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Abstract

Outcrops of Turonian through Campanian strata in the Kaiparowits Plateau of southern Utah provide a unique opportunity to examine both shallow-marine and continental strata within the context of unconformity bounded depositional sequences. This approach provides insights to the evolution of the strata within the plateau as well as regional chrono- and lithostratigraphic relationships within the southwestern Colorado Plateau.

We recognize five unconformity-bounded depositional sequences. These sequences are defined by regional surfaces of erosion that juxtapose amalgamated fluvial deposits over shoreface, alluvial plain, or coal-bearing strata and reflect an abrupt basinward shift in facies tracts. Between these sequence boundaries transgressive and highstand systems tracts are recognized. Transgressive systems tracts are characterized by a progression from amalgamated channel deposits to isolated meanderbelts that have evidence of tidal influence within what is otherwise a wholly alluvial succession. These tidally-influenced fluvial deposits are temporally equivalent to marine maximum flooding surfaces. Early-highstand systems tract deposits are characterized by thick, aggradational shoreface parasequences, thick coal beds, and isolated meanderbelt sandstones encased in thick, fine-grained flood plain strata. Late-highstand systems tract deposits are relatively thin and are characterized by progradational shoreface parasequences, thin, discontinuous coal seams, and fine-grained channel deposits. We interpret these changes in stratigraphic architecture to reflect significant changes in stratigraphic base level that we have correlated to adjacent outcrop belts in the Wasatch Plateau of central Utah, at Black Mesa in northern Arizona, and the San Juan basin in New Mexico. We have compared our estimates of stratigraphic base level with those of Haq et al. (1988) and suggest some modifications in order to account for the observed stratigraphy throughout the southern Colorado Plateau. Recognizing cycles of base-level change has allowed us to synthesize our observations in the Kaiparowits Plateau and extend them across a broad region. Recognition of the relationship between sedimentary architecture and position within a sequence has resulted in a model that allows the geometry and interconnectedness of sedimentary facies to be predicted within the context of parasequence stacking patterns. This provides a useful tool for both exploration and development.

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