THE SILURIAN MULDE EVENT RECORDED IN NIAGARA-LOWER SALINA REEF COMPLEXES IN THE EVAPORITIC MICHIGAN BASIN: A CHEMOSTRATIGRAPHIC APPROACH

The reef complexes of the Niagara-Lower Salina have been studied extensively over the past five decades as they comprise an immense hydrocarbon resource in the Michigan Basin. Recent studies suggest that the transition from normal marine conditions during Niagara carbonate deposition to evaporitic conditions during deposition of the Lower Salina A-1 resulted from local drawdown in the Michigan Basin. Here, we use the inorganic and organic carbon isotope signatures within the carbonates and evaporites, respectively to investigate this abrupt lithologic transition. Carbon isotopes from the Michigan Basin reveal a significant positive isotopic excursion referred to as the Mulde Event, which has been identified globally in Silurian deposits. Understanding how the Mulde Event is expressed in continental interior seaways and in organic carbon has previously been ignored. Our newly acquired isotope data, coupled with previously unpublished δ¹³C_carb data from the Northern pinnacle reef trend along the basin margin, suggest that: 1) the Mulde Event is isotopically recorded in basinal evaporites; 2) deposition of the upper part of Niagara reefs and subsequent A-0 Carbonate occurred during the onset of the Mulde Event; and 3) the Mulde Event is closely tied to a major, sea level fall in the Silurian that resulted from global cooling and glaciation-triggered aridity in the mid-continent.