NEW REGIONAL CORRELATION OF GLACIAL EVENTS AND PROCESSES IN THE INTERLOBATE AREA OF SOUTHERN MICHIGAN AND NORTHERN INDIANA AFTER THE LAST GLACIAL MAXIMUM (LGM)

New geologic mapping by scientists involved in the Central Great Lakes Geologic Mapping Coalition, FEDMAP, STATEMAP, and EDMAP programs has resulted in a reinterpretation of glacial events since the LGM in northern Indiana and southern Michigan. This group has established a regional working stratigraphy, identified a multitude of overlapping and cross-cutting relationships of glacial landforms and landscapes, and developed a relative chronology for ice-margin positions in this interlobate area. The paradigm for geologic mapping includes concepts of the morphosequence, drawing on examples of depositional systems and processes observed in modern glacial environments such as the Bering Glacier in Alaska and outlet glaciers in Iceland. These modern analogs have clear equivalents in the interlobate region where relationships of depositional and erosional features are all connected through a continuum of proglacial, ice-marginal, subglacial, and stagnant-ice processes.

Glacial lobes in the Lake Michigan, Saginaw, Huron, and Erie lake basins converged in northern Indiana and southern Michigan at the LGM. During deglaciation after about 20,000 rad. B.P., the lobes became progressively segregated. The relationships of landforms and landscapes to the sediment record provide a relative chronology for deglaciation of the glacial lobes. Ice-marginal landforms of the Lake Michigan, Huron-Erie, and Erie lobes overlap landforms of the Saginaw lobe. Proglacial meltwater streams that exited margins of the Lake Michigan, Huron-Erie, and Erie lobes crossed a deglaciated terrain previously occupied by the Saginaw lobe. The resulting cross-cutting relationships of proglacial meltwater stream channels and valleys and the overlapping ice-marginal landforms indicate that the Saginaw lobe ice margin retreated from southern Michigan while the Lake Michigan, Huron-Erie, and Erie lobes continued to advance. Proglacial meltwater streams that exited the Erie lobe deposited the last glacial sediment across the interlobate area. Therefore, the regimen of each lobe was asynchronous. The Saginaw lobe margin retreated from southern Michigan before the retreat of the Huron-Erie lobe ice by about 15,000 rad. B.P. and never readvanced into the region again.