GLACIAL LAKE PLAINWELL: SEDIMENTARY AND STRATIGRAPHIC EVIDENCE FOR HIGH-ELEVATION PROGLACIAL LAKES ALONG THE EASTERN MARGIN OF THE LAKE MICHIGAN LOBE IN SOUTHWESTERN MICHIGAN

Multiple low-relief plains composed of fine-grained sediment extend through several counties in southwestern Michigan. Previous studies have recognized these low elevation northeast-to-southwest trending plains as former proglacial lake basins confined between the Valparaiso/Kendall upland and Kalamazoo Moraine of the Lake Michigan Lobe.

Newly collected sedimentologic and stratigraphic data from exposures along the Kendall Upland in southeast Allegan County indicate that a deep, proglacial lake with a minimum elevation of 264 m extended onto and above areas previously interpreted as an end moraine between 14 and 15.5 ka BP. Exposures in gravel pits along the crest and flanks of the upland display a stratigraphic column composed of >3 m of laminated silt and clay overlying ~12 m of eastward dipping gravel foreset beds. The laminated units contain dropstones and gravel units below contain large intraclasts of laminated silt-clay, diamicton and friable sand up to 1 m in diameter. Below the dipping gravel units well-sorted, planar-bedded medium sand forms bottom sets of an undetermined thickness.

Ice-contact marginal positions traced from the proximal side of the Kalamazoo Moraine to a weakly defined grounding-line on the Kendall Upland and a better defined ice-contact position in Allegan County, indicate that the margin of the Lake Michigan lobe near Plainwell, Michigan formed a constricted basin where glacial lake Plainwell developed.

The uppermost facies of laminated silt-clay and the absence of topset beds indicate that the basin deepened rapidly, perhaps as a result of a readvance by the lake Michigan Lobe. The coarse gravel facies intermixed with heterogeneous intraclasts indicate that complex depositional mechanisms were operating along the proximal margin of the ice while the lake was in place. The morphology, sedimentology and stratigraphic sequence observed within the Kendall Upland have many similarities to descriptions of grounding-line fans, ice-contact deltas and morainal banks. Well-defined stream cut terraces indicate that lake levels were high enough to initiate eastward spillway development through the Kalamazoo Moraine.