GENERAL30618 - Laserfiche WebLink

Home Browse Search

-25- Regional Geolo The Somerset Coal Field lies on the southeast margin of the Piceance Basin, and just south of Grand Mesa. The sedimentary strata exposed in the Somerset Coal Field dip at 3 to 5 degrees to the north and northeast. Only minor faulting of limited vertical displacement has been observed in the existing 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. Drill hole data indicates the presence of other faults in the life-of-mine area with similar displacements. One major fault has been encountered in the Somerset Mine. 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 Age 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 Mesa Verde 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 Mesa Verde 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 (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) 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