THE BUCKHORN CONGLOMERATE AS THE UPPER MEMBER OF THE MORRISON FORMATION: NEW EVIDENCE FROM THE TYPE SECTION, CEDAR MOUNTAIN, UTAH

The fluvial-lacustrine deposits of the Upper Jurassic Brushy Basin Member (Morrison Fm.) are abruptly overlain by the discontinuous, coarse-grained fluvial deposits of the Buckhorn Conglomerate (Cedar Mountain Fm.) along the northwestern flank of the San Rafael Swell (east-central Utah). Where present, the contact between the two units is generally interpreted to represent the regional Jurassic-Cretaceous (J/K) unconformity that extends throughout the Mesozoic retro-arc foreland basin in the U.S. western interior. However, there are no reported biostratigraphic data from the interval near the contact. Some workers have reported the contact between the two lithologies to be conformable based on depositional pinchouts of the Buckhorn margins and similarity in clast composition in both units.

Detailed examination of the contact in the vicinity of the type section of the Buckhorn Conglomerate at Cedar Mountain revealed the presence of numerous gutter casts, mud injections into the overlying conglomerate, and conglomerate-filled depressions in the top of the Brushy Basin Member mudstones, which we interpret as dinosaur tracks. All three structures indicate that the Brushy Basin strata were plastic during deposition of the basal Buckhorn. The contact between the two units at the type section is therefore conformable. The structures, composition, and geometry of the conglomerates suggest that the Buckhorn resulted from a major but conformable change in depositional environment and this unit should be viewed as the uppermost member of the Morrison Formation. The J/K unconfomity must lie within or above the Buckhorn.