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The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Biôliotheque nationale du Cmaâa de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microfom, vendre des copies de cette thèse sous paper or electronic formats. la fome de microfiche/iih, de reproduction sur papier ou sur format électronique.

The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts from it Ni la thése ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. ABSTRACT This study examines sediments and landforms in the east- central Taseko Lakes area, southern British.Columbia, and presents a mode1 of landscape evolution during the Late Wisconsinan Fraser Glaciation. Three phases of glaciation are distinguished: i) advance-phase; ii) glacial maximum-phase, and iii) retreat-phase . i) During the advance-phase, topography was a maj or control on local ice-flow, proglacial lake formation and sedimentation. Stratigraphie and geomorphic evidence suggests that southerly drainage was blocked by glaciofluvial outwash and ice in the Big Bar Creek area, where Fraser River is coniined to a canyon in the Camelsfoot Range. A proglacic?l lake system, infomally named glacial lake Camelsfoot, inundated the Fraser Valley to the north, reachi~ga minimum elevation of about 710 m prior to being overridden by ice. Over the Fraser Plateau, the Cordilleran Ice Sheet locally deformed advance- phase glaciofluvial sediments. Deformation is attributed to ductile, then brittle failure, resulting from transpression and loading, as a thin, grounded ice rnargin advanced over saturated sediments, followed by transtension, then transpression, under frozen conditions during early glacier overriding. ii) During the glacial maximum-phase, advance-phase and older deposits were eroded by thicker, wet-based ice, then overlain by Fraser Glaciation till. Two moraine provenances are identified in the area, indicating that glaciers from accumulation areas in the Chilcotin and Camelsfoot ranges were confluent with ice from eastern sources along the Fraser Valley between 5I015'N and 51045'N. By the end of this phase, the Cordilleran Ice Sheet ranged in thickness from approximately 600 m over the plateau to 1000 m in the Fraser Valley. iii) At the onset of retreat-phase, glacier ice blocked southward drainage of Fraser River, damrning a large proglacial lake system north of the area. Southerly drainage of this deglacial lake was established once remnant ice in the Fraser Valley downwasted to between 850 and 760 m elevation. Final drainage was accompanied by disintegration of remnant ice and mass movement of retreat-phase, and older deposits into valleys. These events were followed by decreased sedimentation rates, reflecting lower meltwater volumes and exhaustion of unstable debris. Post-glaciation, valley fil1 was subject to fluvial degradation and terracing.

iii Completion of the thesis would not have been possible without the guidance and financial support of my supervisor, B.E. Broster. I also thank C.J. Hickson of the Geological Survey of Canada for her encouragement and logistical support. Additional financial assistance was received from Supply and Services Canada contract 23254-1-0145/01-XSB and an NSERC operating grant to B.E. Broster.

1 am grateful to R.J. Fulton and S.R. Hicock for their reviews of Huntley and Broster (1994); S.R. Hicock. Géographie physique et Quaternaire associate editors and an anonymous reviewer for comments on Huntley and Broster (1993~);and R.J. Fulton and J.J. Clague for reviewing early versions of the surficial geology maps. 1 also appreciate various discussions with P.T. Bobrows~,V.M. Levson and W.H. Mathews. Occasional draughting and photographic services of Ange1 G6mez and Bob McCulloch were also appreciated. Fieldwork was most ably assisted by Irene Wiggins (nee Alarie). She liked the area so much, she moved there. Many an evening of high-level research was joined by Peter van der Heyden. Steve Metcalfe. Brian Mahoney and Darcy McDonald.

Thanks are extended to Larry and Bev Ramstead and the staff of Gang Ranch. In Fredericton, Victoria and London, liquid sustenance, magical potions and moral support came by way of many a fine individual (again). You al1 know who you are, or you wouldn't be reading this. Distant support was provided by my family. The thesis is dedicated to my Father. Page

TITLE PAGE ...... i ABSTRXT ...... ii ACKNOWLEDGMENTS ...... iv TABLE OF CONTENTS ...... v LIST OF FIGURES ...... ix LIST OF TABLES ...... xiv LIST OF APPENDICES ...... xv

CHAPTER 1 INTRODUCTION ...... 1

1.1 PREVIOUS WORK ...... 1

SCOPE OF STUDY ...... 7

CHAPTER 2 QUATERNARY LANDSCAPES OF SOUTHERN BRITISH COLUMBIA ...... 9

PRE-LATE WISCONSINAN LANDSCAPZS ...... 9

LATE WISCONSINAN LANDSCAPES ...... 13

POST-LATE WISCONSINAN LANDSCAPE ...... 19

CHAPTER 3 LATE WISCONSINAN GLACIER ADVANCE IN THE FRASER VALLEY AND FORMATION OF A PROGLACIAL LAKE SYSTEM ...... 23

INTRODUCTION ...... 20 Physical setting ...... 22 L:~thodsof investigation ...... 22

GLACIAL GEOMORPHOLOGY ...... 26 Page

LITHOSTRATIGRAPHIC DESCRIPTIONS ...... 29 Unit 3a (coarse-gxained member) ...... 33 Unit 3b (fine-grained rnember) ...... 39 unit 4 ...... 41

INTERPRETATION OF LITHOSTRATIGRAPHIC UNITS ...... 43 Unit 3a (coarse-grained member) ...... 43 unit 3b (fine-grained member) ...... 43 Unit 4 ...... 45

DISCUSSION ...... 45 Depositional mode1 ...... 47

CONCLUSIONS ...... 49

CHAPTER 4 LATE WISCONSINAN GLACIER ADVANCE OVER THE FRASER PLATEAU. AND LOCAL DEFORMATION OF ADVANCE-PHASE PROGLACIAL OUTWASH ...... 52

INTRODUCTION ...... 52

THE STUDY ARE24 ...... 54 Stratigraphy and origin of sediments ..... 54 Deformation structures ...... 60

DISCUSSION ...... 63 Deformation mode1 ...... 65

CONCLUSIONS ...... 70

CHAPTER 5 LATE WISCONSINAN GLACIER RETREAT IN MONTANE. PLATEAU AND VALLEY SETTINGS ..... 71 Page

INTRODUCTION ...... 71

THE STüDY ARE24 ...... 73 Physiography ...... 73 Late Wisconsinan glacial geology ...... 75

APPROACH TO STUDY ...... 77

DESCRIPTION OF LITHOSTRATIGRAPHY AND GEOMORPHOLOGY ...... 87 Unit 4a ...... 87 Unit 4b ...... 89 Unit 5a (coarse-grained member) ...... 93 Unit Sb (fine-grained member) ...... 98 Unit 6 ...... 104

INTERPRETATION OF LITHOSTRATIGRAPHY AND GEOMORPHOLOGY ...... 105 Unit 4a ...... 105 Unit 4b ...... 106 Unit Sa (coarse-grained member) ...... 107 Unit 5b (fine-grained rnember) ...... 108 Unit 6 ...... 110

DISCUSSION ...... 111 Deglacial landscape mode1 ...... 111

CONCLUSIONS ...... 119

CHAPTER 6 CONCLUSIONS ...... 123

FRASER GLACIATION ADVANCE-PHASE ...... 123

FRASER GLACIATION MAXIMUM-PHASE ...... 126

vii Page

FRASER GLACIATION RETREAT-PHASE ...... 128

REFERENCES

APPENDICES

VITA

viii LIST OP FIGURBS

Figure Description Page

1.1 Location of Taseko Lakes map area (92-0) and area of investigation ...... 2

1.2 Montane terrain in the Camelsfoot Range ...... 3

1.3 Fraser Plateau looking north frorn Dog Creek Dome ...... 5

1.4 Fraser River at Big Bar Creek ...... 6

2.1 Southern British Columbia ...... 10

2.2 Subdivisions of Quaternary events and deposits in southern British Columbia ...... 11

2.3 Extent of the Late Wisconsinan Cordilleran Ice Sheet ...... 16

3.1 Location of the area of investigation ...... 21

3.2 Physiographic regions in the Gang Ranch area ... 23

3.3 Glacial geomorphology of study area ...... 24

3.4 Till fabric data ...... 25

3.5 Simplified vertical profile logs of reference sections ...... 27

3.6 Fraser Valley in the vicinity of Big Bar Creek . 28 Figure Description Page

Relict periglacial terrain in the eastern Chilcotin Range ...... 30

Pre-Late Wisconsinan multistory gravels and sands (unit 1) ...... 35

Pre-Late Wisconsinan sequence cornprising deformed diamicton, silt and sand (unit 2) ..... 36

3.10 Advance and retreat-phase glaciofluvial sediments in the upper reach of Churn Creek .... 38

3 -11 Basal succession of unit 3b along the lower Churn Creek valley ...... 40

3.12 Structural data for unit 3b in the Gang Ranch area ...... 42

3.13 Mode1 for advance-phase glacial lake formation and deposition ...... 46

Location map of study area and glacial geonorphology of the Big Creek site ...... 53

Detail of Big Creek site section and schematic cross profile along A-A' ...... 55

Pebble fabric and structural data ...... 57

Photograph of dewatering structures ...... 58

Photographs and simplified sketches of deformation structures ...... 61 Figure Deecript ion Page

4.6 Glaciopalinspastic reconstruction of Figure 4.5 ...... 62

Polyphase cleformation mode1 for structures at Big Creek site ...... 66

Region discussed in paper and approximate extent of glacial lake Fraser ...... 72

Principal physiographic elements and nomenclature of east-central Taseko Lake area .. 74

~eriglaciaiterrain on Hungry Mountain ...... 78

Composite vertical profile logs of reference sections ...... 81

Till fabric and palaeocurrent data ...... 82

Indicator clast dispersal limits

~eglacialgeomorphology of the study area ...... 85

Sirnplif ied elevational distribution and drainage directions of meltwater features ...... 86

5.9 Basai till (unit 4a) ...... 88

5.10 Ice-contact colluvium (unit 4b) ...... 90

5.11 Ice-stagnation moraine (unit 4b) ...... 91

5.12 Crevasse fil1 deposits (unit 4b) ...... 92 Figure Oeacripti on Page

Esker ridge ...... 94

Detail of an esker deposit ...... 95

Bedrock-walled meltwater channel ...... 96

Ice-contact and proximal glaciofluvial sediment (unit Sa) ...... 97

Kame deltas at outlet of Canoe Creek an Alkali Creek ...... 99

Channel fil1 deposits (unit Sb) ...... 102

Fine-grained debris-flow (unit Sb) ...... 103

Deglacial palaeohydrogeography. post-glacial maximum phase ...... 113

Deglacial palaeohydrogeography. early ice-proximal phase ...... 115

Deglacial palaeohydroge~graphy.late ice-proximal phase ...... 117

Deglacial palaeohydrogeography. ice-distal phase ...... 120

Landscape evolution mode1 of Taseko Lakes area ...... 124

Map of field log locations ...... 148

Map of palaeoflow measurement location ...... 162 Figure Deecript ion Page

III. 1 Map of structural measurernent locations ...... 174

Map of lithology sample locations ...... 182

Surficial geology of Churn Creek map area ...... Back folder

Sur ficial geology of Empire Valley map area ...... Back folder v. III Surf icial geology of Dog Creek map area ...... Back folder

Surficial geoiogy of Mount Aiex nap area ...... Back folder

xiii LIST OP TABLES

Table Deecriptfon Page

Stratigraphy, descriptions and genetic interpretations of facies associated with individual units ...... 31

Regional correlation of stratigraphie units exposed in Gang Ranch area ...... 34

Range of Quaternary units in major physiographic regions of study area ...... 80

Non-genetic facies code ...... 149

Summary table of palaeoflow data ...... 164

III.1 Summary table of structural data ...... 175

xiv LIST OF APPENDICES

~ppendix Page

APPENDIX FIELD LOGS ...... 146 APPENDIX PALAEOFLOW DATA ...... 161 APPENDIX III STRUCTURAL DATA ...... 173 APPENDIX LITHOLOGY DATA ...... 180 APPENDIX SURFICIAL GEOLOGY MAPS ...... 196 CHAPTER 1 INTRODUCTION

This study concerns the Late Wisconsinan Fraser

Glaciation stratigraphy and geornorphology of an area of highly variable relief lying along the southwestern boundary of the Interior Plateau and Coast Mountains in smthern British Columbia (Figure 1.1). Fieldwork was undertaken in the east-central part of the

Taseko Lakes (92-0) National Topographic System rnap area. The study area has an areal extent of approximately 4900 kd, and includes the Big Bar (92 0/01), Churn Creek (92 0/07), Empire Valley (92 0/08), Dog Creek (92 0/09) and Mount Aiex (92 0/10) map sheets (Figure 1.1).

1.1 PREVIOUS WORK Early reconnaissance investigations of the Taseko Lakes map area (Tipper, 1971a; Heginbottom, 1972), and recent studies of early and late Quaternary sequences along the Fraser and Chilcotin valleys (Mathews and Rouse, 1986; Sles and Clague, 1991) provide fundamental insight into regional ice-flow patterns and depositional environments in the study area. In the south, the Camelsfoot, eastern Chilcotin and Marble ranges are glaciated montane landscapes (Figures 1.1; 1.2). These areas apparently functioned as local centres for ice accumulation during the Fraser, and presumably earlier

Figure 1.2 Glaciated montane landscape of the Camelsfoot Range (location at 51016'N; 122035'W). Similar terrain is found in the east Chilcotin Range. glaciations. To the north. lie the Fraser and Green Timber plateaux (Figures 1.1; 1.3). These are undulating and rolling landscapes with isolated upland areas. Plateau surfaces are blanketed in stony-sand to sandy-loam ground moraine (Heginbottom, 1972). Tipper (1971a) suggests the distribution of flutes and drumlins indicate that ice from the Camelsfoot and Chilcotin ranges coalesced with ice from the Cariboo Mountains along the Fraser Valley between 51°31'PJ and 51047 'N, and that during deglaciation, glaciers from the Cariboo Mountains readvanced into the area, terminating in the Fraser Valley. Morainaî deposits are incised by meltwater charnels and locally overlain by glaciofluvial outwash. Till and glaciofluvial sediments were deposited mostly during the late Wisconsinan Fraser Glaciation (Tipper, 1971a; Heginbottom. 1972). Plateaux and montane areas are dissected to a maximum depth of about 700 m by the south-flowing Fraser River and several major tributaries (Figures 1.1; 1.4). Valleys functioned as effective sediment traps, and were infilled with thick successions of till, diarnicton, stratified gravel, sand and silt. These sequences record at least three phases of glacial lake development during the late Quaterna- (Eyles et al., 1987; Eyles and Clague, 1991) . In addition, the area lies close to the southern limit of a large proglacial lake system that inundated much of the Fraser Basin during deglaciation (Clague, 1987; 1988). Older valley fil1 is incised by Fraser River and its tributaries, and overlain by Figure 1.3 East Fraser Plateau looking north from Dog Creek Dorne toward Chilcotin River and Alkali Creek (location at 51°37'lS; 122010'W). Figure 1.4 Fraser Valley at ~igBar ~reek(location at 51°10'~; 122008'W). View looking south. a variety of terraced Holocene fluvial. allwWrialand aeolian sedirnents (Heginbottom, 1972) .

1.2 SCOPE OF STIJDY This study focuses on: (i) definition of local ice-flow and sediment dispersal patterns of the Fraser Glaciation

Cordilleran Ice Sheet; (ii)determining ice sheet morphology. ice thicknesses and basal thermal regimes; (iii) establishing upper and terminal limits to glaciation; and (iv)delirniting ice retreat patterns. Also. little is known of depositional and erosional processes operating in montane, plateau and valley environments during glaciation. Therefore, the study also attempts to: (v) define former periglacial. glacial and proglacial environments in various physiographic settings; (vi) delimit the areal extent of advance-phase proglacial spillways and lakes on plateaux and in valleys; and ( vii) examine the relationship between ice retreat histos. and evolution of subglacial and proglacial drainage patterns during deglaciation. Chapter 2 outlines the Quaternary stratigraphy and glacial history of southern British Columbia. This brief overview provides a regional context for subsequent chapters. Chapter 3, published partly as Huntley and Broster (1994). establishes the stratigraphy and regional correlation of Quaternary units. in addition to defining glacier dispersal patterns and providing a first order approximation of ice thicknesses in the study area. A mode1 of ice advance and proglacial lake formation is also proposed. In chapter 4, published partly as Huntley and Broster (1993c), polyphase glacigenic deformation structures in advance-phase glaciofluvial sediments provide some insight into basal thermal conditions of grounded ice advancing over the Fraser Plateau. In chapter 5, glacial maximum-. and retreat-phase sediments and landforms provide insight into the limits of glaciation. ice retreat and meltwater drainage patterns. This chapter also relates the final drainage history of the glacial lake system occupying the Fraser Basin. Chapter 6 summarises the Fraser Glaciation history of the east-central Taseko Lakes area, and offers suggestions for further work. Field methods and data are included in appendices following the main text. Surficial geology maps in the back folder also appear as a separate publication (Huntley. 1995). -TER 2

QUATERWARY LhNDSCAPES

OF SOVTHERN BRITISH COL-IA

Quaternary sediments and landforms found in parts of British Columbia, south of latitude 54ON provide a contextual sett ing for unders tanding the relative chronostratigraphy , geomrphology and glacial landscape evolution of a relatively understudied part of the region; the east-central Taseko Lakes area (Figure 2.1).

2.1 PRE-LATE WISCONSINAN LANDSCAPES The landscape of British Columbia reflects development under polycyclic glacial and non-glacial conditions. During the

Quaternary period, much of British Columbia was repeatedly covered by an interconnected system of mountain ice sheets, piedmont lobes and valley glaciers, collectively referred to as the Cordilleran Ice Sheet (Jockson and Clague, 1995). Early Quaternary glacial sediments in the Coast Mountains and central ~nteriorPlateau are locally preserved beneath basalts, potassium-argon-dated between 1.2 and 0.3 Ma (Hickson and Souther, 1984; Mathews and Rouse, 1986; Ryder et al., 1991; Figure 2.2). On southern Vancouver Island and in the Fraser Lowland, possible glacial deposits, including the Westlynn Drift, are found beneath Late Quaternary non-glacial sediments

(Ryder and Clague, 1989; Figure 2.2). However, based on such limited evidence, it is not possible to infer significant ice sheet development at these times.

Late Quaternary sediments record at least three non- glacial periods and two regional glaciations (Figure 2.2).

Since organic detritus radiocarbon-dated at greater than Ca. 43 to 62 ka represent minimum ages, nany older deposits are assigned approximate ages based on relative stratigraphic position (Clague, 1981). In the Thompson Plateau area (Figure 2.1). early fluvial deposits are formally named Westwold Sediments (Fulton and Smith, 1978). Deposits with sirnilar relative stratigraphic position occur on southern Vancouver Island, in the Fraser Lowland (Muir Point Formation and Highbury Sediments, respectively; Hicock and Armstrong. 1983) and the central Interior Plateau (Clague, 1987). Overlying glacial sequences (Figure 2.2) include the Okanagan Centre Drift in the southern Interior Plateau (Fulton and Smith, 1978), and the Dashwood and Semiahmoo drifts on southern Vancouver Island, and in the Fraser Lowland (Hicock and Armstrong, 1983; Ryder and Clague. 1989). The widespread distribution of these deposits suggests that at least one extensive pre-late Wisconsinan ice sheet inundated much of southern British Columbia. Rare, erratic boulders in the Coast Mountains, possibly moved during this glaciation, indicate that ice flowed away from accumulation centres established over the central and northern Interior Plateau (Tipper, 1971b). This suggests the formation of a continental-style ice sheet (cf. Davis and Mathews, 1944). Additionally, deformed diamictons and clastic sediments in valleys throughout the central Interior Plateau are interpreted to have been deposited in supraglacial lakes formed at the end of this glacial cycle (Clague, 1987; Eyles et al., 1987; Eyles and Clague, 1991; Figure 2.2). The transition from pre-late Wisconsinan glacial to non- glacial conditions varies from greater than 58 ka in coastal areas to 43 ka over the Interior Plateau (Ryder and Clague, 1989; Figure 2.2). The range in ages of deposits most likely represents a diachronous boundaq to the onset of non-glacial conditions. In the Thompson Plateau area, Olympia non-glacial interval deposits are formally named the Bessette Sedirnents (Fulton and Smith, 1978). and are contemporary with the Cowichan Head Formation on southern Vancouver Island, and similar sequences in the Fraser Lowland (Clague, 1981; Ryder and Clague, 1989) and Chilcotin and Nechako plateau (Clague, 1987; Harrington et al., 1974). During this non-glacial interval, glaciers were confined to mountain areas, and fluvial, lacustrine and organic sediments were deposited in broad valleys with open forest to shrub-tundra range (Harrington et al., 1996), analogous to modern environmental conditions.

