Post-glacial evolution of Saanich Inlet, British Columbia: results of physical property and seismic reflection stratigraphic analysis

Seismic stratigraphic analysis combined with results from ODP Leg 169S deep ocean drilling within Saanich Inlet to depths greater than 100 m below the seafloor provided an opportunity to study the post-glacial development of the inlet in a quantitative manner. The seismic stratigraphy of Saanich Inlet is consistent with the general stratigraphy established for southwestern British Columbia. Sediment physical property data collected from ODP hydraulic piston cores provide the mechanism with which to correlate the seismic stratigraphy with lithologic descriptions. Acoustic basement (seismic Unit 1) is bedrock, likely Devonian or Carboniferous through Middle and possibly Late Cretaceous Wrangellian Terrane. Acoustic Unit 2 overlies bedrock and is interpreted as till or diamict. It is variable in occurrence and thickness and most prominent in the central portion of the inlet, with a maximum thickness of 247 m. There is no evidence of earlier glacial advances, thus it is likely a Vashon Drift from the latest glaciation (the Fraser Stade). Acoustic Unit 3 correlates with the grey, massive, silty-clay of lithologic Unit II, known regionally as the Capilano Formation, and locally as Victoria clay. It is a glacial-marine sediment resulting from outwash of the deteriorating ice sheet, largely composed of silty mud with beds and stringers of sand and common ice-rafted debris, including some boulders. Sediment of this unit infilled the basin, as evidenced by its thickness (max. 155 m) and its flat-lying upper surface. Its density values show a steep, normal-consolidation profile indicating relatively rapid deposition with no apparent over-consolidation. Hemipelagic and pelagic deposition, represented by Unit 4, became the dominant sedimentation mechanisms in the Holocene, as glacial-marine outwash declined. This transition took place between 14,700 and 12,000 yr bp , correlating with a time of rapid glacier recession and major relative sea level lowering. Relative sea level fell to a low of at least 45 m below present by about 9000 yr bp . This low stand resulted in partial isolation of the inlet and the commencement of anoxic conditions in the deeper portions of the inlet. Unit 4 demonstrates very low bulk density and velocity values as a result of low grain densities, high porosities and in situ gas. It is a flat-lying unit throughout the inlet, pinching out on the steep inlet walls. The unit thickens significantly to the north and then to the west. This thickening