Seismic-stratigraphy of Shuswap Lake, British Columbia, Canada

Shuswap Lake is a glacially-overdeepened 'fiord lake' located in the Shuswap Highlands of southern British Columbia, Canada, and consists of two sub-basins each nearly 60 km long and up to 3 km wide. Single-channel seismic reflection data, collected along 218 km of track line, identifies a tripartite infill stratigraphy up to 800 m thick preserved in bedrock basins eroded as much as 298 m below sea level. A similar tripartite stratigraphy is exposed in nearby outcrops along the valley of the South Thompson River and allows interpretation of age and depositional settings for the infill identified on seismic records. In Shuswap Lake, the lowermost seismic-stratigraphic sequence (SSSI), up to 407 m thick, shows chaotic seismic facies with large-scale diffractions and fills axial parts of V-shaped bedrock basins. Outcrop data suggest that this sequence consists of subaqueously-deposited, ice-contact silts, sands and gravels deposited in a deep (1 km?) ice-frontal lake during late Wisconsin deglaciation (ca. 10 ka). The overlying seismic-stratigraphic sequence (SSSII; up to 403 m thick) shows continuous, high-frequency seismic facies that pass down-basin into transparent (reflection-free) facies, High-frequency facies are interpreted as rhythmically-deposited silts and sands deposited by underflows (varves?); such facies are well exposed along the South Thompson River valley. Transparent 'distal' facies likely record uninterrupted settling from suspended sediment transported down-basin by interflow or overflow plumes and similar deposits are reported as currently forming in nearby Kamloops Lake. A relatively thin (<70 m) postglacial sequence of rhythmically-laminated Holocene silts cm, immediately underlies the modem lake floor. Deposition of very thick late-glacial stratigraphic successions and an absence of older pre-late Wisconsin strata, appears to be a characteristic shared by other narrow, glacially-overdeepened valleys and basins in central British Columbia. This may be the result of scour by subglacial meltwaters and sediment focussing during deglaciation of fiord-like valleys occupied by deep ice-frontal lakes and rapidly retreating ice margins.