CARBON ISOTOPE CHEMOSTRATIGRAPHY OF THE TERRIGENOUS HENSEL FORMATION, CENTRAL TEXAS

The Hensel Formation of Central Texas represents rare terrestrial deposition in an area that, during the Early Cretaceous, was otherwise characterized by marine depositional environments. Its age is constrained by the coeval Glen Rose Formation to span the Aptian-Albian Boundary. To test this, samples were collected approximately every 25 cm to construct a C-isotope curve from sedimentary organic carbon to compare to existing carbon-isotope curves. A total of 93 samples over 30 m were collected from the “Upper Fines” unit of the Hensel Formation near Junction, Texas. The section consists of red calcareous mudstones and tan sandstones near the base to gray, greenish gray, and tan calcareous mudstones, sandstones and limestones toward the top. The section in unconformably overlain by the Cretaceous Fort Terrett Formation. Within red beds, well developed ped structure and burrows indicate soil development. Carbon isotope values ranged from -28.9‰ vs. VPDB to -19.6‰ vs. VPDB. While there is significant scatter, positive and negative trends are correlated to a composite C-isotope curve from marine carbonates in South Texas. The Hensel Formation at this locality is hypothesized to correlate to variations in an overall positive excursion that occurs in the lower Albian. Additional samples from lower in the Hensel Formation will test this correlation. Future work in this project will aim to take advantage of this rare terrestrial unit to elucidate paleotemperature and paleoprecipitatin estimates for the Aptian-Albian.