FORAMINIFERAL ASSEMBLAGES OF THE LIMESTONE BEDS OF THE CENOMANIAN – TURONIAN BRIDGE CREEK LIMESTONE, ROCK CANYON SECTION, PUEBLO, CO: PRODUCTIVITY VS. DILUTION

During the late Cenomanian and early Turonian (~94-93 Ma), the Greenhorn transgression flooded the large asymmetrical foreland basin of western North America to form a shallow seaway. This allowed the deposition of rhythmically bedded sequences including organic-rich strata of the Greenhorn Formation and its equivalents. These strata are linked to the most globally widespread Oceanic Anoxic Event (OAE 2) spanning the Cenomanian – Turonian Boundary (C-T). OAE 2 is a short-lived event (<1 myr) characterized by a significant positive in δ13C of organic carbon and carbonate, and by micro- and macrofaunal extinctions and diversifications. The limestone-marl bedding couplets of the Bridge Creek Member of the Greenhorn Formation are characterized by fluctuations in biofacies and organic carbon. Unlike limestone beds, foraminiferal assemblages from easily disaggregated marlstone and calcareous shale of the Bridge Creek Member at the Rock Canyon section have been qualitatively and quantitatively extensively analyzed in a number of studies. In this study extracted foraminiferal assemblages from limestone beds resulted in a quantitative evaluation of foraminiferal response to cyclically changing conditions that allowed deposition of limestone-marl couplets. The results show no systematic response of foraminiferal assemblages extracted from limestone beds, which may indicate different mechanisms or conditions responsible for depositing these limestones. Furthermore, for any given limestone–marl couplets, there are no major changes in foraminiferal population counts between that of the limestone beds and the adjacent marly intervals. However, there are some minor differences in foraminiferal assemblages that characterize limestone beds from the adjacent marlstone beds. These include disappearance of planktic planispiral morphotypes, slight increase in the proportion of planktic biserial and triserial morphotypes, and slight increase in the proportion of benthics relative to total foraminifera. Such conditions suggest that the limestones may have been (slightly) more productive than the adjacent marlstones. Reduced salinity or greater stratification of the upper water column may have also contributed to the minor differences with assemblages preserved in the marlstones and calcareous shales.