a c i f i c tracks;, and southwards towards A f t o n . The B a t t l e B l u f f i n t r u s i v e complex r e p r e s e n t s the i n t r u s i o n of a l a r g e volume of b a s a l t i c magma, probably at c o n s i d e r a b l e depth. I t may represent the s u b v o l c a n i c e q u i v a l e n t of some of the upper members of the Dewdrop F l a t s Formation v o l c a n i c s . Quaternary (Q) G r a v e l s of the midland s u r f a c e (Qg). In the v a l l e y s north of M a r a . H i l l , the lower sl o p e s are mantled with a s u b s t a n t i a l t h i c k n e s s of a l l u v i a l g r a v e l s . These may represent p r e g l a c i a l or i n t e r g l a c i a l a l l u v i a l fan d e p o s i t s graded to the midland e r o s i o n s u r f a c e of F u l t o n (1975); they predate the c u t t i n g of T r a n q u i l l e Canyon. Later e r o s i o n has i n c i s e d these m a t e r i a l s more or l e s s deeply. Other high a l l u v i a l patches are i n c l u d e d i n other areas. T e r r a c e g r a v e l s (Qtg). Along T r a n q u i l l e R i v e r , f l a t - t o p p e d t e r r a c e s u r f a c e s about 60 metres above present r i v e r l e v e l are u n d e r l a i n by t h i c k w e l l - s o r t e d bedded g r a v e l s . These f i l l a b u r i e d canyon of the T r a n q u i l l e R i v e r , s l i g h t l y east of the present canyon south of Wheeler Mountain. G l a c i a l d r i f t (Qd). Kamloops Lake d r i f t ( F u l t o n 1975) forms a d i s c o n t i n u o u s blanket over the mapped area; i t has been mapped only where i t obscures outcrop. I t i s e x t e n s i v e south of the Thompson R i v e r , where i t c o n t a i n s e r r a t i c s of Eagle H i l l conglomerate t r a n s p o r t e d from the Copper Creek area 30 km northwest. North of the R i v e r , d r i f t forms a k e t t l e d ground north of T r a n q u i l l e , low drumlins on Dewdrop F l a t s , and a wide v a r i e t y of eskers, d r u m l i n o i d s and hummocky ground along the east boundary of the study area. P r o g l a c i a l lakebeds ( Q l b ) . P l e i s t o c e n e l a c u s t r i n e d e p o s i t s of g l a c i a l Lake Kamloops form a t h i n veneer on both s i d e s of the Thompson River trough. I t i s d i s c o n t i n u o u s on the north s i d e , but b l a n k e t s the south s i d e , showing well-developed s t r a n d l i n e s i n the four k i l o m e t r e s west of M i s s i o n F l a t s . In t h i s area, lakebeds e f f e c t i v e l y obscure the bedrock geology. P o s t g l a c i a l r a i s e d d e l t a ( Q d l ) . The main body of the d e l t a of the T r a n q u i l l e R i v e r i s r a i s e d three to s i x metres above present lake l e v e l , marking a higher stand of Kamloops Lake i n e a r l y Holocene time. L a n d s l i d e d e b r i s ( Q l s ) . Three l a r g e l a n d s l i d e s have been i d e n t i f i e d on the south s i d e of the Thompson R i v e r . They occur on a d i p slope of middle and upper T r a n q u i l l e Formation v o l c a n i c and sedimentary rocks which have been oversteepened by g l a c i a l e x c a v a t i o n of the Kamloops Lake trough. The two upper l a n d s l i d e s are composed of middle T r a n q u i l l e Formation d e b r i s , which s l i d on u n d e r l y i n g mudstones; the scarp of the eastern s l i d e i s l o c a l i z e d by a small east-down f a u l t . The lower l a n d s l i d e i s developed i n much-fractured t u f f of the middle-upper member, and e x h i b i t s a f r e s h scarp at 530 metres e l e v a t i o n . At l e a s t two s l i d e s have taken p l a c e here; the younger one i s very r e c e n t . P o t e n t i a l f o r more l a n d s l i d e a c t i v i t y on these slopes i s high. West of Red P o i n t , a l a r g e mass of Rosseau Mountain b r e c c i a has s l i d semi-coherently i n t o the Kamloops Lake v a l l e y from the south face of Rosseau Mountain. T h i s mass s l i d over the h y a l o c l a s t i c southwestern f a c i e s of the member and, i n p a r t , the upper T r a n q u i l l e h y a l o c l a s t i t e s , i n response to an oversteepened s l o p e . P o t e n t i a l f o r r o c k f a l l s and small s l i d e s i s s t i l l present i n t h i s a r ea. A l l u v i a l fans (Qaf). Small p o s t g l a c i a l a l l u v i a l fans have been c o n s t r u c t e d along the northern s i d e of the Thompson River v a l l e y . Larger fans extend southwards a c r o s s Dewdrop F l a t s d e r i v i n g m a t e r i a l from the high south face of the Red P l a t e a u . Fan d e l t a s (Qfd). A l l u v i a l f a n - d e l t a s have been b u i l t i n t o Kamloops Lake by Doherty Creek and other streams c u t t i n g down through the rimming b l u f f s . A l l u v i u m ( Q a l ) . The l a r g e s t area of a l l u v i u m i n the study area i s the d e l t a of the Thompson R i v e r east of T r a n q u i l l e , which has grown westward n e a r l y to the T r a n q u i l l e River r a i s e d d e l t a . Other small areas of u n d i f f e r e n t i a t e d a l l u v i u m u n d e r l i e small a l k a l i l akes south of the r i v e r , and the v a l l e y s of T r a n q u i l l e R i v e r and Cherry Creek. not "been s t u d i e d f o r STRUCTURE P r e - T e r t i a r y S t r u c t u r e Basement s t r u c t u r e i s complex and has t h i s r e p o r t . The presence of northwest-southeast l i n e a r f e a t u r e s has been: noted by Carr and Reed (1976); i n t r u s i o n of the Iron Mask and Cherry B l u f f rocks was l o c a l i z e d by such f e a t u r e s , as was d e p o s i t i o n and . p r e s e r v a t i o n of l a t e r Mesozoic sediments. These sediments may, i n f a c t , represent an e a r l i e r c y c l e of basi n development and t e c t o n i c a c t i v i t y s i m i l a r to that which took p l a c e i n the Eocene. S y n d e p o s i t i o n a l S t r u c t u r e Some s t r u c t u r e s show d e f i n i t e a c t i v i t y d u r i n g the d e p o s i t i o n and e r u p t i o n of Eocene rocks. The Cherry Creek F a u l t Zone l o c a l i z e d hypabyssal bodies which may represent magmas r i s i n g upwards along a leaky a c t i v e f a u l t zone. T h i s zone contin u e s northwest under the main body of Kamloops Lake. To i t s west, no Kamloops Group v o l c a n i c s are preserved. T h i s f a u l t zone formed a t e c t o n i c a l l y a c t i v e southwestern boundary of the T r a n q u i l l e d e p o s i t i o n a l b a s i n ; t h i s i n t e r p r e t a t i o n i s supported by the soft-sediment deformation observed west of Red P o i n t . Minor east-west t r e n d i n g t h r u s t f a u l t s and f o l d s i n and near the f a u l t zone suggest r i g h t - l a t e r a l s t r i k e - s l i p movement. R e l a t i o n s h i p s at Afton mine (summarized i n Preto 1972, and. Carr and Reed 1976) i n d i c a t e subsidence of l a r g e m i n e r a l i z e d blocks of T r i a s s i c rock i n t o the T r a n q u i l l e b a s i n o f f of the upthrown s i d e of high-angle reverse f a u l t s (as shown on F i g . 7). A l s o , the T r a n q u i l l e Formation around the mine i s i n a nonvolcanic d e l t a i c f a c i e s , as d e s c r i b e d above, i n d i c a t i n g that the Afton s t r u c t u r a l zone was a c t i v e d u r i n g T r a n q u i l l e time and represented the southern margin of the T r a n q u i l l e d e p o s i t i o n a l b a s i n . T h i s complex zone f o l l o w s the northern boundary of the Iron Mask B a t h o l i t h with a t h i n selvage of N i c o l a Group rocks from Afton east to the D u f f e r i n area. Immediately east of B a t t l e B l u f f , T r a n q u i l l e sediments are i n f e r r e d to be f a u l t e d down about 120 metres a g a i n s t the east margin of the Cherry B l u f f p l u t o n . T h i s f a u l t i n g took place before the i n t r u s i o n of the B a t t l e B l u f f s i l l s , which pass u n i n t e r r u p t e d through the f a u l t plane. There i s no f i r m evidence f o r other f a u l t s i n the area being a c t i v e d u r i n g d e p o s i t i o n of the Kamloops Group. The anomalous t h i c k n e s s of the Kamloops Group in i t s type area r e l a t i v e to other exposures in the r e g i o n , however, suggests subsidence in the T r a n q u i l l e area. The northern and eastern boundaries of the T r a n q u i l l e b a s i n are not known. The present e a s t e r n edge of T e r t i a r y outcrop does not appear to be the o r i g i n a l boundary, as f i n e - g r a i n e d l a c u s t r i n e sediments are exposed at the bounding f a u l t . However, b a s a l t i c a n d e s i t e flows r e s t d i r e c t l y on basement rocks at Strawberry H i l l east of the North Thompson R i v e r , i n d i c a t i n g a b a s i n boundary at or west of the North Thompson. Syn- to P o s t - d e p o s i t i o n a l S t r u c t u r e F i g u r e s 7 and 8 are s e c t i o n s a c r o s s the T r a n q u i l l e area; F i g . 9 i s an i s o m e t r i c diagram of the i n f e r r e d base of the ; F i g . 7. Cross s e c t i o n of the Afton Mine area south of T r a n q u i l l e ; s e c t i o n LMNOP. F i g . 8. C r o s s - s e c t i o n of the T r a n q u i l l e Canyon graben and Doherty Creek Cone; s e c t i o n ABCD and BE. CHERRYCREEK Foul! : DEWDROP F m m ROAD CREEK Fnnll 1TRANOUILLE (—I CANYON \ grobgn F i g . 9. Isometric diagram of the map ar e a . View i s from the no r t h e a s t ; the east and north w a l l s of the graben have been removed f o r c l a r i t y . Datum i s the i n f e r r e d base of the T e r t i a r y ; no v e r t i c a l e xaggeration. Eocene. The area c o n s i s t s of a l a r g e number of f l a t to g e n t l y t i l t e d and f o l d e d b l o c k s and panels, separated by high-angle f a u l t s . The dominant s t r u c t u r e i s a complex graben ( F i g . .8, F i g . 9) with the g r e a t e s t r e l i e f i n the northern p a r t of the area, along T r a n q u i l l e Canyon, g r a d u a l l y d e c r e a s i n g to the south. South of the Thompson R i v e r v a l l e y , which i s l o c a l i z e d along Eocene f a u l t s , panels d i p g e n t l y to moderately n o r t h . To t h e i r west, a l a r g e e a s t - d i p p i n g panel c o n t a i n s an open s y n c l i n e , while a smaller panel immediately north of Afton i s t i l t e d towards the mine area ( F i g . 7). These panels cascade o f f of the r i g i d b l o c k s of the Iron Mask B a t h o l i t h and the Cherry B l u f f s p l u t o n . These T r i a s s i c - J u r a s s i c i n t r u s i o n s b u t t r e s s the southern and southwestern s i d e s of the subsidence s t r u c t u r e . Northwest-southeast f a u l t s , such as the Road Creek - Coal H i l l f a u l t s , show v a r y i n g degrees and senses of displacement. They represent the dominant f a u l t t r e n d of the southern part of the area, ranging i n azimuth from 130° i n the south to 145° i n the n o r t h . These may i n part be r e a c t i v a t e d Mesozoic f e a t u r e s . More n o r t h e r l y trends are observed i n T r a n q u i l l e Canyon and appear as aeromagnetic l i n e a r s to the northwest. From the r e l a t i o n s h i p s observed i n the T r a n q u i l l e area, and from r e g i o n a l reconnaissance, I i n f e r both the T r a n q u i l l e d e p o s i t i o n a l basin and the T r a n q u i l l e Canyon graben to be e x t e n s i o n a l f e a t u r e s formed by the p u l l i n g apart of blocks w i t h i n a dominant r e g i o n a l s t r i k e - s l i p regime. E a r l y s t r i k e -s l i p a c t i v i t y produced the T r a n q u i l l e lake b a s i n with a compressional southern margin. Continued a c t i v i t y a f f e c t e d the l a t e r volcanism, l e a d i n g to the complex graben s t r u c t u r e observed today. T h i s concept i s developed f u r t h e r i n Chapter I I . : SUMMARY OF GEOLOGIC HISTORY The e a r l i e s t s t r a t a exposed i n the map area r e c o r d the e r u p t i o n i n l a t e T r i a s s i c time of the N i c o l a Group v o l c a n i c s and a s s o c i a t e d v o l c a n i c l a s t i c s . Subvolcanic i n t r u s i o n s formed the Iron Mask B a t h o l i t h (Northcote 1977) and the Cherry B l u f f p l u t o n . In l a t e r Mesozoic time, the Cherry Creek f a u l t zone was rep e a t e d l y active., and l o c a l i z e d the p r e s e r v a t i o n of coarse f l u v i a l conglomerates. At about 52 Ma, subsidence of a block bounded on the southwest and south by the Cherry Creek f a u l t zone and the Afton s t r u c t u r a l zone c r e a t e d the T r a n q u i l l e b a s i n ( F i g . 10a). T h i s b a s i n was occupied by an e x t e n s i v e lake, with one or more d e l t a s b u i l t a g a i n s t and l o c a l l y l a p p i n g over the upthrust southern margin. Rapidly i n c r e a s i n g volcanism i n the area s u p p l i e d abundant p l a g i o c l a s e - r i c h : ash and p a l a g o n i t i c d e b r i s . L o c a l slumps of N i c o l a Group v o l c a n i c b l o c k s and T r a n q u i l l e sediments i n t o the basin a t t e s t to s u b s t a n t i a l r e l i e f and t e c t o n i c a c t i v i t y . B a s a l t i c and ande.sitic volcanism produced p i l l o w e d a n d e s i t e flows, h y a l o c l a s t i t e s , and aquagene t u f f s , l e a d i n g i n upper T r a n q u i l l e time to the f i l l i n g of the lake with d e b r i s and the c o n s t r u c t i o n of a l a r g e t u f f cone of o l i v i n e b a s a l t ( F i g . 10b). Atop t h i s f i l l e d lake b a s i n , b a s a l t i c to a n d e s i t i c , l a r g e l y s u b a e r i a l v o l c a n i c s of the Dewdrop F l a t s Formation were erupted. In the lower Dewdrop F l a t s , the K i s s i c k cone was b u i l t up ( F i g . 10c) j u s t above the water t a b l e , l e a d i n g to mixed s u b a e r i a l - p h r e a t i c b r e c c i a s . F l a t - l y i n g b a s a l t i c a n d e s i t e flows and b r e c c i a s b u i l t s h i e l d s t r u c t u r e s and small cones atop and around t h i s cone. L a t e r the Doherty Creek composite cone a) lower Tranquille Fig.. 10. Paleogeography of. the T r a n q u i l l e area d u r i n g d e p o s i t i o n of the Kamloops Group. Cherry Creek F a u l t Zone i s i n lower l e f t of a l l sketches. Question marks i n d i c a t e the l i m i t s of exposure of rocks of that time p e r i o d . grew to a b a s a l diameter of some 10 km; beginning as a t u f f cone, i t developed t o a mature s u b a e r i a l b a s a l t i c a n d e s i t e to a n d e s i t e volcano, with at l e a s t one s a t e l l i t i c b r e c c i a cone and r e l a t e d h y a l o c l a s t i t e s . '. Contemporaneous with and f o l l o w i n g the development of t h i s cone, t h i c k , f l a t - l y i n g b a s a l t i c a n d e s i t e flows were erupted i n the T r a n q u i l l e Canyon graben to the northeast ( F i g . 8d). These were fo l l o w e d by e r u p t i o n of a t h i c k sheet of a n d e s i t e flow b r e c c i a . F a u l t i n g a s s o c i a t e d with the graben subsidence and r e g i o n a l d e x t r a l shear continued a f t e r e r u p t i o n of the v o l c a n i c s to produce the s t r u c t u r e shown on F i g . 9. In l a t e Eocene to Miocene time, a s u r f a c e of low r e l i e f was developed, on which the P l a t e a u l a v a s , now preserved north of the study area, were d e p o s i t e d about 10 Ma ago. T h i s s u r f a c e was d i s s e c t e d to form a l a t e r , p o s s i b l y P l i o c e n e , midland s u r f a c e . G l a c i a l a c t i v i t y d u r i n g the P l e i s t o c e n e excavated the l a r g e t r a n s v e r s e Thompson R i v e r -- Kamloops Lake trough, i n part along p r e e x i s t i n g l i n e s of weakness such as the Cherry Creek f a u l t zone; e x i s t i n g stream drainage was a l s o r e a l i g n e d . A p r o g l a c i a l lake d e p o s i t e d sediments over the lower p a r t s of the area, and p o s t g l a c i a l d e l t a s have been b u i l t i n t o Kamloops Lake. ALTERATION AND MINERAL POTENTIAL No showings of p o t e n t i a l l y economic m i n e r a l i z a t i o n have yet been l o c a t e d |within the T e r t i a r y of the study a r e a . Most rocks are somewhat a l t e r e d by p a r t i a l d e v i t r i f i c a t i o n of g l a s s , v e s i c l e and f r a c t u r e f i l l i n g , and a l t e r a t i o n of some mafic phenocrysts. C l a y s (Fe-Mg smec t i t e s and m o n t m o r i l l o n i t e ) , opal and c a l c i t e are most abundant. I d e n t i f i a b l e z e o l i t e s occur s p o r a d i c a l l y ; euhedral a n a l c i t e c r y s t a l s were found along the Canadian P a c i f i c , right-of-way at 120°30' W., c h a b a z i t e c r y s t a l s at s e v e r a l l o c a t i o n s i n the C a s t l e Butte b r e c c i a , and f e r r i e r i t e (a M g - z e o l i t e ) at i t s type l o c a l i t y at Red P o i n t . M i c r o s c o p i c z e o l i t e s were found l o c a l l y i n the B a t t l e B l u f f i n t r u s i o n s , and in v e i n c o a t i n g s i n the upper T r a n q u i l l e at K i s s i c k . B r i g h t green amydgules and o c c a s i o n a l d i s s e m i n a t i o n s of c e l a d o n i t e (Fe-Mg mica) are conspicuous i n the Red P l a t e a u member along T r a n q u i l l e Canyon. Sparse geode (quartz, c a l c i t e , opal) occurrences were noted throughout the b a s a l t i c a n d e s i t e flow sequences. Disseminated magnetite i s abundant x i n the Kamloops Group v o l c a n i c s , commonly s u b s t a n t i a l l y a l t e r e d to hematite. P y r i t e has been repo r t e d from f r a c t u r e s (Carr 1975). Magnetic and IP g e o p h y s i c a l techniques are thus d i f f i c u l t or impossible to apply i n t h i s a r e a . Graham and Long (1979) have reviewed and d i s m i s s e d the p o t e n t i a l f o r c o a l resources i n the T r a n q u i l l e b a s i n . Most of the T r a n q u i l l e Formation i s very low i n o r g a n i c matter, as i t was d e p o s i t e d i n an ash-choked l a c u s t r i n e b a s i n with r a p i d d e p o s i t i o n . Thin d e p o s i t s south of the A f t o n s t r u c t u r a l zone, represented by f a u l t s l i c e s w i t h i n the Afton d e p o s i t and at the Guerin Creek o u t l i e r , have been prospected i n the past (Dawson 1895) but are not economic. P l a c e r g o l d has been worked i n T r a n q u i l l e Canyon s i n c e the 1850s (see Dawson 1895). F a i r l y coarse g o l d with accompanying platinum i s s t i l l recovered i n subcommercial q u a n t i t i e s by panning i n a l l u v i u m from Watching Creek and the lower T r a n q u i l l e R i v e r . T e r r a c e g r a v e l s along the lower canyon have a l s o been h y d r a u l i c a l l y mined. GEOLOGIC MAP OF THE TERTIARY ROCKS OF THE AFTON-TRANQUILLE AREA WEST OF KAMLOOPS, BRITISH COLUMBIA DESCRIPTION OF MAP UNITS Quaternary: Qal -- a l l u v i u m Qaf -- a l l u v i a l fan d e p o s i t s Qls -- l a n d s l i d e d e b r i s Qdf — d e l t a fan Qdl — p o s t g l a c i a l d e l t a Qlb -- p r o g l a c i a l lakebeds Qd — d r i f t Qtg -- t e r r a c e g r a v e l s Qg -- g r a v e l s of the midland s u r f a c e U n c o n f o r m i t y T e r t i a r y Kamloops Group (Eocene) Dewdrop F l a t s Formation (Td): Tdo — Opax b r e c c i a (andesite aphanite flow b r e c c i a ) Tdr — Red P l a t e a u -member ( b a s a l t i c a n d e s i t e flows, some sediments) Tdrm — Rosseau Mountain b r e c c i a ( g l a s s y b a s a l t flow b r e c c i a ) Tdrm 1 — h y a l o c l a s t i t e f a c i e s Tdd -- Doherty Creek member ( b a s a l t i c a n d e s i t e flows, intermediate to f e l s i c t u f f s ) Tdc -•- C a s t l e Butte b r e c c i a ( p l a g i o c l a s e - p o r p h y r y a n d e s i t e p h r e a t i c and t r a n s i t i o n a l b r e c c i a ) Tdc 1 •-— h y a l o c l a s t i t e s and aquagene t u f f Tdw -- Wheeler Mountain b r e c c i a Tdw1 -- a n d e s i t e flow b r e c c i a , mudflows Tdw2 -- p h r e a t i c b r e c c i a , flow b r e c c i a , a n d e s i t i c ; Tdw3 -- coarse a n d e s i t e flow b r e c c i a Tdw4 -- b a s a l t p h r e a t i c b r e c c i a Tdm -- Mara H i l l member ( b a s a l t i c a n d e s i t e flows) Tdm1 — a n d e s i t e flow b r e c c i a Tdk -- K i s s i c k b r e c c i a ( b a s a l t i c a n d e s i t e flow and p h r e a t i c b r e c c i a ) Tdk 1 -- t u f f a c e o u s , h y a l o c l a s t i c sediment Tdk 2 -- p h r e a t i c b r e c c i a Tdn -- b r e c c i a of The Nipple ( p o r p h y r i t i c b a s a l t p h r e a t i c b r e c c i a ) T r a n q u i l l e Formation (Tt) Ttu — upper member ( h y a l o c l a s t i t e and aquagene t u f f ) . Ttmu — middle-upper member ( t u f f s , l a c u s t r i n e sediment) Ttm — middle members ( p i l l o w e d a n d e s i t e , b r e c c i a ; deformed sediments i n the Red Po i n t area) T t l -- lower members ( l a c u s t r i n e sediments and t u f f s ) T t l 1 -- l a c u s t r i n e mudstones, t u f f a c e o u s s i l t s o n e s , t u f f T t l 2 — c o a r s e - g r a i n e d a n d e s i t e ; s u b a e r i a l b a s a l t flow at Red Point T t l 3 — s i l t s t o n e , mudstone, t u f f Ttb -- border f a c i e s ( l i t h i c and a r k o s i c wacke, dark s h a l e , l o c a l c o a l ) U n c o n f o r m i t y J u r a s s i c or Cretaceous Mzc -- l i t h i f i e d polymict conglomerate, red-brown matrix U n c o n f o r m i t y Late T r i a s s i c Trn -- N i c o l a Group v o l c a n i c s (meta-andesite, t u f f ) a -- a r g i l l i t e s , probably of T r i a s s i c age I n t r u s i v e Rocks T e r t i a r y T i -- i n t r u s i v e s ; dykes and s u b v o l c a n i c masses, a n d e s i t e to d a c i t e T i 1 — a n d e s i t e dome-plugs and r e l a t e d s i l l s T i 2 -- hypabyssal a n d e s i t e T i b — B a t t l e B l u f f i n t r u s i v e complex (diabase) T i b 1 -- b a s a l s i l l s ; c o a r s e - g r a i n e d south of Kamloops Lake T i b 2 -- middle s i l l s T i b 3 -- upper s i l l s Late T r i a s s i c to E a r l y J u r a s s i c T r i e — Cherry B l u f f p l u t o n ( d i o r i t e , s y e n i t e ) T r i i — Iron Mask B a t h o l i t h ( d i o r i t e , s y e n i t e ) SYMBOLS USED ^ c o n t a c t , long dashes where approximate 111»_i*1111\i -- g r a d a t i o n a l c o n t a c t - V ''.— bedding t r a c e w i t h i n a u n i t (f^>\ -- l a n d s l i d e scarp »-^ a - r "•"•*- - f a u l t ; dashed where i n f e r r e d ; d o tted where concealed; d i p , where known -- dyke of dyke complex; d i p where known s<f — a t t i t u d e of bedding, j o i n t i n g A-^* t r e n d of elongate v e s i c l e s ~- a t t i t u d e of a x i a l planes of minor f o l d s ^ -- d r i l l - h o l e l o c a t i o n iZl -- s h a f t l o c a t i o n (3 O O ~" i s o t o p i c date l o c a t i o n ; paleomagnetic s i t e ; v i t r i n i t e sample s i t e ~~ chemical a n a l y s i s l o c a l i t y ] [ -- t h i n s e c t i o n l o c a l i t y / L - - esker L 1 -CHAPTER 11 . REGIONAL STRATIGRAPHY AND STRUCTURAL SETTING OF THE KAMLOOPS GROUP, SOUTH-CENTRAL BRITISH COLUMBIA SUMMARY The Kamloops Group i s a widespread assemblage of lower to middle Eocene v o l c a n i c and sedimentary rocks i n s o u t h - c e n t r a l B r i t i s h Columbia. In the type area west of Kamloops, the b a s a l T r a n q u i l l e Formation c o n s i s t s of 500 metres of l a c u s t r i n e and d e l t a i c sediments, p i l l o w e d flows and h y a l o c l a s t i t e s . Elsewhere, b a s a l c o a l - b e a r i n g nonvolcanic f l u v i a l and l a c u s t r i n e u n i t s occur, such as the Coldwater Formation at M e r r i t t , the Chu Chua Formation at B a r r i e r e , and the Shorts Creek Formation west of Vernon. O v e r l y i n g these formations are dominantly v o l c a n i c u n i t s . At the type area, the Dewdrop F l a t s Formation i n c l u d e s over 1000 metres of i n t e r s t r a t i f i e d b a s a l t i c a n d e s i t e flows, a n d e s i t i c f l o w - b r e c c i a sheets and cones, b a s a l t i c t u f f r i n g s , and an a n d e s i t i c composite cone. Elsewhere, about 600 metres of f l a t - l y i n g b a s a l t i c a n d e s i t e flows i s the norm, with l o c a l accumulations of flow b r e c c i a . The b a s a l sediments accumulated i n separate fault-bounded basins i n i t i a t e d immediately before the onset of volcanism. These v o l c a n i c rocks f i l l e d the basins and formed a widespread v o l c a n i c blanket which was d i s r u p t e d by continued Eocene f a u l t movement. The numerous basins are l i n k e d by a throughgoing f a u l t network up to 12 k i l o m e t r e s of net r i g h t - l a t e r a l s t r i k e - s l i p d i splacement. INTRODUCTION AND PREVIOUS WORK The e x i s t e n c e of e a r l y T e r t i a r y v o l c a n i c and sedimentary rocks i n s o u t h - c e n t r a l B r i t i s h Columbia has been known s i n c e the f i r s t g e o l o g i c a l reconaissance s t u d i e s (Selwyn 1872, Dawson 1879, 1895). Later r e g i o n a l mapping d e f i n e d t h e i r a r e a l extent in the N i c o l a ( C o c k f i e l d 1948), A s h c r o f t ( D u f f e l l and McTaggart 1952), Vernon (Jones 1959) and Bonaparte Lake (Campbell and Tipper 1971) map-areas. A few areas, l a r g e l y those with p o t e n t i a l c o a l r e s e r v e s , have been s t u d i e d i n g r e a t e r d e t a i l (Uglow 1922, Church 1975, 1979). These s t u d i e s , however, d i d not attempt to unravel the r e g i o n a l d i s t r i b u t i o n of l i t h o l o g i e s and p a l e o t e c t o n i c environments in the Kamloops Group. As a r e s u l t , r e g i o n a l i n v e s t i g a t i o n of the s t r u c t u r e s a f f e c t i n g the Eocene rocks and an understanding of Eocene t e c t o n i c s i n southern B r i t i s h Columbia was not p o s s i b l e . T h i s paper i s based on new s t u d i e s of the s t r a t i g r a p h y and s t r u c t u r e of the Kamloops Group at s e v e r a l areas in the Thompson River v a l l e y . T h i s work, combined with p r e v i o u s work, all o w s the c o m p i l a t i o n of a g e o l o g i c map of the Kamloops Group ( F i g . 11), and a comprehensive c o r r e l a t i o n of s t r a t i g r a p h i c terms. T h i s leads to a s y n t h e s i s of the Eocene s t r u c t u r a l e v o l u t i o n of the r e g i o n . A more d e t a i l e d v e r s i o n of the g e o l o g i c map of F i g . 11 i s F i g . 11c, i n pocket. KAMLOOPS GROUP (Tk): BARRIERE HAT CREEK McABEE SAVONA KAMLOOPS | v \ O N T E L A K E - MERRITT sh-SKULL HILL Fm |Finney Lk beds & 1 rhyolite bc-vo* conies of Battle Ck ms-MOUNT & SAVONAFm d-DEWDROP • FLATS Fm '. tk-TUKTAKAMIN A Breccia v-volcanics v - volcanics ml-MONTE LK Fm (bh-Buse Hill beds) cc-CHU CHUA Fm h IMEDICINE CK F m T lHAT CK COAL FmT Ih-Lower Hot Ck beds mo-McAbee beds a -Arlington porphyry s - sediments and tuff t-TRANQUILLE Fm sc-SHORTS CK Fm cw-COLDWATER Fm VERNON (TerroceMtn.) nbJNASWHITO CK Fm IBOULEAU Rhyolite m-MARRONFm • (KitleyLkMember) oc - ATTENBOROUGH CK Fm SHORTS CK Fm i- intrusive A-provisionally included - provisionally excluded • - excluded sc Fig. 11 (a-),. Geologic map of the Thompson Valley area, showing areas of preserved Kamloops Group rocks. CNC = Central Nicola Complex, OC = Okanagan Complex, SC = Shuswap Complex. Circles = reset dates on metamorphic rocks; squares= significant nonreset dates, triangles . = radiometric date loca l i t i e s from Table 3. Known Eocene faults;are shown by solid bold lines, inferred faults by dashed lines. Mapping in part from MacMillan (1978), Church (1975, 1979a,b), Cockfield (1948), Duffell and McTaggart (1951), Jones (1959) and Campbell and Tipper (1971). (b) Chart showing nomenclature and abbreviations of Kamloops Group and related units. NEW STUDIES OF KAMLOOPS GROUP GEOLOGY Kamloops-Tranquille area ( F i g . 