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Geology of the Apex Mountain group, north and east of the Similkameen River, south-central British Columbia Milford, John Calverley 1984

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GEOLOGY OF THE APEX MOUNTAIN GROUP, NORTH AND EAST OF THE SIMILKAMEEN RIVER, SOUTH-CENTRAL BRITISH COLUMBIA BY JOHN CALVERLEY MILFORD, B.Sc. UNIVERSITY OF CALIFORNIA, LOS ANGELES, 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES (Department of Ge o l o g i c a l Sciences) We accept t h i s thesis as conforming—to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA JULY 1984 (c) JOHN CALVERLEY MILFORD , 1984 86 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 fo r 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 t h e s i s f o r s c h o l a r l y 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 p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of &KD (-0&I S~Cy e ^ C - £ r S The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 -6 (3/81) ABSTRACT The Apex Mountain Group i s an east-dipping sequence of rocks that may represent part of an ancient subduction complex that formed by progressive eastward-directed underthrusting and a c c r e t i o n of successively younger s l i c e s of oceanic sediments and vo l c a n i c rocks. The age of the group ranges from Carboniferous to Middle to Late T r i a s s i c (Ladinian-Karnian); however, no Permian rocks were p o s i t i v e l y i d e n t i f i e d . The youngest rocks l i e i n the western portion of the study area and are s t r u c t u r a l l y below older rocks i n the east. In contrast, l o c a l s t r a t i g r a p h i c r e l a t i o n s h i p s show an opposite sense of facing with t h e i r younging d i r e c t i o n being toward the east. The Apex Mountain Group consists predominantly of pelagic chert and extruded b a s a l t (greenstone). The d e p o s i t i o n a l environment i s i n t e r p r e t e d to be a generally deep (>2 km) open-ocean basin with the volcanic rocks being generated along an a c t i v e l y spreading ocean r i d g e . Quiet conditions p r e v a i l e d ; however, t u r b i d i t y currents were common. L o c a l areas of high r e l i e f such as f a u l t scarps, or ocean ridges are postulated to account f o r the existence of l o c a l r e s t r i c t e d to anoxic subbasins, submarine-fan b r e c c i a , t u r b i d i t e deposits, and o l i s t o l i t h s . At l e a s t part of the Apex Mountain Group was deposited i n shallow water, po s s i b l y along an i s o l a t e d seamount or high standing r i d g e . - i i i -E a r l i e s t recognizable deposition of the Apex Mountain Group began i n the E a r l y Carboniferous. Following deposition the sediments and the associated v o l c a n i c rocks were c a r r i e d eastward (present sense) i n t o an east-dipping subduction zone where successively younger s l i c e s of rock were scraped o f f and accreted below older underthrust s l i c e s . Phase 1 ( F l , F l + ) deformation occurred during t h i s process and i n d u c t i l e zones produced t i g h t f olds with a x i a l surfaces p a r a l l e l with the d i r e c t i o n of shear i n the subduction zone. Ridge spreading, deposition, and subduction continued u n t i l the Middle or Late T r i a s s i c . During l a t e Paleozoic time an oceanic v o l c a n i c arc probably developed east of the Apex Mountain Group and vestiges of the arc or back-arc area may be represented by the Kobau, Anarchist, and Chapperon Groups. In Late T r i a s s i c time the locus of subduction s h i f t e d to a p o s i t i o n west of the Apex Mountain Group where renewed subduction i n i t i a t e d the development of the N i c o l a Group v o l c a n i c arc. Regional u p l i f t and erosion may have occurred as a r e s u l t of the s h i f t and could account f o r a r e g i o n a l unconformity below l o c a l Upper T r i a s s i c c l a s t i c rocks, the l a t t e r representing d e t r i t u s shed from the emerging N i c o l a arc and from eroded upper Paleozoic sequences. Phase 2 (F2) structures i n the study area are p o s s i b l y r e l a t e d to r e g i o n a l s t r i k e - s l i p movement i n post Late T r i a s s i c - E a r l y J u r a s s i c time. In general, the rocks of the Apex Mountain Group have been metamorphosed to lower greenschist f a c i e s . - i v -TABLE OF CONTENTS Page ABSTRACT i i LIST OF FIGURES v i ACKNOWLEDGEMENTS v i i i CHAPTER I - INTRODUCTION 1 A. General Statement 1 B. Purpose of the Study 5 C. Location, Physiography, Surface Geology 6 D. Regional Geologic S e t t i n g 7 1. Middle T r i a s s i c and Older Rocks 9 2. Upper T r i a s s i c and Younger Rocks 11 E. Other E u g e o c l i n a l Assemblages i n Southern B.C 14 1. Eastern Assemblage 14 2. Harper Ranch Group 16 3. Cache Creek Group 17 F. Regional Structure 20 CHAPTER I I - STRATIGRAPHY OF THE STUDY AREA 23 A. Apex Mountain Group • 23 1. Massive and Bedded Chert (Units 2, 3A, 4, 7) 23 2. Volc a n i c Rocks (Units 1, 3, 4A, 6, 8) 37 3. Limestones (Shoemaker Creek, O l a l l a Creek) 48 4. Chert/Breccia Conglomerate (Unit 7A) 55 5. Chert - A r g i l l i t e (Unit 5) 59 B. N i c o l a Group .. 63 C. De p o s i t i o n a l S e t t i n g of the Apex Mountain Group 64 D. Regional Correlations with other E u g e o c l i n a l Assemblages .. 67 - v -Page CHAPTER I I I - STRUCTURE OF THE STUDY AREA 72 A. Folding 72 B. F a u l t i n g 86 C. S t r u c t u r a l I n t e r p r e t a t i o n 88 CHAPTER IV - TECTONIC EVOLUTION AND IMPLICATIONS 94 CHAPTER V - CONCLUSIONS 100 BIBLIOGRAPHY 103 - v i -LIST OF FIGURES Page Figure 1. Location of the Study Area, Tectonostratigraphic Assemblages i n the Canadian C o r d i l l e r a 2 Figure 2. Regional Geology of the Apex Mountain Area 4 Figure 3. Late Paleozoic - Mesozoic Eu g e o c l i n a l Assemblages i n Southern B.C., Northern Washington .. 8 Figure 4. L i t h o l o g y and Structure of the Apex Mountain Group, South-Central B r i t i s h Columbia 24 Figure 5. Apex Mountain Group Cross-Sections 25 Figure 6. S t r u c t u r a l Succession within the Apex Mountain Group 26 Figure 7. Photomicrograph of Carboniferous R a d i o l a r i a n and Sponge Spicule Assemblage 29 Figure 8. Thin Section Photograph Showing Size Gradation of Quartz Grains i n V e i n l e t s of Bedded Chert 31 Figure 9. Thin Section Photograph Showing Preservation of R a d i o l a r i a i n Layers Containing Impurities 35 Figure 10. Thin Section Photograph showing A c t i n o l i t e and P l a g i o c l a s e C r y s t a l s Forming P i l o t a x i t i c Texture i n B a s a l t i c Greenstone ... 39 Figure 11. Trace Element Geochemistry of Apex Mountain Group Greenstones 41 Figure 12. A l k a l i c to T h o l e i i t i c P l o t 41 Figure 13. D i s c r i m i n a t i o n Diagram Ti/100 vs. Zr vs. Y3 42 Figure 14. D i s c r i m i n a t i o n Diagram Ti/100 v s . Zr vs. Sr/2 43 Figure 15. Thin Section Photograph of Unit 4A Showing Amorphous Ash and C r y p t o c r y s t a l l i n e Aggregates of C h l o r i t e ( ? ) or Epidote(?) 46 Figure 16. Thin Section Photograph Showing P l a g i o c l a s e Laths, Amorphous Ash, and Welded Texture i n Ash Flow Tuff 47 Figure 17. Thin Section Photograph Showing T y p i c a l Fauna of Shoemaker Creek Limestone . 50 Figure 18. Photomicrograph of Platform Conodonts Neogondolella c f . N. E x c e l s a ( ? ) , Extracted from Shoemaker Creek Limestone 52 - v i i -Page Figure 19. Thin Section Photograph of Unit 7A 57 Figure 20. Thin Section Photograph of Unit 5 61 Figure 21. Stereonet Plot 74 Figure 22. Stereonet P l o t 75 Figure 23. Enlarged Sketches from F i e l d Notes Showing Dominant Fold Sty l e s 76a Figure 24. Sketch of Photograph of Folded Bedded Chert 79a. Figure 25. Schematic I l l u s t r a t i o n Showing F o l i a t i o n i n Hinges of Minor Folds 80 Figure 26. Sketches from Photographs Showing Two Phases of Folding 83a. Figure 27. Schematic Cross-Sections Showing Mode of Accretion of the Apex Mountain Group 89 Figure 28. Schematic Diagram I l l u s t r a t i n g Development of F l and F l + Folds just before and during Subduction 91 Figure 29. Schematic Map and Sections I l l u s t r a t i n g Proposed Tectonic Model f o r Late Paleozoic through Late T r i a s s i c i n South-Central B r i t i s h Columbia 95 - v i i i -ACKNOWLEDGEMENTS I would f i r s t l i k e to thank Dr. W. C. Barnes and Dr. W. R. Danner. Both provided valuable discussions and rev i s i o n s of the tex t . Dr. Danner a l s o c a r r i e d approximately 400 l b s . of limestone out of the study area and f o r that I am g r a t e f u l . Assistance i n i d e n t i f i c a t i o n of f o s s i l s was also given by Dr. Danner as we l l as the use of his lab to cut t h i n - s e c t i o n s . I would l i k e to thank Dr. H. J . Greenwood f o r his consideration during the f i n i s h i n g stages of the thesis and also Dr. J . V. Ross f o r providing the funds to complete the majority of the work. Chevron Canada Resources Limited i s to be commended for allowing unlimited use of t h e i r f a c i l i t i e s to complete the t h e s i s . S p e c i a l thanks are due to Ruth Santiago for typing and e d i t i n g the t h e s i s . Acknowledgements are i n order for my i n c o r r i g i b l e colleagues; Dave Heberlein, Paul Matysek, and Peter Holbeck, f o r t h e i r enlightening and ent e r t a i n i n g d i s c u s s i o n during my two years at UBC The T e c h n i c a l S t a f f at the UBC Geology Department deserves my thanks, e s p e c i a l l y Ed Montgomery and Bryon Cranston. John Knight and Stanya Horsky gave invaluable assistance on the SEM and XRF apparatus. - i x -Mike Orchard of the G e o l o g i c a l Survey of Canada provided the use h i s m i c r o f o s s i l lab and also i d e n t i f i e d conodonts and r a d i o l a r i a . I would l i k e to thank my f i e l d a s s i s t a n t s ; Scott Henderson, Mike Heckl, and e s p e c i a l l y my s i s t e r , Sara M i l f o r d . F i n a l l y , above a l l , I most g r a t e f u l l y acknowledge my parents f or t h e i r unending love and devotion, and i t i s to them my thesis i s dedicated. - 1 -CHAPTER I  INTRODUCTION A. General Statement The Canadian po r t i o n of the Western C o r d i l l e r a i s divided into f i v e d i s t i n c t geologic and physiographic provinces, each with i t s own p a r t i c u l a r t e c t o n i c h i s t o r y . From east to west these are: Rocky Mountain B e l t , Omineca C r y s t a l l i n e B e l t , Intermontane Be l t , Coast P l u t o n i c Complex, and Insular B e l t (Figure 1, i n s e t ) . In a broad sense the s p a t i a l and temporal r e l a t i o n s h i p s between these b e l t s i s only p a r t i a l l y understood. Close i n s p e c t i o n of the d e t a i l e d r e l a t i o n s h i p s within these b e l t s also reveals many shortcomings i n our attempts to explain e m p i r i c a l data within the context of plate t e c t o n i c s . To f a c i l i t a t e our understanding of the i n t e r n a l r e l a t i o n s h i p s , many workers have subdivided the b e l t s i n t o t e c t o n o s t r a t i g r a p h i c assemblages that represent unique conditions of geologic structure, stratigraphy, sedimentary environment, and tec t o n i c s e t t i n g (Monger, 1977; Monger and P r i c e , 1979; Tipper and others, 1981). Many of these assemblages can be r e g i o n a l l y c o r r e l a t e d , thus g r e a t l y enhancing present geodynamic models of C o r d i l l e r a n e v o l u t i o n . In south-central B r i t i s h Columbia perhaps the most i n t r i g u i n g of these t e c t o n o s t r a t i g r a p h i c assemblages are the vo l c a n i c and sedimentary successions of l a t e Paleozoic and E a r l y Mesozoic age. The complexity -2-F I G U R E 1 L O C A T I O N O F S T U D Y - A R E A (RMB Rocky Mountain Belt; OCB Omineca Crystalline Belt; Int B Intermontane Belt; CPC Coast Plutonic Complex; Ins B Insular Belt) - 3 -and diverse nature of the southern B r i t i s h Columbia assemblages warrants d e t a i l e d studies of each assemblage i n order to accurately i n t e r p r e t t h e i r o r i g i n a l t e c t o n i c s e t t i n g and explain t h e i r present geographic p o s i t i o n s w i t h i n the C o r d i l l e r a . The subject of t h i s study i s the poorly understood succession of rocks north and east of the Similkameen River near Keremeos, previously r e f e r r e d to as the Old Tom, Shoemaker, Bradshaw, and Independence Formations (Bostock, 1941) (Figure 2 ) . Because these formations do not form d i s t i n c t , mappable u n i t s , I w i l l r e f e r to them as the Apex Mountain Group. The e n t i r e study area was previously mapped and studied by H. S. Bostock In the l a t e 1920's and 1930's. His maps and marginal notes were published by the G e o l o g i c a l Survey of Canada i n 1940 and 1941. Neugebauer (1965) described l i t h o l o g i e s and structures i n an area southeast of Apex Mountain, near O l a l l a Creek. Ross and Barnes (1972)-b r i e f l y described rocks of the Apex Mountain Group east of O l a l l a where they i n t e r p r e t e d them to be separated from o v e r l y i n g upper Paleozoic c l a s t i c rocks and limestone by a mid-Paleozoic unconformity. Read and Okulit c h (1977) studied the same rocks as Ross and Barnes and also extended mapping of the Apex Mountain Group over a l a r g e r area near O l a l l a . They assigned a Permo-Triassic age to the unconformity and a Late T r i a s s i c age to the overlying c l a s t i c rocks and limestone. F i e l d mapping was c a r r i e d out during the summer of 1981 at a scale of four inches to the mile. The base map was an enlargement of portions of the Keremeos, Penticton, Ashnola River, and Hedley topographic map sheets (82E/4,5; 92H/1,8). Mapping was supplemented by a e r i a l photographs supplied by the B r i t i s h Columbia M i n i s t r y of Environment. - 4 -d ! SPRINGBROOK FM. (Eocene) I I OKANAGAN INTRUSIVE COMPLEX (L. Jurassic) CD NICOLA GROUP (L. Triassic) I I APEX MOUNTAIN GROUP (Carboniferous to Middle Triassic) I I BLIND CREEK LS. (Carboniferous to Permian) I | KOBAU GROUP (pre-Carboniferous) FIGURE 2. REGIONAL GEOLOGY, APEX MOUNTAIN AREA (From: Little, 1961; Rice, 1947 ) - 5 -B. Purpose of the Study The major concern of t h i s study was to provide a d e t a i l e d account of the stratigraphy and stucture of the mapped area i n order to i n t e r p r e t more accurately the d e p o s i t i o n a l and tectonic environments, and the t e c t o n i c h i s t o r y . E s s e n t i a l l y a l l of the rocks of the Apex Mountain Group are marine sedimentary and v o l c a n i c rocks. A primary obj e c t i v e was to understand t h e i r character and sequence i n time as these are c r i t i c a l f actors i n the evaluation of t h e i r tectonic environment. More s p e c i f i c a l l y , other eugeoclinal rocks of l a t e Paleozoic and Mesozoic age i n south-central B r i t i s h Columbia - the Cache Creek Group, Nicola Group, and Harper Ranch Group - have been suggested by Monger (1982) to represent an ancient subduction/arc/back-arc complex r e s p e c t i v e l y . A determination of the t e c t o n i c and s t r a t i g r a p h i c r e l a t i o n s h i p s of the Apex Mountain Group with these assemblages i s a major concern of t h i s study. F o l d i n g and f a u l t i n g were studied i n order to e s t a b l i s h the s t y l e of deformation and determine the number of phases of deformation present i n the study area. Comparisons of the structure i n the study area with that i n adjacent areas were made to support r e g i o n a l t e c t o n o s t r a t i g r a p h i c c o r r e l a t i o n s . The: r e l a t i v e age of deformation was also determined i n order to r e l a t e deformation to regional orogenic events during the Permo-Triassic Sonoma Orogeny and the Late T r i a s s i c - J u r a s s i c Columbian Orogeny. - 6 -Limestone and chert samples i n the Apex Mountain Group were c o l l e c t e d f o r the purpose of extracting m i c r o f o s s i l s , s p e c i f i c a l l y conodonts and r a d i o l a r i a , which are p a r t i c u l a r l y u s e f u l f o r dating rocks of l a t e Paleozoic and Mesozoic age. Prominent limestones i n Shoemaker and O l a l l a Creeks were s p e c i f i c targets for sample c o l l e c t i o n as these were already known to have yi e l d e d f o s s i l s (Bostock, 1941; Neugebauer, 1965; Read and O k u l i t c h , 1977). The mineralogy, textures, and d e p o s i t i o n a l structures of c l a s t i c sediments and v o l c a n i c rocks, as well as limestones and cherts, were examined i n order to evaluate facing d i r e c t i o n s , sources of c l a s t s , d e p o s i t i o n a l environment, and diagenetic h i s t o r y of the Apex Mountain Group. C. Location, Physiography and Surface Geology The study area l i e s immediately north and east of the Similkameen River between Hedley and Keremeos, south-central B r i t i s h Columbia (Figure 1). The area extends 16 km along P r o v i n c i a l Highway 3, and northward to Apex Mountain i n Apex Mountain P r o v i n c i a l Park. Approximately 85 sq km of t e r r a i n were mapped between l a t i t u d e s 45° 13* N and 49° 24' N and longitudes 119° 52' W and 120° 03' W. The bounds of the study area are within the Princeton and K e t t l e River map areas and the O l a l l a , Hedley, Wolf Creek, and Keremeos map sheets. The area i s within the southern I n t e r i o r Plateau of B r i t i s h Columbia. L o c a l physiography i s characterized by broad upland regions - 7 -of subdued r e l i e f separated by steep-walled, U-shaped v a l l e y s . The upland areas commonly ex h i b i t g l a c i a l features such as cirques, tarn lakes, and moraines, and many exposures exhibit g l a c i a l s t r i a e . The v a l l e y s of the Similkameen River and Keremeos Creek are characterized by steep a l l u v i a l fans and talus cones bordering t h e i r sides and terrace deposits along t h e i r bottoms. Extensive f o r e s t and grass covers most of the study area; however, good exposures can be found along the steep sides of r i v e r and creek v a l l e y s and at higher a l t i t u d e s , where vegetation i s sparse. The a l t i t u d e of the study area ranges from approximately 1500 feet (458 m) above sea l e v e l along the Similkameen River to 7370 fee t (2246 m) at Apex Mountain. D. Regional Geologic S e t t i n g The study area i s located i n the southern part of the Intermontane Belt of the Canadian C o r d i l l e r a (Figure 1, i n s e t ) . The Intermontane B e l t c onsists predominantly of unmetamorphosed to low grade (prehnite-pumpellyite f a c i e s ) sedimentary and vo l c a n i c eugeoclinal s t r a t a of l a t e Paleozoic to E a r l y J u r a s s i c age. Late(?) Mesozoic and E a r l y T e r t i a r y u p l i f t of these rocks i n i t i a t e d the development of successor basins that f i l l e d with marine and nonmarine e l a s t i c s . Mesozoic and T e r t i a r y plutons are abundant, and i n many areas T e r t i a r y plateau volcanic rocks mantle older rocks. Figure 2 shows the general geology of the region immediately surrounding the study area. Figure 3 i s a regional map that shows the general geology of south-central B r i t i s h Columbia. Rocks of the study - 8 -Figure 3. Late Paleozoic - Mesozoic eugeoclinal assemblages in southern B.C., northern Washington. Revised from Tipper and others, 1981; Monger, 1977; Smith, 1979. - 9 -area continue to the north and south where they are intruded by Upper J u r a s s i c granodiorite of the Okanagan In t r u s i v e Complex (Preto, 1973). Upper T r i a s s i c sediments of the N i c o l a Group (Bostock, 1940a; Rice, 1947) occur west of the study area and also to the east (Read and O k u l i t c h , 1977) where they unconformably o v e r l i e presumed upper Paleozoic rocks of the Apex Mountain Group. The youngest rocks are nearly f l a t - l y i n g Eocene conglomeratic and volcanic units of the Springbrook and Marron Formations r e s p e c t i v e l y . L o c a l l y these rocks are found unconformably overlying a l l other rocks. 