SEDIMENT ROUTING APPLIED TO PALEOENVIRONMENTAL RECONSTRUCTION IN THE UPPER FRASER RIVER WATERSHED, BRITISH COLUMBIA
Randy William Dirszowsky
A thesis submitted in confomity with the requirements for the Degree of Doctor of Philosophy, Graduate Department of Geogtaphy in the University of Toronto
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Randy William Dirszowsky, Doctor of Philosophy, 2001. Graduate Department of Geography, University of Toronto.
ABSTRACT
Closely related floodplain and delta sites near Moose Lake, British Columbia
(Canadian Rocky Mountains) are examined for evidence of environmental change and to assess the effects of upstream storage and reworking of sediment on downstream sedimentary records. The simultaneous development of composition-based provenance techniques facilitates the interpretation of downstream sediments in terms of source area, sediment production processes and hydroclimatic forcing.
Initially, Fraser River bed material is used to characterize upstream source areas and to estimate mixing of source materials downstream. Although sedirnentary rocks provide minimal contrast in ternis of geochemical trace elements, it is possible to distinguish sediments €romhnro main watershed source areas based on major elernents associated with carbonate and aluminosilicate bedrock lithologies. Complications arise due to the effects of particle size. hydrodynamic sorting, and transport-related and post- depositional alteration. Currently the Moose River sub-basin contributes a greater proportÏon of the total and fine-grained sedirnent load delivered to Moose Lake than expected based on catchment area. The imbalance is related to greater elevations, runoff, and glacier cover in the sub-basin. A substantiat area of the Moose Lake-Fraser River delta-top floodplain derives from the early half of the Holocene, and is characterized by fragmentary. low-discharge paleochannels containing bed materials characteristic of the uppermost Fraser River.
Most floodplain development since ca. 4 ka BP records persistently higher sediment loads, aggradation and larger, more active channels. Detailed variations in the composition of laterally and vertically accreted floodplain sediments and of channel morphology reflect regionally documented glacier advance and retreat stages. Except for possible till deposits in the northwest, most valley-fiIl below Moose Lake and the delta is glaciolacustrine in origin. deposited rapidly as ice retreated up-valley at the end of the Wisconsinan. A large proportion of the delta was apparently constructed prior to ca. 10 ka BP and is either paraglacial or deglacial in origin. It is estimated that progradation and channel shifting on the delta surface could account for a graduai increase of up to 21% in distal lacustrine accumulation rates over Neoglacial time.
More episodic variations likely occurred due to channel splitting and meander cutoff on the floodplain. I would like to thank Joe Desloges first of al1 for overseeing this project and providing sound advice and funding along the way. Bill Mahaney, Tony Davis. Brian Greenwood. Rorke Bryan, Tony PRce. Larry Band, and Peter Ashmore sewed as reviewers at various times and must be thanked for their constructive comments and suggestions for improving the research design and thesis itself. Bill Mahaney and Tony Davis are to be especially thanked for their sometimes off-base intellectual and moral support. Ron Hancock provided expert knowledge and humour. as well as a nuclear reactor (SLOWPOKE II, fomerly at the University of Toronto). Without access to Bill and Ron's equipment and technical expertise, this project would not have been possible. B.C. Parks kindly granted permission to work in Mt. Robson Provincial Park, and a special word of thanks is due to Wayne Van Velzen, Hugo Mulyk and their staff for many good field ideas and generous logistical support. Greg Redies retrieved samples from the back country when Our own efforts were thwarted. The B.C. Ministry of Transportation generously allowed access to their records in Prince George. and Mark Pratt and Nick Polysou, in particular, are to be thanked. Much of the field work for this project depended on the equipment and expertise of colleagues "out west". Derald Smith, Brandon Beierle and Leif Burge (University of Calgary) made vibracoring possible. I am especially grateful to Brandon. who spent several extra days in the field to assist. Marko Mah (University of Alberta, Geophysics) generously spent a week in the field teaching us the art of seismic surveying as well as many hours post-processing the data. I am of course deeply indebted to my 'reguiaf crew of hardworking field assistants: John Roppa. Angela Vahaviolos and Ross Glenfield each of whom performed well beyond the cal1 of duty. Beth Kawecki, Adam Henley. Bill Dinzowsky, Robert lantria and Adam Horwitz also provided fieid andior lab assistance which I greatly appreciate. l would not have sutvived this endeavour without stimulating discussion, camaraderie and good vibes from some of my friends who are also colleagues: Kyle Hodder. Eliane Raymond, Andrew Stewart and Seem Ahmad; and some of rny friends who are not: Jonathan Seaquist. Michelle Perera and Douglas Converse. Thanks also to Johanne Sanschagrin, Donna Jeynes and Andrea Cottom for essential perspective. Parts of this work are dedicated to Mom. Bean and Erica. ABSTRACT ...... ii ACKNOWLEDGMENTS ...... iv TABLE OF CONTENTS ...... v LIST OF TABLES ...... viii LIST OF FIGURES ...... ix
CHAPTER 1: INTRODUCTION
1. 1 BACKGROUND AND RATIONALE ...... 1 1.2 OBJECTIVES ...... 6 1.3SETTlNG ...... 7 1.4 ORGANIZATION OF THE THESIS ...... Il
CHAPTER 2: SEDIMENT SOURCES AND MIXING IN THE UPPER FRASER RIVER WATERSHED
2.1 INTRODUCTION ...... 13 2.2 STUDYAREA ...... 15 2.2.1 Hydrology ...... 19 2.3 SAMPLING AND ANALYTIC METHODS ...... 19 2.4 RESULTS ...... 22 2.4.1 Fine Grained Geochemistry of River Bed Material ...... 22 2.4.2 Factor Analysis of Geochemical Data ...... 25 2.4.2.1 Raw Elernent Factors ...... 26 2.4.2.2 Adjusted Data Factors ...... 31 2.4.3 Mineralogy and Lithology of River Bed Material ...... 34 2.4.3.1 Clay Fraction Mineralogy ...... 34 2.4.3.2 Clast Llhology ...... 34 2.4.4 Bed Material Mixing at Tributary Junctions ...... 37 2.4.4.1 Distinction of Moose River-Fraser River Sedirnent ...... 37 2.4.4.2 Sediment Mixing Calculations ...... 38 2.4.4.3 Mixing at Upstream Tributary Junctions ...... 41 2.5 SUMMARY AND DISCUSSION ...... 44 2.5.1 Geochemistry of River Bed Sediments and Source Areas ...... 44 2.5.2 Geochemical Source Indicaiors ...... 47 2.5.3 Sediment Mixing at Tributary Junctions ...... 48 2.5.4 Source Area Contributions ...... 50 2.6 CONCLUSIONS ...... 53 CHAPTER 3: StZE-COMPOStTtON REtATiONSHtPS tN AtLUVlAt AND LACUSTRINE SEDIMENTS OF THE UPPER FRASER RIVER. BRITISH COLUMBIA
3.1 INTRODUCTION ...... 55 3.2 SETTING ...... 57 3.3 METHODS ...... 60 3.4 RESULTS ...... 62 3.4.1 Particle Size Characteristics of Analyzed Sediments ...... 62 3.4.2 Downstream Bed Material Variations ...... 66 3.4.3 Geochemical Cornposlion ...... 68 3.4.3 Size-Composition Relations ...... 70 3.5 DISCUSSION ...... 78 3.6 CONCLUSIONS ...... 84
CHAPTER 4: DEPOSITIONAL AND POST-DEPOSITIONAL GEOCHEMICAL AND MlNERALOGlCAL BEHAVIOUR OF SEDIMENTS IN AN ALPINE GEOMORPHIC SYSTEM
4.1 INTRODUCTION ...... 85 4.2 SETilNG ...... 87 4.3 METHODS ...... 90 4.4 RESULTS AND INTERPRETATION ...... 91 4.4.1 Stratigraphy and Sedimentology ...... 91 4.4.2 Geochemistry ...... 97 4.4.2.1 Geochemical Factor Analysis ...... 99 4.4.3 Clay Fraction Mineralogy ...... 102 4.4.4 Downstream and Down-Lake Sedirnent Variations ...... 104 4.4.5 Down-Core Floodplain Sediment Variations ...... 108 4.4.5.1 Core MDCl ...... 108 4.4.5.2 Core MDC3 ...... 110 4.4.5.3 CoreMDC6 ...... 112 4.5 DISCUSSION ...... 112 4.5.1 Modem River and Lake Geochemical Fractionation ...... 114 4.5.2 Post-Depositional Geochemical Alteration of Alluvial Sediments ..... 119 4.6 SUMMARY AND CONCLUSIONS ...... 122 CHAPTER 5: €VOLUTION OF THE MOOSE tAKE DELTA AND ASSûCiATf D VALLEY-FILL: IMPLICATIONS FOR LATE PLEISTOCENE AND HOLOCENE ENVIRONMENTAL CHANGE
5.1 INTRODUCTION ...... 124 5.2 PHYSICAL SETTING ...... 126 5.2.1 Late Quaternary Environment ...... 128 5.3 METHODS ...... 129 5.4 RESULTS ...... 132 5.4.1 Delta Surface Structure and Composition ...... 132 5.4.1.1 Sedimentology ...... 132 5.4.1.2 Cross-Sectional Morphology ...... 138 5.4.1.3 Surface Morphology and Chronology ...... 141 5.4.1.4 Provenance of Paleochannel Sedirnents ...... 144 5.4.2 Delta Sub-Surface ...... 148 5.4.2.1 Seismic Survey ...... 148 5.4.2.2 Borehole Records (B.C. Ministry of Transportation) ...... 152 5.4.2.3 Delta Geometry ...... 154 5.4.3 Holocene Sediment Storage Estimates ...... 157 5.5 DISCUSSION ...... 160 5.5.1 Floodplain Development and Holocene Environmental Change ..... 160 5.5.2 Late Pleistocene Valley-Fill ...... 165 5.5.3 Delta Development and Basin Sediment Yields ...... 170 5.5.4 Implications Holocene Deposition in Moose Lake ...... 172 5.6 CONCLUSIONS ...... 174
CHAPTER 6: SUMMARY AND CONCLUSIONS ...... 177
6.1 General Conclusions ...... 177 6.2 Future Research ...... 181
REFERENCES ClTED ...... 183 LtST OF TABLES
Chapter 2
Table 2.1 Bulk geochemical composition of Fraser River sediments (fine fraction) . 23 Table 2.2 R-mode factor loadings (for raw and adjusted data) ...... 27 Table 2.3 Correlation matrix for sample factor scores and source bedrock type .... 28 Table 2.4 Mineralogy of the clay fraction of Fraser River sediments ...... 35 Table 2.5 Percent lithologic composition of the granule fraction ...... 36 Table 2.6 Calculated mixing proportions in Fraser River sedirnents ...... 40 Table 2.7 Sediment mixing characteristics at upstream tributary junctions ...... 45
Chapter 3
Table 3.1 Fine fraction elernental characteristics (river-fioodplain-lake data) ...... 69 Table 3.2 ANOVA results (composition vs . size separates) ...... 71 Table 3.3 Elemental concentrations in silt and clay fractions ...... 76
Chapter 4
Table 4.1 Size properties of typical Fraser River and Moose Lake samples ...... 95 Table 4.2 Factor loadings and factor correlations for floodplain-river-lake data ... 100 Table 4.3 Clay fraction mineralogy of Moose Lake and Fraser River samples .... 103 Table 4.4 River bed and lake bottom composlion variations ...... 107
Chapter 5
Table 5.1 Provenance indicators and rnixing characteristics of paleochannels .... 146 Table 5.2 Sediment storage estimates: Moose Lake and Fraser River delta ..... 158 Chapter 1
Figure 1.1 Schernatic mode1 of alpine sediment transfers ...... 2 Figure 1.2 Aerial view (northward) of Resplendent Cr . and Reef Icefield ...... 4 Figure 1.3 Aerial view (eastward) of Fraser River and Moose Lake delta ...... 8 Figure 1.4 Location of study area ...... 9
Chapter 2
Figure 2.1 Location of study area and sampling sites ...... 16 Figure 2.2 Surficial and bedrock geology of the upper Fraser River watenhed .... 18 Figure 2.3 CalciumlMagnesium relations for Moose and Fraser river sediments ... 24 Figure 2.4 Factor score plots and spatial relations based or! raw data ...... 30 Figure 2.5 Factor score plots and spatial relations based on adjusted data ...... 33 Figure 2.6 Downstream variations in bulk geochemical composition ...... 42
Chapter 3
Figure 3.1 Location of study area and upper Fraser River watershed ...... 58 Figure 3.2 Location of tributaries and bed material sampling locations ...... 61 Figure 3.3 Size characteristics of upper Fraser River and tnbutary bed material ... 63 Figure 3.4 Size characteristics of river . delta and lake bottom fine fraction ...... 65 Figure 3.5 Downstream particle size variations ...... 67 Fgu~e3.6 a.b Elemental composition according to size separates ...... 73 Figure 3.6 c.d Elemental composition according to size separates ...... 74
Chapter 4
Figure 4.1 Location of study area and sampling sites ...... 88 Figure 4.2 Fraser River floodplain core stratigraphy and radiocarbon dates ...... 92 Figure 4.3 Fine fraction particle size distribution envelopes of sediments studied . . 96 Figure 4.4 Factor score plots and spatial relations ...... 105 Figure 4.5 a Downcore textural and geochemical variations (MDC1) ...... 109 Figure 4.5 b Downcore textural and geochemical variations (MDC3) ...... 111 Figure 4.5 c Down-core textural and geochemical variations (MDC6) ...... 113 Figure 5.1 Location of study area; main features of upper Fraser River basin .... 127 Figure 5.2 Sampling and measurement sites: Moose Lake-Fraser River delta .... 131 Figure 5.3 Delta-top vibracore logs. facies and radiocarbon dates ...... 134 Figure 5.4 Loss on ignition results for delta-top peat sequences ...... 135 Figure 5.5 Longitudinal and cross-sections of alluvial delta-top sediments ...... 139 Figure 5.6 Air-photo showing deMa surface and morpho-stratigraphic zones ..... 142 Figure 5.7 a. b lnterpreted Seismic Reflection Profiles ...... 149 Figure 5.7 c.d lnterpreted Seismic Reflection Profiles ...... 151 Figure 5.8 Longitudinal section of Fraser River/Moose Lake delta ...... 153 Figure 5.9 Schematic valley-fiIl sections inferred from geophysical data ...... 156 1.l BACKGROUND AND RATIONALE The analysis of sedimentary materials facilitates the interpretation and reconstruction of modem, historical and pre-historical environments. Sedimentary deposits directly reflect local landscape evolution and provide proxy indicators of hydroclimatic or geomorphic change and sediment yield regionally. In a terrestrial context, lake bottom sediments are possibly the single most useful deposit type, because of their usuaily continuous, potentially long-term accumulation, and their good potential for preservation (e.g. Desloges and Gilbert, 1995; Leonard, 1997; Lamoureux, 1998; cf Dearing and Foster, 1993). However, a wide range of additional deposit types has yielded useful paleoenvironmental information, and features such as floodplains (e.g. Woodward et aL, 1992). alluvial fans (e.g. Beaudoin and King, 1994), deltas (e.g. Chen et aL. 1995), hillslopes (e.g.Jonasson. 1991). and various glacial landforms (e.g. Mahaney and Kalm. 1995) may offer advantages for paleoenvironrnental interpretation in terms of geographic location and extent. Verifkation of inferred changes is best accomplished when multiple records and proxy indicators are examined in the same environmental context. A combined approach is particularly valuable in alpine systems where diverse depositional environments may be closely linked in an interacting sediment cascade (Luckrnan, 1994). The main components of an alpine sedirnent system are illustrated in Figure 1.l. This deliberately simple model is similar to those of Gurnell (1987), Fenn. (1987) Hicks et al. (1990). and Jordan and Slaymaker (1991), though only the latter two include a lake basin. The model assumes environmental conditions such that al1 or part of the drainage basin is glacier covered at least occasionally, and that sediments may be thought of as either glacial or non-glacial in origin. Non-glacial sedirnent source processes include weathenng, mass wasting. slopewash and fluvial erosion. and have been elaborated and compared in more specific alpine sediment routing rnodels (e.g. Rapp, 1960; Caine, 1974; Jordan and Slaymaker, 1991). Aeolian influx and losses are possible but generally considered of minor NON-GLACIAL EXTRAGLACIAL /--, weathering ISNOWIRAIN SNOW/RAIN 1 .u
mass wasting fluvial transport GLACIAL \""y fluvial transport
crushinglshearing
VEGETATION ]
LACUSTRINE corrasion
1- WATER
Figure 1.1 Simplified alpine ninoff and sedirnent routing model. importance in ternis of the overall sediment budget. at least in humid alpine systems (Caine, 1974). Whether or not mass wasting or slopewash contribute significantly to downstream sediment yields, depends on drainage density and how directly slopes are linked to the stream network (Goudie. 1995). Upland slope sources are expected to decrease in importance downstream as valley gradients decrease. valley bottoms widen and riven develop increasingly complex floodplains (Fig. 1.2). Since fioodplains serve as both short- or long-terni sediment storage sites. it is commonly obsewed that specific sediment yields decrease as drainage basin size increases (Walling and Webb. 1983). A number of alpine sediment routing models have emphasized glacial sources and sinks (0.g. Sugden and John, 1976; Small. 1987). depending on the purpose and scale of the study and the relative balance of glacial and non-giacial surfaces in the drainage basin. While partially contended (e.g. Hicks et al., 1WO), most workers argue or assume that production or modification of sediment by glaciers is proportionally greater than it is for non-glacial mechanisms at least in terms of long-term erosion and long-distance transport (e.g. Bell and Laine, 7985; Harbor and Warburton. 1993). Where detailed suspended sediment measurements are available, higher specific sediment yields from modern glaciated drainage basins have been demonstrated (e.g. Church and Slaymaker, 1989). Although sediment yields may Vary wiih glacial extent and depend on the stage of glacial advance or retreat (Leonard, 1986a,b; 1997). the extent to which glacier activity controls overall sediment loads remains uncertain. Nevertheless, the downstream delivery of fine- grained sediment, assumeci to be sub-gtaeially deri& 'rock flouf, has received most attention in paleoenvironmental studies (Karlén, 1976; Leonard, I986a. b, 1997: Karlkn and Mathews, 1992; Leonard and Reasoner, 1999). Research in both alpine and non-alpine environments has demonstrated that previously glaciated surfaces may continue to release glacial materials for some time following the retreat of an iœ mass (Church et ai., 1989; Ashmore, 1993). Where headwater glaciation or other significant disturbance no longer occurs, increasing downstream specific sedirnent yields are typically observeâ (C hurch and Slaymaker, 1989). As accessible material is g radually reworked, the so-called FIGURE 1.2 North westward view of Resplendent Creek tributary within the Moose River sub-basin of the upper Fraser River watershed. east-central British Columbia. Reef Icefield is visible in background. Strong glacial influence is apparent in the laterally unstable river channel and braided planforni. "paragiacial" effect (Ryder and Chucch. 1972) decreases thraugh time at a rate depending on the basin scale considered (Jordan and Slaymaker, 1991). The alpine sediment routing model defined here teminates at a proglacial lake, which defines the catchment area and is important conceptually because it rnay act as a sediment trap for fine sediment (Hhkanson, 1981). Processes active in glacier-fed lakes are reviewed by Smith and Ashley (1985), and depend on lake morphometry, limnological characteristics, inflow-lake water interactions, and sediment load itself. In general, coarser sediments are deposited at more proximal deltaic sites while glacially derived rock flour is expected to accumulate distally. Sediment concentration and sediment-water density may determine whether sediment is dispersed as overfiows or interfiows, settles from suspension, or is deposited by turbidity currents (Gilbert, 1975; Smith, 1978). Variations in dispersal may depend on circulation patterns in the lake (Gilbert and Shaw, 1981) and the location and strength of inflowing streams (Pharo and Carmack, 1979). In the above context. successful paleoenvironmental reconstruction depends not only on the establishment of the morpho-, litho- and chrono-stratigraphy of deposits, but on the definition of the sediment transfer system and assessrnent of interactions among systern components. Identification of sediment sources is critical if the effects of climate change and upstream storage and reworking are to be distinguished and interpreted. Correlations between source areas and deposits may be addressed statistically or through provenance studies wherein sediment properties are traced directly to source areas or related to source processes (e.g. Cutters et ai., t 988; Wa tting et al.. t 993; Collins et al., t997). Detailed sediment routing studies are not common (cf Jordan and Slaymaker, 1991; Brooks, 1994), but have shown that temporary storage and reworking of sediments or transfer between sinks may be cornplex, especially in larger drainage systems (Walling and Webb, 1983; Dearing and Foster, 1993). In any of these approaches the rnobility of sediments and their affinity for particular depositional sites must be considered. A number of sediment properties have been employed to assess the source, routing and history of sedirnentary materials in drainage basins; for example, texture (vmafl and Ventura. 1995), lithology and mineraiogy (Desloges, 1990; Woodward et al.. 1992). geochemistry (Fralick and Kronberg, 19971, and special properties such as magnetism (Walden et al., 1992). isotopic composition (Hasholt and Walling, 1992) and surface microtextures (Mahaney and Kalm, 1995). Each property offers advantages and disadvantages depending on ease of determination, availability of materials and purpose of the investigation. Geochernistry and mineralogy are of interest for their applicability to srnall particle sizes and small samples, and for their potential to indicate bedrock lithology and weathering history. Disadvantages of a geochemical or mineralogid approach include the potential for chemical change due to weathering or diagenesis, processes that depend on the differential mobility of specific elements and stability of specific minerals (Haughton et al., 1991). 8ulk transport rates and style also differ for sedimentary particles depending on minerai density (e.g. heavy minerals) and particle shape (e.g. micaceous minerals). This is particularly true in fluvial or lacustrine environments where hydrodynamic sorting occurs. These difficulties are confounded by the fact that sediment composition may be related to particle size so that hydrodynamic sorting becomes important regardless of mineral species. If these complicating factors are not addressed in sarnpling and interpretation, it becomes increasingly diRcult to generalize about drainage basin sources based on sediment chemical properties as distance downstream increases.
1.2 OBJECTIVES This thesis addresses the combined problem of sediment routing and paleoenvironmental reconstrudion by examining the central portion of the alpine sediment routing system defined above. Sediment source areas and general routing characteristics are assessed through a consideration of river bed material delivered by major tributaries of the Fraser River upstream of Moose Lake in east- central British Columbia. The fate of these sediments and their utility as provenance and paleoenvironmental indicators are examined as they pass through or are stored in the lower reaches of the river and its floodplain near Moose Lake (Fig.l.3). the Moose Lake-Fraser River delta, and the distal Moose Lake basin. The development of the delta and the evolution of its alluvial cap are examined in detail as long-terni sediment reservoirs and in tans of their paleoenvironmental significance. Specific objectives of the thesis are:
1) to refine current understanding of late Pleistocene and Holocene environmental change in this region of the Canadian Rocky Mountains based on local fioodplain and deltaic sedimentary records,
2) to detenine sediment routing in the upper Fraser River drainage basin and clanfying the nature of hydroclimatic forcing (including glaciation) on sediment production and yield,
3) to assess the importance of upstream (fioodplain and delta) geomorphic and sedimentologic processes on downstream sediment yields and deposition in Moose Lake (the record of which is being investigated independently),
4) to develop provenance techniques based on elemental geochemistry that have not yet been widely employed in Quaternary paleoenvironmental or paleogeographic reconstruction, particularly in areas of sedimentary bedrock.
1.3 SETTING The focus of this study is the Fraser River watershed upstrearn of Moose Lake in east-central British Columbia and adjacent to the Alberta border (Le. eastern Mount Robson Provinciat Park; Fig. t.4). Moose take is t5.4 km2 in area, and its high resolution Holocene sedimentary record is one of the reasons for focusing on this locality (Desloges and Gilbert, 1995; Desloges, 1999; Raymond, 2000). The contributing drainage basin is 1640 km2in area with elevations ranging from 1032 to 3426 rn a.s.1. Glaciers currently occupy -3% of the watenhed by area with most conœntrated in the Moose River sub-basin and in the vicinity of the Mt. Robson massif. Contrasting elevations and glacier cover are thought to be responsible for dissimilar specific stream discharge and specific sediment yields from the Moose River and upper Fraser River sub-basins. FIGURE 1.3 South eastward view of the Fraser River just upstream of Moose Lake, east-central British Columbia. Figure 1.4 Location of study area. The upper Fraser River drainage basin is underlain by Upper Proterozoic to Middle Cambrian sedimentary and metasedimentary rocks including pelites, quartzites, conglornerates and carbonates (Mountjoy, 1980; Murphy, IWO). The regional fold and fault structure results in an uneven distribution of these lithologies across the drainage basin, and the possibility of distinguishing source areas based on sediment composition. The rugged relief and structural orientation of the valleys means that the area contained important sources of 'Montane" ice during the Pleistocene (Bobrowsky and Rutter, 1992). Deglaciation of the main upper Fraser River valley probably took place at or before 11.900 I120 yrs BP (GSC-3885; Levson and Rutter, 1989) shortiy after Cordilleran ice retreated westward from the Jasper area and local ice retreated into tributary valleys (Bobrowsky and Rutter, 1992). Within the upper Fraser River watershed, Tonquin Pass was apparently ice- free some time before 9660 i 280 yn BP (BGS-465; Kearney and Luckman, 1983) while Holocene sedimentation in Moose Lake began some time prior to 9120 I80 yrs BP (T0-6480; Desloges, 1999). Pleistocene glacial deposits currently occupy about 15% of the basin area predorninantly along the main valley floon. The Holocene glacial history of the region has been summarized by Luckman and Osborn (Luckman and Osborn, 1979; Osborn and Luckman, 1988; Luckman, 2000). Following a wann, dry Hypsithemal, climatic deterioration leading to Neoglacial conditions is thought to have begun between 4000 and 5000 yrs BP based on regional treeline and pollen fluctuations (Kearney and Luckman, 1983; Beaudoin, 1986). ovemdden and buried organic materials (Gardner and Jones. 1985), and higher energy lacustrine sedimentatbn (Dirszowsky and Desloges, 1997; Leonard, 1997; Desloges, 1999; Leonard and Reasoner, 1999). Periods of distinct glacier advance appear to include 3700-3500 yrs BP, 3300-2500 yrs BP, ca. 1550 yn BP, AD 1142-1350, the early 18" and early to middle 19" centuries (Luckman, 1986, 1994, 1995, 2000; Luckman et ai., 1993). Desloges and Gilbert (1995) showed that above average sedimentation rates, possibly associated with glacial advance, occurred in the proximal area of Moose Lake between AD 1450-1530, AD 1740-1870, and AD 1910-1 950. Leonard and others (Leonard, l986b, 1997; Leonard and Reasoner, 1999; Luckman, 2000) found a close correspondence between elevated rates of lacustrine sedirnentation and regional glacier advances (ie. 1200's. early 1300's. early 1700's. 1800's) in the much more heavily glaciated Hector Lake drainage basin approximately 170 km to the southeast of Moose Lake.
1.4 ORGANIZATION OF THE THESIS This thesis consists of six chapters, four of which deal with different aspects of sediment routing and paleoenvironmental reconstruction in the upper Fraser River watershed, and which are intended for publication with minor modification. This introductory chapter and Chapter 6 provide an overview and summary of the main results of the project as well as implications and suggestions for further research. Of the four substantive chapters, Chapter 2 deals with the identification of key sediment source areas, and the contemporary production and delivery of sediment from glacial and non-glacial sources in the contributing basin. Effort is devoted to the identification and assessrnent of geochemical indicators useful in discriminating sources in this area of sedimentary bedrock. Chapter 2 also considen how relative source area contributions refiect hydroclimatic conditions in the drainage basin. Chapter 3 considen the relation between particle size and geochemical composition of the fluvial and lacustrine sediments examined in order to assess whether sediment sorting processes and sampling May influence provenance determination. The particle size characteristics of Fraser River bed material and the influence of tributary contributions are also summarized in this chapter. Chapter 4 examines the partlioning of sediments between modem river, ftoodptain and take depositional envimnments in arderto assess whether provenance information is transferable between sites, or whether sediment sorting or depositional or post-depositional alteration of sediments may cornplicate the source signal. Examination of floodplain sediments is intended to assess the utility of this sedimentary environment for paleoenvironmental interpretation and the possibility of post-depositional alteration of sediments over the long-term. Chapter 5 contains a detailed reconstruction of the Moose Lake-Fraser River delta, its alluvial cap and encapsulating valley-fill, and inferences regarding both late Pleistocene and Holoœne environmental change. Use is made of provenance information derived from earlier chapterç. Implicaüons for sediment deposition in Moose Lake and the interpretation of Ïts record are discussed. SEDIMENT SOURCES AND MlXlNG IN THE UPPER FRASER RIVER WATERSHED, EAST-CENTRAL BRITISH COLUMBIA
2.1 INTRODUCTION Inferences concerning past and present geomorphic processes and environmental change are frequently made on the basis of the properîies of sediment, either in active transport (e.g. Walling et al., 1993; Collins et al., 1997a) or in various depositional settings (e.g. Passmore and Macklin, 1994; Collins et al., 1997c; Owens et al., 1999). In the latter case, environmental interpretations are usually based on carefully docurnented lithostratigraphic variations and chronologie control; however, a complete understanding of past sedirnentary environments is possible only if the origin of the sediment is known. In some cases, physical and chernical properties of sedirnent can be linked directly to mode of production, as in the case of quartz microtextures due to glacial erosion (e.g. Mahaney, 1998; Mahaney et al., 1991) or clay mineralogical composition and sediment chemistry due ta weathering (e.g. Mackereth, 1966; Singer, 1984; Curtis, 1990). More generally, provenance techniques atternpt to identify source locations and possible material type, and are thus most commonly based on sediment composition (Walling et al., 1993; Johnsson, 1993). When conditions are favourable, it may be possible to determine source condition, spatial provenance and mode of sediment production frorn the same information. Smith (1978). Leonard (1986a.b) and Leonard and Reasoner (1999), for example, were able to identify glacially derived sediments in an alpine lake because glaciers were associated with carbonate bedrock in the contributing basin. Similady, sediments have been traced to specific land uses andfor fluvial proœsses based on the association of these source types with unique substrates or parent matenals in the contributing basin (Campo and Desloges, 1994; Walling and Woodward, 1995; Owens et aL, 1999; Kelley and Nater, 2000). Alternatively, a nested approach may be adopted in which the source is first narrowed by general area and then further by sub-area or process within the area of interest. For example, Collins et al. (1997a, b, c) found they could better identify sediment source types such as woodland. pasture, cultivated surfaces, and channel banks once they had narrowed the spatial provenance of suspended sediments and floodplain deposits to particular UK sub-basins. Such an approach is especially important in medium to large sale drainage basins where general source types or production mechanisms may be widely distributed in a complex pattern (Walling and Woodward, 1995). The main purpose of this investigation is to establish source areas, sediment routing and sediment mixing behaviour in the upper Fraser River drainage basin of east-central British Columbia. Information on sediment sources and routing specific to the watershed is critical in interpreting changing sediment yields in response to hydro-climatic forcing and glacial activity both recently and through the Holocene. In the past, lithological (e-g. Basu, 1976; James. 1991). mineralogical (e.g. Klages and Hsieh, 1976; Desloges, IWO; Woodward et al., 1992). and geochemical (e.g. Cullers et a/.,1987, 1988; Cullers, 1994; Collins et al., 1997a. b, c; Walling et al., 2000) parameters have al1 found utility in tracing fluvial sedirnents in part depending on the sediment particle sizes of interest. In this study. elemental composition is emphasized for its applicability to fine particle sizes, those which are theoretically most mobile and easily distributed or redistributed in a glacier-fed fluvial system. A broader objective of this paper is to identify the geochernical indicaton of sediment origin that are useful in this area dominated by sedimentary bedrock and which may have broader a pplicability. The high relief upper Fraser River watershed has been subject to ongoing glacier activity sinœ the late Pleistocene. It is hypothesized that this activity is responsible for elevated fine sediment delivery to downstream depositional sites. The lacustrine and alluvial sedimentary records of the upper Fraser River watershed are the subject of ongoing paleoenvironmental investigation, and have pointed toward pronounced variations in glaciation in the drainage basin (Desloges and Gilbert, 1995; Dirszowsky, 1998,2001 ; Desloges, 1999; Luckman. 2000). Emphasis in this study is therefore on discriminating glacial and non-glacial sources and sediments so they may be identified in the paleodeposiüonal record. 2.2 STUDY AREA The area considered in this study is the Fraser River watenhed above Moose Lake, British Columbia and abutting the continental divide to the east. The drainage area (Fig.2.1) is 1640 km2and includes part of the highly glaciated Mt. Robson massif (e.g. Reef Icefield). Several smaller glaciers occur along the divide (e-g. Scarp Glacier and Bennington Glacier) and on isolated peaks in the Selwyn Range to the southwest and unnamed ranges to the northeast. Basin-wide glacier cover is estimated at just over 3%, but the greatest concentrations occur within the Moose River sub-basin (Fig. 2.1). Elevation in the watenhed ranges from 1032 to 3426 m a.s.l., though moût valley-ridge relief ranges from 800-1200 metres. Due to high elevations and the northwest-southeast structural orientation of valleys, the region was an important source of Montane ice during the Pleistocene (Bobrowsky and Rutter, 1992). Geomorphic features near the Jasper town site 30 km east of the study basin and the Athabasca erratics train extending towards Hinton, Alberta, indicate that Montane and Cordilleran ice flowed eastward through the upper Fraser area and across Yellowhead Pass into Alberta during the early and possibly late Wisconsin (Roed et al., 1967; Roed, 1975; Levson and Rutter, 1988). Prior to complete deglaciation, however, ice evidently flowed westward from local centres and deposited the moraine-like material which now impounds Moose Lake (cf. Desloges and Gilbert, 1995). Pleistocene surficial deposits occupy about 16% of the upper Fraser River watershed by area and consist predominantly of outwash and kame materials concentrated along the main valleys. In most valley bottoms, Pleistocene materials have been scoured and ~eplacedor covered over with a relatively thin veneer of Holocene alluviurn and colluvium. Within the watershed. a trellised drainage pattern associated with the local fold-fault structure of the underlying Precambrian and lower Paleozoic bedrock (Mountjoy, 1980; Murphy, 1990) means that several of the most important tributaries join a single 27 km section of the Fraser River trunk. The most important tributary is Moose River which joins the Fraser River about 3 km upstrearn of Moose Lake. The Moose River drainage basin (463 km2total) is more heavily glaciated (-6% by area) than that of the uppermost Fraser River, with most ice concentrated in sub-basins of Resplendent Creek, Upright Creek and Steppe Creek (Fig. 2.1). Major tributaries
joining the main reach of the Fraser River include Sleeper Creek (80 km2 basin), Grant Brook (112 km2 basin), and Ghita Creek (88 km2 basin). Yellowhead Lake effectively traps sediment originating from about 8% of the total watenhed in the area of Rockingham Creek. Geikie Creek drains the more heavily glaciated areas along the continental divide while Tonquin Creek is the main tributary draining the unglaciated areas in the southeast. Bedrock in the drainage basin consists of exposures of upper Proterozoic Miette Group (53% by area), Lower Cambrian Gog Group (26%) and a Cambro- Ordovician sequence of carbonates (21%; see Fig. 2.2; Mountjoy. 1980; Murphy, 1990). Middle Miette strata are dominated by granular conglornerate and sandstones, mainly feldspathic, interspersed with recessive silty chloritic pelites and very rare quartzites and carbonates (white marble, dolomitic sandstone; McDonough and Simony, 1986, 1988). Upper Miette strata consist mainly of aluminous pelites. some calcareous, with minor black Iimestone and coarse sandstone low in concentrations of feldspars and micas (Deschesne, 1990; Deschesne and Mountjoy. 1990). Large-scale thrust faults (e.g. Selwyn Decollement, Rockingham Creek Fault) occur in the Miette strata and are associated with localized graphite- or chlorite-rich phyllites and phyllitic schists. Gog Group material consists almost entirely of quartz arenite, except at the base where beds are more feldspathic, and contain siltstone and shale interbeds (Fritz and Mountjoy, 1975). Carbonates may comprise up to 15% of Gog strata by thickness. Lower to Middle Cambrian carbonate strata contain more limestone than dolomite while calcareous shales and occasional saodstones probably constitute iess than 20% of their thickness (cf. Fritz and Mountjoy. 1975; Mountjoy, 1980). The structural setting and erosional history of the upper Fraser River area has resulted in varying bedrock exposure within the various drainage sub-basins (Fig. 2.2). Since the Fraser River valley follows the relatively young Moose Lake- Chatter Creek fault (Deschesne and Mountjoy, 1990), sub-basins on either side are lithologically distinctive. Those on the south side (e.g. Sleeper Creek, Ghita Creek) drain areas alrnost exclusively underlain by Miette Group rnaterials. Upper Miette strata underlie (out of sequence) Middle Miette strata due tu low angle thnisting, and are thus exposed further to the south along the drainage divide and headwater FIGURE 2.2 Simplifieci surficial and bedrock geology of the upper Fraser River watershed (after Mountjoy, 1980; Murphy. 1990). areas. Sub-basins draining the north and east side of the Fraser River are composed of sequential Middle and Upper Miette strata and. locally, Gog Group strata (i.e. within the Geikie Creek, Cottonwood Creek. and Grant Brook sub- basins). In the Moose River-Resplendent Creek drainage, Lower to Middle Cambrian carbonate strata are important, cropping out over about 61% of the area. These carbonate formations are mapped in only one other sub-basin, Grant Brook, where they comprise only 6% of the area. Miette and Gog group materials crop out in about 7% and 32% of the Moose River drainage.
