SEISMIC-STRATIGRAPHY OF SHUSWAP LAKE, BRITISH-COLUMBIA, CANADA

Shuswap Lake is a glacially-overdeepened 'fiord lake' located in the S huswap Highlands of southern British Columbia, Canada, and consists of two sub-basins each nearly 60 km long and up to 3 km wide. Single-cha nnel seismic reflection data, collected along 218 km of track line, id entifies a tripartite infill stratigraphy up to 800 m thick preserved in bedrock basins eroded as much as 298 m below sea level. A similar t ripartite stratigraphy is exposed in nearby outcrops along the valley of the South Thompson River and allows interpretation of age and depos itional settings for the infill identified on seismic records. In Shus wap Lake, the lowermost seismic-stratigraphic sequence (SSSI), up to 4 07 m thick, shows chaotic seismic facies with large-scale diffractions and fills axial parts of V-shaped bedrock basins. Outcrop data sugges t that this sequence consists of subaqueously-deposited, ice-contact s ilts, sands and gravels deposited in a deep (1 km?) ice-frontal lake d uring late Wisconsin deglaciation (ca. 10 ka). The overlying seismic-s tratigraphic sequence (SSSII; up to 403 m thick) shows continuous, hig h-frequency seismic facies that pass down-basin into transparent (refl ection-free) facies, High-frequency facies are interpreted as rhythmic ally-deposited silts and sands deposited by underflows (varves?); such facies are well exposed along the South Thompson River valley. Transp arent 'distal' facies likely record uninterrupted settling from suspen ded sediment transported down-basin by interflow or overflow plumes an d similar deposits are reported as currently forming in nearby Kamloop s Lake. A relatively thin (<70 m) postglacial sequence of rhythmically -laminated Holocene silts cm, immediately underlies the modem lake flo or. Deposition of very thick late-glacial stratigraphic successions an d an absence of older pre-late Wisconsin strata, appears to be a chara cteristic shared by other narrow, glacially-overdeepened valleys and b asins in central British Columbia. This may be the result of scour by subglacial meltwaters and sediment focussing during deglaciation of fi ord-like valleys occupied by deep ice-frontal lakes and rapidly retrea ting ice margins.