Regional isotopic variations in the Fort Hays Member of the Niobrara Formation, United States Western Interior: Primary signals and diagenetic overprinting in a Cretaceous pelagic rhythmite

Abstract

High-resolution stratigraphic correlation of rhythmically interbedded shale-limestone couplets in the Fort Hays Member of the Niobrara Formation made possible a detailed comparative analysis of individual shale and limestone beds over a large part of the United States Western Interior region. Petrographic and isotopic data suggest that many of the differences observed between shale and limestone beds in the Fort Hays Member are attributable to diagenetic enhancement of originally subtle lithologic variations. Similar oxygen isotopic values from relatively unaltered inoceramid shells, and similar faunal content in limestone and shale beds, indicate that periods of increased runoff and detrital influx into the Western Interior Sea were not accompanied by extensive salinity variations. Inoceramid bivalves in the Fort Hays Member that have been less affected by diagenesis than surrounding matrix provide a sensitive indicator of environmental conditions. Cathodoluminescence and elemental data indicate that elevated levels of manganese occur in generally unaltered inoceramid bivalve remains, mostly in the eastern part of the study area. Manganese enrichment of these inoceramids is associated with increased 18O and decreased 13C contents and is interpreted as primary in origin. Regional east-west gradients in 18O, 13C, and manganese content suggest that the composition of bottom water varied across the Western Interior Sea, possibly in response to a north-ward influx of Tethyan water into the eastern part of the seaway. Similar gradients are observed in shale and limestone beds, indicating that these regional isotopic variations in the Fort Hays Member occurred independently of episodes of increased runoff and detrital influx into the seaway.