GEOPHYSICAL AND STRATIGRAPHIC INVESTIGATION OF THE FORMATION OF THE OTTAWA LAKE BASIN, MONROE COUNTY, MI, USA

The Ottawa Lake basin is an enigmatic feature located on the ancestral Lake Erie (ALE) lake plain in southwestern Monroe County, Michigan, between Toledo, OH and Ann Arbor, MI. The basin itself is roughly rectilinear, about 7 km by 6 km, and appears to be carved out of bedrock, with distinct lineations oriented NW to SE. The basin is only partially filled with a basal diamicton overlying bedrock covered thinly with 1.5 to 2 m of laminated, fossiliferous, silty lacustrine sediments. In relative geomorphic position, the basin is bounded to the west by the Lake Arkona shoreline and to the east by the Lake Warren shoreline in the ALE sequence. This basin was the focus of a graduate field methods course in fall 2014. Prior to this study, it was not clear whether the basin was formed by glacial, glacial-lacustrine, or karstic processes. A detailed Digital Elevation Model (DEM) from recently released Lidar data was developed for the basin and surrounding area, and evaluated in the field for potential processes of formation and to locate geophysical transects. Electrical resistivity and ground penetrating radar transects crossed the western boundary from the higher lake plain into the basin. Stratigraphy exposed in a recently cleared large drainage ditch was evaluated as ground truth. The resistivity profiles strongly supported the working hypothesis that the abrupt western and northern edges of the basin were created by energetic erosion of a dolostone layer underlain by a weak shale bed that was excavated to form the basin. Hummocky landforms in the southeastern part of the basin may be sediment deposited downdrift from that excavation. Several hypotheses were evaluated for the sequence of processes that created the Ottawa Lake basin. The two processes thought to be most likely are catastrophic overland flow of surface water from a glacial outburst flood, or high-pressure sub-glacial flow of water and sediment equivalent to a tunnel channel. Although there is clear evidence of “opportunistic karst” within the basin, for example seasonal draining of water from the present lake with no stream outlet, the overall structure does not appear to be consistent with karstic dissolution.