2.2.1 Hydrology Annual precipitation recorded at Red Pass Junction (western tip of Moose Lake; Fig. 2.1) was recorded between 1931 and 1969 and averaged 742 mm (Environment Canada, 1981). Although precipitation levels in the Rocky Mountains have decreased since that period (Young, 1990), the value is likely an underestimate of basin-wide precipitation due to the higher elevations throughout. Stream discharge data recorded continuously from 1956 to 1995 for the Fraser River at Red Pass (8KA007) and Moose River at Fraser River (8KA008) (Environment Canada, 1W6), reveal a typical nival regime. Peak snow-melt discharges occur in late May and early June. Persistently high discharges, or a second peak. occur through late June or July due to contributions from glacier melt. Mean annual discharge for the Fraser River (including Moose River) and Moose River (alone) are 46.1 m%' and 14.3 m3s" respectively. Average annual runoff is approximatdy 870 mm. Due b siightiy higher ekvations and glacier muer, Moose River contributes 31% of the runoff on average, while draining 27% of the area. During peak flows (Le. mean annual flood), Moose River contributes approximately 36% of total runoff.
2.3 SAMPLING AND ANALYTIC METHODS Direct sampling of sediment sourœ material in a large and mostly inaccessible watershed such as the upper Fraser is difficult; however. an alternative approach to characterizhg sourœ areas is to examine the integrated stream sediment load emanating from sub-basins of interest. In this study four tributary junctions along the main reach of the Fraser River were initially selected to represent basins of distinctive geology and degree of glacier cover. In each case, grab samples were obtained fom the shallow sub-surface layer of exposed sand or gravel ban just upstream, just downstream. and within tnbuiary mouths wherever possible. Additional, more remote reaches or tributaries were sampled where access was possible (Fig.2.1). Only the fine (< 63 ym) fraction was returned to the laboratory for analysis due to its presumed greater mobility, propensity for mixing, and possible utility in later paleoenvironmental interpretation of downstream lake and floodplain deposits. The Moose River-Fraser River junction was examined in detail, with 12 "replicaten samples obtained from each branch plus three sites downstrearn of the junction. Moose River (MW) and upper Fraser River (FRu) samples were obtained at widely spaced positions on lateral or medial bars within 600 m of the junction (see Fig. 2.1 inset). Downstream (FDS) of the junction, replicates were taken at evenly spaced (-50 m) positions along the perimeter or major axis of one medial and two lateral bars up to 4 km away. The collection of replicates was intended to assess sampling error and within-site variability, so each of the above 60 samples was subjected to separate particle size and geochemical analysis. At the 21 sites further upstream sampled, replicates also were obtained in the field to minimize sampling error and maintain comparability, but these were combined for laboratory analysis. Particle size analysis of the < 63 pm fraction of sub-samples was detemined using a Sedigraph 5100 following dispersion using dilute (0.05%) sodium hexametaphosphate, sanification and sieve separation to remove residual sand or gravel material. Duplicate sub-samples were prepared for geochemical analysis. Geochemical analysis of selected major, minor, and trace elements was conducted using Instrumental Neutron Acüvation Analysis (INAA) at the SLOWPOKE nuclear reactor formerly at the University of Toronto. In order to improve representativeness, relatively large sample weights of 750-850 mg were used (Hancock et al., 1988). Elements which produce short-lived radio-isotopes (Le. Al. Ca, CI, Cu, Dy, 1, K. Mg, Na, Ti, and V) were detennined frorn sarnples irradiated serîally for 5 minutes each at a neutron flux of 1.O x 10" n ~m-~s''.After the very short-lived 28AIhad decayed to acceptable levels (approximately 18 minutes), samples were counted for 300 seconds using on-site gamma-ray spectrometen (for K the optimal delay was approximately 24 hours). Relevant gamma-ray peak areas and levels of background radiation were measured, and elemental concentrations calculated based on appropriate standards (Hancock et ai., 1988). The same samples were later batch irradiated for 16 hours at a neutron flux of 2.5 x 10" n cm- *s-',and after a 7 day delay counted for 6000 seconds to determine As, Br, Fe, La, Na, Sb, Sc, Sm, U, and Yb. Sodium was used to cross check with the first analysis. After 14 days the samples were recounted and the concentrations of Ba, Ce, Co. Cr, Cs, Eu, Fe, Hf, Lu, Nd, Ni, Rb, Sc, Sr, Ta, Tb, and Th calculated with Fe and Sc providing a cross check. On average, technical precision is better than 5%. Measurement accuracy appears to be within 10% based on replicate sample analysis and repeated counts. Most error observed dunng analysis could be traced to sample inhomogeneity and sample-detector geometry and thus corrected for by reorienting samples, recounting and averaging the results. Equivaient samples (5- 10 g) were subjected to simple loss on ignition (LOI) procedures in order to estimate organic matter content (Dean, 1974). To complement the geochemical analysis, clay fraction mineralogy and granule (2-4 mm) lithology were determined for selected samples. For mineralogical determinations, ultrasonification and sedimentation were used to separate a c 2 Fm fraction. The resultant slurries were onented on clay tiles by centifugation and x- rayed using Ni-filtered CuKa radiation after air drying, after treatment with ethylene glycol, and after heaüng at 30Q°C,50Q°C, and 550°C (Whittig and Allardice, 1986). Identification of prirnary and clay minerals, facilitated by the above treatments, was accomplished by cornpanson of calculated reflection spacings to standards reported by Brindley and Brown (1980). Limitations to this approach occur where diffraction peaks overlap in mixed samples, particularly at diffraction angles of 27" to 31' 20 (Le. feldspars, pyroxene and micas) and 6" to 9" 28 (i-e.mixed-layer clay minerals), and where minerals occur in small amounts (c 5%; Carroll, 1970). Granule llhology was assessed under 10-30X magnification and quantified using simple particle counts (n = 227-649) following sieve separation. 2.4 RESULTS 2.4.1 Fine Grained Geochemistry of River Bed Material Of the 35 elements identifiable using the INAA methods adopted, 28 were consistently found above detection limits in the majority of samples. Of these, the major elements include Al, Ca, Fe, K, Mg, Mn, Na and Ti. Trace elements include Ba, Co, Cr, Hf, Rb, Sc, Sr, Ta, Tb, Th, U, V, and the Rare Earth Elements (REEs) Ce, Eu, La, Lu, Nd, Sm and Yb. Based on these measurements, the overall composition of the samples may be estimated. and the general distribution of materials in the upper Fraser River watershed assessed. Table 2.1 summarizes the bulk composition of upper Fraser River sediments with average values for al1 samples (n = 81) and those representing the two main sub-basins (Moose River and uppermost Fraser River). Trace elements are more than twice as abundant in uppermost Fraser River samples than in Moose River samples. Similarly, the major elements, other than Si, Ca and Mg are approximately 4X more abundant in uppermost Fraser River samples. As oxide equivalents, these elements account for approximate 8% to 33 % of most samples. Carbonate values indicated in Table 2.1 are based on the assumption that almost al1 of the Ca measured is associated with either calcite (CaCO,) or dolomite (CaMg(CO,),), a possibility suggested by the strong spatial association of Ca and Mg-rich samples with mapped carbonate lithologies. The clear division of Ca and Mg with respect to major sub-basin is illustrated in Figure 2.3 which plots ail samples with respect to position upstream or downstream of the Moose River-Fraser River junction. Calcium values in Moose Rhsuh-basin samples are on average more than 5X those in sarnples from the uppermost Fraser River sub-basin where only minor carbonate bedrock sources occur (e.g. Grant Brook; Fig. 2.2). It is unlikely that al1 Ca in uppemost Fraser River sub-basin samples represents carbonate; however, given the very low concentrations in these samples, a carbonate- assumption introduces little error (< 2%) to bulk composition estimates or sub-basin comparisons. The regression Iine in Figure 2.3 is based on Moose River sub-basin samples only and thus approximates the relationship between Ca and Mg in carbonate materials of the upper Fraser River watershed as a whole. That Fraser River sub- Table 2.1 Proportion of major constituents in upper Fraser River bed material sediments (< 63 prn fraction) based on elemental geochemistry (assumptions and derivations in text).
Mean S.D." S.E. C.V.
Mean S.D. S.E. C.V.
Mean S.D. S.E. C.V.
') 'Silica" is calculated as the resiâual cornpanent and may contain some trace elements. accessory minerals plus intercrystalline &O and organic matter (see text for discussion). ") S.D. = standard deviation: S.E. = standard emr; C.V. = coefficient of variation. t)Organic matter detemined as loss on gnrtion at 550'C for 1 hr (Dean, 1974). / - Fraser River Samples / 0 Moose River Samples / 0 Downstrearn (muced) Sampbs
FIGURE 2.3 Calcium-magnesium relations for upper Fraser River and Moose sub-basin and mixed river bed matenal (< 63 prn fraction). Mg and Ca in Moose River sediments are highly correlated (r = 0.97) and assumed to represent dolomite and calcite almost exclusively. Regression line (dashed where extrapolated) shows that Fraser River and mixed sediments contain Mg from additional sources. basin and mixed downstream samples plot above the line is expected, and implies that additional Mg is present in silicate or other minerals. If so, the regression equation may be used to allocate Mg to either carbonate or non-carbonate (oxide) rninerals and this was done in deriving the bulk compositions in Table 2.1. Similarly, the partitioning of carbonate is based on the availability of Mg and the molar ratios of Ca and Mg in calcite and dolomite. Although limestone is mapped more extensively in the upper Fraser River drainage basin. dolomite appean to be the more abundant carbonate mineral (-60:40) in the < 63 pm fraction of downstream fluvial sediments. Carbonate as a whole clearly makes up the bulk of the fine fraction in sediments sampled from the Moose River sub-basin (up to 75% in Resplendent Creek samples), and on average, carbonates are -63X more abundant in Moose River sub-basin samples than in uppemost Fraser River sub-basin samples. Sample mass not accounted for above should consist mainly of sili~(SiO,) plus additional trace elements (negligible mass), non-silicate minerals (e-g. phosphates), organic matter, and H,O contained within mineral structures. LOI (550°C)results indicate that average organic matter content accounts for no more than 2% of most samples by weight. Inter-crystalline H,O cannot be fully assessed from the available data; however, its presence is a function of clay mineralogy and may range up to 5% by weig ht in clay-rich materials (Dean, 1974). Samples analyzed here contain < 20% clay in al1 but one case. Since minerals other than silicates are known to be rare in the upper Fraser River watershed. silica appears to accwnt for just under 50% of the c 63 ym fraction of fluvial sediments examined, with estimates that range from 15% to 70%. Table 2.1 shows that most variation in the data set is attributable to the carbonate fraction and to a lesser extent the total oxides.
2.4.2 Factor Analysis of Geochemical Data Initial assessment of the geochemical data set revealed a complex set of correlations between elements which wuld reflect combinations of chernical affinity, mineralogid and lithological associations, and other factors such as the spatial distribution of sediments. R-mode factor analysis was employed as a means of clanfying these associations and their relation to drainage basin attributes and source area. The analysis presented here was made with principle axis factor extraction followed by oblique rotation using SPSS. Oblique rotation was used since there is no reason to expect mineralogical or lithological associations should not to be correlated (Tabachnick and Fidell, 1989). The above techniques were first applied to the raw element data to test for general drainage basin associations. Elemental concentrations were then recalculated relative to the non-carbonate non- silica fraction of the sediment to assess whether or not carbonate andior silica overwhelm variation in less abundant sediment constituents and thereby obscure additional source information. In a proportional data set. variation in dominant elements tends to induce correlation among minor constituents (Rollinson. 1993).
2.4.2.1 Raw Element Factors Table 2.2 summarizes raw and adjusted element faaor solutions in ternis of loadings and cornmunalities (i.e. the proportion of an element's variance explained by the factors). In the case of the raw element data. three factors account for the majority of the common variance in the data. Since some correlation exists between Factors land 2 (r = 0.60) and Factors 1 and 3 (r = 0.38). the proportion of variance explained by each factor cannot be detenined exactly. Nevertheless. the factors are well defined by strong and unambiguous loadings, and account for more than 80% of the variance of al1 individual elements other than Hf. Nd and Sr. The first factor is defined by the two most abundant elements measured, Ca and Al, plus seueral less common elements assaciated with one or the other (Table 2.2). The factor is interpreted as representing both carbonate (Ca. Mg and Sr) and aluminosilicate (Al. K, Fe) minerals. with positive and negative loadings reflecting an inverse spatial distribution within the upper Fraser River watershed. This source- related signal is reflected in the factor's strong correlation with Miette Group (positive) and Carbonate (negative) bedrock exposure upstream of each sampling location (Table 2.3). Aluminosilicates denved from Miette lithologies may be primary (Le.feldspars) or secondary (ie. clay minerals), but the latter are most common in the parent rock of the basin. Less abundant elements positively loading on Factor 1 could represent accessory minerals (associated lithologically or spatially) ; however, Table 2.2 Loadings (pattern matrix) for R-mode factors based on raw and adjusted elemental data in upper Fraser River watershed stream bed material samples. RAW DATA NON-CARBONATE NONSlLlCA DATA
Factof Facto f Ct C+ i 2 3 1 2 3 4 5 6
7 Exmdion Method: Principal Aris Factoring; Rotation Methad: Oblimin with Kaiser Nonnalization. 5) Bold indicates hghest loadings used for interpretation. 7) C = extradion cornmunaliaes (proportion of eiements variance accounW for by factors). :) :) SS = sums of squad loadings (approxirnateiy proportional to variance explained). Table 2.3 Speannan's rank correlation coefficients detemined for sample factor scores (c63 pm fraction composition) and the proportion of bedrock type upstream of sample location. RAW DATA NON-CARBONATE NON-SILICA Lithology Factor Factor
Miette Comlation* ,769 A88 AS1 -241 -.573 -.IO7 -.375 -. 150 -.715 Sig. (2-taileâ) .O00 .O00 .O03 -134 .O00 311 .O17 .355 .O00
Go9 Correiation -.350 -.330 -.280 -.181 .236 .457 .225 -.O59 A02 Sig. (2-tailed) .O27 .O37 ,080 .263 .l42 -003 .l63 .Tl7 .O1 0 Carbonate Correlation -.855 0.749 -.369 -A97 ,663 .O00 A41 .179 .729 Sig. (2-tailed) .O00 .O00 .O19 .223 .O00 .999 .O04 .270 .O00 .) in this analysis n=40 since factor scores were not generated where factor elements were missing for a particular sample; bold indicates signficant correlation at 0.01 bvel. the very strong factor loadings suggest the elements are cornponents of the aluminosilicate mineral group, substituted in the case of primary minerals, or adsorbed in the case of clay minerals. The second factor explains alrnost as much variance as the first, and is made up of the REEs measured, plus Ti, Hf, Ta, Th, and U. Similar behaviour amongst the REEs is expected due to strong chemical sirnilarity (e.g. high ionic potential). Along with U and Ta, these elements are widely distributed as trace elements in most crustal materials, but typically concentrate in heavy minerals (McLennan and Taylor, 1985). Thorium, Hf and Ti commonly occur in the heavy minerals monazite, zircon and rutile/ilmenite/titanite respectively (Klein and Hurlburt, 1993). Except under acidic conditions, al1 of the Factor 2 elements are immobile and upon weathering, usually retained in sediments by adsorption on clay minerals (Cullers et al., 1987). It is thus not surprising that Factor 2 correlates moderately with Factor 1, and exhibits similar correlations with the Miette and carbonate lithologies (Table 2.3). The third factor, defined by Co and Mn, explains much less variance than the previous two and behaves independently. Although the two elements are chemically similar in tans of ionic radii and valence state, they cannot be interpreted mineralogically from the available data. The factor is positively, but weakly conelated with Factor 1 and Miette Group bedrock (Table 2.3) and not associated with carbonates. Sarnple factor scores indicate that the above geochemical and lithological associations have strong geographic significance only in the case of Factors 1 and 2. As shown in Figure 2.4, these factors taken together clearly distinguish sediments originating in the Moose River (more abundant carbonate; less abundant REEs) and upper Fraser River (more abundant aluminosilicate; more abundant REEs) sub-basins. Samples from the upper Fraser River exhibit considerably more variation, even in the case of closely spaced bar sites near the Moose River-Fraser River junction. There is a tendency for Fraser River sedirnents just above the junction to score more highly on Factor 1 (greater Al) while those further upstream score more highly on Factor 2 (greater REEs). Among the upstream samples, those from the south and central part of the sub-basin (GCu, GCt, PCt) are rnost GCUO Fraser R. ei ih-kacin miâ-upper SUU-UCIJII I Fraser 0 GCT C PCT
Moose R. A O sub-basin "4 A AyU O a aAA #cl3 O m A
-2 - 1 O 1 2 Factor 1(AI-Ca)
FIGURE 2.4 Factor score plots for upper Fraser River samples based on raw elemental data. Various symbols indicate general sample location relative to Moose River-Fraser River juncüon: Moose River sub-basin (0= upstream; H = at junction); Fraser River sub-basin (O=upstream; 0 = at junction); below juncüon (a = distal: A = proximal). distinctive (greater REEs). Samples from below the Moose River-Fraser River junction appear to show mixed basin affinity with intemediate Factor 1 and 2 scores. Two sarnples (STu. UCU) within this intermediate or "mixedn zone actually originate in the uppemost Moose River basin where carbonate and non-carbonate (Gog) bedrock types are exposed in about equal proportions (Fig. 2.2).
2.4.2-2 Adjusted Data Factors Equivalent factor analysis of elemental concentration in the non-carbonate, non-silica fraction resulted in the extraction of six factors which account for approximately 85% of the total covariance. Because these factors are relatively uncorrelated (r < 0.31 in al1 cases), their relative contribution to covariance explained may be estimated approximately. However, rnuch less of the individual variance of several elements is accounted for by these factors (see cornrnunalities of Ba. Na, Ta and V; Table 2.2). The first adjusted factor, explains almost 40% of the total covariance (Table 2.2). Oefined largely by the REEs, it is structurally identical to Factor 2 of the unadjusted data. Without the diluting effects of carbonate and silica, this REE factor shows no significant correlation with bedrock lithology (Table 2.3). The second factor in this analysis explains approxirnately 17% of the covariance, and is defined by positive loadings of Fe, Mn and Sr, and negative loadings of Cs, Rb and 'non- carbonate' Mg. As a group, Fe, Mn and Sr suggest that the factor may be carbonate-related (Krauskopf, l967), and this interpretation is supported by the sample factor score and lithologic conehtions (Table 2.3). Though not associated with Ca or Mg, this carbonate-related factor appears to preserve some of the carbonate-non-carbonate contrast observed with Factor 1 of the unadjusted raw data. In this analysis the sixth factor explains the next greatest amount of variance (after rotation, -15%). Defined only by Ba and K, its mineralogid significance is uncertain; however, lithological correlation (Table 2.3) suggests that it too is associated with carbonate bedrock or sources in the Moose River sub-basin where Gog lithology is also well represented. The remaining factors (3, 4,5) explain relatively little variance (-7-12%), and basad on only one or two elements each, are difficult to interpret mineralogically. Sinœ bedrock correlations are weak. these factors could be of either very localized or very widespread occurrence, and possibly chernical rather than mineralogical in nature (Le. Cr+Sc. Co-Na). Since Al is by far the dominant element in the non-carbonate, non-silica fraction, it is expected to act as a diluent in the present analysis. This may help explain its apparently independent behaviour and the weak or negative loading of most elements on the factor it defines (Factor 3). The two most discriminating factor score plots based on the non-carbonate non-silica data are shown in Figure 2.5 and reveal more detail in the spatialllithological relations of samples than those based on the raw data. Here. the Moose and Fraser sub-basins are readily distinguished based on Factor 2 (Fig. 2.5a) and Factor 6 (Fig. 2.5b). both apparently indicative of carbonate lithology. Sarnples from below the Moose River-Fraser River junction are difficult to separate from upper Fraser River samples based on these components. Among Moose River sub-basin samples, it appears possible to distinguish upstream. downstream. and to a lesser extent, eastem and western areas based on Factors 1 and 6 (Fig. 2.5b). These divisions do not necessarily correspond to particular sub-basins but do appear to reflect either distinct carbonate lithologies or contributions from carbonate and non-carbonate lithologies in the contributing area. As previously, sample STU from the extreme north of the Mooser River sub-basin is more characteristic of the upper Fraser River sub-basin due to the local occurrence of non-carbonate bedrock. Factor 1 (the REEs) does not distinguish the Moose River and upper Fraser River sub-basins in this analysis; however, as wah the unadjusted data it does appear to distinguish the south-central portion of the upper Fraser River sub-basin (Le. GCU, GCt PCt). Factor 1(REE+)
RCD E
M.R.
- 68 ma 0 GCT
-2 -1 O 1 2 3 4 Factor 1(REE+)
FIGURE 2.5 Factor score plots for upper Fraser River samples based on adjusted (noncarbonate non-silica) elernental data. Various symbols indicate general sample location relative to Moose River-Fraser River junction: Moose River sub-basin (0= upstream; = at juncüon); Fraser River sub-basin (O = upstream; = at junction); below juncüon. (a = distal; A = proximal). 2.4.3 Mineralogy and Lithology of River Bed Material 2.4.3.7 Clay Fraction Mineralogy The mineralogy of the clay-sized (c2 pm) fraction of eight samples representing several sub-basins in the upper Fraser River watenhed is summarized in Table 2.4. Among the primary minerals. quartz is dominant and appears to be more abundant in the Moose River sub-basin samples and the uppemost Fraser River sample (GCu) than in those of other tributaries feeding the main reach of the Fraser River. Calcite and dolomite exhibit the greatest variation and are found in moderate to small amounts in the Resplendent Creek and Moose River samples and Fraser River sediments sampled downstrearn of the Moose River junction. Trace amounts of calcite and dolomite are also found in the Grant Brook sample. Other pn'mary minerals (feldspars and possibly pyroxene) are present in small amounts in samples distributed across the Moose and upper Fraser sub-basins. Gypsum was identfied only in the uppenost Fraser River sample. The abundance of clay minerals varies invenely with that of the primary minerals, especially quartz and the carbonates. Mite is the most abundant clay mineral overall and especially in samples from tributaries entering the Fraser River from the southwest (GCt SCt). Samples from the Moose River sub-basin or downstrearn of the Moose River junction contain less kaolinite, halloysite, illite and vemicu lite than most uppermost Fraser River samples. Within the uppermost Fraser River sub-basin, Grant Brook apparently transports the largest concentration of 1:1 clay minerals (kaolinite plus halloysite) and least amount of illite. These results canfirm the dominance of secondaq aiuminositicates in the finest fraction of bed materials at least, and their general association with Miette Group bedrock.
2.4.3.2 Clast Lithology Representative samples in which the Iithology of the granule fraction was examined are summarized in Table 2.5. Most particles in this size range are lithic fragments that rnay or rnay not be unifom in composition. but may be classified according to four main categories. Fine-grained, clastic lithologies consist of massive appearing light grey siltstone and numerous varîeties of shale disthguished by colour. Limestone and dolomite occur as either light or dark grey massive Table 2.4 Mineralogy of the < 2 Mm fraction of bed material sediments from the upper Fraser River drainage*
Primant Minerals Clay Minerals Sample & Location Q O F P CaDoGy K H UM B V Ch PCU upper Fraser R. xxxxtr-- x xxxxxtrxx GCu upper Fraser R. XXXXxx--- XXXXXXX GCt Ghita CC. xxtrxx--- xx tr xxx x tr x GBt Grant Br. xx - x x tr tr - XXXXXXXX SCt Sleeper Cr. x~trxX--- )a - xxxx x X RCtl Respendent Cr. xxxV X XXXXX- xtfxtr - x MRt Moose R. xxxtrxxxx- x trmcx trx FDS3 Fraser R. (d.s) xxxtrxxxx~- xxxtr- x ') Mineral abundanœ is based on relative peak height on diffractograms: nit (-); minor amount (tr); small amount (x); moderate amount (xx); abundant (xxx). Clay minerals are: kaolinite (K), halloysite (H), illitdmica (IIM), background or mixed-layer. (B), vermiculite 0,chlorite (Ch). Other minerals are: quartz (Q), orthoclase (O), plagioclase feldspar (F), pyroxene (P), calcite (Ca), dolomite (Do), gypsum (Gy). Pyroxene is probably overestimated as residual peak height at 27" to 31" 20 after feldspars and illite accounted for. Sample locations given in Fig. 1. Table 2.5 Percent lithologic composition of the granule fraction (2-4 mm) for selected upper Fraser River and tributary channel bed material samples. Limestone and Shale and Quartzite and Other Sarnple & Location n Oolomite Siltstone Sandstone Metamorphic5 -- .. . - . GCU uppermost Fraser R. ÛBf Grant Br. FRU Fraser R. (above)' RCfl Resplendent Cr. MRT Moose R. (a bove) FOS1 Fraser R. (belo~)~ FOS2 " * " FDS3 " " " *) includes phyllite, mica çchist and intermediate grades; 3 just above the M-se River-Fraser River junction; 5) sites below Moose River-Fraser River junction. particles rarely containing fossils. Quarhite and sandstone fragments identified consist dominantly of mixed, multicrystal or cemented grain foms, except in the uppermost Fraser River material (GCu) where monocrystaline forms are common. Other metamorphics range from dark gray phyllite to micaceous schists. but intermediate grades are common. In one sample group (GBt) schistosity was common in grains classified as quartzite. The few samples tabulated give a good idea of the range of composition found in the coarser fractions of upper Fraser River sediments. The uppermost Fraser River bed material examined here (GCu, FRu) is dominated by metasediments, with quartzite and sandstone at elevated levels downstrearn. Part of this increase may be attributable to contributions from Grant Brook where quartzite is dominant. The Grant Brook sample is also distinctive in having high quantities of shale and siltstone which may correlate with the 1:1 clay minerals. In Moose River sub-basin sedirnents, limestone and dolomite are clearly dominant. especially in the Resplendent Creek sample (RCTI). Samples collected downstream of the Moose River-Fraser River junction contain intermediate quantities of al1 constituents, but in variable proportions. As might be expected due to resistance to weathering, quartzite and silica-rich sandstone are the most widely distributed lithologies in the granule sire range.
2.4.4 Bed Material Mixing at Tributary Junctions 2.4.4.1 Distinction of Moose River-Fraser River Sediments Sediment mixing at the MmeRiver-Fraser Rivw junction is examined in detail because it represents contributions from the largest, most readily distinguishable sub-basin source areas and is least complicated by nearby tributaries. In order to derive estimates of relative sediment contributions from the two sub-basins it is necessary to assess sediment composition variation and sampling error in the vicinw of the junction. At the Fraser River (FRu). Moose River (MRt) and downstream sites (FDS1 and FDS3; n = 12 in al1 cases) average coefficients of variation for al1 elements are 0.22, O. 12, 0.1 9 and 0.1 1 respectively. Higher values occur with particular elements when they are a relatively minor constituent at a particular site (e.g. Ca in Fraser River samples). A one-way analysis of variance (ANOVA) was used to test whether statistically significant differences occur in the average concentrations of elements between sites, and if so, for which geochernical constituents. In the case of the raw elemental data, the nuIl hypothesis (no difference between sites) was rejected for al1 elements other than Hf (a = 0.05). Post-hoc multiple comparisons using Scheffe's test indicate that the sediments of the two tributaries aiways have significantly different concentrations. while downstream (mixed) sediments may not be distinguisha ble. Based on the noncarbonate, non-silica fraction, only 14 elements (Al, Ba, Co, Cr. Cs, Fe, K. Mg, Mn, Na, Rb, Sc, Sr, and V) show significant difierences even behrveen the two tributaries. The above results indicate that many individual elements should be useful in estimating mixing of sediments from the Moose and Fraser tributaries; however, the most useful are those exhibiting maximum between-site differences in coccentration and minimum within-site variability. In order to optimize these characteristics and protect against possible error in any given measurement, discriminant functions (i.e. linear combinations of elements) were derived, which take advantage of the multivariate nature of the data. The aim in this case was not to classify downstrearn samples with respect to a particular source area in the usual sense (discriminant function analysis), but to devise composite indices which exploit the distinctiveness of sub-basin source areas and can be used in mixing calculations. In this study. a stepwise procedure within SPSS was used to identify the 6-8 elements in each data set (adjusted or unadjusted) which best disting uish sources.
2.4.4.2 Sediment Mixing Calcuiations In the simple case of two tributary sources, a mixing mode1 can be defined by the following equation: Ptl Cti + Pn CE = C* (1 where Pt, and P, are the proportions of some sediment component contributed by tributaries #1 and #2 respectively, and C,, , C, and C, are the concentrations of the same sedirnent component measured in sarnples from the two tributaries and from a mixed-sediment site downstream. Given that P, + P, = 1. the above equation can be reamnged to produce: and
In general, the accuracy of proportions determined from eqs. (2) and (3) is maximized when the differenœ between C,, and C, is great; however, the precision of the original measurements must also be considered. Where error in the original measurements is random and independent. standard deviations may be combined in "quadraturen to detenine the enor propagated. In the present case, this appioach is too liberal because C,appears twice in each caiculation. A conservative estimate (i.e. maximum) of the standard deviation of calculated proportions is therefore detemined by directly surnming error associated with the numerator and denominator of eqs. (2) and (3). It is found that where within-site precision is good, the conservative and liberal estimates of error tend to converge. Mixing calculations for Moose River and Fraser River < 63 Vrn fraction bed material based on individual elemental concentrations and discriminant scores are shown in Table 2.6. Overall, a wide range of proportions is obtained depending on the element considered; however, values are consistent where propagated error is le& In the case of the raw data, Al, Ca and especially Na yield the most precise estimates of mixing, with standard deviations (a) ranging from 0.06 to 0.1 3. In contrast, the REEs plus Co, K, Mn, Ta, Tb, Th, Ti and U have standard deviations of the mixing estimates ranging from 0.16 to 0.59. Hafnium concentrations, which failed to distinguish the two tributaries eariier, do not yield valid proportions. When non-carbonate, non-silica data are used to estimate mixing proportions, the precision of the estimates decreases several fold (O up to 4-23), and in most cases invalid results are obtained. The problem is traced to high variabiiity in the Table 2.6 Calculated proportions of c 63 prn fraction bed material derivecl from uppemost Fraser River in samples from two sites downstream of Moose River based on raw and adjusted elernent data.*
ELEMENTS RAW DATA NON-CARBONATE NON- SIUCA
FDS1 FDS3 FDSl FDS3 0.65M.ll 0.66 M.22 0.71 N.10 0.64 I0.47 0.88M. 19 0.71 î0.23 0.73N.23 0.62 IO.% 0.58 M.36 0.64 M.18
0.54a.32 0.53*0.31 0.54 S.42 0.76 20.20 0.81 a.59 0.67iû. t O 0.70 îo.43 0.67 M.27 0.67 î0.20 0.52 20.34 0.83M.25 0.62 M.36 0.56 iû.34 O. 5s a.45 0.46 iCl.33 0.45 M.29 0.17 îû.17 0.38 M.38 0.62 M.15
') calculations were not made for elements exhibiting no significant difference between tributaries; '-' indicates calculated values fall outside valid range 0.0 - 1 .O. 5) D.F.= linear combination of ekments mat maximite differençes behween sub-basins (i-e. discriminant fundions); for raw data: DF = 1 1.9 Ba + 2.3 Ca + 5.1 Mg - 7.1 Nd - 5.1 Sr + 23.0 fi + 8.5 V; for adjusted data: DF = 11.8 Ba - 4.3 Cr + 9.4 Cs + 17-1Fe + 4.0 Mg - 27.1 Sr - 10.9 V. measured or adjusted concentrations and poor contrast between sites. The most useful element in the non-carbonate non-silica fraction is Sr (a = 0.10 - 0.27). Based on discriminant scores, the proportion of total < 63 pm fraction sediment contributed by the uppermost Fraser River ranges from 28 112% at FDSI to 62 11 5% at FDS3 four km downstream of the junction (Table 2.6). These results are comparable to the most precise single-element based estimates with Iittle or no loss of precision. The greater contrast gained by discriminant analysis using several elements more or less offsets the effect of including less precise element measurements in the calculation. Based on raw element concentrations (only short- lived isotope producing elements were analyzed at FDS2), the contribution of uppermost Fraser River sediments at FDS2 appears to be higher than at the surrounding sites (-76%). These varying proportions indicate initial domination of the c 63 vrn fraction by Moose River inputs. but ultimate dominance by uppemost Fraser River sediments further downstream. More thorough mixing of the < 63 Pm fraction downstream of Moose River is indicated by pronounced reductions in the variability of most individual element concentrations measured. Mixing proportions calculated depend on the geochernical fraction considered. The proportion of material contributed by the uppermost Fraser River to the non-carbonate non-silica fraction is greater than it is to the < 63 Fm fraction as a whole: 62 114% and 98 i20% at FDSI and FOS3 respectively. Just as the Moose River sub-basin contributes almost 100% of al1 carbonate rnaterials, most of the fine aluminosilicate material apparently originates in the uppennost Fraser River sub-
2.4.4.3 Mixing at Upstream Tnbutary Junctions The general pattern of < 63 pm bed material mixing in the upper Fraser River drainage basin is summarized in Figure 2.6 depicting downstream variations in bulk geochemistry (carbonate, silica and other oxides) for the two main tributaries. Where available, additional tributary bed material values are plotted adjacent to the main channel trends for cornparison. As found above. the mixing of materials contrasting in ternis of geochemical wnstituents is most dramatic and clearly evident at the Moose River-Fraser River junction. Overall variation is low for al1 a) uppermost Fraser River sub-basin
b) Moose River sub-basin
FmaefR Rmpmdett Cr. Updght Cr. StspQe Cr.
Distance upstream of Moose Lake (km)
FIGURE 2.6 Downstream variations in the bulk geochemical composition of fine bed material load of the Moose and Fraser rivers (m = carbonates. A = silica. = oxides). Composition of tributary sediments shown by larger symbols. constituents in the uppemost Fraser River sub-basin (Fig. 2.6a). However, the pattern suggests that the introduction of sediment containing more oxides and less silica by Ghita Creek and Grant Brook enriches and depletes Fraser River bed material in these two constituents respectively. In contrast, Fraser River values immediately downstream are not consistent with Sleeper Creek values. and straight forward mixing does not appear to occur at this location. The presence of carbonates in the uppermost Fraser River bed material is not consistent with mapped carbonate bedrock. If its presence in uppermost Fraser River bed material is correctly identified (calculated from Ca), it may be derived from unmapped tocalized bedrock sources or from more widely distributed late Pleistocene glacial materials derived regionally. Preliminary investigation has confimed that carbonate bearing late Pleistocene deposits occur at least along the main Fraser River valley. The "disappearance" of carbonate below apparent (minor) sources in the Ghita Creek sub-basin cannot be readily explained from the available data. Introduction of carbonate from the Grant Brook and Moose River sub-basins appear normal. Overall geochemical variations are greater in the Moose River sub-basin than in the uppermost Fraser River sub-basin (Fig. 2.6b). Pronounced downstream increases in carbonate are evident in the upper half of the sub-basin and reflect the presence of non-carbonate llhologies in the northern headwaten and central sub- basin (Fig. 2.2). In the uppermost Moose River sub-basin (STu), where bedrock is comprised of both carbonate and noncarbonate bedrock. carbonate minerals make up much less of the c 63 Fm fraction (9% us. 44% of basin bedrock area). Fudher downstream (UCu), where a minimal ice cover (< 3%) is present on carbonate terrain, carbonate minerals are a more important component of the fine fraction (47% verses 48% basin area). Within the Resplendent Creek sub-basin, where most (88%) of the bedrock is mapped as carbonate and glaciers cover up to 16% of the basin, carbonate minerals make up 51-74% of the c 63 pm river bedload. Downstream, mixed contributions from the Resplendent Creek (RCt) and central Moose River sub-basins appear to first increase silica (at the expense of carbonate), but then restore it in the lowemost reaches. Materials introduced to the Fraser River are thus carbonate-rich. Table 2.7 indicates apparent contributions (mixing ratios) from upstream sources based on bulk geochemical constituents for the tributaries sampled. Results for the Moose River-Fraser River junction are similar to those described previously, though slightly biased toward uppermost Fraser River contributions. All other tributary junction mixing estimates are highly varied and exœed 100% in some cases. Although upstream samples were collected in a similar fashion (later bulked) to those at the Moose River-Fraser River junction, lower contrast between sources and possibly greater within site variability evidently compromises the ability to quantify mixing. Further complications may arise due to the close spacing of tributaries in sorne cases and the possible presence of undocumented (e.g. stream bank) sources. Despite the variable and approximate nature of the estimates, there is an apparent tendency for smaller tributary streams to contribute disproportionately large loads to trunk stream bed material relative to upstream basin area. Sleeper Creek is exceptional in apparently contributing no fine fraction material to the Fraser River.
