CISURALIAN AMMONOIDS OF BUCK MOUNTAIN: PALEOGEOGRAPHIC INDICATORS FOR THE DRY MOUNTAIN TROUGH

Cisuralian ammonoid occurrences in Nevada have been recovered from strata deposited within the Dry Mountain Trough (DMT). The DMT is one of several tectonically induced basins positioned along the western margin of North American. Cisuralian ammonoids and conodonts are abundantly preserved in undifferentiated Riepetown-Pequop Formations (Arcturas Group) at Buck Mountain, White Pine County, Nevada. Abundant and diverse "Buck Mountain" ammonoids (including Uraloceras, Prothalassoceras, Daraelites, Almites, Bransonoceras, Medlicottia, Bamyaniceras, Akmilleria, Agathiceras, Metalegoceras, Crimites, Neocrimites) and conodonts (Sweetognathus whitei, Mesogondollela bisselli) occur in a 60-meter interval of concretionary micritic siltstone near the base of the section. The occurrence of Uraloceras involutum and Sweetognathus whitei support an Artinskian age. S. whitei has been proposed as defining the base of the Artinskian stage (Chuvashov et al., 2002), in the Ural Mountains, Russia and indicates an early Artinskian (Aktastinian) age for the Cisuralian strata at Buck Mountain.

Cisuralian ammonoid faunas appear to be restricted to certain basins along the western United States continental margin where tectonism created basins with suitable habitat conditions. Uraloceras, abundantly preserved at Buck Mountain, has been characterized as a Boreal indicator (Nassichuk, 1965). These occurrences are south of true Boreal regions and a few hundred miles east of localities preserving the abundant Tethyan ammonoid element, Properrinites. This indicates that certain species of Uraloceras occurred in transitional environments where cool-water faunas from the north mixed with warm-water faunas from the south. The Antler highlands may have affected ocean current circulation, creating regions of cool water upwelling and controlled depositional patterns producing sheltered marine environments. Endemism and regional provincialism may have resulted from these conditions. This detailed study of the Buck Mountain fauna enhances the understanding of the paleoecologic and paleobiogeographic dynamics within tectonically initiated basins of the western US.