ISOTOPE GEOCHEMISTRY AND PETROLOGY OF THE LOWER CRETACOUS (ALBIAN) DEQUEEN FORMATION, SOUTHWEST ARKANSAS

Petrographic analysis and stable oxygen and carbon isotopic compositions of carbonate cements and allochems collected from layers of the Lower Cretaceous DeQueen Formation, southwest Arkansas were analyzed and examined to document regional paleoclimate and global climatic changes during deposition of the formation. The sample location is the site of extensive theropod and sauropod dinosaur tracks. The track-bearing nature of these units suggest that early diagenetic cements may be representative of meteoric water. Deposition was interpreted to represent a low subtidal to supratidal paleoenvironment based on the presence of foraminifera, marine bivalve hash, and pelloidal carbonates observed in petrographic thin sections. Currently identified phases of carbonate in thin section include micrite, microspar, marly siltstone and sparry calcite. Preliminary isotopic analyses of different carbonate phases reveal distinct isotopic compositions. For example, from the track-bearing surface sparry calcite is significantly different than micritic and peloidal calcite (δ¹³C = -0.39 ± 0.36‰ VPDB and δ¹⁸O = -4.71 ± 0.11‰ VPDB for sparry calcite versus δ¹³C = -1.80 ± 0.18‰ and δ¹⁸O = -3.82 ± 0.43‰ for micritic and pelloidal calcite). Additional analysis of other carbonate phases will be added to this data set. Data will then be converted to δ¹⁸O_water and added to an existing latitudinal δ¹⁸O groundwater gradient for the Aptian-Albian to improve modeling of the hydrologic cycle during the Cretaceous. A better understanding of the hydrologic cycle during times of greenhouse warming in Earth’s past can provide insight on the role of current and future climate trends.