INVESTIGATION OF CHANGES IN CRETACEOUS PALEOREDOX CONDITIONS DURING OCEAN ANOXIC EVENTS WITHIN THE COLORADO SECTION OF THE WESTERN INTERIOR SEAWAY

Ocean anoxic events (OAE), such as the Cenomanian-Turonian (OAE II) and Coniacian-Santonian (OAE III) events identified within Cretaceous deposits of the Western Interior Seaway (KWIS), are described as periods of profound chemical changes in the ocean water column that resulted in deposition of widespread, thick, organic-rich black shales. Each of these anoxic events is believed to have generated geochemical signatures and coeval depositional facies in temporally equivalent strata across the KWIS and perhaps Proto-Atlantic. The principal hypothesis is that deposition of organic-rich sections was constrained by more localized episodic restrictions of ocean water circulation, stratification, nutrient supply, and volcanism within specific areas of the KWIS, but instead occurred throughout the KWIS and perhaps the Proto-Atlantic Ocean.

Despite being some of the most studied ocean anoxic events in the world, the influence of these ocean anoxic events on the temporal and spatial variations of paleoredox conditions in the Cretaceous water column within the Western Interior Seaway is not well understood. Literature focuses mainly on the controls of vertical variations in geochemistry within a stratigraphic column with little or no reference to spatial variation across the Cretaceous Western Interior Seaway. In this study, both outcrop and core data of the Graneros Shale, Greenhorn Limestone, Carlile Shale and parts of the Niobrara Formation will be analyzed using thin sections, scanning electron microscopy (SEM), x-ray fluorescence (XRF), gas chromatography combustion isotope ratio mass spectrometry (GC/C/IRMS), rock eval pyrolysis, and wireline well log data. Variations in paleoredox conditions that resulted in temporal and spatial differences in source rock quality within the Cretaceous Interior Seaway are to be characterized to better understand the controls on organic carbon distribution within Middle to Upper Cretaceous strata. Once established, controls on organic carbon distribution will be used to develop a regional map of organic facies that will ultimately be used to predict areas favorable for exploration.