2.2 LATE WISCONSINAN LANDSCAPES Late Wisconsinan, Fraser Glaciation deposits date between Ca. 29 and 10 ka (Clague, 1981; Figure 2.2). In the Thompson Plateau area, Fraser Glaciation deposits are included in three units, comprising the Kamloops Lake rift (Figure 2.2; Fulton and Smith, 1978). These units are correlated with the Quadra

Sand, Coquitlam, Vashon and Surnas drifts in coastal British Columbia (Clague, 1981; Ryder et al., l99l). and sequences along major valleys in the central Interior Plateau (Clague, 1987; Eyles et al., 1987; Eyles and Clague, 1991) . The onset of glacial conditions varies front 29 ka in coastal areas to after 22 ka in interior British Columbia (Clague, 1981; Figure 2.2), and probably ref lects a gradua1 climatic deterioration across British Columbia, accompanying cooling of the north Pacific Ocean (Dawson, 1992) and lowering of regional equilibrium line altitudes in the Coast, Cascade,

Columbia and Rocw mountains (Fulton, 1991). Ice accumulation was apparently greatest on ocean-facing dopes of the Coast Mountains where prevailing westerly air was focused. In contrast, the palaeoclimate of the Interior Plateau was probably increasingly dominated by dry arctic air (Dawson, 1992). Although this may have resulted in lower amounts of precipitation, colder temperatures would have ensured that snow was the dominant form of precipitation. In montane areas, srna11 ice fields and cirque glaciers formed at valley heads amalgamated to form reticulate glacier systems that crossed pre-existing mountain drainage divides (cf. Davis and Mathews, 1944). At the southern margin of the ice sheet, several large piedmont glaciers, including the Puget, Okanagan, Columbia and Purcell lobes, advanced to terminal positions in northern Washington, Montana and Idaho between ca. 19 and 14 ka (Waitt, 1980; Ryder and Clague, 1989). In coastal areas, the ice sheet reached its maximum extent after 17 ka (Clague, 1981) . At this time, further westerly advance of the Cordilleran Ice Sheet was limited by calving of ice into the Queen Charlotte Strait (Ryder et al., 1991; Figure 2.3). Along mountain fronts f lanking the Interior Plateau, piedmont glaciers formed as major ice streams spilled out of montane trunk valleys (cf. Davis and Mathews, 1944), coalescing to form a discrete ice sheet after ca. 19 ka (Clague, 1981; Ryder et al., 1991; Figure 2.3). The regional distribution of roches moutonnées, striae and giacially-eroded deposits implies local iceflow was constrained by pre-Fraser Glaciation subglacial topography (cf. Davis and Mathews, 19441, in addition to the presence of a major ice divide between north- and south-flowing ice between latitudes 50° and 52O~(~igure 2.3). The elevation range of nunataks and upper glacial limits (Ryder et al., 1991; Figure 2.3) suggests that by the glacial maximum (between ca. 19 and 14 ka), the Cordilleran Ice Sheet was locally in excess of 2000 m thick. Although low surface gradients may have resulted in more evenly distributed precipitation ovex interior parts of the ice sheet (Davis and Mathews, 1944), accumulation areas remained in montane areas. Rates and patterns of sedirnentation and erosion were profoundly altered during the Fraser Glaciation. In montane valleys, ice advance was accompanied by aggradation of coarse- grained glaciofluvial outwash and ice-contact debris flow (Eyles et al., 19881. In the southern Interior Plateau, finer sediment was rapidly deposited in proglacial lakes typically formed along valleys oriented transverse to advancing glacier margins (Claque, 1987; Ryder et al., 1991) . By the glacial maximum, advance-phase sediments were truncated, then overlain Figure 2.3 Extent of the Late Wisconsinan Fraser Glaciation Cordilleran Ice Sheet in southern British Columbia (after Ryder et al., 1991). Upper limit of Late Wisconsinan glaciation in metres. Location of east-central Taseko Lakes area also shown (cross-hatched box). by basal till. Extensive tracts of glacially-stremlined ground moraine and rarity of ice-thrust features suggest that large areas of basal ice were wam-based, and subglacial substrates remained unfrozen during glaciation (Broster and Clague, 1987; Broster, 1991). Climate warrning apparently began shortly after the Cordilleran Ice Sheet reached its maximum extent (Clague, 1981). The areas most sensitive to late glacial climatic amelioration lay along the western and southern margins of the Cordilleran Ice Sheet, where deglaciation began ca. 13 ka and ended by 11 ka (Clague, 1981; Ryder et al., 1991). At the southern limit of the ice sheet, piedmont lobes damrned proglacial lakes, including glacial lakes Columbia and Missoula (Booth, 1986; Atwater, 1987) . After 19 ka, periodic floating of ice margins destabilised glaciers, triggering episodic catastrophic drawdown of proglacial lakes (Waitt, 1980). Drainage was partly facilitated by jokulhlaups with maximm

peak discharges estimated at 21 x 106 m3 s-1 (Bretz, 1969; Waitt, 1980). Successive drainage events during deglaciation produced a complex system of meltwater spillways, giant ripples and boulder grave1 bars in northern Washington. Coastal areas were rapidly deglaciated as sea-levels rose and glaciers calved back to fjord heads and coastal embayments (Clague, 1989). Isostatic uplift in coastal areas was non-unifom, reflecting complex ice retreat patterns. Deglaciation of the Interior Plateau started after 13 ka and ended by 10 ka (Berger et al., 1987; ~igure2.2). Regional iceflow was likely maintained until the equilibrium line altitude rose above the level of most accumulation areas (Fulton, 1967; 1991). Subsequently, the ice sheet stagnated and upper glacier limits retreated frorn montane sources toward the central parts of the plateau where thicker ice remained. Extensive parts of the Interior Plateau are covered by morainal and ice-contact glaciofluvial sedirnents that attest to the widespread stagnation of the ice sheet during deglaciation (Fulton, 1967). The final phase of ice retreat in the Interior Plateau was apparently interrupted by a late ice advance from the Coast and Cariboo mountains (Tipper, 1971a; 1971b). However, there is scant local climatological data to support this observation, and the geological evidence is ambiguous. Unconsolidated debris exposed upon ice retreat was readily eroded by meltwater and delivered to depressions and valleys. Erosion, transport and deposition rates were initially high, but moderated as sediment supply became exhausted and exposed surfaces stabilised (Church and Ryder, 1972). The distribution of deglacial outwash plains, fans and glaciolacustrine sediments suggest retreating ice and sediment dammed ephemeral lakes in major valleys, including the Thompson, Okanagan, Fraser and Columbia valleys (Mathews, 1944; Fulton, 1965; Shaw, 1977; Clague, 1988; Sawicki and Smith, 1992). During deglaciation, isolated ponded basins evolved toward a regionally-interconnected network of deglacial lakes draining the Interior Plateau. Glaciofluvial and glacial lake deposits were graded to successively lower base-levels, controlled locally by ice-retreat patterns and isostatic rebound (Ryder et al., 1991) . It is uncertain whether lakes in the Interior Plateau drained in a manner to proglacial lakes along the southern margin of the ice sheet. However, since rnany lakes were confined to alpine valleys, glaciers would have destabilised only along narrow margins. In addition. narrow lake basins would have prevented the removal of calved-off ice, so that in situ melting was probably the only mechanism for removing ice (Ryder et al., 1991) .

2.3 POST-LATE WISCONSINAN UNDSCAPES

After Ca. 10 ka, glacial and deglacial environments were superseded by non-glacial coastal. alluvial. fluvial and aeolian depositional regimes (Figure 2.2). In southern British Columbia, alluvial fans and outwash plains were little enlarged after about 6 ka (Ryder. 1971). Early Holocene and older deposits were subsequently dissected by post-glacial rivers. forming prominent valley terraces, mantled by thin-bedded fluvial and aeolian sediments. The vertical and spatial distribution of flüvial terraces along major valleys in the central Interior Plateau suggests that degradation was episodic, starting around 7 ka and reaching present incision rates by 1.2 ka (Ryder and Church, 1985) . CHAPTER 3 LATE WISCONSINAN GLACIER ADVANCE INTO VALLEYS

AND FORMATION OF A PROG&ACXAL LAKE SYSTEM

3.1 INTRODUCTION In southern British Columbia, proglacial lakes were repeatedly formed along major valleys during the advance and retreat of Late Quaternary Cordilleran ice sheets (Mathews, 1944; Fulton, 1965; Shaw, 1977; Clague, 1988; Eyles and Clague, 1991). In the east-central Taseko Lakes area (Figure 3.11, thick, laterally continuous sedimentary succeçsions are exposed along Fraser River and its major tributaries. These deposits record at least three phases of glacial lake formation along major valleys during the Late Quaternary (Eyles et al., 1987: Eyles and Clague, 1991) . At present, the areal extent of palaeoglacial lakes remains undefined. In addition, controls on ice-flow and meitwater drainage, lake formation and deposition in this area are poorly understood. This chapter examines Late Quaternary landforms and sediments, and proposes a depositional mode1 to explain the history of ice-flow and glacial lake formation during the advance-phase of the Late Wisconsinan Fraser Glaciation Cordilleran Ice Sheet.

3.1.1 Physical setting The study area has three distinct physiographic regions (Figure 3.2). In the south, the Camelsfoot, Marble and eastern Chilcotin ranges are montane landscapes with major peaks above 2400 m elevation. To the north lie the Fraser and Green Tixnber plateaux, undulating to rolling plains between about 920 and 1070 m elevation, with isolated hills rising in elevation above 1800 m. These regions are incised to depths between 500 and 700 m by the south-flowing Fraser River and its tributaries.

3.1.2 Methods of investigation Areas of ice accumulation were identified from the distribution of montane glacial landforms, including cirques, arêtes and U-shaped troughs. Glacier dispersal patterns were determined from geomorphic terrain indicators, including roches moutonnées, drumlins, flutes and striae observed during fieldwork and mapped from 1:10 000 and 1:50 000-scale air photographs (Figure 3.3). Palaeoice-flow was also inferred from rose histogram plots of long-axis trends of prolate pebbles and cobbles, sampled in basal tills (Figure 3.4; Appendix II). Upper glacial limits were determined £rom the distribution of trimlines and relict periglacial features

(cf. Garnes and Bergersen, 1980 ; Ballantyne and McCarroll, 1995). Estimates of ice thicknesses were derived by measuring the range in elévations between trimlines, plateau surfaces and valley floors. Figure 3.2 Physiographic regions in the Gang Ranch area. A - Camelsfoot Range; B - Marble Range; C - Chilcotin Range; D - West Fraser Plateau; E - East Fraser and Green Timber plateaux; F - Major river valleys, Solid triangles - major peaks . North1

Figure 3.3 Glacial geomorphology of study area. Limits of glaciation and approximate limits of advance glacial lake formation are also shown. Figure incorporates iceflow data of Tipper (1971a) and Heginbottom (1972) with data collected between 1991 and 1992. 10 20 n=S0 @ # + Striae Distributicn

\wi 1w/

a) Gaspard Creek

-'

b) Gang Ranch e) Alkali Creek

C) Grinder Creek f) Dog Creek

d) Lone Cabin Creek g) Canoe Creek

Figure 3.4 Till fabric data. Also shown are local iceflow directions determined from drumlin and striae orientations (see also Appendix II) . Eight reference sections of valley fil1 were logged in vertical profile along major valleys in the study area

(Figure 3.5; logs A to H; Appendix 1). Section elevations were determined from contours or spot heights on 1: 50 000- scale National Topographie System maps, and measured with an altimeter to an accuracy of f 5 m. ~ithostratigraphicunits were distinguished by outcrop pattern, erosion surfaces, sedimentology and local correlation. Palaeocurrent directions were determined from orientations of channels, trough cross- beds, foreset beds and rose histogram plots of pebble imbrication direction or clast long-axes trends (~ppendix II).

3 .2 GLACIAL GEOMORPHOLOGY Cirques, arêtes and U-shaped troughs are restricted to the Camelsfoot Range and adjacent uplands, indicating that these areas functioned as local centres for ice accumulation during the Fraser Glaciation (Tipper, 1971a; Heginbottom, 1972; Figure 3.3) . In the Camelsfoot Range, valley glaciers flowed northeast to east along structurally-controlled valleys, coalescing with southwest-flowiny ice £rom the

Marble Range (Figures 3.3; 3.4d). In this area, a 700 m deep glacial trough is evidence of a significant south-flowing valley glacier occupying the Fraser Valley (Figure 3.6). In the western Camelsfoot Range, northwest-flowing ice entered the upper Churn Creek basin and was deflected northeastwards by glaciers flowing frorn the eastern Chilcotin

Figure 3.6 Fraser Valley in the vicinity of Big Bar Creek (location at 51010tN; 122004'W). In this area, the south- fiowing Fraser River occupies a 700 m+ deep glacial trough. view looking north. Range (Huntley and Broster, 1993a; Figures 3.3 ; 3.4c, d). Ice subsequently moved into the Fraser Valley, coalescing with Cariboo Mountain ice advancing into Alkali, Dog and Canoe creeks between 51'35'N and 52'N (Figures 3.3; 3.4e to 3.49). Glacial landforms and deposits are rare or absent on peaks and uplands above 2600 m elevation in the Camelsfoot Range, 2290 rn in the eastern Chilcotin Range, and 1980 m over the West Fraser Plateau. in these areas, bedrock is debris- free or buried by relict periglacial landforms, including felsenmeer, solifluxion terraces and polygonal patterned ground (Figure 3.7). Tors also occur along some ridge crests. These observations suggest periglacial conditions prevailed in high montane areas (cf. Garnes and Bergersen, 1980; Ballantyne and McCarroll, 1995), and possibly reflects a stable minimum upper elevation of the Cordilleran Ice Sheet during the Fraser Glaciation (Figure 3.3).

3.3 LtITHOSTRATIGRAPHIC DESCRIPTIONS In montane areas, over plateaux and in upper reaches of tributary valleys, the Quaternary cover ranges in thickness between less than 1 m to approximately 50 m. Frorn 150 and 300 m of Quaternary sediments are exposed in vertical sections along the Fraser Valley. Six units (informally designated units 1 to 6) are identified according to relative stratigraphie position, sedimentology and facies associations (Figure 3.5; Table 3.1). Despite local rernobilisation by landslides and lack of datable horizons, units are Figure 3.7 Relict periglacial terrain exposed above 1980 m elevation in the eastern Chilcotin Range (location at 51°17'N; 122°S5'~). Similar terrain is exposed in the Camelsfoot Range. ltratigraphic Description Inferred origin Relative phase deeignation

Unit 6 Clast-supported diamicton; Laminated Sedirnentation dorninated by mass- Holocene and massive silt, associated with movement, fluvial and alluvial (10 ka - present) terraced gravel and sand processes; some reworking by aeolian processes

Unit 5b Fine-grained member: rhythmically Ice-retreat glaciodeltaic and Fraser Glaciation interbedded silt, sand and glaciolacustrine sediments; Retreat-phase diamicton; minor gravel and sand isolated lakes ponded in valleys (13-10 ka) beds occur as channel lags cut into older sediments

Unit 5a Coarse-grained member: interbedded Ice-retreat glaciofluvial Fraser Glaciation gravel and sand beds with minor sedirnents and reworked till Retreat-phase diamictons (unit 4) (13-10 ka)

Unit 4b Massive and stratified diamicton Supraglacial rneltout till, crevasse Fraser Glaciation fills and ice-proximal colluvium, Retreat-phase deposited during stagnation of the (13-10 ka) Cordilleran ice sheet

Unit 4a Massive diamicton with faceted and Fraser Glaciation till, deposited Fraser Glaciation striated clasts; associated with during advance of the Cordilleran Glacial maximum-phase drumlinised sedirnents (unit 3, 4a) ice sheet; minor deposits are (19-14 ka) and minor gravel and stratified subglacial fluvial deposits diamicton and meltout till Continued over:

Table 3.1 Stratigraphy, descriptions and genetic interpretations of facies associated with uniks. Approximate ages after Clague (1981) and Ryder et al. (1991). Table continued on next page. Stratigraphie Deecription Inferred origin Relative phaee

Continued: Unit 3b Fine-grained member: deformed Ice-advance glaciolacustrine Fraser Glaciation stratified diamicton and silt; sediments deposited in a Advance-phase gravel and sand beds occur as gradually deepening and sediment- (29-20 ka) channel lags cut into other starved ice-dammed proglacial lake; sediments; fines into a partly ice becoming proximal deformed succession of rhythmically interbedded fine sand, silt and clay

Unit 3a Coarse-grained member: interbedded Ice-advance glaciofluvial Fraser Glaciation gravels and stratified diamictons sediments and ice-proximal Advance-phase with minor sand beds debris flow deposits (29-20 ka)

Unit 2 Deformed interbedded stratified Ice-retreat glaciolacustrine Early or pre-Wisconsinan diamicton, silt and sand sediments (A0 ka)

Unit 1 Multistory cobble, grave1 and sand Deposited in a south-flowing Miocene to Quaternary beds; sediments are yellow- fluvial system (10 Ma->40 ka) stained and micaceous

Table 3.1 Stratigraphy, descriptions and genetic interpretations of facies associated with individual units. Approximate ages after Ryder et al. (1991). lithologically distinctive and extensive enough to permit reliable correlation between exposures within and adjacent to the study area (Table 3.2). Pre-late Wisconsinan fluvial (unit 1: Figure 3.8) and deglacial lacustrine sequences (unit 2; Figure 3.9) are presemed along sections of Fraser River and major tributaries (Eyles et al.. 1987; Eyles and Clague. 1991) . Late Wisconsinan Fraser Glaciation advance and retreat-phase sediments (units 3 to 5) are found in al1 physiographic settings (Broster and Huntley, 1992 ) . Holocene. post-glacial fluvial. alluvial and aeolian deposits (unit 6) are mainly restricted to major valleys and overlie earlier units (Figure 3.5). The focus here is on advance-phase seguences of the Late

Wisconsinan Fraser Glaciation (units 3 and 4) ; descriptions of other units are reported elsewhere (Broster and Huntley,

1992; Huntley and Broster, 1993a. 1993b. 1993~).Unit 3 comprises two members: (i) coarse-grained sediments confined to valleys in, or proximal to, the Camelsfoot Range. and along Fraser River; and (ii) fine-grained sediments. restricted to the Fraser Valley and lower reaches of tributaries north of the Camelsfoot Range. Unit 4 occurs in al1 physiographic settings and unconfomably overlies unit 3.

