Benthic foraminiferal change and depositional history across the Cretaceous – Paleogene (K/Pg) boundary in the Brazos River area, Texas

Abstract

The Cretaceous–Paleogene (K/Pg) boundary marks one of the major crises in the history of life on Earth. The cause is widely regarded as a large bolide impact at Chicxulub, Mexico, coincident with a major series of volcanic eruptions on the Deccan Plateau, India. Fieldwork in the Brazos River area of Texas has involved an investigation of the sections on the Brazos River and its tributaries. A previously overlooked K/Pg section (RBS) on the Brazos River was found and contains the most accessible and complete K/Pg boundary succession in the area. The RBS succession provides a clear exposure of the various lithological units within the Paleocene and was used to correlate to the successions in the nearby creeks. The K/Pg boundary is also well-exposed and records an erosional relief of ~1 m, cut into the Maastrichtian mudstone succession, creating a mounded topography. The overlying ‘Event Bed’, containing reworked impact spherules at its base, is shown to infill troughs on this irregular surface. The same features were recorded in tributary creeks, with all previous descriptions of these locations clearly failing to recognise the various sedimentary relationships. Distinct, thin, yellow clay horizons within the uppermost Maastrichtian mudstones are present in some sections. Geochemical analysis and radiometric dating have confirmed these as volcanic ashes, with extracted zircons giving a date of 65.95+0.04 Ma. These ash bands are located in Maastrichtian mudstones just below the K/Pg boundary and the recorded date is, within error, that of the K/Pg boundary. This identification of this latest Maastrichtian volcanic ash negates the suggestion of a pre-K/Pg boundary impact, a pre-extinction impact or multiple impacts. The benthic foraminiferal data generated indicates significant changes in palaeoecology of the benthic foraminifera across the K/Pg boundary. The benthic foraminifera do not experience a mass extinction, unlike the planktic foraminifera, which were significantly affected by the end-Maastrichtian extinction event. The benthic foraminferal assemblage appears to only experience transient, short-lived changes with pulses of agglutinated, elongate and large species in the early Paleocene. Mono-specific samples of Lenticulina rotulata have been analysed for stable isotopes and the data may indicate the presence cyclicity across the K/Pg boundary interval. In the earliest Paleocene significant negative δ18O excursions near the Pα/P1a and NP1/NP2 boundary represents a potential hyperthermal event that may be coeval with the DAN-C2 and Lower C29n events respectively recorded at Gubbio, and in the Atlantic Ocean. A sequence stratigraphy package is determined based on the micropaleontology and sedimentology in the Brazos River area. The latest Maastrichtian is marked by a sealevel rise immediately before the K/Pg boundary. Immediately after the K/Pg event, sealevel fell and is recorded as a change from mid to inner shelf. The condensed unit of the Middle Sandstone Bed (MSB) represents a Transgressive systems tract, with increasing diversity and abundance of benthic foraminifera to the top of the MSB, where maximum abundance and diversity is marks a Maximum Flooding Surface. The interval above indicates sea-level continuing to rise to a mid to outer-shelf setting. These sealevel changes are also recorded at the same stratigraphic level in Alabama, and at more distal K/Pg boundary sections (e.g., Denmark and Tunisia) suggesting that these sealevel changes are eustatically controlled.