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TitlePost-glacial evolution of Saanich Inlet, British Columbia: results of physical property and seismic reflection stratigraphic analysis
AuthorMosher, D C; Moran, K
SourceLate Quaternary sedimentation in Saanich Inlet, British Columbia, Canada: Ocean Drilling Program Leg 169S; by Bornhold, B D (ed.); Kemp, A E S (ed.); Marine Geology vol. 174, 2001 p. 59-77,
LinksAbstract - Résumé
Alt SeriesGeological Survey of Canada, Contribution Series 1999153
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
AreaSaanich Inlet; United States of America
Lat/Long WENS-124.0000 -123.0000 41.0000 33.0000
Subjectsmarine geology; sedimentology; postglacial evolution; seismic interpretations; seismic surveys; seismic profiles; seismic data; deep sea drilling; basins; outwash; glacial deposits; marine sediments; sea level changes; sea level fluctuations
Illustrationslocation maps; geological sketch maps; graphs; seismic reflection profiles; stratigraphic columns
ProgramOcean Drilling Program
AbstractSeismic 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 results from surface water penetration of the Cowichan River sediment plume, terrigenous sediment input from the steep terrain on the west side of the inlet, and tidal residuals forcing sediment laden waters to the western flank of the inlet. Incoherence of seismic reflectors within this unit may be the result of differential thickening, but may also result from occasional debris flows or other mass-wasting processes and fault offsets.