Detailed Thin-bedded Facies Analysis of Mancos C in the Upper Mancos Shale, New Mexico

Abstract

Fine grained sediments were common in epicontinental seas, with shallow slopes, such as the Cretaceous Western Interior Seaway. However, proposed mechanisms for offshore mud transport, such as turbidity currents, tempestites, and hyperpycnal flows, require significant slopes. A core from the Upper Mancos Shale, Mancos C, located in the San Juan Basin of New Mexico was analysed to determine the dominant transport processes bringing sediment offshore. A detailed facies analysis was conducted, over 54 m of slabbed core, using sedimentological data, such as grain size, type of sedimentary structures, bed thickness, lithology, clay content, fossils, ichnofacies, and degree of bioturbation. The facies observed in the core show that multiple processes, including ignitive turbidity currents, hypopycnal and hyperpycnal flows, and tempestites, were responsible for the deposition of the Mancos C core. The resuspension of mud on the inner shelf by storm waves also played a key role in moving mud further offshore. Tidal influence within the Mancos C was relatively small.