SEDIMENTOLOGY OF POSSIBLE SUPER-CRITICAL SEDIMENT GRAVITY FLOW DEPOSITS IN THE UPPERCRETACEOUS MANCOS SHALE, EASTERN UTAH

The Mancos Shale is a marine stratigraphic unit that was deposited in the Western Interior Seaway during Late Cretaceous time. Generally dominated by low-energy mudrocks, anomalous sand-rich intervals within the Aberdeen and Kenilworth Members have been a subject of study since the 1970s. Recent flume work and density-current hydrodynamic studies suggest that sedimentary structures like those seen in the sand-rich intervals can be generated by the bedforms of sub- and supercritical turbidity currents below storm wave base. With these studies in mind, a 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 several weeks, 22 stratigraphic sections were measured through the Hatch Mesa interval documenting the sedimentary structures, grain size, and bedding-thickness characteristics. Hundreds of paleocurrent measurements were also recorded from ripple cross-lamination and sole marks at the base of sandstones. To document the overall depositional architecture of the interval, thousands of photos were taken with a UAV along the 9.8 km outcrop exposure of the Hatch Mesa interval in the study area. Photomosaics were then constructed to study the bedding geometries and compare them with similar flume-generated bedforms and structures. In addition, point-cloud models were created using drone assisted photogrammetry to study the three-dimensional architecture of these intervals.

The photomosaics produced reveal macroforms in the sandstones with amplitudes of ~2 meters and wavelengths of ~40 meters. In addition, 3D point-cloud models reveal that the crests of these macroforms trend NW/SE, an orientation that is roughly perpendicular to measured paleocurrent orientation directed towards the NE. This geometry and lack of distinct cross bedding within the macroforms suggests they may have been produced by large-scale antidunes. Future work will further expand models and photomosaic coverage to the rest of the study area with the hope of yielding more insight into the architectural characteristics of these deposits. This study will ultimately help to better define depositional environments, paleogeography, and the sea level of the Western Interior Seaway during the Late Cretaceous.