UNCOVERING THE PRE-MIDDLE WISCONSINAN GLACIAL HISTORY OF THE WESTERN MARGIN OF THE LAURENTIDE ICE SHEET BASED ON SEDIMENTOLOGICAL PROXY EVIDENCE AND SUBSURFACE MAPPING OF (GLACIO-)FLUVIAL SEDIMENTS (PEACE RIVER AREA, ALBERTA, CANADA)

The pre-Middle Wisconsinan record in the Peace River (Canada) area is commonly recorded by a sand and gravel unit that directly overlies bedrock; however, the depositional history and spatiotemporal relationship of the sand and gravel to the advance and retreat of the Laurentide and Cordilleran Ice Sheets are not well understood. The objective of this study is to understand the sedimentary architecture of the sand and gravel at various scales to reconstruct its depositional history and understand the potential influence of a western and eastern ice margin.

Regional mapping (core, outcrop, and water well lithologs) revealed three gravel-dominated units which overlie distinct bedrock straths positioned above the modern Peace River valley. These units include the Grimshaw gravel (320 m above Peace River), Old Fort gravel (220 m) and Shaftesbury gravel (25 m). Each gravel-dominated unit is overlain by Late Wisconsinan till and, in places, Middle Wisconsinan-aged glaciolacustine sediments. Sedimentological analysis of the Grimshaw gravel identified five facies associations (FAs) in the Grimshaw gravels, which record a west-east transition from a braided river depositional environment to a braidplain delta front setting.

The spatial relationship of the FAs suggests that the Grimshaw gravels braid delta was deposited in a body of water that previously occupied an ancestral Peace River valley, which was likely impounded by the LIS. Incision and partial erosion of the Grimshaw gravel records the retreat of the pre-Middle Wisconsinan LIS margin, resulting in opening of an outlet and lake drainage and incision of the Old Fort valley along the southern margin of the Grimshaw gravel. Based on the relative position of each gravel-dominated unit above the modern Peace River, each of the Grimshaw gravel, Old Fort gravel, and Shaftesbury gravel units are interpreted to record periods of deposition during the evolution of an ancestral Peace River, punctuated by periods of erosion and downcutting during separate interstadials or interglacials. We propose that the Grimshaw braid delta, together with the development of terraced sand and gravel deposits, provides proxy evidence for the presence of an active ice margin in the Peace River area prior to the Middle Wisconsinan.