1. Middle T r i a s s i c and Older Rocks Rocks designated as Middle T r i a s s i c and older were previously mapped by Bostock (1941) and divided i n t o several formations. The Shoemaker Formation consists mainly of ribbon chert with minor t u f f , greenstone, and limestone. Chert, with l e s s e r amounts of greenstone, b r e c c i a , and a r g i l l i t e , and minor limestone form the Independence Formation. The Old Tom Formation was described as being metabasalt and andesite with compositionally r e l a t e d i n t r u s i v e s and minor chert, and the Bradshaw Formation was described as containing a r g i l l i t e , t u f f , q u a r t z i t e , b r e c c i a and andesite. The s i m i l a r nature of these formations warrants t h e i r c o l l e c t i v e designation i n t h i s study as the Apex Mountain Group. Bostock assigned a T r i a s s i c or older age to the above formations based on s t r a t i g r a p h i c r e l a t i o n s h i p s . Recently, most authors (Read and O k u l i t c h , 1977; Monger, 1977) have considered the Apex Mountain - 10 -Group to be Carboniferous i n age based on Danner's suggestion of a probable Pennsylvanian age f o r c r i n o i d a l limestone i n O l a l l a Creek (Neugebauer, 1965). In a d d i t i o n , Read and O k u l i t c h (1977) found middle and l a t e Paleozoic f o s s i l s i n the limestones of Shoemaker Creek. Data from the present study shows the s t r u c t u r a l l y higher O l a l l a Creek limestone to be a Carboniferous o l i s t o l i t h and the Shoemaker Creek limestone to be a reworked deposit that contains Middle T r i a s s i c as well as l a t e Paleozoic fauna (Chapter I I ) . A l a r g e block of upper Paleozoic limestone at B l i n d Creek near Keremeos contains faunas of Mid-Carboniferous and E a r l y Permian(?) age and was emplaced by T e r t i a r y l a n d s l i d i n g (Smith, 1935; Barnes and Ross, 1975; Wilson, 1980). The source t e r r a i n of the limestone remains uncertain; however, Danner (verbal communication, 1982) reports that M i s s i s s i p p i a n fauna In the matrix of the B l i n d Creek limestone are very s i m i l a r to those i n limestone of the Harper Ranch Group. Rocks of the Kobau Group near Mt. Kobau were mapped by Okulitch (1969). The group consists of q u a r t z i t e , p h y l l i t e , greenstone, and minor limestone considered to be pre-Pennsylvanian(?) i n age based on s t r u c t u r a l evidence. O k u l i t c h (1973) has suggested that these rocks may represent the metamorphosed equivalent of the Apex Mountain Group. Okulitch also equated rocks of the Kobau Group with parts of the Anarchist Group i n the southern Okanagan area. Lithology and s t y l e of deformation are s i m i l a r ; however, the - 11 -Anarchist Group i s considered to be Permian i n age based on poor f o s s i l evidence (Waters and Krauskopf, 1941; Rinehart and Fox, 1972). The Chapperon Group, a v a r i a b l y metamorphosed assemblage of a r g i l l a c e o u s sediments and volcanic rocks west of Vernon may also be c o r r e l a t i v e with the Kobau and Anarchist Groups. C o r r e l a t i o n of these groups with the Apex Mountain Group i s discussed i n Chapters I I and IV. 2. Upper T r i a s s i c and Younger Rocks Most of the rocks of t h i s age i n south-central B r i t i s h Columbia belong to the N i c o l a Group. Figures 2 and 3 show the d i s t r i b u t i o n of N i c o l a Group rocks i n t h i s region. Rocks of the Upper T r i a s s i c N i c o l a Group are divided into two d i s t i n c t coeval f a c i e s that are l a t e r a l l y g r a d a t i o n a l . The type l o c a l i t y of the westernmost f a c i e s occurs near N i c o l a Lake, about 65 km southwest of Kamloops. I t consists of b a s a l t i c and a n d e s i t i c flows, b r e c c i a s , t u f f s , r e l a t e d sediments, and reef limestones (Schau, 1970) and i s believed to be representative of an oceanic v o l c a n i c arc complex. Upper T r i a s s i c rocks east of Kamloops co n s i s t of deeper water mudstone, l i t h i c sandstone, v o l c a n i c l a s t i c rocks, conglomerate and limestone; some of the l a t t e r may be o l i s t o s t r o m e s . Within t h i s c l a s t i c f a c i e s there i s a progressive eastward decrease i n the amount of v o l c a n i c l a s t i c m a terial i n the sediments with a corresponding increase i n the amount of terrigenous material (Smith, 1979). This f a c i e s i s thought to - 12 -represent a back-arc basin that developed east of the main v o l c a n i c a r c . Farther east, Upper T r i a s s i c v o l c anic flows l i e unconformably above Permian limestones (Sada and Danner, 1974). Monger (1982) reports that v o l c a n i c rocks of the N i c o l a Group of southern B r i t i s h Columbia also show an eastward gradation from f e l s i c to ultramafic composition. Just northeast of O l a l l a , Upper T r i a s s i c rocks occur above the aforementioned Permo-Triassic unconformity. L o c a l l y , t h i s unconformity has been shown to have up to 500 m of r e l i e f (Ross and Barnes, 1972). The succession consists of a basal chert b r e c c i a followed by calcareous chert wacke, shale, and limestone. These rocks have been c o r r e l a t e d with the N i c o l a Group based on t h e i r s i m i l a r l i t h o l o g y and age (Read and O k u l i t c h , 1977). N i c o l a Group rocks also occur to the west i n the Princeton map area and were mapped and described by Bostock (1940a) and Rice (1947). The N i c o l a Group i n t h i s area consists predominantly of a n d e s i t i c and b a s a l t i c flows, b r e c c i a s , p y r o c l a s t i c rocks, and compositionally r e l a t e d i n t r u s i v e s . Most of the sedimentary rocks l i e below the v o l c a n i c rocks and are much more r e s t r i c t e d i n t h e i r d i s t r i b u t i o n . They consist of tuffaceous a r g i l l i t e , calcareous t u f f , greywacke, v o l c a n i c l a s t i c b r e c c i a , q u a r t z i t e , and limestone. Late T r i a s s i c f o s s i l s have been found at many l o c a l i t i e s (Rice, 1947). The l a r g e s t b e l t of these Upper T r i a s s i c sedimentary rocks occurs near Hedley where they are conspicuously l a c k i n g i n v o l c a n i c m a t e r i a l . - 13 -Bostock (1940a) divided these rocks into several formations: Redtop, Sunnyside, Hedley, and Henry Formation (oldest to youngest). As described by Bostock, these formations consist mainly of q u a r t z i t e , cherty q u a r t z i t e and massive limestone with l e s s e r amounts of s i l i c e o u s a r g i l l i t e , t u f f , b r e c c i a , conglomerate, and impure limestone. These rocks w i l l herein be r e f e r r e d to as the sedimentary f a c i e s of the N i c o l a Group. Sedimentary rocks of the N i c o l a Group occur i n the study area on the west side of Winters Creek, and are described i n Chapter II of t h i s r e p ort. They are separated from the Apex Mountain Group rocks to the east by a steeply dipping f a u l t along Winters Creek. P l u t o n i c and i n t r u s i v e rocks i n the region range i n age from l a t e Paleozoic to Late J u r a s s i c . Minor hornblende-augite s i l l s intrude the Apex Mountain Group near O l a l l a and are presumed to be l a t e Paleozoic i n age based on Read and Okulitch's (1977) report that they cut the Old Tom and Shoemaker Formations but not the N i c o l a Group. Read and O k u l i t c h also report K-Ar ages of two p o r p h y r i t i c s i l l s w ithin the N i c o l a Group rocks near O l a l l a as 149 +5 Ma and 168 +6 Ma ( E a r l y J u r a s s i c ) . Most of the plutonic rocks i n t h i s region belong to the Okanagan I n t r u s i v e Complex, a name given by Preto (1973) to the composite b a t h o l i t h l y i n g between Princeton and Okanagan Lake. These rocks range i n composition from syenite to pyroxenite and p e r i d o t i t e . The more a c i d i c bodies have a K-Ar and Rb-Sr i s o t o p i c age of 156 +6 Ma (Late J u r a s s i c ) and the basic bodies are d i s t i n c t l y older at 165 to 186 Ma (Middle J u r a s s i c ) (Peto and Armstrong, 1976). - 14 -Eocene rocks of the Springbrook and Marron Formations unconformably o v e r l i e the N i c o l a Group and Apex Mountain Group near O l a l l a . D i r e c t l y above the unconformity i s the Springbrook Formation, a f l u v i a l conglomerate that contains angular to subrounded pebbles and cobbles of chert, greenstone, limestone, and t r a c h y t i c syenite set i n a s l i g h t l y calcareous matrix of l i t h i c sandstone. Volcanic rocks of the Marron Formation o v e r l i e the Springbrook Formation and consist of p o r p h y r i t i c basalt and andesite with z e o l i t e - f i l l e d amygdules. This formation a t t a i n s thicknesses up to 1400 m (Church, 1973) and mantles older rocks throughout much of the area of upland t e r r a i n west of the Okanagan V a l l e y . E. Other E u g e o c l i n a l Assemblages i n Southern B r i t i s h Columbia Upper Paleozoic and Mesozoic sequences of eugeoclinal rocks i n the Canadian C o r d i l l e r a have been described by Monger (1977). Figure 3 shows the l o c a t i o n s of southern B r i t i s h Columbia assemblages and i s modified from f i g u r e s i n Monger (1977), Smith (1979), and Tipper and others (1981). B r i e f d e s c r i p t i o n s of these assemblages are given below and are l a r g e l y taken from Monger (1977) and Smith (1979). 1. Eastern Assemblage The Eastern Assemblage forms a discontinuous b e l t that extends for approximately 3200 km from just north of the B r i t i s h Columbia-Idaho border to Alaska. The assemblage contains the easternmost upper Paleozoic eugeoclinal rocks i n the Canadian C o r d i l l e r a . - 15 -In southern B r i t i s h Columbia the assemblage i s represented by the M i l f o r d and Kaslo Groups (Figure 3 ) . Parts of the Mount Ida Group and the Eagle Bay Formation probably represent metamorphosed equivalents of the M i l f o r d and Kaslo Groups (Okulitch and Cameron, 1976) and they are included i n the Eastern Assemblage. The e n t i r e assemblage consists of a lower, mainly sedimentary section o v e r l a i n by an upper v o l c a n i c section of b a s a l t , minor chert, diabase, gabbro, and ultramafic rock. The sedimentary s e c t i o n consists of fine-grained e l a s t i c s and chert with sandstone, conglomerate, f e l s i c to intermediate v o l c a n i c rocks, and minor Upper M i s s i s s i p p i a n to Permian limestones. Gabbro s i l l s i n the sedimentary s e c t i o n are considered to be r e l a t e d to feeders i n the upper v o l c a n i c s e c t i o n . The assemblage l i e s with s t r a t i g r a p h i c and f a u l t e d contact on Devonian and older miogeoclinal rocks l y i n g west of the Rocky Mountain Trench. Most of the assemblage i s lower greenschist grade; however, amphibolite grade i s attained l o c a l l y near i n t r u s i v e bodies (Monger, 1977). In southeast B r i t i s h Columbia, sediments of the M i l f o r d Group l i e s t r a t i g r a p h i c a l l y above older metamorphic rocks of the Lardeau Group and contain c l a s t s of chert, p h y l l i t e , mafic v o l c a n i c s , and Devonian gr a n i t e (Okulitch and others, 1975). Monger and others (1972) suggest that the sediments were derived from lower Paleozoic oceanic crust that was deformed and metamorphosed during the Late Devonian to Middle M i s s i s s i p p i a n Caribooan Orogeny of Douglas (1970). P h y l l i t i c and v o l c a n i c rocks of the Eagle Bay Formation - 16 -may represent portions of t h i s ocean crust as no other pre-M i s s i s s i p p i a n eugeoclinal rocks are p o s i t i v e l y i d e n t i f i e d i n the Intermontane Belt or the Omineca C r y s t a l l i n e B e l t . The upper v o l c a n i c section i s interpreted as oceanic crust; however, the t e c t o n i c s i g n i f i c a n c e of t h i s section i s not completely understood. In northern B r i t i s h Columbia and the Yukon, the volcanic rocks appear to be thrust over underlying miogeoclinal rocks (Monger and others, 1972; Monger, 1977). In other areas of B r i t i s h Columbia, the overlying v o l c a n i c rocks appear to document a gradually deepening basin and eventual extrusion of basalt as they are observed to have a conformable r e l a t i o n s h i p with underlying sediments. 2. Harper Ranch Group Type l o c a l i t i e s of the Harper Ranch Group occur north of the South Thompson River near Kamloops. They have previously been r e f e r r e d to as the Thompson Assemblage (Oku l i t c h , 1979) and Cache Creek Group ( C o c k f i e l d , 1948). Immediately north and east of Kamloops there i s a section dominated by fine-grained v o l c a n i c l a s t i c rocks and containing scattered v o l c a n i c flows and limestone lenses. Conodonts extracted from these limestones have Karnian to Norian ages (Smith, 1979; Monger, 1982). Most authors consider the T r i a s s i c v o l c a n i c l a s t i c s e c t i o n to be part of the N i c o l a Group (Danner i n Smith, 1979). In the Dome H i l l s area north of Kamloops, small, upper Paleozoic - 17 -limestone blocks i n t h i s s e ction are i n t e r p r e t e d as o l i s t o l i t h s , as the surrounding a r g i l l i t e and sandstone matrix contains Late T r i a s s i c conodonts and Permian r a d i o l a r i a (Monger, 1982). On i t s east side, the T r i a s s i c N i c o l a Group section i s i n f a u l t contact with the Harper Ranch Group. Immediately east of the N i c o l a Group i s an approximately 5 km thick sequence of hemipelagic mudstone, sandstone, conglomerate, chert, redeposited t u f f , and Middle Pennsylvanian-Permian limestone (Smith, 1979; Monger, 1982). Farther east the limestones become thicker and more extensive and Permian limestone o v e r l i e s Mississippian-Pennsylvanian limestone (Sada and Danner, 1974, Danner, personal communication, 1984). This l a t t e r s e c t i o n i s unconformably o v e r l a i n on i t s eastern side by v o l c a n i c rocks and minor sediments of the N i c o l a Group. 3. Cache Creek Group Type l o c a l i t i e s of the Cache Creek Group occur near Cache Creek; however, s i m i l a r s t r a t a have long been recognized to occur over much of the length of the western Canadian C o r d i l l e r a . The assemblage forms a b e l t up to 75 km wide that extends from south of the 49th p a r a l l e l i n t o the Yukon and possibly Alaska. T y p i c a l l i t h o l o g i e s i n the Cache Creek Group are mainly ribbon chert and a r g i l l i t e with l e s s e r amounts of massive limestone, mafic v o l c a n i c rocks, and v a r i a b l y serpentinized ultramafic rocks. C l a s t i c - 18 -rocks coarser than a r g l l l l t e are rare and consist of sandstones with chert, a r g i l l i t e , and vo l c a n i c g r a i n s . Massive limestone c h a r a c t e r i s t i c a l l y forms pods up to tens of kilometres long. Some of the limestone bodies are in-place, however some have sheared contacts with surrounding s t r a t a (Danner, verbal communication, 1984). The volcanic rocks are mainly flows and are c h a r a c t e r i s t i c a l l y pillowed. Most of the rocks are of lower greenschist grade. The Cache Creek Group i s distinguished from other eugeoclinal assemblages by i t s d i s t i n c t i v e stratigraphy and paleontology and to a l e s s e r degree by i t s s t r u c t u r a l s t y l e . L i t h o l o g i c a l l y , the assemblage i s di s t i n g u i s h e d by tremendous thicknesses of bedded r a d i o l a r i a n chert, abundant p i l l o w lavas, the presence of blue s c h i s t rocks at several l o c a l i t i e s , and the occurrence of a Tethyan faunal assemblage. F o s s i l s representing ages from E a r l y M i s s i s s i p p i a n to Late T r i a s s i c are found i n carbonates and cherts i n units that have been mapped as the Cache Creek Group, and unique Permian Verbeekinid and Pennsylvanian f u s u l i n i d s form the only known Tethyan assemblage i n the Canadian C o r d i l l e r a . Danner (1965) suggested that these f o s s i l s have strong a f f i n i t i e s with Permian rocks i n Japan, China and Indonesia. Pods of carbonate, greenstone and serpentinized ultr a m a f i c rocks t y p i c a l l y occur within chert and a r g i l l i t e and t h e i r contacts are commonly sheared and f a u l t e d . Apparently t h i s i s the r e s u l t of the o r i g i n a l l e n t i c u l a r and discontinuous nature of the sedimentary and vo l c a n i c units combined with the e f f e c t s of deformation on competency dif f e r e n c e s between the l i t h o l o g i e s . The u l t r a m a f i c rocks are inte r p r e t e d to be fragments of ocean crust - 19 -brought up along major shear zones (Shannon, 1982). Tectonic melanges have also been recognized i n t h i s assemblage (Shannon, 1982). The general d e p o s i t i o n a l environment of the Cache Creek Group i s i n t e r p r e t e d to be an oceanic plateau with s t r a t i f i e d rocks accumulating upon ocean c r u s t . Volcanic seamounts or ridges were present and shallow-water and lagoonal carbonates overlying many basalts i n d i c a t e that these v o l c a n i c platforms may have been l o c a l l y capped by a t o l l s . Smaller bodies of shallow-water carbonate that occur i n brecciated v o l c a n i c s and bedded chert are i n t e r p r e t e d as o l i s t o l i t h s that slumped from a t o l l s into deeper water chert basins (Monger, 1977a), although the cherts were not n e c e s s a r i l y deposited i n deep water. Monger and others (1972) i n t e r p r e t e d the Cache Creek Group to be an accreted t e r r a i n of material scraped o f f downgoing oceanic l i t h o s p h e r e during l a t e Paleozoic and/or e a r l y Mesozoic subduction. K-Ar ages of 211 to 218 Ma (Paterson and Harakal, 1974) from b l u e s c h i s t s i n the Cache Creek Group makes them coeval with upper T r i a s s i c mafic to intermediate arc volcanics of the N i c o l a Group (Monger and P r i c e , 1979). Presumably, u p l i f t and c o oling of the b