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Tracy Brennand

Simon Fraser University, Geography, Faculty Member

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I am a geomorphologist and sedimentologist. My main research interests lie in the processes that shape Earth’s surface, and in particular with the action of glaciers and ice sheets.
Phone: 7787823718
Address: Department of Geography
Simon Fraser University
8888 University Drive
Burnaby, BC, V5A 1S6, CANADA

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Papers by Tracy Brennand

Laurentide meltwater systems: geomorphic and sedimentary evidence

The Nature and Timing of Postglacial Incision of the Nicola River, British Columbia

Abstracts with programs, 2021

Deglacial Style and Pattern of the Last Cordilleran Ice Sheet Near Its Geographic Centre

2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014), Oct 22, 2014

Surficial geology of the Rice Lake area, NTS 31D/1, southern Ontario

Deglacial Style, Pattern and Timing of the Cordilleran Ice Sheet, South-Central Interior British Columbia

GSA Annual Meeting in Seattle, Washington, USA - 2017, 2017

Palaeogeographical reconstruction and hydrology of glacial Lake Purcell during <scp>MIS</scp> 2 and its potential impact on the Channeled Scabland, <scp>USA</scp>

Boreas, Jan 31, 2020

Purcell during MIS 2 and its potential impact on the Channeled Scablands. Large, ice-marginal lak... more Purcell during MIS 2 and its potential impact on the Channeled Scablands. Large, ice-marginal lakes that were impounded by the maximally-extended Cordilleran Ice Sheet (CIS) provided source waters for the extraordinarily large floods that formed the Channeled Scabland of Washington and Idaho, USA. However, flood flows that drained CIS meltwater and contributed to landscape evolution during later stages of deglaciation have hitherto been poorly investigated. This paper provides the first evidence for such a late deglacial floodwater source: glacial Lake Purcell (gLP). Sedimentary evidence records the northward extension of gLP from Idaho, USA into British Columbia, Canada and establishes its minimum palaeogeographical extent. Sedimentary evidence suggests that the deglacial Purcell Lobe was a capable ice dam that impounded large volumes of gLP water. A review of glacioisostatically affected lakes during CIS deglaciation suggests that gLP could have been subjected to tilts ranging from 0->1.25 m km-1. 15 Sedimentary evidence suggests high lake plane tilts (⪆1.25 m km-1) are the most likely to have affected gLP. Using this, the palaeogeography and volume of gLP are modelled, revealing that ~116 km 3 of water was susceptible to sudden drainage into the Channeled Scabland via the Columbia River system. This calculation is supported by sedimentary and geomorphic evidence compatible with energetic flood flows along the gLP drainage route and suggests gLP drained suddenly, causing significant landscape change.

Ice-sheet dynamics and subglacial meltwater regime inferred from form and sedimentology of glaciofluvial systems: Victoria Island, District of Franklin, Northwest Territories

Canadian Journal of Earth Sciences, May 1, 1993

On Victoria Island, tunnel channels, eskers, and associated fans and extended deposits together c... more On Victoria Island, tunnel channels, eskers, and associated fans and extended deposits together constitute channelized glaciofluvial systems. Flutes and drumlinoid ridges, interpreted as residuals left by erosive, catastrophic, subglacial meltwater sheet flows, lie adjacent to these systems. One tunnel channel is described in detail. It exhibits deep scours, a discontinuous thalweg, sculpted margins, and flutes on the downflow side of one wall, features indicative of complex flow and possibly several flow events. The tunnel channel is interpreted as the product of erosion by catastrophic, subglacial meltwater flow in a combined ice – substrate (R/N) channel.Esker sediments and morphology are used to infer details of the depositional environment and meltwater regime. A continuous esker with fans and extended deposits records seasonally controlled discharge events through an R-channel. These features may also suggest a grounding-line environment, thin ice, and localized ice floatation events. Less well connected ridges also record seasonally controlled meltwater rhythms and were produced within a thinning and stagnating ice mass; the depositional environment may have been in a subglacial R-channel or an ice-walled reentrant.Differences in the drainage system associated with each glaciofluvial landform, and temporal disconnection between tunnel channel and esker formation, is also suggested by possible paleoflow reversals between inferred catastrophic and seasonally controlled drainage phases. Changes in ice-sheet profiles between events may have been responsible.

Late-glacial lakes in the Thompson Basin, British Columbia: paleogeography and evolution

