STRATIGRAPHY OF LAKE MICHIGAN LOBE LACUSTRINE SEDIMENTS

Before the final advance of the Lake Michigan Lobe over southwestern Michigan between 14 and 15.5 kyr B.P., at least one large proglacial lake existed between the ice to the west and higher ground to the east. Sedimentologic evidence suggests the presence of this lake shortly after the deposition of the Ganges Till, with lacustrine deposition continuing until the deposition of the Saugatuck Till.

Three cores document the sedimentology of the lacustrine sequence. In the western-most core, the sequence begins with clay and silt directly overlying the Ganges Till, coarsening upward into sand and silt. Following a thin sand and gravel interbed, the sequence continues with fine sand, a second thin gravel layer, and is capped with the Saugatuck Till. A second core consists of medium sand directly overlying the Ganges Till, followed by a thick sequence (~30 meters) of sandy silt to silty sand, immediately overlain by a thin gravel layer, above which lies the Saugatuck Till. The eastern-most core also contains a thick (~30 meters) lacustrine sequence, consisting predominantly of silts and clays. No diamicton is present in this core.

A fining-upward sequence from gravel to distal clay is present in the eastern-most core. The clay is correlatable with gamma-ray logs from regional water wells, and it coarsens to a silt-to-fine sand to the west. This sequence documents the retreat of the Lake Michigan Lobe to the west after the deposition of the Ganges Till. Above this clay unit is a coarsening-upward sequence from distal clay to gravel, interpreted as the readvance of the Lake Michigan Lobe.