The K/Pg boundary at Brazos-River, Texas, USA — An approach by marine palynology

Abstract

Two cores and one outcrop section from the Cretaceous/Palaeogen (K/Pg) boundary interval at the Brazos-River area, Texas, USA, have been high resolution sampled and analysed fully quantitatively by marine palynology. The results have been compared and integrated with data from micropalaeontology, sedimentology and isotope-geochemistry from the same sections. Within all three sections, the K/Pg boundary, defined as the base of the P0 foraminifera zone and the onset of a negative carbon isotope anomaly, closely corresponds to the appearance of lowermost Danian organic-walled dinocysts. Based on various palynological proxies as well as on sedimentologic features, a sequence stratigraphic subdivision of the sections is proposed. From a synopsis of all three sections, a long term, punctuated drop of relative sea-level is indicated, starting at about the top of the Upper Cretaceous CF2 foraminifera zone and reaching a minimum within the upper P1a zone of the Lower Palaeogene. This is followed by a prominent transgression starting within the P1a/P1b boundary interval. The “event deposit” (ED), a siliciclastic unit located well below the base of the P0 foraminifera zone in the present sections, most probably represents a combination of both episodic relative sea-level fall and lag deposit due to initial transgression. It is preceded by significant fluctuations of climatic boundary conditions, with cool climates correlating to heavier δ 13C values and increased p/g ratio of dinocysts, suggestive of higher marine primary productivity. Across the K/Pg boundary, a prominent increase and spike of the warm–temperate waters dinocysts fraction is documented, represented mainly by Trithyrodinium evittii, which is accompanied by climatic warming and an episodic relative sea-level rise. Thus, prominent paleo-environmental changes and sea-level fluctuations precede and coincide with the K/Pg boundary proper. These data are inconsistent with a single “catastrophic” impact as the cause for the K/Pg boundary event, but suggest relative longer term environmental stress as finally leading to End-Cretaceous crisis of the biosphere. Between the ED and the K/Pg boundary proper, the gradual increase to peak abundance in trilete spores demonstrates a significant time lag between these two horizons. According to the distinct distribution of this peak abundance, it cannot be excluded that an impact/tsunami event is related to the lower portion of the ED. In contrast, no significant changes within most palynologic proxies are documented across the yellow clay layer below the ED documented within one of the sections and recently suggested as the original Chicxulub impact ejecta horizon. This questions the impact origin of this horizon or, at least, suggests only little consequences of this event on the ecosystem.