Glacially Overdeepened Troughs and Ice Retreat ‘Till Tongue’ Deposits in Queen Charlotte Sound, British Columbia, Canada

The continental shelf of Queen Charlotte Sound, British Columbia, was inundated by piedmont glaciers which extended to the shelf edge by way of the shelf transverse troughs [Josenhans et al., 1992; Luternauer et al., 1989]. Seismic reflection profiles indicate that glacial erosion overdeepened these troughs and developed a smooth glacial unconformity on bedrock. Glacial and deglacial sediments comprising basal till, multiple ice-contact sequences, and stratified glaciomarine sediments overlie the glacial unconformity and typically attain 50m in thickness. The offshore banks, which separate the troughs, appear to be devoid of glacial deposits and may have remained ice free during the last glaciation. Regional seismic profiles from the trough axes reveal multiple generations of channel fill deposits in channels cut into the Plio-Pleistocene bedrock on the inner shelf. The upper surface of the channel deposits is truncated by the glacial erosional unconformity and overlain by till. These channel deposits have not been sampled and may represent a preglacial subaerial drainage system or possibly subglacial channel sequences that were deposited prior to the last glacial advance.