GLACIAL DISPERSAL AND TILL PROVENANCE IN NORTHERN QUEBEC – NEW CONSTRAINTS ON THE LATE QUATERNARY GLACIAL RECORD OF THE EASTERN LAURENTIDE ICE SHEET

While most of the eastern sector of the former Laurentide Ice Sheet (LIS) is underlain by a single sheet of thin and discontinuous till overlying crystalline rocks, tills of the Abitibi region of Quebec and Ontario are relatively thick and continuous and are capped by glacial Lake Ojibway sediments. Nearly 1150 m of continuous Rotosonic cores provide a comprehensive regional Quaternary record. This record includes 3 regional till sheets separated by 2 units of non glacial sediments. 918 till samples were collected for extensive lithologic, mineralogical and geochemical analyses to establish a provenance record.

The upper till has a characteristic light grey sandy matrix due to derivation from large Archean plutonic bodies lying north of the study area. In contrast, the middle till has a characteristic eastern provenance revealed by its dark greenish gray, slightly calcareous matrix which reflects its high content of debris derived from east-west trending Archean greenstone belts. These two unweathered till sheets overlie organic-bearing lacustrine and alluvial sediments of presumed Sangamonian age. The underlying till, presumably Illinoian in age, is characterized by subtle to moderate weathering features and local provenance.

In an effort to further assess the short and long distance provenance characteristics of the glacial succession, we have made Ar-Ar measurements of hornblende and feldspar grains, and Pb isotope measurements of dated feldspar grains. While all three till units yielded predominantly Archean ages (~ 2.5 Ga) typical of local rocks, the Ar-Ar data from the middle till also yield ages from 1.7-2.4 Ga, which we initially thought might point to a Churchill source and thus a surprising ice-flow direction. The Pb isotope data from the feldspars solve this problem by showing that the Ar-Ar data are most simply explained as Archean with Grenville (~1 Ga) partial overprint. This indicates that the middle till contains grains derived from the distant parautochtonous belt along the Grenville Front, which in turn suggests that this till reflects long-term westward transport during early and middle Wisconsinan stages of the LIS history. Pre-LGM flowlines reconstructed from multiple datasets suggest that the main divide of the eastern LIS during its early development lied slightly east of the Grenville Front.