CORRELATION OF LACUSTRINE SEDIMENTS ASSOCIATED WITH GLACIAL LAKE BENSON AND GLACIAL RIVER WARREN TERRACE SEDIMENTS IN THE MINNESOTA RIVER VALLEY

Glacial Lake Benson formed as an ice marginal lake during the Late Wisconsinan retreat of the Des Moines Lobe in the Minnesota River Valley. Previous studies suggest that Glacial Lake Benson drained shortly after the formation of the Big Stone Moraine and prior to the formation of Glacial Lake Agassiz. The southern outlet of Lake Agassiz was a spillway formed in the Big Stone Moraine, which released outflow as Glacial River Warren. The goal of this study was to determine the relationship between Glacial Lake Benson and Glacial River Warren by studying late the glacial stratigraphy of the Minnesota River Valley.

Three gravel pits in the Minnesota River Valley were located, described and analyzed. Each site was at or near an elevation of 1030ft. Two pits are located just south of the Big Stone moraine near Ortonville, MN. The third pit is approximately 50 miles southeast of the Lake Agassiz spillway, between Montevideo, MN and Granite Falls, MN. Geologically, each pit was located in the Glacial Lake Benson basin and associated with a Glacial River Warren terrace.

All three gravel pits have inter-bedded silt, clay and sand units that are presumed to be Glacial Lake Benson sediments. In the two northern gravel pits these inter-bedded silt, clay and sand layers are overlain with coarse outwash gravel attributed to Glacial River Warren. In one of the northern pits, a layer of till was observed within Glacial lake Benson sediments, suggesting a fluctuating ice margin during Lake Benson time. In the southern gravel pit, the inter-bedded layers are proximal varves. The absence of an upper gravel layer is probably due to excavation during subsequent River Warren events.

The stratigraphic relationships at these three exposures suggest Glacial River Warren formed well after Lake Benson had been established. The relationship of the draining of Glacial Lake Benson and the formation of Glacial River Warren remains the focus of continued studies. Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 0353621).