60th Annual Report on Research 2015

The Cretaceous Period (145-66 Ma) was generally a time of elevated global temperatures superimposed on fluctuating climate regimes and repeated biotic turnover. It recorded several major perturbations of the carbon cycle, characterized by widespread deposition of organic-rich black shale and benthic and photic zone dysoxia to euxinia, termed oceanic anoxic events (OAEs). The last two of OAEs, the Cenomanian-Turonian OAE2 and the Coniacian-Santonian OAE3 are well-preserved in the Western Interior Seaway (WIS) of North America. The expression of these OAEs in the WIS differs both from each other and from contemporaneous open-ocean sections. Despite decades of research in this region, questions remain about the role of oceanographic parameters (sea level, water mass source and character, terrestrial runoff, stratification, productivity, circulation) on the development (or not) of organic-rich shales and anoxic to dysoxic conditions in the WIS. This study utilizes foraminiferal paleoecology and bulk rock geochemical data to address the broad question “How did oceanographic changes effect the development of OAEs in the Western Interior?”

A transect of sites in Texas is used to understand the connection between the well-studied OAE2 interval of the WIS (Greenhorn Formation) and the open ocean, which appears to have been controlled by a sill at the southeast margin of the Comanche platform that was overrun by rising sea level in the late Cenomanian (~94 Ma), ventilating the seaway. The poorly understood development of OAE3 in the central Western Interior is studied in the Niobrara Formation in Colorado, Kansas, and New Mexico and equivalent rocks further west. Foraminiferal trends through this interval show a slow increase in dysoxia prior to the “onset” of OAE3 (~89 Ma) recorded by other proxies. Above this level, following a few declining fluctuations, benthic foraminifera disappear and a stressed planktic assemblage remains unchanged for ~3 million years. This suggests a threshold for anoxia existed in the basin that, once exceeded, pushed the WIS into a new equilibrium. Trends observed during both of these anoxic events suggest that relative sea level change is the principle control on the redox state of the seaway: transgressions are associated with continually improving oxygenation due to improved connections to the open ocean and better ventilation of the seaway, and regressions are associated with deteriorating oxygenation and the deposition of organic carbon due to runoff from both margins, delivery of nutrients, increased productivity, and greater stratification of the water column.

Sea level, coupled with runoff, productivity, water column stratification, and basin restriction, are the primary controls on the development of anoxia in the Western Interior Sea during the Cenomanian-Campanian. The restricted nature of the seaway and its dependence on outside water masses to drive circulation meant that the local sea level forcing greatly overwhelmed any global trends toward increasing or decreasing dissolved oxygen. During the latest Cenomanian OAE2, rising sea level ventilated the seaway during a time when large parts of the open ocean were anoxic and even euxinic. During the Coniacian-Campanian OAE3, falling sea level choked off circulation, driving increased stratification and an expanding oxygen minimum zone, during a time when large parts of the open ocean, particularly the eastern Tethys, were experiencing increasing oxygenation.

During both the Cenomanian-Turonian Greenhorn Cycle and the Turonian-Campanian Niobrara Cycle, rising sea level is associated with increasing ventilation, an abundant, diverse benthic community, and the deposition of carbonate facies, particularly in the central and eastern parts of the seaway. Times of falling sea level are associated with increasing stratification, declining benthic population and diversity, and the deposition of organic-rich facies. During both these sea level trends, the seaway exhibits threshold behavior: during transgression, conditions (salinity and oxygenation, primarily) improve gradually until sea level rise passes some critical threshold to allow an ideal volume of open ocean water into the seaway and geologically instantaneous improvement occurs. This is exemplified by the base of the Bridge Creek Limestone in Colorado (“Bed 63”) and its equivalent, the base of the Scott Ranch Member of the Eagle Ford Shale in west Texas in the sudden abundance of benthic foraminifera (the “Benthonic Zone”) and the change in lithology from organic rich dark shale to organic poor limestone and marl. Multiple transgressive unconformities in the central seaway make a similar horizon more difficult to pinpoint during the transgression of the Niobrara, although the Ft. Hays Limestone Member represents peak transgression or highstand, and the best ventilated conditions of the Niobrara cycle. During regression and deposition of the Smoky Hill Member, the same trend occurs in reverse: conditions deteriorate gradually until sea level fall passes some critical threshold restricting the inflow of open ocean water, and conditions almost instantaneously deteriorate. This is exemplified at the base of the lower shale unit of the Smoky Hill Chalk Member of the Niobrara Formation in Colorado with a sudden increase in TOC and trace metal markers for euxinia and the sudden disappearance of benthic and deeper-dwelling planktic foraminifera.

Orbitally-forced wet/dry climate cycles have long been invoked to explain some or all of the paleoceanographic change in the Western Interior Seaway. There is certainly an orbitally-derived signal recorded in the rocks of the Greenhorn and Niobrara Formations, and in an ice-free world like the Cretaceous, insolation changes were most likely expressed as changes in precipitation. However, these changes appear limited to higher-frequency cycles overprinted on the 4-5 myr 2^nd-order and 1-2 myr 3^rd-order tectonically driven sea level cycles. Shallow-dwelling planktic foraminifera should show a strong response to changes in surface salinity that would accompany increased precipitation and runoff. These taxa do vary but at a high-resolution and low-amplitude, and do not track observed changes in benthic conditions or lithology, suggesting that while regional precipitation did fluctuate enough to imprint on the rock record, it was not the driver of the major observed changes. This interpretation is supported by clay mineralogy, which appears to track changes in the strength of circulation and not freshwater runoff.