Faunal and stratigraphic analysis of the basal Cretaceous-Paleogene (K-Pg) boundary event deposits, Brazos River, Texas, USA

Abstract

A bolide impact ∼66 million years ago near Chicxulub, Yucatan Peninsula, Mexico triggered environmental perturbations on a global scale, leading to a mass extinction at the Cretaceous-Paleogene (K-Pg) boundary. Outcrops on the U.S Gulf Coastal Plain that contain the K-Pg boundary provide a detailed record of environments across this critical transition, but questions remain about the nature and timing of depositional processes that affected the region at the time of impact and mass extinction. We present a new study of coarse-grained K-Pg ‘event deposits’ located at the contact between the fossiliferous Cretaceous Corsicana Formation and the Danian Kincaid Formation, and which outcrop in tributaries along the Brazos River, Falls County, Texas. A generalized succession can be recognized in these deposits. We sampled the basal-most unconsolidated units, Unit I and Unit II, and the Corsicana Formation for macrofaunal and sedimentological data. Unit I is interpreted as a debrite, deposited by a medium – high strength cohesive debris flow initiated by ground shaking and intense seismic activity after the Chicxulub impact. Macrofossil analysis shows a mostly locally derived assemblage. Grain size analysis of non‑carbonate portions of the matrix indicates an identical mean grain size to that of the underlying Corsicana Formation. The chaotic fabric, boulder sized clasts, and muddy matrix support the interpretation of deposition via cohesive debris flow. Unit II is also interpreted as a debrite, deposited by a low-medium strength cohesive debris flow. We propose that this unit was initiated by wave energy from a tsunami or local shelf collapse immediately following impact. Macrofossil analysis of Unit II shows an increase in fauna with a predatory/carnivorous lifestyle, which are interpreted as allochthonous elements derived from shoreward environments and transported across the shelf. The high mud content of the matrix and abrupt pinching out on topographic highs support the interpretation of deposition via a cohesive debris flow for Unit II. Our results indicate that sediment flows were a major driver of mass sediment transport in proximal locations directly following the Chicxulub impact.