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Cache Creek and Nicola Groups near Ashcroft, British Columbia 1978

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CACHE CREEK AND NICOLA GROUPS NEAR ASHCROFT, BRITISH COLUMBIA by JOAN FRANCES GRETTE B.A. , Middlebury C o l l e g e , 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of G e o l o g i c a l Sciences) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA November, 1978 (c) Joan Frances G r e t t e , 1978 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f GeccoC The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V6T 1W5 D a t e APJB/L 2 9 . '9*1  i i ABSTRACT D e t a i l e d mapping near the type area of the Cache Creek Group i n southern B r i t i s h Columbia has l e d to s i g n i f i c a n t changes i n the d i s t r i b u t i o n of Cache Creek and N i c o l a rocks. Much of what was c a l l e d Cache Creek Group i s now considered to be p a r t of the Upper T r i a s s i c N i c o l a Group. Several c r i t e r i a can be used to d i s t i n g u i s h the two groups. These i n c l u d e : 1) l i t h o l o g i c d i f f e r e n c e s , 2) f o s s i l i n f o r m a t i o n , 3) s t r u c t u r a l s t y l e , and 4) metamorphic h i s t o r y . The Cache Creek Group i s subdivided i n t o three mappable, fault-bounded u n i t s and appears to be a t e c t o n i c melange over much of i t s extent. A deformational event produced i s o c l i n a l f o l d s , a p h y l l i t i c f o l i a t i o n i n some l i t h o l o g i e s , and was accompanied by metamorphism w i t h v a r i a b l e pressure-tem- perature c o n d i t i o n s . M i n e r a l assemblages support c o n d i t i o n s from temperatures l e s s than 250°C and pressures of 4 kb or l e s s to t r a n s i t i o n a l b l u e s c h i s t con- d i t i o n s : T = 350° C and P = 6 kb. In c o n t r a s t , the N i c o l a Group i s c h a r a c t e r i z e d by hydrothermal a l t e r a t i o n and the l a c k of a pervasive secondary f a b r i c . I t does not have the bloc k s i n a sheared matrix t e c t o n o - s t r a t i g r a p h i c s t y l e t y p i c a l of the Cache Creek Group. The two u n i t s were brought together along the M a r t e l F a u l t , probably a t h r u s t , during l a t e Lower or e a r l y Middle J u r a s s i c time. Deformation and melange development i n :the Cache Creek Group predates t h i s event. D i s t r i b u - t i o n of Cache Creek and N i c o l a rocks and t h e i r r e l a t i o n s h i p to each other during Upper T r i a s s i c time are s t i l l not c l e a r . TABLE OF CONTENTS Page ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS INTRODUCTION Purpose F i e l d Area Previous Work REGIONAL GEOLOGY General-North America B r i t i s h Columbia Cache Creek Group A t l i n Terrane P i n c h i - S t u a r t Lake Area Type Area N i c o l a Group Ash c r o f t Formation GEOLOGY OF THE STUDY AREA General D i s t r i b u t i o n of Rock Units Cache Creek Group Lower c l a s t i c u n i t Massive mafic v o l c a n i c u n i t Upper c l a s t i c u n i t Age of the Cache Creek Group Sedimentary s t r u c t u r e s and environment of d e p o s i t i o n S t r u c t u r e Metamorphism N i c o l a Group Greenstone u n i t Limestone u n i t Mixed sedimentary and v o l c a n i c rocks Age of the N i c o l a Group St r u c t u r e A l t e r a t i o n J u r a s s i c and Younger Rocks Ash c r o f t Formation Spences Bridge Group Quaternary deposits 11 i i i v v i v i i i 4 6 6 6 8 9 13 14 15 15 15 18 41 42 44 46 49 54 59 61 64 66 68 71 73 73 73 74 75 DISCUSSION Cache Creek Group, Nicola Group and Contact Relationships Tectonic Setting and History SUMMARY AND CONCLUSIONS REFERENCES CITED LIST OF TABLES Page Table I . M i n e r a l o g l c a l Data f o r Cache Creek Greenstones and Greywackes 28 Table I I . Blue Amphibole Probe Analyses 38 Table I I I . Metamorphic M i n e r a l Assemblages i n the Cache Creek 55 Table IV. Rb/Sr Geochronology Data 69 v i LIST OF FIGURES Figure - D „ „ „ 6 Page F r o n t i s p i e c e . Venables V a l l e y and Thompson R i v e r v a l l e y . 1. L o c a t i o n and access of f i e l d area. 2 2. Geologic and physiographic b e l t s of the Canadian C o r d i l l e r a . 5 3. Areas of exposure of Cache Creek and N i c o l a - T a k l a i n B r i t i s h Columbia. 7 4. R e g i o n a l g e o l o g y of the southern Intermontane B e l t . 11 5. Generalized geology of the Venables V a l l e y area. 16 6. Geology of the Venables V a l l e y area. A^pp-eke-fe- Q p CHL 7. Cross s e c t i o n A-A'. in~po .Gke.te. / 8. Cross s e c t i o n B-B'. i n - p e K s k - e t 3 9. Diagrammatic s t r a t i g r a p h i c s e c t i o n of ;the Cache Creek Group. 17 10. Photomicrograph of t y p i c a l p h y l l i t e i n the lower c l a s t i c u n i t of the Cache Creek Group. 20 11. Chaotic f o l d i n g i n Cache Creek p h y l l i t e . 21 12. F i e l d sketches showing f o l d i n g and shearing of p h y l l i t i c f o l i a t i o n i n the Cache Creek Group. 23 13. Photomicrograph of undeformed p l a g i o c l a s e - r i c h greywacke. 24 14. Photomicrograph of l i t h i c fragments i n Cache Creek ... greywacke. 25 15. Rip-up c l a s t s i n Cache Creek greywacke. 27 16. Weakly f o l i a t e d Cache Creek greywacke. 29 17. P r o t o m y l o n i t i z e d Cache Creek greywacke. 30 18. B r e c c i a t e d Cache Creek chert. ' 32 19. M y l o n i t i z e d Cache Creek chert. 33 20. R e c r y s t a l l i z e d r a d i o l a r i a and probable sponge s p i c u l e s i n Cache Creek chert. 34 21. Blue amphiboles i n augite porphyry v o l c a n i c rocks. 37 v i i Figure Page 22. Sodic amphiboles plotted on composition diagrams. 39 23. C a t a c l a s t i c textures i n a l t e r e d augite porphyry. 43 24. Oolites and f u s i l i n i d s i n Cache Creek limestone. 45 25. Soft sediment folds i n fin e grained sediments of the lower c l a s t i c u n i t . 47 26. Rip-up c l a s t s i n sediments of the upper c l a s t i c u n i t . 47 27. S l i d e b r e c c i a i n the upper c l a s t i c unit. 48 28. Stereonet p l o t of s t r u c t u r a l elements i n Cache Creek. 51 29. P-T e q u i l i b r i a at very low grade. 56 30. Enlarged P-T e q u i l i b r i a with reactions from Brown. 58 31. Diagrammatic s t r a t i g r a p h i c section of the Nicola Group. 60. 32. Rb/Sr isochron f or greenstones of the Nicola Group. 70„ 33. Stereonet p l o t of s t r u c t u r a l elements i n Nicola Group. 72 34. Timing of events i n the type area of the Cache Creek. 79 v i i i ACKNOWLEDGEMENTS F i r s t I would l i k e to thank Peter B. Read who helped me f i n d t h i s problem and who gave i n v a l u a b l e a s s i s t a n c e i n the f i e l d and w i t h petrology. A l s o , thank you to Karen Massey who s u f f e r e d the 42° C temperatures of Venables V a l l e y as my f i e l d a s s i s t a n t . K r i s t a S cott provided t e c h n i c a l e x p e r t i s e f o r Rb/Sr geochronology. Lee Pigage, besides l e a d i n g me by the hand on the microprobe, was always happy to answer any questions he could help w i t h . Valuable d i s c u s s i o n s were provided by W.R. Danner, B i l l Travers, Jim Monger, B i l l M c M i l l a n , Randy P a r r i s h and Ian Duncan. Bruce Cameron helped me through m i c r o f o s s i l traumas, and the Geology Department t e c h n i c i a n s a s s i s t e d i n most other c r i s e s . R.L. Armstrong supervised the t h e s i s , provided f i n a n c i a l support, and c r i t i c a l l y read the manuscript. S p e c i a l a p p r e c i a t i o n to Stack and Pat Stackhouse f o r t h e i r p a r t i n my summer i n Venables V a l l e y . 1 INTRODUCTION Purpose Rocks of the Upper P a l e o z o i c Cache Creek Group and Lower Mesozoic N i c o l a Group have f i g u r e d s i g n i f i c a n t l y i n models of the t e c t o n i c h i s t o r y of the Canadian C o r d i l l e r a (Monger and oth e r s , 1972). Yet, e s p e c i a l l y w i t h i n the type area, near Cache Creek, B r i t i s h Columbia, l i t t l e d e t a i l e d work has been published. There has been u n c e r t a i n t y i n d i s t i n g u i s h i n g Cache Creek Group rocks from the N i c o l a Group as l i t h o l o g i e s are q u i t e s i m i l a r . The contact r e l a t i o n s between the two have not been described. The goals of t h i s study are: 1) to describe the Cache Creek Group near i t s type l o c a l i t y i n terms of l i t h o l o g y , s t r a t i g r a p h y , s t r u c t u r e and metamorphism; 2) to determine what c r i t e r i a can be used to d i s t i n g u i s h the Cache Creek and N i c o l a Groups; 3) to study the contact between the two groups; and 4) to provide c o n s t r a i n t s on t e c t o n i c i n t e r p r e t a t i o n s of the Late P a l e o z o i c and E a r l y Mesozoic h i s t o r y of the Intermontane B e l t . F i e l d Area The southernmost exposure of the Cache Creek Group was s e l e c t e d f o r d e t a i l - ed mapping. The area l i e s west of the Thompson Ri v e r midway between the towns of A s h c r o f t and Spences Bridg e , approximately 50°30' N, 121°20' W (Figure 1). Approximately 30 square miles were mapped during June, J u l y , and August of 1975. Access to the area i s easy. I t i s s i t u a t e d along and west of the Trans- Canada Highway and i s crossed by s e v e r a l ranching and logging roads. Q u a l i t y and q u a n t i t y of outcrops depend s t r o n g l y on l i t h o l o g y and e l e v a t i o n . Lime- stone, c h e r t , and greenstone are r e s i s t a n t to weathering i n the dry c l i m a t e and tend to be best exposed w h i l e p h y l l i t e , other c l a s t i c sedimentary rocks, and s e r p e n t i n i t e are poorly exposed. Below about 1300 f e e t the t e r r a i n i s 2 Figure 1 . Location and access of f i e l d area. 3 q u i t e f l a t , and sagebrush, cactus, and shortgrass are the dominant p l a n t l i f e . Sparse rock exposures are l i m i t e d to small knobs, stream g u l l i e s , road cu t s , and some c l i f f s near the r i v e r . At higher e l e v a t i o n s , up to about 2400 f e e t , Ponderosa Pine dominates. Slopes are steeper, but outcrop quan- t i t y i s not much b e t t e r . Above 2400 feet on the slopes west of Venables V a l - l e y , s p r u c e - f i r f o r e s t and long grass undergrowth cover much of the h i l l s i d e . Best outcrop or f l o a t i s found on the north s i d e of s m a l l g u l l i e s s i n c e these south-facing slopes are d r i e r and have l e s s cover. Game t r a i l s which cut through the heavy t u r f can be u s e f u l i n exposing f l o a t . Above e l e v a t i o n s of about 3500 f e e t the slope f l a t t e n s and outcrop becomes extremely sparse. Only o c c a s i o n a l carbonate, greenstone, or chert masses s t i c k out, though l o g - ging cuts r e v e a l the more prevalent p h y l l i t e s and s e r p e n t i n i t e s . Previous Work Selwyn (1872) f i r s t described the Cache Creek and N i c o l a rocks along the Thompson R i v e r . He found f o s s i l s j u s t north of Venables Creek which were then thought to be Devonian to Permian. He t h e r e f o r e c l a s s i f i e d a l l rocks along the west s i d e of the r i v e r as Cache Creek. Dawson (1894) published the f i r s t map that i n c l u d e s the present study area. A l l rocks west of the r i v e r w i t h the exception of a s m a l l area north of Venables Creek was considered to be Cache Creek Group. The Cache Creek i t s e l f was d i v i d e d i n t o a lower u n i t of c l a s t i c sedimentary rocks and green- stones and the upper Marble Canyon Formation w i t h the contact between the two l y i n g along the west s i d e of Venables V a l l e y . Drysdale (1912) changed the l o c a t i o n of t h i s contact to the next v a l l e y west, but otherwise h i s map i s i d e n t i c a l to Dawson's i n t h i s area. The next work was done by D u f f e l and McTaggart (1952). T h e i r map r e v e r t s to that of Dawson. Carr (1962) was the f i r s t to map i n d e t a i l along the highway and r i v e r . He d e l i n e a t e d some 4 l i t h o l o g i e s but d i d not make changes i n the major u n i t s . More r e c e n t l y , Danner (1965, 1975, 1976) has done considerable work on the carbonates i n the Cache Creek Group and has e s t a b l i s h e d t h e i r age and proba- b l e environment of d e p o s i t i o n . Across the Thompson Ri v e r the N i c o l a Group has been mapped i n d e t a i l by McMillan (1974). He i n c l u d e s d e t a i l e d d e s c r i p t i o n s of s e v e r a l measured s e c t i o n s . To the north of t h i s study area, near A s h c r o f t , work i s i n progress by Ladd ( i n press) and Travers (1978) on the Cache Creek, N i c o l a and A s h c r o f t Groups. D e t a i l e d work i n the Cache Creek elsewhere i n B r i t i s h Columbia has been mainly by Monger (1969, 1975a,1977a) i n the A t l i n Area and by Paterson (1973, 1974, 1977a, 1977b), Monger and Paterson (1974) and Paterson and Harakal (1974) i n the P i n c h i - S t u a r t Lake area. REGIONAL GEOLOGY General - North America A eu g e o s y n c l i n a l b e l t c o n t a i n i n g Upper P a l e o z o i c and Lower Mesozoic rocks that i n c l u d e the Cache Creek and N i c o l a Groups runs the l e n g t h of the western North America C o r d i l l e r a . In B r i t i s h Columbia pa r t of t h i s e u g e o s y n c l i n a l region i s c a l l e d the Intermontane B e l t (Figure 2 ) , a r e g i o n of low-grade to unmetamorphosed rocks that l i e s between high-grade c r y s t a l l i n e b e l t s - the Omineca C r y s t a l l i n e B e l t to the east and the Coast P l u t o n i c Complex to the west (Monger and oth e r s , 1972). The Intermontane B e l t i s c h a r a c t e r i z e d by the remnants of an Upper Paleo- z o i c - T r i a s s i c (Monger, 1977b; Paterson, 1977a, 1977b; Travers, 1978) oceanic terrane of ribbon c h e r t , a l p i n e u l t r a m a t i c r o c k s , and b a s i c v o l c a n i c rocks (Monger, 1972, 1977b) allochthonous to North America, and a Lower Mesozoic arc terrane of c a l c - a l k a l i n e to a l k a l i n e v o l c a n i c rocks and r e l a t e d sediment- ary rocks. Upper P a l e o z o i c rocks of oceanic a f f i n i t y belong to the Cache 5 Figure 2. Geologic and physiographic belts of the Canadian Cordillera (from Monger and others, 1972). The belts are 1) Rocky Mountain Belt, 2) Omineca Crystalline Belt, 3) Intermontane Belt, 4) Coast Plutonic Complex, and 5) Insular Belt. Creek Group i n Brit i s h Columbia and are,characterized by .a .distinctive Tethyan f u s i l i n i d fauna (Danner, 1965; Monger arid Ross, 1971; Monger, 1975b) and a pod-like tectono-stratigraphic style, suggesting severe deformation and dismemberment. Travers (1978) calls the Cache Creek near i t s type locality a melange. Blueschist metamorphism of eugeosynclinal strata has been dated i n several l o c a l i t i e s as Late Triassic (Patersdn and Harakal, 1974; Monger, 1977b). No basement for the Cache Creek has been found and most contacts with Lower Mesozoic and other Paleozoic rocks are faults. Upper Paleozoic eugeosynclinal terranes extend discontinuously from Alaska to California. In Alaska the Uyak Formation on Kodiak Island and rocks near Anchorage have Tethyan faunal a f f i n i t i e s (Connelly and others, 1976; Jones and others, 1972). Similar rocks are found in the western Yukon Territory, along the length of Br i t i s h Columbia, and in the San Juan Islands of Washing- 6 ton (Danner, 1974,1976, 1977). In B r i t i s h Columbia the Fergusson and Hoza- meen Groups have been suggested to be the southern extension of the Cache Creek Group (Monger, 1975b, 1977b). B i t s of s i m i l a r rocks are preserved i n the western Cascades of Washington, i n northeast Oregon, i n the Western P a l e o z o i c and T r i a s s i c B e l t of the Klamath Mountains, and along the western edge of the S i e r r a Nevada - Calaveras Formation (Monger, 1975b). Oceanic assemblages i n Alaska and Washington i n c l u d e rocks as young as J u r a s s i c and e a r l y Cretaceous (Monger, 1977b). Mesozoic v o l c a n i c and p l u t o n i c arc rocks w i t h i n the Intermontane B e l t range from mafic through s i l i c i c i n composition. The Upper T r i a s s i c rocks of B r i t i s h Columbia are known as N i c o l a Group i n the south and Takla Group i n the north where volcanism continues i n t o Lower J u r a s s i c time. In southern B r i t i s h Columbia these predominantly v o l c a n i c rocks l i e to the east of the oceanic terrane w h i l e to the north Takla rocks are found to the west of the Cache Creek Group. B r i t i s h Columbia Cache Creek Group Monger (1975a) describes the f o l l o w i n g l i t h o l o g i e s as c h a r a c t e r i s t i c of ,the Cache Creek: ribbon c h e r t , abundant p e l i t i c sediment, a l t e r e d b a s i c v o l c a n i c r o c k s , massive l e n s o i d carbonate, minor c l a s t i c sediment, and a l p i n e u l t r a m a f i c rocks. In B r i t i s h Columbia there are three areas where Cache Creek rocks are exposed: the A t l i n Terrane, the P i n c h i - S t u a r t Lake Area, and the type area at Cache Creek (Figure 3). Cache Creek rocks of the A t l i n Terrane range i n age from M i s s i p p i a n through Permian (Monger, 1975a) w i t h recent reports of Middle T r i a s s i c r a d i o l a r i a n chert (Monger, 1977b). Monger (1977a, 1977b) suggests the d e p o s i t i o n a l environment to be one of 7 Figure 3. Areas of exposure of Cache Creek and Nicola-Takla i n British Columbia. Regions with question marks are tentatively correlated with Cache Creek Group (Monger, 1975b). 