TECTONIC CONTROL OF NESTED SEQUENCE ARCHITECTURE IN THE CASTLEGATE SANDSTONE (UPPER CRETACEOUS), BOOK-CLIFFS, UTAH

The Castlegate Sandstone at its type section, Price Canyon, near Price , Utah, encompasses a single stratigraphic sequence spanning approxima tely 5 m.y. It includes a sandstone member corresponding to a low stan d 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 pas ses laterally into the Sego Sandstone and Neslen Formation, a successi on of at least six higher-frequency sequences of fluvial-estuarine ori gin, The Buck Tongue, a marine shale unit separating the Castlegate Sa ndstone and the Sego Sandstone east of Green River, is erosionally tru ncated below the Sego Sandstone northwest of Trail Canyon. We suggest that the origin of the sequences is related to flexural loading and in traplate stress on two time scales, Eustasy cannot be ruled out, but t here is no independent evidence for this process, The main 5 m.y. sequ ence reflects regional tectonism, with the sandstone member developing at a time of slow subsidence, and the mudstone member reflecting a hi gher long-term subsidence rate, The higher-order sequences nested with in 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 Alien (1993a) , in which ramp type foreland basins are divided into areas of rapid a nd slow subsidence (Zones A and B), We postulate that these zones migr ated basinward and landward in response to variations in long-term sub sidence rate (an effect not predicted in the original model), and can be mapped by reference to the distribution of Type 1 sequence boundari es 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 f olds, faults, and salt diapirs, which we suspect were reactivated by C retaceous tectonism, The high-frequency sequences are within the area of the Paradox Basin, an area that may have been more prone to vertica l structural movements in response to intraplate stresses, Incipient u plift of Laramide structures may also have modified fluvial patterns a nd controlled the orientation of incised valleys on several of the seq uence boundaries.