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Mancos Shale - Metal Balance in the Cretaceous Seaway

Task Objectives

Metalliferous black shales formed periodically through Phanerozoic time in epicontinental basins. Furthermore, synchronous formation of some black shales around the globe indicates widespread dysoxic to anoxic conditions occurred in the world's oceans. These black shales record dramatic changes in the earth's litho-, hydro-, atmo-, and bio-sphere. Yet, the cause of these anoxic events and the source of associated metals in these shales are still not understood.

This task will explore interrelationships between black shale formation, anoxic events, and secular variations of the world's ocean chemistry. This work will initially focus on black shales that formed in the Cretaceous Western Interior Basin and compare them with Paleozoic metalliferous black shales that were studied by the Great Basin and Brines Projects. Data, concepts, and results of chemical/dynamic modeling and laboratory experiments developed by the Brines project will focus data collection to test hypotheses that might account for the anoxia and metal accumulations in the Cretaceous Interior Basin. Mechanisms identified for anoxia, black shale formation, and metal accumulation in these local basins will be applied to the global system to identify processes/conditions that triggered these dramatic periods of global anoxia.

Highlights & Key Findings

The isotopic, petrologic, geochemical, and biologic of black shales associated with Cretaceous anoxic events are strikingly similar to Paleozoic black shales studied under the Brines and Great Basin projects. Systematic variation between Sr, C, and trace metals suggests a common trigger for these periods of black shale depositions and global anoxia. The high degree of preservation and documentation of Cretaceous black shales combined with high-resolution chemostratigraphy has provided important constraints on the genesis of metalliferous black shales. Box modeling of metal and isotopic cycles have shown that current models are unable to explain metal contents present in Cretaceous black shales and global isotopic shifts that accompany their deposition. These results are consistent with models developed under the brines project that show the involvement of basinal hydrothermal (sedex) systems in the formation of black shales and implicates them as triggers of global anoxia. Integration of data and concepts between projects and tasks are laying the foundation to understand the genesis of these metal accumulations and their interrelationships within global systems.

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