8 v o l c a n i c pediments ( a t o l l s ) on which r e e f s have formed. These r e e f s broke o f f from time to time and blocks s l i d i n t o deeper water coming to r e s t among f i n e - g r a i n e d sediments. The s t r u c t u r a l h i s t o r y c o n s i s t s of e a r l y i s o c l i n a l f o l d i n g and a l a t e r b r i t t l e f o l d i n g event (Monger, 1977a, 1977b). Low grade to b l u e s c h i s t meta- morphism accompanied the e a r l i e r deformation which has been t r a d i t i o n a l l y considered post Late Permian and pre Late T r i a s s i c (Monger, 1977a).' I f the new r a d i o l a r i a n ages are v a l i d , t i m i n g of deformation may need r e v i s i o n . In the l a t e r event, during or a f t e r l a t e Middle J u r a s s i c time, l a t e Middle J u r a s s i c and ol d e r s t r a t a were i n v o l v e d i n t h r u s t i n g (Monger, 1975a). The A t l i n Terrane i s fault-bounded. On the northeast the Thiber t Creek F a u l t separates Cache Creek from more deformed and metamorphosed P a l e o z o i c rocks and f a r t h e r south from l e s s deformed Mesozoic rocks. To the southwest the A t l i n Terrane i s t h r u s t over M i d - J u r a s s i c and ol d e r rocks along the Na h l i n F a u l t (Monger, 1977a, 1977b). In the P i n c h i - S t u a r t Lake Area i n the center of B r i t i s h Columbia, t y p i c a l Cache Creek l i t h o l o g i e s are exposed i n a b e l t approximately 450 kil o m e t e r s long (Armstrong, 1949). Though Paterson (1977b) considers some Cache Creek l i t h o l o g i e s to be very s i m i l a r to rocks i n the nearby Takla Group, f o s s i l s i n carbonate rocks range from Pennsylvanian through Permian (Monger, 1975a). In the same region, an o p h i o l i t e sequence i s o v e r l a i n by Upper T r i a s s i c sediments that l o c a l l y c o n t a i n aragonite (Paterson, 1977b). Thus Upper T r i a s s i c rocks have been deformed and metamorphosed w i t h the Cache Creek. On the other hand, d e t r i t a l chromite, presumably derived from u l t r a m a f i c rocks i n the Cache Creek, i s found i n nearby Upper T r i a s s i c sediments (Paterson, 1974) . Therefore the t e c t o n o - s t r a t i g r a p h i c s e t t i n g of the Cache Creek and Takla rocks i s not c l e a r l y defined i n the P i n c h i - S t u a r t Lake Area, and t h e i r ages overlap. 9 Three deformational events are recognized by Paterson (1977b). F i r s t to develop are east - west i s o c l i n a l f o l d s w i t h a x i a l planes p a r a l l e l to compo- s i t i o n a l l a y e r i n g accompanied by b l u e s c h i s t metamorphism K-Ar dated at 211- 218 Ma (Paterson and Harakal, 1974). L a t e r north-south t r e n d i n g , c o n c e n t r i c f o l d s are o v e r p r i n t e d by s t i l l younger Cretaceous to T e r t i a r y warps and k i n k s . L i k e the A t l i n Terrane the P i n c h i - S t u a r t Lake Cache Creek i s f a u l t bound- ed. To the east i s the P i n c h i F a u l t separating oceanic terrane from the Hogem B a t h o l i t h of J u r a s s i c age (Paterson, 1974). There may have been s i g - n i f i c a n t r i g h t - l a t e r a l movement along the P i n c h i F a u l t (Paterson, 1977b). To'the west, northeast dipping f a u l t s place Cache Creek over Upper T r i a s s i c to Lower J u r a s s i c rocks of the S i t l i k a Formation (Paterson, 1974). In the type area (Figure 4 ) , i n c l u d i n g t h i s study, the Cache Creek i s bounded by the Fraser R i v e r F a u l t Zone to the west, beyond which l i e s the Coast P l u t o n i c Complex. In f a u l t contact to the east are T r i a s s i c N i c o l a and younger rocks i n c l u d i n g the Guichon B a t h o l i t h . Other Upper P a l e o z o i c rocks f a r t h e r east are not. c l o s e l y r e l a t e d to the Cache Creek Group. The Eastern P a l e o z o i c B e l t appears to be a d i f f e r e n t d e p o s i t i o n a l realm w i t h a d i f f e r e n t f o r e i g n fauna l e s s t r o p i c a l then the Tethyan fauna i n the Cache Creek Group (Danner, 1976). L i t h o l o g i e s resemble those of the Cache. Creek (Monger, 1975b) and these rocks were mapped as Cache Creek i n e a r l i e r works ( C o c k f i e l d , 1948;.Dawson, 1894). Between the Fraser F a u l t Zone and the Cache Creek Group are rocks of the P a v i l i o n Group, l i t h o l o g i c a l l y s i m i l a r to the Cache Creek but of u n c e r t a i n s t r a t i g r a p h i c p o s i t i o n . I t i s l i k e l y they are a younger f a c i e s of the Cache Creek, but no younger than Middle T r i a s s i c (Campbell and Tipper, 1971). Th e i r contact w i t h the the Cache Creek i s a f a u l t . Of s i m i l a r Middle T r i a s s i c age and west of the Fraser R i v e r F a u l t Zone i s the Fergusson Group (Cameron and Monger, 1971), a s s o c i a t e d w i t h u l t r a m a f i c rocks of Permo-Triassic LEGEND FIGURE 4 T Tertiary - volcanic rocks, some sedimentary rocks UM Upper Mesozoic - sedimentary and volcanic rocks Jurassic - sedimentary rocks N Triassic - Nicola Group P"R Um Permo-Triassic - ultramafic rocks P~^ P» P"^ P Upper Permian, Lower Middle Triassic - Pavillion, Fergusson Groups PCC Pennsylvanian - Permian - Cache Creek Group IP Eastern Paleozoic Belt Coast Plutonic Complex - Cretaceous and younger intrusive and metamorphic rocks Jurassic and Triassic(?) - intrusive rocks Field area location Fraser-Yalakom Fault System 11 Figure 4. Regional geology of the southern Intermontane Belt. 12 age (Campbell and Tipper, 1971). Based on l i m i t e d evidence, Monger (1975b, 1977b) and Cameron and Monger (1971) c o r r e l a t e both P a v i l i o n and Fergusson Groups w i t h the Cache Creek. Probable l a r g e - s c a l e r i g h t - l a t e r a l s t r i k e - s l i p movement along the Fraser F a u l t Zone (Tipper, 1977) places the Fergusson and Cache Creek i n c l o s e r p r o x i m i t y now than during t h e i r d e p o s i t i o n . The Coast P l u t o n i c Complex of dominantly Cretaceous i n t r u s i v e rocks w i t h screens and pendants of country rocks encloses Fergusson Group rocks on the west. Cache Creek rocks w i t h i n the type area c o n t a i n a Middle Pennsylvanian through Permian fauna i n limestone pods and w i t h i n the massive Marble Canyon limestones (Danner, 1976). Travers (1978) has found Middle or Upper T r i a s s i c r a d i o l a r i a i n Cache Creek ribbon c h e r t s . L i t h o l o g i e s are s i m i l a r to those found f u r t h e r north — p e l i t i c sedimentary rocks w i t h pods of limestone, ribbon c h e r t , b a s i c v o l c a n i c rocks, and u l t r a m a f i c rocks - a l l s t r o n g l y deformed i n most exposures. The p o d - l i k e occurrence of shallow water carbonates w i t h r a d i o l a r i a n cherts i s considered to be an o r i g i n a l d e p o s i t i o n a l p a t t e r n by Danner (1975). This i m p l i e s that ribbon cherts are not n e c e s s a r i l y r e s t r i c t e d to deep water c o n d i t i o n s . This environmental i n t e r p r e t a t i o n does not r e q u i r e movement of s l i d e blocks and/or slumping of shallow water sediments i n t o deeper water as was suggested by Monger (1977a, 1977b) f o r the A t l i n r e g i o n . Travers (1978) considers limestone blocks to be allochthonous. Deformation i n the type area has been described only i n general terms. " I n t e n s e l y sheared" (Campbell and Tipper, 1971) or " s e v e r e l y deformed w i t h i n t e r n a l s t r u c t u r e s impossible to s o r t out though w i t h g e n e r a l l y northwest trends and southwest d i p s " ( D u f f e l l and McTaggart, 1952) are the only d e s c r i p - t i o n s a v a i l a b l e . S i m i l a r l y the low grade r e g i o n a l metamophism has not been stu d i e d . Contact r e l a t i o n s h i p s w i t h lower Mesozoic rocks are poorly known; much of t h i s contact i s covered by T e r t i a r y and younger rocks,, The eastern margin 13 of the Cache Creek Group near Cache Creek i s strongly sheared along what Danner (1975) has termed the "Bonaparte Disturbed Zone". Within t h i s zone Travers (1978) has i d e n t i f i e d a f a u l t contact between Cache Creek and Nicola which has apparently had some T e r t i a r y l e f t - l a t e r a l movement but whose e a r l i e r movement h i s t o r y i s unknown. The Guichon Batholith dated at 205 Ma by Rb/Sr (R.L. Armstrong, personal communication, 1978), intrudes N i c o l a rocks. Its s e t t i n g i s s i m i l a r to that of the Hogem Batholith of the Pinchi-Stuart Lake Area, Lowest J u r a s s i c s t r a t a are preserved i n remnants of small basins, the largest of which i s the Ashcroft basin, centered on the town of Ashcroft. In t h i s basin a basal conglomeratic f a c i e s containing c l a s t s of probable Guichon Ba t h o l i t h provenance i s unconformable to disconformable on Nicola volcanic and sediment tary rocks (Frebold and Tipper, 1969). Travers (1978) reports an apparently complete Upper T r i a s s i c through Lower Jurassic section near the town of Cache Creek. Ni c o l a Group Upper T r i a s s i c rocks which run the length of B r i t i s h Columbia are known as the Nicola Group i n the south and Takla Group i n the north. Nicola Group rests unconformably on older rocks (Danner, 1976; Campbell and Tipper, 1970) and consists of shallow marine to subareal c a l c - a l k a l i n e and a l k a l i n e v o lcanic rocks, r e e f o i d a l limestone, and c l a s t i c rocks derived from these. The deposi.tional environment has been interpreted to be a volcanic i s l a n d arc (Monger and others, 1972) with f r i n g i n g carbonate reefs and small basins (Schau, 1968, 1970). Some workers have suggested that a l k a l i n e chemical trends favor a r i f t environment i n t e r p r e t a t i o n (Preto and others, 1975). Nicola rocks of the type area i n the Nicola map sheet (Cockfield, 1948) have been described i n d e t a i l by Schau (1968, 1970), The age there spans 1 4 Late Karnian to Late Norlan and p o s s i b l y as young as P l e i n s b a c h i a n (Preto, 1974). No base i s seen, but to the east N i c o l a i s unconformable on Upper P a l e o z o i c . Schau has d i v i d e d the N i c o l a i n t o two assemblages which he has named P and A based on the dominant phenocryst, p l a g i o c l a s e or a u g i t e . In the lower assemblage (P) p l a g i o c l a s e porphyry flows w i t h a n d e s i t i c to d a c i t i c compositions dominate. Interbedded w i t h and above the flows are sediments derived from the v o l c a n i c s - t u f f , greywacke, a r g i l l i t e , conglomer- ate, b r e c c i a , and some limestone. Dacite plugs i n t r u d e these and tend to l i e along f a u l t s . The upper assemblage (A) i s composed of flows w i t h abundant l a r g e augite phenocrysts and sediment derived from them. Deformation of the N i c o l a i n the type area i s t y p i c a l l y g e n t l e , u p r i g h t , c y l i n d r o i d a l f o l d s and abundant f a u l t i n g . A few overturned f o l d s are found l o c a l l y (Schau, 1968, 1970). A f o l i a t i o n i s developed i n f a u l t zones. Low- grade metasomatic and hydrothermal a l t e r a t i o n i s common. Higher grades of metamorphism are a t t a i n e d c l o s e to i n t r u s i v e bodies. G r a n i t i c rocks s i m i l a r to the N i c o l a and Takla rocks i n chemistry and only s l i g h t l y younger i n age (Travers, 1978; McM i l l a n , 1975) i n t r u d e N i c o l a and Takla rocks. The Guichon B a t h o l i t h and the Hogem B a t h o l i t h are two of the best known. These plutons are not seen to cut the Cache Creek, though Travers (1978) has mapped small stocks c u t t i n g Cache Creek that may be r e l a t e d to the Guichon. Ashc r o f t Formation The A s h c r o f t Formation of Lower J u r a s s i c age unconformably o v e r l i e s the N i c o l a Group and the Guichon B a t h o l i t h . U s u a l l y there i s a b a s a l conglomer- ate c o n t a i n i n g abundant g r a n i t i c d e t r i t u s , probably from the Guichon, and fragments of N i c o l a v o l c a n i c and sedimentary rock. Most of the s e c t i o n i s bl a c k , carbonaceous shale w i t h some s i l t s t o n e , sandstone and minor, impure 15 limestone (McMillan, 1974). Except f o r one questionable block described by Travers (1978), no d e t r i t u s of Cache Creek o r i g i n have been i d e n t i f i e d i n the As h c r o f t Formation. This i s i n con t r a s t to Lower J u r a s s i c boulder conglomerates which cont a i n Cache Creek d e t r i t u s i n the Quesnel-Prince George area (Tipper and Richards, 1976). GEOLOGY OF THE STUDY AREA General D i s t r i b u t i o n of Rock U n i t s F i v e rock;' groups are represented w i t h i n the map area. General bedrock d i s t r i b u t i o n i s shown on Figure 5. Figure 6 ( i n pocket) i s the d e t a i l e d geologic map and Figures 7 and 8 (also i n pocket) are cross s e c t i o n s from t h i s map. The o l d e s t rocks are those of the Cache Creek Group, the focus of t h i s study. Many l i t h o l o g i e s are present and s t r a t i g r a p h y i s complicated by t e c t o n i c d i s r u p t i o n . The age of the Cache Creek here i s probably Late Permian. Upper T r i a s s i c rocks are found along the eastern part of the area where u various l i t h o l o g i e s of the N i c o l a Group are exposed. Lower J u r a s s i c rocks which can be c o r r e l a t e d w i t h the As h c r o f t Formation f u r t h e r north crop out i n two s m a l l areas near the south end of the map area. A f o u r t h u n i t i s the Lower Cretaceous Spences Bridge Group which u n d e r l i e s the western edge of the map area. Covering l a r g e amounts of the bedrock are Quaternary depo- s i t s of va r i o u s o r i g i n s . Cache Creek Group Cache Creek s t r a t i g r a p h y i s obscure because of three f a c t o r s : l ) , p o o r exposure, 2) l e n t i c u l a r nature of o r i g i n a l bedding, and 3) t e c t o n i c d i s r u p - t i o n . The group i s d i v i d e d i n t o three map u n i t s . Figure 9 i s a diagrammatic 16 Figure 5. Generalized geology of the Venables Va l l e y area. 17 COVERED m«tar« . | 0 0 0 Weakly fo l ia ted , f i n e - g r a i n e d s e d i m e n t s Ca rbona te pods and layers Greenstone pods and mass ive unit Chert pods U l t r a m a f i c pods Fo lded phy l l i t es with interbedded s a n d s t o n e , g r e y w a c k e s , c a r b o n a t e s L o w angle f a u l t P o s s i b l e t h r u s t fau l t over N i c o l a Group V- 500 l _ 0 F i g u r e 9 . D i a g r a m m a t i c s t r a t i g r a p h i c s e c t i o n o f t h e C a c h e C r e e k G r o u p . 