2011-01-13_REVISION - C1981038 (8)

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The Mancos Shale is the oldest formation exposed in the region. This unit is composed of over 4,000 feet of gray marine shales and minor interbedded buff sandstones. This unit is highly erodible and unstable. Erosion and over- steepening of slopes in this formation produce the numerous rock falls and landslides observed in the lower North Fork drainage basin. The Mesaverde Formation conformably overlies the Mancos Shale. This formation consists of approximately 2,300 feet of marine and terrestrial sedimentary rocks. The Mesaverde Formation is the coal-bearing formation in the region and is divided into four main members - the Rollins Sandstone, the Lower Coal Bearing (Bowie) member, the Upper Coal Bearing (Paonia) member, and the Barren (Undifferentiated) member. The Rollins Sandstone member is a 120- to 300-foot-thick, massive, cross-bedded, medium- to fine-grained, buff to white sandstone. This sandstone is regionally extensive and resistant in outcrop and forms prominent cliffs. This member is used regionally as a marker horizon to define the top of the Mancos Shale and the bottom of the coal-bearing horizons. The Lower Coal Bearing (Bowie) member consists of 260 to 350 feet of interbedded gray shales, thin to thick lenticular beds of buff-colored, fine- to medium-grained sandstones, and coals. The top of the member is usually capped by a massive buff-colored sandstone up to 90 feet in thickness. This sandstone, however, appears not to be a single persistent bed, but is actually several thick lenticular sandstones occurring at progressively lower stratigraphic horizons from east to west. Three coal beds exist in the Lower Coal Bearing member - the A (Old King) horizon, the B (Somerset) horizon, and the C (Bear) horizon. The A horizon is immediately above the Rollins sandstone and is not currently mined. The B horizon contains two coal seams and occurs about 20 to 120 feet above the Rollins sandstone. The C horizon contains one coal seam that occurs 50 to 100 feet above the B horizon. The Upper Coal Bearing (Paonia) member consists of 200 to 500 feet of gray shales,, interbedded, buff-colored, lenticular sandstones, and coals. The top of this member usually grades into 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. Three coal horizons have been identified in the Upper Coal 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. This horizon was mined in the Bowie No. 1 Mine. The E horizon occurs about 130 feet above the D horizon and contains two coal seams. The F horizon contains two coal seams. Coal seams of the F horizon do not exist to the north of the North Fork of the Gunnison River in thicknesses sufficient for mining. The Barren (Undifferentiated) member of the Mesaverde Formation consists of up to 1,500 feet of terrestrial sedimentary rocks. 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. The Mesaverde Formation is unconformably overlain by the Tertiary Age 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 conglomerate is the basal unit within the formation and is 100 to 200 feet thick. Tertiary igneous intrusive rocks exist within the North Fork drainage basin. A diorite plug about 1,000 feet in diameter outcrops along Hubbard Creek in the SE 1/4 of Section 7, Township 13 South, Range 91 West of the 6th P.M. This may represent the erosional remnants of a volcanic flow feeder. Sills have 13