Persistent Heterogeneity in Bedrock Erosion Rates Beneath the Laurentide Ice Sheet
Abstract
Much of the area formerly covered by the Laurentide ice sheet (LIS) is a landscape shaped by glacial erosion. Landforms found in this area are potentially bedforms in equilibrium with the ice sheet, or relict landforms inherited from preglacial topography. Attainment of steady-state equilibrium between the ice sheet and landscape implies higher total erosion and/or longer-term duration of subglacial erosion relative to a non-equilibrium landscape. Steady-state equilibrium is thus potentially characterized by spatially uniform erosion rates irrespective of bedrock composition during the final stages of glaciation. The record of erosion preserved in Late Wisconsinan tills over an area with two contrasting bed lithologies in the central portion of the former LIS indicates the presence of non-equilibrium landforms, implying limited cumulative bedrock erosion during Pleistocene glaciation. Bedrock east of Lake Nipigon, Ontario consists of Archean greenstones, intruded by a west-dipping Proterozoic diabase sill. Today, the diabase forms a scarp with 50 m of relief over the surrounding greenstone. The last phase of Wisconsinan glaciation advanced westward over the area to the 9.5 ka bp Nipigon moraine. Tills formed during this advance are coarse-textured, derived predominantly from local bedrock, and occur as a thin mantle over bedrock (~1m thick). Diabase forms a distinctive indicator dispersal train. Diabase concentrations in tills overlying the diabase outcrop, and in downice tills overlying greenstone indicate erosion and entrainment rates (eroded and entrained mass per unit bed area per unit flowline length) are 102 higher over the greenstone. Given basal velocities of 10s of m/a during till formation, the indicator dispersal train formed over a few hundred years. From these data, a number of insights into the nature of erosion of hard (crystalline) bedrock by the LIS are evident: Erosion occurred at high rates over short periods of time, suggesting significant bedrock erosion on an ice sheet scale was spatially and temporally restricted. Erosion rates were about 102 higher over the greenstone than the diabase, indicating the persistence of significant bedrock heterogeneity with respect to the erosive power of the ice sheet throughout the period of active erosion. Finally, theory (Hallet, 1979, 1996) predicts higher erosion rates of the diabase due to enhanced plastic flow over the topographically high diabase sill. However, the inverse correlation of measured erosion rates with bedrock mechanical strength indicates bed mechanical strength was the dominant factor controlling erosion rates, implying that erosion had not proceeded to steady-state equilibrium despite >>104 a of continuous glaciation. Contrary to previous studies suggesting that the Canadian Shield was subjected to pervasive and deep erosion by the LIS - 102 m or greater - our data suggest that Pleistocene glaciation may have accomplished little beyond removal of a preglacial cover of soft regolith from hard crystalline bedrock.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.C12A1009L
- Keywords:
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- 1824 Geomorphology (1625);
- 1827 Glaciology (1863);
- 5415 Erosion and weathering;
- 5416 Glaciation