Depositional record of a glacial-lake outburst: Glacial Lake, Souris, North Dakota

Abstract

Glacial Lake Souris, located in what is now North Dakota, was inundated by about 74 km3 of sediment-laden water from the outburst of Glacial Lake Regina (Saskatchewan). Glacial-lake outbursts were common during the late Pleistocene, but detailed studies of outburst-deposited sediments in lake environments have not been done. The purpose of this paper is to document the sedimentologic effects of the outburst on Lake Souris and to develop a depositional model. The outburst-deposited sediments in Lake Souris are grouped into three lithofacies on the basis of textural analyses of subsurface samples and surficial mapping: (1) sand, (2) matrix-rich gravel, and (3) matrix-deficient gravel. Sand is the most common sediment and generally overlies other lithofacies. Lignite particles are present in most sands and, at depth, are significantly larger than other particles. Matrix-rich, lignite-bearing gravel generally occurs at the base of the outburst sections. Matrix -deficient gravel occurs near the ground surface, overlying other outburst lithofacies, in the inlet area. Deposition of the outburst sediment is attributed to three main processes: low-density turbidity currents (the predominant process), high-density turbidity currents, and aggradation in braided rivers. Continuous, low-density turbidity currents caused by the influx of sediment-laden flows and by residual currents from high-density flows characterized the outburst event in Lake Souris. Most high-density flows resulted from the initial influx of the outburst or from continuous avalanching as a result of rapid deposition. Thick, homogenous sections of outburst sediments; a gradual, systematic fining of sediments distally from the Souris spillway; and the scarcity of silt-sized and clay-sized sediment indicate that there was little decrease in flow energy during the outburst. Nonrepetitive vertical sequences, showing little textural variation, indicate that there was little migration of individual depositional processes. The described characteristics may be used to identify and interpret other outburst events in similar settings and to help evaluate the Pleistocene sedimentary record in the marine offshore zone.