PALEONTOLOGY OF THE UPPER DEVONIAN ANTRIM SHALE IN NORTHWEST OHIO: HOW ISOLATED WAS THE MICHIGAN BASIN?

The Upper Devonian Antrim Shale is an important shale gas source rock in the Michigan Basin, primarily composed of siltstones and shales deposited in a dysoxic to anoxic setting. While the lithology of the Antrim has been documented from subsurface cores and drillholes, relatively few fossils have been recovered from the unit. Surface exposures of the Antrim are rare and little is known about this unit in northwest Ohio, on the southeast margin of the Michigan Basin. A large exposure of the Antrim Shale along the Auglaize River in Defiance County, Ohio, was investigated to document its micro- and macrofossils and better constrain its paleoenvironment and age. Rock samples were collected along two transects of the exposure and processed to isolate microfossils. Thin sections of both shale and siltstone samples contain microconchid tubes and cysts of the alga Tasmanites, some crushed and some spherical and infilled with calcite. Hydrogen peroxide digestion residues also contain abundant microconchid tubes and possible otoliths or statoliths. In addition to these microfossils, three macrofossils were recovered from the site. A bivalve fossil is tentatively identified as the epibenthic pterineid Ptychopteria. The presence of the microconchid tubes and bivalve suggest that benthic conditions were not fully anoxic at the time of deposition, at least in the southeast corner of the Michigan Basin. Two terrestrial plant specimens were also collected from the Auglaize River locality. These fossils strongly resemble the lycopsid species Clevelandodendron ohioensis, known from the Famennian Cleveland Shale in northeast Ohio (Appalachian Basin). These plant fossils present two puzzles. First, the Auglaize River exposure was dated by previous workers as Frasnian. If the recovered fossils are C. ohioensis, either the exposure is actually Famennian or the first appearance of C. ohioensis is earlier than previously thought. Second, the nearest source for terrestrial plant material is east of the Appalachian Basin. Either the recovered plant material traveled over 400 kilometers or a terrestrial source closer to the Michigan Basin must have existed during the Late Devonian.