Antrim-Ellsworth-Coldwater shale formations in Michigan

The shale sequences between the overlying Marshall sandstone and the underlying Traverse limestone group were investigated along two lines of sections, one northeast and southwest across the central part of the basin, and the other, north and south along the western side. Samples of cuttings from wells spaced about 10 miles apart were the basis for the study. Three major shale units exist, namely in ascending order, the black or dark brown Antrim, the gray or gray-green Ellsworth, and the gray or blue-gray Coldwater. Evidence was found for the following conclusions. First, the Antrim thins out noticeably from east to west and the upper two-thirds interfingers with the lower half of the Ellsworth. Second, the Ellsworth is confined to the western counties and where studied is almost uniform in thickness. Third, a wedge-like extension of the "Berea" sandstone occurs on the eastern side of the basin. It is thickest along the line of the section at Saginaw Bay and disappears before the middle of the state is reached. A thin gray shale of Coldwater type, above the upper Antrim, has been called the Bedford. The Sunbury, a thin black shale bed, of Antrim type, resting directly on the "Berea," is a mappable unit in the eastern counties only. Several type lithologies are found in many interfingering and separate lenses. Altogether, the lenses build up a shale series of somewhat uniform thickness (approximately 1,500 feet thick) but the separate units, each dominantly of an individual lithology, have interfingering lateral relations to each other.