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Distribution and Depositional Environments of the Westernmost Devonian Rocks in the Michigan Basin
Strata in the vicinity of Milwaukee, Wisconsin, and Chicago, Illinois, constitute the westernmost extension of Devonian rocks in the Michigan Basin. Eifelian-Famennian strata at Milwaukee, where the most complete section occurs, display a general deepening upward transgressive sequence. Lithologically and environmentally these strata resemble their temporal equivalents in southwestern Michigan. The Lake Church Formation (Eifelian) and the Thiensville Formation (lower Givetian?) were deposited under supratidal coastal sabkha to subtidal marine conditions. Features of the sabkha facies include evaporite psueudomorphs, solution collapse breccias, desiccation cracks and fenestrae, and algal mats preserved as bituminous laminae. The Givetian Milwaukee Formation, which unconformably overlies the Thiensville Formation, was deposited during a period of increased clastic influx. Fossils, including bryozoans, brachiopods, pelmatozoans, and fish, are abundant and diverse throughout most of this unit. The uppermost unit, the Antrim Shale (Frasnian-Famennian), represents deeper, mostly anoxic, subtidal conditions. Nearby land, probably the Wisconsin Upland to the west, is suggested by ubiquitous silt-sized quartz grains throughout the section and by rare vascular land plant fragments within the Milwaukee Formation.
In the Chicago area Devonian strata are confined to fissures within the Silurian Racine Dolomite and to the Des Plaines Disturbance, a probable meteor impact structure (McHone, et al., 1986). These dark colored Devonian mudstones, containing a rich conodont fauna, correlate with the Antrim Shale at Milwaukee.
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