3.3.1 Unit 3a (coarse-grained member) Interbedded. poorly-sorted. coarse. massive gravel, subordinate massive sand and stratified diamicton beds are

Figure 3.8 Pre-late Wisconsinan multistory fluvial gravels and sands (unit 1) exposed in the vicinity of Gang Ranch (location at 51°33'~; 122017'W). This sequence is incised by a palaeochannel of Fraser River (contact arrowed), and infilled with pre-late Wisconsinan glaciolacustrlne (unit 21, and late Wisconsinan advance-phase sediments (unit 3b). Figure 3.9 Pre-late Wisconsinan retreat-phase glaciolacustrine sequence (unit 2) comprising deformed diamicton, silt and sand. exposed along lower Churn Creek in the vicinity of Gang Ranch (location at S1°32 IN; 122020 'W). This sequence is unconformably overlain by late Wisconsinan advance-phase sediments (unit 3b). Contact between units 2 and 3b indicated with arrows. exposed in sections up to 50 m thick along montane valley floors in, and adjacent to, the Camelsfoot Range (Figure 3.5; logs A to C; Figure 3.10). Clast imbrication and bedding orientations in gravel and diamicton indicate dominant easterly palaeoflow, with local deviation, toward the Fraser

Valley (Appendix II). In these areas, unit 3a is truncated, locally drumlinised and overlain by massive diamicton (unit 4) and younger sediments (Figure 3.5; Table 3.1). Coarse-grained sedirnents exposed along Fraser River and lower reaches of its tributary valleys infill palaeochannels incised into bedrock or units 1 and 2. These palaeochannels are incised to depths equivalent to present valley floors. Maximum unit thicknesses of 50 to 80 m are attained at tributary outlets and these appear to thin laterally along the Fraser Valley. Basai successions are dominated by channelised interbedded gravel and sand (Figure 3.5; logs A, D, E, G and H). Gravels appear to be finer and better sorted than equivalent sediments in rnontane valleys. Cha~elsGZC infilled with sequences comprising gravel lags, trough- and, or, ripple-bedded sand. Channels and palaeocurrent iridicators are predominantly oriented southward. Above approximately 350 m elevation, gravel and sand is interbedded with stratified diamicton and massive silt (Figure 3.5; logs G and HL This facies change is associated with the appearance of northward palaeoflow indicators in sand beds. Figure 3.10 Advance- and retreat-phase glaciofluvial sediments exposed in the upper reach of Churn Creek (see Figure 3.5; log C for location). The basal 50 m+ comprises coarse-grained facies of unit 3a. Contact with unit 5a is indicated by arrows. 3.3.2 Unit 3b (fine-grained member) In major valleys north of the Camelsfoot Range, unit 3a is gradational with thick successions dominated by diamicton, sand and silt (unit 3b). In the lower reach of Lone Cabin Creek, 20 m of stratified diamicton, massive and laminated silt is truicated and overlain by units 4 and 5 above 570 m elevation (Figure 3.5; log B; Figure 3.11). Similar, but thicker, sequences (between 80 and 120 m) are exposed along the lower reaches of tributaries and the Fraser Valley upstream from Lone Cabin Creek (Figure 3.5; logs D and H). Diamicton and silt beds are channelised and infilled with subordinate grave1 lags and trough or rippled-bedded sand. An upward transition from A and B-type climbing ripples of sand (cf. Ashley et al. , 1982) to draped massive or laminated sand and silt is occasionally observed. Channels, trough-cross beds, ripples and imbricated clasts indicate a dominant palaeoflow toward Fraser River, and a northerly flow along the Fraser Valley. Along much of the Fraser Valley, unit 3b is truncated between approximately 540 and 670 m elevation. However, at Gang Ranch, rhythmically-bedded, laminated fine sand, silt and clay with randomly dispersed, faceted and striated dropstones are preserved above 670 m elevation. This succession occurs up to elevations of about 710 m. and is unconformably overlain by massive diamicton (unit 4; Figure

3.5; log D). Figure 3.11 Basal succession of unit 3b exposed along the lower Churn Creek valley (location at 51032'~;122~20'~). Note that intensity of syndepositional deformation decreases toward top of sequence. Along the lower reaches of Churn and Gaspard creeks, basal exposures of unit 3b are deformed by syndepositional folds and faults. Although fold axial planes show no consistent trend, fault planes generally lie parallel to palaeoflow (Figure 3.12a). The magnitude of faulting decreases above 620 rn elevation. Between 690 and 710 m elevation, unit 3b is deformed by folds with northwest- to southeast-oriented axial planes (Figure 3.12b) . Folds are locally truncat ed and overlain by undeformed diamicton (unit

4).

3.3.3 Unit 4 In rnost valleys, bedrock and older sediments have been eroded or drumlinised and overlain by massive diamic tons ranging in thickness between 0.5 to 15 m. Clasts within this unit are polyrnictic, commonly striated and faceted and supported in a dense, silt- or clay-rich matrix. Prolate pebbles generally show a preferred fabric alignment conforming to local orientations of U-shaped valley troughs, drumlins and striae (Figure 3.4). Diamicton units are locally overlain by less-consolidated stratified and massive diamicton beds, or incised by charnel lag deposits consisting of grave1 and sand. sclast+ orientations

a) Poles to faul planes in basal Unit 3b

n = 50 Striae +Drumlin N

l

-:

b) Poles to fold axial planes in upper Unit 3b

Figure 3.12 Structural data for unit 3b in the Gang Ranch area: a) Poles to faults show with principal current direction derived from 50 long axis clast orientations; b) Poles to fold axial planes show with till fabric data and ice flow detsrmined from drumlin and striae orientations (see also Appendices II and III). 3.4 INTERPRETATION OF LITHOSTRATIGRAPHIC UNITS

3.4.1 Unit 3a (coarse-grained member) In montane valleys, grave1 and sand beds resemble ice- proximal glaciofluvial facies assemblages (Maill, 1977; Rust and Koster, 1984). Palaeocurrent indicators suggest dominant flow toward the Fraser Valley, although variations in clast orientation suggest that streams were braided (Maill, 1977). Associated diamicton beds likely represent subaerial ice- marginal debris-flows (cf. Ryder, 1971; Eyles et al., 1988). Along Fraser River and lower reaches of its tributary valleys, basal gravels and sands are interpreted as fluvial or ice-distal glaciofluvial sediments. Palaeocurrent indicators suggest deposition initially occurred in a south- flowing ancestral stream. Deposition of these sediments occurred following a period of degradation during the last non-glacial interval, which ended with the ancestral Fraser River at a base-level elevation similar to that of present. The appearance of diamicton and silt above approximately 350 m elevation suggests a transition from an ice-distal regime to one dominated by proximal glacial processes. The change is also associated with the appearance of northward palaeoflow indicators, implying impeded southward drainage resulting from periodic blockage by ice and, or, sediment.

3.4.2 Unit 3b (fine-gtained member) Sediments of unit 3b are interpreted to have been deposited in a proglacial lake environment (Eyles and Claque, 1991). The vertical gradation from unit 3a to 3b suggests that glaciofluvial deposition was progressively replaced by glaciolacustrine conditions. Unit 3b reaches a maximum thickness of approximately 220 m at Gang Ranch (Figure 3.5; log D), implying that significant lake development occurred north of the Camelsfoot Range. Along lower tributary reaches, sediment was likely deposited during mass movement down fan-delta foreslopes (cf. Shaw and Archer, 1978; Seigenthaller et al., 1984; Eyles and Clague, 1991) and, or, by waning-stage deposition from turbid underflows (cf. Gustavson et al , 1975; Smith and Ashley, 1985) . Deformation in basal exposures is attributed to channel collapse during incision by underflows or fluctuations in lake levels (cf. Broster, 1991; Eyles and Clague, 1991) . The decrease in intensity of deformation may reflect a reduction in current strength or increasing stability of lake levels above 620 m elevation. In the Fraser Valley, diamicton beds are interpreted as lake-marginal subaqueous debris-flows (cf. Broster and

Hicock, 1985) . Massive and rippled sand beds were likely deposited by north-flowing meltwater or debris-flow-generated underflows (cf. Shaw and Archer, 1978). The transition from ripple- to drape-bedded sands and silts implies episodic dominance of suspension from overflow-interflows over bedload deposition by underflows (Ashley et al., 1982; Smith and Ashley, 1985; Donnely and Harris, 1989). At Gang Ranch, rhythmically-bedded fine sediments with dropstones are interpreted as varves. This suggests that ice- proximal deposition with a limited supply of debris occurred here (cf. Catto, 1987). Fold axial planes in upper varved sections lie orthogonal to local ice-flow directions (Figure

3.12b) , implying deformation was glacigenic (cf. Broster and Clame, 1987; Broster, 1991).

3.4.3 Unit 4 The polymictic, striated or faceted clast assemblage,

supported in a consolidated matrix and preferred fabric

alignrnents (Figure 3.4), suggests that the massive diamicton is a lodgement till (cf. Dreimanis, 1976; Muller, 1983). ~ess-consolidateddiamicton and channel-fil1 sequences likely represent supraglacial meltout till and ice-contact glaciofluvial sedinent facies assemblages (cf. Dreimanis, 1988; Lawson, 1988).

3.5 DISCUSSION In this section, a depositional model of the advance- phase of the Late Wisconsinan Fraser Glaciation in the study area is proposed (Figure 3.13). This model provides an explanarion for proglacial lake formation and sedimentary history in the context of the local glacial history. Figure 3.13 Mode1 for glacial :lake formation and deposition related to the advance of the Cordilleran Ice Sheet during the Fraser Glaciation. For explanation of A to D, see text. Diagrams not to scale. 3.5.1 Depositional mode1 At the onset of the Late Wisconsinan Fraser Glaciation (ca. 29 to 19 Ka; Clague, 1981; Ryder et al., lggl), deposition of glaciofluvial sediments was restricted to ice- proximal montane valleys (Figure 3 .l3, A) . Fluvial, or ice- distal glaciofluvial sedirnents were deposited by a south- flowing ancestral Fraser River draining palaeovalleys incised into bedrock or pre-Late Wisconsinan sediments . In response to continued climatic deterioration, valley glaciers advanced from montane areas over the Fraser and Green Timber plateaux and into lower valley reaches (Figure 3.13, B) . As ice advanced, glaciofluvial and debris-flow activity increased along major tributary valleys (cf. Eyles et al. , 1988 ; Figure 3.10 ) . Ice-proximal glaciof luvial outwash trains and fan-deltas (unit 3a) prograded into the Fraser Valley in the Big Bar Creek area impeding southward drainage. An outlet for northward drainage during this phase has not been identified in the study area, and correlative sediments along the Fraser River in the Williams Lake area indicate southerly flow (Clague, 1986). However, anomalous flow directions may represent localised drainage into isolated basins that developed between prograding outwash fans (Figure 3.13, BI. Glaciers from the Camelsfoot Range and eastern sources subsequently coalesced along the Fraser Valley in the vicinity of Big Bar Creek (Figure 3.6). ponding a proglacial lake north of this area (Figures 3.3; 3.13, C). Limited or no glaciolacustrine sedimentation occurred in montane valleys. North of the Camelsfoot Range, the proglacial lake deepened as ice advanced into lowland areas, so that outwash trains and fan-deltas in tributary valleys were drowned (Figure 3.13, C). Lake development and sedimentation (unit 3b) in ice-distal settings reached a regional minimum elevation of around 620 to 670 m elevation, At Gang Ranch, varved sediments were deposited up to about 710 m elevation (Figure 3.5; log D), suggesting the presence of a remnant lake with a minimum upper level around this elevation (Figure 3.13, D). Dropstones in upper beds of unit 3b suggest that ice advance into the remnant proglacial lake occurred along floated margins. However, glacigenic deformation in the upper 10 to 15 m of unit 3b (Figure 3 .lSb) indicates that glacier margins became grounded as thicker ice overrode the lake, Glacier maximum ice-flow (ca. 19 to 14 Ka;

Clague, 1981; Ryder et al., 1991; Table 3.1) was associated with widespread erosion of older sediments and deposition of lodgement till (unit 4). Correlative glaciolacustrine sediments are exposed along the Chilcotin River and Fraser Valley near Williams Lake (Table 3.2; Clague, 1986; wles and Clague, 1991). In the Quesnel and Prince George area, comparable sediments occur up to elevations of about 690 m, and are overlain by Fraser Glaciation till (Clague, 1988). These observations indicate that prior to being overridden by ice, the proglacial lake likely extended well north of the study area. Trirnline elevations and relict periglacial landforms define the surface of the Cordilleran Ice Sheet in the study area at the end of the Fraser Glaciation maximum (ca. 14 to 13 Ka; Clague, 1981; Ryder et al., 1991) . Their distribution suggests that the ice sheet thinned away from source areas.

Trimiines in the Camelsfoot Range occur around 2600 m elevation, implying an ice sheet thickness in the order of

2000 m in accumulation areas. However, in the upper Churn Creek valley and adjacent port ions of the plateau valley . lower trimlines at approximately 1980 m elevation, suggest ice thicknesses between 600 and 1000 m. Along rnuch of the Fraser Valley, units 3 and 4 were subsequently overlain bydeglacial (unit 5) and post-glacial sediments (unit 6) at the end of the Fraser Glaciation. and during the early Holocene (Table 3.1).

3.6 CONCLUSIONS Six lithostratigraphic units are identified at eight reference sections (units 1 to 6; Figure 3.5; Table 3.1) . These units record multiple glacial and non-glacial depositional events within montane, plateau and valley settings. Complex physiography was a major control on local ice-flow, deposition and lake formation during Fraser Glaciation. Lithostratigraphic and geomorphic evidence suggests that blockage and ponding initially occurred in the Big Bar Creek area. Here, Fraser River is confined to a deeply incised canyon in the Camelsfoot Range (Figure 3.6) . Because the advance-phase proglacial lake originated in this area, it is suggested that it be informally named "glacial lake Camelsfoot". Correlation with similar sequences in the Interior Plateau (Table 3.2) implies that this proglacial lake system may have extended well north of the study area prior to inundation of the Fraser Valley by the Cordilleran Ice Sheet. A composite stratotype (North Arnerican Commission on Stratigraphic Nomenclature, 1983) associated with glacial lake Camelsfoot is exposed along major valleys in the vicinity of Gang Ranch, in the east-central Taseko Lakes area (Figure 3.5). This stratotype comprises laterally and vertically gradational coarse-grained (glaciofluvial) and fine-grained (glaciolacustrine) members (units 3a and 3b, respectively) . These members are unconformably overlain by massive dimicton and subordinate facies (unit 4 ) . A depositional mode1 for units 3 and 4 is proposed (Figure 3.13). involving: (i) early southward drainage of Fraser River and ice accumulation that was limited to southern montane areas; (ii) ice advance into montane valleys with increased ice-proximal glaciofluvial deposition; (iii) blockage of the southward-draining Fraser River in the Big Bar area by glaciofluvial sediments and ice, leading to formation of glacial lake Camelsfoot; (iv) partly floated ice advance into this lake and late grounded overriding by the Cordilleran Ice Sheet; and (v) widespread deposition of Fraser Glaciation tili. Lastly, it is inferred that the Cordilleran Ice Sheet in the Gang Ranch area was in the order of 600 to 2000 m thick; thinner than other regional estimates of ice thickness (cf. Clague, 1989: Fulton, 1991; Ryder et al., 1991) . Physiography and a thin ice sheet likely exerted major controls on regional ice-flow patterns, glacigenic deformation over adjacent parts of the Fraser Plateau (Huntley and Broster, 1993a: 1993~)and the subsequent style of deglaciation (Huntley and Broster, 1993b). CHAPTER 4

LATE WISCONSINAN GLACIER ADVANCE OVER THE FRASER

PLATEAU, AND LOCAL DEFORMATION OF PROGLACIAL OUTWASH

4.1 INTRODUCTION Investigation of glacigenic deformation at several localities in southern British Columbia (Figure 4.la) has provided insight into ice-flow history. basal thermal regimes and patterns of ice retreat during the Late Wisconsinan, and earlier glaciations (Broster et al.. 1979; Broster and Clague, 1987; Eyles et al.. 1987; Broster. 1991; Eyles and Clague, 1991). Deformational features in Late Wisconsinan. Fraser Glaciation sequences were observed at several localities in the east-central Taseko Lakes area (Figure 4.lb; Appendix III). In upland areas, bedrock fractures and till-injection wedges are attributed to overriding by frozen and wet-based glacier ice (Huntley and Broster, 1993~).Along Fraser River and its tributaries. deformation of glaciofluvial and glaciolacustrine sequences is attributed to incision by meltwater, advance of grounded glaciers into valleys (Huntley and Broster. 1994) and loss of support during ice retreat. The significance of these structures with respect to the glacial history of the area is discussed in chapters 3 and 5. This chapter focuses on well- preserved deformation structures in advance-phase glaciofluvial sediments near Big Creek, formed during advance and retreat of the Cordilleran Ice Sheet over the Fraser Plateau (Figure Figure 4.1 Location map: a) study area; b) glacial geomorphology of the Big Creek site, showing study site. Key: location of section A-A' (see Figure 4.2) ; hachured lines - limit of late glacial spillways and glacial lakes; arrows - iceflow inferred from streamlined landf orms and striae; open circles - hummocky ground moraine; solid squares - dynamic glacigenic deformation; solid diamonds - passive glacigenic deformation (see also Appendix III). 4.2 THE STUDY AREA Two regional ice-flow patterns can be recognised over the Fraser Plateau (Figure 4.lb). West of Fraser River, ice from the Chilcotin and Camelsfoot ranges advanced north to northeast. East of Fraser River, ice from sources in the Cariboo Mountains (Tipper, 1971b) generally advanced West to southwest (Figure 4.lb). Elçewhere in the area, glaciofluvial outwash (unit 3a) was deposited ahead of advancing ice margins, then truncated, locally drumlinised and overlain by Fraser Glaciation till (unit 4a; Huntley and Broster, 1994). At the end of glaciation, stagnation of the Cordilleran Ice Sheet resulted in deposition of glaciofluvial sedirnents in contact with, and onlapping, debris-laden remnant ice masses and earlier glacial sediments (Tipper, 1971a; Heginbottom, 1972) .

4.2.1 Stratigraghy and origin of sediments Big Creek is an underfit stream occupying a major deglacial meltwater channel that drained to the Chilcotin Valley (Tipper, 1971a; Figure 4.lb). The spillway dissects a gently undulating, hmocky ground moraine and has exposed diamicton overlying upwards of 15 m of gravels and sands in valley walls [Figure 4.2; Appendix 1). This depositional sequence is further exposed in natural sections, road cuts and borrow pits throughout the area. A borrow pit on the north- facing slope of the Big Creek valley exposes deformed gravels and sands in a section approximately 10 m high and 40 m wide SSE NNW 10m

Sm

Om

- Fraser Till (IaQement and meltout til) 1; 1; A 1 Section w[ - ~waterinpstructures (dast-supporteddiamicton and gravels) ...... - Advance giaciolluvial sedirnents (gravels and sands)

SCHEMATIC PROFILE (A-A') OF BIG CREEK, - Slump Material SHOWlNG POSITION OF SECTION. SEE FIG. 1b FOR LOCATION OF PROFILE. O Pebble fabric site (with letter reference for Fig. 4,3) DIAGRAM NOT TO SCALE AFig' 4.4 Photo site (with figure reference)

Figure 4.2 Detail of Big Creek site section and schematic cross profile along A-A' (see Figure 4.lb). (Figures 4.lb; 4.2). The base of the sequence is not exposed in the borrow pit (Figure 4.2). Gravel beds comprise polymictic, well-rounded clasts ranging in size from 0.5 cm to 5 cm in diameter. Pebble fabrics, measured at 5 m and 0.5 m below the contact with the overlying diamicton unit, exhibit preferred northeast to southwest and east to West alignments, respectively (Figure 4.3a, b). Gravels are interbedded with two deformed, faintly laminated sand units. The lower sand unit attains a maximum thickness of 1 m; while in the upper unit, bedding varies in thickness between 10 and 50 cm. Both sand units display normal grading. This sediment assemblage is regionally widespread and is interpreted as an advance-phase glaciofluvial deposit (unit 3a; Huntley and Broster, 1994). The preferred clast orientation (Figure 4.3a) likely reflects a dominant northeastward palaeoflow direction. In the north-northwest part of the section, gravels and sands are disrupted by gravel- and clast-supported diamicton- filled wedges (Figures 4.2; 4.4). Wedge diamictons and gravels (which are lithologically similar to local gravels) contain rare, small (less than 10 cm long), vertically oriented irregular lenses of sand. Wedge deposits are near-vertical and strike preferentially northeast to southwest (Figure 4.3~).A vertical clast fabric overprints an earlier fabric reflecting inferred palaeoflow (Figure 4.3d). ~orphology,sedimentology and clast fabric of these structures are similar to dewatering structures (Lowe, 1975; Postma, 1983) . Dewaterlng Sltudures Clasl-suppocleddlamlclon and gravels

i \ DHH 197

Figure 4.3 Pebble fabric and structural data plotted as lower hemisphere projections: a) glaciofluvial gravels (2D pebble fabric); b) glaciofluvial gravels (2D pebble fabric); c) dewatering structures (plotted as great circles); d) clast- supported diamicton and gravels in dewatering structures (3D long-axes pebble fabrics); e,f) lodgement till (2D and 3D long- axes pebble fabric); g) meltout till (2D pebble fabric); h) deformation structures, including: fold axes (plotted as great circles), high angle thrusts (solid squares), normal faults (solid circles) and low angle thrusts (solid triangles). Thrusts and faults plotted as poles (see also Appendices II and III). Figure 4.4 Photograph of dewatering structures, with note book for scale (see Figure 4.2 for location). A 3 m thick diamicton unit unconformably overlies glaciofluvial sediments and wedge deposits (Figure 4.2). There is no evidence of subaerial weathering along the contact between diamicton and lower beds. This indicates little or no hiatus between erosion of underlying sediments and deposition of the diamicton. In the lower 1 m of this upper unit, minor warped sub-horizontal sand lenses are interbedded with diamicton. Above this, the diamicton is massive and matrix- supported. Clasts are polymictic, and rounded to sub-rounded with occasional faceted, striated surfaces. Pebble orientations, measured in the diamicton at 0.5 m above the basal contact, demonstrate a strong preferred aliment of prolate pebbles, with clast long-axes oriented predominantly south-southeast to north-northwest (Figure 4.3e). A three- dimensional plot of the clasts (Figure 4.3f) shows a strong unimodal concentration dipping toward the south-southeast. No preferred alignment is seen in the upper part of the diamicton, as demonstrated by the measurements taken 2 m above the basal contact (Figure 4.3g). The diamicton is interpreted as a lodgement till (cf. Dreimanis, 1976) in its lower part, grading upward into supraglacial meltout till (cf. ~reimanis,1988).

