NEW INSIGHTS INTO THE STRATIGRAPHIC FRAMEWORK OF THE MIDDLE-UPPER DEVONIAN TRANSITION IN THE MICHIGAN BASIN

High-resolution stratigraphic correlation of Middle and Upper Devonian strata in the Michigan Basin is hindered by a lack of extensive outcrops, complex lithofacies changes, and limited conodont and brachiopod biostratigraphy. We present a new chronostratigraphic framework based on bio-, sequence- and chemostratigraphic data collected from both core and outcrop (including many type-sections) located in the central, northern, and western portions of the basin. Stratigraphic units included in this study are the constituent formations of the upper Traverse Group in Michigan, the Thiensville and Milwaukee formations of Wisconsin, and the overlying Antrim Formation found throughout the basin. The recognition of third- and, possibly, fourth-order depositional sequences in different facies allows tentative stratigraphic correlations across depositional strike in the basin. A key stratigraphic datum in our correlations is the Taghanic Onlap, a late Middle Devonian transgression that has been recognized globally. Positive excursions in the carbon isotope composition of carbonates have been used to identify Taghanic Biocrisis-equivalent deposits within the basin. The new conodont and brachiopod biostratigraphic data has helped to develop an improved stratigraphic framework. However, more work is needed to reconcile discrepancies. The replacement of endemic taxa by invasive paleoequatorial brachiopods associated with the Taghanic Onlap apparently occurs later in the Michigan Basin than expected in comparison to the better-studied Appalachian Basin, although, this may be an artifact of the incompletely constrained conodont biostratigraphic framework currently available rather than the Michigan Basin serving as a temporary refuge for endemic taxa. Furthermore, but not unexpectedly, the onset of black shale deposition that typifies the Upper Devonian succession in the Michigan Basin appears to begin in the center of the basin during the latest Middle Devonian and to have expanded to the basin margin by the early Late Devonian. This integrated stratigraphic approach utilizing the outcrop and core material holds promise for placing Middle-Upper Devonian Michigan Basin deposits within the global chronostratigraphic framework.