The Cretaceous—Palaeogene boundary on the Brazos River, Falls County, Texas: is there evidence for impact-induced tsunami sedimentation?

Abstract

The sedimenlology and sequence stratigraphy of deposits spanning the Cretaceous-Palaeogene boundary on the Brazos River are described and reinterpreted. A channelled erosion surface (channels up to 1.5 m deep), locally burrowed, at the summit of the Corsicana Clay is identified as a putative sequence boundary, was formed by very late Maastrichtian sea-level fall. The erosion surface is overlain by a transgressive lag conglomerate which incorporates phosphatized hiatus concretions and clasts derived from the Corsicana Clay, set in a shelly, glauconitic sand matrix. As transgression continued, fining-upward sands infilled the channels and progressively onlapped onto the basal disconformity with the Corsicana. The sands have features diagnostic of storm deposition and up to five discrete beds can be identified. Between storm events, the surfaces of successive sands were colonized by infauna to create dwelling burrows, including Ophiomorpha, Thalassinoides and Planolites. Burrows were subsequently truncated by storm-generated erosion. The highest transgressive deposits are calcareous silts, which are overlain by a burrowed omission surface that coincides with the micropalaeontologically defined K-T boundary. The lowest Danian sediments are silty clays deposited during a highstand. Thus, the Brazos succession, rather than representing a single depositional event generated by a tsunami, as has been claimed, was the product of normal shelf processes under the control of sea-level change. Evidence for bolide impact in the Late Maastrichtian here is provided by the occurrence of pseudomorphs after microspherules (presumably microtektites) in the basal conglomerate of the Kincaid Formation, and positive excursions in iridium content close to the micropalaeontological K-T boundary. However, these levels are separated by at least six discrete burrowed horizons which represent a considerable period of time, and it is therefore impossible that the microspherules and iridium were products of the same impact event. Thus, the succession, exposed on the Brazos River, alongside broadly similar successions exposed in northern Mexico, has implications for the currently accepted K-T boundary impact scenario.