Sedimentation in an Alpine Lake During Deglaciation, Okanagan Valley, British Columbia, Canada

Abstract

Four stratigraphic successions in lacustrine sediments exposed beneath prominent terraces in the Okanagan Valley, British Columbia, Canada, are described. Sedimentation processes are deduced from interpretation of vertical and spatial changes in primary sedimentary structure, texture, attitude and thickness of beds, and palaeocurrent. Summer intervals consist of finely graded and massive silts deposited from turbid inflowing meltwater. In addition thick deposits (> 10 cm) of climbing-ripple cross-lamination, and convoluted beds are the result of subaqueous slumps. A mechanism for slumping based on differential compaction of lake bed sediments is proposed. Late summer and winter deposition differs strikingly from the classical varve model. Proximal intervals show rapidly alternating units of structureless sand, current bedded sand and laminated silts. The sands are considered to have been emplaced by slides occurring on delta fronts during drawdown of lake levels at the end of the melt season. Properties of both grain flow and dispersed turbidity underflow deposits are revealed, and a quantitative mechanism for acceleration of grain flows with transition to turbidity flows is presented. Upward thinning of both summer and winter intervals as a result of ice marginal retreat is evident. Final attitudes of the sediment bodies, the deep basins occupied by present day lakes, and terraces underlain by lacustrine deposits are the result of melt out of large masses of glacier ice buried beneath the lacustrine sediments. A model of alpine lake sedimentation based on changing depositional processes with time and distance from the ice margin is presented.