l u e s c h i s t s occurred during the Middle to Late T r i a s s i c Tahltanian Orogeny of Souther (1971). Sparse evidence for t h i s deformational episode i s given by o p h i o l i t i c and chert d e t r i t u s i n Upper T r i a s s i c rocks near P i n c h i Lake (Paterson, 1977), and d e t r i t a l blue amphiboles i n Karnian rocks near Kamloops (Smith, 1979) of which the Cache Creek - 20 -Group i s the only known nearby source. A Cache Creek source f o r the blue amphiboles would suggest that the Cache Creek and N i c o l a Groups were adjacent i n Late T r i a s s i c time. In support of t h i s , Shannon (1982), i n t e r p r e t e d blocks of quartz-eye porphyry i n a Cache Creek Group melange unit as being derived from the N i c o l a Group. The question of whether the two groups were together i n the Late T r i a s s i c i s not, however, c l e a r l y r e s o l v a b l e . The base of the N i c o l a Group i s not exposed i n the Cache Creek area, a l l contacts between the two groups are f a u l t e d , and none of the d i s t i n c t i v e l i t h o l o g i e s present i n the Cache Creek Group have been discovered as exotic blocks i n the N i c o l a Group. F. Regional Structure Upper Paleozoic and lower Mesozoic eugeoclinal assemblages i n south-c e n t r a l B r i t i s h Columbia contain s i m i l a r s t r u c t u r a l sequences and geometries according to Ross (1981) who summarized and described several d i s t i n c t phases of deformation i n these rocks and c o r r e l a t e d them with known orogenic events. Ross (1981) a l s o provided a l o c a l geodynamic model f o r the evolution of these s t r u c t u r e s . According to Ross (1981), the e a r l i e s t recognizable deformation produced i s o c l i n a l f o l d i n g with northerly trending axes. It a f f e c t e d a l l rocks of pre-Late T r i a s s i c age throughout much of the region and resul t e d i n greenschist metamorphism of these rocks. Strongly penetrative Phase 2 structures are only evident i n high grade rocks of the Shuswap Metamorphic Complex east of the Okanagan V a l l e y . They are - 21 -recumbent i n form but more open than Phase 1 structures, and have east-west or northwest-southeast trending axes. Phase 3 deformation produced open f o l d i n g with a x i a l surfaces i n c l i n e d steeply to the north and shallowly plunging east-west trends. Metamorphism associated with t h i s phase i s low-grade and a f f e c t s rocks up to E a r l y J u r a s s i c age on both sides of the Okanagan V a l l e y . Phases 4 and 5 are e s s e n t i a l l y coeval i n Eocene time and are r e l a t e d to regional c r u s t a l extension which r e s u l t e d i n n o r t h e r l y trending f o l d s and normal f a u l t s and subsequent extrusion of c o n t i n e n t a l v o l c a n i c s . According to Ross (1981), f i r s t phase structures were produced as oceanic eugeoclinal sediments and mantle material were obducted onto a west-dipping c o n t i n e n t a l p l a t e , g i v i n g r i s e to e a s t e r l y verging nappes. This event i s c o r r e l a t e d with the Permo-Triassic Sonoma Orogeny. I s o s t a t i c u p l i f t followed by erosion r e s u l t e d i n deposition of Upper T r i a s s i c N i c o l a Group sediments above a regional unconformity, as for example at O l a l l a . S h o r t l y a f t e r the cessation of obduction an e a s t e r l y dipping Benioff zone developed and manifested i t s e l f i n the production of Upper T r i a s s i c -Lower J u r a s s i c arc v o l c a n i c s of the N i c o l a Group. The arc developed west of the obducted t e r r a i n , assuming the two t e r r a i n s are s t i l l i n place r e l a t i v e to one another. This event res u l t e d i n Phase 2 deformation. A d u c t i l e shear zone, the Shuswap Complex, developed continentward of the arc and i t i s here that Phase 2 structures are most apparent. Phase 2 f o l d axes i n i t i a l l y formed at high angles to the e a s t e r l y d i r e c t e d subduction; however, progressive f l a t t e n i n g , - 22 -accompanied by separation along i s o c l i n a l limbs and progressive r o t a t i o n , caused the axes to approach p a r a l l e l i s m with the east-west s l i p d i r e c t i o n (Ross, 1981). Eastward dipping subduction continued but became more and more oblique with time. As a r e s u l t , Phase 3 structures were i n i t i a t e d and most l i k e l y preceded the development of the r e g i o n a l s t r i k e - s l i p motion. Phase 3 deformation i s E a r l y J u r a s s i c and i s c o r r e l a t e d with the Columbian Orogeny. Regional c r u s t a l extension that produced Phases 4 and 5 structures i s interpreted to be the r e s u l t of i n t r o d u c t i o n i n t o the crust of an upper mantle d i a p i r i n Eocene time. - 23 -CHAPTER II STRATIGRAPHY OF THE STUDY AREA Within the study area f i v e major l i t h o l o g i e s are recognized i n the Apex Mountain Group. Together they c o n s t i t u t e 14 s t r a t i g r a p h i c units which form a broad, eastward dipping and facing sequence (Figures 4 and 5 ) . Facing evidence from several l o c a l i t i e s i n d i c a t e s that s t r u c t u r a l l y lower rocks are older than those above; however, t h i s r e l a t i o n s h i p may not hold f o r the e n t i r e succession i n the study area (Section I I I C ) . The rocks w i l l be described i n terms of t h e i r s t r u c t u r a l succession, with rocks i n the west being lowest i n the s e c t i o n (Figure 6). Mapped units on Figure 4 are colored according to t h e i r l i t h o l o g y with chert being orange, greenstone green, c h e r t - a r g i l l i t e grey, chert breccia/conglomerate brown and limestone l i g h t blue. Their frequency i n the study area does not r e f l e c t s t r u c t u r a l r e p e t i t i o n . A. Apex Mountain Group 1. Massive and Bedded Chert (Units 2, 3A, 4, 7) The most common rock type i n the study area i s massive or bedded l i g h t grey to black chert, with red and green v a r i e t i e s being l e s s common. - Many exposures of chert are pervasively f r a c t u r e d such that i n t e r n a l s t r a t i f i c a t i o n i s d i f f i c u l t to i d e n t i f y . However, most of the chert i n the study area i s observed to have a massive texture with s l i g h t or no v i s i b l e laminae. -26-OLALLA CREEK FIGURE 6. S T R U C T U R A L SUCCESSION WITHIN THE A P E X MOUNTAIN G R O U P NOTE In an overs* sense structure*? ragher rocks in the eastern portion ol the study area are older than structure* lower rocks m Ihe west (i e t oldest. 8 youngest) Locs*y. however ndmduel sequences may tace strsbgraphtca*y upward io the east 0 FOSSIL LOCALITY AND AGE FAULTEO CONTACT CHERT MARKER BED GREENSTONE MARKER BED VERTICAL SCALE 1 31.680 1 IN 2 640 FT - 27 -In general, the massive cherts consist almost e n t i r e l y of i n t e r d i g i t a t e d m i c r o c r y s t a l l i n e (0.01 to 0.08 mm) quartz grains. The l a r g e r grains commonly occur i n d i f f u s e zones and they e x h i b i t r e l a t i v e l y s t r a i g h t contacts with adjacent g r a i n s . These zones were at one time probably veins. The presence of larger quartz grains with a mosaic-like texture suggests that moderate amounts of heating, probably due to b u r i a l , and r e c r y s t a l l i z a t i o n have occurred. Samples of massive chert near the granodiorite stock i n the center of the area and also near Winters Creek are notably more r e c r y s t a l l i z e d as they have larger quartz grains and well-developed mosaic texture. The diagenetic and metamprphic h i s t o r y of study area cherts i s discussed f u r t h e r below. Massive cherts commonly contain up to 20% f i n e l y c r y s t a l l i n e , bladed, weakly pleochroic a c t i n o l i t e and hornblende, and/or b i o t i t e , with minor amounts of sodic p l a g i o c l a s e , c h l o r i t e , c a l c i t e and what appear to be f i n e p y r i t e g r a i n s . A brown, amorphous material which occurs as scattered specks and also around the edges of p y r i t e grains i s probably limonite r e p l a c i n g the p y r i t e . A c t i n o l i t e , hornblende and b i o t i t e c r y s t a l s are generally less than 0.02 mm i n length, however, near i n t r u s i v e bodies hornblende dominates and c r y s t a l s range up to 0.1 mm i n length. The presence of these minerals suggests that minor amounts of volcanic m a t e r i a l , probably i n the form of ash or terrigenous c l a y were incorporated with the chert during d e p o s i t i o n . A c t i n o l i t e and hornblende derived from vo l c a n i c material and b i o t i t e derived from p e l i t i c material are suggestive of lower greenschist grade metamorphism. Ghosts of - 28 -r a d i o l a r i a are rare i n massive cherts, but where seen they occur as d i f f u s e spots of quartz and/or chalcedony. Bedded cherts are much l e s s common i n the study area and t y p i c a l l y consist of 2 to 6 cm, l a t e r a l l y discontinuous layers of r a d i o l a r i a n chert separated by less than 1 mm to 25 mm of dark grey to black s i l i c e o u s a r g i l l i t e . Many of the bedded chert sequences are t i g h t l y folded, with f o l d o r i e n t a t i o n s r e l a t e d to tec t o n i c deformation (see Section I I I A ) . Unfolded bedded cherts are also observed. R a d i o l a r i a n e x t r a c t i o n techniques were applied to seventeen samples of bedded chert and well-preserved r a d i o l a r i a were found from samples at two l o c a l i t i e s near O l a l l a Creek i n the easternmost part of the study area (Figure 4 ) . Several species were present and according to D. L. Jones (United States Geological Survey, written communication, 1982), they represent a Carboniferous assemblage (Figure 7 ) . Jones did not i d e n t i f y the In d i v i d u a l species. The contacts of the f o s s i l bearing cherts with adjacent rock units are obscured by cover; thus, t h e i r mutual s t r a t i g r a p h i c r e l a t i o n s are uncert a i n . No evidence of f a u l t i n g , such as sheared rock was seen i n the proximity of these contacts. As w e l l , " e x o t i c " looking blocks were also not observed. Thus, the f o s s i l - bearing cherts are believed to be s t r a t i g r a p h i c a l l y i n place. Jones also reports f i n d i n g cherts containing the same Carboniferous f o s s i l assemblage i n an area near Keremeos, i n rocks mapped as the Old Tom Formation by Bostock (1941). 0.1 mm • Scale Figure 7. Photomicrograph of Carboniferous radiolarian and sponge spicule assemblage (D.L. Jones, written communication, 1982). - 30 -Bedded cherts consist of up to 40% d e t r i t a l microorganism tes t s i n a matrix of c r y p t o c r y s t a l l i n e and amorphous s i l i c a . R a d i o l a r i a are the most common mi c r o f o s s i l s . b u t s i l i c e o u s sponge spicules and s i l l c i f i e d calcispheres are also present. F o s s i l remains are replaced by very f i n e grained quartz and/or chalcedony and contrast with t h e i r enclosing dark-colored c r y p t o c r y s t a l l i n e matrix. Abundant v e i n l e t s are also present i n many of the bedded cherts and they contain w e l l - c r y s t a l l i z e d quartz grains with minor fibrous chalcedony and c h l o r i t e . Most of the v e i n l e t s show a gradation from w e l l - c r y s t a l l i z e d approximately 0.1 mm grains near the middle to approximately 0.01 mm grains along the edges of the v e i n l e t s (Figure 8). These most l i k e l y represent diagenetic migrational pathways of i n t e r s t i t i a l s i l i c e o u s f l u i d s , which were most l i k e l y derived from d i s s o l u t i o n of biogenic opal or l e s s l i k e l y pressure s o l u t i o n . Other v e i n l e t s have sharp boundaries with the adjacent c r y p t o c r y s t a l l i n e matrix and c l e a r l y represent p o s t - l i t h i f i c a t i o n f l u i d migration w i t h i n f r a c t u r e s . M i c r o b r e c c i a t i o n or f r a c t u r i n g may be due to compaction of over l y i n g sediments and could be aided by d i f f e r e n t i a l s o l u t i o n i n adjacent sediments. The dark grey to black c o l o r of chert and associated interbedded a r g i l l i t e i s due to the presence of f i n e l y disseminated p y r i t e and brown to black organic matter. This material also occurs dispersed along v e i n l e t s and as scattered spots as probable i n s o l u b l e residue l e f t a f t e r d i s s o l u t i o n . Red cherts probably owe t h e i r color to abundant minute - 31-r 0.1 mm X-POLARS Figure 8. Thin-section photograph showing size gradation of quartz grains in veinlets of bedded chert. - 32 -f l a k e s of iron-oxide (hematite). At several l o c a l i t i e s black, m e t a l l i c - l o o k i n g , manganiferous cherts were also observed. Metal oxides are presumed to have been exhaled into the seawater from v o l c a n i c vents, and subsequently introduced into the surrounding sediment. The o r i g i n of cherts i n the study area i s best explained by having a background of slow, continuous deposition of biogenic s i l i c e o u s debris over broad areas along with p e r i o d i c blooms of r a d i o l a r i a and other s i l i c e o u s plankton. This would explain the extensive massive chert development i n the area. Superimposed upon t h i s background would be occasional periods of strong i n f l u x e s of a r g i l l i t e . During these periods bedded cherts would develop during r a d i o l a r i a n blooms. The rate and amount of biogenic s i l i c a deposited would be d i r e c t l y r e l a t e d to the amount of dissolved s i l i c a i n the seawater a v a i l a b l e f or uptake by s i l i c a - u t i l i z i n g organisms. S i l i c i o u s planktonic blooms may have been l o c a l l y the r e s u l t of d i r e c t i n t r o d u c t i o n i n t o the seawater of s i l i c a from subaqueous magmas and/or submarine weathering of v o l c a n i c ash (Gibson and Towe, 1971; E l d e r f i e l d and others 1972; Mattson, Pessagno, and H e l s l e y 1972; B e r n o u l l i , 1972). Regional high concentrations of s i l i c a i n marine waters r e s u l t from favorable ocean c i r c u l a t i o n patterns which cause upwelling of nutrient-laden water over broad areas (Berger, 1970; Ramsay, 1973). I t i s f e a s i b l e that t h i s l a t t e r process could operate for periods of time as long as the l i f e of an ocean basin, thus r e s u l t i n g i n t h i c k deposition of chert over large areas. - 33 -I f bedding did develop i n massive cherts, i t may have been destroyed by b i o t u r b a t i o n ; although no evidence of burrowing was seen. Massive cherts are generally l i g h t grey and lack f i n e organic material and p y r i t e ; and were probably deposited i n an oxygenated environment. Bedded cherts i n the study area were n e c e s s a r i l y deposited i n an anoxic or r e s t r i c t e d environment owing to t h e i r abundant included organic carbon and f i n e p y r i t e , and t h e i r lack of b i o t u r b a t i o n . Bedded cherts are much more r e s t r i c t e d i n t h e i r d i s t r i b u t i o n throughout the Apex Mountain Group and are also more l a t e r a l l y discontinuous than massive cherts, suggesting bottom anoxia was only l o c a l l y developed. The absence of lamination, load s t r u c t u r e s , or graded bedding i n the bedded cherts precludes an o r i g i n by current deposition. The lack of s t y l o l i t e s , c l e a r l y sutured quartz grain boundaries and d i r e c t i o n a l f a b r i c s i n cherts precludes pressure s o l u t i o n being a major process during diagenesis; however, d i s s o l u t i o n of r a d i o l a r i a n opal no doubt provided s i g n i f i c a n t amounts of i n t e r s t i t i a l s i l i c e o u s f l u i d s . This may be evidenced by the d i r e c t r e l a t i o n s h i p that e x i s t s between the degree of preservation of r a d i o l a r i a and the amount of m i c r o c r y s t a l l i n e quartz present i n the c h e r t s . The greater the amount of m i c r o c r y s t a l l i n e quartz present, the greater i s the degree of etching ( d i s s o l u t i o n ) of i n d i v i d u a l r a d i o l a r i a . Well-preserved r a d i o l a r i a were found only - 34 -i n samples that showed the l e a s t evidence of d i s s o l u t i o n and migration of s i l i c a such as v e i n l e t s and zones of m i c r o c r y s t a l l i n e quartz. Other f a c t o r s c o n t r o l l i n g the degree of c r y s t a l l i n i t y i n cherts i n c l u d e ; permeability of the sediments, and the presence of organic matter and/or metal oxides. Permeability i s a f f e c t e d by the presence of c l a y minerals which would i n h i b i t the migration of i n t e r s t i t i a l s i l i c e o u s f l u i d s . As w e l l , the presence of organic or metal oxide f i l m s would i n h i b i t nucleation of s i l i c a . Bedded cherts, i n contrast to massive cherts i n the study area c h a r a c t e r i s t i c a l l y contain abundant a r g i l l i t e , metal oxides, and organic matter, and i t i s i n these cherts that preservation of r a d i o l a r i a i s best and c r y s t a l l i n i t y of quartz grains i s l e a s t developed. The l a t t e r i s w e l l - i l l u s t r a t e d by Figure 9 which shows preserved r a d i o l a r i a present i n a layer that contains abundant im p u r i t i e s c o n t r a s t i n g with a "purer" layer i n which preservation of r a d i o l a r i a i s poor and c r y s t a l l i n i t y of quartz grains i s better developed. A l t e r n a t i v e l y , some massive cherts i n the study area may have been subjected to an i n f l u x of water during metamorphism. This would cause oxidation of organic matter thereby leaving s i l i c e o u s debris unprotected from d i s s o l u t i o n . Massive cherts commonly d i s p l a y paper-thin partings of s i l i c e o u s black a r g i l l i t e . The a r g i l l a c e o u s material may be a l l that remains of once t h i c k e r , o r g a n i c - r i c h layers interbedded with chert. - 35 X-POLARS Figure 9. Thin-section photograph showing preservation of radioiaris in layers containing impurities. - 36 -The formation of cherts i n the study area from biogenic opaline s i l i c a can be explained by a "maturation theory" for oceanic chert formation (Heath and Moberly, 1971; Berger and von Rad, 1972; Wise and Weaver, 1974); whereby, given enough time and thermal energy, biogenic s i l i c a i s converted by s o l u t i o n - r e p r e c i p i t a t i o n reactions i n t o c r i s t o b a l i t e and eventually true quartz chert. The absence or r a r i t y of preserved s i l i c e o u s f o s s i l remains i n the end product i s due to near or complete d i s s o l u t i o n of t h e i r opaline t e s t s . This theory a r i s e s from studies of cherts cored from the P a c i f i c Ocean during various legs of the Deep Sea D r i l l i n g P r o j e c t . Massive cherts i n the study area are i n t e r p r e t e d to have undergone greater d i s s o l u t i o n and r e p r e c i p i t a t i o n ( c r y s t a l l i z a t i o n ) than bedded cherts as i n d i c a t e d by t h e i r greater abundance or l a r g e r , w e l l -c r y s t a l l i z e d quartz grains and t h e i r lack or absence of preserved r a d i o l a r i a n remains. This process has been shown experimentally (Mizutani, 1966; Ernst and C a l v e r t , 1969) and e m p i r i c a l l y (Lancelot, 1973; von Rad and Rosch, 1974) to occur quite r a p i d l y ; f o r example, 10 m i l l i o n . years. A s i g n i f i c a n t amount of overburden, on the order of 0.5 to 1.5 km, i s necessary to create high enough temperatures to increase the r e a c t i o n r a t e s . This could have been supplied i n the Apex Mountain Group by accumulation of extruded b a s a l t . Inorganic p r e c i p i t a t i o n of s i l i c a may also occur i n areas of a c t i v e submarine volcanism; therefore, t h i s process must also be considered as a possible mode for l o c a l chert formation i n the study - 37 -area. D e v i t r i f i c a t i o n of volcanic ash and exhalation of s i l i c a - r i c h v o l c anic r e s i d u a l f l u i d s are primary sources of s i l i c a i n some modern marine sediments ( C a l v e r t , 1974; Wise and Weaver, 1974), and some authors f e e l the increased concentrations of s i l i c a could lead to the d i r e c t p r e c i p i t a t i o n of a s i l i c a g e l (Reynolds, 1970; Berger and von Rad, 1972), although the occurrence of such has not been documented i n modern oceans. 