18 s e c t i o n showing these u n i t s and t h e i r r e l a t i o n to each other. Lowest, but not n e c e s s a r i l y o l d e s t , i s a s t r o n g l y deformed, c l a s t i c u n i t c o n s i s t i n g c h i e f l y of p h y l l i t e w i t h minor greywacke and sandstone. V o l c a n i c rocks, limestone l a y e r s and pods, ribbon, chert and s e r p e n t i n i t e masses are pres- ent but l e s s abundant. In the south the o v e r l y i n g u n i t i s composed of a l - . tered b a s a l t i c flows. This u n i t t h i n s to the n o r t h , probably as a r e s u l t of f a u l t i n g . Above the wedge of b a s a l t i s a second c l a s t i c - d o m i n a t e d u n i t that has p r e v i o u s l y been mapped as part of the Marble Canyon Formation ( D u f f e l l and McTaggart, 1952). Present work i n d i c a t e s that limestone i s a minor component, present as small pods i n a sheared p e l i t i c m a trix. Just n o r t h of the f i e l d area, along s t r i k e , massive limestone, probably Marble Canyon Formation, i s exposed on White Mountain. Lower C l a s t i c Unit (Pp, P v c l . P p l . P i s on Figure 6) L i t h o l o g i e s present i n t h i s u n i t i n order of abundance are p h y l l i t e , greywacke, ribbon c h e r t , limestone, v o l c a n i c rocks, v o l c a n i c l a s t i c s e d i - mentary rocks and s e r p e n t i n i t e . Though p h y l l i t e i s very p o o r l y exposed, i t probably u n d e r l i e s most of the area mapped as lower c l a s t i c u n i t . Some good exposures can be found i n g u l l i e s of road c u t s , but outcrops of the lower c l a s t i c u n i t on h i l l s i d e s are u s u a l l y of coarser-grained c l a s t i c rocks, c h e r t , or limestone. P h y l l i t e , however, makes up most of the f l o a t i n the area where f l o a t i s v i s i b l e , i n game t r a i l s or on d r i e r , more open slopes. The p h y l l i t e s are grey to b l a c k , i n some places greenish and weakly band- ed. Thin s t r i n g e r s l e s s than 2-3 mm t h i c k of l i g h t e r c o l o r e d , coarser-grained m a t e r i a l , u s u a l l y s i l t or very f i n e sand, give the p h y l l i t e i t s banded appear- ance. Though these s t r i n g e r s may be r e l a t e d to o r i g i n a l bedding, they no longer can be c a l l e d bedding since they are discontinuous and o c c a s i o n a l l y form s m a l l , r o o t l e s s f o l d s . Banding p a r a l l e l s the p h y l l i t i c f o l i a t i o n i n most..: exposures, but at s e v e r a l outcrops, probably, f o l d hinges, o r i g i n a l bed- ding can be seen i n a c r o s s c u t t i n g r e l a t i o n s h i p to the f o l i a t i o n . U s u a l l y these exposures are i n coarser-grained rocks. In t h i n s e c t i o n the p h y l l i t e s are seen to be s t r o n g l y f o l i a t e d and r e - c r y s t a l l i z e d . The p h y l l i t e s are dominantly very f i n e - g r a i n e d w i t h up'.to 50% p l a g i o c l a s e f e l d s p a r ( a l b i t e ) and very l i t t l e to no quartz. Feldspar and quartz o f t e n occur i n separate patches of i n t e r l o c k i n g grains w i t h c h l o r i t e , + stilpnomelane, and s e r i c i t e wrapping around the patches producing a f l a s e r t e x t u r e (Figure 10). This t e x t u r e supports f i e l d evidence that present l a y e r i n g i s secondary, a r e s u l t of alignment of the p l a t y m i n e r a l s , except where bedding i s preserved i n f o l d hinges. A d i f f e r e n t s o r t of p h y l l i t e i s exposed at the southern end of the area j u s t north of the contact w i t h the Spences Bridge Group and about t h i r t y meters above the highway. Here the p h y l l i t e s are much more d i s r u p t e d and contain abundant but s c a t t e r e d c l a s t s of v a r i a b l e l i t h o l o g i e s . The term "melange" could be used to describe rocks at t h i s exposure. The c l a s t s , up to s e v e r a l centimeters across, i n c l u d e mainly greenstones, but chert and other sediments are present. The p h y l l i t i c f o l i a t i o n i s w e l l developed and i s i n t u r n deformed i n t o sharp chevron f o l d s w i t h v a r i a b l e axes and a x i a l plane o r i e n t a t i o n s . Figure 11 i s a photograph of some of these s t r u c t u r e s . The l o c a t i o n i s shown as I on Figure 6. Many of the f o l d s are r e f o l d e d i n the same manner as the e a r l i e r ones. This seemingly c h a o t i c deformation may be a r e s u l t of pr o x i m i t y to the f a u l t contact w i t h N i c o l a s t r a t a or more l i k e l y , could be c h a r a c t e r i s t i c of much of the lower c l a s t i c u n i t and j u s t poorly exposed elsewhere. Only two other exposures, both i n logging c u t s , show s i m i l a r deformation s t y l e s w i t h i n the p h y l l i t e s . N a t u r a l outcrops of the p h y l l i t e s tend to expose 20 Figure 10. Photomicrograph of typical phyllite i n the lower clastic unit of the Cache Creek Group. Note cataclastic fabric. Augen- li k e patches of feldspar and quartz are surrounded by chlorite, sericite and stilpnomelane. Plane polarized l i g h t . 21 Figure 11. Chaotic folding in Cache Creek phyllite at southern end of the area, locality I on Figure 6. The photograph i s taken looking north-northwest; note hammer near top of outcrop for scale. l e s s deformed p a r t s . Sketches of the deformation s t y l e i n the two a r t i f i c i a l exposures are shown i n Figure 12. In both cases drag f o l d s are a s s o c i a t e d w i t h s t e e p l y d i p p i n g shear zones. The p h y l l i t i c f o l i a t i o n i s i r r e g u l a r l y f o l d e d and sheared. Average s t r i k e of f o l i a t i o n i s N 25° W at l o c a t i o n I I and N 60° W at I I I . F o l d axes p a r a l l e l the s t r i k e but a x i a l planes are curved. Greywacke crops out more than p h y l l i t e i n the lower c l a s t i c u n i t . Occa- s i o n a l l y i t i s seen interbedded w i t h the f i n e r grained sediments, but more of t e n greywacke occurs as pods amidst p h y l l i t e f l o a t . Interbedded greywacke and p h y l l i t e were seen i n l a y e r s as t h i n as 3 cm, but the greywackes may be q u i t e t h i c k i n areas where contacts w i t h the p h y l l i t e are not exposed. In the f i e l d the c l a s t i c nature i s not always evident - greenstone and greywacke are e a s i l y confused. Greywacke i s u s u a l l y green and v a r i e s from massive to deformed, w i t h a well-developed c h l o r i t e s c h i s t o s i t y . G r a i n s i z e i s u s u a l l y l e s s than 2 mm. O c c a s i o n a l l y l a r g e r c l a s t s , s e v e r a l c e n t i - meters i n l e n g t h , s i t i n the f i n e r matrix. These c l a s t s are best observed on cut su r f a c e s . The greywackes are f e l d s p a t h i c . Some co n t a i n as much as 75% p l a g i o c l a s e g r a i n s , euhedral to subhedral i n shape (Figure 13). At l e a s t 30% of every greywacke i s p l a g i o c l a s e . Most of the f e l d s p a r s are now s t r o n g l y s a u s s u r i - t i z e d or completely a l t e r e d to a l b i t e . The o r i g i n a l andesine composition i s preserved i n a few g r a i n s . Zoning and twinning show up i n many g r a i n s . L i t h i c fragments make up most of the remainder of the d e t r i t a l m a t e r i a l . V o l c a n i c rocks are the dominant l i t h i c fragments; chert i s present i n one sample; and another contains abundant carbonate c l a s t s . V o l c a n i c c l a s t s i n - clude p o r p h y r i t i c flow rocks and pieces of p a l a g o n i t e (Figure 14). I t i s r a t h e r unusual to f i n d a piece of g l a s s preserved i n a Permian rock, e s p e c i a l l y s i n c e the rock i s a l t e r e d and deformed, b,ut the fragment i s i s o t r o p i c , brown Figure 12. F i e l d sketches showing folding and shearing of p h y l l i t i c f o l i a t i o n at two road cuts i n the Cache Creek Group. Roman numerals designate locations shown on Figure 6. -24 Figure 13. Photomicrograph of undeformed p l a g i o c l a s e - r i c h greywacke. a) Plane polarized l i g h t ; b) crossed polars. Plag- plagioclase, Qz-quartz. Figure 14. Photomicrographs of l i t h i c fragments in Cache Creek Group greywackes. a) Feldspar porphyry fragment, outlined, b) limestone fragment with f o s s i l and c) palagonite fragment. A l l are taken with plane polarized l i g h t . 26 and has an apparent e l i p t i c a l banded s t r u c t u r e common to pa l a g o n i t e . In one sample a piece of f i n e r grained greywacke occurs as a c l a s t (Figure 15). Quartz i s very r a r e , u s u a l l y only 2-3 gra i n s per t h i n s e c t i o n . A brownish amphibole was observed i n one sample. Though the greywackes themselves have been metamorphosed, there appear to be some c l a s t s d erived from a low-grade metamorphic terrane. Figure 15 shows a d e t r i t a l g r a i n which i s almost com- p l e t e l y bowtie p r e h n i t e . Another s e c t i o n comtains d e t r i t a l epidote. The semi-opaque matrix i s u s u a l l y very f i n e grained and made up of secondary minerals. Metamorphic minerals i n c l u d e c h l o r i t e (found i n a l l samples i n both-the matrix and r e p l a c i n g d e t r i t a l fragments), white mica, c a l c i t e (more prevalent i n the f i n e r grained greywacke), sphene (making up much of the semi-opaque matrix and r e p l a c i n g v o l c a n i c fragments), stilpnome- lane ( o c c u r r i n g w i t h c h l o r i t e i n the matrix of deformed greywacke), pumpel- l y i t e and/or epidote (found both i n the f i n e grained matrix and r e p l a c i n g d e t r i t a l g r a i n s ) . P u m p e l l y i t e i s b r i g h t l y green p l e o c h r o i c and i s t h e r e f o r e of the i r o n r i c h v a r i e t y . Except f o r the d e t r i t a l p r e h n i t e mentioned e a r l i e r , p r e h n i t e occurs i n veins and i s r a r e l y developed w i t h i n the body of the rocks. Table I shows mineralogy of the greywackes. Deformation i n the greywackes v a r i e s from n e g l i g i b l e to str o n g , i n the form of a c a t a c l a s t i c f o l i a t i o n . Figure 13 i s a photomicrograph of one of the r e l - a t i v e l y undeformed greywackes. M a t r i x i s composed of f i n e r and f i n e r grains s i m i l a r i n composition to the l a r g e r c l a s t s . A weakly f o l i a t e d greywacke i s shown on Figure 16. The f o l i a t i o n i s defined by the development of c h l o r i t e i n the matrix and r e c r y s t a l l i z e d d e t r i t a l g r a i n s . Figure 17 shows a more extreme c a t a c l a s t i c f a b r i c , protomylonite, w i t h an almost opaque, r e c r y s t a l - l i z e d and broken fragment zone of f i n e grained m a t e r i a l around the shattered and t e c t o n i c a l l y eroded f e l d s p a r g r a i n s . This v a r i a t i o n i n deformaion w i t h i n the same l i t h o l o g y i s t y p i c a l of the s t r u c t u r a l s t y l e of the Cache Creek 27 Figure 15. a) Thin section of a Cache Creek Group greywacke showing large rip-up clast of siltstone re- deposited while soft. Box i s around approximate location of b) photomicrogragh of a d e t r i t a l prehnite grain. Crossed nicols. se  la  ol o 0 N •H j J •H 4J n •rl cm o 00 a 0) rH 3 3 .< O* o J3 rH •H CM Sample I Lithology Lower Clast i c Unit - 6-18-1 Greywacke f f a a 6-18-2 Greywacke f f a a 6-18-3 Greywacke f f a a a 7-8-2 Greywacke f f a a a 7-20-1 Greywacke f f q aq a a 7-20-6 Greywacke f f a a q 7-20-8 Greywacke f a a a a 7-5-4 Augite porphyry r a av a a 7-5-3B Basalt r a a a 7-5-3A Tuff f r av a 7-6-5 Augite porphyry r q a a 7-22-5 Basalt r rvq av a v 7-20-9 Greywacke f f f q a a a Greenstone Unit 6-9-3 Tuff r av a 7-20-3 Basalt r r q a a aq 7-20-2 Basalt r r vq vq a avq 7-20-4 Basalt r r a a a 7-19-3 Basalt r r q aq a av 6-18-6 Basalt r a a 6-18-5 Basalt r r q q q V Upper Clas t i c Unit 7-25-2 Tuff r r a a 7-12-3 Tuff r a a av 7-25-3C L i t h i c sandstone f r a a 7-25-3B Greywacke f a a f J3 •H ! cu Ol 1 4J 4-> O •H a TI O l l •rl rH j= •H o. ta o. W o 5 r- l a a a a a f a fa a a f a a a a a a a a aq a a f a fa a a a a a a a av a a a a a a a a a a a f a a a a q a q a f av a a r q a r vq vq a r a r q a a aq a aq a av a r a r a a f a a f TABLE I. Mineralogical Data for Greywackes and Greenstones the Cache Creek Group. Key to Mineral Occurrences r - r e l i c t mineral f - d e t r i t a l fragment a - alteration mineral v - vesicle f i l l i n g q - vein mineral Figure 16. Photomicrograph of a weakly foliated Cache Creek Group greywacke. Plane polarized light. 30 Figure 17. Photomicrographs of protomylonitized Cache Creek Group greywacke. Note shattered feldspar grains and fluxion texture i n much of the matrix. Plane polarized l i g h t . 31 and w i l l be discussed l a t e r . Chert, the t h i r d most p l e n t i f u l l i t h o l o g y w i t h i n the lower c l a s t i c u n i t , v a r i e s from l i g h t grey to n e a r l y b l a c k i n c o l o r and i s interbedded w i t h t h i n l a y e r s of b l a c k a r g i l l i t e (ribbon c h e r t s ) . The a r g i l l i t e i s u s u a l l y q u i t e deformed and f r e q u e n t l y squeezed out from'between chert l a y e r s . Chert outcrops are u s u a l l y s m a l l , 6 meters of l e s s i n l a r g e s t dimension. I n v a r i a b l y the chert i s boudined and o c c a s i o n a l l y chevron type f o l d s of sever- a l tens of centimeters amplitude are v i s i b l e . Alignment of a l a r g e number of chert outcrops i n the south end of the f i e l d area suggests there may have been l a r g e r s c a l e boudinage of a s i n g l e h o r i z o n . B r e c c i a t i o n and m y l o n i t i z a - t i o n are a l s o prevalent i n the cherts on macroscopic and microscopic s c a l e s (Figures 18 and 19). Thin s e c t i o n s show some of the cherts to c o n t a i n r a d i o l a r i a and probable sponge s p i c u l e s , but these are too r e c r y s t a l l i z e d to be i d e n t i f i e d and show up as c i r c l e s or e l l i p s e s of s l i g h t l y coarser quartz w i t h i n the f i n e r grained matrix (Figure 20). H y d r o f l u o r i c a c i d etching techniques were used i n an attempt to recover r a d i o l a r i a (Pessagno,1972), but r e c r y s t a l l i z a t i o n has e f f e c t i v e l y destroyed the f o s s i l s . The etching dick.expose t i n y f o l d s of t h i n a r g i l l a c e o u s l a y e r s w i t h i n the c h e r t . Generally the cherts are impure. They conta i n minor amounts of c h l o r i t e w i t h some s e r i c i t e , s c a t t e r e d p y r i t e , and o c c a s i o n a l c l i n o z o i s i t e . The p l a t y m i n e r a l s , i f they are s u f f i c i e n t l y abundant, lend a f o l i a t i o n to the c h e r t . .Carbonates occur as semi-continuous l a y e r s and as pods. The best example of a carbonate l a y e r i s found along the top of the lower c l a s t i c u n i t and can be traced i n t e r m i t t e n t l y f o r 4.5 km. I t i s a s i l t y to sandy limestone w i t h a well-developed f o l i a t i o n : . No f o s s i l s were found. Thin s e c t i o n s show d e t r i t a l m a t e r i a l to be s i m i l a r to that i n the greywackes - p l a g i o c l a s e g r a i n s , some andesine but mostly a l t e r e d to a l b i t e , and l i t h i c fragments, u s u a l l y 32 Figure 18. Brecciated Cache Creek Group chert. Scale i s 5 cm. cm gure 19. Mylonitized Cache Creek chert. 34 Figure 20. R e c r y s t a l l i z e d r a d i o l a r i a and probable sponge s p i c u l e s i n Cache Creek chert. Photomicrograph "a" i s plane p o l a r i z e d l i g h t and "b" with crossed p o l a r s . v o l c a n i c r o c k s , o c c a s i o n a l chert and c a l c i t e g r a i n s . The c l a s t s are surround- ed by c a l c i t e which i s r e c r y s t a l l i z e d i n t o long grains and defines the f o l i a - t i o n . Most of the c a l c i t e i s probably of sedimentary o r i g i n , sand g r a i n s and cement, but some occurs as a replacement of p l a g i o c l a s e . Minor epidote and c h l o r i t e are present. The carbonate pods are much purer limestone and are s t r o n g l y r e c r y s t a l l i z e d and f o l i a t e d . The f o l i a t i o n p a r a l l e l s the length of the pods which i n t u r n p a r a l l e l s the f o l i a t i o n i n the surrounding rocks. These pods vary i n s i z e from a meter or l e s s i n length to a k i l o m e t e r . Unfortunately no f o s s i l s are preserved i n the pods w i t h i n the f i e l d area, though Danner (pers. comm., 1975) f i n d s Permian f u s i l i n e s i n pods to the north. Contact r e l a t i o n s h i p s of the pods w i t h the c l a s t i c rocks surrounding them are not very c l e a r . In some places i t appears as i f the f r i n g e s of the pod are interbedded w i t h the sediments, though i n f o l d i n g i s another p o s s i b i l i t y . U s u a l l y the margins of the pods are more h i g h l y sheared than t h e i r i n t e r i o r s suggesting movement along the contact. However, sheared margins would occur i f the blocks are t e c t o n i c a l l y emplaced or i f they were present p r i o r to tectonism, and deformation was concentrated along the contact zone. Minor v o l c a n i c and v o l c a n i c l a s t i c rocks are present w i t h i n the lower c l a s t i c u n i t . One 2 km long pod i s mapped se p a r a t e l y . TThis pod l i e s along the east s i d e of Venables V a l l e y and i s composed of augite porphyry, some as c r y s t a l and l i t h i c t u f f s . Phenocrysts occur--as s m a l l as 1 mm and l e s s but u s u a l l y are s e v e r a l m i l l i m e t e r s across. Also present are coarser v o l c a n i - c l a s t i c rocks w i t h c l a s t s derived from augite porphyry. O r i g i n a l t e x t u r e s are i n l a r g e part destroyed by l a t e r deformation and r e c r y s t a l l i z a t i o n . A well-developed s c h i s t o s i t y ( c h l o r i t e , a l s o t a l c ? ) i s present i n some of the greenstones of t h i s pod and p a r a l l e l s f o l i a t i o n i n the surrounding p h y l l i t e , w h i l e other greenstones are h a r d l y f o l i a t e d . Metamorphic minerals besides c h l o r i t e i n c l u d e a l b i t e , p u m p e l l y i t e , epidote, stilpnomelane, sphene, white mica, a c t i n o l i t e , and a blue amphibole. The occurrence of t h i s amphir- bole i s at l o c a l i t y IV on Figure 6, j u s t northeast of Venables Lake. The grey-blue to b l u i s h ^ g r e e n , p l e o c h r o i c amphibole grows p a r a l l e l to the foliar-, t i o n on the edges and i n t e r i o r s of a u g i t e grains and i n patches i n the m a t r i x (Figure 21). Microprobe analyses were run on s e v e r a l of these g r a i n s to determine t h e i r composition. Oxide weight percentages of nine elements and the number of ions of each element i n the mineral formula are presented i n Table I I . Two OH molecules were assumed to complete the amphibole s t r u c t u r e . An average formula r e p r e s e n t i n g these analyses i s : '• ^ ^ l . l ' ^ l . l - l ^ ( M % . 