FAUNAL AND STRATIGRAPHIC ANALYSIS OF THE CRETACEOUS - PALEOGENE (K-PG) BOUNDARY EVENT DEPOSIT, BRAZOS RIVER, TEXAS

Approximately 66 million years ago a bolide impact occurred at Chicxulub on the Yucatan Peninsula, Mexico. This impact induced environmental perturbations on a global scale, resulting in a mass extinction event at the end of the Cretaceous period. Locally the impact may have caused mass wasting due to shelf collapse and/or seismic activity, tsunamis, storms surges, and deposition of impact related ejecta. Due to their proximity to the crater outcrops on the U.S Gulf Coastal Plain that contain the K-Pg boundary provide a detailed record of the depositional environments during and after the impact. Our study localities are located along the Brazos River, Falls County, Texas. The Upper Cretaceous is represented by the fossiliferous Corsicana Formation. The Corsicana Formation is overlain by the Danian Kincaid Formation. Yancey and Liu (2013) described an event deposit composed of three distinct facies at the base of the Kincaid. The lowermost facies, Facies A is primarily composed of shell hash in a muddy matrix with mudstone rip-ups. The overlying facies, Facies B, is a semi-laminated shell hash, containing ejecta, in a clay matrix. Both facies are fossiliferous and contain well-preserved mollusks, and vertebrate material. The uppermost event facies, Facies C, is represented by cross-bedded sandstone. Bulk samples (~45 kg each), were collected from the Corsicana Formation, Facies A, and Facies B. Fossils from bulk samples were identified, and analyzed for taphonomic data. Species richness and biodiversity of the Corsicana Formation and Facies A/B were calculated using Simpson’s Diversity index (1-D). Microstratigraphic data from Facies A and Facies B will be used to supplement interpretations made from the macrofaunal analysis. Preliminary results indicate that 1-D=0.86 and 0.94 for Facies A and Facies B, respectively. The Corsicana Formation and Facies A faunal assemblages are dominated by infaunal suspension feeders. Fauna in Facies B do not show any dominant mode of life. Lack of any dominant mode of life and high biodiversity suggest that Facies B contains an allochthonous assemblage, indicating environmental mixing. This may be the result of deposition by a high energy event such as a tsunami or storm surge related to the Chicxulub impact event.