Abstract The Powder River Basin of northeast Wyoming and southeast Montana contains coal resources larger than those of any other basin of comparable size in the United States. These coal resources are developed in 16 strip mines for coal-fired power plants and for coalbed methane in more than 20,000 wells for distribution throughout the country. During this field trip, strip mining methodology and technology at the Wyodak coal mine, and operations at the nearby Wyodak power plant and in coalbed methane fields will be observed and discussed. The power plant utilizes feed coal from the mine and supplies electricity for the City of Gillette and surrounding areas. Coalbed methane development in the basin has coexisted with coal mining since the early 1980s. The same coal beds that are being mined yield gas in the subsurface, which is produced, collected, and moved to pipelines to access the interstate pipeline grid to serve the Rocky Mountains, Midwest, and California customers. Keywords: Powder River Basin, Wyoming, coal, coalbed methane, gas.

Abstract In the Powder River Basin of southeast Montana and northeast Wyoming, coal beds exposed by regional erosion have burned naturally from as early as the Pliocene to the present. Layers of reddish clinker, formed by baking, welding, and melting of sediments above burned coal beds, cover over 4000 km 2 and cap ridges and escarpments throughout the dissected landscape of the Powder River Basin. Fission-track (ZFT) and (U-Th)/He (ZHe) ages of zircon grains from baked sandstones in clinker provide new insights about rates of regional erosion as well as episodic advance of coal fires into hillsides. Older, resistant clinker layers up to 60 m thick, formed by the burning of thick coal beds, cap summits and broad benches. Younger clinker rims, from thinner coals, form ledges on valley sides. ZHe ages of clinker, mainly from the Wyodak-anderson coal zone of the fort Union formation in the Rochelle Hills east of Wright, Wyoming, and from the Wyodak-anderson and Knobloch coal zones in the Tongue River valley near Ashland and Birney, Montana, range from 1.1 Ma to 10 ka. These dates generally agree with ZFT ages of clinker analyzed in the early 1980s, but they are a more precise record of ancient coal fires in the region. Our data indicate 0.2–0.4 km of vertical erosion in the past 1 m.y. Spatial-temporal patterns of clinker ages may prove to be useful in deciphering the patterns of fluvial incision and basin excavation in the Powder River Basin during the late Cenozoic and in weighing the relative importance of uplift, variations in climate, and base-level change.