7 - 3 . 2 ' F e l . 6 - 1 . 8 ' A : L . 2 - . 5 ) 5 S i 8 0 2 2 ' ( O H ) 2 - On the standard sodic amphibole r e c t a n g l e (Figure 22a) t h i s composition p l o t s w i t h i n the magnesioreibekite f i e l d along the c r o s s i t e - m a g n e s i o r e i b e k i t e boundary. The presence of Ca i m p l i e s an a c t i n o l i t i c component, A m i s c i b i l - i t y gap between Na- and Ca-amphiboles has been suggested by-Coleman and Papike (1968) who f i n d glaucophane and a c t i n o l i t e c o e x i s t i n g i n carbonate r i c h sedimentary rocks. The blue amphibole of the present study, however, p l o t s d i r e c t l y i n the center of t h e i r compositional gap (Figure 22b). Brown (1977b) claims that continuous s o l i d s o l u t i o n between c r o s s i t e and a c t i n o l i t e e x i s t s and a l l compositions are s t a b l e w i t h i n the greenschist f a c i e s and lower pressure part of the b l u e s c h i s t f a c i e s . The gap may e x i s t at higher pressures. Amphiboles n e a r l y i d e n t i c a l i n composition to those i n the present study are found i n a b l u e s c h i s t grade o p h i o l i t e from the melange zone i n northern New Caledonia (Black and Brothers, 1978). There r i e b e c k i t i c amphiboles rim r e l i c t igneous pyroxene g r a i n s . The blue amphiboles are f r e q u e n t l y s t r o n g l y zoned; s o d i c - a c t i n o l i t e cores grade out to r i e b e k i t i c rims. The a n a l y s i s which most c l o s e l y resembles the present study i s a sodic a c t i n o l i t e ( a n a l y s i s 13541, p.75 i n Black and Brothers, 1978) l i s t e d on Table I I . In Figure 21. Photomicrographs of blue amphiboles in augite porphyry volcanic rocks. The amphiboles rim or form within r e l i c t augite grains. Some occur in the groundmass. Plane polarized l i g h t . The blue amphiboles are colored to show their distribution. TABLE I I Blue Amphibole Probe Analyses** Oxide Weight % / Ions Per Formula Uni t of Element Na 20 3.27 / 0.89 3.17 / 0.85 3.52 / 1.00 3.99 / 1.14 3.04 / 0.86 3.20 / 0.9*** MgO 14.49 / 3.04 14.35 / 2.97 • 12.68 / 2.77 12.15 / 2.67 14.43 / 3.15 14.70 / 3. 17 A 1 2 0 3 0.99 / 0.16 1.-20 / 0.2 1.37 / 0.24 3.07 / 0.53 1.14 / 0.2 2.40 / 0.41 S i 0 2 57.96 / 8.06 58.71 / 8.14 54.43 / 7.98 53..-76 / 7.92 55.52 / 8.14 53.60 / 7.76 K 20 0.07 / 0.01 0.06 / 0.01 0.07 / 0.01 0.06 / 0.01 0.07 / 0.01 0.05 / 0.01 CaO 7.44 / 1.12 8.23 / 1.22 7.44 / 1.17 8.62 / 1.36 7.29 / 1.15 8.70 / 1.35 TiO 0.14 / 0.02 1.07 / 0.11 2.57 / 0.28 0.21 / 0.02 0.13 / 0.01 0.05 / 0.01 MnO 0.14 / 0.01 0.12 / 0.01 0.12 / 0.02 0.14 / 0.02 0.16 / 0.02 0.30 / 0.04 FeO* 15.05 / 1.77 13.58 / 1.57 13.54 / 1.66 13.63 / 1.68 13.34 / 1.64 15.20 / 1.84 T o t a l 99.55 100.49 95.26 95.64 95.11 98.25 ( l e s s OH) * T o t a l Fe computed as. FeO ** a c c e l e r a t i o n p o t e n t i a l - 15kv specimen current - .25 yamp • beam s i z e - 10 y Bence-Albee data reduction (Bence and Albee, 1968) a f a c t o r s from Albee and Ray (1970) Analyses conducted on U n i v e r s i t y of B r i t i s h Columbia microprobe *** A n a l y s i s 13541 from Black and Brothers (1978) OO 39 N a 2 F e » A l 2 S i 8 0 2 2 ( O H ) 2 100 N a 2 F e ^ F e " 2 S l g 0 2 2 ( O H ) 2 80 60 40 r - T 20 5 0/ CO T i i i I « i a i £ -4 •o < » L 0 N a 2 M g 3 A l 2 S i 8 0 2 2 ( O H ) 2 I i i I c ® «2? o - I _L Composition of a n a l y s i s from present study 20 40 60 80 100 N a 2 M g 3 F e , 2 , S i g 0 2 2 ( O H ) 2 C a 2 ( M g , F e " ) 5 S i 8 0 2 2 ( O H ) 2 Composition of a n a l y s i s from present study N a 2 ( M g , F e " ) 3 A l 2 S i 8 6 2 2 ( O H ) 2 N a 2 (Fe" ,Mg) 3 F e ' 1 .• 2 S i g 0 2 2 ( O H ) 2 F i g u r e 22. Sodic amphibole compositions p l o t t e d on a) standard glaucophane- c r o s s i t e - r i e b e c k i t e rec tang le (Deer and o thers , 1966) and b) an a c t i n o l i t e vs_. sodic amphibole t r i a n g l e from Coleman and Papike (1968). 40 New Caledonia the sod i c amphiboles are greenish r a t h e r than blue. C o l o r l e s s to pale green amphiboles o c c u r r i n g w i t h blue amphiboles i n the present area were not analysed; a d d i t i o n a l microprobe work i s necessary to f u l l y d e s cribe the nature of amphiboles i n t h i s p a r t of the Cache Creek Group. Small pods of v o l c a n i c rocks are found i n the lower c l a s t i c u n i t as w e l l as the l a r g e pod already described. The small pods are b a s a l t i c and i n c l u d e augite porphyry w i t h zoned and twinned a u g i t e ; phenocrysts i n a very f i n e - grained, r e c r y s t a l l i z e d groundmass w i t h abundant a c t i n o l i t e , and amygdaloidal flows w i t h t i n y p l a g i o c l a s e c r y s t a l s s e t i n an extremely f i n e g rained, d e v i - t r i f i e d g l a s s groundmass. V e s i c l e s are c h l o r i t e f i l l e d . Contact r e l a t i o n - ships of these greenstones w i t h the surrounding p h y l l i t e s are not exposed and i t i s not evident whether these greenstones are flows w i t h i n the s e c t i o n , dikes or t e c t o n i c i n c l u s i o n s . U l t r a m a f i c r o c k s , mainly s e r p e n t i n i t e , are s c a t t e r e d throughout the lower c l a s t i c u n i t as small pods, and are found more continuously along the no r t h - ern part of the upper contact of the u n i t . A l a r g e pod pinches out no r t h - ward to a t h i n l a y e r g e n e r a l l y found only i n f l o a t . Although r e l i c t c l i n o - pyroxenes are o c c a s i o n a l l y v i s i b l e i n t h i n s e c t i o n , the u l t r a m a f i c rocks are almost t o t a l l y a l t e r e d to serpentine w i t h magnesite, magnetite and minor chromite. A f o l i a t i o n due to the alignment of serpentine s i m i l a r to the f o l i a t i o n i n the p h y l l i t e s i s developed i n the smaller serpentine pods. P a r t of the l a r g e r u l t r a m a f i c block along the contact between the lower c l a s t i c u n i t and the greenstones, however, i s more massive and i n c l u d e s some p o r p h y r i t i c b a s a l t . The b a s a l t contains a l t e r e d p l a g i o c l a s e phenocrysts set i n a f i n e r mass of f e l d s p a r l a t h s and d e v i t r i f i e d g l a s s . Contacts of the lower c l a s t i c u n i t are unexposed or p o o r l y exposed. No base f o r the u n i t i s seen. The upper contact i s defined by a s e r p e n t i n i t e zone i n the north and greenstones to the south. Shearing has occurred along the contact which appears to dip p a r a l l e l to the r e g i o n a l f o l i a t i o n . The contact i s probably a lowr-angle f a u l t . To the south and along the west and. southwestern margin of the Cache Creek i s a probable f a u l t contact w i t h rocks of the Lower Cretaceous Spences Bridge Group. The contact i s not exposed w i t h i n the area, but Pearson (1974) documents t r u n c a t i o n of z e o l i t e f a c i e s boundaries w i t h i n the Spences Bridge by the contact w i t h the Cache Creek Group. A l s o , low angle f a u l t s i n the Cache Creek are truncated by the contact. Massive Mafic V o l c a n i c U n i t (Pv on Fi g u r e 6) Lying t o p o g r a p h i c a l l y above and i n f a u l t contact w i t h the lower c l a s t i c u n i t i s a great thickness of a l t e r e d mafic v o l c a n i c rocks. In the f i e l d these rocks can only be i d e n t i f i e d as massive, a p h a n i t i c greenstones and resemble the green c l a s t i c sedimentary rocks of the other u n i t s . Fresh surfaces are n e a r l y impossible to produce as these rocks tend to break only along weathered surfaces. O c c a s i o n a l l y a weak f o l i a t i o n can be observed, but f r a c t u r i n g i s much more pr e v a l e n t . In the f i e l d these were grouped by the massive weathering c h a r a c t e r , the greenish c o l o r , and the l a c k of sedimentary features that sometimes show up i n the greywackes. A l l of the greenstones i n t h i s u n i t are b a s a l t i c . In p o r p h y r i t i c members phenocrysts are euhedral augite w i t h or without p l a g i o c l a s e . In one sample a s e r p e n t i n i z e d o l i v i n e phenocryst can be recognized along w i t h a u g i t e and p l a g i o c l a s e . P l a g i o c l a s e of composition A n ^ i s r e l i c t i n one rock, but g e n e r a l l y a l l the f e l d s p a r i s a l t e r e d to a l b i t e . The groundmass i s an intern- l o c k i n g mass of f i n e p l a g i o c l a s e and a u g i t e . Table I contains m i n e r a l o g i c a l d e t a i l s . V e s i c l e s f i l l e d w i t h c a l c i t e , c h l o r i t e , or pumpellyite are surroundr ed by d e v i t r i f i e d glass i n s e v e r a l samples, and the very f i n e grain..size of 42 others makes a flow o r i g i n f o r most of the greenstones seem reasonable. C r y s t a l and l i t h i c t u f f s are present i n smaller q u a n t i t i e s . Flow s t r u c t u r e s , p i l l o w s , or i n d i v i d u a l flow u n i t s could not be d i s t i n g u i s h e d i n the f i e l d . A l t e r a t i o n v a r i e s from one sample to another. C h l o r i t e , a l b i t e and sphene are present i n a l l the greenstones w h i l e p r e h n i t e , p u m p e l l y i t e , c a l c i t e , white mica, and a c t i n o l i t e are v a r i a b l y developed. As s t a t e d above, the greenstones are u s u a l l y massive, but immediately above the contact w i t h the underlying u n i t a protomylonite f a b r i c i s developed. Round grains of augite s i t i n a very f i n e grained, almost opaque, f o l i a t e d m a t r i x (Figure 23). This c a t a c l a s t i c f o l i a t i o n at the base of the greenstone u n i t supports the low-angle f a u l t i n t e r p r e t a t i o n f o r the lower contact, The upper contact i s not exposed anywhere but i s probably f a u l t e d , s i n c e the greenstone u n i t i s cut o f f to the north. Upper C l a s t i c U n i t (Ps, P i s ) As a r e s u l t of very poor exposure due to t h i c k grass, trees and low r e l i e f , rocks i n the area mapped as the upper c l a s t i c u n i t can not be w e l l defined, The u n i t l i e s above the greenstone u n i t to the south and to the north i t i s separated from the u n d e r l y i n g e l a s t i c s by a t h i n s e r p e n t i n i t e b e l t . L i t h o l o g i e s are much the same as i n the lower c l a s t i c u n i t , but deformation and metamorphism seem to be l e s s . Blocks of limestone, greenstone, coarse c l a s t i c rocks - greywacke,. and b r e c c i a - and o c c a s i o n a l l y chert are best exposed. They appear to be surrounded by a s h a l e , c h e r t - a r g l l l i t e , and serpentine matrix which i s only exposed i n f l o a t or i n s c a t t e r e d logging road c u t s . No t r a c e a b l e l a y e r s or s t r u c t u r e s are v i s i b l e . Limestone, one of the best exposed l i t h o l o g i e s , i s d i f f e r e n t from carbon^ ates i n the lower c l a s t i c u n i t . I t occurs as pods, but the pods are not sheared and o r i g i n a l s t r u c t u r e s such as bedding i n some p l a c e s , abundant Figure 23. Cataclastic textures i n altered augite porphyry which l i e s along the contact of the mafic volcanic unit and the lower clastic unit. Pr-prehnite, Pump- pumpellyite, Aug-augite. Plane polarized li g h t . 44 o o l i t e s , and f u s i l i n i d s are preserved. The carbonates are t y p i c a l Marble Canyon limestone, l i g h t grey weathering and coarsely c r y s t a l l i n e . Two pods of t h i s type are exposed on the west s i d e of the high r i d g e . Other limestone exposures are much smaller and are c l o s e l y a s s o c i a t e d w i t h greenstone i n b r e c c i a s . These are probably s l i d e b r e c c i a s and w i l l be d i s - cussed under sedimentary s t r u c t u r e s . C l a s t i c rocks coarser than a r g i l l i t e resemble s i m i l a r l i t h o l o g i e s i n the lower c l a s t i c u n i t . D e t r i t u s i n these greywackes and f i n e sandstones i s composed of v o l c a n i c fragments and augite and f e l d s p a r c r y s t a l s set i n a very f i n e grained matrix of a r g i l l a c e o u s m a t e r i a l , f e l d s p a r , c h l o r i t e , carbonate and s e r i c i t e . P umpellyite occurs i n a fragment i n one sample. Bedding i s not d i s r u p t e d and i s defined by a l t e r n a t i n g bands of coarser and f i n e r mater- i a l . Layers are 2-10 cm t h i c k i n the. few exposures seen. Welded c r y s t a l l i t h i c t u f f s appear to be the major v o l c a n i c component of the upper c l a s t i c u n i t . Augite and p l a g i o c l a s e c r y s t a l s s i t i n a groundmass of d e v i t r i f i e d , welded glass shards. V o l c a n i c fragments are mostly very f i n e grained b a s a l t or n e a r l y opaque pieces of d e v i t r i f i e d g l a s s . A l b i t e , c h l o r i t e , s e r i c i t e , c a l c i t e and minor pumpellyite are the only a l t e r a t i o n m inerals. The c h e r t - a r g i l l i t e and s e r p e n t i n i t e matrix surrounding the b l o c k s of carbonate, greenstone, and coarse c l a s t i c rocks i s exposed only as f l o a t and i n rubble i n l o g g i n g road cuts. Chert and a r g i l l i t e are grey to black and b l a c k r e s p e c t i v e l y and very weakly f o l i a t e d . Contacts between the blocks and matrix are not exposed. Age of the Cache Creek Group F u s i l i n i d s found i n limestone pods of the upper c l a s t i c u n i t (Figure 24) were i d e n t i f i e d by W.R. Danner as Yabeina minuta of Late Permian probably Late Figure 24. Photomicrographs of limestone in the upper clastic unit, Cache Creek Group: a) oolites and b) Upper Permian, probably Late Guadalupian, f u s i l i n i d Yabelna minuta, identified by W.R.Danner. 46 Guadalupian, This age i s t y p i c a l of limestone a m a s s e s ; i n i t h i s p a r t . o f the :; . Cache Creek (Danner, 1976). No i n f o r m a t i o n i s a v a i l a b l e f o r other l i t h o l o - ? gies of f o r limestones of the lower c l a s t i c u n i t . Sedimentary Structures and Environment of D e p o s i t i o n Two models of d e p o s i t i o n a l environment have been presented to e x p l a i n the p o d ^ l i k e occurrence of shallow water carbonates w i t h i n sediments u s u a l l y i n t e r p r e t e d to be of deeper water o r i g i n . From h i s work i n the A t l i n Terrane, Monger (1977a, 1977b) has developed a model of a deep ocean b a s i n i n which seamounts have formed. F r i n g i n g r e e f s are formed on seamounts and a t o l l s and these shallow water carbonates break o f f from time to time and s l i d e i n t o deeper water. They come to r e s t amongst deeper water rocks. Danner (1967, 1975), on the other hand, suggests that these carbonate blocks i n chert are not e x o t i c , but that the cherts and carbonates are interbedded. This r e q u i r e s that ribbon cherts are of very shallow water o r i g i n w i t h i n the Cache Creek Group. In t h i s model l e n s o i d masses of carbonates are i n t e r - preted to be a l g a l mounds. Sedimentary s t r u c t u r e s , l o c a l l y preserved i n areas of l e s s intense deforma- t i o n , r e f l e c t a v a r i e t y of d e p o s i t i o n a l environments w i t h i n the Cache Creek Group. Fine to coarse grained c l a s t i c sedimentary rocks show the most s t r u c t u r e s . Rhythmic l a y e r i n g i n s i l t s t o n e s and greywackes and other s t r u c t u r e s t y p i c a l of the Bouma sequence - graded bedding, laminar bedding, convolute laminations - suggest t u r b i d i t e sedimentation f o r much of the c l a s t i c m a t e r i a l . Soft sediment slump f o l d s (Figure 25) and rip-up c l a s t s (Figures 15 and 26) support g r a i n flow movements. One outcrop at the top of the r i d g e i n the upper c l a s t i c u n i t i s a b r e c c i a c o n t a i n i n g limestone and amygdaloidal and tuffaceous b a s a l t fragments (Figure 27). Chunks of limestone are s e v e r a l centimeters across w h i l e greenstone fragments are l e s s than 2 cm Figure 25. Soft sediment folds i n fine grained sediments of the lower c l a s t i c u n i t , Cache Creek Group. Figure 26. Rip-up clasts i n sediments of the upper c l a s t i c u n i t , Cache Creek Group. Slide breccia i n the upper c l a s t i c unit of the Cache Creek Group. Light grey, recessive weathering fragments are limestone. The remainder i s volcanic material. 