AN INTEGRATED SHELF TO BASIN CHEMO- AND CHRONO-STRATIGRAPHIC CORRELATION OF THE BURNT BLUFF GROUP (BBG) CARBONATES, MICHIGAN BASIN, U.S.A

Scientists have studied carbonate rocks for centuries, yet correlating these strata from the shelf to basin remains problematic. Conventional approaches are routinely used in shallow-water settings where facies changes allow for identification of depositional cycles. It is difficult, however, to identify and correlate depositional cycles in deep-water settings where facies are more homogeneous. Advanced geochemical techniques have recently been used to correlate large-scale carbonate depositional sequences, but correlating individual depositional cycles is still unreliable. In siliciclastic mudrocks, the problem of correlating depositional cycles be tackled by applying chemostratigraphy whereby changes in elemental concentrations are used to infer temporal variations in environmental conditions, such as redox and water depth.

This study integrates core description, energy dispersive x-ray fluorescence spectroscopy (ED-XRF) elemental data, conodont biostratigraphy, strontium (⁸⁷Sr/⁸⁶Sr), and carbon isotopes (δ¹³C) to investigate sequence stratigraphic relationships in the homogeneous lime mudstones of the Burnt Bluff Group (Llandovery, Silurian), Michigan Basin. Systematic variations in continental proxy elements (Al, Si, K, Ti, Zr), which are interpreted to reflect terrigenous input into the basin, can be correlated between the shelf and basin-center. Positive correlations (R²>0.8) between Al-K, Ti-Zr, Al-Si, and Al-Zr are consistent with a continental origin. The covariance between facies and continental proxy element concentrations is interpreted to reflect changes in shore line trajectory, and are used to identify depositional cycles within the BBG.

Three positive carbon isotope (δ¹³C) excursions (CIEs) were also identified in two shelf-position wells, and one positive excursion in a basin-center well. The three CIEs on the shelf are interpreted as the global Early Aeronian, Late Aeronian, and Valgu events. The age of the positive CIE in the basin center is unknown. Forthcoming conodont and strontium (⁸⁷Sr/⁸⁶Sr) data will be used to refine age of the CIEs and thus the stratigraphy of the BBG. This study suggests that integration of various data types can constrain correlations and more specifically depositional cycles in otherwise homogeneous carbonate rocks.