GENERAL33122 - Laserfiche WebLink

Home Browse Search

-30- Regional Geology 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 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 faults have been encountered in the Mt. Gunnison mine 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 Aged sedimentary strata, Tertiary Aged igneous intrusives, and puaternary Aged alluvial and colluvial deposits. A generalized strati graphic column can be found in Figure 2. 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 grey marine shales and minor interbedded buff sandstones. This unit is highly erosive and unstable. Erosion and oversteepening of slopes in this formation produce the numerous rock falls and land- slides observed in the Zower 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. The Rollins Sandstone dfember 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 persistent marker horizon to define the top of the Mancos Shale and the bottom of the coal-bearing horizons.