49 i n diameter. This i s a l s o thought to be a de b r i s f l o w o f f a shallow carbon- ate and b a s a l t i c p l a t f o r m . There i s abundant evidence f o r shallow water c o n d i t i o n s i n the limestone masses of the upper c l a s t i c u n i t . O o l i t e s , f u s i l i n i d s and what may be a l g a l s t r u c t u r e s are present and represent water depths of l e s s than 30 meters. Although very deep water c o n d i t i o n s are not necessary f o r the formation of t u r b i d i t e s , l a c k of shallow water s t r u c t u r e s i m p l i e s they were deposited at l e v e l s below wave base. Apparently interbedded w i t h the debris flows are ribbon c h e r t s . The environment of formation of ribbon cherts i s a f u n c t i o n of many f a c t o r s , but carbonate f r e e r a d i o l a r i t e s are thought to be r e s t r i c t e d to depths below the carbonate compensation depth, probably 2000-3000 meters i n Permian time ( B o s e l l i n i and Winterer, 1975). No shallow water limestones were observed d i r e c t l y interbedded w i t h ribbon cherts i n t h i s area. Contacts of these carbonates are u s u a l l y s t r o n g l y sheared. Carbonate rocks c l e a r l y i n t e r - bedded w i t h the t u r b i d i t e and ribbon chert sequences are c a l c a r e n i t e s , some weakly graded, which are probably t u r b i d i t e s themselves. From evidence i n t h i s area a d e p o s i t i o n a l environment s i m i l a r to Monger's model i s reasonable. Shallow water carbonates form on b a s a l t i c v o l c a n i c sea- mounts that are surrounded by deeper water. The ba s i n i s dominated by chert and a r g i l l i t e sedimentation w i t h i n f l u x e s of coarser m a t e r i a l derived from the v o l c a n i c and carbonate terrane. E x o t i c blocks of limestone w i t h i n the f i n e grained sediments must be s l i d e b l o c k s . In support of t h i s model are p r e l i m i n a r y Mesozoic ages f o r Cache Creek r a d i o l a r i a (Travers, 1978; Monger, 1977b). Thus the s l i d e blocks may be much o l d e r than t h e r m a t r i x i n which they p r e s e n t l y s i t . More f o s s i l data from the non-carbonate rocks are needed to c o n c l u s i v e l y d e f i n e the d e p o s i t i o n a l environment i n t h i s area. S t r u c t u r e A strong p h y l l i t i c f o l i a t i o n i n the c l a s t i c r o c k s , best developed i n f i n e r 50 grained rocks; i s the e a r l i e s t and dominant s t r u c t u r e i n the Cache Creek Group. This f o l i a t i o n i s of v a r i a b l e i n t e n s i t y i n coarser c l a s t i c rocks and absent i n most greenstones. In most cases bedding has been transposed to p a r a l l e l the f o l i a t i o n , and t i n y r o o t l e s s f o l d s , w i t h amplitudes l e s s than a few centimeters, f o u n d . i n ; f i n e r i g r a i n e d ; l i t h o l o g i e s suggests the f o l i a t i o n p a r a l l e l s the a x i a l planes of i s o c l i n a l f o l d s . No l a r g e e a r l y f o l d s were •:' found, but s e v e r a l outcrops c l e a r l y demonstrate c r o s s - c u t t i n g bedding-cleavage r e l a t i o n s h i p s . A b e d d i n g r-cleavage i n t e r s e c t i o n l i n e a t i o n i s developed on f o l i a t i o n surfaces i n the c l a s t i c members. Carbonate and chert responded d i f f e r e n t l y to t h i s deformation. Cherts are s t r o n g l y boudined and b r e c c i a t e d w i t h the a r g i l l i t e l a y e r s almost squeezed out. The long a x i s of the boudins p a r a l l e l s the other l i n e a t i o n . Chert l a y - s ers are probably boudined on a l a r g e r s c a l e . This i s r e f l e c t e d i n the d i s - continuous outcrop p a t t e r n of the c h e r t s . Carbonate, greywacke and green- stone l a y e r s may a l s o have been boudined on a l a r g e s c a l e ; i n some of the carbonate-greenstone b r e c c i a of the upper c l a s t i c u n i t , carbonate c l a s t s have been s t r o n g l y elongated. Chevron f o l d s of one to two meter amplitude are developed i n ribbon cherts i n a few l o c a l i t i e s . No a x i a l plane cleavage cuts these f o l d s , and t h e i r t iming i s unknown. Despite the extreme deformation i n much of the Cache Creek, some p o r t i o n s seem to have been l e f t completely undeformedJ Greywacke pods, mostly i n the upper c l a s t i c u n i t , are u n f o l i a t e d and the sedimentary s t r u c t u r e s are w e l l preserved, even though the pods are apparently surrounded by deformed p h y l l i t e . Some carbonate pods are s t r o n g l y deformed w h i l e others are e s s e n t i a l l y p r i s - t i n e . Figure 28 i s an equal area stereonet p l o t of a l l s t r u c t u r a l elements measured i n the Cache Creek Group. A northwest trend of a l l s t r u c t u r a l e l e - ments dominates. F o l i a t i o n s are c l u s t e r e d around a northwest s t r i k e w i t h a moderate to steep southwest d i p . L i n e a t i o n s p l o t along the northwest trend 51 x Bedding (SQ) ° Foliation (S, ) • Bedding - cleavage intersection lineation ( l f ) " Axis of kink or fold that deforms S, 0 2 ) A Axis of chevron fold in chert Figure 28. Equal area s tereonet p l o t of s t r u c t u r a l elements i n the Cache Creek Group. 52 and plunge moderately. S c a t t e r i n the f o l i a t i o n p l o t i s probably r e l a t e d to l a t e r s u b p a r a l l e l f o l d i n g which i s only l o c a l l y observed and deforms the e a r l i e r f o l i a t i o n and l i n e a t i o n . This event c o n s i s t s of b r i t t l e f o l d i n g and warping w i t h some f r a c t u r i n g along a x i a l planes but no development of a p e n e t r a t i v e a x i a l plane f o l i a t i o n . Folds are f l e x u r a l - s l i p , open, and of small ( l e s s than h a l f a meter) amplitude and wavelength. The best exposure of these f o l d s and t h e i r r e l a t i o n s h i p to the e a r l i e r l i n e a t i o n and f o l i a t i o n i s j u s t west of the f a u l t contact w i t h the N i c o l a Group at the north end of the area. The f o l d s may be r e l a t e d to movement along that f a u l t . Chevron f o l d s deforming the p h y l l i t e at the southern end of the map area (Figure 11) are s i m i l a r and may be of the same age. Ma c r o s c o p i c a l l y the Cache Creek Group appears to be composed of f a u l t - bounded packages of rock. Quite c l e a r l y a low angle f a u l t i s the lower con- t a c t of the greenstone u n i t . The u n i t i s truncated and a strong c a t a c l a s t i c f o l i a t i o n has formed i n some of the lowermost greenstones (Figure 23) and the uppermost members of the lower c l a s t i c u n i t below. This f a u l t p a r a l l e l s the f o l i a t i o n i n the lower c l a s t i c u n i t . Further evidence f o r the f a u l t i s the presence of s e r p e n t i n i t e along the p r o j e c t e d t r a c e to the nor t h . I t i s p o s s i b l e that the two c l a s t i c u n i t s should be considered as r e p e t i t i o n of a s i n g l e u n i t e n c l o s i n g the greenstone u n i t as a fault-bounded wedge. The discontinuous s e r p e n t i n i t e b e l t , d i f f e r e n c e s i n the limestones and i n degree of deformation and a l t e r a t i o n between the upper and lower c l a s t i c u n i t s sup- port the i n t e r p r e t a t i o n of a f a u l t w i t h l a r g e displacement. The only e v i - dence f o r the f a u l t mapped as the upper contact of the greenstone u n i t i s the t r u n c a t i o n of the greenstones at t h e i r northern end. I t i s q u i t e probable that there are s i m i l a r f a u l t s throughout the Cache Creek Group. The l a r g e v o l c a n i c and v o l c a n i c l a s t i c pod north and east of Venables Lake i s probably f a u l t bounded. In many ways the structural style described above f i t s descriptions of tectonic melange as defined by Hsu (1974). Small packages of rock are shear- bounded and there i s great variability in deformation within a small region. Most of the Cache Creek within the f i e l d area is typified by blocks of d i f f e r - ent lithology set in a pervasively sheared matrix. Block size ranges from a few centimeters, as exposed above the highway at the southern end of the area, to more than 5 kilometers, i f the mafic volcanic unit is considered as a block within the deformed sediments,, The next largest block is the 2 km long volcanic and volcaniclastic pod containing the blue amphibole metamorphic assemblage. Sedimentary processes have contributed to the discontinuous nature of many lithologies, but they do not explain the presence of serpen- ti n i t e bodies in close proximity to shallow water carbonates that are mixed into the deformed p e l i t i c matrix containing turbidite deposited clastic beds and bedded chert. The explanation i s more li k e l y tectonic than sedimentary, At least two periods of faulting postdate isoclinal folding, shearing, and low angle faulting. The near ver t i c a l , north-northwest trending Martel Fault which separates Cache Creek and Nicola Groups is the earliest fault after the isoclinal folds, Sense of-movement on this fault cannot be positively deter- mined from this area. Later high angle movement along the Venables Valley fault and several smaller faults offset Lower Jurassic rocks. The Venables Valley fault cuts the Martel Fault, Some of this faulting must be post lower Cretaceous as Spences Bridge Group rocks are in contact with Cache Creek along a fault that parallels the Venables Valley fault. Landslides and slumps along the west side of Venables Valley further complicate structural inter^- pretations in the Cache Creek. One major slide that i s quite clear on areal photographs and in the topography is shown on the map (Figure 6), Rocks exposed in the slide are are limited to ground up phyllite and serpentinite. 54 Metamorphism ' .' The :matrix of greywackes and a l t e r a t i o n of unstable glass and f e l d s p a r i n b a s i c v o l c a n i c rocks best r e f l e c t metamorphic co n d i t i o n s at low temperatures. Table I I I l i s t s seven metamorphic m i n e r a l assemblages observed i n Cache Creek rocks. Assemblages 1-6 are from rocks i n the lower c l a s t i c u n i t , 5 and 6 from the v o l c a n i c and v o l c a n i c l a s t i c pod ( P v c l ) , and assemblage 7 i s from the greenstone u n i t . For determination of temperature the c r i t i c a l minerals are p r e h r i i t e , p u m p e l l y i t e , a c t i n o l i t e and the epidote f a m i l y . Stilpnomelane and the sodic amphibole are the only minerals that a s s i s t i n pressure estimates. Miyashiro (1973), i n reviewing s e v e r a l low grade metamorphic b e l t s describes a sequence of zones from z e o l i t e to greenschist f a c i e s . In K i i P e n i n s u l a , Japan, Zone I I , p r e h n i t e - p u m p e l l y i t e f a c i e s , i n c l u d e s the assemblage q u a r t z , a l b i t e , p r e h n i t e , p u m p e l l y i t e , epidote, and c h l o r i t e . Zone I I I , a t r a n s i t i o n zone, i s marked by the appearance of a c t i n o l i t e and disappearance of p r e h n i t e , and i n Zone IV, greenschist f a c i e s , pumpellyite i s absent and stilpnomelane i s present. The t r a n s i t i o n zone i n the Panoche and Pacheco Pass areas of C a l i f o r n i a contains the assemblage p u m p e l l y i t e , a c t i n o l i t e , c a l c i t e , •>. : epidote and stilpnomelane. Under t h i s c l a s s i f i c a t i o n system assemblages 5 and p o s s i b l y 2 of Table I I I would be i n the t r a n s i t i o n a l zone. Assemblage 3 w i t h p r e h n i t e and p u m p e l l y i t e , no a c t i n o l i t e , f a l l s below the t r a n s i t i o n zone i n the p r e h n i t e - p u m p e l l y i t e f a c i e s , arid 7, w i t h p r e h n i t e and a c t i n o l i t e co- e x i s t i n g , i s probably on the boundary between the two zones. By Miyashiro's d e f i n i t i o n assemblages 1 and 6 f a l l i n the greerischist f a c i e s . Assemblage 4 i s not d i a g n o s t i c . Winkler (1974) marks the t r a n s i t i o n from very low grade to low grade rocks (greenschist f a c i e s ) by the appearance of z o i s i t e / c l i n o z o i s i t e i n s t e a d of i r o n r i c h epidote. Figure 29 summarizes experimentally determined r e a c t i o n e q u i l i b r i a at very low grade. The r e a c t i o n pumpellyite + c h l o r i t e + quartz TABLE I I I Metamorphic M i n e r a l Assemblages i n the Cache Creek Gro Assemblage # Quartz X X X C h l o r i t e X X X X X X X A l b i t e X X X X X X X C a l c i t e X X X X Sphene X X X X X X X White mica X X X X X Epidote X X X X Stilpnomelane X X X X X X A c t i n o l i t e X X X X Subc a l c i c Amphibole X X Pumpellyite X X X Pre h n i t e X X Sample number Assemblage number 6-18-1 1 6- 18-3 2 7- 8-2 3 7-22-5 4 7-5-4 5 7-5-3B 6 6-18-6 7 56 F i g u r e 29. P-T e q u i l i b r i a at very low grade (a f t er W i n k l e r , 1974; p.188, 236). Minera l abbrev ia t ions are Px-pyroxene, Q z - q u a r t z , Pu-pumpel ly i te , C h - c h l o r i t e , Z o - z o i s i t e , A c - a c t i n o l i t e , C c - c a l c i t e , E p - e p i d o t e , P r - p r e h n i t e . 