Canadian Journal of Earth Sciences, Nov 1, 2004

During the decay of the Cordilleran Ice Sheet 10 000-13 000 BP, glacial lakes developed within va... more During the decay of the Cordilleran Ice Sheet 10 000-13 000 BP, glacial lakes developed within valleys that dissect the Interior Plateau of British Columbia. In this paper, we (1) illustrate a procedure for assessing paleo water planes that has general application, (2) document lake paleogeography and evolution in the Thompson Valley, (3) provide new data on the glacio-isostatic response of the central Cordillera, and (4) present new evidence of its late-glacial environment. We employ geomorphology and sedimentology, digital elevation models, and new technologies (differential global positioning systems, ground penetrating radar, and geographic information systems) to refine paleogeographic reconstructions of glacial lakes. Glacial Lake Thompson and Glacial Lake Deadman were ribbon-shaped (width to length ratio ≈ 3:100), deep (>>140 to-50 m) lakes that contained significant water volumes (84-24 km 3). They lengthened to the west and their water level lowered as ice decayed. Final ice dam failure resulted in an-20 km 3 jökulhlaup that eroded bedforms and deposited flood eddy bars within the lake basin, travelled-250 km along the Fraser River system, and may have deposited exotic mud offshore between 10 190 and 11 940 BP. Glacio-isostatic tilts of water planes are among the highest in the world (1.7-1.8 m km-1). Their orientations suggest that ice sheet loads were greater or longerlived to the north-northwest of the study area, lending support to the notion of an ice divide centred on the Fraser Plateau. Résumé : Durant la décroissance de l'inlandsis de la Cordillère, 10 000-13 000 ans avant le présent, des lacs glaciaires se sont développés dans des vallées qui recoupent le plateau intérieur de la Colombie-Britannique. Dans le présent article, nous (1) décrivons une procédure d'application générale pour évaluer les paléonappes d'eau, (2) documentons la paléogéographie et l'évolution des lacs dans la vallée de la rivière Thompson, (3) fournissons de nouvelles données sur la réponse glacio-isostatique de la cordillère centrale et (4) présentons de nouvelles évidences de son environnement tardi-glaciaire. Nous utilisons la géomorphologie et la sédimentologie, des modèles numériques d'élévation et des nouvelles technologies (systèmes de positionnement global différentiels, géoradars et systèmes d'information géographiques) pour préciser les reconstructions paléogéographiques des lacs glaciaires. Les lacs glaciaires Thompson et Deadman avaient une forme rubanée (rapport largeur à longueur~3:100), ils étaient profonds (>> 140 à~50 m) et ils contenaient des volumes d'eau importants (84-24 km 3). Ils s'allongeaient vers l'ouest et leur niveau d'eau s'abaissait au fur et à mesure de la fonte de la glace. Une rupture finale de la digue de glace a causé un jökulhlaup~20 km 3 qui a érodé les fonds rocheux et a déposé des barres de tourbillons de crue dans le bassin du lac; cette masse d'eau s'est déplacée sur~250 km le long du système de la rivière Fraser et aurait pu avoir déposé des boues exotiques au large entre 10 190 et 11 940 ans avant le présent. Les basculements glacio-isostatiques des nappes d'eau sont parmi les plus élevés au monde (1,7-1,8 m km-1). Leurs orientations suggèrent que les charges des feuillets glaciaires étaient plus grandes ou étaient de plus longue durée au nord-nord-ouest de la région étudiée, supportant la notion d'une ligne de partage glaciaire centrée sur le plateau Fraser.

Tunnel channels and associated landforms, south-central Ontario: their implications for ice-sheet hydrology

Canadian Journal of Earth Sciences, Mar 1, 1994

The Peterborough–Trenton drumlin field and associated meltwater erosion marks (s-forms) were inte... more The Peterborough–Trenton drumlin field and associated meltwater erosion marks (s-forms) were interpreted as the products of erosion by turbulent separated flows within a catastrophically released subglacial meltwater sheet, the Algonquin event. Recent theoretical modelling of ice-sheet hydrology suggests progressive channelization of meltwater during the collapse of such a sheet. Inductive testing of this hypothesis using observations in south-central Ontario forms the crux of this paper. A geometric model of the interaction of rough ice base and bed surfaces gives inferred meltwater flow patterns. Such a rough interactive geometry is suggested as the bed is drumlinized, and flows responsible for drumlins may be expected to have eroded similar forms into the ice base. Three landforms in south-central Ontario; namely tunnel channels, megachannels, and late-stage sheet-flow scours, are described, and interpreted as evidence of progressive channelization and flow diversion processes, governed by the geometric interactions between the recoupling ice base and its bed and thermodynamic feedbacks within an increasingly discontinuous meltwater sheet. Some fields of transverse ridges within tunnel channels may record meltwater deposition towards the end of the Algonquin event. By contrast, eskers within tunnel channels record a return to seasonally driven meltwater drainage. The Dummer Moraine and the Oak Ridges complex are integrated into a new landform–vent sequence for south-central Ontario which provides a framework for further field testing.

John Shaw (1943-2018)

Boreas, Jun 11, 2018

Till-forming processes beneath parts of the Cordilleran Ice Sheet, British Columbia, Canada: macroscale and microscale evidence and a new statistical technique for analysing microstructure data

Boreas, Feb 18, 2013

This paper presents the first integrated macroscale and microscale examination of subglacial till... more This paper presents the first integrated macroscale and microscale examination of subglacial till associated with the last-glacial (Fraser Glaciation) Cordilleran Ice Sheet (CIS). A new statistical approach to quantifying till micromorphology (multivariate hierarchical cluster analysis for compositional data) is also described and implemented. Till macrostructures, macrofabrics and microstructures support previous assertions that primary till in this region formed through a combination of lodgement and deformation processes in a temperate subglacial environment. Macroscale observations suggest that subglacial environments below the CIS were probably influenced by topography, whereby poor drainage of the substrate in topographically constricted areas, or on slopes adverse to the ice-flow direction at glacial maximum, facilitated ductile deformation of the glacier bed. Microscale observations suggest that subglacial till below the CIS experienced both ductile and brittle deformation, including grain rotation and squeeze flow of sediment between grains under moist conditions, and microshearing, grain stacking and grain fracturing under well-drained conditions. Macroscale observations suggest that ductile deformation events were probably followed by brittle deformation events as the substrate subsequently drained. The prevalence of ductile-type microstructures in most till exposures investigated in this study suggests that ductile deformation signatures can be preserved at the microscale after brittle deformation events that result in largerscale fractures and shear structures. It is likely that microscale ductile deformation can also occur within distributed shear zones during lodgement processes. Cluster analysis of microstructure data and qualitative observations made from thin sections suggest that the relative frequency of countable microstructures in this till is influenced by topography in relation to ice-flow direction (bed drainage conditions) as well as by the frequency and distribution of voids in the till matrix and skeletal grain shapes.