12). A d e t a i l e d report on the geology of t h i s area i s given i n Chapter I. The l o c a t i o n of the Eocene u n i t s i n the area, together with t h e i r g e n e r a l c h a r a c t e r i s t i c s , i s summarized on Table 2. The Kamloops Group at i t s type l o c a l i t y i s d i v i d e d i n t o two formations; a lower T r a n q u i l l e Formation (the " T r a n q u i l l e beds" of Dawson 1895, and C o c k f i e l d 1948), and an upper Dewdrop F l a t s Formation. The T r a n q u i l l e Formation, up to 450 metres t h i c k , can be d i v i d e d i n t o lower, middle and upper u n i t s i n each of i t s outcrop areas, but these u n i t s are not s t r i c t l y c o r r e l a t i v e . The lower u n i t l i e s unconformably on T r i a s s i c and lower J u r a s s i c rocks, and c o n s i s t s of l a c u s t r i n e sediments and a n d e s i t i c bedded t u f f s . A t y p i c a l s e c t i o n can be found on the n o r t h - d i p p i n g and e a s t - d i p p i n g cuestas near Afton Mine, southwest of Kamloops (as shown on F i g . 12c, segment M-N). In g e n e r a l , the lower u n i t becomes more t u f f a c e o u s upward; t h e . b a s a l l a c u s t r i n e sediments show l i t t l e t r a c e of v o l c a n i c input ( F i g . 13). O l i s t o s t r o m e s of T r i a s s i c v o l c a n i c basement are; present l o c a l l y . T h i s u n i t i s e x t e n s i v e l y i n t r u d e d , near T r a n q u i l l e , by the diabase s i l l s of an the B a t t l e B l u f f complex. The middle u n i t c o n s i s t s of a n d e s i t i c lava flow, o v e r l a i n by a n d e s i t i c and b a s a l t i c t u f f , t u f f a c e o u s l a c u s t r i n e sediment and l o c a l p h r e a t i c b r e c c i a . North of T r a n q u i l l e , well-developed p i l l o w e d a n d e s i t e flows o v e r l i e lower t u f f a c e o u s l a c u s t r i n e mudstones j u s t above r i v e r l e v e l . The upper u n i t c o n s i s t s of approximately 80 metres of p a l a g o n i t i c b r e c c i a , which forms a h y a l o c l a s t i c mound or apron F i g . 12. Map and s t r u c t u r a l s e c t i o n s , T r a n q u i l l e area. (a) G e n e r a l i z e d g e o l o g i c map. (b) S e c t i o n ABCD ac r o s s T r a n q u i l l e Canyon graben and the Doherty Creek cone. (c) S e c t i o n EFGHJ acro s s homoclinal panels of the T r a n q u i l l e Formation and the Afton zone. For symbols see Table 2. above the p i l l o w e d flows. T h i s u n i t passes westward i n t o a t u f f Table 2. Type L o c a l i t i e s and Descriptions of Newly-defined Units, Kamloops Group. Name Type Locality Lat. N Lon. W Lithologies Thickness Symbol i r a n q u l i i e Formation lower middle-upper border Dewdrop Flats Formation Nipple breccia Kissick breccia Mara H i l l member Wheeler Mountain member Red Plateau member Opax breccia Castle Butte breccia Doherty Creek member Rosseau Mountain breccia Battle Bluff intrusions Mt. Savona Formation Battle Creek volcanics McAbee sediments Monte-Lake Formation Tuktakamin breccia various various various NE of Afton p i t Dewdrop Fla t s , NW of Tranquille W of Tranquille River E of Tranquille W, S sides Mara H i l l SW slope Wheeler Mtn. S side Red Plateau W side Opax H i l l S side Red Plateau Doherty Ck headwaters N of Red Point W of Tranquille Crest of Mt. Savona E of Cache Creek McAbee, E of Cache Creek Monte Lake, NW of Wescvold:' Tuktakamin Mtn., E of Westwold 50 43' 120 31' 50°40" 120°30' 50 46' 50°45' 50°44' 50°44 ' 50°46' 50°46' 50°48' 50°46' 50°47' 50°47' 50°44 ' 50°42' 50°49' 50°48' 50°29' 50°28' 120°35' 120°29' 120°29' 120°29' 120°30' 120°33' 120°32' 120°35' 120°41' 120°41' 120°33' 120°4 9' 121°10' 121°10' 119°50' U9°35' l a c u s t r i n e , f l u v i a l , h y a l o c l a s t i c l a c u s t r i n e , tuffaceous pillowed flows, t u f f s h y a l o c l a s t i t e t u f f f l u v i a l , d e l t a i c , tuff volcanics phb of 01-basalt; t u f f r ing flow breccia and phb cone f l a t - l y i n g andes flow+ftb flow breccia and phb;sheet,cone f l a t - l y i n g andes flow+ftb flow breccia, andes, sheet phb, Pl-andesite; large cone andes flow+ftb, t u f f ; stratocone basalt flow breccia,phb; cone diabase s i l l s , irregular intrusionsj boulder cgl, f g l ; coarse bedding andesite flow+ftb; flow breccia lacustrine, h y a l o c l a s t i t e s andesite flow+ftb, f l a t - l y i n g l a t i t i c breccia cone Tt T t l 0-450 m 0-390 m 0-230 m "1 0-250 m J T t u indet. Ttb M0O0 m 0-100 m 190 m 150-300 m Tkm 250 m Twr 350-450 m >300 m To 0-550 m " Tdc to 600 m >100 m Trm Tib >350 a Tms >400 m Tbc 0-550 m Tma >450 m Tml >400 m Ttk Lithology abbreviations: 01 — o l i v i n e ; phb — phreatic, palagonitic breccia; ftb — PI — plagioclase; cgl — conglomerate; f g l -flow-top breccia; andes — andesite; - fanglomerate. F i g . 13. Lower T r a n q u i l l e l a c u s t r i n e sediments, no r t h of Kamloops Lake, B r i t i s h Columbia. r i n g d e p o s i t of o l i v i n e b a s a l t , known i n f o r m a l l y as " b r e c c i a of The N i p p l e " , which may be over 100 metres t h i c k . A separate f a c i e s of the T r a n q u i l l e Formation i s exposed along the southern margin of the outcrop area. I t c o n s i s t s of coarse a r k o s i c - l i t h i c wackes and interbedded grey-black s h a l e , showing f l a s e r bedding and l o c a l l y t h i c k t r a n s p o r t e d c o a l s . T h i s f a c i e s , exposed i n the north w a l l of the Afton mine open p i t and to i t s east, i s i n f e r r e d to mark one or more small f r i n g i n g d e l t a s which grew northward i n t o the T r a n q u i l l e lake (Graham and Long 1979; F i g . 12b and F i g . 14). I t i s deformed by complex reverse f a u l t i n g and slumping i n the Afton s t r u c t u r a l zone, which forms the southern margin of the Eocene b a s i n . O v e r l y i n g the T r a n q u i l l e Formation i s a v o l c a n i c sequence g r e a t e r than 1000 metres t h i c k , here c a l l e d the Dewdrop F l a t s Formation. In the T r a n q u i l l e Canyon area, s i x members have been mapped, i n c l u d i n g the l o c a l N i p p l e b r e c c i a mentioned above. The lowest widespead u n i t i s the K i s s i c k a n d e s i t e b r e c c i a , c o n s i s t i n g of 160 to 190 metres of v a r i a b l y p a l a g o n i t i c b a s a l t i c a n d e s i t e f l o w - b r e c c i a ; primary d i p s i n d i c a t e at l e a s t , one cone s t r u c t u r e . O v e r l y i n g i s the Mara H i l l member, c o n s i s t i n g of up to 300 metres of red-brown thin-bedded b a s a l t i c a n d e s i t e flows and flow-top b r e c c i a s with l o c a l f l o w - b r e c c i a . Above i s the Wheeler Mountain b r e c c i a , 250 metres of a n d e s i t e to b a s a l t flow-b r e c c i a , p h r e a t i c b r e c c i a and mudflows; the Red P l a t e a u member, up to 450 metres of thin-bedded b a s a l t i c a n d e s i t e flows and flow-top b r e c c i a s ; and the Opax b r e c c i a , over 300 metres of g l a s s y , o f t e n banded a n d e s i t e f l o w - b r e c c i a . T h i s sequence forms an e x c e l l e n t s t r a t i g r a p h y , which helps to d e f i n e the complex F i g . 14. Paleogeography of the T r a n q u i l l e b a s i n , west Kamloops, at four times d u r i n g Kamloops Group d e p o s i t i o n . of f a u l t p a t t e r n of the T r a n q u i l l e Canyon graben ( F i g . 12a, F i g . 15) . North and west of Dewdrop F l a t s , quaquaversal d i p s and f a c i e s changes i n d i c a t e a l a r g e (9 km i n diameter) s t r u c t u r e , named the Doherty Creek composite cone ( F i g . 12b). The rocks of t h i s cone are d i v i d e d i n t o three members of the Dewdrop F l a t s Formation. Lowest i s the C a s t l e Butte b r e c c i a , c o n s i s t i n g of over 550 metres of p a l a g o n i t i c and t r a n s i t i o n a l b r e c c i a s of p l a g i o c l a s e - p h y r i c a n d e s i t e ; at i t s base i s a small exposed t h i c k n e s s of h y a l o c l a s t i c d e b r i s . T h i s member marks the b i r t h of the volcano as a t u f f cone b u i l t from a r e s i d u a l T r a n q u i l l e l a k e . O v e r l y i n g i s a t h i c k s u c c e s s i o n of s u b a e r i a l b a s a l t i c a n d e s i t e flows with b a s a l t i c , a n d e s i t i c and d a c i t i c ash, agglomerate and small i n t r u s i v e bodies, c o l l e c t i v e l y r e f e r r e d to as the Doherty Creek member; i t predominates on the western s i d e of the cone s t r u c t u r e . O v e r l y i n g on the west i s the Rosseau Mountain b r e c c i a , a g l a s s y b a s a l t flow b r e c c i a over 100 metres t h i c k . F a i n t bedding i n t h i s u n i t suggests a vent on the western s i d e of the main volcano. C o r r e l a t i o n between the cone sequence and the canyon sequence i s not c e r t a i n . However, C a s t l e Butte b r e c c i a u n d e r l i e s one s e c t i o n of Red P l a t e a u b a s a l t i c a n d e s i t e , suggesting that the l a t t e r i s e q u i v a l e n t i n p a r t to the l i t h o l o g i c a l l y s i m i l a r Doherty Creek member. Quaquaversal d i p s i n b r e c c i a s and flows observed on Hardie H i l l , northwest of the area mapped i n d e t a i l , suggest that a l a r g e cone s t r u c t u r e e x i s t s there a l s o . Mount Savona area ( F i g . 16 map and s e c t i o n ) . The area mapped at 1:50,000 l i e s immediately to the east of F i g . 15. Isometric diagram of the T r a n q u i l l e area, B r i t i s h Columbia. Datum i s the i n f e r r e d base of the Kamloops Group; t h e ea s t e r n w a l l of the graben has been removed f o r v i s i b i l i t y . 1000m 2000'-500m" sea level B Mt. Sovona C D Savona section F i g . 16. Geologic map and s e c t i o n of the Mount Savona a r e a . A b b r e v i a t i o n s : t = t u f f ; f = f e l s i c i n t r u s i v e ; af = a n d e s i t i c flows; fb = flow b r e c c i a ; f g l = Mount Savona Formation. Bold d o t t e d l i n e s = concealed f a u l t s . the s e c t i o n d e s c r i b e d by Dawson (1895) and Rose (1914). The lowest exposed u n i t i s a t u f f a c e o u s complex of indeterminate t h i c k n e s s , which i n c l u d e s bedded and massive a n d e s i t i c t u f f s , r h y o l i t e ash-flow t u f f and a s s o c i a t e d sediments. N e i t h e r the base nor the top i s exposed; northwest and northeast t r e n d i n g shear zones bound the outcrop area. A u n i t of thin-bedded b a s a l t i c a n d e s i t e flows and flow-top b r e c c i a s , g r e a t e r than 200 metres t h i c k , i s i n f e r r e d to o v e r l i e the t u f f s . I t i s followed by about 150 metres of a n d e s i t e f l o w - b r e c c i a , and an equal t h i c k n e s s of b a s a l t i c a n d e s i t e flows. A l l of these u n i t s are cut by r h y o l i t e i n t r u s i o n s which are f r e q u e n t l y banded and t u f f a c e o u s i n appearance, and bear abundant b i o t i t e , quartz and f e l d s p a r phenocrysts. O v e r l y i n g the above u n i t s on a probable unconformity i s the Mount Savona Formation (Table 2), which forms the upper, conspicuous c l i f f s of Mount Savona. T h i s u n i t c o n s i s t s of w e l l -s t r a t i f i e d and extremely coarse conglomerate to b r e c c i a , with boulders up to three metres a c r o s s . L o c a l f i n e r - g r a i n e d i n t e r b e d s approach a coarse sandstone. C l a s t s are composed e n t i r e l y of T e r t i a r y a n d e s i t e s and d a c i t e s . T h i s u n i t i s t e n t a t i v e l y i n t e r p r e t e d as a fanglomerate c o n s i s t i n g of m a t e r i a l reworked from e a r l i e r Kamloops Group d e p o s i t s . The source may be the block east of Durand Creek, which i s s t r u c t u r a l l y high i n r e l a t i o n to the Savona area. S i m i l a r fanglomerates form high b l u f f s on the west s i d e of Deadman Creek some 30 ki l o m e t r e s to the north-northwest, which are repo r t e d to be o v e r l a i n by a n d e s i t e flows ( C o c k f i e l d 1948; D u f f e l l and McTaggart 1952). Rose (1914) and C o c k f i e l d (1948) i n t e r p r e t e d t h i s d e p o s i t as a v o l c a n i c agglomerate, but i t s e x c e l l e n t although coarse bedding and l a t e r a l c o n t i n u i t y suggest a near-source sedimentary o r i g i n r a t h e r than formation from v o l c a n i c e j e c t a . McAbee area ( F i g . 17 map and s e c t i o n ) . E a r l y T e r t i a r y rocks i n t h i s area can be grouped i n t o two i n f o r m a l formations, with e x t e n s i v e l a t e r a l i n t e r t o n g u i n g . The lower sedimentary u n i t , 550 metres t h i c k , i s c a l l e d the "McAbee sediments", a f t e r H i l l s (1965). I t o v e r l i e s T r i a s s i c and e a r l y J u r a s s i c rocks on an unconformity with up to 200 metres of r e l i e f , and c o n s i s t s of a maximum of 550 metres of aquagene m a t e r i a l , i n c l u d i n g some 30 metres of true l a c u s t r i n e sediments west of B a t t l e Creek. Ash from t h i s u n i t y i e l d e d K-Ar dates of about 51 ma (Table 3; H i l l s and Baadsgaard 1967). The dominant l i t h o l o g y of the McAbee sediments i s a p a l a g o n i t i c mudflow and b r e c c i a complex, probably r e p r e s e n t i n g a h y a l o c l a s t i c apron s i m i l a r to the upper u n i t of the T r a n q u i l l e Formation. East of B a t t l e Creek, t h i s l i t h o l o g y i n t e r f i n g e r s with a n d e s i t e flows. Near the bounding f a u l t on the west, are two c l i f f s of columnar-j o i n t e d a n d e s i t e , t e n t a t i v e l y i n t e r p r e t e d as a l t e r e d ash-flow t u f f s ; they pass eastward i n t o a complex admixture of flow b r e c c i a , mudflows and t u f f s . The upper u n i t i s here named the " B a t t l e Creek v o l c a n i c s " , a f t e r good exposures of a n d e s i t i c flows i n B a t t l e Creek Canyon. East of the Canyon, t h i s u n i t c o n s i s t s of over 400 metres of monotonous b a s a l t i c a n d e s i t e flows and flow-top b r e c c i a s . Westward the a n d e s i t e flows give way to over 500 metres of flow b r e c c i a , which t h i c k e n s and becomes more uniform towards the western bounding f a u l t ( F i g . 17). West of the f a u l t , a n d e s i t e F i g . 17. Geologic map and s e c t i o n of the McAbee area. A b b r e v i a t i o n s : T r J i = Guichon Creek i n t r u s i v e s ; mf = mudflows and/or h y a l o c l a s t i t e s ; af = a n d e s i t i c flows; a f t = ash-flow t u f f ( ? ) ; s = sediments; fb = flow b r e c c i a ; f+fb = flows and b r e c c i a s , mixed. Table 3. Radiometric Dates from the Kamloops Sroup, B r i t i s h Columbia. Ident. Min'l %K Ar« SAr« Age(Ha) I Unit H ) | 51.2*1.4! 44.7*1.8 ! 49.8*1.7 52 ±2 51 ±2 49 ±2 46 ±2 49.5*1.7 47.0*1.6 51 • ±2 '• 52 iZ 51: >2 51 12 49 ±2 49:7*1.7 50.5*1.8 42.5*1.7 48.9*1.7 48.8*1.7 47.2*1.6 42.3*1.9 48.7*1.4 48.3*1.7 50.3+1.8 52.3*1.8 48 ±2 Latitude Longitude Analyst/Collector/Source HAT161 Bio HAT36 WR WRD76-2 WR AK636 Plag AK628 Bio AK629 Plag AK117 Bio 46-4 Bio 9-2 WR AK118 AK640 AK641 AK642 AK656 TP32-3 WR Bio Plag San Bio Bio 33-3 BC5 TH3 TH3a TH15 TH16 WR WR WR WR Bio Bio 74-11 Bio Hbl 22498M Bio TER-14 Bio AK149 Bio 6.87 1.39 90 3.15. 0.554 77 2.27 0.445 85 59 •85 74 0.98 5.13 0.515 78 6.07 1.18 88 4.06 0.751 69 5.79 1.05 1.17 86 84 95 86 83 554 90 5.84 1.30 6.78 6.34 6.53 2.83 2.45 0.490 90 2.69 0.500 2.61 0.502 2.58 0.495 6.90 1.284 6.47 1.076 6.81 1.341 82 0.63 0.123 54 7JS 6.98 6.42 1.21 68 88 84 87 77 1.424 1.439 978 84 82 Hat Creek rhyol1te50°40.5' Finney Lk beds 50 46' Cornwall H i l l s 50°40' McAbee #1 ash 50°48' , McAbee #2 ash 50"48' McAbee tZ ash 50°48' Savona flow 50°43.3' Copper Ck pluton 50 51.6' rhyodacite 50°54.3' Ba t t l e B l u f f s i l l 50°44.0' Tranquille ash 50°44' Tranquille ash 50°44' Tranquille ash 50"44' Tranquille ash 50"44' Red Plateau flow 50 45.8' Monte Lake flow 50°29.6' Brasch Ck flow 50°34.0' T r i n i t y H i l l s 50°31.4' T r i n i t y H i l l s 50°31.4' T r i n i t y H i l l s 50°29.5' T r i n i t y H i l l s 50°27.0' Nicola Vly dome 50°11.8' Nicola Vly dyke 50°12' Whiteman Ck stock 50°13l Bouleau r h y o l i t e 50 5.6' Attenborough Ck ash 50° 8.3' 12r34/5'-121*35-' 121°30' 121^10' 121°10' 121°10' 120°49.2' 120°48.0' 120°42.9' 120°33.2' 120°33' 120°33' 120°33' 120°33' 120°33.4' 119°49,9' 119° 3.0' 118°48.0' 118°48.0' 118°53.0' 118°54.4' 121° 1.7' 121° 2' 119°37' 119°37.5' U9°37.5' Harakal/Church/ 1 Harakal/Church/ 1 Harakal/Danner/unpubl1 shed Baadsgaard/Hills/ 3 Baadsgaard/Hills/ 3 . Baadsgaard/Hills/ 3 Baadsgaard/Mathews/ 5 Harakal/Ewing/ 8 Harakal/Ewing/ 8 Baadsgaard/Mathews/ 5 Baadsgaard/Hills/ 3 Baadsgaard/Hills/ 3 Baadsgaard/Hills/ 3 Baadsgaard/Hills/ 3 Harakal/Ewing/ 8 Harakal/Ewing/ 8 Harakal/Mathews/ 6 Harakal/Ma thews/ 6 Harakal/Mathews/ 6 Harakal/Mathews/ 6 Harakal/Mathews/ 6 Harakal/MacMi11 an/ 4 Harakal/MacMi11 an/ 4 Harakal/Soregaroli/unpublished Harakal/Church/ 2 Baadsgaard/Mathews/ 5 A l l ages are r e c a l c u l a t e d ^ V - O.SSlxlO" 1 0 y" 1; = 4.963xl0- 1 0 y' 1; *°K/K =0.01167 atom % 9 \U in r r / c x 10" 5. B?o = b i o t i t e . Plaq = pfagioclase. San = sanidine, Hbl = hornblende, WR = whole rock. Ar* in cc/g x 10" . Bio = b i o t i t e , Plag = plag Sources: 1 = Church (1975); 2 = Church (1979); 3 = H i l l s and Baadsgaard (1967); 4 = MacMillan (1978); 5 - Mathews (1964); 6 = Mathews ( i n prep.); 7 = Wanless and others (1978). p. 24; 8 = t h i s study. flows appear to o v e r l i e the b a s a l unconformity d i r e c t l y , s uggesting that the flow b r e c c i a s were erupted along and/or ponded a g a i n s t the f a u l t zone. Monte Lake area ( F i g . 18 sect i o n ; F i g . 11 map). In t h i s area northwest of Vernon, three formations can be d e f i n e d . At the base i s l e s s than 30 metres of nonvolcanic sand, s i l t and shale which i s c o r r e l a t i v e with the Shorts Creek Formation mapped by Church (1979a,b) to the south. O v e r l y i n g t h i s i s a 600 metre t h i c k sequence of monotonous f l a t - l y i n g b a s a l t i c a n d e s i t e flows and flow-top b r e c c i a s , which are here c a l l e d the Monte Lake Formation, a f t e r e x c e l l e n t c l i f f and roadcut exposures at and near the settlement of Monte Lake. An e x c e l l e n t r e f e r e n c e exposure occurs at the south end of Monte Lake on Highway 97 ( F i g . 19). O v e r l y i n g the Monte Lake v o l c a n i c s i s a c l i f f - f o r m i n g sequence, over 400 metres t h i c k , of west-d i p p i n g f l o w - b r e c c i a s of l a t i t i c composition, here c a l l e d the Tuktakamin b r e c c i a . I t forms the summit of Tuktakamin Mountain, and i s thought to u n d e r l i e s i m i l a r summits to the northwest. The w e s t e r l y d i p i s observed i n each l o c a l i t y , i n d i c a t i n g a s e r i e s of l a r g e e d i f i c e s along - the f a u l t . zone : immediately to t h e : n o r t h e a s t , as i s shown s c h e m a t i c a l l y on F i g . 18. A n d e s i t i c flows and flow-top b r e c c i a s s i m i l a r to the t y p i c a l Monte Lake Formation are observed north and west from Monte Lake i n t o the South Thompson R i v e r v a l l e y , and . probably extend a c r o s s i t to the outcrops northeast of Kamloops. The small o u t l i e r of T e r t i a r y n o r t h - n o r t h e a s t of Kamloops i s composed of b a s a l t i c a n d e s i t e flows unconformably o v e r l y i n g P a l e o z o i c metasediments. South of the South Thompson R i v e r , the 1000m 500m sea level F i g . 18. S e c t i o n through Tuktakamin Mountain east of Monte Lake. A b b r e v i a t i o n s : s = Shorts Creek sediments; af = a n d e s i t i c flows; fb •=. flow b r e c c i a ; PPta = Thompson Assemblage ( O k u l i t c h 1979). F i g . 19. B a s a l t i c andesite flows and flow-top b r e c c i a s , roadcut near Monte Lake, B r i t i s h Columbia. flows pass i n t o small cone s t r u c t u r e s of flow and mudflow m a t e r i a l , the 'Buse H i l l beds', southeast of Kamloops; t h i s area has not been mapped i n d e t a i l . PROPOSALS FOR STRATIGRAPHIC USAGE The mapping of the i n t e r n a l s t r a t i g r a p h y of the Kamloops Group, as d e s c r i b e d above f o r the T r a n q u i l l e , Savona, McAbee and Westwold areas, together with s i m i l a r mapping at T e r r a c e Mountain (Church 1979a,b) and Hat Creek (Church 1975; Kim 1979) allow a r e d e f i n i t i o n and c l a r i f i c a t i o n of Kamloops Group s t r a t i g r a p h i c nomenclature, shown in F i g . l i b . I t i s proposed here that the Kamloops Group be r e s t r i c t e d to v o l c a n i c and sedimentary rocks of lower to middle Eocene age i n south-c e n t r a l B r i t i s h Columbia that are g e n e r a l l y s i m i l a r to the type area at Kamloops and T r a n q u i l l e . T h i s sequence may c o n t a i n l o c a l i n t e r n a l u n c o n f o r m i t i e s and nonvolcanic sediments. From the ages determined by K-Ar (Table 3) and f o s s i l evidence, these rocks are e q u i v a l e n t to the P r i n c e t o n Group to the south, the Marron and White Lake sequences to the southeast, and the Ootsa Lake v o l c a n i c s to the northwest. The d i v i s i o n of the Kamloops Group i n the type area i n t o the T r a n q u i l l e and Dewdrop F l a t s Formations has been d e s c r i b e d above. The T r a n q u i l l e Formation apparently accumulated in. a l o c a l downdropped b a s i n . T h e r e f o r e , although s i m i l a r b a s a l s u c c e s s i o n s are known elsewhere, they should bear separate formation names. Tuffaceous sediments of s i m i l a r f a c i e s to the type T r a n q u i l l e Formation, but not s t r i c t l y c o r r e l a t i v e , are r e f e r r e d to as ' T r a n q u i l l e f a c i e s ' . The Dewdrop F l a t s Formation, on the other hand, probably formed a semicontinuous blanket over much of the southern I n t e r i o r of B r i t i s h Columbia d u r i n g i t s e r u p t i o n . However, in view of the i s o l a t e d p a t t e r n s of outcrop and d i f f i c u l t i e s i n c o r r e l a t i o n , i t i s best to use l o c a l f o r m a t i o n - l e v e l names i n the major outcrop areas. The u n i t s d e s c r i b e d at Hat Creek by Church (1975) have been r e t a i n e d ( F i g . l i b ) , except that the term "Coldwater beds", used by Church f o r the lower f l u v i a l sediments at Hat Creek, should be r e s t r i c t e d to the N i c o l a V a l l e y sedimentary b a s i n s . The lower f l u v i a l sediments at Hat Creek can ; be i n f o r m a l l y termed the "lower Hat Creek beds". These beds, the o v e r l y i n g Hat Creek Coal Formation and the Medicine Lake Formation are i n t e r r e l a t e d , but t h e i r p o s i t i o n r e l a t i v e to the type Kamloops Group i s u n c e r t a i n . They may represent a T r a n q u i l i e - e q u i v a l e n t and Coldwater-e q u i v a l e n t b a s a l sedimentary s u c c e s s i o n , but they are much t h i c k e r and e n t i r e l y n o n v o l c a n i c . Furthermore, G.E. Rouse c o n s i d e r s the palynomorphs from t h e - s u c c e s s i o n to be l a t e Eocene (unpublished) -- s i g n i f i c a n t l y l a t e r than the Kamloops Group, and i n c o n f l i c t with Church's i n t e r p r e t a t i o n of the sur f a c e geology. The Hat Creek sediments cannot be assig n e d to the Kamloops Group u n t i l t h e i r age and s t r a t i g r a p h i c r e l a t i o n s are b e t t e r known. The Finney Lake beds are l i t h o l o g i c a l l y s i m i l a r to the Kamloops Group and of roughly e q u i v a l e n t age (Table 3); they are p a r t of the Kamloops Group. R h y o l i t e domes and necks (Church's " R h y o l i t e formation") that occur between Hat Creek and Cache Creek ( D u f f e l l and McTaggart 1952; Church 1975) are contemporaneous with t y p i c a l Kamloops Group volcanism (see Table 3); they are i n c l u d e d here i n the Kamloops Group. The term "Coldwater Formation" r e f e r s to the nonvolcanic sedimentary s t r a t a p o o r l y exposed i n the M e r r i t t , lower Guichon Creek and Quilchena b a s i n s ( C o c k f i e l d 1948, MacMillan 1979). I t s type area i s at M e r r i t t , at the j u n c t i o n of the Coldwater and N i c o l a R i v e r s ; a r e f e r e n c e s e c t i o n at Coal G u l l y southwest of the town was d e s c r i b e d by Dawson (1895). H i l l s (1965) c o r r e l a t e d these sediments with the T r a n q u i l l e and McAbee sediments on p a l y n o l o g i c grounds, but no K-Ar ages were o b t a i n e d . Dawson (1895) and C o c k f i e l d (1948) i n f e r r e d an unconformity between f o l d e d Coldwater beds and nearby f l a t - l y i n g T e r t i a r y v o l c a n i c s , although they are nowhere in c o n t a c t . Given the complex s y n d e p o s i t i o n a l deformation noted at T r a n q u i l l e and elsewhere, t h i s evidence i s i n s u f f i c i e n t . The Coldwater sediments are t h e r e f o r e i n c l u d e d i n the Kamloops Group (as C o c k f i e l d (1948) •placed them); they are i n f e r r e d to be a nonvolcanic f a c i e s which i s e q u i v a l e n t to the T r a n q u i l l e Formation i n the type area. The Chu Chua and S k u l l H i l l Formations of Uglow (1922) and Campbell and Tipper (1971) are found i n the North Thompson V a l l e y near B a r r i e r e . The Chu Chua i s a c o a l - b e a r i n g sequence l i t h o l o g i c a l l y s i m i l a r to the Coldwater Formation of the M e r r i t t area and the d e l t a i c f a c i e s of the T r a n q u i l l e Formation; f o s s i l s v i n d i c a t e t h e i r rough age e q u i v a l e n c e . The term " S k u l l H i l l " should be r e s t r i c t e d to v o l c a n i c rocks i n the North Thompson v a l l e y of s i m i l a r l i t h o l o g y to the type area southwest of B a r r i e r e . In the Monte Lake area, f o r m a t i o n - l e v e l u n i t s have been introduced above. To the south, Church (1979a) has given i n f o r m a l names i n the Bouleau - Shorts Creek a r e a . The b a s a l Shorts Creek formation and o v e r l y i n g Attenborough Creek formation are t y p i c a l Kamloops Group l i t h o l o g i e s , as are the Bpuleau r h y o l i t e and Naswhito Creek formation. These two sequences are separated by a tongue of K i t l e y Lake t r a c h y t e , a member of the Marron Formation. I propose that the Shorts Creek, Attenborough Creek, Bouleau r h y o l i t e and Naswhito Creek u n i t s be i n c l u d e d i n the Kamloops Group. D e f i n i t e l y a l k a l i n e rocks which are c o r r e l a t i v e with the Marron Formation to the south should be excluded from the Kamloops Group. See Chapter III f o r Kamloops Group geochemistry. As d e f i n e d here, the Kamloops Group rocks form a d i s t i n c t i v e l i t h o l o g i c package extending north from Shorts Creek (Vernon) to the Thompson River and west to Cache Creek. Rocks of s i m i l a r appearance are found i n i s o l a t e d , i n p a r t fault-bounded outcrops east of the L o u i s Creek f a u l t zone, at S q u i l a x , Mount Ida, Enderby and the T r i n i t y H i l l s (Daly 1915; Jones 1959; O k u l i t c h 1979), which have not been studied•• f o r t h i s r e p o r t . Mathews ( i n prep.).; r e p o r t s middle Eocene ages from the T r i n i t y H i l l s and Enderby C l i f f s sequences east of Enderby, as summarized i n Table 3. They i n d i c a t e a g e n e r a l l y s i m i l a r age and a p o s s i b l e equivalence to the Kamloops Group. TECTONIC SETTING During e a r l y to middle Eocene time, small fault-bounded ; basins r e c e i v e d t h i c k accumulations of sediment. In some areas, . volcanism: was simultaneously a c t i v e , producing bedded t u f f s , p i l l o w e d flows and h y a l o c l a s t i t e s (at T r a n q u i l l e , McAbee, and Savona, a l s o east of Deadman Creek, and a concealed basin southwest of Savona (W.J. MacMillan, p e r s . comm., 1979)). In other areas (Chu Chua, Hat C r e e k ( ? ) , Coldwater) volcanism d i d not c o n t r i b u t e to sedimentation; these areas c o n t a i n c o a l and l i g n i t e of economic i n t e r e s t . The a c t i v e fault-bounded r e l a t i o n s h i p i s demonstrated at T r a n q u i l l e , Savona, McAbee and Hat Creek; at T r a n q u i l l e and Hat Creek (Kim 1979), deformation continued, d u r i n g and a f t e r sedimentation. Boundary r e l a t i o n s of the other basins are l e s s c l e a r , but appear to be f a u l t e d . At a s l i g h t l y l a t e r time, the T r a n q u i l l e Canyon graben r e c e i v e d a v o l c a n i c f i l l of over 1 km, s i g n i f i c a n t l y t h i c k e r than known elsewhere. T h i s suggests that the graben developed d u r i n g v o l c a n i c e r u p t i o n , and ponded l a r g e amounts of b a s a l t i c and a n d e s i t i c l a v a and b r e c c i a . Subsidence continued a f t e r e r u p t i o n of a l l of the exposed u n i t s . The sedimentary and v o l c a n i c basins are not simple grabens. At T r a n q u i l l e , the southern b a s i n margin i s a jumble of f a u l t s and slumped b l o c k s , which are the s u r f a c e m a n i f e s t a t i o n s of high-angle reverse f a u l t s (Carr and Reed 1976, a l s o F i g . 