SEDIMENTOLOGY AND STRATIGRAPHY OF SUPER-CRITICAL SEDIMENT GRAVITY FLOW DEPOSITS IN THE UPPER CRETACEOUS MANCOS SHALE, EASTERN UTAH

The Mancos Shale is a marine stratigraphic unit deposited in the Western Interior Seaway during Late Cretaceous time. While generally dominated by low energy, offshore deposits, there are anomalous sand-rich intervals within the Aberdeen and Kenilworth Members of the Mancos that have been a subject of study since the late 1970s. Recently, new flume work and density-current hydrodynamic studies suggest that sedimentary structures like those seen in the Mancos can be generated by the bedforms of sub- and supercritical turbidity currents below storm wave base. With these new studies in mind, a detailed field study of the Upper Mancos was undertaken on sandstones near Hatch Mesa which have been previously correlated with Kenilworth depositional sequences in the Book Cliffs.

Over the course of several weeks, 22 sections were measured detailing and documenting the sedimentary structures as well as grain size and thickness changes while hundreds of paleocurrent measurements were also taken. For larger scale analysis, thousands of photos were taken utilizing a drone to create photomosaics for the 9.8 km long outcrop to study the bedding geometries and compare them with similar flume-generated bedforms and structures. Finally, point cloud models were created using drone assisted photogrammetry to study the three-dimensional architecture of these intervals.

Ultimately, this investigation revealed paleocurrent flow direction and sediment transport was dominantly E/NE with some variability which matches previous studies. While some of the sedimentary structures present matched the flume studies descriptions, the similarities between hummocky cross stratification and combined-flow current ripples with downstream-migrating short antidunes and other supercritical bedforms as well as the presence of soft sediment deformation warrant further investigation. Photomosaics created that delineate the sandstones show pinch and swelling macroforms much like large-scale antidunes. This study is an important step in understanding the field characteristics of these flows and providing more insight into the origins and architecture of similar deposits globally. Finally, it will help to better define depositional environments, paleogeography, and the sea level of the Western Interior Seaway during the Late Cretaceous.