2. V o l c a n i c Rocks (Units 1, 3, 4A, 6 8) The two main types of vo l c a n i c rocks present i n the study area are b a s a l t i c or a n d e s i t i c flows (Units 1, 3, 6, 8) and compositionally r e l a t e d tuffaceous rocks (Unit 4A). Volcanic rocks of Units 1, 3, 6, and 8 t y p i c a l l y appear i n outcrop as massive f i n e l y c r y s t a l l i n e , non-schistose greenstone. V e s i c l e s are conspicuously rare, small, and l o c a l l y are f l a t t e n e d . In Unit 3, at a l o c a l i t y near O l a l l a Creek (Loc. 7212), a pronounced amygdaloidal texture i s displayed, with c a l c i t e - f l l l e d v e s i c l e s up to 7 mm occurring within a dark brown, amorphous matrix of a l t e r e d v o l c a n i c g l a s s , scattered p l a g i o c l a s e laths and minor c h l o r i t e . P i l l o w lavas are generally r a r e ; however, at several outcrops (Loc. 7211) f l a t t e n e d , elongate lenses of greenstone are observed and are int e r p r e t e d as f l a t t e n e d p i l l o w s . The lenses are up to 0.5 m t h i c k and 2 m long and are bounded by t h i n (<2 cm) amorphous brown r i n d s . A flow top i s exposed at one l o c a l i t y i n Unit 3 (Loc. 7215) and i t s surface displays a pronounced ropey texture. The amygdaloidal lava, the pi l l o w s , and the flow top a l l occur within a 1 km square area of Unit 3 and are separated s t r a t i g r a p h i c a l l y by no more than 600 m. These rocks are int e r p r e t e d to be the r e s u l t of l o c a l extrusion of lava i n shallow water, the s i g n i f i c a n c e of which w i l l be discussed below. Both the p i l l o w lavas and flow top face upward toward the east and thus are important s t r u c t u r a l / s t r a t i g r a p h i c i n d i c a t o r s (Chapter I I I ) . The mineralogy of Units 1, 3, 6, and 8 i s f a i r l y uniform; thus, a general d e s c r i p t i o n of the rocks with approximate mineral percentages i s given. Myrmekitic and c l e a r a l b i t e (40%) and c h l o r i t e (20%) are the most abundant minerals and they occur mostly as groundmass m i c r o l i t e s . Pale-green to yellow a c t i n o l i t e (20%), b i o t i t e ( 5 % ) , muscovite (5%), and quartz (5%) also occur as m i c r o l i t e s and together with the p l a g i o c l a s e and c h l o r i t e form c h a r a c t e r i s t i c p i l o t a x i t i c textures (Figure 10). Minor minerals i n these rocks include c l i n o z o i s i t e , hornblende (dark green), sphene, r u t i l e , Ilmenite, c a l c i t e , opaques, and brown amorphous material which i s probably a l t e r e d v o l c a n i c g l a s s . Determinations of p l a g i o c l a s e composition are d i f f i c u l t ; however, most c r y s t a l s have low e x t i n c t i o n angles and r e l i e f i n d i c a t i v e of a l b i t e . In one sample phenocrysts of p l a g i o c l a s e ( l a b r a d o r i t e ? ) are observed to be p a r t i a l l y or wholly replaced. C h l o r i t e occurs along the edges of the phenocrysts, and c l i n o z o i s i t e ( ? ) , a c t i n o l i t e , and small c r y p t o c r y s t a l l i n e aggregates of epidote(?) are v a r i a b l y dispersed along apparent r e l i c t twin boundaries. The r e l i c t twins are broad, suggesting the replaced p l a g i o c l a s e i s l a b r a d o r i t e . Replacement of t h i s sort i s p o s s i b l y r e l a t e d to hydrothermal a l t e r a t i o n of basa l t , X-POCARS PLAME UGHT Rgure 10. Thin-section photograph showing actinoite (pale yellow) and plagioclase crystals forming pilotaxitic texture in basaltic greenstone. - 40 -a common phenomenon at ocean ridges. What may be replaced amygdules are observed as d i f f u s e spots within the dominantly p i l o t a x i t i c textured matrix, and they consist mostly of a c t i n o l i t e and quartz with minor amounts of hornblende and muscovite. In one sample scattered equidimensional 0.1 mm grains of pale green hornblende and a c t i n o l i t e commonly d i s p l a y very t h i n r e a c t i o n rims. Based on t h e i r s i z e , shape and replacement minerals these are l i k e l y replaced augite pyroxene phenocrysts. Larger (up to 2 mm) more prismatic grains that also show r e l i c t ( ? ) carlsbad twinning are probably replaced p l a g i o c l a s e phenocrysts. The abundance of a l b i t e , c h l o r i t e and a c t i n o l i t e and the absence or r a r i t y of hornblende and/or prehnite, pumpellyite, lawsonite or epidote (except as an a l t e r a t i o n mineral) i s t y p i c a l of the a l b i t e -a c t i n o l i t e - c h l o r i t e zone of the lower greenschist metamorphic f a c i e s (Winkler, 1979). These rocks, except where near i n t r u s i v e bodies, were probably subjected to temperatures no greater than approximately 500°C, where a c t i n o l i t e i s replaced by hornblende; and pressures no greater than 6 to 8 k i l o b a r s , where lawsonite could form. Ten samples of mafic v o l c a n i c rock from the study area (see Figure 4 f o r l o c a t i o n s ) were analyzed for trace elements Zr, Y, Sr, Nb and the major element T i by X-ray fluorescence methods. Ratios of t h e i r concentrations were pl o t t e d on binary and ternary diagrams (Figures 12 to 14) f o l l o w i n g the method outlined by Pearce and Cann (1973) fo r d i s c r i m i n a t i n g magma types. This method evolved from studies - 4 1 -SAMPLE Zr (ppm) Ti (ppm) Y (ppm) Nb (ppm) 7215 75 2300 14 10 7237 73 5480 17 9 6191 204 6900 28 16 6203 32 2750 12 4 7211 54 5830 19 6 8102 216 17900 28 28 32015 118 12800 37 12 32014 57 5050 19 8 31912 81 7000 26 9 31913 46 4820 19 6 1 ocean island basalt mean 115 16250 29 32 1 ocean floor basalt mean 92 8350 30 5 FIGURE 11. Trace-element geochemistry of Apex Mountain Group greenstones (values are ±10%, Stanya Horsky, pers. comm., 1982) 1(Pearce and Cann, 1973). «*- alkalic transitional tholeiitic •> island arc low-K thol. & calc-alk basalt—————— Carlsberg Ridge (OF) — Palmer Ridge (OF) Apex Mountain Group* ••• •••••• Mid Atlantic Ridge 45 N(0F) Hawaiian tholeiites (01) i 1 i i i i i i i i i i i i i i i i 0.1 1.0 10 Y/Nb FIGURE 12. Alkalic to tholeiitic plot , adapted from Pearce and Cann, 1973. - 42 -Ti/100 Figure 13. Discrimination diagram, adapted from Pearce and Cann, 1973. (D) ocean island or continental basalt, (B) ocean floor basalt, (A) & (B) low-K tholeiites, (C) & (D) calc-alkali basalt. - 43 -Ti/100 Figure 14. Discrimination diagram, adapted from Pearce and Cann, 1973. (A) low-K tholeiites, (B) calc-alkali basalt, (C) ocean floor basalt. - 44 -of recent v o l c a n i c rocks of known composition and tectonic s e t t i n g but i s extended most a p t l y to ancient volcanic rocks. The method i s based on determinations of r e l a t i v e concentrations of the trace elements Zr, Y, Sr and the major element T i . Unlike most major elements these are high l y immobile and t h e i r o r i g i n a l r e l a t i v e concentrations are r e l a t i v e l y unaffected by hydrothermal and metamorphic processes. Figures 13 and 14 contain the various magma-type f i e l d s defined by Pearce and Cann. Results from t h i s study show the majority of analyzed samples f a l l within the f i e l d of ocean f l o o r b a s a l t s ; that i s , they have high Y/Nb r a t i o s , r e l a t i v e l y normal T i to Zr r a t i o s , and compositions t r a n s i t i o n a l between c a l c - a l k a l i n e basalt and low-potassium t h o l e i i t i c b a s a l t . The compositions are i n d i c a t i v e of plume-type magmas generated at shallow to moderate mantle depths and are t y p i c a l of oceanic spreading ridges (Green and Ringwood, 1969). These r e s u l t s are quite s i g n i f i c a n t as an ocean ridge o r i g i n l i m i t s the te c t o n i c s e t t i n g f o r the basalts of the Apex Mountain Group to e i t h e r an open ocean basin or a back-arc basin (see Chapter IV). One sample has a composition i n d i c a t i v e of an ocean i s l a n d o r i g i n which may be also g e n e t i c a l l y associated with r i s i n g mantle plumes. The occurrence of l o c a l "hot s p o f - r e l a t e d ocean* is l a n d s could be expected as they are common on modern ocean p l a t e s . The majority of v o l c a n i c rocks are inter p r e t e d to be deposited i n water greater than 2 km deep, based on t h e i r general lack ( u s u a l l y <5%) of v e s i c l e s , which indicates the lavas were extruded - 45 -under s u f f i c i e n t h y d r o s t a t i c pressures to keep v o l a t i l e s i n s o l u t i o n (Moore, 1970). This i s supported by the general lack of explosive v o l c a n i c s and p i l l o w lavas, and absence of h y a l o c l a s t i t e s i n the study area* Local amygdaloidal lava and p i l l o w lava i n Unit 3; however, ind i c a t e s that at le a s t some of the volcanic rocks were extruded i n r e l a t i v e l y shallow water as, f o r example, near a seamount or along a high-standing r i d g e . Rocks of Unit 4A are exposed along the northwest side of Shoemaker Creek. They consist mainly of poorly-sorted fragmental t u f f and occasional ash flow t u f f . They are generally paler green i n c o l o r than b a s a l t i c greenstones of the study area and also s l i g h t l y l e s s dense. Fragments make up to approximately 60% of the fragmental t u f f and co n s i s t mostly of broken sodic p l a g i o c l a s e c r y s t a l s with pieces of ba s a l t , quartz, and dark grey chert also present. The fragments are angular to subangular, range In s i z e from 0.5 mm to 2 mm, and are c h a o t i c a l l y oriented within an amorphous dark brown matrix of a l t e r e d volcanic ash. Angular, mafic, cognate l a p i l l i up to 4.0 cm across were also observed at sev e r a l l o c a l i t i e s . In t h i n section a very crudely developed, anastomosing lamination i s formed by discontinuous s t r i n g e r s of c r y p t o c r y s t a l l i n e aggregates of a pale green mineral ( c h l o r i t e or epidote) and amorphous ash (Figure 15). These may represent s a u s s u r i t i z e d p l a g i o c l a s e - r i c h volcanic ash. Ash flow t u f f s consist mostly of approximately 20% broken lath-shaped p l a g i o c l a s e c r y s t a l s i n a dark brown to black c r y p t o c r y s t a l l i n e matrix (ash) (Figure 16). In t h i n - s e c t i o n the t u f f s o c c a s i o n a l l y display welded textures ( l e f t - 4 6 -PLANE LIGHT Pgure 15. Thin-section photograph of Unit 4A showing amorphous ash (dark) and cryptocrystalline aggregates of chiorite(?) or epidote<?). - 47 -X-POLARS Figure 16. Thin-section photograph showing piagioclase laths, amorphous ash, and welded texture in ash flow tuff. - 48 -side of Figure 16). Welding of volcanic ash or glass i n d i c a t e s the deposits were l o c a l l y s t i l l hot when deposited; thus, these deposits are i n t e r p r e t e d as underwater ash flows rather than ash f a l l d eposits. Unit 4A i s i n t e r p r e t e d to have been formed by quiet deposition i n a submarine environment. L i t t l e reworking has occurred as shown by the lack of sedimentary textures and reworked c l a s t i c m a t e r i a l . The v o l c a n i c l a s t i c material was probably vented d i r e c t l y i n t o seawater and subsequently deposited. A nearby source i s postulated based on the presence of angular l i t h i c and c r y s t a l fragments, mafic cognate l a p i l l i and the absence of reworked fragments within the t u f f . Welded textures e i t h e r i n basalt or t u f f are diagnostic evidence of a nearby source of volcanic m a t e r i a l . 3. Limestones (Shoemaker Creek, O l a l l a Creek) The most prominent limestone i n the study area occurs along Shoemaker Creek (Figure 4 ) . It i s moderately r e c r y s t a l l l z e d and contains an abundant and varied fauna. Contacts with over- and underlying units are sharp to gradational and are concordant with surrounding bedding. The more southerly part of the limestone i s 25 to 30 m t h i c k and outcrops can be found along s t r i k e f o r approximately 1.5 km. I t i s s t r u c t u r a l l y o v e r l a i n by massive and bedded dark grey chert. Immediately below the limestone i s a 1 to 5 m t h i c k , discontinuous conglomerate layer with dominantly v o l c a n i c l a s t i c and chert pebbles set i n a g r i t t y calcareous matrix. - 49 -L o c a l l y , the limestone i s gradational with and underlain by calcareous greywacke. Within t h i s limestone there are d i s t i n c t layers and lenses of limestone up to 5 m thick that vary i n amount and s i z e of b i o c l a s t s . The majority of samples c o l l e c t e d were i n t r a m i c r i t e s with some bi o m i c r i t e s and p e l m i c r i t e s . Sparry cement was recognized i n only one sample. M i c r i t e makes up to 40% to 80% of the samples and i n t r a c l a s t s and what are probably l i t h o c l a s t s , are well rounded and t y p i c a l l y contain f o s s i l fragments and o o l i t e s . Occasional angular v o l c a n i c l a s t i c and dark grey chert fragments are present i n t h i s limestone and these, along with i n t r a c l a s t s , are up to 3 cm i n diameter. Allochems include fragments of c r i n o i d stems, brachiopods, pelecypods, bryozoa, t r i l o b i t e s , endothyrid and f u s u l i n i d f o raminifera, stromatoporoids, sponges, ostracods, conodonts, and f i s h teeth (Figure 17) . P e l l e t s and o o l i t e s are also common and some b i o c l a s t s have a l g a l coatings. Pelecypod i n f i l l i n g s are also present. F o s s i l s i n the Shoemaker Creek limestone were studied by Pedder and Mamet ( i n Read and Ok u l i t c h , 1977). Both recognized faunal assemblages of two d i f f e r e n t ages. L i t h o c l a s t s containing reworked stromatoporoids, tabulate c o r a l s (Squameofavosites), and bryozoa were i d e n t i f i e d as Late S i l u r i a n or E a r l y Devonian. L i t h o c l a s t s and matrix containing endothyrid foraminifera were assigned a probable Carboniferous age. The limestone was considered to be a possible reworked deposit although Mamet i d e n t i f i e d i n s i t u sponges of indeterminate age i n the matrix. - So-0.5 mm i Scale Figure 17. Photomicrograph showing typical fauna of Shoemaker Creek Limestone. - 51 -Data gathered from t h i s study confirms that t h i s limestone i s indeed a reworked deposit (see below) and that the age i s Middle-Late T r i a s s i c or younger. Danner (verbal communication, 1981) assigned a p o s s i b l e Late Devonian to Late M i s s i s s i p p i a n age to f o r a m i n i f e r a i n the matrix. Endothyrid foraminifera range into the T r i a s s i c (Danner, verbal communication, 1984); thus, i t i s possible those present i n the Shoemaker Creek limestone are a c t u a l l y T r i a s s i c i n age. M. Orchard (G e o l o g i c a l Survey of Canada, v e r b a l communication, 1982) assigned a Ladinian to Karnian (Middle-Late T r i a s s i c ) age to extracted platform conodonts Neogondolella c f . N.  excelsa(?) (Figure 18). I t i s unknown whether the conodonts came from matrix or l i t h o c l a s t m a t e r i a l ; thus, the minimum age of the limestone i s at l e a s t Ladinian to Karnian. This r e s u l t i s quite s i g n i f i c a n t as the minimum age of the Apex Mountain Group has p r e v i o u s l y been considered as Carboniferous. In l i g h t of this new evidence our i n t e r p r e t a t i o n of the t e c t o n i c evolution of the Apex Mountain Group must be revised (Chapter I V ) . The coexistence of Middle-Late T r i a s s i c and Devonian-Carboniferous fauna i n the matrix i s enigmatic. Although the Paleozoic fauna appear i n the matrix as d i s c r e t e b i o c l a s t s , they must be derived from older limestones. The faunal assemblage and the abundance of o o l i t e s i n the Shoemaker Creek Limestone are i n d i c a t i v e of warm, shallow water, high energy conditions i n a carbonate bank d e p o s i t i o n a l s e t t i n g . The abundance of rounded i n t r a c l a s t s , the wide range of faunal ages and the occurrence of l i t h i c fragments up to 3 cm i n diameter suggest that s i g n i f i c a n t reworking of the sediments occurred under -52-0.2 mm i i Scale Figure 18. Photomicrograph of platform conodonts Neogondolella cf. N. excelsa (?), extracted from Shoemaker Creek Limestone. Age is Ladian-Karnian (M. Orchard, G.S.C. written communication, 1982) 1fish tooth. - 53 -high energy conditions, p r i o r to f i n a l d e p o s i t i o n . I t i s u n l i k e l y that one carbonate bank would e x i s t from S i l u r i a n - Devonian(?) to Late T r i a s s i c time. This along with the f a c t that no f o s s i l s of Permian or E a r l y T r i a s s i c age have been i d e n t i f i e d , suggests that the limestone was at l e a s t p a r t l y derived from u p l i f t e d S i l u r i a n -Devonian(?) and Carboniferous bank deposits i n Middle to Late T r i a s s i c time. The lack of i n t e r n a l s t r a t i f i c a t i o n and sedimentary st r u c t u r e s , the ungraded nature, and the a s s o c i a t i o n of an underlying pebble conglomerate suggest that f i n a l deposition was rapid and i n i t i a t e d by a high density g r a i n flow or t u r b i d i t y c urrent. The coarse-grained limestone l i e s s t r a t i g r a p h i c a l l y and s t r u c t u r a l l y above the underlying pebble conglomerate with the s t r a t i g r a p h i c f a c i n g d i r e c t i o n being toward the east. Older pebblesize l i t h o c l a s t s were probably incorporated into the f i n e r -grained calcareous matrix during f i n a l d e position. Thick