VOLUME OF SEDIMENT IN DEPOSITED IN GLACIAL LAKE NARRAGANSETT DURING LATE WISCONSINAN DEGLACIATION: IMPLICATIONS ON SEDIMENT SOURCE?

The volume of stratified sediment (deltas, lakefloor, and lacustrine fans) deposited in Glacial Lake Narragansett (GLN) during Late Wisconsinan deglaciation calculated using seismic reflection profiles, published Quaternary geologic maps and borehole records exceeds 8.7 x 10⁹ m³. Individual deltas quantified ranged from very small ice marginal deltas (3 x 10⁶ m³) to larger, well-developed delta systems (1.3 x 10⁹ m³). The volume of lakefloor sediment (varves) calculated using seismic reflection profiles (3.5 x 10⁹ m³) is a significant underestimation, as natural gas obscured the seismic record in some areas of the Bay, and some smaller ice-marginal stratified deposits without boreholes were not included in the calculations. The total volume of stratified deposits calculated (8.7 x 10⁹ m³) should be viewed as an extremely conservative estimate; the actual volume of stratified deposits almost certainly exceeds 10 x 10⁹ m³.

This volume of stratified deposits requires excavation of sediment or bedrock erosion >7.5 m ^. m^-2 over the entire 1,200 km² study area. This is more erosion than the limited bedrock exposure ages suggest occurred near the terminal margin during the Late Wisconsinan. If bedrock erosion and/or mobilization of subglacial till did not contribute all of the sediment, then either; a) a significant volume of sediment was transported from outside the local region or b) older glacial deposits provided a source for Late Wisconsinan deglacial deposits. Older deposits could include either Illinoian deglacial deposits or sediment deposited during the Late Wisconsinan advance of the ice sheet. We envision a model where some older deposits survived the advance of the ice sheet. These deposits would have been remobilized during deglaciation providing a supplemental sediment source, to bedrock erosion and subglacial till. This implies, that at least near the terminal margin in southern New England, removal of older deposits and erosion down to bedrock during the Late Wisconsinan advance of the Laurentide Ice Sheet was likely not as complete as often portrayed.