USING TILL PROVENANCE TO UNDERSTAND ICE-SHED EVOLUTION DURING DEGLACIATION

A number of ice streams flowed in the Buffalo corridor in Manitoba (Ross et al., 2009) to the Des Moines lobe (DML) of the Laurentide ice sheet (LIS). Correlation for hundreds of kilometers shows that tills were incorporated in ice stream source areas, as documented in West Antarctica (Vogel et al., 2003), and variability down-ice is restricted to the lower meter and edge of the till and may relate to incorporation by freezing-on or enfolding. Multiple, lithologically distinct till sheets of the DML are reinterpreted as a result of the changing dominance of ice streams as the Buffalo Corridor ice shed evolved. Therefore, the dynamics and interplay of ice streams are the main controls on till lithology and lobe advance.

Does the evolving ice-shed concept apply elsewhere? Perhaps; previous descriptions could be reinterpreted in this context. Johnson and Hansel (1990) detail three lithologically distinct, subglacial lodgement tills in the Wedron Formation that are separated by erosional unconformities and subglacially sorted sand. Their proposed explanations of a change in location of debris entrainment or unroofing of a rock sequence could indicate a reorganization of the tributary ice shed. Heavy minerals in the Huron-Erie-Ontario lobe tills (Hofer and Szabo, 1993) show a shift in provenance to the west over time, another possible example of ice-shed change. We urge a re-examination of till-provenance data for southern Laurentide ice lobes in the context of ice-shed evolution. A better understanding of how the LIS was drawn down is relevant to current ice-sheet behavior and modeling efforts.

Hofer, J.W. & Szabo, J.P. 1993, Port Bruce ice-flow directions based on heavy-mineral assemblages in tills from the south shore of Lake Erie in Ohio: Can. J. E. Sci., v. 30 (6), p. 1236-1241.

Johnson, H.W. and Hansel, A.K. 1990, Multiple Wisconsinan glacigenic sequences at Wedron, Illinois: Sed. Pet., v. 60 (1),p. 26-41.

Ross, M., Campbell, J.E., Parent, M. & Adams, R.S. 2009, Palaeo-ice streams and the subglacial landscape mosaic of the North American mid-continental prairies: Boreas, v. 38 (3), p. 421-439.

Vogel, S.W., Tulaczyk, S., Joughin, I.R., 2003, Distribution of basal melting and freezing beneath tributaries of Ice Stream C: implication for the Holocene decay of the West Antarctic Ice Sheet: Ann. Glac., v. 36 (1), 273-282.