Investigation of Temporal and Spatial Changes in Gretaceous Paleoredox Conditions During Ocean Anoxic Events within the Colorado Section of the Western Interior Seaway

Abstract

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 that are important petroleum source rocks. However, local variations in source rock quality such as total organic matter, organic matter type, thickness, and lateral extent within the KWIS have been observed. The principal hypothesis is that deposition of organic-rich source rocks was constrained by more localized episodic restrictions of ocean water circulation, stratification, nutrient supply, and volcanism within select areas of the KWIS and perhaps Proto-Atlantic Ocean. The influence of the OAE II and OAE III 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 from Colorado will be analyzed using petrographic and geochemical methods. 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 and develop a regional map of organic facies that will ultimately be used to show areas favorable for exploration.

AAPG Datapages/Search and Discovery Article #90351 © 2019 AAPG Foundation 2019 Grants-in-Aid Projects