biostratigraphy and evolution of the Methow Trough, southwestern British Columbia

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Authors O'Brien, Jennifer Ann

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Link to Item http://hdl.handle.net/10150/558073 JURASSIC BIOSTRATIGRAPHY AND EVOLUTION

OF THE METHOW TROUGH, SOUTHWESTERN BRITISH COLUMBIA

by

Jennifer Ann O'Brien

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCES

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 8 7 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillm ent of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided th at accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the m aterial is in the in terests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: 7

APPROVAL BY THESIS DIRECTOR

This fESds hasy been approved on the date shown below:

f P. J./CONEY Date Professor of/Geosciences ACKNOWLEDGMENTS

This thesis represents a study under the joint supervision of

Dr. Jim Monger (Geological Survey of Canada) and Dr. Peter Coney. The thesis topic was suggested by Jim Monger. Interaction with him in and out of the field over the past few years have been invaluable.

Conversations with Peter provided insight and new perspective throughout the study. The project has benefited from their time and support.

Logistical support for fieldwork was provided by the

Geological Survey of Canada in conjunction with Jim Monger's mapping project (GSC project #800029). A fellowship award from Amoco, and research grants from the Geological Society of America and University of Arizona (SOCAL) are also gratefully acknowledged.

Many people have contributed their time, interest and support through the various stages of the project. In the field and out,

Gerry Ray (B ritis h Columbia M inistry of Energy, Mines and Petroleum

Resources) has shared data, ideas and enthusiasm. Geochemical analyses and thin sections of volcanic rocks were also provided through Gerry.

Dr. J. Jeletzky (Geological Survey of Canada - Ottawa) led an enlightening fieldtrip through the Manning Park section. He provided identifications of Buchia and various belemnites and has been a great source of encouragement. 1v

During fieldwork I was ably assisted at various times by Mary

MacLean, Dave Handel and Steve Irw in . Their energy and in te re s t were much appreciated. Access to Carol in Mine and areas north provided through individuals associated with Carol in Mine and Cattermole Timber was much appreciated.

Ammonite id e n tific a tio n s were conducted through the generous and valuable guidance of Dr. H.W. Tipper at the Geological Survey of

Canada (Vancouver). A productive day in the thesis area with Tip provided both new localities and greater appreciation of regional relations.

Dr. Paul Smith (University of B ritis h Columbia) kindly allowed use of his photographic equipment for documentation of ammonite genera. Dr. Terry Poulton (Institute of Sedimentary and Petroleum

Geology) id e n tifie d various bivalves associated with ammonites of the thesis collection and also introduced me to the Manning Park fossil collections of J. Coates. Dr. R. Hall (University of Calgary) provided a second opinion on certain Bajocian ammonite specimens. Dr.

Mike Orchard (Geological Survey of Canada - Vancouver) processed samples fo r conodonts and Fabrice Cordey (U niversite P ierre-et-M arie

Curie, Laboratoire de Stratigraphie) processed samples for radio!arians.

Dr. Karl Flessa has introduced me to a new technique for assessment of paleobiogeography and has increased my awareness of the role of taphonomy in preservation of the fossil record. His approach to the science has been stimulating. In addition to those mentioned above. Dr. George Gehrels, my husband, has been a tremendous resource throughout the p ro je ct. His support and encouragement throughout have played a s ig n ific a n t role in the evolution of the thesis. The extended family have also given generously of their time and energy to create the needed "free time" to complete this phase. . TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS vi 1 i

LIST OF TABLES . . . . x ii

ABSTRACT ...... xi i i

1. INTRODUCTION . . . . . 1

Regional Geology ...... 5 Methow Trough ...... 5 East Boundary of the Methow Trough . . 11 West Boundary of the Methow Trough . . 13 Revised Jurassic Stratigraphic Nomenclature, 15

2. COQUIHALLA AREA 20

Previous Work ...... 21 Jurassic Strata 22 Ladner Group: Boston Bar Formation . . . . 22 Ladner Group: Dewdney Creek Formation . . . 27 Biostratigraphy « ® . ® . . . .„« ® ® «. « . . 35 Intrusive Rocks « . » « ...... * . 38 Structural Relations ...... 38

3. ANDERSON RIVER AREA 41

Jurassic Strata © « ® ® * . . * . . © ® * . . © . ® © ® 42 Ladner Group: Boston Bar Formation ...... 42 Ladner Group: Dewdney Creek Formation ...... 42 Biostratigraphy ...... 44 Structural Relations ...... 49

4. PETROGRAPHY 51

Petrographic methods ...... 52 Petrographic descriptions ...... 58 Boston Bar F o rm atio n ...... 58 Dewdney Creek Formation ...... 60 Thunder Lake sequence ...... 63 Jackass Mountain and Pasayten Groups ...... 66 Discussion ...... 66

v i v i 1

TABLE OF CONTENTS— C o n tinu e d

Page

5. REGIONAL GEOLOGY AND GEOCHEMISTRY OF THE METHOW TROUGH: J URASSIC STRATA o 73

Distribution of Jurassic Units ...... 73 Ladner Group ...... 73 Thunder Lake sequence ...... 80 Dewdney Creek Formation Geochemistry ...... 81

6. TECTONIC IMPLICATIONS OF THE JURASSIC EVOLUTION OF THE METHOW TROUGH ...... 87

Introduction ...... 87 Coeval Stratigraphy ...... 93 "basement" assemblages ...... 97 Jurassic strata ...... 99 DlSCUSSIOn e e e e . e e . . . O o ...... 106

7. CONCLUSIONS ...... 108

APPENDIX A: SYSTEMATIC PALEONTOLOGY ...... 113

APPENDIX B: POINT COUNT DATA ...... 128

APPENDIX C: DESCRIPTION OF MAP UNITS FOR THE COQUIHALLA (FIG. 6) AND ANDERSON RIVER (FIG. 13) AREAS ...... 131

SELECTED BIBLIOGRAPHY ...... 139 LIST OF ILLUSTRATIONS

Page

Figure 1. Location of the Methow Trough (stippled) with respect to regional geologic elements ...... 2

Figure 2. Location of areas mapped in detail in the Methow Trough in southwestern B ritis h Columbia ..... 4

Figure 3. A simplified geologic map of the Methow Trough, illustrating the distribution of Triassic, Jurassic, and units ...... 7

Figure 4. Generalized stratigraphic section of the Methow Trough in southwestern B ritis h Columbia and correlative units in north-central Washington ...... 8

Figure 5. Folds and high angle faults exposed on the north face of Mt. Tulameen, Coquihal1 a area ...... 10

Figure 6. Geologic map and cross sections of the Coquihalla area. (In pocket)

Figure 7. Schematic b io s tra ti graphic section of Jurassic strata exposed in the Coquihal la area ...... 23

Figure 8a. A g ra n itic boulder found in a sheared conglomeratic mudstone o f the Boston Bar Formation ...... 25

Figure 8b. Outcrop of intercalated argillite and siltstone of the Boston Bar Formation exposed in road cuts of the Coquihalla highway east of Ladner Creek ...... 25

Figure 9. Interbedded 1ithofeldspathic sandstone and volcanic-rich conglomerate of basal Dewdney Creek strata ...... 29

Figure 10. Volcanic-rich pebble conglomerate characteristic of the Dewdney Creek Formation ...... 31

v i i i ix

LIST OF ILLUSTRATIONS— Continued

Page

Figure 11. Interbedded tuffaceous greywacke and s iltsto n e is c h a ra c te ris itic of the Oewdney Creek / strata exposed on Mt. Tulameen and Mt. Sm der ( F i g . 6 ) ...... * ...... 33

Figure 12a. Aligned belemnites on the bedding surface of a tuffaceous siltstone found directly west of the summit of Mt. Tulameen ...... 37

Figure 12b. An ammonite (Sonninia (? )) im print was found 2 m above the belemnite surface ...... 37

Figure 13. Geologic map and cross section of the Anderson River area. (In pocket)

Figure 14. Interbedded siltstone and argillite of the Boston Bar Formation « ...... © . © ® « ® . © ® * 43

Figure 15. Andesitic volcanic breccia is characteristic of the upper Dewdney Creek Formation ...... 45

Figure 16. Schematic biostrati graphic section of Jurassic strata exposed in the Anderson River area .... 46

Figure 17. QFL diagram illu s tr a tin g the data points and means of the units of the Methow Trough defined in southwestern B ritis h Columbia .... 55

Figure 18. QFL and QmFLt diagrams superimposed illu s tr a tin g the change in unit mean by considering Qp as a lithic fragment ...... 56

Figure 19. Variation of d e trita l framework components (normalized percentage) up through the stratigraphic section ...... 57

Figure 20. Photomicrograph illustrating fabric characteristic of fine grained strata of the Boston Bar Formation (MV085-119) ...... 59

Figure 21a. Photomicrograph of a typical tuffaceous litho- feldspathic sandstone (MV085-65) of the Dewdney Creek Formation ...... 61 X

LIST OF ILLUSTRATIONS— Continued

Page

Figure 21b. Photomicrograph of MV085-65 in polarized lig h t . . . 61

Figure 22a. Photomicrograph of a Thunder Lake sandstone (MV085-340) ...... 64

Figure 22b. Photomicrograph of a sandstone (MV085-341). from the overlying Jackass Mountain Group ...... 64

Figure 23a. Photomicrograph of MV085-340 in polarized lig h t . . 65

Figure 23b. Photomicrograph of MV085-341 in polarized lig h t . . 65

Figure 24. A comparison of QFL ratio s calculated for the Dewdney Creek Formation ...... 68

Figure 25. Location of Jurassic (solid circles) and Cretaceous (open circles) fossil localities and geochemistry samples (triangles) . . . . . 74

Figure 26. Distribution of Jurassic units in the Methow Trough in southwestern B ritis h Columbia . . . . 75

Figure 27. Andesitic volcanic breccia of Blackmail Peak, Lookout section. Manning Park ...... 77

Figure 28a. Major element discriminant plot of Church (1975) indicates that the samples are predominantly andesite ...... 84

Figure 28b. Data plotted on an a lk a li vs. s ilic a diagram ( Irvine and Baragar, 1971) ...... 84

Figure 28c. An AFM diagram ( Irvine and Baragar, 1971) outlines the sub-alkalic, calc-alkaline composition ...... 84

Figure 29a, Minor element discriminant plots of Floyd and 29b, 29c. Winchester (1978) suggest the samples overlap the andesite and sub-alkaline basalt fields . . 85 x i

LIST OF ILLUSTRATIONS— Continued

Page

Figure 30a, Minor element discriminant plots of Pearce 30b. (1975) suggest affinities of the Dewdney Creek volcanic rocks with those derived from island arcs ...... 86

Figure 31. Location of the Methow Trough with respect to the composite terranes of Monger et a l. (1982) . . . . 88

Figure 32. A terrane map of western North America (from Coney, 1981) that includes elements of the Canadian C ord illera (F ig . 31) in a broader perspective . . . @ . * * » ...... 89

Figure 33. An illustration of various interpretations of the latitudinal positions of major allochthonous terranes with respect to cratonal North America in the Mesozoic ...... 92

Figure 34. Location of coeval assemblages (stippled pattern) with respect to composite terranes of the Canadian C ord illera ...... 95

Figure 35. Generalized biostrati graphic sections for the coeval assemblages of the Canadian C ord illera . . . * . . . * ...... 96

Figure 36. Distribution of Lower and lower volcanic rocks ( shaded areas) in the Canadian Cordillera with respect to the present terrane d is trib u tio n ...... 103 LIST OF TABLES

Page

Table 1 Historical development of Jurassic stratigraphic nomencl ature . . 16

Table 2 Description of grain categories used'for point COUntlng . . 54

Table 3 Calculated mean QFL and QmFLt ratio s ...... 58

Table 4 Major element analyses of Dewdney Creek volcanic

rOCkS o e . o*. o . o o o e e e o o o o . . . . 83

Table 5 Trace element analyses of Dewdney Creek volcanic rocks . . 83

Table 6 The location, terrane affinity, age, and petrology/geochemistry of Lower and Middle Jurassic volcanic assemblages in the Canadian C ord illera . . « ...... 104

xii ABSTRACT

Biostratigraphic studies of Jurassic rocks of the Methow

Trough exposed in southwestern B ritis h Columbia have led to the

redefinition of the stratigraphic nomenclature. Lithostratigraphic

units now recognized include: the Ladner Group which comprises the

Sinemurian(?) to Aalenian(?) Boston Bar Formation and the Aalenian to

Early Bajocian Dewdney Creek Formation, and the Oxfordian to

Kimmeridgian Thunder Lake sequence. Id e n tific a tio n of 5 ammonite genera previously unrecognized w ithin the Ladner Group b etter constrains the age and relation of units.

Lower Middle Jurassic calc-alkaline andesite and associated pyroclastic rocks within the section provides a local source for

volearn'dastic strata. There is little evidence of detrital input

from an eastern margin u n til Late Jurassic deposition of the Thunder

Lake sequence. Coeval assemblages to the east (Ashcroft) and

northwest (Tyaughton) display little or no evidence of proximal

volcanic activity and thus appear not to have evolved in close

association with the Methow Trough.

x iii CHAPTER 1

INTRODUCTION

The Methow Trough comprises a fault-bounded sequence of Lower

Jurassic through Upper Cretaceous sedimentary and volcanic strata that

unconformably overlies Lower Triassic basalt. On the east, the

Pasayten fa u lt separates the trough from Mesozoic c ry s ta llin e rocks of

the Mount Lytton Complex and the Okanogan B atholith. To the west, the

Permian to Jurassic oceanic assemblage of the Hozameen Group is juxtaposed against the Methow strata by the Hozameen fault. To the

northwest, the trough is truncated by the Fraser fault system. A

complex fa u lt system and metamorphic rocks mark the southern lim it of

the trough (F ig . 1) (Barksdale, 1975; Monger, 1970, 1985; Roddick et

al., 1979).

The Methow Trough occupies a key position in the C o rd ille ra ,

outboard of the Intemontane terrane and inboard of the Insular

terrane (Monger et al., 1982; Price et al., 1985). The depositional

history of the basin thus has the potential to record the complex

accretionary interactions between these composite terranes with the

evolving Mesozoic Pacific margin and with each other.

1 2

Lilloer

CPC

\ M L

Vancouver

-CMC

Figure 1. Location of the Methow Trough (stippled) with respect to regional geologic elements. Other stippled areas represent assemblages coeval with the Methow Trough. Abbreviations include: CPC = Coast Plutonic Complex, T = Tyaughton Trough, BR = Bridge River, H = Hozameen, CMC = Cascade metamorphic core, M = Methow Trough, CC = Cache Creek terrane, Q = Quesnel te rran e , ML = Mount Lytton Complex, OB = Okanogan Batholith, FF = Fraser fault, PF = Pasayten fault, HF = Hozameen fa u lt, JMT = Jack Mountain th ru s t, YF = Yalakom fa u lt, SCF = Straight Creek fault, and C = Chilliwack Batholith. 3

The Cretaceous units in northern Washington have received much attention in the past due to better exposure, distinctive stratigraphy, and age. Based on facies relationships, provenance, and current indicators within the Cretaceous section, late Early

Cretaceous deposition is inferred to correspond to the time of accretion of the Insular terrane (Tennyson and Coles, 1978; Davis et a l., 1978; Monger et a l. 1982; Klienspehn, 1985, Trexler, 1985;

Trexler and Bourgeois, 1985). In comparison, the Jurassic stratigraphy is poorly known. Because o f th is and because i t is becoming increasingly apparent that the Jurassic is a critical period in the tectonic evolution of the Cordillera, this study was undertaken through the direction and support of J.W.H. Monger of the Geological

Survey of Canada.

Previous work on the Jurassic strata of the trough exposed in different areas in southwestern British Columbia has led to confusion in the stratigraphic nomenclature (Cairnes, 1924; Coates, 1970; 1974).

This has fru strated an attempt to produce a coherent regional compilation of the Jurassic units (Monger, 1970). The significance of the Jurassic units is therefore difficult to ascertain.

In an effort to clarify the Jurassic stratigraphic nomenclature and d is trib u tio n of u n its, detailed mapping was focused on two areas, the Coquihal1 a (1:25,000) and Anderson River (1:50,000) areas, in the Methow Trough of southwestern B ritis h Columbia where the

Jurassic strata is best preserved (Fig. 2). In the Coquihalla area the base of the Jurassic section is exposed. It is also the area 4

121° 30*

Boston Bor ANDERSON U RIVER VV AREA

COOUIH ALL A AREA

AHope

MANNING PARK — AREA

Figure 2. Location of areas mapped in detail in the Methow Trough in southwestern British Columbia. The Manning Park area is enclosed in a crossed boundary. In the Coquihalla area, regions mapped by: Cairnes (1924) in heavy dots, Anderson (1976) in light dots, Ray (1986a, 1986b) in dashed lin e s , current study in solid lin e s . Anderson River area is also part of current study. 5 where the Jurassic units were defined originally by Cairnes (1924).

The Anderson River area was chosen to provide critical information on

the distribution of Jurassic units in the northern part of the trough.

This area also yielded a relatively abundant Middle Jurassic ammonite fauna, providing much needed b io stratig rap h ic c o n tro l. Reconnaissance mapping in and north o f Manning Park was also undertaken to b etter constrain the distribution of Jurassic units throughout the trough in

southwestern British Columbia.

This thesis is divided into two main sections. A presentation of the Lower and Middle Jurassic biostratigraphy based on mapping completed in the summer of 1985 is included in Chapters 1-5. The

significance of the distribution, age, lithic composition, and • chemistry of these rocks is discussed. Detailed supportive data are

in appendices. The second section is devoted to a biostrati graphic comparison of the Methow trough with coeval assemblages. Through variations and correlations between adjacent coeval assemblages, the

regional tectonic configuration is better understood.

Regional Geology

Methow Trough

Fault-bounded Triassic to Upper Cretaceous sedimentary and

volcanic rocks in southwestern B ritis h Columbia and northern

Washington (Barksdale, 1975) are c o lle c tiv e ly referred to in this

report as strata of the Methow Trough (Fig. 3). These rocks presently

occupy a northwest trending linear belt of 55 km maximum width and

greater than 350 km length (Barksdale, 1975) (Fig. 1). The trough is 6 bounded by a variety of metamorphic, intrusive, and highly deformed sedimentary strata which comprise discrete 1ithostructural packages

(Monger, 1986). Present trough boundaries are the Pasayten fault to the east (Monger, 1970; Barksdale, 1975), the Hozameen fault to the west (Misch, 1966), and the Fraser fa u lt system (Monger, 1985) to northwest. To the south, the Methow strata are juxtaposed against the Okanogan (Remmel) B atholith, Leecher metamorphic complex, and the

Cascade River schist (Roddick et a!., 1979).

The Jurassic stratigraphy of the Methow Trough in British

Columbia includes Lower Jurassic, predominantly argillaceous strata of the Boston Bar Formation. These strata are conformably(?) overlain by and in part coeval with volcanic-rich sedimentary strata, and volcanic rocks of the Dewdney Creek Formation. Both formations comprise the

Ladner Group. In tu rn , these are unconformably overlain by siltsto n e and greywacke of the Late Jurassic Thunder Lake sequence (F ig . 4)

(O'Brien, 1986; see below for a discussion of the revised stratigraphy). In Washington, poorly known Jurassic strata belong to the Newby Group and include argillaceous rocks of the Twisp Formation.

