STRATIGRAPHIC FRAMEWORK OF THE EASTERN MARGIN OF THE LAKE MICHIGAN LOBE

During the Late Wisconsinan, the Lake Michigan lobe advanced into southwestern Michigan from the Lake Michigan basin. The generalized stratigraphic framework determined in a study of Van Buren County, Michigan as part of a USGS STATEMAP project, consists of a basal diamicton (Ganges till; sand and gravel of variable thickness, a lacustrine unit of as much as 20 m thick that is glaciotectonized near the top; and a discontinuous surface diamicton (Saugatuck till) ranging from 0 to 25 m thick. The four terminal "moraines" traditionally mapped in the region, from east to west the Tekonsha, the Kalamazoo, the Valparaiso, and Lake Border, differ morphologically and stratigraphically. The Tekonsha "moraine" is a scarp that truncates southwest trending drumlins of the Saginaw lobe and may be a wave cut or fluvial scarp rather than an ice marginal position. The Kalamazoo is a prominent ridge containing hummocky topography, glaciotectonic thrust masses, and huge glaciofluvial fans that record the termination of a major advance. The Valparaiso is an irregular margin; a distinct N-S trending ridge bordered by outwash fans in southern Van Buren County abruptly ends against a drumlinized upland capped by the Saugatuck till. The upland extends eastward to a band of elevated topography west of the Kalamazoo moraine. Borings and exposures reveal mostly deformed lacustrine sediment and debris flows. The terrain west of the Valparaiso uplands east of the Lake Border moraine consists of a lake plain containing isolated low hills oriented transverse to ice-flow direction composed of glaciotectonically deformed lacustrine sediment, discontinuously capped by the Saugatuck till. The Lake Border moraine is a low, narrow ridge, in which the Saugatuck till is very thick and the underlying lacustrine sediment is thinner than to the east. This distinct ridge does not appear to have ice-marginal fans and other features associated with it. Rapid ice flow and extensive bed deformation by a glacier that had a long-lived proglacial lake between the ice margin and the higher terrain to the east appear to be important in the sediment-landform assemblages in the area.