Triassic Lithostratigraphy and Biostratigraphy North of Currie, Elko County, Nevada
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چکیده
The Triassic section north of Currie in southern Elko County, Nevada, is almost 500 m thick and is assigned to the Thaynes Group and overlying Shinarump Formation of the Chinle Group. The Thaynes Group disconformably overlies the Permian Gerster Formation and is mostly calcareous shale with interbedded wackestones that yield marine invertebrate fossils, especially ammonoids. The Shinarump Formation disconformably overlies the Thaynes Group and is up to 5 m of silica-pebble conglomerate, conglomeratic sandstone and quartzose sandstone containing abundant silicified wood. Overlying siltstone-dominated strata long assigned to the Chinle Group are here considered to more likely be outcrops of the Lower Jurassic Kayenta Formation of the Glen Canyon Group. North of Currie, the Thaynes Group yields ammonoids from three stratigraphic intervals: (1) “Meekoceras assemblage”, upper Smithian Anasibirites kingianus zone; (2) tirolitid assemblage, lower Spathian; and (3) “Keyserlingites” assemblage, middle Spathian. Conodonts from just below the tirolitid assemblage are: Platyvillosus asperatus, Icriospathodus collinsoni, and Novispathodus abruptus, and Columbitella elongate; Triassospathodus ex gr. homeri is associated with the ammonoids, and “Neospathodus” hungaricus occurs above. The conodont succession is correlated with the Columbites beds of early Spathian age.
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Smithian (early Triassic) Ammonoid Biostratigraphy at Crittenden Springs, Elko County, Nevada and a New Ammonoid from the Meekoceras Gracilitatis Zone
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