2.5 SUMMARY AND DlSCUSSlON 2.5.1 Geochemistry of River Bed Sediments and Source Areas In this study R-mode factor analysis was applied to multivariate element data to identify geochemical associations and assess their relationships to bedrock lithologic sources. As Jones and Bowser (1978) point out , whole sediment geochemical data alone is limited in assessing chemical reactivity and the ultimate Win of sediments, but minimal serni-qwntitative mineralogical data such as those collected here confimi and clarify the meaning of the geochemical assemblages observed. The most prominent geochemical groups directly identified in the < 63 pm fraction correspond to calcite plus dolomite and assorted silicate rninerals. As expected, samples dominated by limestone and dolostone (sand, gravel fraction) or calcite and dolomite and related elements (c63 pm fraction) correspond closely to mapped occurrences of Lower and Middle Cambrian carbonate bedrock mainly located in the Moose River sub-basin. While limestone is more abundant and widely distributed in these bedrock sources (Mountjoy, 1980), dolomite appears to be the more common mineral in the c 63 pm fraction of fluvial sediments. Table 2.7 Apparent contribution of upstream sources to downstream Fraser River Moose River fine fraction bed material based on bulk geochemistry
- - Junction Tributary Oh Area Apparent Source (dowammmple) Samplest Upstream Contributions
Sleeper CreeW SCt 8 < 0% Fraser R. (atSCD) GBd 92 > 100%
Grant BrooW GBt Fraser R. (ot GBD) GBu 88 O-65%
. - - -- - Ghita Creekl GCt 12 35% (Sioz; Fraser R. (at GCD) 73'1'0 (oxides) GCU 88 3045%
------Resplertdent CrJ RCtl 24 >100% M00Sû R. (at RCD) RCu 60 Moose RJFraser R. MRt 27 28-32% (aFW) FRu 73 68-72% t) second sampk of each pair is from upstream branch of Fraser River; $) estirnate same or similar for al1 components (carbonates. silica. oxides) unfess otherwise stated presumably due to its relative resistance to physical and chemical breakdown during transport. Because Ca and Mg are so abundant in the carbonate minerals and carbonate-rich sediments examined, they obscure the fact that Sr, Fe, Mn Ba, K and possibly Cr and Sc are also associated with carbonate sources in the watenhed and may define distinct lithologic associations. Several carbonate formations (e.g. Hota, Chetang. Taitei etc.; Fritz and Mountjoy, 1975) crop out locally in the Moose River sub-basin, but it is difficult to assess their significance as sediment sources based on the limited sampling conducted so far. Wiihin the Resplendent Creek sub-basin, sediments recovered near Reef Icefield appear to be enriched in Fe+Mn+Sr and in dolomite, while sediments recovered downstream and toward the west of the sub-basin contain a larger Ba+K component. Although Ba and K are commonly associated with silicate minerals (especially feldspan), they do not appear to be associated with Gog Group outcrops along the southwestern margin of the sub-basin. The Ba+K assemblage does not characterize sediments from Grant Brook, the upperrnost Fraser River to the southeast, or the lower Moose River where Gog Group bedrock occurs extensively upstream. Clastic and nonquartzite rnetamorphic (Le. phyllite, schist) lithologies (sand, grave1 fraction), and primary and secondary aluminosilicate minerals and related elements (c 63 pm fraction) are largely derived from Miette Group bedrock and hence associated with the uppennost Fraser River sub-basin. The dominantly pelitic nature of Miette strata suggests that most of the aluminosilicates in the finer fradion are day minerais. Results €romthe < 2 pm fraction support this conclusion. further indicating that most of the clay minerals likely consist of illite characteristic of lake and marine sediments (Jones and Bowser, 1978) but also probably derived from micaceous schists in this case. The abundanœ of clay minerals is apparently responsible for the association of many trace elements (including the REEs) with the aluminosilicate fraction. Upon weathering release from primary minerals, immobile elements are chacteristically retained in the fine fraction by adsorption to phyllosilicates in approxirnate proportion to their original occurrence (Cullers et aL, 1987). While the REEs covary strongly due to their chemical sirnilarîty, they are quantitatively associated with the alurninosilicate fraction and apparently enriched in some parts of the uppemost Fraser River sub-basin. Where this resuk is observed in the noncarbonate, non-silica fraction, elevated heavy mineral occurrence is also indicated (Taylor and McLennan, 1985). Overall, the abundance of trace elements relative to one another appears to be unifom among the sedimentary rock sources considered here. While primary aluminosilicates (plagioclase, orthoclase and possibly pyroxene) are detected in moderate amounts by XRD, and represented by some of the Al in the geochemical data, these consitituents were not generally observed in the coarse fraction of the upper Fraser River bed materials examined. Since the feldspan occur in significant quantities in some Miette and Gog bedrock types. it appears they either do not survive weathering and fluvial transport in the drainage basin, or they are comminuted and possibly concentrated in the clay and silt fraction by these processes. Silica is on average the most abundant geochemicailmineralogical component of < 63 pm fraction river bed material and is derived from al1 lithologies present in the upper Fraser River watershed. In samples representing source areas mapped as 100% carbonate bedrock, silica constitutes at least 17% of the < 63 prn fraction and usually increases in abundance downstream. Gog Group bedrock appears to be the largest source of coanegrained silica as expected. Elsewhere, sand and finer size fractions are enriched in silica relative to coarser fractions. While this might be expected due to comminution of prirnary alurninosilicates to finer sizes, with the present data this phenornenon cannot be distinguished from the introduction of fine aluminasilicate material €rom tributary streams. 2.5.2 Geochemical Source lndicators The best geochemical, mineralogid or lithological indicators of source are those which occur in different combinations and amounts in specific source areas. To be useful in mixing calculations and to minimize uncertainty and the propagation of error, indicators rnust also Vary relatively little within a source area. In the present context, only the main Moose River and uppemost Fraser River sub-basins exhibit sufficient contrast for reliable mixing estimates, and given the carbonate-silicate contrast between source lithologies, Al and Ca are the single most useful major elements in distinguishing these source areas. Trace elements such as Ba and Ti exhibit greater contrast between sub-basins, but, because they are more variable within sub-basins, yield less reliable mixing estimates. Sodium shows relatively little contrast but yields precise estimates because of very low variability within sub- basins. Many additional elements are usefulonly because they are correlated with Al or Ca, due either to chemicaVmineralogicaI affinity (e.g. Cs, Rb) or to dilution by these elements (cf Rollinson, 1993). These results have important implications for provenance investigations in general since trace elements such as the REEs are commonly used as source indicaiors because by virtue of their general geochernical stability (e.g. Cullers et al.. 1987,1988). Elemental ratios are similarily employed because they are assumed to better preserve differences between sediment sources than single element measurements (e.g. Cullers et al., 1987,1988; McCann, 1991; Fralick and Kronberg, 1997). While true, especially in the case of igneous or metarnorphic terrain, it is also often found that element ratios exhibit relatively narrow ranges, reduced contrast between sources, and greater variability within source areas. In these cases, error propagation may potentially outweigh any other benefits gained. The use of large numbers of elements is not strictly necessary to differentiate and estimate mixing of only two; however, multivariate data is preferable in that it guards against possible measurement error andfor bias in any single variable. The use of linear combinations of variables that maximize the distinction between source materials (i.e. discriminant functions) is one means of extracting the rnost useful information ftom the data set (Cdlins et aL, 1997a, bj. lt was found in this study that increased discrimination of sources compensates for the inclusion of elemental measurements of lower precision. A disadvantage of the discriminant function approach is that the combinations identified are specific to the drainage system considered. 2.5.3 Sediment Mixing At Tributary Junctions In most cases, geochemistry, mineralogy and lithology of river bed matenal showed intemediate mixed composition below confluent branches of the upper Fraser River where sarnpling was carried out. Theoretically, mixtures refiect proportional tn'butary contributions and source area characteristics; however, unifon mixing of sediments is apparently not immediate and the results obtained may be sensitive to sampling location within ban and banks as well as distance downstream. The process and pattern of sediment mixing depend on river hydraulic properties such as discharge, turbulence and channel dimensions, as well as the sediment load, sediment size and bedfon morphology. At the Moose River-Fraser River junction, the most consistent and least variable mixing results based on elemental concentration in the c 63 Fm fraction were obtained at the most downstream location. At tributaries upstream sam ples were usually acquired very close to the junction and more variable, occasionally uninterpretable mixing results were obtained. Based on a limited number of samples, the granule (24 mm) fraction of sediments at the Moose River-Fraser River junction produced variable mixing results depending on lithologic constituent. Quartz composition produced uninterpretable results due to unexpected low values immediately below the junction. and is problematic because the quartz loads of both contributing sub-basins are alike. Consideration of carbonate and metamorphic granules suggests that uppemost Fraser River contribution shifts from -37% at FDSI to -57% at FDS3 downstream, as opposed to 28% and 62% in the c 63 Hm fraction. Both size fractions yield relatively consistent and more unifom results distally; however, at the upstream site granule samples are biased toward the uppemost Fraser River. Since qualitative observation shows that most cobble/gravel material downstream of the junction is in fact derived fmthe Moose River, it appears that the granule size fraction is better represented in uppenost Fraser River sediments pnor to mixing. Most streams tributaries to the main branch of the river are rnodest in size, but energetic due to high relief and steep gradients especially where they enter the main valley. In these cases, coarse material is readily derived from mass wasting of steep mountain dopes and transport distances are short to the basin outiets. As a result, abundant grave1 is introduced to the dominantly sand bed of the Fraser River either directiy or indirectîy via alluvial fans. The dominance of fine tributary sediment just downstream of a junction is likely associated with the reduced mobiiity and more persistent storage of warser materials (cf Church et al., 1991). Based on better agreement of fine and coarse fraction results at the most distal site sampled. it appean that unifom mixing of sediment below the Moose River-Fraser River junction requires 3 km or more of transport. Because of the close spacing of upstream tributaries and constraints placed on sampling. it is not possible to assess downstream mixing distances in general, or to relate them specifically to stream discharge. tributary sediment load or contributing area. The sediment mixing calculations made above assume that al1 sediment originates in one of the two contributing sub-basins at a particular junction. If additional material is introduced to the channel locally via bed or bank erosion for example, it will bias any mixing estimate. Along the upper reaches of the Fraser River. valley widths are reduced and channel shifting results in erosion of late Pleistocene glacial or older Holocene alluvial deposits at some locations. The composition of these materials has not been thoroughly studied. but does differ markedly from the tributary sources in some cases. Carbonate in Fraser River bed material just upstream of Ghita Creek appears to be associated with extensive bank exposures of outwash and till between 23 and 27 km upstream of Moose Lake. The introduction of new material, even upstream of a stream junction, will influence mixing estimates there because bed rnaterial reaching the junction may not be representative of the contributing sub-basin if thorough rnixing and dilution has not yet been achieved. The introduction of carbonate-rich materials from Grant Brook for example appears to influence Fraser River bed material composition downstream of Sleeper Creek. The close spacing of tributaries itself complicates downstream mixing estimates in this case. 2.5.4 Source Area Contributions Despite complex mixing processes. the above results point toward disproportionate < 63 um fraction sediment contributions (per area basis) from the Moose River sub-basin at the present time. Approximately 38% of fine sediments downstrearn of the junction are Moose River in origin while its basin area comprises only 28% of the entire watershed. Limited data on gravei composlion. suspended sediment loads, plus general geomorphic observations indicate that the fine fraction results are a consemative estimate of Moose River's total contribution. Regular sediment load data are not available for the upper Fraser River or its tributaries, but an idea of sediment flux was obtained from spot sampling of suspended sediment in the Moose and uppemost Fraser rivers. Thirteen samples were collected during low flow conditions in late July and early August under both rainy and dry perîods, in years of below average discharge (1994195). After correcting for location relative to gauging stations. including a lag effect on discharge induced by Moose Lake, the load of each stream was estimated from corresponding concentration measurements. Overall suspended sediment load of the Fraser River was found to be about 60% greater than that of the Moose River; however, specific sediment load from Moose River was about 60% greater than that of the Fraser River. Bedload contributions have not been estimated; however. qualitative observations which include extensive alluvial fan development, point toward substantial locally significant inputs from the Moose River sub-basin in the coarser (gravel) fractions. Several factors may explain higher specific sediment yields from the Moose River sub-basin. Modem glacier cover in the upper watershed is -30 km2, concentrated in two large icefields and associated outlet glaciers. Resplendent Creek, and to a lesser extent Moose River downstrearn, are largely braided, indicating high sediment loads and variable discharge. Desloges and Gilbert (1994) argued that upwards of 83% of the fine sediment load of the Lillooet River, British Columbia derived from the relatively restricted glacier covered areas of the basin and this is likely a major factor in the dominance of Moose River sediments within the upper Fraser River watenhed. Additional factors goveming sediment yield from the Moose River sub-basin include greater relie€and elevations. leading to greater precipitation and tunoff (approximately 985 mm in the Moose River sub-basin vs. 850 mm in the Fraser River sub-basin as a whole), as well as elevated levels of mass wasting , slope wash, and strearn energy within the drainage network. Fenn (1987) has suggested that such factors may interact to produce non-linear increases in sediment yields. The relative effectiveness of glaciers and precipitation in sediment production and yield is of interest because both are indicaton of hydroclimate. Although Hicks et al. (1990) have suggested that sediment yields from glaciated basins in New Zealand and elsewhere depend mainly on variations in rainfall, these conclusions have been criticized by Harbor and Warburton (1993) on methodological grounds. Field evidence derived from srnall catchment process studies (e.g. Fenn, 1987) up to continental scale sediment budgets (e.g. Bell and Laine, 1985) typically suggests that glaciers are the more effective geomorphic agent. Data analyzed by Church and others (Church and Ryder, 1972; Church and Slaymaker, 1989; Church et. ai, 1989; Owen and Slaymaker, 1992) show that contemporary glaciers in British Columbia and elsewhere (Gurnell. 1987) generate enough suspended sediment to overwhelrn the downsteam increases in specific yield typically observed where uncansolidated Pleistocene glacial deposits are available for fluvial erosion or reworking. Thus, currently glaciated catchments are thought to resemble other disturbed catchments (e.g. agricultural) that release most sediment from headwater areas and slopes (rather than river channels) and store sediment along field edges, lower hillslopes and floodplains (Walling, 1983; Walling and Webb, 1983). Within the upper Fraser watenhed. it is not possible to definitively identify the factors responsible for sediment production based on the present data; however, evidence does suggest that glacial activity (past or present) is important. Within the Moose River sub-basin, al1 glaciers are currently situated on carbonate terrain (Fig. 2.2) and would have been throughout the Holocene. At the Moose River sub-basin outlet, carbonate minerals account for 62% of the < 63 Pm fraction which is comparable to the sub-basin area mapped as carbonate bedrock (-63%). However. because carbonate lithologies themselves include minerals other than calcite and dolomite (probably < 75% based on samples available), preferential supply of carbonate ta stream sediments is implied by the data. Since carbonates are susceptible to dissolution during transport, especially as fine particles (Brown et al., 1997), they are also likely to be under-represented in stream bed materials downstream. Since carbonate lithologies are at the same time generally resistant to physiwl denudation and stand out in relief thoughout the area. spatially concenhated erosion processes are required to explain enhanœd yields of this material. There is some evidence that in sub-basins containing carbonate substrate but lacking significant glacier cover. carbonate is deficient in river bedload sediments (e.g. 9% glacier cover vs. 44% carbonate bedrock in the uppetmost Moose River sub-basin). Similarly, Moose-Fraser mixing results indicate that as little as 2% of the fine non-carbonate fraction originates in the Moose River sub-basin. where, in contrast with the uppermost Fraser River sub-basin, none of this material is exposed to glacial action. 2.6 CONCLUSIONS Overall. the composition of sediment transported as tributary and main channel bed material load closely reflects bedrock distribution in the Fraser River watershed upstream of Moose Lake and allows characteriration of integrated source areas. Miette Group bedrock consists mainly of petite, sandstone and cong lomerate, and yields aluminosilicate-rich fine sediments disting uished by elevated levels of Al plus related minor or trace elements largely adsorbed to the clay fraction. Lower and Middle Cambrian carbonate lithologies yield sediments enrkhed in Ca, Mg, Sr and, locally, Fe, Mn, Ba and K. Gog Group lithology yields much of the silica found in upper Fraser River bed materials, but wide distribution of this bedrock type and the ubiquity of silica in general limit the use of this material in discriminating source areas. Consemative mixing estimates based on modem downstream alluvial sediments show that the smaller Moose River sub-basin contributes a disproportionate amount ( -38% vs. 28% basin area) of the fine (< 63 pm) fraction. Precise estimates of additional sub-basin contributions would require a more distinct spatial distribution of bedrock lithologies, the absence of secondary mixed sediments (e.g. Pleistocene glacial material), and much more detailed sampling of sediments in channels throughaut the watershed. Determination of proportional contributions is complicated by variable or delayed mixing at tributary junctions depending on local site conditions and the size characteristics of the transported sediments. These problems are minimized at the Moose River-Fraser River junction due to increased valley width and the development of an extensive floodplain. If representative bed material samples are acquired, the best estimates of mixing are obtained using the geochemical components exhibiting maximum contrast between source areas, and minimum variability or measurement error within source areas. In this study of carbonate and clastic sedimentary rocks. Al, Ca and possibly Na, Ba and Ti are found to be most useful. Discriminant functions provide a means of optimizing these conditions at a particular site while taking maximum advantage of multivariate geochemical data. At the present tirne the Moose River drainage basin contributes a greater proportion of the discharge and of the total and fine-grained component of the cornbined Fraser River sedirnent load than would be expected from drainage basin area alone. The imbalance is related to greater relief and ninoff in the Moose River drainage basin as well as to glacier extent and activity. Since difFerences in glacier cover and glacial potential in the two major sub-basins are likely to be persistent (Le. similar in the past). and since relative sediment yields frorn the sub-basins can be determined from sediment composition, a potential indicator of glacier variation during the Holocene is therein available. Specifically, it is expected that Moose River sub-basin sediment yields may have increased relative tu those of the uppenost Fraser River during periods of climatic deterioration when glaciers may have expanded. Wlh appropriate calibration, sediments preserved in downstream sinks such as alluvial fans. floodplains and lakes could provide evidence of these changes. CHAPT ER THREE: SIZE-GEOCHEMICAL COMPOSITION RELATIONS IN ALLUVIAL AND LACUSTRINE SEDIMENTS OF THE UPPER FRASER RIVER, BRITISH COLUMBIA 3.1 INTRODUCTION Provenance techniques have long been used to assess the origin and paleo- environmental or tectonic setting of sediments and sedimentary rocks in large depositional basins (see Haughton et al., 1991; Johnsson, 1993). They have also been adopted, though less systematicalîy, in the study of sediment production and dispersal in a range of modem geomorphic settings, and for Quaternary paleoenvironmental reconstruction at smaller scales (e.g. Klages and Hsiegh, 1975, Desloges, 1990; Collins et al., 1997, 1988; Walling et al., 1999). Traditional provenance methods have emphasized the lithologic and miiierdogic composition of sediments and have necessarily focused on sand or larger size fractions. Most work has focused on understanding the rock record with importafit ancillary studies of modern (Holocene) big river and coastal environrnents (e.g. Potter, 1978; Mazullo and Withers, 1984). The development of geochemical methods has advanced the understanding of sand provenance (Bhatia and Taylor, 1981; Bhatia, 1983; Cullers et al., 1988), and facilitated the use of finer grained materials (McLennan et al., 1993; Fralick and Kronberg, 1997) and a wider range of unconsolidated deposit types (including glacial and lacustrine materials; Cook et al., 1995; Kelley and Nater, 1990) in understanding sediment orîgins and conveyance. Use of provenance to assess modem drainage basin processes (including soi1 erosion and sediment yield) has particularly benefitted from the development of geochemical and related approaches to source discrimination such as the use of magnetic susceptibility (Collins et al., 1997, 1998). Concurrently there has been increasing appreciation that the differing physical and chemical characteristics of mineral grains may lead to important segregation and modification of materials durhg weathering, transport, deposition and diagenesis which rnay complicate or obscure provenance information (Johnsson, 1993). It is generally recognized that particle size and particle composition are interdependent. While a limited number of studies have used particle size itself as a provenance indicator (e.g. Vittori and Ventura, 1995), few studies have exarnined the effect of particle size variations on composition-based provenance determinations (Cullers et al.. 1987). Instead, to compensate for the effects of sediment sorting and grain degradation during transport, many worken have focused on specific size ranges through sub-sampling (McLennan et al., 1993), specific thin-section point count methods (e.g. Cavaua et al., 1993), or the consideration of specific minera1 phases (e.g. resistant heavy minerals) thought to be unaffected by alteration (Morton, 1985; Molinaroli and Basu, 1993). Simple size correction factors also sometimes have been employed (Loring and Rantala, 1992; Collins et al., 1997) usually in the fom of regression models relating clay content to the concentration of a particular compositional constituent. The few studies that have explicitly considered size criteria in assessing relations between modem soils, fiuvial sediments and source lithology have provided varied results. For example, Cullers and others (Cullers et a/., 1987. 1988; Cullers, 1994a) found that the silt and coane clay fraction of fiuvial sediments near granitoid and metamorphic sources in the Wet Mountains, Colorado, and Tobacco Root Batholith, Montana, better reflects the composition and distribution of bedrock upstream than sand does, at least where substantial soi1 weathering has not occurred. Similar relationships appear to hold for sedimentary rocks derived frorn similar igneous and metamorphic sources (Cullers. 1994b, 2000). In contrast, Ca- et al. ((1993)found th& it is the sand composition of upland fluvial sediments which more closely resembles the coarser grained sedimentary rocks of the Northem Apennines, ltaly from which they were derived. To some extent this difference reflects the greater resistenœ of recycled (sedirnentary) rock types to further alteration due to weathering. It also may reflect specific environmental factors such as climate and the nature and distance of transport. While similar factors are thought to acwunt for size-composition relations observed in glacially transported sediments (Shilts and Vagners, 1971; Sugden and John, 1976; Saamisto, 1990), difficulties in provenance determination due to size effeds are more likely to anse in fluvial (and lacustrine) systems where hydrodynamic conditions are likely to be highly variable locally. While much is known about the general patterns and controls on sediment size in streams (e.g. Brierly and Hicken, 1985; Smith and Ferguson. 1995). attempts to model size and especially compositional variations in fluvial systems are in the earliest stages (Piuuto, 1995). The purpose of this paper is to clarify size-composition relations for sediments of the upper Fraser River in east-central British Columbia in order to facilitate ongoing sedimentological and paleoenvironmental investigations (Dirszowsky. 1998, 2001 ; this thesis). The interdependence of size and composition is examined directly for sedirnents emanating from two major sub- basins and deposited in a variety of high energy (channel bars) and low energy (floodplain. lake bottom) sites downstream. Also of interest is the manner in which sediments originating in sub-basins that contrast in tens of source material and sediment delivery mechanism, mix downstream, and how this mixing rnay cornplicate the normally expected downstream fining pattern in larger alpine river systems. The size characteristics of upper Fraser River bed materials are therefore described in some detail. 3.2 SETTING The upper Fraser River in this study drains approximately 1640 km2 upstream of Moose Lake within Mt. Robson Provincial Park, British Columbia (Fig. 3.1). The main reach considered occupies a u-shaped trunk valley that extends approximately 30 km fmm Moose Lake to the continental diuide and served as a conduit for Cordilleran ice flowing eastward across the Rocky Mountains during the Pleistocene (Bobrowsky and Rutter, 1992). Of several tributaries joining the main reach, the Moose River is the largest. It drains 458 km2 to the northwest and joins the Fraser River -4 km upstream of Moose Lake (Fig. 3.1). The Fraser River ûunk valley is marked by several cobble-gravel alluvial fans which serve to partially confine the river to one side of the valley or the other and produce a stepped longitudinal profile. Between fans, valley gradients are low and pronounced meandering has developed. In these short inter-tributary reaches. the Fraser River transports a sandy bedload. The Moose River, in contrast, has steeper Bed Sampie Figure 3.1 Location of study area and bed material sarnpling locations. See Figure 3.2 for detail of main Fraser River reach. gradients overall and a gravel bed along most of its length. Moose River enters the Fraser trunk valley through a steep gorge below which it has constructed the largest of aie alluvial fans (>150 ha). Gravel is thus currently introduced to the Fraser River at the Moose River junction and various points upstream. The upper Fraser River was gauged at Red Pass Oust downstream of Moose Lake) between 1955 and 1996 as was the Moose River just above the Fraser River junction. Both records exhibit peak annual discharges due to snowmelt between late May and early June followed by sustained flow or additional peaks though late June to mid-July due to glacier melt. Mean annual discharge (and mean annual flood) for the Moose and Fraser gauges is 14.3 m3s" (93 m3s") and 46 m3ss (256 m3s-') respectively, indicating that Moose River contributes a slightly greater proportion of discharge relative to its basin area. The difference reflects higher elevations and greater glacier cover within the Moose River sub-basin. Mean annual temperature in the main valley area is estimated at 3.7 OC based on nearby meteorological stations (Jasper, Alberta; Valemount, British Columbia) and local environmental lapse rates. Mean annual precipitation at Red Pass during the period 1931-1969 was 742 mm (Atrnospheric Environment, Sewice, 1981). Average annual runoff for the entire upper Fraser River watenhed based on discharge records is approximately 870 mm. The upper Fraser River drainage basin is characterized by rugged relief and elevations ranging from 1032 to 3426 rn a.s.1. Bedrock consists of Upper Proterozoic through Lower Ordovician clastic and carbonate sedimentary rocks dist&Med such that the majwity of carbonates (95% as mapped; Mountjoy, 1980) outcrop in the Moose River sub-basin. The upper Fraser River drainage basin as a whole may be considered in terms of two main sub-basins contrasting in terms of topography, hydrology and sediment load: Moose River yielding large amounts (relative to area) of gravelly carbonate sediment, and Fraser River with smaller specific yields of sandy non-carbonate sediment (see Chapter 2). 3.3 METHODS Fieldwork and sampling in this study were carried out in two parts. lnitially representative bulk gravel and sand bed material samples were obtained from 21 sites regularly spaced along the main reach of the Fraser River (Fig. 3.2) and from the mouths of major tributaries (Ghita Cr.. Grant Br.. Moose R. and Resplendent Cr.; Fig. 3.1). A sample spacing of approximately 1 km was adopted in the main reach in order to assess the effect of sediment contributions from various tributaries as they join the Fraser River. Samples were consistently obtained from the upstream end of lateral or medial bars under late sumrner low flow conditions after removal of winnowed surface material. Enough material was taken from a 1 m2area so that the largest clast accounted for no more than 5% of the original sample weight. Particle size analysis of the sand and gravel fractions was conducted using a combination of dry sieving in the field (> 12 mm) and wet sieving in the laboratory (Gee and Bauder, 1986). The c 63 Urn grain size distribution was detenined using a Sedigraph 5100 following dispersion with dilute (0.05%) sodium hexametaphosphate, sonification and wet sieve separation. In order to study size-composition relations directiy, size separates ranging from clay to coarse sand were extracted from five samples representative of different parts of the drainage basin using a combination of sieving and sedimentation techniques. Subsequently a second suite of < 63 Mm samples was collected for more detailed geochemical analysis of the fine fraction. This "river-floodplain-laken data set includes additional channel sediments from delta-top locations near Moose Lake (FRGB1 and 3; n = 10). delta top fkxdplain and paleochanne1 samples (padially from cores; n = 36),plus Moose Lake bottom sedirnents (n = 6). Sampling locations and site and deposit characteristics are descrïbed in detail in elsewhere (Chapter 4). Since the c 63 prn fraction was of interest in this case, only matrix material was collected. For geochemical analysis, material was separated by brushing through a 63 pm sieve in order to avoid the use of dispersant. Replicate samples were analysed for grain size by Sedigraph. Bulk geochernical composition was determined by Instrumental Neutron Activation Analysis (INAA) of 750-850 mg samples using the SLOWPOKE Reactor Facility formerîy at the University of Toronto. Thirty-six major and minor elements that produœ either short Figure 3.2 Location of tributaries and bed material sampling locations (gravel and sand bars) along the main reach of the Fraser River upstream of Moose Lake (see Fig. 3.1). Sample codes: FR = Fraser Rier; MR = Moose River; GR = Grant Brook; GB = 'gravel bar'; OW = outwash; TL = till. Slope materials consist of late Pleistocene glacial sediments. outwash or bedrock; downstrearn terraces are alluvial; upstrearn tenaces are alluvial andlor late Pleistocene glacial materials and outwash. or long-lived radioisotopes were measured using a hnro step irradiation procedure described previously (Chapter 2). 3.4 RESULTS 3.4.1 Particle Size Characteristics of Analysed Sediments Active channel sediments (bed material) within the upper Fraser River drainage nehrvork include particles in al1 size ranges up to boulders. the largest sizes restricted to headwater areas. steep tributary reaches and alluvial fans. Detailed particle size distributions were not deterrnined for these high energy sites; however. in glaciated headwaters such as upper Resplendent Creek (RCGB; Fig. 3.1) coarse materials (up to 600 mm) are dominant and subject to transport by melt-related high fiows through laterally unstable channels. Simiiar large caliber inaterial elsewhere (e.g. GRGBI . SCGBI ; Figs. 3.1 and 3.2) represents lag material, possibly relict. and not representative of normal modem fluvial transport. Within the main reach of the upper Fraser River and lower reaches of Moose River and Resplendent Creek, pebbles and granules make up 0-88% of al1 channel materials while sand content ranges from 12.97%. A fine fraction (< 63 pm) is present in al1 samples. typically accounting for 1-5% of the total weight. The majonty of samples are poorly to very poorly sorted (Folk, 1974), having standard deviations greater than 2 phi (Fig. 3.3a). Figure 3.3a compares particle size distributions in ternis of median diameter (D,) and sorting (graphical standard deviation). and discriminates five samples that are unusually fine. These five samples fall enti~elyin the sand and finer fange (silt plus clay, 4-37%), and in al1 but one case are the best sorted. Except for FRGB18, they al1 correspond to valley positions characterized by well developed. low-gradient meanderhg channels upstream of alluvial fans that serve to elevate local base levels (Fig. 3.2). The coanest samples plotted in Figure 3.3a are those recovered from tributaries of the main Fraser River. Figure 3.3b shows size-sorting characteristics for the < 63 Mm fraction of the above samples plus additional tributary and headwater samples for which the coarse fraction was not recovered. Samples previously identified as finest overall are generally the coarsest in the c 63 pm range and this partly reflects their high O Fraser River samples i bibutary sarnples 6 Sorting (phi) Figure 3.3 Particle size charactefistics of upper Fraser River and tributary bed material: (a) complete distributions; (b) c 63 Pm fraction only. Sorting determined as the Graphical Inclusive Standard Deviation of Folk (1974). degree of sorting. Similady, as a consequence of analysing only the c 63 Pm fraction, there is an expected tendency for samples to become less well sorted as average grain size decreases. With the exception of RCGBI, 2, and FRGBI5 (very poorly sorted), al1 samples cluster together in the < 63 Pm fraction. Size characteristics (c63 um) of the second suite of samples collected solely for geochemical-size corn parisons are shown in Figure 3.4. These sam ples include additional active channel sediments, paleochannel sediments, overbank pointbar and quiet water sediments, and lake sediments. The majority of samples fall within a range of size and sorting similar to that described above (Fig. 3.3b); however, some discrimination between deposit types is possible. Modem channel bed materials appear coarsest in the < 63 Pm fraction, and based on sorting it is possible to distinguish those obtained from different sampling locations (e.g.FDSI , FDS3). Paleochannel sedirnents overlap modem channel sediments, but extend the range to finer and less well sorted distributions. It is apparently not possibie to distinguish between vertically and laterally accreted clastic floodplain deposits based on the c 63 Pm fraction. As might be expected, quiet water (organic) samples mostly appear finer than clastic floodplain samples, but sorting is similar in most cases. Except for one neanhore sample (MLEI O), lacustrine sediments are characterized by even finer particle size distributions and cluster closely despite widely spaced sampling locations in Moose Lake. The above variations and ability to distinguish source type are not entirely due to sediment characteristics, but can be traced to sample treatment during analysis as well. The c 63 pm fraction of most sarnples intended for geochemical analysis was isolated by dry sieving. while samples with fine total distributions to begin with (Le.lake, most quiet water and some floodplain samples) were analysed using only Sedigraph (Le. wet). Due to aggregation and adhesion of silts and clays to coarser particles, the former method apparently results in sorne loss of the finer particles from the prepared c 63 Hm fraction. This phenornenon was wnfirmed through replicate wet and dry analysis of selected samples. and is apparent in the slight contrast between modem channel sediments shown in Figures 3.3b and 3.4, wet and dry prepared respectively. In the present study comparing size and composition based on correlation analysis. ske variations related to sarnple O quietwater O paleochannel (wet) . paleochannel (dry] . B modem channel A - lake bottom Sorting (phi) Figure 3.4 Particle size characteristics of the c 63 pm fraction of Fraser River channel, delta top fioodplain, and Moose Lake bottom sediments. treatment enhance rather than obscure the results. Wah the exception of distal lake sedirnents and occasional quiet water and paleochannel sediments, the ability to distinguish sources based only on size characteristics of the c 63 pm fraction is limited, particularly when large numbers of samples are considered. 3.4.2 Downstream Bed Material Variations Gradua1 downstream reductions in overall grain size are only evident across the watershed as a whole. At downstrearn locations, declines are limited to very short reaches (Fig. 3.5). Within the main reach. three zones of relatively fine (Le. sand) sediment (approximately 5. 12, 15 km upstream of Moose Lake) are associated with lower gradient meandering reaches, and considered slightly finer than "normalnfor this section of the Fraser River. Sample FRGB18 (km 18) cornes from a narrow lateral bar within a straight high gradient reach. and probably represents material deposited temporarily. While larger sediment sizes appear to be more typical of the main reach, these are usually associated with large or steep tributary streams andfor localized terraced or entrenched deposits of late Pleistocene or earlier Holocene aga (Fig. 3.5b). Distinct particle size increases occur immediately downstream of Moose River (km 5). Sleeper Creek (km 8), Grant Brook (km 10) and unnamed tributaries #5 (km 15) and #6 (km 7). The effect of tributary #5 is particularly striking given the small size of this watershed and stream , but is attributable to the position of the main Fraser River immediately adjacent to the south valley wall. Under these circumstanœs. coarse streamsediment is introduced di~e&yto the F~ase~River and alluvial fan development is suppressed. Downstream increases in particle size between about km 26 and km 17 (excluding sample FRGB18) are associated with a succession of srnall tributaries and exposures of Pleistocene glacial material along the riverbank. The latter outwash and till deposits (i-e.FROW8, FROW9, FRTLI; Fig. 3.2) are al1 composed of bouldery material and actively eroded by the modem Fraser River. Holocene. probably Neoglacial, fluvial terraces are present at approximately km 7.5 and km 12.5, but only the former contains coarse clasts that noticeably contribute to Fraser River bed material. ,. .. , 7 Il II II I I Il1 1 I F Il II il I I III I I % Kilometers from Moose Lake Figure 3.5 Downstrearn upper Fraser River bed material particle size variations: (a) c 63 (Im fraction only; (b) complete channel deposit. MRGB and GRGB tributary samples also shown; dashed lines indicate trîbutary or channel bank outcrop locations (GB = paleochanneWterrace TL and OW = Pleistoœne till and outwash res pectively). In general, gravel particles are observed in the bed material of the main reach of the Fraser River for 1 to 4 km downstream of the apparent source, but the effects of source on total size distribution may be more extensive. Though generally synchronous, there is a slight tendency for D, particle size variations to persist further downstream of a sediment source than those of the 95th percentile (e.g. km 7-8.km 16-17 Fig. 3.5b). In the case of tributaries, the extent to which gravel persists downstream appears to be related to scale; the effect of the higher discharge and more sediment-laden Moose River is greater than that of tributaries #5 and #6. The effect of non-tributary (i.e. bank) sources appears to be proportional to their spatial extent. Downstream variations within the c 63 vrn fraction are relatively subdued and not necessarily consistent with changes noted in the bed material as a whole (Fig. 3.5a). In some cases, increases in median size of the < 63 pn; fraction are detectable below tributaries (e.g. Moose R.. Grant Br., and unnarned tributary #5); however. elsewhere (e.g. Sleeper Cr. and unnamed tributary #6) downstream variations in the < 63 Mm fraction and whole sample are out of phase. There is no clear relation between these main channel changes and the c 63 pm fraction particle size distribution of the few tributary samples tested (Fig. 3.3b). In al1 cases, c 63 pm fraction variations due to tributary and bank contributions die out before corresponding variations in the coarser fractions. The proportion of silt to clay in upper Fraser River bed materials is generally uniform (-8%). but varies up to 20% in sorne cases. 