This map is a product of field mapping and of photogrammetric mapping using color aerial photographs at a scale of approximately 1:24,000 (Molina, 1983). These geologic investigations were part of a project to map the principal coal beds of large parts of the Powder River basin for national coal resource assessment.
The Ucross-Arvada area is part of the Powder River Basin, a large, north-trending structural depression between the Black Hills on the east and the Bighorn Mountains on the west. Almost all of the study area is within Sheridan and Johnson Counties, Wyoming. Most of the Ucross-Arvada area lies within the outcrop of the Wasatch Formation of Eocene age; the extreme northeast corner falls within the outcrop of the Tongue River Member of the Fort Union Formation of Paleocene age. Within the Powder River Basin, both the Wasatch Formation and the Tongue River Member of the Fort Union Formation contain significant coal resources. The map includes locations and elevations of coal beds at 1:50,000 scale for an area that includes ten 7½-minute quadrangles covering some 500 square miles. The Wasatch Formation coal beds shown (in descending order) are Monument Peak, Walters (also called Ulm 1), Healy (also called Ulm 2), Truman, Felix, and Arvada. The Fort Union Formation coal beds shown (in descending order) are Roland (of Baker, 1929) and Smith.
The assessment of the mineral resource potential of the Routt National Forest and the Middle Park Ranger District of the Arapaho National Forest, Colorado, was conducted so that the mineral resources of the forest can be considered along with other resources in land use planning. This report discusses the potential for as yet undiscovered mineral and energy resources within the forest. Mineral resource information is given in terms of mineral deposit types and their geologic settings, and mineral deposit types are defined by geologic characteristics of known deposits that may occur within or near the forest. All available information regarding mineral deposits or occurrences and energy resources, as of June 1994, was assembled to assess the mineral and energy potential. Geologic maps were compiled and mineral and energy potential maps were also created for deposit types within the forest.
The Big George coal bed, 30 mi (48 km) west of Gillette, Wyoming, is the thickest part of a large Anderson coal deposit. The coal resources of this central core, essentially a single bed of coal up to 202 ft (62 m) thick, End_Page 859------------------------------ were previously estimated at 113 billion short tons. This deposit is in the Paleocene Tongue River Member of the Fort Union Formation; overburden ranges from 700 to 2,400 ft (213 to 732 m). The Big George bed was initially outlined using geophysical logs from nearly 300 oil and gas drill holes. More logs were studied in the northern portion of Big George and as far north as the Montana state line to examine the entire system of coal beds that includes this thick bed. We interpreted geophysical logs primarily for coal and sandstone, digitized lithologic intervals, and generated strip logs of lithologic sequences using a microcomputer. These computer-generated logs were generated in lines of sections, on matching elevations, to reconstruct the stratigraphic framework of subsurface coal in this part of the Powder River basin. The framework was used to trace the interval containing the Anderson deposit into the Decker, Montana, and Recluse, Wyoming, areas. This interval appears to be confined by the Smith coal bed above; the bottom of the interval is less well defined. Lithologic patterns of the framework suggest that a major fluvial channel system defined part of the northwestern boundary of the Big George coal bed. The locations of these channels may have been controlled primarily by Laramide deformation in the Powder River basin. End_of_Article - Last_Page 860------------
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