Quaternary stratigraphy and history of the Ootsa Lake - Cheslatta River area, Nechako Plateau, central British Columbia

Erosion along the Nechako Reservoir and Cheslatta River Spillway has result ed in unusually well-exposed late Quaternary and Holocene stratigraphy. Sur ficial sediments in the study area are mostly products of Late Wisconsinan glaciation. However, evidence for pre-Late Wisconsinan sedimentation has be en found along the shores of the Nechako Reservoir, including till of an ol der glaciation and organic-bearing, blue-grey, lacustrine sediments of prob able Middle Wisconsinan age. Stratigraphic correlation of the lake sediment s suggests that an extensive lake system occurred in the region during the Olympia Nonglacial Interval. Late Wisconsinan ice initially moved along maj or valleys, with glaciofluvial outwash deposited in front of the advancing ice. Advance-phase glaciolacustrine sediments are rare but significant, as slope failures are spatially associated with areas where they are preserved . The distribution of these sediments and associated deltaic deposits indic ates that advance-phase glacial lakes occurred up to approximately 855 m as l, at least several metres above the modern reservoir level. Sediments depo sited in front of the ice margin were overridden during ice advance and are best preserved in large valleys. At the glacial maximum, ice flowed northe asterly throughout the study region. Crag and tails, flutings, and drumlino id ridges with a generally consistent northeast trend are the dominant land forms. Till is the most common Pleistocene surficial sediment, covering app roximately 80% of the area; large areas of exposed bedrock are rare. Late-g lacial glaciofluvial and Holocene fluvial deposits are uncommon and occur m ainly along the Cheslatta River valley.