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Implications of an expanded section and delta deposits in the Cretaceous Straight Cliffs Formation in the northern Kaiparowits Plateau, southern Utah, USA


The Coniacian to Santonian (87.3 – 83.5 Ma) John Henry Member of the Straight Cliffs Formation preserves the transition from fluvial to marginal marine depositional systems along the western edge of the Western Interior Seaway. The John Henry Member preserves fluvial facies in the western Kaiparowits Plateau, and marginal marine facies to the east. However, there is no direct evidence of fluvial output in the marginal marine strata (i.e. clearly defined deltas) throughout much of the plateau. This observation, along with regional paleocurrent analysis, implies that fluvial transport was southwest to northeast, with a westward excursion of the paleoshoreline to the north. This study focuses on an unstudied area in the northeastern corner of the plateau (Buck Hollow) to understand how the fluvial systems are interacting with the marginal marine systems in that location. Stratigraphic analysis in the Buck Hollow area shows a significantly expanded John Henry Member section with a thickness of ∼530 m, which is twice its thickness of ∼230 m in Left Hand Collet 45 km to the south. This increased thickness suggests increased accommodation and high sediment supply, perhaps reflecting increased loading in the central Utah fold and thrust belt to the north, and high rates of sediment delivery from basin-axial fluvial systems. The A sandstone is not present, instead offshore marine deposits suggest that Buck Hollow was distal during A deposition. Above this, the B sandstone interval shows delta influence grading up into tidal deposits. Preliminary observations indicate a wave-dominated delta building into a lagoon and a bay head delta building into an estuary at different points in the John Henry Member at Buck Hollow. These deltaic successions project southward into a hypothesized incised valley system which, interestingly, is oriented roughly parallel to the inferred shoreline. Moving upward the section comprises regressive and transgressive deposits capped by more distal Drip Tank Member (fluvial) than is seen elsewhere. Continued analysis of these differences will contribute to a more detailed understanding of the paleogeography and evolution of the basin during John Henry deposition, and document the connection between fluvial and marginal marine strata with implications for basin to reservoir-scale stratigraphic architecture in analogous systems.