FAUNAL ANALYSIS AT THE CRETACEOUS - PALEOGENE MASS EXTINCTION BOUNDARY, BRAZOS RIVER, TEXAS

Approximately 66 mya, a bolide impact occurred near Chicxulub on the Yucatan Peninsula, Mexico. This event is thought to have caused widespread environmental perturbation, resulting in a mass extinction. Outcrops containing the Cretaceous-Paleogene (K-Pg) boundary on the U.S. Gulf Coastal Plain provide detailed information about depositional environments during and after the impact due to their proximity to the Chicxulub crater. They exhibit a distinct sequence of facies due to unique depositional processes that resulted from the impact. Such processes may have included mass wasting due to shelf collapse and/or earthquake activity, deposition of impact related ejecta (glassy spherules, lapilli, and tektites), tsunamis, and a succession of large storms. We studied sites near the Brazos River, Falls County, Texas. Here the Maastrichtian is represented by the fossiliferous Corsicana Formation. The presence of Discoscaphites iris indicates a latest Maastrichtian age. The Corsicana Formation is overlain by the Kincaid Formation of early Danian age. The base is an event deposit described by Yancey and Liu (2013) comprising three distinct facies. At the base, Facies A is composed primarily of shell fragments in a muddy matrix with mudstone rip-ups. Facies B, positioned above, comprises semi-laminated shell hash and ejecta in a clay matrix. Both facies are fossiliferous and contain well-preserved bivalves, gastropods, ammonites, and vertebrate material. The third event facies is represented by cross-bedded sandstone. A total of six bulk samples of ~6-7 kg each were collected from Facies A and Facies B for analysis. Fossils from these bulk samples have been identified to the lowest taxonomic level possible. Diversity data was analyzed by calculating the species richness and Simpson’s Diversity index for each facies. Detailed observations were made on faunal preservation. A comparison of Facies A, Facies B and the underlying and overlying facies was made using these metrics. It is assumed that Facies A and B are a direct result of the Chicxulub impact. Therefore, the fauna may not record organisms that lived in the area at the time of deposition, but rather those transported into the site. These data can shed important light on the mechanisms related to the K-Pg mass extinction event and related depositional processes.