Investigation of late Maastrichtian faunal and environmental changes in three subsurface wells spanning over 3 km along the Brazos River, Texas, reveals similar minimum diversity high-stress assemblages associated with shallow shelf conditions. Upper Maastrichtian sediments recovered span planktic foraminiferal (CF) zones CF1 to CF4 in well Mullinax-1 to the north and well KT3 at Cottonmouth Creek, and zones CF1-CF2 in Mullinax-3 at Darting Minnow Creek. Biotic stress conditions are demonstrated by the minimum species richness, near exclusion of larger specialized species, dwarfing, and dominance of small generalist taxa.  Faunal assemblages dominated by alternating abundances of the disaster opportunist Guembelitria cretacea (Cushman) (particularly in zones CF4 and CF2-CF1) and heterohelicid species [e.g., Heterohelix globulosa (Ehrenberg), H. planata (Cushman), Paraspiroplecta navarroensis (Loeblich)].. Other small surface and subsurface mixed layer dwellers are rare to common (e.g., hedbergellids, globigerinellids, pseudoguembelinids).

The coincidence of Guembelitria blooms with lithological changes and oxygen and carbon stable isotope excursions may represent discrete episodes of freshwater runoff related to short -term pulses of the latest Maastrichtian (zone CF1-CF2) global climate warming. Climate warming ended in the upper part of zone CF1 with the return to a cooler climate, lower sea level and the formation of incised valleys in a coastal-lagoonal environment. With the subsequent early transgression, incised valleys were infilled by a sandstone complex with reworked impact spherules, as well as lithified clasts with impact spherules up to 80 cm below the K-Pg Boundary. In this environment the shallow inner neritic setting superimposed by sea level and climate changes is the most probable cause for the observed high biotic stress conditions preceding the K-Pg Boundary in the Brazos area.  PDF