SIGNIFICANCE OF INNER SHELF TURBIDITIC-RICH CHANNEL-FILL DEPOSITS, GUNNISON BUTTE TO TUSHER CANYON REGION, BOOK CLIFFS, EASTERN UTAH

Marine mudstone-encased, inner shelf turbiditic channel-fill deposits are concentrated in a 30 to 40 m thick interval in the upper Aberdeen Member (Blackhawk Formation, Campanian), Book Cliffs, eastern Utah. These deposits are 1 to 9 m thick and occur at two distinct levels, forming a series of low lying ridges and hills along the desert floor in the Willow Bend to Gunnison Butte area and along the east side of the Green River at the confluence of Gray and Tusher canyons. Channels are sharp-based and consist of a multistory package of up to five nested channel-fill units, with sharp and erosional contacts. Mudstone-, heterolithic- or sandstone-rich channel-fill deposits are observed, with concave-up beds draping channel margins, low angle accretion surfaces, pinch-and-swell sand body geometries, onlap and downlap.

The dominant facies are pin-striped mudstones, siltstones and very fine- to fine-grained sandstones. Sandstone beds are sharp-based, 1 to 50 cm thick, and are planar- to ripple-laminated, Bouma-like Tbc sandstone beds. Both current and combined flow ripples are observed. Low angle HCS sandstones and rare wave ripple-laminated sandstones suggest deposition above storm wave base. Stacked sets of planar- and ripple-laminated sandstones are interpreted as hyperpycnal-flow derived turbidites or hyperpycnites. The abundance of finely comminuted plant material, low diversity trace fossil assemblage, mild degree of bioturbation, and Inoceramus and Baculites shell fragments indicates fresh water/terrestrial input in a shallow marine environment. Paleocurrent data shows a mean transport direction of N108°E (N=343), which is orthogonal to the upper Aberdeen paleoshoreline trend (N14°E).

The upper Aberdeen channel-fill deposits are interpreted as subaqueous channels that fed sediments from the delta front into the prodelta region. The balance of the sedimentological evidence indicates that the channels were cut and filled in a proximal inner shelf setting by high and low density underflows. Some of these flows appear to be linked to storm activity (i.e. low angle HCS sandstones), while others are linked to river flooding events (i.e. hyperpycnites). Both have the potential to cut scours or channels on the inner shelf, and to transport silt and fine sand into deeper water.