The environment in and around ice-dammed lakes in the moderately high relief setting of the southern Canadian Cordillera

Boreas, Feb 1, 2006

During decay of the Cordilleran Ice Sheet, Â/13 000 Á/10 000 cal. yr BP, numerous ice-dammed, rib... more During decay of the Cordilleran Ice Sheet, Â/13 000 Á/10 000 cal. yr BP, numerous ice-dammed, ribbon-shaped lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia. We describe the pattern and characteristics of lake sediments within the Thompson Valley, propose a palaeoenvironmental model for glacial lakes Thompson and Deadman and explore their implications for the palaeogeography of Cordilleran Ice Sheet decay. Seventeen glaciolacustrine lithofacies are identified within deltas, subaqueous fans and lakebottom beds. Sediments accumulated at high rates and by a diversity of sediment dispersal and depositional processes: hyperpycnal and surge-type turbidity currents, grain flows and debris flows. Megascale subaqueous failures (tens of metres thick) were facilitated by high sedimentation rates. The palaeoenvironmental model highlights: (i) high rates of basin infilling; (ii) the dominant role of tributary rivers, rather than valley-occupying ice, in delivering water and sediment to lakes; and (iii) the role of melt cycles, jö kulhlaups and hyperpycnal flows in sediment delivery. These conditions, in combination with a lack of organics and a fining upward sequence in lake sediments, suggest that glacial lakes Thompson and Deadman were coeval with dwindling plateau ice.

Towards a morphogenetic classification of eskers: Implications for modelling ice sheet hydrology

Quaternary Science Reviews, Feb 1, 2016

Validations of paleo-ice sheet hydrological models have used esker spacing as a proxy for ice tun... more Validations of paleo-ice sheet hydrological models have used esker spacing as a proxy for ice tunnel density. Changes in crest type (cross-sectional shape) along esker ridges have typically been attributed to the effect of changing subglacial topography on hydro-and ice-dynamics and hence subglacial ice-tunnel shape. These claims assume that all eskers formed in subglacial ice tunnels and that all major subglacial ice tunnels produced a remnant esker. We identify differences in geomorphic context, sinuosity, crosssectional shape, and sedimentary architecture by analysing eskers formed at or near the margins of the last Cordilleran Ice Sheet on British Columbia's southern Fraser Plateau, and propose a morphogenetic esker classification. Three morphogenetic types and 2 subtypes of eskers are classified based on differences in geomorphic context, ridge length, sinuosity, cross-sectional shape and sedimentary architecture using geophysical techniques and sedimentary exposures; they largely record seasonal meltwater flows and glacial lake outburst floods (GLOFs) through sub-, en-and supraglacial meltwater channels and ice-walled canyons. General principles extracted from these interpretations are: 1) esker ridge crest type and sinuosity strongly reflect meltwater channel type. Eskers formed in subglacial conduits are likely to be roundcrested with low sinuosity (except where controlled by ice structure or modified by surging) and contain faults associated with flank collapse. Eskers formed near or at the ice surface are more likely to be sharp-crested, highly sinuous, and contain numerous faults both under ridge crest-lines and in areas of flank collapse. 2) Esker ridges containing numerous flat-crested reaches formed directly on the landsurface in ice-walled canyons (unroofed ice tunnels) or in ice tunnels at atmospheric pressure, and therefore likely record thin or dead ice. 3) Eskers containing macroforms exhibiting headward and downflow growth likely record flood-scale flows (possibly GLOFs where a lake can be inferred). These conclusions suggest that esker crest type, sinuosity and geomorphic context, when understood along with sedimentary architecture, largely reflect formational position with respect to the ice-surface. Reconstructions of ice sheet hydrology need to account for variation in esker morphology because basing hydrodynamic inferences on the presence or absence of an esker alone ignores encoded differences in water source, supply, flow magnitude and frequency, and conduit position.

Refining the pattern and style of Cordilleran Ice Sheet retreat: palaeogeography, evolution and implications of lateglacial ice-dammed lake systems on the southern Fraser Plateau, British Columbia, Canada

Boreas, Sep 2, 2014

ABSTRACT Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conc... more ABSTRACT Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conceptualized as being dominated by passive downwasting (stagnation), in part because of the lack of large recessional moraines. Yet, multiple lines of evidence, including reconstructions of glacio-isostatic rebound from palaeoglacial lake shoreline deformation suggest a sloping ice surface and a more systematic pattern of ice-margin retreat. Here we reconstructed ice-marginal lake evolution across the subdued topography of the southern Fraser Plateau in order to elucidate the pattern and style of lateglacial CIS decay. Lake stage extent was reconstructed using primary and secondary palaeo-water-plane indicators: deltas, spillways, ice-marginal channels, subaqueous fans and lake-bottom sediments identified from aerial photograph and digital elevation model interpretation combined with field observations of geomorphology and sedimentology, and ground-penetrating radar surveys. Ice-contact indicators, such as ice-marginal channels, and grounding-line moraines were used to refine and constrain ice-margin positions. The results show that ice-dammed lakes were extensive (average 27 km2; max. 116 km2) and relatively shallow (average 18 m). Within basins successive lake stages appear to have evolved by expansion, decanting or drainage (glacial lake outburst flood, outburst flood or lake maintenance) from southeast to northwest, implicating a systematic northwestward retreating ice margin (rather than chaotic stagnation) back toward the Coast Mountains, similar in style and pattern to that proposed for the Fennoscandian Ice Sheet. This pattern is confirmed by cross-cutting drainage networks between lake basins and is in agreement with numerical models of North American ice-sheet retreat and recent hypotheses on lateglacial CIS reorganization during decay. Reconstructed lake systems are dynamic and transitory and probably had significant effects on the dynamics of ice-marginal retreat, the importance of which is currently being recognized in the modern context of the Greenland Ice Sheet, where &gt;35% of meltwater streams from land-terminating portions of the ice sheet end in ice-contact lakes.

