Tracy A. Brennand

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

Abstract Eskers of south-central Ontario were deposited in closed, subglacial conduits which were continuous (main conduits). This interpretation is supported by: the intimate association of eskers with an anastomosing network of tunnel channels; relatively continuous esker ridges; minimal post-formational disturbance of esker sediments; intercalation of till and stratified sand and gravel; diapiric folding at an esker core; low variability in palaeocurrent direction; and upslope flow paths. Down-esker trends in clast lithology, roundness and sphericity indicate continuous conduits with sediment supply by subglacial deformation of adjacent material into the conduits, melt-out of sediment from the conduit walls, and fluvial resedimentation. Esker ridge morphology is attributed to synchronous erosion, transportation and deposition along the main conduits, such that ridge discontinuities are primarily explained as zones of non-deposition during esker formation. Composite, pseudoanticlinal, and oblique accretion avalanche bed macroforms are identified. Gravel facies within these macroforms were deposited from fluidal flows or hyperconcentrated dispersions. Progradation of macroforms or migration of constituent large bedforms through the main conduits may have temporarily blocked constricted portions of those conduits and acted as possible internal (autogenic) controls on sediment availability and flow resistance. The location of macroforms within conduits was primarily controlled by conduit geometry and sediment availability, and later by feedback between macroform and conduit geometries. Sand and gravel units alternate rhythmically in vertical section. Rhythmicity is interpreted as a response to episodic flood flows, controlled by seasonal changes in water supply, but not necessarily representing annual events. Fans, beads, anabranched reaches and extended, hummocky deposits are intimately associated with the main esker ridges. They are interpreted in terms of subglacial cavities and localized flotation zones, connected to the main conduits during flood events. In-phase wave structures are the products of hydraulic jumps in hyperconcentrated flood flows at flow expansions into swells within the main conduits, into cavities connected to the main conduits, or at a grounding line. Esker ridges record only the most powerful flood events, whereas fans and beads record flow events in finer detail, being primarily depositional reservoirs. Laterally fining deposits record the waning stages of conduit operation.

Deglacial meltwater drainage and glaciodynamics: inferences from Laurentide eskers, Canada

Abstract This paper evaluates current knowledge of Laurentide eskers in Canada in the light of developments in glacier hydrology and glacial sedimentology. Questions regarding the morpho-sedimentary relations of eskers, the synchroneity and operation of R-channel systems, the role of supraglacial meltwater input and proglacial water bodies, the controls on esker pattern, and the glaciodynamic condition of the ice sheet at the time of esker formation are discussed. A morphologic classification of eskers is proposed. Five types of eskers are identified and investigated. Type I eskers likely formed in extensive, synchronous, dendritic R-channel networks under regionally stagnant ice that terminated in standing water. Type II eskers likely formed in short, subaqueously terminating R-channels or reentrants close to an ice front or grounding line that may have actively retreated during esker sedimentation. Type III eskers plausibly formed in short R-channels that drained either to interior lakes in, or tunnel channels under, regionally stagnant ice. Type IV eskers may have formed as time-transgressive segments in short, subaerially terminating R-channels (or reentrants) that developed close to the ice margin as the ice front underwent stagnation-zone retreat or downwasted and backwasted regionally (stagnant ice); however, formation in synchronous R-channels cannot be discounted on the basis of reported observations. Type V eskers may have formed in H-channels that terminated subaerially. The spatial distribution of these esker types is discussed. The factors that determined Laurentide R-channel pattern and operation were likely a complex combination of (i) supraglacial meltwater discharge, (ii) the number and location of sink holes, (iii) the ice surface slope, thickness and velocity, and (iv) the permeability, topography and rigidity of the bed. These factors cause and respond to changes in ice dynamics and thermal regime over the glacial cycle.

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

Abstract Interlobate moraines have been defined by their relationshio 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 (OAAB) 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 metlwater 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.

A review of open-channel megflood depositional landforms on Earth and Mars

Large freshwater floods on Earth in recent times and in the Quaternary have often been associated with catastrophic out-bursts of water from lakes impounded by glacial-ice or debris (such as moraine). In either case, large-scale depositional sedimentary landforms are found along the courses of the floodwaters. On Mars, similar floods are believed to have resulted from catastrophic efflux of water from within the Martian surface. Within the Martian flood tracts, landforms have been imaged that appear similar to those identified on Earth. These are primarily suites of giant bars – “streamlined forms” – of varying morphology that occur primarily as longitudinal features within the floodways and along the margins as well as in areas of the floodways that were sheltered from the main flow. In addition, flow-transverse bedforms within the floodways have been identified as giant sedimentary dunes or antidunes. Information concerning the flood hydraulics that created these forms may be deduced from their location and plan view morphology. Some other fluvial landforms which have been associated with megafloods on Earth have yet to be identified on Mars. The examples from Earth are described, so as to spur the search for further water-lain landforms on Mars

Measuring (subglacial) bedform orientation, length, and longitudinal asymmetry – Method assessment

Geospatial analysis software provides a range of tools that can be used to measure landform morphometry. Often, a metric can be computed with different techniques that may give different results. This study is an assessment of 5 different methods for measuring longitudinal, or streamlined, subglacial bedform morphometry: orientation, length and longitudinal asymmetry, all of which require defining a longitudinal axis. The methods use the standard deviational ellipse (not previously applied in this context), the longest straight line fitting inside the bedform footprint (2 approaches), the minimum-size footprint-bounding rectangle, and Euler’s approximation. We assess how well these methods replicate morphometric data derived from a manually mapped (visually interpreted) longitudinal axis, which, though subjective, is the most typically used reference. A dataset of 100 subglacial bedforms covering the size and shape range of those in the Puget Lowland, Washington, USA is used. For bedforms with elongation > 5, deviations from the reference values are negligible for all methods but Euler’s approximation (length). For bedforms with elongation < 5, most methods had small mean absolute error (MAE) and median absolute deviation (MAD) for all morphometrics and thus can be confidently used to characterize the central tendencies of their distributions. However, some methods are better than others. The least precise methods are the ones based on the longest straight line and Euler’s approximation; using these for statistical dispersion analysis is discouraged. Because the standard deviational ellipse method is relatively shape invariant and closely replicates the reference values, it is the recommended method. Speculatively, this study may also apply to negative-relief, and fluvial and aeolian bedforms.

The Deglacial Landscape of the Southern Fraser Plateau, British Columbia

Until recently, the prevailing view of late glacial Cordilleran Ice Sheet (CIS) decay over the interior plateaus was one of regional stagnation attributed to a rapid rise in equilibrium line altitude (ELA). A new model of late glacial CIS decay has emerged from recent landform mapping and classification across the southern Fraser Plateau, facilitated by the availability of higher resolution terrain data, geophysical surveys and sedimentology at available exposures. Patterns of palaeoglacial lake evolution and moraines suggest an active ice margin systematically retreating northwestward back towards the Coast Mountains, consistent with the pattern of eskers, ice-marginal channels, and reconstructed glaci-isostatic tilt from palaeolake shorelines. Lake effects likely enhanced glacier retreat, at least partially decoupling it from broader climate patterns. Ice-margin retreat was accompanied by thinning recorded by the presence of nested ice-marginal channels and supraglacial eskers. A rapidly rising ELA and lake effects enhanced melt rates allowing supraglacial lakes to form on the ice surface and drain through crevasses or moulins forming subglacial meltwater corridors and glacial lake outburst flood eskers. Pockets of localized stagnation, supported by the presence of hummocky terrain, crevasse-fill ridges and ice-walled canyon eskers, existed during regional retreat across the southern Fraser Plateau.