Rhys Hamlyn, Kévin Boulesteix , Kevin G. Taylor, Stephen S. Flint, Rhodri M. Jerrett

Mudstone studies that use a large spacing (>5km) between datapoints provide little insight into the architecture and depositional process variability at kilometre scale. This often leads to the assumption that mudstones are laterally homogenous at kilometre and sub-kilometre scales. To better understand the lateral variability of mudstones at kilometre scale, the Mancos Shale in the eastern part of Utah, USA (Book Cliffs) was examined. The Mancos Shale was deposited on a storm-dominated, low-angle shallow-marine ramp along the western shoreline of the Western Interior Seaway. Here, a combined petrographic, sedimentological and sequence stratigraphic approach was used to analyse two 43-47 m thick time-equivalent sections at a 1.6km across-strike spacing. Oriented samples were collected at 2m intervals through each measured section. Five 5m – 12m thick coarsening-upward parasequences were identified. The lower four parasequences are interpreted to be the distal deposits of the updip Aberdeen Member and the fifth parasequence is interpreted to be the basal distal parasequence of the overlying Kenilworth Member. Depositional processes including turbidity currents, linked debris flow/turbidity currents, wave-enhanced sediment-gravity flows, hyperpycnal flows and suspension settling have been identified from the samples collected. The products of these flows (beds) stack to form bedsets which display an upward increase in the lateral continuity of beds and a coarsening-upward trend. This is interpreted to reflect a change from occasional current-deposited beds with depositional breaks to more frequent varying energy conditions with fewer hiatuses. These bedsets show considerable spatial and temporal variability which is interpreted to be a result of subtle seafloor topography. Based on the bedset and parasequence stacking patterns and the temporal variability of depositional processes, the lower four parasequences are split into two parasequence sets. The lower parasequence set is interpreted to be storm-wave-dominated with fluvial influence and the upper parasequence set lacks the fluvial influence.