Late Pleistocene debris-flow deposits in large glacial lakes in British Columbia and Alaska

Abstract

The movement of Late Wisconsin (<30,000 B.P.) ice-sheet lobes counter to the direction of regional drainage ponded large lakes (up to 5000 km 2) in both the Upper Fraser River Valley in British Columbia and the Cooper River Valley in Alaska. Along the Fraser River, near Quesnel, glaciolacustrine sediment gravity-flow complexes contain thick (up to 10 m) and laterally extensive (up to 3 km) diamict units containing large rafts of poorly lithified Tertiary rocks. Diamicts offlap and thin away from Tertiary bedrock highs and were emplaced by subaqueous debris flows. These flows probably evolved from slumps and slides associated with the downslope failure of unstable Tertiary bedrock slopes triggered by rising lake levels, a process akin to “first-filling” slope failures around the margins of man-made lakes. In Alaska, along the Copper River Valley, massive and stratified diamicts were also deposited by debris flows and are interbedded with thin-bedded turbidites, poorly sorted gravels and normally graded diamicts. Debris flow deposits show basal grooves, internal compressional structures and rafted clasts that protrude from bed tops. Flows originated by slumping of rapidly deposited and underconsolidated glacial sediments around the basin margins; an important factor in flow generation has been the combination of steep substrate slopes and frequent earthquake shock. Observations in both British Columbia and Alaska stress the significance of sediment gravity-flow processes acting to deposit “glacial” sedimentary successions in large ice-contact lake basins.