Volcanic rocks of the middle member of the overlying Buck Mountain

Formation (Newby Group) are considered Early Cretaceous in age

(Maurer, 1958). The basal member of the formation has not been dated, however, and may be in part correlative with the Dewdney Creek

Formation (Barksdale, 1975; Tennyson and Coles, 1978). 7

Pasayten Group Jackass Mountain Gp. Thunder Lake seq. CAROLIN MINE Ladner Group Spider Peak Fm.

o Cretaceous Fossil Localities • Jurassic Fossil Localities (This Study)

B m j ISH_COLUM BIA 4 9 ------v WASHINGTON

EE^Midnight Peak Fm. ZzjWinthrop Fm. Virginian Ridge Fm. :==; Harts Pass Group Iffl Newby Group

Figure 3. A simplified geologic map of the Methow Trough, illustrating the distribution of Triassic, Jurassic, and Cretaceous units. Note that the Newby Group distribution may include Cretaceous strata. New fossil localities described in this study are included. Compiled from Monger, 1970, Coates, 1974, Roddick et a l . , 1979 and O'Brien, 1986. 8

BRITISH COLUMBIA WASHINGTON PASAYTEN MIDNIGHT GROUP PEAK FORMATION

WINTHROP JACKASS FORMATION MOUNTAIN VIRGINIAN GROUP RIDGE FORMATION

HARTS PASS GROUP

BUCK MOUNTAIN FORMATION

THUNDER _J LAKE SEQUENCE

NEWBY GROUP

DEWDNEY CREEK FORMATION LADNER TWISP BOSTON GROUP FORMATION BAR FORMATION

Lithologies argillite, slate tuffaceous siltstone groywacke 0o3 conglomerate volcanic breccia v volcanic flow rocks greenstone • diagnostic SPIDER Contacts PEAK unconformity FORMATION -----1 gradational

Figure 4. Generalized stratigraphic section of the Methow Trough in southwestern British Columbia and correlative units in north-central Washington. Time scale from Palmer (1982). Compiled from Coates (1974), Trexler (1985), and O'Brien (1986). 9

Unconformably above the Jurassic section are marine arkose, conglomerate and siltstone characterized by an abundance, of crystalline detritus. These rocks, referred to as the Jackass

Mountain Group in southwestern British Columbia, are in part overlain by and in part coeval with nonmarine sedimentary strata with abundant chert detritus and volcanic rocks of the Pasayten Group (Coates, 1970;

1974). Cretaceous strata in Washington include the Buck Mountain

Formation, Harts Pass Group, V irginian Ridge, Winthrop, and Midnight

Peak Formations (Barksdale, 1975; Tennyson and Coles, 1978) (Fig. 4).

Detailed stratig rap h ic studies in Washington have led to a b etter understanding of complex facies relationships between east (Harts Pass

Group and Winthrop Formation) and west (Virginian Ridge Formation) derived sediments in the evolving Cretaceous Trough. To enable better correlation between Methow strata north and south o f the border, a more detailed stratigraphic analysis of the Cretaceous rocks in

B ritish Columbia is necessary (J.W.H. Monger, pers. comm., 1987).

Strata of the Ladner Group are inferred to unconformably overlie Lower Triassic(?) greenstone, gabbro, and minor tuffaceous sedimentary rocks of the Spider Peak Formation (Cairnes, 1924; Ray,

1986). The Spider Peak Formation lo c a lly crops out in the Coquihal 1 a area, exposed within strands of the Hozameen fault along the western margin of the Trough. It is not known to occur elsewhere.

Strata of the Methow Trough have undergone at le a s t two phases of deformation (Fig. 5) and low grade metamorphism (lower greenschist). Dominant structures include northwest-trending, east 10

Figure 5. Folds and high angle faults exposed on the north face of Mt. Tulameen, Coquihal1 a area. 11

vergent thrust faults and shallowly plunging folds that parallel the bounding fa u lts (F ig . 3 ). These are in ferred to have formed during deformation that initiated in mid-Cretaceous time (Coates, 1974;

McGroeder and Mohrig, 1987). Due to the absence of angular unconformities within the Jurassic and Early Cretaceous section earlier regional deformation is difficult to identify (Coates, 1974;

O'Brien, 1986), however, local disruption is evident within the

Jurassic strata (Ray, 1986b). Late stage dip-slip and strike-slip faults disrupt the continuity of older structures.

East Boundary of the Methow Trough

The present eastern boundary of the trough, the Pasayten

fault, is a vertical to steeply southwest-dipping structure. It is

truncated to the north by the early Tertiary Fraser fault system and

is lo s t to the south where gneissic rocks of the Okanogan Batholith come in contact with high grade metamorphic rocks of the Cascade metamorphic core (F ig . 1) (Barksdale, 1975; Roddick e t a l ., 1979). In

Washington, Barksdale (1975) documented normal movement along

this structure with the final displacement post-Paleocene and pre-

Holocene. He cited the study of Lawrence completed in 1968 (Lawrence,

1978) which suggests rig h t la te ra l s tr ik e -s lip motion of Middle and

Late Cretaceous age followed by left lateral motion. Although left

lateral motion is uncharacteristic for faults within the region,

recent work by Miller (1986) on the Yalakom fault, a possible northern

continuation of the Pasayten fa u lt (Kleinspehn, 1985), also suggests

an early le ft lateral component of displacement. 12

In southwestern British Columbia, Rice (1947) describes a depositions! unconformity along the inferred fault trace of the

Pasayten fa u lt between Cretaceous strata of the trough and c ry s ta llin e rocks d ire c tly to the east. Recent palynological studies o f the overlying sedimentary rocks show a middle Eocene age for the strata above crystalline rocks of the Eagle Complex (C. Greig, pers. comm., 1987). This preliminary study better constrains a minimum age for movement along the Pasayten fault. The depositions! relationship is not recognized elsewhere.

Based on a lack of both appropriate source areas for the

Cretaceous strata and correlatable units across the inferred structure, Coates (1970) suggests modest (approxim ately-100 km) rig h t- lateral strike slip motion on the Pasayten fault along with a large amount of dip slip displacement. Recent mapping of the Hope map area by Monger (pers. comm., 1985) shows th at the trace of the Pasayten fault is sinuous. The sinuosity is due either to folding or to being cut by a series of northeast trending faults. Clearly the nature and history of th is fundamental boundary has not been resolved.

Mesozoic meta-plutonic rocks of the Mount Lytton Complex crop out east of the Pasayten fault in British Columbia. K-Ar and U-Pb dates from the complex range in age from 250 Ma to 140 Ma (Monger,

1985), but the complex in tru sive relatio n s between suites of different age have yet to be described in detail. Current work on the southern end of the crystalline complex by Greig (pers. comm., 1987) establishes the presence of a younger structural and intrusive entity

(approximately 140 - 110 Ma) referred to as the Eagle Complex (Fig. 13

1). East of the Pasayten fault in Washington, the Okanogan (Remmel)

Batholithic Complex is described as a belt of granitoid gneiss and associated intrusive rocks with a similar range of ages (Barksdale,

1975).

West Boundary of the Methow Trough

The Hozameen fa u lt, the present western boundary of the trough in southwestern British Columbia, varies in orientation along strike

(Rice, 1947; Ray, 1986). Although presently a near vertical structure, it is generally believed to have been a low angle thrust la te r deformed to its present o rien tation (Ray, 1986). Serpentine is locally associated with this structure in the Coquihal1 a area

(Cairnes, 1924; Ray, 1986). In Washington, the southern continuation of the Hozameen fa u lt has tra d itio n a lly been mapped as the Jack

Mountain thrust (Misch, 1966). Recent work, however, demonstrates that the Hozameen fault actually cuts the Jack Mountain thrust. It is further postulated that the Hozameen is part of a Late Cretaceous-

Pal eogene o b liq u e-slip fa u lt system of unknown displacement

(McGroeder, 1987).

Permian to Middle Jurassic chert, greenstone, argillite and rare with associated ultramafic bodies constitute the

Hozameen Group, a highly deformed, probable oceanic assemblage juxtaposed against the Methow strata by the Hozameen fault (Haugerud,

1985). The North Creek Volcanics, Jack Mountain P h y llite and E lijah

Ridge Schist occur west of the Methow s trata in Washington along with a host of Late Cretaceous and Tertiary intrusive rocks (Misch, 1966; 14

Roddick et a l . , 1979). West of the Hozameen Group are paragneiss and orthogneiss of the Custer-Skagit Gneiss and a variety of metamorphic rocks which comprise the Cascade core (Misch, 1966; Cowan and Potter,

1986).

Right-lateral movement on the early Tertiary Fraser River

Fault System (FRFS) is estimated to be approximately 90 km based on the correlation of structures and displaced mid-Cretaceous and older units across this system (Kleinspehn, 1985; Monger, 1985). Eocene strata are deformed w ithin the system and the 01igocene Chilliwack batholith cuts across the fault near the international boundary. The southern continuation of the FRFS in northern Washington is the

S traight Creek fa u lt (Misch, 1966; Vance and M u ller, 1981; Monger,

1985). The FRFS truncates the Methow Trough north of Boston Bar, although Methow strata and their meta-equivalents are caught up between strands of the fault and can be recognized north of Lilloet.

The Tyaughton Trough constitutes the northwestern continuation of the

Methow Trough across the FRFS (Fig. 1 ).

A correlation between the Tyaughton and Methow Troughs was o rig in a lly proposed by Jeletzky and Tipper, (1968). The Tyaughton

Trough, as defined by Jeletzky and Tipper (1968), comprises all upper

Middle Jurassic to Upper Cretaceous strata which unconformably(?)

overlie the Late Triassic to Middle Jurassic Tyaughton Group. A basin

reconstruction based on the facies distribution of Lower Cretaceous

strata of the Tyaughton and Methow Troughs suggests a minimum of 110

km of dextral translation on the FRFS (Kleinspehn, 1985). This

reconstruction aligns the Yalakom and Ross Lake fa u lts whereas 15

Monger's reconstruction (1985) aligns the Yalakom and Hozameen faults.

In e ith e r case, the Tyaughton and Methow Troughs are re-aligned to form a continuous depositional basin in Cretaceous time.

Revised Jurassic Stratigraphic Nomenclature

Revisions to the original Jurassic stratigraphic nomenclature defined by Cairnes (1924) has caused confusion in the consistent recognition of Jurassic 1ithostratigraphic units throughout the trough in southwestern B ritis h Columbia (Table 1) (Coates, 1970; Monger,

1970; Ray, 1986b), A combination of paleontological studies and detailed and reconnaissance mapping in the trough during the summer of

1985 enables better definition of Jurassic units and hence, more applicable revisions of the stratigraphic nomenclature (Table 1)

(O'Brien, 1986). The applicability of the revisions is illustrated within the text in Chapter 5 where a compilation of the Jurassic units of the trough in southwestern B ritis h Columbia aids in a b e tte r understanding of the stratigraphic relations within the section. 16

Table 1. Historical development of Jurassic stratigraphic nomenclature.

AGE Cairnes (1924) Coates (1970) Current Study

Ber \ Tth Kim "-Thunder Lake s .— Oxf c r y s ta l-lith ic s ilts to n e , s iltsto n e Civ tu f f sandstone sandstone Bth Dewdney Creek S. Dewdney Creek Gp. Dewdney Creek JFm. Baj ,— ,—._, """volcanic breccia'. Aal volcanic breccia. lava, sediments.__ Toa argillite, grey- lava, tuffaceous —cirgi H i t e , " s i l t - Plb wacke, conglomerate sediments stone, greywacke Sin Ladner Series Ladner Group conglomerate Het Boston Bar Fa.

Nor Car greenstone Lad Cache Creek Series greenstone,gabbro Ans sediments Scy Spider Peak Fin.

Prior to detailed mapping by Coates (1970, 1974) in Manning

Park (Fig 2.), few diagnostic fossils had been collected from the

inferred Jurassic strata. In the Coquihalla area, Cairnes (1924)

subdivided the Jurassic strata into the Ladner Series and the Dewdney

Creek Series. The Ladner Series was included predominantly

argillaceous strata with minor intercalations of tuffaceous wacke and

conglomerate at the base and top of the section. The overlying

Dewdney Creek Series, believed to be in part coeval with the Ladner

Series, was characterized by coarser grained tuffaceous strata.

Within the section Cairnes (1924) reported the occurrence of Jurassic-

Cretaceous bivalves and belemnites. The discovery of an arnioceratid 17 ammonite in flo a t w ithin the upper member of the Dewdn'ey Creek Series suggested an (Sinemurian) age for the unit (Cairnes,

1924). Later work by Cairnes (1944) and Rice (1947) did not better constrain the age of the Jurassic units.

Two sections measured and collected by Coates in the Manning

Park area provided most of the biostrati graphic control for the Early and Middle Jurassic rocks (Frebold et a l 1969; Coates, 1974) prior to this study. The strata crop out in two structurally controlled belts. Characteristic of the eastern belt is the section exposed in readouts along the Lookout Road ju s t north of Manning Park Lodge.

Ammonites, found mainly as imprints on bedding surfaces of tuffaceous siltstone intercalated with volcanic-rich wacke and pebble conglomerate, are the most common megafossil noted. They are reported to be characteristic of the middle Bajocian (early Bajocian) (Frebold et al., 1969).

The second section crops out on a ridge just south of Silver

Daisy Mountain in the western belt and is referred to as the Divide section. It comprises two lithologic units. A lower unit consists of argillite and siltstone and contains late (Dumorteria levesquei zone) and e arly Bajocian (Aalenian) ammonites. The upper unit comprises volcanic sandstone with ammonite genera th a t are similar to the Lookout fauna and indicative of a middle Bajocian

(early Bajocian) age. Thus ammonites collected from Ladner strata in the Manning Park area indicate marine deposition from la te Toarcian to early Bajocian time.

Coates (1970) included a ll argillaceous and volcanic strata of 18

Early and early Middle Jurassic age in the Ladner Group with complete disregard of the lithostratigraphic subdivisions of Cairnes (1924).

Upper Jurassic siltsto n e and fin e grained sandstone recognized to unconformably o verlie the Ladner rocks of the western b e lt were referred to as the Dewdney Creek Group in acknowledgement of Cairnes original age interpretation of the Dewdney Creek Series in the

Coquihalla area.

During mapping by the writer in 1985, significant new fossil lo c a litie s were found in the Anderson R iver, Coquihalla and Manning

Park areas (Fig. 3). A total of thirteen Jurassic ammonite localities vastly improves the age control w ithin the section and provides the necessary tools for correlation of facies equivalent units throughout the trough. Although the preservation is moderate to poor, genus level identification is possible in all but two cases and species level "or species affinity in some. Of the nine genera retrieved and described below (see also Appendix A ), only one is documented as occurring in the Manning Park area (Coates, 1974).

All ammonites are representative of the Aalenian and early

Bajocian and indicate an early Middle Jurassic age for the volcanic rocks o rig in a lly designated as the Dewdney Creek Series by Cairnes

(1924). Because of this, all Middle Jurassic volcanic strata within the trough are herein referred to as the Dewdney Creek Formation of the Ladner Group. The Dewdney Creek strata are considered a Formation within the Ladner Group because underlying argillaceous strata of the

Ladner Group are in part coeval with and lateral equivalents of the volcanic rocks. Furthermore, significant accumulations of 19 argillaceous rocks indistinguishable from typical Ladner a r g illite s occur within the Dewdney Creek section, being particularity evident in the Anderson River area.

The Upper Jurassic s iltsto n e and sandstone which disconformably o verlie the western b e lt of the Ladner Group in Manning Park are

inform ally referred to as the Thunder Lake sequence in th is report

(previously the Dewdney Creek Group of Coates (1970)). Both its age

(Jeletzky et a l., in prep.) and lithology (Coates, 1974; see also

Petrography section below) distinguish i t from the Ladner Group. I t

is, however, remarkably similar to overlying Hauterivian strata of the

Jackass Mountain Group and should thus be considered in any further

studies of Cretaceous strata. CHAPTER 2

COQUIHALLA AREA

The Coquihalla area, originally mapped by Cairnes (1924), has

received more attention than any other area of the Methow Trough in

southwestern British Columbia, primarily due to the occurrence of gold

(Cairnes, 1924; Anderson 1976; Ray et a l . , 1985; Ray 1986a, 1986b).

The gold occurs both w ithin the basal Ladner and in quartz veins genetically related to the Hozameen fault system. Also of

significance in the area is the reported unconformable relationship preserved between Jurassic rocks of the Ladner Group and an underlying greenstone unit (Cairnes, 1924; Anderson, 1976; Ray, 1986).

Detailed mapping (1:25,000) in the Coquihalla area was

undertaken in the summer of 1985 (Fig. 6 - in pocket. Appendix C).

Mapping extended eastward to the Chuwanten fault, an interpreted east-

vergent thrust (Coates, 1970) which juxtaposes Jurassic strata over

Cretaceous units. The late Eocene Needle Peak Pluton forms the

northeastern boundary and the Hozameen fa u lt the western boundary.

Mapping around Carol in mine was lim ited to more of a reconnaissance

fashion as Ray (1986a; 1986b) has recently completed detailed mapping

and sampling in the area. The Coquihalla highway, in construction

during the study, provides excellent access to the map area.

20 21

Previous Work

Cairnes (1924) distinguished two Jurassic units in the

Coquihalla area, the Jurassic Ladner Series and the Jurassic-

Cretaceous Dewdney Creek Series. He fu rth er subdivided the Ladner

Series into two members. The lower member comprised predominantly

argillite (slate) intercalated near the base of the section with

tuffaceous wacke and conglomerate and constituted over 90% of the

total outcrop of the Ladner. The upper member was characterized by

conglomerate, tuffaceous greywacke, and slate. Unconformably above

the Ladner Series, tuffaceous strata referred to as the Dewdney Creek

Series crop out. South of the Coquihalla River, these two units were mapped as cropping out in a broad syncline centered on Dewdney Creek.

Only the Ladner Series was shown to crop out north o f the r iv e r .

In the vicinity of the present Carol in Mine (Fig. 6), Cairnes

(1924) described a probable unconformity which separates sediments of

the Ladner Series from the "Cache Creek Series". This contact has

been studied more recently by Anderson (1976) and Ray (1986a; 1986b).

Anderson (1976) described the rocks underlying the Ladner Group as

being predominantly gabbroic units intruded by sheeted dikes, and

suggested th at these rocks represent oceanic crust. South of the

Coquihalla River Anderson described an unconformable relationship with

fine-grained sediments of the Ladner Group overlying a le n tic u la r

fragmental volcanic unit built on the inferred oceanic crust of the

Hozameen Group (Anderson, 1976).

Through detailed mapping north and south of the Coquihalla 22 river in combination with geochemistry, Ray (1986a, 1986b) distinguished the underlying oceanic assemblage of gabbro, massive and pillowed greenstone, aquagene breccia and rare volcanic sandstone from Hozameen strata. An Early(?) Triassic age suggested for the strata is based on rare conodonts collected from a chert conglomerate in the upper part of the section. The maximum stratigraphic thickness of the unit, referred to by Ray (1986) as the Spider Peak Formation, is 500 m. I t is considered d is tin c t from the Hozameen Group, both in relative proportions of lithic constituents and major and minor trace element geochemistry on metabasalt (Ray, 1986b).

Although the Ladner Group and Spider Peak Formation presently are structurally juxtaposed (Fig. 6), a locally developed conglomerate near the base of the Ladner Group contains abundant clasts sim ilar in lithology to the Spider Peak Formation. Ray (1986b) also reports the sporadic occurrence of a preserved basal conglomerate. On the basis of these observations, the Ladner Group is inferred to depositionally overlie the Spider Peak Formation.

Jurassic strata

Ladner Group: Boston Bar Formation

Slaty rocks of Cairnes' lower Ladner unit are well exposed in readouts along the Coquihalla highway and comprise a d is tin c tiv e lithologic unit mappable throughout the western half of the Methow

Trough. The sediments are primarily mudstone and siltstone in tercalated with fin e grained greywacke (F ig . 7 ). Massive 23

STRATIGRAPHIC FOSSILS ▲ LITHOLOGY AGE NOMENCLATURE (l o c a l it y ) Jackass Cretaceous Mountain Group

Bajocian Sonninia ? — E Dewdney O Q Creek O . — ------— ^ Tmetoceras A a lenian ro Formation C l 3 (C-II8653) :• ••• :••• •• *. • o ° o * o ° o ° o 0 V CD

L. 0) — — — •— — c Toarcian TJ E Boston O O O ^ _L _L _L A Bar Wevla o (\J • ' — • — * — • — Formation iil; 'i_- Pliensbachian » —- — • ■— « — '■ • ■ ■ • —

Sinemurian > Spider Peak Triassic vz ,v A v Formation

Figure 7. Schematic b io s tra ti graphic section of Jurassic strata exposed in the Coquihalla area. 24

conglomerate lenses occur in the basal part of the section and organized pebbly mudstones (Walker and Mutti, 1973) are a minor constituent throughout. Together, the thickness of these strata is estimated to be 2000 m.

