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_P2_ <br />horizon occurs about 130 feet above the "D" horizon and contains two coal <br />seams. This horizon is currently being mined at the Hawk's Nest Mine and at <br />the Blue Ribbon Mine. It may be mined at the Mt. Gunnison Mine and at the <br />Bear No. 3 Mine in the future. The "F" horizon contains two coal seams and <br />presently is only mined at the Mt. Gunnison Mine. Coal seams of the "F" <br />horizon do not exist to the north of the North Fork in thicknesses sufficient <br />tur wininy. <br />Tiie barren (Unaiffierentiated) member of the Mesaverde Formation <br />consists of up to 1,600 feet of terrestrial sedimentary rocks. This <br />unit consists of fine-grained, buff-colored, lenticular sandstones, <br />gray shales and thin lenticular coal beds. The sandstones predominate <br />and are highly lenticular, discontinuous and of limited lateral extent <br />in outcrop (Johnson, 1948). <br />The Mesaverde Formation is unconformably overlain by the Tertiary Age <br />Rudy or Wasatch Formation. This formation consists of red to <br />buff-colored shales, red sandstones, and red to gray conglomerates. <br />The sediments of this formation are weathered volcanic rocks. The Ohio <br />Creek conglomerate is the basal unit within the formation and is 100 to <br />200 feet thick. <br />Igneous intrusive rocks exist within the North Fork Drainage Basin. A <br />diorite plug about 1,000 feet in diameter outcrops along Hubbard Creek <br />in the SE 1/4 of Section 7. This may represent the erosional remnants <br />of a volcanic flow feeder. Sills have injected the Lower Coal-Bearing <br />member, particularly the "B" and "C" seams. These sills consist of <br />diorite and appear to have their source to the northwest of Terror <br />Creek. <br />The alluvium of the North Fork of the Gunnison River consists of <br />Quaternary Age deposits of mixed coarse sand, cobbles and boulders. <br />These coarse sediments are composed primarily of igneous and <br />metamorphic rock types, and have their source area in the headwaters <br />and upper reaches of the North Fork. This coarse alluvium is capped by <br />finer sands and silts. The North Fork alluvium in the area of the <br />Hawk's Nest, Bear, Mt. Gunnison and Somerset Mines is fairly narrow in <br />width and between 50 to 70 feet thick. About a mile below the town of <br />Somerset, Colorado, the width of alluvium increases while the thickness <br />of alluvium decreases to about 35 feet. <br />Description of the Ground Water Regime <br />Four categories of potential aquifers occur in the Somerset Coal <br />Field. These are: the alluvial and terrace deposits associated with <br />the North Fork of the Gunnison River; the localized, shallow alluvium <br />along creeks tributary to the North Fork; the discontinuous, lenticular <br />and laminar sandstones of the Mesaverde Formation; and, the Rollins <br />sandstone. <br />