DEPOSITIONAL TRENDS IN SILURIAN STRATA OF THE WESTERN MICHIGAN BASIN

Silurian rocks on the west side of the Michigan Basin (Wisconsin-Illinois) exhibit a series of conspicuous north-south lithostratigraphic changes. First recognized by Chamberlin (1877), this pattern is not aligned precisely with the existing structural configuration of the basin. Rather, it reflects the impact of temporal variations in sea level along with the changing configuration of the proto-Michigan Basin on adjacent Silurian shelf areas. In this region, the maximum thickness of Silurian rocks parallels the western shore of Lake Michigan. However, the age and character of these rocks change significantly from north to south. In the north, the section is dominated by about 200 m of early Silurian (Rhuddanian- Aeronian) marginal marine (hypersaline) carbonates, possibly representing the continuation of a northern shallow and slowly subsiding basin that was already active during the late Ordovician. To the south, in east-central Wisconsin, most units begin to thin depositionally. This southward thinning is notably enhanced in Ozaukee and Milwaukee Counties where a NE-SW scarp was formed by erosional truncation related to a prominent late Aeronian-early Telychian unconformity. Farther south, in Illinois, depositional thinning of early Silurian rocks continues, and sediments become more normal marine. This southward trend is interrupted around Chicago where older strata infill local large-scale erosional topography on the top of the Ordovician Maquoketa Group. In Kankakee County, Illinois, the general thinning pattern terminates when these rocks reach a thickness of 10 m, which remains constant throughout most of the Midwestern U.S. to the south. In this area, about 200 m of younger Silurian rocks (late Llandovery, Wenlock, Ludlow) overlie the Aeronian-Telychian unconformity and are inversely proportional to the thickness of older strata along this trend. In contrast to older Silurian strata, these younger rocks represent more normal-marine shelf environments marked by reefs and carbonate banks, which rim the proto-Michigan Basin to the east. These banks project westward into southeastern Wisconsin, following in part the older scarp trend, as well as into northeastern Illinois. Gaps in the bank near Milwaukee permitted the farthest westward deposition of true Silurian basinal sediments.