12c). The b a s i n ' s southwestern margin i s a broad, b r a i d e d , i n t r u s i v e -l i n e d f a u l t zone which i s one of the throughgoing f a u l t s of the area. At Hat Creek, where d r i l l i n g has d e f i n e d s t r u c t u r e s i n the Hat Creek Coal Formation, the sediments are moderately to t i g h t l y f o l d e d , cut by d i p - s l i p f a u l t s with a l t e r n a t i n g senses of displacement and by s t r i k e - s l i p f a u l t s (Kim 1979). Th e r e f o r e , although these basins are subsidence s t r u c t u r e s , they are u n l i k e l y to have been formed by simple e x t e n s i o n . Rather, they show some s i m i l a r i t y to the sedimentary basins of southern C a l i f o r n i a (Crowell 1974). The small Eocene basins are t i e d together by l a r g e throughgoing f a u l t s which form prominent topographic furrows i n the Thompson V a l l e y r e g i o n . Where they have been s t u d i e d , these f a u l t zones are one to three k i l o m e t r e s wide, composed of numerous interweaving s t r a n d s , and marked by complex shearing and l o c a l a s s o c i a t e d t h r u s t s . A t y p i c a l example i s the Cherry Creek f a u l t zone west of Kamloops. S e v e r a l prominent v a l l e y segments, covered with Quaternary m a t e r i a l , appear to be l o c a l i z e d by these f a u l t s ( p a r t s of North Thompson v a l l e y , the v a l l e y s between B a r r i e r e and Vernon, the c e n t r a l p a r t of Kamloops Lake, Deadman Creek, and Durand Creek east of Mount Savona). V o l c a n i c cone s t r u c t u r e s are a l i g n e d along these f a u l t zones (the Tuktakamin b r e c c i a , the Buse H i l l cones, and the McAbee f l o w - b r e c c i a s ) and some c o n t a i n T e r t i a r y i n t r u s i v e s and sediments not c o r r e l a t i v e with m a t e r i a l o u t s i d e the f a u l t zone (Cherry Creek f a u l t zone, and f a u l t s south of McAbee). Apparent d i p - s l i p movement on these zones i s l o c a l l y over 500 metres, but can change markedly or reverse i t s sense along i t s le n g t h (Cherry Creek f a u l t , p o s s i b l y the McAbee f a u l t ) . F a u l t trends are dominantly north-south and northwest-southeast. Dominant s t r i k e - s l i p motion on these major f a u l t s best accounts f o r the f e a t u r e s o u t l i n e d above. D i p - s l i p motion, although l o c a l l y important, i s probably a secondary f e a t u r e . The sma l l sedimentary and v o l c a n i c depressions occupy niches w i t h i n the throughgoing system of f a u l t s , e i t h e r i n t r i a n g u l a r areas between s p l a y i n g f a u l t s ( T r a n q u i l l e , Deadman Creek, and southwest of Savona), along the si d e of a major f a u l t bend (McAbee), or i n an area l y i n g between o f f s e t f a u l t s as a p u l l -a p a r t basin ( T r a n q u i l l e Canyon, Chu Chua?, M e r r i t t , and Hat Creek?). These l o c a t i o n s are c o n s i s t e n t with those expected from s p l a y i n g networks of s t r i k e - s l i p f a u l t s (Lensen 1958; Wilcox and others 1973) •. The a l t e r n a t i n g t e n s i o n and compression noted at Hat Creek and T r a n q u i l l e are c o n s i s t e n t with a dominantly t r a n s c u r r e n t regime (Crowell 1974). A r i g h t - l a t e r a l sense of motion i s i n d i c a t e d by the d i r e c t i o n of graben e l o n g a t i o n (about north-south) and the northward-directed reverse f a u l t i n g at A f t o n . Such motion i s c o n s i s t e n t with r e g i o n a l r i g h t - l a t e r a l shear along the F r a s e r and other f a u l t zones i n the Canadian C o r d i l l e r a d u r i n g l a t e s t Mesozoic and e a r l y Cenozoic time (Monger and P r i c e 1979; see Chapter IV, below). Another t e c t o n i c c u r i o s i t y e x i s t s i n the Thompson River r e g i o n . Eocene-reset K-Ar metamorphic dates on Mesozoic igneous and metamorphic rocks have been obtained i n two areas ( r e s e t dates are shown by a c i r c l e d R on F i g . 11). Reset dates are found to the south and east of the map-area in rocks of the Shuswap and Okanagan Complexes (Mathews 1976; O k u l i t c h 1979). Young dates have been r e p o r t e d from the C e n t r a l N i c o l a Complex (the C e n t r a l N i c o l a B a t h o l i t h of C o c k f i e l d 1948) between Kamloops and M e r r i t t (Preto and others 1979); p r e l i m i n a r y Rb-Sr dete r m i n a t i o n s suggest a Mesozoic age f o r the p r o t o l i t h (R.L. Armstrong, pe r s . comm., 1980). These r e s e t t e r r a n e s have a r e g i o n a l d i s t r i b u t i o n i n broadly l i n e a r n o r t h - n o r t h e a s t t r e n d i n g b e l t s through southern B r i t i s h Columbia and the northwestern U n i t e d S t a t e s ; they are i n f e r r e d (Chapter IV) to represent metamorphic core complexes (Coney 1979) which re c o r d roughly east-west c r u s t a l e x t e n s i o n . The C e n t r a l N i c o l a Complex ( F i g . 11) extends roughly n o r t h -northeast from N i c o l a Lake to a p o i n t some 20 km south-southwest of Kamloops. I t f i t s i n t o the system of throughgoing f a u l t s d e s c r i b e d above, being n e s t l e d between the Cherry Creek F a u l t extension on the north and a strong northwest-southeast l i n e a r d e p r e s s i o n on the south. Thus, three l a r g e e x t e n s i o n a l or i n f e r r e d e x t e n s i o n a l f e a t u r e s of Eocene age -- the T r a n q u i l l e Canyon graben, the C e n t r a l N i c o l a Complex, and the M e r r i t t basin — form a broad arc from northwest to southeast i n s o u t h - c e n t r a l B r i t i s h Columbia. The three main c l a s s e s of t e c t o n i c f e a t u r e s d e s c r i b e d above form a c o n s i s t e n t p a t t e r n , which can be sketched as i n F i g . 20. The e n t i r e region i s d i v i d e d i n t o a number of b l o c k s , separated by e i t h e r o b l i q u e - s l i p f a u l t s or e x t e n s i o n a l s t r u c t u r e s . I f we assume that the s t r i k e - s l i p f a u l t s represent c o n s e r v a t i v e block margins, a c r o s s which no net c r u s t a l s h o r t e n i n g or e x t e n s i o n took p l a c e , i t i s p o s s i b l e to c o n s t r u c t a displacement v e c t o r diagram such as F i g . 21 by t r e a t i n g each f a u l t boundary as an azimuth of h o r i z o n t a l displacement. The r e s u l t i n g diagram g r a p h i c a l l y shows the e f f e c t of Eocene-active tectonism on the Thompson R i v e r region — a net northwestward t r a n s l a t i o n of the western end r e l a t i v e to the e a s t e r n end. The amount of Eocene I I F i g . 20. I d e a l i z e d sketch of major Eocene; t e c t o n i c f e a t u r e s i n . s o u t h - c e n t r a l B r i t i s h Columbia. Throughgoing f a u l t s , e x t e n s i o n a l grabens, and core complexes d i v i d e the area i n t o many b l o c k s . A b b r e v i a t i o n s : HH = Hardie H i l l ; GM = Greenstone Mountain; AH = Arrowstone H i l l s ; FM = Forge Mountain; CR - C l e a r Range; MN = M e r r i t t North; IM = Iron Mountain. : F i g . 21. Displacement v e c t o r diagram f o r s o u t h - c e n t r a l B r i t i s h Columbia M e s o z o i c - T e r t i a r y s t r i k e - s l i p f a u l t i n g . : See te x t f o r assumptions. GCB — Guichon Creek B a t h o l i t h ; OKW --Okanagan West; MI — Mount Ida; see F i g u r e 20 f o r other a b b r e v i a t i o n s . Numbers are on a r e l a t i v e s c a l e . displacement i s unknown, as no l a t e r a l l y d i s p l a c e d f e a t u r e s have yet been recog n i z e d . I f , however,' we assume a reasonable displacement of about one ki l o m e t r e a c r o s s the T r a n q u i l l e Canyon s t r u c t u r e s , t o t a l e l o n g a t i o n i s on the order of 12 km northwest-southeast. T h i s s t y l e and d i r e c t i o n of e l o n g a t i o n i s c o n s i s t e n t with P r i c e ' s (1979) model f o r p e r v a s i v e shear between d i v e r g i n g s t r i k e - s l i p f a u l t s . SUMMARY OF GEOLOGIC HISTORY OF THE KAMLOOPS GROUP At about 53 Ma, s t r i k e - s l i p movement was i n i t i a t e d along a network of s p l a y i n g and i n t e r c o n n e c t e d f a u l t s , some of which may have had a p r i o r Mesozoic h i s t o r y of movement. As a consequence of l o c a l c r u s t a l e x t e n s i o n , some blocks subsided and r e c e i v e d up to 500 metres of f l u v i a l , d e l t a i c and l a c u s t r i n e sediments. Volcanism began c o n c u r r e n t l y with subsidence i n the T r a n q u i l l e and McAbee areas, choking the sedimentary basins with ash, p i l l o w e d flows and h y a l o c l a s t i t e s ( F i g . 14).. Other basins to the northeast and southwest were e i t h e r s l i g h t l y o l d e r or more d i s t a n t from the e a r l y c e n t r e s of volcanism, and r e c e i v e d nonvolcanic sediment i n c l u d i n g l o c a l l y s i g n i f i c a n t c o a l zones. Increased v o l c a n i c a c t i v i t y f i l l e d the bas i n s and formed a blanket over much of the r e g i o n . Low b a s a l t i c a n d e s i t e s h i e l d e d i f i c e s were most abundant, but l a r g e a n d e s i t i c volcanoes formed l o c a l l y , with t h e i r c e n t r e s o f t e n a l i g n e d by the throughgoing f a u l t s . More than 1400 metres of Dewdrop F l a t s Formation v o l a n i c rocks accumulated i n the s u b s i d i n g T r a n q u i l l e Canyon graben. Coarse fanglomerates developed along some of the f a u l t zones. Simultaneously with f a u l t i n g , sedimentation and volcanism, metamorphic core complexes rose both l o c a l l y and r e g i o n a l l y as a deep-seated response to c r u s t a l e xtension i n the s t r i k e - s l i p regime (Coney 1979). The l a t e s t age of f a u l t movement i s not known. Most s t r i k e -s l i p movement on the major f a u l t s to the south appears to have ended by 42 Ma (Monger and P r i c e 1979). The e n t i r e r e gion was l a t e r eroded to a low r e l i e f s u r f a c e before the e r u p t i o n of the l a t e Miocene to P l i o c e n e P l a t e a u b a s a l t s . CHAPTER I I I . PETROLOGY AND GEOCHEMISTRY OF THE KAMLOOPS GROUP VOLCANICS, BRITISH COLUMBIA SUMMARY The Kamloops Group i s a h i g h - a l k a l i c a l c - a l k a l i n e v o l c a n i c s u i t e of lower to middle Eocene age, widespread i n s o u t h - c e n t r a l B r i t i s h Columbia. Members of the s u i t e range from high-K b a s a l t through a n d e s i t e to r h y o l i t e . The s u i t e i s c h a r a c t e r i z e d by high K20, Sr and Ba, but low Zr, T i and Ni c o n c e n t r a t i o n s , only moderate Ce enrichment, and l i t t l e or no Fe enrichment. S i m i l a r s u i t e s i n c l u d e the A b s a r o k a - G a l l a t i n s u i t e i n Wyoming and the lower San Juan (Summer Coon) s u i t e i n Colorado. K20 content at 60% s i l i c a i n c r e a s e s r e g u l a r l y eastward a c r o s s southern B r i t i s h Columbia. The chemical data support the subduct i o n - r e l a t e d c o n t i n e n t a l arc o r i g i n of the Kamloops Group v o l c a n i c s . 8 7 S r / 8 6 S r i n i t i a l r a t i o s are about 0.7040 i n the western h a l f , and about 0.7060 i n the ea s t e r n h a l f of the study area. No d i f f e r e n c e i n chemistry or mineralogy marks t h i s sharp t r a n s i t i o n . The high e r , e a s t e r n r a t i o s are due to i s o t o p i c exchange with or a s s i m i l a t i o n of ol d e r c r u s t . The v o l c a n i c rocks c o n s i s t i n the main of a u g i t e - p i g e o n i t e a n d e s i t e s ranging from 52% to 62% s i l i c a , with subordinate q u a n t i t i e s of o l i v i n e - a u g i t e - p i g e o n i t e b a s a l t and b i o t i t e r h y o d a c i t e and r h y o l i t e . The a n d e s i t e s and b a s a l t s were d e r i v e d by a combination of low-pressure 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 , h i g h e r - p r e s s u r e 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 , " and v a r i a b l e p a r e n t a l magmas, while low-pressure 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 p l a g i o c l a s e , b i o t i t e and a p a t i t e accounts f o r the genesis of the r h y o l i t e s . The p a r e n t a l magmas were high-K b a s a l t s and b a s a l t i c a n d e s i t e s with high K, Sr and Ba. The immediate source of these magmas i s i n f e r r e d to have been an a l k a l i - e n r i c h e d hydrous p e r i d o t i t e with n e i t h e r p l a g i o c l a s e nor garnet present in the residuum. INTRODUCTION The Kamloops Group i s a widespread, one- to t h r e e - k i l o m e t r e t h i c k sequence of lower to middle Eocene v o l c a n i c and sedimentary rocks i n s o u t h - c e n t r a l B r i t i s h Columbia. The Kamloops v o l c a n i c s form p a r t of a b e l t of Eocene v o l c a n i c rocks extending from the C h a l l i s and Absaroka v o l c a n i c s of Wyoming and Idaho northwestward to the B r i t i s h Columbia-Yukon border. Although the e x i s t e n c e of the Kamloops v o l c a n i c s has been recognized s i n c e the work of Dawson (1895), t h e i r chemistry and mineralogy have not p r e v i o u s l y been s t u d i e d over a l a r g e r e g i o n . T h i s paper presents the r e s u l t s of a study of the p e t r o l o g y and geochemistry of the Kamloops Group v o l c a n i c s , based on an ext e n s i v e c o l l e c t i o n at t h e i r type area, r e g i o n a l sampling and c o l l e c t i o n of data i n the l i t e r a t u r e . The r e g i o n a l s t r a t i g r a p h y and geochronology of the Kamloops Group was the subject of the prev i o u s chapter. DATA COLLECTION AND ANALYSIS Samples were c o l l e c t e d d u r i n g two seasons of mapping and reconnaissance of the Kamloops Group and c o r r e l a t i v e rocks i n southern B r i t i s h Columbia. Based on t h i n s e c t i o n s t u d i e s , a set of 73 samples was s e l e c t e d f o r major-element chemical a n a l y s i s by X-ray f l u o r e s c e n c e spectrometry, using the g l a s s d i s c technique of N o r r i s h and Hutton (1969), as adapted f o r use at the U n i v e r s i t y of B r i t i s h Columbia. Sodium was analysed on pressed powder p e l l e t s . From t h i s set of an a l y s e s , 32 samples were s e l e c t e d f o r trace-element a n a l y s i s by X-ray f l u o r e s c e n c e using pressed powder p e l l e t s , as w e l l as determination of f e r r o u s i r o n and C02 us i n g wet chemical and i g n i t i o n techniques (the above techniques and o p e r a t i n g c o n d i t i o n s are d e t a i l e d i n Berman 1979). D u p l i c a t e samples have been run at each stage of a n a l y s i s ; the r e s u l t s are g e n e r a l l y i n c l o s e agreement. Major and t r a c e element a n a l y s e s f o r s e l e c t e d samples i s presented i n Table 4, along with a n a l y t i c a l e r r o r s . Ten samples have a l s o been analyzed f o r st r o n t i u m i s o t o p e s (Table 5). Sample l o c a l i t i e s are shown on F i g . 22. Samples were : s e l e c t e d on the b a s i s of fr e s h n e s s , g e o l o g i c i n t e r e s t , and s t r a t i g r a p h i c coverage. A l t e r a t i o n v a r i e s from s l i g h t to severe, being at a maximum i n the p a l a g o n i t i c b r e c c i a s of the lower u n i t s i n the; type a r e a ; Some saponite and/or c e l a d o n i t e a l t e r a t i o n of the groundmass, and a l t e r a t i o n of o l i v i n e ( i f present) are widespread. Table 4. Chemistry of Kamloops Group and Related Volcanic Rocks. Sample Unit TP43-1 Tdn TP 4-1 4-2 Tdm Tdc TP33-1 5-6 Tdc Tdrm 5-7 Tdrm 8-3 Tdo Tdo TP 9-3 10-4 1-1A TP22-1 Tib Tib Ti Ti 49.71 55.94 53.21 64.41 0.98 0.64 1.08 0.66 15.24 16.56 15.70 14.29 4.13 2.42 3.78 3.42 4.83 2.09 3.25 1.39 0.18 0.11 0.11 0.07 7.42 1.21 4.65 2.27 8.43 5.73 7.40 3.70 3.12 4.62 3.40 2.98 2.19 5.11 1.84 3.55 0.42 0.22 0.51 0.29 3.04 0.93 1.60 0.79 0.96 0.20 1.84 1.53 0.32 2.45 0.28 0.70 SiO, : TiO, Al 6 HnO : MgO CaO • Na-0 K,6 P2°5 : h;o-c6 2 Total Rb Sr Ba Ce Nd • Y Zr Cr -V Ni K/Rb Rb/Sr Lat. Lon. 1' 50.26 52.79 -53.44: 52.23 0.98 1.24 1.24 1.27 14.55 16.05 19.69 18.20 3.05 6.99 3.34' 5.88 1.27 3.21 8;41 0.15 0.07 •0.11. 0.12 -. 10.06 4.28 1.85 2.79 7.40 7.48 6.73 6.74 2.31 3.79 3.23 3.32 2.75 2.98 2.70 2.66 0.56 0.80 0.62 0.62 2.14 0.71' ': 1.54 2.18 0.51 1.10 1.14' 1.66 0.23 0.23 1.21 101.48 99.92 99.41 100.21 101. 59. 46. : 806. 1704. 1352. 1311. 2480. 2700. 42. 222. 74. 27. 77. 34. 21. 25. 23. 109. 234. 166. 300. 174. 14. 212. 183. 188. 215. 85. • 4.9 226. 419. .'125 .035 1487. .034 50°45' 50°44'-; 5 0 > ' 5 0 > ' 120°31' 120°28' 120°37' 120°35' 50.85 1.09 16.19 56.07 1.02 15.59 2.59 3.79 0.11 4.54 6.12 2.21 2.20 0.47 4.65 0.65 0.00 100.26 100.43 9.29 0.17 5.98 8.18 2.79 2.52 0.49 2.02 0.62 60.03 57.46 0.88 0.96 15.61 15.66 1.18 , „ 4.78 0.11 0.13 3.19 3.67 5.41 5.70 3.54 3.36 2.57 3.05 0.42 0.53 1.54 0.79 0.34 0.34 0.00 100.13 99.42 101.48 98.45 98.65 100.21 54, 945. 1250. 84. 30. 17. 200. 123. 152. 47. 83. 604. 1170. 75. 29. 20. 230. 81. 133. 35. 338. 257. .057 .137 70. 100. 50. 87. 1060. 557. 1050. 450. 1290. 1470. 1400. 1190. 22. 41. 70. 43. 13. 21. 30. 19. 19. 27. 17. 16. 89. 211. 167. 202. 175. 10. 133. 55. 204. 62. 182. 109. 36. 7.8 42.; 18. 260. 386. 305, 339. .066 .197 50°47' 50°47' 50°49' 50°48' 50°44 • 50°44' 120°41' 120°40' 120°32' 120°32' 120°34' 120°33' .048 .193 50°43' 50°43' 120°29' 120°28' Analyses by X-ray fluorescence, uui Y C ! a iijr ui o n n s n LOlumoia, iy/9-1980. Table .4 .(continued) 16-1 f i 70.01 0.30 14.04 2.91¬0.03 0.86 2.12 2.17 4.34 0.13 2.30 1.79 16-5 a f t 17-2 fb 27-5 Tml 27-9 Ttuk 40-7 dyke 36-2 dac 9-2 rd 77.74 0.20 10.89 0.47 1.92 0.05 0.36 0.97 2.14 4.67 0.09 0.46 0.18 0.16 .57.92 0.97 15.48 : 3.11 3.59 0.12 3.69 6.08 2.53 2.66 0.40 : 3.24 0.54 0.37 52.21 1.02 13.84 4.80 3.47 0.12 6.47 8.42 1.94 2.67 0.45 1.92 1.24 0.30 57.92 1.11 17.33 3.04 2.03 0.09 1.79 4.47 2.28 6.62 0.49 1.17 1.09 0.44 47.22 1.26 16.23 3.30 4.97 0.15 6.74 8.00 2.43 3.38 1.16 1.77 0.37 2.31 62.94 0.48 17.88 5.24 0.14 0.36 4.40 4.70 2.12 0.33 1.27 0.42 100.98 100.51 101.10 99.26 100.12 99.81 100.28 161. 159. 880. 17. 10. 16. 164. 22. 32. 6.2 241. 1.013 72. 793. 1340. 60. 30. 21. 192. 81. 152. 40. : 307. .090 57. 841. 1140. 66. 29. 26. 176. 408. 177. 200. 389. 50°44' 50°44'- 50°42' 120°48' 120°49'. 120°48' .068 238. 1486. 2730. 143. 45. 25. 296. 46. 144. 36. 231. .160 72. 2405. 5550. 408. 112. 30. 322. 112. 181. 93. 388. .030 50°28' 50°28' 50°56' 119°38' 119°35' 118°27' std.dev. 1 72.10¬0.24 13.14 1.22 0.53 0.04 0.78 1.22 2.14 4.88 0.10 1.70 1.47 0.34 0.21 0.01 0.18 0.12 0.20 0.004 0.05 0.05 0.07 0.03 0.01 0.02 0.02 0.02 99.98 152. 3. 752. 9. 1880. 100. 21. 1. 10. 5. 11. 3. 161. 5. 6. 12. 28. 5. 2.7 2. 267. .202 49°23' 50°54' 120°30' 120°43' : Table 5. Strontium Isotope Data for Volcanic Rocks of the Kamloops Group, B r i t i s h Columbia, Sample Unit Location Rock Type Rb Sr 87 R b /86 Sr 8 7 S r / 8 6 S r ( 8 7 S r / 8 6 S r ) o TP 9-3 Ba t t l e B l u f f i n t r u s i o n 50°44.38' 120°33.76' N w diabase 70. 1060, 0.1910 0.7043 0.7042 31-1 Ba t t l e b l u f f intrusion 50°44.42' 120°33.82' N W syenite 231. 825. 0.8100 0,7049 0.7043 TP 43-1 Nipple breccia Dewdrop Flats Fm. 50°45.29' 120°31.35' N W o l i v i n e basalt 101. 806. 0.3625 0.7049 0.7046 4-2 Castle Butte breccia Dewdrop Flats Fm. 50°46.14' 120°36.63' N' W plagioclase andesite 46. 1352. 0.0984 0.7040 0,7040 8-3 Opax breccia Dewdrop Flats Fm. 50°48.58' 120 32.05' N w glassy andesite 83. 604. 0.3975 0.7036 0.7034 17-2 flow breccia Savona area 50°42.12' 120°47.58' N U andesite' 72. 793. 0.2616 0.7042 0.7040 16-5 ash-flow t u f f Savona area 50°43.56' 120°48.60' N W r h y o l i t e 161. 159. 2.9297 0.7065 0.7044 27-5 Monte Lake Fm. 50°27.92' n9 038.15' N W ba s a l t i c andesite 57. 841. T>1961 0.7080 0.7078 27-9 Tuktakamin breccia 50°27.79' 119°34.79' N W glassy l a t i t e 238. 1486. 0.4634 0 .7063 a 7060 40-7 Three Valley Gap dyke 50°55.7' 118°27.4' N W basalt 72. 2405. 0.0871 0 .7059 0 7058 WRD -76-2 Cornwall H i l l s (collected by W.R. Danner) 50°40' 121°30' N W r h y o l i t e 56. 1028. 0.1563 0 .7038 0.7037 Notes: Rb and Sr values determined by X-ray fluorescence. Sr isotopic composition was measured on unspiked single-filament samples at the University of B r i t i s h Columbia. ( Sr/ Sr) values are i n i t i a l r a t i o s calculated with a Rb decay constant of 1.42 x 10" 1 1 v" 1 and an assumed age of 50 Ha. ^Rexmoun McABEE V "GARFIELD £ ^ THREE VALLEY GAP 3 SAVONA WESTWOLD ^S7^ Monte Lake f f l t * T l i < t a k a m i n M t n % ^ 1>% NAP IER L A K E Okanagan Lake jjP q W H I T E L A K E P PRINCETON [i9_°N British Columbia Washington — REPUBLIC-50 km 120°W. 118'W, F i g . 22. L o c a t i o n map f o r chemical analyses of Eocene v o l c a n i c rocks i n southern B r i t i s h Columbia; Hat Creek data from Church (1975b). KAMLOOPS GROUP: A HIGH-ALKALI CALOALKALINE SUITE Major elements. The v a r i a t i o n of major elements with s i l i c a i s shown in F i g . 23. The p l o t of t o t a l a l k a l i vs. s i l i c a ( F i g . 23b) shows that the Kamloops Group v o l c a n i c s have a high a l k a l i c ontent, r e l a t i v e to average c a l c - a l k a l i n e v o l c a n i c s . T h i s i s r e f l e c t e d i n high normative o r t h o c l a s e i n the CIPW norm and s a n i d i n e i n the Rittmann norm. Thus, many of these rocks would be c o n s i d e r e d a l k a l i n e under s e v e r a l p e t r o l o g i c c l a s s i f i c a t i o n s , such as Kuno (1966) or M i y a s h i r o (1978). Although the Kamloops Group trends p l o t i n the a l k a l i n e f i e l d on a a l k a l i - s i l i c a diagram, no other i n d i c a t o r shows any a l k a l i n e c h a r a c t e r f o r the vast bulk of the Kamloops Group. A p l o t of normative s a t u r a t i o n vs. d i f f e r e n t i a t i o n index (DI) ( a f t e r Coombs, 1963) shows a s t r o n g and c o n s i s t e n t o v e r s a t u r a t e d d i f f e r e n t i a t i o n t rend from b a s a l t to r h y o l i t e ( F i g . 24). A l / S i r a t i o s are c o n s i s t e n t l y low, and f o l l o w a c a l c - a l k a l i n e t r e n d ( a f t e r 'Church, 1975a). The major-element c l a s s i f i c a t i o n scheme of I r v i n e and Baragar (1971) g i v e s c o n s i s t e n t l y c a l c - a l k a l i n e i n d i c a t o r s (save for a l k a l i c o n t e n t ) , and the trace-element c l a s s i f i c a t i o n of Winchester and F l o y d (1977) does the same. Peacock i n d i c e s from each area are c o n s i s t e n t l y 58 to 62, that i s , c a l c - a l k a l i c to c a l c i c . Four samples appear to show some a l k a l i n e c h a r a c t e r ; the l a t i t i c Tuktakamin b r e c c i a on the e a s t e r n s i d e of the study area, one a n d e s i t e at Kamloops (the Mara H i l l member), and two samples of a l a t e - s t a g e s y e n i t e d i f f e r e n t i a t e of K - r i c h diabase s i l l s at Kamloops. A l l other samples f a l l w i t h i n the main Kamloops Group sequence. The n o n - t h o l e i i t i c c h a r a c t e r of the s e r i e s i s shown by low Ti02 contents ( F i g . 23i) and the lack of Fe enrichment as £ 6 -"'- — 1 P P P e Alkaline o • A P . • •— Tholeiitic 0 P P e * * * * O « 6 2 S SO 60 weight V. SiO: 10 70 eo 50 60 70 weight V. SiO? •4 ° u. '2 ? •0 h2 2.0-0,04 0 ° * » . ° ' p p 3 o # o p • , • « . . . • S Cfe • * • * = 0 o e 3 • ° o . P p o pA -a p * • 0* . '.V'. • ° • ° * 1.5 •I.Oo •0.0 50 60 70 80 F£ g9*1 '"f$\ Major-element v a r i a t i o n diagrams f o r . Kamloops Group and c o r r e l a t i v e v o l c a n i c rocks. S p i n d l e s = ; s y e n i t i c segregates of B a t t l e B l u f f i n t r u s i o n s ; open c i r c l e s = i n t r u s i v e s at Kamloops, c l o s e d c i r c l e s = v o l c a n i c s at Kamloops; other, symbols as i n F i g . 22. Boundaries on Na20 + K20 vs. Si02 graph are from Kuno (1966), s e p a r a t i n g Japanese t h o l e i i t i c , c a l c -a l k a l i n e and a l k a l i n e t r e n d s . F i g . 24. S i l i c a s a t u r a t i o n (expressed as q+(hy./2)) vs. Thornton - T u t t l e d i f f e r e n t i a t i o n index f o r Kamloops Group rocks, compared with the Absaroka and Highwood trends (data from Iddings 1913) and the Japanese c a l c - a l k a l i n e t r e n d . QB = quartz b a n a k i t e , Sho = shoshonite, Ban = banakite, Abs = a b s a r o k i t e , a n a l y s e s from the Absaroka Range, Wyoming (Iddings 1913). Diagram a f t e r Coombs (1963). Fig. 25. MgO vs. Fe203* for Kamloops Group volcanic rocks, compared to Cascades calc-alkaline trend and Japanese pigeonitic (arc-tholeiitic) trend (after Jakes and G i l l 1970). shown on an MgO - Fe203* p l o t ( F i g . 25). The Kamloops Group v o l c a n i c s are most s i m i l a r to the Absaroka main t r e n d ( G a l l a t i n Range; Iddings 1913) of Wyoming and Montana, and the San Juan Mountains (Summer Coon volcano; Z i e l i n s k i and Lipman 1976) of Colorado; some comparisons are shown in F i g . 26a, as w e l l as on F i g . 24. The Absaroka shoshonite s e r i e s and the Highwood (Montana) a l k a l i n e f i e l d are shown f o r comparison. Note t h a t , except f o r a few anomalous samples, Kamloops Group v o l c a n i c s are not s h o s h o n i t i c i n chemi s t r y . Chemical analyses of Kamloops G r o u p - c o r r e l a t i v e rocks i n southern B r i t i s h Columbia and northern Washington have been repo r t e d from the White Lake B a s i n , B r i t i s h Columbia (Marron, Marama and White Lake Formations; Church 1973) and the Republic Graben, Washington ( S a n p o i l and r e l a t e d v o l c a n i c s ; Pearson and Obradovich 1977). Four major- element analyses of P r i n c e t o n o b a s i n v o l c a n i c rocks were a l s o made f o r t h i s study. F i e l d s of these s u i t e s of Eocene v o l c a n i c rocks are p l o t t e d on F i g . 26. Church (1973) has d i v i d e d the v o l c a n i c rocks of the White Lake Basin i n t o three s e r i e s : a n d e s i t e - r h y o l i t e ('A'), t r a c h y t e ('B'), and t e p h r i t e - p h o n o l i t e ( ' C ) . The 'A' s e r i e s , t y p i f i e d by the Park R i l l a n d e s i t e a n alyses shown on F i g . 26, i s c l o s e l y s i m i l a r to the a n d e s i t e s of the Kamloops Group. The 'B' and 'C s e r i e s form part of the C o r y e l l a l k a l i n e p r o v i n c e , which o v e r l a p s and h y b r i d i z e s with the f i r s t s e r i e s at White Lake and to the e a s t . S i m i l a r rocks are not found i n the Kamloops Group except f o r the Tuktakamin b r e c c i a . The P r i n c e t o n and Republic v o l c a n i c rocks are more s i l i c i c and s o d i c than most of the Monte Lake v o l c a n i c s (Kamloops Group); QB, Sho, Ban, Abs as i n F i g . 24. y l = Yellow Lake p h o n o l i t e of s e r i e s ' C , n l , k l Nimpit Lake and K i t l e y Lake t r a c h y t e s of s e r i e s 'B', Park R i l l = Park R i l l a n d e s i t e of s e r i e s 'A', Marron Formation, White Lake Basin (data from Church 1973). Republic data from Pearson and Obradovich (1977); Summer Coon data from Z i e l i n s k i and Lipman (1976). {a)_ K20 vs. Si02. {b)_ Na20 vs. K20. Black square = Tuktakamin b r e c c i a . Kamloops Group, but are otherwise s i m i l a r . Trace elements. The Kamloops Group v o l c a n i c rocks are c h a r a c t e r i z e d by t h e i r high contents of Sr and Ba ( F i g . 