Regional Geology
The Powder River Coal Basin, of northeastern Wyoming, lies entirely within the boundaries of the Powder River structural and topographic basin. Coal-bearing strata range in age from Upper Cretaceous in the Mesa Verde Formation to Eocene in the Wasatch Formation. The Powder River Coal Basin covers parts of Campbell, Converse, Crook, Natrona, Niobrara, Johnson, and Weston Counties and is the largest coal basin in Wyoming.

Sandstone is a major lithologic component of the overburden in the mine Area. Siltstone constitutes approximately 15 percent of the overburden by volume. Like the mudstone, it is uniformly textured with 20 to 55 percent silt, and generally 20 to 60 percent clay, the remainder being sand. It is light to medium gray in color and slightly more consolidated than mudstone. Like other overburden lithologic units, the siltstone is discontinuous and occurs interbedded with mudstone and sandstone. In some areas, it overlies coal.

Property Geology
The economically mineable coal in Campbell County occurs within the Tongue River Member of the Fort Union Formation. The Wyodak coal seam occurs at the top of the Fort Union Formation and is overlain by the Wasatch Formation. The coal is low sulfur, low ash, and is subbituminous C in rank. Surface mineable coal deposits occur along the north-northwesterly striking subcrop of the Wyodak coal seam. The coal seam subcrops on the eastern edge of the lease and dips about two to three ........

The Black Thunder LOM Plan projects mining from three pits, North, West, and South, through 2036. The pits are typically 200 to 230 feet wide, with pit lengths ranging from 4,310 feet to 16,906 feet in the LOM Plan. The typical pit configuration is an initial truck/shovel pass for prestrip, since the draglines cannot handle the total depth of overburden. In some areas, coal (Wyodak Rider seams) is encountered in the prestrip and where quality is acceptable, it is mined. Most of the overburden is blasted, although there are some unconsolidated areas where blasting is not required.

The mining method at the Black Thunder Mine is surface mining utilizing draglines and truck/shovel mining equipment. The Black Thunder Mine is mining the Upper and Main splits of the Wyodak Seam and parting interval within the seam utilizing draglines, shovels, front-end loaders, trucks, dozers, or scrapers in three long pits.

The Black Thunder Mine currently operates a fleet of four draglines and nine shovels for overburden removal and four shovels for coal removal from three pits. The shovels develop a series of benches that range from 50 to 100 feet to prepare a bench for the draglines. A fleet of seven diesel-powered drills create 12-1/4-inch boreholes at a 60 degree angle for cast-blasting the overburden 150 feet above the coal. The draglines remove the overburden above the coal from a bench created by the dozers, after the cast-blast. After the overburden is removed by the ........

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