Abstract

The Cedar Mountain Formation has been the focus of numerous paleontological and geological studies aimed at understanding Early Cretaceous dinosaurs and tectonics. A major problem in these studies is that the contact between the Upper Jurassic Morrison and Lower Cretaceous Cedar Mountain Formations is notoriously difficult to identify where the Buckhorn Conglomerate is absent. Our stratigraphic and sedimentologic analysis of these formations in Utah and Colorado permits accurate recognition of this previously enigmatic contact. The top of the Morrison Formation is marked by increases in chert-pebble lags and channelized conglomerates overprinted by paleosols that exhibit iron concentrations, manganese-coated grains, intense red-purple-green mottling, and bleaching. These features indicate a period of reduced accommodation in the Tithonian.

Where the basal Cedar Mountain Formation consists of the Buckhorn Conglomerate or a pebbly mudstone (debritic facies) of the Yellow Cat Member, comprising slightly reworked, angular fragments of Morrison Formation regolith, the formational contact occurs at the base of these members. The Buckhorn Conglomerate and the debritic Yellow Cat pebbly mudstone facies (1) interfinger with each other, indicating time equivalancy, (2) typically grade upsection into sandstones, and (3) comprise a depositional system that locally removes part or all of the paleosol that elsewhere caps the Morrison Formation. In the absence of the Buckhorn and debritic Yellow Cat facies, a fine-grained, maroon, calcareous paleosol marks the base of the Cedar Mountain Formation.

A calcrete/silcrete paleosol complex, indicative of a multi-million-year depositional hiatus, commonly overprints the top of the Buckhorn Conglomerate, portions of the Yellow Cat Member, and in some cases, the Morrison Formation paleosol. Above the calcrete, the Cedar Mountain Formation consists of fine-grained overbank deposits with caliche nodules or calcrete horizons (Ruby Ranch Member) and isolated to amalgamated sandstone lenses and beds.