MACROFOSSIL ASSESSMENT OF THE CRETACEOUS-PALEOGENE (K-PG) BOUNDARY DEPOSITS BRAZOS RIVER, TEXAS: IMPLICATIONS FOR DEPOSITIONAL PROCESSES AND MASS EXTINCTION

Approximately 66 million years ago the Chicxulub bolide impact triggered global environmental perturbations, leading to a mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary. Proximal to the crater in the Gulf of Mexico, the impact likely caused mass wasting due to shelf collapse and/or seismic activity, tsunamis, storms, and rapid deposition and reworking of ejecta. K-Pg boundary outcrops on the U.S Gulf Coastal Plain may thus provide a detailed record of depositional environments and the effects of impact-related processes on marine biota. Along the Brazos River, Texas, the K-Pg boundary occurs between the fossiliferous late Maastrichtian Corsicana Formation and Danian Kincaid Formation. Previous authors have described an event deposit composed of three units coincident with the K-Pg transition. Unit A is composed of shell hash in a muddy matrix with mudstone rip-ups. The overlying Unit B is a semi-laminated shell hash, containing impact ejecta, in a clay matrix. Both units are fossiliferous and contain well-preserved mollusks, the subject of this study. The uppermost Unit C is represented by unfossiliferous cross-bedded sandstones and interbedded siltstones. We conducted a faunal analysis to elucidate depositional processes of these units using bulk sediment and systematically collected fossil samples. Faunal data from Unit A, Unit B and the Corsicana Formation were evaluated relative to one another using multivariate analysis (non-parametric multidimensional scaling – NMDS), diversity estimates, and quantitative analyses of molluscan modes of life. High-resolution stratigraphic logs, grain size analysis, and point counts provide additional paleoenvironmental information. Data suggest that Units 1 and 2 represent allochthonous deposition, with faunal distributions different to the underlying Corsicana Formation, such as the sudden appearance and abundance of the bivalve Vetericardiella webbervillensis. A two-fold increase in gastropod species richness in Unit 2 also suggests derivation from a distinctly different paleoenvironment. Unit 1 is considered to be the result of localized mass flows related to seismic activity following the Chicxulub impact. Unit 2 may be the result of higher energy depositional process, such as a tsunami or the effects of large post-impact storms.