GENERAL54560 - Laserfiche WebLink

Home Browse Search

1. Description of the Hydrologic Environment a. Regional Geology The Blue Ribbon Mine lies on the southwestern margin of the Piceance Basin, a part of the Colorado Plateau province. The steep slopes of the stream valleys and the instability of the rock strata in the North Fork of the Gunnison 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 "Geological Hazards, North Fork Gunnison River Valley, Delta and Gunnison Counties, Colorado." The Blue Ribbon Mine extracted coal from the E seam of the Upper Coal Member of the Mesaverde Formation. The Upper Coal Member ranges from 400 to 600 feet thick in the Somerset Coal Field, also known as the North Fork-Minnesota Creek or Paonia Coal Field. The rocks exposed in the Somerset Coal Field consist of the Mancos Shale and the coal-bearing Mesaverde Formation of Upper Cretaceous Age; and the Ohio Creek Conglomerate, the Wasatch Formation and the Quartz monzonite porphyry of Early Tertiary Age. