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~' <br />2. Regional Geology <br />The Somerset Coal Field lies on the southeast margin of the Piceance <br />Basin, and just south of Grand Mesa. The sedimentary strata exposed in <br />the Somerset Coal Field dip at 3 to 5 degrees to the north and northeast. <br />Only minor faulting of limited vertical displacement has been observed <br />in the existing Blue Ribbon, Bear, and Hawk's Nest Mines. However, in <br />the Orchard Valley Pdine, a fault with a displacement of 50 feet was <br />encountered during mining. Drill hole data indicates the presence of <br />other faults in the life-of-mine area with similar displacements. One <br />major fault has been encountered in the Somerset Mine. The faults which <br />have been encountered in existing mines tend to be high angle normal <br />faults. <br />The steep slopes of the stream valleys and the instability of the rock <br />strata in the North Fork Drainage Basin has contributed to numerous <br />landslides, mud flows and rock falls. These mass wasting features have <br />been mapped by W.R. Junge of the Colorado Geological Survey and <br />published as an open file report, entitled "Geologic Hazards, North Fork <br />Gunnison River Valley, Delta and Gunnison Counties, Colorado". <br />Geologic units exposed in the North Fork Drainage Basin consist of Late <br />Cretaceous to Early Tertiary Age sedimentary strata, Tertiary Age <br />igneous intrusives, and Quaternary Age alluvial and colluvial deposits. <br />The units are described below in ascending order. <br />The Mancos Shale is the oldest strata exposed in the region, and is of <br />Late Cretaceous Age. This unit is composed of over 4,000 feet of gray <br />marine shales and minor interbedded buff sandstones. This unit is <br />highly erodible and unstable. Erosion and oversteepening of slopes in <br />this formation produce the numerous rock falls and landslides observed <br />in the lower North Fork Drainage Basin (Junge, 1978). <br />The Mesa Verde Formation is of Late Cretaceous Age and conformably <br />overlies the Mancos Shale. This formation consists of approximately <br />2,300 feet of marine and terrestrial sedimentary rocks. The Mesa Verde <br />Formation is the coal-bearing formation in the region and is divided <br />into four main members; the Rollins sandstone, the Lower Coal Bearing <br />(Bowie) member, the Upper Coal Bearing (Paonia) member, and the Barren <br />(Undifferentiated) member (Johnson, 1948), <br />The Rollins sandstone member is a 120 to 200 foot thick, massive, <br />cross-bedded, medium to fine-grained, buff to white sandstone. This <br />sandstone is regionally extensive and resistant in outcrop and forms <br />prominent cliffs. This member is used regionally as a marker horizon to <br />define the top of the Mancos Shale and the bottom of the coal-bearing <br />horizons. <br />33 <br />