27). T y p i c a l b a s a l t i c a n d e s i t e s range from 700 to 1000 ppm Sr and 1000 to 2000 ppm Ba; some v o l c a n i c s r i c h e r i n Sr and Ba are a l s o found. These high a l k a l i n e e a r t h contents are g r e a t e r than many c a l c - a l k a l i n e arc assemblages, although some c o n t i n e n t a l margin assemblages show s i m i l a r abundances. The San Juan (Summer Coon) a n d e s i t e s are s i m i l a r i n t h i s regard to the Kamloops Group v o l c a n i c s and the Park R i l l a n d e s i t e at White Lake. The a l k a l i n e t r a c h y t i c and p h o n o l i t i c s e r i e s of the White Lake area have much higher contents of Sr and Ba, as shown on F i g . 26. Trace element abundances f o r other elements are d i s p l a y e d as a f u n c t i o n of s i l i c a content i n F i g . 28. Rb r i s e s s t e a d i l y with s i l i c a , f o l l o w i n g K as an incompatible elment. Zr and ( l e s s c e r t a i n l y ) Ce a l s o i n c r e a s e s t e a d i l y to 60-65% s i l i c a , a f t e r which they decrease i n abundance. Sr and Ba are n e a r l y constant i n c o n c e n t r a t i o n (with some s c a t t e r ) to 65% s i l i c a , d e c r e a s i n g above t h i s ; Sr contents decrease more s t r o n g l y than Ba. N i , Cr and V decrease throughout d i f f e r e n t i a t i o n ; Cr and Ni show a wide s c a t t e r . In a b s o l u t e abundances, Cr and Ni are s u b s t a n t i a l l y higher than the usual c a l c - a l k a l i n e v a l u e s , and Zr and Ce are somewhat h i g h e r . A l l of the trace-element v a l u e s , however, are w e l l w i t h i n the range of c a l c - a l k a l i n e v a l u e s , e s p e c i a l l y those fo r 'High-K' or ' h i g h - a l k a l i ' a n d e s i t e s (Condie 1976, Whitford and Jezek 1979). The trace-element contents of the Eocene v o l c a n i c s i n B r i t i s h Columbia i s c l o s e l y s i m i l a r to those of the Summer Coon v o l c a n i c s i n Colorado ( Z i e l i n s k i and Lipman 1976), ppm Ba F i g . 27. Sr vs. Ba f o r Kamloops Group v o l c a n i c rocks, and comparison with Marron v o l c a n i c s (Church 1973), Summer Coon v o l c a n i c s , " and v a r i o u s c i r c u m - P a c i f i c s u i t e s . Java data from W h i t f o r d and Jezek (1979); Namosi ( F i j i ) data from G i l l (1974). Arrows poin i n the d i r e c t i o n i n i n c r e a s i n g s i l i c a . I n v e r t e d t r i a n g l e s = averages from T a y l o r (1969) and Condie (1976); cab = c o n t i n e n t a l a l k a l i b a s a l t ; hab = high-alumina b a s a l t ; l k a = low-potassium a n d e s i t e ; hka = high-potassium a n d e s i t e ; sho shoshonite. Other symbols as i n F i g s . 22 and 23. sow 100 low 10-iow 1CH KXH i 0 • 4i * • V. * * 50 60 ' 70 weight V. Si02 60 e 1005-10D J 50 weight V. S i 0 2 70 » 1 = 1 • • • o * ° 0-A *\. 0 0 0 1. -% • o * J" „ • 0 0 0 e -> 1 . 1 3 • ) 1 . 0 3 A _ 0 • * D • -T-0 c o F i g . 28. Trace-element Group v o l c a n i c r o c k s . Symbols v a r i a t i o n with s i l i c a f o r Kamloops as i n F i g s . 20 and 21. except f o r lower c o n c e n t r a t i o n s of l i g h t REE. SPATIAL AND TEMPORAL VARIATIONS The Kamloops Group and c o r r e l a t i v e v o l c a n i c s show strong, r e g u l a r chemical v a r i a t i o n a c r o s s southern B r i t i s h Columbia, d e s p i t e t h e i r p e t r o g r a p h i c s i m i l a r i t y . The more s i g n i f i c a n t of these v a r i a t i o n s are shown[on F i g . 29. A value of percent potash at 60% Si02 (K 6 0) has been c a l c u l a t e d f o r four s i t e s w i t h i n the Kamloops Group, i n c l u d i n g the Hat Creek analyses of Church (1975b). A d d i t i o n a l s i n g l e analyses of the Rexmount p l u t o n and the Three V a l l e y Gap dyke .can be p r o j e c t e d with s u b s t a n t i a l l y l e s s c e r t a i n t y , to y i e l d approximate K 6 0 v a l u e s . Values are a l s o shown f o r the P r i n c e t o n : and Park R i l l v o l c a n i c s ; i n B r i t i s h Columbia, and the Eocene v o l c a n i c s i n northern Washington. K 6 0 shows a pronounced p r o g r e s s i o n - from west-southwest to e a s t - n o r t h e a s t , and may contoured as shown on F i g . 29a. T h i s p a t t e r n agrees with those r e p o r t e d f o r v a r i o u s s u b d u c t i o n - r e l a t e d arc complexes by Hatherton and Di c k i n s o n (1969) and other workers; i t s t r o n g l y suggests a l i n k a g e of the Eocene v o l c a n i c s to an e a s t - d i p p i n g subduction zone o f f the P a c i f i c coast of North America. There i s no evidence i n t h i s area f o r an im b r i c a t e subduction zone, as p o s t u l a t e d . f a r t h e r south by Lipman and others (1972). Use of the K 6 V - depth r e l a t i o n , as c a l c u l a t e d by Condie (1973), g i v e s apparent subduction zone depths of from 200 to 340 k i l o m e t r e s f o r the Kamloops Group proper, and 145 km f o r the P r i n c e t o n v o l c a n i c s . Other s p a t i a l v a r i a t i o n s can be noted. Values f o r Fe203* at 65% Si02 ( F e 6 5 ) and MgO at 65% Si02 (Mg 6 5) show systematic v a r i a t i o n i n southern B r i t i s h Columbia ( F i g . 29b). The main body F i g . 29. S p a t i a l v a r i a t i o n of chemical and i s o t o p i c parameters a c r o s s the Eocene a r c , southern B r i t i s h Columbia and northern Washington. (a)_ K20 at 60% Si02 (K 6 0) . (b)_ Fe203* and MgO at 65% Si02 ( F e ' s and M g 6 5 ) ; Mg v a r i a t i o n underscored. (c)  8 7 S r / 8 6 S r i n i t i a l r a t i o s . Data f o r southern B r i t i s h Columbia from t h i s study, Armstrong and Peto ( i n press) and unpublished data of R.L. Armstrong. of the Kamloops Group has v a l u e s of 5.2 and 2.2, r e s p e c t i v e l y , whereas Hat Creek and P r i n c e t o n have lower Mg 6 5 v a l u e s , and the P r i n c e t o n and Park R i l l v o l c a n i c s have lower F e ' 5 v a l u e s . These v a r i a t i o n s imply that the degree of d i f f e r e n t i a t i o n at 65% s i l i c a i s g r e a t e r i n the southwest and west than i n the no r t h e a s t , perhaps a r e f l e c t i o n of a s l i g h t l y more t h o l e i i t i c c h a r a c t e r to the seaward si d e of the a r c . The strontium i s o t o p i c data r e p o r t e d here, together with the data of Armstrong and Peto ( i n p r e s s ) , d e f i n e an i s o t o p i c d i s c o n t i n u i t y w i t h i n Eocene magmas that runs roughly north-south through Kamloops ( F i g . 29c). West of t h i s boundary, i n i t i a l 8 7 S r / 8 6 S r r a t i o s range from 0.7038 to 0.7042, whereas to the east , they range from 0.7058 to 0.7078. No change i n Sr content or other chemical or p e t r o g r a p h i c change appears to mark t h i s d i s c o n t i n u i t y . A s i m i l a r boundary f o r Mesozoic and Cenozoic igneous rocks i n the northwestern United S t a t e s has been i n t e r p r e t e d to d e l i n e a t e the western edge of Precambrian c o n t i n e n t a l c r u s t (Armstrong and others 1977). At Kamloops, s t r a t i g r a p h i c c o n t r o l and petr o c h e m i c a l coverage are s u f f i c i e n t to note chemical v a r i a t i o n s through the v o l c a n i c p i l e ( F i g . 30). The o l d e s t v o l c a n i c s are a n d e s i t e s ( T r a n q u i l l e Formation). Volcanism became more b a s a l t i c with time, accompanied by an i n c r e a s e i n Sr and Ba, and i n c l u d i n g the anomalously a l k a l i n e Mara H i l l member. Volcanism r e t u r n s to a more a n d e s i t i c and c a l c - a l k a l i n e c h a r a c t e r i n the upper Dewdrop F l a t s Formation. The l a t e s t known Eocene a c t i v i t y i n t h i s area (not shown) i s the i n t r u s i o n of r h y o d a c i t e . p F i g . 30. S t r a t i g r a p h i c v a r i a t i o n of s e l e c t e d elements w i t h i n the type Kamloops Group at Kamloops, B r i t i s h Columbia. Trace elements are given as l o g a r i t h m s . PETROLOGY AND DIFFERENTIATION TRENDS The most abundant l i t h o l o g y of the Kamloops Group v o l c a n i c s i s a u g i t e - p i g e o n i t e - l a b r a d o r i t e a n d e s i t e , ranging i n composition from 52% t o 60% s i l i c a . C l o t s of c l i n o p y r o x e n e c r y s t a l s (both a u g i t e and p i g e o n i t e ) , and s t e l l a t e p l a g i o c l a s e aggregates are common throughout the s u c c e s s i o n (Fig.: 31). Euhedral o l i v i n e phenocrysts are observed i n the more mafic' samples ( l e s s than 55% S i 0 2 ) , while h i g h l y o x i d i z e d and resorbed hornblende needles are o c c a s i o n a l l y found i n samples with from 55% to 67% Si02. Euhedral magnetite a l s o forms phenocrysts i n some samples. The groundmass t y p i c a l l y c o n s i s t s of small p l a g i o c l a s e (andesine) l a t h s with subhedral to anhedral i n t e r s t i t i a l a u g i t e and p i g e o n i t e , magnetite and r e s i d u a l g l a s s . T r a c h y t i c t e x t u r e i s w e l l developed i n the t h i n , f l u i d flows which c h a r a c t e r i z e much of the Kamloops Group, o f t e n g i v i n g r i s e to megascopic flow-banding. P l a g i o c l a s e l a t h s i n flow and p h r e a t i c b r e c c i a u n i t s , on the other hand, are e i t h e r unaligned or p o o r l y a l i g n e d . P l a g i o c l a s e i s normally zoned in most i n s t a n c e s , and i s commonly o s c i l l a t o r y . O c c a s i o n a l samples are reverse zoned, however. O s c i l l a t o r y zoning of c l i n o p y r o x e n e i s a l s o common. One flow-b r e c c i a at Kamloops (the Opax b r e c c i a ) i s very poor i n phenocrysts, g l a s s y and commonly banded, and may represent a mixing of d a c i t i c magma with more t y p i c a l a n d e s i t e . F e l s i c d i f f e r e n t i a t e s , r h y o d a c i t e and r h y o l i t e , occur l o c a l l y but i n small volume. They t y p i c a l l y c o n t a i n phenocrysts of euhedral b i o t i t e , subhedral p l a g i o c l a s e l a t h s , a n h e d r a l resorbed q u a r t z , and anhedral potassium f e l d s p a r . The groundmass. i s t y p i c a l l y an anhedral intergrowth of q u a r t z , a l b i t e and' F i g . 31. T y p i c a l Kamloops Group b a s a l t i c a n d e s i t e , showing a g g l u t i n a t e c l i n o p y r o x e n e phenocrysts and s t e l l a t e p l a g i o c l a s e l a t h s i n a t r a c h y t i c matrix. Crossed n i c o l s . potassium f e l d s p a r ^ I n t r u s i v e rocks have a l s o been examined at Kamloops. An a u g i t e - p i g e o n i t e diabase s i l l complex (te B a t t l e B l u f f diabase) shows hypidiomorphic t e x t u r e , with e x t e n s i v e overgrowths of : o l i g o c l a s e on c a l c i c andesine p l a g i o c l a s e l a t h s ( F i g . 32), and subhedral pyroxenes. Syenite d y k e l e t s , i n the upper p a r t s of two of the s i l l s , are two to f i v e centimetres wide and are a l i g n e d p a r a l l e l and p e r p e n d i c u l a r to the upper s i l l c o n t a c t . They c o n s i s t of an anhedral intergrowth of o r t h o c l a s e with some c a l c i t e , which surrounds small euhedra of aegerine-rimmed a u g i t e , b i o t i t e and p l a g i o c l a s e . The presence, of p i g e o n i t e throughout the sequence i s remarkable, as most t y p i c a l c a l c - a l k a l i n e sequences c o n t a i n hypersthene as the Ca-poor pyroxene. P i g e o n i t e i s t y p i c a l l y a s s o c i a t e d w i t h t h e Fe-enrichment of t h o l e i i t i c d i f f e r e n t i a t i o n ; however, the Kamloops Group v o l c a n i c s do not show Fe-enrichment. I t i s noteworthy in t h i s regard, that the Summer Coon s e r i e s i s r e p o r t e d to c o n t a i n c l i n o p y r o x e n e and very l i t t l e or no orthopyroxene (Lipman 1968), although the presence or absence of p i g e o n i t e was not e s t a b l i s h e d t h e r e . • The r e l a t i v e .proportions of phenocrysts i n a t y p i c a l Kamloops Group d i f f e r e n t i a t i o n sequence are shown i n F i g . 33a; the CIPW and Rittmann norms c a l c u l a t e d f o r the same sequence are shown on F i g . 33b. In g e n e r a l , the normative and modal m i n e r a l o g i e s agree w e l l , once the o c c u l t phases Q, Or and C (or San and Cd) are c o n s i d e r e d . The norms show n e a r l y constant percentages of p l a g i o c l a s e and potash f e l d s p a r from 50% to 66% s i l i c a , with slowly d e c l i n i n g pyroxene content, as i s observed I l l F i g . 32. B a t t l e B l u f f diabase, Kamloops, B r i t i s h Columbia, showing an o l i g o c l a s e groundmass ( b r i g h t ) o p t i c a l l y continuous with l a b r a d o r i t e l a t h s . Crossed n i c o l s . F i g . 33. Composite Kamloops Group v a r i a t i o n diagram, l a r g e l y from Kamloops. (a) Phenocryst mode, i n percent of t o t a l p h e n o c r y s t s . (b) CIPW norm. (c) Rittmann norm. Black area i n each diagram r e p r e s e n t s minor phases (magnetite, a p a t i t e , i l m e n i t e , e t c . ) . i n the phenocryst mode. Above 65% s i l i c a , a sharp r e d u c t i o n i n p l a g i o c l a s e and the e l i m i n a t i o n of pyroxenes are noted, both i n the norms and i n the phenocryst mode. These o b s e r v a t i o n s , together with the trace-element v a r i a t i o n diagrams d e s c r i b e d above, i n d i c a t e that there are two s t y l e s of d i f f e r e n t i a t i o n i n the Kamloops Group; one a c t i v e from 50% to 65% Si02, and another from 65% to 75% Si02. F e l s i c d i f f e r e n t i a t i o n . The d i f f e r e n t i a t i o n process from 65% to 75% s i l i c a can be c o n v i n c i n g l y modelled by low-pressure f r a c t i o n a t i o n of the observed phenocryst phases. Least-squares l i n e a r programming ( a f t e r Wright and Doherty 1970), using mineral compositions e i t h e r determined o p t i c a l l y ( f o r p l a g i o c l a s e ) or from the Rittmann norm ( f o r mafic p h e n o c r y s t s ) , suggests that about 45% 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 a mixture of 70% p l a g i o c l a s e , 25% b i o t i t e , and minor q u a r t z , K - f e l d s p a r , a p a t i t e and oxides ( F i g . 34, l a s t step) can account f o r the gene r a t i o n of r h y o l i t e from a t y p i c a l Kamloops Group a n d e s i t e . Support f o r t h i s model a l s o e x i s t s i n the t r a c e element data; the content of Sr, f o r i n s t a n c e , i s g e n e r a l l y reduced i n the r h y o l i t e ( p l a g i o c l a s e s egregation) whereas Ba i s l e s s a f f e c t e d ( l i t t l e K - f e l d s p a r s e g r e g a t i o n ) . The marked r e d u c t i o n i n Ce c o n c e n t r a t i o n in the r h y o l i t e s i s unusual, and may r e f l e c t the se g r e g a t i o n of l i g h t REE-capturing minerals such as a p a t i t e . N i , Cr and V a l l d e c l i n e , r e f l e c t i n g capture by b i o t i t e or other mafic m i n e r a l s . A s i m i l a r process was i n f e r r e d by Z i e l i n s k i and Lipman (1976) f o r the genesis of r h y o l i t e s at Summer Coon volcano. S h a l l o w - l e v e l d i f f e r e n t i a t i o n to r h y o l i t e was uncommon i n S i 0 2 DEEP 51% TP 9-3 F=48% / 48% Hb ( 1.5%Cpx\ 1.5%0l V 1%M.I.A\ S=1.6 M 5 2 % F = 5 8 % . - W 24% Hb 7%Bt 10%Pl 58°/o 0.8% I, A S=73 TP 39-3 6 1 V o F=51°/o/" 8" 3 23%Hb/ 24% PI I 1.5P/oM.l\ S=0,3 <J 66% TP 22-1 SHALLOW F=97% 3% Cpx S=2,9 75% 9-2 F=56% 11% Bt 30% PI 1% Q.San 1,7% I, A S=3,0 F=3A% 25% Cpx 15%0l 27% PI 1.3%M.I, A S=12 10-4 F i g . 34. Acceptable models f o r deep and shallow hydrous c r y s t a l l i z a t i o n of Kamloops Group v o l c a n i c rocks as d e r i v e d from l e a s t - s q u a r e s mixing c a l c u l a t i o n s . M = magnetite; I = i l m e n i t e ; A = a p a t i t e ; S = s e n s i t i v i t y of s o l u t i o n ( l e s s than one i s e x c e l l e n t , 1 to 5 good, 5 to 20 f a i r , more than 20 poor); F = percent of l i q u i d remaining. the Kamloops Group as a whole, although i t d i d occur l o c a l l y . T h i s i m p l i e s that there was a general lack of p e r s i s t e n t h i g h -l e v e l magma chambers i n which the a n d e s i t i c magma c o u l d evolve to r h y o l i t e . Those chambers that d i d e x i s t e i t h e r remixed with more mafic m a t e r i a l , or q u i c k l y vented to the s u r f a c e . The r a r i t y of s t a b l e h i g h - l e v e l magma chambers i s c o n s i s t e n t with the e x t e n s i o n a l . t e c t o n i c regime i n f e r r e d on r e g i o n a l s t r a t i g r a p h i c and s t r u c t u r a l grounds. The Kamloops Group v o l c a n i c s were probably erupted w i t h i n two to four Ma i n a complex t e c t o n i c regime of : s t r i k e - s l i p f a u l t i n g and e x t e n s i o n a l basin formation .(Chapter II). The e x t e n s i o n a l s e t t i n g w i t h i n an a c t i v e c o n t i n e n t a l a r c , and the i n f e r r e d high magma production r a t e s seem to be consonant with the p e t r o l o g i c f e a t u r e s noted above. Maf i c to intermediate d i f f e r e n t i a t i o n . The s t y l e of d i f f e r e n t i a t i o n i s l e s s c l e a r i n the dominant high-K b a s a l t s and an d e s i t e s of the Kamloops Group. The n e a r l y constant normative abundance of p l a g i o c l a s e , and i t s i n c r e a s i n g modal abundance, i n d i c a t e that p l a g i o c l a s e was not e f f i c i e n t l y f r a c t i o n a t e d from these magmas, d e s p i t e i t s abundance as phenocrysts. D e c l i n i n g pyroxene contents support pyroxene f r a c t i o n a t i o n , but the presence of modal hornblende h i n t s at d e e p e r - l e v e l hydrous 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 not r e f l e c t e d i n the anhydrous norms. The l e a s t - s q u a r e s mixing program was used to explore p o s s i b l e c r y s t a l l i z a t i o n paths i n the d i f f e r e n t i a t i o n sequence of F i g . 33. Two pyroxene-dominated types of models were t e s t e d , one c a l l i n g f o r f r a c t i o n a t i o n ('shallow'), the other f o r hornblende - dominated f r a c t i o n a t i o n ('deep'). Hornblende and p l a g i o c l a s e compositions f o r the l a t t e r case were estimated from the "wet" p l u t o n i c Rittmann norm. The r e s u l t s of t h i s study are shown on F i g . 34. From t h i s i t can be seen that s e v e r a l processes c o u l d be at work. Low-pressure, s u b v o l c a n i c c r y s t a l l i z a t i o n y i e l d s a c c e p t a b l e models f o r d i f f e r e n t i a t i o n f o r the two Opax samples, TP 39-3 and 8-3 (by 3% pyroxene f r a c t i o n a t i o n ) , and f o r the genesis of magma 5-6 from i t s p o s s i b l e mafic p r e c u r s o r s . Pyroxene-based c r y s t a l l i z a t i o n cannot, however, l i n k the e n t i r e s u i t e . Hornblende-based c r y s t a l l i z a t i o n y i e l d s f e a s i b l e s o l u t i o n s f o r the. d e r i v a t i o n of magma 5-6 from b a s a l t , and f o r t h e d e r i v a t i o n of the d a c i t e TP 22-1 from a n d e s i t e 8-3. TP 39-3, however, can only be d e r i v e d from sample 5-6 with d i f f i c u l t y , whatever : the mode of f r a c t i o n a t i o n . Some a l t e r n a t i v e such as v a r i a b l e p a r e n t a l magmas or heterogeneous contamination must be invoked to e x p l a i n some of the observed v a r i a t i o n s . Trace-element data do not support extended 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 at shallow depths. Ce c o n c e n t r a t i o n s r i s e from 50% to 54% s i l i c a , then remain e s s e n t i a l l y c o n s t a n t . Sr remains n e a r l y constant throughout the a n d e s i t e s . Ni and Cr show an i r r e g u l a r drop from 50% to 65% s i l i c a , p o s s i b l y accompanied by V, which might be taken to i n d i c a t e f r a c t i o n a t i o n of mafic m i n e r a l s . However, elements such as Sr, Ba and Ce, which should be a c t i n g as incompatible elements i n the absence of c r y s t a l l i z i n g phases, do not f o l l o w Rb i n a steady i n c r e a s e , but remain v i r t u a l l y c o n s t a n t . K/Rb r a t i o s (about 300) f a l l i r r e g u l a r l y with Si02, which i s c o n s i s t e n t with amphibole f r a c t i o n a t i o n . However, the a v a i l a b l e set of trace-element data, taken together, seem to i n d i c a t e that simple 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 from a s i n g l e b a s a l t i c parent magma cannot by i t s e l f account f o r Kamloops Group v a r i a b i l i t y , even at one l o c a l i t y . A v a r i e t y of p a r e n t a l magmas i s necessary. MAGMA ORIGINS The Kamloops Group magmas (before s h a l l o w - l e v e l 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 ) were b a s a l t s to b a s a l t i c a n d e s i t e s , with Si02 ranging from 48% to about 55%. They had moderate A l and Na contents and high K, Sr and Ba. The low 8 7 S r / 8 6 S r i n i t i a l r a t i o s in the west i n d i c a t e that the Sr i s from a "young" or low Rb/Sr source. The higher Sr i s o t o p i c values i n the east i n d i c a t e e i t h e r e x t e n s i v e i s o t o p i c exchange or contamination with ' o l d ' c r u s t a l Sr, or a higher Rb/Sr r a t i o i n an ' o l d ' magmatic source r e g i o n . High Sr and Ba contents are c h a r a c t e r i s t i c of both r e g i o n s . The broad o u t l i n e s of an a c c e p t a b l e source region can be sketched. The high Y contents' and moderate Ce/Y r a t i o s i n d i c a t e that garnet was not a r e s i d u a l phase i n the source r e g i o n . The high Sr contents l i k e w i s e r u l e out p l a g i o c l a s e as a r e s i d u a l phase. The general c h a r a c t e r of the trace-element d i s t r i b u t i o n i s most c o n s i s t e n t with amphibole and o l i v i n e as r e s i d u a l phases, although pyroxenes are a l s o p e r m i s s i b l e . The source r e g i o n , then, was presumably a p e r i d o t i t e , without garnet," and with high contents of Sr, Ba and K. Sources w i t h i n the lower c r u s t , or w i t h i n the b a s a l t l a y e r of a subducting oceanic c r u s t , are u n l i k e l y , given the evidence f o r the l a c k of garnet and p l a g i o c l a s e i n the source r e g i o n . Hornblende and s p i n e l p e r i d o t i t e s have been shown (see W y l l i e 1979) to be unstable at depths g r e a t e r than 70-90 k i l o m e t r e s , where they transform to g a r n e t - b e a r i n g p e r i d o t i t e . The base of the Eocene c r u s t of' the area was u n l i k e l y to have been l e s s than 35 km deep. P a r t i a l m e l t i n g to produce the Kamloops Group magmas, then, o c c u r r e d between about 35 km and 90 km. As p o i n t e d out above, however, the apparent depth to subduction zone value f o r the c e n t r e of the Kamloops Group arc i s about 250 km. T h i s c o n t r a d i c t i o n s t r o n g l y suggests a m u l t i s t a g e h i s t o r y f o r the genesis of the magmas, i n which v o l a t i l e f l u i d s came from depth to f e r t i l i z e a zone, 35-90 km deep, at which magmas were subsequently generated. Such a model i s s i m i l a r to the one proposed by Ringwood (1974) f o r p r o d u c t i o n of c a l c - a l k a l i n e magmas at subduction zones. I t should be noted a l s o that the apparent depth to subduction zone shows a marked discrepancy with present-day depths underneath the v o l c a n i c f r o n t of c i r c u m - P a c i f i c a r c s , which average around 100-150 km. There are two p o s s i b l e ways to r e s o l v e t h i s d i s c r e p a n c y . In the f i r s t , the K 6'-depth r e l a t i o n i s c o n s i d e r e d f a i r l y a c c u r a t e , and the Eocene arc was indeed b u i l t over an anomalously deep subducted s l a b . In t h i s case, the high a l k a l i and a l k a l i n e e a r t h contents may r e f l e c t i n c r e a s e d scavenging l e n g t h by the m a g m a - i n i t i a t i n g f l u i d s (Best 1975). In the second case, the arc was b u i l t over a more t y p i c a l subducted s l a b . In t h i s case, high a l k a l i and a l k a l i n e e a r t h contents may r e f l e c t higher c o n c e n t r a t i o n s of these elements w i t h i n the mantle o v e r l y i n g the s l a b . In e i t h e r case, the high c o n c e n t r a t i o n s of Sr and Ba may be i n f l u e n c e d by the composition of the subducted s l a b , e s p e c i a l l y the subducted sediments. CONCLUSION O p t i c a l petrography and major-element and trace-element geochemistry of the Kamloops Group v o l c a n i c s of s o u t h - c e n t r a l B r i t i s h Columbia can be combined with r e g i o n a l g e o l o g i c s t u d i e s to y i e l d a h y p o t h e t i c a l h i s t o r y of p e t r o g e n e s i s and e r u p t i o n . The Kamloops Group magmas were i n i t i a t e d by f l u i d s r i s i n g from a subducted oceanic s l a b at about 52 Ma. These f l u i d s , bearing K, Rb, Sr and Ba from the c o n t i n e n t a l l i t h o s p h e r i c mantle, con c e n t r a t e d them between 35 and 90 km depth. There, r e a c t i o n with mantle p e r i d o t i t e l e d to the generat i o n of high-K b a s a l t and b a s a l t i c a n d e s i t e magmas. These magmas ascended i n t o the c r u s t , and react e d with i t so t h a t , by the time they reached the su r f a c e , they had a c q u i r e d a v a r i a b l e S r - i s o t o p i c s i g n a t u r e , r e f l e c t i n g the presence of ' o l d ' lower c r u s t and/or mantle to the e a s t , and 'young' ( P a l e o z o i c or Mesozoic) c r u s t and/or mantle to the west. Deep hornblende f r a c t i o n a t i o n m o d i f i e d the magma compositions. Most of the magmas were erupted with only minor h i g h - l e v e l , p y r o x e n e - p l a g i o c l a s e c r y s t a l l i z a t i o n . L o c a l l y , and perhaps somewhat l a t e r , more p e r s i s t e n t h i g h - l e v e l magma chambers were occupied, and d i f f e r e n t i a t i o n 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 produced small amounts of r h y o l i t i c magma. CHAPTER IV. PALEOGENE TECTONIC EVOLUTION OF THE PACIFIC NORTHWEST SUMMARY Three con s e c u t i v e t e c t o n i c regimes are recorded i n the Paleogene of the North American C o r d i l l e r a between 40° and 60° l a t i t u d e . The Paleocene (65-53 Ma) regime produced c r u s t a l s h o r t e n i n g along the e a s t e r n edge of the C o r d i l l e r a , as w e l l as a weak magmatic arc i n B r i t i s h Columbia. The lower and middle Eocene regime (53-42 Ma) produced a robust v o l c a n i c arc superimposed on a network of s t r i k e - s l i p and normal f a u l t s and l a r g e n o r t h - n o r t h e a s t t r e n d i n g metamorphic t e r r a n e s showing Eocene r e s e t K-Ar dates as a consequence of u p l i f t and c o o l i n g i n a t e c t o n i c s e t t i n g of c r u s t a l e x t e n s i o n . The Late Eocene and Oligocene regime (42-30 Ma) produced a north-south Cascade v o l c a n i c arc south of 50° l a t i t u d e , and t e c t o n i c quiescence to the n o r t h and e a s t . T h i s sequence of events i s c o n s i s t e n t with the f o l l o w i n g t e c t o n i c model. In Paleocene time, continuous subduction took place from 40° to 60° l a t i t u d e ; low-angle subduction south of 49° c r e a t e d the Laramide magmatic gap. At about 53 Ma, amalgamation of the P a c i f i c , Kula and northern F a r a l l o n p l a t e s c r e a t e d a P a c i f i c - N o r t h America transform boundary north of 47° l a t i t u d e . Transform motion was i n part taken up on the F r a s e r - T i n t i n a s t r i k e - s l i p system, which t r a n s e c t e d the v o l c a n i c arc produced from the remnant of the subducted s l a b . At about 42 Ma, i n l a n d t ransform motion ceased, and a l l transform motion took p l a c e on the Queen C h a r l o t t e transform. South of 47°, subduction continued with a steady steepening of the subduction angle from north to south with time. INTRODUCTION In the P a c i f i c Northwest (here d e f i n e d as the North' American C o r d i l l e r a from 42° to about 60° north l a t i t u d e ) , Paleogene rocks are widespread and l i t h o l o g i c a l l y . and s t r u c t u r a l l y v a r i e d . In p a r t i c u l a r , geochronologic work i n the l a s t two decades (such as Mathews 1964; McDowell 1971; Armstrong 1975) has o u t l i n e d an i n t e n s e , s h o r t - l i v e d Eocene magmatic and t e c t o n i c event from 53 to.42 Ma. T h i s event i s i n l a r g e p a r t separable from the regimes preceding and following.. The v a r i e d and w e l l - p r e s e r v e d rock r e c o r d of the Paleogene should be amenable to p a l e o t e c t o n i c i n t e r p r e t a t i o n , and be c o n s i s t e n t with what i s known of o f f s h o r e p l a t e motions during the e a r l y T e r t i a r y . T h i s paper i s an attempt to make such a r e c o n s t r u c t i o n . -It. reviews and r e i n t e r p r e t s , what i s known of Paleogene geology and t e c t o n i c s in the area, and o f f e r s a s y n t h e s i s of the Paleogene e v o l u t i o n of the P a c i f i c Northwest. EARLY TERTIARY ARC SYSTEMS Paleocene (65-53 Ma) A northwest-southeast t r e n d i n g b e l t of p l u t o n s y i e l d i n g Paleocene dates e x i s t s i n western B r i t i s h Columbia and Washington ( F i g . 