Palimpsest subglacial meltwater corridors of the last cordilleran ice sheet: Geomorphology, stratigraphy and insights into subglacial meltwater dynamics during deglaciation

Abstract

Recent releases of high-resolution elevation data in British Columbia have revealed several previously unidentified meltwater-derived landforms associated with the last Cordilleran Ice Sheet (CIS). Prominent among these are small subglacial meltwater corridors (sSMCs): morphologically distinct channels up to 10s of km long that are eroded into glacial sediments, with varying intra-corridor landform assemblages. Intra-corridor landforms identified include curvilinear ridges, eskers and multi-ridged fans. The sSMCs are one or more orders of magnitude smaller than the previously described Chasm and Green Lake meltwater corridors, the latter two formed from an extremely large supraglacial lake outburst flood. Results from geomorphological, sedimentological and near-surface geophysical methods show that an erosional unconformity cut into the regional till sheet forms the base of each sSMC. This unconformity extends from the channel boundary and includes intra-corridor curvilinear ridges and erosional remnants, consistently topped by a 0.2–0.3 m thick concentration of cobbles. A discontinuous channel-wide fill (5–15 m thick) buries some curvilinear ridges and is sometimes capped by an intra-corridor esker. Multi-ridged fans occurring downflow from braided outwash plains postdate the sSMCs and all other intra-corridor landforms. We interpret the sSMCs as palimpsest landforms that developed over several melt seasons through the continuous reoccupation by subglacial meltwater flow (and later proglacial streams). Each trough found lateral to a curvilinear ridge represents an individual subglacial meltwater channel incised over one or more melt seasons. The broad corridor fill records higher-than-average melt season discharge within portions of the sSMC, burying or further eroding curvilinear ridges and potentially forming eskers during late-stage waning flows. Results from this study indicate that the last CIS had an active and dynamic subglacial drainage system during deglaciation.