ORGANIC CARBON ISOTOPE CHEMOSTRATIGRAPHY OF THE YUCCA FORMATION, BIG BEND RANCH STATE PARK

The Yucca Formation is a Lower Cretaceous sedimentary unit found across West Texas. The Cretaceous Period was a time of major global climate change, and the Yucca Formation could record some of those major climatic shifts. However, age constraint must be improved and geochemical climate proxies employed to better understand any possible recorded climatic shifts within the Yucca Formation. The goals of this project are to 1. Better constrain the age of the Yucca Formation using stable isotope geochemistry, specifically organic carbon stable isotope chemostratigraphy, 2. Correlate the Yucca Formation with strata of the similar age using chemostratigraphy.

163 samples were collected from Big Bend Ranch State Park (BBRSP) to determine the d¹³C value of bulk sedimentary organic matter. C-isotope values range from -27.02 to -18.42‰. Carbon isotope excursions that are associated with Aptian-Albian Boundary are identified as well as some associated with Oceanic Anoxic Events (1a and 1b). This allows us to conclude that the Aptian-Albian boundary is recorded within the Yucca Formation strata. Based on this correlation and the currently accepted Geologic Time Scale boundary, a date of 113.2 Ma is assigned to 71 meters above the base of the section exposed in the Lower Shutup of the Solitario in Big Bend Ranch State Park. Regional correlation of the Yucca Formation to other chemostratigraphic records from other Cretaceous strata in the suggests that the Yucca Formation in BBRSP is time equivalent to the Sligo, Pine Island, James, Bexar, and the lower portion of the Glen Rose Formation on the Comanche Platform.