35a). Most of these p l u t o n s l i e i n the e a s t e r n h a l f of the Coast C r y s t a l l i n e Complex of B r i t i s h Columbia. From north to south, rep o r t e d Paleocene plutons a r e : s e v e r a l i n the A t l i n area, northwestern B r i t i s h Columbia (Bultman 1979) and the adjacent Whitehorse area of the Yukon (Morrison and others 1979); the Quottoon p l u t o n (Armstrong and Runkle 1979); small stocks i n the north- south Babine i n t r u s i v e b e l t of c e n t r a l B r i t i s h Columbia (Carter 1974); p l u t o n s i n the Anahim Lake map-area (Church 1973; C a r t e r 1974); the Styx Creek p l u t o n (Woodsworth 1979); the Bendor p l u t o n s (Woodsworth 1977); the Yale i n t r u s i o n s (Richards and White 1970) and the C l a r k Mountain plutons i n the Holden area of Washington (Cater and Crowder 1967). No Paleocene v o l c a n i c s have been rep o r t e d from t h i s r e g i o n . Paleocene v o l c a n i c ash i s , however, known i n the Sustut basin of n o r t h - c e n t r a l B r i t i s h Columbia (Eisbacher 1973). Reported Paleocene magmatic a c t i v i t y i s d i s c o n t i n u o u s and f a r i n l a n d south of l a t i t u d e 48°. The Adel Mountains v o l c a n i c s of western Montana give an age of 65 Ma (Chadwick 1972). V o l c a n i c s and p l u t o n s i n the L i t t l e Rocky Mountains of c e n t r a l Montana are dated as 58 to 66 Ma (Hearn and others 1978). A l k a l i c p lutons of the Black H i l l s i n South Dakota and Wyoming range from 59 to 49 Ma (McDowell 1971). Voluminous Paleocene magmatism i n the western U n i t e d S t a t e s i s r e s t r i c t e d to the Colorado and Arizona-New Mexico segments (Snyder and others Paleocene igneous and tectonic . Slcko m . i tafftprophyres s u V Roe :g<fe^-..»H;lden Little Rocky Mtn» id. t I Add Mfns UjlrlW Umpqua •> r ~ * 2 , T " ' ~~ ggbine Line ' v v , v v y v , • Oe&a'Lake Eocene igneous J-56* i i I late Eocene-Oiiaocene j nnecuscndtectonic I56' V, OLYMPIC MT? xHeJIJ&lig «r »aiiaL*iieji-T-rt>o;)f;«/s / ^ i V. i \ 1 finger Ohonqpeccsh ( C o w / r a s s E^v, 'v. Helena Vlrgin-a " City v>'v V K V v v v ** i — ' 1 vj^ Mcades _ !-'* " Great 9qjirifx-lis I v v v vv ' v v v Kamloops X To* in a w w 'Barron Mi BllCJuift x3'dev»Sanpoit b _^ "SilverPk, !! V ^ Teanov^  Crescent hfGHWOOO ! Tillamook xSiletz R ' x Umpqua Clarno' - V'C h Qv' SH V - J . . » V V V v v | ! ii.2' i j Susrof Eocene faults, basins Tyee Reurnoy F i g . 35. Igneous and t e c t o n i c elements of the Paleogene of the P a c i f i c Northwest. V o l c a n i c f i e l d names are i n lower case, p l u t o n names are i n lower case and u n d e r l i n e d , sedimentary b a s i n s are i n lower case i t a l i c s (waved r u l e = marine, s t i p p l e = nonmarine), igneous p r o v i n c e s are i n upper case. For sources see t e x t . ( A ) — Paleocene (65-53 Ma) igneous and t e c t o n i c elements. ( B ) — Eocene (53-42 Ma) igneous elements. ( C ) — Late Eocene and Oligocene (42-30 Ma) igneous and t e c t o n i c elements. ( D ) — Eocene f a u l t s and sedimentary basins ( f a u l t names: HO = Hope-Osborn, E = E n t i a t , SC = S t r a i g h t Creek, P = Pasayten, F = F r a s e r , NT = North Thompson, RMT = Rocky Mountain Trench). A l l maps are r e s t o r e d to pre-B a s i n and Range c o n f i g u r a t i o n , a f t e r Armstrong and Suppe (1973) and Armstrong and oth e r s (1977). 1976). I t i s i n f e r r e d that the b e l t of Paleocene dates i n B r i t i s h Columbia and Washington marks the s i t e of intermediate to s i l i c i c a r c - r e l a t e d i n t r u s i o n d u r i n g the p e r i o d 65 to 53 Ma. T h i s arc extended from l a t i t u d e 48° north beyond l a t i t u d e 60°, and represented the c o n t i n u a t i o n of l a t e Cretaceous magmatism i n the Coast C r y s t a l l i n e Complex. South of l a t i t u d e 48°, magmatism was d i s c o n t i n u o u s and f a r i n l a n d ; t h i s r e g ion formed p a r t of the "Laramide magmatic gap" noted by p r e v i o u s workers ( B u r c h f i e l and Davis 1972; Snyder and others 1976; Armstrong 1979). The lack of v o l c a n i c rocks along the Paleocene arc i s presumably due to subsequent u p l i f t and e r o s i o n of the Coast Mountains. There are comparatively few Paleocene dates from the P a c i f i c Northwest as a whole (see G r i f f i t h s 1977 and Armstrong 1979). T h i s was taken by Vance (1977) to support Paleocene' transform f a u l t i n g i n the area. However, the dates that are repor t e d , coupled with the low sample d e n s i t y i n much of the Coast C r y s t a l l i n e Complex, suggest that a weak magmatic arc d i d e x i s t at that time. Eocene (53-42 Ma) E x t r u s i v e and i n t r u s i v e igneous rocks of Eocene age are r e p o r t e d from most p o r t i o n s of the P a c i f i c Northwest. They form a narrow (about 70 km) b e l t north of 56° l a t i t u d e along the A l a s k a - B r i t i s h Columbia boundary, a 250-km wide b e l t from 56° to about 48° l a t i t u d e , and a broad (500-800 km) f i e l d from 48° to 42° l a t i t u d e ( F i g . *35b). In northern B r i t i s h Columbia, most Eocene igneous rock i s i n t r u s i v e , l o c a t e d along the e a s t e r n margin of the Coast C r y s t a l l i n e Complex ( C h r i s t o p h e r 1973; C a r t e r 1974; Smith 1977; Bultman 1979). V o l c a n i c s are preserved l o c a l l y (Sloko v o l c a n i c s ; Souther 1977). In c e n t r a l and southern B r i t i s h Columbia, intermediate to f e l s i c v o l c a n i c rocks are more e x t e n s i v e . Large plutons are r e s t r i c t e d to the e a s t e r n margin of the Coast C r y s t a l l i n e Complex (Woodsworth 1977; Ti p p e r 1978) and the e a s t e r n part of the C o r y e l l p r o v i n c e . In c e n t r a l B r i t i s h Columbia, Eocene v o l c a n i c s of the Ootsa Lake Group are known (Church 1972; Eis b a c h e r 1973; Souther 1977), a s s o c i a t e d with abundant small stocks of the Babine i n t r u s i o n s (Carter 1974). Near l a t i t u d e 51°, intermediate v o l c a n i c s of the Kamloops Group are dated as Eocene (Mathews 1964; H i l l s and Baadsgaard 1967; see Chapter I I ) . These rocks i n t e r f i n g e r southward with a l k a l i n e and c a l c -a l k a l i n e v o l c a n i c s of the Marron Formation (Mathews 1964; Church 1973, 1979); v o l c a n i c s and sediments o v e r l y i n g the Marron are a l s o Eocene (Church 1973, 1979). The a l k a l i n e rocks of t h i s area are comagmatic with the C o r y e l l i n t r u s i v e s of southernmost B r i t i s h Columbia ( L i t t l e 1961; Church 1973; Ross 1974), of e a r l y Eocene age. East of the broad v o l c a n i c areas, lamprophyres of Eocene age have been r e p o r t e d (Wanless and others 1972,1973; P a r r i s h 1979) . In the northwestern U n i t e d S t a t e s , Eocene v o l c a n i c and r e l a t e d i n t r u s i v e a c t i v i t y was e x t e n s i v e and voluminous (Armstrong 1979). A n d e s i t e s and d a c i t e s of the S a n p o i l v o l c a n i c s i n t e r f i n g e r northwards with the Marron v o l c a n i c s ; they y i e l d e a r l y to middle Eocene dates, as do the u n d e r l y i n g O'Brien Creek, and o v e r l y i n g Klondike Mountain Formations (Pearson and Obradovich 1977). A s s o c i a t e d Eocene p l u t o n s have a l s o been reported; (Engels and others 1976). In northwestern Washington, Eocene magmatism i s recorded i n the Holden area (Duncan H i l l p l u t o n , Old Gib v o l c a n i c s ; Cater and Crowder 1967), the Mount P i l c h u k stock (Yeats and Engels 1971) and the S i l v e r Peak v o l c a n i c s (Gresens and others 1977). Middle to l a t e Eocene v o l c a n i c s occur i n the Puget Group of the S e a t t l e area (Buckovic 1979). E a r l y phases of the C h i l l i w a c k b a t h o l i t h may be as o l d as 49 Ma (Misch 1966; R. L. Armstrong, To the p e r s o n a l communication). south, the a n d e s i t i c C l a r n c v o l c a n i c s of c e n t r a l to 48 Ma (Swanson and Robinson J . A. Vance, pe r s o n a l Oregon have given ages .from 42 1968; Enlows and Parker 1972; communication). In Idaho, the c a l c - a l k a l i n e C h a l l i s v o l c a n i c s cover a wide area; a s s o c i a t e d p l u t o n s , such as the Sawtooth B a t h o l i t h , have a l s o been dated (Armstrong 1974,1975; Siems and Jones 1977). The Absaroka v o l c a n i c s to the east are c o r r e l a t i v e (Chadwick 1970,1972; Smedes and Prostka 1972); minor a c t i v i t y i n the Absaroka f i e l d continued to 36 Ma (Love and others 1976). East and northeast of the C h a l l i s and Absaroka f i e l d s are a l k a l i n e volcanic, and p l u t o n i c rocks of the Highwood p r o v i n c e ( C e n t r a l Montana pr o v i n c e of Larsen 1940; Lipman and others 1972). Eocene plutons of p o t a s s i c i n t e r m e d i a t e to f e l s i c rock core many of the mountain complexes of c e n t r a l Montana (Marvin and others 1973; Hearn and o t h e r s 1978; Snee and S u t t e r 1979). In the Black H i l l s a l k a l i n e ; p l u t o n i s m continued i n t o Eocene time (McDowell 1971). A small amount of a l k a l i n e magma formed the Wyoming (Pekarek and others 1974). Ra t t l e s n a k e H i l l s i n c e n t r a l Other Eocene plutonism i s recorded from the west coast of Vancouver I s l a n d ( T o f i n o p l u t o n , 50 Ma; Carson 1972). South of 42°, Eocene v o l c a n i c s are not r e p o r t e d except f o r i s o l a t e d areas i n Colorado and New Mexico (see Snyder and others 1976). It i s i n f e r r e d that a continuous v o l c a n i c and p l u t o n i c arc e x i s t e d d u r i n g Eocene time from l a t i t u d e 42° north to northernmost B r i t i s h Columbia. T h i s arc was narrowest i n the north, where l a t e r u p l i f t has removed most of the v o l c a n i c cover. In c e n t r a l and southern B r i t i s h Columbia/ i t was some 250 k i l o m e t e r s wide; southward i t widened to 500-800 k i l o m e t r e s i n Oregon, Idaho, Wyoming and Montana, then ceased a l t o g e t h e r . T h i s magmatic arc was of c a l c - a l k a l i n e c h a r a c t e r throughout, except f o r two a l k a l i n e p r o v i n c e s , the C o r y e l l i n southernmost B r i t i s h Columbia, and the Highwood in c e n t r a l Montana. T h i s e x t e n s i v e magmatic a c t i v i t y i s the C h a l l i s v o l c a n i c episode of Armstrong (1979). In most areas, v o l c a n i c a c t i v i t y appears to have, reached a maximum at 48-51 Ma, and d i e d away more or l e s s r a p i d l y before about 42 Ma. Late Eocene and Oligocene (42-30 Ma) Rocks from t h i s time span occur i n three geographic groupings; a north-south b e l t . f r o m 50° l a t i t u d e south beyond 42°, a northwest-southeast l i n e c l o s e to the c o n t i n e n t a l margin north of 49°, and s c a t t e r e d d a t e s . i n the intermontane region ( F i g . 35c). Plutons of Late Eocene and Oligocene age. are r e p o r t e d from southernmost B r i t i s h Columbia and northern Washington; the H e l l s Gate p l u t o n (42 Ma; Wanless and others 1973), Needle Peak pl u t o n (39 Ma; Monger 1970), p a r t of the C h i l l i w a c k B a t h o l i t h (38 Ma, Misch 1966), and the Index p l u t o n (34 Ma; Yeats and Engels 1971). South of l a t i t u d e 48°, v o l c a n i c s of t h i s age are widespread i n Washington (Naches Formation, 39-41 Ma, Tabor and F r i z z e l l 1979; Ohanapecosh Formation, younger than 38 Ma; J . A. Vance, p e r s o n a l communication; also. Fiske. and others 1963; Goble v o l c a n i c s , about 40 Ma; Beck and Burr 1979).•Voluminous magmatic a c t i v i t y i n Oregon and northern C a l i f o r n i a began by 35 Ma (western Cascades, Hammond 1979, Smith 1979; John Day Formation, Swanson and Robinson 1968); in those same areas, l i t t l e v o l canism i s recorded i n the 8 m.y. p r e c e d i n g . Volcanism i n the Great Basin (northern Nevada and adjacent areas) i s about 36 to 40 Ma i n age (Stewart and C a r l s o n J976; Snyder and others 1976). S c a t t e r e d p l u t o n s are recorded along the c o n t i n e n t a l margin north of 49°. Small quartz d i o r i t e p l u t o n s are s c a t t e r e d across Vancouver I s l a n d (Carson 1972), and a l a r g e r body i s found i n the c e n t r a l Queen C h a r l o t t e I s l a n d s (Wanless and others 1969). In southeastern A l a s k a , small stocks host porphyry molybdenum d e p o s i t s (Quartz H i l l p l u t o n s , 30 Ma; Hudson and others 1979). In Montana and Wyoming/minor volcanism o c c u r r e d d u r i n g Late Eocene and Oligocene time. The Helena f i e l d (37 Ma) and the V i r g i n i a / C i t y f i e l d (33 Ma) erupted i n Montana (Chadwick 1978). A date of 36 Ma i s r e p o r t e d from the southeastern Absaroka f i e l d (Love and others 1976). I t I s i n f e r r e d t h a t the north-south alignment of ages south of 50° l a t i t u d e marks the i n c e p t i o n of the Cascade v o l c a n i c a r c . T h i s a r c , w e l l e s t a b l i s h e d by 35 Ma, i s at a 30 o-40° angle to the.arc. t r e n d of the Eocene, although the two o v e r l a p and are i n d i s t i n g u i s h a b l e i n northern Washington (Vance 1979). T h i s arc appears to have terminated northward at about 48°-50° l a t i t u d e d u r i n g Late Eocene and Oligocene time. North of t h i s , magmatic a c t i v i t y took p l a c e only c l o s e to the c o n t i n e n t a l margin and i n low volume. j Summary E a r l y T e r t i a r y magmatism i n the P a c i f i c Northwest showed three d i s t i n c t but s h o r t - l i v e d phases. The f i r s t (65-53 Ma) was an igneous arc extending northwest from northwestern Washington, with d i s c o n t i n u o u s magmatism i n Montana and South Dakota. The second (53-42 Ma) was marked by a robust magmatic arc t r e n d i n g northwest-southeast from l a t i t u d e 60° to l a t i t u d e 42°, dominantly c a l c - a l k a l i n e but c o n t a i n i n g two a l k a l i n e p r o v i n c e s . A c t i v i t y i n t h i s arc began about 53 Ma, reached i t s maximum i n t e n s i t y at 48 to 50 Ma, and decayed to 42 Ma. The t h i r d (42-30 Ma) was marked by the i n c e p t i o n of the north-south Cascade arc south of 50° l a t i t u d e and l o w - i n t e n s i t y plutonism along the c o n t i n e n t a l margin north of 49°. EARLY TERTIARY FAULTING, FOLDING AND BASIN DEVELOPMENT Paleocene Evidence f o r Paleocene t e c t o n i c a c t i v i t y i s l a r g e l y c o n f i n e d to the e a s t e r n edge of the C o r d i l l e r a ( F i g . 35a). North of 46° l a t i t u d e , t h i n - s k i n n e d deformation continued from Late Cretaceous through Paleocene time i n the northern Rocky Mountains (Monger and P r i c e 1979). S e v e r a l t h r u s t s cut Paleocene f l u v i a l sediments of the Paskapoo Formation and the Wapiti Group (shown on Tipper 1978); the time of l a s t movement on these t h r u s t s i s not known. At 56° l a t i t u d e , the Sustut and S i f t o n nonmarine successor b a s i n s r e c e i v e d sediments from l a t e Cretaceous to middle Eocene time (Eisbacher 1973). South of 46°, t h i n - s k i n n e d t h r u s t i n g took place i n the S e v i e r system, or Idaho-Wyoming t h r u s t b e l t (Armstrong 1968; Dorr and others 1977). The t h i c k - s k i n n e d Laramide u p l i f t s to the east were a l s o a c t i v e i n the Paleocene. Movement on the Wind Ri v e r t h r u s t continued i n t o e a r l y Eocene time (Dorr and others 1977). C l i m a c t i c deformation of the Sweetwater u p l i f t o ccurred in l a t e s t e a r l y Eocene, ending about 50 Ma (Love 1970). L a t e s t movement on the Laramide Elkhorn t h r u s t i n c e n t r a l Colorado has been bracketed between 49 and 56 Ma (Marvin and others 1978). West of the Rocky Mountains, Paleocene tectonism i s obscure. The Chuckanut nonmarine sediments of northwestern Washington have been c o n s i d e r e d to be of Paleocene age (Weaver 1937; M i l l e r and Misch 1963), but much of the great t h i c k n e s s of m a t e r i a l may be e a r l y Eocene (51 Ma;Vance and Naeser 1977, F r i z z e l l 1979). A small amount of Paleocene f l u v i a l sediment i s p r e s e r v e d i n the Methow trough of n o r t h - c e n t r a l Washington (Pipestone Canyon Formation; Barksdale 1975). The presence of other Paleocene sedimentary basins or Paleocene t e c t o n i c f e a t u r e s has not been demonstrated i n the P a c i f i c Northwest; some f e a t u r e s , however, may be masked by l a t e r Eocene a c t i v i t y . The observable t e c t o n i c r e c o r d i n d i c a t e s that Paleocene c r u s t a l s h o r t e n i n g took p l a c e along the e n t i r e e a s t e r n margin of the P a c i f i c Northwest. T e l e s c o p i n g of the sedimentary s u c c e s s i o n took p l a c e i n the Rocky Mountain and S e v i e r f o l d and t h r u s t b e l t s . Basement sh o r t e n i n g took p l a c e i n the Laramide basement-cored u p l i f t s , and presumably i n the h i n t e r l a n d of the t h r u s t b e l t s , although d i r e c t evidence i s l a c k i n g . No other i n f e r e n c e s about Paleocene t e c t o n i c s can be drawn from present knowledge. In p a r t i c u l a r , there i s no d i r e c t evidence f o r or a g a i n s t Paleocene s t r i k e - s l i p f a u l t i n g i n the P a c i f i c Northwest, such as suggested by Vance (1977). Eocene S i g n i f i c a n t tectonism and basin development o c c u r r e d i n lower and middle Eocene time i n most p a r t s of the P a c i f i c Northwest ( F i g . 35d). Evidence f o r the nature of t h i s episode comes l a r g e l y from three a r e a s : southern B r i t i s h Columbia, the North Cascades of Washington, and southwestern Oregon. In southern B r i t i s h Columbia, small fault-bounded sedimentary basins developed i n lower Eocene time. The T r a n q u i l l e basin r e c e i v e d about 500 metres of l a c u s t r i n e and volcanogenic m a t e r i a l (dated at 50 Ma; Mathews 1964; see Chapter I I ) . S i m i l a r accumulations o c c u r r e d to the north (Chu Chua Formation; Campbell and T i p p e r 1972) and to the west (McAbee sediments, 50 Ma; H i l l s and Baadsgaard 1967). M a t e r i a l probably of an e q u i v a l e n t age occurs at M e r r i t t (Coldwater beds; C o c k f i e l d 1948) and Hat Creek (Church 1975). These occurrences are d i s c u s s e d at g r e a t e r l e n g t h i n Chapter I I . T h e i r geometry, s t r u c t u r e and s t r a t i g r a p h y suggest that they formed w i t h i n an i n t e r l o c k i n g s t r i k e - s l i p and d i p - s l i p f a u l t network marked: by l o c a l grabens, and rapid;changes i n both t h i c k n e s s and f a c i e s . Other sedimentary basins occur to the south at P r i n c e t o n (48 Ma; H i l l s and Baadsgaard 1967) and White Lake (47 Ma; Church 1973). They appear to be c o n t r o l l e d by north-south t r e n d i n g subsidence s t r u c t u r e s s i m i l a r to Hat Creek (Church 1975, 1979). Other major t e c t o n i c f e a t u r e s of southern B r i t i s h Columbia are the metamorphic t e r r a n e s with Eocene r e s e t K-Ar dates. These t e r r a n e s , found i n the e n t i r e P a c i f i c Northwest, w i l l be d i s c u s s e d i n a separate s e c t i o n . The northern Cascades of Washington experienced a complex s e r i e s of t e c t o n i c events d u r i n g e a r l y and middle Eocene time.' A t h i c k sequence of nonmarine sediments (Swauk and Chuckanut Formations) i s i n p a r t of e a r l y Eocene age (Vance and Naeser 1977; F r i z z e l l 1979). T h i s sequence i s deformed by t i g h t c o n c e n t r i c f o l d s (Foster 1960; M i l l e r and Misch 1963) which are c r o s s c u t and o v e r l a i n by 47-Ma Teanaway b a s a l t ( F o s t e r 1960; Tabor and F r i z z e l l 1979). The f o l d axes are northwest-trending i n the northwestern Cascades, become n e a r l y north-south to the south, then swing n e a r l y east-west i n the Swauk outcrop area east of the S t r a i g h t Creek F a u l t ; i n the l a t t e r area the Teanaway dike swarm c u t s f o l d axes at r i g h t angles (Fo s t e r 1960). A f t e r the Teanaway volcanism, nonmarine sediments of the Roslyn Formation were d e p o s i t e d . These sediments c o r r e l a t e with the t h i c k graben f i l l of the Chiwaukum graben, and the Puget Group d e l t a i c sediments to the west (Buckovic 1979). T u f f i n the graben f i l l y i e l d s z i r c o n f i s s i o n - t r a c k dates from 47 to 40 Ma (Vance and Naeser 1977; F r i z z e l l 1979). The S t r a i g h t Creek F a u l t runs north-south through the North Cascades, and has been i n f e r r e d to be the s i t e of r i g h t - l a t e r a l displacement (Misch 1966; Monger and P r i c e 1979). The timing of t h i s movement has been d i s p u t e d . Vance (1977) suggests that t h i s movement was pre-Eocene, with Eocene d i p - s l i p r e a c t i v a t i o n of the zone. F r i z z e l l (1979), on the other hand, c o n s i d e r s 160 k i l o m e t r e s of motion to have occ u r r e d a f t e r Chuckanut-Swauk d e p o s i t i o n — n e a r l y as much as Misch (1977) proposed f o r t o t a l movement on the f a u l t (about 180 km). V e r t i c a l Chuckanut-e q u i v a l e n t conglomerates are r e p o r t e d w i t h i n the northern ex t e n s i o n of the f a u l t zone at Hope, B r i t i s h Columbia (Read 1960; McTaggart 1970). S e v e r a l i n f e r e n c e s can be drawn regarding e a r l y T e r t i a r y t e c t o n i c s i n the North Cascades. The Teanaway di k e complex r e p r e s e n t s west-northwest / e a s t - s o u t h e a s t e x t e n s i o n , while the immediately preceding Swauk f o l d b e l t r e p r e s e n t s north-south compression. These events are compatible with a s i n g l e s t r e s s regime, with p r i n c i p a l compression s l i g h t l y east of north. A l t e r n a t e compression and r i f t i n g i n d i c a t e an exchange of the p o s i t i o n s of intermediate and l e a s t p r i n c i p a l s t r e s s e s at about 47 Ma. T h i s o r i e n t a t i o n of s t r e s s e s i s roughly c o n s i s t e n t with r i g h t - l a t e r a l s t r i k e - s l i p movement on the S t r a i g h t Creek F a u l t . The swinging of Chuckanut-Swauk f o l d axes i n t o the l i n e of the S t r a i g h t Creek F a u l t suggests r o t a t i o n of s t r e s s e s and/or s t r u c t u r e s c o n s i s t e n t with r i g h t - l a t e r a l movement on the f a u l t . S i m i l a r sigmoidal p a t t e r n s are seen i n the o l d e r t e r r a i n s i n southern B r i t i s h Columbia (see T i p p e r 1978). A l t e r n a t i v e l y , the change i n f o l d axes may represent b u t t r e s s i n g a g a i n s t the Mount S t u a r t B a t h o l i t h to the n o r t h e a s t ( J . A. Vance, p e r s o n a l communication). In southwestern Oregon, a complex middle Eocene d e l t a i c and t u r b i d i t i c : sequence (Tyee and Flournoy Formations) was b u i l t over deformed e a r l y Eocene Roseburg Formation d i s t a l t u r b i d i t e s and e a r l y Eocene Umpqua b a s a l t s .(Baldwin 1974; P e r t t u and Benson, 1980). Deformation of the Roseburg Formation i n d i c a t e s s y n d e p o s i t i o n a l f o l d i n g and net northeast-southwest shear ( P e r t t u and Benson, 1980). T h i s shearing d i e s out g r a d u a l l y upward i n t o north-down d i p - s l i p f a u l t i n g , as noted along the Canyonville. f a u l t zone ( P e r t t u 1976); the middle Eocene Flournoy Formation passes unbroken over t h i s zone. D e l t a i c and marine d e p o s i t i o n continued through middle Eocene time. In northwestern Oregon and western Washington b a s a l t i c seamounts grew to and above sea l e v e l during t h i s time (Snavely and others 1968). The Tyee d e l t a was succeeded i n l a t e Eocene time by marine to d e l t a i c s t r a t a of the Coaledo and Spencer Formations (Baldwin 1974). Post-Tyee r o t a t i o n . o f over 70° c l o c k w i s e has a f f e c t e d ; t h e Oregon Coast Range and the Klamath Mountains (Simpson and: Cox 1977); about 40° of t h i s o c c u r r e d before the e r u p t i o n of the Yachats b a s a l t s at 39 Ma (Simpson and Cox 1977). A throughgoing system of s t r i k e - s l i p f a u l t s was probably a c t i v e d u r i n g Eocene time n o r t h of l a t i t u d e 47°. S e v e r a l f e a t u r e s d i s t i n g u i s h i n d i v i d u a l f a u l t s i n t h i s system. T h e i r s t r a i g h t ground t r a c e i s o f t e n composed of many bra i d e d f a u l t segments with a t o t a l width of one to four k i l o m e t r e s . The f a u l t s show v a r i a b l e d i p - s l i p o f f s e t s of Eocene s t r a t a , f r e q u e n t l y l o c a l i z i n g Eocene pocket basins and e r u p t i v e c e n t e r s . They are g e o m e t r i c a l l y r e l a t e d to e x t e n s i o n a l s t r u c t u r e s , such as grabens and metamorphic r e s e t t e r r a n e s (see below). They are continuous over tens to hundreds of k i l o m e t r e s . They show minor s t r u c t u r e s , p e r v a s i v e shearing and s u b h o r i z o n t a l s i i c k e n s i d e s , which i n d i c a t e s t r i k e - s l i p movement. The major throughgoing f a u l t s with i n f e r r e d Eocene s t r i k e - s l i p motion are shown on F i g . 35d. From north to south, these i n c l u d e ten f a u l t systems. 1. The Hope-Osborn f a u l t system of northern Idaho, i s r e l a t e d to an o f f s e t of Eocene metamorphic r e s e t t e r r a n e ( M i l l e r and Engels 1975) and -is i n f e r r e d to have Cretaceous to Eocene r i g h t - l a t e r a l displacement. 2. The north-south S t r a i g h t Creek F a u l t has been d i s c u s s e d above. Although i s cut l o c a l l y by post-40 Ma b a t h o l i t h s , i t can be t r a c e d northwards i n t o B r i t i s h Columbia (McTaggart 1970), where i t merges with the Pasayten f a u l t . 3. The E n t i a t f a u l t i s the northwest-southeast s t r a i g h t f a u l t which forms the northeast boundary of the post-45 Ma Chiwaukum graben. I t can be t r a c e d northwestward i n t o the Marblemount zone of Misch (1966,1980) and i n t o the S t r a i g h t Creek F a u l t (see the c o m p i l a t i o n of Haugerud 1980). I t s s t r a i g h t t r a c e and c o n t i n u i t y suggest an e a r l i e r , p o s s i b l e Eocene, s t r i k e - s l i p h i s t o r y before i t s known d i p - s l i p motion. 4. The Pasayten f a u l t trends northwest-southeast and forms the n ortheast boundary of the Cretaceous-Paleocene Methow trough (Barksdale 1975). The age of main motion i s p o o r l y understood, but d i p - s l i p motion o c c u r r e d d u r i n g or a f t e r the Paleocene. It j o i n s with the S t r a i g h t Creek F a u l t north of Hope, B r i t i s h Columbia (Mon.ger 1970). 5. The F r a s e r F a u l t Zone i s a northwest-southeast t r e n d i n g b r a i d e d f a u l t zone which has been t r a c e d from the Hope area to Quesnel, B r i t i s h Columbia. Eocene sediments and v o l c a n i c s have been trapped and f o l d e d i n t h i s zone west of Hat Creek ( D u f f e l l and McTaggart 1952). 