Institut de Ciències de la Terra "Jaume Almera," CSIC
Geologica acta vol:7 issue:1-2 pages:11-34
The Antioch Church core from central Alabama, spanning the Cretaceous-Paleogene (K-P) boundary, was investigated by a multi-proxy approach to study paleoenvironmental and sea level changes within the wellconstrained sequence stratigraphic setting of the Gulf of Mexico margin. The Antioch Church core comprises the Maastrichtian calcareous nannoplankton Zone CC25 and the Danian Zones NP1 to NP4 corresponding to the Maastrichtian planktonic foraminifera Zones CF3 and the Danian Zones P1a to P2. Facies shifts from a Maastrichtian siliciclastic to a mixed siliciclastic-carbonate depositional system during the late Danian. Sedimentary proxies indicate that depositional settings changed between littoral (foreshore) and inner and middle neritic (offshore transition zone). Four sedimentary sequences, each encompassing LST, TST, and HST were identified. Estimated water depths by using benthic foraminiferal assemblages were not exceeding 20-40 m for the Maastrichtian and 0-40 m for the Danian sequences. The succession of facies shifts within systems tracts can be very well disentangled by major and trace element data as well as by various element ratios including Zr/Rb, (Zr+Rb)/Ca, and Sr/Ca. By applying element stratigraphy, the ambiguities of the natural gamma ray log –with peaks associated either with maximum flooding surfaces or with silty lag deposits (“placer silts”) during the late regressive HST– are resolved. In addition, the Zr/Rb ratio provides a good proxy for monitoring grain size distribution
and sorting effects. According to the Antioch Church core data, the K-P boundary is associated with a sandstone event bed that includes ejecta spherules from the Chicxulub impact. However, the genesis of the K-P event bed, whether lowstand, tempestite- or tsunami-related, cannot be resolved from this core. In terms of clay mineralogy, the studied interval is characterized by a steady increase in smectite that parallels a decrease in kaolinite with the latter disappearing about two My after the K-P boundary during Biozone NP2. This change in the clay mineral assemblage, which is almost independent of lithology, may suggest a long-term shift from stable, tropical warm and humid climates during the latest Maastrichtian to warm climate with alternating humid and arid seasons in the middle Danian.