Frontier Formation, Wyoming and Adjacent Areas

Abstract

The Frontier formation in its type area in southwestern Wyoming consists of about 2,000 feet of sandstone and siltstone interbedded with softer units of shale and mudstone containing minor beds of sandstone, coal, bentonite, and porcellanite. At Cumberland Gap, Wyoming, 15 miles south of Frontier, the lower half of the Frontier formation consists of non-marine sandstone, siltstone, mudstone, and water-laid volcanic rocks with some coal, carbonaceous shale, and limestone. The upper half of the formation consists of marine sandstone, siltstone, and shale, and, near the top, a 260-foot non-marine unit of sandstone, mudstone, and some coal. The formation is underlain by the marine Aspen shale of late Early Cretaceous (Albian) age and is overlain by 6,000 feet of marine Hillia d shale of middle Late Cretaceous (Coniacian-Santonian) age, which in turn is overlain by 4,000 feet of largely non-marine Adaville formation. The non-marine lower half of the Frontier formation can not be precisely dated, but it is probably Cenomanian. The marine beds in the upper half of the formation are dated as of late Greenhorn (Turonian), early Carlile (Turonian), and early Niobrara (Coniacian) ages. The non-marine unit near the top of the formation is believed to be of late Carlile age (Turonian). Northward from the type area the Frontier formation does not appear to change greatly as far as Fontanelle Basin, but the upper half of the overlying Hilliard shale changes to sandstones and shales of Adaville aspect. Still farther north in Snider Basin a relatively thin equivalent of the typical Frontier formation can be identified, but it is overlain by a great thickness of sandstone and shale that is not greatly different from the Frontier. Surface measurements suggest as much as 13,000 feet, but there may be some duplication of beds not readily seen on the surface. The uppermost part has not yielded fossils, but the main part of these beds contains Niobrara fossils, and the whole sequence above the Aspen has been called Frontier by some authors. Though the name Frontier was first applied in southwestern Wyoming, its use has spread over most of Wyoming and into northern Utah, northwestern Colorado, southeastern Idaho, and southern Montana. The formation passes eastward into dominantly marine rocks with fewer and thinner beds of resistant sandstone. Inasmuch as the sandstone beds that locally form the top of the Frontier formation pass eastward into shale, the upper limit of the formation changes in age. Ordinarily one of three large units of sandstone forms the top of the formation. The youngest sandstone unit, of early Niobrara age, is tongue-shaped, extending from the southwest and northwest corners of Wyoming into the central part of the state. Sandstone beds of about middle Carlile age form the top of the Frontier formatio for some distance eastward in Wyoming, northeastern Utah, and northwestern Colorado, beyond the eastern limits of the sandstone beds of early Niobrara age. In a much smaller area in north-central Wyoming and south-central Montana sandstone beds of pre-Carlile age form the top of the Frontier formation. Comparisons of the Frontier formation at many widely separated localities with the standard Great Plains Cretaceous sequence reveal many apparently local breaks in the Frontier sections. In central Wyoming, rocks equivalent to the lower part of the Carlile shale of the Black Hills seem to be either missing or represented in the Frontier formation by abnormally thin lithologic units. Probably as more sections are studied in detail many time-gaps will be discovered. Much remains to be done before the details of Frontier paleogeography can be worked out.