Transgressively-Modified Coarse-Grained Sandstones and Turbidite-Rich Shelf Deposits, Upper Aberdeen to Lower Kenilworth Stratigraphic Interval, Campanian Book Cliffs, Utah: Evidence for Tectonically-Driven Sedimentation Patterns
Simon A.J. Pattison
Brandon University, Brandon, MB
Marine mudstone-encased, inner shelf turbidite bodies are concentrated in a 70 to 90 m thick stratigraphic interval that spans the upper Aberdeen and lower Kenilworth members (Blackhawk Formation, Campanian), Book Cliffs, eastern Utah. The mid-point of this interval (i.e. Aberdeen-Kenilworth contact) is marked by isolated pods of coarse-grained sandstones. These coarse-grained pods are a few meters thick and have a limited areal extent. Angular mudstone clasts, granules, pebbles, bone fragments, shell debris (Baculites, Scaphites, Placenticeras, Inoceramus, bivalves), wood fragments (with a Teredolites ichnofacies) and fish teeth are concentrated in the upper part of each body. The well defined lag deposit, coupled with the blanketing by marine mudstones, provides evidence for transgressive ravinement. This in turn implies that the shoreface (i.e. wave base) passed through this area, both during the falling stage of sea level and subsequent transgression. Paleocurrent data indicates a mean transport direction of N145°E (N=53) and therefore the lowstand shoreline was southeast of the study area. The thin and patchy occurrence of the coarse-grained bodies reveals that the depth of fluvial incision on the exposed shelf was relatively shallow. Interfluves are marked by iron-rich siltstones and mudstones, with shell fragments and discontinuous lenses of very fine- to fine-grained sandstones. A short-term pulse of tectonic uplift and subsidence explains the introduction of the coarse-grained sediments into the basin, transgressive modification of the falling stage/lowstand deposits, and stratigraphic proximity to the turbidite-rich shelf deposits (i.e. generation of shelf turbidites and concentration in a narrow stratigraphic interval).