3.4.3 Geochemical Composition INAA yielded data on 36 elements; however, 6 elements were dropped from the analysis where a large proportion of samples rneasured near or below instrumental detection limits. Some elements near detection limits in occasional samples (e-g.Ca. Cs) were retained if this was due to low concentrations rather than high background interference. Major and trace elements are Iisted in Table 3.1 showing average concentrations in the river-fioodplain-lake sample suite and precision based on counting statistics and detector properties. For the majority of elements, precision is within 5% of the mean value or much better. An indication of Table 3.1 Fine (< 63 pm) fraction elemental characteristics of river-floodplain-lake sample suite (n=52). Major Elements (%) Minor and Trace Elements (ppm) 1) mean concentration of element in samples; 2) staoôard deviation of concentralion; 3) coefficient of variation; 4) average counting precision of al1 composition rneasurements for this element glven as % of concentration; 5) Kolmogorov-Smlrnov test statistic for nonnality, ns = not significant at a = 0.05 the accuracy of measurement was obtained from repeated counts and found to be within 10% for the majorii of samples. Where poor reproducibility was encountered, it was due to inhomogeneity of generally coarser samples and sample- detector geometry. In these cases, additional counts were made and the values averaged. Kolrnogorov-Srnirnov tests (Stevens, 1996) indicate that approxirnately one half of al1 elernent concentrations are not normally distributed (a = 0.05) within the < 63 Vrn fraction sample suite (Table 3.1). Most distributions show some tendency to be right skewed, indicating the presence of samples with unusually high concentrations of certain elements. Frequency distributions of the modern river samples from this sample suite tend to be bimodal. a reflection of differences between the two main bar sampling locations. Among lake samples, MLEIO from the delta foreslope tends to be distinct and more closely resembles Fraser River samples. 3.4.4 Size-Composition Relations Relations between particle size and geochemical composition are examined here in two ways: first through direct chemical analysis of size separates from the five independent riverbed samples, and then by correlation analysis of elemental concentration according to eleven % phi size classes within the c 63 Pm fraction for the river-floodplain-lake sample suite. Size fractions examined by direct chemical analysis include coarse sand (0+1@), medium sand (la -2@),fine sand (241-341), very fine sand fw4@, and coarsest silt (44t 4.5@ abtained by mechanical separation, plus "fine silt" (4.54I -9a) and clay (~94I)obtained through repeated sedimentation and decantation. Decantation yields a pure clay sample; however. the %ne silt" neœssarily contains residual clay. Analysis of variance techniques were used initially to explore the extent to which differences in composition occur between the particle size separates examined. Sinœ MANOVA strongly rejected the nuIl hypothesis that elemental composition is the same for al1 size fractions. multiple univariate tests (ANOVA) were employed in order to identify which elements and which sizes may be responsible for the difference. Table 3.2 gives results only for those elements which Table 3.2 ANOVA results for compositional differences between size fractions in active channel bed sediments. Gmup Mean9 ------coarse medium fine very coarse fine cfaY SPSS' sand sand sand fine siR siit calculated Sand a NOTES: 1) Only signifiant results are shown. For each element a standard critical value (a)of 0.05 was considered appropriate; however. more stringent 'nominaf values were used to campensate for inflated Type-l enor inhemnt in rnultipk tests (Stevens, 1986). For 30 repeated tests a nominal a = 0.002 is approximateîy equal to an adual a = 0.04, nominal a = 0.003 is approximately equal to a = 0.09; 2) ln al1 cases the day separate is stgnificantly different frorn separates indicated Wh + based on multiple cornparisons (Scheffe's Test; nominal a = 0.05). 3) alt but two of the original 30 element sarnple distributions tested posRNe for equal variance between groups (Levene's Test; nominal a = 0.05). 4) All but five of the original 30 elernents sample distributions were distinctly non-normal; however, identical results were obtained using nonnally d~tnbutedlog-transfomecl variables. had significant differences at the approximate 95% (a = 0.05) level (nominally higher significanœ levels were used as critical values because of inflated Type4 error due to repeated testing; Stevens, 1996; see Table 3.2 notes for statistical assumptions). Of the 30 elements considered, only eight (As, Ba, Co, Cr, Cs, Fe, Na, and V) show a strong 'dependency' on size according to these relatively stringent statistical criteria. If the levels of significance adopted are relaxed somewhat, K. Mn, and Rb also show statistical difierences between size groups. Post-hoc multiple cornparisons (Scheffe's test) indicate that in al1 cases, rejection of the nuIl hypothesis is attributable to differences between the clay fraction and most of the other size classes. Additional insight is gained by examining the sizecornposition relations directly with respect to the source of the sediments or tributaries involved. Figure 3.6 shows representative elemental patterns and variations with selected source specific trends superirnposed. Figure 3.6a contains most of the elements (Na, Co etc.) with statistically significant size class differences as determined above, and clearly shows the pronounced enrichment in the clay fraction that is responsible. Similar trends are apparent in the case of Al, Mn. the REEs, and related elements (19 in total; Fig. 3.6b) except that somewhat elevated concentrations also occur in certain sand fractions in some samples. Hafnium and Ta (Fig. 3.6~)are distinct in exhibiting low concentrations in the clay fraction relative to coaner fractions, specifically in the Fraser River samples. In the case of the Moose River sample (and to a lesser extent the Resplendent Creek sub-basin sample), al1 of the elements describeci above (Fig. 3.6~1,b, c) are extremely rare except in the clay fraction. Calcium, Mg and Sr trends (Fig. 3.6d) wntrast with those above and highlight differences between Fraser River and Moose River samples. In the case of Ca and Sr, the Moose River and especially Resplendent Creek samples exhibit gradua1 decreases in concentration as particle size decreases; Mg follows a similar trend, but with lower concentrations in the coarsest fractions. All three elements are rare in Fraser River samples except in the clay fraction. This tendency is the reverse of that for the elements descnbed previously. a) Na* Co* (also As*, Ba*, Cr*, Cs*) FIGURE 3.6a,b Representative variations in elemental composition according to size. Main graph depicts size fractions analysed separately. lnset gives conesponding Speamm rank correlations for elementat concentration in % size classes (x-axis) within the c 63 Pm fraction. Elements with similar behaviour indicated in brackets; asterix indicates elements identified by ANOVA to have significantly dEferent clay fractions (Table 3.2). Large F=lower Fraser River sample (FRGB5); small F=upper Fraser River sample (FRGBPO); D=Fraser River sample below Moose River (FRGBP); M=Moose River sample (MRGB1); R=Resplendent Creek sample(RCGB1). d) Ca (aiso Sr) Size Class Size Class FIGURE 3.6c,d Representative variations in elernental composlion according to size. Main graph depicts size fractions analysed separately. lnset gives corresponding Speaman rank correlations for elernental concentration in % @ size classes (x-mis) within the < 63 prn fraction. Elements with similar behaviour indicated in brackets; asterix indicates elements identifieci by ANOVA to have significantly different clay fractions (Table 3.2). Large F=lower Fraser River sample (FRGBS); small F-upper Fraser River sample (FRGB20); D=Fraser River sample below Moose River (FRGBZ); M=Moose River sample (MRGB 1); RlResplendent Creek sample(RCGB1). It is evident from Figure 3.6 that variations in composition are not necessarily a linear function of particle size (or vice versa), but that the two variables are interdependent. The complex variations that result, combined with differences in pattern and overall concentrations between sources, reduces the discriminating power of ANOVA and limits the value of simple statistical tests. While statistical significanœ therefore cannot be properly assessed with the available data, it is clear that most elements exhibit size-composition interaction. Of particular interest are the pronounced differences that occur between siM and clay fractions. Table 3.3 summarizes silt and clay fraction concentrations of the representative elements in the Moose River and Fraser River samples in order to assess the sensitivity of the c 63 pm fraction to shifts in particle size distribution. The silt values tabulated are weighted averages of the coarse silt and fine silt fractions. The relative percent change from nearly pure silt tc pure clay is calculated for the Moose River and Fraser River samples individuaiiy, and also assuming a 5050 combination of materials. Mixing ratios calculated for the real downstream sample (D) analysed in this study imply a greater proportion of Fraser River sediment in the mix consistent with observations in Chapter 2 based on more rigourous sampling of sediments in the vicinity of the Mooser River-Fraser River junction. Differences of this magnitude (50:50 vs. 60:40) have only a minor effect on the siit to clay changes shown in Table 3.3. For most elements, large relative increases in concentration are observed, particularly in the Moose River sample where silt fraction concentrations are low. In the case of Ca, Mg and Hf, opposing tendencies in the Moose River and Fraser River samples lead to virtually no change in the combined estimate. Relatively conservative changes in Al and Ta concentrations reflect small changes in the Fraser River sample. lnset graphs in Figure 3.6 show correlation coefficients for each % @ size class with respect to concentrations of the representative elements identified above. This correlation approach gives a good indication of whether elements show affinity for specific fine fraction particle sizes, but, as previously. suffen from problems of inflated Type-l error due to repeated testing (over 500 correlations are calculated; Stevens, 1996), and the possibility of correlations induced proportional (size and composition) data (Rollinson, 1993). The latter problem is partly evident Table 3.3 Relative concentrations of representative elements in the < 63 Pm fraction of Moose River and Fraser River bed sediments. Element Tnbutary Concentration Concentration Relative l ncrease (WJP) in Silt' in Clay' Tributary Mixed Samplet Fraser 25,3Oppm Moose 46,550ppm Fraser 67.00pprn Moose 38.1Oppm Fraser 9.96% Moose 8.1 1% Fraser 1,787ppm Moose 1,063pprn Fraser 18.20ppm Moose 6.8lppm Fraser 1.05ppm Moose 2.48pprn Fraser 3.68pprn Moose 6.81ppm Fraser 3.29% Moose 15.90% Mg Fraser 1.60% 1.99% Moose 4.78% 4.33% - - 7) assumes 5050 mixture of tributary çornponents in the tendency for opposing correlations in the silt verses clay fractions. Here the positive correlation is taken to be the true correlation and used for interpretation. The majority of elernents do not appear to correlate strongly with a particular size class of the c 63 Mm fraction at a nominal significanœ level of 99.9% (a = 0.001) and this is bom out by visual inspection of the inset graphs in Figure 3.6. A number of elernents (Al, As, Co. Cr, Cs. K, Ti, and V) show a some affinity for very fine silt and coarse clay (entering on the 8.5@ -9.0@ size interval), while Fe and especially Mn show affinities for slightly smaller sites. Calcium is associated with medium silt while Hf and Lu show an affinity for the coaner silt size. The concentrations of the REEs other than Lu, plus related heavy elements (Ta, Th, and U) and Na, are apparently unrelated to particle size within the c 63 pm fraction. The alkaline earths (Ba. Mg, and Sr) plus Rb and Sc also Vary independently of particle size based on stringent statistical rejection criteria (Fig. 3.6); however, based on the pattern of correlation coefficients, Ba. Rb and Sc do appear to associate with the clay fraction (as with the Al group) while Mg and Sr associate with the silt fraction (as does Ca). The < 63 pm correlation patterns are in general agreement with the size separate analysis conducted above. They appear to imply a weaker interdependence of size and composition in the < 63 pm fraction, which may, in part, result frorn the relatively unifom size distribution of samples (Fig. 3.4). Aluminum and Ti exhibit dearer variation here than in the size separate analysis where no statistically significant differences were noted. In the case of Al, it is clear that sampie source uafiahility (Fig. 3.6b) previously masked compositional differences between the finer class sizes and this rnay also be the case with Ti. Sodium and rnost of the REEs show dramatically less variation between the silt and clay fractions in the correlation analysis. The reason for the discrepancy is not obvious but may be due to using separate sample sets. In the direct chernical analysis (Fig. 3.6a) the single Moose River sarnple exhibited unusually high values of Na, but otherwise more modest differences. This would appear to suggest unusual site characteristics and possible contamination of the sediment or sarnple. 3.5 DISCUSSION The above analysis demonstrates the existence of a complex interdependence of size and composition for various groups of elements th&, in part, depends on source (tributary samples) and thus dominant lithology. Most elements other than Ca, Mg and Hf show a tendency to concentrate in both the < 63 pm and one or more sand (or warser) fractions in most samples examined. In the c 63 pm fraction of modem upper Fraser River samples, Hf, Ta and especially Al and related elements (Fig. 3.6) show an afinity for silt, though overall (river-floodplain- lake data), Al is most closely associated with fine silücoarse clay. Sodium, Co, Mn, Fe, the REEs and related elements tend to concentrate in the clays in al1 materials analysed. Among the carbonate-related elements, Ca and Sr appear to decrease with decreasing particle size in carbonate-rich (i.e. Moose River) samples, whereas Mg (partly dolomite) tends to concentrate in fine sand and silt. The patterns observed may, in part, be explained by generally accepted ideas of downstream particle/mineral breakdown and attrition in which less resistant materials gradually concentrate in finer size fractions or disappear with greater distance from their source (Basu, 1976; Johnsson, 1993). In sediments derived directly from igneous and metamorphic rocks, larger near-source clasts typically consist of lithic fragments that eventually break down and are replaced by monomineralic grains in the sand fractions (Basu, 1976; Cullers et al., 1997). Quartz and feldspars typically dominate the sand fraction with subordinate heavy mineral grains depending on the overall lithology (i.e. mafic verses felsic). In the case of upper Fraser River sediments, lithic fragments derived €romsedimentary and metasedimentary rocks (Le. recycled) are abundant and consist dominantly of phyllite and quartzite which rnay contain micaceous minerals and exhibit varying degrees of schistosity (Chapter 2). Siltstone and shale are also present but in lesser amounts indicative of their lower resistance to breakdown and more limited distribution in the drainage basin. It appears that coarse phyllosilicate/ aluminosilicate-bearing fragments largely account for elevated levels of Al, K and related elements in the sand fractions examined, particularly in uppermost Fraser River and mixed Moose River and Fraser River samples. Ordinarily, fine-grained aluminum-rich materials might be expected to concentrate in fine or intermediate sizes while resistant quartz persists in warser fractions. Here however, the relative depletion of most elements in the fine sands signifies dilution by silica. Concentration of quartz in this intenediate fraction may be related to the relatively fine-grained nature of most quarhite observed in basin bedrock and stream bed material, and rapid cornminution of Al-bearing materials to even finer sizes once transport begins. Limestone and quartzite dominate the coarse sediment load of Moose River (and Resplendant Creek) and this explains the concentration of Ca and Sr in the coanest analysed fractions. The gradua1 decrease in concentration with size rnay be explained by continuous abrasion enhanced by chemical weathering during transport (Cevassa et al., 1993). Brown et al., (1997) demonstrate that solution of calcite, for example in glacier meltwater, may be rapid and usually increases as particle size decreases (specific surface area increases). Dolomite is more resistant to both abrasion and solution, and thus appears to be more common than calcite in upper Fraser River bed materials despite being less common in source rocks (Chapter 2). Greater resistance to breakdown may account for the concentration of Mg in the fine sandlcoarse silt fraction of Moose River samples along with quartz. Hafnium, Ta and to a lesser extent the REEs (e.g. Dy) and related elements are sirnilar to silica in that they concentrate in one or more of the intemediate size fractions (e-g.fine sands or coarse silts). Similar patterns have been noted and discussed by Cullers and others (Cullers et al., 1987; Cullers, 1994a). and attributed to the presence and behaviour of heavy minerais that tend to concentrate these elements (Mclennan and Taylor, 2985). üühiie most heavy and maftc minerais are rapidly degraded during transport, several more resistant phases (e.g. ilmenite, monazite, zircon) tend to persist in certain size fractions. In the case of the granitoid and metamorphic source lithologies studied by Cullen (1994a), Hf (in general associated with zircon) and the heavy REEs concentrate in the siltlcoarse clay fraction, but in theory this depends on distance transported. The distances of transport considered in this study are similar to those considered by Cullers (1994a; i.e. 10's of km) and the fractions enriched in Hf and the REEs are comparable. Pronounced sample to sample pattern variability in the case of Hf could reflect variable transport distances within the upper Fraser River drainage basin and renewed inputs from tributary streams. Variabilkty might also result from the relative ranty of heavy minerals in the recycled sedirnents of the upper Fraser River watershed. Mineralogically, the clay fraction of modem upper Fraser River samples is dominated by quartz and assorted clays, with subordinate feldspars, carbonate and possibly pyroxene, depending on tributary source (Chapter 2). Clay minerals (especially illite) dominate the finest fraction of the floodplain and lake sediments (Chapter 4). In the clay and probably the fine silt fractions, it thus appears that primaiy and especially secondary aluminosilicates account for elevated levels of Al, K and elernents which may substitute for them in the minerai lattice or sheet structure of these minerals (e.g. Fe and Mg). The exceptionally high trace element concentrations observed in the clay fraction are in tum attributable to adsorption on clay mineral surfaces (Hakanson and Jannson, 1983; Cullers et al., 1987; cf Warren and Zimmerman, 1993). In materials relatively rich in mafic heavy minerals, Cullers et al. (1987) found that 50% or more of the REEs and trace elements such as Sc and Co were extractable from the clay fraction and thus had been adsorbed. In the recycled sedirnents of the upper Fraser River where heavy minerals are relatively lacking, the importance of adsorption is expected to be much higher. Exact clay mineralogy determines the overal capacity of this phase to retain cations (Birkeland, 1999); however. the particular combination of cations retained depends on their availability (Le.source) and relative mobilitylsolubility, and thus on environmental conditions such as pH (Culie~set al., lm?),eH (Engstrorn and Wright, 1984), salinity and biological activity (Anikiev et al, 1997). While the relatively immobile REEs, Co and Sc etc. occur in al1 mineral gtoups to some degree, most accumulating in the c 63 pm fraction are nomally derived from the weathering of weaker heavylmafic minerals parücularly in upstream locations nearest to the source (Cullers et al., 1987). In more distal locations, the concentration of these elements is expected to decrease in the < 63 pm fraction as more resistant silicic minerals (e.g. feldspars) eventually break down and contnbute additional elements (Ba, Rb etc.). Cullers and others (Cullers et a/., 1987, 1988; Cullers, 1994; Rollinson, 1993) have observed that the balance of element loss from coarse fractions and element concentration or retention in finer fractions results in the silt/coarse clay fraction of fluvial sediment being the most representative of source rocks at relatively proximal sites. In upper Fraser River sediments, this relation likely applies, but may be complicated by the presenœ of carbonates (Cullers, 2000) and the relatively low concentrations of trace elernents (Cavazza et al., 1993). Further complications may anse due to the presence of oxides and oxyhydroxides which concentrate the redox dependent elements Fe and Mn. In upper Fraser River sediments, these elements show an affinity for slightly finer sizes than the majority of elements concentrating in the fine fraction. This suggests that oxides and oxyhydroxides are on average smaller than clay minerals in these sediments which might be expected if these constituents are newly foming (precipitating) at some sites especially in the lake or fioodplain (Engstom and Wright, 1984). While the size-composition relations described here are broadly consistent with those observed in drainage systems elsewhere, they rnay not be entirely due to fluvial transport processes alone. The concentration of many elementslminerals in coarse and fine size fractions (bimodal distributions), and in particular the concentration of quartz and dolomite (Mg) in the fine sands and coane silts has also been widely observed in glacially transported sediments (Dreimanis and Vagners, 1971; Sugden and John, 1976; Shilts and Kettles, 1990; Saarnisto, 1990). In glacial sediments the bimodal distribution is attributed to the self-limiting crushing/abrasion of lithic fragments on the one hand, and their decived mineral grains of characteristic composition and strength on the other. While the distance of transport tends to determine which size mode is dominant, the "terminal gradesnthemselves are characteristic of the particular rock or mineral type. Since glaciers are currentiy active in the headwaters of vatious upper Fraser River tributaries (notably Resplendent Creek within the Moose River sub-basin), a bundant fine-grained, g lacially crushed matenal is continuously introduced to the fluvial system and transported downstream. Addlional materials derived from Pleistocene glaciers continue to be reworked by the upper Fraser River and tributaries at a limited number of sites (Fig. 3.5). The question anses to what extent inherited glacial size-composlion relations may be preserved during fluvial transport. The continuous decrease in calcite concentration with decreasing size in carbonate- rich samples (Fig. 3.6d) suggests dominantly fluvial control; however, this may not apply to more resistant dolostone and quartz materials. Assessing the glacial versus fluvial origin of these particles would require more thorough sampling and the examination of surface microtextures (e.g. Mahaney and Kalm, 1995). The size-composition relations observed in the upper Fraser River sediments apparently depend on the bulk composition of the material examined and are thus distinct in sediments derived from the Moose River and uppermost Fraser River sub- basins. In the former case, carbonate particles dorninate al1 size fractions along with quark. Clay and oxide minerals in the clay size fraction allow concentration of Al. Fe, Mn, Na and most trace elements, and pronounced increases are observed in al1 cases. In uppermost Fraser River sediments where carbonates are lacking. coarse size fractions contain abundant primary aluminosilicates while the finest fraction has abundant clay minerals. As a consequence, the concentration of Al and related elements changes relatively little between the silt and clay fractions compared to adsorbed trace elements. As mobile cations Ca and Mg also increase in the fine fraction of uppenost Fraser River sediments, but to a lesser degree. Sediments downstream of the Moose River-Fraser River junction represent an approximate 40160mixture of Moose River and Fraser River sedimants (depending on distance downstream; Chapter 2), and the size-composition relations observed are intermediate in character. In the c 63 pm fraction of mixed sediments mat elements continue to show modest to large increases in the clay versus silt fractions. In the case of Ca, Mg, Ta and Hf, opposing tendencies in Moose River and Fraser River derived materials tend to balance, resulting in Iittle difference in composition between the silt and clay fractions. This relation might be expected to shift through time and according to deposit location due to changing source contributions and vanable mixing (Chapter 2). The one rnixed riverbed sample for which fractions were separately analyseci appean to bear slightly more resemblance to uppermost Fraser River sediments. The pattern of c 63 pm fraction size- composition relationships for the river-floodplain-lake data set containing sediments deposited over the longer ten, appears to exhibit Moose River characteristics to a greater extent. Size characteristics of bed material in the main reach of the Fraser River above Moose Lake are highly variable overall. The expected downstream fining and occurrence of a graveCsand transition is complicated by the introduction of dominantly coarse-grained material frorn small but high-gradient, near-source tributary streams (cf Sambrook-Smith and Ferguson, 1995; Knighton, 1999). The construction of alluvial fans results in local variation in base level and stream gradient. and rnay or may not buffer the main river from contrasting tributary inputs (cf Jones, 2000). Coarse-grained material is also introduced to the main channel due to localized reworking of Holocene fluvial terrace and late Pleistocene glacial and glaciofluvial deposits. As a consequence. the coarse fraction of bed material, representing actual bed load in most cases, is typically increased for up to -4 km downstream depending on tributary size, local gradient, combined discharge, and nature of material introduœd. In comparison, variations in < 63 pm fraction (wash load) size distribution are modest and appear to be damped out more rapidly as mobile sub-populations are readily mixed. Both size distribution and composition in this fraction thus reflect relative source area contribution in a relatively unbiased manner (Pinuto. 1995). It is noteworthy that the differencesin elemental concentration between the sediments of the Moose and Fraser rivers sampled just upstream of their junction are comparable in magnitude to the individual differences between the silt and clay fiacüons f Fig. 3.6). Size Merences slightly dominate in the case of Na, Fe, Mn, Co and related tram elements, while source contrasts are greater in the case of Ca. Mg, Al and related elements. While generally uniform. the proportion of clay to silt in modem upper Fraser River samples occasionally varies by up to 20% (absolute). Similar variations occur in the river-floodplain-lake sediment data set as a whole. While source related variations in composition may thus be expected to dominate most alluvial and possibly lacustrine sediments, careful consideration of possible size effects are required if reliable provenance information is to be derived. 3.6 CONCLUSIONS There are definite size1composition dependencies among the geochemical constituents analyzed in this study which may be related to, (1) the relative resistance of parent lithologies andlor mineralogy to breakdown during fluvial transport (especially in the case of coarse particle sizes), and (2) the presence of abundant secondary minerals (clays, oxides) for the most part also derived from source rocks. Numerous trace elements (alkaline, alkaline earth, metals, REEs) concentrate in the clay fraction by adsorption following release due to weathering. Carbonate minerals may be subject to additional breakdown due to chemical weathering during transport. The size fraction in which certain constituents concentrate is partly a function of transport distance and complicated by the continuous addition of 'fresh' material from small but high gradient tributary streams. Some size-composition patterns may be inherited from glacially derived sediments, but this requires further testing to confin. These patterns are consistent with those observed elsewhere, but exhibit variability unique to the dominant sedimentary lithology and local basin processes and morphology. While composition thus detemines size characteristics of the sediment. from a sampling perspective, transportderived sizes dictate sediment composition and must be considered in provenance determination. Geochemical analysis of the fine fraction only (< 63 prn) compensates for most of the size dependence related to transport distance and especially local hydrodynamic sorting. Within this fraction size dependence occun, but is more consistent and more easily accounted for. Since furthec subdivision of the c 63 prn fmction dufing routine analysis is dificult, use of the < 63 pm fraction for provenance detenination is recommended. Fortunately, downstream size variation of the < 63 pm fraction of upper Fraser River bed material is minor relative to variations in the overall bed material load. It is nevertheless advisable to assess c 63 pm fraction size variations before attempting to infer provenance from these sediments. particular in the vicinity of tributary streams or in depositional environments away from the main channel (ie. backwater, floodplain, lake). Similarly, the extent of transport should be considered or held constant. DEPOSITIONAL AND POST-DEPOSITIONAL GEOCHEMICAL AND MINERALOGICAL BEHAVIOUR OF SEDIMENTS IN AN ALPINE GEOMORPHIC SYSTEM 4.1 INTRODUCTION The analysis of sedimentary materials and environments serves a number of functions in reconstrucüng modem, historical and paleoenvironmental conditions. Sedimentary deposits directly reflect local landscape evolution and provide proxy indicators of regional hydroclimatic or geomorphic change and sediment yield. In a terrestrial context, lake-bottom sediments are possibly the most useful deposit type because of their usually continuous accumulation pattern, their preservation potential, and their possible longevity (Dearing and Foster, 1993; Fostsr, 1995). Nevertheless, a wide range of deposl types including floodplains (e.g. Woodward et al., 1992; Collins et al. . 1997; Owens et al., 1999) has yielded useful paleoenvironmental information, and the best inferences are drawn when multiple records andlor proxy indicators are examined and cross-checked in the same environment (Luckrnan. 1994). Such an approach is particularly valuable in alpine systems where diverse deposlional environments may be closely juxtaposed. In such cases sucœssful paleoenvironmental reconstruction depends not only on the establishment of morpho-, litho- and chrono-stratigraphy, but also on the identification and assessrnent of interactions among system components, and between these and the hydroclimatic driving forces and sediment release mechanisms. In lake and floodplain studies, identification of sediment sources is critical if the effeds of climate change on sedimentation are to be distinguished and interpreted. In lakes, sediments are allochthonous (allogenic), autochthonous (endogenic), or authigenic. Allochthonous materials, originating outside of the lake basin. are generally considered to be most important in ternis of abundance, especially in alpine andlor glacier-fed systems (Jones and Bowçer, 1978; Engstrorn and Wright, 1984). Correlations between drainage basin source areas and deposits are often addressed statistically or through provenance studies wherein sediments or their properties are traœd directly to source areas andfor source processes (e.g. Walling et al., 1993; Collins et al., 1997). Detailed sediment routing studies are less common. but have shown that interactions between system components involving temporary storage and reworking may be complex. especially in larger drainage systems (Schumm, 1981; Walling and Webb, 1983; Brooks, 1994). In al1 of these approaches the mobility of sediments and affînity for particular depositional sites should be taken into account but typically are not. As provenance indicators. the geochemistry and mineralogy of sediments are of interest for their applicability to small particle sires and srnall samples. and for their potential to indicate bedrock lithology and weathering history. Fine-grained materials are the most likely to be found in, and transfened between. a wide range of depositional environments. Disadvantages of a geochemical or rnineralogical approach to tracing sources include the potential for chemical change due to weathering or other chemical processes such as pedogenesis or diagenesis, that depend on the differential mobility of specific elements and stability of specific minerals (Haughton et al.. 1991). Where the final concentration of any sediment constituent is suficiently altered by these processes. paleoenvironmental and provenance information may be destroyed. The problem is most serious where mobile elements or compounds make up a large proportion of the sediment because their behaviour necessarily affects relative concentrations of less abundant components in normal geochemical data (Rollinson, 1993). Transport modes and depositional style also difier for sedhntary padicles &pendhg on mineral de- (e.g. heavy minerals) and particle shape (e.g. micaceous minerals) in both fluvial and lacustrine systems. These difficutties are confounded by the fact that sediment composition is frequently related to particle size (Chapter 3), so that hydrodynamic sorting becomes important regardless of mineral species. If these factors are not addressed in sampling and interpretation, it becomes increasingly difficult to generalize about drainage basin chatacteristics based on the chemical properties of sediments the more widely separated they are in time and space. While the basic problems/processes of geochemical alteration of lake sediments have long been known (e.g. Mackereth, 1966; Jones and Bowçer, 1978; Engstrom and Wright, 1984) and exploited to some extent as paleoenvironmental indicators in their own nght (e.g. Sasseville and Norton, 1975; Lewis and Weibezahn, 1981), rarely have these been considered in the context of glacier-fed alpine lakes and drainage basins (cf Kennedy and Smith, 1978). Alpine lakes are generally considered to be oligotrophic and in many cases dominated by inputs of glacially derived "rock flour" (e.g. Karién, 1981; Smith, 1982; Smith and Ashley, 1985; Leonard, 1997). Application of geochemical provenance in a fluvial context is relatively recent, particularly in upland environments (e.g. Cullen et al., 1987, 1988; Collins et al., 1997; Walling et al., 1998). In most studies, only minor consideration has been given to weathering of source materials. fluvial sorting of particles, particle size complications, and the effects of post-depositional alteration The purpose of this paper is to assess the nature and properties of sediments in three closely related depositional environments of the upper Fraser River watershed, British Columbia: the active channel of the Fraser River, its floodplain in the vicinity of Moose Lake. and Moose Lake itself. The mûin objectives are to assess the partitioning and sorting of sediments among the sites in tens of sedirnent particle size and geochemical composition, and where possible to assess the effect of post-depositional alteration on these sediments. By evaluating local sediment routing and postdepositional behaviour, the potential for reconstruction of local and regional paleoenvironments and watenhed history from these depositional environments is enhanced. The work is part of a more extensive and detailed investigation of late Pleistocene and Holocene paleoenvironmental change and effects on sedirnent touting and valley-fil evolution in the upper Fraser River watenhed, British Columbia (Desloges and Gilbert, 1995; Dinzowsky, 1998. 2001 ; this thesis). 4.2 SETTING The focus of this study is the upper Fraser River where it drains through Moose Lake in east central British Columbia near the Alberta border (Fig. 4.1). Moose Lake (elevation 1032 m a.s.1.) is large (15.4 km2) and deep (-40% of its area is greater than 80 m) for a lake so close to the continental divide, and defines a watershed 1640 km2 in area. The iake has received a continuous supply of FIGURE 4.1 Location of the study area. Top inset shows Fraser River drainage basin configuration and location of ice cover (dark shading). In sample designations, MLE = Moose Lake grab samples; MLC = Moose Lake core sample; FDS = Fraser River bed material sample; MDC = deka-top aresample. sediment since deglaciation of the area approximately 10.000 yrs BPI and has an estimated trap efficiency of 95% (Desloges and Gilbert, 1995; Desloges, 1999). Over the same timespan. delta progradation has allowed extension of the Fraser River more than 4 km into the lake basin, producing an extensive delta-top floodplain characterized by lateral and vertical accretion deposits. Meander scars and pointbar deposits derived from the Fraser River account for approximately 40% of the visible delta surface area. Most of the delta surface lies within a few metres of lake level and is characterized by extensive open water. sedge-willow marsh, and peat accumulation. Thus. most of the floodplain currently receives sediment only during perîodic overbank flow events with extent and amount depending on position relative to the active channel. The study area lies in the Temperate Dry lnterior Mountain climate zone (Slaymaker, 1990). but experiences above average precipitation and runoff due to the high elevations and glacier cover (-3%). Mean annual temperatures (1961- 1990) are estimated to be 3.2'C based on nearby stations at Jasper (60 km east) and Valemount (100 km west) and local environmental lapse rates (Atrnospheric Environment Service. 1993; Luckman. 1997). Mean annual precipitation recorded at Red Pass (west end of Moose Lake) during the period 1931-1969 was 742 mm, but this is likely an overestimate of present precipitation (see Chapter 2). Stream discharge data for the Fraser River at Red Pass and Moose River adjacent to the study area both indicate that peak snowrnelt discharges occur in late May and early June following winter low flow conditions (Environment Canada, 1996). Discharge levels are maintained or peak again from late June to midJuly due to glacier melt. Average annual runoff for the entire upper Fraser watershed is estimated at 870 mm (1A8xI 07m3) indicating relatively high precipitation, low evaporative losses and high net glacier melt contributions from headwater areas (Chapter 2). The water table in the Moose Lake delta area is at or near the ground surface during most of the year. Some drying which occurs during late winter coincides with freezing of the surface and maximum snow pack accumulation. The catchment feeding Moose Lake and the delta floodplain is characterized by rugged relief of up to 2400 m. The major tributary valleys are aligned dominantly northwest-southeast due to the fold and thrust fault structure of the Main Ranges of the Rocky Mountains (Fig. 4.1, inset; Mountjoy, 1980). Bedrock consists of Upper Proterozoic pelites. sandstones and granular conglomerates (Miette Group), Lower Cambrian quartz arenites (Gog Group), and part of a Cambro-Ordovician sequence of carbonates (mainly limestone; Dechesne. IWO; Dechesne and Mountjoy. IWO). Miette Group lithologies dominate the drainage basin (-53% of area), especially in the southern and eastem sectors. Carbonates (-21 % of area), are almost exclusively confined to the Moose River sub-basin to the northwest. while Gog Group crops out sporadically in several parts of the drainage basin northeast of the Fraser River. Late Pleistocene surficial deposits of outwash, glaciolacustrine sediments and minor till. as well as recent alluvium are extensively distrîbuted along the main valley. 4.3 METHODS Three groups of samples were obtained in the field representing each of the depositional environments of interest. On the floodplain, three former pointbars were vibracored in a line roughly parallel to present channel location (MDC samples; Fig. 4.1). Cores (10 cm diameter) which varied between 420 cm and 620 cm in length were later split, logged and sampled at intervals of approximately 30 cm, an interval that captures al1 of the major facies changes and provides a complete record of deposition. Three conventional "C dates were obtained from hoof the cores to establish an approximate chronology. The modem river channel was sampled at three medial or pointbar locations on the delta as matrix grab sarnples during law flaw conditions At locations FRSI & 3 (F@ 4-11, fiue individual samples were obtained at 50 m intervals parallel to the channel; at FDS4 a single composite sample was assembled from evenly spaced points across a relatively small river mouth bar. Lake bottorn sediments (MLE sarnples; Fig. 4.1) were sampled using an Ekman dredge deployed from a small boat, except in one case (MLCI) where a vibraconng procedure was used. Samples were sub-sampled using a splitter for particle size, loss on ignition (LOI), and elemental and rnineralogical determinations. Detailed particle size determinations were made on a set of 72 sub-samples. In a few cases, sand plus granules and pebbies, were separated by dry sieving, while the siit-plus-clay fraction (<63pm) was wet sieved and analysed using a Sedigraph 5100. In most cases, pretreatment with hydrogen peroxide to remove organics was unnecessary, but dilute (0.05%) sodium hexarneta phosp hate solution and an ultrasonic bath were used to disperse particles (Gee and Bauder, 1986). Organic matter and carbonate content were estimated from LOI after 1 hour at 550°C and 1100°C respectively (Dean, 1974). These procedures may slightly underestirnate clay and overestimate organic matter content in some samples. Mineralogy was determined for the <2 pm fraction of eight samples using X- ray diffraction (XRD) procedures described in Chapter 2. In conjunction with XRD. dehydration plus glycolation was used to identify clay mineral species. lnterpretation of diffractograrns was based on mineral identification standards compiled by Brown and Brindley (1980) and quantified based on relative peak heights. For detailed geochernical analysis. silt-plus-clay samples were separated by brushing through a 63 pm sieve so that use of dispersant was avoided. The c 63 pm fraction was used to minimize compositional variations attributable to size differences (seeChapter 3). and replicate samples were subjected to Sedigraph analysis to assess whether size distributions were comparable. Geochemical results were obtained by neutron activation (INAA) of 750-850 mg samples using the Slowpo ke Reactor Faciltty formeriy at the University of Toronto. Thirty-six elernents which produce either short or long-lived radioisotopes were detemined using a two step irradiation procedure described by in Chapter 2 and by Hancock et a/. (1988). Of these elements, six were dropped from fudher analysis because large numbers of samples measured near or below detection limits. Based on instrumental precision and replicate analysis of sample material, accuracy of the INAA concentration results is wnsidered to be within 10% (Chapter 2). 4.4 RESULTS AND INTERPRETATION 4.4.1 Stratigraphy and Sedimentology Scroll patterns and meander structures which characterize large sections of the delta-top fioodplain are well represented in the core sections (Fig. 4.2). The stratigraphy in al1 cases is made up of single or multiple fining upward sequences MDCG FIGURE 4.2 Moose Lake-Fraser River delta-top floodplain core stratigraphy and location of radiocarbon dated materials. typical of lateral and vertical accretion, but overlain by quiet water deposits and accumulations of peat. Lateral accretion deposits account for 20% to 50% of each core, and are cornprised of horizontally bedded or poorly structured medium-fine to coane gravelly sands (up to 40% granules). Layers of fine sand occur occasionally, but overall, texture is uniform within a given channel sequence. Overlying laminated sands up to 1 m in overall thickness represent elher bar-top or near-channel overbank deposits. These grade into, or are replaced upward by, more distal overbank deposits composed of silty fine sand to clayey silt layen (el mm to 10 cm thick). These fine-grained overbank (vertical accretion) deposits are interspersed with abundant. largely detrital, organic material, and occasionally interrupted by medium to coarse sand layers (up to 15 cm in thickness) representing scour and fiIl events probably associated with extrerne floods. All together, clastic lateral and vertical accretion materials constitute approximately 55% to 60% of the fioodplain sediments in the cores exanined. Uppermost near channel overbank sediments eventually give way to massive or faintly laminated silts and clayey silts indicative of quiet water deposition. These increase in organic content upward until they are replaced somewhat abruptly with minerogenic peat (organic rnatter content up to 90%). In the case of MDCl (Fig. 4.2), the vertical accretionlquiet water sequence is transitional, less organic-rich (~20%).and represents infilling of an abandoned channel by more energetic events. In cores MDC3 and MDC6 and elsewhere the upper sequence indicates elevated water levels due to delta subsidence andlor lake level rise. and more gradua1 mineral sedimentation leading ta conditions optimal for marsh uegetation and peat accumulation (see Chapter 5). Limited chronologie control for the above deposits is provided by three 14C dates (Fig. 4.2). The MDCl fioodplain sequence is most complete since the core extends into rhythrnically laminated silts and fine sands representing seasonal and sub-seasonal delta foreset accumulation (Chapter 5). The lower paleochannel sediments, in erosional contact with the delta materiais. appear to have been emplaced about 3280 I60 yrs BP (WAT-3045; 3550 I80 calibrated yrs BP), as detennined from detrital wood found just above the delta-fioodplain contact. Organic material sufficient for conventional radiocarbon dating was not available from the base of the upper paleochannel; however, the transition between lateral and vertical accretion, and therefore channel abandonment is dated at 1830 k 90 yn BP (WAT-3046; 1750 i 130 calibrated yn BP). The average rate of channel infilling is approximately 0.93-1 .O3 mm yf'. Core MDC6, taken from the central area of a second pointbar more distant from the modern channel, yielded a near basal date of 3550 k 80 yrs BP (BGS-2059; 3835 k 100 calibrated yrs BP). The third core (MDC3) was not dated directly; however, based on consideration of delta evolution as a whole the paleochannel at the base is probably on the order of 7000-7500 years old (Chapter 5). The modem river channel sediments examined were taken from two mid- channel bars and one pointbar within the delta reach (Fig. 4.1). Overall particle size distributions of modem bar materials are similar to those of the paleochannels. but more clearly show a tendency to fine downstream across the deRa (from sandy grave1 to fine sand; Table 4.1). Samples taken and/or sub-sampled for compositional analysis were derÏved from the matrix or sandy areas of upper bar surfaces, and therefore represent sediments deposited after recent high-flow events (May-July, 1996). The downstream fining trend continues into Moose Lake (Table 4.1) as fine sands and coarse silts are first deposited from suspension or by delta foreslope avalanching and turbidity currents of varying strengths. Sam ple MLE 10, from approximately 500 m offshore, consists of massive to weakly graded sands and silts represenüng no more than a few years of accumulation. More distal lake bottom sediments (>2km down-lake) consist of rhythmically laminated to massive silts or clayey silts depositeâ mainfy hmsuspension. Downlake fining is apparent but less consistent within the narrower size range (Table 4.1). The varved character of some of these sediments (see Desloges and Gilbert, 1995). means that the grab samples examined span approximately 24 to 32 years of accumulation. Sirnilarly. core sample MLCldO is estimated at approximately 300 yean in age. Particle size distributions of the < 63 Pm fraction of representative floodplain sediments are shown in Figure 4.3a. The majority of the lateral and vertical accretion sediments form a narrowly defined envelope within the coaner part of the range. Within this envelope. vertically accreted materials are on average slightly coarser than laterally accreted materials. It should be noted that the size range Table 4.1 Sediment size properties of Fraser River channel and paleochannel, and Moose Lake bottom samples. - -- Sarnple Distance Texture Gravel(%) Sand (%) Silt (%) Clay (%) O, (vm) (km)' FDS 1 -3.5 sandy gravel 65 34 1.1 0.3 (6)- 4200 FDS3 -1.4 sandy grave1 52 47 1.6 0.2 (1 5) 2200 FOS4 0.0 fine sand O 89 9.9 1.1 (10) 125 MDCl a n.a. çoarse sand 0.9 04 13.1 3.5 (24) ma. MW1b na. cciarse sand 1 .O 94 3.9 0.8 (17) ma. Mm3 ma. medium sand O 98 1 .8 0.2 (10) ma. MLElO 0.5 sandy silt O 20 76 4 (5) 20 ME2 3.0 clayey siit O O 73 27 4.5 MLES 4.7 dayey si& O O 74 26 4.5 MLE6 6.2 clayey siit O O 65 35 3.1 MLE7 7.9 dayey sin O O 67 33 3.2 MLC 1-O 7.6 clayey siit O O 74 27 4.0 MLC1-50 7.6 dayey siit O O 68 33 3.6 ME8 9.6 clayey siit O O 63 37 2.8 distanœ upstream or down-iake of river mouth; " bracketed values indicate proportion of < 63 Pm fraction. Grain Size (mm) FIGURE 4.3 Particle size distribution envelopes of fine (~63pm) fraction: (a) delta- top floodplain sediments. and (b) modem channel and lake bottom deposls. A srnall number of vertical and lateral accretion samples are shown in (a) that fall outside the main envelope (shaded; see text for discussion). depicted here is biased towards coarser sizes due to the dry sieving procedure used to isolate the < 63 pm fraction (Chapter 3). Either as aggregates or by adherence to coarser particles, a small portion of the clay fails to pass the sieve. The true range of the channel-related < 63 pm fraction sediments examined is indicated by the extended (shaded) envelope in Figure 4.3a based on representative samples that were fine enough overall for complete Sedigraph analysis or that were separately wet sieved. Sediments characterizing quiet water deposits were in most cases detemined by Sedigraph only. Size distributions of these sediments are more variable and extend the range of corresponding channel and related sediments. Bias toward coarse sizes in the < 63 Urn fraction narrows the range of geochemical determination of some elements (Chapter 3), but does not alter interpretations made below. Figure 4.3b shows the < 63 pm fraction distributions of modem river and lake sediments. In this size range modem river sediments are simiiar to, but slightly coarser than comparable paleochannel sediments pretreated in the same manner. Moose Lake bottom sediments, most of which represent complete particle size distributions and were Sedigraphed only, also fom a narrowly defined envelope in the fine fraction, distinctly finer than any modem or paleochannel material but comparable to some delta-top quiet water sediments. The one exception is sample MLE10 from the delta foreslope, which bears a greater resemblance to river channel sediments than to more distal lake sediments. Overall, only six out of 46 non-lake samples fell outside of the narrowly defined channel-related sediment size range enuelopes (Figs. 4.3a,b). 4.4.2 Geochemistry Thirty elements could be reliably determined using the INAA procedures employed in this study (see Chapter 2). In uppemost Fraser River samples, Ca occasionally measured close to detection limits. but because this is due only to low concentrations in sarnples from this part of the drainage, the element is retained without introducing significant error to the analysis. The behaviour of Ca is in fact of primary interest because it derives almost exclusively from the Moose River sub- basin and is thus a key provenance indimtor in this system. The major elements (> 0.1 %; Rollinson, 1993) determined in this study include: Al, Ca, Fe, K, Mg, Na, and Ti ranging in concentration from c 0.3% to -7% on average (up to 21% maximum in the case of Ca). Minor and trace elements detected include As, Ba, Co, Cr, Cs, Hf. Mn, Rb, Sc, Sr, Ta, Tb, Th, Ut VI and the rare earth elements (REEs) Ce, Dy, Eu. La, Lu, Nd, Sm. and Yb. Concentrations of these constituents range from c 0.4 ppm to 630 ppm on average and in several sarnples Mn could be considered a major elernent. Carbon, Si and O are not detected using the INAA methods adopted, but dominate most samples depending on the relative abundance of carbonate and silicate minerals. Initial assessment and correlation analysis of the geochemical data revealed close relationships between many of the elements. In order to clanfy these associations and at the same time reduce the data set to a more manageable and interpretable number of variables, R-mode factor analÿsis was employed using SPSS (Stevens, 1996). R-mode factor analysis identifies groupiiigs of related variables based on common variance (i.e. covariance), but does not necessarily imply importance in tans of abundance. It was inlially assumed that elemental variation would reflect chemical similanty andlor affinity and therefore rnineralogical associations, but that lithological variations might also play a role. Because such groupings could be naturally correlated, oblique rotations were applied to the basic factors in order to maximize the variance accounted for. Departures from normality in the elemental data set are not considered important because statistical inference is not a part of the technique, and because parametric (Pearson's) and non- parametrie f Speamants)conelaüon matrices were found b be st~cturallyidentical. Following standard procedures (Tabachnick and Fidell. 1989), a number of extraction and rotation methods, variable combinations, and parameter settings were explored in order to test the robustness of the factor solutions and to aid in interpretation of the results. Where sample size was large (ie.the complete river- floodplain-lake data set) al1 methods converged on four factors capable of explaining more variance than any single variable. 4.4.2.1 Geochemical Factor Analysis Factor loadings and correlations are presented in Table 4.2. Since the factors are only weakly correlated, the sums of squares of loadings gives an approximate idea of the variance explained by each factor despite the oblique rotation. Most elernents measured have strong and generally unambiguous loadings. The elements Sr, Ti and U do not load highly on any factor indicating that they behave independently among the elements determined. Factor 1 explains approximately 36% of the common variance and is characterized by large positive loadings of the REEs and to a lesser extent Hf, Ta, Th, and Yb. This grouping is expected based on the similar chemical properties (e.g. high ionic potential) and behaviour of these elernents, and their universal distribution in rock-forming and secondary silicate minerals. The REEs are in general more concentrated in heavy minerals or as adsorbed cations on clay rninerals (Cullers et al., 1987,1988; this thesis, Chapten 2 8 3). The fact that Hf, Ta, Th and Yb loadings are similar and somewhat smaller may reflect chemical differences of mineralogical significance. These elements characteristically concentrate in specific heavy minerals such as zircon, monazite and columbite (Cullers et al., 1987; Klein and Hurlburt, 1993). Factor 2 accounts for almost as much of the common variance as Factor 1 (-34%). It is largely defined (negative loadings) by the major elements Al and K plus nurnerous trace elements and thus appears to represent aluminosilicate minerals. Feldspars are relatively rare in the upper Fraser River drainage, particularfy in fluvial sediments, but they are present in rnino~to trace amounts in the c 63 urn fraction analysed (see below). As expected. clay minerals are far more abundant in the e 63 Hm fraction and illite (or mica) in particular probably dominates the factor. Of the trace elements associated with Factor 2, Cr and V are chemically similar to Al, while Ba, Cs, Rb and Sr are similar to K. In feldspars these trace elements would be expected to substitute for the major elements in the crystal lattice. In clay rninerals, substitution depends on species; however, because these trace elements are largely immobile they also may be retained by adsorption (Hakanson and Jansson, 1983). Factor 2 elements are in fact largely correlated with the fine siltlcoarse clay size fraction (see Chapter 3). Table 4.2 Factor loadings (pattern matnx) and cortelations for river- floodplain-lake data set (n = 52). Element Factor* -- 1 (REE) 2 (Al, K) 3 (Fe. Mn) 4 (Ca, Mg) Factor 1 1.O00 Factor 2 -0.178 1.O00 Factor 3 0.211 -0.141 1 .O00 Factor 4 4.321 O. 143 O. 107 1.000 ') Extraction method is Principal Axis Factoring; rotation method is Oblimin with Kaiser Normalization. $) SSL = sum of squares of loadings; since factors are slightiy correlated, these give only an approximate idea of the (proportion 09 variance accaunted for Factor 2 is also defined by weaker positive loadings of Ca and Hf. As stated earlier, Hf is typically associated with zircon and other heavy minerals, and its negative correlation with the aluminosilicate related elements in Factor 2 may be due to source area lithological contrasts in the drainage basin (see Chapter 2). This explanation also applies to Ca, which occurs mainly in combination with carbonate in upper Fraser River and especially Moose River sediments. The upper Fraser River drainage basin is comprised dominantly of fine-grained clastic sedimentary rock juxtaposed with carbonate and quartzite lithologies in some sub-basins. As sediments from these sources are mixed in various proportions, the relative abundance of each constituent varies inversely with the others. Although Hf (zircon) is a minor component in ternis of abundance, and probably associated with coarse clastic sedimentary rock (including quartzite), it appears to correlate with the carbonate bedrock spatially (Chapter 2). Factor 3 accounts for approximately 16% of the common variance. It is defined by the major elements Fe and Mn and by the minor elements As and Co. Most Fe and Mn in the c 63 pm fraction is thought to be associated with clay minerals andlor oxyhydroxides (see Chapter 3) which may be derived directly from fine-grained sedimentary rocks or from soils in the drainage basin. Factor 3 most likely arises because of the similar redox behaviour of the two elements and their potential mobiltty under the anoxic conditions (reduced state) typically encountered in the marshMoodplain and lake environments studied. Subsequent oxidation may lead to local reprecipitation as oxides/oxyhydroxides which would explain occasional mottling in the lake and Mplainmateriais. The redox uariability of Fe and Mn (and possibly Co) is not as important in modern (oxygenated) fluvial sediments where these elements tend to correlate with the aluminosilicate phase (Chapter 2). The combination of major and minor elements in Factor 3 suggests that part of the association is due to the presence of sulfide andior mafic silicate minerals; however, such minerals are probably rare in the drainage basin. Traœ amounts of pyroxene may be present in the clay fraction. but its detection is uncertain (see below). No concentrated suifide or mafic silicate minerals were observed in the coarse fraction of the riverbeci material examined, but could occur as constituents of commonly encountered metasedimentary particles (Chapter 2). Factor 4 accounts for slightly less variance than Factor 3 (-1 3%). It is defined by the major elements Ca. Mg (positive loading) and Na (negative loading). lnterpretation in th is case is less certain, but the assemblage is probably attributable to the sirnilar chemistry of the three cations. The inverse loading of the elements may reflect either isomorphous substitution within rninerals such as plagioclase or else cornpetition for exchange sites on clay and humus particles. The inverse relationship between the cations could indicate spatial lithological contrasts as in the case of Factor 2 (carbonate verses aluminosilicate minerals): however, there are no obvious bedrock distributions in the drainage basin to acwunt for this. The association of Ca and Mg suggests that Factor 4 may be carbonate-related even though carbonate variation is partially accounted for by Factor 2. Since the two factors show only a weak correlation (Table 4.2), Ca and Mg as represented by Factor 4, are probably only indirectly related to carbonate (i-e.as the source of cations). Strontium, commonly found in carbonates and substituting for Ca in general, is better correlated with Factor 2 than Factor 4 (Table 4.2). 4.4.3 Clay Fraction Mineralogy Mineralogy of the clay (< 2 vm) fraction of selected samples was analysed in order to clam the geochemical relationships. Relative abundances shown in Table 4.3 are based on comparison of nomalized peak heights for the principal minerals detected. Detection Iimits for well-crystallized material using the XRD techniques adopted are approxirnately 5% concentration (Carroll, IWO). Arnong the primary rninerals detected. quartz is generally the most abundant, reflecting its resistance to weathering and abrasion, and its widespread occurrence in basin source rocks (Gog and some Miette sandstones in particular). Orthoclase, plagioclase feldspar, and probably pyroxene (see Table 4.3) are present mostly in small to trace amounts and are chiefly derived from middle and upper Miette sandstones and conglomerates. The carbonate minerals calcite and dolomite are present in all samples in quantities comparable to the feldspan. Arnong the 1:1 clay minerals. kaolinite is most abundant (small to rnoderate amounts) and present in al1 samples. Halloysite is present in some samples but not others. Among the 2: 1 clay minerals. illite is by far the most abundant and is the Table 4.3 Mineralogy of the <2 Pm fraction of sediments from Moose Lake and Fraser River fioodplaina. Other Minerals Sample Age 11@18@ K H HM Sm B V Ch Sp Q O F Pb Ca Do FDS3 modern 0.73 x x x - tr -- x - xxx tr x xx x x MDC 1-50 300 yrsc <0.01 xx -- xxx tr x tr xx x x x tr tr x tr MDC1-100 1000 yrs 0.12 xtrxxxxxxx xx x x x tr tr MDCI-300 3000 yrs 0.36 x tr xxx - xtrxx xx x x tr x tr MDCI -500 3600 yrs 0.15 x -- xxx tr x x x x xxxxxxx MLEIO modern 0.13 x x xxx tr tr x x tr xx tr x x tr tr MLC1-O modern 0.08 x x xxx tr x xxx x - x tr tr tr tr tr MLCI -50 300 yrsd 0.33 xx x xxx tr x xx x x x tr tr tr tr tr a) Mineral abundance is based on relative peak height: ni1 (-); minor amount (tr); small amount (x); rnoderate amount (xx); abundant (xxx). Clay minerals are: kaolinite (K), halloysite (H), illitelmica (IIM), smectite (Sm),background or mixed-layer (B),vemiculite (V), chlorite (Ch), sepiolite (Sp). Other minerals are: quartz (Q), orthoclase (O), plagioclase feldspar (F), pyroxene (P), calcite (Ca), dolomite (Do), gypsum (Gy). b) identification of pyroxene is uncertain; it is estimated approximately as the residual peak at d (A) value 3.15-3.25 once mica and feldspars are accounted for. c) MDCI ages based on interpolation from "C dates assuming constant accumulation in vertical accretion and quiet water deposits; d) MLCl ages based on annual layers. most abundant mineral overall. Chlorite and mixed-layer clay minerals (part of background in Table 4.3) are present in al1 samples in uniformly small amounts. Smectite and usually veniculite are present in lesser amounts and less consistently. The clay minerals detected are wnsidered to be detrital in the sediments examined and indicative of varying source rock type (Le. Miette Group shales, phyllite and schist; see Chapter 2), mixing. and possibly sorting during transport. There are no obvious variations in clay mineral concentration as a function of deposit age. Advanced weathering products or precipitates such as gibbsite (AI(OH),) or hematite (Fe@,) are not identifiable in any of the samples, but possibly occur in an amorphous state. Relative to al1 others, the modern Fraser River sample (FDS3) is dominated by primary rninerals and contains the most quartz despite having the finest < 2 Mm grain size distribution (Table 4.3). Floodplain sediments from core MDCl contain only moderate amounts of quartz and a relative reduction in most other primary minerals in most samples. Most of these losses are covered by illite; however. smectite, vermiculite, mixed-layer clay minerals, and amorphous material are also elevated in the floodplain sediments. Halloysite disappears to some extent in the floodplain samples. While Moose Lake sample MLEI O (delta foreslope) closely resembles the floodplain samples mineralogically, more distal lake sediments (MDCI) are distinct in having moderate to high amounts of vemiculite in addition to abundant illite. Enrichment of the clay minerals is evidently made at the expense of primary minerals such as quartz noted upstream. 4.4.4 Do wnstmam and Down-Lake Sediment Variations Figure 4.4 illustrates the geochemical variation present in the river-floodplain- lake data set in ternis of sample factor scores. Sinœ the factors are well defined chemically, and/or in ternis of source area, the Figure 4.4 plots indicate spatial variations in geochemistry. Based on the first three factors (-85% of the variance represented) there are obvious contrasts between lake and river-plus-floodplain sites. MANOVA of factor scores with respect to location indicates that these differenœs are in fact statistically significant (a=0.001) in the case of Factors 1 and 2. Possible trends within the lake are evident (highlighted by arrows), but subject to 1 8 FOS1 0 FDS3 8 Lake 3 MDCl i A MDC3 A I MDCG -3 -2 -1 O 1 2 3 Factor 2 (AI+K) b) MDCl MDC3 MDC6 -2 I I . I I I v I I -3-2-10123456 Factor 3 (Fe+Mn) FIGURE 4.4 Spatial trends in sample factor scores for most of river-fioodplain-lake data set. Factor scores were not calculated for sarnples where concentration data for one or more elements were not available. (a) Factor 1 (REEs) vs. Factor 2 (AI+K) representing alurninosilicate-carbonate contrasts in the drainage basin. (b) Factor 1 (REEs) vs. Factor 3 (Fe+Mn) representing elernents with mobility dependent on redox conditions. some variabiltty. In conjunction with lake site MLElO on the delta foreslope (500 m from the river mouth), modem river samples also suggest a downstream trend if sample FDS4 is ignored. Sample FDS4 exhibls unusually high factor scores which rnay reflect more restricted sampling at this small river mouth bar site. Floodplain core samples, including lateral and vertical accretion sediments, exhibit a relatively high degree of variability encornpassing and expanding the range of modern river samples. Table 4.4 illustrates downstream. down-lake and cross-floodplain (Le. modern sediments only) geochemical variations in more detail, summarizing concentrations of the key elements identified through factor analysis. The REEs, represented by La, exhibit a fairly consistent increase in concentration downstream and down-la ke where samples are spaced more widely. Here and elsewhere, sample FDS4 appears to be an outlier, and due to complexities of fiuvial mixing in conjunction with the limited sample base, downstream changes should be viewed with caution (Chapter 2). Aluminosilicate minerais, including the clays and represented by Al (Table 4.4). also show a pronounced increase downstream and into Moose Lake. While strearn samples show greater contrast here than with the REEs, lake samples are strikingly uniform more than -3 km from the river mouth. The carbonate phase (Ca and LOI) follows an inverse trend relative to Al and La, and is clearly subject to greater variability from site to site. Loss on ignition values are both higher and more variable than those of Ca reflecting the additional degradation of dolomite and expulsion of clay minera1 lattiœ water respectively; since the latter paFUy depends on ciay mineral species, it does not necessarily follow total clay content (Table 4.4; last column). The ratio CalMg follows a trend similar to Ca alone, suggesting a preferential loss of Ca downstream and down-lake. All of the carbonate indices appear to increase slightly in the most distal lake sedirnents. lron increases downstream and into the lake where, as in the case of Al, it appears to be quite uniform. Manganese follows a similar trend, but with greater variability. The single modem floodplain sample analysed (MDCI -O) contains amounts of Fe and Al similar to modem channel sediments, but lesser concentrations of carbonate, Mn and the REEs. Table 4.4 Measured distribution of major c 63 Pm fraction geochemical constituents and indices along river bed and lake bottom and expected mean lake value based on particle size differences. MDC1-O FDS 1 FOS3 FDS4-1 MLE-1O MLE-1 ML€-2 MLE-3 MLE4 ML55 MLE-6 MLC1-O MtE-8 - 11.3 2.4 13.5 1.O - 1260 37 River Mead 55 6.2 9.2 16.5 - 2.9 590 10.3 Lake Mean' 93 11.1 2. O 3.8 - 4.2 1260 28.7 Expeded5 85 7.0 9.2 - - 3.6 745 - LOI = loss on ignition at 1000'C; equivaient calcite content may be estimated as Cd0.40 or L0110.44 (assurning no dolomite or lattis water present). t)mean based on FOS1 and FDS3; $) mean based on ME3through MLE8; 5) expectsd value based on partide sue relations given in Chapter 3 and discussed in text. 4.4.5 Do wn-Core Floodplain Sediment Vadations Floodplain sediment geochemistry is again considered in tens of concentrations of the key representative elements in part because factor scores could not be calculated for samples where one or more elemental measurements was missing (and are in general more difFïcult to interpret). However. factor score variations ciosely correspond to those of the main elements represented in the majonty of cases. Down-core variations are presented in Figure 4.5 and summarized in the following sections. 4.4.5.1 Core MDCl Clay content in the MDCl profile is highly uniform (-8%), implying that particle size should not be influencing sediment chemistry (Fig. 4.5a). Calcium closely matches LOI in al1 but one sample (350 cm) and indicates that carbonate content is quite variable. If deltaic sediments at the base of the core are disregarded, an overall decrease in concentration through tirne is apparent. Carbonate maxima occur within the two laterally accreted channel layers and a minerakich peat layer at 50 cm depth. The Ca/Mg ratio shows a remarkably close agreement with Ca alone. Aluminosilicates represented by Al (Fig. 4.5a) are moderately variable within the core particularly in the upper section, but show a general tendency to increase upward. Except with the deltaic sediments at the base of the core, the pattern is the inverse of that of carbonate, but more subdued. The REEs and related elements (La is plotteci) exhibit relatively low variation in meMDCl despite accounting for most of the covariance in the overall river-floodplain-lake data set. Again disregarding the deltaic sediments, REE values appear to decrease upward and are certainly lower on average in the younger (c 1750 yn) channel and overbank deposits than the older (c 3550 yn). Depressed REE values occur in the lower part of the laterally accreted sediments, while substantially higher values occur within the deltaic sands. lron and Mn both appear to decrease slightly upward through the alluvial sequence; however, this tendency is overwhelmed by high variability through the middle section and high values within the laterally accreted and lower marsh sediments. Variability is particularly high in the case of Mn. % finef % % ratio % PPm % wm FIGURE 4.5a Downcore textural and geochemical variations in floodplain core MDCI. LA = laterally accreted sediments; VA = vertically accreted sediments. 4.4.5.2 Corn MDC3 Fine-fraction clay content within this core is uniform (5-1 5%) except for one sample near the base of the marsh sediments (234 cm) where values increase to > 50% (Fig 4.5b). Calcium is nearly constant through the clastic vertical accretion sediments, but increases slightly in marsh and laterally accreted sediments at the top and base of the core respectively. The LOI pattern closely follows that of Ca and thus carbonate minerals, except in the case of the clay-rich marsh sample where high values are observed. The anomalous LOI value at -240 cm depth (540% higher) in this case partly represents the loss of lattice water from clay minerals (cf Dean, 1974) and actually offsets the expected sizedependent decrease in carbonate (Chapter 3). In this core, factor scores did not always match changes in the dominant minerals. implying underlying differences in the relationships between the elements measured relative to other sediments studied. The most obvious geochernical differenœ in this core relative to the others is the lower concentration of Ca (carbonate) relative to Al (aluminosilicate), particularly through the middle section. Related to the lack of carbonate is the fact that Mg more closely follows Al in al1 but the uppermost two sarnples. The CalMg ratio again closely follows Ca and is thus uniformly low except in the uppermost two and lowemost samples. As expected, the aluminosilicate (Al) pattern is largely the inverse of the carbonate (Ca) pattern with exceptions at 202 cm (too high) and 280 cm (too low). The REEs (La) and related elements exhibit a pronounced up-core decrease. but with unusually high levels in and just below the Wood deposit" at -265-285 cm. lron and Mn approximately follow the aluminosilicates except for lower values beneath the flood deposit and exceptionally high values at the base of the core. lron and Mn concentrations in this basal sample (-14% and 3400ppm respectively) are the highest obsenred in the entire river-floodplairt-lake data set. FIGURE 4.5b Downcore textural and geochemical variations in fioodplain wre MDC3. LA = laterally accreted sediments; VA = vertically accreted sediments; F D = flood deposk 4.4.5.3 Core MDC6 In core MDC6 clay content of the < 63 pm fraction is more variable than in the previous cores, but does not appear to complicate the LOI values which closely parallel those of Ca (Fig. 4.5~)and Mg (not shown). Carbonate levels are exceedingly high (up to 50% calcite equivalent) in the upper part of the lateral accretion deposit. The low value at the base of the deposit is associated with a separate grave1 bed. Above the channel sediments, carbonate content decreases steadily upward to relatively constant low values (-2%) in the lower minerogenic marsh sediments. Calciumlmagnesium ratios again closely parallel Ca and LOI but with relatively subdued variations. In core MDC6 the inverse relationship between aluminosilicates and carbonates is plainly seen, with Al increasing upward particularly through the vertically accreted floodplain and manh sediments. The change in aluminosilicates relative to carbonates is less pronounced in the laterally accreted sediments despite large changes in carbonate content. The REEs and related elements follow Al closely except in two samples (200 cm and 420 cm) where values are elevated. lron concentrations are nearly constant in the laterally accreted sediments and exhibit a graduai but subtle decrease above. Manganese follows a similar pattern in the lower core but appears depleted relative to Fe in the lower marsh sediments. There is no obvious relationship between Fe and Mn and the other elements in this core. 4.5 DISCUSSlON Within the known bedrock context of the upper Fraser River drainage basin. bulk geochemistry of the alluvial, deltaic and lacustrine sediments examined may be interpreted and understood in ternis of a limited number of mineral groups. In particular, the sedimentary lithologies of the upper Fraser River basin appear to yield a fine (< 63 vm) fraction containing, in approximate order of abundance: quartz, calcite/dolomite (0.6: 1), clay minerals. feldspars, oxideloxyhydroxides (Fe, Mn) and accessory heavy minerals not identiiied directly other than pyroxene. Analysis of the geochemical variability (correlation and factor analysis) of the c 63 Urn fraction sediments reveals that only a sub-set of the above minerals behave FIGURE 4Sc Down-core textural and geochemical variations in fioodplain core MDC6. LA = laterally accreted sediments; VA = vertïcally accreted sediments. distinctly in the drainage in terms of geographic origin and mixing or sorting of sediments as they are transported though the alpine fluvial system (see Chapter 2). Specifically, only two major sub-basin areas (Moose River and upperrnost Fraser River) are reliably distinguished in downstream delta and lake area sediments based almost entirely on the contrast between bedrock substrates with carbonate and aluminosilicate mineral and elemental affinities. In terms of abundance, the bulk composition of Moose Lake and associated delta-top alluvial sed iments reflects source area llhology and mineralogy, and a large portion (-35%) of the geochemical covariability measured in samples derives from changes in the sediment supplied to the sites. This is consistent with the view that glacier-fed alpine lakes are oligotrophic and dominated by allochthonous sedimentation (e.g. Karlén, 1976; Leonard, 1997). It is also expected in the case of the floodplain which receives significant inputs of fluvial clastic sediments relatively directly. A large arnount of geochemical variability apparently arises due ;O the chemical properties and character of the sediments that may not have mineralogical significance and are independent of sediment source. The largest proportion of covariability in the data set described anses from the close chemical similarities of the REEs. While the comlation pattern of these elements points towards a fractionation into at least two phases (i.e. adsorbed cations and heavy mineral species; cf Cullers et al., 1987), the chemical relationship is dominant in the bulk samples analysed and the elements define a single factor. Geochemical variability in Moose Lake and Fraser River sediments also arises due to the similar mobility of specific groups of elements (Le. Ca, Mg, Na, and Fe, Mn) during transport and following deposlion. 4.51 Modem River and Lake Geochemical Fractionation All of the major c 63 pm fraction constluents examined show well defined trends downstream and down Moose Lake. The alumino~ilicates~Fe, Mn (probably as oxides), and the REE increase approximately 80-140% (relative) overall, while carbonate decreases by about 85% in the case of calcite (Mg decreases by approximately 2796, but less is related to carbonate). While consistent wlh the overall trend, the two main Fraser River sample groups are inadequate to define a definite downstream change within the 4 km reach. While the anomalous river mouth sample is probably not representative, substantial site-to-site variation in river bed material composition near the Moose River and other tributary junctions of the upper Fraser River has been documented (Chapter 2). Despite its relative mobility, the < 63 prn fraction requires up to at least 3 km of transport before complete mixing is achieved (Chapter 2) depending on the sediment load and stream discharge characteristics of the tributaries. In contrast, lake sediments in medial and distal locations are much more consistent in ternis of composition and particle size, and as a group, are distinctly different from the riverbed samples. Proximal lake sediments have intermediate composition suggesting a gradua1 systematic transition. Given that particle size distributions overall and within the ~63Hm fraction exhibit systematic changes from river to lake (Le. increasing clay content), it is suggested that size and composition are related. Significant relationships between particle size and concentration of most elements analysed in Fraser River and Moose Lake sediments have been documented (Chapter 3) that are related to mineralogic control on particle weathering and abrasion and to the neofornation of clay minerals and oxidesloxyhydroxides. In the present context. hydrodynamic sorting determines particle size distributions and thus may influence the chemistry and mineralogy of the sediments sampled. The extent to which this explains observed shifts in composition may be assessed approximately by applying previously detemined sizecomposition relations based on silt venus clay content (Chapter 3). In addition to sample specific values. Table 4.4 contains mean elemental concentrations for the main Fraser River samples and distal lake samples. Also given are the expected lake sediment elemental concentrations based on the corresponding 18.5% (absolute) increase in clay content. The estimate is based on observed differences between separate silt and clay fractions and assumes that riverbed material entering Moose Lake comprises a 5050 mix from the Moose River and Fraser River. This balance is approximately true of modem sediment entering the lake (Chapter 2) and is important because observed size-composîtion relations are to some extent defined by the dominant mineralogy of the sample. Although the size relations are based on only a few samples and subject to considerable uncertainty (Chapter 3), it is apparent that expected composition changes based on fining of the sediment considerably underestimate the observed concentration of La, Al, Fe and Mn in most cases (by 9%, 35%, 17% and 41% respectively). Since Ca content does not change significantly from silt to clay in unifomly mixed Moose and Fraser river samples, the observed Moose Lake values are unexpectedly low (an average absolute decrease of -7% is recorded). Apparently, additional processes within the lake are operating to alter Ca and carbonate in the sediment composition. The expectation that Ca concentration would not change represents the balance of two sets of conditions in the Moose River and Fraser River sediments respectively. In Moose River, sediments that are dorninated by carbonate mineralogyAithology pmgressively decrease in Ca content with particle size, and this is thought to represent the increasing loss of calcite through solution (Brown et al., 1997). If clay minerals are present in the finest fraction, soma of the Ca is likely retained there by adsorption. Similarly, adsorption probakly explains relative increases in Ca observed in the clay fraction of Fraser River samples where calcite is rare and more abundant clay mineral particles provide cation exchange sites. It is reasonable to expect that the dissolution process responsible for loss of < 63 pm calcite and dolomite in Moose River bed material continue to operate as sediment is transported further downstrearn. into and within Moose Lake. Detailed assessrnent of dissolution processes in the Fraser River and Moose Lake requires determination of carbonate equilibria and saturation conditions (Kelts and Hsu, 1978},which haç not yet been undertaken. However, several lines of evidence support a solution hypothesis: (1) Ca concentrations in Moose River, Fraser River and Moose Lake surface water samples are undersaturated with respect to pure water at ambient environmental temperatures (cf. Brown et ai., 1997); (2) Moose River, Fraser River and Moose Lake surface water samples readily dissolve reagent grade CaCO, in the laboratory; (3) calcite/dolornite ratios are lower in Fraser River sediments than in source rocks of the drainage basin (Fritz and Mountjoy, 1975; Mountjoy, 1980; this thesis, Chapter 2); (4) even assuming the possibility that Moose River waters are occasionally saturated with respect to carbonate (cf Brown et aL, 1997). mixture with Fraser River water would ensure that water is undersaturated as it enters Moose Lake. In a regional survey of the hydrogeochemisty of the Fraser River, Cameron et al. (1995) reported highly undersaturated conditions at a site upstream of Moose Lake, and at the town of McBride approximately 150 km downstream. Once in the lake. fine particles could remain in the water column for a substantial period of time, increasing the oppominity for solution particularly during winter (Dell, 1973). The presence of colder hypolimnic water during summer would further promote dissolution as carbonate particles approach the lake bottom or following deposition (Kelts and Hsu, 1978; Gilbert and Leask, 1981). Interestingly, Kennedy and Smith (1977) concluded that carbonate solution (or precipitation) processes were not important in controlling sediment composition in Bow and Hector lakes also fed from glaciated, dominantly carbonate sources in the Canadian Rockies. A number of lake, drainage basin and sediment characteristics may explain the different processes inferred for Hector and Bow lsikes and for Moose Lake. Though abundant, carbonate is a less important constituent of Fraser River and Moose Lake sediments than it is of Bow and Hector lake sedirnents. Moose Lake is larger and more distal relative to carbonate sources in its drainage basin. Moose Lake-F raser River source carbonates contain a smaller proportion of dolomite (less soluble than calcite) than those of Bow and Hector lakes (Mountjoy, 1980). Kennedy and Smith (1977) argued that systematic down-lake increases in the calcite/dolomite ratio were due to more rapid settling of the latter denser and possibly coarser mineral. While diffe~entialsettiifg wouM be expected in Moose Lake as well, systematic changes are not observed in the CafMg ratio of distal lake sediments, except that 1 closely follows Ca concentration. This suggests only that the more mobile element is preferentially lost from calcite (the CaMg ratio is exaggerated since a greater proportion of Mg is non-carbonate). The simplifying assumption of a 50:50 mixture of Moose and Fraser river sediments delivered to the floodplain and lake does not necessarily imply a 5050 association of Ca with carbonates and aluminosilicates (adsorbeci). The extent of loss of Ca from river to lake clearly exceeds 50%. Since LOI results are in good agreement with Ca deterrninations, it appears not only that carbonate solution is the dominant mechanism responsible for Ca loss, but that most Ca in Moose Lake is in fact associated with carbonate. While the exact mineralogical associations of Ca cannot be distinguished with the available data, it appears that Moose River sediments may be preferentially represented in distal Moose Lake andlor that Fraser River sediments also contain minor carbonate in the < 63 Pm fraction, derived either from largely unmapped bedrock sources or else late Pleistocene glacial deposits. If carbonate solution is important in Moose lake. its loss from the sediments could partially account for the increases observed in the other major constituents. Assuming al1 Ca represents calcite, the obsewed absolute loss of -7% Ca (-18% calcite) would theoretically lead to a 23% relative increase in al1 other constituents. This increase is sufficient to explain the observed gains in La and Fe, but not Al and Mn. An excess of Al might be explained by a shift in composition in < 63 pm sediments from primary to secondary aluminosilicates (i.e.feldspars to clay minerais) due either to mechanical sorting or even hydrolysis reactions (Thomas, 1969; cf Bowser and Jones, 1978). but it is difficult to quantify. In contrast. an excess of Mn is readily explained by redox conditions/processes characteristic of lake bottorn environments. Both Fe and Mn become mobile under reducing conditions which commonly develop upon accumulation and burial of lake bottom sediment under the influence of microbial activity and decomposition of organic matter (Jones and Bowser, 1978). Dissolved Fe2+and Mn2+ ions may then undergo translocation via molecular diffusion or pore water movement until saturation of interstitial water or reprecipitation of oxides occurs. uwally near the sediment water interface. Typically. a cycle is set up leading to Fe and especially Mn enrichment of surface sediments or. depending on accumulation rates, preservation of Fe+Mn enriched layers at depth (Jones and Bowser, 1978; Engstrom and Wright. 