Sedimentary and stratigraphic evidence for subglacial flooding, south-central Michigan, USA

Abstract

The glacial landscape in south-central Michigan is a dissected surface with streamlined, residual, and upland blocks surrounded by lowland valleys dominated by outwash. The upland surface has ridges, hills, and hummocks composed of boulder gravel, which are transitional to drumlins in a down-ice direction, and is dissected by tunnel channels with eskers within them. Sedimentary descriptions from 14 gravel pits indicate a consistent stratigraphy across a width of 15 km. The upland consists of bedrock covered by a lower till with subglacial melt-out or lodgment characteristics. Above the lower till is a sand-and-gravel facies that passes upward into a boulder-gravel facies. Nestled within depressions on the boulder-gravel surface is a discontinuous facies of laminated sand and silt. Above the gravel is an upper till with supraglacial melt-out and flow till characteristics. Analyses of the till and gravel indicate an upward change in clast lithology from locally derived carbonates to clastics and finally to crystallines from the lower till through the gravel facies into the upper till. The transition mimics the bedrock lithology up-ice of the study area, and is explained by meltwater erosion of bedrock, previously deposited subglacial sediment, and overlying debris-rich basal ice. The systematic change in clast lithology and the stratigraphic position of the gravel facies between a subglacial and supraglacial till strongly suggests that the gravel facies sediment was deposited subglacially and that the entire till–gravel–till sequence resulted from the same glacial advance.