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-20- Description of the Hydrologic Environment - Regional Geology (Cont'd) mined. The "B" horizon contains two r_oal seams and occurs about 20 to 120 feet above the Rollins sandstone. This lioi-izon is presently mined by U.S. Steel at the Somerset Mine and may be mined at the Mt. Gunnison Mine. The "C" horizon contains one coal seam that occurs 50 to 100 feet above the "B" horizon. This has been mined by U.S. Steel and is currently mined at the Bear mines. It may be mined at the Mt. Gunnison Mine in the future. The Upper Coal-Bearing (Paonia) member consists of 200 to S00 feet of gray shales, interbedded, buff-colored, lenticular sandstones, and coals. The top of this member is generally considered to be capped by a massive, cliff-forming sandstone. However, like the similar sandstone at the top of the Lower Coal member, this sandstone is not a single persistent bed. Dri11 hole data for both the Mt. Gunnison and the Somerset life-of-mine areas show this sandstone unit to occur as several lenticular sandstones at slightly different stratigraphic horizons. Three coal horizons have been identified in this member; the "D" (Oliver) Horizon, the "E" (Hawk's Nest) horizon, and the "F" horizon. The "D" horizon occurs directly above the "massive" sandstone of the Lower Coal-Bearing member and contains three seams. Thi horizon is currently being mined in the Orchard Valley Mine. It may be mined in the future at the Hawk's Nest Mine, at the Mt. Gunnison Mine, and by Bear Coal in the Bear No. 3 hfine. The "E" horizon occurs about 130 feet above the "D" horizon and contains two coal seams. This horizon is currently being mined at the Hawk's Nest Mine and at the Blue Ribbon Pfi ne. It may be mined at the Mt. Gunnison Mine and at the Bear No. 3 Cline in the future. The "F" horizon contains two coal seams and presently is only mined at the h1t. Gunnison t9ine. Coal seams of the "F" horizon do not exist to the north of the North Fork in thickness sufficient for mining. The Barren (Undifferentiated) member of the Mesaverde Formation consists of terrestrial sedimentary rocks to 1,600 feet in thickness. This unit consists of fine-grained, buff-colored, lenticular sandstones, gray shales and thin lenticular coal beds. The sandstones predominate and are highly lenticular, discontinuous and of limited lateral extent in outcrop (Johnson, 1998). The Mesaverde Formation is unconformably overlain by the Tertiary aged Rudy or Wasatch Formation. This formation consists of red to buff-colored shales, red sandstones, and red to gray conglomerates. The sediments of this formation are weathered volcanic rocks. The Ohio Creek congolmerate is the basal unit within the formation and is 100 to 200 feet thick. Igneous intrusive rock exist within the North Fork Drainage Basin. A diorite plug about 1,000 feet in diameter outcrops along Hubbard Creek in the SE-'s of Section 7. This may represent the erosional remnants of a volcanic flow feeder. Sills have injected the Lower Coal-Bearing member, particularly the "B" and "C" seams. These sills consist of diorite and appear to have their source to the northwest of Terror Creek. The alluvium of the North Fork of the Gunnison River consists of Quaternary Aged deposits of mixed coarse sand, cobbles and boulders. These coarse sediments are composed primarily of igneous and metarrarphic rock types, and have their source area in the headwaters and upper reaches of the North Fork. This coarse alluvium is capped by finer sands and silts. The North Fork alluvium in the area of the Hawk's Nest, Bear, tot. Gunnison and Somerset mines is fairly narrow in width and between SO to 70 feet thick. About a mile below the town of Somerset, Colorado, the width of alluvium increases while the thickness of alluvium decreases to about 35 fee