MID-CARBONIFEROUS AMMONOID BIOSTRATIGRAPHY, SOUTHERN NYE COUNTY, NEVADA: IMPLICATIONS OF THE FIRST NORTH AMERICANHOMOCERAS

Abstract

Abstract The Scotty Wash Formation on the Nevada Test Site (NTS), southern Nye County, Nevada has produced the first North American representatives of the globally significant index ammonoids Homoceras s.s. and Isohomoceras s.s. and contains the only ammonoid succession across an uninterrupted mid-Carboniferous boundary sequence known in North America. Four ammonoid assemblages can be recognized at NTS that are homotaxial with the reference successions for the middle and upper Arnsbergian (E2) and Chokierian (H1) Stages, Namurian Series, in western Europe, and their equivalents worldwide. The upper Mississippian (Chesterian) portions of the NTS sections yield assemblages referable to a Eumorphoceras girtyi Ammonoid Biozone, representing the middle Arnsbergian Stage (E2b), followed by a Delepinoceras thalassoide Ammonoid Biozone, equivalent to the upper Arnsbergian Stage (E2c). The latter ammonoid biozone also occurs in the Imo and Rhoda Creek Formations of Arkansas and Oklahoma, requiring reassignment of ...