STRATIGRAPHIC ANALYSIS OF THE CRETACEOUS STRAIGHT CLIFFS FORMATION, NORTH KAIPAROWITS PLATEAU, SOUTHERN UTAH

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 of a depositional system 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 in the Left Hand Collet area, implying 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 Buck Hollow in the northeastern corner of the plateau to understand if 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 ~582 m, which is two to three times its thickness 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. The A – F sandstone intervals used to characterize the John Henry to the south are difficult to apply because of the expanded section. Six flooding surfaces are seen within the section; future analysis will work to assess the shoreline trajectories and stacking patterns as well as correlations to the south. Preliminary observations indicate a bay head delta building into an estuary in the middle of the John Henry Member at Buck Hollow. This succession projects southward into a hypothesized incised valley system which, interestingly, is oriented roughly parallel to the inferred shoreline. The A sandstone is not present, instead offshore marine deposits suggest that Buck Hollow was distal during A deposition. Above this, the section comprises progradational 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, document the connection between fluvial and marginal marine strata, and provide stratigraphic information about a new area.