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The Big Horn Coal basin is located within the topographic and structural basin of the same name and is defined by the limits of the Upper Cretaceous Mesaverde Formation in northwestern Wyoming and the Eagle Sandstone in south-central Montana. Eight coalfields in Wyoming and two in Montana occur in this large, asymmetrical, northwest-trending synclinal coal basin. While coal-bearing rocks are found in the Cloverly, Frontier, Lance, Willwood, Tatman, Judith River, Fort Union, Meeteetse, Eagle Sandstone, and Mesaverde formations, only the last four of these formations contain coal beds of economic significance.
The coal in this basin ranges in rank from high volatile C bituminous (based primarily on resistance to weathering) to subbituminous B coal. In general, the Mesaverde and Eagle coals are highest in heat content, averaging over 10,500 Btu/lb; the Fort Union coals in the Red Lodge-Bear Creek and Grass Creek fields average about 10,200 Btu/lb and are second highest in heating value. The Meeteetse Formation contains coals that average 9,800 Btu/lb, the lowest heating values in the basin. An average heating value for all coal in the basin is slightly less than 10,000 Btu/lb. The average sulfur content of all coals in this basin is less than 1%, with a range of 0.4 to 2.2%. For coals of economic interest in the basin, the Forth Union coals in the Grass Creek field are lowest in sulfur conte t, averaging 0.4%; the Fort Union coals in the Red Lodge-Bear Creek field are highest in sulfur, averaging about 1.9%.
Coal mining in the Big Horn Coal basin began in the late 1880s in the Red Lodge field and has continued to the present. Almost 53 million tons of coal have been mined in the basin; nearly 78% of this production (41 million tons) is from bituminous Fort Union coal beds in tho Red Lodge-Bear Creek and Bridger coal fields, Montana. Nearly all the production in Wyoming has been from Mesaverde coals in the Gebo field. Most of the coal was mined underground and was used by the railroads; maximum production for the entire basin was attained in 1920, when 2.4 million tons were produced. The only coal activity in the Big Horn basin at present is a small strip mine operated by Northwestern Resources at Grass Creek, Wyoming.
Original in-place resources for the Big Horn Coal basin are given by rank of coal: 1,265.12 million tons of bituminous coal resources have been calculated for the Silvertip field, Wyoming, and the Red Lodge-Bear Creek and Bridger fields, Montana; 563.78 million tons of subbituminous resources have been calculated for the remaining Wyoming coal fields. Remaining recoverable bituminous resources in coal beds over 28 in. (71 cm) thick and at depths less than 1,000 ft (300 m) are estimated at 400 million tons. Remaining recoverable subbituminous resources in coal beds over 5 ft (1.5 m) thick and under less than 1,000 ft (300 m) of overburden are estimated at 41.7 million tons, of which 18.6 million tons is considered strippable.
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