Basal member. Coarse grained strata are best exposed near

Carol in Mine (Ray, 1986b). A distinctive lensoidal conglomerate unit, the "basal" conglomerate of Ray (1986b), outcrops north of Carol in

Mine operations. Greenstone and gabhro d etritu s lo c a lly predominate within this unit, although chert (<5%), granitic (5%), volcanic(<20%), and carbonate ( %5) detritus is always present in minor proportions.

These angular to well-rounded clasts are generally less than 20 cm in diameter and are supported in a light green weathering, fine grained sandstone m atrix. The matrix is d is tin c tiv e in that i t contains coarse monocrystalline quartz presumably derived from quartz porphyry volcanic clasts.

The "basal" conglomerate is also reported to be preserved south of Coquihalla River, 1 km northwest of Mt. Snider (Fig. 6), where a conglomeratic horizon up to approximately 8 m thick over!ies(?) the Lower(?) Triassic chert-bearing conglomerate of the

Spider Peak Formation (Ray, 1986b). This horizon tapers extremely rap id ly north and south and contains limestone blocks greater than 1 m in diameter. Volcanic detritus predominates (>75%) and granitic d etritu s (<5%) is locally present. The unit is very poorly sorted with sub- to well-rounded clasts in a complete spectrum of sizes.

M atrix also appears to vary from a coarse-grained wacke to a mudstone.

The contact with the underlying strata is not exposed and the unit 25

Figure 8a. A granitic boulder found in a sheared conglomeratic mudstone of the Boston Bar Formation. Photo was taken near Carol in Mine. Figure 8b. Outcrop of intercalated argillite and siltstone of the Boston Bar Formation exposed in roadcuts of the Coquihalla highway east of Ladner Creek. Section is intruded by a dike related to the Needle Peak Pluton. 26

appears more similar to conglomeratic horizons higher in the section.

Up section from the basal conglomerate are polym ictic

conglomerate lenses that comprise approximately 10% of the section,

interbedded with tuffaceous wacke and a rg illite . The conglomerates contain well-rounded pebble to boulder size clasts of volcanic,

granitic and carbonate debris (Fig. 8a). Although locally clast

supported, this unit is generally more of a conglomeratic mudstone in which the matrix is sheared. G ranitic clasts sampled north of the

Carol in Mine operations are cu rrently being dated by U/Pb methods.

Limestone clasts sampled fo r conodonts were barren (M.J. Orchard,

pers. comm., 1987).

Tuffaceous wacke and arg illite occur in laterally continuous

thin to medium beds. The sand/shale ratio averages approximately 3/10

in this part of the section. Sedimentary structures are common within

the coarser strata and include cross and graded bedding, load casts,

rip-up clasts, rare flame and ball and pillow structures, scour, and

occasional slump features.

Overlying s tra ta . The basal member of the Ladner Group

becomes finer grained upward, passing into siltstone, argillite, and

minor fine-grained wacke which are rhythmically intercalated in thin

to medium beds (F ig . 8b). These s trata are more typical of the Boston

Formation exposed elsewhere. The sand/shale ratio decreases upward

and averages approximately 1/10 in this part of the section. Bases of

beds are generally sharp and the units appear to be laterally

continuous where cover and deformation have not obscured the outcrop

relations (ie. east of Ladner Creek). Sedimentary structures are 27 relatively uncommon in this unit and are limited to graded bedding and cross laminations observable in the coarser grained strata. Along the

Coquihalla highway, the transition to these finer grained strata can be observed in roadcuts west and east of Ladner Creek.

Ladner Group: Dewdney Creek Formation

Depositionally above the Boston Bar sediments are the coarser, immature, volcanic-rich strata of the Dewdney Creek Formation. This contact is gradational and is best exposed on the northwest flank of

Mt. Snider (F ig . 6 ). The Dewdney Creek Formation comprises three distinct volcanic-rich, sedimentary members in the Coquihalla area.

From oldest to youngest these include: 1) carbonate-bearing conglomerate, tuffaceous greywacke, and minor a rg illite , 2) volcanic- rich pebble-breccia conglomerate, tuffaceous wacke and minor a r g i l l i t e , and 3) tuffaceous greywacke and siltsto n e and minor argillite and volcanic-rich pebble conglomerate. A fourth member, characterized by volcanic breccia, tuff, and cobble conglomerate occurs along the eastern boundary of the map area. These informal members, in part facies equivalent, are different from the original subdivision of the Dewdney Creek Series into three members characterized by crystal-lithic tuff and minor argillaceous strata

(Cairnes, 1924). The thickness of the Dewdney Creek Formation is estimated to be 2500 m based on structure sections (Fig. 6).

Basal Member. On the divide between C edarflat and Dewdney

Creeks, the tran sitio n from the Ladner Group to the Dewdney Creek

Formation is marked by a buff weathering conglomerate that occurs.as a 28 massive lenticular bed with a maximum thickness of 4 m (Fig, 6).

Interbedded rusty weathering a rg illite , tuffaceous dark grey-green greywacke, and volcanic-rich pebble conglomerate occur in laterally discontinuous, thin to massive beds above the basal conglomerate.

This overlying sequence attains a maximum thickness of 150 m.

The basal conglomerate is a very poorly sorted u n it which coarsens up-section but does not display internal stratification or other sedimentary structures. It is mainly matrix supported although clast supported pockets occur throughout. Matrix is a fine grained greywacke. Angular to sub-rounded clasts reach a maximum diameter of approximately 20 cm. The detritus is predominantly felsic volcanic rocks (quartz porphyry and aphanitic "cherty" rocks) (80%) and siltstone-argillite (15%) with minor amounts of distinctive "gritty" carbonate (<5%). Belemnite casts are present. A carbonate sample processed for conodonts was barren (M.J. Orchard, pers. comm., 1987).

North of Carol in Mine (Fig. 6), the transition is marked by a black, sheared pebble mudstone which lo c a lly becomes a m atrix- supported pebble conglomerate. This conglomeratic mudstone, which constitutes approximately 30% of the section, contains elongate siltstone clasts, local well-rounded carbonate and felsic volcanic clasts, detrital quartz, and wood and rare bivalves. This unit is intercalated with pebble conglomerate (25%), tuffaceous greywacke

(25%), and minor argillite (<10%). The clast supported pebble conglomerate comprises predominantly volcanic detritus with sedimentary and rare granitic fragments also noted. It displays well Figure 9. Interbedded 1ithofeldspathic sandstone and volcanic-rich conglomerate of basal Dewdney Creek s tra ta . Photo was taken northeast of Pipestem mine. 30

developed internal s tr a tific a tio n , c la s t im brication, and channel

cuts (Fig. 9).

A poorly sorted, massive conglomerate lense similar to the

basal conglomerate described above also occurs within this section.

Carbonate clasts up to 1 m in diameter collected for radio!arians and

(or) conodonts did not y ie ld (F. Gordey, pers. comm., 1987 and M.J.

Orchard, pers. comm., 1987). This unit, estimated to be approximately

150 m in this area, is preserved as part of the eastern limb of a

truncated syncline (Fig. 4).

Overlying strata. Overlying the basal member are massive

lenticular beds of coarse volcanic-rich 1ithofeldspathic wacke that

grade into pebble breccia-conglomerate (Fig. 10). These distinctive epiclastic strata comprise approximately 70% of the overlying strata

in th is area. Subordinate units include medium bedded, ivory weathering tuffaceous siltstone (25%) and minor, discontinuous lenses

of argillite (<5%). The massive (up to 5 m thick), volcanic-rich beds

of clast-supported conglomerate form re sis ta n t ridges which weather

b u ff/g rey.

Angular to subrounded clasts w ithin the conglomerate consist

predominantly of grey and white felsic volcanic detritus with

plagioclase and quartz phenocrysts (75%), dark brown and green

andesitic feldspar porphyry volcanic rock fragments (20%), pinkish-

white banded rhyolite (<5%), tuffaceous siltstone (<5%0, and rare

limestone and granitic fragments. Belemnite fragments and casts are

present and commonly concentrated in horizons. Where volcanic clasts

predominate, the conglomerate locally appears breccia-like. Clast 31

Figure 10. Volcanic-rich pebble conglomerate characteristic of the Dewdney Creek Formation. 32

- diameter averages 5 cm. Coarse-mode grading and internal

stratification are rarely observed, Channel cuts are common, Clasts

are imbricated in rare instances. The maximum thickness of this unit

is 2000 m. This unit tapers to the north and is truncated to the

south by northeast- and north-trending fa u lts .

Exposed on Mt. Tulameen and Mt. Snider is a heterogenous unit

that appears to be in part coeval with, and a more distal equivalent

of, the predominantly coarse grained strata described above. This

unit comprises interbedded tuffaceous siltstone (35%),

feldspatholithic-1ithofeldspathic wacke (30%), crystal tuff (20%),

volcanic-rich pebble conglomerate (15%), conglomeratic mudstone (5%),

arg illite (5%), and rare calcareous horizons. The maximum thickness

of this unit is 1500 m.

Tuffaceous siltstones crop out in ivory weathering, thin to

medium beds that appear laterally continuous (Fig. 11). Dark green

weathering fine to coarse grained greywackes occur intercalated in

medium to massive beds. Abundant sedimentary structures in these

sediments include ball and pillow structures, flames, load casts,

channel cuts, and grading. Slump and fold structures associated with

soft sediment deformation characterize the finer grained strata.

Pebble conglomerates are similar in composition to those described

above but generally are more thinly bedded.

Rare limestone horizons, each approximately 20 cm thick, were

discovered ju s t west of Mt. Tulameen summit. Chip samples collected

for conodonts/radiolaria were barren (M.J. Orchard, pers.comm., 1987).

Crystal t u f f occurs as massive ( average 2-3 m) interbeds exposed on 33

Figure 11. Interbedded tuffaceous greywacke and siltsto n e is c h aracteristic of Dewdney Creek strata exposed on Mt. Tulameen and Mt Snider (Fig. 6). 34

the conical peak ju s t west of the Tulameen summit. I t is a dark grey weathering andesitic(?) unit with plagioclase and rare augite

phenocrysts. This unit is not recognized elsewhere, although augite

phenocrysts characterize andesite fragments of the Blackwall Peak

(Manning Park) breccia.

On the northwestern flank of Mount Snider, argillaceous strata

of the Ladner Group grade upward into th is heterogeneous Dewdney Creek

unit. Intercalated pebble conglomerate, tuffaceous siItstone and wacke in equal proportions characterize the transition. The contact

is located approximately where arg illite within the section becomes a

d is tin c tly minor component. An unconformity has not been recognized.

The absence of limestone-bearing conglomerate characteristic of the

basal Dewdney Creek Formation to the east, and lack of s ig n ific a n t

amounts of volcanic-rich pebble-breccia conglomerate, suggest that

telescoping of the strata juxtaposed partial (?) lateral equivalents in

this area (Fig. 6).

Volcanic rocks. Caught between strands of the Chuwanten fa u lt

in the Treasure Mountain area (Fig. 6) is a section of andesitic

volcanic breccia, volcanic-rich cobble conglomerate, crystal-lithic

tuff and tuffaceous sediments. Based on lithic similarities with

Middle Jurassic volcanic strata both north in the Anderson River area

(Chapter 3) and in the Manning Park area (Chapter 5) th is section is

considered to be part of the Dewdney Creek Formation. The strata are

d is tin c t from Cretaceous rocks in the area (Chapter 4) which are

characterized by cobble conglomerates with abundant foliated granitic

detritus (>30%) and a lithofeldspathic matrix with detrital muscovite. 35

Volcanic-rich cobble conglomerate that is clast supported forms massive ( approximately 5 m thick) le n tic u la r horizons which outline the trace of the Chuwanten fault on the ridge northwest of

Treasure Mountain (Fig. 6). The sub- to well-rounded clasts are predominantly green and maroon andesitic feldspar porphyry and fine­ grained felsic (cherty(?)) volcanic rocks, along with a few (<10%) unfoliated granitic clasts. Clasts range up to 15 cm in diameter.

The matrix is a dark green weathering 1ithofeldspathic wacke. There is, however, an unusually high proportion of quartz detritus within this unit (Appendix 2).

Volcanic breccia crops out primarily as a structureless mass between two conglomeratic horizons on the western flank of Treasure

Mountain. It weathers dark green and purple, with angular clasts up to 20 cm noted. Ivory weathering tuffaceous siltstone interbedded with volcanic-rich pebble conglomerate and feldspatholithic wacke similar to strata described above account for approximately 60% of this unit. Minimum thickness of the unit is 300 m.

Biostratigraphy

Poor biostrati graphic control in the Coquihalla area led

Cairnes (1924) to conclude that the Ladner Series was Jurassic and the

Dewdney Creek Series was Jurassic-Cretaceous in age. More recent mapping in the area within predominantly argillaceous strata of the

Ladner Group yielded only species of the bivalve Buchia in a greywacke u nit near Pipestem mine (Ray, 1986) and the bivalve Weyl_a discovered by the writer in 1985. Ray's (1986) Buchia are indicative of a Late 36

Jurassic (Oxfordian) age. East-northeast of the Buchia locality and separated from it by a series of north- and northwest-trending faults, the bivalve Weyla was retrieved. This occurrence indicates an Early

Jurassic age (Sinemurian to Toarcian) for these strata (Damborenea and

Mancenido, 1979).

Fossils retrieved from the Dewdney Creek section south of the

Coquihalla River include belemnites indicative of a general Jurassic-

Cretaceous age and two new localities that contain poorly preserved ammonite imprints. From a sheared arg illite directly above limestone­ bearing conglomerate of the basal Dewdney Creek member, imprints of the genus Tmetoceras(?) (C-118653) (H.W.Tipper and P.L.Smith, pers. comm., 1985) were retrieved (F ig . 6) which indicates a minimum age of

Aalenian for this transitional unit.

The second locality with diagnostic fossils is located near the peak of Mt. Tulameen, near where Cairnes (1924; identification by

McClearn) reported the Sinemurian arnioceratid occurrence within float

(F ig . 6 ). Both belemnites and rare ammonite imprints were found in place within the section near the peak (Figs. 12a, 12b). Although the ammonites cannot be identified with certainty, they resemble early

Bajocian Sonninids collected from the Anderson River area (see below)

(H.W. Tipper, pers. comm., 1986). The "arnioceratid" collected by

Cairnes has been misplaced (T.P. Poulton, pers. comm., 1986) and is therefore not available for re-evaluation. It should be noted, however, that arnioceratids and sonninids have been confused in another study by McClearn (Tipper, pers. comm., 1986). 37

Figure 12a. Aligned belemnites on the bedding surface of a tuffaceous siltsto n e found d ire c tly west of the summit of Mt. Tulameen. Figure 12b. An ammonite (Sonninia sp.(?)) imprint was found 2 m above the belemnite surface! 38

Intrusive Rocks

The Needle Peak Pluton, a granodioritic to quartz monzonitic in trusive body (Monger, 1970) has yielded 40 Ma (Wanless e t a l ., 1967) and 45 Ma (K-Ar ages on hornblende) (C.Greig, pers. comm., 1987). It intrudes Jurassic strata in the northeast part of the map area and occupies more than 200 square km (Ray, 1986) of the Methow Trough.

Massive sills and dikes interrupt the country rock for more than a 2 km radius about this intrusion. Numerous smaller intrusive bodies of variable composition and commonly characterized by abundant acicular hornblende occur within the study area. No effort to differentiate these bodies has been made, however Ray (1986b) discusses criteria for recognition of intrusives associated with the Needle Peak Pluton.

Structural Relations

The main structure developed in the Jurassic section in this area comprises a N30°W-trending syncline originally recognized by

Cairnes (1924). This feature plunges to the southeast, exposing older rocks to the north. Uplift and warping of the strata associated with intrusion of the Needle Peak Pluton enhances this trend. In d e ta il, however, the structure is much more complicated than that illustrated by Cairnes (1924) (Figs. 6 and 13). For this reason, thickness estimates of Cairnes' Jurassic units are considered maximums.

Fold hinges are generally not preserved at outcrop scale, but medium scale folds are inferred from dips of units and facing direction within the section. The amplitude of these folds is v aria b le, ranging from 500 m to 1 km. T ig h t, sm all-scale folds plunge 39 at shallow angles (generally less than 10°) to the northwest or southeast. They are asymmetric (generally with steep eastern limbs) and occur with amplitudes of tens of meters. Exposures on the north face of Mt. Tulameen (F ig . 4) best illu s tr a te the complex nature of the structure.

Two major fa u lts , the Chuwanten and Hozameen fa u lts , p arallel the trend of the dominant folds within the area. The subvertical

Chuwanten fault juxtaposes Jurassic strata structurally over

Cretaceous rocks. The fault is imbricated and does not contain breccia or gouge. SIickenslides Were not seen. East of this fault within the trough, no Jurassic rocks have been identified and therefore any lateral offset along the Chuwanten would be difficult to document.

The Hozameen fa u lt occurs as two main splays th at are subvertical (Ray, 1986a). The Coquihalla serpentine belt is bounded to the west by the Hozameen Group and to the east by the Spider Peak

Formation. Further east the contact between the Spider Peak Formation and the Ladner Group is a covered interval. Movement along the contact between Spider Peak and Ladner rocks is inferred from the attitude and appearance of the adjacent strata of each unit. It is difficult, however, to estimate the offset, nature, and age of the structure. Despite this observation, the Spider Peak Formation likely represents "basement" to the Methow s trata as inferred by Ray (1986b).

NW-trending faults are cut by N30°E-trending subvertical, . fa u lts which appear to have both appreciable dip- and s trik e -s lip components (Fig. 6). One of these structures is observed to bound the 40

Needle Peak Pluton for much of its SE side, which indicates that final movement on these faults is post-Late Eocene. North-trending vertical dip-slip(?) faults are observed to cut all structures and represent the latest phase of deformation recognized within the area. CHAPTER 3

ANDERSON RIVER AREA

In the Anderson River area, east and southeast of Boston Bar, the Methow Trough is s tru c tu ra lly narrowed to less than 4 km in width

(Fig. 3). This area was firs t mapped by Cairnes (1944) and shown to be predominantly marine and nonmarine(?) Lower Cretaceous sediments of the Jackass Mountain Group. Monger (1970) subdivided these rocks into

Lower Cretaceous marine strata of the Jackass Mountain Group and nonmarine strata of the Pasayten Group (R ice, 1947).

Reconnaissance mapping in the area in the summers of 1982 through 1985 (J.W.H. Monger, pers. comm., 1985) documented the occurrence of both the Pasayten and Jackass Mountain s trata as well as a thick, fault-bounded section of the Dewdney Creek Formation.

Stratigraphic relations between units are not preserved and sections are structurally repeated. It is from this area, however, that c r itic a l age constraints have been determined for Dewdney Creek strata, as they contain a relatively abundant, moderately preserved marine fauna (see below and Appendix A ). Fossil c o lle c tin g and reconnaissance mapping (1:50,000) of the Dewdney Creek s trata was undertaken in the summer of 1985 (F ig . 13 - in pocket. Appendix C).

Active logging roads provide reasonable access to th is forested area.

41 42

Jurassic strata

Ladner Group: Boston Bar Formation

The Boston Bar Formation in the Anderson River area crops out as a fault-bounded sequence of slate with minor intercalations of fine-grained wacke. This incomplete section is approximately 700 m thick with the base and top of the formation truncated. It does represent, however, typical exposures of Ladner Group without the lo c a lly s ig n ific a n t basal conglomeratic horizons. Bedding relationships are generally obscured by a slaty cleavage. Where observed, however, bedding dips to the northeast (approximately 30°)

(Fig. 14). •

Ladner Group: Dewdney Creek Formation

Dewdney Creek strata outcrop in two fault-bounded panels, separated by a section of Jackass Mountain conglomerate (Fig. 14).

The western panel comprises a section of predominantly tuffaceous, fine to coarse-grained, medium to thick bedded greywacke, with massive lenses of volcanic-rich pebble conglomerate and rare arg illite.

Approximately 770 m of section is exposed. Stratigraphic relations and relative proportions of these lithologies are similar to those of the distal Dewdney Creek strata in the Coquihalla area. Covered in tervals separate these strata from Ladner slate to the west and

Jackass Mountain conglomerate to the east. The strata are deformed in 43

Figure 14. Interbedded siltstone and argillite of the Boston Bar Formation. Photo was taken at Dubariceras locality, 10 km north of Boston Bar along Highway 1. 44

open folds observable on outcrop scale.

