Paper No. 7
Presentation Time: 10:30 AM
PROVENANCE OF NEAR-SURFACE GLACIAL SEDIMENTS IN MINNESOTA
GOWAN, Angela S. and THORLEIFSON, L. Harvey, Minnesota Geological Survey, University of Minnesota, 2609 West Territorial Road, St. Paul, MN 55114-1009, gowa0001@umn.edu
Quaternary strata in Minnesota largely consist of till and associated sorted sediments. As is the case elsewhere, this till presumably consists of a mix of eroded bedrock, at varying distance from source, eroded residuum, and reworked sediments. Although the clay, silt, and finer sand-sized fraction of till is more challenging to trace to source, aside from some characterizations of bulk composition and distinctive tracers, the coarser sand and gravel fractions can be categorized lithologically and often associated with known bedrock sources. With respect to bulk composition, abundant rocks such as red volcanics in Minnesota tills are attributable to the Lake Superior basin, while igneous and metamorphic rocks are derived from the Canadian Shield, carbonate rocks are potentially derived from Manitoba or Hudson Bay, and abundant shale clasts in till of the Red River and Minnesota River valleys have a source in North Dakota and Manitoba.
Distinguishing between a Hudson Bay and Manitoba provenance for some till sheets, or components thereof, has been a longstanding problem. To some extent, it has been known on the basis of distinctive erratics that debris from Hudson Bay is present across the region, at least at trace levels. Much recently-acquired data is now available to more precisely identify specific source areas, at least for near surface till.
To distinguish between the two known carbonate sources, calcite and dolomite content of the silt and clay matrix indicates that Hudson Bay till is more calcite-rich, while southern Manitoba till is generally more dolomite-rich. The more dolomite-rich and also shale-bearing till of the Des Moines lobe is thus attributed to sources in the northwest. In central Minnesota, calcareous, but largely shale-free, till of the Wadena lobe is also dolomite rich, so the carbonate component is also attributed to sources in the northwest, in this case shale-free sources, although dilution has caused Wadena carbonate contents to be lower than Des Moines.
