Sedimentary architecture of a glaciolacustrine braidplain delta: proxy evidence of a pre-Middle Wisconsinan glaciation (Grimshaw gravels, Interior Plains, Canada)

Abstract

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.