Abstract:
Saanich Inlet on southern Vancouver Island is a deep near-shore marine basin with anoxic bottom waters and sediments that preserve a detailed record of postglacial marine and terrestrial environments. Cores collected by ODP Leg 169S reveal a massive, silty clay unit 40-50 cm thick, radiocarbon dated between 10,100-10,500 years old. This deposit is marked by a very sharp lower contact with laminated sediments, and an abundance of reworked Tertiary palynomorphs. Deposition of this unit has been interpreted as resulting from massive floodwater discharges caused by the collapse of glacial dams in the Fraser River drainage of mainland British Columbia. A mainland source of sediment is postulated by clay mineralogy, and the Fraser River source is defined by palynology, which matches the Tertiary pollen in the clay layer to sedimentary rocks of the Fraser Lowland near Vancouver. The upper Paleocene to lower Oligocene Huntingdon Formation (including former Kitsilano and Burrard Formations) is the most likely source for most of the microfossils, which include Carya, Pterocarya, Juglans, Ulmus, Fagus, Tilia, Intratriporopollenites, Osmunda, and other pollen and spores. Some contributions from the Miocene Boundary Bay Formation are also likely, based mainly on abundant Cedrus-type pollen. The erosion of Fraser Lowland bedrock by massive floodwaters of glacial origin, followed by transport across southern Georgia Strait in a sediment plume, is the best explanation for the "flood clay deposit" and its enclosed Tertiary palynomorphs in Saanich Inlet.