Stratigraphy and paleontology of the cretaceous-tertiary boundary, Big Bend National Park, Texas

Abstract

An exposure of the Javelina and Black Peaks Formations north of Grapevine Hills in Big Bend National Park, Brewster County, straddles the Cretaceous-Tertiary (K/T) boundary. Mapping and section measurement resolved a pronounced change in mudstone color and contact form interpreted as different preserved soil profiles reflecting an increase in rainfall and cooling of the environment in the early Paleocene. With the intent to provide a useful field marker, and through comparison of these distinctive paleosol horizons, the Javelina-Black Peaks Formational boundary is now described at the top of the former "sandstone-dominated" portion of the Javelina Formation. Redefinition of this boundary adds over 100 meters of strata to the base of the Black Peaks Formation over original type section definition. As a result, the K/T boundary lies within the lower Black Peaks Formation. The Grapevine Hills section contains the most tightly limited K/T boundary section in Big Bend Park, based on the position of Cretaceous dinosaur and Paleocene vertebrate remains. Unlike other documented marine KfT boundary sections, this terrestrial K/T section cannot be recognized on the basis of stratigraphic details such as an unconformity or a "megawave" deposit as might have been caused by a bolide impact in the Caribbean at the close of the Cretaceous. The K/T interval has been refined by fossil remains to a section of 4 meters of strata which show an unusual lithology and fossil abundance seen nowhere else in the field area.