The eastern panel is a predominantly volcanic u n it characterized by coarse volcanic breccia, volcanic flow rocks, volcanic-rich pebble to cobble conglomerate, and fossiliferous tuffaceous siltstone and sandstone. In most areas, tuffaceous sediments and volcanic-rich pebble conglomerate predominate near the base whereas the upper h a lf of the section is massive volcanic breccia

(Fig. 15) and minor volcaniclastic strata capped by an andesitic volcanic flow approximately 5 m thick. A covered interval separates the top of the section from Cretaceous sediments to the east. On

Stoyoma Creek road, this section structurally overlies Jackass

Mountain conglomerate.

The section is approximately 1000 m thick in this area whereas fu rth er to the south in the East Anderson River area the eastern panel

"expands" to include a thicker section of tuffaceous s tra ta .

Andesitic volcanic breccia is localized in lenticular massive units and volcanic flows are not seen. Just north of the confluence of the

East Anderson and Anderson Rivers, the two Dewdney Creek fault-bounded sequences are inferred to merge. South of that point, no Cretaceous rocks are recognized except directly west of the Pasayten fault.

Biostratigraphy

Due to structural complexity and poor exposure in the Anderson

River area, stratigraphic relations are difficult to constrain. The faunas therefore provide critical age determinations for construction of a schematic stratigraphic section necessary for structural 45

Figure 15. Andesitic volcanic breccia is characteristic of the upper Dewdney Creek Formation. The u n it is well exposed on Stoyoma Creek road. 46

STRATIGRAPHIC LITHOLOGY FOSSILSA NOMENCLATURE] (l o c a l it y ) AGE

FAULT

A v v v v v ? Chondroceras r , A 7 D ? ^ A O o Stephonoceros (C-II865I) Early Witchellia Bajocian •o60. o 0 d - ? o : ° Dewdney U-T—7-T^5AJ (c-11827 2) Sonninia Creek (C-II8 27 2^ E o Posidonia o Formation fC-118676) o Ervcitoides C\J ••• Aal eni an CL fc -118676) =j Tmetoceras 7 ^ <7^V ^ V 4 7 o ---- — — (c-88070,C-88069 0-118672,0-118651

FAULT

Figure 16. Schematic b iostratigraphic section of Jurassic strata exposed in the Anderson River area. 47 interpretations (Fig. 16). Age constraints on the Dewdney Creek

Formation in this area provide a basis for correlation with units in the Coquihalla area and to the south. The only fossils collected from slaty Ladner strata were collected by J.W.H. Monger in 1984 from a locality north of Boston Bar on Highway 1. This locality yielded relatively well preserved specimens of the Tethyan ammonite genus

Dubariceras (GSC Loc. C-087352). This is the first documentation of

PIiensbachian strata in the Methow Trough and is therefore of particular significance.

Eight ammonite localities with relatively abundant specimens were found w ithin the Dewdney Creek section. The ammonites are preserved in tuffaceous siltstone and fine to medium grained greywacke and are commonly associated with various bivalves (T.P.

Poulton, pers. comm., 1986) and abundant woody debris. Two distinct ammonite associations are represented in five of these localities.

The other three localities contain poorly preserved fragments which cannot be identified with confidence to the genus level.

The first association is characterized by the occurrence of the ammonite genus Tmetoceras. Tmetoceras, previously reported from the Manning Park section (Frebold e t a l ., 1969), is the most abundant, well preserved ammonite collected from the Anderson River area. It occurs exclusively in two out of four localities (C-88069, C-88070)

(Plate 1, fig s . 1 -3 ). In one lo c a lity Tmetoceras is found in association with a probable Plannamatoceras or Hammatoceras imprint

(C-118672) (Plate 1, fig. 4) (H.W. Tipper, pers. comm., 1986) and in another with Erycitoides(?) and the bivalve Myophorella (C-118651) 48

(T.P. Poulton, pers. comm.» 1986). Tmetoceras 1s generally restricted

in European sections to the Scissum zone of the Aalenian stage,

however, in North America i t is believed to occur throughout the

Aalenian (Appendix A; H.W. Tipper, pers. comm., 1986).

The second association is represented in only one locality.

It is characterized by a more diverse fauna with a minimum of two

genera, Witchellia and Sonninia (Euhoploceras) (C-117272). An attempt

at subgenus/species level identification was frustrated by the moderate preservation of the material and the confusion surrounding

the Sonninid classification. There is, however, clearly more than one

species of Sonninia represented (Plate 3, Figs. 1-3). This fauna is

ch ara c te ris tic of the early Rajocian in both European and North

American c la s s ific a tio n s , s p e c ific a lly the local Acanthodes zone

(Appendix A).

Three localities in the area contain fragments of different

genera from the main associations discussed above, but were not as

well preserved. The first includes the bivalve Posidonia (H.W.

Tipper, pers. comm., 1985) with deformed imprints of Erycitoides

(Plate 1, figs. 5, 6). A probable Aalenian age is assigned to this

locality. The second includes fragments of two different

Stephanoceratids, clearly indicative of the mid-Bajocian. Both

collections are from isolated outcrops of highly sheared argillite, a

rock type known to occur throughout the section. There is, therefore,

no stratigraphic control for these occurrences.

The third locality is from a tuffaceous siltstone directly

below a belemnite-bearing volcanic pebble conglomerate on the Stoyoma 49

Creek road. This unit is overlain hy volcanic breccia and andesitic flow rocks. A poorly preserved partial specimen (venter) of the ammonite Chondroceras(?) was retrieved (H.W. Tipper pers. comm.,

1985).

Structural Relations

Predominant structures in the area are northwest-trending faults that separate various lithologic units within the trough.

These fa u lts p a ralle l the boundary structures of the Methow Trough

( i . e . Pasayten and Hozameen fa u lts ) and must have accommodated significant dip-slip motion to achieve the present distribution of units (Fig. 13). Late, ME-trending faults are inferred to cut these

MW-trending faults, including the Pasayten fault(?), however, limited exposure severely constrains documentation of the structural relations. The M-trending Anderson River fault of Ray (1986a) that cuts the Hozameen fault represents the latest phase of brittle deformation. Abundant M-trending 1 inears identified on high level aerial photographs have little documented offset and do not appear to represent significant structural features.

Moderately tight, minor folds with amplitudes of less than 10 m are preserved in tuffaceous strata of the Dewdney Creek section south of the East Anderson River (F ig . 13). The folds plunge uniformly to the southeast at approximately 10°. To the north, the structural style changes to more large scale (>50 m) open folds. The poor understanding of the continuity and relation of these structures in the area precludes reasonable thickness estimates for the section. 50

The well-developed, subvertical slaty cleavage imposed on the

Ladner Group is best exposed in roadcuts on the main Anderson River road. A near horizontal stretching 1ineation in the slaty cleavage is approximately parallel to the regional trend (i.e. N20°W). The intensity of deformation decreases rapidly to the east (Fig. 13), thus the fabric is attributed at least in part to early Tertiary movement on the Fraser fault. The Hozameen fault is truncated by the Fraser fault approximately 2 km south of the Anderson River area (Fig. 13). CHAPTER 4

PETROGRAPHY

Petrographic studies of the Methow strata are limited. Theses carried out at the University of Washington (Tennyson, 1972; Cole,

1973; Tennyson, 1974; T re x le r, 1985) comprise most studies which are

based on the work of Barksdale (1975). Others are currently in progress (V. Todd, pers. comm., 1987). Published data from c o rre la tiv e and (or) coeval units in the Tyaughton Trough are even more scarce (Kleinspehn, 1985). Most studies tend to focus on the

stratigraphy of Cretaceous strata (Cole, 1973; Tennyson, 1974;

Kleinspehn, 1985; Trexler, 1985) due to better exposure of the

Cretaceous section. Petrographic work on the less well known Jurassic

units includes studies which are limited in field studies of the

Jurassic section (Tennyson, 1972; Cole, 1973). The stratigraphic

position of analyzed samples and their significance are therefore

difficult to ascertain.

Any tectonic scenario developed for the Methow Trough

(Tennyson and Cole, 1978; Trexler and Bourgeois, 1986) is constrained

by the petrographic data presented in the above mentioned studies.

Petrographic studies therefore represent important contributions to

our understanding of the evolution of the trough.

51 52

From observing the Jurassic lithologies in outcrop, it is readily apparent that the detritus is dominantly volcanic. The rocks, therefore, seem unlikely candidates for traditional point count methods as the primary function of detrital mode data is to enable provenance interpretations to be made (Dickinson and Suczek, 1979).

Furthermore, due to in c ip ie n t low grade metamorphism, the grains are, at best, difficult to count. Despite these considerations, however, point counts were undertaken to enhance fie ld observations and u n it descriptions. Normalized detrital modes (ie. QFL) and particularly the variatio n in accessory framework minerals document the subtle distinction of petrofacies within the Jurassic section.

Petrographic methods

More than 70 thin sections of Ladner and Dewdney Creek strata sampled north of the international boundary were examined. For comparison, 2 of the Thunder Lake sequence, 15 of the Jackass Mountain

Group, and 4 of the Pasayten Group were also included. From the comparison charts of Folk (1968), visual estimates of the percentage of matrix material, detrital grain framework mode, degree of sorting, sp h ericity, and roundness were noted for each section. D iffic u lty was often encountered in discerning primary matrix and in identifying altered clasts due to diagenesis and/or incipient low grade

( prehnite/pumpel 1y it e ) metamorphism. This lim ited the number of countable thin sections to those in which grain boundaries were well preserved, alteration was at a minimum and matrix content constituted less than 25% of the total section. 53

Of the 90 sections studied, 28 thin sections of fine- to medium-grained (mean grain size estimated to be 4mm) sandstones were

selected as suitable for point counting. These sections include 20

Dewdney Creek, 1 Thunder!ake, 4 Jackass Mountain, and 3 Pasayten

representatives. The Ladner Group samples were e ith er too fin e ­

grained or too altered to provide sections suitable for counting.

The sections were stained for potassium feldspar and counted

using the Gazzi-Dickinson point count method (Dickinson, 1970;

Ingersoll et a l., 1984). A summary of components and criteria for

their recognition are included in Table 2. For each section, greater

than 400 points were counted to maximize r e lia b ilit y (Van Der PI as and

Tobi, 1965). Point intervals were a minimum of .2mm greater than the

average grain size of each section. Appendix B contains the

normalized raw point counts to framework percent which excludes m atrix, cement, in tra c la s ts , and opaques.

Data from the representative thin sections is displayed on

ternary discriminant plots (Figs. 17, 18) to illustrate different

characteristics of the units studied (Dickinson and Suczek, 1979).

The mean ratios for each unit calculated for the ternary diagrams are

included on Table 3. For further emphasis of subtle petrographic

variations w ithin the section, a bar graph which displays the

variation of framework minerals is shown in Figure 19. 54

TABLE 2. Description of grain categories used for point counting.

GRAIN CATEGORY DESCRIPTION

Monocrystalline low relief, straight to slightly undulose quartz (Qm) extinction, inclusions and/or dust trails common; common grain embayments distinguish volcanic quartz.

Polycrystalline quartz in a microcrystalline or cryptocrystalline quartz (Qp) mosaic ( ie . c h e rt); meta-si 1 iceous volcanic fragments may not have been consistently distinguished.

Plagioclase (P) low relief, commonly twinned, compositional zonation, uneven extinction; often "moth eaten", albite replacement, sericitized.

Potassium both orthoclase and microcline are stained yellow feldspar (K)

Muscovite clear, moderate relief, cleavage and high birefringence

Heavy Minerals includes pyroxene (most common) and rare epidote, amphibole, o liv in e (? ), sphene.

Opaques includes organic matter and pyrite

Volcanic rock felsitic, micro!itic, lathwork and vitric fragments (Lv) volcanic fragments; igneous texture recognized in altered fragments; includes meta-equivalents.

Sedimentary rock fine grain sandstone, s iltsto n e and mudstone; fragments (Ls) includes meta-equivalents.

Carbonate rock - high birefringence, generally well-rounded fragments (Lc)

Cements primary clay and carbonate rarely distinguished.

Matrix generally secondary and includes: chlorite, zeolite (heulandite/laumontite), quartz, potassium feldspar, prehnite, pumpellyite, albite

Intraclasts argillaceous rip-ups 55

Q

ASAYTEN GROUP ® Unit Mean

JACKASS MOUNTAIN GROUP

THUNDER LAKE * SEQUENCE * l DEWDNEY CREEK • FORMATION

Figure 17. QFL diagram illu s tr a tin g the data points and means of the units of the Methow Trough defined in southwestern British Columbia. UNIT

Pasayten Jackass Mm. Thunder Lake Dewdney Ck.

RECYCLED

OROGEN

DISSECTED

TRANSITIONAL

UNDISSECTED

Figure 18. OFL and OmFLt diagrams superimposed illu s tr a tin g the change in unit mean by considering Qp as a lithic fragment. Provenance fie ld s re fe r to the QFL diagram. 57

Kp (3)

Kjm (2)

JKs i (4)

mJdcv (4)

mJdcsS (1 0 )

mJdccg (4)

mJdc (1 )____ Map 10 30 5 15 .015 .04$ .01 .03 .015 .049 Units Qm QP K F Auqite Mica

Figure 19. Variation of d e trita l framework components (normalized percentage) up through the strati graphic section. 58

Table 3. Calculated mean QFL and QmFLt ra tio s .

UNIT N QFL QmFLt

Pasayten Group 3 60/23/17 41/23/36

Jackass Mountain Gp. 2 50/34/16 29/34/37

Thunder Lake/Hauterivian 4 9/51/40 7/51/42

Dewdney Creek volcanic 3 2/62/36 1/62/37

Dewdney Creek distal 10 5/44/51 3/44/53

Dewdney Creek proximal 4 2/47/51 1/47/52

Dewdney Creek basal 1 0/55/45 0/55/45

Petrographic Descriptions

Boston Bar Formation

Petrographic studies of the basal Ladner Group indicate that

the sediments are predominantly very fin e grained, submature

feldspathic sandstones and a r g illit e s . The samples are moderately to

highly altered with chlorite, cal cite, prehnite, and pumpellyite the

common alteration products. Less common authigenic constituents

include albite and sen*cite. In proximity to the Needle Peak Pluton,

andalusite, biotite and rare garnet porphyroblasts characterize the

contact metamorphosed Ladner sediments. Secondary sulphides ( i . e .

p y rite) are also a common accessory component. A well developed

foliation is present in all fine grained lithologies, while those

samples taken in close proximity to the Hozameen fault display a 59

Figure 20. Photomicrograph illustrating fabric characteristic of fine grained strata of the Boston Bar Formation (MV085-119). (Plane light, magnification 2X, abbreviations as in Table 2.) 60 crenulated foliation (Fig. 20). In coarser grained samples of the

Boston Bar Formation, visual estimates of framework grains include monocrystalline quartz (< 10%), plagioclase (< 50%), volcanic rock fragments (< 40%), plutonic detritus (< 10%), and sedimentary rock fragments (< 10%). Chert, potassium feldsp ar, or pyroxene d etritu s that are present as minor components in the overlying Dewdney Creek strata were not observed. Volcanic rock fragments were predominantly micro!itic, with felsitic grains a minor but common constituent.

Sedimentary rock fragments include mudstone intraclasts and foliated s ilts to n e . Carbonate clasts are also a subordinate component.

Although no point counts were attempted on Boston Bar samples, visual estimates suggest that on ternary diagrams the Boston Bar Formation sections would define a sim ilar fie ld to the Dewdney Creek Formation representatives (i.e. volcanic arc-derived).

Dewdney Creek Formation

Coarse-grained, volcanic-rich strata of the Dewdney Creek

Formation are quite distinct within the Methow section. This heterogeneous formation, subdivided into 3 informal sedimentary members in the Coquihalla area, is characterized by volcanic-rich immature fe ld sp ath o lith ic sandstone (F ig . 21) and lith o fe ld sp a th ic breccia-like conglomerate. Crystal-!ithic tuff, andesitic breccia, and volcanic flow rocks comprise significant but minor constituents within the section. The observation that the Dewdney Creek Formation in the

Coquihalla area is predominantly epiclastic is contrary to petrographic descriptions of the units by Cairnes (1924), who 61

Figure 21a. Photomicrograph of a typical tuffaceous 1ithofeldspathic sandstone (MV085-65) of the Dewdney Creek Formation. (plane lig h t, magnification 10X, abbreviations as in Table 2.) Figure 21b. Photomicrograph of MV085-65 in polarized lig h t. 62 identified most rocks of the Dewdney Creek Series as verities of tuff. Glass shards, pumice and vesicles were not present in most thin sections. Alteration of the matrix to chlorite, zeolites

(heulandite/laumontite(?)), quartz and potassium feldspar is prevalent.

The sediments are rich in both volcano!ithics and plagioclase

(andesine) with each constituting an average of approximately 50% of the to tal framework population. Again, the dominant volcanic species that comprises up to 95% of the total lithic component is micro!itic grains. Lathwork, vitric and felsitic volcanic fragments are always present in minor proportions but together never constitute greater than approximately 15% of the total lithic population. The sections are poor in both mono- and p o lycrystallin e quartz and potassium feldspar. Monocrystalline quartz is predominantly volcanic in origin and accessory chert is recognized in approximately half of the sections

The basal Dewdney Creek member marks a petrologic and petrographic transition. Common quartz predominates over other quartz species. Plagioclase comprises greater than 50% of the framework grains. Carbonate clasts are a distinctive but subordinate component considered to be lithic fragments (Lc) rather than intraclasts.

Megascopic carbonate clasts characterized by the presence of fragmented crinoid columnals and rare solitary corals are dissimilar to the rare unfossiliferous calcareous lenses recognized within the

Jurassic section.

In contrast to this, samples from overlying Dewdney Creek strata contain greater than 50% volcano!ithic detritus. 63

Monocrystalline quartz is predominantly volcanic in origin and accessory pyroxene (augite) is found in every thin section. The presence of well-rounded, rare chert fragments in most samples is also significant. On ternary plots (Figs. 17, 18) the Dewdney Creek samples d elim it a fie ld c h ara c te ris tic of sediments derived from a tra n s itio n al volcanic arc, which suggests local or in te rm itte n t exposure of cogenetic plutons (Dickinson and Suczek, 1979; Dickinson et a l., 1982). Alternatively, the samples may represent mixed provenance with d etritu s from local volcanic sources dominant over marginal source terranes.

Thunder Lake sequence

The Thunder Lake sequence and Hauterivian Jackass Mountain

Group are considered together. An outcrop studied in detail along the northwest shore of Thunder Lake illu s tr a te s that the lith o lo g ie s are indisting uishable. All are fin e-g rain ed , submature fe ld sp ath o lith ic sandstones (F ig . 22, 23). Normalized d e trita l framework modes are s ta tis t ic a lly sim ilar although differences in accessory framework grains are noted. While the Thunder Lake sample contains 2.5% pyroxene, the Jackass sections contain mica ( approximately 1%), pyroxene and (or) hornblende, which comprise up to 13% of one section

(MV085-284). Whether these differences are s ig n ific a n t or not is difficult to ascertain in view of the limited number of samples from these u n its. On ternary plots (F ig . 17, 18) the Thunder Lake and

Hauterivian samples also occupy part of the field indicative of.a transitional arc but the detrital composition is clearly distinct from 64

Figure 22a. Photomicrograph of a Thunder Lake sandstone (MV085-340). (plane light, magnification 10X, abbreviations as in Table 2.) Figure 22b. Photomicrograph of a Hauterivian sandstone (MV085-341) from the overlying Jackass Mountain Group, (plane lig h t, magnification 10X, abbreviations as in Table 2 .) 65

Figure 23a. Photomicrograph of MY085-340 in polarized lig h t. Figure 23b. Photomicrograph of MV085-341 in polarized lig h t. Abbreviations as in Table 2. 66

Dewdney Creek samples (F ig . 19).

Jackass Mountain and Pasayten Groups

The younger Jackass Mountain Group lithologies are distinct from the Upper Jurassic and Lower Cretaceous sediments described above in that the quartz content increases significantly (Figs. 17-19).

