Ordovician and Silurian graptolite succession in the Trail Creek area, central Idaho; a graptolite zone reference section

Abstract

The graptolite-bearing sequence in the vicinity of Trail Creek, Idaho, consists of an unusually large number of zones virtually continuously exposed in a homoclinal section of siliceous shale. Eight Ordovician zones have been distinguished in the Phi Kappa Formation. These include the Early Ordovician Zones of Didymograptus protobifidus and Isograptus, the Middle Ordovician zones of Glossograptus hincksii, Nemagraptus, Climacograptus bicornis, and Climacograptus tubuliferus, and the Late Ordovician Zones of Pleurograptus linearis and Dicellograptus ornatus. The Early and Middle Silurian Zones of Monograptus convolutus and Cyrtograptus overlie this Ordovician succession. The relative completeness of the graptolite succession from late Early Ordovician through Late Ordovician becomes apparent when it is compared to other known graptolitic sequences of western North America. Thus it is a potentially useful reference section for at least the southern part of the North American Cordillera. The species Leptograptus demissus and the subspecies Phyllograptus anna prolatus are new. The species Cyrtograptus kirki Ruedemann is revised. ^ Elevation approximately B.400 ft 'Bulman's (1970) terminology and classification have been used in the following descriptions.