FORAMINIFERAL ASSEMBLAGES AND PALEOCEANOGRAPHY OF THE UPPER CRETACEOUS NIOBRARA CYCLE, MESA VERDE, COLORADO

The late Turonian-early Campanian Niobrara Cycle of the Western Interior Seaway is the longest second-order cycle in the upper Cretaceous. Samples were collected from a 680 m section of the Mancos Shale at Mesa Verde National Park, Colorado, proximal to the western edge of the seaway. Three third-order transgressive-regressive (T-R) cycles are recognized within the larger Niobrara Cycle spanning a period of ~9m.y. The upper Turonian Juana Lopez and Montezuma Valley Members of the Mancos Shale represent the first and longest T-R cycle, while the Coniacian- middle Santonian Smoky Hill and the middle Santonian–basal Campanian Cortez Members record progressively smaller T-R cycles. The overlying Point Lookout Sandstone defines the end of the cycle. Due to a major unconformity between the Montezuma Valley and Smoky Hill members, additional foraminifera from the uppermost Turonian–lower Coniacian Rebecca Bounds core in western Kansas were analyzed.

All three transgressions of the Niobrara Cycle are also recognized in Utah. Data from both the Mesa Verde site and the Rebecca Bounds core have been correlated to other localities within the northern (Manitoba) and southern (Texas) portions of the basin.

High planktic:benthic ratios (p:b) mark the transgressions of the Niobrara Cycle at Mesa Verde and reflect the advance of warm Tethyan water masses into the seaway. Increased abundances of benthic infaunal taxa such as Neobulimina also coincide with the Coniacian transgression. Like modern buliminids, the marked increase in this benthic taxon suggests the development of low oxygen conditions. This is further supported by the association with elevated levels of TOC, indicative of poorly oxygenated bottom waters. Dysoxic to anoxic conditions in the seaway are also indicated by foraminiferal-free zones in the transgressive upper Santonian part of the Cortez Member as well as the small size of the foraminifera throughout the section. Dysoxia in the seaway was caused by the incursion of an oxygen minimum zone with rising sea level, or by enhanced salinity stratification, particularly during regression. Widespread dysoxia/anoxia during the Coniacian-Santonian part of the Niobrara Cycle is an expression of Oceanic Anoxic Event 3 of the central Atlantic-Caribbean region.