Study of Sedimentation and Stratigraphy of Lower Mississippian in Western Michigan

Abstract

The lower Mississippian of western Michigan is composed of the Coldwater, Ellsworth, and the upper part of the Antrim formations. These formations in the western part of the state differ so markedly from the formations of the eastern side of the state that different basins and times of sedimentation are suggested. The basal member of the is red fossiliferous limestone with some red shale; several very fossiliferous zones rich in crinoids, ostracods, and bryozoans with some brachiopods, gastropods, cephalopods, and pelecypods occur locally; clay-ironstone concretions are more abundant in the section discussed than elsewhere in the state; and the Coldwater lime which in many places is light gray dolomite rather than limestone, has embedded crystals of secondary dolomite and pepperings of glauconite. The Ellsworth is present only in the western part of the state and is gray and gray-green shale with gray dolomite zones. Small pyrite concretions and some spore cases are present throughout the formation. In places near the contact of the Ellsworth and there is a buff oolitic limestone in which the central core of the oolites is light brown dolomite. Where the oolites are absent the zone is brown crystalline dolomite which in many places produces gas. Stratigraphically this zone is at the horizon of the Berea sandstone of the Michigan basin but lithologically it is dolomite containing grains of rounded frosted sand. The green shales of the Ellsworth grade into, and alternate with, the black shales of the Antrim so that it is difficult to determine the true top. Since the lower Mississippian does not crop out in the area discussed, all correlation must be made from well logs and samples. Recent drilling developments have added considerable information and future work will undoubtedly define the paleogeography.