Use of geophysical logs in recognizing depositional environments in the Tongue River Member of the Fort Union Formation, Powder River area, Wyoming and Montana

The environmental conditions under which rocks in the Paleocene Tongue River Member of the Fort Union Formation were deposited in the Powder River area, Wyoming and Montana, can be determined using geophysical logs with some limitations. It is widely recognized that gamma ray and density logs are useful in identifying thickness and stratigraphic position of coal beds. In addition, gamma ray and electrical resistivity logs can be used to infer conditions of transportation and deposition of sandstones, siltstones, and other rock types. In particular, intensity responses of the gamma ray and resistance logs provide a clue to variations of grain size such as fining-upward and coarsening-upward characteristics of fluvial channel and crevasse splay deposits, respectively. These signatures in the geophysical logs are readily observed for some beds; for other beds however, the depositional conditions are difficult to determine because the beds do not produce clear-cut log-response patterns. Thus,. analysis of the environments of deposition of detrital rocks in drill holes can be made more accurate by a study of stratigraphically equivalent intervals in outcrops near drill-hole sites.