57 goes to z o i s i t e + a c t i n o l i t e can be used as a maximum temperature f o r Cache Creek rocks i n Venables V a l l e y . Assemblage 5 c l e a r l y l i e s on the low tempera- tur e s i d e of that r e a c t i o n , and 1 and 7 are on the low temperature s i d e of the r e a c t i o n p r e h n i t e + c h l o r i t e goes to pumpellyite + a c t i n o l i t e + quartz. I r o n r i c h epidote r a t h e r than z o i s i t e i n other assemblages i n d i c a t e s those rocks a l s o are below the boundary between rocks of low grade and very low grade. In a more recent work, Brown (1977a) presents c a l c u l a t e d r e a c t i o n s i n t h i s pressure and temperature range. Assumptions of f i x e d compositions f o r many of these minerals were necessary f o r the c a l c u l a t i o n s , and the l o c a t i o n s of the r e a c t i o n curves are s t i l l estimates. F i g u r e 30 enlarges the area of i n t e r e s t from Figure 29 and i n c l u d e s r e a c t i o n s from Brown. Several assemblages from Table I I I can be given estimated P-T f i e l d s based on these r e a c t i o n s . Assem- blages 3 and 7, discussed e a r l i e r , are "plotted below the p r e h n i t e + c h l o r i t e breakdown, w i t h 7 at higher temperature due to the presence of a c t i n o l i t e . C o - e x i s t i n g c a l c i t e and epidote i n assemblage 1 puts i t below c a l c i t e + epidote + pumpellyite + a c t i n o l i t e + Ĥ O + C02" Lack of c a l c i t e and epidote together i n 5 and 6 and c o - e x i s t i n g pumpellyite and a c t i n o l i t e i n 5 puts them above the same r e a c t i o n . Pressure c o n d i t i o n s are hot as w e l l c o n t r o l l e d . Stilpnomelane probably can be used as an i n d i c a t i o n of f a i r l y high pressure as i t i s r a r e i n mediun pressure rocks and very r a r e i n rocks of low pressure metamorphism (Miyashiro, 1973). Sodic amphibole a l s o can suggest high pressure c o n d i t i o n s , but only the a d d i t i o n a l presence of aragonite or lawsoraite i s d i a g n o s t i c f o r high pressure. Lawsonite i s more prevalent i n A l - r i c h rocks; those c o n t a i n i n g the blue amphiboles here are b a s i c i n composition, p o s s i b l y even s i l i c a d e f i c i e n t , which favors blue amphibole at lower pressure. Brown (1977b) presents a t e n t a t i v e c o r r e l a t i o n of pressure to c r o s s i t e component i n Ca-amphiboles. In h i s model the sodic amphiboles of t h i s study, c o n t a i n i n g approximately one Na i n the MA s i t e , would occur at pressures of about 6 kb. These pressures are 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 Figure 30. Enlarged P-T e q u i l i b r i a from Figure 29 with react ions from Brown (1977a). M i n e r a l abbreviat ions as on Figure 29 with Hem-hematite and C r o s s - c r o s s i t e . oo 59 c o n d i t i o n a l on the presence of i r o n oxide i n the r e a c t i o n assemblage; i r o n oxide i s absent i n the present study. S i m i l a r amphiboles found i n the North- ern New Caledonia melange zone areiestlmate'd to have formed at P-T. c o n d i t i o n s of 350°C and .7 kb (Black and Brothers, 1978), but j a d e i t i c pyroxene i s a l s o i n that mineral a s s o c i a t i o n . That i n d i c a t e s higher pressure than i n the present Study. Thus an estimate of 5 to 6 kb pressure f o r rocks i n the Venables V a l l e y area c o n t a i n i n g sodic amphiboles and stilpnomelane i s considered reasonable. The assemblages of Table I I I are p l o t t e d on Figure 30 at ..pressures ..betweem.4 .land 6 kb. Assemblages 2 and 4 do not c o n t a i n d i a g n o s t i c minerals f o r t h i s p l o t . From Figure 30 i t i s apparent that metamorphic c o n d i t i o n s are not constant throughout even t h i s s m a ll area of Cache Creek rocks. D i f f e r e n t blocks w i t h i n the sheared m a t r i x c o n t a i n metamorphic assemblages r e f l e c t i n g a temperature range of 100° C and a p o s s i b l e 1 to 2 kb pressure d i f f e r e n c e . N i c o l a Group In the Venables V a l l e y area the N i c o l a Group can be d i v i d e d i n t o three map u n i t s . The f i r s t , dominantly greenstones, contains andesite, d a c i t e , r e l a t e d h i g h - l e v e l i n t r u s i v e bodies and sedimentary rocks derived from the v o l c a n i c rocks. Thick, massive to bedded limestone makes up the second map u n i t and the t h i r d i s a mixture of sedimentary rocks and some greenstones. The t h i r d u n i t includes a r g i l l i t e , t h i n bedded limestone, minor v o l c a n i c rocks, abundant green chert ( s i l i c e o u s t u f f ) and coarser c l a s t i c rocks. Figure 31 i s a diagrammatic s t r a t i g r a p h i c s e c t i o n of the N i c o l a and o v e r l y i n g J u r a s s i c rocks. The northern exposure of the second u n i t was mapped as Ni c o l a , " but most of the f i r s t and t h i r d u n i t s have p r e v i o u s l y been mapped as part of the Cache Creek Group (Carr, 1962; D u f f e l l and McTaggart, 1952). Comparison of s t r a t i - g raphic, s t r u c t u r a l and metamorphic c h a r a c t e r i s t i c s w i t h the Cache Creek Group, l i m i t e d f o s s i l evidence and a Rb/Sr isochron support an Upper T r i a s s i c age t y p i c a l of the N i c o l a Group f o r t h i s s e c t i o n . A S H C R O F T NICOLA ^ ERODE D t u p m e t e r s C a r b o n a t e C a r b o n a t e w i t h c h e r t n o d u l e s C a l c a r e o u s s a n d s t o n e S a n d s t o n e S h a l e or a r g i l l i t e C o n g l o m e r a t e C l a s t s of intrusive o r i g i n Intrusive, m a i n l y q u a r t z - e y e p o r p h y r y S i l i c e o u s g r e e n t u f f s F l o w s , a n d e s i t e to d a c i t e ;„yy, Undivided g r e e n s t o n e s p F o s s i l l o c a l i t y U n c o n f o r m i t y v ^ r ^ S h e a r e d c o n t a c t 300 L 0 Figure 31. Diagrammatic s t r a t i g r a p h i c s e c t i o n of the N i c o l a Group and Ashcro f t Format ion. 61 Greenstone Unit (TRv on Figure 6) Flows, i n t r u s i v e rocks, p y r o c l a s t i c and e p i c l a s t i c rocks of t h i s u n i t are complexly interbedded and i n t e r f i n g e r i n g as sketched on Figure 31, I n d i v i d u a l l i t h o l o g i e s appear to have l i m i t e d l a t e r a l extent. Poor exposure and minor f a u l t s c o n t r i b u t e to the d i f f i c u l t y of t r a c i n g l a y e r s . I t i s o f t e n d i f f i c u l t to d i s t i n g u i s h fragmental t e x t u r e s i n the f i e l d unless a c l e a n , smooth-weather- ed surface i s present. Even then rocks types can be ambiguous, P o r p h y r i t i c andesite u n d e r l i e s much of the eastern p a r t of the area and i s best exposed i n g u l l i e s above the Trans-Canada Highway. In outcrop the andesite is. blue green to grey w i t h white to p i n k i s h , subhedral to g l o b u l a r f e l d s p a r phenocrysts. The a p h a n i t i c groundmass contains segregations of epidote or i s cut by epidote v e i n s . S t r u c t u r e s such as p i l l o w s and flow tops were not observed, but o c c a s i o n a l l y l a r g e blocks of sedimentary rocks (one oyer a meter long) are caught up i n flows. Under the microscope p l a g i o c l a s e phenocrysts, o r i g i n a l A n ^ composition preserved i n some g r a i n s , are seen to be dominantly a l b i t e i n composition w i t h considerable epidote and s e r i c i t e replacement. The phenocrysts comprise an average of 20% of the rock and s i t i n a very f i n e grained, i n t e r l o c k i n g mosaic of r e c r y s t a l l i z e d f e l d s p a r , mainly a l b i t e w i t h traces of K - f e l d s p a r , and other a l t e r a t i o n minerals r-. c h l o r i t e , s e r i c i t e , epidote, and sometimes c a l c i t e . No primary mafic minerals are preserved, Though most of the andesites are not f o l i a t e d , there i s considerable evidence of s t r a i n i n the rocks. Kinked twins i n p l a g i o c l a s e grains are p l e n t i f u l , and phenocrysts show p o l y g o n a l i z a t i o n and r e c r y s t a l l i z a t i o n along ol d and new g r a i n margins, L o c a l l y a weak f r a c t u r e cleavage i s developed, D a c i t i c rocks are found throughout the greenstone u n i t but are most plenti<- f u l and best exposed along the r i d g e east of Venables V a l l e y , In the f i e l d , rocks of t h i s composition are e a s i l y i d e n t i f i e d by the presence of abundant quartz , u s u a l l y i n the form of s p h e r i c a l to e l l i p s o i d a l , c l e a r to smoky pheno^- 62 c r y s t s (quartz eyes), 1^6 mm i n diameter. Feldspar phenocrysts are absent to more p l e n t i f u l than the quartz eyes. The groundmass, where unsheared, i s a p h a n i t i c and s i l i c e o u s , g e n e r a l l y l i g h t green i n c o l o r . L o c a l l y the groundmass i s bleached to very pale green or white. These bleadhed zones are m i n e r a l i z e d and c o n t a i n abundant p y r i t e . Ratios of phenocrysts to groundmass, s i z e of phenocrysts, and other macro- scopic t e x t u r e s r e f l e c t a v a r i e t y of v o l c a n i c - p l u t o n i c processes i n v o l v e d i n the formation of these quartz r i c h rocks. Numerous text u r e s can be observed by t r a v e r s i n g the r i d g e east of Venables V a l l e y . J u s t south of the road c r o s s - i n g the r i d g e , v o l c a n i c b r e c c i a crops out. Fragments are almost e x c l u s i v e l y quartz-eye porphyry and are surrounded by a matrix of s i m i l a r m a t e r i a l - quartz c r y s t a l s and green a p h a n i t i c matrix/groundmass. This may be an e x p l o s i o n b r e c c i a deposited near a v o l c a n i c vent. S i l i c e o u s t u f f s represent more d i s t a n t deposits r e l a t e d to e x p l o s i v e volcanism. Quartz c r y s t a l s i n the tuffaceous rocks are u s u a l l y l e s s than 2 mm across and f e l d s p a r i s a b s e n t l A p h a n i t i c groundmass comprises more than 95% of these rocks, and i n many places they look megascopically l i k e pale green chert w i t h s c a t t e r e d round quartz c r y s t a l s and o c c a s i o n a l fragments of s i m i l a r a p h a n i t i c m a t e r i a l . Farther north along the r i d g e both f e l d s p a r and quartz phenocrysts are present and make up a l a r g e r percentage of the rock. Scattered fragments of quartz and q u a r t z - f e l d s p a r porphyries occur i n these r o c k s , but the general appearance i s of flows r a t h e r than b r e c c i a s . S t i l l f a r t h e r north rocks of apparently the some composition are d e f i n i t e l y i n t r u s i v e i n o r i g i n . Two miles north of the northern edge of the f i e l d area, a c o a r s e l y c r y s t a l l i n e quartz .dio- r i t e crops.out. Mafics are a l t e r e d to c h l o r i t e , and epidote i s abundant. This quartz d i o r i t e i s probably the i n t r u s i v e e q uivalent of the e x t r u s i v e and pyro- c l a s t i c q u a r t z - r i c h rocks. Thus the q u a r t z - r i c h rocks show i n an s m a l l area the volcano^-plutonic nature of the N i c o l a Group. Along f a u l t zones o r i g i n a l igneous t e x t u r e s have been ove r p r i n t e d by a shear 63 f o l i a t i o n . L o c a l l y a q u a r t z - s e r i c i t e s c h i s t i s developed. The f o l i a t i o n wraps around the quartz phenocrysts and fragments. Rocks w i t h t h i s f o l i a t i o n have a r a t h e r lumpy appearance. In t h i n s e c t i o n s of the d a c i t i c rocks the round to e l l i p t i c a l quartz pheno- c r y s t s can be seen to have p a r t i a l l y resorbed margins and some quartz over- growths. In more deformed samples the c r y s t a l s are i n t e r n a l l y s h a t t e r e d . Feldspar phenocrysts, i f u n a l t e r e d , are p l a g i o c l a s e w i t h an average composition of andesine ( A n ^ ) . Feldspar i s g e n e r a l l y a l t e r e d to a l b i t e or pseudomorphed by e p i d o t e , c a l c i t e and f i n e grained a l b i t e . The groundmass, much the same as that i n the andesites, i s a mosaic of f e l d s p a r , quartz, c h l o r i t e and s e r i c i t e w i t h v a r i a b l e amounts of epidote, c a l c i t e , K -feldspar and some sphene. More a l t e r e d v a r i e t i e s contain more K - f e l d s p a r . Included fragments show up as patches of s l i g h t l y d i f f e r e n t g r a i n s i z e . Margins of the fragments are i n - d i s t i n c t . E p i c l a s t i c rocks derived from the v o l c a n i c and i n t r u s i v e rocks make up the remainder of the greenstone u n i t . These i n c l u d e conglomerate, b r e c c i a , grey- wacke, l i t h i c sandstone and s i l t s t o n e . One of the best conglomerate exposures i s along the Trans-Canada Highway j u s t north of the greenstone-limestone contact. W e l l rounded cobbles of p o r p h y r i t i c andesite are n e a r l y i n d i s t i n g u i s h a b l e from the f i n e r grained, f e l d s p a r r i c h m a trix. The rocks have been a l t e r e d and the m a t r i x i s as r e s i s - tant to weathering as the v o l c a n i c cobbles. Epidote occurs i n veins and patches w i t h i n both c l a s t s and matrix. Another conglomerate, the only one w i t h i n the greenstone u n i t c o n t a i n i n g c l a s t s d erived from a non-volcanic source, caps the 2700 foot high h i l l northwest of the limestone-greenstone contact. There 2-3 cm long fragments are limestone, black p e l i t i c rocks and abundant l i g h t green to white t u f f . The matrix i s quartz and f e l d s p a r r i c h and w e l l indurated. V o l c a n i c b r e c c i a s are interbedded w i t h the andesites and d a c i t e s . The 64 angular to subrounded fragments are mainly pieces of v o l c a n i c rocks quartz- eye porphyry, f e l d s p a r porphyry, s i l i c i f i e d t u f f , a p h a n i t i c greenstone >- but elongate pieces of green sandstone and greywacke are a l s o present. The b r e c c i a s are poorly sorted but have l i t t l e very f i n e m a t e r i a l . Most of the fragments are at l e a s t s e v e r a l m i l l i m e t e r s i n diameter and range up to s e v e r a l c e n t i - meters across. "In both conglomerates and b r e c c i a s c l a s t s do not weather out i n r e l i e f . F i n e r grained e p i c l a s t i c r o c k s , sandstones and greywacke, are u s u a l l y darker green than most of the v o l c a n i c r o c k s , but s i l t s t o n e s are f a i r l y l i g h t green i n c o l o r . M i c r o s c o p i c examination shows l i t h i c fragments are c l e a r l y d e r i v e d from v o l c a n i c rocks i n the N i c o l a Group. Round quartz grains are p l e n t i f u l ; there i s a l s o c o n s i d e r a b l e subrounded a l b i t e g r a i n s . C a l c i t e , e pidote, and s e r i c i t e r e p l a c e the f e l d s p a r s , and c h l o r i t e and very f i n e f e l d s p a r comprise the matrix. No metamorphic minerals l i k e p r e h n i t e , p u m p e l l y i t e , or a c t i n o l i t e are present i n any of the greenstones s t u d i e d . Contacts between the greenstone u n i t and other u n i t s are exposed i n only two p l a c e s , both w i t h the o v e r l y i n g limestone. That contact appears to be d e p o s i t i o n a l , as bedding o r i e n t a t i o n s i n the limestone are s u b p a r a l l e l to the contact. Whether t h i s i s conformable i s unknown because of l a c k of bedding i n the greenstones. Exposures of the contact are i n two prospecting p i t s . In both p i t s the rocks are sheared p a r a l l e l to the contact. C h l o r i t e forms a s c h i s t o s i t y i n the greenstone. This shearing probably represents l a t e r movement along a d e p o s i t i o n a l contact r a t h e r than a l a r g e f a u l t displacement. A l l other e x t e r n a l contacts of the greenstone u n i t are s t e e p l y d i p p i n g f a u l t s . Limestone U n i t (TRls) Two s u b s t a n t i a l bodies of massive limestone have been d i s t i n g u i s h e d as a separate map u n i t w i t h i n the N i c o l a . The l a r g e s t caps the h i l l northeast of Venables Creek. The second forms c l i f f s i n the next creek south. They are 65 mapped on the b a s i s of l i t h o l o g y and are not n e c e s s a r i l y of p r e c i s e l y the same age or s t r a t i g r a p h i c p o s i t i o n . Thickness of the limestone i s v a r i a b l e , and the northern body appears to pinch out towards the east. The limestone i s l i g h t grey weathering and dark blue to bl a c k on a f r e s h surface. Much i s massive, f i n e to medium grained, but some i s ' l i g h t and dark banded. Compositional v a r i a t i o n s i n c l u d e a range from almost pure carbonate to very sandy limestone. Scattered through much of these limestones are r e d - brown weathering chert nodules. In some places the nodules are a l i g n e d and may represent o r i g i n a l interbedded l a y e r s , 2-3 cm t h i c k . These l a y e r s are now dis r u p t e d and boudinaged and o c c a s i o n a l l y form small f o l d hooks of chert f l o a t i n g i n carbonate. Dark grey l a y e r s i n the f i n e grained, laminated v a r i e t i e s are due to concentrations of carbonaceous m a t e r i a l . As much as 15 to 20% of the banded limestones i s qu a r t z , u s u a l l y of the same g r a i n s i z e as the carbonate but o c c a s i o n a l l y i n patches which may have been l a r g e r g r a i n s now r e c r y s t a l l i z e d . A few l a r g e r quartz g r a i n s remain, but the margins of those grains are p o l y - gonalized. The limestone u n i t i n c l u d e s carbonate sands, e a s i l y i d e n t i f i e d i n t h i n s e c t i o n . D e t r i t a l limestone grains 1^3 mm across have curved or bent twins and are darker i n c o l o r than the r e c r y s t a l l i z e d cementing m a t e r i a l , A minor part of the carbonate sands i s other d e t r i t u s - quartz and f e l d s p a r g r a i n s , u s u a l l y much smaller than the carbonate fragments. B i o c l a s t i c limestones are present i n the northern limestone mass down i n Venables Creek. There limestones have been sheared and r e c r y s t a l l i z e d , and c r i n o i d s are the only i d e n t i f i a b l e f o s s i l preserved. Several samples from d i f f e r e n t s i t e s i n the u n i t were d i s s o l v e d i n an attempt to separate conodonts w i t h no success. The general l e n s o i d shape and the v a r i e t y of l i t h o l o g i e s - c a l c a r e n i t e s to b i o c l a s t i c to m i c r i t e w i t h areas of organic laminae >- suggest a probable r e e f - s h e l f , o r i g i n . 