Based solely on d e trita l framework mode calcu latio n s, however, they are difficult to distinguish from Pasayten Group lithologies. This observation fu rth er emphasizes the problems with the presently defined

Cretaceous units north of the international boundary discussed in

Chapter 1. Petrographic studies south of the boundary are suggestive of d is tin c tiv e petrofacies which cannot be broken out from the Jackass

Mountain and Pasayten Groups as presently mapped.

The Jackass Mountain section includes a variety of sediments which vary from a d is tin c tiv e submature, micaceous boulder conglomerate with an abundance of granitic clasts to a very fine grained, submature fe ld sp ath o lith ic sandstone. In contrast, medium grained, mature, chert-bearing quartzofeldspathic arenites are considered ch ara c te ris tic of the Pasayten Group. As mapped, however, the Pasayten Group also includes a variety of sedimentary units which overlap in composition with the Jackass Mountain Group (Maclean,

1986).

Discussion

Although the variation of petrofacies through the Jurassic 67

section is subtle, it is distinct and important. The submature, fine­

grained tu rb id itic sequence c h ara c te ris tic of the basal Ladner Group

represents sediments derived from an arc terran e. The in flu x of

immature, proximal volcanic-rich sediments and volcanic rocks in the

early Middle Jurassic suggests that sedimentation became dominated by

local volcanic activity. Relatively mature Upper Jurassic-Lower

Cretaceous sediments that contain a minor plutonic component signal a

change in sedimentary regime controlled by an increase in uplift and

erosion rates, likely associated with deformation.

The fields defined in ternary diagrams by the detrital mode data for different Jurassic units are in general agreement

with previous work despite the fact that those data sets were likely

derived from samples collected from a combination of a ll currently

defined Jurassic units (Fig. 24). Because of the overwhelming influx

of volcanic detritus within the Jurassic section, the subtle variation

illu s tra te d in accessory framework grain compositions is more

significant. The ternary diagrams, useful for general provenance

analyses, do not illustrate any significant variation in sedimentation

during the Jurassic.

Sedimentary strata of the Ladner Group can be characterized in

the facies terms and associations originally defined by Mutti and

Ricci-Lucchi (1972), modified subsequently by Walker (1978). Chaotic

deposits of bouldery mudstone and poorly sorted conglomerate which

characterize the basal Ladner member are interpreted to represent

slump products (Facies F). These lensoidal deposits occur within

interbedded a r g illit e and sandstone sequences best described as the 68

Q

A Dewdney Creek Ladner (Tennyson, 1972) X Dewdney Creek 8 Mean □ Ladner ■ Mean

Dewdney Creek Mean Field outlined (Current study)

TRANSITIONAL

UNDISSECTED

A A h

Figure 24. A comparison of OFL ratios calculated for the Dewdney Creek Formation. Triangles are Dewdney Creek/Ladner samples from Tennyson (1972) (no mean c alcu lated ). X's are Dewdney Creek samples of Cole (1974) with the mean value in a box. Squares are Ladner samples of Cole (1974) with the mean filled in. The current study Dewdney Creek mean is a diamond with the field of values outlined. 69

finer-grained members (Facies D and E) of a classic Bouma turbidite

sequence. The stratigraphic relations observed in this basal section

are considered c h ara c te ris tic of submarine fan associations,

particularily a middle fan association. The overlying rhythmic

interbeds of laterally continuous arg illite and less common fine­

grained sandstone constitute a pelitic-arenaceous sequence (Facies D)

considered characteristic of an outer fan association.

Estimates of the d e trita l framework mode and m aturity

of representative samples indicate that these Sinemurian(?) to

Aalenian(?) feldspathic sediments most closely resemble distal

derivatives of a transitional volcanic arc (Dickinson et a l., 1982).

Paleocurrent indicators within this part of the section are rare and

difficult to interpret due to structural complexity within the area.

Previous workers, however, favour an eastern derivation for the

sediments and in te rp re t the Ladner sediments to represent an eas te rly-

derived turbidite prism (Coates, 1970, 1974; Ray, 1986b). Ray (1986b)

further suggests th at the lower coarse c la s tic member of the Ladner

Group represents a deep-water channel slope or tu rb id ite fan deposit.

From data presented in this study, the Lower Jurassic strata

are considered characteristic of the more distal portions of submarine

fan deposits. The vertical succession of interpreted middle fan

deposits overlain by outer fan deposits suggests the occurrence of

syndepositional basin subsidence and/or marine transgression. The

lack of more proximal sediments within the Early Jurassic section

fru strates any attempt to d elim it an eastern margin to the

depositional basin. 70

The basal member of the Dewdney Creek Formation, characterized by massive, poorly sorted conglomerate lenses and associated coarse grained sandstone (Facies A), indicates a significant change in the deposit!"onal environment. With no angular unconformity recognized at the contact with underlying strata, the influx of these coarse-grained strata can be explained either by a rapid marine regression and/or transgression of submarine fans. The transgression of fans may in part be a response to uplift and erosion of marginal source terrane(s) and is certainly in part due to intrabasinal volcanism.

Calc-alkaline andesitic lavas associated with massive lenses of volcanic breccia, poorly sorted conglomerate and finer grained tuffaceous strata (Facies A) are recognized in local concentrations along the axis of the trough and represent the eastern-most outcrops of the Dewdney Creek Formation. The distribution of these proximal deposits in association with locally significant volcanic flow rocks suggests that volcanic centers were active w ithin the depositional basin during the Aalenian(?) and early Bajocian. (Coates, 1970;

O'Brien, 1986).

Regional lateral continuity is displayed by more distal equivalents of the volcanic s tra ta . These e p icla stic rocks comprise interbeds of volcanic-rich pebble conglomerate and lithofeldspathic- feldspatholithic sandstone in varying proportions and minor argillite.

These sediments are inferred to have been derived predominantly from local volcanic centers on the basis of both compositional similarity and immaturity of the detritus.

Jurassic and Early Cretaceous strata of the Methow Trough were 71 previously interpreted to have accumulated in a west-facing forearc basin (Dickinson, 1976; Tennyson and Coles, 1978) related to the Late Triassic Nicola arc. As discussed above, it is clear that the Jurassic section does not constitute a forearc basin in the strict sense (Dickinson, 1976) because: 1) deposition of the Sinemurian(?) to early Bajocian strata occurred primarily post-Nicola magmatism and 2) intercalated calc-alkaline lavas are present in the section in association with proximal pyroclastic deposits.

The lack of Lower Jurassic proximal strata makes it difficult to reconstruct an eastern margin for the depositional basin during this time in te rv a l. Perhaps in part due to the overwhelming volume of volcanic detritus introduced into the section in the early Middle

Jurassic, there is little evidence of detrital input from marginal sources. Thus, throughout deposition of the Ladner Group there is no direct evidence of an eastern margin to the depositional basin. This changes in Late Jurassic time when the more mature, relatively quartz- rich strata of the Thunder Lake sequence are deposited.

From the ternary plots both the Dewdney Creek Formation and

Thunder Lake/Hauterivian strata are in ferred to represent sediments derived from a tra n s itio n al arc. The increase of quartz and presence of minor mica in the Late Jurassic/Early Cretaceous samples, however, suggests a tran sitio n in sedimentary regime. Overlying Jackass

Mountain Group sediments characterized by abundant plutonic d etritus appear to be derived from a dissected arc while the Pasayten Group strata were lik e ly derived from a recycled orogen.

A change in tectonic configuration in the Early Cretaceous has 72

been well documented (Tennyson and Coles, 1978; Trexler and I Bourgeois, 1985), however th is study emphasizes the dynamic evolution

of the trough throughout the Jurassic. Petrographic studies combined with biostrati graphic data indicate that two sources for volcanic

detritus were available during deposition of Jurassic strata, rather

than continuous erosion of a marginal volcanic arc to the Jurassic

Methow basin. CHAPTER 5

REGIONAL GEOLOGY AND GEOCHEMISTRY OF THE METHOW TROUGH: JURASSIC STRATA

Reconnaissance mapping (1:50,000) throughout the trough in southwestern B ritis h Columbia concentrated on the d is trib u tio n of

Jurassic units to provide continuity between areas mapped in more d e t a il„ S ig n ific a n t new fossil lo c a litie s and geochemical analyses of the Dewdney Creek volcanic rocks better constrain facies relationships within and between these units (Fig. 25). The resulting compilation of the distribution of the Jurassic units better illustrates the structural style and complex facies relationships characteristic of the Jurassic section (Fig. 26).

Distribution of Jurassic units

Ladner Group

Manning Park. Coates (1970, 1974) recognized two d is tin c t belts of Lower and lower Middle Jurassic rocks in the Manning Park area th at he referred to as the Ladner Group. The fault-bounded eastern belt comprises coarse volcanic breccia, breccia-conglomerate, tuff, andesitic lavas and minor argillite overlain by a "turbidite" V sequence. The turbidite section comprises rhythmic interbeds of fine breccia, sandstone and minor arg illite. The early Bajocian ammonite

73 74

Figure 25. Location of Jurassic (solid circles) and Cretaceous (open circles) fossil localities and geochemistry samples (triangles). Figure 26. Distribution of Jurassic units in the Methow Trough of southwestern British Columbia.

LEGEfiD

Tv Coquihalla and related volcanic rocks (22Ma)

Tqm Needle Peak pluton (45 Ma)

Ks undifferentiated Cretaceous strata

uJ$ Thunder Lake sequence

mJv. Dewdney Creek volcanic rocks and associated sediments

mJs Dewdney Creek e p ic la s tic strata

Us Boston Bar Formation a rg illite, greywacke, and conglomerate

Tv Spider Peak Formation greenstone, gabbro, and sediments

Mzs Hozameen Group greenstone, chert, and a r g illit e

Mzu Coquihalla serpentine belt

Mzgd Mt. Lytton meta-plutonic complex 75

i i. i S r * ■ - • .

.?irS::^^-irrrr-=PULP ( HARVESTING — \ ri)M i m

'J ) ■■-

mmwiiV >•

\ ;:o \ x ''- LTT2" ..; S 7. C.v- X .-Q..>X— ^ z 2. \ —

, ■ > ' • ptutotfr^* X -Sr rzv:fa;y . / T - „ X 3 — i,--^r , s ^ t - X‘ ■ mi ;T»"^ j l f m ^c- r CoquThalla K.s Vblcanics - X z ^ / - i-f— rr. :J*. • ix v i / i , 1' a i;v , v— L \x~4^nwv'-' < x x z - ^ #% :_ r— X , ~ < < i r*

->e

x k ^ x , ^ -

--X ^ : N\ G :.,>

^ I-

Figure 26. Distribution of Jurassic units in the Methow Trough in southwestern British Columbia. 76

genus Stephanoceras is found on bedding planes of fine-grained

sandstone within this unit. Possible fragments of the late Toarcian

ammonite genus Grammoceras( ? ) , Dum ortieria(?) or Catullocerasf?) were

reported from the base of Blackwall Peak, a massive lense of coarse

volcanic breccia near the structural base of the eastern belt (Frebold

et a l ., 1969). The eastern b e lt is well exposed on the Lookout road,

north of Manning Park Lodge.

The western belt, considered the more distal Ladner

representative, comprises arg illite, tuffaceous siltstone and

sandstone, and minor pebble-rich strata. The "type" section exposed

on the ridge south of Silver Daisy Mountain is subdivided into an

argillaceous unit and an upper turbidite unit. Ammonites collected

from the section range in age from la te Toarcian to mid-Bajocian with Bajocian genera restricted to the coarser grained upper unit

(Frebold et al., 1969). Upper Jurassic sediments are reported to

overlie the section either disconformably or with slight angular

discordance.

During fieldwork in 1985, both belts of "Ladner" strata in

Manning Park were re-examined. At the structural base of the section

along the Lookout Road, the ammonite genus Oppelia (Appendix A, Plate

3, Fig. 4) was collected directly below andesitic breccia of Blackwall

Peak (Fig. 27). This occurrence indicates an early Bajocian age"for

volcanism in the area and further suggests that the proximal volcanic

and sedimentary strata of the eastern belt are restricted to the early 77

Figure 27. Andesitic volcanic breccia of Blackwall Peak, Lookout section. Manning Park. 78

Bajocian. If this is the case, then Lower Jurassic strata of the western belt can no longer be considered distal equivalents of eastern belt strata (Coates, 1970; 1974).

Due to loss of the original ammonite collection from the base of Blackwall Peak, the late Toarcian(?) age cannot be reassessed (T.P.

Poulton, pers. comm., 1986). It is significant to note, however, that north of the Manning Park area the initiation of volcanism is also recognized as being early Bajocian in age. Although this observation does not preclude an earlier (late Toarcian(?)) volcanic episode within the trough, lithic similarity, facies relationships, and stratigraphic/structural position of the Blackwall Peak volcanic breccia support an early Bajocian maximum age for the unit. The eastern b e lt is thus herein considered representative of the uppermost member of the Dewdney Creek form ation.

The western b e lt is well exposed along the highway ( i . e .

Skagit Bluffs), directly east of Snass Creek. The section coarsens upward and comprises w ell-indurated tuffaceous s i!ts to n e , sandstone and volcanic-rich pebble conglomerate similar in character to sediments of the Dewdney Creek Formation in the Coquihal1 a area. The

Boston Bar Formation is not recognized in this area. Upper Jurassic

Buchia-bearing greywacke occurs near the top of the exposed section.

Overlying strata that contain a mid- fauna (Eulytoceras)

(Jeletzky, 1970) are lithically indistinguishable from the Upper

Jurassic strata. No angular unconformities are recognized throughout the Jurassic section.

South of the highway, on the ridge d ire c tly north of Thunder 79

Lake, volcanic-rich pebble conglomerate, 1ithofeldspathic sandstone and minor breccia occur in isolated outcrops above an argillaceous section and below a well exposed Jurassic-Cretaceous section. The contact between predominantly argillaceous rocks and those considered part of the Dewdney Creek Formation is not exposed but appears to be gradational. Thus the western belt comprises both subdivisions of the

Ladner Group, although the lower u nit is truncated and the basal strata are nowhere exposed.

The large scale, SE plunging synclinal structures recognized by Coates west of the Chuwanten fa u lt in the Manning Park area can be extended to the north where progressively older strata are exposed.

This trend is enhanced by upwarp associated with the Needle Peak pluton and NE-trending dip-slip (and strike-slip) faults. In proximity to the Fraser fault (near Boston Bar), however, later dip- and strike-slip faults cut these large scale folds. Smaller scale structures are relatively more abundant in the older strata, giving the general appearance of being more deformed. With no recognized angular unconformities between the Jurassic units and between the Jurassic and lowest Cretaceous unit, this observation is difficult to reconcile unless it is attributable to variation in competency between lith o lo g ie s .

North of Manning Park. Critical traverses on Mt. Dewdney and

Snass Mountain further constrain the distribution of the Ladner Group.

On the west flank of Mt. Dewdney and directly east of the inferred trace of the Hozameen fault, the Dewdney Creek Formation is overlain conformably by Upper Jurassic and Cretaceous strata. On Snass 80

Mountain, the imbricated Chuwanten fault is well preserved. Massive granitoid-bearing conglomerate beds to the east are juxtaposed against a structureless mound of volcanic breccia-conglomerate, similar in lithic character and structural position to Blackwall Peak in Manning

Park. Further east, a repetition of these units occurs (J.W.H.

Monger, pers. comm., 1986).

South of the International boundary. Jurassic rocks of the western b e lt extend south into north-central Washington, where they are mapped as part of the Newby Group (Twisp Formation). The eastern belt is truncated directly south of the boundary. Lithologic descriptions of the lowest member of the Buck Mountain Formation of the Newby Group are sim ilar to the Dewdney Creek formation exposed in the eastern belt. Based on stratigraphic relations with the

Hauterivian middle member and on the occurrence of a Cretaceous(?) belemnite in the lowest member, however, the Buck Mountain Formation appears restricted to the Early Cretaceous (Maurer, 1958; Barksdale,

1975). Lower Middle Jurassic volcanic rocks and associated pyroclastics and volcanic-rich sediments have not been recognized in the Methow Trough in Washington, thus there does not appear to be an equivalent of the Dewdney Creek Formation.

Thunder Lake Sequence

Upper Jurassic rocks are well exposed along the northwest shores of Thunder Lake. An abundant, well-preserved Late Jurassic fauna was collected from fine-grained sandstone together with J.A.

Jeletzky, T.P. Poulton, and A. Zeiss (Jeletzky et al., in prep.) 81

Lithically similar strata that disconformably overlie the Jurassic section contain a well-preserved Hauterivian Inoceramus fauna (J.A.

Jeletzky, pers. comm., 1985). This relationship is also observed along eastern exposures of the Skagit Bluffs described above and further north on Mt. Dewdney. Upper Jurassic rocks are not recognized north of Mt. Dewdney with the exception of Buchia-bearing greywackes reported by Ray (1986) north of Carol in Mine. These greywackes occur in a structurally attenuated argillite-dominated section. It is, therefore, difficult to assess their stratigraphic position.

Dewdney Creek Formation Geochemistry

Whole rock and trace element analyses on 8 Dewdney Creek volcanic samples from the Anderson River and Manning Park areas were provided by G.E. Ray and the B ritis h Columbia M inistry of Energy,

Mines and Petroleum Resources (Fig. 25) (Tables 4 and 5 ). Major oxide data displayed on a three-axis orthogonal plot (Church, 1975) indicate that the volcanic samples are andesitic in composition (Fig. 28a).

Through more traditional AFM and alkali vs. silica plots (Irvine and

Baragar, 1971) the andesites are shown to be calc-alkalic and generally subalkalic (Fig. 28b, 28c).

Trace element discriminant plots generally support major oxide data, however, the samples tend to overlap the andesite and basalt fields rather than being confined to the andesite field (Fig. 29a-c).

This is likely attributed to varying degrees of major element mobility during low grade metamorphism of the section (Garcia, 1978). Trace element data is further employed to indicate the genetic association 82 of the samples (Fig. 30a, 30b). Both plots indicate that the Dewdney

Creek volcanic rocks most closely resemble island arc derivatives. 83

Table 4. Major element analyses of Dewdney Creek volcanic rocks.

# Si 02 Ti02 A1203 Fe203 FeO MgO CaO Na20 K20 MnO LOI

212 46.74 1.11 17.06 12.83 9.48 5.26 6.76 3.91 0.75 .24 4.70

400 53.53 0.66 17.45 6.56 3.86 5.79 6.40 4.84 0.04 .15 4.36

412 55.05 0.86 18.00 8.76 4.06 4.38 5.45 4.96 0.33 .15 1.97

341 52.67 0.73 18.37 7.77 4.59 6.10 3.31 4:35 1.49 .12 4.17

5 57.30 0.64 16.19 7.26 5.19 4.11 4.41 4.69 0.53 .14 2.84

6 51.59 0.66 19.43 6.12 2.81 2.63 7.27 5.09 0.48 .13 3.38

7 52.75 0.67 17.43 8.33 2.81 4.41 7.89 4.91 0.15 .12 3.35

8 53.91 0.71 19.53 7.30 4.27 3.17 7.39 3.11 0.69 .12 1.89

Table 5. Trace element analyses of the Dewdney Creek volcanic rocks.

# Ti Rb Sr Y Zr Nb Ba Mi Cr 1 P205

212 6100 18 780 20 48 4 500 <100 0.15

400 3900 2 380 18 48 4 120 <100 0.19

412 4500 8 530 20 51 4 200 <100 0.15

341 3900 18 510 22 87 4 380 <100 0.17

5 <7 402 29 66 5 202 24 59 0.09

6 <7 175 28 68 6 76 21 18 0.12

7 <7 216 25 57 <4 64 53 81 0.12

8 <7 850 20 74 <4 379 19 20 0.11 84

No20+K20 0 5 10 15 35

0 30-

0 25- - i ^20-L, . basalt------\ * 4- - 1 ; °I5 -

rB ANDESITE------1, \ ^ i ,z 5

0 ‘ •I .2 .3 « Al203/S.02

FeO

Figure 28a. Major element discriminant plot of Church (1975) indicates that the samples are predominantly andesite. Figure 28b. Data plotted on an a lk a li vs. s ilic a diagram (Irv in e and Baragar, 1971). Figure 28c. An AFM diagram (Irvine and Baragar, 1971) outlines the sub-alkalic, calc-alkaline composition. 85

,60 - -

56 --

SUB ALKALINE

72

68 --

64 ..

