--> Abstract: Enrichment of Organic Carbon and Carbonate in the Upper Cretaceous Niobrara Formation, Western Interior Basin: The Role of Siliciclastic Flux, by Locklair, Robert E. and Bradley B. Sageman; #90071 (2007)

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Enrichment of Organic Carbon and Carbonate in the Upper Cretaceous Niobrara Formation, Western Interior Basin: The Role of Siliciclastic Flux

Locklair, Robert E. and Bradley B. Sageman
Northwestern University, Evanston, IL

     The Cretaceous is characterized by intervals of widespread organic carbon enrichment in marine sediments and relatively high sea levels. The Upper Cretaceous is noted for expansive deposits of coccolith-rich sediment that accumulated in deep ocean basins, flooded shelves, and epicontinental seaways to produce calcareous oozes, chalks and marls. Both organic-rich and carbonate-rich marine sediments were deposited in the Western Interior basin of North America during the Late Cretaceous. Although there are many organic-rich units in the Cretaceous offshore section of the Western Interior, there are relatively few that are both organic-rich and carbonate-rich. The Upper Turonian-Lower Campanian Niobrara Formation has the highest average carbonate content of the Cretaceous section and is also one of the most organic-rich. The confluence of high TOC (source rock) and high carbonate content (brittleness) plays a key role in making the Niobrara Fm. an economic gas play. Understanding the controls on and variability in carbonate and TOC may help in the development of better production models for other post-Paleozoic gas shales. In this study, we characterize organic and carbonate content of the Niobrara Fm. and evaluate the primary depositional controls on enrichment. Accumulation rates are also calculated via frequency analysis of rhythmic bedding identified throughout the formation. We also focus on the role of siliciclastic supply as a source of dilution. Siliciclastic flux is evaluated in terms of sediment supply, transport pathways, and depocenters. Mud flux in the offshore is further scrutinized in the context of relative sea level change and multi-millennial changes in climate.

 

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah