ABSTRACT

The Uinta Formation in northeastern Utah was the original basis of the Uintan land mammal “age.” Magnetostratigraphic studies were conducted in four sections in the northeastern, north-central, and northwestern Uinta Basin. The uppermost Evacuation Creek Member of the Green River Shale, and all of unfossiliferous Uinta Formation unit “A,” was of normal polarity. This normal interval probably correlates with Chron C21n (46.3-47.8 Ma), as originally suggested by Prothero and Swisher (1992). Most of Uinta “B” was reversed (= Chron C20r, 43.8-46.3 Ma). A short normal zone spanning upper Uinta “B” and lower Uinta “C” probably correlates with Chron C20n (42.5-43.8 Ma). The upper part of the Uinta “C” and the lowermost portion of the Duchesne River Formation were also reversed (= C19r, 41.4-42.5 Ma), with normal (= C19n, 41.1-41.4 Ma) and reversed (= C18r, 40.0-41.1 Ma) magnetozones in the higher part of the Brennan Basin Member.

Although the original biostratigraphic data for most Uinta Basin collections are very poor, distinctions between the faunas of Uinta “B1,” “B2,” and “C” are possible. Uinta “B1” (the “Metarhinus zone” of Osborn, 1929) spans the interval 45–46 Ma, and is characterized by overlapping ranges of the brontotheres Sthenodectes and Metarhinus, the rhinocerotoids Hyrachyus eximius, Forstercooperia grandis and Triplopus obliquidens, and the agriochoerid oreodont Protoreodon parvus. Uinta “B2” (the “Eobasileus-Dolichorhinus zone” of Osborn, 1929), including White River Pocket, spans the interval 43-45 Ma, and is characterized by the overlapping ranges of the brontotheres Sphenocoelus, Metarhinus, Eotitanotherium, the chalicothere Eomoropus, the horse Epihippus gracilis, the creodont Oxyaenodon, and the artiodactyls Diplobunops, Oromeryx, and Leptotragulus.