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Cache Creek and Nicola Groups near Ashcroft, British Columbia Grette, Joan Frances 1978

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CACHE CREEK AND NICOLA GROUPS NEAR ASHCROFT, BRITISH COLUMBIA  by  JOAN FRANCES GRETTE B.A. , M i d d l e b u r y C o l l e g e , 1974  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department o f G e o l o g i c a l S c i e n c e s ) We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d  THE UNIVERSITY OF BRITISH COLUMBIA November, 1978 (c)  Joan Frances G r e t t e , 1978  In 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  fulfilment  an a d v a n c e d d e g r e e a t  of  the L i b r a r y I further for  shall  the U n i v e r s i t y  make i t  agree that  by h i s r e p r e s e n t a t i v e s . of  extensive  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  this  written  thesis for  It  Department of  f i n a n c i a l gain shall  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 , Canada V6T 1W5  Date  APJB/L 2 9 . '9*1  I agree  r e f e r e n c e and copying of  this  that  not  copying or  for  that  study. thesis  by t h e Head o f my D e p a r t m e n t  i s understood  permission.  the requirements  B r i t i s h Columbia,  freely available for  permission for  of  or  publication  be a l l o w e d w i t h o u t  my  ii  ABSTRACT D e t a i l e d mapping near t h e type a r e a o f t h e Cache Creek Group i n s o u t h e r n B r i t i s h Columbia has l e d t o s i g n i f i c a n t changes i n t h e d i s t r i b u t i o n o f Cache Creek and N i c o l a r o c k s .  Much o f what was c a l l e d Cache Creek Group i s now  c o n s i d e r e d t o be p a r t o f t h e Upper T r i a s s i c N i c o l a Group. can be used t o d i s t i n g u i s h t h e two groups.  Several  criteria  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 history. The Cache Creek Group i s s u b d i v i d e d i n t o  t h r e e mappable, f a u l t - b o u n d e d  u n i t s and appears t o be a t e c t o n i c melange o v e r much o f i t s e x t e n t .  A  d e f o r m a t i o n a l 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 p r e s s u r e - t e m perature conditions.  M i n e r a l assemblages s u p p o r t c o n d i t i o n s from temperatures  l e s s than 250°C and p r e s s u r e s o f 4 kb o r l e s s t o t r a n s i t i o n a l b l u e s c h i s t cond i t i o n s : T = 350° C and P = 6 kb. I n c o n t r a s t , t h e N i c o l a Group i s c h a r a c t e r i z e d by h y d r o t h e r m a l a l t e r a t i o n and the l a c k o f a p e r v a s i v e secondary f a b r i c .  I t does n o t have t h e b l o c k s i n  a sheared m a t r i x 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 o f t h e Cache Creek Group. The two u n i t s were brought t o g e t h e r a l o n g t h e M a r t e l F a u l t , p r o b a b l y a t h r u s t , d u r i n g l a t e Lower o r e a r l y M i d d l e J u r a s s i c t i m e .  D e f o r m a t i o n and  melange development i n :the Cache Creek Group p r e d a t e s t h i s event.  Distribu-  t i o n o f Cache Creek and N i c o l a r o c k s and t h e i r r e l a t i o n s h i p t o each o t h e r d u r i n g Upper T r i a s s i c t i m e a r e s t i l l n o t c l e a r .  TABLE OF CONTENTS Page ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS  11  iii v vi viii  INTRODUCTION Purpose F i e l d Area P r e v i o u s Work REGIONAL GEOLOGY G e n e r a l - N o r t h 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 A r e a Type A r e a N i c o l a Group Ashcroft Formation GEOLOGY OF THE STUDY AREA G e n e r a l D i s t r i b u t i o n o f Rock U n i t s 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 o f t h e Cache Creek Group Sedimentary s t r u c t u r e s and environment o f d e p o s i t i o n Structure Metamorphism N i c o l a Group Greenstone u n i t Limestone u n i t Mixed s e d i m e n t a r y and v o l c a n i c r o c k s Age o f t h e N i c o l a Group Structure Alteration J u r a s s i c and Younger Rocks Ashcroft Formation Spences B r i d g e Group Quaternary d e p o s i t s  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  T a b l e I I . B l u e Amphibole Probe A n a l y s e s  38  Table I I I . Metamorphic M i n e r a l Assemblages i n the Cache Creek  55  Table IV.  69  Rb/Sr Geochronology Data  vi  LIST OF FIGURES Figure  - D „ „ „ Page  6  Frontispiece.  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 a c c e s s o f f i e l d a r e a .  2  2.  G e o l o g i c and p h y s i o g r a p h i c Cordillera.  5  3.  b e l t s o f t h e Canadian  Areas o f exposure o f 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 o f the s o u t h e r n Intermontane B e l t .  11  5.  Generalized  16  6.  Geology o f t h e Venables V a l l e y a r e a .  geology o f t h e Venables V a l l e y a r e a .  A^pp-eke-feQ 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'.  in-peKsk-et  9.  Diagrammatic s t r a t i g r a p h i c s e c t i o n o f ;the Cache Creek Group. 17  10. 11. 12.  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 o f t h e Cache Creek Group. C h a o t i c f o l d i n g i n Cache Creek p h y l l i t e . F i e l d s k e t c h e s showing f o l d i n g and s h e a r i n g o f p h y l l i t i c f o l i a t i o n i n t h e Cache Creek Group.  20 21 23  13.  P h o t o m i c r o g r a p h o f undeformed p l a g i o c l a s e - r i c h greywacke.  24  14.  P h o t o m i c r o g r a p h o f 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.  Protomylonitized  30  18.  Brecciated  19.  M y l o n i t i z e d Cache Creek c h e r t .  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 p r o b a b l e sponge s p i c u l e s i n Cache Creek c h e r t .  34  B l u e amphiboles i n a u g i t e p o r p h y r y v o l c a n i c r o c k s .  37  21.  Cache Creek greywacke.  Cache Creek c h e r t .  '  32  3  vii Figure  Page  22.  S o d i c amphiboles p l o t t e d  23.  Cataclastic  24.  Oolites  25.  Soft  on c o m p o s i t i o n diagrams.  39  t e x t u r e s i n a l t e r e d a u g i t e porphyry.  43  and f u s i l i n i d s i n Cache Creek l i m e s t o n e .  45  sediment f o l d s i n f i n e g r a i n e d sediments o f 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 o f the upper c l a s t i c u n i t .  47  27.  Slide breccia  i n the upper c l a s t i c u n i t .  48  28.  S t e r e o n e t p l o t of s t r u c t u r a l elements i n Cache Creek.  51  29.  P-T  56  30.  E n l a r g e d P-T  e q u i l i b r i a with reactions  31.  Diagrammatic  stratigraphic  32.  Rb/Sr i s o c h r o n f o r greenstones of the N i c o l a  33.  S t e r e o n e t p l o t o f s t r u c t u r a l elements i n N i c o l a  34.  Timing of events i n the type a r e a of the Cache Creek.  e q u i l i b r i a a t very low grade.  section  from Brown.  o f the N i c o l a  Group.  Group. Group.  58 60. 70„ 72 79  viii  ACKNOWLEDGEMENTS F i r s t I would l i k e t o thank P e t e r B. Read who h e l p e d 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 p e t r o l o g y . thank y o u t o Karen Massey who s u f f e r e d t h e 42° C temperatures V a l l e y as my f i e l d a s s i s t a n t . Rb/Sr geochronology.  Also,  o f Venables  K r i s t a Scott provided technical expertise f o r  Lee P i g a g e , b e s i d e s l e a d i n g me by the hand on t h e  m i c r o p r o b e , was always happy t o answer any q u e s t i o n s he c o u l d h e l p w i t h . V a l u a b l e d i s c u s s i o n s were p r o v i d e d by W.R. Danner, B i l l T r a v e r s , J i m Monger, B i l l M c M i l l a n , Randy P a r r i s h and I a n Duncan.  Bruce Cameron h e l p e d me through  m i c r o f o s s i l traumas, and t h e 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 s u p e r v i s e d t h e t h e s i s , p r o v i d e d f i n a n c i a l  and c r i t i c a l l y read t h e m a n u s c r i p t .  support,  S p e c i a l a p p r e c i a t i o n t o Stack and P a t  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 o f t h e Upper P a l e o z o i c Cache Creek Group and Lower M e s o z o i c 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 o f t h e t e c t o n i c h i s t o r y o f the C a n a d i a n C o r d i l l e r a (Monger and o t h e r s , 1972).  Yet, especially within  t h e type a r e a , near Cache Creek, B r i t i s h C o l u m b i a , l i t t l e d e t a i l e d work has been p u b l i s h e d .  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 r o c k s from the N i c o l a Group as l i t h o l o g i e s a r e q u i t e s i m i l a r .  The  c o n t a c t r e l a t i o n s between t h e two have not been d e s c r i b e d . The g o a l s o f t h i s study a r e : 1) t o d e s c r i b e t h e Cache Creek Group n e a r i t s type l o c a l i t y i n terms o f 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) t o d e t e r m i n e what c r i t e r i a can be used t o d i s t i n g u i s h t h e Cache Creek and N i c o l a Groups; 3) t o s t u d y the c o n t a c t between the two groups; and 4) t o p r o v i d e 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 o f t h e L a t e P a l e o z o i c and E a r l y M e s o z o i c h i s t o r y o f the Intermontane B e l t .  F i e l d Area The southernmost exposure o f 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 a r e a l i e s west o f t h e Thompson R i v e r midway between the towns  of A s h c r o f t and Spences B r i d g e , a p p r o x i m a t e l y 50°30' N, 121°20' W ( F i g u r e 1 ) . A p p r o x i m a t e l y 30 square m i l e s were mapped d u r i n g June, J u l y , and August of 1975. Access t o t h e a r e a i s easy.  I t i s s i t u a t e d a l o n g and west o f t h e T r a n s -  Canada Highway and i s c r o s s e d by s e v e r a l r a n c h i n g and l o g g i n g r o a d s .  Quality  and q u a n t i t y o f o u t c r o p s 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-  s t o n e , c h e r t , and g r e e n s t o n e a r e r e s i s t a n t t o w e a t h e r i n g i n t h e d r y c l i m a t e and tend t o be b e s t exposed w h i l e p h y l l i t e , o t h e r c l a s t i c sedimentary r o c k s , and s e r p e n t i n i t e a r e p o o r l y 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, life.  c a c t u s , and s h o r t g r a s s a r e the dominant p l a n t  Sparse r o c k exposures a r e l i m i t e d t o s m a l l knobs, stream g u l l i e s ,  c u t s , and some c l i f f s near the r i v e r . 2400 f e e t , Ponderosa P i n e dominates. t i t y i s not much b e t t e r . ley,  road  A t h i g h e r e l e v a t i o n s , up t o about S l o p e s a r e s t e e p e r , but o u t c r o p quan-  Above 2400 f e e t on t h e s l o p e s west of V e n a b l e s V a l -  s p r u c e - f i r f o r e s t and l o n g g r a s s undergrowth c o v e r much of t h e  hillside.  Best o u t c r o p o r f l o a t i s found on the n o r t h s i d e of s m a l l g u l l i e s s i n c e these s o u t h - f a c i n g s l o p e s a r e d r i e r and have l e s s c o v e r .  Game t r a i l s w h i c h c u t  through t h e heavy t u r f can be u s e f u l i n e x p o s i n g f l o a t .  Above e l e v a t i o n s  of about 3500 f e e t the s l o p e f l a t t e n s and o u t c r o p becomes e x t r e m e l y Only o c c a s i o n a l c a r b o n a t e , g r e e n s t o n e ,  sparse.  o r c h e r t masses s t i c k o u t , though l o g -  g i n g c u t s r e v e a l the more p r e v a l e n t p h y l l i t e s and  serpentinites.  P r e v i o u s Work Selwyn (1872) f i r s t d e s c r i b e d the Cache Creek and N i c o l a r o c k s a l o n g Thompson R i v e r . then thought  the  He found f o s s i l s j u s t n o r t h of Venables Creek w h i c h were  t o be Devonian t o Permian.  He t h e r e f o r e c l a s s i f i e d a l l r o c k s  a l o n g the west s i d e of the r i v e r as Cache Creek. Dawson (1894) p u b l i s h e d t h e f i r s t map area.  t h a t i n c l u d e s the p r e s e n t  study  A l l r o c k s west of the r i v e r w i t h the e x c e p t i o n of a s m a l l a r e a n o r t h  of Venables Creek was i t s e l f was  c o n s i d e r e d t o be Cache Creek Group.  The Cache Creek  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  r o c k s and  green-  stones and t h e upper M a r b l e Canyon F o r m a t i o n w i t h the c o n t a c t between the two l y i n g a l o n g 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 c o n t a c t to the next v a l l e y w e s t , but o t h e r w i s e h i s i s i d e n t i c a l t o Dawson's i n t h i s a r e a . and McTaggart (1952). the f i r s t t o map  T h e i r map  The next work was  done by D u f f e l  r e v e r t s t o t h a t o f Dawson.  i n d e t a i l a l o n g the highway and r i v e r .  map  C a r r (1962) was  He d e l i n e a t e d some  4 l i t h o l o g i e s b u t d i d not make changes i n t h e major u n i t s . More r e c e n t l y , Danner (1965, 1975, 1976) has done c o n s i d e r a b l e work on t h e c a r b o n a t e s i n t h e Cache Creek Group and has e s t a b l i s h e d t h e i r age and p r o b a b l e environment o f d e p o s i t i o n .  A c r o s s t h e Thompson R i v e r the N i c o l a Group  has been mapped i n d e t a i l by M c M i l l a n (1974). d e s c r i p t i o n s o f s e v e r a l measured s e c t i o n s .  He i n c l u d e s d e t a i l e d  To t h e n o r t h o f t h i s study a r e a ,  near A s h c r o f t , work i s i n p r o g r e s s by Ladd ( i n p r e s s ) and T r a v e r s (1978) on t h e 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 t h e Cache Creek e l s e w h e r e i n B r i t i s h Columbia has been m a i n l y by Monger (1969, 1975a,1977a) i n t h e A t l i n A r e a and by P a t e r s o n (1973, 1974, 1977a, 1977b), Monger and P a t e r s o n (1974) and P a t e r s o n and H a r a k a l (1974) i n t h e P i n c h i - S t u a r t Lake a r e a .  REGIONAL GEOLOGY General - North America A e u 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 M e s o z o i c r o c k s t h a t i n c l u d e t h e Cache Creek and N i c o l a Groups runs t h e l e n g t h o f the w e s t e r n N o r t h America C o r d i l l e r a .  I n B r i t i s h Columbia p a r t of t h i s  r e g i o n i s c a l l e d the Intermontane B e l t  eugeosynclinal  ( F i g u r e 2 ) , a r e g i o n o f low-grade t o  unmetamorphosed r o c k s t h a t l i e s between h i g h - g r a d e c r y s t a l l i n e b e l t s - t h e Omineca C r y s t a l l i n e B e l t t o t h e e a s t and t h e Coast P l u t o n i c Complex t o t h e west (Monger and o t h 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 t h e remnants o f an Upper P a l e o z o i c - T r i a s s i c (Monger, 1977b; P a t e r s o n , 1977a, 1977b; T r a v e r s , 1978) o c e a n i c t e r r a n e of r i b b o n 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 r o c k s (Monger, 1972, 1977b) a l l o c h t h o n o u s t o N o r t h A m e r i c a , and a Lower M e s o z o i c a r c t e r r a n e o f c a l c - a l k a l i n e t o a l k a l i n e v o l c a n i c r o c k s and r e l a t e d sedimentary rocks.  Upper P a l e o z o i c r o c k s of o c e a n i c a f f i n i t y b e l o n g t o t h e 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 B r i t 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 s t y l e , suggesting severe deformation and dismemberment.  Travers (1978) c a l l s the Cache Creek near i t s type  l o c a l i t y 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 f o r the Cache Creek has been found and most  contacts with Lower Mesozoic and other Paleozoic rocks are f a u l t s . 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 i n the western Yukon Territory,  along the length of B r i t i s h Columbia, and i n the San Juan Islands of Washing-  6 ton  (Danner, 1974,1976, 1977).  I n B r i t i s h Columbia t h e F e r g u s s o n and Hoza-  meen Groups have been suggested t o be the s o u t h e r n e x t e n s i o n of the Cache Creek Group (Monger, 1975b, 1977b).  B i t s of s i m i l a r r o c k s a r e p r e s e r v e d i n  the w e s t e r n Cascades of Washington, i n n o r t h e a s t 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 M o u n t a i n s , and a l o n g t h e w e s t e r n edge of the S i e r r a Nevada - C a l a v e r a s F o r m a t i o n (Monger, 1975b).  Oceanic  assemblages i n A l a s k a and Washington i n c l u d e r o c k s as young as J u r a s s i c and e a r l y C r e t a c e o u s (Monger,  1977b).  M e s o z o i c v o l c a n i c and p l u t o n i c a r c r o c k s w i t h i n the Intermontane B e l t range from m a f i c through s i l i c i c i n c o m p o s i t i o n . The Upper T r i a s s i c r o c k s of B r i t i s h Columbia a r e known as N i c o l a Group i n the s o u t h and T a k l a Group i n the  n o r t h where v o l c a n i s m c o n t i n u e s i n t o Lower J u r a s s i c t i m e .  In southern  B r i t i s h Columbia t h e s e p r e d o m i n a n t l y v o l c a n i c r o c k s l i e t o the e a s t of the o c e a n i c t e r r a n e w h i l e t o the n o r t h T a k l a r o c k s a r e found t o the west of the Cache Creek Group.  B r i t i s h Columbia Cache Creek Group Monger (1975a) d e s c r i b e s 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: r i b b o n 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 , m a s s i v e l e n s o i d c a r b o n a t e , minor c l a s t i c sediment, and a l p i n e ultramafic rocks.  I n B r i t i s h Columbia t h e r e a r e t h r e e a r e a s where Cache  Creek r o c k s a r e exposed: the A t l i n T e r r a n e , the P i n c h i - S t u a r t Lake A r e a , and the  t y p e a r e a a t Cache Creek ( F i g u r e 3 ) . Cache Creek r o c k s o f the A t l i n T e r r a n e range i n age from M i s s i p p i a n through  Permian (Monger, 1975a) w i t h r e c e n t r e p o r t s of M i d d l e T r i a s s i c c h e r t (Monger,  radiolarian  1977b).  Monger (1977a, 1977b) s u g g e s t s the d e p o s i t i o n a l environment t o be one of  7  Figure 3.  Areas of exposure of Cache Creek and Nicola-Takla i n B r i t i s h Columbia. Regions with question marks are t e n t a t i v e l y correlated with Cache Creek Group (Monger, 1975b).  8 v o l c a n i c pediments ( a t o l l s ) on w h i c h r e e f s have formed.  These r e e f s b r o k e  o f f from time t o time and b l o c k s s l i d i n t o deeper w a t e r coming t o 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 o f 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). morphism accompanied  Low grade t o b l u e s c h i s t meta-  t h e e a r l i e r d e f o r m a t i o n w h i c h has been t r a d i t i o n a l l y  c o n s i d e r e d p o s t L a t e Permian and p r e L a t e 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 a r e v a l i d , t i m i n g of d e f o r m a t i o n may need r e v i s i o n .  In  the l a t e r e v e n t , d u r i n g o r a f t e r l a t e M i d d l e J u r a s s i c t i m e , l a t e M i d d l e J u r a s s i c and o l 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 T e r r a n e i s f a u l t - b o u n d e d . On t h e n o r t h e a s t t h e T h i b e r t Creek F a u l t s e p a r a t e s Cache Creek from more deformed and metamorphosed P a l e o z o i c r o c k s and f a r t h e r s o u t h from l e s s deformed M e s o z o i c r o c k s .  To t h e southwest  the A t l i n T e r r a n e i s t h r u s t over M i d - J u r a s s i c and o l d e r r o c k s a l o n g t h e Nahlin Fault  (Monger, 1977a,  1977b).  I n t h e P i n c h i - S t u a r t Lake A r e a i n t h e c e n t e r o f B r i t i s h C o l u m b i a , t y p i c a l Cache Creek l i t h o l o g i e s a r e exposed i n a b e l t a p p r o x i m a t e l y 450 k i l o m e t e r s l o n g (Armstrong, 1949).  Though P a t e r s o n (1977b) c o n s i d e r s some Cache Creek  l i t h o l o g i e s t o be v e r y s i m i l a r t o r o c k s i n t h e nearby T a k l a Group,  fossils  i n c a r b o n a t e r o c k s range from P e n n s y l v a n i a n t h r o u g h Permian (Monger,  1975a).  I n t h e same r e g i o n , 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 t h a t l o c a l l y c o n t a i n a r a g o n i t e ( P a t e r s o n , 1977b).  Thus Upper  T r i a s s i c r o c k s have been deformed and metamorphosed w i t h t h e Cache Creek.  On  the o t h e r hand, d e t r i t a l c h r o m i t e , presumably d e r i v e d from u l t r a m a f i c r o c k s i n t h e Cache Creek, i s found i n nearby Upper T r i a s s i c sediments  (Paterson,  1974) . T h e r e f o r e t h e 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 o f t h e Cache Creek and T a k l a r o c k s i s n o t c l e a r l y d e f i n e d i n t h e P i n c h i - S t u a r t Lake A r e a , and t h e i r ages o v e r l a p .  9 Three d e f o r m a t i o n a l events a r e r e c o g n i z e d by P a t e r s o n (1977b).  F i r s t to  develop a r e e a s t - west i s o c l i n a l f o l d s w i t h a x i a l p l a n e s p a r a l l e l t o compos 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 a t 211218 Ma ( P a t e r s o n and H a r a k a l , 1974).  Later north-south trending, concentric  f o l d s a r e o v e r p r i n t e d by s t i l l younger Cretaceous t o 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 T e r r a n e t h e P i n c h i - S t u a r t Lake Cache Creek i s f a u l t bounded.  To the e a s t i s t h e P i n c h i F a u l t s e p a r a t i n g o c e a n i c t e r r a n e from t h e  Hogem B a t h o l i t h o f J u r a s s i c age ( P a t e r s o n , 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 a l o n g t h e P i n c h i F a u l t ( P a t e r s o n , 1977b). To'the west, n o r t h e a s t d i p p i n g f a u l t s p l a c e Cache Creek over Upper T r i a s s i c t o Lower J u r a s s i c r o c k s o f t h e S i t l i k a F o r m a t i o n ( P a t e r s o n , 1974). I n t h e type a r e a ( F i g u r e 4 ) , i n c l u d i n g t h i s s t u d y , t h e Cache Creek i s bounded by t h e F r a s e r R i v e r F a u l t Zone t o the w e s t , beyond w h i c h l i e s t h e Coast P l u t o n i c Complex.  I n f a u l t c o n t a c t t o the east a r e T r i a s s i c N i c o l a  and younger r o c k s 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  r o c k s f a r t h e r e a s t a r e n o t . c l o s e l y r e l a t e d t o t h e Cache Creek Group.  The  E a s t e r n P a l e o z o i c B e l t appears t o be a d i f f e r e n t d e p o s i t i o n a l r e a l m 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 t h e Tethyan fauna i n t h e Cache Creek Group (Danner, 1976).  L i t h o l o g i e s resemble those o f t h e Cache. Creek  (Monger, 1975b) and t h e s e r o c k s 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 t h e F r a s e r F a u l t Zone and t h e Cache Creek Group a r e r o c k s of t h e 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 t o t h e Cache Creek but of u n c e r t a i n stratigraphic position.  I t i s l i k e l y they a r e a younger f a c i e s o f t h e  Cache Creek, but no younger t h a n M i d d l e T r i a s s i c (Campbell and T i p p e r , 1971). T h e i r c o n t a c t w i t h t h e t h e Cache Creek i s a f a u l t .  Of s i m i l a r M i d d l e  T r i a s s i c age and west of t h e F r a s e r R i v e r F a u l t Zone i s t h e 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 r o c k s o f P e r m o - T r i a s s i c  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 T r i a s s i c - P a v i l l i o n , 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 T i p p e r , 1971).  Based on l i m i t e d e v i d e n c e , Monger (1975b,  1977b) and Cameron and Monger (1971) c o r r e l a t e b o t h P a v i l i o n and  Fergusson  Groups w i t h the Cache Creek.  strike-slip  Probable l a r g e - s c a l e r i g h t - l a t e r a l  movement a l o n g the F r a s e r F a u l t Zone ( T i p p e r , 1977) Cache Creek i n c l o s e r p r o x i m i t y now Coast P l u t o n i c Complex o f dominantly  p l a c e s the Fergusson  than d u r i n g t h e i r d e p o s i t i o n . Cretaceous  and pendants of c o u n t r y r o c k s e n c l o s e s Fergusson  The  i n t r u s i v e rocks w i t h  Pennsylvanian  through Permian fauna i n l i m e s t o n e pods and w i t h i n t h e m a s s i v e M a r b l e Travers  screens  Group r o c k s on t h e west.  Cache Creek r o c k s w i t h i n t h e type a r e a c o n t a i n a M i d d l e  l i m e s t o n e s (Danner, 1976).  and  Canyon  (1978) has found M i d d l e o r Upper T r i a s s i c  r a d i o l a r i a i n Cache Creek r i b b o n 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 n o r t h — p e l i t i c sedimentary  r o c k s w i t h pods of l i m e s t o n e , r i b b o n  c h e r t , b a s i c v o l c a n i c r o c k s , and u l t r a m a f i c r o c k s - a l l s t r o n g l y deformed i n most exposures.  The p o d - l i k e o c c u r r e n c e of s h a l l o w w a t e r carbonates  with  r a d i o l a r i a n c h e r t s i s c o n s i d e r e d t o 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).  T h i s i m p l i e s t h a t r i b b o n c h e r t s a r e not n e c e s s a r i l y r e s t r i c t e d  t o deep w a t e r c o n d i t i o n s .  This environmental  movement of s l i d e b l o c k s and/or slumping deeper water as was Travers  suggested  i n t e r p r e t a t i o n does not r e q u i r e  of s h a l l o w w a t e r sediments i n t o  by Monger (1977a, 1977b) f o r the A t l i n r e g i o n .  (1978) c o n s i d e r s l i m e s t o n e b l o c k s to be a l l o c h t h o n o u s .  Deformation  i n the type a r e a has been d e s c r i b e d o n l y i n g e n e r a l  " I n t e n s e l y sheared"  (Campbell  and T i p p e r , 1971)  terms.  o r " 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 i m p o s s i b l e to s o r t out though w i t h g e n e r a l l y northwest t r e n d s and southwest d i p s " ( D u f f e l l and McTaggart, 1952) tions available.  are the only d e s c r i p -  S i m i l a r l y the low grade r e g i o n a l metamophism has not been  studied. Contact r e l a t i o n s h i p s w i t h lower Mesozoic  r o c k s a r e p o o r l y known; much of  t h i s c o n t a c t i s covered by T e r t i a r y and younger r o c k s , , The e a s t e r n margin  13 of t h e Cache Creek Group near Cache Creek i s s t r o n g l y sheared a l o n g what Danner (1975) has termed t h e "Bonaparte D i s t u r b e d Zone". Travers  (1978) has i d e n t i f i e d a f a u l t  Within t h i s  zone  c o n t a c t between Cache Creek and N i c o l a  which has a p p a r e n t l y 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 B a t h o l i t h dated a t 205 Ma by Rb/Sr communication,  1978), i n t r u d e s N i c o l a r o c k s .  (R.L. Armstrong, p e r s o n a l  I t s s e t t i n g i s s i m i l a r to that  of the Hogem B a t h o l i t h o f t h e P i n c h i - S t u a r t Lake A r e a , are p r e s e r v e d i n remnants  Lowest J u r a s s i c  strata  o f s m a l l b a s i n s , the l a r g e s t of which i s the  A s h c r o f t b a s i n , c e n t e r e d on the town o f A s h c r o f t .  In t h i s b a s i n a b a s a l  c o n g l o m e r a t i c f a c i e s c o n t a i n i n g c l a s t s of p r o b a b l e Guichon B a t h o l i t h provenance i s unconformable t o d i s c o n f o r m a b l e on N i c o l a v o l c a n i c and sediment t a r y rocks  ( F r e b o l d and T i p p e r , 1969).  complete Upper T r i a s s i c  Travers  (1978) r e p o r t s an a p p a r e n t l y  through Lower J u r a s s i c s e c t i o n near the town of  Cache Creek.  Nicola  Group  Upper T r i a s s i c r o c k s which run the l e n g t h o f B r i t i s h Columbia a r e known as the N i c o l a Group  i n the south and T a k l a Group  r e s t s unconformably on o l d e r rocks  i n the n o r t h .  Nicola  Group  (Danner, 1976; Campbell and T i p p e r , 1970)  and c o n s i s t s of s h a l l o w marine t o s u b a r e a l c a l c - a l k a l i n e and a l k a l i n e v o l c a n i c r o c k s , r e e f o i d a l l i m e s t o n e , and c l a s t i c r o c k s d e r i v e d from t h e s e .  The  deposi.tional environment has been i n t e r p r e t e d t o be a v o l c a n i c i s l a n d a r c (Monger  and o t h e r s , 1972) w i t h f r i n g i n g carbonate r e e f s and s m a l l b a s i n s  (Schau, 1968, 1970). favor a r i f t  Some workers have suggested t h a t a l k a l i n e c h e m i c a l t r e n d s  environment i n t e r p r e t a t i o n  (Preto and o t h e r s ,  1975).  N i c o l a rocks of t h e type a r e a i n the N i c o l a map sheet ( C o c k f i e l d , 1948) have been d e s c r i b e d i n d e t a i l by Schau  (1968, 1970),  The age t h e r e spans  1 4  L a t e K a r n i a n t o L a t e N o r l a n 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 1974).  (Preto,  No base i s seen, b u t t o t h e e a s t N i c o l a i s unconformable on Upper  Paleozoic.  Schau has d i v i d e d t h e N i c o l a i n t o two assemblages w h i c h he has  named P and A based on t h e dominant p h e n o c r y s t , p l a g i o c l a s e o r a u g i t e . t h e l o w e r assemblage  (P) p l a g i o c l a s e p o r p h y r y f l o w s w i t h a n d e s i t i c t o  d a c i t i c c o m p o s i t i o n s dominate.  I n t e r b e d d e d w i t h and above t h e f l o w s a r e  sediments d e r i v e d from t h e v o l c a n i c s - t u f f , greywacke, a r g i l l i t e , a t e , b r e c c i a , and some l i m e s t o n e . along f a u l t s .  In  The upper assemblage  conglomer-  D a c i t e p l u g s i n t r u d e t h e s e and tend t o l i e (A) i s composed o f f l o w s w i t h  abundant  l a r g e a u g i t e p h e n o c r y s t s and sediment d e r i v e d 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 . locally  (Schau, 1968, 1970).  A few o v e r t u r n e d f o l d s a r e found  A f o l i a t i o n i s d e v e l o p e d i n f a u l t zones.  grade metasomatic and h y d r o t h e r m a l a l t e r a t i o n i s common.  Low-  H i g h e r grades o f  metamorphism a r e a t t a i n e d c l o s e t o i n t r u s i v e b o d i e s . G r a n i t i c r o c k s s i m i l a r t o t h e N i c o l a and T a k l a r o c k s i n c h e m i s t r y and o n l y s l i g h t l y younger i n age ( T r a v e r s , 1978; M c M i l l a n , 1975) i n t r u d e N i c o l a and T a k l a r o c k s .  The Guichon B a t h o l i t h and t h e Hogem B a t h o l i t h a r e two o f  the b e s t known.  These p l u t o n s a r e n o t seen t o c u t t h e Cache Creek, though  T r a v e r s (1978) has mapped s m a l l s t o c k s c u t t i n g Cache Creek t h a t may be r e l a t e d to the Guichon.  Ashcroft Formation The A s h c r o f t F o r m a t i o n o f Lower J u r a s s i c age unconformably o v e r l i e s t h e N i c o l a Group and t h e Guichon B a t h o l i t h .  U s u a l l y t h e r e i s a b a s a l conglomer-  a t e 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 , p r o b a b l y from t h e G u i c h o n , and fragments o f N i c o l a v o l c a n i c and s e d i m e n t a r y r o c k .  Most o f t h e s e c t i o n i s  b l a c k , carbonaceous s h a l e w i t h some s i l t s t o n e , sandstone and m i n o r , impure  15 l i m e s t o n e ( M c M i l l a n , 1974). Except  f o r one q u e s t i o n a b l e b l o c k d e s c r i b e d by T r a v e r s  (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 t h e A s h c r o f t F o r m a t i o n . i s i n c o n t r a s t t o Lower J u r a s s i c b o u l d e r conglomerates  This  w h i c h c o n t a i n Cache  Creek d e t r i t u s i n t h e Q u e s n e l - P r i n c e George a r e a ( T i p p e r and R i c h a r d s , 1976).  GEOLOGY OF THE STUDY AREA  G e n e r a l D i s t r i b u t i o n o f Rock U n i t s F i v e rock;' groups a r e r e p r e s e n t e d w i t h i n t h e map a r e a . d i s t r i b u t i o n i s shown on F i g u r e 5.  G e n e r a l bedrock  F i g u r e 6 ( i n pocket) i s the d e t a i l e d  g e o l o g i c map and F i g u r e s 7 and 8 ( a l s o i n p o c k e t ) a r e c r o s s s e c t i o n s from t h i s map.  The o l d e s t r o c k s a r e those o f t h e Cache Creek Group, t h e f o c u s o f  t h i s study.  Many l i t h o l o g i e s a r e p r e s e n t and s t r a t i g r a p h y i s c o m p l i c a t e d by  tectonic disruption.  The age o f t h e Cache Creek h e r e i s p r o b a b l y L a t e Permian.  Upper T r i a s s i c r o c k s a r e found a l o n g t h e e a s t e r n p a r t o f t h e a r e a where v a r i o u s l i t h o l o g i e s o f t h e N i c o l a Group a r e exposed. w h i c h can be c o r r e l a t e d w i t h t h e A s h c r o f t F o r m a t i o n  Lower J u r a s s i c r o c k s f u r t h e r n o r t h crop o u t  i n two s m a l l a r e a s near t h e s o u t h end o f t h e map a r e a . the Lower Cretaceous of t h e map a r e a .  u  A fourth unit i s  Spences B r i d g e Group w h i c h u n d e r l i e s t h e w e s t e r n  C o v e r i n g l a r g e amounts o f t h e bedrock a r e Quaternary  edge depo-  s i t s of v a 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 o f t h r e e f a c t o r s : l ) , p o o r exposure, tion.  2) l e n t i c u l a r n a t u r e o f o r i g i n a l b e d d i n g , and 3) t e c t o n i c d i s r u p -  The group i s d i v i d e d i n t o t h r e e map u n i t s .  Figure 9 i s a  diagrammatic  16  Figure  5.  Generalized  geology o f the Venables V a l l e y  area.  17  Weakly f o l i a t e d , f i n e - g r a i n e d Carbonate  pods  and layers  Greenstone  pods  and massive  Chert  sediments  unit  pods  Ultramafic  pods  F o l d e d p h y l l i t e s with i n t e r b e d d e d greywackes, carbonates Low  COVERED  m«tar« . |  V-  Possible  thrust  angle  sandstone,  fault  f a u l t over N i c o l a  Group  0 0 0  500  l_ 0  Figure 9.  Diagrammatic  stratigraphic  section  of  the Cache  Creek  Group.  18 s e c t i o n showing t h e s e u n i t s and  t h e i r r e l a t i o n to each o t h e r .  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 l i m e s t o n e l a y e r s and pods, r i b b o n , c h e r t and  sandstone.  Volcanic rocks,  s e r p e n t i n i t e masses a r e p r e s -  ent but l e s s abundant.  I n 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.  T h i s u n i t t h i n s to the n o r t h , p r o b a b l y as a r e s u l t o f  faulting.  