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The A-1 Carbonate is the primary hydrocarbon source rock and an important reservoir component of the Silurian (Niagaran) pinnacle reef complexes in the Michigan Basin. The geology of the A-1 Carbonate, however, is not widely known because the majority of published research about this hydrocarbon system focuses on the pinnacle reefs. To gain a better understanding of the sedimentology and stratigraphy of the A-1 Carbonate, we integrated data from slabbed core, thin section petrography, gamma-ray logs, and energy-dispersive X-ray fluorescence spectrometry (ED-XRF). Thirteen distinct lithofacies within the A-1 Carbonate are recognized, with inferred depositional environments ranging from intertidal-sabkha to deep basin. The recognition of deep-water lithofacies contrasts significantly with previous interpretations of the A-1 Carbonate as a shallow, peritidal deposit.

Lithofacies stacking patterns and ED-XRF elemental trends within the A-1 Carbonate are consistent with basinwide sea-level fluctuations that resulted in deposition of three major stratigraphic units, called the Lower A-1 Carbonate, Rabbit Ear Anhydrite, and Upper A-1 Carbonate. The basal part of the Lower A-1 Carbonate was deposited during a basinwide transgression, as evidenced by deep-water pelagic carbonate accumulation in the basin center, lithofacies that become progressively muddier from bottom to top, and higher concentrations of Si, Al, and K upward, which are interpreted to reflect the influx of continental sediments. The subsequent highstand deposits of the upper part of the Lower A-1 Carbonate are characterized by a decrease in Si, Al, and K, coupled with a shallowing-upward facies succession consistent with increased carbonate production rates. The Rabbit Ear Anhydrite, which bifurcates the Upper and Lower A-1 Carbonate units, exhibits a variety of anhydrite fabrics across a wide range of paleotopographic settings within the basin. The Rabbit Ear Anhydrite is interpreted to reflect a time-correlative sea-level drawdown, which caused basin restriction, gypsum deposition, and elevated concentrations of redox-sensitive elements, such as Mo and Ni. The Upper A-1 Carbonate represents sedimentation during another major basinwide transgression that culminated in the deposition of shallow-water microbialites on the crests of previously exposed Niagara reef complexes. Similar to the Lower A-1 Carbonate, the base of the Upper A-1 Carbonate exhibits elemental signatures indicative of continental influence, whereas the overlying highstand deposits are characterized by more normal marine conditions and lower concentrations of Si, Al, and K.

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