TESTING THE DEVELOPMENT OF δ13C GRADIENTS ACROSS A PROXIMAL TO DISTAL TRANSECT OF AN ORDOVICIAN-SILURIAN CARBONATE SHELF IN CENTRAL NEVADA

Interpretation of ancient δ¹³C records from carbonate strata can be informed by consideration of sedimentary basin geometry, paleobathymetry, and local water mass circulation. Strata comprising a Late Ordovician-Early Silurian carbonate shelf that developed on the passive continental margin of western North America are exposed in fault-bounded blocks across central Nevada as a result of Cenozoic continental extension. Rocks collected from South Egan Range, Pancake Range, and Lone Mountain were analyzed for inorganic carbon isotope ratios. Strata from these three ranges span a transect of depositional environments from near shore to farther offshore settings. A well-developed sequence stratigraphic framework provides an independent method for correlation between the ranges.

During the Early Silurian, normal faulting or large-scale slope failure resulted in the transformation of the carbonate shelf geometry from a shallow ramp to a rimmed platform. This transition may have resulted in a shift from open ocean circulation on the shelf to a more restricted ocean circulation system. In many modern carbonate platforms isotopic gradients of dissolved inorganic carbon (DIC) develop as local carbon cycling occurs in restricted near shore settings, altering the δ¹³C compared to open ocean settings. In an attempt to examine whether similar processes were recorded in the geochemistry of Paleozoic carbonate strata, we tested the development of δ¹³C gradients across a paleogeographic transect of an Ordovician-Silurian carbonate shelf in response to the change in basin geometry. The evolution of the carbonate shelf during the Early Silurian helps identify the effects of restricted ocean circulation on a Paleozoic inorganic carbon isotope record.