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 t h a t  has p r e v i o u s l y been mapped as p a r t of t h e Marble Canyon F o r m a t i o n and McTaggart, 1952).  (Duffell  P r e s e n t work i n d i c a t e s t h a t l i m e s t o n e i s a minor  component, p r e s e n t as s m a l l pods i n a sheared p e l i t i c m a t r i x .  Just north  o f the f i e l d a r e a , a l o n g s t r i k e , massive l i m e s t o n e , p r o b a b l y M a r b l e Canyon Formation,  i s exposed on White M o u n t a i n .  Lower C l a s t i c U n i t (Pp, P v c l . P p l . P i s on F i g u r e  6)  L i t h o l o g i e s p r e s e n t i n t h i s u n i t i n o r d e r of abundance a r e  phyllite,  greywacke, r i b b o n c h e r t , l i m e s t o n e , 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 s e d i mentary r o c k s and s e r p e n t i n i t e . Though p h y l l i t e i s v e r y p o o r l y exposed, i t p r o b a b l y u n d e r l i e s most o f the a r e a 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 o u t c r o p s o f the lower c l a s t i c u n i t on  hillsides  are u s u a l l y of coarser-grained c l a s t i c r o c k s , c h e r t , or limestone.  Phyllite,  however, makes up most of t h e f l o a t i n the a r e a 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 s l o p e s . The p h y l l i t e s are grey to b l a c k , i n some p l a c e s g r e e n i s h and weakly banded.  T h i n 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 , c o a r s e r - g r a i n e d  m a t e r i a l , u s u a l l y s i l t or v e r y f i n e sand, g i v e the p h y l l i t e i t s banded appearance.  Though t h e s e 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 b e d d i n g , they  no  l o n g e r can be c a l l e d bedding s i n c e they a r e d i s c o n t i n u o u s 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 t h e p h y l l i t i c f o l i a t i o n i n  most..: exposures, b u t a t s e v e r a l o u t c r o p s , probably, f o l d h i n g e s , o r i g i n a l bedd i n g 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 t o t h e f o l i a t i o n .  Usually  t h e s e exposures a r e i n c o a r s e r - g r a i n e d r o c k s . I n t h i n s e c t i o n t h e p h y l l i t e s a r e seen t o be s t r o n g l y f o l i a t e d and r e crystallized.  The p h y l l i t e s a r e dominantly v e r y 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 v e r y l i t t l e t o no q u a r t z . q u a r t z o f t e n o c c u r i n s e p a r a t e patches  F e l d s p a r and  of i n t e r l o c k i n g g r a i n s w i t h c h l o r i t e ,  + s t i l p n o m e l a n e , and s e r i c i t e wrapping around t h e patches p r o d u c i n g 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 t h a t present l a y e r i n g i s secondary, where bedding  a r e s u l t o f alignment o f t h e p l a t y m i n e r a l s ,  except  i s preserved i n f o l d hinges.  A d i f f e r e n t s o r t o f p h y l l i t e i s exposed a t t h e s o u t h e r n end o f t h e a r e a j u s t n o r t h o f t h e c o n t a c t w i t h t h e Spences B r i d g e Group and about t h i r t y meters above t h e highway.  Here t h e p h y l l i t e s a r e much more d i s r u p t e d and  c o n t a i n abundant b u t s c a t t e r e d c l a s t s o f v a r i a b l e l i t h o l o g i e s . "melange" c o u l d be used t o d e s c r i b e r o c k s a t t h i s exposure.  The term  The c l a s t s ,  up t o s e v e r a l c e n t i m e t e r s a c r o s s , i n c l u d e m a i n l y g r e e n s t o n e s , b u t c h e r t and o t h e r sediments a r e p r e s e n t .  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 . The  F i g u r e 11 i s a photograph o f some o f t h e s e s t r u c t u r e s .  l o c a t i o n i s shown as I on F i g u r e 6.  the same manner as t h e e a r l i e r ones.  Many o f t h e f o l d s a r e r e f o l d e d i n  T h i s seemingly  c h a o t i c d e f o r m a t i o n may  be a r e s u l t o f p r o x i m i t y t o t h e f a u l t c o n t a c t w i t h N i c o l a s t r a t a o r more l i k e l y , c o u l d be c h a r a c t e r i s t i c o f much o f t h e lower c l a s t i c u n i t and j u s t p o o r l y exposed  elsewhere.  Only two o t h e r exposures, b o t h i n l o g g i n g c u t s , show s i m i l a r s t y l e s w i t h i n the p h y l l i t e s .  deformation  N a t u r a l o u t c r o p s of t h e p h y l l i t e s tend t o expose  20  Figure 10. Photomicrograph of typical p h y l l i t e i n the lower c l a s t i c unit of the Cache Creek Group. Note cataclastic fabric. Augenl i k e patches of feldspar and quartz are surrounded by chlorite, s e r i c i t e and stilpnomelane. Plane polarized l i g h t .  21  Figure 11.  Chaotic folding i n Cache Creek p h y l l i t e at southern end of the area, l o c a l i t y 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 d e f o r m a t i o n s t y l e i n the two  artificial  exposures a r e shown i n F i g u r e 12.  I n b o t h cases d r a g f o l d s a r e 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 a t l o c a t i o n I I  and N 60° W a t I I I .  F o l d axes p a r a l l e l t h e s t r i k e but a x i a l p l a n e s a r e  curved. Greywacke c r o p s out more than p h y l l i t e i n t h e 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 i n t e r b e d d e d w i t h the f i n e r g r a i n e d s e d i m e n t s , but more o f t e n greywacke o c c u r s as pods amidst p h y l l i t e f l o a t .  I n t e r b e d d e d 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 q u i t e t h i c k i n a r e a s where c o n t a c t s w i t h the p h y l l i t e a r e not  exposed.  I n the f i e l d t h e c l a s t i c n a t u r e i s not always e v i d e n t - g r e e n s t o n e greywacke a r e e a s i l y c o n f u s e d .  Greywacke i s u s u a l l y green and v a r i e s  massive t o deformed, w i t h a w e l l - d e v e l o p e d c h l o r i t e s c h i s t o s i t y . s i z e i s u s u a l l y l e s s t h a n 2 mm.  and from  Grain  Occasionally larger clasts, several  meters i n l e n g t h , s i t i n t h e f i n e r m a t r i x .  be  centi-  These c l a s t s a r e b e s t observed  on c u t s u r f a c e s . The greywackes a r e f e l d s p a t h i c .  Some c o 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 , e u h e d r a l to s u b h e d r a l i n shape ( F i g u r e 13). 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 a r e now  t i z e d or completely a l t e r e d to a l b i t e . p r e s e r v e d i n a few g r a i n s .  A t l e a s t 30% of every strongly  saussuri-  The o r i g i n a l a n d e s i n e c o m p o s i t i o n i s  Zoning and t w i n n i n g 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 r o c k s a r e the dominant l i t h i c fragments; c h e r t i s p r e s e n t i n one sample; and a n o t h e r c o n t a i n s abundant c a r b o n a t e c l a s t s .  Volcanic clasts i n -  c l u d e p o r p h y r i t i c f l o w r o c k s and p i e c e s of p a l a g o n i t e ( F i g u r e 14).  It is  r a t h e r u n u s u a l t o f i n d a p i e c e o f g l a s s p r e s e r v e d i n a Permian r o c k , e s p e c i a l l y s i n c e the r o c k 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 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 at two road cuts i n the Cache Creek Group. Roman numerals designate l o c a t i o n s 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 p o l a r i z e d l i g h t ; b) crossed p o l a r s . P l a g p l a g i o c l a s e , Qz-quartz.  Figure 14. Photomicrographs of l i t h i c fragments i n 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 t o p a l a g o n i t e . sample a p i e c e o f f i n e r g r a i n e d greywacke o c c u r s as a c l a s t  (Figure 15).  Quartz i s v e r y r a r e , u s u a l l y o n l y 2-3 g r a i n s p e r t h i n s e c t i o n . amphibole was observed  i n one sample.  I n one  A brownish  Though t h e greywackes themselves have  been metamorphosed, t h e r e appear t o be some c l a s t s d e r i v e d from a low-grade metamorphic t e r r a n e .  F i g u r e 15 shows a d e t r i t a l g r a i n w h i c h i s almost com-  p l e t e l y bowtie p r e h n i t e . The  Another s e c t i o n comtains d e t r i t a l e p i d o t e .  semi-opaque m a t r i x i s u s u a l l y v e r y f i n e g r a i n e d and made up o f  secondary m i n e r a l s .  Metamorphic m i n e r a l s i n c l u d e c h l o r i t e (found i n a l l  samples i n b o t h - t h e m a t r i x and r e p l a c i n g d e t r i t a l f r a g m e n t s ) , w h i t e m i c a , c a l c i t e (more p r e v a l e n t i n t h e f i n e r g r a i n e d greywacke), sphene (making up much o f t h e semi-opaque m a t r i x and r e p l a c i n g v o l c a n i c f r a g m e n t s ) ,  stilpnome-  l a n e ( o c c u r r i n g w i t h c h l o r i t e i n t h e m a t r i x o f deformed greywacke), pumpell y i t e and/or e p i d o t e d e t r i t a l grains).  (found b o t h i n t h e f i n e g r a i n e d m a t r i x and r e p l a c i n g  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 t h e i r o n r i c h v a r i e t y .  Except f o r t h e 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 o c c u r s i n v e i n s and i s r a r e l y developed T a b l e I shows m i n e r a l o g y Deformation  w i t h i n t h e body o f t h e r o c k s .  o f t h e greywackes.  i n t h e greywackes v a r i e s from n e g l i g i b l e  of a c a t a c l a s t i c f o l i a t i o n .  F i g u r e 13 i s a p h o t o m i c r o g r a p h o f one o f t h e r e l -  a t i v e l y undeformed greywackes.  M a t r i x i s composed o f f i n e r and f i n e r g r a i n s  s i m i l a r i n composition to the l a r g e r c l a s t s . shown on F i g u r e 16.  t o s t r o n g , i n t h e form  A weakly f o l i a t e d greywacke i s  The f o l i a t i o n i s d e f i n e d by the development o f c h l o r i t e  i n t h e m a t r i x 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 .  F i g u r e 17 shows a more  extreme c a t a c l a s t i c f a b r i c , p r o t o m y l o n i t e , w i t h an almost opaque, r e c r y s t a l l i z e d and b r o k e n fragment zone o f f i n e g r a i n e d m a t e r i a l around t h e s h a t t e r e d 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 o f t h e s t r u c t u r a l s t y l e o f t h e Cache Creek  27  Figure 15.  a)  Thin section of a Cache Creek Group greywacke showing large rip-up clast of s i l t s t o n e redeposited 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.  r-l  la se  J3 •H  o  jJ •rl 00 3 .<  Sample I  0 •H  cm  a  N 4J  •H  0) 3  rH  O*  !  •H  ol  n  o  J3 rH  o  cu 4J O TI •rl  o. W  CM  Ol 4-> •H O rH  ta o  1 a  ll j=  o.  Lithology  Lower C l a s t i c Unit Greywacke 6-18-1 Greywacke 6-18-2 Greywacke 6-18-3 Greywacke 7-8-2 Greywacke 7-20-1 Greywacke 7-20-6 Greywacke 7-20-8 Augite porphyry 7-5-4 Basalt 7-5-3B Tuff 7-5-3A Augite porphyry 7-6-5 Basalt 7-22-5 Greywacke 7-20-9 Greenstone U n i t 6-9-3 Tuff 7-20-3 Basalt 7-20-2 Basalt Basalt 7-20-4 7-19-3 Basalt 6-18-6 Basalt Basalt 6-18-5 Upper C l a s t i c Unit 7-25-2 Tuff Tuff 7-12-3 L i t h i c sandstone 7-25-3C 7-25-3B Greywacke  •H  TABLE I.  5  Mineralogical  Data f o r Greywackes and Greenstones -  f f f f f f f r r fr r f  r r r r r r  r r f f  r f  r r r r r r  r r  f f f f  a a a a f q aq f a a a av a a a av a q rvq av a fq  a a a a a a a a a a a a a  av a vq a aq a  a a a a a a  aq avq a av  q  q  V  a a a a  a a a a  q vq q q  a a a q a a  v a  a  a a a a  a a  a a a  a a a  a fa a a aq fa a av  a a q q  a  av  av f  q vq  vq  a aq  q aq  av a a a  a a a a a a a a a a a a a  a a a  f f  a a a a a a a  f  a a a a a a a  a  r r r r  a  r r f f  Key to Mineral Occurrences  f  f  a a  the Cache Creek Group.  r f a v q  -  r e l i c t mineral d e t r i t a l fragment a l t e r a t i o n mineral vesicle f i l l i n g v e i n mineral  Figure 16. Photomicrograph of a weakly foliated Cache Creek Group greywacke. Plane polarized l i g h t .  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 d i s c u s s e d  later.  C h e r t , 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 t o n e a r l y b l a c k i n c o l o r and l a y e r s of b l a c k a r g i l l i t e deformed and  (ribbon cherts).  The  i s interbedded with t h i n  a r g i l l i t e i s usually  f r e q u e n t l y squeezed out from'between c h e r t  quite  layers.  C h e r t o u t c r o p s a r e 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 d i m e n s i o n . I n v a r i a b l y the c h e r t i s boudined and  o c c a s i o n a l l y chevron type f o l d s of  a l tens of c e n t i m e t e r s a m p l i t u d e are v i s i b l e . c h e r t o u t c r o p s i n the s o u t h end  A l i g n m e n t of a l a r g e number of  of the f i e l d a r e a s u g g e s t s t h e r e may  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 . t i o n are a l s o p r e v a l e n t (Figures  18 and  B r e c c i a t i o n and  have  mylonitiza-  i n the c h e r t s on m a c r o s c o p i c and m i c r o s c o p i c  scales  19).  T h i n s e c t i o n s show some of the c h e r t s t o c o n t a i n r a d i o l a r i a and sponge s p i c u l e s , but  ( F i g u r e 20).  probable  these a r e too r e c r y s t a l l i z e d t o be i d e n t i f i e d and  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 c o a r s e r matrix  grained  H y d r o f l u o r i c a c i d e t c h i n g t e c h n i q u e s were used i n an  e f f e c t i v e l y d e s t r o y e d the f o s s i l s .  The  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  chert.  the c h e r t s a r e impure.  has  e t c h i n g dick.expose t i n y f o l d s of  They c o n t a 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 minerals,  show  q u a r t z w i t h i n the f i n e r  attempt t o r e c o v e r 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  Generally  sever-  occasional c l i n o z o i s i t e .  The  i f they are s u f f i c i e n t l y abundant, l e n d a f o l i a t i o n to the  .Carbonates o c c u r as s e m i - c o n t i n u o u s l a y e r s and  as pods.  The  platy  chert.  b e s t example  of a c a r b o n a t e l a y e r i s found a l o n g the top of the l o w e r c l a s t i c u n i t  and  can be t r a c e d i n t e r m i t t e n t l y f o r 4.5  km.  with a well-developed foliation:.  f o s s i l s were found. T h i n s e c t i o n s show  No  I t i s a s i l t y t o sandy l i m e s t o n e  d e t r i t a l m a t e r i a l t o be s i m i l a r t o t h a t i n the greywackes - p l a g i o c l a s e some a n d e s i n e but m o s t l y a l t e r e d to a l b i t e , and  grains,  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 p r o b a b l e sponge s p i c u l e s i n Cache Creek c h e r t . Photomicrograph " a " i s p l a n e p o l a r i z e d l i g h t and "b" w i t h c r o s s e d 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 c h e r t and c a l c i t e g r a i n s .  The c l a s t s a r e s u r r o u n d -  ed by c a l c i t e w h i c h i s r e c r y s t a l l i z e d i n t o l o n g g r a i n s and d e f i n e s t h e f o l i a tion.  Most o f t h e c a l c i t e i s p r o b a b l y o f s e d i m e n t a r y  and cement, b u t some o c c u r s as a replacement  o r i g i n , sand g r a i n s  o f p l a g i o c l a s e . Minor  epidote  and c h l o r i t e a r e p r e s e n t . The carbonate pods a r e much p u r e r l i m e s t o n e and a r e 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 t h e l e n g t h o f t h e pods w h i c h 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. from a meter o r l e s s i n l e n g t h t o a k i l o m e t e r .  These pods v a r y i n s i z e  U n f o r t u n a t e l y no f o s s i l s a r e  p r e s e r v e d i n t h e pods w i t h i n t h e f i e l d a r e a , though Danner ( p e r s . comm., 1975) f i n d s Permian f u s i l i n e s i n pods t o t h e n o r t h . Contact r e l a t i o n s h i p s o f t h e pods w i t h t h e c l a s t i c r o c k s s u r r o u n d i n g are not very c l e a r .  them  I n some p l a c e s i t appears as i f t h e f r i n g e s o f t h e pod  are interbedded w i t h the sediments,  though i n f o l d i n g i s another  U s u a l l y the margins o f t h e pods a r e more h i g h l y sheared s u g g e s t i n g movement a l o n g t h e c o n t a c t .  possibility.  than t h e i r  interiors  However, sheared margins would o c c u r  i f t h e b l o c k s a r e t e c t o n i c a l l y emplaced o r i f they were p r e s e n t p r i o r t o t e c t o n i s m , and d e f o r m a t i o n was c o n c e n t r a t e d a l o n g t h e c o n t a c t zone. M i n o r v o l c a n i c and v o l c a n i c l a s t i c r o c k s a r e p r e s e n t w i t h i n t h e lower clastic unit.  One 2 km l o n g pod i s mapped s e p a r a t e l y . TThis pod l i e s  t h e e a s t s i d e o f Venables V a l l e y and i s composed o f a u g i t e p o r p h y r y , as c r y s t a l and l i t h i c t u f f s .  Phenocrysts  along some  o c c u r - - a s s m a l l as 1 mm and l e s s  but u s u a l l y a r e s e v e r a l m i l l i m e t e r s a c r o s s .  Also present are coarser v o l c a n i -  c l a s t i c r o c k s w i t h c l a s t s d e r i v e d from a u g i t e p o r p h y r y .  