chert sequences above and fur t h e r below the limestone i n d i c a t e f i n a l d e p o s i t i o n occurred i n quiet, probably much deeper water. To the north the Shoemaker Creek Limestone i s broken by f a u l t i n g and i s s l i g h t l y more r e c r y s t a l l i z e d . The b i o c l a s t s are l a r g e r , and l i t h i c fragments are also more common. Faunal assemblages are the same as to the south; however, no conodonts were found i n the northernmost part of the limestone. Well rounded i n t r a c l a s t s are also sparse toward the north, perhaps r e f l e c t i n g l e s s reworking. Some l i t h o c l a s t s also contain abundant smaller, rounded i n t r a c l a s t s that are s i m i l a r to matrix material i n the southernmost part of the limestone. Both parts of the limestone - 54 -are i n t e r p r e t e d to have formed i n the same manner. Although the present s t r a t i g r a p h i c p o s i t i o n s are complicated by f a u l t i n g , based on the s i m i l a r i t i e s i n thickness, fauna present, reworked nature, and proximity, both parts are interpreted to be the same limestone. Two prominent limestone outcrops occur at the 5500 and 5000 foot l e v e l s of O l a l l a Creek (Upper and Lower Limestones r e s p e c t i v e l y ) . Both are highly r e c r y s t a l l i z e d and the only i d e n t i f i a b l e f o s s i l remains are c r i n o i d columnals that range up to 2 cm i n diameter and make up to 75% of the rock. Danner (verbal communication, 1981) suggested a Carboniferous age based on the s i z e of the c r i n o i d columnals. The matrix of both limestones c o n s i s t s of secondary red jasper that has probably replaced f i n e grained carbonate; however^ much of the higher e l e v a t i o n limestone consists of pure white marble. Both limestones occur as large, angular, discordant blocks l y i n g with sharp contacts In greenstone. A small portion of the lower limestone contains bedding which s t r i k e s to the northwest and dips to the northeast. Nearby bedded cherts s t r i k e c o n s i s t e n t l y to the northeast and dip to the southeast; however, jumbled and discordant r e l a t i o n s h i p s are also observed i n nearby c h e r t s . The lower limestone block i s approximately 25 to 30 m t h i c k and 150 m long; the upper limestone i s about twice t h i s s i z e . Several smaller blocks were observed around the edges of the l a r g e r blocks. The limestones are i n t e r p r e t e d to be at d i f f e r e n t s t r a t i g r a p h i c i n t e r v a l s based on d i f f e r e n c e s i n adjacent u n i t s . - 55 -The abundance of c r i n o i d stems i n these limestones i s i n d i c a t i v e of a warm-water, carbonate bank de p o s i t i o n a l environment. However, based on t h e i r blocky and discordant nature and the presence of associated smaller blocks they are interpreted as o l i s t o l i t h s . They must have been f a i r l y coherent or l i t h i f i e d during f i n a l deposition due to t h e i r a n g u l a r i t y and unfragraented nature. Therefore, t h e i r s t r a t i g r a p h i c p o s i t i o n i s a r e s u l t of post-depositional u p l i f t , followed by gravity-induced mechanical f a i l u r e and redeposition as o l i s t o l i t h s . Blocky f a i l u r e may have occurred s h o r t l y a f t e r i n i t i a l d e p osition as cementation of limestone operates quickly In warm water. Other limestones i n the study area occur as scattered pods, l e n t i c u l a r lenses, or blocks within other u n i t s , u s u a l l y greenstone. A l l of these are r e c r y s t a l l i z e d to such a degree that no i n t e r n a l character Is recognized. They range i n s i z e from 1 m to about 10 m across. Some of these limestones appear to be part of a normal s t r a t i g r a p h i c sequence; however, they were not mapped as separate u n i t s due to e i t h e r t h e i r discontinuous nature or t h e i r l i m i t e d exposures. Small, angular blocks of limestone i n greenstone were l i k e l y incorporated during extrusion of lava. L e n t i c u l a r lenses and pods may represent pelagic or reworked accumulations ponded i n topographic depressions. 4. Chert Breccia/Conglomerate (Unit 7A) Exposures of Unit 7A occur on Beaconsfield Mountain (Figure 4 ) . They co n s i s t of 60% to 80% poorly sorted, w e l l - c r y s t a l l i z e d angular - 56 -grey chert and rare volcanic c l a s t s set i n a m i c r o c r y s t a l l i n e matrix of b i o t i t e muscovite and quartz m i c r o l i t e s (Figure 19). Matrix grains are 0.01 mm and smaller and the assemblage of quartz, muscovite, and b i o t i t e i s i n d i c a t i v e of low to medium grade metamorphism of p e l i t i c rocks, and r e f l e c t s the proximity of Unit 7A to the g r a n o d i o r i t e stock (Unit 9) i n the middle of the study area. Chert c l a s t s range from pure quartz to those with abundant m i c r o l i t i c a c t i n o l i t e and hornblende to r a d i o l a r i a n chert. As w e l l , i n d i v i d u a l quartz grains i n the chert c l a s t s range from 0.01 to 0.05 mm i n s i z e . These c h a r a c t e r i s t i c s r e f l e c t the d i f f e r e n t types of chert present i n the study area and show that e a r l y l i t h i f i c a t i o n of cherts was prevalent. The unit i s o v e r l a i n and underlain by massive and bedded chert. On the southern slope of Beaconsfield Mountain Unit 7A i s 60 to 120 m t h i c k . Here c l a s t s are well rounded and range from coarse sand to pebble s i z e (1 to 10 mm). Near the summit of Beaconsfield Mountain the unit abruptly thickens to about 600 m over an i n t e r v a l of 60 to 85 m. The upper contact remains almost p a r a l l e l to overlying bedding; however, the lower contact becomes, i n places, h i g h l y discordant with underlying chert (Figure 4 ) . On the northwest slope of Beaconsfield Mountain both contacts are s u b p a r a l l e l to adjacent bedding and the unit thins gradually towards, the Apex townsite. Within the t h i c k e s t parts of Unit 7A c l a s t s a t t a i n s i z e s up to 10 cm across and are very angular and unsorted. Many of the l a r g e r c l a s t s of chert are rectangular, i n d i c a t i n g they were derived from a l r e a d y - l i t h i f i e d bedded chert. Near the upper contact c l a s t s are notably smaller and more rounded. - 57 -0.1 mm PLANE LIGHT Figure 19. Thin section photograph of Unit 7A. Light colored grains are chert clasts. - 58 -This unit i s in t e r p r e t e d as a submarine fan deposit. The large s i z e and ang u l a r i t y of c l a s t s , the ungraded nature, as well as t h e i r d i r e c t l i t h o l o g i c r e l a t i o n s h i p with underlying chert suggest continuous rapid deposition without s i g n i f i c a n t transport, i n a high energy environment. Deposition was probably accomplished by successive high-density debris flows. Smaller c l a s t s i n the thinner portions of the un i t near the upper contact are representative of lower energy regimes present on the shallow i n c l i n e s near the upper portions of the fan. The lower contact i s a paleo-erosional surface and the abrupt- change i n r e l i e f (approximately 400 m) of t h i s contact along the Beaconsfield Mountain summit i s probably due to a pal e o - f a u l t scarp. The map pattern of Unit 7A i s s i m i l a r to a l o n g i t u d i n a l c r o s s - s e c t i o n through a submarine fan with transport d i r e c t i o n being towards the northeast (Figure 4 ) . This unit also provides an Important s t r a t i g r a p h i c r e l a t i o n s h i p as i t faces upward to the east. The submarine fan o r i g i n of Unit 7A and the considerable r e l i e f of the underlying e r o s i o n a l surface are markedly s i m i l a r to chert breccia/conglomerate ov e r l y i n g the Permo-Triassic unconformity near O l a l l a (Chapter I ) . Clasts of Unit 7A are the same (chert) as those at O l a l l a and are approximately the same s i z e . However, c l a s t s are more angular i n Unit 7A and the matrix i s p e l i t i c as opposed to g r i t t y . The O l a l l a unit i s also o v e r l a i n by c l a s t i c rocks and i t s age i s Karnian-Norian, whereas Unit 7A, i n the study area, i s over-and underlain by massive chert and i t s age i s pre-Late T r i a s s i c . The units are not l i k e l y c o r r e l a t i v e ; however, they do represent very s i m i l a r d e p o s i t i o n a l s e t t i n g s . - 59 -Exposures of f i n e r grained chert breccia/conglomerate also occur within other units as t h i n , i n d i s t i n c t lenses up to 5 m t h i c k . Because of t h e i r i n d i s t i n c t character and t h e i r discontinuous nature they were not mapped separately. Clasts i n these deposits range up to 10 mm across and are subangular to well rounded. Sorting i s generally poor and some of these deposits are more properly classed as greywackes or pebbly mudstones. These are probably the r e s u l t of minor t u r b i d i t y currents or debris flows within submarine channels. 5. C h e r t - A r g i l l i t e (Unit 5) Approximately 500 m of Unit 5 are exposed near the mouth of Shoemaker Creek. The unit consists mainly of dark grey bedded chert with i n t e r l a y e r e d massive to crudely f i s s i l e black a r g i l l i t e which has l o c a l l y developed hackly s l i c k e n s i d e s . I n d i v i d u a l a r g i l l i t e layers are up to 40 m t h i c k and contain abundant t h i n -bedded chert layers or nodules, and elongate bedded chert blocks up to 10 m across. Bedding i n these blocks i s s u b p a r a l l e l to bedding i n the surrounding a r g i l l i t e . Limestone blocks up to 1 m across, chert pebble lenses and subangular boudinaged layers of greywacke up to 0.5 m across are also present within a r g i l l a c e o u s l a y e r s . The a r g i l l i t e contains s i l t to pebble-size, subangular to rounded, l e n t i c u l a r grains of c r y p t o c r y s t a l l i n e chert that o c c a s i o n a l l y contain flakes of c h l o r i t e and muscovite. I n d i v i d u a l chert grains are suspended within and separated by anastomosing f i l m s of dark, very f i n e , amorphous cla y or mica and opaque organic - 60 -m a t e r i a l (Figure 20). This crudely laminated f a b r i c i s probably due to mechanical breakup of chert layers during compaction, dewatering, or d i s s o l u t i o n ; followed by mechanical r o t a t i o n of the chert fragments into t h e i r s u b p a r a l l e l arrangement p a r t i a l l y during b u r i a l , or more prominently during tectonic deformation. The l a t t e r could account for the rounded nature of some g r a i n s . Pressure s o l u t i o n i s ruled out as a mechanism to create t h i s f a b r i c as there i s no evidence of s t y l o l i t e s or grain growths. Unit 5 has several gradational aspects. A r g i l l i t e s show a progressive upward increase i n grain s i z e to s i l t i n the upper parts of the u n i t . Cherts show a gradation from dominantly black a r g i l l a c e o u s cherts i n the lower parts of the unit to purer, l e s s a r g i l l a c e o u s cherts In the upper part. A general upward increase i n green color of a r g i l l i t e s r e f l e c t s an increase i n f i n e grained v o l c a n i c m a t e r i a l . This increase i s perhaps a precursor to l a t e r , t h i c k , f i n e , v o l c a n i c l a s t i c deposition represented by Unit 4A (Figure 6 ) . The presence of amorphous organic matter, the f i n e grained nature of the a r g i l l i t e s and the abundance of i n t e r c a l a t e d chert i s i n d i c a t i v e of quiet deposition i n a r e s t r i c t e d basin. Although a r g i l l i t e s are probably terrigenous, the lack of coarse e l a s t i c s except as large blocks precludes a nearby source. Unit 5 i s truncated along s t r i k e by a f a u l t northwest of Shoemaker Creek and the unit does not appear on the other s i d e . The f a u l t i s - 61 -PLANE UGHT Figure 20. Thin section photograph of Unit 5 showing chert grains suspended within argillite. - 62 -i n t e r p r e t e d as a paleo-scarp and deposition of Unit 5 i s interpreted to have been within a l o c a l f a u l t bounded sub-basin. This i s a common phenomenon along the flanks of modern ocean ridges (Garrison, 1974). Blocks of chert and limestone w i t h i n Unit 5 probably broke of f the steeper margins of the basin as a r e s u l t of l o c a l tectonism. Chert pebble lenses and boudinaged greywacke layers probably represent small t u r b i d i t y current channel deposits. The upward increase i n g r a i n s i z e and decrease i n a r g i l l a c e o u s material could represent i n f i l l i n g of the sub-basin and le s s r e s t r i c t e d c o n d i t i o n s . F o l i a t i o n i n bedded cherts and a r g i l l i t e s i s p a r a l l e l to bedding even i n the hinge regions of minor f o l d s . The absence of s l a t y cleavage i n a r g i l l i t e layers at an acute angle to bedding suggests dewatering of the sediments occurred p r i o r to deformation. This would lower pore pressures and allow platy c l a y minerals and p o s s i b l y chert grains to mechanically rotate perpendicular to the load pressure. As such, t h i s process would i n h i b i t the development of s l a t y cleavage during any subsequent compressive deformation. This process normally occurs at low pressures and temperatures and probably occurred at the same time or s l i g h t l y a f t e r d i s s o l u t i o n and c r y s t a l l i z a t i o n began i n associated chert l a y e r s . As discussed i n Chapter I I I , the major development of l a y e r - p a r a l l e l f o l i a t i o n and s l i c k e n s i d e s i n bedded c h e r t / a r g i l l i t e i s most l i k e l y due to l a t e r t e c t o n i c a l l y - r e l a t e d deformation. - 6 3 -B. N i c o l a Group Exposures of the N i c o l a Group are found west of Winters Creek where they form a w e l l - s t r a t i f i e d , westward dipping, homoclinal sequence. Approximately 3 km of Upper J u r a s s i c i n t r u s i v e s (Unit 9) crop out between these rocks and well-dated (Late T r i a s s i c ) N i c o l a Group sediments near Hedley (Figure 2). Bostock (1940a) c o r r e l a t e d the rocks near Winters Creek with the N i c o l a Group based on s i m i l a r i t y of l i t h o l o g y and a t t i t u d e . No f o s s i l s were found i n these rocks during t h i s study; thus, t h e i r c o r r e l a t i o n with the N i c o l a Group near Hedley i s uncertain. However, s i m i l a r i t i e s of l i t h o l o g y and nature of the sediments suggest a close c o r r e l a t i o n . The N i c o l a Group west of Winters Creek consists mainly of w e l l -s t r a t i f i e d , v a r i c o l o r e d q u a r t z i t e s with interbedded l i g h t - c o l o r e d , impure marble, dark grey meta-pelites, and rare pebble conglomerate lenses. No f a c i n g i n d i c a t o r s were observed i n these rocks. A l l of the rocks are thermally metamorphosed and show a gradation from h i g h l y r e c r y s t a l l i z e d textures adjacent to Unit 9 to fine-grained hornfels f a r t h e r away across s t r i k e . Highly r e c r y s t a l l i z e d q u a r t z i t e s with no v i s i b l e s tructures are the most common rock type w i t h i n about 300 m of Unit 9. L o c a l skarn i s present and contains garnet porphyroblasts up to 15 mm In diameter. Farther away bedding i s preserved. Quartzites t y p i c a l l y show 1 to 6 cm t h i c k banding and consist of 75% to 90% m i c r o c r y s t a l l i n e , r e c r y s t a l l i z e d and i n t e r d i g i t a t e d quartz grains with l e s s e r amounts of b i o t i t e , muscovite, and opaques concentrated w i t h i n d i s t i n c t bands and laminae. P e l i t i c hornfels contains mostly quartz with l e s s e r amounts of b i o t i t e , muscovite, and p o i k i l o b l a s t i c - 64 -c o r d i e r i t e ; an assemblage which i s common i n amphibolite grade hornfelses (Winkler, 1979). Convoluted laminae are preserved i n some meta-pelites and are p o s s i b l y the r e s u l t of p a r t i a l l i q u i f a c t i o n during t u r b i d i t y current deposition, deformation during dewatering, or s o f t sediment slumping. I t i s d i f f i c u l t to determine i f q u a r t z i t e s i n the N i c o l a Group i n the study area are r e c r y s t a l l i z e d cherts or quartz sandstones. Their r y t h m i c a l l y bedded nature as well as the existence of interbedded p e l i t i c layers strongly suggest that they were once bedded c h e r t s . Convolute laminae i n meta-pelites are also suggestive of quiet water conditions, unless they res u l t e d from t u r b i d i t y currents. Rare pebble conglomerate lenses are probably the r e s u l t of minor t u r b i d i t e d e p o s i t i o n . N i c o l a sediments at Winters Creek resemble those at Hedley i n gross l i t h o l o g y (see Chapter I for d e s c r i p t i o n of Hedley sequence), w e l l -s t r a t i f i e d nature, and s c a r c i t y of interbedded extrusive v o l c a n i c s . The o r i g i n of the sequence i n the study area i s probably s i m i l a r to that proposed by Rice (1947) f o r N i c o l a sediments at Hedley; that i s , quiet deposition i n a moderate to deep, extensive marine basin. C. .Depositional S e t t i n g of the Apex Mountain Group The general d e p o s i t i o n a l s e t t i n g of the Apex Mountain Group i n the study area i s i n t e r p r e t e d to be a quiet, deep-water open ocean basin. P e l a g i c and hemipelagic cherts and a r g i l l i t e s predominate along with - 65 -extruded, subaqueous volcanic flows and minor p i l l o w lavas which have compositions equivalent with modern ridge-generated ocean f l o o r b a s a l t s . P a l e o n t o l o g i c a l data i n d i c a t e deposition may have been more or l e s s continuous from Carboniferous to at l e a s t Late T r i a s s i c (Karnian-Norlan) time, although there i s a conspicuous absence of d e f i n i t i v e Permian and E a r l y T r i a s s i c f o s s i l s . Sources of f i n e terrigenous debris were remote or i s o l a t e d from the basin of deposition, as indicated by t h e i r scarceness. The average depth of the basin i s v a r i a b l e but believed to be greater than 2 km. Areas of high r e l i e f such as an ocean ridge or l o c a l scarps account for t u r b i d i t y current deposited pebble conglomerates, submarine fan breccia, t u r b i d i t e limestone, and o l i s t o l i t h s i n the area. L o c a l non-ridge ocean i s l a n d s (seamounts) may also have been present and could account for terrigenous m a t e r i a l . Rounded and angular c l a s t s of chert i n t u r b i d i t e conglomerates are noteworthy as they i n d i c a t e that some pelagic sediments were u p l i f t e d ( p o s s i b l y by ridge transform f a u l t i n g ) and were also competent enough to allow abrasion i n a higher energy environment than that i n which they were deposited. R i f t i n g along the flanks of a spreading ridge may have also l ed to ponding of sediments i n l o c a l , r e s t r i