66 The lower contact of the northern limestone has already been described as a sheared d e p o s i t i o n a l contact. The b a s a l contact of the southern limestone mass i s a f a u l t against younger rocks. The t r a n s i t i o n upwards from r e l a t i v e l y pure limestone to interbedded c l a s t i c r o c k s , c h e r t , and limestone of the o v e r l y i n g mixed u n i t i s c l e a r l y exposed i n both limestone bodies. Mixed-Sedimentary and V o l c a n i c Rocks (TRs, T R s l , TRsv) The remainder of the N i c o l a Group i s lumped as one u n i t . Most of the l i t h o l o g i e s found i n the other u n i t s r- limestone, s i l i c e o u s v o l c a n i c r o c k s , v o l c a n i c l a s t i c rocks, some ande s i t e , conglomerate - are found i n t h i s u n i t . Black a r g i l l i t e i s a l s o present. T y p i c a l of much of the upper u n i t i s interbedded limestone and greenstone. The greenstone i s l a r g l y s i l i c e o u s , a p h a n i t i c , and pale green i n c o l o r , prob- ably t u f f . Beds range from 5-15 cm i n thickness and tuffaceous greenstone l a y e r s are commonly laminated. Limestone l a y e r s are u s u a l l y medium grey weathering except i n patches of skarn where they have been a l t e r e d and r e - c r y s t a l l i z e d . Nearly white, sugary textured marbles w i t h coarse g r a i n s i z e i n these zones are as s o c i a t e d w i t h c h l o r i t i z e d and/or s i l i c i f i e d , bleached greenstones. Garnet and epidote have formed from interbedded p e l i t i c rocks i n one of these skarn zones. Limestone l a y e r s , up to s e v e r a l meters t h i c k , can sometimes be traced f o r s e v e r a l hundred meters before they pinch out or are truncated by f a u l t s , Layering w i t h i n these carbonates on a 3r5 cm s c a l e i s defined by a l t e r - n a t i n g coarse sandy and f i n e grained bands. Coarse l a y e r s can be b i o c l a s t i c and may show grading. One of the best exposures of t h i s l i t h o l o g y i s a l i t t l e over one k i l o m e t e r up Venables Creek from the highway. There, bedded limestones and c a l c a r e n i t e s are interbedded w i t h a few t u f f l a y e r s . Greenstones i n the upper u n i t are muctT'the same as those already described i n the lower u n i t . Some -more extensive and b e t t e r exposed greenstone bodies are mapped separately (TRsv). Pale green ch e r t , t y p i c a l l y i n beds 5-10 cm t h i c k w i t h exceptions up to a few meters t h i c k , i s the most common. The chert i s a s i l i c e o u s t u f f , very f i n e grained w i t h no phenocrysts. Quartz, f e l d s p a r and minor amounts of c h l o r i t e and s e r i c i t e are v i s i b l e i n t h i n sec- t i o n . Greenstone conglomerates up to a meter t h i c k contain andesite, d a c i t e and tuffaceous fragments as w e l l as some g r a n i t i c d e t r i t u s . The source f o r the g r a n i t i c m a t e r i a l may be the high l e v e l plutons described i n the green- stone s e c t i o n . Generally the g r a n i t i c fragments are very quartz r i c h . A few are much l e s s a l t e r e d ; primary hornblende i s preserved. These c l a s t s could be derived from the Guichon B a t h o l i t h . A conglomerate of t h i s d e s c r i p t i o n i s exposed along the highway south of Venables Creek. North of Venables Creek, above the highway, the lower c l i f f s are pre- dominantly very f r e s h andesite. Euhedral, white f e l d s p a r phenocrysts, 1-2 mm across, s i t i n an intermediate to dark green a p h a n i t i c groundmass. Upwards, more s i l i c e o u s greenstones and a l t e r e d andesites give way to banded sediment ' tary rocks and minor limestone pods. C l a s t i c rocks other than greenstone conglomerate and t u f f i n c l u d e a r g i l l i t e , some sandstone and greywacke, and considerable amounts of conglomerate derived from non-greenstone ter r a n e . A r g i l l i t e s are b l a c k , s i l i c e o u s and w e l l bedded. Sandstone and greywacke are green, v o l c a n i c - d e r i v e d and very s i m i l a r to those i n the greenstone u n i t . Conglomerates r e f l e c t the v a r i e t y of l i t h o l o g i e s i n the N i c o l a Group. C l a s t s i n c l u d e green s i l i c e o u s t u f f , a p h a n i t i c purple v o l c a n i c fragments, a r g i l l i t e , and limestone, w i t h quartz, f e l d s p a r , and c a l c i t e grains and a r g i l l a c e o u s m a t e r i a l making up the sm a l l e r f r a c t i o n . No- where was d e t r i t u s of Cache Creek o r i g i n i d e n t i f i e d . The upper contact of these bedded rocks i s an angular unconformity w i t h a Lower J u r a s s i c b a s a l conglomerate which can be c o r r e l a t e d w i t h the Ashc r o f t Formation to the nor t h . The unconformity i s best exposed i n Venables Creek 68 about one k i l o m e t e r upstream from the highway. Small, open f o l d s i n bedded limestone are truncated by the conglomerate. Age of the N i c o l a Group Two age determinations support a Mesozoic r a t h e r than P a l e o z o i c age f o r t h i s map u n i t . A Rb/Sr date was obtained on a s u i t e of 6 samples from the greenstone u n i t . Four of these are from w i t h i n the f i e l d area and two are from the quartz d i o r i t e j u s t north of the f i e l d area. Table IV l i s t s a n a l y t i c a l data f o r the samples. They have a good spread i n Rb/Sr r a t i o s arid d e f i n e an isochron (Figure 32). The c a l c u l a t e d date i s 196 + 15 Ma w i t h an i n i t i a l r a t i o of .7043 + 2 somewhat d i f f e r e n t from values obtained from the Guichon B a t h o l i t h , 205 + 10 and ,7034 (R.L.Armstrong, pers comm. 1978). The i n i t i a l r a t i o , .7043, i s s l i g h t l y higher than the mean value f o r arcs (Faure and P o w e l l , 1972) and may i n d i c a t e some contamination very l i k e l y w i t h Sr from seawater. A 196 Ma age f o r the N i c o l a i s a b i t younger than expected f o r time of formation as the N i c o l a i s dated by f o s s i l s as Karnian to Norian. The isochron date may r e f l e c t the hydrothermal a l t e r a t i o n event, A Late T r i a s s i c age of formation and e a r l y J u r a s s i c a l t e r a t i o n i s reasonable. Only one f o s s i l c o l l e c t i o n was obtained from the N i c o l a . I t i n c l u d e s microfauna from the bedded limestones below the unconformity i n Venables Creek, In a c o l l e c t i o n of conodont fragments, B.E.B. Cameron of the G e o l o g i c a l Survey of Canada i d e n t i f i e d one specimen of Enantiognathus z i e g l e r i (Diebel) which ranges i n age from lower T r i a s s i c to Upper Karnian (Mosher, 1968) and three specimens of Neospathodus sp. c f . N_. newpassensis Mosher found i n Lower Karnian, p o s s i b l y ranging down to lower Middle T r i a s s i c (Mosher, 1968). Cameron assigns the assemblage a probable Upper T r i a s s i c (Karnian) age. D e p o s i t i o n a l Environment of the N i c o l a Group Submarine v o l c a n i c r o c k s , abundant v o l c a n i c - d e r i v e d c l a s t i c m a t e r i a l and TABLE IV Rb/Sr Geochronology Data Sample # L i t h o l o g y L o c a l i t y L a t i t u d e Longitude ppm Sr ppm Rb Rb/Sr R b ^ r 8 6 Sr?>86 / S r 6-5-4 Dacite V 50°32'37" 121°19'5" 68.0 33.0 0.485 1.404 .7079 6-5-2 A l t e r e d d a c t i e VI 50°32'27" 121°19'1" 91.0 24.8 0.273 0.790 .7062 11-4-1 Quartz d i o r i t e V I I 50 O33'7 M 121°17'48" 82.1 13.5 0.164 0.476 .7063 6-13-4 A l t e r e d d a c i t e V I I I 50°32'39" 121°17'50n 23.2 58.0 2.49 7.22 .7244 4-141 I n c l u s i o n i n 4-14R o f f map 50°40'40,r: 121°22'50" 126 7.9 0.062 0.181 .7047 4-14R Quartz d i o r i t e o f f map 50°40'40" 121°22*50" 170 12.7 0.075 •0.216 .7046 Analytical Techniques ; Rb and Sr concentrations were determined by replicate analysis of pressed powder pellets using X-ray flour'- escence. U.S. Geological Survey rock standards were used for calibration; mass absorption coefficients were obtained from Mo Ka Compton scattering measurements, Rb/Sr ratios have a precision of 2% (1 a) and concen^- trations a precision of 5% ( l a ) , Sr isotopic composition was measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer (60° sector, 30 cm radius, solid source) i s of U.S. National Bureau of Standards design, modified by H, Faul. Data aquisition i s digitized and automated using a 86 88 NOVA computer. Experimental data have been normalized to a Sr/ Sr ratio of 0.1194 and adjusted so that the 87 86 NBS standard SrCO (SRM987) gives a Sr/ Sr ratio of ,71022*2 and the Eimer and Amend Sr a ratio of 0.70800 87 86 ±2. The precision of a single Sr/ Sr ratio i s 0.00013 (1 a), Rb/Sr dates are based on a Rb decay constant of 1,42 x 10 ̂ y l. The regressions are calculated according to the technique of York (1967), 70 71 carbonate s h e l f to reef f a c i e s rocks of the N i c o l a Group are t y p i c a l of a v o l c a n i c i s l a n d a r c . Rapid f a c i e s changes and r e s t r i c t e d l a t e r a l extenti.df. most l i t h o l o g i e s probably r e f l e c t s m a l l basins and r a p i d , i r r e g u l a r periods of u p l i f t . No subareal v o l c a n i c rocks are present w i t h i n the area,, but c l a s t s i n conglomerates r e f l e c t subareal volcanism nearby. Rounded boulders i n some conglomerates suggest some d e t r i t a l m a t e r i a l has had f l u v i a l t r a n s p o r t . I n f l u x e s of d e t r i t a l m a t e r i a l , i n c l u d i n g tuffaceous l a y e r s , i n t e r r u p t e d carbonate sedimentation. T u r b i d i t y currents redeposited c l a s t i c sediments and r e e f - d e r i v e d fragments. S t r u c t u r e F a u l t i n g i s the dominant deformation of the N i c o l a Group. Major f a u l t s trend north-northwest and are v e r t i c a l . Most of these appear to b,e block f a u l t s w i t h normal o f f s e t . The other major f a u l t i s the M a r t e l F a u l t which forms the boundary between Cache Creek and N i c o l a s t r a t a . Near t h i s f a u l t and s u b p a r a l l e l to i t i s a f r a c t u r e cleavage to s c h i s t o s i t y i n the N i c o l a green- stone. F o l i a t i o n i s weakly developed or absent outside f a u l t zones. Figure 33 presents s t r u c t u r a l Elements measured i n the N i c o l a . Bedding a t t i t u d e s are northwest t r e n d i n g and moderately southwest d i p p i n g . Small f o l d s , u s u a l l y drag f o l d s along minor f a u l t s or d u c t i l e f o l d s i n limestone, have a f a i r l y c o n s i s t e n t northwest-southeast trend w i t h moderate to shallow plunges. No a x i a l plane f o l i a t i o n i s developed. This southwest t i l t i n g of the N i c o l a s e c t i o n could be a r e s u l t of i n t r u s i o n of the Guichon B a t h o l i t h . To the east across the Thompson R i v e r , l a r g e f l a t i r o n s of N i c o l a dip to the west away from the Guichon contact. However.^ the Lower J u r a s s i c A s h c r o f t Formation i s a l s o t i l t e d to the southwest, so t h i s t i l t i n g may be post lower J u r a s s i c . On the f l a t i r o n to the east, west vergent f o l d s are developed i n s e v e r a l - meter-.thick bedded carbonate l a y e r s ; contacts w i t h over and u n d e r l y i n g rocks are not f o l d e d . Apparently the carbonate behaved i n a d u c t i l e f a s h i o n under 72 Bedding Foliation — limestone and clastic units Foliation — greenstone unit Fold axes Figure 33. Equal area stereonet p l o t of s t r u c t u r a l elements i n the N i c o l a Group. 73 shear s t r e s s which has allowed the o v e r l y i n g s e c t i o n to move downslope r e l a t i v e to the rocks underlying the carbonate. Movement was taken up i n the limestone. Though not c l e a r l y demonstrated i n the present area, f o l d s i n the limestones are thought to be of s i m i l a r o r i g i n . A l t e r a t i o n Secondary minerals w i t h i n the greenstones i n c l u d e epidote, a l b i t e , c a l c i t e , K - f e l d s p a r , and c h l o r i t e . Some areas are s t r o n g l y s i l i c i f i e d , bleached and u s u a l l y p y r i t i z e d . At one l o c a l i t y w i t h i n the upper sedimentary u n i t a garnet- epidote skarn i s developed. These are a l l hydrothermal a l t e r a t i o n s probably r e l a t e d to f l u i d movement during i n t r u s i o n of the Guichon B a t h o l i t h . Such a l t e r a t i o n i s s i m i l a r to the f e l d s p a t h i z a t i o n zone i n the Wairaki geothermal f i e l d of New Zealand (Miyashiro, 1973). I t i s c h a r a c t e r i z e d by a l b i t i z a t i o n and replacement of p l a g i o c l a s e by K - f e l d s p a r , l o c a l c a l c i t e a l t e r a t i o n and abundant p r e c i p i t a t i o n of s i l i c a . This occurs at temperatures of 230-250° C. Small skarns are a l s o developed where dikes crosscut limestone l a y e r s . Epidote and more coarsely c r y s t a l l i n e c a l c i t e replace the f i n e grained grey limestone. J u r a s s i c and Younger Rocks Ashcroft Formation Rocks of J u r a s s i c age are exposed i n two sm a l l areas, the l a r g e r along Venables Creek and the smaller patch s l i g h t l y northeast (Figures 5 and 6). The Venables Creek exposure d i s p l a y s a more complete s e c t i o n . A b a s a l conglomerate c o n t a i n i n g abundant g r a n i t i c cobbles unconformably o v e r l i e s f o l d ed N i c o l a sedimentary rocks. The g r a n i t i c c l a s t s , probably of Guichon provenance, are accompanied by N i c o l a d e t r i t u s , p r i n c i p a l l y greenstone fragments. There i s no Cache Creek d e t r i t u s . C l a s t s i n the conglomerate are rounded to subrounded and range i n s i z e from 15 cm boulders to f i n e sand. The matrix and some sand 74 grains are calcareous. This conglomerate resembles some of the N i c o l a conglom- erates but i s d i s t i n g u i s h e d by the abundant g r a n i t i c m a t e r i a l and l a c k of a l t e r a t i o n of matrix. C l a s t s weather out i n r e l i e f as opposed to c l a s t s w i t h i n N i c o l a conglomerates. Conglomerate grades r a p i d l y upward i n t o c a l c a r e n i t e and sandy, f o s s i l i f e r - ous limestone, These are interbedded w i t h and grade i n t o b lack s h a l e , common i n the Ashcroft Basin (Frebold and Tipper, 1969;.Travers, 1978). The e n t i r e exposed s e c t i o n i s 200 meters t h i c k . F o s s i l c o l l e c t i o n s from the sandy limestones and c a l c a r e n i t e s were i d e n t i - f i e d by T.P, Poulton of the G e o l o g i c a l Survey of Canada. They i n c l u d e b i v a l v e s Weyla (?) sp. i n d e t , . Camonectes (?) sp. i n d e t . , and Pleuromya (?) sp, i n d e t , plus T e r e b r a t u l i d brachiopods, i n d e t , and a s o l i t a r y c o r a l , i n d e t , Poulton suggests a probable Lower J u r a s s i c age, The J u r a s s i c rocks are preserved i n two down-dropped f a u l t b l o c k s . In both cases the western contact i s a f a u l t which juxtaposes J u r a s s i c s t r a t a against massive N i c o l a limestone. The s t r a t a d ip southwest towards the f a u l t s . A f r a c t u r e cleavage i s developed i n some of the sandstones. I t i s s u b p a r a l l e l to the f a u l t on the west s i d e of the s e c t i o n and probably developed at the time of f a u l t i n g . Spences Bridge Group The lower Cretaceous Spences Bridge Group i s i n f a u l t contact w i t h the Cache Creek Group along the western edge of the f i e l d area. I t i s very poorly exposed, but three outcrops of z e o l i t i z e d , p o r p h y r i t i c andesite support the l o c a t i o n of the contact drawn by D u f f e l l and McTaggart (1952) and Pearson (1974), The group i s made up of an d e s i t e , d a c i t e and some r h y o l i t e which have undergone z e o l i t e a l t e r a t i o n and gentle warping ( D u f f e l l and McTaggart, 1952). 