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

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Erosion along the Nechako Reservoir and Cheslatta River Spillway has resulted in unusually well-exposed late Quaternary and Holocene stratigraphy. Surficial sediments in the study area are mostly products of Late Wisconsinan glaciation. However, evidence for pre-Late Wisconsinan sedimentation has been found along the shores of the Nechako Reservoir, including till of an older glaciation and organic-bearing, blue-grey, lacustrine sediments of probable Middle Wisconsinan age. Stratigraphic correlation of the lake sediments suggests that an extensive lake system occurred in the region during the Olympia Nonglacial Interval. Late Wisconsinan ice initially moved along major 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 indicates that advance-phase glacial lakes occurred up to approximately 855 m asl, at least several metres above the modern reservoir level. Sediments deposited in front of the ice margin were overridden during ice advance and are best preserved in large valleys. At the glacial maximum, ice flowed northeasterly throughout the study region. Crag and tails, flutings, and drumlinoid ridges with a generally consistent northeast trend are the dominant landforms. Till is the most common Pleistocene surficial sediment, covering approximately 80% of the area; large areas of exposed bedrock are rare. Late-glacial glaciofluvial and Holocene fluvial deposits are uncommon and occur mainly along the Cheslatta River valley.