c t e d sub-basins. P e l a g i c cherts and f i n e terrigenous a r g i l l i t e s only require quiet conditions for deposition; however, t h e i r a s s o c i a t i o n with sparsely v e s i c u l a r basalt and d i s t a l t u r b i d i t e deposits, suggests deposition at near-abyssal depths (2-4 km) or at moderate depths (<2 km) at long distances from a source of coarse terrigenous sediments. Sparse f o s s i l evidence shows a general westward younging of Apex Mountain sediments which i s i n agreement with an i n t e r p r e t a t i o n of an a c t i v e l y spreading - 66 -ridge l y i n g to the west ( i n the present sense) during deposition (see Chapter I I I ) . Sedimentological evidence i s inconclusive as to whether t h i s ridge was within a back-arc or open ocean te c t o n i c s e t t i n g , however, the general lack of v o l c a n i c l a s t i c rocks precludes a back-arc s e t t i n g . Further d i s c u s s i o n of t e c t o n i c s e t t i n g for the Apex Mountain Group i s given i n Chapter IV. The base of the Apex Mountain Group i s not exposed and, other than Silurian-Devonian(?) l i t h o c l a s t s In the Shoemaker Creek limestone, c l a s t i c rocks show no evidence of an older source t e r r a i n . The Apex Mountain Group i s int e r p r e t e d to have been deposited d i r e c t l y upon oceanic c r u s t . A quiet, moderate to deep ocean environment i s also proposed for the N i c o l a Group In the study area, and there i s evidence to suggest the youngest Apex Mountain Group rocks and the N i c o l a rocks represent a t r a n s i t i o n a l succession. N i c o l a Group qu a r t z i t e s west of the Winters Creek f a u l t have very s i m i l a r l i t h o l o g y and s t r i k e d i r e c t i o n as those of the Apex Mountain Group on the east side of the f a u l t . The Apex Mountain Group rocks d i f f e r i n being well-bedded and having e a s t e r l y d i p s . Although the two groups are f a u l t separated and t h e i r contact was not observed, t h e i r marked s i m i l a r i t y of l i t h o l o g y and o r i e n t a t i o n suggests t h e i r deposition may be s p a t i a l l y and temporally r e l a t e d . This contrasts with the s i t u a t i o n at O l a l l a where a d i s t i n c t e r o s i o n a l unconformity separates Apex Mountain Group rocks from overlying coarse marine e l a s t i c s of presumed N i c o l a Group (Ross and Barnes, 1972). The ove r l y i n g rocks are i n t e r p r e t e d as a submarine canyon i n f i l l i n g . If the rocks above the unconformity are c o r r e l a t i v e with the N i c o l a Group, they must represent a d i s t i n c t f a c i e s change from the rocks at Winters Creek and Hedley. - 67 -D. Regional C o r r e l a t i o n s with Other Eug e o c l i n a l Assemblages Co r r e l a t i o n s of the Apex Mountain Group with other eugeoclinal assemblages are based on age, geographic p o s i t i o n , l i t h o l o g y and s t r u c t u r a l s t y l e . Such c o r r e l a t i o n s must be approached with caution as reg i o n a l v a r i a t i o n s i n structure and metamorphic grade are common i n southern B r i t i s h Columbia. L i t h o l o g i c a l l y s i m i l a r successions often d i f f e r widely i n age and poor faunal records within each assemblage preclude good c o r r e l a t i o n s . The known time of deposition of the Apex Mountain Group ranges from E a r l y Carboniferous to at l e a s t Ladinian-Karnian. A minimum age i s given by the ove r l y i n g sediments of the N i c o l a Group near O l a l l a (Karnian-Norian). The r e l a t i o n s h i p s between the Apex Mountain and N i c o l a Groups are discussed i n Section ID. Other assemblages p o s s i b l y temporally c o r r e l a t i v e with the Apex Mountain Group include the Kobau Group, Chapperon Group, Harper Ranch Group, Cache Creek Group, and parts of the Eastern Assemblage (Figure 3). A l l of these have ages and gross l i t h o l o g i e s s i m i l a r to the Apex Mountain Group and t h e i r present r e l a t i v e geographic posi t i o n s are presumably not due to major transcurrent f a u l t i n g . The Kobau and Anarchist Groups consist of low-grade metamorphic rocks derived from a succession of eugeoclinal sedimentary and basic v o l c a n i c rocks ( O k u l i t c h , 1973; Fox and others, 1977). Metamorphism i s more Intense i n the Kobau Group and has obscured sedimentary structures making determinations of d e p o s i t i o n a l environment d i f f i c u l t ; - 68 -however, Ok u l i t c h (written communication, 1984) believes abundant v o l c a n i c l a s t i c material may be present i n the Kobau Group. The Kobau Group i n North-Central Washington also contains abundant p y r o c l a s t i c sediments (Rinehart and Fox, 1972). The Anarchist Group consists of a r g i l l i t e , greywacke, conglomerate, metachert, basic v o l c a n i c s , v o l c a n i c l a s t i c rocks, and limestone of probable Permian age although f o s s i l s are generally sparse and poorly preserved (Rinehart and Fox, 1972; Waters and Krauskopf, 1941). Limestone units range from b i o c l a s t l c to m i c r i t i c and contain shallow water faunas. In north - c e n t r a l Washington the Kobau Group and a t h i c k sequence of metavolcanic rocks (Palmer Mountain Greenstone) may o v e r l i e the Anarchist Group along a possible unconformity. Thus, the maximum age of the u n f o s s i l i f e r o u s Kobau Group would be probably Late Permian (Rinehart and Fox, 1972). Ross and Barnes (verbal communication, 1982) beli e v e the Anarchist and Kobau Groups s t r i k e into one another i n t h i s area and are probably the same age. Okulitch (1973) instead suggested the Kobau underlies the Anarchist Group and i s mid-Paleozoic i n age, based on s t r u c t u r a l evidence l i n k i n g the Kobau Group to a pre-Anarchist deformational events The Anarchist Group i n Washington i s also p a r t l y o v e r l a i n unconformably by Upper T r i a s s i c sediments of the Cave Mountain Formation. These rocks are s i m i l a r to the sedimentary sequence at Hedley, and Rice (1947) considered them to be part of the sedimentary f a c i e s of the N i c o l a Group. Fox and others (1977) propose an ocean basin environment i n a back-arc t e c t o n i c s e t t i n g f o r the Anarchist and Kobau Groups. Based on t h e i r s i m i l a r i t y i n age and l i t h o l o g y , r e l a t i o n s h i p with the - 69 -ove r l y i n g N i c o l a Group, and present geographic proximity, I i n t e r p r e t the Kobau, Anarchist and at l e a s t the upper Paleozoic parts of the Apex Mountain Group to have been deposited i n the same d e p o s i t i o n a l environment (ocean basin) and po s s i b l y i n the same basin. As with the Apex Mountain Group the te c t o n i c s e t t i n g (back-arc, open ocean) i s uncert a i n . The Kobau and Anarchist Groups may also have been deposited i n an i s l a n d - a r c s e t t i n g . Other successions i n the Eastern Assemblage po s s i b l y c o r r e l a t i v e with the Apex Mountain Group include the M i l f o r d and Kaslo Groups, and t h e i r metamorphosed equivalents ( O k u l i t c h and Cameron, 1976) the Eagle Bay Formation and the Mount Ida Group. P h y l l i t e , p h y l l i t i c sandstone, chert, and conglomerate of the M i l f o r d Group unconformably o v e r l i e miogeoclinal rocks of the Lardeau Group (Monger, 1977). Limestones within the M i l f o r d Group are Carboniferous to Permian i n age (Wheeler, 1968), and the group grades upward in t o dominant greenstones of the Kaslo Group. Monger (1977) i n t e r p r e t s these rocks to be deposited i n a gradually deepening ocean basin, i n a s e t t i n g perhaps marginal to an ocean basin fa r t h e r west which may be represented by the Kobau, Anarchist and Apex Mountain Groups. The Chapperon Group consists of v a r i a b l y metamorphosed p h y l l i t e , q u a r t z i t e , greenstone, chert, and limestone, and has been assigned a Middle M i s s i s s i p p i a n age based on observed conodonts (Cameron, 1972). The age, l i t h o l o g y , and s t r u c t u r a l s t y l e are very s i m i l a r to the Apex Mountain Group and O k u l i t c h (1973) has co r r e l a t e d them with the Kobau Group. Their r e l a t i o n s h i p with overlying rocks i s also s i m i l a r to that of the Apex Mountain Group at O l a l l a . A succession of a r g i l l i t e , t u f f , greywacke, c a l c a r e n i t e , and conglomerate unconformably o v e r l i e s the - 70 -Chapperon Group near Salmon River and t h e i r age, based on observed conodonts, i s Late Karnian (Okulitch and Cameron, 1976). The conglomerate unit occurs immediately above the unconformity and contains c l a s t s of the Chapperon Group. Read and O k u l i t c h (1977) c o r r e l a t e d the overlying rocks with the basal sedimentary f a c i e s of the N i c o l a Group near Hedley. Both successions near Salmon River are l i t h o l o g i c a l l y s i m i l a r to the Apex Mountain and N i c o l a Group rocks i n the study area and they are very l i k e l y c o r r e l a t i v e . The Harper Ranch Group d i f f e r s most from the Apex Mountain Group by c o n s i s t i n g dominantly of marine v o l c a n i c l a s t i c rocks and mudstone. Smith (1979) in t e r p r e t e d the Harper Ranch Group to have been deposited i n a marine basin d i s t a l to a westerly l y i n g v o l canic a r c. Age of the basin i s M i s s i s s i p p i a n to Permian. Younger sediments (Karnian-Norian) to the west are f a c i e s equivalent with the lowermost d i s t a l arc sediments of the N i c o l a Group. The older rocks i n the Harper Ranch Group are coeval with parts of the oceanic Cache Creek Group (Smith, 1979) and probably the Apex Mountain Group, although d e f i n i t e Permian and Lower T r i a s s i c rocks were not observed i n the Apex Mountain Group. The Harper Ranch and the Apex Mountain Groups may also be i n part, l a t e r a l l y equivalent f a c i e s . Their age and oceanic o r i g i n are the strongest evidence f o r equivalence. As w e l l , t h e i r geographic p o s i t i o n s r e l a t i v e to the N i c o l a Group and t h e i r present geographic proximity also suggest possible equivalence. Upper Paleozoic c r i n o i d a l limestone i n the Harper Ranch Group and the Apex Mountain Group may i n d i c a t e s i m i l a r d e p o s i t i o n a l environments, thereby supporting an i n t e r p r e t a t i o n of a close s p a t i a l r e l a t i o n s h i p at the time of deposition. Danner (verbal communication, 1982), also reports that the limestones i n the Harper - 71 -Ranch Group contain a warm water, s u b t r o p i c a l fauna. Karnian and Norian marine v o l c a n i c l a s t i c and c l a s t i c rocks of the N i c o l a Group northeast of Kamloops and those of the N i c o l a Group near Hedley are probably l a t e r a l l y equivalent f a c i e s . The Cache Creek Group (Pennsylvanian-Upper T r i a s s i c ) consists mainly of pelagic chert and limestone interbedded with submarine volcanic rocks and associated platform carbonate. The volcanic rocks have compositions s i m i l a r to modern ocean i s l a n d basalts such as those of Hawaii or the Galapagos (Shannon, 1982); however, some greenstones have ocean f l o o r a f f i n i t i e s . The geochemical d i f f e r e n c e s between the greenstones of the two groups are r e f l e c t e d m i n e r a l o g i c a l l y . Greenstones of the Apex Mountain Group are dominated by plagioclase and those of the Cache Creek Group are dominated by clinopyroxene, the l a t t e r r e f l e c t i n g also t h e i r genetic r e l a t i o n s h i p with deep r i s i n g mantle plumes (hot spots) as opposed to shallow to moderate depth plumes of the Apex Mountain Group. A deep, open ocean basin with l o c a l high r e l i e f i s proposed by Shannon f o r the d e p o s i t i o n a l environment of the Cache Creek Group. Sequences of t h i n l y laminated black a r g i l l i t e and chert, with a lack of carbonate except as blocks or as limestone t u r b i d i t e s , are representative of the deeper parts of the basin and carbonate bank limestone, a l g a l limestone, and greenstone with i n t e r p l l l o w carbonate represent shallower portions. L i t h o l o g i c s i m i l a r i t i e s with the Apex Mountain Group are s t r i k i n g and the d e p o s i t i o n a l environments are the same except the Apex Mountain Group i n the study area lacks ocean i s l a n d v o l c a n i c s . The ages of both groups are also the same, suggesting they were once part of the same large, open ocean basin. - 72 -CHAPTER I I I STRUCTURE OF THE STUDY AREA As seen from the maps of Bostock (1940a,b, 1941), and Rice (1947), the rocks of the Apex Mountain Group and N i c o l a Group i n south-central B r i t i s h Columbia are folded on a r e g i o n a l s c a l e . Within the study area, the documentation of map-scale s t r u c t u r a l elements i s hampered by poor exposures, and a lack of coherent s t r a t i g r a p h y , marker beds, and top i n d i c a t o r s . Minor outcrop-scale f o l d s are scarce i n the Apex Mountain Group and only one macroscopic f o l d i s recognized i n the study area. F a b r i c elements such as penetrative cleavage, s c h i s t o s i t y , and mineral l i n e a t i o n s are absent; thus, unless a c t u a l refolded folds are observed, i t i s d i f f i c u l t to determine unequivocally the number of phases of deformation present. S t r a i n evaluations were not undertaken due to the homogeneity of i n d i v i d u a l u n i t s and lack of s t r a i n markers. J o i n t i n g Is common but extremely v a r i a b l e . Faults were recognized by o f f s e t s of l i t h o l o g i c u n i t s , the occurrence of topographic i r r e g u l a r i t i e s along t h e i r t r a c e , and by lineaments on a e r i a l photographs. A. F o l d i n g The broadly folded map pattern of l i t h o l o g i c u nits i n the study area i s representative of only one d e f i n i t e phase of f o l d i n g as no other macroscopic f o l d hinges were recognized. Rocks i n the northern part of the study area dip moderately to the southeast and those i n the southern part dip moderately to the northeast o u t l i n i n g an openly - 73 -folded synformal structure that i s most l i k e l y c y l i n d r i c a l i n geometry. Poles to bedding l i e on a very d i f f u s e great c i r c l e , the pole of which ( f o l d a x i s ) plunges moderately to the northeast (070/50° estimated) (Figure 21). The s c a t t e r i n bedding a t t i t u d e s may r e f l e c t o r i g i n a l sedimentary o r i e n t a t i o n s or r e f o l d i n g . Fold axes of several minor f o l d s c l u s t e r about the estimated major f o l d axis (Figure 22); however, they are r e l a t e d to a separate phase of f o l d i n g which i s discussed below. The o r i e n t a t i o n of the a x i a l plane i s broadly defined by the map pattern of l i t h o l o g i c units and the s t r u c t u r a l data which i n d i c a t e a northeasterly s t r i k e and a dip of approximately 60° to the southeast. Minor f o l d a x i a l planes do not coincide with t h i s o r i e n t a t i o n and are a l s o i n t e r p r e t e d to be r e l a t e d to a d i f f e r e n t phase of f o l d i n g . A l l of the minor f o l d s i n the Apex Mountain Group occur i n sequences of bedded chert and a r g i l l i t e . Thin bedded cherts and a r g i l l i t e s of Unit 5 are the most t i g h t l y folded and e x h i b i t a wide range of deformational s t y l e s which commonly vary along the a x i a l trace of s i n g l e f o l d s . In terms of the dominant f o l d s t y l e , asymmetric chevron folds with Interlimb angles between 45° and 60° are the most common along with chevron f o l d s which have been f l a t t e n e d to form t i g h t e r , closed and i s o c l i n a l f o l d s with hinges that are more rounded (Figure 23). Interlimb angles vary along a x i a l surfaces with i n c r e a s i n g l y t i g h t e r f o l d s being present towards the cores of folded sequences. I n d i v i d u a l chert layers form predominantly Ramsay (1973) Type l b ( p a r a l l e l ) and a l s o Ic f o l d s that show a s l i g h t thickening i n t h e i r hinge region. D u c t i l e a r g i l l a c e o u s layers form Ic and Type II ( s i m i l a r ) - 74 -N i • • 1 ~ •• F2 Fold axis 070/50 • Poles to bedding • FT and F1 + Axial planes Figure 21. - 75 -Figure 23. Enlarged sketches from field notes showing dominant fold styles. Upper fold is F1+, bottom fold is F1. Arrows on overlay indicate sense of vergence. - 77 -f o l d s . The a x i a l planes of minor folds dip toward the east, northeast, and southeast. As w e l l , asymmetric f o l d s d i s p l a y both westerly and e a s t e r l y senses of vergence. The s i g n i f i c a n c e of these i s discussed on page 90. The r e s t r i c t i o n of minor f o l d s to sequences of interbedded chert and a r g i l l i t e i s believed to be due to the abundance of a r g i l l i t e and also t h e i r t h i n l y bedded nature. The spectrum of f o l d s t y l e s encountered i s r e f l e c t i v e of v a r i a b l e amounts of s t r a i n allowed by v a r i a b l e thickness d i f f e r e n c e s and strong d u c t i l i t y contrasts between i n d i v i d u a l chert and a r g i l l i t e l a y e r s . Where the thickness of i n d i v i d u a l chert layers i s greater than approximately f i v e times that of interbedded a r g i l l i t e layers chevron f o l d s are the most common f o l d s t y l e . Chert layers commonly show a s l i g h t thickening i n the hinge regions and are usually accompanied by thinning and attenuation or boudinage on one or both limbs. The long axes of boudins are coincident with associated minor f o l d axes. The hinges i n chert layers are also often collapsed or bent inward toward the a x i a l surface. A r g i l l i t e layers show l i t t l e or no thickening i n the hinges of these f o l d s . T h i c k l y bedded (>8 cm) c h e r t / a r g i l l i t e sequences are rare but when folded have more open geometries and greater amplitudes and wavelengths than the bedded v a r i e t i e s . Where the thickness of chert layers i s less than approximately f i v e times that of associated a r g i l l i t e l a y e r s , greater amounts of s t r a i n have been allowed as evidenced by t h e i r t i g h t e r f o l d s t y l e s and more rounded - 78 -hinges. Folded chert layers d i s p l a y " f l o a t i n g " i s o c l i n a l hinges and transposed l a y e r i n g i n a d d i t i o n to s l i g h t l y thickened hinges and attenuated and/or boudinaged limbs (Figure 24). A r g i l l i t e layers tend to have thickened hinges i n these f o l d s . As mentioned i n Chapter I I , the f o l i a t i o n i n the a r g i l l i t e layers i s p a r a l l e l to bedding even i n the hinges of minor f o l d s . The o r i e n t a t i o n of t h i s bedding plane f o l i a t i o n i s schematically i l l u s t r a t e d i n Figure 25. The f o l i a t i o n Is roughly perpendicular to the a x i a l plane at the