Original textures  a r e i n l a r g e p a r t d e s t r o y e d by l a t e r d e f o r m a t i o n 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 the greenstones  ( c h l o r i t e , a l s o t a l c ? ) i s p r e s e n t i n some o f  o f 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 t h e s u r r o u n d i n g  p h y l l i t e , w h i l e o t h e r greenstones  are hardly f o l i a t e d .  Metamorphic m i n e r a l s  b e s i d e s 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 , e p i d o t e , s t i l p n o m e l a n e , sphene, w h i t e m i c a , a c t i n o l i t e , and a b l u e amphibole.  The o c c u r r e n c e of t h i s amphir-  b o l e i s a t l o c a l i t y IV on F i g u r e 6, j u s t n o r t h e a s t of Venables Lake.  The  g r e y - b l u e t o 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 t o 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 g r a i n s and i n patches i n the m a t r i x ( F i g u r e 21). determine  M i c r o p r o b e a n a l y s e s were run on s e v e r a l of t h e s e g r a i n s t o  t h e i r composition.  Oxide weight p e r c e n t a g e s  of n i n e  elements  and the number of i o n s of each element i n the m i n e r a l f o r m u l a a r e p r e s e n t e d i n Table I I . structure.  Two  OH m o l e c u l e s were assumed to complete  the amphibole  An average f o r m u l a r e p r e s e n t i n g these a n a l y s e s i s : '• ^ ^ l . l ' ^ l . l - l ^  ( M  %.7-3.2' l.6-1.8' F e  A : L  .2-.5 5 8 22' )  S i  0  ( O H )  2-  On the s t a n d a r d s o d i c amphibole r e c t a n g l e ( F i g u r e 22a) t h i s c o m p o s i t i o n p l o t s w i t h i n the m a g n e s i o r e i b e k i t e f i e l d a l o n g 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 miscibil-  i t y gap between Na- and Ca-amphiboles has been suggested by-Coleman and (1968) who sedimentary  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 c a r b o n a t e rocks.  Papike  rich  The b l u e amphibole of the p r e s e n t s t u d y , however, p l o t s  d i r e c t l y i n t h e c e n t e r o f t h e i r c o m p o s i t i o n a l gap  ( F i g u r e 22b).  Brown (1977b)  c l a i m s t h a t c o n t i n u o u s 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 and a l l c o m p o s i t i o n s a r e s t a b l e w i t h i n the g r e e n s c h i s t f a c i e s and p r e s s u r e p a r t of the b l u e s c h i s t f a c i e s .  The gap may  exists  lower  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 c o m p o s i t i o n to those i n the p r e s e n t  study  a r e 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 n o r t h e r n New  C a l e d o n i a ( B l a c k and B r o t h e r s , 1978).  r e l i c t igneous pyroxene g r a i n s .  There r i e b e c k i t i c amphiboles  The b l u e 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 c o r e s grade out to r i e b e k i t i c r i m s . which most c l o s e l y resembles ( a n a l y s i s 13541, p.75  rim  The  analysis  the p r e s e n t study i s a s o d i c a c t i n o l i t e  i n B l a c k and B r o t h e r s , 1978)  l i s t e d on T a b l e I I .  In  F i g u r e 21. Photomicrographs of blue amphiboles i n augite porphyry volcanic rocks. The amphiboles rim or form within r e l i c t augite grains. Some occur i n the groundmass. Plane polarized l i g h t . The blue amphiboles are colored to show t h e i r d i s t r i b u t i o n .  TABLE I I B l u e Amphibole Probe A n a l y s e s * * Oxide Weight % / Ions P e r Formula U n i t o f Element  2  3.27 / 0.89  MgO  14.49 / 3.04  Na 0  3.17 / 0.85  3.52 / 1.00  3.99 / 1.14  3.04 / 0.86  3.20 / 0.9***  14.35 / 2.97 • 12.68 / 2.77  12.15 / 2.67  14.43 / 3.15  14.70 / 3. 17  1.37 / 0.24  3.07 / 0.53  1.14 / 0.2  2.40 / 0.41  0.99 / 0.16  1.-20 / 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 0 2  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  15.05 / 1.77  13.58 / 1.57  13.54 / 1.66  13.63 / 1.68  13.34 / 1.64  15.20 / 1.84  95.26  95.64  95.11  98.25  A1 0 2  Si0  2  FeO*  3  Total 99.55 ( l e s s OH)  100.49  * 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 c u r r e n t - .25 yamp • beam s i z e - 10 y Bence-Albee d a t a r e d u c t i o n (Bence and A l b e e , 1968) a f a c t o r s from A l b e e and Ray (1970) A n a l y s e s conducted on U n i v e r s i t y o f B r i t i s h Columbia microprobe ***  Analysis  13541 from B l a c k and B r o t h e r s  (1978) OO  39  Na Fe»Al Si 0 2  2  8  2 2  (OH)  100  Na Fe^Fe" Sl 0 (OH)  2  2  T  T  80  i i i I i i  60 CO  40  20  I  <  2  3  2  8  2 2  2 2  2  4  a  -I  o c  Composition of a n a l y s i s from p r e s e n t study  ®  «2?  20  Na Mg Al Si 0  g  •o «  i i  5  0  £-  I  0/  r-  2  (OH)  »  L  40  _L  60  80  100  Na Mg Fe 2  ,  ,  3  2  Si 0 g  2 2  (OH)  2  2  Ca (Mg,Fe") Si 0 (OH) 2  5  8  2 2  2  Composition of a n a l y s i s present study  Na  2  (Mg,Fe") Al Si 6 3  2  8  2 2  (OH)  N a (Fe" ,Mg) F e ' .• S i g 0 ( O H )  from  1  2  2  3  2  2 2  2  F i g u r e 22. S o d i c amphibole c o m p o s i t i o n s p l o t t e d on a) s t a n d a r d glaucophanec r o s s i t e - r i e b e c k i t e r e c t a n g l e (Deer and o t h e r s , 1966) and b) an a c t i n o l i t e vs_. s o d i c amphibole t r i a n g l e from Coleman and P a p i k e (1968).  40 New C a l e d o n i a  t h e s o d i c amphiboles a r e g r e e n i s h r a t h e r than b l u e .  Colorless  to p a l e green amphiboles o c c u r r i n g w i t h b l u e amphiboles i n t h e p r e s e n t were n o t a n a l y s e d ; a d d i t i o n a l m i c r o p r o b e work i s n e c e s s a r y  to f u l l y  area  describe  t h e n a t u r e o f amphiboles i n t h i s p a r t o f t h e Cache Creek Group. S m a l l pods o f v o l c a n i c r o c k s a r e found i n t h e lower c l a s t i c u n i t as w e l l as t h e l a r g e pod a l r e a d y d e s c r i b e d .  The s m a l l pods a r e b a s a l t i c and i n c l u d e  a u g i t e p o r p h y r y w i t h zoned and twinned a u g i t e ; p h e n o c r y s t s  i n a very  g r a i n e d , 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 f l o w s 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 e x t r e m e l y 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 .  s h i p s o f these greenstones w i t h t h e s u r r o u n d i n g  fine-  amygdaloidal  f i n e grained, deviContact  relation-  p h y l l i t e s a r e n o t exposed  and i t i s n o t e v i d e n t whether these g r e e n s t o n e s a r e f l o w s w i t h i n t h e s e c t i o n , dikes or tectonic 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 , m a i n l y s e r p e n t i n i t e , a r e s c a t t e r e d t h r o u g h o u t t h e lower c l a s t i c u n i t as s m a l l pods, and a r e found more c o n t i n u o u s l y a l o n g t h e n o r t h e r n p a r t o f t h e upper c o n t a c t o f t h e u n i t .  A l a r g e pod p i n c h e s  ward t o a t h i n l a y e r g e n e r a l l y found o n l y i n f l o a t .  Although  out n o r t h -  relict  clino-  pyroxenes a r e 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 , t h e u l t r a m a f i c r o c k s a r e a l m o s t t o t a l l y a l t e r e d t o s e r p e n t i n e w i t h m a g n e s i t e , m a g n e t i t e and minor chromite. A f o l i a t i o n due t o t h e a l i g n m e n t o f s e r p e n t i n e s i m i l a r t o t h e f o l i a t i o n i n t h e p h y l l i t e s i s developed i n t h e s m a l l e r s e r p e n t i n e pods.  Part of the  l a r g e r u l t r a m a f i c b l o c k a l o n g t h e c o n t a c t between t h e lower c l a s t i c u n i t and the g r e e n s t o n e s , basalt.  however, i s more m a s s i v e and i n c l u d e s some p o r p h y r i t i c  The b a s a l t c o n t a i n s a l t e r e d p l a g i o c l a s e p h e n o c r y s t s  set i n a finer  mass o f 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 . C o n t a c t s o f t h e lower c l a s t i c u n i t a r e unexposed o r p o o r l y exposed. base f o r t h e u n i t i s seen.  No  The upper c o n t a c t i s d e f i n e d by a s e r p e n t i n i t e  zone i n t h e n o r t h and greenstones  to the south.  S h e a r i n g has o c c u r r e d  t h e c o n t a c t w h i c h appears t o d i p p a r a l l e l t o t h e r e g i o n a l f o l i a t i o n . c o n t a c t i s p r o b a b l y a lowr-angle  The  fault.  To t h e s o u t h and a l o n g t h e west and. southwestern  margin o f t h e Cache  Creek i s a p r o b a b l e f a u l t c o n t a c t w i t h r o c k s o f t h e Lower Cretaceous B r i d g e Group.  along  Spences  The c o n t a c t i s n o t exposed w i t h i n t h e a r e a , b u t P e a r s o n  (1974)  documents t r u n c a t i o n o f z e o l i t e f a c i e s b o u n d a r i e s w i t h i n t h e Spences B r i d g e by t h e c o n t a c t w i t h t h e Cache Creek Group.  A l s o , low a n g l e f a u l t s i n t h e  Cache Creek a r e t r u n c a t e d by t h e c o n t a c t .  M a s s i v e M a f i c V o l c a n i c U n i t (Pv on F i g u r e 6) L y i n g t o p o g r a p h i c a l l y above and i n f a u l t c o n t a c t w i t h t h e lower u n i t i s a g r e a t t h i c k n e s s of a l t e r e d m a f i c v o l c a n i c r o c k s . t h e s e r o c k s can o n l y be i d e n t i f i e d as m a s s i v e , resemble t h e green c l a s t i c sedimentary  clastic  I n the f i e l d  a p h a n i t i c greenstones and  rocks of the other u n i t s .  Fresh  s u r f a c e s a r e n e a r l y i m p o s s i b l e t o produce as these r o c k s tend t o break o n l y a l o n g weathered s u r f a c e s .  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 o b s e r v e d ,  but f r a c t u r i n g i s much more p r e v a l e n t . massive weathering  I n t h e f i e l d these were grouped by t h e  c h a r a c t e r , t h e g r e e n i s h c o l o r , and t h e l a c k o f sedimentary  f e a t u r e s t h a t sometimes show up i n t h e greywackes. A l l o f t h e greenstones phenocrysts  i n t h i s unit are b a s a l t i c .  a r e e u h e d r a l a u g i t e w i t h o r w i t h o u t p l a g i o c l a s e . I n one sample  a s e r p e n t i n i z e d o l i v i n e phenocryst plagioclase.  c a n be r e c o g n i z e d a l o n g w i t h a u g i t e and  P l a g i o c l a s e o f c o m p o s i t i o n A n ^ i s r e l i c t i n one r o c k , b u t  generally 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 . l o c k i n g mass o f f i n e p l a g i o c l a s e and a u g i t e . details.  I n p o r p h y r i t i c members  The groundmass i s an intern-  Table I contains m i n e r a l o g i c a l  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 g l a s s i n s e v e r a l samples, and t h e v e r y f i n e g r a i n . . s i z e of  42 o t h e r s makes a f l o w o r i g i n f o r most of the greenstones seem r e a s o n a b l e . C r y s t a l and l i t h i c t u f f s a r e p r e s e n t i n s m a l l e r q u a n t i t i e s .  Flow  structures,  p i l l o w s , o r i n d i v i d u a l f l o w u n i t s c o u l d 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 t o a n o t h e r .  C h l o r i t e , a l b i t e and sphene  a r e p r e s e n t 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 , w h i t e m i c a , and a c t i n o l i t e a r e v a r i a b l y developed.  As s t a t e d above, the  greenstones a r e u s u a l l y m a s s i v e , but i m m e d i a t e l y above the c o n t a c t w i t h the u n d e r l y i n g u n i t a p r o t o m y l o n i t e f a b r i c i s developed.  Round g r a i n s of  a u g i t e s i t i n a v e r y f i n e g r a i n e d , almost opaque, f o l i a t e d m a t r i x ( F i g u r e 23).  T h i s c a t a c l a s t i c f o l i a t i o n a t the base of the g r e e n s t o n e u n i t s u p p o r t s  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 l o w e r c o n t a c t , i s not exposed  The upper c o n t a c t  anywhere but i s p r o b a b l y f a u l t e d , s i n c e the g r e e n s t o n e  unit  i s cut o f f to the n o r t h .  Upper C l a s t i c U n i t ( P s , P i s ) As a r e s u l t of v e r y poor exposure due t o t h i c k g r a s s , t r e e s and low  relief,  r o c k s i n the a r e a mapped as the upper c l a s t i c u n i t can not be w e l l d e f i n e d , The u n i t l i e s above the greenstone u n i t t o the s o u t h and t o t h e n o r t h i t i s s e p a r a t e d 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 a r e much t h e same as i n the lower c l a s t i c u n i t , but d e f o r m a t i o n and metamorphism seem to be l e s s .  B l o c k s of l i m e s t o n e , g r e e n s t o n e ,  coarse  c l a s t i c r o c k s - greywacke,. and b r e c c i a - and o c c a s i o n a l l y c h e r t a r e b e s t 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 ,  s e r p e n t i n e m a t r i x which i s o n l y exposed road c u t s .  and  i n f l o a t or i n s c a t t e r e d l o g g i n g  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 a r e v i s i b l e .  L i m e s t o n e , one of the b e s t exposed a t e s i n the lower c l a s t i c u n i t .  l i t h o l o g i e s , i s d i f f e r e n t from  carbon^  I t o c c u r s as pods, but the pods a r e 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 c l a s t i c unit. Pr-prehnite, Pumppumpellyite, Aug-augite. Plane polarized l i g h t .  44 o o l i t e s , and f u s i l i n i d s are p r e s e r v e d .  The  c a r b o n a t e s are t y p i c a l  Canyon l i m e s t o n e , l i g h t grey w e a t h e r i n g and c o a r s e l y c r y s t a l l i n e . of  Marble Two  pods  t h i s type are exposed on the west s i d e of the h i g h r i d g e . Other l i m e s t o n e exposures  greenstone  i n breccias.  cussed under sedimentary  are much s m a l l e r and are c l o s e l y a s s o c i a t e d w i t h  These a r e p r o b a b l y s l i d e b r e c c i a s and w i l l be structures.  C l a s t i c r o c k s c o a r s e r than a r g i l l i t e resemble lower c l a s t i c u n i t .  s i m i l a r l i t h o l o g i e s i n the  D e t r i t u s i n t h e s e greywackes and f i n e sandstones i s  composed o f v o l c a n i c fragments  and a u g i t e and f e l d s p a r c r y s t a l s s e t i n a v e r y  f i n e grained m a t r i x 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 , and s e r i c i t e .  dis-  P u m p e l l y i t e o c c u r s i n a fragment i n one sample.  carbonate  Bedding i s  n o t d i s r u p t e d and i s d e f i n e d by a l t e r n a t i n g bands o f c o a r s e r and f i n e r material.  L a y e r s 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 . of  A u g i t e 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  d e v i t r i f i e d , welded g l a s s s h a r d s .  V o l c a n i c fragments  are mostly very  f i n e g r a i n e d b a s a l t o r n e a r l y opaque p i e c e s of d e v i t r i f i e d g l a s s .  Albite,  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 p u m p e l l y i t e are the o n l y a l t e r a t i o n minerals. 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 m a t r i x s u r r o u n d i n g the b l o c k s o f  c a r b o n a t e , g r e e n s t o n e , and coarse c l a s t i c r o c k s i s exposed o n l y as f l o a t i n r u b b l e i n l o g g i n g road c u t s .  Chert and a r g i l l i t e  b l a c k r e s p e c t i v e l y and v e r y weakly  foliated.  and  are grey to b l a c k and  C o n t a c t s between the b l o c k s  and m a t r i x are not exposed.  Age o f the Cache Creek Group F u s i l i n i d s found i n l i m e s t o n e pods o f the upper c l a s t i c u n i t ( F i g u r e 24) were i d e n t i f i e d by W.R.  Danner as Y a b e i n a minuta o f L a t e Permian p r o b a b l y L a t e  Figure 24. Photomicrographs of limestone i n the upper c l a s t i c unit, Cache Creek Group: a) oolites and b) Upper Permian, probably Late Guadalupian, f u s i l i n i d Yabelna minuta, i d e n t i f i e d by W.R.Danner.  46 Guadalupian,  T h i s age i s t y p i c a l of l i m e s t o n e 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 o t h e r l i t h o l o - ?  g i e s o f f o r l i m e s t o n e s of the l o w e r c l a s t i c  unit.  Sedimentary S t r u c t u r e s 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 p r e s e n t e d t o e x p l a i n the p o d ^ l i k e o c c u r r e n c e of s h a l l o w w a t e r c a r b o n a t e s 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 t o be of deeper w a t e r o r i g i n .  From h i s work i n t h e A t l i n  T e r r a n e , 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 a r e formed on seamounts and  a t o l l s and t h e s e s h a l l o w water c a r b o n a t e s break o f f from time t o t i m e and s l i d e i n t o deeper w a t e r .  They come t o r e s t amongst deeper water r o c k s .  Danner (1967, 1975), on the o t h e r hand, s u g g e s t s t h a t t h e s e c a r b o n a t e b l o c k s i n c h e r t a r e not e x o t i c , but t h a t the c h e r t s and c a r b o n a t e s a r e i n t e r b e d d e d . T h i s r e q u i r e s t h a t r i b b o n c h e r t s a r e of v e r y s h a l l o w w a t e r o r i g i n w i t h i n t h e Cache Creek Group.  