Two Cordilleran and three Laurentide glacial advances are recorded in Quaternary sediments and landforms in the Peace River valley, northeast British Columbia. The advances are inferred from fluvial gravels, glaciolacustrine sediments, and tills within nested paleovalleys excavated during three interglaciations and from the distribution of granitoid clasts derived from the Canadian Shield. Till of the last (Late Wisconsinan) Laurentide glaciation occurs at the surface, except where it is overlain by postglacial sediments. The advance that deposited this till was the most extensive in the study area, and the only advance definitively recognized in western Alberta south of the study area. Late Wisconsinan Cordilleran till has not been found in the study area, but Cordilleran and Laurentide ice may have coalesced briefly during the last glaciation. Support for this supposition is provided by the inferred deflection of Laurentide flutings to the southeast by Cordilleran ice. The earliest Laurentide advance may have been the least extensive of the three Laurentide events recognized in the study area. Erratics attributed to this advance occur only east of the Halfway River – Beatton River drainage divide.
In the past, researchers have disagreed over the maximum extent of the Cordilleran Ice Sheet in the Peace River valley during the Late Wisconsinan. Some workers argued that Cordilleran ice reached beyond the Rocky Mountains and briefly coalesced with the Laurentide Ice Sheet on the westernmost Interior Plains. In contrast, others asserted that Cordilleran ice did not reach beyond the eastern front of the Rocky Mountains. Stratigraphic interpretation of three sections within a Middle Wisconsinan paleovalley and re-examination of a previously published regional stratigraphic framework show that western-sourced ice (likely the Cordilleran Ice Sheet) extended east of the mountain front during the Late Wisconsinan, prior to the incursion of the Laurentide Ice Sheet into the area. This conclusion has implications for Cordilleran Ice Sheet reconstruction and modelling, and provides insight into the interactions between the Cordilleran and Laurentide ice sheets during the last glaciation.
The erosional nature of glacial systems commonly results in removal of direct evidence of previous glaciation (e.g. till and moraine). Therefore, reconstruction of former ice‐margin positions may rely, in part, on indirect (proxy) evidence from the sedimentary record. This study examines the facies and sedimentary architecture of a pre‐Middle Wisconsinan sand and gravel deposit (the ‘Grimshaw gravels’), which is positioned between areas where previous stratigraphical investigations have identified single (Late Wisconsinan) and multiple (pre‐Middle to Late Wisconsinan) glaciation by the Laurentide Ice Sheet. Five facies associations (FAs) are characterized within the deposit, which, together with the sedimentary architecture, record a transition from a braided river environment in the west (FA1‐3) to a gravelly braidplain delta front in the east (FA4 and 5). We propose that the Grimshaw gravels braid delta formed at the margin of a body of water that occupied the ancestral Peace River valley, probably impounded by the LIS; hence, the Grimshaw braid delta provides proxy evidence of the presence of an ice margin (previously unrecognized) in the Peace River lowland prior to the Middle Wisconsinan. This study provides further understanding of the origin of the Grimshaw gravels deposit, allowing re‐evaluation of previous models of formation. These findings offer insight into the glacial history of the southwestern margin of the LIS, and may help to refine ice‐sheet reconstructions spanning the Wisconsinan glaciation.
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