This deposit resembles ice-stagnation moraine with a hummocky surface expression. This unit is correlated with Late

Wisconsinan Fraser Glaciation till (unit 41, exposed elsewhere in the Taseko Lakes area (Huntley and Broster, 1993% 1994) . Glacial landforms in the vicinity of the study area indicate a northward ice-flow direction (Huntley and Broster,

1993a). The preferred alignment of clasts with strong south- southeast dip (Figure 4.3f) suggests lodgement deposition from northwest flowing ice (Dowdeswell and Sharpe, 1986). The minor deviation from regional ice-flow may reflect local topographie control on glacier flow direction. The absence of a preferred fabric in the upper till unit (Figure 4.3g) may be attributed to clast realignment during meltout.

4.2.2 Deformation structures

The majority of deformation structures are observed specifically within sand units. The areal extent of deformation cannot be accurately assessed because of poor exposure elsewhere along the Big Creek valley. Deformation structures recognised include: folding, high-angle reverse faults (thrusts), normal faults, low-angle reverse faults (thrusts) and warping of fault and fold planes (Figure 4. Sa,b) . Warping of the overlying till unit is also observed. The cross-cutting relationships of these structures (Figure 4.5) allow reconstruction of the original bedding geornetry and identification of a relative sequence of deformational events (Figure 4.6a to 4.6d) . The earliest deformation structures (Dl) are folds with northeast- to southwest-oriented axial planes, dipping to the southeast (Figures 4.3h: 4.5a). Bedding along fold limbs is boudined and the intensity of folding increases over the length of the section (Figure 4.2). Folding reaches a maximum amplitude of 2 m in the north-northwest where it is disrupted by dewatering structures (Figures 4.2; 4.4). Folds are disturbed by high-angle thrusts (late Dl) striking northeast to Figure 4.5 Photographs and simpified sketches of deformation structures: a) early Dl folding offset by late Dl thrusts, a D2 normal fault and D3 lm angle thrusts; b) late Dl high angle thrusts in a fold limb, offset by D2 and D3 structures. Arrows indicate sense of mot ion. 0' 0' a) LATE Dl: High angle thrusî piopagltlon b) 02: Normal frultlng and truncrtion in folded rands of 01 toucturir

3: Low angle thrusts and truncatio of 01-02 strucîuro8

Figure 4.6 Glaciopalinspastic reconstruction of Figure 4.5b. based on cross-cutting relationship of glacigenic deformation structures: a) Dl structures: folds and high angle thrusts; b) D2 structures: normal faults; c) D3 structures: low angle thrusts; D4 structures: warping. southwest, and dipping southeast (Figures 4.3h; 4.5a,b; 4.6a). Thrust décollements are observed only within the sand beds and displacements rarely exceed 1 m. Folds and thrusts are cross- cut by northeast- to southwest-striking normal faults (D2). dipping northwest (Figures 4.3h; 4.6b). Individual fault displacements do not exceed 2 m. Irregular lenses of sand occur along fault planes (Figure 4.2). Folds. high-angle thrusts and normal faults are further cross-cut in the south-southeast part of the section by low-angle thnists (D3), dipping southwest (Figures 4.3h; 4.5b; 4.6~).Thrusting is partly focused along northeast to southwest-striking décollements and reactivated high-angle thrust planes (Figure 4.6~).Rare grave1 clasts have been incorporated along thrust planes. Displacement of sand beds by low-angle thrusts rarely exceeds 10 cm. Al1 structures are truncated and overlain by lodgement and meltout till. A slight upward warping of fold axes, fault planes and the overlying till unit (Figure 4.2) indicates a final phase of deformation (D4; Figure 4.6d).

4.3 DISCUSSION Unconsolidated sediments and bedrock typically preserve deformation structures relating to the regional glacial history of an area (Brcster et al., 1979; Hicock and Dreimanis. 1985; Broster. 1991). In such areas. it is possible to distinguish between deformation produced by dynamic and passive application of glacigenic stresses (Croot, 1988; Aber et al., 1989). The former relates to deformation imparted by loading and overriding glacier ice, whereas the latter refers to deformation associated with the relaxation of stress during deglaciation (Broster and Clague, 1987; Broster and Burke, 1990). Deformation style is dependent upon the basal thermal regime and ice-flow characteristics of the overriding glacier, and rheology of the bed material (Boulton, 1981; Beget, 1986; Hicock et al,, 1989) . Current models of Late Wisconsinan ice sheets envisage a zone behind the glacier terminus where thin ice is below pressure-melting point (Le. cold-based) and the basal substrate is frozen (Moran et al., 1980; Tsui et al., 1988). In this zone, glacigenic deformation may be facilitated through differentials in viscosity between partly-frozen substrate and moving ice, and, or by slip along shear planes (Echelmeyer and Zhonxiang, 1987). Semi-ductile and brittle transpressional and transtensional structures (including folds, thrusts and normal faults) may be produced during advance of a frozen glacier margin over an area (Dreimanis, 1976). ~p-glacierof this zone, thicker, faster-flowing ice is generally at pressure-melting point (Le. warm-based; Tsui et al,, 1988). Under such conditions, glacier advance-phase and subglacial rnorainal deposits become erosionally streamlined in the direction of local ice-flow and subglacial meltwater-flow (Shaw, 1983; Shaw and Kvill, 1985). Subglacial deposits may be further rnodified and deformed during unloading associated with ice retreat, or by mass-movement and fluvial processes in proglacial settings. Here, a four part mode1 is proposed based on concepts discussed above, in addition to stratigraphic and cross-cutting relationships of sediments and deformation structures exposed at this site (Figure 4.7). Although the absolute timing of events is not known, the absence of later glacigenic sedirnents suggests that deposition and deformation at this site are related to the Late Wisconsinan Fraser Glaciation (cd. 29 to 10 Ka; Clague. 1981; Ryder et al., 1991). Limitations are placed upon the mode1 because thermal regimes and ice-flow dynamics of the Fraser Glaciation Cordilleran Ice Sheet are poorly understood.

4.3.1 Deformation mode1 Deformation may be attributed to permafrost activity in a proglacial environment. In the east-central Taseko Lakes area, relict polygonal and patterned ground above an elevation of 1980 rn indicates permafrost activity under glacier-free conditions above this elevation (Huntley and Broster, 1994). This observation attests to the localised presence of permafrost within the area during the Fraser Glaciation. However, the morphology of most structures resembles those produced in response to glacier overriding (Croot, 1988; Broster. 1991) rather than a permafrost origin (cf. Mathews and Mackay, 1960; Johnson, 1990). Deformed advance-phase glaciofluvial sediments are eroded and overlain by relatively undisturbed Fraser Glaciation till. Fabric and structural data further corroborate a glacigenic origin for deformation. Although drurnlins suggest that later ice-flow over the area was northward (Tipper, 1971a; Huntley and Broster, 1993a: Figure 4.2). the preferred fabric in lodgement till (Figure 4.3e, f) implies that glaciers initially advanced in a north-northwest NNW 0.lqrmaUon Phum 1 (01) 9 Coid-ômad gkdw mugin rdvrnms ovor uturrtrd g~rd~(~uvhisedirnents.

NNW ~~romutlonpnrm II (~2) 9 Ext.nrkn.l (lorr Md mas fldd tnnr(at.d lnto (i0t.n mat rdlmwib produces normal Iaultr.

NNW ûeformatlon Phue III (03) 1) Afriv8l of rrilrm-brud Ici Wind 1- 1- hmmugin. Compressional strrrr fldd rwrt8biish.d duo to dHfw«~Url fiow ratu of cold and wum-brd W. II) Cornprosalon produces low angle thnim. - Low angk thrusts Mi) Localkd groundwatof (Iow along thnist planas. d iconn. hr) Incroasd basil siidin rates livour Cold-ôasod ko rosion or gusionud? udimants and IoUg«Mnt ot UiI. --r Groundwater flow d NNW Oelomirtion Ph- IV (01) i) Wldwprord sUgnaUon of debris-lader Ico producos pr ut UH and hurnmocûy ground moraine. II) Lou 01 ovofburdrn pr~uro Wggem unloading wHhin s.dimenU. iii) Incision of sec!lon by Iito glacial uid pOSlgkd8l Big ctmk

f Uniording Not to scalo

Figure 4.7 Polyphase deformation mode1 for structures at Big Creek site. direction. Al1 fold axes are transverse to inferred local ice- flow, and poles to faults lie parallel to this direction (cf. Broster and Clague, 1987; Figure 4.3h). The earliest phase of deformation was transpressional and produced folds, dewatering structures and high-angle thrusts. The alignment of fold axes transverse to the inferred ice-flow direction is consistent with a "push-from-the-rear" mode1 (cf. Croot, 1987). To this end, this phase of deformation may have occurred in an ice-marginal setting. Boudined sand in fold limbs indicates that sediments were ductile and wet during initial transpression of the sediment pile. With ice advancing over saturated glaciofluvial sediments, increased porewater pressure in gravels, confined by less porous sands, would have facilitated initial Eolding and loading of the sediment pile (Figure 4.7a) . Transpression of saturated sediments may also have enhanced groundwater flow. In the north-northwest part of the section, folded sand units are disrupted by northeast- to southwest-oriented wedge-shaped dewatering structures. Within the wedges, remnants of bedding are seen as small irregular sand lenses. The preferred alignment and vertical pebble fabric preserved in these structures (Figure 4.3c,d) suggest localised groundwater escape occurred along zones of low hydrostatic pressure; for instance around glacier rnargins (cf. Broster, 1991). Elsewhere in the gravels, the shift in preferred alignment and increase in scatter of clast orientations (~igure 4.3b) may reflect volumetric adjustments within the dewatering sediment pile under a transpressive stress regime during ice advance. Folds are cross-cut by high-angle thrusts, suggesting a change from a ductile to brittle deformational regime during transpression. This change is consistent with sedirnent consolidation due to progressive dewatering of the sediment pile (Broster, 1991; Hicock and Dreimanis, 1992). Thrusting can also be explained if sediments were frozen, The simplest method to freeze then thrust sediments in a transpressive regime would be to assume that the advancing margin of the ice sheet in the Big Creek area was thin and cold-based (cf. Moran et al., 1980). In this way, transpressive stress imparted by ice-flow could be translated into compacting partly frozen basal sediments. Early transpressional structures are cross-cut by normal faults, indicating a change to deformation in a transtensional regime. Normal faulting may reflect brittle failure of the overridden, partly-frozen sediment pile during continued advance of a cold-based glacier margin over the site (cf. Moran et al., 1980; Broster and Clague, 1987). However, irreguiar sand lenses along normal fault planes suggest the presence of minor water-flow focused along developing faults at this time. Limited exposure prevents observations on how faulting was facilitated at depth. In other studies, the presence of a basal décollement may have Eacilitated faulting (Croot, 1988). Normal faults rnay have an unexposed listric component; this is not depicted in the mode1 (Figure 4.7b). Dl and D2 structures are offset by low-angle thrusts, indicating a change to a transpressional stress regime (Figure 4.7~).Transpression of sediments could have occurred if rapid flowing, wam-based ice was confined behind a slower moving margin, grounded over a partly-frozen bed (Figure 4.7~). Thrusts are partly propagated by décollement along reactivated high-angle thrust planes. Failure may have been facilitated by residual groundwater confined along fracture planes (cf. Broster. 1991). It is not known how thrusting was propagated at depth, although individual faults may have combined to form an up-glacier listric décollement. Glaciofluvial sediments and Dl to D3 structures are eroded and overlain by till. The absence of subaerial weathering along the erosional contact suggests truncation of the glaciofluvial unit and subsequent till deposition occurred subglacially. This sequence of events is consistent with the arriva1 of thicker, faster-flowing, wet-based ice behind a thin, cold-based glacier margin (cf. Moran et al., 1980). Elsewhere in the area, drumlinised terrain indicates that regionally, widespread wet- based depositional and erosional regimes were associated with Fraser Glaciation maximum-phase ice-flow. The final phase of deformation involved the upward-flexing of faults, folds and overlying till, and suggests vertical unloading of the sediment pile. Elsewhere, post-glacial deformation structures have been attributed to release of residual glacigenic stresses, for instance a loss of confining pressure upon glaciation (cf. Broster and Burke, 1990) . passive deformation was not associated with fluvial entrenchment of advance-phase glacial deposits at this site (cf. Broster and Clague, 1987) . Thus, the final phase deformation likely occurred in response to deglaciation (Figure 4.7d).

4.4 CONCLUSIONS Four phases of glacigenic deformation can be recognised and attributed to movement of grounded glacier ice over the West Fraser Plateau. A mode1 of polyphase glacigenic deformation is proposed, based on cross-cutting relationships in glacial sediments. The majority of deformation (Dl to D3; Figure 4.7) appears to have occurred during the advance-phase of the Late Wisconsinan Fraser Glaciation. The pattern of deformation suggests that wet sediments were dewatered, consolidated and then partly frozen during ice advance. As a consequence, sediments record a history of ductile, brittle and semi-brittle failure resulting from: (i) horizontal transpression and vertical loading during initial glacier advance into the area: (ii) translation of lateral transtensional, then transpressional stress into partly frozen basal sediments; (iiil subsequent erosion of underlying units and till deposition under wet-based glacial conditions. This sequence is consistent with ice-marginal and subglacial deformation by a thin, cold-based margin of the Cordilleran Ice Sheet, and subsequent erosion by thicker, wet-based ice (cf. Moran et al., 1980) . These events were followed by (iv) vertical relaxation in response to deglacial unloading, during ice-stagnation at the end of the Fraser Glaciation. CHAPTER 5

LATE WISCONS INAN GLACIER RETREAT IN

MONTANE, PLATEAU AND VALLEY SETTINGS

5.1 INTRODUCTION Tbroughout southern British Columbia (Figure 5.la), Late Wisconsinan Fraser Glaciation deposits attest to widespread stagnation of the Cordilleran Ice Sheet and formation of proglacial lakes along major valleys during deglaciation

(Mathews, 1944; Fulton, 1967; Tipper, 1971b; Clague, 1988; Eyles and Clague, 1991). Lake-surface and drainage outlet elevations were controlled by retreating ice margins, glacial outwash and Iandslide debris (Fulton, 1969; Broster and

Clague, 1987; Sawicki and Smith, 1991). As a result, many deglacial lakes were short-lived and drained catastrophically (Booth, i986; Atwater, 1987). Glacial valley fills were subsequently degraded as the fluvial landscape became established in the early Holocene (Ryder et al., 1991). Figure 5.lb shows the potential extent of a large proglacial lake in central British Columbia formed at the end of the Fraser Glaciation (Clague, 1988) , informally referred to as "glacial lake Fraser". Recent studies indicate that glacial lake Fraser was dammed by ice in the Fraser Valley south of Quesnel, and inundated the Fraser drainage basin to a maximum elevation of about 800 m elevation (Clague, 1987; 1988; Plouffe; 1991; Huntley et al. 1996). Deposition of a thin sequence of rhythmically-bedded clay and silt couplets indicate the lake system was sediment-starved in comparison with older glacial lakes in the region (Eyles and Clague, 1991), and was possibly extant for 100 years or less, draining prior to 10 ka (Berger et al., 1987; Clague, 1988).

The palaeohydrology of the glacial lake Fraser system remains poorly understood. in this chapter, Late Wisconsinan, Fraser Glaciation morainal and retreat-phase sediments and landforms in the east-central Taseko Lakes area are used to interpret the history of ice retreat and meltwater drainage at the southern limit of glacial lake Fraser (Figure 5.lb). This history attempts to define (i)glacial limits of the Cordilleran Ice Sheet during deglaciation; (ii) patterns of ice dispersal and glacier retreat; and (iii) areal extent, elevational range and drainage patterns and history of meltwater spillways and deglacial lakes in the area.

5.2 THE STUDY ARBA 5.2.1 Phyaiography The east-central Taseko Lakes area lies along the southwestern boundary of the Interior Plateau (Holland, 1980), and includes the southern and western limits of the Fraser and Green Timber plateaux, respectively (Figure 5.2). Plateaux are undulating to rolling plains, flanked along their southern margins by uplands and montane foothills of the eastern Chilcotin, Camelsfoot and Marble ranges (Figure 5.2). Figure 5.2 Principal physiographic elements and nomenclature of the east-central Taseko Lakes area. A - Camelsfoot Range; B - Marble Range; C - Chilcotin Range; D - West Fraser Plateau: E - East Fraser and Green Timber plateaux; F - major river valleys . The area is drained by tributaries to the south-flowing Fraser River. In the area, Fraser River occupies a 500 to 1000 m deep valley following the strike of the Fraser fault system (Mathews and Rouse, 1984); related splay faults partly control tributary valley trends. The east Fraser and Green Timber plateaux are drained by Alkali, Dog and Canoe creeks.

These underfit streams occupy valleys that hang up to 350 rn above the present Fraser Valley floor. West of the Fraser Valley, Churn, Gaspard and Lone Cabin creeks drain parts of the West Fraser Plateau, northern Camelsfoot and eastern Chilcotin ranges. These creeks occupy valleys that are incised to depths conformable with the present Fraser Valley floor. The West and northwest Fraser Plateau is drained by Big Creek which bas formed a northeast-trending valley confluent with Chilcotin River.

5.2.2 Late Wiaconainan glacial geology Late Wisconsinan, Fraser Glaciation sediments and landforms occur in al1 physiographic settings. Cirques, arêtes and U-shaped valley troughs are found in the Camelsfoot and eastern Chilcotin ranges, indicating these areas were local centres for ice accumulation during the Fraser Glaciation (Huntley and Broster, 1993a; 1994). Glaciers from these sources flowed northeast to east, coalescing along the Fraser Valley between 51'35'N and 52'N with west-flowing ice from the Marble Range and Cariboo Mountains (Huntley and Broster, 1994) . During glacier advance, montane valleys and adjacent parts of the West Fraser Plateau were aggraded with . glaciofluvial outwash, eroded, then overlain by Fraser Glaciation till (Huntley and Broster, 1994). North of the Camelsfoot Range, glaciofluvial and glaciolacustrine sediments were deposited along the Fraser and tributary valleys (Eyles and Clague, 1991; Huntley and Broster, 1994). Dropstones in upper beds of lake sediments suggest ice advance into the proglacial lake occurred along floated margins. However, glacigenic deformation of upper beds indicates that grounded ice eventually overrode the lake and infilled the Fraser Valley (Huntley and Broster, 1994). Equivalent sequences in the Chilcotin and Taseko river valleys (Sles and Clague, 1991; Plouffe et al., 1996), along Fraser River between Williams Lake and Prince George (Clague, 1987; 1988) and in the Nechako River valley (Levson et al.,

1996), indicate that prior t3 the Fraser Glaciation maximum, an advance-phase glacial lake system (glacial lake Camelsfoot; Huntley and Broster, 1994) possibly occupied an area equivalent to that of glacial lake Fraser (Figure 5.2). Outlets for this proglacial lake have yet to be identified. Deformation structures and erosional glacial landfonns provide some insight into the basal thermal regime of that part of the Cordilleran Ice Sheet occupying the area. In upland valleys, till injection wedges in glacially-fractured bedrock suggest advancing glaciers were alternately cold- and wet-based (Huntley and Broster, 1993~).Over the plateau, transtensional and transpressional structures in advance- phase outwash are compatible with overriding by a thin cold- based grounded ice margin, followed by erosion and lodgement of till by thicker, wet-based ice (Huntley and Broster, 1993b). However, it is uncertain if wet-based conditions resulted in enhanced ice-flow rates during the glacial maximum-phase . Uplands above 1980 rn elevation, including Blackdome, Hungry Mountain, Piltz Peak and Mount Wales are locally mantled by felsenmeer, polygonal patterned ground and solifluxion terraces, and tors occur along some ridge crests (Figure 5.3). The elevation range between relict periglacial features and glaciated terrain is interpreted to represent a stable minimum upper altitude (trimline) of the Cordilleran

Ice Sheet (cf. Garnes and Bergersen, 1980; Ballantyne and McCarroll, 1995) at the close of the Fraser Glaciation maximum (ca. 14 to 13 ka; Huntley and Broster, 1994). Upper limits to the ice sheet in the area, determined from trimline elevations, ranged from 2600 m in the Camelsfoot Range to 1980 m in the Piltz Peak area. Ice sheet thicknesses around this time probably varied from about 2000 m in montane axeas to 1000 m along major valleys and 600 m over the plateaux (Huntley and Broster, 1994).

