SEQUENTIAL ARCHITECTURE IN A PLUVIAL SUCCESSION - SEQUENCE STRATIGRAPHY IN THE UPPER CRETACEOUS MESAVERDE GROUP, PRICE CANYON, UTAH

Five unconformity-bounded sequences have been documented within a 1300 m thick succession of predominantly fluvial Upper Cretaceous and Lowe r Tertiary deposits in the mice Canyon area of the westernmost part of the Book Cliffs, east-central Utah, U.S.A. This area experienced majo r tectonically induced changes in the paleogeography during deposition . The earliest fluvial deposits in the study area were formed in a rel atively simple foreland basin setting in front of the Sevier Orogenic Belt, and contain more marine influence than previously recorded. Foll owing a gradual termination of thrusting, sedimentation became increas ingly controlled by uplift of the San Rafael Swell to the southeast. O ur studies suggest that the fluvial systems in the upper part of the s uccession alternated between being confined within a valley open towar ds the north and being more directly eastward flowing. On the basis of the observed sequences, an idealized model of alluvial sequences, who se internal architecture can be related to a fall-rise-fall cycle of t he stratigraphic base level, has been established. Upward or downward changes in the position of this base level dictate the creation and de struction of accommodation on the alluvial plain and therefore exert a direct control on the sequential architecture and sandstone body geom etry of fluvial successions. In our model, the basal sequence boundary is overlain by an amalgamated fluvial sandstone sheet. The sheet is s ucceeded by a fining-upward, more mudstone-rich level with more isolat ed sandstone bodies, and this level may culminate in a marine or lacus trine transgression. The upper part of the sequence may show a coarsen ing-upward trend heralding the next phase of base-level fall and seque nce-boundary generation. Use of the suggested model has the potential to refine existing lithostratigraphic schemes and, given the higher re solution and more detailed correlation, may significantly improve pale ogeographic reconstructions and aid in prediction of potentially hydro carbon-bearing reservoirs. A revised and refined lithostratigraphy has been established on the basis of alluvial sequence analysis, The lowe r half of the succession forms the youngest part of a major eastward a rd-prograding elastic tongue, the Mesaverde Group, and is Campanian. W e divide this part into the Blackhawk, Castlegate, and Price River For mations. The rest of the succession is Maastrichtian to Paleocene and is referred to the North Horn Formation. This formation was deposited in an intermontane setting.