Linked Silurian carbon cycle perturbations, bursts of pinnacle reef growth, extreme sea-level oscillations, and evaporite deposition (Michigan Basin, USA)

Abstract

Integration of sedimentologic, chemostratigraphic, biostratigraphic, and geochronologic data identifies the profound influence that sudden bursts of carbonate sedimentation, sea-level oscillation, and hypersalinity, associated with Ireviken and Mulde carbon cycle perturbations, had on the evolution of the Michigan Basin during the Silurian. Conodonts and carbon isotope (δ13Ccarb) stratigraphy of the Lockport and Engadine Dolomites constrain the rapid build-up and progradation of a thick biohermal wedge along the basin margins to the Sheinwoodian-age Ireviken Excursion, a recently identified period of global reef growth. This event significantly changed the hydrologic architecture of the basin, ultimately resulting in restricted seawater exchange that drove evaporite deposition. Zircon UPb ages of ~428 Ma from bentonites near the base of the evaporite-bearing Salina Group, together with δ13Ccarb profiles transecting the basin, constrain the onset of hypersalinity to initiation of the late Homerian Mulde Excursion. Importantly, the double δ13Ccarb peak of this event parallels a complex pattern of dramatic sea-level oscillations, on the order of 100 m, recorded within the A-0 to A-2 evaporites and associated pinnacle reefs. Ascending δ13Ccarb values were accompanied by sea-level fall, karstification, and the formation of evaporites in the basin center. Intra-excursion sea-level rebounds triggered exceptionally rapid upward pinnacle reef growth. Subsequent sea-level fall again exposed the ramp and pinnacle reefs resulting in basin-wide restriction and massive salt deposition that, ultimately, encased the reefs in salt. The return to baseline δ13Ccarb values was accompanied by flooding and carbonate sedimentation.Our integrated approach, with high-resolution δ13Ccarb stratigraphy from closely spaced (intra-basinal) sections as a central component, enables finely tuned chronostratigraphic correlation, recognition of unconformity-bound facies packages (i.e., systems tracts), and constrains variations in sea level and sedimentation rates. These findings have immediate applications to predictive static reservoir models for the evaluation of the reefs as repositories for anthropogenic CO2 sequestration, gas storage reservoirs, and hydrocarbon exploration. Ultimately, our findings demonstrate that a cascade of rapid environmental changes, including pulses of carbonate sedimentation, high-amplitude sea-level oscillations, and evaporite deposition, were associated with the Ireviken and Mulde perturbations of the Silurian carbon cycle.