Regional-scale meltwater erosion and deposition patterns, northern Quebec, Canada

Annals of Glaciology, 1996

t in le rn a ti o na l G lac iologica I Soc ie ty Regional-scale tneltwater erosion and depositio... more

Comparisons between macro- and microfabrics in a pebble-rich, sandy till deposited by the Cordilleran Ice Sheet

Boreas, Apr 15, 2015

ABSTRACT It is standard practice to measure particle fabrics in glacial studies to infer palaeo-i... more ABSTRACT It is standard practice to measure particle fabrics in glacial studies to infer palaeo-ice flow directions and processes of till formation but few studies examine the relationship between particle fabrics at different (i.e. the macro- and micro-) scales. This knowledge is critical to inform the utility of the methods and limitations of associated interpretations. Micro- (sand grain) and macro- (pebble) fabrics of pebble-rich, sandy subglacial till (Kamloops Lake till) deposited by the Cordilleran Ice Sheet, south-central British Columbia are compared to assess their similarities and differences, and therefore their utility for understanding subglacial processes. Before comparisons are made, the data are tested for robustness by assessing various controls (e.g. sampling face orientation, number of particles measured, statistical variation due to sampling effects, particle shape, size and concentration) on particle fabrics. A new method of microfabric analysis is applied that involves the identification and delineation of distinct clusters of similarly-oriented sand grains in order to compare them to macrofabrics and inferred ice flow directions. Results show that microfabrics, on their own, are an unreliable indicator of ice flow direction in Kamloops Lake till in the study area and should not be used as a substitute for macrofabric data, as they likely record late-stage microscale strain patterns and pore water flow in addition to till deposition and deformation by overriding ice. We suspect that this would also be the case for coarse-grained till elsewhere. Our findings suggest that till microfabric interpretations should always be made after assessing corresponding macrofabric data alongside sedimentological and structural observations.

Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia, Canada and northern Washington State, USA

Quaternary Science Reviews, Nov 1, 2009

Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated i... more Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300 m below sea level. Over 600 m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform-substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts. During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s). Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.

Transient subglacial hydrology of a thin ice sheet: insights from the Chasm esker, British Columbia, Canada

Quaternary Science Reviews, Dec 1, 2012

Glacier drainage systems are never steady state; instead they fluctuate over time and space in re... more Glacier drainage systems are never steady state; instead they fluctuate over time and space in response to variation in water input. In order to improve numerical models of glacier hydrology it is important to fully understand the processes controlling subglacial drainage system evolution, but contemporary ice sheet beds are typically inaccessible. Thus, esker distribution, morphology, and sedimentology have been used to infer the dynamics and hydrology of former ice sheets. However, debate remains as to the processes responsible for esker formation and most theoretical investigations have assumed that they formed due to processes that operated beneath thick ice despite many field investigations to the contrary. We investigate esker formation during the final stages of the thin, inactive, and rapidly decaying Cordilleran Ice Sheet (CIS) in interior British Columbia, Canada. A combination of geomorphological, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) data suggest esker ridge sedimentary architecture is consistent with synchronous formation during a glacial lake outburst flood (GLOF). These data reveal esker ridge deposition most likely took place within an ice tunnel that evacuated late-waning stage flow, following erosion and partial fill of a broader meltwater corridor. Esker ridge sedimentary architecture reveals this depositional environment was dynamic, reflecting complex interaction between ice thickness, ice structure, ice tunnel geometry, flow conditions, and sediment supply. Under these thin, inactive ice conditions ice tunnel location was initially governed by structural weaknesses in the ice and/or equipotential gradient. Because creep closure rates were low, the ice tunnel evolved through feedbacks between conduit growth via frictional melting/mechanical ice excavation, and conduit closure due to sediment infilling, rather than ice creep. This resulted in a nonuniform ice tunnel that enlarged in an upglacier direction and locally unroofed to form open channels during its final stages. These data suggest that eskers may record unsteady, rather than steady flows in a drainage network and so the assumptions often made when using eskers to inform hydrological components of ice sheet models may not be applicable to all ice sheet settings.

The Harricana glaciofluvial complex, Abitibi region, Quebec: its genesis and implications for meltwater regime and ice-sheet dynamics

Sedimentary Geology, Apr 1, 1996

Interlobate moraines have been defined by their relationship to adjacent landforms and sediments,... more Interlobate moraines have been defined by their relationship to adjacent landforms and sediments, with minimal reference to any genesis suggested from their own geomorphology and sedimentology. A case in point is the "Harricana interlobate moraine". The Harricana complex, a relatively continuous, linear accumulation of glaciofluvial sediments, is investigated in terms of its geomorphology, sedimentology, stratigraphic context, and landform associations for the portion between latitudes 48"N and 50"N. The complex is narrower to the north than to the south. In the narrower northern part, better rounding and poorer preservation of less resistant clasts suggest more vigorous transport in a narrower conduit; the inverse in the wider southern part favours less vigorous flows and higher deposition rates. These inferences, together with unidirectional paleoflows towards the ice margin and the relatively continuous upslope path of the southward-widening complex, favour synchronous formation of the Harricana complex in a continuous closed conduit, beneath an ice sheet which thinned southward. Subaqueous fans and grounding-line deposits with fine gravel and sandy in-phase wave structures in the south, may have been deposited in subglacial cavities adjacent to the complex, or later deposited in reentrants in calving ice fronts. The basic building blocks of the Harricana complex are gravel facies which indicate generally high energy, but unsteady, transport. These facies are arranged into macroforms. Composite and oblique-accretion, avalanche-bed (OAAEl) macroforms are attributed to deposition by unsteady and non-uniform flows in conduit enlargements. Pseudoanticlinal macroforms were deposited in relatively narrow conduit segments of uniform width. Gravel-sand couplets register flow unsteadiness, perhaps related to variations in supraglacial meltwater supply. The origin of the Harricana complex as a mainly synchronous subglacial landform is linked to formation of adjacent streamlined bedforms and bedrock erosional marks by meltwater outburst floods. The complex is located where the orientation of these subglacial bedforms indicate strong flow convergence. Post-flood, ice sheet collapse along this convergence zone initiated redirection of ice flow, resulting in cross-cutting striae. Later subglacial meltwater systems followed new hydraulic gradients toward this trough of thinner ice, forming a major conduit and depositing the Harricana glaciofluvial complex.

