Sedimentological, paleontological, and geochemical evidence suggests that the platform was drowned near the C/T boundary as a result of changing environmental conditions. Pelagic-rich strata overlying the drowning surface are highly bioturbated, and concentrations of redox-sensitive elements (Mn, Fe, Cr, Cu, and Zn) indicate deposition under well-oxygenated conditions. The pelagic sediment contains abundant calcispheres, which are interpreted to be opportunistic organisms that inhabit eutrophic, but unstable, environments. Elevated concentrations of Ba, P, and Si (in the form of chert nodules) observed in the pelagic facies are typical of modern and ancient upwelling systems. Petrographic evidence for oxygen deficiency is limited to the lower slope region. The warm mid-Cretaceous clim te and a global rise in sea level resulted in changes in oceanic circulation patterns, increased organic productivity, and a shift in the trophic structure to one that favored planktonic productivity over benthic productivity. Pelagic deposition persisted for approximately 1 to 1.5 m.y., longer than the OAE. The time difference between the end of the OAE and the reestablishment of a shallow, benthic platform is attributed to increased accommodation caused by the Turonian sea-level rise and tectonic subsidence.