Late Pleistocene glacimarine record in Saanich Inlet, British Columbia, Canada

Abstract

Abstract Late Pleistocene deposits were cored at two sites in Saanich Inlet and provide a record of the sedimentary environment during deglaciation of this fjord. At Site 1033 at the southern end of the fjord, 59.3 m of Late Pleistocene sediment was obtained in a hole drilled to 105.1 mbsf. At Site 1034, 4.8 km to the north, a hole was drilled to 118.2 mbsf, recovering 49.8 m of Late Pleistocene sediment. Sediments were deposited in this deep (>200 m) fjord basin, as a lobe of the Cordilleran ice sheet retreated at the end of Fraser Glaciation. This is the first recovery of basin-fill sediments from the Pleistocene retreat of the Cordilleran ice sheet. Lithofacies analysis, particle size measurements and 17 AMS 14 C dates on marine shells or wood fragments are used to reconstruct the glacimarine sedimentary environment. Sediments are dense, gray to olive-gray terrigenous silty clay with thick, graded sand beds and interlaminated fine sand and silt with mud. Scattered dropstones, clusters of pebbles, and pods of sand and granules are common. Bioturbation, indicated by disruption of laminae and horizontal burrows or black mottles is minimal to moderate, and decreases downcore. A calving tidewater glacier retreated northward through Saanich Inlet, however the ice-contact sediments are beneath the depth cored at both sites. The glacier retreated rapidly and the basin was open to marine exchange through Satellite Channel prior to 13,270±60 14 C yr BP, the oldest date from the cores. Deposition of Lithofacies Association I occurred in an iceberg-zone depositional environment dominated by turbid meltwater plumes and intense iceberg rafting within approximately 10 km of the glacier terminus. After the glacier grounded at the head of Cowichan Bay, Lithofacies Association II was deposited in the ice-distal depositional environment by turbidity currents that by-passed the sill and turbid meltwater plumes. Lithofacies Association III was deposited under post-glacial conditions at the end of the Pleistocene.