High resolution analysis of unit bar and dune accretion patterns of the Ferron Sandstone Member of the Mancos Shale, Utah

Point bars and braid bars in early studies were interpreted to be fundamental bars in river systems, however, newer studies suggest that unit bars are fundamental bar units and act as a core element for the amalgamation and growth of attached dune sets. Nevertheless, the identification of unit bars in the ancient fluvial system is not well studied. Plan view exposure of a Cretaceous meandering channel belt in the Ferron Sandstone Member provides an ideal example to examine the facies architectural elements of a scroll bar in this channel belt. It provides an opportunity to assess how well drone images can be used to investigate unit bars by high-resolution mapping of numerous rib and furrow structures and ripples. Overall, 100 drone images were taken, with 302 rib and furrow structures identified and 310 paleocurrents measured from these images in three study area. From a detailed quantitative analysis, five large-scale rib and furrow structures 210-360 cm wide were identified, along with smaller and medium-scale features 30-170 cm in width that were interpreted as dune sets. The sizes of large-scale rib and furrow structures are six times larger than the median size. The pattern in rib-width distribution was similar in multiple study areas, where small-scale dune sets accreted and attached to one or two large-scale unit bars that formed earlier. In addition, there was a systematic change in paleocurrent distribution from NNW in study areas 1 and 2 to NNE in study area 3.