I n t h i s model l e n s o i d masses of c a r b o n a t e s a r e i n t e r -  p r e t e d 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 p r e s e r v e d i n a r e a s of l e s s i n t e n s e deformat 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.  F i n e t o c o a r s e g r a i n e d c l a s t i c s e d i m e n t a r y r o c k s show the most  structures.  Rhythmic l a y e r i n g i n s i l t s t o n e s and greywackes and o t h e r  s t r u c t u r e s t y p i c a l of the Bouma sequence - graded b e d d i n g , l a m i n a r b e d d i n g , c o n v o l u t e l a m i n a t i o n s - suggest t u r b i d i t e s e d i m e n t a t i o n f o r much of the clastic material.  S o f t sediment slump f o l d s ( F i g u r e 25) and r i p - u p  ( F i g u r e s 15 and 26) s u p p o r t g r a i n f l o w movements.  clasts  One o u t c r o p a t the top  o f the r i d g e i n t h e 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 l i m e s t o n e and a m y g d a l o i d a l and t u f f a c e o u s b a s a l t fragments ( F i g u r e 2 7 ) .  Chunks of l i m e s t o n e  a r e s e v e r a l c e n t i m e t e r s a c r o s s w h i l e g r e e n s t o n e fragments a r e l e s s than 2 cm  Figure 25.  Soft sediment folds i n f i n e grained sediments of the lower c l a s t i c u n i t , Cache Creek Group.  Figure 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 , Cache Creek Group.  S l i d e b r e c c i a 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 v o l c a n i c m a t e r i a l .  49 i n diameter.  T h i s i s a l s o thought  t o be a d e b r i s f l o w o f f a s h a l l o w  carbon-  a t e 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 s h a l l o w water c o n d i t i o n s i n t h e l i m e s t o n e masses o f t h e 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 a r e p r e s e n t and r e p r e s e n t w a t e r depths o f l e s s than 30 meters. A l t h o u g h v e r y deep water c o n d i t i o n s a r e n o t n e c e s s a r y f o r t h e f o r m a t i o n o f turbidites,  l a c k o f s h a l l o w water s t r u c t u r e s i m p l i e s they were d e p o s i t e d a t  l e v e l s below wave base. ribbon cherts.  Apparently interbedded w i t h the debris flows are  The environment o f f o r m a t i o n o f r i b b o n c h e r t s i s a f u n c t i o n  o f many f a c t o r s , b u t carbonate  f r e e r a d i o l a r i t e s a r e thought  t o be r e s t r i c t e d  to depths below t h e c a r b o n a t e compensation d e p t h , p r o b a b l y 2000-3000 meters i n Permian time ( B o s e l l i n i and W i n t e r e r , 1975). observed  No s h a l l o w w a t e r l i m e s t o n e s were  d i r e c t l y interbedded w i t h ribbon cherts i n t h i s area.  these c a r b o n a t e s  a r e u s u a l l y s t r o n g l y sheared.  Contacts of  Carbonate r o c k s c l e a r l y  inter-  bedded w i t h t h e t u r b i d i t e and r i b b o n c h e r t sequences a r e c a l c a r e n i t e s , some weakly graded, w h i c h a r e p r o b a b l y t u r b i d i t e s  themselves.  From e v i d e n c e i n t h i s a r e a a d e p o s i t i o n a l environment s i m i l a r t o Monger's model i s r e a s o n a b l e .  S h a l l o w water carbonates  mounts t h a t a r e surrounded  by deeper w a t e r .  form on b a s a l t i c v o l c a n i c s e a -  The b a s i n i s dominated by c h e r t  and a r g i l l i t e s e d i m e n t a t i o n w i t h i n f l u x e s o f c o a r s e r m a t e r i a l d e r i v e d from the v o l c a n i c and c a r b o n a t e  terrane.  E x o t i c blocks of limestone w i t h i n the  f i n e g r a i n e d sediments must be s l i d e b l o c k s . p r e l i m i n a r y Mesozoic 1977b).  I n s u p p o r t o f t h i s model a r e  ages f o r Cache Creek r a d i o l a r i a  ( T r a v e r s , 1978; Monger,  Thus t h e s l i d e b l o c k s may be much o l d e r than t h e r m a t r i x i n w h i c h they  presently s i t .  More f o s s i l d a t a from t h e non-carbonate r o c k s a r e needed t o  c o n c l u s i v e l y d e f i n e t h e d e p o s i t i o n a l environment i n t h i s a r e a .  Structure A s t r o n g p h y l l i t i c f o l i a t i o n i n t h e c l a s t i c r o c k s , b e s t developed  i n finer  50 g r a i n e d r o c k s ; i s t h e e a r l i e s t and dominant s t r u c t u r e i n t h e 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 g r e e n s t o n e s .  I n most cases bedding has been t r a n s p o s e d  t o p a r a l l e l t h e 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 a m p l i t u d e s l e s s t h a n a few c e n t i m e t e r s , 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 s u g g e s t s t h e 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, b u t s e v e r a l o u t c r o p s c l e a r l y demonstrate c r o s s - c u t t i n g relationships.  A beddingr-cleavage  bedding-cleavage  i n t e r s e c t i o n l i n e a t i o n i s d e v e l o p e d on  f o l i a t i o n s u r f a c e s i n t h e c l a s t i c members. Carbonate and c h e r t responded d i f f e r e n t l y t o t h i s d e f o r m a t i o n .  Cherts  a r e s t r o n g l y boudined and b r e c c i a t e d w i t h t h e a r g i l l i t e l a y e r s almost out.  The l o n g a x i s o f t h e boudins p a r a l l e l s t h e o t h e r l i n e a t i o n .  e r s a r e p r o b a b l y boudined on a l a r g e r s c a l e . continuous outcrop p a t t e r n of the c h e r t s .  squeezed  Chert l a y - s  This i s r e f l e c t e d i n the d i s -  C a r b o n a t e , greywacke and g r e e n -  s t o n e 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 o f t h e c a r b o n a t e - g r e e n s t o n e b r e c c i a o f t h e upper c l a s t i c u n i t , c a r b o n a t e c l a s t s have been s t r o n g l y e l o n g a t e d .  Chevron f o l d s o f one t o two meter a m p l i t u d e a r e  d e v e l o p e d i n r i b b o n c h e r t s i n a few l o c a l i t i e s .  No a x i a l p l a n e c l e a v a g e c u t s  these f o l d s , and t h e i r t i m i n g i s unknown. D e s p i t e t h e extreme d e f o r m a t i o n i n much o f t h e Cache Creek, some p o r t i o n s seem t o have been l e f t c o m p l e t e l y undeformedJ  Greywacke pods, m o s t l y i n t h e  upper c l a s t i c u n i t , a r e u n f o l i a t e d and t h e s e d i m e n t a r y s t r u c t u r e s a r e w e l l p r e s e r v e d , even though t h e pods a r e a p p a r e n t l y surrounded by deformed Some c a r b o n a t e pods a r e s t r o n g l y deformed  phyllite.  while others are e s s e n t i a l l y  pris-  tine. F i g u r e 28 i s an e q u a l a r e a s t e r e o n e t p l o t of a l l s t r u c t u r a l measured i n t h e Cache Creek Group. ments dominates.  elements  A northwest trend of a l l s t r u c t u r a l  ele-  F o l i a t i o n s a r e c l u s t e r e d around a n o r t h w e s t s t r i k e w i t h  a moderate t o s t e e p southwest d i p .  L i n e a t i o n s p l o t along the northwest  trend  51  F i g u r e 28.  x  Bedding (S )  °  Foliation  •  Bedding - cleavage  "  Axis of kink or fold that deforms  A  Axis of chevron fold in chert  Q  (S, ) intersection  lineation S,  (l ) f  0 ) 2  E q u a l a r e a s t e r e o n e t 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  subparallel  f o l d i n g which i s o n l y l o c a l l y observed and deforms t h e e a r l i e r f o l i a t i o n and lineation.  T h i s event c o n s i s t s o f b r i t t l e f o l d i n g and w a r p i n g w i t h some  f r a c t u r i n g a l o n g a x i a l p l a n e s b u t no development o f 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 .  F o l d s a r e f l e x u r a l - s l i p , open, and o f s m a l l ( l e s s than h a l f  a meter) a m p l i t u d e and wavelength.  The b e s t exposure o f these f o l d s and t h e i r  r e l a t i o n s h i p t o t h e 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 o f t h e f a u l t c o n t a c t w i t h t h e N i c o l a Group a t t h e n o r t h end o f t h e a r e a . be r e l a t e d t o movement a l o n g t h a t f a u l t .  The f o l d s may  Chevron f o l d s deforming t h e p h y l l i t e  at t h e s o u t h e r n end o f t h e map a r e a ( F i g u r e 11) a r e s i m i l a r and may be o f the same age. M a c r o s c o p i c a l l y t h e Cache Creek Group appears t o be composed o f f a u l t bounded packages o f r o c k .  Q u i t e c l e a r l y a l o w a n g l e f a u l t i s t h e l o w e r con-  t a c t o f t h e greenstone u n i t .  The u n i t i s t r u n c a t e d and a s t r o n g 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 o f t h e lowermost greenstones the uppermost members o f t h e l o w e r c l a s t i c u n i t below. the f o l i a t i o n i n t h e lower c l a s t i c u n i t .  ( F i g u r e 23) and  This f a u l t  parallels  Further evidence f o r the f a u l t i s  the p r e s e n c e o f s e r p e n t i n i t e a l o n g t h e p r o j e c t e d t r a c e t o t h e n o r t h .  It is  p o s s i b l e t h a t t h e two c l a s t i c u n i t s s h o u l d be c o n s i d e r e d as r e p e t i t i o n o f a s i n g l e u n i t e n c l o s i n g t h e greenstone u n i t as a f a u l t - b o u n d e d wedge.  The  d i s c o n t i n u o u s 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 t h e l i m e s t o n e s and i n degree o f d e f o r m a t i o n and a l t e r a t i o n between t h e upper and lower c l a s t i c u n i t s 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 large displacement.  sup-  The o n l y e v i -  dence f o r t h e f a u l t mapped as t h e upper c o n t a c t o f t h e greenstone u n i t i s t h e t r u n c a t i o n o f t h e greenstones a t t h e i r n o r t h e r n end. I t i s q u i t e p r o b a b l e t h a t t h e r e a r e s i m i l a r f a u l t s throughout t h e 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 n o r t h and e a s t o f  Venables Lake i s p r o b a b l y 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 v a r i a b i l i t y i n deformation within a small region. Most of the Cache Creek within the f i e l d area i s typified by blocks of d i f f e r ent lithology set i n 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 i s considered as a block within the deformed sediments,, The next largest block i s 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 serpent i n i t e bodies i n close proximity to shallow water carbonates that are mixed into the deformed p e l i t i c matrix containing turbidite deposited c l a s t i c beds and bedded chert. The explanation i s more l i k e l y tectonic than sedimentary, At least two periods of faulting postdate i s o c l i n a l folding, shearing, and low angle faulting.  The near v e r t i c a l , north-northwest trending Martel Fault  which separates Cache Creek and Nicola Groups i s the earliest fault after the i s o c l i n a l 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 i n 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 i n the Cache Creek. One major slide that i s quite clear on areal photographs and i n the topography i s shown on the map  (Figure 6), Rocks  exposed i n the slide are are limited to ground up p h y l l i t e and serpentinite.  54 Metamorphism ' .' The :matrix of greywackes and a l t e r a t i o n of u n s t a b l e g l a s s and f e l d s p a r i n b a s i c v o l c a n i c r o c k s b e s t r e f l e c t metamorphic c o n d i t i o n s a t low T a b l e I I I l i s t s seven metamorphic m i n e r a l assemblages observed rocks.  Assemblages 1-6  i n Cache Creek  a r e from r o c k s 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 greenstone u n i t .  temperatures.  ( P v c l ) , and assemblage 7 i s from the  For d e t e r m i n a t i o n of temperature t h e c r i t i c a l m i n e r a l s  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 e p i d o t e f a m i l y .  Stilpnomelane  the s o d i c amphibole a r e t h e o n l y m i n e r a l s t h a t a s s i s t i n p r e s s u r e  and  estimates.  M i y a s h i r o (1973), i n r e v i e w i n g s e v e r a l low grade metamorphic b e l t s a sequence of zones from z e o l i t e to g r e e n s c h i s t f a c i e s .  are  describes  In K i i Peninsula,  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 , e p i d o t e , 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 d i s a p p e a r a n c e and i n Zone IV, g r e e n s c h i s t f a c i e s , p u m p e l l y i t e i s absent and i s present.  The  of p r e h n i t e ,  stilpnomelane  t r a n s i t i o n zone i n the Panoche and Pacheco Pass areas  C a l i f o r n i a c o n t a i n s 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 , ••>. e p i d o t e and s t i l p n o m e l a n e .  of :  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 T a b l e 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 coe x i s t i n g , i s p r o b a b l y on the boundary between the two zones. d e f i n i t i o n assemblages 1 and 6 f a l l i n the g r e e r i s c h i s t  facies.  By M i y a s h i r o ' 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 v e r y low grade t o low grade r o c k s  ( g r e e n s c h i s t 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.  F i g u r e 29 summarizes e x p e r i m e n t a l l y determined r e a c t i o n  e q u i l i b r i a a t v e r y low grade.  The r e a c t i o n p u m p e l l y i t e + c h l o r i t e +  quartz  TABLE I I I Metamorphic M i n e r a l Assemblages i n t h e Cache Creek Gro  Assemblage # Quartz  X  Chlorite  X  X  X  X  Albite  X  X  X  X X  Calcite  X X  Sphene  X  White m i c a  X  Epidote  X  X  Stilpnomelane  X  X  Actinolite  X  X X  X  X  X X  X  X  6-18-1 6- 18-3 7- 8-2 7-22-5 7-5-4 7-5-3B 6-18-6  X  X  X  X  X  X  X  X X X  X  X  X X  X  X  X X X X  Prehnite  Sample number  X  X X  S u b c a l c i c Amphibole Pumpellyite  X  X X  Assemblage number 1 2 3 4 5 6 7  X  56  F i g u r e 29.  P - T e q u i l i b r i a a t v e r y low grade ( a f t e r W i n k l e r , 1974; p . 1 8 8 , 236). Mineral abbreviations are Px-pyroxene, Qz-quartz, Pu-pumpellyite, 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 , Ep-epidote, Pr-prehnite.  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 Creek r o c k s i n V e n a b l e s V a l l e y . t u r e s i d e of t h a t r e a c t i o n , and  f o r Cache  Assemblage 5 c l e a r l y l i e s on t h e low tempera1 and 7 a r e on t h e low temperature  s i d e o f 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 t o p u m p e l l y i t e + a c t i n o l i t e + q u a r t z .  Iron  r i c h e p i d o t e r a t h e r than z o i s i t e i n o t h e r assemblages i n d i c a t e s t h o s e r o c k s a l s o a r e below the boundary between r o c k s of low grade and v e r y low  grade.  I n a more r e c e n t work, Brown (1977a) p r e s e n t s 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 p r e s s u r e and temperature  range.  Assumptions of f i x e d c o m p o s i t i o n s  f o r many  of these m i n e r a l s were n e c e s s a r y 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 e n l a r g e s t h e a r e a of i n t e r e s t  from F i g u r e 29 and i n c l u d e s r e a c t i o n s from Brown. Table I I I can be g i v e n e s t i m a t e d P-T  S e v e r a l assemblages from  f i e l d s based on these r e a c t i o n s .  b l a g e s 3 and 7, d i s c u s s e d e a r l i e r , a r e "plotted below the p r e h n i t e + breakdown, w i t h 7 a t h i g h e r temperature  due t o t h e presence  Assem-  chlorite  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 e p i d o t e i n assemblage 1 p u t s i t below c a l c i t e + e p i d o t e + p u m p e l l y i t e + a c t i n o l i t e + H^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 p u m p e l l y i t e and a c t i n o l i t e i n 5 p u t s them above the same r e a c t i o n . P r e s s u r e c o n d i t i o n s a r e 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 h i g h p r e s s u r e as i t i s r a r e i n mediun p r e s s u r e r o c k s and v e r y r a r e i n r o c k s of low p r e s s u r e metamorphism ( M i y a s h i r o , 1973).  S o d i c amphibole a l s o can suggest h i g h p r e s s u r e c o n d i t i o n s , but o n l y  the a d d i t i o n a l presence pressure.  Lawsonite  o f a r a g o n i t e o r lawsoraite i s d i a g n o s t i c f o r h i g h  i s more p r e v a l e n t i n A l - r i c h r o c k s ; those c o n t a i n i n g the  b l u e amphiboles h e r e a r e b a s i c i n c o m p o s i t i o n , p o s s i b l y even s i l i c a w h i c h f a v o r s b l u e amphibole a t lower p r e s s u r e .  deficient,  Brown (1977b) p r e s e n t s a  t e n t a t i v e c o r r e l a t i o n o f p r e s s u r e t o c r o s s i t e component i n Ca-amphiboles.  In  h i s model t h e s o d i c amphiboles of t h i s s t u d y , c o n t a i n i n g a p p r o x i m a t e l y one  Na  i n the M  A  s i t e , would occur a t p r e s s u r e s of about 6 kb.  These p r e s s u r e s  are  200  F i g u r e 30.  250  300  350  400  E n l a r g e d P - T e q u i l i b r i a from F i g u r e 29 w i t h r e a c t i o n s from Brown (1977a). M i n e r a l a b b r e v i a t i o n s as on F i g u r e 29 w i t h 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 t h e presence of i r o n o x i d e i n t h e r e a c t i o n assemblage; i r o n o x i d e i s absent i n t h e p r e s e n t s t u d y .  