A STRAY SANDSTONE FINDS A HOME: TIME EQUIVALENCY OF THE HATCH MESA SUCCESSION AND THE LOWER KENILWORTH MEMBER, BOOK CLIFFS, UTAH

Isolated or stray sandstone bodies of enigmatic origin are scattered throughout the Mancos Shale in eastern Utah and western Colorado. One example is the marine mudstone-encased Hatch Mesa succession (lower Kenilworth Member, Campanian), which is a 6 to 20 m thick, turbiditic sandstone body that is located 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, and is weakly coarsening-upward. These attributes, in combination with locally abundant hummocky cross stratified sandstones, a paucity of wave ripples in the background fair-weather deposits, carbonaceous matter in all facies, a lack of clinoforms, and a low-diversity and low-abundance trace fossil suite, suggest deposition as a storm-influenced prodelta turbidite complex between fair-weather and storm wave base. High-resolution outcrop data reveals that the Hatch Mesa succession is time-equivalent to Kenilworth parasequence 2 (KPS2). Paleocurrent data indicates that the Hatch Mesa succession and lower Kenilworth Member turbidity currents flowed N72°E. This is orthogonal to the strike of the KPS2 paleoshoreline trend (i.e. N18°W). The paleogeographic reconstruction shows that the Hatch Mesa succession was deposited at least 16 km basinward of the time-equivalent lower shoreface deposits, in approximately 20 m of water. A three-component model, consisting of delta-front, subaqueous channel, and prodelta turbidite deposits, is proposed to explain the depositional environment and setting of the Hatch Mesa succession. All three components are observed in the lower Kenilworth Member to upper Aberdeen Member stratigraphic interval. The results of this study indicate that shallow marine facies models should be revised to include marine mudstone-encased prodelta turbidite complexes, thus adding one more possibility to the diverse suite of interpretations used to explain the generation and preservation of isolated marine sandstone bodies. These results also shed new light on the stratigraphic position and depositional setting of the Mancos Shale-encased, isolated sandstone bodies of the Prairie Canyon Member in eastern Utah and western Colorado.