The Black Mesa Basin of northeastern Arizona contains the stratigraphic record of the transgression and regression of the Greenhorn Sea during late Cenomanian to late middle Turonian time. The transgressive phase of the cycle is much thinner than the regressive phase. Foraminiferal data suggest that peak transgression of the Greenhorn Sea occurred during mid-early Turonian to earliest middle Turonian time ( Mammites nodosoides to perhaps basal Collignoniceras woollgari Biozone) in this area of the interior seaway. The Cenomanian/Turonian boundary interval is marked by dynamic changes in the population structure of planktonic foraminifera and in foraminiferal abundance. It is suggested that an oxygen minimum zone (OMZ) within the water column was established in the Black Mesa area during this time, based on the marked increase in relative abundance of Heterohelix. The OMZ gradually waned through the time of peak transgression. Clay mineral, sedimentological, and foraminiferal data suggest regional climatic warming and increased weathering accompanying the northward advance of warm, normal marine waters during transgression. Cooling, decreased intensity of chemical weathering (leaching), and a slight decrease in oceanic salinity in the Black Mesa area characterized the subsequent regression. These data are also used to reinterpret Upper Cretaceous depositional history in the Black Mesa Basin.