Macroforms, large bedforms and rhythmic sedimentary sequences in subglacial eskers, south-central Ontario: implications for esker genesis and meltwater regime

Sedimentary Geology, Jun 1, 1994

ABSTRACT

Laurentide meltwater systems: geomorphic and sedimentary evidence

The Nature and Timing of Postglacial Incision of the Nicola River, British Columbia

Abstracts with programs, 2021

Deglacial Style and Pattern of the Last Cordilleran Ice Sheet Near Its Geographic Centre

2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014), Oct 22, 2014

Surficial geology of the Rice Lake area, NTS 31D/1, southern Ontario

Deglacial Style, Pattern and Timing of the Cordilleran Ice Sheet, South-Central Interior British Columbia

GSA Annual Meeting in Seattle, Washington, USA - 2017, 2017

Palaeogeographical reconstruction and hydrology of glacial Lake Purcell during <scp>MIS</scp> 2 and its potential impact on the Channeled Scabland, <scp>USA</scp>

Boreas, Jan 31, 2020

Purcell during MIS 2 and its potential impact on the Channeled Scablands. Large, ice-marginal lak... more Purcell during MIS 2 and its potential impact on the Channeled Scablands. Large, ice-marginal lakes that were impounded by the maximally-extended Cordilleran Ice Sheet (CIS) provided source waters for the extraordinarily large floods that formed the Channeled Scabland of Washington and Idaho, USA. However, flood flows that drained CIS meltwater and contributed to landscape evolution during later stages of deglaciation have hitherto been poorly investigated. This paper provides the first evidence for such a late deglacial floodwater source: glacial Lake Purcell (gLP). Sedimentary evidence records the northward extension of gLP from Idaho, USA into British Columbia, Canada and establishes its minimum palaeogeographical extent. Sedimentary evidence suggests that the deglacial Purcell Lobe was a capable ice dam that impounded large volumes of gLP water. A review of glacioisostatically affected lakes during CIS deglaciation suggests that gLP could have been subjected to tilts ranging from 0->1.25 m km-1. 15 Sedimentary evidence suggests high lake plane tilts (⪆1.25 m km-1) are the most likely to have affected gLP. Using this, the palaeogeography and volume of gLP are modelled, revealing that ~116 km 3 of water was susceptible to sudden drainage into the Channeled Scabland via the Columbia River system. This calculation is supported by sedimentary and geomorphic evidence compatible with energetic flood flows along the gLP drainage route and suggests gLP drained suddenly, causing significant landscape change.

Ice-sheet dynamics and subglacial meltwater regime inferred from form and sedimentology of glaciofluvial systems: Victoria Island, District of Franklin, Northwest Territories

Canadian Journal of Earth Sciences, May 1, 1993

On Victoria Island, tunnel channels, eskers, and associated fans and extended deposits together c... more On Victoria Island, tunnel channels, eskers, and associated fans and extended deposits together constitute channelized glaciofluvial systems. Flutes and drumlinoid ridges, interpreted as residuals left by erosive, catastrophic, subglacial meltwater sheet flows, lie adjacent to these systems. One tunnel channel is described in detail. It exhibits deep scours, a discontinuous thalweg, sculpted margins, and flutes on the downflow side of one wall, features indicative of complex flow and possibly several flow events. The tunnel channel is interpreted as the product of erosion by catastrophic, subglacial meltwater flow in a combined ice – substrate (R/N) channel.Esker sediments and morphology are used to infer details of the depositional environment and meltwater regime. A continuous esker with fans and extended deposits records seasonally controlled discharge events through an R-channel. These features may also suggest a grounding-line environment, thin ice, and localized ice floatation events. Less well connected ridges also record seasonally controlled meltwater rhythms and were produced within a thinning and stagnating ice mass; the depositional environment may have been in a subglacial R-channel or an ice-walled reentrant.Differences in the drainage system associated with each glaciofluvial landform, and temporal disconnection between tunnel channel and esker formation, is also suggested by possible paleoflow reversals between inferred catastrophic and seasonally controlled drainage phases. Changes in ice-sheet profiles between events may have been responsible.

Late-glacial lakes in the Thompson Basin, British Columbia: paleogeography and evolution