60

56 - ■

52 .. SUB ALKALINE 4 8 -. BASALT

44

40

LOG(NbA)

N

*2 andesite * * /

basalt

SUB alkaune basalt

LOG (Nb/Y)

Figure 29a, 29b and 29c. Minor element discriminant plots of (Floyd and Winchester, 1978) suggest the samples overlap the andesite and subalkaline basalt fie ld s . In 29b and 29c, samples 7 and 8 were below the detection lim it. 86

N-MORB

LOG ppm Cr

WP8

MORB

LOG ppm Zr

Figure 30a and 30b. Minor element discriminant plots of Pearce (1975) suggest affinities of the Dewdney Creek volcanic rocks with those derived from island arcs. CHAPTER 6

TECTONIC IMPLICATIONS OF THE JURASSIC EVOLUTION OF THE METHOW TROUGH

Introduction

A v arie ty of tectonic fragments are caught between the

Intermontane and Insular composite terranes of the Canadian Cordillera

(Monger et a l 1982; Monger and Berg, 1984; Price et a l., 1985) (Fig.

31 and.32). These comprise p rim arily fault-bounded packages of la te

Paleozoic and Mesozoic volcanic and sedimentary rocks of oceanic and arc a f f in it y which are variab ly deformed. The Methow Trough and its probable northern counterpart, the Tyaughton Trough (Jeletzky and

Tipper, 1968), represent the most inboard of fragments caught between the composite terranes, cropping out along the western margin of the

Intermontane terrane. They are separated from the terrane by a system of fa u lts (Pasayten, Fraser and Yalakom fa u lts ) which have documented s trik e -s lip displacements (Monger, 1985; Kleinspehn, 1985; Lawrence,

1978). The character and complete chronology of displacement along these faults is, however, not fully understood.

The location of the eastern margin of the Insular terrane is currently a subject of debate. Tipper (1984) or Cameron and Tipper

(1985) would place the boundary as fa r east as the Pasayten-Fraser-

Yalakom system, therefore including rocks of the Tyaughton and Methow

87 | Intermontane terrane j%| Methow trough TL "terrane fragments" Coast Plutonic Complex — Insular terrane

500 km

Figure 31. Location of the Methow Trough with respect to the composite terranes of Monger et a l . (1982) 89

Figure 32. A terrane nap of western North America ( from Coney, 1981) that includes elements of the Canadian C o rd illera (F ig . 31) in a broader perspective. Terranes referred to in the text include: A - Alexander, W - Wrangellia, St - Stikine, C - Cache Creek, Q - Quesnel, and S - Sonomia. 90 as distal components of the Insular terrane. Other workers favour a margin obscured by the Coast Plutonic Complex c itin g the differences between Triassic rocks of the Insular terrane and tectonic fragments as having incompatible tectonic h isto ries (Coney e t a l . , 1980; Monger,

1986; Potter, 1986; Rusmore, in press; Rusmore et a l., in press).

The Intermontane terrane is considered to have accreted to the

North American margin in the lower Middle Jurassic (Monger, 1984 and references therein) while affects of the accretion of the Insular to the Intermontane terrane are clearly in evidence by late Early

Cretaceous (Jeletzky and Tipper, 1968; Davis et a l., 1978; Tennyson and Cole, 1978; Monger, 1984; Trexler and Bourgeois, 1985). The la titu d e at which these accretionary events took place remains unresolved.

A re-evaluation of paleomagnetic data from intrusive rocks in the Canadian C o rd illera suggests accretion of both the Intermontane and Insular terranes at the present latitude of Baja California (less than 30° north latitude) with significant (approximately 2,500 km) northward translation of the entire block after mid-Cretaceous time

(Irving et al., 1985). This interpretation is difficult to reconcile with in terp retatio n of geologic structures in the Canadian

Cordillera (Gabrielse, 1985; Monger, 1985; Struik, 1986; Price and

Carmichael, 1986). The paleomagnetic interpretation also conflicts with paleobiogeographic interpretations of Jurassic ammonite distributions (Tipper, 1981; Callomon, 1984; Taylor et al., 1984;

Smith and Tipper, 1986) (Fig. 33). 91

For the Pliensbachian stage of the Jurassic, Tipper (1981) and Smith and Tipper (1986) re-align the latitudinally controlled

Boreal-Tethyan boundary (recognized as a zone of mixing o f realm- specific faunas) between each terrane with reference to the craton.

From this reconstruction, the terranes are inferred to occupy the . present latitudes between northern Nevada and the international boundary. Post-Piiensbachian northward translation of the terranes increases passing outboard from the craton, estimated at 'approximately

500 km for Quesnel terrane, 1,800 km for Stikinia and 2,400 km for

Wrangellia.

Using a similar method of reconstruction, Taylor et a l. (1984) conclude that the components of Wrangellia are north of 3 0 °north latitude by Bajocian time. A summary of the distribution of Middle and Late Jurassic ammonite faunas by Callomon (1984) fu rth er suggests that there has been little or no latitudinal offset of suspect terranes with respect to the craton since the late Middle Jurassic

(post Bajocian). This implies that any northward translation indicated by the Early Jurassic (Pliensbachian) faunal distributions must have occurred primarily during the interval between the Pliensbachian and

Bathonian stages.

Further conflict with Irving et al.'s interpretation was raised by May and Butler (1986) in their reassessment of Jurassic paleopoles for North America. Revision of the Jurassic apparent polar wander path (APWP) for North America indicates th at North America was at more southerly paleolatitudes during Jurassic tim e. A comparison 92

PALEOMAGNETICS

LATE TRIASSIC - EARLY JURASSIC MIOGEOCLINE ______(St and O) ______(May and Butler, 1986) _____

&/E\ Cratonic /.? ( —1 Boreal * vQjtZjCraton

PALEOBIO GEOGRAPHY

EARLY JURASSIC North W.St.O

(Smith and Tipper,1986)

America

PALEOMAGNETICS CRETACEOUS y W,CPC,St /

(Irving et al.,1985)

600km

Figure 33. An illustration of various interpretations of the latitudinal position of major allochthonous terranes with respect to cratonal North America in the Mesozoic. 93

of paleomagnetic data from the Intermontane composite with the revised

APWP suggests that both Quesnel and S tik in ia were in th e ir

approximate present position relative to the craton during the

Jurassic. In order to then accommodate the conclusion of Irving et

a l . (1985), the terrahes must tran slate south 2,500 km between Late

Jurassic and mid-Cretaceous time, and then return to their present

position by early Tertiary.

Preliminary paleomagnetic results on the Asitka Group of

S tik in ia which suggests that S tik in ia was in its present la titu d in a l position relative to the craton in the Early Permian (Irving and

Monger, 1987) fu rth er necessitates post Late Jurassic southward

translation of the terranes in order to accommodate both data sets.

Structures necessary to accommodate this potential translation have not been identified.

Through a stratig rap h ic comparison of the Jurassic Methow

Trough and coeval assemblages in the Canadian C o rdillera the

relationship between the Methow and the various adjacent terranes can

be better constrained. This information proves critical in evaluation

of regional correlations and existing tectonic models that are based

in part on paleomagnetic and paleobiogeographic interpretations (Davis

et al., 1978; Monger et al., 1986; Mortimer, 1987; Umhoefer, 1987;

Rusmore et al., in press).

Coeval Stratigraphy

Jurassic marine sedimentary and volcanic sequences are common

components of suspect terranes of the Canadian Cordillera. Detailed 94 b io s tra ti graphic studies of various assemblages coeval with the Methow

Trough considered in this discussion include: the Ashcroft Group

(Frebold and Tipper, 1969; Travers, 1978; 1982; Arthur, 1985); the

Bowser Basin (Eisbacher, 1974; 1977; Smith et a l ., 1984; Thomson,

1985; Thomson e t a l . , 1987); Tyaughton Trough (Tozer, 1982; Jeletzky and Tipper, 1968; Tipper, 1978; O'Brien, 1985; Rusmore, in press);

Methow Trough (Frebold e t a l . , 1969; Coates, 1970, 1974; O'Brien, 1986 and this re p o rt); the Bridge River-Hozameen assemblage (P o tte r, 1986;

Rusmore et a l., in press); the west side of Harrison Lake (Crickmay,

1962; Arthur, 1986 and pers. comm.); and the Queen Charlotte Islands

(Tipper and Richards, 1976; Cameron and Tipper, 1985). The Fernie basin (Frebold, 1957, 1966, 1976; Hall and Stronach, 1981) is also included.

The Fernie basin is considered a Jurassic cratonal basin while a ll other assemblages are (suspect) terrane s p e c ific . The

Ashcroft sediments depositionally overlie Triassic rocks of the Nicola

Group (Travers, 1978) and are therefore linked to the Intermontane terrane (Quesnel). Likewise, the Bowser basin overlies Triassic and

Jurassic rocks of the Stikine terrane, northern Intermontane terrane

(Gabrielse, 1977). The Jurassic rocks of the Queen Charlotte Islands demonstrably overlie Triassic strata of inferred Insular stratigraphy

(Cameron and Tipper, 1985). The Harrison, Tyaughton and Methow strata

rest unconformably on Triassic volcanic and sedimentary assemblages of unknown affinity (Arthur, 1986; Rusmore, in press; Ray, 1986).

A comparison of generalized stratig rap h ic sections of these

Mesozoic assemblages (F ig . 35) illu s tra te s marked s im ila ritie s between 95

"Eastern" Assemblages Intermontane terrane Bridge River-Cadwallader Coast Plutonic Complex Insular terrane

QUEEN CHARLOTTE |K ISLANDS

FERNIE BASIN

500 km

Figure 34. Location of coeval assemblages (stippled pattern) with respect to composite terranes of the Canadian C o rd ille ra . 96

INBOARD ......

:-v ; * ! r:V ;V V

V-V- i H I # m

5 ^ ^ *- ‘ P 4 BRIDGE RIVER HARRISON ME T m Ow TYAUCHTON FERNICASHCROFT m o z a m e e n CHARLOTTES

Figure 35. Generalized biostrati graphic sections for the coeval assemblages of the Canadian C o rd ille ra . F ille d c irc le s indicate fossil control within each assemblage. 97 them. The predominance of c la s tic sedimentation punctuated by at least two regionally significant hiatuses is apparent. No marked angular unconformities occur within the Jurassic sections although the possibility of a minor angular unconformity between Bajocian and

Cal 1ovian/Oxfordian rocks in the Queen Charlotte Islands (Cameron and

Tipper, 1985) and between Bajocian and Callovian rocks in the Ashcroft

Group (Travers, 1982) has been reported. All the basements to these assemblages are volcanic and sedimentary sequences of Triassic age, however, recent work illustrates significant variations in the age, lithology and chemistry of these Triassic units.

Of the coeval sequences mentioned above, those assemblages most likely deposited proximal to or actually interconnected with the

Methow Trough are the Ashcroft and Tyaughton assemblages. The

Harrison Lake sequence, outboard of these assemblages and presently separated from them by oceanic strata of the Bridge River/Hozameen terrane and the in tru sive and metamorphic rocks of the northern

Cascade core, is also included for comparison.

Triassic "basement" assemblages

At least part of the interpreted basement to the Methow

Trough, exposed in the Coquihal1 a area, is the Spider Peak Formation

(Ray, 1986). This Early(?) Triassic sequence of greenstone, gabbro and minor clastic strata presently is structurally juxtaposed against the Ladner Group. S im ila ritie s between the d e trita l components within the basal section of the Ladner Group and lithologies of the Spider

Peak Formation suggest a depositional contact between the two. 98

Geochemistry of the Spider Peak basalts resembles oceanic ridge-type subalkaline basalts. Early(?) Triassic conodonts were retrieved from a chert conglomerate horizon at the top of the Spider Peak section

(Ray, 1986).

The contact between the Cadwallader and overlying Tyaughton

Group in the Tyaughton Trough is everywhere a fa u lt. I t is an inferred unconformity on the basis of petrologic sim ilarities between c la s tic sediments of the two groups (Rusmore, 1985, in press). The

Cadwallader Group is subdivided into two formations. The lower Pioneer formation comprises pillow basalt, flow and breccia. The conformably overlying Hurley, formation is a predominantly volcanic-rich sedimentary sequence. Geochemistry on the Pioneer basalts show close resemblance to island arc tholeiite with affinities to those formed in backarc basins. Early Mori an conodonts were collected from the

Hurley formation (Rusmore, in press).

A disconformity to slight angular unconformity separates

Ashcroft sediments from the Nicola Group in some areas whereas a

"chaotic zone of intense deformation" (Travers, 1978) occurs between the two in other areas. The Nicola Group comprises upper Carnian to lower Sinemurian volcanic flows and breccia, crystal lithic tuffs, limestone, and argillite (Monger, 1985). The Nicola volcanics can be subdivided into an east and west facies on the basis of lithology and geochemistry. The western facies is characterized by feldspar porphyry (low potassium feldspar phase) while the eastern facies is characterized by augite and bladed-feldspar porphyry (potassic). The bladed-feldspar porphyry and augite porphyry are distinctly potassic. .99 shoshonltic volcanic rocks, considered indicative of being subduction related (Mortimer, 1986) further supporting the inference that the

Nicola represents a west-facing arc. Associated plutonic rocks range in age from 192 to 214 Ma (K-Ar and Rb-Sr) (Armstrong, in press).

The Middle Triassic Camp Cove Formation is separated from the overlying Harrison Lake Formation by an unconformity. Crickmay (1962) described the Triassic(?) Camp Cove Series as a sequence of arg illite, quartzite, chert, and conglomerate that contains clasts of upper

Paleozoic carbonate. A more recent study by Arthur (1986) describes the Camp Cove Formation as siliceous a rg illite , plagioclase porphyry flow rocks and tuffaceous sandstone. Middle Triassic radio!aria and conodonts have been retrieved from siliceous a r g illit e (A rthur, 1986).

Strata that comprise each of these Triassic "basement" assemblages are d is tin c t in age, lith o lo g y , and chemistry. These assemblages are also d is tin c t from typical Insular (W rangellia) .

Triassic strata which includes Middle Triassic argillaceous strata overlain by a thick upper Ladinian to upper Carnian sequence of tholeiitic pillow lavas and flows in turn overlain by Norian carbonate and c la s tic strata (Jones et a l ., 1977; Monger, 1984). D iffe re n t geochemical signatures of the volcanic rocks indicate evolution in various tectonic settings that precludes close association of the tectonic fragments prior to Early Jurassic sedimentation.

Jurassic strata

All four Jurassic sequences are characterized by' a basal and

(or) Lower Jurassic conglomerate overlain predominantly by argillaceous strata of Early Jurassic age. The initiation of Jurassic 100 deposition varies between the Sinemurian and PIiensbachian stages. A comparison of the d e trita l composition of the conglomerates illustrates some variation in provenance, although volcanic detritus appears to dominate in each. Lower Middle Jurassic s trata of each assemblage display considerable variatio n in lit h ic d e ta il. Upper

Middle-Upper Jurassic sediments o verlie each assemblage e ith e r disconformably or with slight angular discordance.

Basal conglomerate. As described in previous sections, a lenticular conglomerate unit no greater than 10 m thick occurs at the structural base of the Ladner Group. Angular to well-rounded clasts are predominantly greenstone with varying amounts of chert (locally predominant), crystalline, acid volcanic, and carbonate detritus. The conglomerate varies from clast to matrix supported and contains clasts which average approximately 20 cm but range up to over 1 m in diameter. This unit was deposited earlier than the PIiensbachian stage based on the occurrence of the ammonite genus Dubariceras from strata above the basal Ladner member.

Lower Jurassic conglomerate of the Tyaughton Group is lithologically indistinguishable from Upper Triassic conglomerate described by Tozer (1967). It comprises a poorly sorted, but generally well-rounded pebble conglomerate with predominantly volcanic detritus (60%), quartz and chert (25%) and minor arg illite, c ry s ta llin e and metamorphic clasts (Tipper and Richards, 1976;

O'Brien, 1985). The matrix contains abundant angular plagioclase.

The second cycle detritus is believed to have been derived from the

Cadwallader Group (Rusmore, in press). The conglomerate contains the 101

Early Jurassic bivalve Weyla and early Sinemurian ammonite (Badouxia) fragments (O'Brien, 1985)„

The basal conglomerate of the Ashcroft Group is a massive (>50 m), poorly sorted pebble conglomerate with angular to subrounded clasts of volcanic porphyry and volcanic!astic rocks (60-70%), felsic granite (20%), and sedimentary clasts (10-20%) which include sandstone, argillite and quartz (Arthur, 1985). Travers (1978) also reports the occurrence of dark chert, greenstone, ultramafic and limestone detritus. The interpreted primary source for this unit was the underlying Nicola Group and the Guichon Creek B atholith which is unroofing during deposition of the Ashcroft sediments (Preto et a l.,

1979). Upper Pliensbachian sandstone overlies the conglomerate conformably (Arthur, 1985).

The unconformity between the Camp Cove and Harrison Formations is also marked by a conglomerate (originally included in the Camp Cove

Series of Crickmay (1962)). The pebble conglomerate consists of well- rounded clasts of chert, carbonate and volcanic detritus within a poorly sorted medium to coarse grained feldspatholithic sandstone.

Carbonate clasts contain abundant fossils including bryozoa, crinoid fragments, brachiopods and schwagerinid fusulinids. Chert clasts contain both ra d io !aria and conodonts of probable Middle to Late

Triassic age (Arthur, 1986). Bryozoa and schwagerinid fusulinids are a distinctive component of the Chilliwack Group exposed in the northern

Cascades ( Cascade Core of Figure 1 ). Lower Toarcian a r g il l i t e overlies the conglomerate conformably. 102

Overlying strata. Interbedded argillite and siltstone with lesser amounts of tuffaceous sandstone and pebble mudstone comprise the Lower Jurassic rocks of the Methow Trough. Visual estimates of the average detrital mode for the various lithologies suggests close resemblance to arc-derived sediments. Proximal strata within the section are distinctly absent. A change in depositional regime is signaled by the introduction of coarse, volcanic-rich elastics during the Aalenian stage. The sediments are immature, with predominantly arc-derived detrital components and minor amounts of well-rounded granitic and chert detritus. Intrabasinal andesitic volcanism causes extreme lateral facies variation within the epi- and pyroclastic section throughout the Aalenian and early Bajocian.

Geochemistry of the Middle Jurassic volcanic rocks resembles those derived from island arcs. A comparison of coeval volcanic assemblages throughout the Canadian C o rd illera indicates that the

Dewdney Creek volcanics are most similar to the western facies of the

Telkwa Formation ( Hazel ton Group) of S tik in ia (F ig . 33) (Table 6)

(Souther, 1977). The Telkwa Formation is interpreted to represent a south(? )west-facing arc (Souther, 1977). Whether the Methow rocks represent a southern extension of the Hazel ton arc or are unrelated remains difficult to resolve due to the ambiguity of the Early and early Middle Jurassic regional tectonic configuration. 103

\X

Figure 36. Distribution of Lower and lower Middle Jurassic volcanic rocks ( shaded areas) in the Canadian C o rd illera with respect to the present terrane distribution. 104

Table 6. The location, terrane affinity, age, and petrology/geochemistry of Lower and Middle Jurassic volcanic assemblages in the Canadian C o rd ille ra .

GROUP/FORMATION LOCATION AGE PETROLOGY

Rossi and Group southeast B.C. Sin-Toar + augite- and plagio- (Quesnel) Baj clase-phyric alkaline basalt, subalkaline andesite

Hazel ton Group central B.C. Telkwa Fm. west (S tik in ia ) PIiens-Baj subalkaline-calc- alkaline andesite. Telkwa Fm. east basalt, alkaline andesite, basalt. Nilkitkw a PIiens-Aal volcanogenic sediments breccia. Ankwel1 Toar alkaline basalts

Toodogone vole north-central Toar-Baj andesitic-basal tic B.C. and older flows, pyroclastics. (S tik in ia ) (?) s iltsto n e

Dewdney Creek southwest B.C. Aal-early subalkaline-calc- Fm. (Methow) Baj alkaline andesite, basalt

Harrison Lake southwest B.C. T o ar(?)- agglomerate, andesitic (?) Baj(?) fl ow

Yakoun Group Queen Charlotte andesitic agglomerate. Richardson Bay Is. (Wrangellia) early Baj breccia, tuff. Fm. sandstone

Bonanza Group Vancouver Island . Sin-Toar basalt-rhyodacite. (Wrangellia) breccia, t u f f , minor in tercalated sediments 105

Lower and Middle Jurassic rocks of both the Tyaughton and

Ashcroft assemblages are predominantly fine grained marine sediments and do not include lavas and significant pyroclastic deposits.

