DIAGNOSIS OF UTAH'S ELUSIVE EARLY CRETACEOUS CEDAR MOUNTAIN – LATE JURASSIC MORRISON FORMATIONAL CONTACT AND THE INTERFINGERING OF THE BUCKHORN CONGLOMERATE AND THE YELLOW CAT DEBRITIC FACIES

The Cedar Mountain Formation contains the most diverse Early Cretaceous faunas known from North America and is a key to deciphering early Cretaceous tectonics. A major obstacle to both paleontologic and geologic studies in the formation is that the contact between the Late Jurassic Morrison and Early Cretaceous Cedar Mountain formations is notoriously difficult to identify, especially when Buckhorn Conglomerate is absent. New stratigraphic and sedimentologic analyses of these formations in Utah and Colorado permits accurate recognition of this previously enigmatic contact. When intact, the top of the Morrison Formation is marked by redoximorphic paleosols that exhibit iron concentrations, manganese-coated grains, intense red-purple-green mottling, and bleaching. Upsection increases in chert-pebble lags and channelized conglomerates within the paleosol section indicate a period of reduced accommodation in the Tithonian.

Where the basal Cedar Mountain Formation consists of the Buckhorn Conglomerate or a debritic, pebbly mudstone facies (Yellow Cat Member of the Cedar Mountain Formation), the bottom of these units represent the base of the formation. The Buckhorn Conglomerate and the debritic Yellow Cat pebbly mudstone facies: (1) interfinger with each other indicating time equivalency, (2) usually grade upsection into sandstones, (3) locally cut out the paleosol at the top of the Morrison, and (4) represent different facies of a complex degradational system. In the absence of the Buckhorn and debritic Yellow Cat facies, a fine grained maroon paleosol marks the base of the Cedar Mountain Formation.

An array of caliche/calcrete/silcrete paleosol complexes, some indicative of a multi-million-year depositional hiatuses, overprints the top of the Buckhorn Conglomerate and portions of the Yellow Cat Member as well as the intact top of the Morrison Formation. Above this intermittent calcrete zone, the Cedar Mountain formation consists of fine-grained overbank deposits with caliche nodules or caliche horizons (Ruby Ranch Member) and sporadic sandstone lenses and beds.