DEPOSITIONAL HISTORY OF THE LAKE ST. CLAIR DELTA, MICHIGAN, USING GEOPHYSICS AND CORES

Lake St. Clair, located between Michigan and Ontario, is the smallest of the Great Lakes. The evolution of Lake St. Clair has been influenced by Late-Wisconsin ice re-advances, isostacy, and changes in outlet points of the Great Lakes system during deglaciation. A shift to the Port Huron outlet during the Nipissing-I highstand around 5500 years BP, prior to which Lake St. Clair was a separate basin, resulted in Lake Huron/Michigan draining through the St. Clair and Detroit Rivers to Lake Erie. Loss of stream competence at the inflow of Lake St. Clair led to the formation of a deltaic system, which currently has a surface area of approximately 230 km². Compared to the other Great Lakes, the postglacial history of the lake and depositional history of the delta are poorly studied. We used coring, ground-penetrating radar, and electrical resistivity methods to characterize this deltaic system. Our results show four main depositional phases. The sequence starts with Wisconsin-age glacial till (Phase I), followed by late glacial (glacio-) lacustrine deposits (Phase II). Phase III is an Early Holocene swamp environment (Scirpus & Brassicaceae seeds were dated at 9620 +/- 50 years BP). Phase IV is a typical coarsening upward delta progradation sequence with lacustrine clays, silts and sands. A paleosol (Eleocharis & Scirpus seeds dated at 1630 +/- 50 years BP) in the upper part of the deltaic sequence indicates a possible lake-level low.