6. A complex network of f a u l t s i n southern B r i t i s h Columbia (Chapter II) l o c a l i z e s sedimentary basins and v o l c a n i c c e n t e r s . T h i s network i s bounded on the northeast by the North Thompson f a u l t zone (Campbell and T i p p e r 1972) and i t s e x t e n s i o n s . These Eocene s t r u c t u r e s are l a r g e l y concealed to the northwest, but may j o i n with the F r a s e r F a u l t Zone or the northern Rocky Mountain Trench. The network terminates to the southeast at the Shuswap-Okanogan metamorphic r e s e t t e r r a n e ( d e s c r i b e d below). 7. The Bowron R i v e r F a u l t , i s a northwest-southeast s t r a i g h t f a u l t extending southeast from the northern Rocky Mountain Trench along the e a s t e r n boundary of the Eocene Bowron Ri v e r c o a l f i e l d . 8. The c e n t r a l Rocky Mountain Trench i s a broad l i n e a r furrow running southeast from the northern Rocky Mountain Trench to the Canoe River area i n e a s t - c e n t r a l B r i t i s h Columbia. I t has the same appearance as the northern Trench, although i t i s of d i f f e r e n t o r i e n t a t i o n . The Rocky Mountain Trench south of Canoe Ri v e r i s q u i t e d i f f e r e n t i n nature; s t r i k e - s l i p movement has not o c c u r r e d there ( P r i c e 1977). 9. S e v e r a l northwest-trending f a u l t s , i n c l u d i n g the P i n c h i f a u l t zone, l i e p a r a l l e l to and southwest of the northern Rocky Mountain Trench. Mesozoic motion on these zones has a l r e a d y been proposed ( G a b r i e l s e and Mansy 1978; Monger and P r i c e 1979) but Eocene a c t i v i t y i s p e r m i s i b l e and c o n s i s t e n t with other a c t i v i t y . 10. The northern Rocky Mountain Trench ( G a b r i e l s e and Dodds 1977) runs northwest from e a s t - c e n t r a l B r i t i s h Columbia to approximately l a t i t u d e 60°. I t i s r e l a t e d to the T i n t i n a Trench on the north, on which 450 k i l o m e t r e s of r i g h t - l a t e r a l displacement s i n c e 80 Ma has been i n f e r r e d (Tempelman-Kluit and others 1976). In B r i t i s h Columbia, the Trench l o c a l i z e s the Cretaceous-Eocene S i f t o n B a sin, and bounds the Eocene - r e s e t Wolverine metamorphic complex on the northeast ( P a r r i s h 1979). I t i s impossible with the a v a i l a b l e data to f i r m l y e s t a b l i s h the timing and magnitude of movements along these f a u l t s . The t o t a l post-80 Ma displacement has been estimated i n the Yukon and i n northern Washington. The establishment of Eocene displacement comes from t h e i r r e l a t i o n s h i p to pocket b a s i n s , as mentioned above, and to metamorphic r e s e t t e r r a n e s . Late Eocene and Oligocene T e c t o n i c elements of these ages are l a r g e l y r e s t r i c t e d to the P a c i f i c coast south of l a t i t u d e 49° ( F i g . 35c). There, d e l t a i c to marine sediments were d e p o s i t e d i n Late Eocene and Oligocene time (Puget Group, Buckovic 1979; C o w l i t z Formation, Armentrout and others 1980; Coaledo Formation, Baldwin 1974). The Oregon Coast Range was r o t a t e d about 40° before 39 Ma, as mentioned above. L e f t - l a t e r a l movement on the Leech River F a u l t began a f t e r 40 Ma ( F a i r c h i l d 1979) and may be r e l a t e d to the a r c h i n g and eastward movement of the Olympic Mountains (Tabor and Cady 1978). Elsewhere, a number of grabens and half- g r a b e n s are known. The Quesnel b a s i n , c o n t a i n i n g about 150 metres of c o n t i n e n t a l s t r a t a (W. H. Mathews and G. E. Rouse, i n p r e p a r a t i o n ) i s l o c a l i z e d along the F r a s e r F a u l t Zone, suggesting l o c a l d i p - s l i p r e a c t i v a t i o n . West-down l i s t r i c normal f a u l t s cut e a r l i e r t h r u s t s in the Rocky Mountains of southern B r i t i s h Columbia and Montana ( P r i c e and Mountjoy 1970), c r e a t i n g h a l f - g r a b e n s which are f i l l e d in p a r t by the Late Eocene and Oligocene Kishinehn Formation (Jones 1969). These f a u l t s may represent postorogenic extension a s s o c i a t e d with l a t e subsidence of an E o c e n e - u p l i f t e d c r y s t a l l i n e h i n t e r l a n d . . t e r r a n e ( M i l l e r and by Coney 1979; the EARLY TERTIARY RESET TERRANES Large areas of high-grade metamorphic rock i n the P a c i f i c Northwest y i e l d i s o t o p i c dates of 40 to 55 Ma. These dates are g e n e r a l l y , d i s c o r d a n t ; b i o t i t e K-Ar y i e l d s : E o c e n e dates while hornblende K-Ar and/or whole-rock Rb-Sr y i e l d Mesozoic dates at the same l o c a t i o n . F i g . 36 shows those t e r r a n e s i n which K-Ar ages have been r e s e t as d e f i n e d by a survey of e x i s t i n g i s o t o p i c data. T h e i r boundaries are shown at the approxmate 60-Ma "chrontour" ( l i n e of equal r e s e t b i o t i t e K-Ar d a t e ) . From south to n o r t h , these major t e r r a n e s a r e : the Idaho r e s e t t e r r a n e (Armstrong 1974, 1975); the P r i e s t R i v e r Engels 1975; a l s o c a l l e d the S e l k i r k t e r r a n e new name ( a f t e r the P r i e s t R i v e r metamorphics of Daly 1912) i s suggested here to a v o i d c o n f u s i o n with, the l a r g e S e l k i r k s t r u c t u r a l f e a t u r e s i n Canada); the Okanogan t e r r a n e i n c e n t r a l Washington (Fox and others 1977); the Skagit t e r r a n e (Misch 1966; Engels and others 1976); the Shuswap. t e r r a n e (Ross 1974; Medford 1975); the Kootenay Lake terrane 1 (W. H. Mathews, pe r s o n a l communication); the C e n t r a l N i c o l a t e r r a n e (Preto and others 1979; R. L. Armstrong, p e r s o n a l communication); the Wolverine t e r r a n e ( P a r r i s h 1976,1979) and the Skeena t e r r a n e (Hutchinson and others 1979; Berg and others ;:1978) . The major t e r r a n e s south of 54° are elongate s l i g h t l y east of north, and step en echelon along a broad northwest-trending zone. The areas of r e s e t t i n g roughly correspond with areas of s i l l i m a n i t e - g r a d e rocks, but t h i s correspondence i s not exact ( f o r example, P a r r i s h 1976). The c o n t a c t s of these r e s e t t e r r a n e s with a d j o i n i n g \Skeena (V, 48 Ma Wolverine 45 Ma ! l I i Eocene reset terranes ^ = 1 ^ = 2 « \ / - /, Cent. ...... . \ \shuswap NlcolaO J I 4 9 M a \ II _ \ . ) i v\Kootenay ' -1 V Skagit S ftL . 1. Priest River 45 Ma , '^ |40Ma I I >— F i g . 36. Eocene r e s e t metamorphic t e r r a n e s . Ages are the average minimum age of b i o t i t e K-Ar r e s e t t i n g . For sources see t e x t . Boundaries: 1 = r e s e t t i n g f r o n t , 2 = denudation f a u l t , 3 = throiighgoing f a u l t , 4 = unconformity, 5 = type not determined or i n f e r r e d c o n t a c t . nonreset rocks f a l l i n t o four types. In the f i r s t , there i s a continuous s t r a t i g r a p h y a n d / s t r u c t u r e a c r o s s the boundary, with a r e s e t t i n q f r o n t marked by a r a p i d s u c c e s s i o n of chrontours; t h i s i s seen in the Wolverine t e r r a n e ( P a r r i s h 1976) and the P r i e s t R i v e r t e r r a n e ( M i l l e r and Engels 1975). In the second, m y l o n i t i c f l a t f a u l t s or denudation f a u l t s separate r e s e t metamorphics from nonreset sediments and v o l c a n i c s . T h i s i s r e p o r t e d from the P r i e s t River t e r r a n e (Newport f a u l t ; M i l l e r 1971), the northern Shuswap complex (Read 1976) and northern Washington (Cheney 1979). In the t h i r d c l a s s , r e s e t areas are bounded: by s t e e p l y - d i p p i n g f a u l t s , some o.f which have i n f e r r e d s t r i k e - s l i p motion; t h i s i s r e p o r t e d from the Okanagan f a u l t system (Church 1975), the .borders of the Republic graben (Muessig 1967) and the Skagit t e r r a n e (Misch 1966). In the f o u r t h case, younger rocks o v e r l i e the r e s e t t e r r a n e on an unconformity, as noted along the margins of the Columbia River P l a t e a u and the Snake River P l a i n . Nonreset Eocene and e a r l i e r rocks which are i n f e r r e d to l i e atop the r e s e t t e r r a n e are known from the Midway-Greenwood area of B r i t i s h Columbia (Monger 1967) and the adjacent Republic area of Washington (Muessig 1967). Both areas show profound e x t e n s i o n , evidenced by profuse normal f a u l t s of both high and low angle, and e x t e n s i v e r o t a t i o n of f a u l t b l o c ks ( E . I r v i n g , P.Schwimmer, p e r s o n a l communications). Along the western margin of the Shuswap t e r r a n e , s l i d e b r e c c i a and fanglomerate w i t h i n the middle Eocene s t r a t a i n d i c a t e an u p l i f t e d source area to the east (Monger 1967; Church 1973). I t i s i n f e r r e d that these r e s e t t e r r a n e s are metamorphic core complexes, s i m i l a r to those d e s c r i b e d by Coney (1979). The presence of denudation f a u l t s , sharp thermal g r a d i e n t s shown by r e s e t t i n g , and high-grade metamoirphic rocks i s s i m i l a r to the core complex of the Basin and Range country. The metamorphic r e s e t t e r r a n e s d i f f e r from the l a t t e r only i n t h e i r age; a l l known core complexes south of the Snake River P l a i n were u p l i f t e d and reset i n the l a t e Oligocene or Miocene (Coney 1979). S e v e r a l mechanisms have been proposed f o r producing these r e s e t t e r r a n e s . Purely hydrothermal mechanisms have been suggested ( L i t t l e 1960, Medford 1975, Hyndman 1978) to be a s s o c i a t e d with intense Eocene magmatism. C r i s s and T a y l o r (1979) have o u t l i n e d l a r g e hydrothermal systems in the Idaho t e r r a n e with oxygen i s o t o p e s . T h i s mechanism may l o c a l l y be important; but the amount of Eocene magmatism in many of these t e r r a n e s , notably the northern Shuswap, northern P r i e s t River and Wolverine t e r r a n e s , i s minor, and probably i n s u f f i c i e n t to generate the e x t e n s i v e r e s e t t i n g . Furthermore, t h i s model does not e a s i l y e x p l a i n the presence of denudation f a u l t s and r e l a t e d core complex deformation. Other mechanisms i n v o l v e the u p l i f t of hot metamorphic i n f r a s t r u c t u r e over a broad a r e a . T h i s might be achieved i n a compressional environment by c r y s t a l l i n e t h r u s t s r e l a t e d to Rocky Mountain t h r u s t i n g , but the l a t t e r i s probably e a r l i e r than the observed r e s e t t i n g . The a l t e r n a t i v e i s u p l i f t i n an e x t e n s i o n a l environment by d u c t i l e normal f a u l t i n g as proposed by Davis and Coney (1979) f o r the Basin and Range core complexes. I favor t h i s model f o r the genesis of the e a r l y T e r t i a r y r e s e t t e r r a n e s , as i t i s c o n s i s t e n t with the extension seen elsewhere in~ southern B r i t i s h Columbia (noted by P r i c e 1979) and the lack of known Eocene compressional f e a t u r e s i n most areas of the P a c i f i c Northwest. The p r e f e r r e d model, then, f o r r e s e t t e r r a n e e v o l u t i o n c a l l s f o r l a r g e areas of r a p i d u p l i f t of hot metamorphic rock and a s s o c i a t e d i n t r u s i v e bodies, with t e c t o n i c and e r o s i o n a l denudation i n an e x t e n s i o n a l regime, dur i n g e a r l y and middle Eocene time. I f the Davis and Coney model i s v a l i d , we may expect that the long a x i s of the core r e p r e s e n t s the in t e r m e d i a t e s t r e s s a x i s (the a x i s of e l o n g a t i o n of the megaboudinage), and that the extension which generated the P a c i f i c Northwest core complexes was d i r e c t e d west-northwest/east-southeast. The eri echelon geometry strengthens t h i s i n f e r e n c e , as does the c l o s e r e l a t i o n s h i p of the core complexes with major f a u l t s , which i s explored below. DISCUSSION AND SYNTHESIS The Paleogene geology of the P a c i f i c Northwest, as d e s c r i b e d above, c o n s i s t s of a s u c c e s s i o n of magmatic a r c s , and a system of f a u l t s and f o l d s a s s o c i a t e d with basins and metamorphic core complexes. Each of these c a t e g o r i e s y i e l d s c l u e s to the nature and cause of Paleogene t e c t o n i c a c t i v i t y . Paleogene magmatic b e l t s The evidence f o r three d i s t i n c t magmatic b e l t s i n the e a r l y T e r t i a r y has been o u t l i n e d above, together with i n f e r e n c e s concerning t h e i r geometries. If we assume, as a c o r o l l a r y of p l a t e - t e c t o n i c theory, that these three l i n e a r , g e n e r a l l y c a l c - a l k a l i n e magmatic a r c s mark a subjacent subducted s l a b of oceanic l i t h o s p h e r e , then a h i s t o r y of the geometry of that s l a b can be determined. In the northwestern United S t a t e s , s c a t t e r e d , mostly a l k a l i c , Paleocene magmatism in the i n t e r i o r was succeeded by i n t e n s e , widespread, c a l c - a l k a l i n e C h a l l i s episode (Armstrong 1979). T h i s was f o l l o w e d by the development of the north-south Cascade arc and the Great Basin v o l c a n i c f i e l d s . I t i s reasonable to i n f e r t h a t Paleocene magmatism marks the presence of a very s h a l l o w l y d i p p i n g subjacent s l a b which n e a r l y excluded magmatic a c t i v i t y . The e x t e n s i v e Eocene arc suggests that the subducted s l a b s t i l l dipped at a low angle beneath the c o n t i n e n t , but now p e r m i t t e d magmatic a c t i v i t y north of 42°. The Late Eocene and Oligocene a r c , on the other hand, i m p l i e s a moderately e a s t - d i p p i n g subducted s l a b north of l a t i t u d e 42°. These i n f e r e n c e s are those of Snyder and others (1976) and Cross and P i l g e r (1978), and are compatible with the e a r l y T e r t i a r y magmatic h i s t o r y of the southwestern United S t a t e s d e s c r i b e d by Coney and Reynolds (1977) and K e i t h (1978). In B r i t i s h Columbia, a weak Paleocene p l u t o n i c arc gave way to a robust v o l c a n o - p l u t o n i c arc somewhat to the east i n the e a r l y and middle Eocene. T h i s volcanism ceased by the l a t e Eocene, and no Oligocene magmatic arc can be d e f i n e d , except f o r near-margin plutonism. T h i s h i s t o r y suggests that a subducted s l a b was present beneath B r i t i s h Columbia through Paleocene and e a r l y to middle Eocene time. T h i s i n t e r p r e t a t i o n i s at v a r i a n c e with that of Vance (1977), who q u e s t i o n s the e x i s t e n c e of the Paleocene a r c , proposing i n s t e a d a transform margin d u r i n g that time. F u r t h e r geochronologic work in B r i t i s h Columbia i s necessary to r e s o l v e t h i s q u e s t i o n . In any case, the subducted s l a b ceased to produce magmas by 45-42 Ma, and no Oligocene subducted s l a b can be i n f e r r e d north of 50°; t h i s was noted i n a general f a s h i o n by G r i f f i t h s (1977). Eocene t e c t o n i c elements Evidence has been presented i n a p r e v i o u s s e c t i o n f o r a s i g n i f i c a n t episode of s t r i k e - s l i p f a u l t i n g with concurrent b a s i n development, f o l d i n g and l o c a l r i f t b a s a l t s . A l s o d i s c u s s e d above were the broad areas of Eocene metamorphic r e s e t t i n g , which are i n f e r r e d to represent metamorphic core complexes formed by west-northwest/east-southeast extension a c r o s s a d u c t i l e metamorphic i n f r a s t r u c t u r e . One model for r e l a t i n g a s t r i k e - s l i p f a u l t system with metamorphic core complexes i s presented on F i g . 37. In t h i s model, two primary d e x t r a l s t r i k e - s l i p f a u l t s of d i v e r g e n t o r i e n t a t i o n e x i s t , with the f a u l t on the west being the F i g . 37. Block diagram of conceptual model showing the r e l a t i o n s h i p between s t r i k e - s l i p f a u l t i n g and formation of grabens and metamorphic core complexes. I n s e r t at lower l e f t shows a plan view (L) and a schematic displacement v e c t o r diagram (R). southward-continuing f e a t u r e . Displacement, on the other f a u l t (between b l o c k s C and E) must then be taken up through extension roughly p e r p e n d i c u l a r to i t s t r e n d . T h i s i s accommodated through l i s t r i c normal f a u l t i n g and graben development i n the b r i t t l e s u p e r s t r u c t u r e , and d u c t i l e spreading, growth f a u l t i n g and a s s o c i a t e d u p l i f t i n the mobile i n f r a s t r u c t u r e . Secondary s t r i k e - s l i p f a u l t s produce a s s o c i a t e d extension i n minor grabens and metamorphic core complexes. I n t e g r a t i o n i n t h i s manner of the i n f e r r e d s t r i k e - s l i p f a u l t system and the metamorphic core complexes i n the P a c i f i c Northwest leads to the network shown on F i g . 38a. Thus the northern boundary of the Shuswap r e s e t t e r r a n e i s the c e n t r a l Rocky Mountain Trench, and i t s southern boundary i n f e r r e d to be the Pasayten f a u l t . The Hope-Osborn f a u l t system of northern Idaho l i k e w i s e forms the northern boundary of the Idaho r e s e t t e r r a n e . Movement on the Pasayten f a u l t i m p l i e s a l a r g e , concealed e x t e n s i o n a l area beneath '. the ; Columbia P l a t e a u , s e p a r a t i n g the northern Cascades from n o r t h e a s t e r n Oregon and western Idaho. The need f o r the l a t t e r . h a s been mentioned by Davis and others (1978) i n another c o n t e x t . By i g n o r i n g secondary f a u l t s and r e s e t t e r r a n e s , we can s i m p l i f y t h i s t e c t o n i c model to. a f i v e - b l o c k scheme. Assuming that most of the motion on the bounding f a u l t s was s t r i k e - s l i p and that block r o t a t i o n was small (which i s supported by the s t r a i g h t n e s s of the major f a u l t s ) , a displacement v e c t o r diagram can be c o n s t r u c t e d ( F i g . 38b) by u s i n g the f a u l t azimuths :as s l i p _ v e c t o r s between b l o c k s . T h i s diagram c o n t a i n s an i n t e r n a l check (the azimuth of the F r a s e r F a u l t Zone) and i s probably F i g . 38a. Eocene f a u l t s and metamorphic r e s e t t e r r a n e s i n the P a c i f i c Northwest. Double l i n e s are the boundaries of the blo c k s used i n the f i v e - b l o c k model; block names as i n F i g . 38b. F i g . 38b. Maximum Eocene (or middle Cretaceous to Eocene) l a t e r a l displacements c a l c u l a t e d from a f i v e - b l o c k model. For d i s c u s s i o n see t e x t . t r u e to a f i r s t approximation. I f we assume f u r t h e r that a l l of these f a u l t s and e x t e n s i o n a l zones were a c t i v e c o n c u r r e n t l y , and that the estimates of motion on the T i n t i n a Trench (450 km) and the S t r a i g h t Creek F a u l t (180 km) s i n c e the middle Cretaceous are v a l i d , the amounts of displacement a c r o s s each of the other boundaries may be c a l c u l a t e d ; they are shown on F i g . 38b. Note that t h i s model p r e d i c t s about 75 k i l o m e t r e s of net c r u s t a l e x tension a c r o s s the Shuswap t e r r a n e from middle Cretaceous to l a t e Eocene time; that i s , about 35% to 50% c r u s t a l e x t e n s i o n . .As mentioned before, i t i s by no means proven that t h i s magnitude of displacement o c c u r r e d d u r i n g the Eocene. The lack of known Paleocene basin development in the Southern Intermontane block suggests, however, that the minor s t r i k e - s l i p f a u l t s i n t h i s area were not then a c t i v e . T h i s might be taken to extend to the major f a u l t s and core complexes as w e l l . Furthermore, some displacement on t h i s system must be Eocene, to account f o r the broad core complexes, the e x t e n s i v e basin development and l i s t r i c normal f a u l t i n g of that age. T h i s minimum displacement i s d i f f i c u l t to determine; i t must be estimated from the minimum amount of extension across the Shuswap needed to produce r e s e t t i n g and l i s t r i c normal f a u l t i n g . I c o n j e c t u r e that 10%-20% might be a reasonable minimum, with 50% p e r m i s s i b l e d u r i n g the Eocene. According to t h i s c o n j e c t u r e , a minimum of 20% and a maximum of 100% of the t o t a l s t r i k e - s l i p and e x t e n s i o n a l a c t i v i t y shown in F i g . 38b marks Eocene tectonism. S y n t h e s i s During the Paleocene a subducted s l a b extended c o n t i n u o u s l y from northern B r i t i s h Columbia to Mexico. In the U n i t e d S t a t e s , t h i s s l a b dipped s h a l l o w l y beneath the c o n t i n e n t , producing a broad area of l i t t l e or no magmatic a c t i v i t y and pronounced Laramide f o r e l a n d deformation (Cross and P i l g e r 1978). The" c r u s t was d r a m a t i c a l l y shortened along the e n t i r e landward edge of the C o r d i l l e r a d u r i n g t h i s time. In the lower and middle Eocene, a subducted s l a b s t i l l e x i s t e d i n a l l areas. North of 42°, a l a r g e f l a r e u p of magma gene r a t i o n occurred, marking the p a r t i a l c o l l a p s e of the s h a l l o w l y d i p p i n g s l a b i n the Idaho-Montana area (Snyder and others 1976; Armstrong 1979). T h i s magmatism was superimposed on a throughgoing system of d e x t r a l s t r i k e - s l i p f a u l t s , metamorphic core complexes and volcanosedimentary basins which suggests net r i g h t - l a t e r a l displacement of 100-500 km. In the Late Eocene and Oligocene there was no subducted s l a b north of 50°; to the south, the subducted s l a b continued to steepen and c o l l a p s e (Coney and Reynolds 1977; K e i t h 1978; D i c k i n s o n 1979), forming the north-south Cascade arc and the Great Basin v o l c a n i c f i e l d s . An o f f s h o r e transform margin i s i n f e r r e d n o r t h of 50°, based on the lack of both arc magmatism and onland deformation. The newly - a c c r e t e d Coast Range oceanic t e r r a n e r o t a t e d some 40° c l o c k w i s e before 39 Ma (Simpson and Cox 1977; J . M a g i l l , p e r s o n a l communication). There are three mechanisms f o r g e n e r a t i n g l a r g e s t r i k e - s l i p systems such as that of the P a c i f i c Northwest. The f a u l t i n g c o u l d represent a t r a n s c u r r e n t system r e l a t e d to c o l l i s i o n of a c o n t i n e n t a l block, as the C e n t r a l A s i a f a u l t s are r e l a t e d to the c o l l i s i o n of In d i a (Molnar and Tapponier 1975). Or, i t c o u l d represent a s u b d u c t i o n - r e l a t e d t r a n s c u r r e n t f a u l t system r e s u l t i n g from o b l i q u e subduction, as i n the western P a c i f i c ( F i t c h 1972). A l t e r n a t i v e l y , i t c o u l d represent an onland transform margin s i m i l a r to the San Andreas f a u l t system of C a l i f o r n i a . The f i r s t mechanism does not appear to apply to the P a c i f i c Northwest during the e a r l y T e r t i a r y , d e s p i t e an e a r l i e r a c c r e t i o n a r y h i s t o r y (Monger and P r i c e 1979). The second and t h i r d mechanisms produce the two models which are i l l u s t r a t e d i n F i g . 39. Both models have s i m i l a r c o n d i t i o n s at 60 Ma (Paleocene, F i g . 39a) and 38 Ma ( l a t e s t Eocene, F i g . 39d). The o v e r a l l geometry and t i m i n g of spreading r i d g e s and r i d g e r e o r i e n t a t i o n or e x t i n c t i o n i n the P a c i f i c i s taken from Byrne (1979). The two models d i f f e r i n e x p l a i n i n g the events between about 55 Ma and 42 Ma. The f i r s t model ( s u b d u c t i o n - r e l a t e d t r a n s c u r r e n t f a u l t i n g ) r e s u l t s from the assumption that the Eocene arc was a product of an a c t i v e Eocene subduction zone. The c r o s s c u t t i n g of the arc and the s t r i k e - s l i p regime must then i n d i c a t e / t r a n s c u r r e n t f a u l t i n g due to o b l i q u e subduction ( F i g . 39b). / / I n d i r e c t plate-motion evidence may support the e x i s t e n c e of o b l i q u e subduction, as I have argued p r e v i o u s l y (Ewing 1979b). T h i s model, however, s u f f e r s from two drawbacks; the lack of an adequate i n i t i a t i n g mechanism f o r a f a u l t system not c o n f i n e d to the a r c , and the need to allow about f i v e m i l l i o n years lag-time between changes i n p l a t e geometry and changes i n arc p a t t e r n s (as noted by Snyder and others 1976). The second model (transform f a u l t i n g ) c o n s i d e r s the Eocene Fig. .39. An interpretation of early Tertiary plate tectonics in the Pacific Northwest. Vertical ruled areas are magmatic arcs, shaded areas are metamorphic core complexes, wavy lined areas are turbidite fans.. Small barbs outline the extent of presently preserved magnetic lineations on the Pacific plate. Oceanic plate boundaries after Byrne (1979):. A) Paleocene. B) Eocene, model I. C) Eocene, model II. D) Late Eocene and Oligocene. arc i n B r i t i s h Columbia to be d e r i v e d from a headless subducted s l a b which remained from a p r e v i o u s l y a c t i v e subduction zone ( F i g . 39c). At 55-57 Ma the P a c i f i c p l a t e s underwent a d r a s t i c r e o r g a n i z a t i o n , r e s u l t i n g i n the amalgamation of the P a c i f i c , Kula and ( t h i s model proposes) the northern end of the F a r a l l o n p l a t e s . Thus the subduction zone o f f B r i t i s h Columbia ceased to be a c t i v e at that time, and a l a r g e l y t ransform P a c i f i c - N o r t h America boundary was e s t a b l i s h e d north of about l a t i t u d e 48°. Much of t h i s transform motion was taken up on the s t r i k e - s l i p and e x t e n s i o n a l network which t h i s paper i n f e r s ; other motion o f f s h o r e accounts f o r the 350 km of Eocene displacement observed on the D e n a l i F a u l t Zone of Alaska (Lanphere 1978). At 45-42 Ma transform motion ceased i n l a n d , and was e n t i r e l y taken up o f f s h o r e on the Queen C h a r l o t t e transform system. T h i s i s the p r e f e r r e d model, as i t a l l o w s a g e o l o g i c a l l y reasonable lag-time between p l a t e geometry changes and arc t r a n s i t i o n s , and accounts f o r the broad o u t l i n e s of the t e c t o n i c s of the P a c i f i c Northwest using only one major p l a t e r e o r g a n i z a t i o n . Future work w i l l t e s t these models, and h o p e f u l l y b r i n g us c l o s e r to an understanding of the g e o l o g i c h i s t o r y of the Paleogene of the P a c i f i c Northwest. CONCLUSION T h i s t h e s i s was c o n s t r u c t e d by examining at v a r i o u s s c a l e s the problems of the Kamloops Group. Geologic mapping e s t a b l i s h e d a b a s i c i n t e r n a l s t r a t i g r a p h y of a complex s t r u c t u r a l h i s t o r y i n the Kamloops area. The main s t r a t i g r a p h i c u n i t s and t e c t o n i c elements r e l a t e d to the Kamloops Group were examined i n a r e g i o n a l g e o l o g i c i n v e s t i g a t i o n . M i c r o s c o p i c and chemical i n v e s t i g a t i o n s determined the nature and r e g i o n a l v a r i a t i o n of the magmatic s u i t e . And f i n a l l y , wide-ranging c o m p i l a t i o n and s y n t h e s i s generated a t e c t o n i c and c h r o n o l o g i c context i n t o which the o b s e r v a t i o n s may be p l a c e d . Future work w i l l c e r t a i n l y r e v i s e and modify the c o n c l u s i o n s given here. 