THE CIRCULAR FEATURE IN OTTER LAKE (BENZIE COUNTY) MI IS NOT AN IMPACT CRATER: A GROUNDWATER SPRING VENT HYPOTHESIS

Otter Lake (Benzie County, Michigan) is sufficiently shallow for features on the lake bottom shallower than the Secchi depth to be visible in the orbital satellite images. The (presently underwater) circular lake-bottom "landforms" at the east end of Otter Lake viewed in orbital satellite images since at least early 1993 look (at least superficially) like an impact crater ~15 m diameter with (concentrically outward) a raised rim, light proximal ejecta, and dark proximal ejecta. This presentation explores an alternative hypothesis for the Otter Lake feature.

Published reports for Otter Lake and the surrounding area describe local aquifer material as thick glacial till and outwash sand with some heterogeneously distributed gravel, and local groundwater upwelling into surface water. Numerous groundwater upwelling sites were reported in or very near the bed of Otter Creek. Sand and gravel with local springs occur along the northern and eastern shoreline of Otter Lake. A ~6m (20ft) diameter groundwater spring vent was documented offshore in eastern Otter Lake. Although the discharge of the large spring vent was not measured, discharge was observed.

We infer that the ~1 m diameter circular features in the shallow offshore east of the large circular feature in the orbital satellite image with the most favorable resolution and illumination are small shallow underwater counterparts of the previously reported onshore local springs. We infer that the ~15 m circular feature offshore in the orbital satellite image is the previously reported spring vent and the ring of sediment redistributed around it by the discharge of the vent. The discharge of the large spring vent is hypothesized to at least intermittently have local discharge high enough to entrain (stir up and remobilize) grains as large as sand size, a process known to occur in similar settings elsewhere in Michigan. The coarsest of the mobilized grains (sand-size) are deposited near the edge of the vent's flow, evidently building up a shallow ring just outside the volume of sediment suspension over the vent, and any smaller grains present remain in suspension longer and settle farther away from the vent. Redistribution of sediment of up to sand size by a perennial spring vent is sufficient to account for the observed lake-bottom features the orbital satellite images from Otter Lake.