Canadian Journal of Earth Sciences, Nov 1, 2004

During the decay of the Cordilleran Ice Sheet 10 000-13 000 BP, glacial lakes developed within va... more During the decay of the Cordilleran Ice Sheet 10 000-13 000 BP, glacial lakes developed within valleys that dissect the Interior Plateau of British Columbia. In this paper, we (1) illustrate a procedure for assessing paleo water planes that has general application, (2) document lake paleogeography and evolution in the Thompson Valley, (3) provide new data on the glacio-isostatic response of the central Cordillera, and (4) present new evidence of its late-glacial environment. We employ geomorphology and sedimentology, digital elevation models, and new technologies (differential global positioning systems, ground penetrating radar, and geographic information systems) to refine paleogeographic reconstructions of glacial lakes. Glacial Lake Thompson and Glacial Lake Deadman were ribbon-shaped (width to length ratio ≈ 3:100), deep (>>140 to-50 m) lakes that contained significant water volumes (84-24 km 3). They lengthened to the west and their water level lowered as ice decayed. Final ice dam failure resulted in an-20 km 3 jökulhlaup that eroded bedforms and deposited flood eddy bars within the lake basin, travelled-250 km along the Fraser River system, and may have deposited exotic mud offshore between 10 190 and 11 940 BP. Glacio-isostatic tilts of water planes are among the highest in the world (1.7-1.8 m km-1). Their orientations suggest that ice sheet loads were greater or longerlived to the north-northwest of the study area, lending support to the notion of an ice divide centred on the Fraser Plateau. Résumé : Durant la décroissance de l'inlandsis de la Cordillère, 10 000-13 000 ans avant le présent, des lacs glaciaires se sont développés dans des vallées qui recoupent le plateau intérieur de la Colombie-Britannique. Dans le présent article, nous (1) décrivons une procédure d'application générale pour évaluer les paléonappes d'eau, (2) documentons la paléogéographie et l'évolution des lacs dans la vallée de la rivière Thompson, (3) fournissons de nouvelles données sur la réponse glacio-isostatique de la cordillère centrale et (4) présentons de nouvelles évidences de son environnement tardi-glaciaire. Nous utilisons la géomorphologie et la sédimentologie, des modèles numériques d'élévation et des nouvelles technologies (systèmes de positionnement global différentiels, géoradars et systèmes d'information géographiques) pour préciser les reconstructions paléogéographiques des lacs glaciaires. Les lacs glaciaires Thompson et Deadman avaient une forme rubanée (rapport largeur à longueur~3:100), ils étaient profonds (>> 140 à~50 m) et ils contenaient des volumes d'eau importants (84-24 km 3). Ils s'allongeaient vers l'ouest et leur niveau d'eau s'abaissait au fur et à mesure de la fonte de la glace. Une rupture finale de la digue de glace a causé un jökulhlaup~20 km 3 qui a érodé les fonds rocheux et a déposé des barres de tourbillons de crue dans le bassin du lac; cette masse d'eau s'est déplacée sur~250 km le long du système de la rivière Fraser et aurait pu avoir déposé des boues exotiques au large entre 10 190 et 11 940 ans avant le présent. Les basculements glacio-isostatiques des nappes d'eau sont parmi les plus élevés au monde (1,7-1,8 m km-1). Leurs orientations suggèrent que les charges des feuillets glaciaires étaient plus grandes ou étaient de plus longue durée au nord-nord-ouest de la région étudiée, supportant la notion d'une ligne de partage glaciaire centrée sur le plateau Fraser.

Tunnel channels and associated landforms, south-central Ontario: their implications for ice-sheet hydrology

Canadian Journal of Earth Sciences, Mar 1, 1994

The Peterborough–Trenton drumlin field and associated meltwater erosion marks (s-forms) were inte... more The Peterborough–Trenton drumlin field and associated meltwater erosion marks (s-forms) were interpreted as the products of erosion by turbulent separated flows within a catastrophically released subglacial meltwater sheet, the Algonquin event. Recent theoretical modelling of ice-sheet hydrology suggests progressive channelization of meltwater during the collapse of such a sheet. Inductive testing of this hypothesis using observations in south-central Ontario forms the crux of this paper. A geometric model of the interaction of rough ice base and bed surfaces gives inferred meltwater flow patterns. Such a rough interactive geometry is suggested as the bed is drumlinized, and flows responsible for drumlins may be expected to have eroded similar forms into the ice base. Three landforms in south-central Ontario; namely tunnel channels, megachannels, and late-stage sheet-flow scours, are described, and interpreted as evidence of progressive channelization and flow diversion processes, governed by the geometric interactions between the recoupling ice base and its bed and thermodynamic feedbacks within an increasingly discontinuous meltwater sheet. Some fields of transverse ridges within tunnel channels may record meltwater deposition towards the end of the Algonquin event. By contrast, eskers within tunnel channels record a return to seasonally driven meltwater drainage. The Dummer Moraine and the Oak Ridges complex are integrated into a new landform–vent sequence for south-central Ontario which provides a framework for further field testing.

John Shaw (1943-2018)

Boreas, Jun 11, 2018

Till-forming processes beneath parts of the Cordilleran Ice Sheet, British Columbia, Canada: macroscale and microscale evidence and a new statistical technique for analysing microstructure data

Boreas, Feb 18, 2013

This paper presents the first integrated macroscale and microscale examination of subglacial till... more This paper presents the first integrated macroscale and microscale examination of subglacial till associated with the last-glacial (Fraser Glaciation) Cordilleran Ice Sheet (CIS). A new statistical approach to quantifying till micromorphology (multivariate hierarchical cluster analysis for compositional data) is also described and implemented. Till macrostructures, macrofabrics and microstructures support previous assertions that primary till in this region formed through a combination of lodgement and deformation processes in a temperate subglacial environment. Macroscale observations suggest that subglacial environments below the CIS were probably influenced by topography, whereby poor drainage of the substrate in topographically constricted areas, or on slopes adverse to the ice-flow direction at glacial maximum, facilitated ductile deformation of the glacier bed. Microscale observations suggest that subglacial till below the CIS experienced both ductile and brittle deformation, including grain rotation and squeeze flow of sediment between grains under moist conditions, and microshearing, grain stacking and grain fracturing under well-drained conditions. Macroscale observations suggest that ductile deformation events were probably followed by brittle deformation events as the substrate subsequently drained. The prevalence of ductile-type microstructures in most till exposures investigated in this study suggests that ductile deformation signatures can be preserved at the microscale after brittle deformation events that result in largerscale fractures and shear structures. It is likely that microscale ductile deformation can also occur within distributed shear zones during lodgement processes. Cluster analysis of microstructure data and qualitative observations made from thin sections suggest that the relative frequency of countable microstructures in this till is influenced by topography in relation to ice-flow direction (bed drainage conditions) as well as by the frequency and distribution of voids in the till matrix and skeletal grain shapes.

