Mineral resources of the Seven Blackfoot Wilderness Study Area, Garfield County, Montana

Abstract

This report assesses the identified mineral resources (known) and mineral resource potential (undiscovered) of parts of the Blackfoot Wilderness Study Area (MT-024-657A and MT-024-657B; 1,160 and 4,550 acres, respectively). The areas are individually referred to as segments A and B, respectively. The segments are collectively referred to as the Seven Blackfoot Wilderness Study Area, the or simply the study The area is located about 30 miles northwest of Jordan, Mont., in the northern Great Plains physiographic province, on the northwestern flank of the Williston Basin. The wilderness area has tracts (fig. 1) containing 746,300 short tons of demonstrated subeconomic resources of coal (fig. 2); of this amount, 273,025 short tons of coal may not be of resource quality due to possible high ash contamination as indicated from analyses of weathered surface exposures; these analyses however, may not be representative of the unweathered coal. The wilderness area has other occurrences of noneconomic coal in beds too thin to be considered a resource. The wilderness area also has mineral occurrences of kaolinite, montmorillonite, placer gold, and sand and gravel; concentrations of these mineral commodities are small, impure, and are not considered to be resources. Parts of the wilderness area have (1) a high mineral resource potential for coal, (2) a moderate mineral resource potential for coal and kaolinite, and (3) a low mineral resource potential for montmorillonite, sand, gravel, and placer gold (fig. 1). The entire area has (1) a moderate energy resource potential for oil and gas, (2) a low mineral resource potential for all metallic minerals and bentonite, and (3) a low geothermal energy resource potential (fig. 1). Character and Setting The area is located about 30 mi (miles) northwest of Jordan, Mont. (fig. 3), in the northern Great Plains physiographic province, north-central Montana, on the northwestern flank of the Williston Basin (fig. 4). The regional topography is characterized by rolling prairies, broad shallow valleys, and scattered buttes. In contrast to the regional setting, the area is deeply incised by the Missouri River and its tributaries. The resulting rugged badland topography, known as the Missouri Breaks, is characterized by numerous deep canyons separated by steep, narrow ridges. The area is accessible by gravel roads from Jordan. Upper Cretaceous (see geologic time chart in appendix for relative ages) rocks exposed in, and immediately underlying, the area include, in ascending order, the Judith River Formation (subsurface only), Bearpaw Shale, Fox Hills Sandstone (including the Colgate Member of the Fox Hills Sandstone), and Hell Creek Formation. Tertiary rocks exposed in the area include the Tullock Member of the Fort Union Formation. In the area, Upper Cretaceous and Tertiary strata are nearly horizontal. Landslide deposits of Quaternary age flank valley walls and intertongue with Quaternary alluvial deposits that cover valley floors in and adjacent to the wilderness area. Deposits of glacial till of Pleistocene age occur in thin isolated patches scattered on higher surfaces along the western boundary of segment A. The Judith River Formation, Bearpaw Shale, and Fox Hills Sandstone were deposited in intertonguing offshore-marine, nearshore-marine, and continental environSeven Blackfoot Wilderness Study Area D1 | a o o (0 (0 (/) 10 7° 30 ' 47 °3 5' T2 1 N \-f c \. R 33 E A P P R O X IM A T E B O U N D A R Y O F S E V E N B LA C K F O O T W IL D E R N E S S . S T U D Y A R E A ( M T -0 2 4 -6 5 7 A )