Abstract

A regional surface cross section from the southern Big Horn Basin through Hells Half Acre in the eastern Wind River Basin, to Glenrock in the Powder River Basin, was measured and plotted. It illustrates a correlation based on upper flood plain (coarse grained massive sandstone channels), lower flood plain (less massive, finer grained, more bentonitic bank overflow shales and crevasse splays), and swamps (lignite and chocolate brown shales). Pipys (tubular concretions formed by ancient ground water flows) and cross bedding indicate a strong easterly flow of rivers in an area beginning to respond to the inchoate Laramide Orogeny. Basin forming and internal drainage were not decisive till the Upper Cretaceous ended, some 65 MM years ago. A model of richest fossil plant distribution illustrates the multi-cycles of lush vegetation, swamps, and streams that explain the great numbers of dinosaurs in this formation. A subsurface-surface structural profile across the eastern Wind River Basin demonstrates the relationship of massive sandstone units to the basin axis, the truncation at the top, and oil and gas productive potential by stratigraphic thinning undip.

The Lance Formation in central Wyoming may be divided into three broad litholigic units distinctive enough to correlate as members. They are a lower coaly and sandy informal unit, (Glenrock facies), a middle sandy unit (Hell's Half Acre Member), and an upper bentonitic shale unit (Monkey Mountain Member).