The environment in and around ice-dammed lakes in the moderately high relief setting of the southern Canadian Cordillera

Boreas, Feb 1, 2006

During decay of the Cordilleran Ice Sheet, Â/13 000 Á/10 000 cal. yr BP, numerous ice-dammed, rib... more During decay of the Cordilleran Ice Sheet, Â/13 000 Á/10 000 cal. yr BP, numerous ice-dammed, ribbon-shaped lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia. We describe the pattern and characteristics of lake sediments within the Thompson Valley, propose a palaeoenvironmental model for glacial lakes Thompson and Deadman and explore their implications for the palaeogeography of Cordilleran Ice Sheet decay. Seventeen glaciolacustrine lithofacies are identified within deltas, subaqueous fans and lakebottom beds. Sediments accumulated at high rates and by a diversity of sediment dispersal and depositional processes: hyperpycnal and surge-type turbidity currents, grain flows and debris flows. Megascale subaqueous failures (tens of metres thick) were facilitated by high sedimentation rates. The palaeoenvironmental model highlights: (i) high rates of basin infilling; (ii) the dominant role of tributary rivers, rather than valley-occupying ice, in delivering water and sediment to lakes; and (iii) the role of melt cycles, jö kulhlaups and hyperpycnal flows in sediment delivery. These conditions, in combination with a lack of organics and a fining upward sequence in lake sediments, suggest that glacial lakes Thompson and Deadman were coeval with dwindling plateau ice.

Towards a morphogenetic classification of eskers: Implications for modelling ice sheet hydrology

Quaternary Science Reviews, Feb 1, 2016

Validations of paleo-ice sheet hydrological models have used esker spacing as a proxy for ice tun... more Validations of paleo-ice sheet hydrological models have used esker spacing as a proxy for ice tunnel density. Changes in crest type (cross-sectional shape) along esker ridges have typically been attributed to the effect of changing subglacial topography on hydro-and ice-dynamics and hence subglacial ice-tunnel shape. These claims assume that all eskers formed in subglacial ice tunnels and that all major subglacial ice tunnels produced a remnant esker. We identify differences in geomorphic context, sinuosity, crosssectional shape, and sedimentary architecture by analysing eskers formed at or near the margins of the last Cordilleran Ice Sheet on British Columbia's southern Fraser Plateau, and propose a morphogenetic esker classification. Three morphogenetic types and 2 subtypes of eskers are classified based on differences in geomorphic context, ridge length, sinuosity, cross-sectional shape and sedimentary architecture using geophysical techniques and sedimentary exposures; they largely record seasonal meltwater flows and glacial lake outburst floods (GLOFs) through sub-, en-and supraglacial meltwater channels and ice-walled canyons. General principles extracted from these interpretations are: 1) esker ridge crest type and sinuosity strongly reflect meltwater channel type. Eskers formed in subglacial conduits are likely to be roundcrested with low sinuosity (except where controlled by ice structure or modified by surging) and contain faults associated with flank collapse. Eskers formed near or at the ice surface are more likely to be sharp-crested, highly sinuous, and contain numerous faults both under ridge crest-lines and in areas of flank collapse. 2) Esker ridges containing numerous flat-crested reaches formed directly on the landsurface in ice-walled canyons (unroofed ice tunnels) or in ice tunnels at atmospheric pressure, and therefore likely record thin or dead ice. 3) Eskers containing macroforms exhibiting headward and downflow growth likely record flood-scale flows (possibly GLOFs where a lake can be inferred). These conclusions suggest that esker crest type, sinuosity and geomorphic context, when understood along with sedimentary architecture, largely reflect formational position with respect to the ice-surface. Reconstructions of ice sheet hydrology need to account for variation in esker morphology because basing hydrodynamic inferences on the presence or absence of an esker alone ignores encoded differences in water source, supply, flow magnitude and frequency, and conduit position.

Refining the pattern and style of Cordilleran Ice Sheet retreat: palaeogeography, evolution and implications of lateglacial ice-dammed lake systems on the southern Fraser Plateau, British Columbia, Canada

Boreas, Sep 2, 2014

ABSTRACT Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conc... more ABSTRACT Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conceptualized as being dominated by passive downwasting (stagnation), in part because of the lack of large recessional moraines. Yet, multiple lines of evidence, including reconstructions of glacio-isostatic rebound from palaeoglacial lake shoreline deformation suggest a sloping ice surface and a more systematic pattern of ice-margin retreat. Here we reconstructed ice-marginal lake evolution across the subdued topography of the southern Fraser Plateau in order to elucidate the pattern and style of lateglacial CIS decay. Lake stage extent was reconstructed using primary and secondary palaeo-water-plane indicators: deltas, spillways, ice-marginal channels, subaqueous fans and lake-bottom sediments identified from aerial photograph and digital elevation model interpretation combined with field observations of geomorphology and sedimentology, and ground-penetrating radar surveys. Ice-contact indicators, such as ice-marginal channels, and grounding-line moraines were used to refine and constrain ice-margin positions. The results show that ice-dammed lakes were extensive (average 27 km2; max. 116 km2) and relatively shallow (average 18 m). Within basins successive lake stages appear to have evolved by expansion, decanting or drainage (glacial lake outburst flood, outburst flood or lake maintenance) from southeast to northwest, implicating a systematic northwestward retreating ice margin (rather than chaotic stagnation) back toward the Coast Mountains, similar in style and pattern to that proposed for the Fennoscandian Ice Sheet. This pattern is confirmed by cross-cutting drainage networks between lake basins and is in agreement with numerical models of North American ice-sheet retreat and recent hypotheses on lateglacial CIS reorganization during decay. Reconstructed lake systems are dynamic and transitory and probably had significant effects on the dynamics of ice-marginal retreat, the importance of which is currently being recognized in the modern context of the Greenland Ice Sheet, where &gt;35% of meltwater streams from land-terminating portions of the ice sheet end in ice-contact lakes.