1984). The exact pattern would depend on the sedirnent delivered, organic matter concentrations, lake and interstitiai water chemistry (e.g. sulfate may promote FeS formation). sediment accumulation rates, permeability. dewatering on compaction etc., and may therefore be highly cornplex. Some of these effects may be responsible for the high site-to-site variability in Fe and Mn concentration observed in Moose Lake. Manganese concentration and variability are expected to be greater because of this elernent's higher solubility relative to Fe (Callendar and Granina, 1997). While certainly among the most important, there is no reason to assume that the chemical processes discussed above are the only ones operating to differentiate sediments within Moose Lake and in upstream environments. The quantitative estimates made are first approximations only. Preferential settling of heavy minerals (and some REEs) or quartz (silica) relative to clay minerals (adsorbed elements). or differentiation of clay minerals (e.g. vemiculite vs. illite) due to hydrodynamic sorting or chemical alteration may also be expected, and could contribute to the observed trends. Since compositional data are interdependent, uncertainty is inherent in bulk geochemical data. Assessrnent that is more exact can only be made through more specific (e.g. extractive) chemical analysis and characterization of water column and lake bottom conditions especially over tirne. 4.5.2 Post-Depositional Geochemical Alteration of Floodplain Sediments Floodplain sediment structure and composition in the vicinity of Moose Lake are highly variable and jointly explained by differing modes of deposition over spans of time during which source material characteristics may be subject to change (Le. 1000's of years). The dominant modes of deposition represented by floodplain facies are within-channel lateral accretion and overbank vertical accretion on floodplain surfaces and low-lying marsh areas. Since particle size control on elemental concentration can be ruled out in most cases (e.g. MDCI), gradua1 changes in the composition of core çediments through time and cont~astsbetween distinct strata are most likely attributable to varying mixtures of sediment derived from distinct sources in the fluvial system. Although channel materials are progressively laid down as point ban are laterally accreted, as represented in narrow (-1 0 cm) core sections. most paleochannel sediments are expected to represent nearly instantaneous deposition from a single source. In some cases where discrete beds are observed. multiple stages of channel migration may be recorded in which sediments may be distinct (e-g. lower MDC3). In contrast, while vertically accreted materials are composed of materials deposited during multiple disctete events, thick deposits represent accumulation over potentially long periods of time during which sediment sources in the drainage basin may be subject to change. In general, carbonate-related elements derived from Moose River headwaters are expected to Vary inversely with aluminosilicate related elements more characteristic of the upper Fraser River (see Chapter 2) and this appears to be the case through rnost of the delta floodplain cores. Over the long terni, REE patterns appear to match those of carbonate, suggesting spatial correlation of sources at a general level. In some cases where the aluminosilicates and carbonates do not exhibit the expected inverse relationship, dilution by an unmeasured constituent (i.e. silica) is implied which could result from sorting and comminution of less resistant minerals during fluvial transport. The deltaic sediments of MDCI . for example, exhibit low concentrations of both aluminosilicate, carbonate indicaton, and appear to be silica-rich relative to overlying alluvium. Since the REEs respond to this mixture by increasing in abundance, they appear to be associated with heavy minerals rather than clay minerals and related to the silica fraction. In theory, laterally accreted sediments represented in core sections correspond to near instantaneous deposition during which upstream sources remain constant; however, as pointed out earlier, site-to-site variation within the river may be quite high especialiy downstream of tributary junctions. The extent of differential rnixing and concentration of specific chemical elements or minerals at a point within the channel have not been investigated, but are expected to increase where minerals of varying specific gravity or distinctive shape are present. Though heavy minerals are relativefy rare in upper Fraser River sediments. high concentrations of Fe, Mn and REEs in some modem channel (e.g. FDS4) and paleochannel (e.g. MDC3 base) samples may indicate preferential deposition of these constihients. Given the non-unifonn stratigraphy and inherited composition of the floodplain sediments, postdepositional change or chemical diagenesis is difficult to recognize. However, large diifferences in composition within laterally accreted beds (e-g.MDCI upper channel; MDC6 channel) and in some cases vertically accreted beds. point toward de positional or postdepositional processes unrelated to sediment source supply. As with lake sediments, highllow Mn and Fe values (e.g. lower marsh, MDCI) could reflect changing redox conditions and ion mobility. In the fioodplain environment, translocation processes are not expected to operate as systematically as at the lake bottom. While anoxic conditions may be maintained by high rates of organic decomposition and persistent waterlogging in most of the sediments examined, the occasional introduction and circulation of meteoric water via more permeable sediments at depth as well as water table fluctuations within the near-surface marsh sediments would tend to oxidize and immobilize Fe and Mn locally. Carbonate equilibria in the floodplain environment also would be cornplex. In theory. the introduction of meteoric waters may be expected to promote carbonate dissolution and leaching of Ca2+or Mg2+ions along preferred, largely lateral, flow paths. Unusually low Ca concentrations in more permeable lateral accretion deposits (e.g. -250cm. MDCI ; -420cm. MDC6) might be explained in this way. In the absence of fresh meteoric waters. saturation of interstitial waters with respect to carbonate would occur quickly to inhibit further solution. Since clay minerals are present even in the carbonate-rich sediments examined, Ca2+and Mg2+ions may be retained by adsorption and tend to dominate exchange sites where solution occun. Thus Ca and Mg rnay persist in the < 63 Mm fraction even after finegrained carbonate particles are dissolved; both ions would continue to be supplied from abundant and longer lived coarse carbonate particles. If the above reasoning is correct and accounts for the few atypical samples analysed, it appears that post-depositional alteration and authigenic processes are more a function of floodplain architecture and inherited composition than they are of age. The gemal lack of carbonate in the &est sedhents of axe MDC3 is probably a reflection of Fraser River contributions following Ca. 7000 yn BP. Sediments younger than Ca. 3850 yrs BP (MDCI, MDC6, upper MDC3) appear to have been derived from more carbonate-rich sources in the Moose River sub-basin. Cores MDCl and MDC6 indicate that carbonate-rich Moose River contributions decreased following channel deposition between ca. 3545 ynBP and 3835 yrs BP, but increased again by ca. 1750 yn BP. The upper channel-fiIl of core MDCl indicates further decreases in carbonate contribution following ca. 1750 yn BP. but recently renewed supply, possibly coincident with Little Ice Age glacial activity in the Moose River sub-basin. Within core MDCI , clay mineralogy appean to Vary slightly, but this could easily be attributed to source variation. Minimal change in the percent clay fraction through most of the cores argues against significant hydrolysis and formation of clay minerals, and thus against ver-strong leaching of the floodplain deposits. This is expected in a largely waterlogged environment and consistent with the persistenœ of carbonates in most of the floodplain sediments. 4.6 SUMMARY AND CONCLUSIONS While closely related spatially and geornorphologically, the Fraser River, delta-top floodplain and Moose Lake environments studied, exhibit important differenœs in tenns of sediment texture. structure and composition arising from physical sorting mechanisms and geochemical alteration. The Fraser River floodplain in the vicinity of Moose Lake has a complex structure characteristic of lower gradient, meandering stream and pointbar development. While overall textural properties of the sediments Vary significantly, fine fraction (4 63 pm) size distributions are mostly uniform and similar to modem Fraser River bed material. Lacustrine sediments beyond the delta foreslope are unifomly finer than the alluvial sedirnents both overall and within the fine fraction. Bulk geochemical analysis of al1 sediments reflects the limited number of mineral species derived from the sedimentary mcks of the drainage basin: quartz. clay minerals, feldspars. calcite/dolomite, oxides/oxyhydroxides, and probably some heavy minerals. In ternis of overall abundance, sediments in ali three depositional environments reflect the dominant source lithologies of the contributing basin, with approxirnatefy 113 of sample geochemicat conefation afising due to difierences in the balance of carbonate versus aluminosilicate source areas which may change through time. Long-terni sediment variation in the floodplain cores examined indicates that carbonate contributions from the Moose River sub-basin have increased through the Holocene. possibly due to the expansion of glaciers during the Neoglacial. Since approximately 3500-3800 yrs BP, carbonate contributions from the Moose River sub-basin have apparentiy decreased then increased at least twice (i.e. prior to ca. 1750 yn BP and prior to the Little Ice Age). Additional geochemical variability appears in the sediments examined due to the chernical affinity of the REEs which tend to concentrate in either heavy minerals (relatively rare) or as adsorbed cations in clay rninerals. Relatively less variability anses due tu the similar geochemical behaviour of two groups of elernent: Ca. Mg and Na which are generally highly mobile, and Fe and Mn which are mobile under reducing conditions. Wihin the part of the upper Fraser River drainage system examined. geochemical variation is greatest between modem river and lake environments. Aluminum, REEs, Fe and Mn are dominantly associated with finer grained sediments and thus partially enriched in distal lake sediments. Variability arises due to the association of these elements with different mineral phases (e.g. secondary venus primary aluminosilicates and heavy minerals) which may behave differently during transport and deposition. Important differences between modern river and lake sedirnent also occur due to the solution of carbonate minerals (especially calcite) prior to deposition. Calcite plus dolomite loss apparently reduces sediment mass by more than 20% (absolute) between the two environments. In Fraser River floodplain sediments. geochernicai patterns indicate that post- depositional alteration of minerals containing mobile elements may be of concern. but is probably more a function of sediment structure and groundwater movement than of tirne. This requires that care be taken in interpreüng floodplain sediments in terms of provenance. but also implies that the lake record once established may remain relatively intact. Sediment structure and texture in Moose Lake Vary only slightly. Other than early compaction and dewatering, movernent of interstitial water is likely to be unifonn and minimal in the clay-rich sediments of most of the lake. EVOLUTION OF THE MOOSE LAKE DELTA AND ASSOCIATED VALLEY-FILL: IMPLICATIONS FOR LATE PLEISTOCENE AND HOLOCENE ENVIRONMENTAL CHANGE. 5.1 INTRODUCTION The problem addressed in this paper is one of refining paleoenvironmental reconstruction for the Canadian Rocky Mountains in a moderate size drainage basin (1640 km2). Deep water or distal lacustrine deposits such as those of Moose Lake, Mount Robson Provincial Park, potentially offer a continuous and high resolution record of sediment production in the contributing basin and can therefore provide a framework for understanding environmental change (Desloges and Gilbert, 1995; Desloges, 1999). However. because lake basin sedirnents integrate the effects of sediment routing though the contributing watenhed. % is not a simple matter to interpret either direct (e.g. discharge) or indirect (e-g.glacier activity. climate change) causes of changes in levels of sedirnentation. For example, higher rates of input could relate to one or more of: increased glacial erosion, higher rates of transport associated with increased runoff, or new access to and depletion of sediments stored in upstream sediment sinks. While changes in production and hydrology are often cited as causes of variation in sediment yield (Desloges and Gilbert. 1994; Leonard, 1997), changes to up-basin sinks are rarely measured directly (cf Jordan and Slaymaker. 1991; Brooks, 1994). The research presenfed here focuses on the problem of upstream sediment storage by characterizing and quantifying the largest single sediment sink upstream of the present Moose Lake basin, the Fraser River delta and its associated alluvial materials. An attempt is thus made to reconstnict the geomorphic history of the Moose Lake delta. its overiying floodplain, and underlying late Pleistocene/Holocene valley-fill. and to infer environmental change from these deposits of the upper Fraser River drainage basin. The exercise is partly intended to identify potential impacts of delta growth and floodplain evolution on downstream sediment yields rewrded in the distal sediments of Moose Lake. In alpine lake environments the architectural development of deltas is controlled mainly by the fluvial regime and lake basin geometry. The size, shape and bathymetiy of the lake determine the growth rate of the delta and its overall dimensions (e.g. thickness; Postrna, 1995). In conjunction with the varying influences of fluvial and lacustrine sedimentation proœsses, these factors also detemine the relative extent of delta physiographic zones, including the delta plain, delta front. delta slope, and prodelta areas (Orton and Reading, 1993; Postma, 1995). The delta's planform and the pattern of sediment dispersal in the lake depend on the stability and configuration of the distributary system. In alpine and other glacially influenced environments, steep gradients and laterally unstable stream channels are the nom (Gustavson et al., 1975; Pickrill and Irwin, 1983; Smith and Jol, 1997) leading to unifom sediment dispersal and essentially arcuate deltas (e.g. Smith, 1978; Weirich, 1985). As stream gradients and bedload to total load ratios decrease downstream, more widely spaced stable distributary channels may develop which prograde into the receiving basin, at rates potentially independent of each other (Bhattacharya and Walker, 1992; Hutchinson et al., 1995). Since these sediment point sources are subject to lateral movement, water currents and sedirnent dispersal patterns in the lake may undergo change over the long tem. In alpine environments, such dispersal is frequently limited by valley wall and shoreline configuration (e.g. Hamblin and Camack, 1978; Pharo and Carrnack, 1979). In addition to their role as a zone of sediment storage, deltaic deposits incorporate important geomorphic records in theu awn right providing insight into tectonic setting and water or sea level change (e.g. Hart et al., 1995; Chen et al., 1997), and more localized variations in hydroclirnatic forcing and drainage basin characteristics (e.g. Smith; 1975; Kostaschuk and Smith, 1983; Kalmar, 1995, Hutchinson et al.. 1995). In these studies, it is the sub-aerial delta plain and its sediments that yield most insight into Holocene variations in sediment input due mainly to accessibility of sediments, greater preservation of datable materials. and the availability of well established alluvial facies models (Miall, 1992). In this study, environmental change is inferred from river channel morphology as well as from changes in composition of laterally and vertîcally accreted floodplain sediments. In so doing, the utility of sediment provenance as an indicator of hydroclimatic change in the upper Fraser River watershed is assessed. 5.2 PHYSICAL SETTING Moose Lake is located along the upper Fraser River approxirnately 30 km west of the Alberta border in Mount Robson Provincial Park, British Columbia (Fig. 5.la). Just under 10 km in length, the lake covers an area of approximately 15.4 km2and reaches depths of over 80 metres (Desloges and Gilbert, 1995). The contributing basin includes the combined drainages of the uppermost Fraser and Moose rivers which converge just upstream of the lake (Fig. 5.1 b). The total drainage area is 1640 km2,460 km2of which are drained by Moose River. The drainage basin is underlain by a mixture of Proterozoic to middle Cambrian sedimentary and metasedimentary bedrock (Mountjoy, 1980; Murphy. 1990). Carbonate bedrock accounts for approximately 21 % of total basin area, but is confined to the northwest almost entirely within the Moose River sub-basin. Pelites, quartzites, sandstones and conglomerate of the Miette and Gog groups dominate the remainder of the watershed, with younger rocks (upper Miette and Gog) generally exposed in the northeast (Dechesne and Mountjoy, 1990). The Moose Lake-Chatter Creek fault controls the location of the uppermost Fraser River valley (Dechesne and Mountjoy, 1990); however, othet than slight valley asyrnmetry (steeper southwestern slopes), valley and lake morphology is mainly attributable to the style of Pleistocene glaciation and late glacial and HoIocene fluvial actiuity. Moose Lake occupies a bedrock basin padially in-filled with deglacial sediment. Lake level is controlled by late Pleistocene glacial deposits at the western end of the basin (Desloges and Gilbert. 1995; Desloges, 1999). The Fraser River delta extends approximately 3.5 km frorn the vicinity of the Moose River-Fraser River junction to Moose Lake and sub-aqueously another 2 km. The Moose River alluvial fan foms an integral part of the delta plain and dominates its upstream end (> 25% area; Fig. 5.1~). The Moose River currently joins the Fraser River adjacent to the southwest valley wall where the latter river is confined by fan FIGURE 5.1 Location of study area (a), main features of the upper Fraser River drainage basin above Moose Lake (b) and geomorphology of the Moose Lake- Fraser River delta surface (c). growth. From there. the combined Fraser River channel with associated levee and point bar features fans out across the delta and culminates in a simple "bird's foot" extension into the lake. Mean annual precipitation and mean annual temperature at Moose Lake are estimated at approximately 750 mm and 3.2'C respectively (see Chapter 2). Glacier cover in the drainage basin is about 3% overall, with concentrations in the upper Moose River and Resplendent Creek sub-basins. The Fraser River was gauged at the outlet of Moose Lake between 1956 and 1996. Mean annual discharge and mean annual fiood for this period (expressed as mean daily flows) are 46.7 m3s" and 256 m3s-'respectively. The Moose River. previously gauged just upstream of the Fraser River, contributes approximately one third of the total discharge. Peak discharges are associated with snowmelt in late May or early June. and glacier melt through mid-July and August (Raymond. 2000). 5.2.1 Late Quaternary Environment Deglaciation of the main upper Fraser River valley probably took place at or before 1 1,900 I120 ynBP (GSC-3885; Levson and Rutter, 1989) shortly after Cordilleran ice retreated westward from the Jasper area. and local "Montanenice retreated into tributary valleys (Bobrowsky and Rutter. 1992). Within the upper Fraser River watershed, Tonquin Pass was apparently ice-free sometime before 9660 k 280 yrs BP (BGS-465;Keamey and Luckman. 1983). This does not imply that adjacent tributary valleys were clear at this time; however. Osbom and Luckman (1988)cite evKlence th& late Wisconsin glaciers in this pad of the Rocky Mountains had retreated to approximately their modem positions before ca.10, 000 yrs Bi? Holocene sedimentation in Moose Lake began some time prior to 9120 f 80 yrs BP (TO-6480; Desloges. 1999). The onset of Neoglacial conditions in the area is thought to have begun between 4000 and 5000 yn BP based on regional treeline and pollen fluctuations (Keamey and Luckman, 1983; Beaudoin. 1986), overridden and buried organic materials (Gardner and Jones. 1985). and higher energy lacustrine sedimentation (Dirszowsky and Desloges, 1997; Leonard. 1997; Desloges. 1999; Leonard and Reasoner, 1999). Local and regional Neoglacial advances (e.g. Robson Glacier, Bennington Glacier and Peyto Glacier) have been inferred from 14C and dendrochronological dating of overridden forests and moraine formation (Luckman, 1995; Luckman et al., 1993), and supported by tree ring width and densiometric chronologies at Robson and Bennington glaciers (Luckman, 1994). Periods of distinct glacier advance appear to include 3700-3500 yrs BP, 3300-2500 yrs BP, Ca. 1550 yrs BP, AD 1142-1350, and during the early 18mand early to middle 1grn centuries. Additional penods unfavourable for tree growth occurred from AD 900- 950, AD 1340-1360, AD 1440-1460, and AD 1560-1620; however, tree ring chronologies for the area are cunently limited to the last millennium (Luckman, 2000). Retreat of glaciers from late 1grn century maximum to modern positions is well known (Luckman, 1986), and although punctuated by numerous stand-stills and/or limited re-advances, appears to be ongoing (Luckman, 1995). Desloges and Gilbert (1995) showed that above average sedimentation rates occurred in the proximal area of Moose Lake between AD 1450-153Q, AD 1740- 1870, and AD 1910-1 950. Sedimentation in the distal area of Moose Lake is not entirely consistent with the proximal pattern. above and below average rates occurring at apparently much higher frequency prior to AD 1130 and after AD 1430 (see Luckman, 2000). If increased sediment yields are attributable to increased glacial erosion, increased ice extent or glacier retreat as hypothesized by Smith and Ashley (1985) and Leonard (1986a), it appean that such events are not always recorded in Moose Lake in a straight forward manner. Leonard and others (Leonard, 1986a, b, 1997; Leonard and Reasoner, 1999; Luckman, 2000) found a doser correspondence between eleuated rates of lacustrine sedimentation and regionai glacier advances (i.e. 1200's, early 1300ts, early 1iOO's, 1800's) in the much more heavily glaciated Hector Lake drainage basin approximately 170 km to the southeast of Moose Lake. 5.3 METHODS Delta plain alluvial materials were examined and sampled in cutbank exposures, by hand augering and through vibraconng. The latter was carried out over a one week period in March 1996 and yielded 10 cores (10 cm diameter) ranging from 2.1 to 7.7 metres in length. The technique produced significant compaction (possibly up to 1 m) only in organic sediments and peat horizons. Disturbance to mineral sediments was minimal even along core margins. Three of the vibracores were of sufficient length to fully penetrate the delta's alluvial cap and up to two metres of underlying lacustrine delta materials. Hand auger penetration was usually limited by grave1 or solid refusal. Coring was sited to sample al1 major architectural elements of the delta surface apparent in aerial photographs in approximate proportion to their areal extent, and to cover as much of the delta area as possible (Fig. 5.2). Field work in 1995i96i97 was supplemented by hand and power auger logs produced by the B.C. Ministry of Transportation during preliminary surveys for Highway #16 in 1966167. In the vicinity of Moose River, two 50 m deep drill logs obtained during bridge construction provide the rnost complete "directn sampling of the delta subsurface. Additional sub-surface data were obtained from limited shallow seismic reflection profiling at four sites where surface conditions allowed deployrnent of equipment. Seismic data were obtained using a 12 channel seismograph, a 24 geophone receiver array with roll-along switch, and a 7 kg hamrner source.Profiling was conducted end-on with initial 72 m offsets and a receiver-shot station spacing of 2 m. This configuration yielded 35 m common midpoint profiles with up to 6 fold coverage. Due to the low energy sound source and high background noise from trafic, 40-50 hammer blows were stacked per record to achieve optimum signal bridge construction. Post-processing, including velocity analysis. filtering, static and normal move-out corrections and stacking were camed out by M. Mah at the Department of Geophysics, Uniuersity of Alberta. Assuming a dominant response frequency for the hammer source of about 100 Hz (see Boyce and Koseoglu, 1996) and measured sound velocities ranging from 800-2300 ms", expected reflection resolution is 2-6 metres or better. Vibracores were split in the laboratory, logged and sub-sampled for particle size and geochemical analysis. Particle size was determined using standard sieve and Sedigraph techniques for the warse and fine (< 63 pm) fractions respectively. Organic matter and carbonate content were estimated by loss on ignition (LOI) after 1 hour at 550° C and 1100° C (Dean, 1974). In order to relate paleochannel sediments to source areas, selected samples were analyzed for mineralogical FIGURE 5.2 Sampling or rneasurement locations on the Fraser River delta and Moose River alluvial fan. Contour interval is 100 ft (30 m). Dashed lines indicate location of sections in Figs. 5.5 and 5.8. Hand augering and cut-bank examination specifically for this project (solid circles) is supplemented by similar records obtained from the B.C. Ministry of Transportation (open circles). Similarly, vibracores (Cl-Cl 1) are supplemented by drill logs (C12615) produced during highway and bridge construction. and geochemical composition using X-ray diffraction (XRD) and neutron activation (INAA) techniques. Details of these procedures are given in Chapters 2, 3 and 4. The lithology of sediments in the pebble range was detenined under low magnification and summarized as particle counts. Finally, four samples of detrital wood from 3 cores were radiocarbon dated to establish an approximate chronology of floodplain and delta development. 5.4 RESULTS 5.4.1 Delta Surface Structure & Composition 5.4.1.1 Sedimentology Sediments examined from the upper 8 m of the Fraser River delta lakeward of the Moose River alluvial fan comprise four distinct facies or facies assemblages: horizontally bedded sands and gravels (laterally accreted pointbar), organic-rich laminated sands, silts and clays (vertically accreted overbank and marsh sediments), rhythmically bedded sands and silts (delta foresets), and d iamict (debris flow). Lateral accretion, vertical accretion and marsh sediments typically occur in fining upward sequences, which in many cases can be correlated with meander scan and paleochannels visible in the floodplain. Lateral accretion and vertical accretion facies are dominant in most of the vibracores (Fig. 5.3), cutbank and auger samples examined, particularly in the western and southern areas of the delta surface. The delta foreset facies. was observed only in the lowest section of the three longest cores recovered, whereas diamict was observed definitely only once in 10 vibracores and 55 auger sampies (i.e. Cg). Lateral accretion materials range from poorly sorted coane gravel to medium sand, the latter more comrnon in down-valley examples. Silt plus clay content typically ranges from about 1% to 8% in gravel and sand unes respectively, but values as high as 40% are occasionally observed. Sedimentary structures visible in core and cutbank are limited to faint, slightly inclined planar bedding or lamination in sand, and multiple beds of varying grain size in gravel. In two basal units examined (C7, FP1; Figs. 5.2, 5.3), channel gravels are thinly (2-5 cm) and more regularly bedded, and relatively devoid of c 63 prn material. The position of these two coring sites near former shorelines identifiecl on aerial photographs. and cornparison with auger samples examined along the modem shoreline (FPO; Fig. 5.2), suggest that these lateral accretion materials have been partially reworked by wave activity prior to burial by additional fluvial (overbank) sedimentation. Clastic vertical accretion materials consist of horizontally laminated silts and fine to medium sands with finely dispersed organics andlor discrete organic interbeds and occasional coarse detritus. Sirnilar but more fine-grained and organic-rich (up to 14%) materials occur in the upper part of core Cl (Fig. 5.3).in this case due to infilling of an abandoned channel. lnfill and clastic vertical accretion sediments in most cases grade upward into quiet water marsh facies. In auger and cutbank profiles documented near the modern channel, the fine stratified overbank materials are capped by relatively coarse, well sorted, sand beds indicative of flooding andlor levee construction. A second overbank facies (sequence) in delta surface sediments consists of organic-rich clayey silt and silt evidently deposited in low energy standing water or marsh environments. It is most wrnrnonly found in the upper section of core and auger samples located distal to the modem river channel. areas lying up to -2 m below more active parts of the floodplain. This facies sequence invariably begins with massive silts up to 72 cm thick, but relatively low in organic matter (LOI = 5- 30%). Eventually the silts grade, somewhat rapidly, into more organic peats (LOI = 10-90%). Core Cl is distinct in that the basal silts of the upper sequence are more clearly laminated. probably due to gradua1 abandonment and ongoing, episodic overbank flow. The peat sequenees, composed mainly of sphagnum and sedge, are characterized by varying degrees of decornposition and bulk density both of which generally decrease upward. Bulk density and degree of decomposition also Vary on average between cores, and are greatest in C4 and C3 and least in C9 and Cl0. All peat sequences contain occasional diffuse or well defined silt or clayey siR layers or similarly textured transitional (often interbedded) organic-inorganic zones. Figure 5.4 shows LOI results for cores containing thick (long-lived) organic-fich sequences at the surface (C3,C4, C6, Cg). Loss on ignition at 550°C reveals variations in organic matter content that may or may not correspond to visually discemable minerogenic layers (light bands or dark lines in Figures 5.3 and 5.4). Similarly, C8 casa Qrnl rn P PO- Dawnstrearn < Vpstrearn FIGURE 5.3 Delta-top vibracore logs showing main facies assemblages and location of radiocarbon dated samples. C4 Organics (%) s=3 20 40 w 80 100 ...... O-. -+- -r/-- Carbonate (%) C6 C9 Organics (46) Organics (%) FIGURE 5.4 Loss on ignition resutts for vibracores containing thick organic-rich vertically accreted sequences. Organic matter content is assumed proportional to LOI at 550°C; carbonate content is assumed equivalent to residual LOI at 11OO°C. Prominent carbonate peaks discussed in text are numbered from top to bottom. carbonate content (LOI at 1100°C)variations do not necessarily correspond to those of obvious minerogenic sediment input. Excluding slightly elevated levels at the surface, each sequence contains five or more carbonate peaks of varying intensity. In core C6, peaks are least well defined and least consistent. and the lowermost peak is in fact associated with the laterally accreted materials at the base. In core Cg,the lowest (fiffh) peak is also associated with channel sediments. In cores C4 and especially C3, five well defined carbonate peaks occur entirely within the fine-grained organic-nch sequence. Additional carbonate variations associated with older minerogenic, laterally and vertically accreted sediments occur lower in these cores, but are relative su bdued. The LOI results suggest that the especially prominent siM layer (9 to 25 cm thick) that occurs within 55 cm of the surface in six out of the ten cors sequences is not the same; it may correspond to either the first or second carbonate peak. That inorganic layers generally decrease in thickness and particle size while increasing in organic content away from the modem channel zone, indicates that overbank sedimentation processes are the major control on the organic-inorganic ratio. However, variation in peak heights points to changing channel position and overbank flood dynamics through time. The relative proportion of lateral and vertical accretion (excluding peat) in al1 cores, varies somewhat around 1: 1 . However, fining upward sequences appear to fall into main two categories based on lateral accretion thicknesses and thus paleochannel dimensions. About 213 of the lateral accretion deposits documented in cores (and those associated with the present riuer channel) range in thickness from 100 to ~150cm. The rest fall into a narrower range of 50-80 cm. There is no obviously consistent relationship between thickness and texture (Le.grave1 vs. sand); however, younger lateral accretion sediments in core C9 (Fig. 5.3) are notable in their limited thickness and relatively coarse texture. Long term floodplain aggradation is indicated by stacked fining upward sequences in three of the ten wres examined (i.e. Cl. C4. Cg). The lower sequence in these cases is usually truncated, with thin vertical accretion units apparently scoured and replaced by overfying laterally accreted rnaterials. Contacts between lateral accretion and underlying delta sediments also ap pear to be distincüy erosional. The third facies identified consists of horizontal laminae ranging in texture from clayey silt to fine sand (bulk analysis) and interpreted as resulting from delta foreset sedimentation. Laminae are generally grouped in a rhythmic pattern of alternating relatively coarse and fine material. The lowest part of each rhythrnic cycle typically consists of one relatively thick fine sand layer, either massive in appearance or wlh intemal parallel bedding. Occasional coarse detrital organic material also may be present. The basal layer is ovedain by multiple fine silt laminae with upward increasing clay content. The sequence is usually capped by one or more dark coloured organic-rich laminae before the next cycle begins at an abrupt but apparently conformable boundary. Contacts within cycles range from sharp to diffuse depending on textural conttast. Overall, 'rhythrnite" thickness ranges from 1.5 cm to 15 cm; however, where cycles are best defined they average around 6 cm. The rhythmite sequences observed in cores C5 and C7 tend to fine upwards; however, this is not the case in core Cl and therefore not considered typical. Shallow surface cores from Moose Lake near the mouth of the Fraser River reveal the same larninated structure, but are too short to show the rhythmic structure. While superficially resernbling single turbidity current deposits. variability and contrast between well defined intemal laminae indicate that individual cycles probably result from multiple depositional events. The rhythmic nature of the deposits suggests a seasonal variation in sediment input to the delta foreslope consistent with the presence of vawes in distal lake sediments. Lack of interna1 grading and the presence of occasional parallei bedding suggest that the basal sand layen are due to quasi-continuous underfiows active early in a hydrologic year (i.e. spring snowrnelt; cf. Gilbert, 1975). Sediment gravity flow may also be involved, but is likely less important given the low angle of the delta front (c 3') and near horizontal bedding of the sediments sarnpled. Finer laminae higher in a cycle are probably deposited frorn suspension or by occasional turbidw currents related to summer glacier melt or precipitation later in the year and eventually more gradua1 settling during winter. Large variations in laminae and overall rhythrnite thickness depend on drainage basin sediment yield to the lake. proximity to the river mouth at the tirne of deposition, and sediment dispersal pattern on the delta front. All rhythmically bedded sediments examined would have been deposited in relatively shallow water (c 5 m) with cores Cl and C7 representing sites more proximal to the mouth. The final facies identified in delta-top sediments is the pebbly, silty sand diamict present only in core C9 (Fig. 5.3). It occurs as two distinct layen of 40 and 50 cm thickness, separated by 5 cm of medium fine sand, streaked in appearance. and inclined at about 30°. The diamict layers are highly consolidated, and include detrital wood and "rip-upn clasts of silty sand. The assemblage is interpreted as a debris fiow or flows, consistent with site position adjacent to the steep vailey wall where slope failures could trigger such events (Fig. 5.2). The assemblage is underlain by poorty sorted gravel, more angular than observed elsewhere; however, it is unclear if the gravel and overlying diamict are derived from related processes. The debris flow apparently terminated in standing water or produced ponding (by damming river fiow for example) which allowed the accumulation of massive clayey silt just above. 