5.3 APPROACH TO STüDY Six lithostratigraphic units have been identified in the area (Table 5.1) and are partly described elsewhere (Broster Figure 5.3 Periglacial terrain above 2250 m elevation on Hungry Mountain (location at 51017'N; 122O54'W). Note tors at spur crests. Thick felsenmeer deposits occur at the summit of Piltz Peak and Mount Wales in centre horizon. Relict periglacial terrain is also exposed on the summit of Blackdome and in the eastern Chilcotin and Camelsfoot ranges. and Huntley, 1992; Huntley and Broster, 1993a, 19935, 1994). This chapter focuses on the lithostratigraphy and geomorphology of glacial maximum and retreat-phase sequences related to the Late Wisconsinan Fraser Glaciation (units 4 and 51, and early post-glacial sediments (unit 6). Composite and reference sections were logged throughout the area, and measured with a 30 rn tape or by altimeter to an accuracy of f 5 m (Figure 5.4; Appendix 1). In addition, the terrace and fan surface elevations were also measured at several locations (Figure 5.4). Elevations were determined from spot heights and contours on 1:SO 000-scale National Topographic System maps. Units were distinguished by field observation of sedimentological characteristics including: erosion surfaces, fissility, jointing, deformation structures and field estimates of degree of consolidation, percentage matrix content and texture. Palaeoice-flow directions were inferred by measuring long-ais orientations of samples of 50 prolate pebble- to cobble-size clasts in basal till exposures, and £rom orientations of drumlins, flutes, roches moutonées and striae (Tipper, 1971a; Heginbottom, 1972;

Huntley and Broster , 1994 ; Appendix II ) . Palaeocurrent directions were inferred from charme1 and esker trends, orientations of climbing ripple sequences, trough-cross, and foreset beds, in addition to pebble orientation data (Appendix II). Correlation of units was determined by comparing relative stratigraphie position, sedirnentology, facies associations and surface expression of deposits. i L ? m m -:%a .- E ers 5 I :SE I E=1 gggg%=- & 2 5 .; c "0~05?2L z g

3 irj au Ur: ' -4-03 Ç4XdX

Morainal provenances and limits to the ice sheet were partly inferred from clast dispersa1 patterns (Figure 5.6; Appendix IV). Marble Canysn Formation limestones, Jackass Mountain Group and Silverquick Formation conglomerates, and Piltz Peak and Mount Wales plutonic complex rocks were selected as indicator lithologies since they had restricted exposure with respect to previously identified ice source areas (cf. Broster and Huntley, 1995; Appendix IV). Percentage distributions were detemined by field estimates or counts of 50 to 100 pebble to cobble-size clasts (Figure 5.6; ~ppendixIV). Recessional palaeoice surfaces, meltwater drainage patterns and deglacial lake limits were defined by mapping the distribution and elevation range of morainal deposits, meltwater spillways, kames, fans and kame-deltas (Figures 5.7; 5.8). Potential ice thicknesses were derived from the differences in elevation between inferred upper ice surfaces and minimum altitude of glacial erosion surfaces and basal till deposits. Surficial geology and geomorphology were mapped from 1:50 000 and 1:10 000-scale airphotos (BC5242; BC87072. respectively) and ground checked during fieldwork. Mapping followed a convention similar to British Columbia mapping standards (Howes and Kenk, 1988 ; Huntley , 199 5 ; Appendix V) .

1 22fJ30~ 122 O km l0

Kama turre end L,, Esker (tlow known. kame-delta % flow uncertrin~ Meltweter channei ce-stagnation moraine ~asrmovrmrnt \ ~owknown. (IOW uncenain)

Figure 5.7 Deglacial geomorphology of the study area (after Huntley, 1995; see also Appendix V). Figure 5.8 Simplified elevational distribution and drainage directions of meltwater features in the study area. Figure also shows approximate stable ice surfaces inferred frorn geomorphic data (see Figure 5.7). 5.4 DESCRIPTION OF LITHOSTRATIGRAPHY AND GEOMORPHOLOGY 5.4.1 Unit 4a In al1 physiographic settings below inferred glacial limits, bedrock and advance-phase sequences (unit 3) are eroded and overlain by massive, matrix-supported diamicton deposits (Figure 5.91, containing subordinate interbeds of stratified diamicton, grave1 and sand. Deposits have undulating to rolling upper surfaces reflecting underlying topography, or are streamlined in the direction of local palaeoice-flow (Huntley and Broster, 1994; Huntley, 1995). Diamicton matrices comprise consolidated silt and clay, with sculpted intradeposit bedding surfaces, moderate to strong, bedding-parallel fissility and jointing. Oxidation, if present, is predominantly confined along joints or fissility planes and discolours matrices orange to red-brown. East of Fraser River, diamicton matrices are locally weakly reactive with dilute hydrochloric acid, indicating a detectable carbonate content. Pebbles and cobbles range from subrounded to siibangular. Prolate clasts usually have striated, faceted surfaces and show a preferred alignrnent subparallel to palaeoice-f low (Figure 5.5a, b,cl d). For the three indicator lithologies, maximum float concentrates in, or close to, bedrock source areas. Concentrations decrease to minimum values in areas distal to source and toward the

Fraser Valley (Figure 5.6) . Figure 5.9 Massive, matrix-supported diamicton basal (lodgement) till (unit 4a) (location at 5 lSSoS7 'W). 5.4.2 Unit 4b Over plateaux, unit 4a is, locally, conformably overlain by undulating or hummocky deposits of massive and stratified matrix-supported diamicton (Figures 5.7; 5.10). The latter is distinguished from underlying units by often pervasively oxidised, poorly-consolidated, silt- and clay-deficient matrices. Clasts are predominantly pebbles and cobbles with weak fabric development (Figure 5.5h). Erratic boulders, up to 2 m in diameter, are exposed at the surface, forming linear trains along some valley sides (Figures 5.7; 5.11). Chilcotin Group basalt boulders are the dominant erratic lithology. In valleys, unit 4b is also locally confined to en echelon linear ridges mostly lying perpendicular to drumlin and striae trends (Figure 5.7). Ridge dimensions range from 1 to 5 m in height and 200 to 300 m in length. At the outlets of the Koster, Churn, Gaspard and Alkali creek valleys

(Figure 5.12), ridges have a distinctive stratigraphy consisting of a core of massive diamicton, gradationally overlain by coarse-grained, stratified diamictons with gravel interbeds. Although ridge cores are sedimentologically similar to underlying unit 4a diamicton, prolate clasts show some degree of re-alignment subparallel to ridge trends (Figure S.Se,f,g). Other ridges are cornposed primarily of coarse, poorly-sorted gravel and coarse-grained sand. Grave1 clascs are generally aligned subparallel to ridge orientations. Below 610 m elevation in the Gang Ranch and Figure 5.10 Stratified, clast-supported diamicton interpreted as ice-contact colluvium (unit 4b) in eskew complex 10 km southeast of Mount Alex (location at 51°35'~; 122033'~). Figure 5.11 Ice-stagnation moraine (unit 4b) north of Dog Creek Dome (location at 51038'N; 122°08'W). Note linear train of limestone erratics (arrowed). Figure 5.12 Crevasse fil1 deposits (unit 4b) at outlet of Gaspard Creek, 3 km north of Gang Ranch (location at 51034'~; 122024'W). Note basalt erratics along ridge crests. Gaspard Creek areas, depressions between ridges are draped by thin-bedded silt and erratic boulders (Figure 5.12).

5.4.3 Unit Sa (coarse-grained member) In upland areas and over the plateaux, between 1970 and 1550 m elevation, unit Sa is preserved in relict bedrock- walled channels incised across cols and spur crests, hanging in anomalous settings on valley walls (Figure 5.15). From 1525 to 1160 ml unit 5a is confined to sinuous, single or anastomosing ridges with symmetrical cross-profiles, ranging in height from 2 to 5 m. and rarely exceed 1 km in length (Figure 5.13); and sediment-walled channels, oriented perpendicular, or at an angle, to hill-slopes and valley walls (Figures 5.7; 5.16). Beiow this elevation range, unit 5a forrns val*-marginal terraces, fans and conical mounds, graded to a minimum altitude of approximately 850 m. Most ridges are constructed of normal-graded sand, grave1 (Figure 5.14) and interbeds of stratified diamicton resembling unit 4a. Basal exposures of channel, terrace, fan and mound-fills are dominantly composed of planar-bedded or massive, clast-supported gravels, and unit 4b diamictons. Grave1 beds are predorninantly composed of rounded, polymictic pebbles and cobbles. Coarse beds typically grade into massive, planar, trough-cross-stratified and ripple-bedded sands. Contacts are gradational, with sand infilling the open framework of upper parts of underlying beds. Sequences generally fine upward, and diamicton beds become rare toward Figure 5.13 East-trending esker, 5 km northeast of Blackdome (location at 51°26 IN; lZ023 'W). Figure 5.14 Detail interbedded grave1 and sand (unit sa) exposed in an esker deposit in the vicinity of China Gulch (location at 51021'~;122003IW). Note that the sequence is partly deformed by a normal fault. Fault plane oriented para1 le1 I ridge f lank. Figure 5.15 Northeast-trending bedrock-walled meltwater chamel 2 h east of Mount Waies at an elevation of 1680 m (location at 51024'~;122050'W). Figure 5.16 Ice-contact and proximal glaciofluvial sediment (unit 5a) exposed in a valley-marginal kame terrace, northern Green Timber Plateau (approximate location at 51°26 'N; 1220W) . Note passive deformation structures attributed to melting of stagnant ice mass confined in valley. the top of unit Sa. A maximum thickness of 30 m for unit 5a is attained in the middle reach of Churn Creek (Figure 5.4; log E). Palaeoflow indicators in sand beds of ridge deposits are occasionally oriented counter to present drainage directions. Gravel clasts are predominantly aligned subparallel to ridge and channel trends (Figure 5.5i.j,k,l). Bedding is locally deformed by post-depositional transtensional faults, with failure planes lying subparallel to ridge flanks and valley walls (Figure 5.14; Figure 5.16; Appendix III).

5.4.4 Unit Sb (fine-grained member) Unit Sa is correlative with sequences dominated by finer-grained sediments (unit 5b) at tributary outlets and along the Fraser Valley. Distinctive sediment and landform assemblages occur east and west of the Fraser Valley. East of Fraser River, in the lower Canoe Creek, valley-marginal fans are composed of planar- and foreset-bedded sand and gravel, dipping dom-valley between 10° and 20° (Figure 5.4: log GL Locally, these beds are truncated. and overlain by 1 to 3 rn thick sequences of massive cobbles and boulders. Fans are graded to a maximum elevation of 850 m. Raised embankments at the tributary outlet (Figure 5.17. A) consist of about 60 m of massive, planar and trough-cross-bedded sand, with subordinate interbeds of gravel and diarnicton deposited upon glacially-scoured bedrock (Figure 5.4; log HL These landforms have constructional planar surfaces facing up- Figure 5.17 a) Kame deltas at outlet of Canoe Creek (location at 51°27'N; 1220101W).Upper terraced platform graded to 760 m elevation, lower platform is graded to 610 rn elevation; b) kame delta at outlet of Alkali Creek, graded to 610 m elevation (location at 51°44'~; 122°20'W). 100 valley (Figure 5.l7a) . In contrast , embankment surfaces facing the Fraser Valley are undulating and mantled by-unit 4 diamictons. Embankments are graded to a maximum elevation of

760 m. Finer sediments, graded to an elevation of 610 m, blanket the valley floor at the tributary outlet (Figure

5.17, A). The northernmost embankment and valley fil1 is

entrenched by Canoe Creek. Between Canoe and Alkali creeks, unit 5b is confined to terraces along the east wall of the Fraser Valley (Figure

5.7 ) . Terraces are formed where meltwater spillways from adjacent parts of the east Fraser Plateau join the valley (Figure 5.4; log B). In the lower Dog Creek valley, stratified diamictons, gravels and subordinate sands are deposited as fans. In these areas, finer-grained sedirnents are partly overlain by massively-bedded gravels up to 2 m thick. Terraces and fans are graded to an elevation of 760 m (Figure 5.4; reference elevation B).

At Alkali Creek (Figure 5.4; log A), unit 5b forms an embankment at the tributary outlet that is rnorphologically similar to those at Canoe Creek (Figure 5.17b). Unit 5b overlies advance-phase sediments (unit 3b) and unit 4a above 545 m elevation. The basal 25 rn consists of cyclically- interbedded planar and massive sands, silts and subordinate stratified diamictons. Pebble- to cobble-size dropstones and diamicton lenses are randomly dispersed in the bottom 15 m. Above approximately 570 m elevation, this sequence is overlain by 25 rn of massive-bedded sands. These beds are scoured by numerous shaliow channels (Figure 5.18) and infilled with silt-rich stratified diamicton, massive gravel, trough-cross- and ripple-bedded sands. Channel trends, trough-cross and ripple-drift orientations indicate a dominant palaeoflow toward the Fraser Valley. Between about 580 and 595 m elevation, bedding is deformed by syndepositional folds, normal faults and small-scale loading features. Although fold axial planes show no consistent trend, fault planes generally lie parallel to valley axes (Appendix III). Deformed sediments are partly overlain by foreset-bedded sand and gravel. Foresets dip toward the

Fraser Valley. Above 595 m, finer-grained sediments, including planar-bedded silt, sand and stratified diamicton dominate (Figure 5.19). These beds dip gently toward the

tributary valley axis and Fraser Valley (Figure 5.5~).The upper surface of the embankment is graded to 610 m elevation. The embankment is incised by Fraser River and a west- trending, meandering palaeovalley, presently drained by the underfit Alkali Creek. West of Fraser River, in the Gang Ranch area, southeast- trending palaeochannels are incised to a depth of about 570 m elevation into advance-phase lake deposits (unit 3b) and unit

4a diamicton. The basal 25 m of channel infill comprises interbedded stratified diarnictons, massive sands and silts (Figure 5.4; logs C and F). ~nderlyingbeds are occasionally disrupted by ball-and-pillow and flame structures. Rare dropstones are also randomly dispersed throughout lower beds Figure 5.18 Southeast-trending channel fills, comprising massive-bedded grave1 and trough and ripple-bedded sand at about 570 m elevation (unit 5b), at outlet of Alkali Creek (see Figure 5.4, log A for location) . Figure 5.19 Fine-grained matrix-supported diamicton interpreted as a debris flow at about 600 m elevation (unit Sb), at outlet of Alkali Creek (see Figure 5.4, log A for location) . (Figure 5.4; log F). This sequence is truncated at about 595 m elevation, then overlain by 10 to 15 m of trough- and foreset-bedded gravel, ripple and planar-bedded sand, and subordinate massive silt. Palaeocurrent indicators are consistent with flow toward, and southward along, the Fraser Valley (Figure 5.5m,n). In exposures along the lower Churn Creek valley, unit 5b is deformed by abundant syndepositional folds and faults at about 580 m elevation (Figure 5.4; log F). Fault planes and fold plunge orientations are consistent with extension into valleys (Appendix III). Sirnilar successions are exposed in fans formed at the outlet of Empire Valley and along the lower reach of Lone

Cabin Creek (Figure 5.4; log L). In the Lone Cabin Creek valley, unit 5b overlies retreat-phase glaciofluvial sediments (unit Sa) above 570 m elevation. The lower 25 m consist of multiple thin silt and sand interbeds with dropstones. Above 595 m elevation, the sequence comprises about 15 m of planar-bedded sand and gravel. These successions are also graded to about 610 rn elevation.

5.4.5 Unit 6 In montane and upland areas, unit 6 forms fan and blanket deposits of massive and stratified clast-supported diamicton locally overlying beàrock and unit 4. Basal contacts are commonly erosional and may contain rip-up clasts of underlying beds. On steeper dopes, deposits are generally coarser-grained, matrix-deficient and include subangular, cobble- to boulder-size clasts of local provenance. Along valleys, unit 5 and older deposits are incised and terraced by post-glacial streams. Gently sloping fans, composed of 1 to 5 rn of interbedded silt, sand, grave1 and diamicton, are formed on many terraces. Massive deposits of silt and fine-grained sand form cliffhead dunes along many terrace edges. River terraces are graded to successively lower elevations, and banked against bedrock, older valley- fil1 or landslide debris. Mass movements range in areal extent from under 500 rn2 in tributary valleys to greater than 5 km2 along the Fraser Valley (Figure 5.7).

5.5 INTERPRETATION OF LITHOSTRATIGRAPHY AND GEOMORPHOLOGY

5.5.1 Unit 4a Massive diamictons are interpreted as tills deposited by lodgernent at the base of sliding wet-based ice (cf. Muller, 1983: Dreimanis, 1988). Advance-phase deposits (unit 3; Huntley and Broster, 1994) and tills are locally drumlinised, which indicates that wet-based conditions persisted after deposition. Subordinate interbeds may represent basal meltout till and subglacial fluvial deposits (cf. Shaw, 1985). Stratigraphie position suggests unit 4a was deposited prior to the retreat-phase of Late Wisconsinan Fraser Glaciation, and is probably correlative with other Fraser Glaciation tills in southern British Columbia (Fulton and Smith, 1978; Clague, 1987; Eyles and Clague, 1991; Huntley and Broster, 1994). Two moraine provenances are identified frorn clast dispersal patterns (Figure 5.6). Marble Canyon Formation limestone was dispersed and deposited by ice flowing West over the eastern Fraser and Green Timber plateaux. Secondly, Jackass Mountain Group and Silverquick Formation float, £rom the eastern Chilcotin and Camelsfoot ranges, and granitic float, frorn Piitz Peak, Mount Wales and Mount Alex, was transported northwest over the western Fraser Plateau. Decreasing concentration of indicator lithologies probably reflects dilution attributed to an increase in basal debris concentration toward glacier margins and comminution in direction of transport. Minimum concentrations represent a first order approximation of clast dispersal limits for indicator lithologies over the Fraser and Green Timber plateaux, and along the Fraser Valley (Figure 5.6).

5.5.2 Unit 4b Stratigraphie position suggests that unit 4b was deposited after the Fraser Glaciation maximum. Undulating and hummocky diamicton deposits are interpreted as ice-stagnation moraines, comprising supraglacial meltout tills and ice- proximal colluvium (cf. Dreimanis, 1988; Lawson, 1988). As such, moraines were formed by rneltout of englacially- and supraglacially-transported debris and mass rnovement around retreating glacier rnargins. Diarnicton-filled ridges resemble crevasse fills (Tipper, 1971a; Heginbottom, 1972), found at debris-covered glacier margins (cf. Sharp, 1985). Crevasse fills in the Fraser Valley appear to be polyphase features, and are morphologically similar to grounding-line features found in ice-damrned lake basins (Booth. 1986) . Crevasse fil1 orientations are consistent with an extending ice-flow pattern £rom west-draining tributary valleys. This inferred ice-flow pattern is supported by other indicator data (Huntley and Broster, 1994). The absence of discrete erosion surfaces and similarity in sedimentology and clast orientation data (Figure 5.se, f,g) suggest that diamicton cores are partly derived from squeezing of basal till (unit 4a) into crevasses opened at the bottom of an occasionally floated ice margin (cf. Beudry and Prichonnet. 1995). Ridges, with wimowed upper sections, or with significant clastic components display structures indicative of glaciofluvial reworking, although palaeoflow direction is uncertain. Drapes of silt and erratics suggest periods of deposition in quiescent water and undemelt from debris-laden ice.

