SEQUENCE STRATIGRAPHY OF THE JURASSIC CURTIS, SUMMERVILLE, AND STUMP FORMATIONS, UTAH-COLORADO

The Middle-Upper Jurassic Curtis, Stump, and Summerville formations of Utah and Colorado contain marine and tidal facies deposited at the southern margin of the Western Interior seaway during Callovian-Oxfordian time. Collectively, these rocks represent one complete depositional sequence and demonstrate the utility of sequence stratigraphy in regional stratigraphic correlations.

In northeast Utah and northwest Colorado, the Curtis Formation contains the transgressive systems tract of the depositional sequence. In this area, tidal channel deposits of the lower Curtis Formation are contained in erosional valleys cut into the underlying Entrada Formation during lowstand development of a regional sequence-bounding unconformity (J3). These incised channels are immediately overlain by fining-upward, tidal flat and storm-influenced shallow marine deposits that make up the upper part of the transgressive systems tract. The Curtis Formation is capped by a conglomeratic lag at the contact with the overlying Stump Formation. This contact represents the surface of maximum marine flooding across the region. The coarsening-upward, shallow marine rocks of the Stump Formation are interpreted as highstand systems tract deposits. The depositional sequence culminated with nonmarine deposition of the overlying Morrison Formation during Late Oxfordian time.

In central Utah, incised tidal-channel deposits of the lower part of the Curtis Formation are immediately overlain by a transgressive lag and the regional maximum flooding surface. Above this transgressive surface, storm- and tidally-influenced shallow marine rocks of the upper Curtis Formation are transitional with tidal flat deposits of the Summerville Formation. As such, in central Utah, the majority of the Curtis-Summerville stratigraphic interval was deposited during the highstand systems tract of the depositional sequence. In the area surrounding the San Rafael Swell, the sequence is capped by the Morrison Formation along a contact, that in places, displays angular truncation of the underlying Summerville Formation. This apparent sequence-bounding unconformity, however, grades into a conformable contact between the Stump and Morrison formations towards the north.