RE-WRITING THE CRETACEOUS-PALEOGENE BOUNDARY EVENTS IN TEXAS: NEW SECTIONS AND REVISED MICROPALEONTOLOGICAL INTERPRETATIONS (Invited Presentation)

Fieldwork over the last three years in the Brazos River area, Falls County, Texas, has located a number of completely new Cretaceous–Paleogene (K/Pg) boundary sections in an area that has been intensively studied in the last thirty years. One of these, at River Bank South (RBS), exposes the most complete K/Pg succession in the area with almost all the key events clearly displayed. Suites of closely spaced samples from River Bank South, Cottonmouth Creek, Darting Minnow Creek, other Brazos River sections (RB-1 to RB-5) and new sections at River Bank North (RBN) and River bank West (RBW) have facilitated an investigation of benthic and planktic foraminifera, ostracods, calcareous nannofossils and dinoflagellate cysts. These newly-generated micropaleontological data have been integrated into a synthesis of the latest Maastrichtian and earliest Paleocene events and a robust biostratigraphy constructed. Using these data, we present a revised interpretation of the K/Pg boundary events in Texas and show how they relate to other areas (e.g., Demerara Rise, El Kef, Gubbio, Maastricht and Stevns Klint). The K/Pg boundary that we use follows the ICS definition for the base of the Paleocene: the first evidence of an event related to the Chicxulub impact. The tsunami from this impact scoured the Maastrichtian sea floor across Texas, creating a topography that was partially in-filled by a locally-derived mudstone conglomerate, a re-worked spherule-rich bed and a succession of storm-derived, hummocky cross-stratified sands with mudstone inter-beds. The spherule-rich bed is un-graded, rich in shell debris and containing re-worked foraminifera, calcareous nannofossil and ichthyolith fragments. In the RBS section, the Maastrichtian mudstones remained exposed on the sea floor and, at the K/Pg boundary these were colonized by juvenile oysters. Petrological work and radiometric dating has confirmed that there are a number of volcanic ashes in the succession, one of which has previously been used as evidence for a pre-extinction impact events or part of a suite of multiple impact events. Our work suggests the presence of only one impact, which was coincident with the major biotic changes of the K/Pg boundary event.