STRONTIUM ISOTOPE RATIOS FROM THE EARLY EOCENE GULF COAST – INFLUENCE OF SALINITY AND POTENTIAL FOR AGE CONTROL

Refinements in the early Paleogene strontium isotope seawater curve have increased the usage of ⁸⁷Sr/⁸⁶Sr ratios for age control and correlation of marine carbonates. In shelf settings, the potential influence of freshwater influx on ⁸⁷Sr/⁸⁶Sr ratios must be taken into account, as runoff typically will have a more radiogenic signature distinct from the marine reservoir. This influx will cause deviations from marine values when mixed. The utility of ⁸⁷Sr/⁸⁶Sr ratios for chemostratigraphy depends on consistency of ratios across the shelf; therefore, it is important to constrain the range of variation seen in contemporaneous deposits. Here, we combine strontium isotope ratios with oxygen isotope data to evaluate the influence of paleosalinity on the chemistry of shell material and to assess the potential of ⁸⁷Sr/⁸⁶Sr for age control in shelf settings.

Shells of the aragonitic bivalve Venericardia were collected from the early Eocene (~55 Ma) Bashi Formation, a geographically extensive, fossiliferous, shoreface deposit composed of unlithified, glauconitic sands. Samples were collected across ~1000 km of the shelf from Texas to Georgia to assess spatial variability within and between localities along the paleoshoreline. Preliminary results suggest an average strontium isotope ratio of 0.70777 along nearly the entire shelf, consistent with the marine seawater ratio for the earliest Eocene (53.8 – 55.8 Ma). The exceptions are data from Texas, which fall well below the marine seawater ratio for this time and are lower than any other Cenozoic ratios. The departure of Texas samples from the expected ratio suggests the mixing of fresh water draining a basin characterized by non-radiogenic rocks. The TX Coastal Plain contains extensive exposures of Cretaceous marine carbonates with comparatively low ⁸⁷Sr/⁸⁶Sr ratios, the likely source of nonradiogenic Sr. Oxygen isotope values from Texas bivalves are much lower than those from elsewhere on the shelf, and the Bashi in TX is part of a larger deltaic system, supporting the inference of freshwater mixing in this region. Contrary to earlier work, we find a high degree of consistency in ⁸⁷Sr/⁸⁶Sr ratios in areas free of independent evidence for brackish water, and suggest that ⁸⁷Sr/⁸⁶Sr ratios are likely to be quite useful for age control in this setting.