Date of this Version
Bulletin of the American Museum of Natural History Volume 178, article 3, pages 215-368, figures 1-42
Late Tertiary Valentine and Ash Hollow formations of the Ogallala Group in north-central Nebraska contain two previously unnamed fossiliferous ash-bearing members. These, with four published members, provide a stratigraphic framework for large collections of fossils in the Frick Collection in AMNH and other institutions.
The Cornell Dam Member (New) in the basal Valentine Formation has salient lithic features and geologic relationships not found in other members of the Valentine. Basal channel sand disconformably overlying the Rosebud Formation contains macro- and microvertebrate fossils (Norden Fauna, New) that also show the ecological and faunal distinction of this member. Fission track dates suggest that Valentine sediments spanned one and perhaps three million years.
The Merritt Dam Member (New), of late Clarendonian to late Hemphillian age, disconformably overlies the Cap Rock Member of the referred Ash Hollow Formation. The Merritt Dam Member is less cliff forming than the Cap Rock Member, contains more volcanic ash and local channel and pond sediments. Tectonic readjustment caused deep channel erosion through the Ogallala into Arikaree rocks on the east flank of the Chadron Arch and eastward into the Cap Rock Member and the Valentine Formation. Sediments filling some of these channels contain vertebrate fossils overlain by vitric tuffs with a fission track date of 9.5 ± 0.8 Ma.
The paleogeomorphology of the Ogallala Group and its depositional framework is the product of overlapping alluvial fans of at least three paleodrainage systems which filled pre-existing valleys and spread sediments over a vast Great Plains area in Nebraska and South Dakota. In north-central Nebraska widespread aggradation and two short periods of degradation occurred during the Valentinian. Gradual aggradation during the early Clarendonian was followed by intermittent aggradation and degradation during the late Clarendonian and Hemphillian.
The stratigraphic allocation and history of 98 collecting localities and documentation of 90 holotypes of fossil vertebrates and 13 plants provide a firm base for continued research.
The principal aquifer in the Ogallala is the Crookston Bridge Member of the Valentine Formation.