LATE ORDOVICIAN AND EARLY SILURIAN GLACIOEUSTATIC SEA LEVEL CHANGES AND CARBON ISOTOPE CHEMOSTRATIGRAPHY

Here we present new carbon isotope data from eight stratigraphic sections of the Upper Ordovician Ely Springs Dolostone and the Lower Silurian Laketown Dolostone from the central and eastern Great Basin. These strata were deposited in shallow marine environments subject to sea level fluctuations likely driven by the waxing and waning of Gondwanan glaciers. The δ¹³C curves provide tie points for intrabasinal and global correlation, including the first phase of the end-Ordovician Hirnantian positive carbon isotope excursion. Previous workers developed a sequence stratigraphic framework for the succession based on facies stacking patterns and recognition of subaerial exposure surfaces, including the unconformity marking the Ordovician-Silurian system boundary. The carbon isotope records are tied to this independent sequence stratigraphic model and available biostratigraphy, providing an opportunity to test high-resolution δ¹³C correlations between the sections. The δ¹³C values range between -2‰ and +3‰ at each section, reaching an apex within the uppermost sequence preserved in the Ordovician strata (sequence O5). While many chemostratigraphic tie points are consistent with the sequence stratigraphic framework and biostratigraphic constraints, others are not. These discrepancies may result from development of lateral δ¹³C gradients in semi-restricted water masses during deposition, post-depositional alteration of the δ¹³C signal during dolomitization and burial, and/or difficulties with assigning sequence boundaries in these strata.