SURFICIAL GEOLOGICAL MAPPING IN ALLEGAN COUNTY, MICHIGAN USA: LANDFORMS AND SEDIMENTS OF THE LAKE MICHIGAN LOBE FORMED DURING THE LATE WISCONSIN DEGLACIATION

Surficial and 3D geological mapping by the Michigan Geological Survey in Allegan County, Michigan provides insight into sediments, landforms, and stratigraphy formed by the Lake Michigan lobe (LML) during deglaciation. The maximum extent of the LML is marked by the Kalamazoo moraine-outwash-fan complex in the southeastern corner of Allegan County. This complex is composed of interbedded sand, gravel, and diamicton that formed as an ice margin and outwash fans, which carried meltwater to the east. In central Allegan County, Valparaiso moraines, spillway channels, streamlined uplands, and glaciolacustrine-capped morainal uplands are common landforms. In western Allegan County the Lake Border moraine consists of one or two ridges with areas of ground moraine or outwash between ridges. Extensive areas of the county are covered with organic sediments, legacy sediment-alluvium, and coastal and inland eolian dune sediments.

Sediments and stratigraphic data from cores confirm that the Saugatuck till is the surface till in Valparaiso and Lake Border moraines and uplands. Overlying the surface till locally is thin glaciolacustrine sediments of at least two glacial lakes at elevations of ~264 m (~866 ft; possibly Glacial Lake Plainwell) and ~224-230 m (~735-755 ft; possibly Glacial Lakes Madron and Dowagiac), which formed during retreat from the Kalamazoo position. Glacial Lake Chicago sediments occur at elevations of ~201 to ~199 m (~660-650 ft) in western Allegan County and these levels were likely reached during and after the advance to the Lake Border position during the Glenwood phase. Below Saugatuck till and surface sediments is a thick, >20 m (>65 ft), package of glaciolacustrine sediments of Glacial Lake Milwaukee. This extensive glacial lake existed before the advance to the Kalamazoo position during the Crown Point phase. Previous research and cores have also shown that glaciotectonic deformation of outwash and Glacial Lake Milwaukee sediments occurred during drumlin formation in central Allegan County during the Crown Point advance. The Ganges till (Late Wisconsin) and Glen Shores till (Pre-Illinoian) occur in the subsurface below GLM sediments.