Biostratigraphy and Ammonoid Fauna of the Lower Jurassic (Sinemurian, Pliensbachian and Lowest Toarciar) of Eastern Oregon and Western Nevada

Abstract

<p>Over 2000 ammonite specimens from the Lower Jurassic of Oregon and Nevada are identified, described, and their stratigraphic ranges determined. At least 66 species are represented, distributed amongst 32 genera or subgenera and 9 families. Luningiceras is erected as a new subgenus of Acanthopleuroceras (type species A. (L.) pinnaforme n. sp.). Paltechioceras depressum, P. gettyi and Acanthopleuroceras (A.) mulleri, are described as new. Hyperderoceras and Asteroceras (Eparietites) are also probably represented by new species but the nature of the material does not warrant the establishment of new names.</p> <p>The following assemblage zones are erected spanning the Sinemurian (excluding the lowest Sinemurian), Pliensbachian, and lowest Toarcian Stages: A. ceratitoides, P. harbledownense, P. rothpletzi, Acanthopleuroceras (Luningiceras) pinnaforme, D. dayiceroides, R. fannini, and Tiltoniceras propinquum. The zones are used to correlate the contrasted facies of Oregon and Nevada which are respectively eugeosynclinal and miogeosynclinal in aspect. The age of all the lithostratigraphic units discussed is either refined or revised. In the southern part of the Nevada study area, a regression occurred during the late Carixian and early Domerian which correlates with a time of transgression in the Suplee-Izee area of east-central Oregon. The Lower Jurassic sequences of the two states accumulated on separate microplates but the coincidence of regression and transgression suggests microplate interaction and hence, geographic proximity. Marine deposition was continuous at Westgate, Nevada but the presence of vertically imbedded ammonites and other lines of evidence suggest a shallow water environment of deposition. Analysis of the orientation of ammonite planes of bilateral symmetry indicate a palaeocurrent from the northwest.</p> <p>The Lower Jurassic ammonites of Oregon and Nevada show Tethyan affinities. The Pliensbachian faunas of the northeast Pacific show a tripartite division into Boreal, mixed, and Tethyan faunas as in Europe but the transition is from west to east rather than from south to north. This pattern is caused by a northward transportation of the Wrangellia terrane relative to the North American plate since the Early Jurassic.</p>