Abstract

The depositional history of the “lower Sundance” Formation (Upper Jurassic-Callovian) of the eastern Wyoming region is interpreted in terms of a transgressive-regressive sedimentologic cycle. The transgressive Canyon Springs Sandstone consists dominantly of large-scale cross-bedded sands which contain many features suggestive of deposition as large, tidally influenced submarine sand waves. Thinner, flat-bedded sandstones which are locally oolitic, formed on the crests of these sand bodies in higher flow regime and locally shoaling conditions. Silts and shales accumulated in inter-sand wave swales. The overlying Stockade Beaver Shale represents an offshore marine deposit which is transitional upward into the regressive Hulett Sandstone. In the northern Black Hills region, the flaser-bedded transition facies is overlain by a coarsening-upward sequence of bioturbated, small-scale cross-bedded, and low-angle laminated sands inferred to represent a prograding barrier island. Silty and sandy lagoonal sediments overlie the beach deposits. The equivalent sequence in the southern Black Hills is thinner, and the transition facies, which here contains structures indicative of emergence, is overlain by the silty and sandy lagoonal facies. The Hulett of south-central Wyoming consists of a silty cross-bedded facies interpreted as an offshore bar complex, which becomes more shaly westward. Redbeds of the Lak Member form the top of the sequence. Stratification of any kind is absent in this unit. The red color of the rock, its gradational position above a barrier complex in a regressive sequence and total lack of fauna suggest a non-marine origin. While the genesis of the redbeds is problematical, its silty texture and lack of stratification suggest an eolian loess origin as plausible.