STRATIGRAPHY OF THE DEVONIAN ANTRIM SHALE ON THE SOUTHEASTERN MARGIN OF THE MICHIGAN BASIN

The Late Devonian Antrim Shale Formation is a widespread, black shale-dominated unit in the Michigan Basin that rarely outcrops, but has been studied extensively in the subsurface. Member-level stratigraphic subdivision of the Antrim is well-defined throughout the basin with the exception of the southern and southeastern margin, where stratigraphic correlation and nomenclature is hindered by confusion associated with “state-line stratigraphy” (the margin comprises parts of Michigan, Ohio, and Indiana, as well as the Province of Ontario). This study extends Antrim Shale subunits to southeastern Michigan Basin using a combination of gamma log correlation, lithostratigraphy, and chemostratigraphy.

Gamma log correlation along a transect from the depositional center of the basin to the southeastern margin shows that the calcareous mudstones of the “Traverse Formation” and subunits of the Antrim (Norwood, Paxton, Lachine, and upper members, as well as the Ellsworth Shale), can all be traced, and thin, from the basin center to the margin. These units can also be traced using gamma logs, and maintain a relatively consistent thickness, along a transect on the southeastern margin of the basin that includes wells in northeastern Indiana, northwestern Ohio, and southeastern Michigan.

Litho- and chemostratigraphic study of the Antrim Shale in the Schmucker 3-30 core (northwestern Ohio, southeastern Michigan Basin margin) shows remarkable similarity to basin-center Antrim. Norwood strata in the core consist of dark gray and black pyritic shale. The contact with the overlying Paxton is sharp in the core, with Paxton lithology piped into the underlying Norwood within burrow fills. Paxton strata consist of variably calcareous and pyritic, heavily bioturbated, fossiliferous gray mudstone. Cored Lachine strata consist of dark gray and black pyritic shale.

This study provides a better understanding of the stratigraphy of the Antrim Shale, and demonstrates that the stratigraphic framework of the basin center can be confidently applied to the southeastern margin. State geological surveys in the study area should therefore review their Antrim Shale stratigraphic nomenclature with the goal of eliminating state-line stratigraphic inconsistencies.