LANDFORMS AND STRATIGRAPHIC ARCHITECTURE OF THE SAGINAW LOBE TERRAIN, SOUTHERN CALHOUN COUNTY, MICHIGAN

Current and recent 7 1/2 minute quadrangle mapping projects in Calhoun County, Michigan done through the USGS STATEMAP and Great Lakes Geologic Mapping Coalition, lie within a large drumlin field formed by the Saginaw Lobe of the Laurentide Ice Sheet. Approximately 35 rotosonic and Geoprobe test borings, most reaching bedrock, provide the first regional stratigraphic framework for this area. Two advance/retreat cycles are interpreted from the subsurface data. The bedrock surface, which appears to be eroded and tectonized by the first of the two advances, is commonly overlain by a dense, gray diamicton. The diamicton is overlain by a sequence of sand and gravel units of variable thickness, probably deposited during ice retreat. A second advance is represented by a sandy, yellow-brown diamicton, which forms the core of the drumlins. Active ice retreat from this advance resulted in the deposition of discrete outwash fans associated with ice-contact topographic scarps. In several places tunnel valleys terminate at the fan apex and ice-marginal scarp. These fan heads represent short-lived ice-marginal positions formed as the ice front retreated through the area. The outwash fan deposits partially bury some of the drumlins by filling in swales between them, thus reducing the original height of the drumlins. LIDAR DEMs show that the traditionally mapped Tekonsha “moraine” consists of drumlinized upland tracts and fan heads and is therefore not a terminal or recessional moraine. The final glacial event evident in the landscape is a series of meltwater channels associated with westward flow of meltwater, including events of high discharge, from the Huron-Erie Lobe, across the recently deglaciated Saginaw Lobe terrain. The sand and gravel unit between the two diamictons, and the surficial outwash fans constitute the main aquifers in this intensely irrigated agricultural area in places where the bedrock consists of the Mississippian Coldwater Shale.