S i m i l a r amphiboles  found i n t h e N o r t h -  e r n New C a l e d o n i a melange zone a r e i e s t l m a t e ' d t o have formed a t P-T. c o n d i t i o n s of  350°C  and .7 kb ( B l a c k and B r o t h e r s , 1978), b u t 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 . Study.  That i n d i c a t e s h i g h e r p r e s s u r e than i n t h e p r e s e n t  Thus an e s t i m a t e o f 5 t o 6 kb p r e s s u r e f o r r o c k s i n t h e Venables  a r e a c o n t a i n i n g s o d i c amphiboles  Valley  and s t i l p n o m e l a n e i s c o n s i d e r e d r e a s o n a b l e .  The assemblages of T a b l e I I I a r e p l o t t e d on F i g u r e 30 a t ..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 F i g u r e 30 i t i s apparent throughout  t h a t metamorphic c o n d i t i o n s a r e n o t c o n s t a n t  even t h i s s m a l l a r e a of Cache Creek r o c k s .  Different blocks within  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 t o 2 kb p r e s s u r e d i f f e r e n c e .  N i c o l a Group In units.  t h e Venables V a l l e y a r e a t h e N i c o l a Group can be d i v i d e d i n t o t h r e e map The f i r s t , d o m i n a n t l y g r e e n s t o n e s , c o n t a i n s a n d e s i t e , 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 b o d i e s and sedimentary rocks.  r o c k s d e r i v e d from t h e v o l c a n i c  T h i c k , m a s s i v e t o bedded l i m e s t o n e makes up t h e second map u n i t and t h e  t h i r d i s a m i x t u r e o f sedimentary  r o c k s and some g r e e n s t o n e s .  The t h i r d  unit  i n c l u d e s a r g i l l i t e , t h i n bedded l i m e s t o n e , minor v o l c a n i c r o c k s , abundant green c h e r t ( s i l i c e o u s t u f f ) and c o a r s e r c l a s t i c r o c k s .  F i g u r e 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 o f t h e N i c o l a and o v e r l y i n g J u r a s s i c r o c k s . The n o r t h e r n exposure o f t h e second u n i t was mapped as N i c o l a , " but most o f 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 p a r t o f t h e Cache Creek Group ( C a r r , 1962; D u f f e l l and McTaggart, 1952).  Comparison of s t r a t i -  g r a p h i c , 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 t h e Cache Creek Group, l i m i t e d f o s s i l e v i d e n c e and a Rb/Sr i s o c h r o n s u p p o r t an Upper T r i a s s i c age t y p i c a l of t h e N i c o l a Group f o r t h i s s e c t i o n .  ERODE D  ASHCROFT  NICOLA  Carbonate  ^  Carbonate  with  Calcareous  t u p  chert  nodules  sandstone  Sandstone Shale  or argillite  Conglomerate Clasts  of intrusive  Intrusive, Siliceous Flows, ;„yy, p  quartz-eye  green  tuffs  andesite  to  Undivided Fossil  mainly  origin porphyry  dacite  greenstones  locality  Unconformity v^r^ meters  contact  300  L  Figure  Sheared  0  31.  Diagrammatic s t r a t i g r a p h i c Ashcroft  Formation.  s e c t i o n of  t h e N i c o l a Group and  61 Greenstone U n i t (TRv on F i g u r e 6) F l o w s , i n t r u s i v e r o c k s , p y r o c l a s t i c and e p i c l a s t i c r o c k s of t h i s u n i t a r e complexly  i n t e r b e d d e d and i n t e r f i n g e r i n g as s k e t c h e d on F i g u r e 31,  l i t h o l o g i e s appear t o have l i m i t e d l a t e r a l e x t e n t .  Individual  Poor exposure and minor  f a u l t s c o n t r i b u t e t o 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 often d i f f i c u l t  to d i s t i n g u i s h f r a g m e n t a l t e x t u r e s i n t h e f i e l d u n l e s s a c l e a n , smooth-weathered s u r f a c e i s p r e s e n t .  Even then r o c k s types can be ambiguous,  P o r p h y r i t i c a n d e s i t e u n d e r l i e s much of the e a s t e r n p a r t of the a r e a i s b e s t exposed i n g u l l i e s above the Trans-Canada Highway.  and  In outcrop  the  a n d e s i t e i s . b l u e green t o grey w i t h w h i t e t o p i n k i s h , s u b h e d r a l to g l o b u l a r feldspar phenocrysts.  The a p h a n i t i c groundmass c o n t a i n s s e g r e g a t i o n s of  e p i d o t e or i s cut by e p i d o t e 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 f l o w tops  were not observed, but o c c a s i o n a l l y l a r g e b l o c k s of sedimentary  rocks  (one  oyer a meter l o n g ) a r e caught up i n f l o w s . 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 ^  p r e s e r v e d i n some g r a i n s , a r e seen t o be dominantly c o n s i d e r a b l e e p i d o t e and s e r i c i t e replacement.  composition  a l b i t e i n composition w i t h  The p h e n o c r y s t s  comprise  an  average of 20% o f the r o c k and s i t i n a v e r y f i n e g r a i n e d , 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 , m a i n l y a l b i t e w i t h t r a c e s of K - f e l d s p a r , and o t h e r a l t e r a t i o n m i n e r a l s r-. c h l o r i t e , s e r i c i t e , e p i d o t e , and sometimes calcite.  No p r i m a r y m a f i c m i n e r a l s a r e p r e s e r v e d ,  Though most o f t h e a n d e s i t e s a r e not f o l i a t e d , t h e r e i s c o n s i d e r a b l e evidence of s t r a i n i n the r o c k s . p l e n t i f u l , and p h e n o c r y s t s old  and new  g r a i n margins,  Kinked twins i n p l a g i o c l a s e g r a i n s are  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  L o c a l l y a weak f r a c t u r e c l e a v a g e i s d e v e l o p e d ,  D a c i t i c r o c k s a r e found throughout  the greenstone  u n i t but a r e most plenti<-  f u l and b e s t exposed a l o n g the r i d g e e a s t of V e n a b l e s V a l l e y , r o c k s of t h i s c o m p o s i t i o n a r e e a s i l y i d e n t i f i e d by t h e presence  In the  field,  of abundant  q u a r t z , u s u a l l y i n the form of s p h e r i c a l t o 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 ( q u a r t z e y e s ) , 1^6 mm  i n diameter.  more p l e n t i f u l than the q u a r t z eyes.  F e l d s p a r p h e n o c r y s t s a r e absent  to  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 . i s b l e a c h e d t o v e r y p a l e green o r w h i t e .  These bleadhed  L o c a l l y t h e groundmass zones a r e 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 . R a t i o s of p h e n o c r y s t s  t o groundmass, s i z e of p h e n o c r y s t s , and o t h e r macro-  s c o p i c 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 p r o c e s s e s i n v o l v e d i n the f o r m a t i o n of t h e s e q u a r t z r i c h r o c k s .  Numerous t e x t u r e s can be  by t r a v e r s i n g t h e r i d g e e a s t of V e n a b l e s V a l l e y . 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 o u t . q u a r t z - e y e porphyry  and a r e surrounded  J u s t s o u t h of t h e r o a d c r o s s -  Fragments a r e almost  exclusively  by a m a t r i x o f s i m i l a r m a t e r i a l - q u a r t z  c r y s t a l s and green a p h a n i t i c matrix/groundmass. b r e c c i a d e p o s i t e d near a v o l c a n i c v e n t . deposits r e l a t e d to e x p l o s i v e volcanism. r o c k s a r e u s u a l l y l e s s than 2 mm  observed  T h i s may  be an e x p l o s i o n  S i l i c e o u s t u f f s r e p r e s e n t more d i s t a n t Quartz c r y s t a l s i n t h e t u f f a c e o u s  a c r o s s and f e l d s p a r i s a b s e n t l  Aphanitic  groundmass comprises more than 95% of t h e s e r o c k s , and i n many p l a c e s they m e g a s c o p i c a l l y l i k e p a l e green c h e r t w i t h s c a t t e r e d round q u a r t z c r y s t a l s  look 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 . F a r t h e r n o r t h a l o n g the r i d g e b o t h f e l d s p a r and q u a r t z p h e n o c r y s t s p r e s e n t and make up a l a r g e r percentage  of the r o c k .  are  S c a t t e r e d fragments of  q u a r t z and q u a r t z - f e l d s p a r p o r p h y r i e s o c c u r i n t h e s e r o c k s , but the g e n e r a l appearance i s of f l o w s 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 n o r t h r o c k s of  a p p a r e n t l y t h e some c o m p o s i t i o n a r e 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  n o r t h of t h e n o r t h e r n edge of the f i e l d a r e a , a c o a r s e l y c r y s t a l l i n e q u a r t z . d i o r i t e c r o p s . o u t . M a f i c s a r e a l t e r e d t o c h l o r i t e , and e p i d o t e i s abundant.  This  q u a r t z d i o r i t e i s p r o b a b l y t h e i n t r u s i v e e q u i v a l e n t 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 r o c k s show i n an s m a l l a r e a  t h e v o l c a n o ^ - p l u t o n i c n a t u r e of the N i c o l a Group. A l o n g 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 o v e r p r i n t e d by a shear  63 foliation.  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.  wraps around the q u a r t z p h e n o c r y s t s  and fragments.  The  foliation  Rocks w i t h t h i s  foliation  have a r a t h e r lumpy appearance. In  t h i n s e c t i o n s o f the d a c i t i c r o c k s the round t o e l l i p t i c a l q u a r t z pheno-  c r y s t s can be seen to have p a r t i a l l y r e s o r b e d margins and some q u a r t z o v e r growths.  I n more deformed samples the c r y s t a l s a r e i n t e r n a l l y s h a t t e r e d .  F e l d s p a r p h e n o c r y s t s , 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 o f andesine  (An^) .  composition  F e l d s p a r 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 o r pseudomorphed  by e p i d o t e , c a l c i t e and f i n e g r a i n e d a l b i t e .  The groundmass, much the same as  t h a t i n the a n d e s i t e s , i s a mosaic o f f e l d s p a r , q u a r t z , c h l o r i t e and  sericite  w i t h v a r i a b l e amounts o f e p i d o t e , c a l c i t e , K - f e l d s p a r and some sphene. a l t e r e d v a r i e t i e s c o n t a i n more K - f e l d s p a r . patches o f 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 .  More  I n c l u d e d fragments show up  as  M a r g i n s o f the fragments are i n -  distinct. E p i c l a s t i c r o c k s d e r i v e d from the v o l c a n i c and i n t r u s i v e r o c k s make up remainder o f the greenstone  unit.  wacke, l i t h i c sandstone and  siltstone.  One  These i n c l u d e conglomerate,  the  b r e c c i a , grey-  o f the b e s t conglomerate exposures i s a l o n g the Trans-Canada Highway  j u s t n o r t h o f the g r e e n s t o n e - l i m e s t o n e  contact.  W e l l rounded c o b b l e s o f  p o r p h y r i t i c a n d e s i t e 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 g r a i n e d , feldspar r i c h matrix. t a n t to w e a t h e r i n g  The r o c k s have been a l t e r e d and the m a t r i x i s as  as the v o l c a n i c c o b b l e s .  patches w i t h i n b o t h c l a s t s and m a t r i x . w i t h i n the greenstone  Another conglomerate,  and  the o n l y  one  u n i t c o n t a i n i n g c l a s t s d e r i v e d from a n o n - v o l c a n i c  caps the 2700 f o o t h i g h h i l l n o r t h w e s t There 2-3  Epidote occurs i n v e i n s  resis-  source,  of the l i m e s t o n e - g r e e n s t o n e  contact.  cm l o n g fragments a r e l i m e s t o n e , b l a c k p e l i t i c r o c k s and  abundant  l i g h t green to w h i t e t u f f .  The m a t r i x i s q u a r t z and  f e l d s p a r r i c h and  well  indurated. V o l c a n i c b r e c c i a s are i n t e r b e d d e d w i t h the a n d e s i t e s and d a c i t e s .  The  64 a n g u l a r to subrounded fragments a r e m a i n l y p i e c e s of v o l c a n i c r o c k s eye p o r p h y r y , f e l d s p a r p o r p h y r y , s i l i c i f i e d  quartz-  t u f f , a p h a n i t i c greenstone  e l o n g a t e p i e c e s of green sandstone and greywacke a r e a l s o p r e s e n t . a r e p o o r l y s o r t e d but have l i t t l e v e r y f i n e m a t e r i a l .  >- but  The b r e c c i a s  Most o f t h e fragments  a r e a t 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 a c r o s s . " I n b o t h conglomerates in  and b r e c c i a s c l a s t s do not weather out  relief. F i n e r g r a i n e d e p i c l a s t i c r o c k s , sandstones  and greywacke, a r e u s u a l l y d a r k e r  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 a r e f a i r l y l i g h t green i n color.  M i c r o s c o p i c e x a m i n a t i o n shows l i t h i c fragments a r e c l e a r l y d e r i v e d  from v o l c a n i c r o c k s i n the N i c o l a Group.  Round q u a r t z g r a i n s a r e p l e n t i f u l ;  t h e r e 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 p i d o t e , and  s e r i c i t e r e p l a c e t h e f e l d s p a r s , and c h l o r i t e and v e r y f i n e f e l d s p a r comprise the m a t r i x .  No metamorphic m i n e r a l s 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  a r e p r e s e n t i n any of the greenstones C o n t a c t s between the greenstone  studied.  u n i t and o t h e r u n i t s a r e exposed i n o n l y  two p l a c e s , b o t h w i t h t h e o v e r l y i n g l i m e s t o n e . That c o n t a c t appears t o be d e p o s i t i o n a l , as bedding the c o n t a c t . bedding pits.  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  Whether t h i s i s conformable  i n the greenstones.  i s unknown because of l a c k of  Exposures of the c o n t a c t a r e i n two p r o s p e c t i n g  I n b o t h p i t s the r o c k s a r e sheared p a r a l l e l t o the c o n t a c t .  forms a s c h i s t o s i t y i n t h e greenstone.  This shearing probably represents  movement a l o n g a d e p o s i t i o n a l c o n t a c t r a t h e r than a l a r g e f a u l t A l l o t h e r e x t e r n a l c o n t a c t s of t h e greenstone  Limestone U n i t Two  Chlorite later  displacement.  u n i t are s t e e p l y dipping f a u l t s .  (TRls)  s u b s t a n t i a l b o d i e s o f m a s s i v e l i m e s t o n e have been d i s t i n g u i s h e d as  a s e p a r a t e map Venables Creek.  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 t h e h i l l n o r t h e a s t of  The second forms c l i f f s i n t h e next c r e e k s o u t h .  They a r e  65 mapped on t h e b a s i s o f l i t h o l o g y and a r e n o t n e c e s s a r i l y o f p r e c i s e l y t h e same age o r 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 t h e n o r t h e r n body appears t o p i n c h out towards t h e e a s t . The  l i m e s t o n e i s l i g h t grey w e a t h e r i n g  surface. banded.  Much i s m a s s i v e ,  and dark b l u e t o b l a c k on a f r e s h  f i n e t o medium g r a i n e d , b u t some i s ' l i g h t and dark  C o m p o s i t i o n a l v a r i a t i o n s i n c l u d e a range from almost pure  t o v e r y sandy l i m e s t o n e . brown w e a t h e r i n g  carbonate  S c a t t e r e d through much o f these l i m e s t o n e s a r e r e d -  chert nodules.  I n some p l a c e s t h e nodules a r e a l i g n e d and  may r e p r e s e n t o r i g i n a l i n t e r b e d d e d l a y e r s , 2-3 cm t h i c k .  These l a y e r s a r e  now d i s r u p t e d and boudinaged and o c c a s i o n a l l y form s m a l l f o l d hooks o f c h e r t f l o a t i n g i n carbonate. Dark grey l a y e r s i n t h e f i n e g r a i n e d , l a m i n a t e d v a r i e t i e s a r e due t o c o n c e n t r a t i o n s o f carbonaceous m a t e r i a l .  As much as 15 t o 20% o f t h e banded  l i m e s t o n e s i s q u a r t z , u s u a l l y o f t h e same g r a i n s i z e as t h e carbonate b u t o c c a s i o n a l l y i n patches w h i c h 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 q u a r t z g r a i n s remain, b u t t h e margins o f those g r a i n s a r e p o l y gonalized. The  l i m e s t o n e u n i t i n c l u d e s c a r b o n a t e sands, e a s i l y i d e n t i f i e d i n t h i n  section.  D e t r i t a l l i m e s t o n e g r a i n s 1^3 mm a c r o s s have c u r v e d o r bent t w i n s  and a r e d a r k e r i n c o l o r than t h e 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  p a r t o f t h e carbonate sands i s o t h e r d e t r i t u s - q u a r t z and f e l d s p a r g r a i n s , u s u a l l y much s m a l l e r than t h e carbonate  fragments.  B i o c l a s t i c l i m e s t o n e s a r e p r e s e n t i n t h e n o r t h e r n l i m e s t o n e mass down i n V e n a b l e s Creek.  There l i m e s t o n e s 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.  S e v e r a l samples from  d i f f e r e n t s i t e s i n t h e u n i t were d i s s o l v e d i n an attempt w i t h no s u c c e s s .  t o s e p a r a t e conodonts  The g e n e r a l l e n s o i d shape and t h e v a r i e t y o f l i t h o l o g i e s -  c a l c a r e n i t e s t o b i o c l a s t i c t o m i c r i t e w i t h areas o f o r g a n i c laminae >- suggest a probable reef-shelf, o r i g i n .  