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

摘要

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 (delta C-13(carb)) 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 U-Pb ages of similar to 428 Ma from bentonites near the base of the evaporite-bearing Salina Group, together with delta C-13(carb) profiles transecting the basin, constrain the onset of hypersalinity to initiation of the late Homerian Mulde Excursion. Importantly, the double delta C-13(carb) 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 delta C-13(carb) 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 delta C-13(carb) values was accompanied by flooding and carbonate sedimentation.