Abstract

The Castlegate Sandstone at its type section, Price Canyon, near Price, Utah, encompasses a single stratigraphic sequence spanning approximately 5 m.y. It includes a sandstone member corresponding to a lowstand systems tract, consisting of braided-fluvial sheet sandstones, and a mudstone member, in which shales are more abundant and some evidence of tidal influence is present. This is the transgressive to highstand systems tract. From near Trail Canyon eastward the mudstone member passes laterally into the Sego Sandstone and Neslen Formation, a succession of at least six higher-frequency sequences of fluvial-estuarine origin. The Buck Tongue, a marine shale unit separating the Castlegate Sandstone and the Sego Sandstone east of Green River, is erosionally truncated below the Sego Sandstone northwest of Trail Canyon. We suggest that the origin of the sequences is related to flexural loading and intraplate stress on two time scales. Eustasy cannot be ruled out, but there is no independent evidence for this process. The main 5 m.y. sequence reflects regional tectonism, with the sandstone member developing at a time of slow subsidence, and the mudstone member reflecting a higher long-term subsidence rate. The higher-order sequences nested within the third-order sequence east of Trail Canyon are interpreted as a basinal response to episodes of crustal shortening on a 10 5 yr time scale. This study amplifies the model of Posamentier and Allen (1993a), in which ramp-type foreland basins are divided into areas of rapid and slow subsidence (Zones A and B). We postulate that these zones migrated basinward and landward in response to variations in long-term subsidence rate (an effect not predicted in the original model), and can be mapped by reference to the distribution of Type 1 sequence boundaries in the higher-order sequences. Differences in sequence architecture east and west of Trail Canyon may have been amplified by differences in crustal rheology. The sequence architecture changes at the boundary of the underlying Paleozoic Paradox Basin, a zone of NW-SE-oriented folds, faults, and salt diapirs, which we suspect were reactivated by Cretaceous tectonism. The high-frequency sequences are within the area of the Paradox Basin, an area that may have been more prone to vertical structural movements in response to intraplate stresses. Incipient uplift of Laramide structures may also have modified fluvial patterns and controlled the orientation of incised valleys on several of the sequence boundaries.

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