Abstract

This report is a review for the non-specalist geologist of current knowledge of the post-Eocene continental Tertiary deposits of the western United States. The area covered extends from the High Plains geomorphic province westward to include the Rocky Mountains and the Great Basin. The subject is presented in two parts, of which this is the first.

This first part traces the history of geological exploration of the High Plains from its inception in the 1860's to the present. Historically, the emphasis of study has been in the specialized field of vertebrate (especially mammalian) paleontology. It was in the classical area of western Nebraska and adjoining regions that much of the modem chronostratigraphic classification of the Middle Tertiary, the North American Land Mammal Ages, (NALMA) was developed and extended elsewhere. The relationship of the NALMA to the rock stratigraphic section of this province is described from an historical perspective and is reviewed critically in terms of proper application of widely accepted rules of stratigraphic nomenclature. Much of the lithostratigraphic terminology that has been used in this province appears flawed in a technical nomenclature sense. These apparent flaws are treated at length and remedies are suggested. As a case in point, the name Ogallala Formation (or Group) lacks validity and it is recommended that it be replaced with its senior synonym Ash Hollow Formation. There is no acceptable scientific defense for retaining current usage.

The mid-Tertiary lithostratigraphic units lend themselves to a tripartite grouping: These consist of. (1) a lower unit characterized by several formations of relatively broad areal extent which consist of loessic deposits having a very high volcaniclastic composition and includes the White River and Arikaree Groups; (2) a middle grouping of formations (Runningwater, Box Butte, Sheep Creek, Olcott, Sand Canyon and Valentine) which occur as a complex of elongate, narrow stream channel deposits which have only a minor tuffaceous content; and (3) a thin but very extensive deposit of typically sandy and gravelly fluvial beds of the Ash Hollow Formation.