66 The  lower c o n t a c t o f t h e n o r t h e r n l i m e s t o n e has a l r e a d y been d e s c r i b e d as  a sheared d e p o s i t i o n a l c o n t a c t .  The b a s a l c o n t a c t of t h e s o u t h e r n  mass i s a f a u l t a g a i n s t younger r o c k s .  limestone  The t r a n s i t i o n upwards from  r e l a t i v e l y pure l i m e s t o n e t o i n t e r b e d d e d c l a s t i c r o c k s , c h e r t , and l i m e s t o n e of t h e 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 b o t h l i m e s t o n e b o d i e s .  Mixed-Sedimentary and V o l c a n i c Rocks (TRs, T R s l , TRsv) The remainder o f t h e N i c o l a Group i s lumped as one u n i t .  Most of t h e  l i t h o l o g i e s found i n t h e o t h e r u n i t s r- l i m e s t o n e , 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 r o c k s , some a n d e s i t e , conglomerate - a r e found i n t h i s u n i t . Black a r g i l l i t e i s also present. T y p i c a l of much of t h e upper u n i t i s i n t e r b e d d e d l i m e s t o n e and The greenstone ably t u f f .  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 p a l e green i n c o l o r ,  Beds range from 5-15 cm i n t h i c k n e s s and t u f f a c e o u s  prob-  greenstone  l a y e r s a r e commonly l a m i n a t e d . Limestone l a y e r s a r e u s u a l l y medium grey weathering  except i n patches o f s k a r n where they have been a l t e r e d and r e -  crystallized.  N e a r l y w h i t e , sugary t e x t u r e d marbles w i t h c o a r s e g r a i n s i z e  i n these zones a r e a s 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 , greenstones.  bleached  Garnet and e p i d o t e have formed from i n t e r b e d d e d p e l i t i c r o c k s i n  one o f these s k a r n  zones.  Limestone l a y e r s , up t o s e v e r a l meters t h i c k , can sometimes be t r a c e d f o r s e v e r a l hundred meters b e f o r e they p i n c h out o r a r e t r u n c a t e d by f a u l t s , L a y e r i n g w i t h i n t h e s e carbonates  on a 3r5 cm s c a l e i s d e f i n e d by a l t e r -  n a t i n g c o a r s e sandy and f i n e g r a i n e d bands. and may show g r a d i n g .  Coarse l a y e r s can be b i o c l a s t i c  One of t h e b e s t exposures o f 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 t h e highway. l i m e s t o n e s and c a l c a r e n i t e s a r e i n t e r b e d d e d w i t h a few t u f f  There, bedded  layers.  Greenstones i n t h e upper u n i t a r e muctT'the same as t h o s e a l r e a d y d e s c r i b e d i n t h e lower u n i t .  Some -more e x t e n s i v e and b e t t e r exposed greenstone  bodies  a r e mapped s e p a r a t e l y (TRsv).  P a l e green c h 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 e x c e p t i o n s up t o a few meters t h i c k , i s the most common. c h e r t i s a s i l i c e o u s t u f f , v e r y f i n e g r a i n e d w i t h no p h e n o c r y s t s .  The Quartz,  f e l d s p a r and minor amounts o f c h l o r i t e and s e r i c i t e a r e v i s i b l e i n t h i n s e c tion. Greenstone conglomerates  up t o a meter t h i c k c o n t a i n a n d e s i t e , d a c i t e  and t u f f a c e o u s 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 t h e h i g h l e v e l p l u t o n s d e s c r i b e d i n t h e greenstone s e c t i o n .  G e n e r a l l y the g r a n i t i c fragments a r e v e r y q u a r t z r i c h .  a r e much l e s s a l t e r e d ; p r i m a r y hornblende be d e r i v e d from the Guichon B a t h o l i t h .  i s preserved.  A few  These c l a s t s c o u l d  A conglomerate o f t h i s d e s c r i p t i o n  i s exposed a l o n g the highway south o f Venables Creek. N o r t h o f Venables Creek, above the highway, the lower c l i f f s a r e p r e dominantly  very f r e s h a n d e s i t e .  Euhedral, white f e l d s p a r phenocrysts,  a c r o s s , s i t i n an i n t e r m e d i a t e t o dark green a p h a n i t i c groundmass. more s i l i c e o u s greenstones  1-2 mm  Upwards,  and a l t e r e d a n d e s i t e s g i v e way t o banded sediment '  t a r y r o c k s and minor l i m e s t o n e pods. C l a s t i c r o c k s o t h e r 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 c o n s i d e r a b l e amounts o f conglomerate d e r i v e d from non-greenstone t e r r a n e .  A r g i l l i t e s a r e b l a c k , s i l i c e o u s and w e l l bedded.  Sandstone and greywacke a r e green, v o l c a n i c - d e r i v e d and v e r y s i m i l a r t o those i n the greenstone  unit.  the N i c o l a Group. v o l c a n i c fragments,  Conglomerates r e f l e c t t h e v a r i e t y o f l i t h o l o g i e s i n  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 p u r p l e argillite,  and l i m e s t o n e , w i t h q u a r t z , f e l d s p a r , and  c a l c i t e g r a i n s and a r g i l l a c e o u s m a t e r i a l making up the s m a l l e r f r a c t i o n . where was d e t r i t u s o f Cache Creek o r i g i n  No-  identified.  The upper c o n t a c t o f these bedded r o c k s i s an a n g u l a r u n c o n f o r m i t y w i t h a Lower J u r a s s i c b a s a l conglomerate w h i c h can be c o r r e l a t e d w i t h the A s h c r o f t Formation  to the n o r t h .  The u n c o n f o r m i t y  i s b e s t exposed i n Venables Creek  68 about one k i l o m e t e r upstream from the highway. l i m e s t o n e a r e t r u n c a t e d by t h e  S m a l l , open f o l d s i n bedded  conglomerate.  Age of t h e N i c o l a Group Two  age d e t e r m i n a t i o n s s u p p o r t a M e s o z o i c r a t h e r t h a n P a l e o z o i c age f o r  t h i s map  unit.  A Rb/Sr d a t e was  o b t a i n e d on a s u i t e of 6 samples from t h e  greenstone u n i t . Four of t h e s e a r e from w i t h i n the f i e l d a r e a and two a r e from t h e q u a r t z d i o r i t e j u s t n o r t h of t h e f i e l d a r e a .  T a b l e IV l i s t s  analytical  d a t a f o r t h e samples.  They have a good spread i n Rb/Sr r a t i o s arid d e f i n e an  i s o c h r o n (Figure 32).  The c a l c u l a t e d d a t e i s 196 + 15 Ma w i t h an  initial  r a t i o of .7043 + 2 somewhat d i f f e r e n t from v a l u e s o b t a i n e d from t h e Guichon B a t h o l i t h , 205 + 10 and ,7034 (R.L.Armstrong, p e r s comm. 1978).  The  initial  r a t i o , .7043, i s s l i g h t l y h i g h e r than t h e mean v a l u e f o r a r c s (Faure and P o w e l l , 1972) and may  i n d i c a t e some c o n t a m i n a t i o n v e r y l i k e l y w i t h Sr from  seawater. A 196 Ma age f o r t h e N i c o l a i s a b i t younger than expected f o r time of f o r m a t i o n as the N i c o l a i s dated by f o s s i l s as K a r n i a n to N o r i a n . i s o c h r o n d a t e may  r e f l e c t the hydrothermal a l t e r a t i o n event,  The  A Late T r i a s s i c  age of f o r m a t i o n 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 r e a s o n a b l e . Only one f o s s i l c o l l e c t i o n was  o b t a i n e d from t h e N i c o l a .  I t includes  m i c r o f a u n a from the bedded l i m e s t o n e s below the u n c o n f o r m i t y i n Venables  Creek,  I n a c o l l e c t i o n of conodont f r a g m e n t s , B.E.B. Cameron of the G e o l o g i c a l Survey o f Canada i d e n t i f i e d one specimen  of E n a n t i o g n a t h u s z i e g l e r i  ( D i e b e l ) which  ranges i n age from lower T r i a s s i c t o Upper K a r n i a n (Mosher, 1968) and t h r e e specimens  of Neospathodus sp. c f . N_. newpassensis Mosher found i n Lower K a r n i a n ,  p o s s i b l y r a n g i n g down to lower M i d d l e T r i a s s i c the assemblage a p r o b a b l e Upper T r i a s s i c  D e p o s i t i o n a l Environment  (Mosher, 1968).  Cameron a s s i g n s  ( K a r n i a n ) age.  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  Rb^ 86  Sr?>86 /Sr  0.485  1.404  .7079  24.8  0.273  0.790  .7062  82.1  13.5  0.164  0.476  .7063  23.2  58.0  2.49  7.22  .7244  121°22'50"  126  7.9  0.062  0.181  .7047  121°22*50"  170  12.7  0.075  •0.216  .7046  Sample #  Lithology  Locality  Latitude  Longitude  ppm S r  ppm Rb  Rb/Sr  6-5-4  Dacite  V  50°32'37"  121°19'5"  68.0  33.0  6-5-2  Altered dactie  VI  50°32'27"  121°19'1"  91.0  11-4-1  Quartz d i o r i t e  VII  50 33'7  121°17'48"  6-13-4  Altered  VIII  50°32'39"  121°17'50  4-141  I n c l u s i o n i n 4-14R  o f f map  50°40'40  4-14R  Quartz d i o r i t e  o f f map  50°40'40"  dacite  O  M  ,r:  n  r  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 of 1,42 x 10 ^ y . l  Sr/ Sr ratio i s 0.00013 (1 a),  Rb/Sr dates are based on a Rb decay constant  The regressions are calculated according to the technique of York (1967),  70  71  carbonate s h e l f t o r e e f f a c i e s r o c k s of the N i c o l a Group a r e t y p i c a l of a volcanic island arc.  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 p r o b a b l y r e f l e c t s m a l l b a s i n s and r a p i d , i r r e g u l a r p e r i o d s of u p l i f t .  No s u b a r e a l v o l c a n i c r o c k s a r e p r e s e n t w i t h i n the area,, but  i n conglomerates conglomerates  r e f l e c t s u b a r e a l v o l c a n i s m nearby.  suggest  clasts  Rounded b o u l d e r s i n some  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 t u f f a c e o u s l a y e r s , i n t e r r u p t e d carbonate s e d i m e n t a t i o n . reef-derived  T u r b i d i t y c u r r e n t s r e d e p o s i t e d c l a s t i c sediments and  fragments.  Structure F a u l t i n g i s the dominant d e f o r m a t i o n of t h e N i c o l a Group. trend north-northwest w i t h normal o f f s e t .  and a r e v e r t i c a l .  Major f a u l t s  Most of these appear t o b,e b l o c k f a u l t s  The o t h e r major f a u l t i s the M a r t e l F a u l t w h i c h forms  t h e 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 t o i t i s a f r a c t u r e c l e a v a g e t o s c h i s t o s i t y i n the N i c o l a greenstone.  F o l i a t i o n i s weakly developed  o r absent o u t s i d e f a u l t  zones.  F i g u r e 33 p r e s e n t s s t r u c t u r a l Elements measured i n the N i c o l a . a t t i t u d e s a r e northwest  t r e n d i n g and m o d e r a t e l y  Bedding  southwest d i p p i n g .  Small  folds,  u s u a l l y drag f o l d s a l o n g minor f a u l t s o r d u c t i l e f o l d s i n l i m e s t o n e , have a f a i r l y c o n s i s t e n t northwest-southeast No a x i a l p l a n e f o l i a t i o n i s developed.  t r e n d w i t h moderate t o s h a l l o w  plunges.  T h i s southwest t i l t i n g of the N i c o l a  s e c t i o n c o u l d be a r e s u l t o f i n t r u s i o n of the Guichon B a t h o l i t h .  To  the  e a s t a c r o s s 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 d i p t o t h e west away from the Guichon c o n t a c t .  However.^ the Lower J u r a s s i c A s h c r o f t F o r m a t i o n i s  a l s o t i l t e d t o t h e southwest,  so t h i s t i l t i n g may  be p o s t lower  On t h e f l a t i r o n t o t h e e a s t , west v e r g e n t f o l d s a r e developed meter-.thick bedded carbonate a r e not f o l d e d .  Jurassic. i n several-  l a y e r s ; c o n t a c t s w i t h over and u n d e r l y i n g r o c k s  A p p a r e n t l y t h e c a r b o n a t e behaved i n a d u c t i l e f a s h i o n under  72  Bedding Foliation — limestone Foliation — greenstone Fold  F i g u r e 33.  and clastic units unit  axes  Equal area stereonet p l o t of s t r u c t u r a l the N i c o l a Group.  elements i n  73 s h e a r s t r e s s w h i c h has a l l o w e d t h e o v e r l y i n g s e c t i o n t o move downslope to  the rocks underlying the carbonate.  relative  Movement was taken up i n t h e l i m e s t o n e .  Though n o t c l e a r l y demonstrated i n the p r e s e n t a r e a , f o l d s i n the l i m e s t o n e s are  thought t o be o f s i m i l a r  origin.  Alteration Secondary m i n e r a l s w i t h i n the greenstones i n c l u d e e p i d o t e , a l b i t e , K - f e l d s p a r , and c h l o r i t e . usually pyritized.  calcite,  Some areas a r e s t r o n g l y s i l i c i f i e d , b l e a c h e d and  A t one l o c a l i t y w i t h i n t h e upper s e d i m e n t a r y u n i t a g a r n e t -  epidote s k a r n i s developed.  These a r e a l l h y d r o t h e r m a l a l t e r a t i o n s p r o b a b l y  r e l a t e d t o f l u i d movement d u r i n g i n t r u s i o n o f 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 t o the f e l d s p a t h i z a t i o n zone i n t h e W a i r a k i geothermal f i e l d o f New Z e a l a n d ( M i y a s h i r o , 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 o f 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 .  T h i s o c c u r s a t temperatures o f 230-250° C.  S m a l l s k a r n s a r e a l s o developed where d i k e s c r o s s c u t l i m e s t o n e l a y e r s . E p i d o t e and more c o a r s e l y c r y s t a l l i n e c a l c i t e r e p l a c e t h e f i n e g r a i n e d grey limestone.  J u r a s s i c and Younger Rocks Ashcroft Formation Rocks o f J u r a s s i c age a r e exposed i n two  s m a l l areas, the l a r g e r along  Venables Creek and the s m a l l e r p a t c h s l i g h t l y n o r t h e a s t ( F i g u r e s 5 and 6 ) . Venables Creek exposure d i s p l a y s a more complete s e c t i o n .  The  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 c o b b l e s unconformably o v e r l i e s f o l d e d N i c o l a sedimentary rocks.  The g r a n i t i c c l a s t s , p r o b a b l y o f Guichon provenance, a r e  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. no Cache Creek d e t r i t u s .  There i s  C l a s t s i n the conglomerate a r e rounded t o subrounded  and range i n s i z e from 15 cm b o u l d e r s to f i n e sand.  The m a t r i x and some sand  74 grains are calcareous.  T h i s conglomerate resembles some o f t h e N i c o l a conglom-  e r a t e s b u t 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 o f a l t e r a t i o n of m a t r i x . within Nicola  C l a s t s weather o u t i n r e l i e f  as opposed t o c l a s t s  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 l i m e s t o n e ,  These a r e i n t e r b e d d e d w i t h and grade i n t o b l a c k s h a l e , common  i n t h e A s h c r o f t B a s i n ( F r e b o l d and T i p p e r , 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 t h e sandy l i m e s t o n e s 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, P o u l t o n o f t h e G e o l o g i c a l Survey o f 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 (?) s p , i n d e t , p l u s T e r e b r a t u l i d b r a c h i o p o d s , 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 p r o b a b l e Lower J u r a s s i c age, The J u r a s s i c r o c k s a r e p r e s e r v e d i n two down-dropped f a u l t b l o c k s . cases t h e w e s t e r n  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 a g a i n s t  massive N i c o l a l i m e s t o n e .  The s t r a t a d i p southwest towards t h e f a u l t s .  f r a c t u r e c l e a v a g e i s developed to  I n both  i n some o f t h e sandstones.  A  I t i s subparallel  t h e f a u l t on t h e west s i d e o f t h e s e c t i o n and p r o b a b l y developed  a t the  time o f f a u l t i n g .  Spences B r i d g e Group The lower Cretaceous  Spences B r i d g e Group i s i n f a u l t c o n t a c t w i t h t h e  Cache Creek Group a l o n g t h e w e s t e r n edge o f t h e f i e l d a r e a .  I t i s very poorly  exposed, b u t t h r e e o u t c r o p s o f z e o l i t i z e d , p o r p h y r i t i c a n d e s i t e s u p p o r t t h e l o c a t i o n o f t h e c o n t a c t drawn by D u f f e l l and McTaggart (1952) and Pearson (1974),  The group i s made up o f a n d e s i t e , d a c i t e and some r h y o l i t e w h i c h  have undergone z e o l i t e a l t e r a t i o n and g e n t l e w a r p i n g 1952).  ( D u f f e l l and McTaggart,  75 Quaternary D e p o s i t s Q u a t e r n a r y r o c k s o f s e v e r a l o r i g i n s c o v e r much o f the f i e l d a r e a . g l a c i a l f l u v i a l deposits f i l l i s shown on the map  the lower Thompson R i v e r v a l l e y .  Terraced  Quaternary  ( F i g u r e 6) o n l y i n areas l a c k i n g o u t c r o p o r mappable  f l o a t of o l d e r r o c k s .  DISCUSSION Cache Creek Group, N i c o l a Group and C o n t a c t R e l a t i o n s h i p s I n t h i s a r e a the Cache Creek Group appears to be a melange.  It is 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 w i t h lower Mesozoic r o c k s .  The  group d i s p l a y s a sheared p e l i t i c  contact  matrix  c o n t a i n i n g b l o c k s o f v a r y i n g s i z e , l i t h o l o g y , age, and metamorphic grade. B l o c k s found w i t h i n the the f i e l d a r e a range i n l e n g t h from a few to g r e a t e r than 5 k i l o m e t e r s .  L i t h o l o g i e s r e p r e s e n t e d as b l o c k s i n c l u d e  s h a l l o w water l i m e s t o n e , a u g i t e p o r p h y r y  f l o w s and t u f f a c e o u s r o c k s , r i b b o n  c h e r t , s e r p e n t i n i t e and c l a s t i c sedimentary p r i m a r i l y d i s c e r n a b l e i n greenstones may  centimeters  rocks.  