SEQUENCE STRATIGRAPHY AND CONTROLS ON FLUVIAL ARCHITECTURE IN THE STRAIGHT CLIFF FORMATION, SOUTHEASTERN UTAH

Preliminary results suggest that the John Henry Member of the Straight Cliffs Formation in the Kaiparowits Plateau (south central Utah) contains two additional sequence boundaries than formerly thought. Previous interpretations describe only one sequence boundary within the John Henry Member located near its base. It was identified by a change in facies; specifically fluvial overlying lower shoreface. In the eastern Kaiparowits Plateau, this sequence boundary is represented by a correlative conformity composed of shoreface packages. A similar pattern is seen in the middle of the John Henry Member; estuarine facies overly shoreface deposits that transition into consecutive lower shoreface packages towards the east. This is potentially an additional sequence boundary within the John Henry. Secondly, the uppermost sandstone unit within the John Henry appears to be nonmarine in nature along the eastern border of the Kaiparowits Plateau, as evidenced by channel lags and high-angle trough cross-beds. This interpretation differs from previous analyses that interpret this unit as a marine sandstone. If this sandstone body is in fact fluvial, it overlies marine sands and muds and thus represents a second additional sequence boundary within the John Henry Member. Ongoing work will verify these findings and trace these and previously interpreted sequence boundaries into nonmarine strata. The Straight Cliffs preserve the transition between these two environments and correlations will be made by physically walking out surfaces within dip-oriented canyons. Moreover, microanalyses of mudstones and coals within the nonmarine are anticipated in order to obtain more precise locations of nonmarine sequence boundaries. These data will provide an extremely accurate correlation between these two environments; enabling an analysis of the relationship between sea level and fluvial architecture as well as an assessment of existing fluvial architecture models.