ANALYSIS OF BASAL TILL DEPOSITS BIG STONE MORAINE, STEVENS COUNTY, MINNESOTA AND ROBERTS COUNTY, SOUTH DAKOTA

The Big Stone Moraine is the last major constructional feature of the Des Moines lobe prior to the formation of Lake Agassiz. The Big Stone Moraine formed during multiple phases of deposition either as: recessional deposits or as “nested” readvance deposits. The goal of this study is to analyze the basal till sediments of the Big Stone Moraine to determine if significant changes in source sediments occur during multiple depositional events.

Models of the formation of the Big Stone Moraine cite variations in till lithology and grain size between at least two depositional phases (“inner” and “outer” Big Stone Moraine). Grain size variation appears to result from incorporation of lakes sediments deposited immediately after the formation of the “outer” Big Stone Moraine. Till lithology changes have been suggested to be the result of changing provenance due to varying flow patterns of the Des Moines lobe – perhaps evidence of ice stream activity. Analysis of basal till sediment lithology for this study were undertaken to determine if lithologic variations were due to Des Moines flow dynamics or the excavation and incorporation of older sediments.

Samples were collected on the Big Stone Moraine along ice marginal drainages. The outcrops observed provide stratigraphic and sedimentologic evidence of Des Moines Lobe readvances. Clast counts, grain size analysis, and provenance studies will determine if the Des Moines Lobe changed flow patterns or eroded older sediments during the formation of the Big Stone Moraine

Research for this study was funded by a grant from the N.S.F.-R.E.U Program (NSF-EAR 1262945).