5.4.7.2 Cross-Sectional Morphology Figure 5.5 contains sectional views of the upper delta sediments based on selected core and auger logs running down-valley (approxirnately parallel to Highway #16) and cross-valley (see Fig. 5.2 for locations). The general distribution and spatial association of channel-related facies (section 1), quiet water or marsh- related facies (section II), alluvial fan (section III), and delta facies (section IV) are clearly shown. As seen prevbusly, lateral accretion and clastic vertical accretion sequences are most prevalent down-valley and to the west and south of the sampling area. The limited number of cores allows only minimal demarcation of former channel position (fine dashed Iines; Fig. 5.5); however, the occurrence or preservation of channel or near-channel sediments appears to be fragmentary, especially to the southeast. In addition to their dominant occurrence to the north and east of the delta surface, vertically accreted quiet water sediments (section II) apparently occur to greater depths in this area. The upvalley end of the delta-top is dominated by coarse alluvial fan material (section Illa) associated with the Moose River. These sediments extend for at least 600 rn under the finer floodplain a) Longitudinal Sedim (W-E): llla i - Channel Dominated II - Quiet Water ûominated llla Moose River Alluvial Fan N - Delta Foresets lllb - Fan Extension b) Upstrearn Cross-Sedion (N-S): c) hvnstream CrossSection (N-S): I Hwy 16 I rrW FIGURE 5.5 Longitudinal and cross-sections of alluvial delta-top sediments and distribution of main facies assemblages; locations shown in Fig. 5.2. Fine dashed lines indicate correlative beds and former channel boundaries. Coarser dashed lines indicate uncertain donboundaries. and marsh sediments (section Illb), but were not sampled or could not be distinguished in the more distal cores. The cross-valley sections shown include two auger records (FP4, FP11) in which solid refusal was encountered at shallow depth. The similar position of the auger sites relative to the southem delta margin suggests that the hard material encountered is associated with the adjacent mid-valley "south- ridgen (see Fig. 5.2); however, it is unknown whether this feature consists of bedrock. coarse unconsolidated glacial or glacially modified sediments. or both. In either case these materials limit the extent of delta-top alluvial materials just below the surface (Fig. 5.5). Floodplain and marsh sediments are thinnest in the southeast where they overlie the alluvial fan complex; however, it is evident that delta-top sediments in general (including fan materials) thin towards Moose Lake. Although delta topography was not surveyed in detail, part of this thinning (-3 rn surface topography) is attributable to fan growth and proximal floodplain aggradation required to maintain river channel dope as the delta progrades (cf Jordan and Slaymaker, 1991). While stacked channel sequences provide direct evidence of aggradation, that the alluvial delta-top profile also thins lakeward from below (-7 m) indicates an apparent rise in the delta topseVforeset interface (and lake level) as the delta prograded. This possibility might be expected if the uppemost Fraser River valley has been subject to isostatic adjustrnent during the Holocene; however. such adjustment is unconfimed. An apparent rise in the topseüforeset interface may also be attributable to subsidence of older areas of the delta, expected to occur over the hgtem because of Ming,compresUon and dewateting of relatiuely fine-grained and thus more porous sediments (Scott and Fisher, 1969). This process would partially account for the pronounced contrast between well defined levee surfaces. continuously supplied with coarse-grained overbank sediment. and low-lying marshy areas of the delta away from modem (or recent) river channel positions. It would also account for widespread submergence of floodplain features and for the bird's foot style extension of the river mouth (Coleman, 1988). While subsidence would be expected to decrease gradually through tirne. renewed subsidence and compaction of the delta surface of up to 6 m were observed during highway fiIl and construction activities during the late 1960's (B.C. Ministry of Transportation files). This lowering can be only partially explained by the compaction of low bulk density organic deposits. 5.4.1.3 Surface Morphology and Chronology Based on surface morphologic expression and vibracore evidence. including 14C dates, the Fraser River delta-top may be considered in terms of stages of development and corresponding rnorphostratigraphic or depositional zones (Fig. 5.6). In Figure 5.6, zones designated A and B refer to the northeast section of the delta and correspond to low-lying marshy areas and the thick sequences of quiet water vertical accretion deposits identified earlier (Fig. 5.5). Zones C-G dominate the southern and western areas of the delta, fanning outward from the Moose River- Fraser River junction toward Moose Lake. Except immed iately adjacent to Moose Lake, they correspond to the channel-dominated areas identified in section and are characterized by conspicuous meander scars at the surface. Zones A and B together account for approximately 113 of the delta-top. and partially overlap Moose River alluvial fan sediments. All sediments identified at the surface in these zones consist of either peat or massive to very faintly laminated. organic-rich clayey silt to silt. In Zone A. faint meander scars reflect the presence of isolated channel sediments found at depth mostly at the lower limited of coring or augering (>2 m). A 14C date from fine-grained clastic vertical accretion deposits in core C4 (330 cm; Fig. 5.3) indicates that river channels were active in the area before and after 6334 2190 yn BP (BGS-2058; 7220 k190 calibrated yrs BP). and tbtmarsh sedimentation hes likefy occuned continuously thfough much of the Holocene. Most of the channel deposits (laterally accreted beds) identified in cores from Zone A (C3, C4, Cl0) appear to be thinner than average. or else compound (e.g. lower C3). Zone B is defined by a single, straight and relatively wide paleochannel, shallow in section (Cg; Fig. 5.3) and overlying older Zone A type quiet water and possibly channel sedirnents as well as debris fiow deposls. The Zone B paleochannel originates at the Moose River fan (and fan-top Moose River paleochannels) and follows the northeast valley side until truncated by a paleochannel defining Zone C. FIGURE 5.6 Aerial photograph (BC 79169 048) showing modem Fraser River/Moose Lake delta surface and morpho-stratigraphie or depositional zones discussed in text. A cross-ccutting relationship indicates that the meander of Zone C predates that of Zone D to the southeast in ternis of abandonment, but the two meanders are closely related spatially and their development likely overlapped chronologically. As indicated by wood enclosed in the lower sequence of core C6 (Fig . 5.3), the centrally located lateral accretion sedirnents of Zone C were ernplaced Ca. 3545 + 80 yrs BP (BGS-2059; 3835 r 100 calibrated yrs BP). If an average modem rate of channel migration of 0.3 myr' were applicable (Kawecki,1995). then completion of the meander might have occurred approximately 2800 yrs BP, followed by its abandonment and the final deveiopment of Zone D. The paleochannel of Zone D is notable in its narrow width and limited thickness of laterally accreted sediments (C5, Fig. 5.3) relative to most others. Zone E comprises a well-defined rneander core anti paleochannel set apparently abandoned at ca. 1830 I90 yn BP (WAT-3046; 1750 i 30 calibrated yrs BP) based on organic materials recovered from the lowermost channel fiIl (Cl, 180 cm; Fig. 5.3). Pnor to abandonment, drainage through the Zone E paleochannel evidently became divided, with part of the fiow re-occupying channels in Zones C and D. Following abandonment, fluvial activity in the central area of the delta shifted to Zone F. that associated directly with the modem Fraser River channel. Zone F is readily defined by forest cover due to persistent overbank sedimentation and thus dry, elevated levee and meander scroll surfaces. The older meander core and paleochannel of Zone E have similar dimensions to those of Zone FI but are lower in elevation and consequently marshy in character. A precise toposraphic survey was not conducted; however, it is evident in the field and from aerial photographs that fioodplain elevation increases across the four younger fluvial zones (C to F), and thus through time. That zones E and F are underlain by multiple lateral and vertical accretion deposits confimis that several metres of aggradation has occurred in these two zones. Wood recovered directly from laterally accreted sediments at the base of core Cl (460 cm; Fig. 5.3) and dated at Ca. 3280 r 60 yrs BP (WAT-3045; 3545 + 80 calibrated yn BP), indicate that former channels in the area were active during the same period as those of zones C and Dl and imply that general aggradation began somewhat later than this. Despite general aggradation through the, meanden of zones D, E and F appear to be ingrown based on surface rnorphological expression and depth to lateral accretion materials (e-g.C8-Cl, FP3-C5; Fig. 5.5~). Zones G and H lack fluvial deposits and are closely associated with Moose Lake in tems of position and elevation. They are characterized by open water, variable marsh vegetation and massive, organic-rich, fine-grained (silt to silty sand) sediments over fine to medium (reworked) andfor fine (deltaic) sands (e.g. C7, Fig. 5.3). While Zone H merges gradually with the fiuvial zones described above, Zone G is separated by a -1 m high step and ridge feature defining a former shoreline (beach and bem) that post-dates fluvial activity in Zone C. A similar beach and bem currently separate Zone G from Moose Lake so that sedimentation occurs in standing water only following occasional flooding of the Fraser River. The modem shoreline is linked to the Fraser River by a relatively narrow distributary channel that evidently resulted from a breach in the main channel bank and levee. It appears that sediment delivered via this channel as it episodically prograded into Moose Lake, resulted in the creation of a series of small spits before it completely sealed off the resulting lagoon (i.e.Zone G). A similar process occun in Zone H with sediments supplied by the main Fraser River channel. in this case closure of the lagoon is prevented by the presence of a small valley-side creek. 5.4.7.4. Provenance of Paleochannel Sediments It has previously been demonstrated (Chapter 2) that two major sub-basins of the upper Fraser Riuer watershed (Le.those of the Fraser River and Moose River; Fig. 5.1) can be readily distinguished based on mineral composition andfor bulk elernental composition of fluvial sediments. Using a simple mixing model, it is therefore possible to calculate the percent contribution (within sub-fractions) of each of the two source basins to downstream deposits. At the present tirne, specific sediment yields from the Moose River sub-basin are relatively high, and this is attributed to both greater ninoff and glacial activity in this part of the watenhed. Given this response, it is expected that Moose River sediment yields may have increased relative to those of the Fraser River during periods of climatic deterioration (colder, wetter) when glaciers may have expanded. This possibility is evaluated here by assessing the sediment composition of the paleochannels preserved in the Fraser River delta-top. Table 5.1 shows selected lithologic and geochemical composition data for the coarse and fine fraction of lateral accretion sediments identified in the delta-top cores (Fig. 5.3) and which may be used to assess provenance. Average modern bed rnaterial composition for the Moose River and Fraser River just upstream of their junction. as well as three sites 1.2, 1.7 and 2.5 km downstream, are also tabulated for comparison. Lithology of the granule (2-4 mm) fraction is classified according to four main groups indicative of different bedrock types in the drainage basin: carbonate (limestone and dolostone); fine-grained clastic (shale and siltstone); guartzite (monocrystalline and complex grains); and, other metamorphics including phyllite. schist and any of the above lithologies if foliation or schistosity is evident. The < 63 Mm fraction was analyzed in tens of elemental concentration (Chapter 2) and selecteci elements applied to provenance determination. Two important complications anse in inferring source from deposited sedirnents: dîfierential downstream mixing (Chapters 2 & 3). and depositional or postdepositional alteration (Chapter 4). The latter problem applies especially to carbonate and quartz (silica) sediments, which respectively tend to decrease and increase in concentration through time due to differing resistance to weathering and abrasion. The problem may be partially addressed by considering non-carbonate, non-silica sub-fractions separately. and here the fine clastic-plus-metamorphic particles of the granule fraction (MIXI; Table 5.1) and SrlU ratios of the < 63 pm fraction f MW)are examined. The latter index is one of the few th& combines relatively immobile elements with sufficient inter-basin contrast to compensate for natural variability and sampling error (see Table 5.1 notes). While not a simple problem, differential mixing occurs such that, on average, sediment contributed by the higher energy Moose River concentrates near the point of initial mixing, but decreases in abundance downstream (approximately 10% per 1000 rn in the coarse fraction and 25% per 1000 m in the fine). To compensate for non-source-related changes, as well as bias introduced by considering specific sub-fractions, indices were derived (FRI, FR2; Table 5.1) which relate apparent mix in paleochannel or lateral accretion sediments to the Table 5.1 Provenance indicators and mixing characteristics of Fraser River deita- top paleochannels. Granule Uthology Matrix Geoc hemisûy Coreor Depth Positiong Zone Carbonate FineClastic Quarkite OtheP Mixl' Index Mi%$ Index PI^ (cm) (ml (16) (Sc) (%) (36) (%) FRlt (%) FR2' C 1 240 1917 E 10 5 (15)$ 56 29 90 1.0 89 1.2 C 1 395 1917 E 6 0 (0) 62 32 106 1.2 71 1 .O C3 458 1585 A O 7 (26) 73 20 79 0.9 88 1.3 C4 280 1704 A O 1 (7) 85 14 99 1.2 96 1.3 C4 380 1704 A 14 5 (22) 64 18 83 1.0 86 1.2 C5 35023860 2 6 (13) 53 39 93 1.0 102 1.2 C6 340 2620 C 70 4 (100) 26 O O 0.0 O 0.0 C7 180 2748 CIG 19 7 (21) 47 27 84 0.9 92 1 .O C8 110 2407 E 13 7 (23) 57 23 82 0.9 48 0.6 C9 270 2343 0 67 5 (100) 27 O O 0.0 16 0.2 Mo098 S~C O F 34 14 (100) 5 1 O River Fraser sfc O F O 4 (6) 30 66 River FOS1 sfc 1150 F 27 10 (21) 26 37 84 62 FDS2 sfc 1704 F 23 9 (24) 40 29 8 1 FOS3 sfc 2513 F t5 4 (10) 42 38 96 98 ') Position is straight line distance from Mwse River-Fraser River jundion (since true downstream distance cannot be determined for paleochannels and pmserved lateral amtion deposits. ") Other includes phylliie. schist and intemediate metamorphic grades. 5) Apparent percent contribution by the Fraser River (see Chapter 2 for derivation): Mixl based on fine dastic and other metamorphic granules only; MDd basad on SrfU ratios in the non-carbonate. non-silica fraction only. Because carbonate and silica are dominant in most samples. their variation influences that of less abundant elements in raw concentration data. Because carbonate and silica are also subject to variations unrelated to source mixing (0.g. differentiai solution and abrasion) it is necessary to consider orner elemenb separately. Of those detectable with the neutron activation analysis used. Sr (associateci with cabnate but relaüuety immobile) is one of the few with inter-basin differences sgnificantly greater than natural within-basin variability. postdepositional change and sampling error (Chapter 2). Inter-basin contrast is enhanced by amparison with U which is relativeiy uniforni arnong ail samples tested regardless of source. Calculated values 5.4.2 Delta Sub-Surface 5.4.2.1 Seismic Sunmy Seismic survey was restricted to solid ground at open. dry sites on the Moose River alluvial fan and the compacted shoulder of Highway #16 (Fig. 5.2). Two profiles (SI, S2) on the Moose River fan provide a relatively continuous partial cross-valley section. while the two down-valley oriented sites adjacent to the roadway (S3,S4) are more widely separated. Signal penetration and return ranged from 105-250 metres depending on surface conditions. seismic velocities and ground roll and air-wave interference. Although signal amplitude was minimal in most cases. numerous reflectors were detected which appear to be correlated and associated with three main facies units. The absence of coherent reflectors in the upper 25 m means that delta-top sediments, including alluvial fan deposits and road-fill, are not resolved using the acquired data. Best penetration and clearest signal return were obtained at sites S1 and S3, the former providing the most complete pichire of the delta sub-surface. In profile SI, the lowest reflector at 220 rns (two-way travel time; Fig. 5.7a) is well defined and interpreted as the contact between bedrock and overlying unconsolidated materials. The calculated depth to bedrock of ca. 250 m is consistent with mid-valley depths to bedrock (> 160 m) inferred from lake sub-bottom acoustic hagery approximately 10 km down-valley (Desloges and Gilbert, 1995; see below). At site SI bedrock is overlain by a 73 m (145-220 ms) sequence (Une 1) of relatively strong and regularly spaced (c 15 rn) reflectors. the lowest of which are best defined. Seismic velocities within this unit appear to be greater than 2200 ms*'. Within the 35 m length of I tower G.L. O 5 10 15 20 25 30 35 b) S2 - "Moose City Transecf O 5 10 15 20 25 30 35 Distance (m) FIGURE 5.7 a,b lnterpreted seisrnic refiecüon profiles showing 3 main facies discussed in text (see Fig. 5.2 for profile locations). G.L.= glaciolacustrine. Approximate depths to prominent reflectors are given to left of diagrams (non-linear scale). Reflections in lower secüons of (a) are ground roll andlor air wave interference. the profile, not al1 reflectors are continuous; however, this could result from variable signal attenuation by overiying maferiaiis. A second acoustically stratified unit (II) approximately 150 rn thick (40-145 ms) overlies the first and rises to within 27 rn of the surface. Seismic velocities in Unit II range from approximately 1800 ms-' to 2400 ms-'. While Unit II reflectors are weaker. less continuous and appear less closely spaced. they are shilar in overall appearance and geometry to those below. The boundary between the two stratified units is also not sharp, suggesting a gradua1 transition between the two sediment masses. The upper 27-33 m of profile SI are devoid of strong reflectors and appear to make up a homogeneous Unit (111) corresponding to undifferentiated delta sediments. Seismic velocities in this unit are relatively low at approxirnately 1300 ms-' to 1800 ms". A 6 m rise in elevation along the base of the Unit III and in the upper layers of the stratified Unit II appears to be continued in seismic profile S2 approximately 100 m to the northeast. At site S2, useful retums were obtained only to 130 m depth (Fig. 5.7b), possibly due to the attenuating effect of the occurrence of drier, coarser grained surface materials. Several relatively pronounced refiectors between 33 m and 21 17 m, spaced approximately 10 m apart correspond to the upper section of Unit II at site SI. The more detailed S2 image shows stratification more clearly and reveals srnall scale relief of up to several metres along sorne of the major contacts. In contrast, the contact with the overlying homogeneous Unl III at 33-35 m is more nearly horizontal. Seismic profile S3 aligned down-valley 1.2 km west of SI and S2 reveals three sedirnentary units similar in appearance to those of SI,but of lesser thickness overall (Fig. 5.7~).In this case the lower stratîfîed Unl I accounts for approximately 40 rn of sediment (124-1 59 ms); however, it is uncertain if the well defined reflector at 160 ms (139 m) represents the lower lirnit of the profile (nearer the valfey side) since interference masked signal retum beyond this point. Reflectors in the lower unit are exceptionally clear, straight and conünuous, and exhibit an apparent dip of 10' toward the northwest. At this location, the overlying less strongly stratified Unit II is approximately 74 m thick (40-129 ms). Intemal reflectors appear more closely spaced (4-8 m) than at SI and S2,again relatively straight and with an apparent dip I Lower G.L. 111 Deka Mm- 79m + 105m + 5 10 15 20 25 30 35 Distance (m) FlGURE 5.7 c,d lnterpreted seismic reflection profiles showing 3 main facies discussed in text (see Fig. 5.2 for profile locations). G.L.= glaciolacustnne. Approximate depths to prominent reflecton are given to left of diagrams (non-linear scale). Refiections in lower sections of (d) are ground roll andlor air wave interference. (10-1 5") towards the northwest. The contact with the overlying homogeneous Unit III apparently dips at approximately 13' in the sarne north westward direction. The upper unit thus varies from 27 to 34 m (38-48 ms) in thickness. At site S4 oniy the upper Unit III and part of the middle Unit II are imaged since clear signal retums were limited to 105 m (180 ms; Fig. 5.7d). In this case the upper unit (III) measures approximately 64 m (124 ms) in thickness, and its lower boundary is neariy horizontal. Within Unit II, only two major reflecton were detected, occurring at 79 m and 105 m depth and slightly undulating. 5.4.2.2 Borehole Records Additional information on the sub-surface in the vicinity of sites SI and S2 was obtained from B.C. Ministry of Transportation boreholes drilled during construction of the Moose River bridge in the late 1960's. The two most useful holes (here designated Cl2 and C13,45 m apart) were located 350 m to the northwest of sites SI and S2. In contrast with the seismic profiles, only 42-46 m of unconsolidated sediment are present above bedrock at this location. The apparent bedrock surface gradient between the two drill sites is 15" north westward while that between the drill and seismic survey sites averages approximately 28" towards the southeast. The latter value is high but falls well within the range of bedrock gradients observed on surrounding valley sides and in lake sub-bottom acoustic profiles (see below). The shallow depth and compressed stratigraphy of the cores is apparently related to their position near the north valley side, and the extension of a bedrock high below the surface. The stratigraphy of the two wres is depicted in Figure 5.8a (upper right). At the southeastem site, intact bedrock is overlain by about 1.5 m of fragmented bedrock plus 1.4 rn of inorganic silt (mud). Otherwise the two logs are similar and begin with 15-19 m of interbedded silty sands and gravels, most of which are poorly sorted. Above the sand and gravel unit, 4.6-5.3 m of inoganic silt and clay are recorded, followed by 10.5-1 1.7 m of silty sand. No information is available on the interna1 structure of these materials. The final, uppermost unit consists of 10.2-12.4 m of poorly sorted gravel or silty gravel, which additional vibracorhg and hand FIGURE 5.8 Longitudinal secüon of Fraser RivediMoose Lake delta and component sediments. (a) main facies interpreted with lower boundaries of foreset and bottomset beds interpolated from Moose River bridge logs and lake morphology (dashed lines), and from seismic profiles and lake morphology (solid lines). DïfFerences anse from proxirnity survey sites to valley sides (see Fig. 5.2 for locations). (b) proposed extent of Neoglacial, Hypsithermal and paraglacial or deglacial sediments; Neoglacial sediment extent based on delta surface morphology, solid and dashed Iines show past delta front positions obtained assuming Neoglacial rates applied throughout the Holocene; shaded Hypsitherrnal area assumes HypsithemaVNeoglacial ratio (40/60) similar to that found in distal Moose Lake sediments. See text for explmation and discussion. augering reveal is crudely bedded and overlain locally (e.g. Cl1, C14,Cl 5; Fig . 5.2) by fining upward laminated sands and organic-rich silts or woody peats. Although detailed sedimentology is not available. the Moose River bridge sediments may be tentatively interpreted from location and stratigraphic position, and partially correlated with the seismic profiles. The upper gravelly unit of the Moose River bridge cores clearîy represents alluvial fan deposits (coarse channel and fine overbank) of the Moose River apparently deposited on the surface of the Fraser River delta, but too thin to be resolved on the seismic profiles. The coarsening upward (clay-silt to silty sand) rniddle sequence of the Moose River bridge section is infened to have resulted from deltaic (fine bottomset and coarse foreset) sedimentation similar to that currently operating in Moose Lake. These deltaic and alluvial fan sediments together correlate mainly with the uppermost unit (III) of the four seismic profiles; however, given the paucity of interna1 reflectors in the upper part of Unit 11 (-27 m to 60 m), it is possible that the claylsilt layer of the cores corresponds to the uppermost part of this unit. Although significantly thinner, the lower sand and gravel layen of the Moose River bridge boreholes apparently correspond to seismic units II andlor 1. 5.4.2.3 Delta Geometry Figure 5.8a represents a simplified down-valley sectional view of the upper valley-fiIl from Moose River to Moose Lake based on the available data. The lower limits of deltaic and lacustrine sediments (foreset and bottomset beds respectively) are depicted by two sets of Ifws in the diagram. The uppar (dashed)set represents a straight-line interpolation of the foreseübottornset and lower bottomset boundaries based on the Moose River bridge logs and lake bottomlsub-bottom topography (a-a'; see Fig. 5.2). Given the shallow depth of the valley side boreholes, this interpolation and implied delta geometry are most likely a minimum estimate of delta vertical extent. The interpolation agrees well with boundary locations in seismic profile S3 sited along the same down-valley transect, but underestimates boundary depths in the case of seismic profile S4 further to the northwest. Depth to bedrock is thus apparently reduced along the northeast margin of the valley, at least behnreen sites S3 and Cl2/13 adjacent to the culmination of the Rainbow Range structural block (see Fig. 5.2). The lower set of iines (solid) in Figure 5.8a depicts lower deltaic and lacustnne boundaries based on lake bottom morphology plus seismic profiles SIISZ and S4, corresponding to more central and, in the latter case, down-valley locations (a-an;see Fig. 5.2). This reconstruction depicts probable maximum delta thicknesses away from valley side walls, and is more representative approaching Moose Lake. Using either interpolation, a substantial lakeward thickening of the delta foreset body is indicated. Figure 5.9 contains schematic representations of valley-fiIl morphology aligned cross-valley (at Moose River and 7 km down Moose Lake). The most striking feature of the up-valley (at Moose River) cross-section is the apparent over- deepening of the central valley (sub-surface) relative to ejrposed valley-sidz bodrock slopes (Fig. 5.9a.b). This is especially notiœable on the better constrained northeast valley side where a pronounced break in slope is implied at approximately 990 m a.s.1. The fom of the bedrock on the south side of the valley is less certain because lower valley-side slopes adjacent to the delta are mantled by unconsolidated material with moraine-like surface expression. Seismic Unit III (delta and alluvial fan materials) coincides with the widest section of the sub-surface profile. Details of the down-valley section (Fig. 5.9~)are based on lake sub-bottom acwstic p~ofiles(Fig 5.M; also Fig. 3 in Desloges and Gilbert, 1995). The down- lake profile contains three obvious acoustic facies units which partially correspond to the up-valley seismic units. The lowest acoustic unit, here designated Unit O is distinctive in that it appears dense and stnictureless relative to stratified materials above it. While overlying stratified units appear to make up one body of sediment (approximately 35 rn thick), an upward decrease in the strength and regularity of reflectors suggests that the middle acoustic unit corresponds to both stratified seismic units (1 and II). The stratified acoustic materials are thus designated Units 1/11 (combined) to coincide with the seismic units. Acoustic Unit III represents Holocene. deep water lacustrine sedimentation (Desloges and Gilbert, 1995), and FIGURE 5.9 Valley-fiIl cross-sections inferred from seismic and acoustic imagery, borehole logs and 1:50,000 scale NTS topographic maps. (a) detailed section at Moose River; (b) general cross-section at Moose River; (c) detail cross-section 7m down-lake from current shore line; (d) sub-bottom acoustic image detail, 7 km down- lake from current shoreline, from Desloges and Gilbert (1995). while different in character (i.e. silt/clay). corresponds to the deltaic sediments of seismic Unit Illlupper Unit II in ternis of time of formation and the lake depositional system (Le. distal verses proximal). Acoustic Unit III is more opaque and finely stratified in the uppenost and lowerrnost several metres. The up-valley and down-valley cross-sections both exhibit valley centre over- deepening; however, down-valley this occurs at two levels (i.e. -1 030 m a.s.1. or lake level; -930-940 m a.s.1.; Fig. 5.9~).The two cross-sections also contrast in ternis of depth and thickness of sediments those down-valley being less. Since Desloges and Gilbert (1995) assume constant sound velocities (1460 m s-') in the fine-grained sediments below Moose Lake, down-valley depths (to bedrock in particular) are probably underestimated (by up to - 30 m; cf. Gilbert. 1997). Shallower depths than indicated are consistent with down-valley increases in bedrock elevation behnreen the two sites and also evident from other acoustic records near the distal end of the lake (see Desloges and Gilbert, 1995, Fig 3c). Nominally. the implied average bedrock gradient between the up-valley and down- valley sites is approximately 0.5" dipping eastward. Thickness, but not necessarily volume, of valley-fiIl sediments approximately triples over the same distance. 5.4.3 Holocene Sediment Storage Estimates Sediment storage estimates for the Moose LakelFraser River delta (Table 5.2) were made based on the sectional data described above (Figs. 5.5 and 5.8), corelauger logs, and particle size analysis of representative sediments. Detailed composition of delta topsetlalluvial fiIl was determined based on 36 unifomly spaced corelauger logs totaling 126 m in length. Segments of core record were categorized according to eight textural classes ranging from clayey silt to gravelly sand, and each class characterized in ternis of actual sandlgravel. silt and clay content based on 45 separate particle size analyses. Peat sections containing varying amounts (1949%) of silt and clayey silt. were considered separately. Calculation of arealvolume and average composition by weight was made separately for sections of fiIl identified earlier (Fig. 5.5) as channel dominated Table 5.2 Holocene sediment storage estimates for Moose Lake and the Fraser River delta Moose Lake bottom': 1.6x1O1'kg (27Oh) (al fine fraction)' (37%) Subdelta (bottomset): 0.47 x 10" kg (8%) (al1 fine fracti~n)~ (11 Oh) Delta Foreset: 3.4 x 1O" kg (57%) 2.1 x 10" kg fine Fraction (49%) (average 6 1% fine) FloodplainlDeIta Topset 0.48 x 10" kg 0.13 x 10" kg fine fraction section 1% 0.08 x 10" kg (average 27% fine) section Il 0.09 x 10" kg (8%) (3%) section Illa,b 0.31 x 1O" kg Total: 6.0 x 10" kg 4.3 x 10" kg ') Measured to average depth of Ilm previously interpreted as Iimit of normal Holocene sedimentation (Desloges and Gilbert, 1995) 5) Oeita sub-ôottom assurned similar 10 distal lake bottom sedirnents; sue distributions in Chapter 4. t)Massivolume estimate is a minimum sinœ based on otraight-line interpolation of lower contact (see Fig. 5.7 and discussion in text). $) Sections delineated in Fig. 5.7: sedion I is channel dominated: sedion II is quiet -ter dominated; section III is Moose Ritalluvial fan (section 1), quiet water dominated (section II) and alluvial fan (section Illa,b). Delta foreset material, mostly silt and fine sand, was characterized based on 12 samples taken from the nearshore lake bottom and the lower section of two cores. Distal lake bottom sediments were analysed by Desloges and Gilbert (1995) and in Chapter 4. and found to consist almost entirely of silt and clay. Volumes of deltaic sediment were initially calculated from cross-sectional area and an average delta width of 1370 m. This approach yields good estimates of the topset/alluvial fiIl where valley width is relatively constant with depth (see Fig. 5.9),but is conservative in the case of delta foreset and sub-bottom sediments where lower boundaries are based on the straight-line interpolation shown in Fig. 5.8. Conservative estimates are reported here due to the uncertainty involved in estimating the threadimensional geometry of the sub-surface. Non-delta lake bottom sediment volumes were based on lake area beyond the foreset slope and an average Holocene sediment thickness of 11 m reported by Desloges and Gilbert (1995). Volumes were finally converted to mass estimates assuming bulk densities of 750 kg mW3for peat, 1500 kg mJ for relatively fine-grained alluviaVtopset and foreset materials other than peat, 1800 kg rnJ for gravelly alluvial fan materials, and 1250 kg m') for lake bottom and buried bottomset sediments. These values were arrived at through consideration of selected core measurements, typical porosities of fine and coarse grained sediments and bulk densities reported in the literature. Table 5.2 clearly shows that delta foreset sedimentation accounts for the bulk (- 57%) of deltaicAacustrine deposition in the Moose Lake basin. Foresets account for a slightly lower fraction (-49%) of the c 63 Pm sediment considered alone; however, this is likely an underestimate given the conservative volume estimate plus the fact that foreset texture was estimated from uppermost (proximal) foreset sediments only. Distal lake bottom sediments account for approximately 27% and 37% of total and c 63 Urn deltaidlacustrine deposition respectively; delta topsetlalluvial sediments account for approximately 8% and 3% respectively. Most of the fine-grained materials of the topset alluvium are conœntrated in the Fraser River floodplain (30% coarse. 58% fine, 12% organic by weight) as opposed to the Moose River alluvial fan (94% coarse. 6% fine. negligible organics). 5.5 DISCUSSION 5.5.1 Floodplain Development and Holocene Environmental Change The alluvial cap of the Moose Lake-Fraser River delta preserves a record of deposition and reworking of sediments spanning most or al1 of the Holocene. A substantial area to the northeast (Zone A/B) derives from the early half of the Holocene. for some time before and after 6330 yrs BP. These early Holocene sediments may have been more extensive originally, but have been partially reworked or are now buried by younger Fraser River sediments and to a lesser extent Moose River alfuviaf fan sediments to the southeast. The alluvial fan currently extends completely across the valley and confines the Fraser River against the south valley side. Most of the fan's growth probably occurred during the earliest part of the Holocene under strongly paraglacial conditions (Church and Ryder, 1972; Church and Slaymaker, 1989). and as a result. the fan underlies a large proportion of the Zone Ai6 floodplain. Though most cores contain laterally accreted beds. the fragmentary occurrence of paleochannels and lack of meander scrolls in Zone NBindicate both the incornplete reworking of the floodplain, and substantial overbank quiet water accumulation. Small channel dimensions (narrow widths; thin lateral accretion deposits) relative to the modem Fraser River indicate relatively low levels of discharge. With the exception of the oldest laterally accreted sediments sampled (Ce460cm; also the coarsest and thickest), the bed material load of early Holocene time appean to have bem chafacterized by relatively littie contribution from the Moose River basin or at least from sub-areas characterized by carbonate bedrock. Superimposed channels and thick vertical accretion sequences in Zone A/B reflect substantial aggradation during and since the early Holocene due in part to long terni subsidence. In consequence, much of the delta has becorne a long terni sink for mainly fine-grained sedirnents that othenMse would have reached Moose Lake. These areas also provide a relatively continuous long term record of sediment fluxes and organic matter accumulation. The paleochannel of Zone B is closely associated with the cornplex of early Holocene delta-top sediments. but is distinctive in ternis of composition. morphology and degree of preservation. The paleochannel runs along the northeast edge of the delta. apparently originating at the western margin of the Moose River alluvial fan where faint channel swrs are visible beneath forest vegetation. In contrast with other paleochannels in this area of the delta. the Zone B channel contains little or no material derived from the uppermost Fraser River. As its low sinuosity and high widthldepth ratio are also comparable to those of the modem Moose River. it appears the paleochannel represents a direct extension of the Moose River to Moose Lake. active some time during the middle Holocene. Relatively clear surface expression (preservation) suggests a comparatively young age. limited by subsequent fluvial activity in Zone C (ca. 2.8-4.0 ka BP). The separate delivery of (distinctive) Moose River and uppennost Fraser River sediments to Moose Lake that must have occurred during this stage of delta floodplain evolution is similar to that of the Lillooet and Green rivers at Lillooet Lake described by Gilbert (1975). At Moose Lake. most floodplain development since ca. 4000 yrs BP has been limited to the southern and western areas of the delta. The earliest recorded sediments of the period (3560 r 100 yrs BP) occur within 1 km of Moose Lake and mark the central portion of a meander loop (Zone C) that developed over several hundred years perhaps as late as 2800 yrs BP. During this time. the nver shifted over 400 m northward and then entered Moose Lake adjacent to the northeast valley side. The paleochannel and inner meander are obscured due to moderate subsidence and infitting, but resernbte modem river equivatents in temof channet and meander dimensions (see Fig 5.6). Prior to ca. 3560 yrs BP, the river apparently carried bed material with a large (relative to modern) and increasing component of Moose River sediment. Moose River contributions at Ca. 3560 yrs BP are the highest documented; however, based on meander and channel morphology, recombined Moose River and Fraser River flow probably occurred at this tirne. Contemporaneous to slightly younger nver activity is represented by a second meander loop (Zone D) and laterally accreted sediments preserved in the sub- surface further to the south (Zone E). It is not possible to determine the exact sequence of events at this time because of the fragmentary and overlapping nature of the deposits. The buried Zone E sediments dated at 3280 î 60 yrs BP is similar in thickness to those of modem and Zone C channels, but slightly finer in texture. The source signature of these sediments exhibits slightly greater Fraser River influence than modem channel sediments, so evidently, Moose River contributions had decreased by this time. The decrease is further indicated by the Zone D meander loop where even finer, laterally accreted sediments (C5-350 cm) also have a modem source signature and the preserved paleochannel is unusually narrow (see Fig. 5.6). The age of this low discharge paleochannel is unknown. but must be less than that of Zone C to the north. The most prominent paleo-meander loop on the Roodplain (Zone E) was cut off Ca. 1830 î 90 yn BP. Prior to this event. some water and sediment was evidently reintroduced to the older Zone D and Zone C paleochannels to the northwest via a breach in the channel bank. For several hundred yean leading up to ca. 1830 yrs BP, Moose River contributions appear to have been above modern values but decreasing. At the time of abandonment, the river channel had dimensions similar to the modem Fraser River channel (see Fig. 5.6) with slightly finer textured sediments. Sediment composition was also similar to today. During the last 1750 years the Fraser River has continuously occupied and shifted within a relatively narrow zone along the southwestern margin of the delta. The largest meander loop. closest to Moose Lake. appean to have shifted northward approximately 400-500 m during that time, suggesting average migration rates (0.22-0.28rn yr-') less than modem rates of 0.29-0.40 (cf. Kawecki, 1995). Details of river change over the last 1830 years cannot be ascertained because the recent pointbars of Zone F were not accessible for coring. It appears that throughout this most recent period, and probably during the evolution of Zone E as well (perhaps 2500-3000 years in total), the point of entry of the Fraser River into Moose Lake has remained relatively constant, and this has facilitated the pronounced extension of the leveed bird's foot channel. The timing of subordinate distributary development and associated shoreline advancement is unknown. Since maximum lateral shear stresses and flood thalweg location are expected to occur just downstrearn of a meander apex, the levee breach responsible for distributary formation may have occurred when the final meander loop was in a more southerly position approximately 750-1 000 yean ago. Overall reduced Moose River sediment contributions to the older areas of the delta are consistent wiih the general idea of a wamer and dner 'Hypsithemal" period characterizing most of the early Holocene (Heusser, 1956; Osborn and Luckrnan, 1988; Leonard and Reasoner. 1999; Desloges, 1999). To the extent that a strong correlation exists between carbonate bedrock and the occurrence of glaciers in the Moose River basin and upper Fraser River watershed in general, it appean that in the early Holocene, glaciers were reduced or even absent. The almost complete absence of glaciers in some watershed that currently contain glacier ice in the Canadian Rockies has been proposed but not confimed (e.g. Luckman and Osborn, 1979; Desloges, 1999). Because of higher elevations and greater relief, the Moose River sub-basin sediment yield is expected to be more sensitive than the uppennost Fraser River sub-basin to changes in temperature, precipitation, runoff. and vegetation cover as well as glaciation. Higher sediment yields would be expecteâ to accompany elevated precipitation, runoff or glacier activity, but because these may be positively correlated, it is difficult to distinguish unique causal effects. Since early to middle Holocene floodplain morphology at Moose Lake also points toward lower levels of discharge, it remains uncertain to what extent glaciers specifically played a rote in early Holocene sediment Ruxes. Later fluvial activity and related deposits of the Moose Lake delta-top floodplain are more obviously associated with glacier fluctuations known to have occurred regionally. Overall, the Neoglacial Penod has been characterized by a shift of fluvial activity frorn the northeast side to the southwest side of the upper Fraser River delta in the vicinity of the Moose Lake shoreline. The earliest part of the Neoglacial record documented from channel-related sediments is that leading up to ca. 3560 yn BP (3840 calibrated yrs BP). during which time Moose River contributions to downstream bed material load were higher than present and increasing. This increase ceased and bed load became upperrnost Fraser River dominated (apparently more so than today) shortly after ca. 3280 yrs BP (3550 calibrated yrs BP). The increased Moose River sediment yield at this time likely reflects pronounced climatic deterioration associated with the "Peyto" glacier advance (nominally ca. 3300-2800 and Ca. 2500 yrs BP; Luckman et al., 1993). The shift to increased uppermost Fraser River sediment composition observed here is consistent with the apparent break in the Peyto event, suggesting that glaciers became inactive or actually retreated for a time. Slightly older 14Cages deterrnined in this study could reflect minor residual contamination of samples due to carbonate reworking in the contributing basin and sediments. While earliest Neoglacial climatic deterioration (Le. Boundary Advance; Luckman et al., 1993) is well known from glacier forefield (Gardner and Jones, 1985) and lacustrine (Dirszowsky and Desloges, 1997; Leonard and Reasoner, 1999; Desloges, 1999) evidence, it is not identified in the laterally accreted sediments of the Moose Lake delta floodplain. Sometime after Ca. 3280 ynBP and the post-Peyto wan phase, general aggradation of the fioodplain began and river channel characteristics and sediment composition tended towards values characteristic of modem (post-Little Ice Age) conditions. lncreased or increasing sediment yields through this period are consistent with the gradvally deteriorating clirnatic conditions and multiple glacial advances that culminated in the late 19"' century (Luckman, 2000). Limited channel incision recorded by the three rnost recent meander and paleochannel sequences of zones DI E and F may represent reduced sediment yields defining minor climatic improvements between glacial advances. However. only once were Moose River contributions greater than present identified for this period (specifically leading up to ca. 1830 yrs BP or 7750 calibratecl yrs BP). At ca. t 830 yrs BPI sediment yields and climatic conditions were apparently similar to today. Specific glacier advances known regionally (i-e.unnarned advance ce. 1500 yn BP; Cavell Advance beginning ca. 1000 ynBP; Luckman et al., 1993; Luckman. 