5.5.3 Unit 5a (coarse-grained member) Unit 5a sediments are interpreted to represent subglacial esker fills or proglacial outwash deposited in meltwater spillways, kame terraces and conical kames. Stratigraphie position implies that unit 5a was deposited during deglaciation, and contemporaneously with unit 4b. Meltwater features are graded to successively lower base-levels that are conformable with estimated elevations of recessional ice surfaces inferred from the distribution of ice-stagnation moraines (Figure 5.8). Palaeoflow indicators in eskers suggest many functioned alternately under vadose and phreatic conditions. Seasonal fluctuation in both subglacial and proglacial meltwater discharge and sediment availability is suggested by cyclical interbedding of gravels, sands and diamictons. Gradational contacts, normal grading and interstitial sand in grave1 beds are indicative of deposition from suspension under waning-flow conditions (Smith and Ashley, 1985). An ice-contact or ice-proximal depositional setting for basal parts of subaerial sequences is implied by the presence of ice-contact colluvium interbeds (unit 4b). Upward-thiming and -fining of sequences indicate that sedirnent transport and depositional rates decreased over tirne, and likely represents change to ice-distal conditions. ~eformationis attributed to loss of confinement during collapse of ice-walled charnels and fluvial incision of valley fil1 embankments (cf. Broster and Clague, 1987).

5. S. 4 Unit Sb (fine-grained member) Unit Sb is correlative with retreat-phase glaciofluvial sediments (unit Sa). These sediments are interpreted to represent retreat-phase fan-deltaic, kame-deltaic, lacustrine, fluvioglacial and alluvial fan sequences deposited in ice- and sediment-dammed basins at tributary outlets, and spillway channels and karne terraces along the Fraser Valley. Upper surfaces of deposits are conformable with stable rennant ice surfaces in the Fraser Valley at 850 760 and 610 m elevation, inferred from elevational range of ice stagnation moraines and crevasse fills (Figure 5.7). Basal sequences, dominated by diamicton and planar- bedded sands with dropstones, resemble waterlain debris-flows and related traction deposits deposited in subglacial or ice- contact settings (Shaw, 1985). Sediment was most likely derived from undermelting of periodically floated debris- laden basal ice, unstable kame-delta foreslopes, and saturated debris remobilised around pond margins (cf. Evenson et al., 1977; Broster and Hicock, 1985). Silts indicate the periodic dominance of deposition from suspension during periods of low meltwater and sediment influx (Donnely and Harris, 1989) . Overlying sands and gravels are similar to ice-proximal sequences deposited £rom sediment-laden meltwater entering ice-ponded basins (cf. Shaw and Archer, 1978; Seigenthaller et al., 1984). Channelisation of beds between elevations of 570 and 600 rn is interpreted to represent episodic drainage events and basin infilling during ice retreat (cf. Booth, 1986) . An increase in aggradation rates is indicated by an abrupt upward-coarsening in fan- deltaic sequences in eastern tributaries and progradation of delta foresets £rom tributaries toward the Fraser Valley. Palaeocurrent indicators suggest meltwater drainage and sediment transport was directed locally toward basin centres formed at tributary outlets, and periodically southward along the Fraser Valley (Figure S. Sm,n, o,p) . Deformation of sediments at about 580 m elevation likely records loss of support during disintegration of remnant ice in the Fraser Valley and fluvial incision (cf. Broster and Clague, 1987). The vertical and lateral gradation from chamel-fil1 and deltaic sequences to finer, planar-bedded fluvioglacial and alluvial fan sediments implies decreasing sediment delivery, water depth and stream cornpetence, most likely in an ice-distal depositional setting.

5.5.5 Unit 6 Stratigraphie position suggests sediments comprising unit 6 were deposited during and immediately following deglaciation. Deposits in upland areas are interpreted to have been deposited by subaerial frost-shatter and gravity- induced mass movement in proglacial and non-glacial environments, and include hillslope colluvium and talus. In valley settings, fine-grained sediments are interpreted as post-glacial alluvial, fluvial and aeolian deposits. Terraces are interpreted to reflect periods of fluvial aggradation in response to temporary blockage of southerly drainage by landslides, followed by stream incision. As such, they reflect the adjustment of the post- glacial landscape to progressively lower regional base- levels . 5.6 DISCUSSION This section describes the deglacial landscape history of east-central Taseko Lakes in four phases (cf. Fulton,

1967 ; Garnes and Bergersen, 1980). Late Wisconsinan Fraser glacial maximum and retreat-phase sediments and landforms are used to mode1 the limits of the Cordilleran Ice Sheet, patterns of ice-retreat and meltwater drainage during deglaciat ion. Timing of deglacial phases is explicitly relative since datable material was not found in the area. The chronology of events presented is based on correlation between similar units at equivalent stratigraphie levels, cross-cutting relat ionships in sediinentary and def ormation structures, and identification of landform sequences. Correlative sediments in the Fraser Valley north of the study area (Berger et al., 1987; Clague, 1988) and in south-central British Columbia (Fulton and Smith, 1978) suggest deglaciation of the study area began after the Cordilleran Ice Sheet reached its maximum extent (ca. 14 and 13 ka), and was complete by ca. 10 ka.

5,6.1 Deglacial landscape mode1 ~t the onset of deglaciation in the area, the regional equilibrium line altitude was probably still lower than the elevation of cirques in montane source areas (cf. Fulton, 1991), so that the Cordilleran Ice Sheet remained active (Figure 5.20). As a result, basal tills and related subglacial sediments (unit 4a) were deposited over an erosion surface ranging in altitude between about 1000 m over the plateaux to 540 m in the Fraser Valley. This phase is analogous to the active-ice phase of Fulton (19671, where regional ice-flow was maintained but diminished as ice thinned . Two moraine provenances are identified from clast dispersal patterns in basal till (Figure 5.6). West of the Fraser Valley, drumlin orientations, striae trends and clast dispersal patterns indicate conglomerate and granitic clasts were dispersed by piedmont and valley glaciers flowing north to northwest over the Fraser Plateau. East of the Fraser Valley, morainal deposits are distinguished by the presence of limestone clasts and carbonate-rich matrices, deposited by ice flowing westward over the eastern Fraser and Green Timber plateaux (Broster and Huntley, 1995) . Moraine limits lie along the Fraser Valley between 51'35'N and 52'N (Figures 5-6;5.20). During this phase, a stable upper glacial limit, or trimline (cf. Garnes and Bergersen, 1980; Ballantyne and McCarroll, 1995) appears to bave ranged in elevation £rom 2600 m in the Camelsfoot Range to 1980 rn in the Piltz Peak area. Potential ice thicknesses in the study area ranged from approxlmately 600 m over the west Fraser Plateau to greater than 1000 m along the Fraser Valley (Huntley and Broster, 1994a). The range of trirnline elevations indicates the ice sheet sloped northward at between 0.6O to 0.80. Surface Figure 5.20 Conceptual mode1 depicting the deglacial paiaeohydrogeography of east -central Taseko Lakes . POS t -glacial maximum phase (a). Figure incorporates ice-flow and geomorphic data discussed in text and earlier figures. 114 gradients of this magnitude imply that the terminal margin of the ice sheet was situated approximately 150 km north of the study area, between Quesnel and Williams Lake (Figure 5.lb). At this time, remnant ice in the Fraser Valley dammed southerly drainage, impounding the glacial lake Fraser system to elevations between 800 and 700 m (Clague, 1988, Plouffe, 1991: Huntley et al., 1996; Figure S.lb). Drainage outlets during this phase have yet to be identified. As montane and upland areas became deglaciated, the ice sheet split into different branches confined to valleys and plateaux (Figure 5.21). This phase is equivalent to the transitional upland phase of deglaciation (Fulton, 1967). Supraglacial till, crevasse fills, erratics and proglacial

colluvium (unit 4b) fom distinctive undulating and hmocky moraines deposited from debris-covered stagnant ice during this phase (Figure 5.7). These deposits were readily remobilised by meltwater focused in ice-marginal spillways and along kame terraces (Figures 5.16; 5.21). Hydrologic

continuity between ice-free areas was likely maintained by eskers and supraglacial channels. Inferred palaeocurrent

directions in unit Sa (Figure 5.5i. j, k,1) and charnel orientations (Figure 5.8) indicate early meltwater drainage and sediment transport was directed north to northwest across Fraser River toward the Chilcotin River valley (Figures 5.1; 5.21). As ice surfaces fell below 1160 rn elevation, drainage was captured by the Fraser Valley, and was probably Figure 5.21 Conceptual mode1 depicting the deglacial palaeohydrogeography of east-central Taseko Lakes. Early ice- proximal phase (b). Figure incorporates geomorphic data discussed in text and earlier figures. coincident with inundation of deglaciated reaches of the Chilcotin and Fraser valleys south of Williams Lake by. glacial lake Fraser (cf. Clague, 1987; 1988; Figure 5.lb). This phase probably corresponds to the onset of Fulton's (1967) stagnant-ice phase (Figures 5.8; 5.22). Initially, rernnant ice occupied the Fraser Valley to an elevation of approxirnately 1030 m, and rneltwater was diverted north toward the confluence of the Fraser and Chilcotin valleys (Figure 5.8). However, potential ice thickness in the Fraser Valley of about 600 m still prevented significant southward drainage of the glacial lake at this tirne. Once recessional ice surfaces downwasted below 850 m elevation, the lower reaches of Canoe, Dog and Alkali creeks became ice- free, so that rernnant ice in the Fraser Valley impounded interlobate glacial lakes at tributary ourlets (Figure 5.22). Sediment, eroded £rom adjacent deglaciated parts of plateau, was deposited at kame deltas formed at tributary outlets, and along kame terraces and spillways in the Fraser Valley, graded to elevations of 850 and 760 ni. These elevations are conformable with inferred surface altitudes of recessional ice surfaces in the Fraser Valley and the glacial lake Fraser system north of the area (Clague,

1988; Plouffe, 1991; Huntley et al., 1996). Initially, an hydraulic connexion between glacial lake Fraser and interlobate lakes appears to have been maintained by south- draining meltwater overflow routes along the Fraser and Empire valleys (Figures 5.2; 5.22). However, grounding-line Figure 5.22 Conceptual mode1 depicting the deglacial palaeohydrogeography of east-central Taseko Lakes. Late ice- proximal phase (c). Figure incorporates geomorphic data discussed in text and earlier figures. crevasses in the Fraser Valley, indicate that as remnant valley ice downwasted to equal lake-surface altitudes, ice margins were periodically floated. In response to this event, Fraser Glaciation till and ice-proximal kame-delta sediments were incised to a minimum elevation of approximately 570 m (Figure 5.4; log F). This elevation is interpreted to represent the minimum altitude of subglacial drainage in the Fraser Valley during floating of remnant ice (cf. Booth, 1986). Unit 5b channel-fil1 sequences indicate southerly underflow drainage was episodic and maintained under aggradational conditions. Crevasse fills. with winnowed surfaces and significant clastic component, suggest underflow was also partly focused through crevassed ice on valley margins. Mass movement structures and upward coarsening of ice-proximal sequences are attributed to higher rates of sediment remobilisation during final disintegration of remnant valley ice and drainage of the glacial lake Fraser system. Following ice disintegration. remaining ice did not significantly affect evolving fluvial drainage along the Fraser and tributary valleys (Figure 5.23). This phase is equivalent to the dead-ice phase of Fulton (1967). Finer. sorted sediment deposited in terraces and fans along valleys were aggraded to a minimum elevation of about 610 m elevation (Figure 5.23). These deposits reflect decreasing sediment delivery rates and water depth in an ice-distal setting (cf. Church and Ryder, 1972) . Subsequently, deglacial valley fil1 sequences were remobilised by landslides and incised by post- glacial streams- Successively lower regional terraces levels apparently reflect an increasing frequency of temporary blockage of southerly drainage by landslides and fluvial incision during the early Holocene.

5.7 CONCLUSIONS Late Wisconsinan, Fraser Glaciation sediments and landforms in the east-central Taseko Lakes area provide insight into the history of ice retreat, deposition and meltwater drainage at the southern limit of a large deglacial lake system, informally referred to as glacial lake Fraser. In the area, Fraser Glaciation till comprises two members. Unit 4a was deposited subglacially, principally by lodgement and basal meltout processes. Two components are recognised: the "Fraser moraine", deposited by piedmont and valley glaciers from the Camelsfoot and Chilcotin ranges; and the "Green Timber moraine", deposited by ice from the Marble Range and Cariboo Mountains. A moraine limit lies along the Fraser Valley between 51'35'N and 52'N. Unit 4b was deposited during ice-retreat and comprises supraglacial meltout till, crevasse fil1 deposits and ice-contact colluviurn. These sediments locally form ice-stagnation moraines in plateau and valley settings. Unit 5 comprises two members: a coarse- grained member. consisting of ice-contact and ice-proximal glaciofluvial outwash in upland valleys and over the plateaux; and a fine-grained member. consisting of ice- AUrn (dH 3 -4 U U *rn cd a-ri 4 a,a tnxaaa QJL, proximal deltaic, kame-deltaic sediment, and ice-distal fluvioglacial outwash and alluvial fan sediments, is restricted to the lower reaches of tributaries, and along the Fraser Valley. During deglaciation, the Cordilleran Ice Sheet appears to have sloped northward at between 0.60 to 0.80. ~t the onset of deglaciation in the area, the ice sheet apparently terminated between Williams Lake and Quesnel (cf. Clague, 1987; 1988). Southerly drainage of Fraser River was prevented at this time, and glacial lake Fraser inundated the Fraser Basin to elevations between 800 and 700 rn (Clague, 1988; Huntley et al., 1996). This suggests that glacial lake Fraser was initially drained through outlets north of the study area during early deglaciation (cf. Plouffe, 1991). Southerly drainage began once ice in the Fraser Valley reached surface elevations conformable with lake surface altitudes. Around this time, interlobate deglacial lakes were inpounded in the lower reaches of eastern hanging valleys by remnant ice from west~ïlysources (cf. Booth, 1986). Early southerly drainage was maintained along subaerial overflow channels along the Fraser and Empire valleys. However, drainage also occurred as episodic underflow events

(jokulhlaups ?) beneath stagnant crevassed ice (cf. Booth, 1986). Scoured advance-phase sediments, rarity of Fraser Glaciation till and rapidly deposited retreat-phase outwash along the Fraser Valley, south of the area, may be additional evidence of high-magnitude late glacial drainage events. Following the disintegration of remnant valley ice bodies, fluvioglacial and alluvial processes dominated, and the Fraser Valley was aggraded to an approximate elevation of 610 m. Lower sedimentation rates partly reflect lower rneltwater volumes and exhaustion of unstable debris sources (cf. Church and Ryder, 1972). Stepwise graded profiles of deglacial sequences imply that the area was not affected by isostatic rebound (cf. Sawicki and Smith, 1991). The end of deglaciation was defined by onset of degrading conditions associated with establishment of fluvial drainage in the early Holocene. CHAPTER 6 CONCLUSIONS

The east-central Taseko Lakes area is a polycyclic landscape developed under glacial and non-glacial conditions operating during the Quaternary (Tipper, 1971a; Heginbottorn, 1972; Mathews and Rouse, 1986; Eyles and Clague, 1991). In this study, sediment and landform assemblages found in the area are used to mode1 landscape evolution during the Late Wisconsinan Fraser Glaciation (Figure 6.1). ûver this glacial cycle, fluvial and glacial depositional and erosional environments were principally controlled by climate and topography. The absence of organic detritus in glacial deposits in the area limits the resolution of timing for events. Events proposed by this study are presented in the framework of a relative chronology, based on dated Fraser Glaciation sediments from adjacent parts of southern British Columbia (Fulton and Smith, 1978; Berger et al., 1987; Clague, 1988).

6.2 FRASSR GLACIATION ADVANCB-PHASE (ca. 29 to 20 ka) i) Fraser Glaciation sediments and landforms occur in al1 physiographic settings. Cirques. arêtes and U-shaped troughs are restricted to the Carnelsfoot and eastern Chilcotin ranges, and indicates these areas were local centres for ice accumulation at the onset of the Fraser Glaciation (Huntley and Broster, 1993a; 1994). In a manner similar to south- central British ~olumbia(Fulton and Smith, 1978), valley glaciers probably did not advance beyond montane areas until after 21 ka (Figure 6.1). The advance-phase corresponds to the alpine phases of glaciation (Davis and Mathews, 1944). ii) During glacier advance, montane valley floors and adjacent parts of the West Fraser Plateau were aggraded with coarse-grained glaciofluvial outwash and ice-proximal colluvium (unit 3a) . North of the Camelsfoot Range, finer- grained glaciofluvial and glaciolacustrine sediments (unit 3b) were deposited along the Fraser, and tributary valleys (Huntley and Broster, 1993a; 1994) . iii) Southerly drainage of Fraser River was initially impeced by ice and sediment south of Lone Cabin Creek, in an area where the Fraser Valley dissects the Camelsfoot and Marble ranges. This event irnpounded a proglacial lake ("glacial lake Camelsfoot"; Huntley and Broster, 19941, which possibly inundated the Fraser drainage basin to a minimum elevation of 710 rn before the Fraser Glaciation maximum-phase. Dropstones in upper beds of unit 3b suggest glaciers from southwestern sources advanced into the glacial lake along floated margins. However, glacigenic deformation of upper beds of unit 3b indicate that glacier margins in the Fraser valley became grounded as ice finally overrode glacial lake Camelsfoot (Huntley and Broster, 1994). iv) The palaeogeography of the glacial lake Camelsfoot system remains poorly defined and drainage outlets during ice advance have yet to be identified. Further investigation of advance-phase glaciofluvial, glaciodeltaic and glaciolacustrine sequences along valleys in central and northern parts of the Interior Plateau (cf. Levson et al. , 1996) is required in order to establish whether glacial lake Camelsfoot did inundate the Fraser Basin prior to the Fraser Glaciation maximum.

v) It is not known how glacial lake Camelsfoot af fected the ice-flow dynamics of the Cordilleran Ice Sheet north of the study area. It is possible that glaciers advancing into the lake were periodically destabilised in a manner similar to southern parts of the ice sheet (cf. Waitt, 1980; Atwater, 1987). At this tirne, floated ice rnargins may have rapidly

advanced, or possibly surged over the lake. If water was confined subglacially following inundation of the Interior Plateau by the ice sheet, then glacially-stremlined terrain possibly reflects regional subglaciai meltwater drainage and ice-flow patterns (cf. Shaw, 1983; Shaw and Kvill, 1984) .

6.3 FRASER GLACIATION MAXIMUM-PHASE (ca. 19 to 14 ka) i) Glaciers from montane source areas in the Camelsfoot and eastern Chilcotin ranges flowed northeast to east, coalescing along the Fraser Valley, between 51'35'N and 52'N, with west- flowing ice from the Marble Range and Cariboo Mountains (Huntley and Broster, 1993a; 1994). By this time, the Cordilleran Ice Sheet appears to have been continuous, although ice-flow was still controlled by pre-late Wisconsinan topography (cf. Davis and Mathews, 1944; Figure 6.1). There is no evidence to suggest full development.of a continental-style ice dome in the study area at this time (cf. Davis and Mathews, 1944; Tipper, 1971b) . Neither was evidence found of a late glacier advance from the Cariboo Mountains prior to the end of the Fraser Glaciation (c£. Tipper, 1971a; 1971b). ii) In upland valleys, till injection wedges in glacially- fractured bedrock suggest valley glaciers were alternately cold- and wet-based. Over the plateau, transtensional and transpressional structures in advance-stage glaciofluvial outwash are compatible with overriding by a thin, cold-based grounded ice margin (Huntley and Broster, 1993~).After the arriva1 of thicker, wet-based ice, the deformed outwash was eroded. This event was followed by lodgement of Fraser Glaciation tiil (unit 4a). ~rumlinisationof till implies that wet-based conditions and enhanced basal ice-flow rates persisted after deposition. iii) Fraser Glaciation till (unit 4a) deposited during the advance- and maximum-phases has two components. The "Fraser moraine" includes float derived £rom the eastern Chilcotin and Camelsfoot ranges, Piltz Peak, Mount Wales and Mount Alex. The float was deposited by piedmont and valley glaciers flowing north to northeast, over the western Fraser Plateau and into the Fraser Valley. The "Green Timber moraine" was 128 deposited by ice flowing westward over the eastern Fraser and Green Timber plateaux into the Fraser Valley. Terminal moraine lirnits support the presence of an ice confluence along the Fraser Valley in the study area. An effective way to delimit the extent of the "Fraser" and "Green Thber moraines" might be to examine and compare the geochemistry of till matrices.

