THE EARLY CRETACEOUS BUCKHORN PALEOVALLEY: IMPLICATIONS FOR SEVIER FORELAND BASIN DEVELOPMENT
The Early Cretaceous Buckhorn Conglomerate lies between fine-grained low-gradient fluvial deposits of the Late Jurassic Morrison Formation and the Cedar Mountain Formation (CMF), recording a distinct shift in fluvial style and dispersal mechanisms across the Colorado Plateau. Hypotheses for the cause of this shift include 1) a period of regional uplift, 2) a drop in Early Cretaceous sea level, or 3) onset of activity in the Sevier fold and thrust belt in western Utah.
Contrary to earlier studies that interpret Buckhorn deposition as a widespread sheet gravel in the foreland basin region, isopach and paleocurrent data reveal a well-defined paleovalley trending northeastward across the San Rafael Swell and extending to Dinosaur National Monument. The paleovalley is further constrained by the distribution of coeval palustrine and pedogenic calcrete wherever the Buckhorn facies is absent, indicating depositional stasis on the landscape surrounding the paleovalley.
Trough-cross bedded pebble- to cobble-conglomerate channels of the Buckhorn indicate deposition in a braided fluvial system, in sharp contrast to the underlying meandering fluvial deposits of the upper Morrison Formation (Brushy Basin member) and the overlying anabranching river system of the Ruby Ranch member of the CMF. Buckhorn clast sizes with D90 ~ 23-50 mm correspond to a regional paleoslope of 3 - 5 x 10-3. Clast composition and particle size in the Buckhorn remain constant at corresponding stratigraphic sections throughout the paleovalley, suggesting an extensive progradational fluvial system.
Sandstone, fine-pebble conglomerate, and paludal and pedogenic carbonates immediately overlying the Buckhorn in this paleovalley indicate a transition in foreland basin development. Regional subsidence, due to Sevier uplift west of the depositional basin, resulted in a shift from a progradational to a retrogradational fluvial system, characterized by finer grained deposits. Development of oncoids within a pebble conglomerate channel overlying the Buckhorn implies a low-energy paludal-fluvial deposition. Following incision and filling of the Buckhorn paleovalley, the regional foreland basin underwent a subsidence event and Buckhorn deposits were buried by the widespread Ruby Ranch member of the CMF.