DEPOSITIONAL ARCHITECTURE AND SEQUENCE STRATIGRAPHY OF THE LOWER CRETACEOUS DAKOTA FORMATION, NORTHERN UINTA BASIN, UTAH AND COLORADO

The late Albian Dakota Formation in the northern Uinta Basin consists of fluvial channel, overbank, estuarine, and marine deposits. Based on the architectural arrangement of channel and estuarine/overbank deposits observed in outcrops, the Dakota Formation contains two depositional sequences. The first Dakota sequence (Kd1) is up to 40 m thick and consists of fluvial channel sandstones and conglomerates, as well as overbank and estuarine sandstone, siltstone, and mudstone. The lowstand fluvial strata of the Kd1 interval were deposited above a regional erosion surface incised as much as 20 m into the underlying Lower Cretaceous Cedar Mountain Formation. Fluvial channel sandstone, overbank mudstone and estuarine sandstone/mudstone in the upper parts of the Kd1 interval were deposited during transgressive and highstand systems tracts of the sequence.

The second Dakota sequence (Kd2) is ~8-45 m thick, and contains lithologies similar to the Kd1. The Kd2 was deposited above a surface that was incised up to ~30 m into the underlying Kd1 during development of a lowstand unconformity during latest Albian time. In rare instances, the entire Kd1 is eroded and the Kd2 rests directly on the Cedar Mountain Formation. In these areas, laterally and vertically amalgamated Kd2 sandstones/conglomerates form valley-fill complexes up to 1 km wide and 30 m thick. These paleovalley fills represent the lowstand and early transgressive systems tract deposits of the sequence. The upper part of the Kd2 consists of up to 15 m of tidal and marine sandstone, conglomerate, mudstone, and shale deposited during the late stages of the Kd2 transgressive systems tract. The late Albian/early Cenomanian Mowry Shale overlies the Dakota Formation across the study area. In outcrop, the Mowry Shale is 25-35 m thick and is composed of siliceous marine shale, carbonaceous mudstone, siltstone, and bentonite beds. These deposits correlate with upward-coarsening shoreface siltstones and sandstones in the subsurface portion of the study area, and represent highstand marine deposits of the Kd2 depositional sequence.