W i t h i n these b l o c k s ,  and greywackes, metamorphic c o n d i t i o n s  v a r y 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 o f sedimentary There i s abundant evidence o f slumping as c o a r s e r m a t e r i a l was  vs_. t e c t o n i c p r o c e s s e s  i s not  clear.  and d e b r i s f l o w s i n the c l a s t i c  rocks  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 i n v o k e d to e x p l a i n the p r e s e n c e of s h a l l o w w a t e r , r e e f o i d a l l i m e s t o n e masses amongst deeper water c h e r t s (Monger, 1977b). been proposed, the r e e f s a r e developed  I f , as  has  on a v o l c a n i c p i l e , b a s a l t b l o c k s  c o u l d 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 t o e x p l a i n a r e  d i f f e r e n c e s i n metamorphism w i t h i n r o c k s of the same c o m p o s i t i o n , the p r e s e n c e of t r a n s i t i o n a l b l u e s c h i s t grade.  the  especially  There i s some e v i d e n c e o f a  low grade metamorphic source t e r r a n e s i n c e d e t r i t a l p r e h n i t e and p u m p e l l y i t e a r e found. case.  Cache Creek Group r o c k s a r e p r o b a b l y t h e s o u r c e r o c k s  in-this  More h i g h l y metamorphosed r o c k s a r e 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 o t h e r u l t r a m a f i c b o d i e s . The l a r g e u l t r a m a f i c mass t h a t forms the c o n t a c t between the two  clastic  u n i t s i s q u i t e c l e a r l y emplaced a l o n g a f a u l t , but s m a l l e r 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 t h e Cache Creek Group, on.the o t h e r hand, a r e not c h a o t i c as they a r e i n a t y p i c a l melange.  Over most o f the a r e a f o l i a t i o n  maintains  a f a i r l y c o n s t a n t o r i e n t a t i o n , though i t weakens upwards and t o the west. The p o d s - i n - m a t r i x s t y l e does p e r s i s t  throughout.  I n c o n t r a s t , the N i c o l a Group, w h i c h has p r e v i o u s l y been m i s t a k e n l y i d e n t i f i e d as Cache Creek Group i n t h i s a r e a , does not have the same t e c t o n o stratigraphic style.  There a r e 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 o f poor o u t c r o p , some l i t h o l o g i e s l o o k l i k e i s o l a t e d b l o c k s ; but t h e r e i s no sheared m a t r i x .  No r e g i o n a l f o l i a t i o n i s d e v e l o p e d ;  i s l o c a l i z e d a l o n g f a u l t zones. f a u l t s and a r e b e s t developed  Shear f o l i a t i o n and s c h i s t o s i t y  i n r o c k s near the M a r t e l F a u l t .  deformation parallel 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 movement r a t h e r than the wide P-T  ranges found i n the Cache Creek Group.  At o u t c r o p s c a l e c l a s t i c r o c k s i n the N i c o l a Group a r e e a s i l y w i t h weakly deformed Cache Creek r o c k s of the same l i t h o l o g y .  confused  However, i f a  l a r g e enough a r e a 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 s h o u l d c l e a r l y s e p a r a t e the two groups.  rocks  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 d a t a . the v o l c a n i c r o c k s i n the Cache Creek Group f o r  More such d a t a i s needed 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 r o c k s do n e c e s s a r i l y h o l d f o r o t h e r areas of t h e Cache Creek.  not  I n the P i n c h i - S t u a r t  Lake A r e a , P a t e r s o n  (1977b) f i n d s L a t e K a r n i a n t o E a r l y N o r i a n T a k l a sediments  c o n t a i n i n g metamorphic a r a g o n i t e and s p e c u l a t e s t h a t a u g i t e p o r p h y r y v o l c a n i c r o c k s i n v o l v e d i n b l u e s c h i s t metamorphisn c o u l d be c o r r e l a t e d w i t h the abundant a u g i t e p o r p h y r y o f the T a k l a Group.  S i m i l a r v o l c a n i c rocks with l a r g e  augite phenocrysts  a r e common i n the N i c o l a Group o u t s i d e the p r e s e n t  study  area  1970;  the  (Schau, 1968,  Campbell and T i p p e r , 1971;  P r e t o , 1974).  At  same time most o f the v o l c a n i c and v o l c a n i c l a s t i c r o c k s of t h e Cache Creek Group i n . t h i s a r e a a r e a u g i t e p o r p h y r y .  I n f a c t the h i g h e r p r e s s u r e meta-  morphic assemblage i s developed i n a b l o c k o f a u g i r e p o r p h y r y f l o w s and There a r e no d a t a t h a t r e q u i r e the age o f these to be P a l e o z o i c .  tuffs.  They c o u l d  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 d e f o r m a t i o n Travers  and metamorphism.  (1978) f i n d s T r i a s s i c  A l s o o n l y 30 k i l o m e t e r s to the n o r t h  (Ladinian or Karnian)  r a d i o l a r i a i n Cache Creek  cherts. As p r e s e n t l y mapped the two groups a r e s e p a r a t e d Fault.  I t i s now  by a f a u l t , the M a r t e l  v e r t i c a l t o s t e e p l y west d i p p i n g , appears t o t r u n c a t e  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 a r e a s 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.  I n o t h e r l o c a l i t i e s , n o t a b l y the south end of  the  f i e l d a r e a , 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 r o c k s . Its  present  steep d i p c o u l d 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 p o s t  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 t o the southwest, s u b p a r a l l e l to the average d i p i n the Cache Creek. ing  i s removed and  If this  tilt-  the N i c o l a and A s h c r o f t s t r a t a a r e r e t u r n e d t o 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 a g a i n s t N i c o l a i s v e r y s i m i l a r to the P i n c h i F a u l t w h i c h forms the boundary o f the Cache Creek Group i n the P i n c h i - S t u a r t Lake A r e a 1973,  1977b).  Paterson  eastern  (Paterson,  (1977b) s u g g e s t s 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 c u r r e n t movement i n L a t e T r i a s s i c t o E a r l y J u r a s s i c time on t h e P i n c h i F a u l t . I t has been suggested  (P.B. Read, p e r s . comm., 1975) t h a t t h e M a r t e l F a u l t  i s t h e s o u t h e r n e x t e n s i o n o f t h e P i n c h i F a u l t System. I n t h a t case one might propose s t r i k e - s l i p movement on t h e M a r t e l F a u l t i n e a r l y M e s o z o i c time u s i n g Paterson';s i d e a .  However, t h e r e i s no c o n c r e t e e v i d e n c e i n t h e  Venables V a l l e y a r e a t o support t h e p r e s e n c e o f t h e M a r t e l F a u l t i n t h e e a r l y Mesozoic. T e c t o n i c S e t t i n g and H i s t o r y L i t t l e h a r d d a t a a r e a v a i l a b l e from t h i s study on t h e t i m i n g o f e v e n t s i n the type a r e a o f t h e Cache Creek Group. c a o n t a c t s and f o s s i l c o n t r o l .  There i s a l a c k o f s t r a t i g r a p h i c  An approximate time-space p l o t can be drawn  ( F i g u r e 34) by i n c o r p o r a t i n g d a t a from P a t e r s o n and H a r a k a l (1974) f o r t h e time o f b l u e s c h i s t metamorphism, and d a t a from T r a v e r s (1978) f o r e x t e n s i o n o f Cache Creek d e p o s i t i o n up i n t o t h e T r i a s s i c .  Timing o f f a u l t i n g e v e n t s  cannot be w e l l documented b u t may be f a i r l y c o n t i n u o u s from Late' T r i a s s i c t o Tertiary. From M i s s i s s i p p i a n time t o as l a t e as t h e Upper T r i a s s i c t h e Cache Creek o c e a n i c assemblage was f o r m i n g , presumably  on o c e a n i c c r u s t (Monger, 1972,  1975a, 1977b) and d u r i n g t h e P a l e o z o i c i t l a y f a r from N o r t h A m e r i c a .  In  Upper T r i a s s i c ( K a r n i a n ) time t h e N i c o l a a r c 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 r o c k s which a r e thought t o r e p r e s e n t an e a r l i e r a r c (Danner, 1977).  A t t h e same time Cache Creek Group r o c k s were p r o b a b l y b e i n g deformed  and moved toward t h e c r a t o n . foliation.  T h i s produced  t h e i s o c l i n a l f o l d s and p e r v a s i v e  T h r u s t 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 d e f o r m a t i o n p r o b a b l y was  i n i t i a t e d during t h i s Late T r i a s s i c period.  In latest Triassic or early  J u r a s s i c time t h e Guichon B a t h o l i t h i n t r u d e d and p o s s i b l y t i l t e d  Nicola  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 i n 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 f o r 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 r o c k s , b e s t developed a l o n g t h e M a r t e l  F a u l t , i s a p p a r e n t l y t h e same age as h y d r o t h e r m a l a l t e r a t i o n . r o c k s do n o t appear t o have been a f f e c t e d by t h e h y d r o t h e r m a l  Cache Creek event.  One  would expect Cache Creek r o c k s t o be s i m i l a r l y a l t e r e d i f they were a d j a c e n t to t h e N i c o l a a t t h e time t h e b a t h o l i t h was emplaced. F o l l o w i n g i n t r u s i o n o f t h e Guichon,  t h e n e x t event was t h e b e g i n n i n g o f  d e p o s i t i o n o f t h e A s h c r o f t F o r m a t i o n i n Lower J u r a s s i c t i m e , l i k e l y S i n e murian (H.W. T i p p e r , p e r s . comm., 1975).  That a l s o marks t h e time o f expo-  s u r e o f t h e Guichon B a t h o l i t h and l o c a l l y , ..the end o f v o l c a n i s m .  Farther  n o r t h and near t h e N i c o l a t y p e a r e a , N i c o l a v o l c a n i s m c o n t i n u e d i n t o t h e J u r a s s i c (Campbell and T i p p e r , 1971; T i p p e r and R i c h a r d s , 1976; P r e t o , 1974). Timing o f events f o l l o w i n g d e p o s i t i o n o f t h e lowermost A s h c r o f t s t r a t a i s not e a s i l y determined  from t h e p r e s e n t s t u d y .  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 p o s t lower Cretaceous  faulting.  Later folds  i n Cache Creek r o c k s , b r i t t l e f o l d s t h a t  f o l d the e a r l i e r f o l i a t i o n , are  p r o b a b l y r e l a t e d t o l a t e r f a u l t : movement.  V o l c a n i s m was renewed i n e a r l y  C r e t a c e o u s , A p t i a n time ( D u f f e l l and McTaggart, 1952), when Spences B r i d g e Group a n d e s i t e , d a c i t e , and r h y o l i t e were d e p o s i t e d umconformably on o l d e r rocks. Some o f t h e f a u l t i n g i n t h i s a r e a may be r e l a t e d t o movement a l o n g t h e F r a s e r and Yalakom F a u l t s . meters has been proposed Eisbacher,  R i g h t l a t e r a l movement o f s e v e r a l hundred k i l o -  f o r b o t h ( T i p p e r , 1977; T i p p e r and R i c h a r d s , 1976;  1977).  I n c o n v e n t i o n a l p l a t e t e c t o n i c models, t e c t o n i c melange, w h i c h i n c l u d e s t h r u s t s h e e t s and shows areas o f h i g h - p r e s s u r e 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 d e v e l o p i n g i n s u b d u c t i o n zones ( D i c k i n s o n , 1972). H i g h e s t p r e s s u r e c o n d i t i o n s a r e g e n e r a l l y found on t h e a r c s i d e o f t h e melange wedge.  I n t h e p r e s e n t study h i g h e s t p r e s s u r e assemblages a r e found  i n t h e s t r u c t u r a l l y lowest p a r t o f t h e Cache Creek which now s i t s on t h e  81 e a s t e r n s i d e of t h e Cache Creek Group.  T h i s 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 s u b d u c t i o n , f a c i n g o f the arc  i s not  clear.  T e c t o n i c models t h a t i n c l u d e Cache Creek r o c k s o f t h e type a r e a eastward  T r a v e r s , 1978).  As i n the P i n c h i - S t u a r t Lake  ( P a t e r s o n , 1977b), t h e r e are s p a t i a l problems w i t h t h i s model.  Creek melange i s now to  suggest  s u b d u c t i o n of an o c e a n i c 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; Area  concurrent  Cache  a d j a c e n t t o the a r e complex and as c l o s e as 3 k i l o m e t e r s  the Guichon B a t h o l i t h , the r o o t o f the N i c o l a a r c .  than s t a n d a r d models o f an a r c - t r e n c h system.  T h i s i s much c l o s e r  There i s c o n s i d e r a b l e specu-  l a t i o n as t o whether the a l k a l i n e N i c o l a v o l c a n i c r o c k s r e p r e s e n t a t r u e arc.  A l k a l i n e c h e m i c a l t r e n d s a r e thought  and o t h e r s , 1975;  to i m p l y a r i f t i n g o r i g i n ( P r e t o  H o l l i s t e r and o t h e r s , 1975).  Y e t a l k a l i n e r o c k s can  develop  i n a r c s where t h e r e i s a v e r y low convergence r a t e , l e s s than 2 cm/yr ( M i y a s h i r o , 1973), so these c h e m i c a l t r e n d s do not p r e c l u d e an a r c 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  s o l v e d 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 t o have c o n s i d e r a b l e d i s p l a c e ment .  SUMMARY AND CONCLUSIONS I n t h e Venables V a l l e y a r e a , t h e southernmost  exposure o f type Cache Creek  Group r o c k s , t h e Cache Creek i s thought t o be a t e c t o n i c melange. i s o l a t e d b l o c k s s i t i n a deformed c h e r t - a r g i l l i t e m a t r i x .  Exotic,  Some b l o c k s ,  e s p e c i a l l y l i m e s t o n e and p o s s i b l y g r e e n s t o n e , a r e thought t o be emplaced as s l i d e b l o c k s , as t h e r e i s abundant e v i d e n c e o f d e b r i s f l o w s and t u r b i d i t y c u r r e n t d e p o s i t s i n f i n e r g r a i n e d c l a s t i c sedimentary r o c k s .  Other  exotic  b l o c k s , n o t a b l y s e r p e n t i n i t e and v o l c a n i c r o c k s showing h i g h p r e s s u r e metamorphic 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; stone i s c l e a r l y f a u l t bounded.  one l a r g e mass o f green-  Metamorphic c o n d i t i o n s , though p o o r l y  documented, may range from temperatures below 250° C w i t h p r e s s u r e s p r o b a b l y 4 kb o r l e s s , t o p r e s s u r e s a p p r o a c h i n g 6 kb and temperatures near 350° C. Rocks o f most s e v e r e d e f o r m a t i o n and h i g h e s t metamorphic grade a r e found i n . t h e eastern," s t r u c t u r a l l y l o w e r p a r t o f t h e Cache Creek Group.  Only one  f o s s i l age, L a t e Permian, was o b t a i n e d f o r a Cache Creek l i m e s t o n e b l o c k . N i c o l a r o c k s 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 o f a p e r v a s i v e f o l i a t i o n , by h y d r o t h e r m a l , a l b i t e e p i d o t e a l t e r a t i o n r a t h e r t h a n dynamothermal metamorphism, and by t h e l a c k o f a sheared p e l i t i c m a t r i x and exotic blocks.  R a p i d f a c i e s changes o c c u r , 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 e x t e n t , b u t t h e r e i s no i n d i c a t i o n o f i s o l a t e d b l o c k s , and t h e r e a r e no u l t r a m a f i c r o c k s i n t h e N i c o l a Group.  V o l c a n i c r o c k s i n c l u d e a n d e s i t e and  d a c i t e r a t h e r t h a n b a s a l t s seen i n t h e 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 g r e e n s t o n e s i s l i k e l y t h e time o f h y d r o t h e r m a l a l t e r a t i o n r e l a t e d t o i n t r u s i o n o f t h e Guichon  Batholith.  Today t h e c o n t a c t between t h e two groups i s a v e r t i c a l t o s t e e p l y west d i p p i n g f a u l t , named t h e 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 t o t h e southwest and i f t h i s t i l t i n g i s removed t h e f a u l t becomes a t h r u s t .  Movement i s p o s t Lower J u r a s s i c .  B r i t t l e d e f o r m a t i o n i n the Cache Creek i s p r o b a b l y r e l a t e d i n p a r t to movement a l o n g t h i s f a u l t . Tectonic  models i n which the Cache Creek i s i n t e r p r e t e d as a s u b d u c t i o n  complex r e l a t e d to the N i c o l a a r c a r e too s i m p l i f i e d . transcurrent  Concurrent and l a t e r  f a u l t i n g and t h r u s t f a u l t i n g have c o m p l i c a t e d the o r i g i n a l  s p a t i a l arrangement of the Cache Creek and N i c o l a Groups. tectonic puzzle  can be s o l v e d  f o r the Cache Creek Group. cherts,  a d d i t i o n a l q u a n t i t a t i v e d a t a must be o b t a i n e d  These i n c l u d e p a l e o t o l o g i c d a t i n g o f r a d i o l a r i a n  i s o t o p i c and c h e m i c a l d a t a f o r v o l c a n i c r o c k s ,  e s p e c i a l l y augite  prophyry v o l c a n i c s , b e t t e r p r e s s u r e - t e m p e r a t u r e i n f o r m a t i o n conditions,  B e f o r e the e n t i r e  and i s o t o p i c d a t i n g o f metamorphic events.  f o r metamorphic  REFERENCES CITED  A l b e e , A.L. and Ray, L., 1970, C o r r e c t i o n f a c t o r s f o r e l e c t r o n probe microsa n a l y s i s o f s i l i c a t e s , c a r b o n a t e s , p h o s p h a t e s , and s u l f a t e s ; A n a l y t i c a l Chem., V o l ! 42, p.1408-1414. 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