AN ISOLATED, STORM-INFLUENCED, PRODELTA TURBIDITE COMPLEX IN THE LOWER KENILWORTH MEMBER, HATCH MESA, BOOK CLIFFS, UTAH

The marine mudstone-encased, Hatch Mesa succession (lower Kenilworth Member, Campanian, Upper Cretaceous) is located in the southern part of the Book Cliffs, approximately 15 km east of Green River, Utah. Relative to most other marine mudstone-encased isolated sandstone bodies, the Hatch Mesa succession has an abundance of Bouma-like turbidite beds, a greater proportion of mudstone and siltstone, is thinner (6-20 m), and is weakly coarsening-upward. Background deposits consist of non- to mildly-bioturbated, pin-striped mudstones and sandy siltstones, with a paucity of wave ripples, suggesting deposition below fair weather wave base. In contrast, the sandstones represent high energy event beds that punctuate the quiet water, background conditions. These sandstones have a great diversity of vertical stratification styles and sedimentary structures, and are dominated by turbidite beds, with lesser amounts of storm-generated beds. Wave-modified turbidites, hummocky cross stratified sandstones, classical turbidites, and hyperpycnal flow-derived turbidites are all observed within the Hatch Mesa succession. These attributes, in combination with locally abundant carbonaceous matter, convolute bedding, pinch and swell geometries, a lack of clinoforms, and a low diversity/abundance trace fossil suite, suggest deposition in a prodelta environment, between fair weather and storm wave base.

Regional correlation has demonstrated that the Hatch Mesa succession is time equivalent with the second parasequence in the Kenilworth (KPS2), which is characterized by inner shelf turbidite bodies at a number of other localities. Deposition occurred at least 15 km basinward of the time equivalent KPS2 lower shoreface deposits in a mud-rich prodelta setting. A three-component shoreface-to-shelf model, consisting of a delta front, subaqueous channel and a prodelta turbidite complex, is proposed to explain the depositional setting and environment of the Hatch Mesa succession. Shallow marine facies models should be revised to include isolated prodelta turbidite bodies in some inner shelf settings.