Evidence for rapid sedimentation in a tunnel channel, Oak Ridges Moraine, southern Ontario, Canada

In south-central Ontario, a Late Wisconsinan regional unconformity consisting of tunnel channels and drumlinized till crops out north of Lake Ontario. The tunnel channels are locally infilled and the unconformity buried by sediment of the Oak Ridges Moraine. Based on seismic reflection profiling and drillcore, the tunnel channels are known to continue beneath the moraine. Detailed sedimentological analysis of ∼300 m of continuous core from two drillholes located ∼7 km apart in subparallel tunnel channels identified three facies. The gravel facies is 17 m thick and occurs in only one of the cores. It directly overlies the unconformity and may include a number of upward-fining units. Seismic reflection data suggests the gravel forms stacked gravel mesoforms or quasi-horizontal gravel layers (bed load sheets deposited from fluidal flows). Along the deep channel axis, the graded massive sand facies up to 37 m thick is the most common facies and consists of silty, medium sand with minor coarse sand. Strata are reverse-graded, normal-graded, or massive with isolated silt intraclasts and evidence locally for dewatering. This facies is interpreted to have been deposited from hyperconcentrated dispersions downflow of a hydraulic jump (or major channel confluence). The third facies consists of medium-scale and small-scale cross-stratified sand. Medium-scale cross-stratification occurs predominantly in the lower 37 m of the core and is interbedded with the graded massive sand facies; small-scale cross-stratified sand is progressively more common upward in core. Medium- and small-scale cross-stratified sands was deposited, respectively, by subaqueous dunes and ripples formed in dilute fluid flows. The complete succession is interpreted to have been deposited very rapidly within a subglacial tunnel channel before discharge ceased along the channel. Deposition followed closely, and in part coincided with rapid expansion of the channel by erosion in a hydraulic jump or at a major channel confluence along the grounding line of a subglacial lake. Scour into unconsolidated sediment contributed to the sediment flux and quickly overloaded the flow with suspended sediment, which in turn resulted in extremely high rates of sedimentation immediately downflow. Such depositional conditions support the notion that tunnel channels in the study area formed and/or served as conduits for subglacial jökulhlaup discharges, and that the Laurentide Ice Sheet most probably did not generally drain by steady state processes, but instead by short-lived catastrophic events.