75 Quaternary Deposits Quaternary rocks of s e v e r a l o r i g i n s cover much of the f i e l d area. Terraced g l a c i a l f l u v i a l deposits f i l l the lower Thompson River v a l l e y . Quaternary i s shown on the map (Figure 6) only i n areas l a c k i n g outcrop or mappable f l o a t of o l d e r rocks. DISCUSSION Cache Creek Group, N i c o l a Group and Contact R e l a t i o n s h i p s In t h i s area the Cache Creek Group appears to be a melange. I t i s a t e c t o n i c u n i t , r a t h e r than a s t r a t i g r a p h i c u n i t , everywhere i n f a u l t contact w i t h lower Mesozoic rocks. The group d i s p l a y s a sheared p e l i t i c m a t r i x c o n t a i n i n g blocks of v a r y i n g s i z e , l i t h o l o g y , age, and metamorphic grade. Blocks found w i t h i n the the f i e l d area range i n length from a few centimeters to greater than 5 k i l o m e t e r s . L i t h o l o g i e s represented as blocks i n c l u d e shallow water limestone, augite porphyry flows and tuffaceous r o c k s , ribbon ch e r t , s e r p e n t i n i t e and c l a s t i c sedimentary rocks. W i t h i n these b l o c k s , p r i m a r i l y d i s c e r n a b l e i n greenstones and greywackes, metamorphic c o n d i t i o n s may vary from about 250° C, 4 kb to c o n d i t i o n s t r a n s i t i o n a l to b l u e s c h i s t f a c i e s , T = 350° C, P = 5.5-6 kb. The r e l a t i v e importance of sedimentary vs_. t e c t o n i c processes i s not c l e a r . There i s abundant evidence of slumping and d e b r i s flows i n the c l a s t i c rocks as coarser m a t e r i a l was brought i n t o p r o x i m i t y w i t h p e l a g i c sediments. This mechanism has been invoked to e x p l a i n the presence of shallow water, r e e f o i - d a l limestone masses amongst deeper water cherts (Monger, 1977b). I f , as has been proposed, the r e e f s are developed on a v o l c a n i c p i l e , b a s a l t blocks could a l s o be l a r g e l y slumped m a t e r i a l . More d i f f i c u l t to e x p l a i n are the d i f f e r e n c e s i n metamorphism w i t h i n rocks of the same composition, e s p e c i a l l y the presence of t r a n s i t i o n a l b l u e s c h i s t grade. There i s some evidence of a low grade metamorphic source terrane since d e t r i t a l p r e h n i t e and pumpellyite are found. Cache Creek Group rocks are probably the source rocks i n - t h i s case. More h i g h l y metamorphosed rocks are l i k e l y t e c t o n i c a l l y emplaced. A s i m i l a r mechanism i s needed f o r s e r p e n t i n i t e and other u l t r a m a f i c bodies. The l a r g e u l t r a m a f i c mass that forms the contact between the two c l a s t i c u n i t s i s q u i t e c l e a r l y emplaced along a f a u l t , but smaller masses cannot be t i e d to known f a u l t s . S t r u c t u r e s i n the Cache Creek Group, on.the other hand, are not c h a o t i c as they are i n a t y p i c a l melange. Over most of the area f o l i a t i o n maintains a f a i r l y constant o r i e n t a t i o n , though i t weakens upwards and to the west. The pods-in-matrix s t y l e does p e r s i s t throughout. In c o n t r a s t , the N i c o l a Group, which has p r e v i o u s l y been mistakenly i d e n t i f i e d as Cache Creek Group i n t h i s area, does not have the same tectono- s t r a t i g r a p h i c s t y l e . There are r a p i d l a t e r a l changes i n l i t h o l o g y , and because of poor outcrop, some l i t h o l o g i e s look l i k e i s o l a t e d b l o c k s ; but there i s no sheared matrix. No r e g i o n a l f o l i a t i o n i s developed; deformation i s l o c a l i z e d along f a u l t zones. Shear f o l i a t i o n and s c h i s t o s i t y p a r a l l e l f a u l t s and are best developed i n rocks near the M a r t e l F a u l t . Hydrothermal a l t e r a t i o n i s a l s o l o c a l l y v a r i a b l e , but r e f l e c t s d i f f e r e n c e s i n f l u i d move- ment r a t h e r than the wide P-T ranges found i n the Cache Creek Group. At outcrop s c a l e c l a s t i c rocks i n the N i c o l a Group are e a s i l y confused w i t h weakly deformed Cache Creek rocks of the same l i t h o l o g y . However, i f a l a r g e enough area i s mapped, the c o n t r a s t i n g t e c t o n i c s t y l e of these rocks should c l e a r l y separate the two groups. Other means f o r d i f f e r e n t i a t i o n i n c l u d e f o s s i l i n f o r m a t i o n , l i m i t e d though i t i s , the absence of u l t r a m a f i c masses i n the N i c o l a Group, and i s o t o p i c data. More such data i s needed f o r the v o l c a n i c rocks i n the Cache Creek Group f o r comparison. These d i s t i n c t i o n s between N i c o l a and Cache Creek Group rocks do not n e c e s s a r i l y h o l d f o r other areas of the Cache Creek. I n the P i n c h i - S t u a r t Lake Area, Paterson (1977b) f i n d s Late Karnian to E a r l y Norian Takla sediments c o n t a i n i n g metamorphic aragonite and speculates that a u g i t e porphyry v o l c a n i c rocks i n v o l v e d i n b l u e s c h i s t metamorphisn could be c o r r e l a t e d w i t h the abun- dant augit e porphyry of the Takla Group. S i m i l a r v o l c a n i c rocks w i t h l a r g e augite phenocrysts are common i n the N i c o l a Group o u t s i d e the present study area (Schau, 1968, 1970; Campbell and Tipper, 1971; P r e t o , 1974). At the same time most of the v o l c a n i c and v o l c a n i c l a s t i c rocks of the Cache Creek Group i n . t h i s area are a u g i t e porphyry. In f a c t the higher pressure meta- morphic assemblage i s developed i n a block of augire porphyry flows and t u f f s . There are no data that r e q u i r e the age of these to be P a l e o z o i c . They could be T r i a s s i c and p a r t of the N i c o l a Group as w e l l as i n v o l v e d w i t h Cache Creek deformation and metamorphism. Also only 30 k i l o m e t e r s to the n o r t h Travers (1978) f i n d s T r i a s s i c ( L a d i n i a n or Karnian) r a d i o l a r i a i n Cache Creek c h e r t s . As p r e s e n t l y mapped the two groups are separated by a f a u l t , the M a r t e l F a u l t . I t i s now v e r t i c a l to s t e e p l y west d i p p i n g , appears to truncate l i t h o l o g i e s i n the N i c o l a Group, but i n some areas s u b p a r a l l e l s f o l i a t i o n i n the Cache Creek Group. In other l o c a l i t i e s , notably the south end of the f i e l d area, t h i s f a u l t appears to cut the f o l i a t i o n of Cache Creek rocks. I t s present steep dip could be a r e s u l t of r e g i o n a l t i l t i n g i n post lower J u r a s s i c time s i n c e the e n t i r e N i c o l a and A s h c r o f t s e c t i o n i s t i l t e d to the southwest, s u b p a r a l l e l to the average dip i n the Cache Creek. I f t h i s t i l t - i n g i s removed and the N i c o l a and A s h c r o f t s t r a t a are returned to h o r i z o n t a l , the M a r t e l F a u l t becomes a t h r u s t f a u l t p l a c i n g f o l i a t e d Cache Creek over N i c o l a and A s h c r o f t beds (as suggested by Ladd, i n p r e s s ) . On a l a r g e r s c a l e , the M a r t e l F a u l t j u x t a p o s i n g Cache Creek i n the west against N i c o l a i s very s i m i l a r to the P i n c h i F a u l t which forms the eastern boundary of the Cache Creek Group i n the P i n c h i - S t u a r t Lake Area (Paterson, 1973, 1977b). Paterson (1977b) suggests l a r g e s c a l e r i g h t l a t e r a l t r a n s - 78 current movement i n Late T r i a s s i c to E a r l y J u r a s s i c time on the P i n c h i F a u l t . I t has been suggested (P.B. Read, pers. comm., 1975) that the M a r t e l F a u l t i s the southern extension of the P i n c h i F a u l t System. In that case one might propose s t r i k e - s l i p movement on the M a r t e l F a u l t i n e a r l y Mesozoic time using Paterson';s i d e a . However, there i s no concrete evidence i n the Venables V a l l e y area to support the presence of the M a r t e l F a u l t i n the e a r l y Mesozoic. Tectonic S e t t i n g and H i s t o r y L i t t l e hard data are a v a i l a b l e from t h i s study on the timing of events i n the type area of the Cache Creek Group. There i s a l a c k of s t r a t i g r a p h i c caontacts and f o s s i l c o n t r o l . An approximate time-space p l o t can be drawn (Figure 34) by i n c o r p o r a t i n g data from Paterson and Harakal (1974) f o r the time of b l u e s c h i s t metamorphism, and data from Travers (1978) f o r extension of Cache Creek d e p o s i t i o n up i n t o the T r i a s s i c . Timing of f a u l t i n g events cannot be w e l l documented but may be f a i r l y continuous from Late' T r i a s s i c to T e r t i a r y . From M i s s i s s i p p i a n time to as l a t e as the Upper T r i a s s i c the Cache Creek oceanic assemblage was forming, presumably on oceanic c r u s t (Monger, 1972, 1975a, 1977b) and during the P a l e o z o i c i t l a y f a r from North America. I n Upper T r i a s s i c (Karnian) time the N i c o l a arc became a c t i v e , b u i l d i n g up on Upper P a l e o z o i c rocks which are thought to represent an e a r l i e r arc (Danner, 1977). At the same time Cache Creek Group rocks were probably being deformed and moved toward the craton. This produced the i s o c l i n a l f o l d s and pervasive f o l i a t i o n . Thrust f a u l t i n g and t r a n s i t i o n a l b l u e s c h i s t metamorphisn accom- panied t h i s deformation. Melange development and deformation probably was i n i t i a t e d during t h i s Late T r i a s s i c p e r i o d . In l a t e s t T r i a s s i c or e a r l y J u r a s s i c time the Guichon B a t h o l i t h intruded and p o s s i b l y t i l t e d N i c o l a v o l c a n i c s w h i l e c o n t r i b u t i n g to hydrothermal a l t e r a t i o n of the N i c o l a 79 Figure 34, Timing of events in the type area of the Cache Creek. Broken lines indicate possible continuation of events. Data for Fraser Fault System from Eisbacher (1977) and for the Yalakom Fault from Tipper and Richards (1976). Radiometric age for the Guichon Batholith from R.L. Armstrong (pers. comm., 1978); blueschist metamorphic date from Paterson and Harakal (1974); alteration date of Nicola from present study. 80 Group. The f o l i a t i o n i n N i c o l a rocks, best developed along the M a r t e l F a u l t , i s apparently the same age as hydrothermal a l t e r a t i o n . Cache Creek rocks do not appear to have been a f f e c t e d by the hydrothermal event. One would expect Cache Creek rocks to be s i m i l a r l y a l t e r e d i f they were adjacent to the N i c o l a at the time the b a t h o l i t h was emplaced. Fol l o w i n g i n t r u s i o n of the Guichon, the next event was the beginning of d e p o s i t i o n of the Ashc r o f t Formation i n Lower J u r a s s i c time, l i k e l y Sine- murian (H.W. Tipper, pers. comm., 1975). That a l s o marks the time of expo- sure of the Guichon B a t h o l i t h and l o c a l l y , ..the end of volcanism. Farther north and near the N i c o l a type area, N i c o l a volcanism continued i n t o the J u r a s s i c (Campbell and Tipper, 1971; Tipper and Richards, 1976; P r e t o , 1974). Timing of events f o l l o w i n g d e p o s i t i o n of the lowermost Ashcroft s t r a t a i s not e a s i l y determined from the present study. There was abundant post Lower J u r a s s i c f a u l t i n g and f u r t h e r post lower Cretaceous f a u l t i n g . L a t e r f o l d s i n Cache Creek rocks, b r i t t l e f o l d s that f o l d the e a r l i e r f o l i a t i o n , are probably r e l a t e d to l a t e r f a u l t : movement. Volcanism was renewed i n e a r l y Cretaceous, A p t i a n time ( D u f f e l l and McTaggart, 1952), when Spences Bridge Group and e s i t e , d a c i t e , and r h y o l i t e were deposited umconformably on o l d e r rocks. Some of the f a u l t i n g i n t h i s area may be r e l a t e d to movement along the Fraser and Yalakom F a u l t s . Right l a t e r a l movement of s e v e r a l hundred k i l o - meters has been proposed f o r both (Tipper, 1977; Tipper and Richards, 1976; Eisbacher, 1977). In conventional p l a t e t e c t o n i c models, t e c t o n i c melange, which i n c l u d e s t h r u s t sheets and shows areas of high-pressure low temperature metamorphic c o n d i t i o n s , i s i n t e r p r e t e d as developing i n subduction zones (Dickinson, 1972). Highest pressure c o n d i t i o n s are g e n e r a l l y found on the arc s i d e of the melange wedge. In the present study highest pressure assemblages are found i n the s t r u c t u r a l l y lowest p a r t of the Cache Creek which now s i t s on the 81 eastern s i d e of the Cache Creek Group. This may be because the s e c t i o n has been t i l t e d and might not be r e l a t e d to the o r i g i n a l c o n f i g u r a t i o n . Thus i f the Cache Creek melange i s r e l a t e d to subduction, f a c i n g of the concurrent arc i s not c l e a r . Tectonic models that i n c l u d e Cache Creek rocks of the type area suggest eastward subduction of an oceanic assemblage underneath the N i c o l a v o l c a n i c arc.(Monger and o t h e r s , 1972; Travers, 1978). As i n the P i n c h i - S t u a r t Lake Area (Paterson, 1977b), there are s p a t i a l problems w i t h t h i s model. Cache Creek melange i s now adjacent to the are complex and as c l o s e as 3 k i l o m e t e r s to the Guichon B a t h o l i t h , the root of the N i c o l a arc. This i s much c l o s e r than standard models of an a r c - t r e n c h system. There i s c o n s i d e r a b l e specu- l a t i o n as to whether the a l k a l i n e N i c o l a v o l c a n i c rocks represent a true ar c . A l k a l i n e chemical trends are thought to imply a r i f t i n g o r i g i n (Preto and o t h e r s , 1975; H o l l i s t e r and o t h e r s , 1975). Yet a l k a l i n e rocks can develop i n arcs where there i s a very low convergence r a t e , l e s s than 2 cm/yr (Miyashiro, 1973), so these chemical trends do not preclude an arc i n t e r - p r e t a t i o n f o r the N i c o l a v o l c a n i c s . The space problem noted above can be solved by a l l o w i n g the M a r t e l t h r u s t f a u l t to have considerable d i s p l a c e - ment . SUMMARY AND CONCLUSIONS In the Venables V a l l e y area, the southernmost exposure of type Cache Creek Group rocks, the Cache Creek i s thought to be a t e c t o n i c melange. E x o t i c , i s o l a t e d blocks s i t i n a deformed c h e r t - a r g i l l i t e matrix. Some b l o c k s , e s p e c i a l l y limestone and p o s s i b l y greenstone, are thought to be emplaced as s l i d e b l o c k s , as there i s abundant evidence of debris flows and t u r b i d i t y current deposits i n f i n e r grained c l a s t i c sedimentary rocks. Other e x o t i c b l o c k s , notably s e r p e n t i n i t e and v o l c a n i c rocks showing high pressure meta- morphic c o n d i t i o n s might be t e c t o n i c a l l y emplaced; one l a r g e mass of green- stone i s c l e a r l y f a u l t bounded. Metamorphic c o n d i t i o n s , though poorly documented, may range from temperatures below 250° C w i t h pressures probably 4 kb or l e s s , to pressures approaching 6 kb and temperatures near 350° C. Rocks of most severe deformation and highest metamorphic grade are found i n . t h e eastern," s t r u c t u r a l l y lower p a r t of the Cache Creek Group. Only one f o s s i l age, Late Permian, was obtained f o r a Cache Creek limestone block. N i c o l a rocks can be d i s t i n g u i s h e d from Cache Creek by t h e i r l a c k of a pervasive f o l i a t i o n , by hydrothermal, a l b i t e epidote a l t e r a t i o n r a t h e r than dynamothermal metamorphism, and by the l a c k of a sheared p e l i t i c m atrix and e x o t i c b l o c k s . Rapid f a c i e s changes occur, and some l i t h o l o g i e s have l i t t l e l a t e r a l extent, but there i s no i n d i c a t i o n of i s o l a t e d b l o c k s , and there are no u l t r a m a f i c rocks i n the N i c o l a Group. V o l c a n i c rocks i n c l u d e andesite and d a c i t e r a t h e r than b a s a l t s seen i n the Cache Creek. A Rb/Sr i s o t o p i c date of 196 + 15 Ma f o r N i c o l a greenstones i s l i k e l y the time of hydrothermal a l t e r a t i o n r e l a t e d to i n t r u s i o n of the Guichon B a t h o l i t h . Today the contact between the two groups i s a v e r t i c a l to s t e e p l y west dipp i n g f a u l t , named the M a r t e l F a u l t . , The: e n t i r e Cache Creek through Lower J u r a s s i c s e c t i o n i s t i l t e d to the southwest and i f t h i s t i l t i n g i s removed the f a u l t becomes a t h r u s t . Movement i s post Lower J u r a s s i c . B r i t t l e deformation i n the Cache Creek i s probably r e l a t e d i n part to move- ment along t h i s f a u l t . Tectonic models i n which the Cache Creek i s interpreted as a subduction complex rel a t e d to the Nicola arc are too s i m p l i f i e d . Concurrent and l a t e r transcurrent f a u l t i n g and thrust f a u l t i n g have complicated the o r i g i n a l s p a t i a l arrangement of the Cache Creek and Nicola Groups. Before the e n t i r e tectonic puzzle can be solved a d d i t i o n a l quantitative data must be obtained for the Cache Creek Group. 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