bottom of the hinge and i s progressively aligned into a more s u b p a r a l l e l r e l a t i o n s h i p with the a x i a l plane toward the top of the hinge. This i s not s l a t y , a x i a l plane cleavage. Although material has flowed i n t o the hinge, presumably as a r e s u l t of f l a t t e n i n g , s l a t y cleavage p a r a l l e l to the a x i a l plane has not developed. The reason for t h i s i s believed to be r e l a t e d to previous f o l d i n g . I n i t i a l f o l d i n g of undeformed c h e r t / a r g i l l i t e sequences may have involved buckling with f l e x u r a l s l i p occurring within a r g i l l a c e o u s l a y e r s . This could create a strong f a b r i c of s l i p planes p a r a l l e l to bedding even i n the hinges. During progressive deformation s t r a i n would p r e f e r e n t i a l l y be taken up along these s l i p planes; and unless l a t e r deformation involved intense shearing or high c o n f i n i n g pressures combined with thermal metamorphism, i t i s possible that a secondary f a b r i c such as a x i a l plane s l a t y cleavage would not be imprinted on the e a r l i e r f a b r i c . L o c a l l y developed b e d d i n g - p a r a l l e l s l i c k e n s i d e s i n a r g i l l a c e o u s layers l i k e l y represent vestiges of the e a r l y formed s l i p planes. Minor f o l d s i n the study area were undoubtedly subjected to a c e r t a i n amount of shearing during progressive deformation. Evidence for t h i s kind of deformation -79-Figure 24. Sketch of a photograph of folded bedded chert (loc. 5285). Folds are F1 and F1 + folds. Note clockwise senses of vergence on folds and thrust faults. Also note floating isoclinal hinges, variable fold styles, attenuated limbs, transposed layering, asymmetric geometries ,and thrust faults(?) subparaiiel to bedding and axial planes. - 30 -Figure 25. Schematic illustration showing foliation in hinges of minor folds. - 81 -i s given by asymmetric geometries, disrupted hinges, transposed l a y e r i n g , thickened hinges, and f l a t t e n e d limbs. However, s l a t y a x i a l plane cleavage did not develop. Based on the above di s c u s s i o n i t i s believed that the e a r l i e s t f o l d i n g i n the Apex Mountain Group involved buckling with f l e x u r a l s l i p along l a y e r i n g as the dominant f o l d mechanism. P a r a l l e l (Type lb) symmetric folds would have formed during t h i s deformation. Concentric chevron f o l d s d i s p l a y i n g s l i g h t l y thickened hinge regions (Figure 23) and in t e r l i m b angles le s s than 60° (Ramsay, 1974) i n d i c a t e that some f l a t t e n i n g was also involved. As deformation progressed simple shear eventually became the dominant mechanism and s i m i l a r (Type II) asymmetric f o l d s were formed with a x i a l planes i n c l i n e d p a r a l l e l with the d i r e c t i o n of shear. Thus, the v a r i a b i l i t y of f o l d s t y l e s along a x i a l surfaces and the presence of extension r e l a t e d structures (boudinage) are probably the r e s u l t of the combined e f f e c t s of d i f f e r e n t i a l f l a t t e n i n g during e a r l i e s t f o l d i n g , and shearing deformation during the l a t e r phases of progressive f o l d i n g . In a l l cases throughout the study area the f o l d axes and a x i a l planes of the above-mentioned minor folds are p a r a l l e l . As such, both s t y l e s of f o l d i n g are believed to be the r e s u l t of a s i n g l e progressive deformation phase. E a r l i e r formed p a r a l l e l folds w i l l be termed F l f o l d s and t h e i r later-formed counterparts ( s i m i l a r f o l d s ) w i l l be termed F l + f o l d s . A p l o t of poles to minor f o l d a x i a l planes shows some s c a t t e r but i s coincident with the d i f f u s e great c i r c l e formed by the poles to bedding - 82 -(Figure 21). This suggests that F l and F1+ minor f o l d s were folded along with bedding by a l a t e r phase which may have also contributed to c l o s i n g of the e a r l i e r f o l d limbs, thereby creating the nearly-coplanar r e l a t i o n s h i p between the early phase a x i a l planes and bedding. Minor f o l d axes form a d i f f u s e 40° to 50° small c i r c l e d i s t r i b u t i o n about the derived major f o l d axis suggesting that l a t e r f o l d i n g , termed F2, was concentric. Accordingly, the open f o l d i n g phase, discussed on page 73, would represent the l a t e r deformation. Minor folds at four l o c a l i t i e s c l e a r l y show open, concentric f o l d s superimposed upon e a r l i e r t i g h t or i s o c l i n a l f o l d s (Figure 26). These l a t e r phase folds are not pervasive and ac t u a l examples are only l o c a l l y developed. The a x i a l plane o r i e n t a t i o n s of measured F2 folds are v a r i a b l e and not c l o s e l y coincident with that of the major open f o l d o u t l i n e d i n the study area (Figure 21). This may be inherently due to the small number of measured refolded f o l d s or to fanning of minor f o l d a x i a l planes about an axis p a r a l l e l to the major f o l d a x i s . The l a t t e r has been shown to occur i n folded bedded chert sequences i n Nevada ( M i l l e r and others, 1982). Measured F2 f o l d axes are also highly scattered about the derived F2 a x i s ; however, some of the F l and F l + minor f o l d axes which do approximate the estimated major f o l d axis may i n fact be F2 axes even though they were not recognized as such i n the f i e l d . C e r t a i n l y the s i m i l a r i t i e s i n s t y l e of both phases, and Inherent v a r i a t i o n s i n a l l the f o l d i n g make t h i s p o s s i b l e . As mentioned previously, minor t i g h t and i s o c l i n a l f o l d s which do not d i s p l a y a two-phase f o l d i n g r e l a t i o n s h i p o f t e n contain i n d i r e c t evidence of r e f o l d i n g or progressive deformation such as disrupted hinges, transposed l a y e r i n g , thickened hinges, and f l a t t e n e d limbs. Minor f o l d s d i s p l a y i n g two phases of f o l d i n g may have -83--83-Figure 26. Sketches from photographs showing two phases of folding. Overlay shows F1 + and F2 axial planes and sense of shear of phase 2. - 84 -developed only where e a s i l y folded, i s o c l i n a l F l + f o l d s were o r i g i n a l l y present, thus explaining t h e i r scarceness. F l and F l + folds apparently formed by v a r i a b l e , f l e x u r a l - s l i p (along layering) to shear mechanisms with most of the s t r a i n taken up i n a r g i l l a c e o u s l a y e r s . The geometry of F l and F l + minor f o l d s was c o n t r o l l e d by the l i t h o l o g i e s present. F2 f o l d s probably formed by f l e x u r a l - s l i p only, and Involved buckling, at a minor s c a l e , only of previous i s o c l i n a l f o l d s . This i n d i c a t e s that t h e i r geometry was c o n t r o l l e d by previous f o l d s with s l i p occurring along a r g i l l a c e o u s l a y e r i n g between i n d i v i d u a l i s o c l i n a l F l f o l d s . In summary, the main evidence favoring two separate phases of deformation are: 1. Poles to bedding define an F2 f o l d axis that i s most e a s i l y r e l a t e d to the open synform o u t l i n e d by the pattern of map-scale l i t h o l o g l c u n i t s ; 2. Poles to F l and F l + minor f o l d a x i a l planes have a planar d i s t r i b u t i o n that i s coincident with bedding poles and normal to the derived F2 a x i s , suggesting F l and F l + f o l d s are refolded; 3. Minor F l and F + f o l d axes form a d i f f u s e small c i r c l e d i s t r i b u t i o n about the derived F2 axis, suggesting that F2 f o l d i n g i s c o n c e n t r i c . This i s consistent with the observed s t y l e of F2 f o l d s . Evidence against the presence of two phases i s the absence of map-scale F l f o l d hinges and the I n a b i l i t y of minor f o l d data from actual refolded f o l d s - 85 -to c l e a r l y i l l u s t r a t e the presence of two phases. The absence of map-scale F l hinges may be due i n part to a low regional plunge; however, Bostock (1940b) c l e a r l y shows t i g h t f o l d hinges i n the Apex Mountain Group southwest of Keremeos. Read and Ok u l i t c h (1977) also report a nearly h o r i z o n t a l axis f or i s o c l i n a l f o l d i n g i n the Apex Mountain Group near O l a l l a . The s c a t t e r i n minor f o l d data may be the r e s u l t of primary sedimentary o r i e n t a t i o n s , l o c a l v a r i a t i o n s i n the amount and d i r e c t i o n of s t r a i n , and d i s r u p t i o n by f a u l t i n g . For the above reasons, I conclude the favoring evidence i s more convincing and accordingly, supports a conclusion of two phases of deformation i n the Apex Mountain Group. F l and F l + deformation occurred while the sediments were s t i l l s o f t enough to behave In a d u c t i l e fashion (except i n l o c a l areas of high s t r a i n ) . Pre-deformational dewatering may have occurred during b u r i a l and compaction; i n h i b i t i n g the development of s l a t y cleavage. Therefore, the observed f o l i a t i o n i n a r g i l l i t e s i s thought to be due l a r g e l y to f l e x u r a l s l i p and simple shear during F l and F l + deformation which produced a strong f a b r i c of l a y e r - p a r a l l e l s l i p planes. F2 deformation further deformed the f o l i a t i o n along with bedding into i t s present form. An a l t e r n a t e explanation for the presence of minor folds i n bedded chert and a r g i l l i t e i s by penecontemporaneous s u r f i c i a l downslope slumping. T y p i c a l c r i t e r i a f o r the recognition of s u r f i c i a l slump deformation include the r e s t r i c t i o n of f o l d s to s i n g l e beds, the upsection truncation of folds along a sedimentary contact, chaotic f o l d o r i e n t a t i o n s and undeformed burrows c u t t i n g across deformed material (Hobbs, Means and Williams, 1976). Other than a few chaotic folds i n bedded chert, none of these i n d i c a t o r s were recognized i n minor f o l d s of the study area. It i s also highly u n l i k e l y that random slumping events could account for the f a i r l y consistent o r i e n t a t i o n of minor f o l d a x i a l planes. - 86 -B. F a u l t i n g The most prominent f a u l t s i n the study area are east and southeast trending, nearly v e r t i c a l f a u l t s . Their occurrence i s recognized by o f f s e t s of l i t h o l o g i c contacts, prominent c l i f f s that occur along the s t r i k e of many of these f a u l t s , and lineaments on a e r i a l photographs. Actual displacements are d i f f i c u l t to determine and only crude estimates can be made. S t r i k e separation values range from 200 m to 1000 m. Assuming only v e r t i c a l motion has occurred then displacements would be up to 1200 m. These f a u l t s crosscut a l l rock units and f o l d i n g i n the Apex Mountain Group. Their genetic r e l a t i o n s h i p with F2 f o l d s and t h e i r a s s o c i a t i o n with regional deformational events i s discussed i n Section IIIC and Chapter IV. They may be re l a t e d to f a u l t i n g near O l a l l a where an e a s t e r l y s t r i k i n g , s u b - v e r t i c a l f a u l t cuts the Nicola and Apex Mountain Groups but not the Eocene Springbrook Formation (Bostock, 1941; Read and O k u l i t c h , 1977). A v a r i a b l y trending, nearly v e r t i c a l f a u l t separates Units 3 and 4 i n the c e n t r a l part of the map area (Figure 4 ) . In many places t h i s f a u l t i s delineated by conspicuous notches across ridges and the occurrence of springs along i t s trace. In a d d i t i o n , many l o c a l i t i e s show rocks on e i t h e r side of the f a u l t to be high l y sheared. The shear planes ge n e r a l l y dip to the east and southeast at angles s l i g h t l y steeper than those i n associated bedding. Often, however, the shear planes are nearly v e r t i c a l and at several l o c a l i t i e s they dip steeply to the west and northwest, opposite to the d i r e c t i o n of bedding d i p . The trace of the f a u l t i s c u r v i l i n e a r , and p a r a l l e l s the larg e - s c a l e openly folded pattern of l i t h o l o g i c u n i t s , suggesting t h i s f a u l t pre-dates and - 87 -was a f f e c t e d by F2 f o l d i n g . The above r e l a t i o n s h i p s are also seen along other chert/greenstone contacts and also within i n d i v i d u a l u n i t s , suggesting that many of the t h i n and discontinuous units i n the study area owe t h e i r s t r a t i g r a p h i c p o s i t i o n to c l o s e l y spaced imbrication s u b p a r a l l e l to bedding. F a u l t s are d i f f i c u l t to map due to the lack, of marker beds and poor exposure; thus, the a e r i a l extent and amount of o f f s e t i s unknown. These f a u l t s are most l i k e l y r e l a t e d to F l b r i t t l e deformation (Section I I I C ) . A northeast trending nearly v e r t i c a l f a u l t near Winters Creek separates the Apex Mountain and N i c o l a Groups i n the study area (Figures 4 and 5 ) . Although l i t h o l o g i e s on e i t h e r side of the f a u l t are s i m i l a r (Chapter I I ) , there i s a d e f i n i t e change i n dip d i r e c t i o n across the f a u l t . In a d d i t i o n , rocks adjacent to the f a u l t are h i g h l y sheared l o c a l l y . Apex Mountain Group rocks east of the f a u l t have e a s t e r l y dips and N i c o l a Group rocks on the west side dip to the west. Bostock (1941) inter p r e t e d the f a u l t to be a broken a n t i c l i n a l a x i s , although Nicola Group equivalent rocks are not found east of the f a u l t . Throughout t h i s region the contact between the two groups follows a l i n e of i n t r u s i o n and f a u l t i n g (Bostock, 1941; Rice, 1947) i n d i c a t i n g that the contact may be a major f a u l t that has served as a conduit f o r l a t e r ( J u r a s s i c ) i n t r u s i o n . Small displacement f a u l t s are also observed within i n d i v i d u a l outcrops of folded bedded chert (Figure 24). Movement on these f a u l t s probably occurred along with i n t e r l a y e r s l i p to compensate for d i f f e r e n t i a l s t r a i n . They are believed to have developed as thrust f a u l t s during F l + deformation. - 88 -C. S t r u c t u r a l I n t e r p r e t a t i o n Facing i n d i c a t o r s i n the study area, such as erosion surfaces, pillows and flow tops, show the s t r u c t u r a l l y highest rocks to be also s t r a t i g r a p h i c a l l y highest (Chapter I I ) . Evidence of age r e v e r s a l s i n the sequence, however, suggest that the observed eastward dipping homoclinal sequence i s not e n t i r e l y a continuous succession. Middle to Upper T r i a s s i c limestone i n Shoemaker Creek and p o s s i b l e T r i a s s i c rocks just east of Winters Creek contrast with s t r u c t u r a l l y higher, Carboniferous r a d i o l a r i a n cherts near O l a l l a Creek and Keremeos (Chapter I I ) . The evidence, though sparse, r e f l e c t s an o v e r a l l increase i n age from s t r u c t u r a l l y lower rocks i n the west to s t r u c t u r a l l y higher rocks i n the east and c o n f l i c t s with s t r a t i g r a p h i c evidence given by f a c i n g i n d i c a t o r s such as p i l l o w s , flow tops, and t u r b i d i t e deposits, which a l l face s t r a t i g r a p h i c a l l y upward to the east. A possible explanation that accommodates both of these age r e l a t i o n s h i p s i s that the Apex Mountain Group i s an accretionary subduction complex s i m i l a r to those discussed by Seely and others, (1974); Karig and Sharman (1975); and Moore and Karig (1976). Accordingly, the Apex Mountain Group would represent a s e r i e s of stacked accretionary wedges b u i l t up within the fore-arc region of an under-thrust ocean margin (Figure 27). Sediments and volcanic rocks would be deposited within an a c t i v e l y spreading ocean basin. New oceanic crust would be generated along a westward l y i n g spreading center (ridge) thus creating an eastward increase i n age away from the r i d g e . As the oceanic lithosphere moved eastward into the east-dipping subduction zone, successive s l i c e s or wedges of sediments and volcanic rocks would - 89 -Figure 27. Schematic cross-sections showing (A) mode of accretion of the Apex Mountain Group. Note age relationships within the forearc region . (B) Rotation of accretion wedges into steeper inclinations during continued subduction and accretion, - 90 -be scraped o f f , below the leading edge of the upper plate, and thrust below older, underthrust s l i c e s . This would produce an east-dipping sequence of rocks with an o v e r a l l increase i n age up-structure, with l o c a l l y (within i n d i v i d u a l s l i c e s ) s t r u c t u r a l l y higher rocks being the youngest. The apparent absence of Permian and Lower T r i a s s i c rocks i n the sequence could be due to complete subduction of the rocks of th i s age. The deformational s t y l e of the Apex Mountain Group i s consistent with e a s t e r l y d i r e c t e d subduction implied by the above i n t e r p r e t a t i o n . Tight F l + minor f o l d s , which developed i n d u c t i l e sequences, have a x i a l surfaces which dip to the east, p a r a l l e l i n g the d i r e c t i o n of shear i n the subduction zone. A counterclockwise sense of vergence (as viewed to the north) should develop i n fo l d s associated with an east-dipping subduction zone. As mentioned previously, and as seen on Figures 23 and 24, a clockwise sense of shear i s observed i n several of the minor folds of the study area. Where present, minor f o l d s with clockwise vergence, as viewed to the north, are inte r p r e t e d to be subsidiary folds on the overturned limb of a much larger i n c l i n e d , t i g h t or i s o c l i n a l f o l d which was not recognized by mapping. A h o r i z o n t a l compressional regime could be expected i n front of the subduction zone (Figure 28), and i t i s here that i n i t i a l buckling of the bedded sequences may have occurred ( F l f o l d i n g ) . A f t e r the sediments entered the subduction zone and began the subduction process, a shearing regime would e x i s t and would produce asymmetric s i m i l a r f o l d s - 91 -HORIZONTAL COMPRESSION Figure 28. Schematic diagram illustrating development of F1 and F1 + folds just before and during subduction. - 92 -(F1+ f o l d s ) . The o r i g i n a l f o l d axes of both F l and F l + f o l d s were probably h o r i z o n t a l and trended roughly north-south. North-south trending shear fractures and f a u l t s which developed i n b r i t t l e zones, dip v e r t i c a l l y or steeply to the east and probably represent imb r i c a t i o n , associated with underthrusting, of larger wedges of rock. These f a u l t s and shear zones are generally more steeply i n c l i n e d than bedding and i n several l o c a l i t i e s have dip d i r e