Regional-scale meltwater erosion and deposition patterns, northern Quebec, Canada

Annals of Glaciology, 1996

t in le rn a ti o na l G lac iologica I Soc ie ty Regional-scale tneltwater erosion and depositio... more

Comparisons between macro- and microfabrics in a pebble-rich, sandy till deposited by the Cordilleran Ice Sheet

Boreas, Apr 15, 2015

ABSTRACT It is standard practice to measure particle fabrics in glacial studies to infer palaeo-i... more ABSTRACT It is standard practice to measure particle fabrics in glacial studies to infer palaeo-ice flow directions and processes of till formation but few studies examine the relationship between particle fabrics at different (i.e. the macro- and micro-) scales. This knowledge is critical to inform the utility of the methods and limitations of associated interpretations. Micro- (sand grain) and macro- (pebble) fabrics of pebble-rich, sandy subglacial till (Kamloops Lake till) deposited by the Cordilleran Ice Sheet, south-central British Columbia are compared to assess their similarities and differences, and therefore their utility for understanding subglacial processes. Before comparisons are made, the data are tested for robustness by assessing various controls (e.g. sampling face orientation, number of particles measured, statistical variation due to sampling effects, particle shape, size and concentration) on particle fabrics. A new method of microfabric analysis is applied that involves the identification and delineation of distinct clusters of similarly-oriented sand grains in order to compare them to macrofabrics and inferred ice flow directions. Results show that microfabrics, on their own, are an unreliable indicator of ice flow direction in Kamloops Lake till in the study area and should not be used as a substitute for macrofabric data, as they likely record late-stage microscale strain patterns and pore water flow in addition to till deposition and deformation by overriding ice. We suspect that this would also be the case for coarse-grained till elsewhere. Our findings suggest that till microfabric interpretations should always be made after assessing corresponding macrofabric data alongside sedimentological and structural observations.

Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia, Canada and northern Washington State, USA

Quaternary Science Reviews, Nov 1, 2009

Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated i... more Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300 m below sea level. Over 600 m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform-substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts. During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s). Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.

Transient subglacial hydrology of a thin ice sheet: insights from the Chasm esker, British Columbia, Canada

Quaternary Science Reviews, Dec 1, 2012

Glacier drainage systems are never steady state; instead they fluctuate over time and space in re... more Glacier drainage systems are never steady state; instead they fluctuate over time and space in response to variation in water input. In order to improve numerical models of glacier hydrology it is important to fully understand the processes controlling subglacial drainage system evolution, but contemporary ice sheet beds are typically inaccessible. Thus, esker distribution, morphology, and sedimentology have been used to infer the dynamics and hydrology of former ice sheets. However, debate remains as to the processes responsible for esker formation and most theoretical investigations have assumed that they formed due to processes that operated beneath thick ice despite many field investigations to the contrary. We investigate esker formation during the final stages of the thin, inactive, and rapidly decaying Cordilleran Ice Sheet (CIS) in interior British Columbia, Canada. A combination of geomorphological, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) data suggest esker ridge sedimentary architecture is consistent with synchronous formation during a glacial lake outburst flood (GLOF). These data reveal esker ridge deposition most likely took place within an ice tunnel that evacuated late-waning stage flow, following erosion and partial fill of a broader meltwater corridor. Esker ridge sedimentary architecture reveals this depositional environment was dynamic, reflecting complex interaction between ice thickness, ice structure, ice tunnel geometry, flow conditions, and sediment supply. Under these thin, inactive ice conditions ice tunnel location was initially governed by structural weaknesses in the ice and/or equipotential gradient. Because creep closure rates were low, the ice tunnel evolved through feedbacks between conduit growth via frictional melting/mechanical ice excavation, and conduit closure due to sediment infilling, rather than ice creep. This resulted in a nonuniform ice tunnel that enlarged in an upglacier direction and locally unroofed to form open channels during its final stages. These data suggest that eskers may record unsteady, rather than steady flows in a drainage network and so the assumptions often made when using eskers to inform hydrological components of ice sheet models may not be applicable to all ice sheet settings.

The Harricana glaciofluvial complex, Abitibi region, Quebec: its genesis and implications for meltwater regime and ice-sheet dynamics

Sedimentary Geology, Apr 1, 1996

Interlobate moraines have been defined by their relationship to adjacent landforms and sediments,... more Interlobate moraines have been defined by their relationship to adjacent landforms and sediments, with minimal reference to any genesis suggested from their own geomorphology and sedimentology. A case in point is the "Harricana interlobate moraine". The Harricana complex, a relatively continuous, linear accumulation of glaciofluvial sediments, is investigated in terms of its geomorphology, sedimentology, stratigraphic context, and landform associations for the portion between latitudes 48"N and 50"N. The complex is narrower to the north than to the south. In the narrower northern part, better rounding and poorer preservation of less resistant clasts suggest more vigorous transport in a narrower conduit; the inverse in the wider southern part favours less vigorous flows and higher deposition rates. These inferences, together with unidirectional paleoflows towards the ice margin and the relatively continuous upslope path of the southward-widening complex, favour synchronous formation of the Harricana complex in a continuous closed conduit, beneath an ice sheet which thinned southward. Subaqueous fans and grounding-line deposits with fine gravel and sandy in-phase wave structures in the south, may have been deposited in subglacial cavities adjacent to the complex, or later deposited in reentrants in calving ice fronts. The basic building blocks of the Harricana complex are gravel facies which indicate generally high energy, but unsteady, transport. These facies are arranged into macroforms. Composite and oblique-accretion, avalanche-bed (OAAEl) macroforms are attributed to deposition by unsteady and non-uniform flows in conduit enlargements. Pseudoanticlinal macroforms were deposited in relatively narrow conduit segments of uniform width. Gravel-sand couplets register flow unsteadiness, perhaps related to variations in supraglacial meltwater supply. The origin of the Harricana complex as a mainly synchronous subglacial landform is linked to formation of adjacent streamlined bedforms and bedrock erosional marks by meltwater outburst floods. The complex is located where the orientation of these subglacial bedforms indicate strong flow convergence. Post-flood, ice sheet collapse along this convergence zone initiated redirection of ice flow, resulting in cross-cutting striae. Later subglacial meltwater systems followed new hydraulic gradients toward this trough of thinner ice, forming a major conduit and depositing the Harricana glaciofluvial complex.

Macroforms, large bedforms and rhythmic sedimentary sequences in subglacial eskers, south-central Ontario: implications for esker genesis and meltwater regime

Sedimentary Geology, Jun 1, 1994

ABSTRACT