Geochemical and Petrologic Investigation of Cemented Glacial Outwash at Lake Wabasis, Michigan

The Pleistocene glacial deposits covering the upper US Midwest shape regional landscapes and hold important groundwater resources. Very rarely these sediments are well cemented with carbonate, of which outcrops have been noted sporadically and often anecdotally. As a whole these geologically young rocks have received very little scientific attention. Of particular interest are the calcite cements within, which preserve geochemical and petrographic information about when and how they formed. Their stable isotope chemistry may also provide information about past environmental conditions.

We specifically focus on a unique west Michigan locality at Lake Wabasis with a ~50 m exposure of ~0.5 to ~3-m-thick cemented glacial outwash. This is the first scientific study of these rocks, which have previously been described colloquially as a “limestone cave”. We will present preliminary field observation, petrographic, XRD, and stable isotope data from a suite of five samples of rock and one sampled ~2 cm speleothem. We use these data to evaluate whether cementation was very recent (with vadose cement and approximately modern stable isotope geochemistry), or instead occurred earlier during the Holocene/deglacial when the water table was likely higher (possibly resulting in phreatic cements and/or more negative δ18O). This work will place this unique outcrop in geologic context, enhance understanding of variability within glacial aquifers, and perhaps assist with evaluations of modern and past hydrologic models.