PROVENANCE AND FACIES CHANGES ACROSS THE UPPER CRETACEOUS BLACKHAWK-CASTLEGATE SEQUENCE BOUNDARY (SOUTHERN WASATCH PLATEAU, SALINA CANYON, UTAH, USA)

One of the most studied and controversial nonmarine sequence boundaries is the contact between the Upper Cretaceous Blackhawk and Castlegate formations in the Book Cliffs of central Utah. At this contact, fluvial foreland-basin deposits transition from isolated, lithic-rich channels of the Blackhawk Formation to amalgamated, quartz-dominated channels of the Castlegate Sandstone. In the northern Book Cliffs, near Price, Utah, previous studies have shown that Castlegate deposition resulted from tectonic uplift and unroofing in the Sevier thrust belt. In the southern Wasatch Plateau near Salina, Utah, the Blackhawk-Castlegate contact and provenance are relatively uncharacterized. We present results from a field investigation across the Blackhawk-Castlegate boundary in Salina, Utah, aimed at constraining the stratigraphic position of provenance changes, facies changes, and the sequence boundary in this region. We evaluated facies and provenance for sand bodies throughout the Blackhawk-Castlegate transition zone using measured sections, paleocurrent data, and thin-section sandstone-composition data.

In Salina Canyon, the Blackhawk Formation comprises isolated channel facies surrounded by laterally continuous floodplain facies. The Castlegate Sandstone includes multistory channel deposits with rare, laterally discontinuous floodplain deposits. The transition between the two fluvial styles occurs abruptly 10m above the major cliff-slope break in this region. Preliminary sandstone-composition data suggest that composition changes from sublitharenitic, carbonate-grain-rich sandstone in the Blackhawk Formation to quartz arenite sandstone in the Castlegate; this transition also occurs 10m above the major cliff break, coincident with the lithofacies change. These results contrast with previous interpretations in this area that map the sequence boundary and provenance change at the base of the cliff-slope break, and contrasts with published provenance data in Price Canyon which show the provenance shift to quartz arenite occurring 20m above the Castlegate sequence boundary. These results suggest lateral variations in the timing of source-area unroofing along the Sevier thrust front with respect to the timing of the formation of the Blackhawk-Castlegate sequence boundary.