Sequential architecture in a fluvial succession; sequence stratig
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摘要

Five unconformity-bounded sequences have been documented within a 1300 m thick succession of predominantly fluvial Upper Cretaceous and Lower Tertiary deposits in the Price Canyon area of the westernmost part of the Book Cliffs, east-central Utah, U.S.A. This area experienced major tectonically induced changes in the paleogeography during deposition. The earliest fluvial deposits in the study area were formed in a relatively simple foreland basin setting in front of the Sevier Orogenic Belt, and contain more marine influence than previously recorded. Following a gradual termination of thrusting, sedimentation became increasingly controlled by uplift of the San Rafael Swell to the southeast. Our studies suggest that the fluvial systems in the upper part of the succession alternated between being confined within a valley open towards the north and being more directly eastward flowing. On the basis of the observed sequences, an idealized model of alluvial sequences, whose internal architecture can be related to a fall-rise-fall cycle of the stratigraphic base level, has been established. Upward or downward changes in the position of this base level dictate the creation and destruction of accommodation on the alluvial plain and therefore exert a direct control on the sequential architecture and sandstone body geometry of fluvial successions. In our model, the basal sequence boundary is overlain by an amalgamated fluvial sandstone sheet. The sheet is succeeded by a fining-upward, more mudstone-rich level with more isolated sandstone bodies, and this level may culminate in a marine or lacustrine transgression. The upper part of the sequence may show a coarsening-upward trend heralding the next phase of base-level fall and sequence-boundary generation. Use of the suggested model has the potential to refine existing lithostratigraphic schemes and, given the higher resolution and more detailed correlation, may significantly improve paleogeographic reconstructions and aid in prediction of potentially hydrocarbon-bearing reservoirs. A revised and refined lithostratigraphy has been established on the basis of alluvial sequence analysis. The lower half of the succession forms the youngest part of a major eastward-prograding clastic tongue, the Mesaverde Group, and is Campanian. We divide this part into the Blackhawk, Castlegate, and Price River Formations. 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.

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