CHICXULUB IMPACT AND THE K/T MASS EXTINCTION

New cores and outcrops from the Brazos River, Falls County, Texas, reveal the stratigraphic and temporal separation between the K/T boundary and event deposit and led to the discovery of the original Chicxulub impact ejecta layer. Stratigraphy, sedimentology, mineralogy and geochemistry reveal the nature and sequence of these three events. The original Chicxulub ejecta layer consists of a 3 cm yellow clay, or cheto smectite derived from alteration of glass with relic glass compositions similar to Chicxulub impact spherules in NE Mexico and Haiti. The cheto smectite is interbedded in undisturbed late Maastrichtian mudstone of zone CF1, 45 cm below the event deposit, which indicates that the Chicxulub impact predates the K/T boundary by about 300 ky. This is consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1. The event deposit infills a channel, first with a a transgressive lag conglomerate of locally derived clay and mudstone clasts, but also lithified clasts with impact spherules from the original layer. Above it are three units of upward fining glauconitic, shelly, spherule-rich sands, followed by cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with recolonization of the ocean floor by invertebrates between storms. Above the event deposit normal shallow neritic clay- and siltstone sedimentation returns with high stress late Maastrichtian microfossil assemblages, small shells and burrows infilled with framboidal pyrite indicative of low oxygen conditions. The K/T boundary is 80 cm above the event deposit. The Brazos River sequences thus reveal a similar sequence of events as in Mexico, with the Chicxulub impact ~300 ky prior to the K/T boundary, the event deposit marking a sea level fall followed by transgression, and the K/T boundary event and Ir anomaly marking a second impact.