ABSTRACT

Earlier stratigraphic work had predicted that at the type section of the Castlegate Formation, the Castle Gate, near Price, Utah, the unit consists of two sequences separated by a sequence boundary representing approximately one million years of unrecorded time. Although the type section is well exposed, it consists of a monotonous succession of braided fluvial sandstones and no obvious boundary can be identified using facies criteria--this is a good example of a "cryptic sequence boundary." Petrographic data indicate, however, a significant change in detrital composition 20 m above the base of the section, at a through-going erosion surface that is therefore interpreted as the sequence boundary.

Revised sequence correlations, together with other petrographic data and regional paleocurrent patterns, provide the basis for a model of the paleogeographic evolution of the area. Rocks assigned to the Castlegate Sandstone comprise two or possibly three sequences formed at times of slow regional subsidence. Erosional sequence boundaries and tilts in paleoslope between each sequence record thrust loading and unloading of the basin and the growing influence of intrabasinal upwarps, movement of which was beginning to be affected by Laramide movements toward the end of Castlegate sedimentation.