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T h o l e i i t i c and a l k a l i c b a s a l t s of the Eocene S i l e t z River V o l c a n i c s , Oregon Coast Range. American J o u r n a l of Science, 266, pp. 454-481. Snee, L.W., and S u t t e r , J.F. 1979. K-Ar geochronology and major element geochemistry of p l u t o n i c and a s s o c i a t e d v o l c a n i c rocks from the southeastern Pioneer Mountains, Montana (a b s . ) . G e o l o g i c a l S o c i e t y of America, A b s t r a c t s with Programs, 11, p. 302 Snyder, W.S., D i c k i n s o n , W.R., and Silberman, M.L. 1976. T e c t o n i c i m p l i c a t i o n s of space-time p a t t e r n s of Cenozoic magmatism in the western U n i t e d S t a t e s . E a r t h and P l a n e t a r y Science L e t t e r s , 32, pp. 91-106. Souther, J.G. 1977. Volcanism and t e c t o n i c environments i n the Canadian C o r d i l l e r a — a second look; .in Baragar, W.R.A., Coleman, L.C., and H a l l , J.M., eds., :. V o l c a n i c regimes i n Canada. G e o l o g i c a l A s s o c i a t i o n of Canada S p e c i a l Paper 16, pp. 3-24. Stevenson, L.S. 1939, Rhyodacite from the T r a n q u i l l e P l a t e a u , B r i t i s h Columbia. American M i n e r a l o g i s t , 24, pp. 446-447. Stewart, R.J. 1976. T u r b i d i t e s of the A l e u t i a n a b y s s a l p l a i n . Mineralogy, provenance and c o n s t r a i n t s f o r Cenozoic motion of the P a c i f i c p l a t e . G e o l o g i c a l S o c i e t y of America B u l l e t i n , 87^ pp. 793-808. Stewart, J.H., and C a r l s o n , J.E. 1976. Cenozoic rocks of Nevada: Nevada Bur. Mines and Geology Map 52, 4 sheets. Swanson, D.A., and Robinson, P.T. 1968. Base of the John Day Formation i n and near the Horse Heaven mining d i s t r i c t , n o r t h - c e n t r a l Oregon; in G e o l o g i c a l r e s e a r c h 1968. Chapter D. United S t a t e s G e o l o g i c a l Survey P r o f . Paper 600-D, pp. D154-D161. Tabor, R.W., and Cady, W.M. 1978. 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W i l s o r i r M.V.H. 1977. Middle Eocene freshwater f i s h e s from B r i t i s h Columbia. L i f e Sciences C o n t r i b u t i o n , Royal O n t a r i o Museum, no. 113, 61p. Winchester, J.A., and F l o y d , P.A. 1977. Geochemical d i s c r i m i n a t i o n of d i f f e r e n t magma s e r i e s and t h e i r d i f f e r e n t i a t i o n products using immobile elements: Chemical Geology, 20, pp. 325-343. Woodsworth, G.J. 1977. Pemberton (92J) map-area. G e o l o g i c a l Survey of Canada, o p e n - f i l e r eport OF-482. ( Woodsworth, G.J. 1979. Metamorphism, deformation and plutonism in the Mount R a l e i g h pendant, Coast Mountains, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada B u l l e t i n 295, 58pp. Wright, T.L., and Doherty, P.C. 1970. A l i n e a r programming and l e a s t - s q u a r e s computer method f o r s o l v i n g p e t r o l o g i c mixing problems. G e o l o g i c a l S o c i e t y of America B u l l e t i n , 81, pp. 1995-2008. W y l l i e , P.J. 1979. Magmas and v o l a t i l e components. American M i n e r a l o g i s t , 64, pp. 469-500. Yeats, R.S., and Engels, J.C. 1971. Potassium-argon ages of plut o n s i n the Skykomish - S t i l l a g u a m i s h areas, North Cascades, Washington; jjn G e o l o g i c a l r e s e a r c h 1971. United Sta t e s G e o l o g i c a l Survey P r o f e s s i o n a l Paper 750-D, pp. D34-D38. Z i e l i n s k i , R.A., and Lipman, P.W. 1976. Trace-element v a r i a t i o n s at Summer Coon volcano, San Juan Mountains, Colorado, and the o r i g i n of c o n t i n e n t a l - i n t e r i o r a n d e s i t e . G e o l o g i c a l S o c i e t y of America B u l l e t i n , 87, pp. 1477-1485. APPENDICES APPENDIX 1. NOTES TO ACCOMPANY THE "GEOLOGIC MAP OF THE KAMLOOPS LAKE AREA, BRITISH COLUMBIA" A g e o l o g i c map of the Kamloops Lake area ( F i g . . 40, i n pocket), has been compiled at the s c a l e of 1:50,000. I t i n c l u d e s the d e t a i l e d mapping which has been repo r t e d elsewhere i n t h i s t h e s i s (Chapters I and I I ) , and r e g i o n a l reconnaissance along most of the roads i n the area. Other d e t a i l e d mapping used in the c o m p i l a t i o n i n c l u d e s : Northcote (1977) f o r the Iron Mask i n t r u s i o n s southwest of Kamloops; MacMillan (1978) f o r the southwest corner of the map; Morrison (1970) for the middle Carabine Creek area; and Betmanis (1972) f o r Cherry B l u f f . The r e g i o n a l g e o l o g i c base i s from C o c k f i e l d (1948) and Mathews (unpublished MS map). The map shows the s t a t u s of g e o l o g i c knowledge in the area as of August, 1980. Aeromagnetic maps f o r the area have been s t u d i e d ( G e o l o g i c a l Survey of Canada, 1965, 1968a,b,c). Each of the major s t r a t i g r a p h i c u n i t s i n the area has a d i s t i n c t i v e aeromagnetic s i g n a t u r e : Tmv (Miocene P l a t e a u v o l c a n i c s ) — a v e r a g e i n t e n s i t y (I) = 2300 ft; anomaly amplitude (A) = 600 ^, wavelength C\) = 3-4 km; unoriented anomalies. Td, Tev (Kamloops Group v o l c a n i c s ) — I = 3200 If ; A = 1000¬1500 fl"; X= 1~4 km. The three major l i t h o l o g i e s are d i s t i n c t ; b a s a l t i c a n d e s i t e flows have the h i g h e s t i n t e n s i t i e s (Tdr, Tdm, Tdd), flow b r e c c i a s have intermediate i n t e n s i t i e s (Tdo, Tdw, Tdk), and p h r e a t i c b r e c c i a s the lowest i n t e n s i t i e s (Tdn, Tdc). Tm (Mount Savona Fm) -- low i n t e n s i t i e s which l a r g e l y mask the u n d e r l y i n g a n d e s i t e flows at Mount Savona. Tt ( T r a n q u i l l e Fm) — I = 2900 A l e s s than 100 IS. Low i n t e n s i t y , f l a t f i e l d . T i b ( B a t t l e B l u f f i n t r u s i o n s ) -- s l i g h t e x p r e s s i o n as magnetic highs south of Kamloops Lake; masked by adjacent Cherry B l u f f p l u t o n . Mzv, Mzc (Carabine Creek v o l c a n i c s , Eagle H i l l conglomerate) — I = 2700 T; A = 400 ft . G e n e r a l l y low i n t e n s i t y , f l a t . T r J i (Iron Mask i n t r u s i o n s ) — I = 3500 \; A = 2000-2500JT ; \ - 1-2 km. Very high i n t e n s i t i e s and anomalies, r e f l e c t i n g l a r g e magnetite c o n c e n t r a t i o n s i n the p l u t o n s . Trn ( N i c o l a Group v o l c a n i c s ) — I = 2700 t; A = 100-400 \ , /\ =4-8 km. G e n e r a l l y low i n t e n s i t y f i e l d with g e n t l e anomalies. PzMzs (sediments) ~ I = 2500 ^; A = 100-400 . Low i n t e n s i t y , f l a t f i e l d . L i n e a r s were found i n a l l u n i t s o l d e r than the Miocene. They are marked by elongate anomalies and sharp f i e l d g r a d i e n t s . Anomalies caused by poor r e c o r d i n g on a f l i g h t l i n e were discarded.. F a l s e lows caused by r e l i e f (the canyon e f f e c t ) was a l s o d i s c o u n t e d . The remaining anomalies and l i n e a r s are p l o t t e d on the map. The l i n e a r s , together with the mapped f a u l t s , o u t l i n e a rhombic p a t t e r n i n the map ar e a . The stro n g e s t l i n e a r s t rend NW-SE, with very common l i n e a r s t r e n d i n g N-S. T h i s i s a r e f l e c t i o n on a smal l e r s c a l e of the r e g i o n a l f a u l t p a t t e r n of Eocene and Re-Eocene f a u l t s throughout the Thompson V a l l e y area (Chapter I I ) . In p a r t i c u l a r , the rhombic p a t t e r n i n the T r a n q u i l l e R i v e r outcrop area supports the c o n t e n t i o n made elsewhere that the T r a n q u i l l e Canyon s t r u c t u r e r e p r e s e n t s a complex p u l l - a p a r t s t r u c t u r e i n a s t r i k e - s l i p , regime. APPENDIX 2. NOTE ON THE PALEOMAGNETISM OF THE KAMLOOPS GROUP AT KAMLOOPS, BRITISH COLUMBIA A paleomagnetic study of the Kamloops Group was undertaken i n c o o p e r a t i o n with Myrl Beck and h i s graduate students at Western Washington U n i v e r s i t y , Bellingham, Washington. B r i a n Globerman and Peter Schwimmer from Bellingham v i s i t e d the Kamloops area i n J u l y 1979, and d r i l l e d t h i r t e e n paleomagnetic s i t e s , with an average of 7 to 8 samples per s i t e . The l o c a t i o n s of these s i t e s are given on Table 6, and shown on F i g . 41. Core a t t i t u d e s were measured by both sun compass and Brunton compass. The cores were analyzed at ; Western Washington U n i v e r s i t y by Peter Schwimmer ( s i t e s 8 - 13) and the author ( s i t e s 1 - 7). T i l t c o r r e c t i o n s (TC on Table 6) were taken from measurements on a d j o i n i n g s t r a t a , the a t t i t u d e of colonnade columnar j o i n t i n g , or by i n f e r e n c e from the g e o l o g i c map. A l l samples were demagnetized by a l t e r n a t i n g f i e l d (AF) techniques to a l e v e l of 100-500 o e r s t e d , as determined by p i l o t s t u d i e s . The experimental data obtained to date are presented on Table 6. The t i l t - c o r r e c t e d magnetic f i e l d d i r e c t i o n s f o r the AF-c l e a n e d Kamloops Group sap l e s ( w i t h ; S i t e 12 r e j e c t e d ) are shown in F i g . 41. The number of a c c e p t a b l e s i t e s i s too low, and the sampling b i a s towards the B a t t l e B l u f f and K i s s i c k s i l l s t oo g r e a t , to be sure that the average f i e l d d i r e c t i o n i s w e l l d e s c r i b e d . Furthermore, no r e v e r s a l s : have been encountered, i n the accepted s i t e s . Some c o n c l u s i o n s , however, can be drawn the evidence at hand. — The two Red P l a t e a u s i t e s show i n c l i n a t i o n s that f r om are anomalously shallow as compared to the expected Eocene f i e l d Table 6.. Preliminary. Paleomagnetic Results, Kamloops Group Volcanics at Kamloops, B.C. Si t e Latitude Longitude 1 * " ~ 2 3 4 5 6 7 8 9 10 11 12S 12G 13 D PC n 6.47 58.4 8 3.95 157. 8 4.33 2.54 14 9. 305. 7 10 3.69 180. 8 5.63 77.2 8 4.87 92. 9 az./dip Pol. u n i t rock acc. 50 41.4' 4i.4: 1 43.4' 4 8 .3 ' 48.5' 44 .4 • 44.1' 43.5' 43.5' 42.2' 39.S* 43.2' 43.2' ;44.0' 120 34.3' .34.0' 29.6 ' 33.3' • 33.4' 33.9' 33.8' 29.7' 29.7' 34.7' 32.9' 28.4 ' 28:4 ' 31.2' 327.78 347.54 349.61 332.46 329.56 327.86 331.77 73.30 69.25 .65.37 38.29 36.27 60.91 64.55 281.08 77.64 345.39 355.39 342.53 33.00 333.21 52.40 66.92 59.90 82.76 51.14 2.91 386. 5.19 104. 3.39 190. 22.98 19. 6.33 245. 10.22 38. 57/18 70/12 . 50/06 180/06 0/0 58/14 354/16 60/12 150/20 95/20 0/0 0/0 97/27 12.98 14.33 359.61 329.95 329.56 351.39 339.56 65.58 64.83 61.95 43. 55 36.27 53.02 49.32 T i b 2 T i b 3 T i 1 N S N N Tdr M Tdr N N T i b T i b 2 s i l l s i l l s i l l flow flow s i l l s i l l s i l l 347 84 81.50 N m- 1 s i l l / 359 88 71.03 N T i 1 s i l l / 38 22 62.48 K Ti-cck s i l l ? / 342 53 59.9 0 M T i dyke « 33 00 82.76 R T i dyke x 359 27 61.20 V Ttu p i l f l o w / 50 Ma pole for North America i s . 78C-N, 137°W. Paleolatitude. for Kamloops i s 54.08°.N. Expected f i e l d d i r e c t i o n i s D = 341.04O, i = 70.09°.: A l l samples subjected-to AF demagnetization at 200 to 500 oe. Symbols: D = d e c l i n a t i o n , I = inclination'} a g 5=radius; of c i r c l e of confidence at 95%, p r e c i s i o n parameter; n .= number of samples,-az./dip c o r r e c t i o n ; D ,1 = corrected d e c l i n a t i o n , i n c l i n a t i o n ; geologic map; ace. = a c c e p t a b i l i t y . azimuth and dip of t i l t P ol. = p o l a r i t y ; unit = units Samples c o l l e c t e d July, 1979, by P. Schwimmer and B. Globerman, Western Washington and T. Ewing, University of B r i t i s h Columbia. Samples analyzed by T. Ewing and P. Schwimmer at Western Washington Un i v e r s i t y . University. I on F i g . 41. Stereographic p l o t of magnetic f i e l d d i r e c t i o n s f o r Kamloops Group s i t e s l i s t e d i n Table 6. Black square = expected Eocene f i e l d d i r e c t i o n . d i r e c t i o n (the black square i n F i g . 41) and the other Kamloops Group samples. These two samples are s t r a t i g r a p h i c a l l y c l o s e ; they may record a b r i e f e x c u r s i o n r e l a t i v e to the other s i t e s . no l a r g e r o t a t i o n of the Kamloops Group can be s u b s t a n t i a t e d at t h i s time. If there was r o t a t i o n , i t was l e s s than about 25°. — i n c l i n a t i o n s of the m a j o r i t y of samples are s l i g h t l y shallower than the expected i n c l i n a t i o n , but no northward t r a n s l a t i o n can be proven. Such t r a n s l a t i o n i s probably not more than a few degrees. -- i t i s s i g n i f i c a n t that the d e c l i n a t i o n f o r s i t e 11 (a c o l u m n a r - j o i n t e d b a s a l t west of Afton Mine) i s 25° f a r t h e r east (clockwise) than any other a c c e p t a b l e s i t e . T h i s b a s a l t i s l o c a t e d w i t h i n the Cherry Creek f a u l t zone which, I have contended elsewhere, was the l o c u s of d e x t r a l s t r i k e - s l i p movement during the Eocene. Blocks w i t h i n t h i s f a u l t zone would be expected to show clo c k w i s e r o t a t i o n ; S i t e 11 supports t h i s . I t i s p o s s i b l e that h i g h e r - l e v e l AF or thermal demagnetization may improve the s c a t t e r . The h i g h F e 3 / F e 2 r a t i o s encountered i n the Kamloops Group (Chapter I I I ) suggest that a hematite secondary component may be present i n the ma g n e t i z a t i o n . The low-AF demagnetization that was performed may not have e l i m i n a t e d such a secondary, although i t d i d e l i m i n a t e v i s c o u s magnetization (VRM) i n the present f i e l d . The sampling b i a s and small number of samples can, however, only be overcome with a more thorough o f f - r o a d sampling program. APPENDIX 3. NOTE ON A VITRINITE STUDY OF THE KAMLOOPS GROUP In order to gain f u r t h e r understanding of the thermal h i s t o r y of the Kamloops Group sediments, samples were s e l e c t e d and prepared f o r determination of mean maximum r e f l e c t a n c e (Rc) of v i t r i n i t e p h y t o c l a s t s . Eleven samples were subjected e i t h e r to f l o t a t i o n by tetrabromoethane or to d i s s o l u t i o n i n h y d r o f l u o r i c a c i d . Only s i x samples c o n t a i n e d s u f f i c i e n t r e c o v e r a b l e organic matter f o r r e f l e c t a n c e d e t e r m i n a t i o n . F i v e of these r e s u l t s are shown on Table 7, together with values from other s t u d i e s i n the Eocene rocks of southern B r i t i s h Columbia. Samples which show evidence of contact metamorphism are not l i s t e d ; the s i x t h r e s u l t of t h i s study (from lower T r a n q u i l l e Formation sediments between the lower and middle B a t t l e B l u f f s i l l s ; R„ = 1.10%) f a l l s i n t h i s category. Too few s a p l e s are a v a i l a b l e to d i r e c t l y determine a geothermal g r a d i e n t i n the Kamloops area. Those rare sequences which c o n t a i n s u f f i c i e n t o rganic matter f o r a sequence of d e t e r m i n a t i o n s are e x t e n s i v e l y i n t r u d e d by s i l l s (Creaney 1979). Furthermore, the depth of b u r i a l as a f u n c t i o n of time i s somewhat i n d e f i n i t e , owing to the complex s t r u c t u r a l h i s t o r y of the area. I t i s u n l i k e l y that much improvement can be made i n the T r a n q u i l l e v i t r i n i t e data set i n the f u t u r e . A few o b s e r v a t i o n s can be made regarding the d i s t r i b u t i o n of background r e f l e c t a n c e v a l u e s i n southern B r i t i s h Columbia (see F i g . 42). The T r a n q u i l l e v a l u e s , although they are widely s c a t t e r e d , c l u s t e r about a mean value of 0.62%. T h i s value i s only s l i g h t l y lower than those from Tulameen (average = 0.67%) and M e r r i t t (about 0.73%). Hat Creek values are s i g n i f i c a n t l y Table 7 . V i t r i n i t e Reflectance of Kamloops Group and cor r e l a t i v e sedimentary successions, southern B r i t i s h Columbia.  Sample Location Unit Co H ector /Analyst KAMLOOPS -TRANQUILLE: 6-7 15-1 34- 1 Afton 394/78 388/78 395/78 383/78 384/78 357/78 355/78 364 / 78 362/78 363/78 359/78 366/78 367/78 KERRITT: 35- 3 N e r r i t t TULAMEEN: Red Point, on CNR •Mara H i l l , i n mudflow Guerin Creek Afton Mine GG1, 70.4 m GG1, 72.9 m GG1, 162.8 m GG1, 166.0 m: GG1, 166.4 m GG6, 593.8 m GG6, 596.5 m Afton Mine Coal Gully seam outcrop North end South end average HAT CREEK: (?74, 1426' #75-73, 671' surface 037, 1535' (775-35, 484' f?75-57, 1337 1#75-73, 422' surface Ttm Tdm Ttb Ttb Ttb 0.69 0.25 0.4 0.71 0.62 0.55 0.46 0.54 0.55 0.62 0.62 0.85 0.61 0.61 0.92 0.77 0.73 TColdwater 0.76 0.71 TAllenby THat 50' 43 6 50 0.62-0.65 0.86 0.67 Creek 0.46 0.41 0.37 0.47 0.39 0.49 0.47 0.42-0.43 Ewing / Bust i n , UBC Ewing / Bustin, UBC Ewing / Bustin, UBC / Pearson, BCMM Creaney / Creaney, GSC / Ewing / Bustin, UBC HacMillan / Pearson, BCMM Williams S Ro Ross Church Church / Bustin / Bustin / Bustin / Bustin / Bustin / Bustin / Bustin / Bustin R n o i I I Sources:- Ewing/Bustin = present study; Williams & Ross = Williams and Ross (1979); Creaney / Creaney = Creaney (1979); Church / Bustin - mentioned i n Church and others (1979); /Pearson and /Bustin = personal communications, unpublished data. M q x . T (instantaneous), 150*200° 250° 300e 35,0 • Constant T for 50 m.y. 40°60° 80° 100° l. I i » • i i_ 120° _l Tulameen Hat Creek Merr itt .00 1 1 ll ,l / r> c r ill Tranquille 1.4 AO .60 .80 1.0 1.2 Vc Ro F i g . 42. Histograms of v i t r i n i t e r e f l e c t a n c e values from Table 7, by area. Temperature s c a l e s at the top are v i s u a l l y estimated from graphs i n B o s t i c k (1973). lower (average = 0.44%).. T h i s may be due e i t h e r to (a) a low thermal g r a d i e n t i n the Hat Creek area s i n c e the Eocene, or (b) a l a t e r time of d e p o s i t i o n , as proposed by G.E. Rouse. The u n c e r t a i n a f f i n i t i e s of the Hat Creek u n i t s have been d i s c u s s e d in Chapter 11. Two temperature s c a l e s are shown in F i g . 42; one assuming a constant temperature s i n c e 50 Ma, and the other an 'instantaneous' thermal event (both v i s u a l l y estimated from graphs i n B o s t i c k 1973) . For the average Kamloops sample, no temperature higher than about 220°C c o u l d have been experienced, judged from the l a t t e r s c a l e . To use the former s c a l e , more i n f o r m a t i o n i s r e q u i r e d . The p o s i t i o n of the T r a n q u i l l e samples ( l a r g e l y from the Afton Mine area) r e l a t i v e to the sub-Miocene e r o s i o n s u r f a c e i s approximately 0.76 km. I f t h i s depth had been maintained s i n c e the Eocene, and the present-day geotherm as measured i n the Eocene v o l c a n i c s at White Lake (34.7°C/km) i s assumed, then a r e f l e c t a n c e of about 0.53% would r e s u l t . Conversely, using: the observed r e f l e c t a n c e and the White Lake geotherm, the average b u r i a l depth s i n c e 50 Ma would be 1.5 to 1.6 km (seer. F i g . 43). The known t h i c k n e s s of the Kamloops Group which c o u l d have o v e r l a i n the T r a n q u i l l e samples i s about 1.0-1.2 km. T h e r e f o r e , f o r any reasonable c o n f i g u r a t i o n of the A f t o n / T r a n q u i l l e time-depth c u r v e ( F i g . 43), the average 1.5-1.6 kil o m e t r e depth cannot be achieved. I t i s t h e r e f o r e concluded that Eocene geothermal g r a d i e n t s were s i g n i f i c a n t l y higher at Kamloops than t h e v a l u e s recorded at present at White Lake.. T h i s l a t t e r g r a d i e n t , approximately the Basin and Range geotherm, i s i t s e l f Time (Ma) 2.5J F i g . 43. Time-depth curve f o r T r a n q u i l l e Formation samples. (a) shallow b u r i a l case; T r a n q u i l l e sediments not i n T r a n q u i l l e Canyon graben. (b) deep b u r i a l case; f u l l t h i c k n e s s of Kamloops Group o v e r l i e s T r a n q u i l l e samples. H o r i z o n t a l l i n e i s the constant depth needed to produce the observed r e f l e c t a n c e v a l u e s, using the White Lake geotherm of Jessop and Judge (1971). anomalously high f o r c o n t i n e n t a l areas. Eocene values at T r a n q u i l l e were s u b s t a n t i a l l y higher than t h i s . T h i s i s not s u r p r i s i n g f o r an area of a c t i v e arc volcanism (Chapter I I I ) . A s i m i l a r a n a l y s i s may a l s o h o l d f o r M e r r i t t , when the data become a v a i l a b l e . Hat Creek escaped t h i s high geothermal g r a d i e n t ; indeed, these values do not even r e f l e c t the post-Miocene u p l i f t . T h i s strengthens the case f o r l a t e r d e p o s i t i o n of the Hat Creek sediments. The A l l e n b y Formation at Tulameen shows the e f f e c t s of high thermal g r a d i e n t s , d e s p i t e i t s p o s i t i o n high i n the s t r a t i g r a p h i c s u c c e s s i o n (above the main v o l c a n i c sequence) and westward of the Eocene arc a x i s . Miocene thermal metamorphism by o v e r l y i n g flows may be i n d i c a t e d . APPENDIX 4. ROAD LOG, KAMLOOPS / TRANQUILLE FIELD TRIP D i s t (km) 0.0 S i x m i l e Point overlook on north of Trans-Canada Highway, j u s t east of roadcuts i n N i c o l a Group v o l c a n i c s . View N, NE, E over Kamloops Lake ( F i g . 44). The NW-SE t r e n d i n g p o r t i o n of the lake below us i s c o n t r o l l e d by the Cherry Creek f a u l t zone. The f a u l t c o ntinues SE up Cherry Creek v a l l e y as a 1-2 km wide f a u l t zone, f o l l o w e d by the Highway. The type area of the Kamloops Group l i e s northeast of t h i s f a u l t . To the N i s the i n f e r r e d Hardie H i l l s t r a t o c o n e ; to the NE i s the Doherty Creek s t r a t o c o n e , exposed on the f o r e s t e d b l u f f s . I t s vents are l o c a t e d i n the e r o s i o n a l basins carved i n t o the b l u f f s by Doherty Creek. Dips west of the vent are unif o r m l y W, with minor ch a n n e l i n g . The v o l c a n i c rocks are l a r g e l y a n d e s i t e flows, a n d e s i t e to d a c i e t t u f f s , and a n d e s i t i c b r e c c i a s . Rosseau Mountain i n the foreground to the NE i s u n d e r l a i n by b r e c c i a s which may have formed a s a t e l l i t e cone. A l a r g e p o s t g l a c i a l s l i d e mass of t h i s b r e c c i a forms a h i l l y area on the shore of the l a k e . At lake l e v e l are very good exposures of the T r a n q u i l l e Formation, here forming a west-dipping homocline. The sequence begins at the E as dominantly ' lakebed sediments c o n t a i n i n g a few o l i s t o s t r o m e s of N i c o l a Group m a t e r i a l . Upwards i t i s r a p i d l y dominated F i g . 44. View northeast from S i x m i l e Point overlook on the Trans-Canada H i g-hway we s t of Kami oops. West f l a n k of Doherty" Creek cone. F i g . 45. View showing east f l a n k of nor t h from Cherry B l u f f s E s t a t e s road, the Doherty Creek cone. by h y a l o c l a s t i c . t u f f s , i s i n t e r r u p t e d by a b a s a l t flow and v o l c a n i c b r e c c i a s . Much of the middle T r a n q u i l l e i s i n v o l v e d i n l a r g e - s c a l e soft-sediment slumping; t h i s u n i t forms the r e s i s t a n t c l i f f i n the c e n t r e of the sequence. East of the T r a n q u i l l e exposures, and i n f a u l t c o n t a c t , are N i c o l a Group v o l c a n i c s and T r i a s s i c -J u r a s s i c p l u t o n i c rocks of the Cherry B l u f f s p l u t o n . T h i s m a t e r i a l forms the lower b l u f f s on both s i d e s of Kamloops Lake, i n c l u d i n g B a t t l e B l u f f N of the l a k e , and Cherry B l u f f s S of the l a k e . To the SE are cuestas b u i l t on E - d i p p i n g lower T r a n q u i l l e Formation t u f f a c e o u s sediment, and B a t t l e B l u f f s i l l s . D r i v e E and SE along the Trans-Canada Highway, onto g l a c i a l lakebeds. 7.5 G a g l i a r d i Road to r i g h t ( S ) . Continue ahead. Cross Cherry Creek. The h i l l to l e f t (N) i s N i c o l a Group v o l c a n i c rock. The highway now continues SE along the Cherry Creek f a u l t zone. The l a r g e h i l l to the north i s Roper H i l l , u n d e r l a i n by the Cherry B l u f f s p l u t o n . The Glen Iron mine was l o c a t e d to the north; i t s u p p l i e d magnetite ore i n the l a t e 19th century. 11.7 Turn l e f t i n t o Cherry B l u f f s E s t a t e s . Continue up the paved e n t r y road, which f o l l o w s the b a s a l contact of the T e r t i a r y , here formed by coarse i n t r u s i v e diabase to the r i g h t . 12.5 View to l e f t (N) at bend i n the road; Red P l a t e a u north of Kamloops Lake, showing the east f l a n k 1 of the Doherty Creek cone ( F i g . 45). We are now on r o l l i n g benchland at about 2000' e l e v a t i o n ; B a t t l e B l u f f to the NNE i s north of the l a k e . The low h i l l s d i r e c t l y to the north i n the foreground are T e r t i a r y diabase of the b a s a l B a t t l e B l u f f s i l l , surrounded by Cherry B l u f f s d i o r i t e and monzonite. To the south, the h i l l i s composed of diabase with l a b r a d o r i t e c r y s t a l s to 3 cm. 12.9 Road j u n c t i o n at the f i r s t a l k a l i l a k e ; continue l e f t on g r a v e l . Ahead i s a columnar s i l l i n the b a s a l s i l l complex. 13.4 S a l soda Pond; Stop and turn-. To the N and W, T r i a s s i c -J u r a s s i c Cherry B l u f f s : p l u t o n outcrops are. j o i n t e d monzonite and d i o r i t e , forming rounded knobs. ; To the south, the columns mark a w e l l d e f i n e d s i l l in the b a s a l diabase complex. To the southeast i s a twin cuesta formed by the middle and upper s i l l s of the B a t t l e B l u f f complex i n t h i s a r e a. Behind to the E are grassy slopes of cuestas formed by r e s i s t a n t t u f f a c e o u s sediments i n the lower T r a n q u i l l e s u c c e s s i o n . A l k a l i l akes are frequent i n the Kamloops area. Salsoda 'Lake was mined from 1931 to 1935 f o r sodium carbonate; the remains of sheds and mining s c a r s can s t i l l be seen. Up the v a l l e y to the E i s Saltwort Pond, c o n t a i n i n g 100,000 to 200,000 T of sodium sulphate ( C o c k f i e l d , 1948, p.146). Return to road j u n c t i o n at f i r s t a l k a l i lake 13.9 Turn l e f t (S). on paved road. 14.2 Low outcrops of coarse diabase to the l e f t ( E ) ; b l u f f s ahead are the middle and upper s i l l s . To the r i g h t (SW), the t r e e l e s s h i l l i s T e r t i a r y w i t h i n the Cherry Creek f a u l t zone. Greenstone Mountain ( N i c o l a Group v o l c a n i c s ) on the s k y l i n e . 14.65 16.2 J u n c t i o n ; bear r i g h t on broad g r a v e l road. Basal s i l l crops :• out. Stop . B a s a l T r a n q u i l l e lakebeds exposed i n roadcut below the middle B a t t l e B l u f f s i l l . The exposures are t y p i c a l of the lower T r a n q u i l l e Formation. Here a 37-sequence i s sandwiched between sequence i s deformed by two cm coarsening-upward muds and s i l t s . The a n t i t h e t i c reverse f a u l t s with about 10-20 cm displacement. V i t r i n i t e from t h i s l o c a l i t y g i v e s a value of 1.10% R , which i s high f o r the area; the c o a l rank has probably been r a i s e d by the a d j o i n i n g s i l l s . Above the roadcut, the middle s i l l can be examined; the co n t a c t with the sediments i s obscured by t a l u s . To the west i s a l i m i t e d view of the Cherry Creek f a u l t zone. Turn; r e t u r n to road j u n c t i o n at a l k a l i lake (18.55), bear l e f t and r e t u r n to Trans-Canada Highway. 19.75 Turn l e f t (S) onto Trans-Canada Highway (CAUTION), and d r i v e southeast along the Cherry Creek f a u l t zone. 20.3 Ahead are knobs, furrowed by strands of the Cherry Creek f a u l t zone. The l e f t knob i s the T e r t i a r y i n t r u s i v e seen p r e v i o u s l y ; i t i s p e r v a s i v e l y sheared and cut by c r o s s - f a u l t s . To i t s r i g h t are N i c o l a Group v o l c a n i c s . F u r t h e r strands are i n d i c a t e d by topography to continue 0.8 km southwest. 21.75 To l e f t ( E), a view of the h i l l s along the E s i d e of the f a u l t zone; h i l l s of N i c o l a Group v o l c a n i c s i n the foreground and i n the f a u l t zone. High diabase cuestas l i e behind. 