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-18- Description of the Hydrologic Environment - Regional Geology (Cont'd) Only minor faulting of limited vertical displacement has been observed in the existing Somerset, Blue Ribbon, Bear and Hawk's Nest mines. However, in the Orchard Valley Mine, a fault with a displacement of 50 feet was encountered during mining and drill hole data indicates the presence of other faults in the Life-of-mine area with similar displacements. No other major faults have been encountered in mines in the region to date. The faults which have been encountered in existing mines tend to be high angle normal faults. The steep slopes of the stream valleys and the instability of the rock strata in the North Fork Drainage Basin has contributed to numerous landslides, mud flows and rock falls. These mass wasting features have been mapped by W.R. Junge of the Colorado Geological Survey and published as an open file report, entitled "Geologic Hazards, North Fork Gunnison River Valley, Delta and Gunnison Counties, Colorado." Geologic units exposed in the North Fork Drainage Basin consist of Late Cretaceous to Early Tertiary Age sedimentary strata, Tertiary Age igneous intrusives, and Quaternary aged alluvial and colluvial deposits. A generalized stratigraphic column of Late Cretaceous units can be found in Figure 3. The units are described below in ascending order. The Mancos Shale is the oldest strata exposed in the region, and is of Late Cretaceous age. 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 oversteepening of slopes in this formation produce the numerous rock falls and landslides observed in the lower North Fork Drainage Basin (Junge, 1978). The Pfesaverde Formation is of Late Cretaceous age and 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 for- mation 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 (Johnson, 1948). The. Rollins sandstone member is a 120 to 200 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) members consist of 260 to 350 feet of inter- bedded 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 horizons 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