THE DEVONIAN SUCCESSION IN WISCONSIN: INSIGHTS FROM CONODONT BIOSTRATIGRAPHY AND CARBONATE CARBON ISOTOPE CHEMOSTRATIGRAPHY

Integration of new and previously published conodont data with new carbonate carbon isotope data from drill cores helps place the Devonian succession in Wisconsin within the global event and chronostratigraphic framework. Devonian rocks are known from the southeastern portion of Wisconsin (Milwaukee and Ozaukee counties) with an outcrop belt measuring ~15 km wide by ~60 km long. These dolostone and dolomitic shale units were deposited at or near the western edge of the Michigan Basin, dip to the east below Lake Michigan, and are covered by glacial deposits making natural outcroppings rare. The Devonian succession in Wisconsin includes, from oldest to youngest, the Middle Devonian Lake Church, Thiensville, and Milwaukee Formations, and the Late Devonian Antrim Formation. Conodonts from the Lake Church type section place these rocks within the Eifelian Tortodus kockelianus Zone; however, carbonate carbon isotopic data from three core successions containing ‘Lake Church’ rocks suggests more biostratigraphic work is needed. None of these carbon isotope profiles contain excursions, yet the absolute isotopic values observed are different in each core (approximately -0.3, +0.2, or +0.5‰). One explanation for this disagreement is that rocks called the Lake Church Formation are not all the same age, which is suggested by the observed lithologic variability and ~12 meters of relief over 0.6 km on the unconformable contact with underlying Silurian strata. Although no conodont data are available for the Thiensville Formation, stratigraphic position and positive excursions in the carbon isotopic profile suggest that these rocks record the Givetian Taghanic Crisis. The Milwaukee Formation contains conodonts spanning the Givetian Schmidtognathus hermanni through Skeletognathus norrisi Zones, and the carbon isotope profile records the positive excursion associated with the Geneseo Event. The Antrim Formation in Wisconsin has been assigned to Frasnian MN 4 (Palmatolepis transitans) Zone, suggesting that the positive excursion in the carbon isotope profile of this unit is associated with the Timan Event. Together, use of conodont biostratigraphic and carbon isotope chemostratigraphic data allows high-resolution placement of the Wisconsin Devonian succession within the global chronostratigraphic framework.