Open File Report 2012-16

Open File Report 2012-16

OFR 2012-16

Surficial Geology and Quaternary History of the Southern Lesser Slave Lake Area (NTS 83O South)

Author(s) Pawley, S. Atkinson, N. Date 2012-11-06

This report describes the distribution, character, and origin of surficial sediments south of Lesser Slave Lake in the Lesser Slave River (NTS 83O/SE) and Faust (NTS 83O/SW) map areas and outlines the Quaternary history of the region. This region consists of a narrow lowland that extends parallel to the lake and separates uplands rising to the north and south.

Widespread deposits of till and associated streamlined glacial landforms document the southward advance of Laurentide ice across the southern Lesser Slave Lake area. A succession of up to three till facies, in places overlying preglacial fluvial deposits indicates that the Lesser Slave basin and the surrounding uplands of the Swan Hills and Pelican Mountains were inundated by the Laurentide Ice Sheet during late Wisconsinan glaciation.

The Laurentide Ice Sheet retreated from the southern Lesser Slave Lake area at ca. 12 ka BP. This retreat is recorded by a widespread suite of ice-marginal and proglacial sediment-landform associations, including large areas of well-developed doughnut moraine. The association of doughnut moraine with glaciofluvial landforms supports the interpretation that deglaciation was characterized by widespread ice stagnation, in part associated with the melting of residual ice masses that persisted after regional deglaciation.

Following regional retreat of the Laurentide Ice Sheet, the postglacial evolution of the Lesser Slave Lake area has been influenced by the accumulation of fine-grained alluvium, eolian sediment, and extensive peatlands, which mantle the glaciolacustrine deposits, particularly across the lowlands of the Lesser Slave Plain. Upland areas have been further influenced by slope failures and mass movements, with extensive colluvial deposits resulting from deep-seated landslides that fringe the steep slopes of the Pelican, Flat Top and House mountains, and Grizzly Ridge.