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The Upper Proterozoic Maraghan formation of the Murdama group is subdivided into five lithofacies units over an area of about 750 sq km near An Najady, in the northern Arabian Shield. The Maraghan formation is composed mainly of sandstone, siltstone, shale, and lesser amounts of limestone, dolomite, and carbonate-cemented sandstone, and becomes finer grained upward. The base of the Maraghan is not exposed in the study area. The lowest exposed unit, lithofacies A, is dominated by coarse-, medium-, and fine-grained sandstone and lesser amounts of siltstone, shale, and interbedded limestone and dolomite. Beds in lithofacies A form fining-upward sequences that decrease in maximum grain size upward. Lithofacies B, which gradationally overlies lithofacies A, is dominantly a fine-grained sequence that is characterized by siltstone and shale, with lesser amounts of fine-grained sandstone. Beds of limestone and dolomite are rare; some zones of medium-grained sandstone occur locally. Lithofacies C overlies lithofacies B with a gradational contact and shows a west-to-east facies change. In the western part of the area, lithofacies C is fine-grained sandstone, siltstone, and shale, containing locally thick zones of brecciated limestone and dolomite; a pinkish tan dolomite is at the base, and a thick gray dolomite pod at Jabal Rahail forms a local carbonate unit that may have been tectonically thickened. In the eastern part of the area, sandstones are coarse-and medium-grained; fining-upward cycles prevail, and carbonate occurs as thin, lenticular beds at the base of cyclic beds. Locally, sections as thick as 10 m are dominated by coarse-or medium-grained sandstone. Lithofacies C appears to thicken toward the east. Lithofacies D, which gradationally overlies lithofacies C, is composed mainly of medium-and fine-grained sandstone, siltstone, and shale. Carbonate-bearing beds are rare, although carbonate pods along the bounding thrust fault in the northwest part of the basin may be part of lithofacies D. Fining upward cycles dominate clastic rocks. In western exposures siltstone and shale are dominant, but in eastern exposures medium-and fine-grained sandstone are more common. Lithofacies E, the uppermost of the facies units mapped, is composed mainly of graded sequences of coarse-and medium-grained sandstone, fine-grained sandstone, siltstone, and shale. Limestone and dolomite occur as cement in sandstone beds or as lenticular concretions.

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The information on these maps is preliminary and questionable areas have not been field checked for accuracy. The mapping was done at a level of detail that was compatible for compilation, at 1:250,000 scale, of The Butte 2° quadrangle, Montana. Slightly more detailed study was made on the overthrust-fault zone of the Garnet Range, however. These maps do not represent the standards of mapping common to U.S.G.S. 1:24,000 quadrangles, and these maps should be considered as rough-draft field compilations.

The Butte 1 °X2° quadrangle in west-central Montana was investigated as part of the U.S. Geological Survey's Conterminous United States Mineral Assessment Program (CUSMAP). These investigations included geologic mapping, geochemical surveys, gravity and aeromagnetic surveys, examinations of mineral deposits, and specialized geochronologic and remote-sensing studies. The data collected during these studies were compiled, combined with available published and unpublished data, analyzed, and used in a mineral-resource assessment of the quadrangle. The results, including data, interpretations, and mineralresource assessments for nine types of mineral deposits, are published separately as a folio of maps. These maps are accompanied by figures, tables, and explanatory text. This circular provides background information on the Butte quadrangle, summarizes the studies and published maps, and lists a selected bibliography of references pertinent to the geology, geochemistry, geophysics, and mineral resources of the quadrangle. The Butte quadrangle, which includes the worldfamous Butte mining district, has a long history of mineral production. Many mining districts within the quadrangle have produced large quantities of many commodities; the most important in dollar value of production were copper, gold, silver, lead, zinc, manganese, molybdenum, and phosphate. At present, mines at several locations produce copper, molybdenum, gold, silver, lead, zinc, and phosphate. Exploration, mainly for gold, has indicated the presence of other mineral deposits that may be exploited in the future. The results of the investigations by the U.S. Geological Survey indicate that many areas of the quadrangle are highly favorable for the occurrence of additional undiscovered resources of gold, silver, copper, molybdenum, tungsten, and other metals in several deposit types.