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-zo- <br />The proposed Mt. Gunnison No. 1 Mine is an underground mine within the <br />F coal seam. Mining is to progress from the portals in the valley of <br />the North Fork of the Gunnison River up the dip of the F seam under the <br />watershed of Minnesota Creek. The extraction of coal will be by <br />continuous mining methods. Controlled subsidence will occur during <br />pillaring. <br />Development waste rock and processing waste from the Mt. Gunnison mine <br />will be disposed of in two disposal piles, a small initial pile located <br />on the bench below the portals, which will be active during the first <br />5-year permit period, and a larger pile located above the portal bench, <br />which is proposed to be permitted in 1985. <br />Load out facilities for the Mt. Gunnison Mine will be located along the <br />North Fork of the Gunnison River. <br />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 <br />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 Mine, a fault with a displacement of 50 feet was <br />encountered during mining and 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 <br />which have been encountered in existing mines tend to be high angle <br />normal 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 <br />Fork 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 <br />deposits. A generalized stratigraphic column of Late Cretaceous units <br />can be found in Figure 3. The units are described below in ascending <br />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 />