METEORIC DIAGENESIS OF SEQUENCE BOUNDARIES IN LATE ORDOVICIAN-EARLY SILURIAN DOLOMITES OF THE GREAT BASIN, NEVADA

Eustatic sea level fluctuations in the late Ordovician and early Silurian accompanied the waxing and waning of continental ice sheets over Earth’s south pole and the first mass extinction of the Phanerozoic Eon. The Great Basin contains miogeoclinal carbonate sedimentary rocks deposited on the western margin of Laurentia that record these fluctuations as a series of shallowing upwards sequences of bioturbated, cross-bedded, and laminated dolomite facies. Sequence boundaries display lithologic evidence of meteoric diagenesis. Meteoric flushing by isotopically depleted fluids has elsewhere been implicated to explain systematic carbon and oxygen isotope covariation beneath sequence boundaries in carbonate shelf successions. Remineralization of terrestrial biomass is hypothesized to be the origin of carbon isotope depletion in these meteoric waters. Here we present new high-resolution inorganic carbon isotope data from Great Basin Ordovician-Silurian strata and pair those records with organic carbon isotope curves. Variations between organic and inorganic isotope curves clarify the origin and extent of geochemical alteration due to meteoric diagenesis along sequence boundaries in these strata. Microfacies petrography, trace elemental analysis, cathodoluminescence, and electron backscatter imaging support chemostratigraphic analyses and are used to separate the effects of meteoric diagenesis from other diagenetic signals. Integrating these techniques with lithologic observations at outcrop and hand sample scale allows multiple independent assessments of the effects of meteoric diagenesis on the upper Ordovician - lower Silurian carbonates of the Great Basin, further constraining the diagenetic history and chemostratigraphic records of the region.