1995) were not specifically identified in the laterally accreted deposits of the Moose Lake delta-top floodplain. In contrast. long-lived vertically accreted deposits from the delta's older manhy areas are more continuous and may record al1 significant phases of climatic deterioration and glacier advance. From the prelirninary assessrnent of sediment composition based on LOI, five prominent carbonate peaks denoting increased Moose River contributions may be correlated tentatively with al! five known glacier advances (Le. Boundary, Peyto, unnamed, Cavell, and Little Ice Age maxima). Dating of peat sequences to confimi and constrain these events chronologically has not yet been camed out. The magnitude of climatic fluctuation also may not be easily assessed here since LOI peaks and inorganic accumulation in general are influenœd by changing fioodplain configuration and sedirnent delivery processes. Overall, Neoglacial advance and retreat stages identified in the region appear to be recorded but not perfectly delineated in the delta-top sediments. In this context, the contribution of glacial activity to sub-basin sediment yields is more certain and this is reflected in consistent lithologicaVmineralogical composition in the coarse and fine fractions of the bed material sampled, and sensitive representation in fine-grained overbank deposits. While delivery of coane material is largely a function of stream cornpetence and thus discharge or runoff levels, fine material is expected to correlate more closely with glacial activity due to sediment abrasion and the production of 'rock flour' (Karlén, 1976; Matthews et al.. 2000). This distinction is evident in the vertically accreted overbank sequences where lower organic content (inorganic peaks) due to higher discharge and flood frequency does not necessarily coincide with higher carbonate content probably due to glacier production in the Moose River sub-basin. 5.5.2 Late Pleistocene Valley-Fill The alignrnent and to some extent the cross-sectional asymmetry of the Fraser River valley in the vicinity of Moose Lake are structurally controlled by the Moose Lake-Chatter Creek fault (Dechesne and Mountjoy. 1990); upstrearn of Moose Lake the southwest valley walls are slightly steeper on average. In contrast, over-deepening of the valley centre and stepped valley-wall cross-profiles point to the effects of two or more styles of glaciation. In the vicinity of Moose Lake, prominent breaks in valley-side dope occur at approximately 1,350 rn a.s.1. (300 m above cuvent valley floor) and between 1,030-1,060 rn a.s.1. at or just below the modem valley floor and lake level. The simplest explanation for this profile would be that inset troughs were produced by persistent but reduced ice flow from relatively localized sources between episodes of full glaciation such as the Last Glacial Maximum (LGM). It has been suggested that maximum ice thicknesses in the area rnay have reached 2400 m (Roed. 1975) at which time Cordilleran ice fiowed eastward through the Fraser River valley and across Yellowhead Pass (Roed et al., 1967; Bobrowsky and Rutter. 1992). In contrast, local valley glaciers active between regional advances rnay have flowed westward, down gradient toward the Rocky Mountain Trench (Desloges and Gilbert. 1995). Ice fiow directions in the upper Fraser River valley would have been complicated by the fact that generally eastward flowing Cordilleran ice orig inating west of the Rocky Mountain Trench probably entered the Fraser Valley at points both upstream and downstream of Moose Lake at different times (cf Roed et al., 1967). During full glacial conditions (e.g. early Late Wisconsin; Bobrowsky and Rutter, 1992), ice emanating from the Moose River valley apparently joined main eastward flow via pronounced breaches in the upper valley-side several kilometres west of the present valley outlet. The convergence of large eastward flowing glaciers rnay be responsible for some of the valley widening in the vicinity of the Moose River-Fraser River junction. particularly along the southwest side of the valley. It also rnay be associated with formation of the mid-valley "south ridgen running adjacent to the delta (see Fig. 5.2). The composition of the ridge is unknown (i.e. bedrock vs. unconsolidated debris); however, its streamlined morphology strongly suggests that it is glacial in origin and rnay have resulted from flow separation at the valley junction dunng full glacial condlions. Topographic expression of the ndge has been enhanced by the more localized glacial down- cutting of the central vaIfey fioor aiong ifs norfhern fiank. Greater depths to bedrock observed in the southeastem end of the Moose Lake (bedrock) basin below the delta rnay indicate enhanced glacial scour at the valley junction. This would be expected from the combination of Moose River valley and Fraser River valley ice flow. and consequent increases in shear stress, pressure melting and flow velocities. Ice convergence at this location near the modem valley junction, would have involved srnaller. early and late-stage glaciers and north westward flow within the Fraser River valley. That bedrock elevations in the central valley increase towards the northwest suggests that basal shear stresses were quickly dissipated as the valley widened and ice thinned. Valley-fiIl of glacial origin at the northwestern end of the Moose Lake basin indicates that north westward flowing ice terminated for some time at that location (Desloges and Gilbert. 1995). Bedrock cross-profiles below Moose Lake reveal a final break in valley side slope at 930 m a.s.1. that defines a narrow gorge approximately 70 m in depth. While the pronounced v-shaped profile of the gorge does not preclude a glacial origin (Eyles and Menzies, 1983), the possibility of fluvial erosion is also raised. Fluvial erosion of this magnitude would require both a low level westward draining outlet to the basin and that most of the observed valley cross-section predates at least the LGM. Since bedrock elevations at the northwestern end of the Moose Lake basin are obscured by overlying coarse-grained , acoustically opaque sediments, details of fluvial or glacial activity in the valley predating the LGM remain to be clarified. The sedirnentary valley-fiIl examined in the vicinity of Moose Lake consists of ihree main parts: the massive acoustic Unit O (northwest of basin only); the larger body of mainly stratified material of seismidacoustic units I and II; and the recent (Holocene) deltaic and lacustrine sediments of seismic/acoustic unit II1. Little is known of the sedimentology of the deeper sub-surface materials except that they consist of sand and grave1 interbeds in the vicinity of the Moose River bridge and largely of finer grained (acoustically transparent) materials below Moose Lake to the northwest. The widespread, large-scale acoustic and seismic stratification of most of the sediments, plus their basinal context indicates that much of the Moose Lake area valley-fiIl is of glaciolacustrine origin. Seismic velocities in the southeastem end of the basin are generally lower than would be expected in tills or other sub- glacially deposited materials subjected to significant compaction (cf Eyles et al., 1990). Similar deposits have been identified and desctibed in the fiil of numerous valley systems of the western Cordillera. Based on their context. the large volumes of sediment involved, and geophysical and correlative stratigraphic evidence, most deposits have been attribut4 to late Pleistocene deglaciation and rapid sediment release into iœ marginal or proglacial lakes (e.g. Shaw and Archer, 1977; Eyles et ai.. IWO; Mullins et al., 1991, Desloges and Gilbert, 1995; Spooner and Osborn, 2000; Eyles et al., 2000). In some studies of western Cordilleran valley-fills, sediments at depth have been linked at least tentatively to full glacial conditions (Le. possible tiiis) or else earlier Pleistocene glaciations in general (e.g. Eyles et al., IWO). In the Moose Lake setting, this interpretation might apply in the northwestem part of the basin in the case of the lowerrnost acoustic unit (O), where the massive, relatively dense appearance and possibly coarser texture of the unit points toward direct glacial deposition. A glacial origin is consistent with the identification of moraine-like ridges below glaciolacustrine sedirnents near the northwestern margin of the basin and correlative sub-aerial ridges damrning the lake at its cuvent level (Desloges and Gilbert. 1995). The stratigraphic position and internat appearance of the sub-bottom ridges is similar to the basal Unit (O) described here, indicating a probable genetic relationship. It appean that while bedrock scour and sediment evacuation prevailed in the southeastem end of the basin, decreased shear stresses below a thinning ice margin allowed or promoted the accumulation of sub-glacial (lodgment) and ice marginal debris to the northwest. While the general character of the two glaciolacustrine units (1 and II) is sirnilar, there is a definite upward shift frorn well defined seismic/acoustic stratification to stratification that is less pronounced, less regular and less frequent. Recognizing that reflectors do not correspond one-for-one to actual sediment beds, the two units apparently record changing styles and decreasing rates of sedimentation, which would be expected at or near an actively retreating ice margin or due to waning sediment supply to the lake. Desloges and Gilbert (1995) inferred mat gtaciofacustnne sedirnents in aie extreme northwestern of the Mobse Lake basin were deposited in close association with (sub-aqueous) recessional moraines as the ice began to retreat from that area. Based on stratigraphic position, it is likely that the lower glaciolacustnne Unl I identified here represents similar ice proximal (ice contact) sedimentation corresponding to retreat of ice across the full basin. The actual processes operating at this time could range from sub-glacial or sub-aqueous 'outwash" (Eyles and Menties. 1983; Eyles et al., 1990) to rapid deposition from suspension of relativeiy coane and poorly sorted material adjacent to or below the (fioating?) iœ (Hicks et al., 1990). While sedimentation rates would have been high, this phase of deposition would have been relatively short-lived as long as the ice cantinued to retreat. Quick, continuous retreat is suggested by the relative thinness of the lower glaciolacustrine Unit (1). Further from the ice front, or as ice retreated from a particular location, one would expect sedimentation to shift to settiing from suspension of finer and finer material via overflows or interflows, and increasingly toward emplacement by far reaching turbidity cunents depending on specific sediment loads and waterlsediment density (Gustavson. 1975). The result would be thin-bedded, possibly rhythmic, sedirnents decreasing in (bed) thickness and particle size away from the ice-margin or later input streams. Such sediments would appear relatively transparent on seismic and acoustic imagery and thus may account for most of seismidacoustic Unit II. Since al1 of the above styles of sedirnentation rnay occur over varying distances depending on the strength of flow. size and concentration of sediment, and rate of source retreat, the transition between the lower and upper glaciolacustrine units may range from abrupt to transitional as appern to be the case. The thickness of the glaciolacustrine units (1 and II) increases from -50 m in the northwest to > 220 m in the southeast, more than compensating for increased depths to bedrock. If ice contacted the lake the entire time the glaciolacustrine units were being deposited, gradua1 southeastward thickening most likely implies that ice retreat rates decreased as the basin was exposed and infilled. Based on the occurrence of probable recessional moraines, Desloges and Gilbert (1995) proposed a mode1 for the distal basin arguing that retreat rates would have inevitably decreased as iœ backed away from higher ground and the basin widened. However, since the basin nanows again in the southeast. it appean that decreasing retreat rates may also have depended on glaciologie factors such as mass balance, ice flow dynamics and sediment supply. The possibility also exists that not al1 of the glaciolacustrine sediments (e-g. those of upper Unit II) were deposited while ice remained in contact with the lake. If ice had retreated from the lake basin at some point, dom-lake accumulation rates would have decreased as a simple function of inflow proximity. Rates of sedimentation in modem iœ contact (e.g. Gustavson, 1975; Hicks et al., 1990) and proglacial (e.g. Gilbert and Shaw, 1980) lakes and fjords (e-g. Cowan 1?O et al., 1988; Smith. 1990) may be on the order of several metres per year, decreasing rapidly away from the sediment source. Since rates of sedimentation in comparable late Pleistocene settings should be somewhat higher (e-g.Ashley. 1 975; Shaw and Archer, 1977; Eyles et al., 1990). overall glaciolacustrine sediment thickness in the Moose Lake basin suggests that exposure of the whole lake basin may have taken no more than a few decades. 5.5.3 Delta Development and Basin Sediment Yields The upper seismic/acoustic Unit III corresponds to lacustrine and delta sediments more typical of modem (and Holocene in general) conditions. Excluding delta-top alluviurn, delta foreset sediments were sampled infrequently but indicate relatively low energy conditions (Le. mainly silts) between site Cl and Moose Lake (see Fig. 5.2). It is evident from the age and style of fioodplain development on the delta surface, that much of the delta's overall volume in fact dates from the early Holocene and possibly earliest Holocene or late Pleistoœne. Delta surface areas dating from the early HoloceneMypsithenal extend from the Moose River alluvial fan to within 1 km of Moose Lake, at least on the northeast side of the valley (see Fig. 5.6). This implies that at most 1.3 km of delta progradation, including the bird's foot channel extension, occurred dunng the Neoglacial Period. Figure 5.8b shows hypothetical delta front positions at 7000 and 10,000 yrs BP back-extrapolated on the assumption that delta progradation has always occurred at Neoglacial rates and with constant foreslope gradients. While these positions are very approximate and depend on the depth interpolation used (valley centre vs. offcentre; solid vs. dashed lines), they imply that a minimum of 11-38% of delta volume from Moose River to Moose Lake (2438% of equivalent surface area) was already in place at the start of the Holocene (ca.10,000 yrs BP). The values are minimum estimates because, (a) the eastern limit of the Moose Lake basin has not been idenüfied so the full extent of early delta construction is not known, and (b) high Neoglacial progradation rates were used to account for low energy Hypsithermal progradation. The location and nature (Le. sharp verses gradual) of the transition frorn high- energy late Pleistoceneiearly Holocene to low-energy Hypsithermal sedimentation cannot be precisely identified. In distal Moose Lake, corresponding transitional materials (mainly interbedded sands and silts) have been identified through coring (Desloges, 1999). but the stratigraphic context and total thickness of these sediments are uncertain. Massive fine-grained sediments which certainly represent Hypsithermal sedimentation make up approxirnately 40% of the post-glacial (Le. Hypsithermal plus Neoglacial) sequence. Lake-wide applicability of this proportion is unknown and does not necessarily apply to deltaic sedimentation governed by distinct processes involving coarser sediments. However. if the 40% figure is adopted as representative. it irnplies that low-energy Hypsithermal sediments account for at most 16-24% of delta volume. Amved at in this way. deglacial and paraglacial sediments combined would then account for at leasi 41-59% of total delta volume and extend to within 1.6-1.8 km of Moose Lake at the mouth of Fraser River. Extensive early Holocene delta growth beyond that associated with deglaciation is consistent with the idea of paraglacial sedimentation first discussed by Church and Ryder (1972). According to this model. high but gradually declining rates of sedimentation result from the reworking of large amounts of unstable sedirnent on unvegetated surfaces left by large Pleistocene glacien. The efFect. whose duration depends on the scale of basin considered (Church and Slaymaker. 1989), is generally manifested in widespread alluvial fan development. valley-bottom aggradaüon and subsequent terrace devetopment as seen ttiroughout the western Cordillera (Church and Ryder. 1972; Ryder and Church, 1986; Beaudoin and King, 1994). Comparable features are evident in the upper Fraser River valley and include the Moose River alluvial fan. an integral component of early delta-top fioodplain development. That large volumes of deltaic sediment were already in place by 10.000 yrs BP suggests that some of the sediments of seismic/acoustic Unit III may be deglacial rather than paraglacial, and correspond to the retreat of glaciers up-valley after having left the lake basin. This possibility is consistent with the glaciolacustrine valley-fiIl model proposed above in which lacustrine seismic/acoustic Units I and II correspond to ice-contact and icegroximal (but in-lake) sedimentation. Further clarification of local deglaciation history requires direct dating, or at least direct examination. of the sediments making up the valley-fiIl units. particularly near facies transitions and in the unmapped southeastem portion of the greater Moose Lake basin. The model by which formerly glaciated and ice scoured valleys of the Western Cordillera are rapidly infilled with large volumes of glaciolacustnne sediment during deglaciation has now been widely documented (e-g. Shaw and Archer, 1977; Eyles et al. 1990, 1991, 2000; Mullins el al.. 1991; Desloges and Gilbert. 1991). Though on a somewhat smaller scale, the Moose Lake basin illustrates that similar processes occur in comparatively high elevation upland valleys and may be even more extensive than previously thought. It has been pointed out by several researchers that late Pleistocene glacial sediments are frequently subject to later fluvial reworking and thereby contribute significantly to interglacial (i.e. Holocene) sediment fluxes (e.g. Ryder and Church, 1986; Brooks. 1994). Such remobilization is probably most significant where former glaciolacustrine deposition occurred at high levels due to ice stagnation or retreat resulting in the damming of normal drainage pathways (e.g. Eyles and Clague, 1991; Sawicki and Smith. 1992; Huntley and Broster, 1997). In the upper Fraser River valley, higher terraces exist but are attributable to ice-marginal kame deposition and largely inaccessible to later river erosion. Lower terraces resulting from degradation of valley-floor glaciolacustrine and glaciofluvial materials are relativety restricfed in extent. Whife g taciotacustrine vatley-fills certainfy represent a large component of sediment flux associated with phases of Pleistocene glaciation, those deposited in overdeepened basins such as that below Moose Lake are not available for reworking during interglacials. 5.5.4 Implications for Holocene Deposition in Moose Lake Sediment storage estimates derived earlier (Table 5.2) clearly show the dominance of deltaic sedimentation in the Moose Lake reservoir and point toward possible geomorphic controts on distal lake sedimentation during the Holocene. Considering the probable Neoglacial and Hypsithermal sediments only, the ratio of distal lacustrine storage to delta or proximal lacustrine storage (including sub-delta; Table 5.2) is approximately 1:1.4 (or 1:0.9 if only the c 63 pm fraction is considered). Considerable uncertainty exists in these estimates because of problems in defining the surface chronology and in delineating the lower and lateral limits of each sediment mass (Fig. 5.8); the volume of distal lake sediment for example is assumed the same for these estimates and those made earlier. lgnoring deglacial and paraglacial sediments, delta foresets no longer account for the bulk of fine sedirnent stored in the system; however, the amount is still substantial. The proportion of fine sediments stored in the alluviaVtopset body (excluding the Moose River fan), on the other hand, remains low at about 4%. Despite its relatively small part of the sediment budget, the alluvialltopset component is of concem because of its propensity for reworking and downstream re-release of sediments at a variety of time scales. The role of proximal sedirnent storage must be considered in interpreting sedimentation rates in the distal lake basin. Sediment accumulation in distal Moose Lake depends not only on the amount of sediment transferred from the delta, but also on the mode of dispersal within the lake. Dispenal patterns depend on a variety of factors including sedimenhater characteristics, mode of transport (e.g. OV~~OWSvs. turbidity currents), lake currents (related to Coriolis effect, wind and water inputs), and finally, the point of entry to the lake. The latter factor is directly related to delta progradation and lateral distributary shifting. During the Hypsithermal, delta progradation apparently amounted to about 600 m. while channel migration may have spanned the entire valteywiûth (4300m). White channet migration was tikely slow, Ît was punctuated by channel avulsion which may or may not have affected the position of the river mouth. Notable changes to sediment delivery occurred later in the Hypsithermal when the Moose River followed a independent course along the northeast side of the valley. During Neoglacial time. the recornbined Moose-Fraser River channei shifted first to the north (ca. 4000-2800 BP), then more suddenly to the southwest side of the valley. While subject to ongoing shifting and meander cutoff processes. the mouth of the river has probably maintained the same cross- valley position since that time. However, during the last 3000 to 4000 yrs, approximately 1000 m of progradation and the river mouth extension have occurred. The above changes in delta morphology cmbe cornpared to modem Moose Lake sedimentation patterns in order to assess possible impacts on the long terni distal sedimentation record. As expected in a simple elongate lake basin. the dominant pattern observed in modem Moose Lake is one of decreasing particle size and sedirnentation rates away from the point of sediment input (Desloges and Gilbert; 1995; Raymond. 2000; this thesis, Chapter 4). Gradual down-lake change is interrupted occasionally by above or below average rates associated with sediment focusing due to variations in water depth, bottom topography, and turbidity current dynamics. A secondary pattern of cross-lake thickening (southwest to northeast) also occurs, meaning that observed down-lake trends are somewhat sensitive to lateral sampling position (cf. Evans and Church, 2000). From approximately 2 to 7 km down-lake, where annually laminated sediments (Desloges and Gilbert, 1995) allow precise chronologic control. recent sedimentation rates average 1.9 mm yr' but decrease at approximately 0.17 to 0.24 mm yr' per km distance down-lake (the range resuits from considering 20 year and 10 year periods respectively). The proportional effect of this decrease increases down-lake from approximately 6%-9% to 9-15%). While these rates are themselves subject to change through tirne, and sensitive to location considered. they may be used to infer the impact of changing sedirnent source location. Specifically, the cumulative effect of progradation and lateral shifting over the last -3100 years on sedimentation at a point in the lake now located 7 km from the river mouth. would be an increase in sedimentation rate of 17-21 % or more. Alrnost al1 of this increase would be due to delta progradation. The accompanying lateral channel shift of approximately 850 m would only be important at locations nearer to the delta. Additional effects of delta evolution on lake sedimentation will be examined in a future discussion of both records. 5.6 CONCLUSIONS The alluvial cap of the Moose Lake-Fraser River delta preserves a record of deposition and reworking of sediments spanning rnost of the Holocene. A substantial area of the delta-top floodplain to the northeast derives from the early half of the Holoœne and is characterized by fragmentajy, low discharge paleochannels with bed material characteristic of the uppenost Fraser River sub- basin. Most floodplain development since about 4000 yrs BP has been Iimited to the westem and southern areas of the delta and records persistently higher sediment loads, aggradation and larger more active channels. At least twice during this period (leading up to 3560 yn BP and before 1830 yrs BP), the river transported a greater proportion of Moose River derived sediment than at present. The above changes reflect overall shifts from wam, dry Hypsithemal conditions to cooler, wetter Neoglacial climatic conditions defined regionally. and confirm the hypothesis that the Moose River and uppemost Fraser River respond differently to climate change. Due to higher elevations, precipitation and glacier cover, Moose River sub-basin sedirnent yields apparently increase during periods of climatic deterioration and probable glacier advance. Variations in sediment composition (both granule lithology and fine-fraction geochemistry) and channel morphological development during the Neoglacial reflect some regionally documented glacier advance and retreat stages, but channel preservation is too fragmentary to delineate these exactly wlhout more extensive sampling. In contrast, the carbonate and organic matter content of long-lived and more continuous vertically accreted quiet water deposits apparently records al1 of the major glacier advances known to have occurred regionally. Variation in the ratio of organic to inorganic sediment in these overbank deposits may reflect changing discharge regime in the upper Fraser River watershed (independently of glacial activity), but also depends on evolving floodplain configuration and sediment detivery p rocesses. A thick valley-fiIl sequence underlies Moose Lake and the Fraser River delta within a bedrock basin produced by Pleistocene glacial scour. Except for possible till deposits at the northwest end of the basin. most valley-fiIl sediments are glaciolacustrine in origin, and were probably deposited rapidly as ice retreated from the basin and/or up-valley at the end of the Wisconsin. In conjunction with similar (larger) deposits found elsewhere in the westem Cordillera, these materials represent a significant component of sediment flux and denudation associated with Pleistocene glaciaUinterglacial cycles. A large proportion of the Moose Lake-Fraser River delta itself was apparently constructed pnor to cal O ka BP and is thus paraglacial andlor deglacial in origin. The HypsithetmaüNeoglacial wrnponents of the delta evidently exceed equivalent distal lake sediments in ternis of overall sediment mass (by approximately 40%). A smaller, but significant portion (-47%) of the equivalent fine (c 63 vm) fraction lake sediment budget is similarly stored in the delta. lnterpretation of the distal lacustrine record must consider both long and short term storage and reworking of deltaic sediments. Progradation and channel shifting alone could account for graduai increases of up to 21 % in distal lake accumulation rates over Neoglacial time. Episodic variations in sediment flux also would be expected due to geomorphic phenomena such as channel splitting and meander cutofT particularly near the Moose Lake shoreline. Evolution of the Moose Lake delta and Fraser River floodplain docurnented here provides the basis for detailed interpretation of the distal Moose Lake record. 6.1 GENERAL CONCLUSIONS While lacustrine sedimentary environments provide some of the most long- lived and continuous terrestrial archives of environmental change information available, they also record complex signals integrating the effects of catchment sediment delivery processes as well as hydroclimatic forcing. The present study addresses this problem by examining closely related landforms and deposits within a single alpine sediment delivery system, and by developing means of relating and tracing downstream sediments to more distant upstream sources and production mechanisrns. Demonstrating the general utility and feasibility of a multi-site, multi- proxy approach to identifying and understanding environmental change based on sedimentary deposits is the primary focus of this research even though the environmental indicaton identified and reconstructions made are specific to the Fraser River watershed upstream of Moose Lake. In the upper Fraser River watershed, the composition of sediment transported as tributary and main channel bed material load closely reflects bedrock distribution. However, discrimination of source areas is a function of their lithological distinctiveness and wmplicating factors which include the influence of particle size, hydrodynamic sorting and mixing, and transport-related and post-depositional alteration. Until now, these factors have seldom been considered in detail in provenance-based investigation of environmental change (c. f. Walling et ab, 1993; Collins et al., 1997~).In the present context, the fine (< 63 pn) fraction is of greatest interest and utility because of its high mobility and widespread dispersal and storage in the full range of depositional sites examined. Moreover, the fine fraction has a geochemical signature that is easily deterrnined using INAA techniques and provides a better reflection of the range of source materials present. even in small samples. By considering just the fine fraction, problems related to size-composition fractionation, mineralogical wntrol and sampling method are minimized or may be modelled when comparing materials from different depositional environments. If active channel materials are usecl to characterize source areas, however, site selection must account for the fact that complete mixing of the fine fraction may require transport distances of several kilometres. Postdepositional alteration of sediments and the movement or translocation of sorne geochemical elements were not tested here. but despite this. major changes to bulk sediment composition appear to be minimal in the water-saturated river. floodplain and lacustrine deposits examined. Of greater concern is the downstream loss of carbonate-related elements due to solution during transport, particularly where finer sediments are transferred frorn fluvial to lake-bottom environments. This generally unappreciated loss of sediment mass (up to 20% in the present case) may have important implications for sediment yield estimates and paleoenvironmental inferences based on lake sediments in areas of carbonate terrain (e.g. Kennedy and Smith. 1977; Leonard. 1986al 1997; Desloges and Gilbert. 1995; Leonard and Reasoner, 1999). As at other locations (e.g. Cullen. 1994b, 2000; Cavana et al., 1993), the sedimentary rocks of the upper Fraser River area provide minimal contrast in terms of geochemical trace elements. especially in comparison wlh igneous and rnetamorphic source materiais (e.g. Cullers. et al.. 1987. 1988; Cullers. 1994a). However, it is possible to reliably distinguish and estimate mixing of sediments from two main upper Fraser River source areas based on major and minor elements associated specifically with carbonate and aluminosilicate bedrock lithologies. and wlh greater precision than has previously been possible (e.g. Leonard, 1986a, b, 1997; Desloges, 1999). It must be appreciated that spatial discrimination of this type depends on the uneven distribution of lithologies within the drainage basin. At the p~esenttime, the Moose River sub-basin contributes a greater proportion of the discharge and of the total and fine-grained component of the combined Fraser River sedirnent load than would be expected from drainage basin area alone. The imbalance is related to greater relief, runoff and glacier cover in the Moose River sub-basin. The spatial association of carbonate-rich stream sediment load, glacier cover and carbonate bedrock exposure indicates that glaciers are likely the most important source of fine-grained fiuvial sediment in the basin. This is despite the high resistance of this rock type to erosion and susceptibility to solution loss. In this regard, the upper Fraser River watenhed is similar to other drainage basins in the Canadian Cordillera where strong glacial influence on sediment yield has been observed (Church and Slayrnaker, 1989; Church et al., 1989) or inferred (Leonard, l986a, b; Desloges and Gilbert, 1994). Since differences in glacier cover and thus glacier erosion potential among sub-basins of the upper Fraser River watershed are likely to have persisted through the Holocene, the valley and deltaic alluvial fills are indicaton of long-terni glacier fluctuation. Detailed examination of the Moose Lake-Fraser River delta surface based on surface morphology, auger and vibracore logs and cutbank sections reveals a record of deposition and reworking of sediments spanning most or al1 of the Holocene. By combining geomorphic interpretation of river channel and overbank deposits with measurements of sediment composition and provenance, a coherent record of broader long-terrn environmental change is produced that is consistent with regional glacier and climate change chronologies. A substantial area of the floodplain to the northeast of the delta derives from the early half of the Holocene (the Hypsithermal) and is characterized by fragmentary, low-discharge channels with bed materials more characteristic of the uppermost Fraser River sub-basin. Most floodplain development since about 4000 yrs BP (onset of the Neoglacial) has been limited to the western and southem areas of the delta and records persistently higher sediment yields and aggradation by larger, more active, channels. Variations in latetally accreted sediment composition and channel morphological development at this time probably reflect regionally documented glacier advances, but the evidence and dating control is fragmentary. In contrast. long-lived, vertically accreted deposits from the delta's older, low-lying areas contain well defined (as yet undated) variations in carbonate and o~ganicmatter content that correiate visually with al1 known glacier advances (Le. Boundary, Peyto, unnamed, Cavell and Little Ice Age; Luckman, 1986. 1994, 1995, 2000; Luckman et al., 1993). Overall. the floodplain evidence appean to confirm the association of glacial activity and elevated yields of carbonate-rich sediment from the Moose River sub-basin. With appropriate calibraiion, sediments preserved in other downstream sinks such as alluvial fans and proximal (deltaic) or distal lake basins could provide similar evidence of past glacier activity. Back extrapolation of inferred delta progradation rates based on the floodplain chronology developed here suggests that a large proportion of delta (foreset) sedimentation occurred during the earliest Holocene. The role of deltas in paraglacial and possibly deglacial sediment storage and yield has not been widely documented, but the geomorphic records presented here and elsewhere (e.g. Kostaschuk and Smlh, 1983) appear to be consistent with observations of elevated, early Holocene paraglacial sedimentation based on alluvial fans and terraces (e.g. Church and Ryder, 1972; Ryder and Church, 1985). Further information on delta and associated valley-fiIl development was obtained by seismic survey, and combined with previously obtained lake sub-bottom acoustic data (Desloges and Gilbert, 1995). These data provide insight into the nature of late Pleistocene environmental conditions and highlight the rapid deposition of large quantities of glaciolacustrine sediment comparable to that documented in lower elevation fjord lakes of the western Cordillera (Eyles et al., 1990, 2000; Mullins et al.. 1991 ). Valley-fiIl in the vicinity of Moose Lake illustrates that similar deglacial processes operate in upland and lowland locations, and that this cornponent of Pleistocene glacial sediment flux is even more extensive than previously thought. The substantial storage of fine (4 63 pm) as well as coane (63ym - 24 mm) sediment in delta foreset and topset alluvial beds has important implications for the interpretation of more distal lacustrine and other downstream sedimentary records. The long-term storage of fine sediment in the body of the delta may account for up to 50% of that reaching Moose Lake and thus reduces distal lacustrine sedimentation by this amount on average. The abstraction of fine sediment and its storage in the delta-top floodpiain is much less (-3%); however. based on modern down4ake accumulation rates, a graduai inc~easein distal hke sedimentation of up to 15% through the Neoglacial is probable due simply to documented river mouth advance. More extreme short-term fluctuations in sediment delivery down-lake are expected due to inferred river channel splitting and shifting, and floodplain reworking. The exact effects on accumulation are difficult to predict without more detailed floodplain reconstruction. Upstream geornorphic controls on downstrearn sediment yields of this type partially account for the usually poor correlation (? c 30%) of lacustrine sedimentation rates and hydroclimatic variables such as stream discharge or summer temperature (e.g. Desloges and Gilbert, 1995; Lamoureux. 1998). 6.2 FUTURE RESEARCH The results of this study suggest directions for further research, including refinernents to the methods developed and extension or application of the findings. Having established the geomorp hic and sedimentologic history of the Moose Lake- Fraser River delta and derived new proxy records of regional environmental change, it is now possible to assess in more detail the high resolution distal record of Moose Lake (Desloges and Gilbert. 1995; Desloges, 1999; Raymond, 2000). This analysis should proceed through direct comparison of fluvial and deltaic sediment transfer and lake accumulation chronologies, application of provenance techniques to lake sediments, and comparison of independent delta and lake proxy records. Application of provenance techniques to lacustrine sediments would be enhanced through further study of the sedirnent alteration that occurs during transport and deposition in the lake. Further research might include more thorough documentation of lake water column and bottom sediment chemistry in order to assess how carbonate loss is distributed spatially. In the absence of such refinernents, long-term temporal variations in bottom sediment composition rnay be calibrated against known glacial fluctuations (e-g. modern verses Little Ice age conditions). In keeping with the rnulti-site, multi-proxy theme of this research, additional depositional sites and proxy indicators (e.g. upland moraine chronologies) should be exarnined in the upper Fraser River watershed. The evidence presented in this study contributes to the debate regarding geomorphic controls on denudation and sediment yield, and points toward the important role of glacial efosionin alpine environments. 8ecause high relief in the Moose River sub-basin might contribute to greater sediment yield in several ways, more thorough investigation is required to definitively identify and potentially quantify particular sources. One way in which the methods employed here might be extended is through examination of river bed material upstream and in closer proximity to suspected glacial sources. While labour intensive, the direct sampling and analysis (using identical methods) of suspended sediment would assist in determining the timing of sediment release with respect to meteorological conditions. It would then be possible, for example, to distinguish material evacuated from hillslopes from material released by glaciers, since the former may be more closely related to precipitation events. Assessrnent of the lake record would also assist in distinguishing sediment source type though the cornparison of variations in composition, texture and apparent accumulation rates, assuming these respond uniquely to varying sedirnent production and transport conditions (e.g. glacial erosion verses runoff and stream discharge). To cornplement these methods, additional sediment properties indicative of mode of production (e.g. surface micro- textures) also could be examined. While some of the findings of this study are of local significance only, the techniques usedldeveloped may be applied to additionai geomorphic settings, perhaps better suited to answering specific research questions. For example, provenance signals may be more discrirninating in areas of mixed plutonic, volcanic and sedimentary bedrock such as found in the coastal mountain ranges of British Columbia. In this setting, both trace element composition and bedrock exposure should Vary to a greater extent and in potentially useful geographic configurations. Correspondingly, more heavily glaciated basins may provide greater insight into questions of geornorphic control and sediment yield. 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