PETROLOLOGY OF THE CRETACEOUS STRAIGHT CLIFFS-WAHWEAP FORMATIONS TRANSITION, SOUTHERN UTAH

Five stratigraphic sections were measured and 68 petrographic samples were collected in the upper part of the Straight Cliffs Formation (John Henry and Drip Tank members) and the lower part of the overlying Wahweap Formation on the margins of the Paunsaugunt Plateau including Bryce Canyon National Park (BCNP). These sections and petrographic samples were then compared to samples taken from two measured sections near the type section of the Drip Tank Member of the Straight Cliffs Formation on the Kaiparowits Plateau. Petrographic results show that the lithics in the Drip Tank Member are dominated by chert (>95%) and that the Wahweap lithics are dominated by carbonates (>85%). The chert to carbonate ratio observed within the top 5 meters of the Drip Tank varies widely, consistent with previous work indicating a change in provenance from the Mongollon Highlands to the Sevier orogenic belt marked by the mixing of sediment types. These observations were then applied to a section located approximately 15 km west of BCNP in Hillsdale Canyon along the western margin of the Paunsaugunt Plateau. This section consists of a series of conglomerates rather than the single cliff forming conglomerate that marks the top of the Drip Tank Member elsewhere and as such the boundary between the Straight Cliffs and Wahweap formations in this area has been problematic. This study suggests that petrography can be used to distinguish the Wahweap conglomerates from and Straight Cliffs conglomerates based on the lithic content. The Drip Tank Member of the Straight Cliffs Formation in Hillsdale Canyon can be tentatively assigned a thickness of 74 m, consistent with the 40-132 m thicknesses observed on the eastern margins of the plateau.