ASSESSING SUBSTRATE GENESIS AND MECHANISMS OF FAST ICE FLOW ALONG THE SOUTHERN MARGIN OF THE CORDILLERAN ICE SHEET, BRITISH COLUMBIA, CANADA

Zones of fast ice flow in past ice sheets are often inferred from broad tracts of streamlined landforms (e.g. drumlins, mega-lineations). Such inferences assume that i) pervasive deformation of bed sediment (till) is mainly responsible for fast ice flow and, ii) bed deformation creates streamlined forms. Inferences of fast ice flow seldom include field evidence supporting these assumptions.

We evaluate the field evidence for fast ice flow around the Okanagan Valley, British Columbia, Canada. The Okanagan Valley is a 250 km-long, linear, North-South trending valley bounded by plateaus to the East and West. Along-valley ice streams have been proposed where plateau ice may have converged toward the valley. Field evidence in the valley fails to support this proposition as i) streamlined bedforms are few and tend to occur in bedrock and, ii) till is rare in the stratigraphic record. However, streamlined bedforms and till are abundant on the plateaus; hypothetically, these may record fast ice flow.

The plateau landsystem contains drumlins eroded in bedrock, diamicton, and diamicton interbedded with sand and gravel. Diamictons are poorly consolidated and contain mainly sand and gravel. Silt and clay are rare and are concentrated in laminations within the diamicton. Clast fabric and shape data suggest these are not primary deposits. The material is interpreted as a ‘hybrid till’ resulting from lodgement followed by melt-out and squeeze-flow into water-filled subglacial cavities and localized deformation and poreflow at sticky spots.

Drumlins exhibit crescentic scours, en echelon arrangement, and enhanced development over positive bedrock steps. Sediment and bedrock drumlins develop in association with tunnel channels crossing topographic highs. Sediment characteristics, drumlin morphology and form continuity in the landscape are consistent with meltwater erosion in a broad underburst.

The plateau landsystem highlights the importance of subglacial water storage and release in till genesis and the formation of streamlined bedforms. This landsystem may record a type of fast ice flow controlled by changes in basal water pressure rather than by pervasive deformation of ice sheet substrate.