Tyaughton Group sediments range in age from middle(?) Hettangian to early Bajocian and are characterized by a fining upward marine clastic sequence of lithic pebble conglomerate, feldspatholithic wacke and siltstone with intercalated calcareous concretionary horizons and ash

(H.W. Tipper, pers. comm., 1985; O'Brien, 1985). Callovian(?) to

Barremian fine grained clastic sediments of the Relay Mountain Group are thought to o verlie the Tyaughton Group unconformably although the contact is everywhere recognized as a fault (Jeletzky and Tipper,

1968; Rusmore et al., in press).

Ashcroft rocks comprise a fin in g upward sequence of upper

Pliensbachian and middle Bajocian conglomerate, sandstone and shale of the informally designated Thompson formation. The Thompson formation is overlain by predominantly fine-grained Upper Jurassic strata with a Callovian conglomerate marking the unconformity (Travers, 1978;

Arthur, 1985).

The Jurassic Harrison Lake sequence differs substantially from the Ashcroft and Tyaughton sections in that the upper Lower and Middle

Jurassic section is dominated by volcanic and pyroclastic rocks of the

Harrison Lake Formation. Argillaceous strata which o v erlie the basal conglomerate characterize the base of the section. Volcanism is considered to have been initiated in mid-Toarcian time (Arthur, 1986).

Intercalated arkose, tuff, sandstone, and argillite of the Middled?)

Jurassic Echo Island Formation (Crickmay, 1930; Arthur, 1986) 106 conformably overlie the Harrison volcanics. Unconformably above are

Callovian tuffaceous sediments of the Mysterious Creek Formation.

Discussion

The conclusion that the Jurassic Methow assemblage was not closely connected with presently adjacent coeval assemblages

(Tyaughton, Ashcroft, and Harrison), is illustrated by lithostratigraphic evidence outlined above. Notable differences in

Triassic basement age, lithologies, and geochemistry of volcanic rocks and in the character of overlying Lower Jurassic strata indicate that local tectonic processes were controlling deposition in each assemblage. There is no compelling stratigraphic evidence to link these assemblages through the Early Jurassic. Furthermore, the distinct lack of volcanic rocks and associated pyroclastics in both the Ashcroft and Tyaughton sections suggests that a close association is u n likely between these assemblages and the Methow Trough in the

Early and early Middle Jurassic.

In contrast, both the Harrison and Methow sections include significant accumulations of pyroclastic and volcanic rocks, suggestive of intrabasinal volcanism. Despite the similar stratigraphy shared between the Harrison and Methow, however, i t seems u n likely that these two assemblages evolved together. Differences in

Triassic basements, provenance of the basal conglomerate, age of initiation of volcanism, and lack of faunal similarities are all significant. Perhaps more important, however, is the occurrence of the Hozameen-Bridge River te rra n e , an interpreted collapsed ocean 107 basin, and the northern continuation of metamorphic and c ry s ta llin e rocks of the Northern Cascades, interpreted as a zone of considerate crustal shortening, between the two assemblages (P o tte r, 1986; Monger,

1986).

Upper Middle and Upper Jurassic fine-grained clastic rocks which unconformably o verlie Lower and lower Middle Jurassic strata in each basin represent similar facies. These may constitute an overlap assemblage.

By removing Late Cretaceous and e arly T e rtiary rig h t-la te ra l displacement on the Fraser-Straight Creek and Yalakom-Hozameen faults, the resulting pre-Cenomanian configuration places the Tyaughton Trough outboard and to the northwest of the Methow Trough (Kleinspehn, 1985).

Any older movements on structures are d ifficult to constrain.

Jurassic reconstructions of a continuous Methow-Tyaughton basin are thus conjectural. Broad stratig rap h ic s im ila ritie s between the two assemblages provide weak evidence of association as a ll Jurassic assemblages discussed above (F ig . 35) r e fle c t generally s im ilar sedimentary histories. CHAPTER 7

CONCLUSIONS

This biostrati graphic study of Jurassic rocks of the Methow

Trough provides data on the early history of a critical tectonic fragment in the northern Cordilleran collage. In its present position outboard of the Intermontane and inboard of the Insular composite terranes, the Methow Trough records the interplay of tectonic processes during Mesozoic accretion, amalgamation, and translation along the evolving North American margin. Through detailed and reconnaissance mapping and p etrologic, paleontoTogic, and geochemical studies the distribution, age, and lithic character of the Jurassic section has been refined.

1. The Jurassic stratigraphic nomenclature is refined to include the Sinemurian(?) to early Bajocian Ladner Group and the disconformably overlying Oxfordian to Tithonian Thunder Lake sequence.

The Ladner Group is further subdivided into the Sinemurian(?) to

Toarcian(?) Boston Bar Formation and the overlying Aalenian(?) to early Bajocian Dewdney Creek Formation. The two formations may be in part coeval.

2. The Boston Bar Formation comprises two informal members,

a lower coarse-grained sequence of conglomerate, greywacke, and

108 109 argillite, and an upper unit of laterally continuous, interbedded argillite, siltstone and fine-grained greywacke. The formation is equivalent to the west facies of the Ladner Group recognized in

Manning Par (Coates, 1974). The lower member is structurally juxtaposed against greenstone of the Early(?) Triassic Spider Peak

Formation in the Coquihalla area. An original deposit!onal relationship is inferred, however, based on lithic components of

"basal" conglomerate. Initiation of deposition of the Ladner Group is poorly constrained, but it must be older than Pliensbachian and younger than Early Triassic.

3. The Dewdney Creek Formation comprises a d is tin c t unit

Subdivided into three, volcanic-rich epiclastic members based on facies distributions observed in the Coquihalla area. A fourth, volcanic member, fault-bounded throughout the trough, comprises calc- alkaline andesitic lavas, breccia, and pyroclastic deposits associated with minor amounts of intercalated sediments. This member of the

Dewdney Creek Formation is equivalent to the east facies of the Ladner recognized in Manning Park (Coates, 1974).

4. Ammonites retrieved from the Dewdney Creek Formation range in age from Aalenian to early Bajocian, with the youngest genera collected from the Manning park area. Newly documented genera that occur w ithin the Dewdney Creek section include: Plannamatoceras,

Erycitoides, Sonninia, Witchellia, and Oppel ia . Other genera collected for this study and previously described from the Manning

Park collections include Tmetoceras and Stephanoceras. All are cosmopolitan genera with the exception of two Tethyan ammonite genera 110

(Dubariceras and Eudmetoceras(? )) previously reported.

5. The disconformably overlying Thunder Lake sequence is characterized by relatively mature, fine to medium-grained sandstone.

It is lithologically indistinguishable from overlying Hauterivian strata of the Jackass Mountain Group. The u n it is distinguished from the underlying Ladner sediments by its m aturity and a d e trita l mode that includes a significant increase in monocrystalline quartz.

6. The Jurassic section has undergone two observable phases of deformation as well as low grade (prehnite-pumpel1yite maximum) burial (?) metamorphism. In lower Ladner strata this is characterized in thin section by a crenulated cleavage which is best developed in close proximity to the Hozameen fault system. More massive, competent units of the Dewdney Creek Formation outline the northwest-southeast shallowly plunging, asymmetric folds. These are associated with eastward thrusting on the Hozameen and Chuwanten fa u lts which occurred post-Hauterivian deposition. This deformation dominates the map pattern. At least two phases of brittle deformation (northeast­ trending right lateral dip-slip faults and north-south normal (?) fa u lts ) synchronous w ith , and follow ingf?) intrusion o f the Needle

Peak pluton (i.e . 45 Ma) have also occurred.

7. Detrital modes determined for both formations of the

Ladner Group and the Thunder Lake sequence suggest a f f in it ie s with sediments derived from a tran sitio n al volcanic arc. There are, however, differences in accessory d e trita l components, m aturity and facies relationships that are distinct. The moderate maturity displayed by the sediments and facies relations within the Boston Bar I l l

Formation suggest deposition on the d istal edge of a submarine fan in a subsiding basin. There are no equivalent proximal facies recognized and thus the eastern margin of the basin is obscured fo r Early

Jurassic deposition.

. The influx of significant accumulations of locally derived volcanic-rich sediments, pyroclastics and calc-alkaline lavas signals a change in depositional regime from the relatively quiescent deposition of the lower Ladner Group. Proximal sedimentation and active volcanism characterize the section and indicate a shallowing trend of the depositional basin with little evidence of a marginal source. R elatively mature, quartzose Thunder Lake sediments are inferred to represent the earliest identifiable derivatives of a c ry s ta llin e (eastern) marginal source terran e.

8. Lower and lower Middle Jurassic strata probably do not represent forearc deposits of the Nicola arc.

9. The lack of preserved angular unconformities between the

Jurassic units suggests a lack of deformation associated with the evolving depositional regime.

10. Triassic basement strata and the Early and e arly Middle

Jurassic depositional history of the Methow Trough are distinct from adjacent coeval assemblages, including the Tyaughton, Ashcroft, and

Harrison Lake assemblages. Most significant is intrabasinal, early

Middle Jurassic volcanism within the Methow trough which does not appear to be recorded in e ith e r the Tyaughton or Ashcroft assemblages.

This suggests that there was not a close association between these assemblages p rio r to Late Jurassic tin e . 112

10. Upper Middle-Upper Jurassic and Lower Cretaceous strata unconformably above these various assemblages represent s im ila r facies and suggest either common tectonic controls on sedimentation and (or) a closer association of the terrane fragments by that time. These strata might be considered -an overlap assemblage fo r the T ria s s ic - early Middle Jurassic fragments. APPENDIX A

SYSTEMATIC PALEONTOLOGY

Ammonites collected during mapping are part of the Geological

Survey of Canada (GSC) collections stored at the Institute of

Sedimentary and Petroleum Geology in Calgary, A lberta.

Identification of the specimens was untaken by the writer in the summer of 1986. The GSC number fo r each co llection corresponds to those specimens collected from a given u n it (Figs. 7, 16). All new fossil localities in the trough are included on Figure 25.

Measurements on the specimens were not made because of d isto rtio n and poor preservation due to deformation. Representative specimens from most taxa described below are figured at the end of the text.

Assemblages zones refered to in the text are listed in the following tab le.

113 114

Table Al. Correlation of local and European assemblage zones for the late Aalenian and early Bajocian.

EUROPE SOUTH ALASKA WESTERN NORTH AMERICA

(Hall and Westermann, 1981) (Taylor, in review)

Chondroceras oblatum OBLATUM

HUMPHRIESIANUM

Stephanoceras kirschneri KIRSCHNERI

SAUZEI

Parabigotites crassicostatus CRASSICOSTATUS

LAEVIUSCULA

BURKEI

OVAL IS Docidoceras widebayense

ACANTHODES

DISCITES

Eudmetoceras amplectens PACKARDENSIS

CONCAVUM Erycitoides bowel 1i H. MOWICHENSIS

MURCHISONAE A. SPARSICOSTATA

OPALINUM 115

ORDER AMMOMOIDEA ZITTEL, 1884

SUBORDER AMMONITINA HYATT, 1889

SUPERFAMILY HILDOCERATACEA HYATT, 1867

FAMILY HILDOCERATACEA HYATT, 1869

SUBFAMILY TMETOCERATINAE SPATH, 1936

GENUS Tmetoceras BUCKMAN, 1892

Tmetoceras scissuro (BENECKE) 1865

Plate 1, figures 1, 2, and 3

*1865 Ammonites scissum BENECKE, Geog. Pal. B e itr ., 1, p. 170, p i.

6, figs. 4a-b.

1964 Tmetoceras (Tmetoceras) scissum (BENECKE), Westermann, B u ll, of Am. Paleo., v. 47, No. 216, p. 428, pi. 72, figs. l-2c, text figs

32 and 34. (see synonomy)

1969 Tmetoceras c f . T. scissum (BENECKE), Frebold et a l ., GSC Paper

67-10, p. 21, pi. 1, figs. 1-5.

1973 Tmetoceras scissum (BENECKE), Imlay, USGS Prof. Paper 756, p.

59, pi. 2, figs. 1-6.

M aterial: Aproximately 35 specimens were collected from 5 lo c a litie s .

Those specimens collected from tuffaceous graywackes in the Anderson

River area (GSC numbers C-118672, C-88069, C-88070, C-118651) were either imprints or incomplete internal molds with venters slightly flattened but preserved. The 2 specimens from a sheared argillite in the Coquihalla area are distorted imprints (C-118653). 116

Description: Specimens are evolute with a subcircular whorl section and rounded umbilical w alls. Moderately dense, re c ti radiate ribs rise from the lower flank and curve forward over the venter, terminating at a mid-ventral groove. Constrictions are noted on 2 specimens.

Discussion: TmetoCeras occurs exclusively in 3 lo c a litie s (C-88069,

C-88070, C-118653). In locality C-118672 a poorly preserved fragment of a Plannamatoceras(?) (Poulton, pers. comm., 1986) or

Hammatoceras( ?) (Tipper, pers. comm., 1986) is found in association with Tmetoceras. In locality C-118651 it occurs with Erycitoides(?).

Occurrence: Tmetoceras is a cosmopolitan genus common to many

Aalenian marine sedimentary sections in the Cordillera (Taylor et a l.,

1984). Tmetoceras is documented to occur in the Manning Park area from the

Divide section (Frebold et a l . , 1969). Both specimens from the

Manning Park area are fragmentary and poorly preserved, therefore the collections from the Anderson River area provide conclusive evidence for the presence of this genus within the Trough. In the Taseko Lakes area, Tmetoceras is reported in association with Erycites aff. E. howel1i (Frebold et al., 1969).

Age: Westermann (1964) notes that Tmetoceras is associated with

Praestrigites cf. P. deltotis and Erycities/Abbasites in a sequence overlain by beds containing Sonninia (Euhoploceras) , Witchellia and

Docidoceras. This relationship indicates that Tmetoceras occurs through to latest Aalenian. Imlay (1973) reports that Tmetoceras is associated with Praestrigites, PIannamatoceras and Docidoceras, 117 indicative of the European Concavum Zone. Taylor (in press) notes that T. scissum occurs with Abbasites sparsicostata, an association indicative of the local sparsicostata faunizone.

In Europe, the first occurence of Tmetoceras is in the L. jurense Zone in NW Europe and the Late Toarcian in the central

Apenninnes in association with Dumorteria or C atulloceras. The la te s t occurence of Tmetoceras is in the L. murchisonae Zone and perhaps the

G. concavum Zone.

FAMILY PHYMATOCERATIDAE HYATT, 1887

SUBFAMILY HAMMATOCERATINAE BUCKMAM, 1887

GENUS Hammatoceras(?) or Planamatoceras(?)

P1annamatoceras(?) sp.

Plate 1, figure 4

Material: One poorly preserved imprint from locality C-118672.

Description: A moderately evolute(?), keeled specimen with moderately dense, slightly prorsiradiate ribbing. Ribbing appears to fade on outer whorl. .

Discussion: Extremely poor preservation frustrates identification although volution, presence of keel and ribbing do suggest affinities with either Hammotoceras or Plannamatoceras species.

Occurrence: PIannamatoceras is considered a probable Tethyan form and is not a common taxon in the Canadian C o rd ille ra . 118

Age: Plannamatoceras is considered generally an Early Bajocian form, but in association with Tmetoceras it must represent the latest

Aalenian.

GENUS Erycitoides WESTERMANN, 1964

Erycitoides sp.

Plate 1, Figures 5 and 6

*1964 Erycitoides (Kialagvikites) kialagvikensis (White) Westermann, p. 392, p is. 62, 63.

1969 Erycites kialagvikensis (White) Frebold et al., pi. 1, figs.,

6, 7.

Material: From one locality on the East Anderson River (C-118676) 4 complete imprints and 3 fragments were re triev e d . Possible fragments of th is genus were also collected from lo c a lity C-118651.

Description: Midvolute specimens, umbilical wall steep, with an abruptly rounded umbilical wall. No keel is observed. Moderately dense, primary ribs (ie. 12 per half whorl) originate at the umbilical wall. They rise rectiradiately to the mid-flank then curve forward over the ventro-lateral shoulder. Ribs bifurcate at the mid flank.

Ribbing strengthens ventrally. No tubercles are noted.

Discussion: The ribbing p attern , volution and absence of a keel suggest strong s im ila ritie s with Erycitoides (Westermann, 1964; Imlay,

1984), however, deformation has obscured d etails making species level identification impossible. It occurs in association with the bivalve 119

Posidonia (H.W. Tipper, pers. comm., 1985) in one locality and likely with Tmetoceras in the other.

Occurrence: Erycitoides is a widespread taxon. Locally it is documented to occur in the Manning Park area and elsewhere in the

Tyaughton Trough. I t is considered a "Bering" form (d is trib u tio n restricted to the northern Pacific) that occurs as far south as eastern Oregon (Taylor et a l., 1984).

Age: Erycitoides is known to range from the European Concavum (la te

Aalenian) to the Discites Zone (early Bajocian), however, in association with Posidonia and Tmetoceras(? ), Erycitoides in this study is considered representative of the local Erycitoides Zone.

Ammonite sp. indet.

Plate 2, figure 1

Material: A flattened partial cast of the outer whorl of a specimen from locality C-117272.

Description: Specimen has a keel. Ribs rise from tubercles on umbilical shoulder and bifurcate "tuning fork style" at the lower mid­

flank. Strength of ribbing remains constant as ribs rise radially then curve forward at ventro-lateral shoulder.

Discussion: Fragment is sim ilar in appearance to the Hammatoceras

figured in the Treatise but due to its association with other

Sonninids that is considered an unlikely possibility. It compares 120 well with a middle whorl of Sonninia (Euhoploceras) dominans (Imlay,

1973, plate 12, fig. 5) but doesn't really fit descriptions of

Sonninids well. It is likely of the subfamily Hammatoceratinae.

Age: Hammatoceratids range from Upper Toarcian to the Lower Bajocian therefore indicating a general Lower to early Middle Jurassic age.

FAMILY SONNINIDAE HYATT, 1867

GENUS SONNINIA BAYLE, 1879

SUBGENUS S. (EUHOPLOCERAS) BUCKMAN, 1913

Sonninia (Euhoploceras) sp. aff. S. polyacantha (Waagen)

Plate 2, figure 2

1973 Sonninia (Euhoploceras) polyacantha (Waagen), Imlay, USGS Prof.

Paper 756, p i. 18, 19; p i. 20, fig s . 1, 5-7; p i. 21, fig s . 8 ,9 . (see synonomy p. 64).

Material: One large, incomplete imprint found at locality C-117272.

Description: Moderately evolute specimen with a round umbilcal wall.

No venter preserved. Strong, recti radiate ribs rises from nodes at umbilical wall. There is a distinct variation of rib coarseness and intercostal space.

Discussion: Specimen most closely resembles S. polyacantha figured by

Imlay (1973). It occurs with various other Sonninids (see below).

Absence of preserved keel and whorl section frustrates a positive identification. 121

Occurence: Euhoploceras is considered a cosmopolitan genus. S. polyacantha is reported to occur in Oregon.

Age: S. polyacantha is representative of the local acanthodes zone.

Sonninia sp.

Plate 3, figures 1 and 2

Material: The most common species retrieved from locality C-117272 is represented by 3 flattened, three-dimensional specimens as well as approximately 10 im prints. All the material is deformed s lig h tly .

Description: Specimens are mid-volute with rounded umbilical wall. A keel is present. Fine ribs rise from umbilicus slightly rursiradiate then curve forward at ventro-1ateral shoulder. On the outer whorl, ribbing is faint on lower flank and strenghthens ventrally. Forms are non-tuberculate and all are less than 40mm in diameter.

