Abstract

The Curtis and Summerville Formations comprise an unconformity bounded interval between the underlying Entrada Sandstone and overlying Morrison Formation. They comprise the upper part of the Middle Jurassic (Callovian) San Rafael Group exposed along the San Rafael Swell in east-central Utah. The sedimentary history of the Curtis was initiated by marine flooding followed by coastal onlap of a partially stable, epicratonic platform near the eastern margin of the Cordilleran mobile belt. Tidal-and wave-currents transported sediment in nearshore shelf and marginal marine settings at the southeastern limit of the Middle Jurassic sea. Farther east, an emergent coastal plain was sustained by coastal sabkha and eolian processes. The sandstone-mudstone facies and composite sandstone facies of the Curtis indicate a progressive decrease in water depth, and increase in current energy and sand transport on a subtidal to intertidal platform. Deposition of the rippled silty facies in the Curtis marks a transition from sand-rich, high energy conditions to silt- and mud-rich, restricted, evaporative, hypersaline conditions during deposition of the Summerville. Reddish-brown silty facies and gypsiferous facies in the Summerville were deposited in the supratidal stage of the upward-shallowing cycle in the Curtis-Summerville interval. Spring tides, storm/hurricane washover, and wadi flooding built a siliciclastic coastal sabkha. Hereafter, withdrawal of the Middle Jurassic sea from Utah was succeeded by erosion and terrestrial sedimentation in the Morrison Formation.