c t i o n s opposite of bedding, suggesting that imbrication was v a r i a b l y oriented with respect to the general d i r e c t i o n of shear i n the subduction zone or, a l t e r n a t i v e l y , shear was l o c a l l y v a r i a b l y oriented. As such, some of these f a u l t s may be a n t i t h e t i c to the major thrust f a u l t s . These f a u l t s may also have been l o c a l l y rotated upward into steeper i n c l i n a t i o n s during progressive underthrusting and a c c r e t i o n (Figure 27) as t h i s i s documented to have occurred i n other ancient subduction complexes (Moore and Wheeler, 1978). In t h i s regard, the associated s t r a t a would also be rotated i n the same manner. West dipping bedding, however, i s observed only l o c a l l y i n the study area and i s not consistent over broad areas. The f a u l t zones, except where they are a f f e c t e d by F2 f o l d i n g , are unfolded and are also seen to disrupt previously folded sequences, suggesting that once s t r a i n was l o c a l i z e d , i t was taken up p r e f e r e n t i a l l y within these zones during continued deformation. This probably i n h i b i t e d the development of lar g e - s c a l e F l and F l + f o l d s and could explain t h e i r apparent absence i n the study area. F2 deformation r e s u l t e d from roughly northwest-southeast compression and i s most e a s i l y r e l a t e d , to regional deformation i n Late T r i a s s i c - E a r l y J u r a s s i c time (see Chapter I V ) . East and southeast trending f a u l t s could be r e l a t e d - 93 -to t h i s deformation, based on t h e i r o r i e n t a t i o n , and i n d i c a t e that embrittlement of already l i t h i f i e d rocks occurred with f o l d i n g during t h i s phase of deformation. The type of f a u l t s present and t h e i r mode of occurrence, as well as f o l d s t y l e s and geometry, and r e l a t i v e d i s t r i b u t i o n of f o l d axes are a l l s t r i k i n g l y s i m i l a r to structures described i n the Kodiak melange of Alaska (Moore and Wheeler, 1978), the Golconda Allochthon i n Nevada ( M i l l e r and others, 1982), and the Klamath Mountains of C a l i f o r n i a (Wright, 1982). A l l of these t e r r a i n s are int e r p r e t e d to have been deformed i n subduction-related accretionary wedge environments. CHAPTER IV TECTONIC EVOLUTION AND IMPLICATIONS The i n t e r p r e t a t i o n of the Apex Mountain Group as a subduction-r e l a t e d accretionary wedge complex agrees with observed i n t e r n a l geologic r e l a t i o n s h i p s . However, the r e l a t i v e sense of timing of the implied subduction event and i t s s p a t i a l r e l a t i o n s h i p with the paleotectonic environments and present geographic p o s i t i o n s of other s i m i l a r t e r r a i n s i n south-central B r i t i s h Columbia must also be considered. Ridge spreading and subduction were probably occurring before the onset of deposition of the Apex Mountain Group; however, no record of t h i s i s known to be preserved i n south-central B r i t i s h Columbia. Subduction and a c c r e t i o n of the group probably began i n the M i s s i s s i p p i a n and/or Pennsylvanian s h o r t l y a f t e r deposition as some of the sediments were s t i l l somewhat soft when deformed (Chapter I I I ) . This implies the ridge and subduction zone were i n r e l a t i v e close proximity. The youngest known deformed sediments are Ladinian-Karnian i n d i c a t i n g that subduction and a c c r e t i o n continued u n t i l at l e a s t the Middle or Late T r i a s s i c . During l a t e Paleozoic time a v o l c a n i c arc would be expected to develop east of the Apex Mountain Group (Figure 29A). In the absence of any continental sequences i n south-central B r i t i s h Columbia t h i s arc can be assumed to be an oceanic a r c . The arc or i t s back-arc area i s probably represented by the Kobau, Anarchist and, possibly, the Chapperon Group. Fox and others (1977) proposed a back-arc s e t t i n g f or the Anarchist and Kobau Groups. The presence of abundant v o l c a n i c l a s t i c sediments i n these groups supports t h i s i n t e r p r e t a t i o n . As mentioned i n Chapter I I , the marked l i t h o l o g i c - 95 -CREEK 1 . H A R P ^ n R , ? „ N C H A ~GROUP * GROUP ° * o r # \ * • Kamloops \ \ * U \ f •Salmon River \ PRESENT \ _ * NORTH \ * 1 APEX L 1 1 MOUNTAIN L A olalla 1 GROUP | ~J L V* * * r KOBAU/ 1* * ANARCHISTI A * GROUPS J t J * ' # 1 1 # * S 1 * * HARPER RANCH o r * GROUP CACHE 1 # CREEK <\ # * # GROUP * i * * \ • Kamloops ^ \ * \ * * \ V* * \ \ \ * Salmon River W r ? * \\ * \ r t * \ ' 1 1 * n I r* 1 i 1 NICOLA 1? \ 1 + ^ \ • Olteilla 1 • * APEX MOUNTAIN - P" GROUP ~ ^KOBAU/ + * v i ANARCHIST r {GROUPS + APEX MOUNTAIN GROUP SUBDUCTION COMPLEX | Late Paleozoic-Middle Triassic Late Triassic * # Active oceanic arc amt Oceanic lithosphere * > Remnant oceanic arc > Subduction zone ^ Ocean spreading ridge m+ Voteanigenic sediment — Plate motion direction transport ° o Ocean islands Figure 29A,B. Schematic map and sections illustrating proposed tectonic model for Late Paleozoic through Late Triassic in south-central British Columbia. - 96 -s i m i l a r i t i e s of these groups with the Apex Mountain Group also allow speculation that they have an open ocean-floor o r i g i n . The s t r u c t u r a l r e l a t i o n s h i p s i n these groups, however, do not suggest they were part of an accretionary wedge complex. Further geochemical study of the basalts i n these areas might resolve t h i s ambiguity. Sometime during the Middle or Late T r i a s s i c , the locus of subduction s h i f t e d to a p o s i t i o n west of the Apex Mountain Group and eventually resulted i n the development of N i c o l a Group volcanic arc i n the Late T r i a s s i c (Figure 29B). The s h i f t i n subduction i s invoked to account for the present r e l a t i v e geographic p o s i t i o n s of the two groups. The basal sedimentary f a c i e s of the N i c o l a Group at Hedley and Winters Creek, i n the study area, probably represent the t r a n s i t i o n from dominantly open-ocean, r e l a t i v e l y v o l c a n i c - f r e e sedimentation to that of i s l a n d arc sedimentation dominated by v o l c a n i c l a s t i c rocks. The westward s h i f t of subduction may have caused regional u p l i f t and associated erosion, and i n south-central B r i t i s h Columbia may account for the r e g i o n a l unconformity below Upper T r i a s s i c sediments at O l a l l a and Salmon River, and also r e g i o n a l Late T r i a s s i c - E a r l y J u r a s s i c deformation reported by Ross (1981). Upper T r i a s s i c c l a s t i c sediments i n northern Washington (Cave Mountain Formation), and at O l a l l a and Salmon River that occur d i r e c t l y above the unconformity l i k e l y represent d e t r i t u s shed from the emerging arc and eroded older s t r a t a . A somewhat s i m i l a r s e r i e s of events i s recorded i n the Cache Creek, Nicola and Harper Ranch Groups farther to the north and assuming an absence of major s t r u c t u r a l breaks between these rocks and those discussed above, t h e i r respective t e c t o n i c h i s t o r i e s should be r e l a t e d . The Apex Mountain Group i s most c l o s e l y r e l a t e d i n age, d e p o s i t i o n a l and t e c t o n i c environment, and t e c t o n i c h i s t o r y to the Cache Creek Group. Both are considered to be accreted t e r r a i n s of generally deep, open-ocean - 97 -material scraped o f f dovragoing oceanic lithosphere during l a t e Paleozoic and/or e a r l y Mesozoic subduction. S t r u c t u r a l l y , the Cache Creek Group i s characterized by obducted sequences, tectonic melanges, and fault-bounded pods of deeply-seated u l t r a m a f i c rock (Chapter I ) . This contrasts with the Apex Mountain Group and i s a t t r i b u t e d to c o l l i s i o n of ocean islands or seamounts with the subduction zone (Shannon, 1982). The Apex Mountain Group probably consisted of r e l a t i v e l y unobtrusive ocean-floor sequences and was accreted with l i t t l e or no attendant tectonic mixing or obduction. Based on the above t e c t o n i c models and also faunal evidence (Chapters I and I I ) , i t appears the two groups were widely separated during deposition and deformation and c o n s t i t u t e d unique paleogeographic s e t t i n g s . They were, however, probably deposited on the same oceanic plate and were mutually a f f e c t e d , though at d i f f e r e n t times, by subduction of the p l a t e . Sparse s t r a t i g r a p h i c evidence i n d i c a t e s the Cache Creek and N i c o l a Groups were not adjacent u n t i l Late T r i a s s i c time (Shannon, 1982; Smith, 1979), with the N i c o l a arc developing In response to subduction of the Cache Creek Group (Figure 29B). This i s contemporaneous with the westward s h i f t of subduction i n the south and subsequent development of the N i c o l a arc i n that area. Perhaps the c o l l i s i o n of the Cache Creek and Harper Ranch Groups i n i t i a t e d the stepping out of the subduction zone to the south. Based on the above models, i t appears that the Cache Creek, Nicola and Apex Mountain Groups had reached t h e i r present r e l a t i v e p o s i t i o n s by the l a t e s t T r i a s s i c . Karnian-Norian v o l c a n i c l a s t i c rocks northeast of Kamloops represent the associated back-arc f a c i e s of the N i c o l a Group (Chapters I and I I ) and are l i k e l y , i n part, l a t e r a l f a c i e s equivalent with the coeval c l a s t i c rocks above the r e g i o n a l unconformity at Salmon River, O l a l l a , and Cave Mountain. Upper Paleozoic rocks i n the Harper Ranch Group are coeval with older rocks i n - 98 -the Cache Creek and Apex Mountain Groups. They probably represent a back-arc f a c i e s of an e a r l i e r pre-Nicola v o l c a n i c arc that developed i n the l a t e Paleozoic during subduction of pre-Cache Creek sediments (Monger, 1977). Widely d i f f e r e n t Permian fauna i n the Cache Creek and Harper Ranch Groups nec e s s i t a t e them being f a r apart at that time. The Harper Ranch and Apex Mountain Groups may have l a i n on the same side of the pre-Nicola arc suggesting that t h e i r upper Paleozoic and Lower Mesozoic sediments were deposited within the same back-arc basin. Unless the Apex Mountain Group was deposited i n a starved portion of the basin, the notable absence of thi c k successions of immature, arc-derived v o l c a n i c l a s t i c sediments precludes t h i s i n t e r p r e t a t i o n . More l i k e l y , the older Harper Ranch sediments are i n part l a t e r a l l y f a c i e s equivalent with possible ocean-arc t e r r a i n s of the Chapperon, Kobau and Anarchist Groups. Further study of the geochemistry of the basalts i n these sequences could greatly enhance our i n t e r p r e t a t i o n of the i n t e r e l a t i o n s between these groups. I propose that F l and F l + structures i n the Apex Mountain Group were formed during e a s t e r l y d i r e c t e d subduction and are probably not rel a t e d to s i m i l a r north-south trending, t i g h t to i s o c l i n a l f o l d i n g i n other areas of south-central B r i t i s h Columbia. The l a t t e r (Phase 1 of Ross, 1981), are a t t r i b u t e d to e a s t e r l y d i r e c t e d obduction i n Permo-Triassic time (Sonoma Orogeny) and evidence, given by Ross, of t h i s phase, i s not seen i n the Apex Mountain Group. Ross' Phase 2 s t r u c t u r e s , where developed, are generally east-west trending. They are the r e s u l t of east-directed subduction during Late T r i a s s i c - E a r l y J u r a s s i c time. This corresponds with the subduction zone s h i f t discussed previously and post-dates accretion and F l deformation i n the Apex Mountain Group. L o c a l l y , Phase 2 structures of Ross are north-south trending and i t i s possible that l a t e s t F l deformation i n the - 99 -Apex Mountain Group i s coeval with very e a r l y Phase 2 deformation of Ross. F2 structures i n the study area are co r r e l a t e d with Ross' Phase 3 s t r u c t u r e s . They r e s u l t e d from o r i g i n a l l y eastward d i r e c t e d subduction becoming more oblique with time u n t i l s t r i k e - s l i p motion developed. East and southeast trending f a u l t s i n the study area p o s s i b l y represent conjugate f a u l t s developed during t h i s deformation. Ross c o r r e l a t e d t h i s phase with the very e a r l y part of the Columbian Orogeny i n post-Late T r i a s s i c - E a r l y J u r a s s i c time. A l t e r n a t i v e l y , the east and southeast trending f a u l t s may have formed as r a d i a l f a u l t s during i n t r u s i o n of the granodiorite stock (Unit 9) i n the Late J u r a s s i c . This event; however, would not account for the large, F2 open f o l d i n the study area. Based on t h e i r s i m i l a r i t y of s t y l e , o r i e n t a t i o n , and timing of development, Phases F l and F2 i n the study area are equated with the f i r s t two phases of deformation recognized i n rocks now designated as the Apex Mountain Group near O l a l l a (Read and O k u l i t c h , 1977). - 100 -CHAPTER V CONCLUSIONS 1. The Apex Mountain Group consists of f i v e major l i t h o f a c i e s : massive and bedded chert, greenstone, chert breccia, a r g i l l i t e , and limestone. Together they form a broadly folded, east-dipping sequence that has an o v e r a l l increase i n age towards s t r u c t u r a l l y higher rocks i n the east. This contrasts with l o c a l s t r a t i g r a p h i c r e l a t i o n s h i p s which show s t r u c t u r a l l y highest rocks to be younger than those below. Both r e l a t i o n s h i p s are accommodated i n , and support the i n t e r p r e t a t i o n that the group represents at l e a s t part of an ancient subduction complex that formed by progressive eastward-directed underthrusting and a c c r e t i o n of successively younger s l i c e s of oceanic sediments and volcanic rocks. 2. The above i n t e r p r e t a t i o n implies the existence of an oceanic plate moving eastward r e l a t i v e to the North American craton. Volcanic rocks i n the study area have trace-element compositions i n d i c a t i v e of an ocean-ridge o r i g i n ; thus they provide i n d i r e c t evidence for movement of an ocean plate and also ridge spreading as a p o t e n t i a l d r i v i n g mechanism for plate motion. 3. The d e p o s i t i o n a l environment of the Apex Mountain Group i s inte r p r e t e d to be a gene r a l l y deep, open-ocean basin. Quiet conditions p r e v a i l e d and t u r b i d i t y currents were common. Lo c a l areas of high r e l i e f such as f a u l t scarps, or ocean ridges were present and account for the existence of l o c a l r e s t r i c t e d sub-basins, submarine-fan b r e c c i a , t u r b i d i t e deposits, and o l i s t o l i t h s . Anoxic conditions developed i n r e s t r i c t e d - 1 0 1 -sub-basins as i n d i c a t e d by the abundance of preserved organic m a t e r i a l i n a r g i l l a c e o u s rocks. Fine terrigenous material i n the succession i n d i c a t e s a possible nearby land mass which may have been i s o l a t e d ocean i s l a n d s . Regional ocean c i r c u l a t i o n may have provided nutrient-laden waters and could account f o r t h i c k , a e r i a l l y extensive deposits of b i o l o g i c a l l y accumulated chert. Water depth i s considered to be greater than 2 km, based p r i m a r i l y on the absence of v e s i c l e s or explosive textures i n the majority of volcanic rocks. The presence of shallow, warm water-fauna i n deeper water limestone, as well as o l i s t o l i t h s , t u r b i d i t e deposits, l o c a l amygdaloidal lava, and p i l l o w lava i n d i c a t e shallow water deposition occurred l o c a l l y . The maximum and minimum ages of the Apex Mountain Group, based on faunal ages i n limestones and chert, are E a r l y Carboniferous and Ladinian-Karnian r e s p e c t i v e l y . Deposition, subduction, and a c c r e t i o n of the group i s considered to be continuous throughout t h i s time; however, the absence of Permian and E a r l y T r i a s s i c f o s s i l s may i n d i c a t e a period when the rocks of Permian and E a r l y T r i a s s i c age were f u l l y subducted and not accreted. Subduction of the Apex Mountain Group i n l a t e Paleozoic time probably i n i t i a t e d the development of an associated oceanic-arc l y i n g to the east. The Kobau and Anarchist Groups of south-central B r i t i s h Columbia and northern Washington have l i t h o l o g i c a f f i n i t i e s with i s l a n d - a r c t e r r a i n s and t h i s , along with t h e i r present geographic p o s i t i o n , suggests they are remnants of the ancient arc. - 102 -6. In Middle to Late T r i a s s i c time subduction of the Apex Mountain Group i s i n t e r p r e t e d to have ceased and renewed subduction began along a zone l y i n g to the west. This event i s documented by rocks of the N i c o l a Group. The basal sedimentary f a c i e s of the N i c o l a Group, which i s seen near Winters Creek, records the t r a n s i t i o n from ocean-f l o o r sedimentation, s i m i l a r to that i n the Apex Mountain Group, to v o l c a n i c a l l y dominated, subduction-induced, i s l a n d - a r c sedimentation i n the upper part of the N i c o l a Group. D e t r i t u s shed from the west-lying arc i s represented by Upper T r i a s s i c c l a s t i c rocks l y i n g unconformably above the Apex Mountain Group near O l a l l a . 7. Two separate phases of deformation are recognized i n the Apex Mountain Group. The f i r s t phase of deformation was a progressive phase rel a t e d to the subduction process. F l f o l d s formed In response to h o r i z o n t a l compressive s t r a i n as the sediments approached the subduction zone. Open, p a r a l l e l f o lds r e s u l t e d . During actual subduction the F l f o l d s were deformed i n t o t i g h t e r , asymmetric s i m i l a r s t y l e f o l d s ( F l + ) i n response to shear p a r a l l e l to the d i r e c t i o n of subduction. North-south trending shear f r a c t u r e s and f a u l t s are interpreted to be the r e s u l t of imbrication i n b r i t t l e zones during subduction. S t r a i n was l o c a l i z e d along these zones once they formed and further f o l d i n g was i n h i b i t e d . F2 deformation i s c o r r e l a t e d with oblique northwest-southeast compression during the e a r l y part of the Columbian Orogeny i n Late T r l a s s i c - E a r l y J u r a s s i c time. This phase accounts for the large northeast-trending f o l d i n the study area and p o s s i b l y east and southeast trending v e r t i c a l f a u l t s . - 103 -BIBLIOGRAPHY Barnes, W.C., and Ross, J.V., 1975, The Bli n d Creek Limestone, Keremeos, B r i t i s h Columbia: Structure and Regional Tectonic S i g n i f i c a n c e , Canadian Journal of Earth Science, 12, pp. 1929-1933. 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