6.4 FRASER GLACIATION RETREAT-PHASE (ca. 13 to 10 ka) i) This phase probably began sometirne before 11 ka and ended by 10 ka (cf. Fulton and Smith, 1978; Berger et al., 1987; Clague, 1988). As the regional equilibrium line altitude rose above the elevation of montane accumulation centres, glaciers were cut off from sources and stagnated in situ (cf. Fulton, 1967; Tipper, 1971b; Figure 6.1) . ii) At the onset of deglaciation in the study area, upper limits to the Cordilleran Ice Sheet, determined from the distribution of relict periglacial terrain and trimline elevations, ranged from 2600 m elevatior? in the Camelsfoot Range to 1980 m in the Piltz Peak area (Huntley, 1994). Ice sheet thicknesses at the onset of this phase probably varied from about 2000 rn in montane areas to 1000 m along major valleys and 600 rn over the plateau (Huntley and Broster, 1994). iii) Stable ice surfaces apparently sloped gently northward at 0.60 to 0.80 during deglaciation. Surface gradients of this magnitude imply that at the onset of deglaciation in the study area, the terminus of the Cordilleran Ice Sheet was situated approximately 150 km to the north. At this time, glacier ice occupying the Fraser Valley prevented southerly drainage and an extensive proglacial lake system was impounded in the Fraser drainage basin north of the study area, infomally referred to as "glacial lake Fraser" (Figure 6.1). iv) O~tletsfor glacial lake Fraser during early deglaciation have yet to be identified. Recent studies indicate early drainage may have occurred at the northern limit of the lake system (Plouffe, 1991; Levson et al., 1996). ~nvestigationof potential outlets in these areas will provide invaluable information on lake hydrology and ice retreat history at the onset of deglaciation north of the study area.

V) During deglaciation, supraglacial meltout till, crevasse fil1 deposits and ice-contact colluvium (unit 4b) were deposited, forming ice-stagnation moraines. In ice-contact and ice-proximal settings, the abundance of meltwater ensured rapid redistribution of morainal deposits (cf. Church and Ryder, 1972). Eroded sediment was deposited subglacially in eskers; or subaerially in ice-directed meltwater spillways and ice-marginal kame terraces (unit Sa). Fan-deltas and karne-deltas formed in interlobate basins, where meltwater was irnpounded along the lower reaches of Canoe, Dog and Alkali creeks (unit 5b) (cf. Booth, 1986).

vi) Retreat-phase glaciofluvial and glaciolacustrine sediments in the study area are graded to elevations of 850, 760 and 610 m, which implies an hydraulic connexion with glacial lake Fraser. The step-like elevational range of graded landforms suggests glacial isostatic tilting had little impact in the area (cf. Sawicki and Smith, 1992). Rather, it suggests that as remnant ice in the Fraser Valley downwasted to between 850 and 760 m elevation, episodic southerly drainage of glacial lake Fraser began. Drainage apparently occurred as high-magnitude events. It was facilitated by ice-marginal meltwater overfiow charnels, as well as subglacially beneath periodically-destabilised, grounded, remnant valley ice (cf. Booth, 1986). Drainage was accompanied by disintegration of ice and numerous mass- rnovements along valleys. Examination of retreat-phase outwash sequences along the Fraser Valley south of the area should provide further insight into the final drainage history of glacial lake Fraser. vii) These latter events were followed by decreased sedimentation rates, reflecting lower meltwater volumes and exhaustion of unstable debris (cf. Church and Ryder, 1972). The post-glacial landscape was characterised by the dominance of degrading conditions associated with establishment of fluvial drainage patterns by early Holocene tirne. RBPERENCBS

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Shaw, J. and Kvill, D. 1984. A glaciofluvial origin for drumlins of the Livingstone Lake Area, Saskatchewan. Canadian Journal of Earth Science, v.21. pp.1442-1459.

Smith, N. D. and Ashley. G.M. 1985. Proglacial lacustrine environment. In Glacial sedimentary environments. Edited by G.M. Ashley, J. Shaw and N.D. Smith. Society of Economic Palaeontologists and Mineralogisrs, Short Course 16, pp.135-216.

Tipper, H.W. 1971a. Glacial geomorphology and Pleistocene history of Central British Columbia. Geological Survey of Canada, Bulletin 196, 89 p.

Tipper, H.W. 1971b. Multiple glaciation in central British Columbia. Canadian Journal of Earth Sciences, v. 8, pp.279-298. Tipper, H.W. 1978. Taseko Lakes ( 920) map-area. Geologicai Survey of Canada, Open File 534.

Tsui, P.C., Cruden, D.M. and Thompson, S. 1988. Ice-thrust terrains and glaciotectonic settings in central Alberta. Canadian Journal of Earth Sciences, v. 26, pp. 1308-1318.

Valentine, K.W.G., Watt, W. and Bedwany, A.L. 1987. Soils of the Taseko Lakes area, British Columbia. Agricul ture Canada, British Columbia soi1 survey, Report 36, 131 p. van der Heyden, P. and Metcalfe, S. 1992. Geology of the Piltz Peak plutonic cornplex, northwestern Churn Creek map area, British Columbia. In Current Research, Part A, Geological Survey of Canada, Paper 91-lA, pp.113-119.

Waitt, Jr., R.B. 1980. About forty last-glacial Lake Missoula jokulhlaups through southern Washington. Journal of Geology, v. 88, pp. 653-679.

Watt, W. and Bedwany, A. 1979. Soils and landforms of Taseko Lakeç map area, British Columbia Ministw of Environment Resource Analysis Branch, 92-0/07, 92-0/08, 92-0/09, 92- 0/10 map sheets. APPENDIX I

FIELD LOGS

The study was undertaken as part of the Frontier Geoscience programme (Hickson, 1990; Hickson et al., 1991). The purpose of research was to examine late Quaternary sediments and landforms in the east-central Taseko Lakes area, southern

British ~olumbia(see also Figure 1.1). The study area included the Big Bar (92 0/01), Churn Creek (92 0/07), E2npire Valley (92

0/08), Dog Creek (92 0/09) and Mount Alex (92 0/10) National

Topographie Systern (NTS) map sheets (see also Figure 1.1) . Fieldwork during the summers of 1991 and 1992 was based out of a camp established at Gang Ranch, 60 km south of

Williams Lake (see also Figure 1.1). Montane and upfand areas, and large tracts of the Fraser and Green Tirnber plateaux were accessed by four-wheel drive vehicles on public and private roads, or by foot dong cattle and animal trails through open forest. Acceçs to remote areas was provided by helicopter or on horseback . Field station locations were located by triangulation with major landforms or dead reckoning on air photos, then plotted on a traverse map. Key sections, examined in Stream banks, terraces, landslide headscarps, road cuts and borrow pits, were logged in vertical profile at 28 sites throughout the area (Figure 1.1). Section heights were measured using a 30 m tape, or with an altimeter to an accuracy of about 5 m. Altirneter readings were calibrated twice daily to a datum elevation of 640 m at Gang Ranch, or at local NTS spot heights. Facies assemblages were described in logged profiles. Specific characteristics recorded included: type of facies, unit thickness, types of contacts, interna1 structures, textural properties (including relative percentage of matrix and clasts), clast properties (dominant shape, roundness and size), and other distinguishing criteria (striated surfaces and faceting). Sediments were described using non-genetic coding similar to Eyles et al. (1983) (Table 1.1), then given an interpreted origin. Stratigraphie units were defined for laterally and vertically continuous facies assemblages by presence of major unconformities (see also Tables 3.1; 3.2; 5.1). fi-C

k-ÉJ Facies code Description

Laminated silt and clay Massive silt and clay

Planas-bedded sand Ripple-bedded sand Trough-bedded sand Massive sand

Planar-bedded gravel Trough-bedded gravel Foreset-bedded gravel Massive gravel Massive boulder

Dm Massive matrix-supported diamicton Dms Stratified matrix-supported diamicton DCS Stratified clast-supported diamicton Dcm Masshe clast-supported diamicton l~dditionalqualifier

Dropstones Id I Table 1.1 Non-genetic facies code (after &les et al., DHH 1-

îœdiW lacustrine sediment (unit 5b)

bdgement dill (unit 4a) mm Lodgernent till and subgladal ludd sedimerit (unit &A)

DHH in DHW 179 DHH 196 OHH 197 DHH lm DHH 199 OHH PO DHW 176 m

OHH 216

lce-proximal glaciofluvial sediment (unit 54

mGrn Ice-proxhd dluuiurn (unit 3a) om iie om la

OHM 1%

Lodgernerit till (unit 4a) OHH la1

DHH 194

om 146 sedimerit (unit 2)

om iu

DHH 315 a 71ornebes

om147

OHH 195

Ice-distal fluvioglacial and sa lacustrine sediment (unit Sb) un

Iœ-proximd glaciofkiuial secüment (unit 3a)

OHH 219 Dmn Lodgement 13 (unit 4a)

[knRm, Iœ=pro~dcduviumrd bôgemmt till (unit 3a and unit 4a)

Do Hiidop cdfuviurn (unit 6)

Sm bproxirnal deîtaic sediment (unit Sb) am

Gf Ice-proximal de&& sediment (unit 5b) Gm Hillslope colluvium (unit 6)

be-proximalkame- sedimerit (unit 5b) mm Ice-proximal glaciolacustrine Dtm sedimsnt (unit 3bl

OnffGm Glaciol- sedinseni (mit 2)

&Gm Rniidseament(uiii1)

Sm Aliuvid fan sedmerit (unit 6) Dmn Lodgement till (unit la) Hiîmwhtbic sediment (unit Sb)

Dnis,Sm, Ftn

Fiuw al sedimerrt (unit 1 ) 9 DHH Id0

-tael glacidluvial sedimerit (unit 3a)

Rn,Dcs Fluvid and icedsial fluvbgîacial Seament (unit 3a) APPENDIX II PALABOFLOW DATA

Several types of palaeoflow data were collected at 82 locations (Table 11.1; Figure 11.1). and then plotted as unidirectional and bidirectional rose histograms using the

Rosym pro ject ion programme. Stable palaeocurrent directions in units 1, 3 and 5 were inferred from long-axis trends of 50 elongate pebble- to cobble-size clasts and imbrication dip-directions of platy clasts. These data were evaluated against palaeoflow directions inferred from channel and esker trends, trough cross-beds, clirnbing ripples and foreset beds. Approximations of sediment mass-transport direction were determined from long-ais dip direction trends of 50 prolate clasts in diamictons interpreted as debris flows (units 2, 3b and Sb). Palaeoiceflow directions were inferred from the dominant trend(s) of long-axes rneasurements of 50 prolate clasts sampled in diamicton

identified as basal till (unit 4a). These data were evaluated against iceflow orientations determined from indicator landforms including drumlins, flutes and striae. Fabric measurements were also taken in material identified as supraglacial meltout till and crevasse fills (unit 4b).

Sample ünit Facies Orientation data Local landfonn Type of plot

Unit 1 Trough-bedded gravel Imbricated clasts Unidirectional Unit 1 Massive grave1 Clast long-axes Bidirectional Unit 1 Stratified diamicton Clast long-axes ~alaeochannel Unidirectional Unit 1 Trough-bedded gravel Imbricated clasts Unidirectional Unit 1 Massive gravel Bidirectional

Unit 2 Stratified diarnicton Clast long-axes Unidirectional Unit 2 Stratified diamicton Clast long-axes Palaeochannel Unidirectional Unit 2 Stratified diamicton Clast long-axes Palaeochannel Unidirectional Unit 2 Stratified diarnicton Clast long-axes Unidirectional

Unit 3a Massive grave1 Clast long-axes Bidirectional Unit 3a Massive grave1 Clast long-axes Bidirectional Unit 3a Massive grave1 Clast long-axes Bidirectional Unit 3a Trough-bedded gravel lrnbricated clasts Palaeochannel Unidi rect ional Unit 3a Massive grave1 Clast long-axes Bidirectional Unit 3a Massive grave1 Clast long-axes Bidirect ional

Unit 3b Stratified diamicton Clast long-axes Unidirectional Unit 3b Stratified diarnicton Clast long-axes Pa laeochannel Unidirectional

Unit 4a Massive diamicton Clast. long -axes Bidirectional Unit 4a Massive diamicton Clast long-axes Roche moutonee Bidi rect ional Unit 4a Massive diamicton Clast long-axes Striae Bidirectional Unit 4a Massive diamicton Clast. long-axes Bidirectional Unit 4a Massive diarnicton Clast long-axes Bidirectional Unit 4a Massive diamicton Clast long-axes Bidirectional Unit 4a Massive diamicton Clast long-axes Drumlin Bidirect ional Unit 4a Massive diamicton Clast long-axes Bidirectional Unit 4a Massive diamicton Clast long -axes Flute Bidirectional Coatinued overt 8-10 mit Facies Orientation data Local landform Type of plot Continud: 123 Unit 4a Massive diamicton Clast long-axes Bidirectional 131 Unit 4a Massive diamicton Clast 1 ong - axes Roche moutonee Bidirectional 138 Unit 4a Massive diarnicton Clast 1ong - axes Flute Bidirectional 142 Unit 4a Massive diamicton Clast long-axes Druml in Bidirectional 144 Unit 4a Massive diarnicton Clast long-axes Bidirectional 147 Unit 4a Massive diarnicton Clast long-axes Drurnlin Bidirectional 151 Unit 4a Massive diamicton Clast long-axes Flute Bidirectional 163 Unit 4a Massive diamicton Clast long-axes Striae Bidirectional 168 Unit 4a Massive diarnicton Clast long-axes Druml in Bidirectional 171 Unit 4a Massive diarnicton Clast long-axes Striae Bidirectional 172 Unit 4a Massive diamicton Clast long-axes Bidirectional 174 Unit 4a Massive diamicton Clast long-axes Bidirectional 177 Unit 4a Massive diamicton Clast 1ong - axes Bidirectional 179 Unit 4a Massive diamicton Clast long-axes Bidirectional 180 Unit 4a Massive diamicton Clast long-axes Roche moutonee Bidirectional 181 Unit 4a Massive diamicton Clast long-axes Bidirectional 182 Unit 4a Massive diamicton Clast long-axes Bidirectional 183 Unit 4a Massive diamicton Clast long-axes Druml in Bidirectional 184 Unit 4a Massive diairiicton Clast long -axes Flut e Ridi rect ional

Unit 4b Massive diarnicton Clast long-axes Drumlin Bidirectional Unit 4b Massive diamicton Clast 1ong - axes Druml in Bidirectional Unit 4b Massive diamicton Clast long -axes Flute Bidirectional Unit 4b Massive diamicton Clast long-axes Druml in Bidirect ional Unit 4b Massive diamicton Clast long -axes Druml in Bidirectional Unit 4b Massive dianiicton Clast long-axes Druml in Bidirectional Unit 4b Massive diamicton Clast long-axes Flute Bidirect ional Unit 4b Massive diamicton Clast 1 ong-axes Flute Bidirectional Unit 4b Massive diamicton Clast long-axes Bidirectional Cont inued over t acEacÉcaco5 COOCOOOCE 0 -4 .ri 0 -4 .4 -4 0 0 C

rnm 111111rn

cn 111 cn VI Cl) m 5 (O Q E X C , Drumlin

Roche mouton&

., Striae

Meltwater channel - or esker trend

om 213 Sarnple number

UNIT 1 UNlT 2

UNlT 3b

UNlT 4a

UNlT 4b

UNIT Sa

UNIT Sb APPENDIX III

STRUCTURAL DATA

Structures attributed to dynamic and passive glacigenic deformation (cf. Broster and Clague, 1987) were examined in advance- and retreat-phase sequences at 16 locations (Table 111.1: Figure 111.1). Structural orientation data were plotted using the Stereom projection programme. Planar data, including orientation trends of glacier advance-phase till injection wedges and glacier retreat-phase mass-movement failure planes are shown as plots of dip and dip direction. Dewatering features, fault planes and fold axes in advance-phase glaciofluvial deposits (unit 3a) are plotted as great circles. Fault planes and fold axial planes in advance- phase glaciolacustrine deposits (unit 3b) are plotted as poles to great circles. n--

I;I !-

R CU- C Samgh Unit Structure8 obeerved Local landforai Type of plot

Fraser Glaciation advànce-phase:

Bedrock Till injection wedges Roche moutonee Dip & dip direction Bedrock Till injection wedges Roche moutonee Dip & dip direction Bedrock Till injection wedges Druml in Dip & dip direction Bedroc k Till injection wedges Flute Dip & dip direction

Unit 3a Dewatering structures Drumlin Great circles Unit 3a Reverse faults (Dl) Druml in Great circles Unit 3a Folds (Dl) Drurnlin Great circles Unit 3a Normal faults (D2) Druml in Great circles Unit 3a Reverse faults (D3) Drumlin Great circles

Unit 3b Normal faults Palaeochannel Poles to fault planes Unit 3b Folds Striae; drumlin Poles to fold axial planes

Fraser Glaciation retreat-phase and post-glacial:

189 Units 1-6 Normal faults Mass movement Dip & dip direction 190 Units 1-6 Normal faults Pa laeochannel Dip & dip direction 191 Units 1-6 Normal faults Pa laeochannel Dip & dip direction 192 Units 1-6 Normal faults Palaeochannel Dip & dip direction 193 Units 1-6 Normal faults Palaeochannel Dip & dip direction Table 111.1 Sumrnary table of structural data. Striae ,, Meltwaterchannel or esker trenâ

DM( zso Sam ple location

FRASER GLACIATION ADVANCE-PHASE

Dynamic deformation FRASER GLACIATION ADVANCE-PHASE Dynamic deformation* FRASER GLACIATION ADVANCE-PHASE

Passive deformation Dynamic deformation

FRASER GLACIATION RETREAT-PHASE

Passive deformation FRASER GLACIATION RETREAT-PHASE

Passive deformation APPENDIX IV

L 1THOLOGY DATA

Upper and terminal lirnits of the Cordilleran Ice Sheet and rnorainal provenances in the study area were partly determined from analysis of clast content in diamictons interpreted as basal tills (unit 4a; Fraser Glaciation till). In the study area, the Fraser fault system juxtaposes Triassic phyllites and cherts, and linestone of the Permian Marble Canyon Formation east of the Fraser Valley against

Mesozoic and early Tertiary rocks to the West (Tipper, 1978 ; Mathews and Rouse, 1984; see also Figure 5.6). Cretaceous conglomerates (Jackass Mountain Group and Silverquick Formation) and Middle Jurassic siltstones, argillite and volcaniclastic sediments are exposed only in the Camelsfoot and eastern Chilcotin ranges and middle reach of Churn Creek

(Hickson, et al., 1991; Mahoney et al., 1992; see also Figure 5.6) . To the north, Eocene rhyolites, pyroclastic rocks and maf ic to intermediate flows bury an early Tertiary palaeosurface (Hickson, 1992). Early Cretaceous quartz diorite and granite (Piltz Peak and Mount Alex plutonic complexes; van der Heyden and Metcalfe, 1992) exposed around Piltz Peak, Mount Wales and Mount Alex (see also Figure 5.6) were topographic highs on this palaeosurface. Adjacent to palaeohighlands, Miocene and early Quaternary fluviatile sediments, tephra and coal are preserved in palaeobasins formed along the Fraser Valley (Mathews and Rouse, 1984). North of the Carnelsfoot and Marble ranges, older rocks are capped by an extensive cover of Pliocene to early Quaternary plateau basalt flows (Chilcotin Group; Mathews, 1989). Three rock types with restricted exposure with respect to ice source areas were selected as lithologic indicators (see also Figure 5.6). (i} Massive, micritic limestone of the Permian Marble Canyon Formation and subordinate contemporary limestones of the Cache Creek complex, exposed east of Fraser River. (ii) Chert pebble and volcanic clast conglomerates of the Cretaceous Jackass Mountain Group and Silverquick Formation, exposed in the southwest quadrant of study area (Jackass Mountain Group) and along the Churn Creek valley (Silverquick Formation) (see also Figure 5.6). (iii) Granodiorite, diorite and quartz-diorite of the Lower Cretaceous and Jurassic Piltz Peak and Mount Alex formations, exposed in the west-central part of the study area (see alss Figure 5.6). At 99 locations (Figure IV.l), percentage contents were estimated by field counts, or determined by counting 100 pebble to cobble-size clasts dry sieved from till samples in the laboratory. Clasts were grouped according ta lithology and percentage distributions of indicator float then presented as pie charts.

