Abstract:
The Ocean Drilling Program (ODP) drilled two sites using the advanced hydraulic piston corer in Saanich Inlet, a silled fjord in southeastern Vancouver Island, British Columbia, Canada. Although, the primary target was to obtain a complete varved Holocene record, the Late Pleistocene sediments below were also cored. 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. Lithofacies analysis, particle size measurements and 14 AMS C14 dates on bivalves 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 C-14 yr BP, the oldest date from the cores. Turbid meltwater plumes, turbidity currents and intense iceberg rafting are dominant sedimentary processes. A maximum sediment accumulation rate of 0.62 m/yr occurs over a 12 m interval of core with cross bedded and graded sand beds, probably deposited by sediment gravity flows originating from the sill at the fjord mouth. Lithofacies in Saanich Inlet are similar to those in modern glacial fjords in temperate southern Alaska and therefore facies models from Alaska may be used to reconstruct the Late Pleistocene in B.C.