Discussion: Specimens share characteristics of Sonninia modesta figured in Iml ay (1973, figs. 7-10). A three dimensional fragment of an outer whorl could conceivably be the outer whorl of this species. The amount of flattening and deformation of the specimens restricts species identification.

Occurrence: Sonninia is a cosmopolitan genus. It has been reported throughout the Cordillera. This study represents the firs t documented occurrence of Sonninia within the Methow Trough. 122

Age: Sonninids are characteristic of the early Bajocian up to the

European sauzei Zone and in western North America, the acanthodes and. burkei zones.

W itch ellia sp.

Plate 3, figure 3

Material: A partial cast of an outer whorl of one specimen was collected from lo c a lity C-117272.

Description: The specimen has rounded umbilical w alls and a triangular whorl section. It is bisulcate with a prominent keel.

Strong primary ribs rise from the umbilicus and curve forward on to the venter. Intercostal space equals ribbing size.

Discussion: Without more of the specimen preserved it is difficult to characterize this specimen, however, by comparison with figured specimens for the Lower Bajocian it resembles most Closely Witchel1ia sutneroides Westermann n.sp. (Westermann, 1969, p. 116, plate 28, fig .

1) in whorl section and ribbing.

Occurrence: The genus Witchellia is also considered a cosmopolitan taxon. This is the first documented occurrence of this genus in the

Methow trough.

Age: W. sutneroides is representative of the local widebayense Zone of south Alaska (Early Bajocian). Witchellia species generally occur within the European laeviuscula zone. Other species from the Wide 123

Bay lo c a lity overlap th is age range however.

SUPERFAMILY HAPLOCERATACEAE ZITTEL, 1884

FAMILY OPPELIIDAE BONARELLI, 1894

GENUS OPPELIA WAAGEN, 1869

Oppelia cf. 0. subradiata (J. de C. SOWERBY)

Plate 3, figure 4

Material: A partial three dimensional specimen from a tuffaceous siltsto n e u n it at the base of Blackwall Peak in Manning Park ( C-

118664)

Description: The specimen is likely involute with a compressed shell.

Ribs rise from upper mid-flank and are absent(?) on lower fla n k.

Recti radiate in itially, the ribbing becomes strongly prorsiradiate at the ventro-1ateral shoulder. The venter bears a keel although the probable la te ra l fla tte n in g of the specimen has lik e ly pronounced i t .

Discussion: The presence of a pronounced k e e l, the lack of ribbing on the lower flank and the whorl section (assuming that lateral fla tte n in g was minimal) allow a close comparison with 0. subradiata.

Its general appearance is similar to the genotype of 0. subradiata (J. de C. Sowerbyi) (Arkel1, 1951, p. 50, 51, text fig. 11). The ribbing pattern also bears close resemblance to W itch ellia sp. but the whorl section precludes i t from th is genus. 124

Occurrence: Oppelia is another cosmopolitan genus. 0. subradiata is reported from the Snowshoe Formation of Eastern Oregon by Iml ay

(1973). This is the firs t documented occurrence of Oppelia within the

Methow Trough.

Age: 0. subradiata is indicative of the local widebayense Zone.

SUPERFAMILY STEPHAMOCERATACEA Neumayr, 1875

FAMILY STEPHAMOCERATIDAE Neumayr, 1975

Stephanoceras sp.

Material: One partial imprint retrieved from shale in the East

Anderson River area (C-118681),

Description:. Fragment illu s tra te s a tuberculated specimen at m id-flank. Ribs trifu rc a te at the tubercles. Venter, whorl section, and other important character!'sites are not preserved.

Discussion: With such a poor fragment, the generic level id e n tific a tio n remains s lig h tly suspect, however, r e la tiv e ly few genera display such d is tin c tiv e ribbing.

Occurrence: Stephanoceras is a cosmopolitan genus common throughout

Bajocian strata of the Cordillera. It previously has been documented from the Manning Park (Frebold e t a l ., 1969).

Age: The presence of Stephanoceras indicates a mid-Bajocian age, specifically the local S. kirschneri and C. oblatum zones. PLATE 1

(All figures are natural size)

Figure 1, 2, 3 Tmetoceras scissum (BENECKE)

la. C-88069; lateral view of an internal mold fragment

lb. C-88069; venter

lc . C-88069; whorl section

2. C-88070; lateral view of an internal mold and imprint

3. C-117272; lateral view of a slightly deformed internal mold

Figure 4 Plannamatoceras(?) sp.

C-118672; lateral view of a latex cast

Figure 5, 6 Erycitoides sp.

5. C-118676; lateral view of an internal mold

6. C-118676; lateral view of a latex cast 125

PLATE I PLATE 2

(All figures are natural size)

Figure 1 Ammonite sp. indet.

C-117272; lateral view of an internal cast

Figure 2 Sonninia (Euhoploceras) sp. a ff. S.

polyacantha

C-117272; lateral view of a latex cast 126 PLATE 2 PLATE 3

(All figures are natural size)

Figure 1, 2 Sonninia sp.

la. C-118651; lateral view of an internal cast

lb. C-118651; venter

2. C-117272; lateral view of an internal cast

Figure 3 W itch ellia sp.

3a. C-117272; lateral view of an internal cast (fragment)

3b. C-117272; venter

3c. C-117272; whorl section

Figure 4 Oppel ia c f . 0_, subradiata

4a. C-118664; lateral view of an internal mold

4b. C-118664; venter

4c. C-118664; whorl section 127

PLATE 3 lb

3b APPENDIX B

POINT COUNT DATA

The following tables present normalized point count data for framework grains. The data from thin sections are lis te d from oldest

( ie . Ladner Group) to youngest (Pasayten Group). Subheadings of the different units correspond to specific map units that the sample represents (F ig . 4 and 12; Appendix 3 ). Qpl refers to chert, Qp2 is polycrystalline quartz and includes meta-siliceous volcanic, plutonic, and metamorphic d e tritu s . Other abbreviations are keyed to Table 2.

Thin sections without potassium feldspar stain have * before the sample number.

SECTION N Qm Qpl Qp2 P . KF Lv Ls Lc Lt Mica Hvs Lv:Lt K:F

DEWDNEY CREEK FORMATION mjdc MV085-68 427 0 0 0 229 0 229 180 8 0 188 0 10 .96 0 mJdccg MV085-31A 434 0 0 3 189 0 189 210 31 0 241 0 1 .87 0

MV085-65 364 5 4 0 157 0 157 185 5 0 190 3 5 .974 0

*MV085-71 402 9 2 3 127 0 127 245 7 0 252 0 9 .972 0

MV085-73 411 0 2 0 253 0 253 148 5 0 153 0 1 .967 0

128 129

SECTION N Qm Qpl 0p2 P KF Lv Is Lc Lt Mica Hvs Lv:Lt K:F mJ dcs MV085-61A 438 2 1 6 218 5 223 184 18 0 202 1 3 .91 .02

*MV085-115 434 2 1 5 178 0 178 216 20 0 236 0 12 .915 0

MV085-116 411 7 0 2 94 17 111 281 6 1 288 0 3 .98 .15

MV085-313 422 9 1 2 191 0 191 200 14 0 214 0 5 .93 0

MV085-324 413 3 # 1 6 127 0 127 238 4 0 242 0 6 .98 0

MV085-331 411 4 1 3 179 2 181 216 5 0 221 0 1 .98 .01

MV085-197 403 3 3 7 206 0 206 165 15 1 181 0 3 .912 0

MV085-L2 440 7 0 0 251 0 251 158 14 0 172 0 4 .92 0

MV085-D2 461 11 2 2 170 0 170 227 41 0 268 0 6 .84 0

MY085-285 436 58 2 9 227 0 227 138 0 2 140 0 0 .99 0 mJdcv MV085-386 420 0 0 4 233 0 233 174 3 0 177 0 6 .98 0

*C-118672 427 7 3 7 234 0 234 168 6 0 174 0 2 .97 0

*MV085-92 320 1 0 1 121 0 121 194 1 0 195 0 2 .99 0

MV085-93B 425 0 0 0 288 0 288 96 4 2 102 0 35 .941 0

MV085-219 497 96 2 47 305 0 305 29 10 0 39 6 2 .74 0 130

SECTION N Qm Qpl Qp2 PKF Lv Ls Lc Lt Mica Hvs Lv:Lt K:F

THUNDER LAKE SEQUENCE

MV085-339A 444 15 8 23 234 0 234 145 8 0 153 0 11 .948 0

JACKASS MOUNTAIN GROUP

IKs MV085-340 400 10 3 5 182 0 182 188 1 4 193 0 7 .974 0

MV085-284 459 27 3 0 201 12 213 142 8 0 150 4 62 .947 .056

MV085-343A 468 48 1 3 239 3 242 156 5 10 171 1 2 .912 .012

Kjm MV085-105 448 134 23 66 75 0 75 102 37 10 149 1 0 .685 0

MV085-135 412 109 0 88 207 0 207 0 0 0 0 56 8 0 0

JACKASS MOUNTAIN AND PASAYTEh1 GROUPS Kjmp

MV085-215 413 170 56 73 6 0 6 35 73 0 108 0 0 .324 0

MV085-106A 450 168 4 70 113 0 113 48 37 0 85 5 5 .565 0

MV085-96 412 174 7 36 169 0 169 20 4 0 24 2 0 .833 0 APPENDIX C

DESCRIPTION OF MAP UNITS FOR THE COQUIHALLA (FIG. 6) AND ANDERSON RIVER (FIG. 13) AREAS

Qs UNCONSOLIDATED SEDIMENTS (QUATERNARY): These include s i l t , sand, and gravel characteristic of fluvial deposits.

Tqm NEEDLE PEAK PLUTOM (LATE EOCENE): Massive, medium grained, hornblende-biotite quartz monzonite of the Needle Peak pluton intrudes sediments of the Methow Trough and the northwest trending Chuwanten

Fault. It has an areal extent of approximately 200 km and forms the southern boundary of the Anderson River map area (F ig . 13). The intrusion has been dated at 40 Ma (Wanless et a ! . , 1967) and 45 Ma (C.

Greig, pers. comm., 1987) using K-Ar determinations. Northeast- and north-trending faults cut the pluton in the Coquihalla area (Fig.4).

Tg (TERTIARY): Small in tru sive bodies and dikes with a spectrum of compositions (Cairnes, 1924; Ray, 1982) are located throughout the trough, often spatially associated with the northeast­ er north-trending Tertiary faults. In the Coquihalla area felsic sills and dikes are considered genetically related to the Needle Peak

Pluton (Fig. 6) (Cairnes, 1924; Ray, 1982 and 1986).

131 132

Kjmp JACKASS MOUNTAIN AND PASAYTEN GROUPS (CRETACEOUS):

U ndifferentiated marine and nonmarine sandstone, shale, and conglomerate of the Pasayten and Jackass Mountain Groups. The

Cretaceous units can be distinguished from the Jurassic stratigraphy by their maturity and composition.

Kjm JACKASS MOUNTAIN (EARLY CRETACEOUS): Massive cobble conglomerate with minor intercalated fine-grained sediments is characteristic of the Aptian-Albian(?) Jackass Mountain Group and is mapped separately where possible. The conglomerate is generally clast supported and contains a predominance of well-rounded clasts of granitic rock. Other common detritus includes intermediate volcanic rocks, siltstone and chert. Clasts up to 1.5 m have been noted, however the average size is approximately 20 cm. Matrix of the conglomerate is a medium-grained feldspatholithic wacke which contains detrital muscovite. Minor siltstone and sandstone occur interbedded with this unit.

In the Coquihalla area, this unit structurally underlies

Dewdney Creek strata against the Chuwanten fault (Fig. 6). In the

Anderson River area two fault-bounded conglomeratic sequences crop out

(Fig. 13). Although both are assigned to Kjm, the western unit varies in composition from the typical Jackass Mountain conglomerate described above. I t contains a predominance of volcanic

(predominantly andesitic) clasts with chert and crystalline detritus subordinate. Clast size is generally less than 20 cm. The eastern 133 fa u lt block, in which both conglomerate and un d ifferen tiated

Cretaceous sediments outcrop, has the more typical clast composition described above. Without further study of the petrology and distribution of the Cretaceous units it is difficult to determine whether this is due to stratigraphic position or a local variation in provenance.

mJdcv DEWDNEY CREEK FORMATION (AALENIAN(?) - EARLY BAJOCIAN):

Andesitic breccia, volcanic rich pebble conglomerate, and minor andesite and tuffaceous siltstone characterize the volcanic member of the Dewdney Creek Formation. This unit is recognized along the structural axis of the basin (ie. directly west of the Chuwanten fault) (Fig. 6 and 13).

The breccia contains fragments up to 1 m in diameter although the average fragment size is generally less than 20 cm. The breccia occurs in massive, structureless mounds with no lateral continuity.

Minor interbedded tuffaceous siltstone and volcanic-rich pebble conglomerate bearing marine fossils are other common constituents.

The clast supported conglomerates are massive and laterally discontinuous. Moderately rounded clasts are predominantly intermediate to acidic volcanics with rare quartz and granitic detritus noted. Andesite lavas are rarely preserved. In the Anderson

River area, a massive (approximately 5 m), vesiculated, andesitic flow overlies volcanic breccia. Coates (1974) also describes intercalated volcanic flows associated with volcanic breccia and tuffaceous sediments in the Manning Park area. 134

mJdcs DEWDNEY CREEK FORMATION (TOARCIAN?, AALENIAN AND EARLY

BAJOCIAN): Intercalated feldspatholIthic siltstone and sandstone characterize this unit. In outcrop, the sediments are generally thin- to medium-bedded. Sedimentary structures observed include ball and pillow structures, load casts, cross laminations, and soft sediment slump and fold structures. Most ammonites have been retrieved from this u n it. These immature sediments have undergone both diagenesis and low grade metamorphism as evidenced by the presence of abundant zeolites (laumontite/heulandite?), chlorite and epidote. Minor argillite and conglomerate occur interbedded with the siltstone. The conglomerate is matrix supported and contains volcanic and siltstone intraclasts. Rare argillite outcrops are sheared and often not laterally continuous, in part due to deformation effects.

Jdccg DEWDNEY CREEK (AALENIAN AND BAJOCIAN): This u n it is characterized by a restricted, lenticular basal limestone conglomerate

(Coquihalla area) (Fig. 6) overlain by volcanic rich pebble conglomerate interbedded with minor volcanic breccia (Anderson River area) (Fig. 13), argillite and feldspatholithic wacke. The limestone conglomerate varies from matrix to clast supported and contains a range in c la s t diameter from less than 5 cm to 1 m. Other clasts include intermediate to felsic volcanic rocks, siltstone and granitic fragments. Distinctive limestone-bearing conglomerate appears re s tric te d to the proximal contact between Ladner argillaceous strata and the Dewdney Creek Formation. 135

The more extensive volcanic-rich pebble conglomerates are clast supported with sub- to well-rounded clasts of intermediate to silicic volcanics (65%), siltstone/fine grained wacke (25%), monocrystalline quartz (5%), crystalline fragments (<5%) and limestone

(<5%). Quartz is well-rounded relative to volcanic clasts suggesting a mixed provenance. Matrix contains abundant plagioclase and is best referred to as a feldspatholithic sandstone. Channel cuts are common features of the basal contact of the massive (average thickness approximately 2 meters) conglomerate beds and coarse-mode normal grading and stratification can be observed. Conglomerate units are generally lenticular. Belemnites are relatively common within this unit. This unit is best exposed in the Coquihallai area on the ridge between C edarflat and Dewdney Creeks (F ig . 6 ).

Jsv (JURASSIC?) - East of the Chuwanten fault in the Coquihalla area, volcanic breccia occurs similar in character to the volcanic breccia of the Dewdney Creek Formation (mJdev) (F ig . 6 ). In contrast, however, the breccia is a relatively minor constituent of a sequence that contains medium- to thick-bedded tuffaceous wacke and massive volcanic-rich cobble conglomerate. Sub- to well-rounded pebble to cobble size clasts of intermediate to felsic volcanic (80%), granitic

(10%) and sedimentary (10%) detritus characterize this clast supported conglomerate. The conglomerate is unlike any other unit mapped within the Methow Trough. These sediments in general are more mature and contain a higher quartz content than other Jurassic units, but are unlike Cretaceous units (characterized by detrital mica, chert). Mo 136 fossils have been retrieved from this unit so the Jurassic assignment is tentative.

JT_ BOSTON BAR FORMATION (SI-MEMURIAN(?) - AALENIAM(?)): Marine clastic fine-grained sediments are the characteristic rock type of the

Boston Bar Formation. Moderately sorted 1ithofeldspathic siltstones and wacke crop out in thin to medium beds intercalated with arg illite.

Laterally continuous beds generally display sharp bases. Sedimentary structures within the more coarse members include cross 1 ami nations, normal grading and rare load casts. Elongate rip-up clasts are noted in the basal portion of some sandstone beds.

Intercalations of laterally discontinuous conglomerate occur in the basal strata in the Coquihal1 a area (Fig. 6). Two different conglomerate members are distinguished primarily by matrix composition. The matrix of the conglomerate which occurs near the base of the section is a 1ithofeldspathic medium-grained wacke. The conglomerate changes laterally from being matrix to clast supported and is poorly sorted. Sub- to well-rounded clasts include intermediate and silicic volcanic rocks and minor constituents of siltstone, carbonate and crystalline detritus. The conglomerate higher up in the section contains similar clasts which are matrix supported. The matrix is a sheared arg illite. The average monocrystalline quartz content is relatively high (Qm 15%) in comparison to units included w ithin the Dewdney Creek Formation.

The interpreted base of the Ladner Group is exposed in this area near the Carol in Mine (Fig. 6). There the Ladner Group is 137

described as resting unconformably on the meta-volcanic and

sedimentary sequence of the Spider Peak Formation (Ray, 1986). Near

the Hozameen fa u lt, the rocks have been p enetratively deformed such

that bedding is completely obliterated and the argillite becomes a

slate with subhorizantal lineations that display.a similar trend to

the structural grain of the map area (N20 W).

Tsp SPIDER PEAK FORMATION (EARLY(?) TRIASSIC): Ray (1986) describes

the Spider Peak Formation as a predominantly volcanic unit which

comprises gabbro, massive and pillowed greenstone, aquagene breccia

and minor volcanic rich sediments. It crops out in a fault-bounded

panel between serpentine (g e n e tica lly related to the Spider Peak (?))

and the Ladner Group in the Coquihalla area (Fig. 6). Few Early(?)

Triassic conodonts were retrieved from a lo c a lly developed chert

conglomerate horizon at the top of the section (Ray, 1986). The

Spider Peak Formation is interpreted to represent basement of the

Methow Trough (Cairnes, 1924; Ray 1986).

PJh HOZAMEEN GROUP (PERMIAN - MIDDLE JURASSIC): Deformed

greenstone, chert, limestone and argillite unit with no internal

stratigraphy. Correlated with the Bridge River Group to the north

west of the Fraser fa u lt system.

Mzs SERPENTINE (MESOZOIC): Serpentine crops out between the east

and west strands of the Hozameen fault in the Coquihalla area (Fig.

6), attaining a maximum thickness of greater than 2 kilometers. The

presence of gabbroic intrusive rocks similar to those within the 138

Spider Peak Formation are found w ithin the u nit (Ray, 1986).

Mzgdn Ml. LYTTOM COMPLEX (MESOZOIC): The meta-plutonic complex

directly east of the Pasayten Fault is subdivided into an older

northern part and a younger, more uniformly deformed southern part

(the Eagle Complex) (Monger, 1985). Granodiorite, hornblende gneiss, migmatite and pegmatite comprise the southern portion. A well-

developed foliation parallels the regional trend (i.e . approximately

N20 W). K-Ar and U-Pb dates are reported to range from 250 Ma to 140

Ma (in Ray, 1986). U-Pb date on a migmatite suggests the age of the

fabric is 135 Ma, or early Cretaceous (Monger, 1985). A masters

thesis in progress at the University of B ritis h Columbia (Grieg, pers.

comm., 1986) should better constrain the age and genetic history of

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— m ______- FIGURE 6. GEOLOGIC MAP AND CROSS SECTIONS OF THE COQUIHALLA AREA FIGURE 13. GEOLOGIC MAP AND CROSS SECTION OF THE ANDERSON RIVER AREA