DEPOSITION IN AN ESKER, BEAD AND FAN COMPLEX, LANARK, ONTARIO, CANADA

A glaciofluvial landform complex comprising a single esker ridge proximally, multiple ridges and beads in the middle section, and a complex of ridges and fans distally is mapped and described. We present detailed successions in the associated sediment, showing sedimentary structure, grain size, and palaeocurrent estimates. Sediment in the esker is mainly coarse grained and varies from boulders with a fine-sand matrix deposited at maximum discharges to diamicton interpreted as a product of ice pressing under low discharges. Early stage deposition in the esker at high flow was from suspension and bed load; intermediate stage deposition was mainly from bed load; and late stage deposition was from hyperconcentrated suspension. The esker rests in a bedrock channel which functioned as a Nye channel prior to esker formation. The channel surface carries a variety of fluvial erosional marks. Short distributary ridges of the ridges and beads complex contain mainly cobbles and boulders, but beads are mainly in sand with rapid down-flow fining and change in sedimentary structure. Faulting and overfolding of bead sediment suggest a subglacial origin. Deposition of the esker/fan complex was highly variable in space and time; down-fan sedimentary changes, with transitions from gravel to cross-laminated fine sand in a few tens of metres, indicate rapid loss in competence. Fan location lateral to the esker indicates a broadening of the zone of subglacial meltwater with time. Deposition from suspension and steep-sided scours are interpreted to result from deposition and erosion by hyperconcentrated flows. A model of underflows, with submerged hydraulic jumps, spilling laterally from the esker tunnel into subglacial cavities, explains the morphological and sedimentary characteristics of the fans. Detailed analysis of small-scale fining-upwards sequences in the fans and beads indicates deposition by a large number of flow events. These events are explained by glacier hydraulics, whereby periodic separation of the glacier from its bed caused water with a high suspended sediment concentration to spill out from the esker tunnel into adjacent subglacial cavities. Evolution of the subglacial drainage system indicates a complexity of subglacial sedimentation near to a grounding line which is not readily apparent from observations at modern glaciers terminating in relatively deep water.