Abstract

Isolated sandstone bodies encased in marine mudstones have proved difficult to explain, especially those that are not easily incorporated into conventional facies models. The Hatch Mesa succession (Campanian, lower Kenilworth Member) is a marine mudstone-encased, turbiditic sandstone body, 6 to 20 m thick, that is exposed along a 7-km-long outcrop belt, approximately 15 km east of Green River, Utah, U.S.A. Various interpretations of depositional environment and regional correlation have been proposed over the past 20 years. A sedimentological analysis of the Hatch Mesa succession suggests deposition as a storm-influenced, prodelta turbidite complex on the shallow inner shelf, between fair-weather and storm wave base. This interpretation is corroborated by the high-resolution outcrop correlation and subsequent paleogeographic reconstruction, which indicates deposition 16 to 21 km basinward of the time equivalent lower-shoreface deposits, in about 20 m water depth. A variety of mechanisms are capable of generating instability in the delta front and triggering the turbid flow of sediments into deeper water, including storm events, river flooding, high rates of sedimentation, or earthquakes. This explains the complex mixture of event beds in the Hatch Mesa succession, which are dominated by wave-modified turbidites. 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.