APPENDIX V

SURFICIAL GEOLOGY MAPS Surficial deposits and geomorphology were mapped at a scale of 1:50 000 over the Churn Creek (92 0/07; V.11, Empire Valley (92 0/08; V.111, Dog Creek (92 0/09; V.111) and Mount

Aiex (92 0/10; V.IV) NTS map sheets (see also Figure 1-11. Surficial geology maps in the back folder are in part derived from compilation of earlier 1:250 000-scale surficial geology maps (Tipper, 1971a; Heginbottorn. 1972), 1:250 000 soils maps (Valentine et al., 1987) and 1:50 000-scale soi1 and landform maps (Watt and Bedwany. 19751, in addition to fieldwork and terrain analysis undertaken as part of this study. Maps depict homogenous or composite assemblages of sediment as terrain polygons, and landforms as onsite symbols. Polygons were delimited according to the degree of similarity or dissimilarity of terrain characteristics. A minimum polygon

size of Ca. 1 cm2 was adopted, equivalent to a ground area of 25 hectares. Foilowing recent mapping convention, polygons were labeled using a nomenclature modified frorn the Terrain

Classification System for British Columbia (Howes and Kenk, 1988). For this study, coding was simplified to include type of surficial material (genesis) and surface expression (landfom and, or material thickness), as well as accomrnodating information on stratigraphy and composite units. On site symbols were added to indicate specific landforms not delimited by terrain polygons or letter symbols. PLEASE NOTE=

Ovenize maps and chans are filmed in sections in the foliowhg manner:

LEFT TO RIGHT, TOP TO BO'il'OM, WITH SMAU OVERLAPS

The foilowing map or chvt has ken rehdin its entirety at the end of thdissertation (no

Blad< and white photographic PME (17" x 23") are available for an additionai charge.

SURFICIAL GEOLOGY OF THE CHURN CREEK MAP-AREA, BRITISH COLUMBIA

MAP UNK LETTER NOTATION

SWLE TERRAIN UNIT SYSTEM surficial material surface expression -~b-

Composite units - Upper component (Cv) stratigraphically ovedies the lower R component (R) ~blFab- First component (Mb) more extensive than the latter component (Pb)

SURFICIAL MATERIAL

C - COLLUVIAL Diamict and gravel transpoited and deposited by gravitational processes. lncludes ice- contact colluvium, talus, landslide, debris flow and other mass wastage products. F - FLUVIAL Gravel ana sand transported and deposited by rivers. Includes fioodplains, river terraces, deltas and alluvial fans. L - LACUSTRINE Fine sand, silt and diamicton settled from suspension and gravity flows. Contains variable thicknesses of organic deposits; Fa - GLACIOFLUVIAL Gravel, sand and subordinate diamicton depositec; in association with glacier ice. lncludes kettled oufwash, kame terraces, rneltwater channels and eskers. La- GLACIOLACUSTRINE Lacustrine diamicton. silt and subordinate gravel and sand deposited in association with glacier ice. Sequences display slump structures. icsrafted dropstones, terraces. kettles and strandlines, M - MORAINE Diamicton (MgemeM and'meltout till) deposited directly by glaciers. Generaliy consists of well-campactecl material with variable structure and texture. lncludes moraines, drumlins, f lutes .qnd - ridges. R - BEDROCK Outcop covered by less than 50 cm of unconsolidated material.

i.. b - blanket-(>1 rn) v -.veneer (< 1 m) 1.- fan t - terraced r - rage *--K. h - hummoclcy d,-defomed p- patternal

*

GEOLOGICAL SURVEY OF CANADA

-- - t - tepa&&- p- patterned

GEOMORPHK: SYMBOCS Glacial trough \b$\ Moraine ridges Drurnlin (flow direction 0 Kettle holes known) OO Esker (flow Ice fbw direction determined .b, direction known) from till fabric Meltwater channel Glacial flute \ \ Terrace Striae (flmdirection assumed) Landslide

Erratics

SURFICIAL GEOLOGY NOTE The Chum Creek map area comprises three physiographic regions: a) South and southeast portions are montane landscapes. Above 6300 ft (1980 m), bedrock (R) is locally mantled by patterned ground (Cp), felsenmeer-(Cb), protalus ramparts (Ct) and solifluxion tenaces. mese units are interpreted as a relict permafrost terrain formed above a stable minimum surface elevation of the Cordilleran Ice Sheet during the Fraser Glaciation. At lower elevations, bedrock is covered by Fraser Glaciation till (Mb, Mv). late or postglacial debris flows (Cv) and talus (Cf): Glacial troughs, cirques, flutes, dnimlins and striae indicare that ice flow was generally northeast; b) A moderatefy unduiating portion of the Fraser Plateau. ranging m elevation from 4400 ft (1341 m) to 1900 fi (1524 m). flanks montane areas. The plateau is covered by Fraser Glaciation till (Mb), deglacial stagnation (Mh) ~ndridge (Mt) moraines; c) Montane and plateau areas are dissectecl by Chum Creek and its tributaries. Fraser Glaciation advance- stage glaeiofluvial sediments - (p) are confined to the Churn Creek valley. This unit is truncated, locally drumlinised and overlain by Fraser Glaciation till (Mb) and retreat- stage glacioflwial sediments (F%). Fraser Glaciation units. postglacial debris flows (Cv) and alluvial sediments (Ft) are extensively incised and terraced. Fluvial sediments (Fb) and landslide debris (C) are confined to the present valley floor. A late glacial mdtwater aystem, confined to tributary valleys and the adjacent portions of the plateau, drained northwest to Big Creeli and northeast toward Chum Creek. The distribution of relici channels and eskers reflects the pattern of ice decay and orientation of regional structural lineaments. Fra& Glaciation retreat-stage glaciofluvial sediments (FGb,F%. Pr) are overlain by postglacial lacustrine (Lb) and fluvial units (Fb).

WBLIOGRAPHY IBr-r B.E. and Huntley;-D.H.

WtItIUAIUI I &UmRWa. ? 1 UD~UW -8 w i~i~~ipiunwuwrr ri. .UV--. p--.iY-.. .-. .-.. . -,....,, abovea stabb minimum surface elevation of the Cordilleran Ice Sheet dufig'the Fraser Glaciation. At lower'elevé&ts, bed- is covered by Fraser Glaciation till (Mb. Uv),lale or postglacial debfis Hows (Cv) and talus (Cf),Glacial troughs. cirques, flutes, drumlik and stfiae indicate that ice flow was generalîy noroieast: b) A moderatefy undubting portion of the Fraser Plateau, ranging in elevation from 4400 ft (1341 m) ta 5000 ft (1524 m), f lanks muntane areas. The plateau is covered by Fraser Glaciation till Pb), deglacial stagnation (Mh) and ridge (Ur) moraines; c) Montana and plateau areas are disSectd by Chum Creek and its tributaries. Fraser Glaciation advance- stage gheioflwiai sediments -(p)are confined to the Chum Creek valley. This unit is tmncated, locally drumlinised and overlain by Fraser Glaciation till (Mb) and retreat- stage @acioflwialsediments (Pb). Fraser Glaciation units, postgtacial debris f lows (Cv) and alluvial sediments (Ft) are extensively incised and terraced. Fluvial sediments (Fb) and landsfide debris (Cf are confined to the present valley floor. A late glacial rneitwater systern. confined to tributary valleys and the adjacent portions of the plateau, drained northwest to Big Creek and noitheast toward Chum Creek. The distribution of rekt channels and eskers reflects the pattern of ice decay and orientation of regional strridtwal lineaments. Fraser Glaciation retreat-stage glaciofluvial sediments (ÇOb, F%, For)are overlain bv ~ostrrlaciallacustrine (Lbl and fluvial mits (Fbl.

Broster S.E. and Huntley, D.H. 1992: Quatemary stratigraphy in the east-central Taseko Lakes area, British Columbia. In Current Research. Part A; Geological Survey of Canada, Paper 92- 1A, p. 237-24 1. Heginbottom, J.A. 1972: Suificial geology of Taseko Lakes map area, British Columbia. Geological Suwey of Canada. Paper 72-14,9 p. Howes, O.E. and Kenk, E. 1988: Terrain classification system for British Columbia (Second Edition). MO€ Manuat 1O. HunUey, D.H. and Broster, BE. 1993: Glacier flow patterns of the Cordilleran Ice Sheet during the Fraser Glaciation of the Taseko Lakes area, British Columbia. Current Research, Part A: Geological Suwey of Canada, Paper 93-IA, p. 167-172. nppt, H.W. 1971: Glacial geomorphology and Pleistocene history of central British Columbia. Geologicaf Suwey of Canada, Bulletin 196,89 p.

> L, , : . . Mapping by David H. Huntley (Department of Geology. The University of New-Brunswick, .' P.O. Box 4400, Fredericton, New Brunswick, E3B 5A3) as pan of Ph.D. research. . . -s . . Support was received from Dr. C.J. Hickson (Geobgical Suniey of Canada. Cotdilleran -- ;OMS~~;Project no. 890039) and Dr. B.E. Broster (Department of Geology, University 01 Y' r ri-. A --'7: ;~ew- Brunswick; Contract' r~o.2a54-1-01'1145Mt-XSB). An earlier version of this map .I- . h. . wés reviewed by Dr. RJ. FU~Iand- . Dr. J.J. Claque.

. .

CHURN- CREEK LxLmokPLAluD~~m BRITISH COLUMBIA COIDMBIE-BRITANNI1 CREEK

Oversize maps and chara are hlmed in sections in the foiiowirig manner:

LEFTTO RIGHT, TOP Tû BOTTOM, WiTH SMAU OVERLAPS

The foUowing map or chM has ken refilmed in its entiny at the end of this dissenation (not avaiiab1e on midche). A xerogqhic reproduction has ken provided for pape copies and Q iwned into the inside of the back cover.

Black and white photographie prints (17" x 23") are available for an additional charge.

-

1 SURFICIAL-. GEOLOGY -OFTHE . rEMPtftEVALLEY WAP AREA, BRITISH COLUMBIA-

: - - - .-.., , - , . . 3:: -. . -. . . 1; :-., ;;::&---\.;,!.:% ...- .. ..SC -- , . A:,-. .?.:, .;%.. , r'. . ?- .f .' ...... ; . .. _ --- :l"f:. +-:-y.-.;' -.:. . ..-- - -; . ' ' . ' - ----..&....-;, -.fiw-. * ~ .-:.@ml.;- : - -. ..: ,- .,.:a. ': ....- - ..,

PLEASE NOTE=

OversLe maps and chanr are hdin sections in the foiiouiirig manner:

LEFTTO RIGHT, TOPTO BOTTOM, WITH SMAU OVERLAPS

The foiiowing map or chan has been refilmed in its entircy at the end of this dissertation (not avaiîable on micrdche). A xerographic reproduction hrs ken provided for paper copies aad is insened into the inside of the back cover.

Bladr and white photographie priw (17" x 23") are available for an additional charge.

SURFICIAL GEOLOGY OF THE DOG CREEK MAP AREA, BRITISH COLUMBIA

MAP UNIT LMRNOTATKM

SIMPLE TERRAIN UNIT SYSTEM surficial material surface expression -~b-

- Upper component (Cv) stratigraphically overlies the lower component (R) - Fïrst cornponent (Mb) more extensive than the latter component (Pb)

SURFICIAL MATERIAL

C - COLLUVIAL Diamicton and gravel transported and deposited by gravitational processes. lncludes ice-contact colluvium, talus, landslide, debris flow and other mass wastage products. F - FLUVIAL Gravel and sand transported and deposited by tiers. lncludes floodplains, river terraces, deltas and alluvial fans. L - LACUSTRINE Fine sand, silt and diamicton settled from suspension and gravity flows. Contains variable thicknesses of organic deposls. F" - GLACIOFLUVIAL Grave!, sand and subordinate diarnicton deposited in association with glacier ice. lncludes kettled outwash, kame terraces, meltwater channels and eskers. Lo- GLACIOLACUSTRINE Lacustrine diamicton, silt and subordinate gravel and sand deposited in association witl glacier ice. Sequences display slump structures, ice-rafted dropstones. terraces. kettles and stmndlines. *- ' U - MORAINE --. Xamict (lodgement and mehout till) deposited directly by glaciers. Generalty consists fi well-compcte6~hteriaiwithvariable structure and texture. lncludes moraines. jrurnlins, flutes and ridges. R - BEDROCK Dutcrop cwered by lsss than 50 cm of unconsoüdated material.

SURFACE EXPRESSION"

t - terraced

-

GEmOGICAL SURVEY Of CANADA

Glacial tmugit \\\\ Moraine ridges Drumlin (flow direction O Kettle hales knwn) 00 Esker (flow Ice flow direction determined direction knom) frmtill fabric

Glacial flute

Striae (flow direction assumed)

Erratics

SURFICIAL GEOLOGY NOTE The Dog Cmk maparea comprises three physiographic regions with distinct landfonn and sediment assemblages: a) Upland areas are f~undin the west and southeast, and reach a-maximum devation of -5400 ft (1646 nif muth of Dog Creek. In these amas, bedrock (R) is mantled by Fraser Glaciation till (Mb.Mv). stagnation moraine (Mh), bte or postgtecial debris flows (Cv) and talus-(Cf). Glacial troughs, dnimlins and striae indicate regknat ice flaw hto the Fraser Valley; b) The gently to moderately undulating Fraser Plateau, ranging in elevation from 2500 ft (762 m) to 3500 ft (1067 m). flanks upland areas. The plateau k covered by Fraser Glaciation till (Mb). which is localîy and overlain by stagnation moraine (Mh). Morainal units are incised by metwvater channels that drained toward major valleys. Glaciofluvial sediments (Pb, fi. Pr) are confined ta channel floors. The distribution of relict channels and eskers reflects the pattern of ice decay over the_ plateau; c) Upland and plateau areas are dissected by the Fraser River and -stributahs.Patae&annels exposed along the raser, Churn and Gaspard valleys are -~tinto bedrock. (R) or defomed prslate Wisconsinan glaciolacustrine sediments (cd).and infilled with- Fraser Glaciation advance-stage glaciolacustrine sediments (0. These units are tiuncated, localty defoned and overbin by Fraser Glaciation till (Mb). Fraser Glàciaiian retreat-stage kame depasits (F'k). g laciof luvial sediments (fi)and deb& flows (Cv, Ct) are confîned to valley sides. Retreat-stage ghciolacustrine sequences (~9)are restricted to.the mouths of Alkali. Dog and Chum creeks. suggesting sediments were deposited in interlobate. glacial lakes foned in these areas. Ridge moraines (Mr) lyiig adjacent to tributary conf luenœs are interpreted as crevasse-f ills deposited-alongglacial lake margins. Fraser Glaciation units. postglacial debris flouus, (Cv) and alluvial sediments (R) are extensively incised and terraced. Fluvial sediments (Fb) and landsliôe debris (C)are confined to contemporaryvalfey floois.

by sta~àtionmoraine (Mh). Morainal units are incised by mettwater channels thaî drained toward.mejor vali6ys. Glaciofluvial sedhants (Pb. F%, fi)are confined to chanml fbors. The distriôution of relict channels and eskers refbthe pattern of iœ decay over the plateau; c) Uphd and plateau areas are dissected by the Fraser River ard 6 tributa&. Palaeochannek ex@oseddong the Fkser, Chum and ~aspardvalleys are-ait into bedrock. (R) or deformed prelate Wisconsinan glaciolacustrine sediments (Lod).and hfilled with Fraser Glaciation advance-stage glaciolacustrine sediments (L?. These un& are tnincated, Wly deformed and overfah by Fraser Glaciation tal (Mb). Fraser ~laciaiiinretreat-stage kame deposits (m). glaciofhMaf sediments (F%) and dekis flows (Cv. Ct) are confined to valley sides. Retreat-stage glaciolacustrine seqümces (L?) are restricted to-the mwths of Alkali, Dog and Churn creeks. suggesting sediments were. deposited in interlobata glacial lakes fomed m these areas. Ridge moraines (Mr) tyhg adjacent to tributary confluences are interpreted as crevasse-filk depabited.along glacial lake margins. Fraser Glaciation units, postgtacial debris flows. (Cv) and alluvial sedirnents (R) are extensively incised and terraced. Fluvial sediments (Fb) end landslide debris (C) are confined to cdnternporary valiey floon.

Broster B.€. and Huntley, D.Hw 1992: Quatemary stratigraphy in the east-central Taseko Lakes area. British Columbia. la Current Research, Pad A; Geological Survey of Canada. Paper 92-1A. p. 237- 241. Eyles, N. and Clague, J.J. 1991: Glaciolacustrine sedimentation during advance and retreat of the Cordilleran Ice Sheet in central British Columbia. Géographie physique et Quaternaire, v. 45, p. 317-332. Hainbottom, J.A. 1972: Surficial geology of Taseko ~akesmap area, British Columbia; Geological Survey of Canada. Paper 72-14. 9 p: Howes, D.E. a. Kenk. Ew 1988: Terrain classification system for British Columbia (Second Edition); MO€ Manual 1O. Tipper, HwWw - 1971: Glacial geomorphology and Pleistocene history of central British Columbia. Geoiogicat Survey of Canada, Bulletin 196, 89 p. happing by David H. Huntley (Departmekt of Geolagy. The University of New Brunswick P.O. .Box 4400, Fredericton, New Brunswick, E3B 5A3) as part of Ph.D. research was received from Dr. C.J. Hickson (Geological Survey of Canada, Cordillerai Project no. 890039) and Dr. B.E. Broster (Department of Geology, University a Contract no. 7232254-1-011 145/01 -XSB). An earlier version of this mq Dr. R.J. Fulton and Dr. J.J. Clague.

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DOG CREEK tIt.rnoET LAND DfSI'RICF BRITISH COLUMBIA COLOMBIE-BR1

il. r CREEK 3T LAND DI-ICT COLOMBIE-BRITANNIQUE

Oversizc maps and charts are hedin sections in the following mamer:

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SURFICIAL GEOLOGY OF THE MOUNT ALEX MAP-AREA, BRITISH COLUMBIA

SIMPLE TERRAlN UNIT SYSTEM surf icial material surface expression + LMb-1

Composite units (& - Upper component (Cv) stratigraphically overlies the lower R component (R) MblFGb - First component (Mb) more exlensive than the latter component (FGb)

SURFICIAL MATERIAL

C - COLLUVIAL . Diamicton and gravel transported and deposited by gravitatiunal processes. lncludes ice-contact colluvium, talus, landslide, debris flow and other mass wastage products. F - FLUVIAL Gravel and sand transported and deposited by rivers. Includes floodplains, river terraces, deltas and alluvial fans. L - LACUSTRINE ,- Fine sand, silt'and diamicton settled from suspension and gravit~flows. Contains variable thicknesses of organic deposits. F" - GLACIOFLUVIAL Gravel,sand and subordinate diamicton deposited in association with glacier ice. lncludes kettled outwash. kame terraws, meltwater channels and eskers.

La - GLACIOLACUSTRINE t Lacustrine diamicton. silt and subordinate gravel and sand deposited in association with glacier ice. Sequences display shimp structures. ice-rafted dropstones, terraces. kettles-and strandlines. M - MORAINE Diamicton (lodgement and meltout till) deposited directly by glaciers. Generally consists of well-compacted material with variable structure and texture. lncludes moraines, drumlins, flutes and ridges. R - BEDROCK Oytcrop covered by less than 50 cm of unconsolidated material.

SURFACE EXPRESSION b - blanket (z 1 m) v - veneer (< 1 m) f - fan t - terraced r - ridge h - hummocky d - deformed p- patterned Natural Resources Ressources natu rolles l*lCanada Canada

MISSION GEOLOGIQUE DU CANADA

MOUNT ALEX LELAK)ET LAND DISI'RICT BRITISH COLUMBIA COLOMB IE-BRIT )ET LAND DISTRfCî i COLOMBIE-BRITANNIQUE