Sedimentology and extinction patterns across the Cretaceous-Tertiary boundary interval in east Texas

Abstract

Biostratigraphic analysis of planktic foraminifera and calcareous nannofossils, as well as sedimentological and geochemical studies, suggests that the K-T boundary interval on the Brazos River in Falls County, Texas, has a complete Cretaceous section but that a hiatus probably exists in the earliest Palaeocene, representing as little as 35,000 years of time. Planktonic foraminifera, calcareous nannofossils and molluscs decline abruptly in diversity at the K-T boundary in conjunction with an increase in iridium levels. Molluscan assemblages are diverse and dominated by suspension feeders up to the K-T contact but are replaced by a very low diversity assemblage of predominantly deposit-feeders and carnivores in the Early Palaeocene. When both microfauna and macrofauna are considered, species that lived or fed in the water column (planktonic foraminifera, calcareous nannofossils, suspension feeding bivalves) suffered the highest mortality while benthic, dominantly deposit-feeding or carnivorous taxa (benthic foraminifera, carnivorous and deposit-feeding gastropods and bivalves) were less affected. Although the extinction appears massive and abrupt on the whole, some minor faunal irregularities (abnormal forminiferal tests and a slight molluscan diversity drop) occur about one meter below the K-T boundary. We don't as yet know whether these changes are part of normal background environmental fluctuations or whether they are related to the K-T mass extinction event.