ABSTRACT: Quadra Sand: a study of the late Pleistocene geology and geomorphic history of coastal southwest British Columbia

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Abstract

Quadra Sand is a late Pleistocene lithostratigraphic unit with widespread distribution in the Georgia Depression, British Columbia and Puget Lowland, Washington. The unit consists of horizontally and cross-stratified, well-sorted sand, minor silt, and gravel. it is overlain by till and related glacial sediments deposited during the Fraser Glaciation and is underlain by fluvial, estuarine, and marine sediments deposited during the preceding nonglacial interval. The unit is part of an apparently unbroken stratigraphic succession which records the major climatic oscillations of late Pleistocene time: till deposited during a pre-Fraser glaciation; glaciomarine sediments laid down during the subsequent transition to nonglacial conditions; marine, estuarine, and fluvial sediments deposited during the Olympia nonglacial interval; outwash deposited during the following nonglacial-glacial transition; and till deposited under full glacial conditions of the Fraser Glaciation. Stratigraphic evidence, paleocurrent data, sand mineralogy, and radiocarbon dates indicate that Quadra Sand was deposited progressively down the axis of the Georgia Depression and Puget Lowland from source areas in the Coast Mbuntains to the north and northeast. The unit is markedly diachronous; it is older than 29 ,000 radiocarbon years at the north end of the Strait of Georgia but is younger than 15,000 years at the south end of Puget Sound. Aggradation of Quadra Sand is thought to have been climatically induced. The initial influx of sand into the Georgia Depression probably occurred during a period of climatic deterioration at the onset of the Fraser Glaciation. The sand was deposited, in part, as distal outwash aprons at successive positions in front of, and perhaps along the margins of, glaciers moving from the coast Mountains into the Georgia Depression and Puget Lowland during late Wisconsin time. After deposition at a site, but before burial by ice, the sand was dissected by meltwater and the eroded detritus was transported farther down the basin to sites where aggradation continued. Quadra Sand buried older fluvial and estuarine deposits which, in turn, were laid down over marine sediments filling much of the Strait of Georgia. The present patchy distribution of Quadra and older sediments is due, in large part, to scour by glaciers at the height of the Fraser Glaciation.

Bibliography of Canadian Geomorphology