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.