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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 those formations to the upper Arnsbergian Stage (E2c). The appearance of the Isohomoceras subglobosum Ammonoid Biozone marks the base of the Chokierian Stage (H1a) at NTS. The zonal name-bearer continues into lower Pennsylvanian (Morrowan) strata, where it joins the Homoceras coronatum coronatum Ammonoid Biozone assemblage in an interval equivalent to the upper Chokerian Series (H1b). A pronounced unconformity at NTS separates the Scotty Wash Formation from the overlying Tippipah Limestone, which contains a fifth ammonoid assemblage characterized by Cancelloceras cf. C. elegans that is equivalent to the Yeadonian Stage (G1), Namurian Series, of western Europe.
The conodont succession recovered from the ammonoid-bearing sections at NTS allows refined correlation of the Arnsbergian and Chokierian Stages with the Mid-Carboniferous Global Stratotype Section and Point (GSSP) at nearby Arrow Canyon, Nevada, and the North American midcontinent. The Lower Rhachistognathus muricatus Conodont Biozone of western North America is equivalent to the upper Arnsbergian Stage (E2c), and must include the upper portion of the Adetognathus unicornis Conodont Biozone as recognized in the midcontinent. The Upper R. muricatus Conodont Biozone is equivalent to that portion of the Chokierian Stage (H1a) below the appearance of Declinognathodus noduliferus, marking the mid-Carboniferous boundary horizon, including some of the R. primus Conodont Biozone as used in the North American midcontinent. The intercontinental mid-Carboniferous boundary, drawn at the appearance of D. noduliferus, does not correspond to the Arnsbergian-Chokierian Stage boundary (E2c-H1a) that is defined by the appearance of Isohomoceras subglobosum. A significant break occurs in the Arrow Canyon GSSP less than 4 m above the position of the mid-Carboniferous boundary, where Chokerian strata (H1) are probably succeeded by Kinderscoutian strata (R1). Higher at Arrow Canyon, the position of the Scotty Wash-Tippipah unconformity juxtaposes Kinderscoutian and Yeadonian (G1) strata and the entire upper Namurian Series is limited to no more than 54 m.
Comparison of Eurasian and North American ammonoid assemblage compositions suggests that at least intermittent faunal interchange persisted between the two regions until at least the close of the Chokierian.
Definition of the Mississippian-Pennsylvanian boundary, which has never been defined faunally in either type area, to correspond to the intercontinental mid-Carboniferous boundary would be compatible with relationships known in the Chesterian and Morrowan type areas.
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