GEOPHYSICAL ANALYSIS OF OTTAWA LAKE AND RELATIONSHIP TO GLACIAL LAKE ARKONA, MONROE COUNTY, MICHIGAN

Ottawa Lake is one of a series of NW-SE oriented linear features in southwestern Monroe Co., Michigan. The lake sits on the SE edge of the Michigan basin, across Silurian-Devonian Limestones, Dolostones, and Sandstones. Ottawa Lake is also highly ephemeral and appears to be an infiltration point into the local groundwater system. Immediately west lies the northern extent of the Bowling Green Fault. The western edge of Ottawa Lake basin includes sections interpreted as the glacial Lake Arkona shoreline. Three geologic questions were asked about these features. Do the lakes in Whiteford Township trend similarly to the local structure of SE Michigan and NW Ohio? What processes formed Ottawa Lake? What geophysical and lithological evidence supports this hypothesis?

Previous work by the author correlate these lineations with the declination of the regional structure. Results indicate a widespread series of joint sets throughout NW Ohio and SE Michigan, trending NW-SE, NE-SW, and NNW-SSE. Historic field data and modern geospatial analysis of Ottawa Lake support the interpretation of structural control.

Ottawa Lake and the surrounding linear features were originally part of a regional series of joints, widened by subglacial waters, and possibly by loading and offloading of the Late Wisconsinan Ice Sheet. As the ice retreated, these widened joints were plugged by glacial till, creating a series of palimpsest karst features. Geophysical measurements in the lakebed and along the western boundary indicate a sudden change in topography, matching a notable dip in bedrock elevation. The bedrock surface is ~7 m higher on the western boundary of Ottawa Lake than in the lakebed. The thickness of deposits atop the western rim range from 1.2-3 m; sediments within the basin range from 1.4-5.5 m thick. This suggests the glacial deposits somewhat match the topography of the underlying bedrock surface.

An exposure in a drainage ditch located in the NW corner of the lake revealed a series of NW-SE trending glacial striations. The striae were crosscut by a series of solution-widened grikes, and topped with ~2.8 m of till. While the elevation of the western boundary agrees with the elevation of glacial Lake Arkona, no notable deposits of coastal sediments have been found near to Ottawa Lake, indicating a possible erosional coastline.