22.25 J u n c t i o n with Beaton Lake road. Roadcuts are i n Mesozoic v o l c a n i c s and sediments f o r the next 0.3 km. Note on l e f t , N i c o l a v o l c a n i c s t h r u s t over a red-brown conglomerate. T h i s conglomerate i s s i m i l a r to ones i d e n t i f i e d along the f a u l t f o r s e v e r a l m i l e s (Carr and Reed 1976); s i m i l a r rocks a l s o occur northwest of Kamloops. Thrust motion i s c o n s i s t e n t with r i g h t - l a t e r a l motion on the Cherry Creek f a u l t zone. 24.0 J u n c t i o n with Dominic Lake / Paska Lake road to r i g h t ( S ) . Quaternary g r a v e l s exposed in roadcuts through canyon; N i c o l a v o l c a n i c s and i n t r u s i v e s a c r o s s the creek. 25.4 Approximate j u n c t i o n of the Afton s t r u c t u r a l zone with the Cherry Creek f a u l t zone. The Cherry Creek zone continues to the SE, l a r g e l y d r i f t - c o v e r e d , forming the approximate SW boundary of the Iron Mask b a t h o l i t h . To the north, note the E - d i p p i n g panel of diabase s i l l s , lower T r a n q u i l l e sediments and t u f f s forming c u e s t a s . To the northeast i s a SE-dipping cuesta of l i g h t pink-grey a n d e s i t e , forming the high edge of the Afton p a n e l . 26.1 Roadcuts of T e r t i a r y b a s a l t i c a n d e s i t e . I t i s not c o r r e l a t a b l e with the T e r t i a r y s t r a t i g r a p h y to the N, and i s probably an i n t r u s i v e w i t h i n the Afton and Cherry Creek zones. 27.4 J u n c t i o n , A f t o n p i p e l i n e road to l e f t (N). To r i g h t (S), view of Afton Mines; c o n c e n t r a t o r and smelter to SE are b u i l t on Iron Mask i n t r u s i v e r o c k s . The P i p e l i n e road roughly f o l l o w s the f a u l t s e p a r a t i n g the e a s t - d i p p i n g panel we have seen from a n o r t h - d i p p i n g panel of lower and middle T r a n q u i l l e Formation, v i s i b l e to the n o r t h e a s t , which l a c k s diabase i n t r u s i o n s . 28.2 Turnout on l e f t (N); Stop (CAUTION). From here i s a view of the n o r t h - d i p p i n g cuesta of the lower T r a n q u i l l e Formation to the N. The base of the S-f a c i n g slope i s c l o s e to the b a s a l unconformity of the T r a n q u i l l e Formation. We have been f o l l o w i n g the Afton s t r u c t u r a l zone, named a f t e r the best exposures of i t i n the Afton Mines open p i t immediately to the S. T h i s zone forms the S boundary of the T r a n q u i l l e b a s i n . I t i s a complex of r e v e r s e and normal f a u l t s , i n t e r p r e t e d by Carr and Reed (1976; Reed, pe r s . comm.) as a r e v e r s e -f a u l t e d b a s i n margin with e x t e n s i v e slumpint of N i c o l a and Iron Mask rocks i n t o the T r a n q u i l l e sediments. On the N w a l l of the Afton p i t a d e l t a i c f a c i e s of the T r a n q u i l l e Formation i s exposed. I t i s a l s o seen in d r i l l core nearby (Graham and Long 1979) and i n outcrop on the h i l l to the N. T h i s f a c i e s c o n s i s t s of l i t h i c - a r k o s i c wackes, black laminated shales and small pods of c o a l ; graded bedding, t u f f c l a s t s and slump f e a t u r e s were noted by Graham and Long. T h i s d e l t a i c sequence has been in t r u d e d by s i l l s (Creaney 1979) and l o c a l l y f o l d e d (Reed, p e r s . comm.), i n response e i t h e r to reverse f a u l t i n g or to l a r g e slump masses. The d e l t a i c sequence appears to i n t e r f i n g e r r a p i d l y northward i n t o l a c u s t r i n e and t u f f a c e o u s m a t e r i a l . 29.8 Afton Mine t u r n o f f to r i g h t (S). H i l l s on r i g h t are p a r t of the Iron Mask B a t h o l i t h . The view N ( l e f t ) shows the n o r t h - d i p p i n g panel w e l l ; the cuesta of the middle T r a n q u i l l e i s v i s i b l e . The middle T r a n q u i l l e t u f f s form an unstable oversteepened d i p slope north i n t o the Kamloops v a l l e y ; three l a r g e l a n d s l i d e s have been mapped in t h i s a r e a . The Afton s t r u c t u r a l zone continues NE, then turns E. The road leaves the zone and c r o s s e s higher ground u n d e r l a i n by the Iron Mask B a t h o l i t h . 31.3 Kamloops c i t y l i m i t s . 31.75 Polygon Pond, an a l k a l i lake c o n t a i n i n g magnesium and sodium sulphate, to the r i g h t ( S ) . Note the s a l t polygons. 32.55 Roadcut i n Iron Mask rocks to the l e f t (N). Ahead i s Iron Mask H i l l ; the dumps of the o l d Iron Mask copper mine are v i s i b l e . Copper was f i r s t l o c a t e d i n t h i s area i n 1896, touching o f f the Kamloops copper boom. The Iron Mask, one of the l a r g e s t and l o n g e s t - i i v e d o p e r a t i o n s , was worked from 1901 to 1928, producing over 5,000,000 l b Cu, 3600 oz. Au, and 41,000 Ag ( C o c k f i e l d 1948, p.147). 33.0 Iron Mask Lake to r i g h t (S), a sodium sulphate l a k e . Old foundations of the Iron Mask m i l l can be seen. 34.8 J u n c t i o n , Lac Le Jeune road to r i g h t (S); 1.9 km up the road i s an e x c e l l e n t View of the T r a n q u i l l e area. At the j u n c t i o n : the h i l l s to the r i g h t (S) are of Iron Mask rocks. Ahead (E) at 11:00 i s Mount D u f f e r i n , with r a d i o towers. Mount D u f f e r i n i s a NE-dipping cuesta on p h r e a t i c b r e c c i a s which are o v e r l a i n by a n d e s i t e flows of the Mara H i l l member, Dewdrop F l a t s Formation. T h i s cuesta has a long d i p slope to the NE which overhangs the Weyerhaeuser pulp m i l l at M i s s i o n F l a t s . F o r t u n a t e l y , t h i s slope appears to be r e l a t i v e l y s t a b l e i n the absence of c l a y - r i c h i n t e r l a y e r s . 35.9 Highway c r o s s e s Coal H i l l f a u l t . 37.0 Low h i l l ahead (E) i s N i c o l a Group a n d e s i t e , l y i n g S of the main Afton zone. To the R are low roadcuts through the d r i f t i n t o N i c o l a v o l c a n i c s , here red-green p l a g i o c l a s e - p o r p h y r y a n d e s i t e flows and mudflows. Copper showings have been noted by Gordon White, B.C.M.M. 37.7 Roadcut to l e f t (NE), i n e p i d o t e - r i c h N i c o l a Group v o l c a n i c s . 38.75 Cross Guerin Creek; W entrance of Aberdeen H i l l s s u b d i v i s i o n ; Coal H i l l to S. About halfway up the h i l l , c o a l was d i s c o v e r e d i n the 1880s by Joseph Guerin, a l o c a l rancher. E x p l o r a t o r y s h a f t s were sunk by two f i r m s , and G.M. Dawson r e p o r t e d on the prospect, but no s i g n i f i c a n t tonnage was produced. The : area appears to be a : fault-bounded o u t l i e r of T r a n q u i l l e Formation sediments and v o l c a n i c s east of the Coal H i l l f a u l t . Recent t r e n c h i n g lower down re v e a l s complex s t r u c t u r e s i n v o l v i n g T e r t i a r y and T r i a s s i c s t r a t a , s i m i l a r i n s t y l e to the r e l a t i o n s at Afton Mine. From here i n t o Kamloops, N i c o l a v o l c a n i c s are exposed near the highway. 39.45 J u n c t i o n , B.C. Hwy. 5 on Continue ahead. r i g h t (S), to M e r r i t t . : 40.: Approximate c r o s s i n g of D u f f e r i n f a u l t , the eastern l i m i t of the T e r t i a r y S of the Thompson R i v e r . N i c o l a v o l c a n i c s and sediments to the E f o r s e v e r a l m i l e s . 40.8 J u n c t i o n with bypass; take business route to l e f t (N). 41.9 M c G i l l Road c r o s s i n g ; Cariboo C o l l e g e to W, S a h a l i Centre M a l l to E. 42.3 Turn l e f t (CAUTION) i n t o Overlanders Overlook; Stop . View to N and W. To the W, the f i r s t rock r i d g e i s N i c o l a v o l c a n i c rock. Behind i t i s Mount D u f f e r i n , showing a NE d i p s l o p e on Mara H i l l a n d e s i t e flows. To i t s S i s a r i d g e of i n t r u s i v e m a t e r i a l w i t h i n the Afton s t r u c t u r e zone, then Coal H i l l , whose summit i s i n the Iron Mask B a t h o l i t h . To the NW> Mara H i l l i s the f i r s t f o r e s t e d h i l l behind the Thompson V a l l e y ; Wheeler Mountain and Opax H i l l l i e behind. Far to the NW i s the T r a n q u i l l e P l a t e a u , formed by Miocene p l a t e a u b a s a l t s which unconformably o v e r l i e the Eocene s u c c e s s i o n . To the NNW i s the B a t c h e l o r Range, which i s mostly u n d e r l a i n by a r g i l l i t e s of probably T r i a s s i c age. There ar patches of reddened, w e l l - i n d u r a t e d conglomerate probably c o r r e l a t i v e with s i m i l a r , pre-Eocene m a t e r i a l i n the Afton p i t and along th Cherry Creek f a u l t zone. To the N i s the North Thompson R i v e r v a l l e y . I t may be c o n t r o l l e d by a major throughgoing f a u l t , but evidence i s c u r r e n t l y l a c k i n g . To the NE, Mount Paul i s an Iron Mask-like i n t r u s i v e i n t o T r i a s s i c a r g i l l i t e s ; these are separated by a NW-SE through f a u l t (Smith 1978) from upper P a l e o z o i c a r g i l l i t e s and limestones (Danner 1978). Mounts L o l o and F l e e t on the s k y l i n e are g r a n i t i c i n t r u s i o n s i n t o the P a l e o z o i c m a t e r i a l . Return to main highway, turn l e f t (CAUTION) towards downtown. 43.9 J u n c t i o n with T h i r d Ave; turn l e f t (N) at l i g h t . 44.2 J u n c t i o n with V i c t o r i a St.; turn l e f t (W) on V i c t o r i a , through c e n t r a l business d i s t r i c t . 45.35 J u n c t i o n with M i s s i o n F l a t s Road; bear . - l e f t on Overlander Bridge approach. S h o t c r e t e d roadcuts are i n T r i a s s i c ( ? ) a r g i l l i t e s . The bridge c r o s s e s the Thompson River immediately below the j u n c t i o n of the North and South Thompson R i v e r s . The c o l o u r c o n t r a s t between the r i v e r s i s due to the sediment-trap a c t i o n of Shuswap Lake. The d i f f e r e n c e i n sediment l o a d p e r s i s t s to the head of Kamloops Lake to the west, and i s r e s p o n s i b l e f o r the asymmetric d e l t a t h e r e . 46.0 Turnoff to North Kamloops business d i s t r i c t ; continue ahead. 46.8 Fortune J u n c t i o n . Continue ahead of T r a n q u i l l e Road. 47.6 B r o c k l e h u r s t shopping c e n t r e . View to S i s of Mt. D u f f e r i n ; note the t h i c k a n d e s i t e s l a b above the pulp m i l l . 50.6 A i r p o r t t u r n o f f to l e f t from T r a n q u i l l e Road; B r i t i s h Columbia M i n i s t r y of Energy, Mines and Petroleum Resources o f f i c e s . Stop b r i e f l y . From t h i s area are wide View s to S and W. To the SE, one looks down the D u f f e r i n f a u l t , with Kamloops Group v o l c a n i c s downfaulted a g a i n s t N i c o l a v o l c a n i c s . To the S, note s t r a n d l i n e s of the g l a c i a l Kamloops Lake. There are sparse outcrops of T r a n q u i l l e lakebeds and t u f f s , cut by p e r v a s i v e NW-SE shears on l i n e with the Coal H i l l and Dewdrop Creek f a u l t s . To the SW, the N-dipping panel NE of Afton can be seen. An o l d , f o r e s t e d l a n d s l i d e i s v i s i b l e high on the h i l l s l o p e . An unvegetated s l i d e extends to r i v e r l e v e l f a r t h e r to the W. S t i l l f a r t h e r W, the d i p s l o p e of the E- d i p p i n g panel can be seen. On both s i d e s of the Kamloops Lake v a l l e y are the rugged b l u f f s of Cherry B l u f f s i n t r u s i v e rock. The N-di p p i n g cuestas N of B a t t l e B l u f f are diabase s i l l s . To the NW i s Mara H i l l , with badlands cut i n t o upper T r a n q u i l l e and lower Dewdrop F l a t s Formations. The c r e s t of the h i l l i s i n the Mara H i l l member; to the r i g h t , i t i s i n a f l o w - b r e c c i a f a c i e s , to the l e f t i n i n t e r l a y e r e d flows and flow-top b r e c c i a s c h a r a c t e r i s t i c elsewhere. To the N, the small gulch marks the E boundary of the T e r t i a r y . To i t s E are n e a r l y h o r i z o n t a l flows and t u f f s of the N i c o l a Group, with a r g i l l i t e s to the NE. 52.75 Bear l e f t (W) on T r a n q u i l l e Road, around r e f i n e r y . 54.2 H i l l s to NW are i n upper T r a n q u i l l e s t r a t a , l a r g e l y concealed. 55.8 Turn r i g h t on road c r o s s i n g CN t r a c k s . Immediately turn l e f t through gate (please c l o s e ) onto one-lane road to small g r a v e l p i t . D r i v e past s t e e p l y S-dipping SE-t r e n d i n g d a c i t e dyke. 56.2 Stop in g r a v e l p i t , south foot of Mara H i l l . Park for a h i l l climb of about 500'. The best climb from here i s to the NW, s i d e h i l l i n g a cross a small a l l u v i a l fan to an ochre patch ( p a l a g o n i t i c ash of the T r a n q u i l l e Formation). Climb the low r i d g e u n t i l past the treacherous dip-slOpe of the dyke. Veer r i g h t i n t o small draw to view p i l l o w e d a n d e s i t e s of the middle T r a n q u i l l e . P i l l o w s are i n d i s t i n c t here, but c h i l l e d margins can be t r a c e d . Climb out through the draw, bearing l e f t to a View atop a bench cut i n upper T r a n q u i l l e h y a l o c l a s t i t e s , showing c h a r a c t e r i s t i c p a l a g o n i t i c a l t e r a t i o n to ochre Fe-Mg s m e c t i t e . M u l t i t u d e s of dykes cut t h i s u n i t to the E and N, o f t e n bordered by c h a l c e d o n y - c a l c i t e -z e o l i t e v e i n s . To., the W i s a s p e c t a c u l a r view ( F i g . 3, i n p a r t ) . The c l i f f s are developed i n K i s s i c k f l o w - b r e c c i a , cut by a composite, c o l u m n a r - j o i n t e d dyke complex. Note the l a y e r i n g i n the white-grey b r e c c i a s ; they i n d i c a t e : a primary d i p of about 30° to the south. The b r e c c i a c l a s t s are m o n o l i t h o l o g i c a n d e s i t e s , with inconspicuous p l a g i o c l a s e phenocrysts. P h r e a t i c i n f l u e n d e i n c r e a s e s to the N and E, marked by ochre p a l a g o n i t i c a l t e r a t i o n and i n c r e a s i n g g r a n u l a t i o n of the matrix. The e n t i r e K i s s i c k : e d i f i c e i s b u i l t conformably on the h y a l o c l a s t i t e mudflows of the upper T r a n q u i l l e with no e r o s i o n a l .break. The l i t h o l o g y of the h y a l o c l a s t i t e s can be examined here. A short t r i p u p h i l l leads to the base of the K i s s i c k b r e c c i a s . Return to v e h i c l e s by about the same route. Return to road. 56.6 Cross CN t r a c k s , turn r i g h t (W) on ^ T r a n q u i l l e Road. 57.4 to r i g h t (N), e x c e l l e n t View of badlands on the south face of Mara H i l l . Upper T r a n q u i l l e h y a l o c l a s t i t e s with l o c a l interbedded t r a c t i o n sediments form s p e c t a c u l a r ochre outcrops, o v e r l a i n by grey K i s s i c k b r e c c i a . The dark s u r f a c e s are faces of composite dykes. To the NW i s the 'Ribrock' i n t r u s i o n , of f i n e -g r a i n e d a n d e s i t e ; the road passes i t f o r the next m i l e . Note the conspicuous r i b s ; they are dish-shaped F i g . 46. View west from bench south of Mara H i l l , above T r a n q u i l l e road. s t r u c t u r e s , formed by c l a y - a l t e r e d v s . Carbonate-a l t e r e d a n d e s i t e . ' These f e a t u r e s are p e r p e n d i c u l a r to c o o l i n g j o i n t s , which fan out from f o c i on the s u r f a c e of the dome-shaped, intrusion... They appear to r e s u l t from rhythmic crack propagation and a s s o c i a t e d a l t e r a t i o n during c o o l i n g . S i m i l a r e f f e c t s are seen on other i n t r u s i o n s i n the ar e a . 58.7 Turnout on l e f t ; Stop ( o p t i o n a l ) . Above here, two l a r g e and many small s i l l s i n t r u d e the upper T r a n q u i l l e ; some are exposed in roadcuts to %he E and W. In the draws, one can see upper and lower c h i l l e d margins and complex, t e r m i n i of v a r i o u s s i l l s . Around the bend to W are r a i l r o a d cut? . i n p i l l o w e d a n d e s i t e and p i l l o w brecc i a . To the south i s the Thompson River d e l t a . Across the v a l l e y i s a recent l o w - e l e v a t i o n l a n d s l i d e which reaches the Canadian P a c i f i c r i g h t - o f - w a y . The l a r g e : c l i f f s at lake l e v e l to the SW are i n t r u s i v e diabase : o v e r l a i n by i s o c l i n a l l y - f o l d e d , w e l l - i n d u r a t e d slumped sediments, showing northeastward t r a n s p o r t . To the NE i s a view of the Ribrock i n t r u s i o n , which i s r e l a t e d to the s i l l s . 59.2 NNW t r e n d i n g a n d e s i t e - d a c i t e dyke i s exposed to N. 59.3 J u n c t i o n ; turn r i g h t on Red Lake Road ( g r a v e l ) ; T r a n q u i l l e Road conti n u e s l e f t to T r a n q u i l l e School, b u i l t on the p o s t g l a c i a l d e l t a of the T r a n q u i l l e R i v e r . We w i l l be s k i r t i n g the landward edge of t h i s d e l t a . j 59.75 Pruden Pass Road to r i g h t (N); c ontinue ahead. 60.0 Low a n d e s i t e outcrops, probably i n t r u s i v e i n t o the T r a n q u i l l e Formation. 60.3 M i n i s t r y of Highways g r a v e l p i t to r i g h t (N), i n t e r r a c e g r a v e l s which f i l l the p r e g l a c i a l channel of the T r a n q u i l l e R i v e r . Note the l a r g e - s c a l e c r o s s bedding. Ahead, ochre s o i l s mark the h y a l o c l a s t i t e s of the upper T r a n q u i l l e Formation; K i s s i c k b r e c c i a tops the b l u f f beyond. 60.8 Crossroads with T r a n q u i l l e Farm road. To the r i g h t (N), a locked gate at 0.3 km. A 15-minute walk up the creek leads to a very good outcrop of p i l l o w e d a n d e s i t e with interbedded sediments (Note: t h i s i s T r a n q u i l l e School p r o p e r t y ) . 61.2 c r o s s T r a n q u i l l e R i v e r . 61.4 View S, W, NW. To W, B a t t l e B l u f f s i l l s are w e l l exposed j u s t north and east of B a t t l e B l u f f . 62.0 At bend, p h r e a t i c b r e c c i a and mudflows of the N i p p l e member o v e r l i e upper T r a n q u i l l e mudflows. They i n t e r f i n g e r here and to the N. 62.55 Stop ( o p t i o n a l ) at roadcut of a t y p i c a l phenocryst-poor N i p p l e b r e c c i a . From base to top a r e : B a s a l t i c a n d e s i t e aphanite, f r a c t u r e d , with Qz-Cc vei n s b l o c ks of b a s a l t i c a n d e s i t e in t u f f a c e o u s matrix; p h r e a t i c b r e c c i a ; b l o c k s to 5 metres b r e c c i a , becoming more t u f f a c e o u s upward crudely-bedded p h r e a t i c b r e c c i a , fragments to 8 cm, bands of aquagene t u f f massive grey a n d e s i t e , mafic phenocrysts, c l a y -carbonate a l t e r a t i o n , r i b b e d ; i n t r u s i v e ? The h i l l cap i s K i s s i c k f l o w - b r e c c i a . To the south, note the panels of T r a n q u i l l e and lower Dewdrop F l a t s sediments and v o l c a n i c s a c r o s s the v a l l e y . Immediately to the W, the v a l l e y of Dewdrop Creek separates the b r e c c i a s exposed at t h i s cut from diabase s i l l s of the B a t t l e B l u f f i n t r u s i o n s and lower T r a n q u i l l e l a c u s t r i n e sediments. The t r a c e of the Dewdrop Creek F a u l t can be t r a c e d southeast and i s a l i g n e d with the Coal H i l l F a u l t . 63.0 Low outcrops of N i p p l e b r e c c i a . B a t t l e B l u f f s i l l s to l e f t (W). 63.8 Bend in road; take Dewdrop F l a t s Road s t r a i g h t ahead (W), over embankment. 64.5 Minor road to r i g h t (N); continue ahead. H i l l s to S are the d i p s l o p e of the B a t t l e B l u f f s i l l s , here d i p p i n g N about 10°. 65.3 Stop , at small road l e a d i n g l e f t ( S ) . We are on the lower slopes of a l a r g e a l l u v i a l fan which i s fed from the Red P l a t e a u b l u f f s to the N. To the NW i s the Doherty Creek s t r a t o c o n e . I t s c e n t r e i s a l a r g e e r o s i o n a l embayment i n the b l u f f s which forms te headwaters of Doherty Creek. To i t s west, west-dipping flows and b r e c c i a s continue f o r three km. On the east s i d e of the vent, the cone i s l a r g e l y composed of e a s t - d i p p i n g C a s t l e Butte b r e c c i a , which forms the c l i f f s to NW and N. T h i s b r e c c i a i s c o a r s e l y p l a g i o c l a s e - p h y r i c , l a r g e l y p h r e a t i c , with a s s o c i a t e d mudflows and t r a c t i o n sediments. These p h r e a t i c b r e c c i a s f l a r e out to the W, t h i c k e n i n g and c o a r s e n i n g towards the vent. Upwards they g r a d u a l l y l o s e t h e i r p h r e a t i c c h a r a c t e r , and are o v e r l a i n by s u b a e r i a l a n d e s i t e flows and flow-top b r e c c i a s of the Doherty Creek member. The Doherty Creek str a t o c o n e began as a p h r e a t i c vent; today i t i s marked by h y a l o c l a s t i c sediments and b r e c c i a s i n the Doherty Creek b a s i n . I t p r o g r e s s i v e l y b u i l t up and out, changing c h a r a c t e r i n t o a t u f f cone of p h r e a t i c b r e c c i a , then to a s u b a e r i a l composite cone dominated by a n d e s i t e and b a s a l t i c - a n d e s i t e flows and a s s o c i a t e d b r e c c i a s and mudflows. The cone s t r u c t u r e i s cut o f f to the NE by the Dewdrop Creek f a u l t . On the other s i d e are c l i f f s of f l a t - l y i n g a n d e s i t e flows and b r e c c i a s of the Red Pl a t e a u member. These probably c o r r e l a t e with the upper s u b a e r i a l flows of the Doherty Creek member. These flows form the red c l i f f s to the N and NNE. To the NE are coarse f l o w - b r e c c i a s which t h i n r a p i d l y t o the east; they are c o r r e l a t e d with the Wheeler b r e c c i a . The h i l l i s a p i e - s l i c e s e c t i o n of a b r e c c i a cone, the other s e c t i o n s of which have been eroded away or f a u l t e d out. Low h i l l s to the W are of T r i a s s i c N i c o l a v o l c a n i c s . A f a u l t of indeterminate magnitude may run beneath the a l l u v i a l Dewdrop F l a t s v a l l e y . To the S i n the foreground i s a small sodium sulphate l a k e . Behind the lake are h i l l s of B a t t l e B l u f f diabase, which o v e r l i e T r i a s s i c - J u r a s s i c i n t r u s i v e rocks of the Cherry B l u f f s p l u t o n ( a c c e s s i b l e from d i r t t r a c k l e a d i n g S from t h i s s t o p ) . Turn; r e t u r n to Red Lake Road. 66.75 Red Lake Road; turn l e f t , u p h i l l . For next 0.3 km, roadcuts are i n b a s a l t p h r e a t i c b r e c c i a which i s e i t h e r Nipple or upper Wheeler. 67.7 H i l l s on both s i d e s are K i s s i c k f l o w - b r e c c i a ( o p t i o n a l s t o p ) . Ahead i s view of Mara H i l l . A s c i s s o r s f a u l t d i s p l a c e s the capping a n d e s i t e flows, and i s r e f l e c t e d i n the geomorphology. 68.3 Road c r o s s e s bench on Mara H i l l a n d e s i t e . Good view to NW of the f l o w - b r e c c i a cone in Wheeler b r e c c i a . 68.8 Ahead i s the N i p p l e , composed of E - d i p p i n g o l i v i n e -b a s a l t p h r e a t i c b r e c c i a . 69.6 Turn r i g h t (E) to viewpoint. 69.8 Stop at viewpoint. View of lower T r a n q u i l l e Canyon, and a chance to examine the N i p p l e b r e c c i a . Ascend r i d g e to r i g h t (S) f o r best views and outcrops. Here, N i p p l e b r e c c i a i s w e l l exposed. Note the w e l l - p r e s e r v e d o l i v i n e and c l i n o p y r o x e n e phenocrysts in the c l a s t s , and the t y p i c a l ochre a l t e r a t i o n of the matrix caused by Fe-Mg sm e c t i t e . Steep v a l l e y w a r d d i p s may be primary. D o w n h i l l , the b r e c c i a i n t e r f i n g e r s with h y a l o c l a s t i c mudflows of the upper T r a n q u i l l e Formation. The source of t h i s b r e c c i a i s unknown; the l i t h o l o g y suggests a t u f f r i n g with a concealed source to the W. In the canyon, note the b u r i e d T r a n q u i l l e River canyon p a s s i n g east of the present gorge. The o l d watercourse i s completely f i l l e d by t e r r a c e g r a v e l s , which form conspicuous c l i f f s below. The lower T r a n q u i l l e Canyon has been e x t e n s i v e l y p l a c e r mined si n c e 1860; both g o l d and platinum are s t i l l found i n the upper p a r t s of the Canyon. On the h i l l s to the e a s t , from N to S: Massive grey c l i f f s to the N are Opax f l o w - b r e c c i a , the youngest e x t r u s i v e u n i t preserved here, marking the core of the T r a n q u i l l e Canyon graben. I t o v e r l i e s and i s f a u l t e d a g a i n s t red-brown Red. P l a t e a u flows and flow-top b r e c c i a s , exposed on O p a x . H i l l to the N. Opax b r e c c i a forms a t h i n capping of Wheeler Mountain to the NE; the bulk of the mountain i s Red P l a t e a u a n d e s i t e . On the west s i d e of Wheeler Mountain, n o r t h -d i p p i n g sheets of grey-brown: Wheeler b r e c c i a are u p f a u l t e d , and form l a r g e c l i f f s , d i p p i n g i n t o the canyon. Below them are red-brown a n d e s i t e s of the Mara H i l l member, which r i s e southward to the c r e s t of Mara H i l l to the ESE. Grey-brown K i s s i c k b r e c c i a u n d e r l i e s the Mara H i l l member, and forms c l i f f s i n the canyon : d i r e c t l y below. Note a l s o the e x t e n s i v e development of g r a v e l s at about 2500' to 3000' e l e v a t i o n mantling the shoulders of Wheeler Mountain, Opax H i l l and. Mara H i l l . Turn;' r e t u r n to Red Lake Road; 69.95 Turn r i g h t (N) on Red Lake Road, which now descends i n t o the canyon. ; H i l l s to W are i n p h r e a t i c b a s a l t b r e c c i a of the Wheeler b r e c c i a , which resembles the N i p p l e b r e c c i a we have j u s t examined. Road bends r i g h t ; roadcuts i n Wheeler b r e c c i a , here g l o s s y - b l a c k a n d e s i t e aphanite f l o w - b r e c c i a , with blocks from 3 to 7 cm, l i t t l e m a trix. Lookout to r i g h t ( E ) ; Stop ( o p t i o n a l ) . View of T r a n q u i l l e Canyon and W a t e r f a l l Creek to E, and chance to examine roadcuts of Wheeler b r e c c i a . To SE, note the Wheeler b r e c c i a c l i f f s descending to r i v e r l e v e l , then r i s i n g to viewpoint. Continue N on Red Lake Road. Roadcuts are now i n Red P l a t e a u a n d e s i t e flows and flow-top b r e c c i a s tor the next s e v e r a l k i l o m e t r e s . The road f o l l o w s a narrow bench through the canyon, formed in part by t e r r a c e g r a v e l s . The canyon i n t h i s s t r e t c h approximately f o l l o w s the west f a u l t of the T r a n q u i l l e Canyon graben c o r e . J u n c t i o n with d i r t road to r i g h t ( E ) ; turn on d i r t road. Bear l e f t at road j u n c t i o n at 0.15 km. Stop ; View of j u n c t i o n of T r a n q u i l l e R i v e r and Watching Creek. The road c o n t i n u e s to r i v e r l e v e l , but i s s u i t a b l e f o r jeeps o n l y . On road and i n the gorge below are good exposures of Red P l a t e a u a n d e s i t e flows. To the E i s a view of c l i f f s of Opax b r e c c i a o v e r l y i n g Red P l a t e a u flows. The same r e l a t i o n i s seen to the N. Below i s the j u n c t i o n of the c r e e k s . Gold and platinum values appear to be c o n f i n e d to Watching Creek above here. They may stem from the A l l i e s g old prospect on Cannel Creek,.a t r i b u t a r y to the north, or from ?Mesozoic u l t r a m a f i c s i n the upper C a n n e l l Creek area. These areas (see C o c k f i e l d 1948) mark the northern boundary of the T r a n q u i l l e Canyon graben complex. Turn; r e t u r n to Red Lake Road. Turn r i g h t (N) on Red Lake Road; roadcuts in Red P l a t e a u a n d e s i t e . Roadcuts in Red P l a t e a u a n d e s i t e flows, here with a patchy emerald-green c e l a d o n i t e a l t e r a t i o n , and chalcedony amygdules. B a i l e y bridge a c r o s s T r a n q u i l l e R i v e r ; B.C. F o r e s t S e r v i c e s i t e on l e f t (SW); Stop . Here are e x c e l l e n t water-washed exposures of flow and b r e c c i a u n i t s w i t h i n the Red P l a t e a u member. The Red Lake Road conti n u e s NW with poor to f a i r exposures f o r approximately 20 km to the settlement of Red Lake. Connections can be made to Copper Creek and Savona. Turn; r e t u r n to Kamloops. End of l o g . 

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