FORAMINIFERAL RECORD OF OCEANOGRAPHIC AND BIOTIC CHANGES IN THE WESTERN INTERIOR SEAWAY BEFORE AND DURING OAE3

Global perturbations of the carbon cycle, characterized by widespread deposition of organic-rich black shale and a positive excursion in carbon isotope curves, occurred intermittently throughout the Cretaceous period. The last of these Oceanic Anoxic Events (OAEs), OAE3, is both more limited (with black shale deposition mainly in restricted basins and isotope excursions of 0.5-1.0‰) and more protracted (>1 myr) than earlier OAEs. This interval also coincides with increased carbonate productivity and chalk deposition in many shallow seas. Both of these characteristic attributes, organic carbon deposition and chalk deposition, occur in the Turonian-Campanian Niobrara Formation of the US Western Interior Seaway, making it an ideal location to study the ways in which changes in biota and ocean circulation record the development of OAE3. We present data on foraminiferal paleoecology and biostratigraphy from three sites (an outcrop of the Niobrara-equivalent Austin Chalk in Terrell Co., TX, the USGS Portland Core in Fremont, CO, and a composite outcrop section in western KS) to quantify paleoenvironmental variability across the Niobrara Formation and understand how circulation or runoff/precipitation changes may have lead to increased organic matter preservation in the Western Interior Sea.

The lithology of the Niobrara Formation (especially carbonate content) varies strongly both east-west and north-south, with the purest chalks deposited in a belt from west Texas to central Kansas. Foraminiferal paleoecology varies greatly across the Niobrara as well, and is often tied to lithological changes. For example, chalk units in the Fort Hays Limestone Member have a higher percentage and diversity of benthic foraminifera than the thin interbedded dark gray shales; the chalks also have a higher percentage of biserial planktic foraminifera, a generalist group that typically dominates low-diversity assemblages, suggesting a high productivity setting. The OAE3 interval of the Smoky Hill Chalk Member can be generally described as being dominated by a low diversity planktic assemblage with very few to no benthic foraminifera. This is coincident with increased organic matter content, and suggests dysoxic to anoxic conditions on the seafloor created by increased stratification.