NEW RECORD OF AN ABUNDANT AMMONITE ASSEMBLAGE FROM THE LATEST CRETACEOUS CORSICANA FORMATION, BRAZOS RIVER, TEXAS. IMPLICATIONS FOR THE CRETACEOUS–PALEOGENE (K–PG) MASS EXTINCTION EVENT IN THE GULF OF MEXICO

Outcrops located along the Brazos River, Texas, which contain the Cretaceous-Paleogene (K–Pg) boundary have been extensively studied for the last 30 years, and provide critical insight into the timing and nature of the mass extinction event 66 million years ago. New investigations of the Corsicana Formation at two sites (Darting Minnow and Cottonmouth Creek) have resulted in the discovery of an abundant ammonite assemblage directly below the K–Pg boundary. In-situ ammonites range to within 10 cm of the ‘clastic unit’ which marks the boundary at Brazos. Findings of Discoscaphites iris confirm the presence of the highest Maastrichtian ammonite range zone in North America, and with calcareous nannofossil biostratigraphy allow for precise dating of the fauna. Additional age constraints are provided by a bentonite ash layer 30 cm below the K–Pg boundary at Cottonmouth Creek. The Brazos ammonite fauna is dominated by large examples of the genera Eubaculites and Discoscaphites, with rare Sphenodiscus and Gaudryceras – the first record of this genus in the Maastrichtian of the United States. Jaws and juvenile specimens are also present, indicating this represents a living community. Specimens exhibit well-preserved shell material allowing for determination of water temperatures during the latest Cretaceous. These data can be compared with those from other fossil groups, as well as sedimentary geochemistry (mercury concentrations), to explore global climate trends and the effects of volcanism leading up to the mass extinction event and Chicxulub bolide impact. The composition of the Brazos fauna suggests this assemblage was living in deeper water compared to those of the eastern Gulf, while the presence of Gaudryceras indicates persistent links to the Pacific and Tethys oceans. Ammonites thrived during the latest Maastrichtian in the western Gulf Coast, apparently contradicting hypotheses for an early disappearance of the group at low latitudes prior to the K–Pg extinction.