A GLACIOFLUVIAL SEDIMENTARY RECORD OF ACTIVE ICE TERMINATION IN SOUTH-CENTRAL MICHIGAN, USA

A glaciofluvial sedimentary record of active ice termination in south-central Michigan, USA Glacial stratigraphic investigations in south-central Michigan suggest that the deglacial sedimentary record was dominated by subglacial meltwater activity followed by a poorly recorded ice-margin retreat and/or stagnation. The study area is on a regional topographic high of upland blocks and ridges composed of till and gravel. Moreover, it is dissected by tunnel channels with eskers, and surrounded by lowland valleys with outwash. Sedimentary descriptions from 14 gravel pits indicate a consistent stratigraphy across a width of 15km. The upland consists of a lower till with subglacial melt-out or lodgment characteristics over bedrock. Above the lower till is a sand-and-gravel facies that passes upward into a boulder-gravel facies with overlying pockets of discontinuous laminated sand and silt. The uppermost facies is the upper till with supraglacial melt-out and flow till characteristics. Clast lithology changes from locally derived carbonates to clastics and finally to crystallines from the lower till through the gravel facies into the upper till, mimicking the bedrock lithology up-ice of the study area. The lower tills record an ice margin south of the study area and the intertill gravel records glaciofluvial activity. A subglacial rather than subaerial glaciofluvial environment is favored because of the flatbedded nature of the gravel, the gravel is found on the uplands, and its stratigraphic position integrated with the clast lithology data. The boulder-gravel sheet was subsequently incised by a tunnel-channel flow that deposited composite boulder-gravel bedforms with a hummocky morphology. The hummocks most likely record waning flow within the tunnel channel and the up-to-2m of overlying diamicton actually lessens the hummocky relief on the landscape, rather than creating it. Final deglaciation in the region is recorded by outwash in the lowlands, and by small ridges and kames in the uplands.