Late Quaternary sediments and geomorphic history of northern Vancouver Island, British Columbia

Abstract

Materials from two glacial intervals and one nonglacial interval have been identified on northern Vancouver Island, British Columbia. The oldest Pleistocene unit, termed "older drift," consists of glaciomarine silt and clay >38 000 years BP in age that overlie a till that has only been recorded in well logs. "Older drift" is tentatively correlated with Dashwood drift of the Semiahmoo Glaciation (early Wisconsin or older). No sediments of the succeeding Olympia nonglacial interval (mid-Wisconsin) have been found in the area. It is thought that this interval was characterized by a period of degradation in which Olympia-age sediments were deposited in transient sedimentary environments and subsequently eroded, in part during the Fraser Glaciation (late Wisconsin). The youngest Pleistocene unit, termed Port McNeill drift, includes advance deposits, till, and deglacial sediments, all deposited during Fraser Glaciation. Ice of this glaciation did not cover most of northern Vancouver Island until after 20 600 ± 330 years BP. At the maximum, which probably occurred about 15 000 years ago, Coast Mountain ice coalesced with and overrode Vancouver Island ice, and flowed in a westerly to northwesterly direction across northern Vancouver Island. Deglaciation commenced prior to 12 930 ± 160 years BP and possibly as early as 13 630 years ago on the eastern coast. Maximum sea level during and immediately following deglaciation was about 92 and 20 m elevation on the east and west coasts, respectively. This suggests that ice thickness at the Fraser maximum decreased westward across the study area. Deposits of Recent time include colluvial sediments formed by weathering and mass movement processes, alluvial fan and floodplain deposits, eolian sands associated with active beaches on the west coast, and organic deposits.