BENTHIC FORAMINIFERAL CHANGE ACROSS THE CRETACEOUS – PALEOGENE BOUNDARY (K/PG) AT BRAZOS RIVER, TEXAS

The Cretaceous – Paleogene boundary (K/Pg) is one of the major crises in the history of life on Earth. The cause is widely regarded as a combination of degassing of the Deccan Traps and a bolide impact in Chicxulub, Mexico. The Brazos area in Texas represents an area affected by the proposed “tsunami” from the impact. New samples and core material from Texas provide beautiful, fresh, fossiliferous material across the impact level and the “event bed”. The benthic foraminifera are utilised to document the changing environment of the boundary interval and provided a new interpretation of the paleoecology of the succession as well as the relationship of the benthic foraminifera to the previously well documented planktic foraminifera is also achieved. Samples from the Brazos River sections were analysed for this study. Benthic foraminifera across the K/Pg boundary at Brazos do not demonstrate a “mass extinction” event across the boundary, unlike the planktic foraminifera. The assemblage is dominated by rounded species, specifically Lenticulina rotulata, Robulus spp. and Cibicides spp. Agglutinated species are the only group to be affected across the boundary with the disappearance of taxa such as Clavulinoides spp. and Tritaxia spp. Approximately 1 metre above the K/Pg boundary the assemblage of the larger size fractions (<500µm and 500µm-250µm) is totally dominated by large uniserial elongate benthic taxa. Specifically Nodosaria affinis, Vaginulina cretacea, Marginulina spp. and Dentalina spp. The percentage of elongate benthic taxa becomes more abundant the further up the succession. The occurrence of these species coincides with significant numbers of large, flat lenticulinids (undescribed) as well as a dramatic increase of ostracods and other macrofauna in this interval. A proportion of the ostracods from this interval show circular borings on the carapace that are probably the result of predation from gastropods. The smaller size fractions (250-150µm and 150µm-63µm) are dominated by rounded benthic foraminifera such as Gyroidina spp., Cibicides spp., Eponides spp. These patterns continue up to the top of the succession, approximately 2 metres above the K/Pg boundary, and appear to show a dominance of infaunal taxa in the larger size fractions and a dominance of epifaunal taxa in the smaller size fractions.