Mesozoic stratigraphy of the Hawthorne and Tonopah quadrangles, Nevada

Abstract

Mesozoic rocks of the Hawthorne and Tonopah quadrangles include Triassic and Lower Jurassic formations that occur in isolated areas in the different mountain ranges of the area and have a maximum thickness of 30,000 feet. The mapped formations are (from top to bottom): The lithology, areal extent, fauna, and correlation of each formation is described. Rich fossil collections from the area furnished much material new to this continent. The Claraia and Proptychites assemblages from the Lower Triassic Candelaria formation are earlier than the Meekoceras fauna, which has been heretofore known as the oldest Triassic ammonite fauna of this continent. The fossils from the Grantsville and the Excelsior formations, although differing from one another, both show some affinity to the European Muschelkalk fauna. Reef-building corals from the Luning formation are correlated with the Karnic of the European section and not the Noric as was previously held by other writers. A new Gumbelites fauna above the Tropites assemblage in the Luning probably represents a transitional Karnic-Noric fauna. Three zonal assemblages were recognized in the Gabbs formation: Zone of Sagenites giebeli , zone of Pinacoceras mettemichi , and the zone of Pteria contorta (or Choristoceras marshi ). All three are new to North America. The Choristoceras fauna is of particular interest, as marine rocks equivalent to the Rhaetic of the European section heretofore have not been recognized in North America. These Rhaetic beds are overlain with a gradational contact by the lowermost Jurassic beds containing the ammonites Psiloceras, Waehneroceras , and Euphyllites . These ammonites likewise have not been hitherto recognized on this continent. The gradational contact between the Triassic and Jurassic systems in western North America permits a revision of the previously held views that “nowhere on the continent is there any latest Triassic or earliest Jurassic sediment.” The sudden change in most of the area from marine deposition of the Sunrise and older formations to subaerial and volcanic debris of the Dunlap is ascribed to acute diastrophism during the latter part of the early Jurassic.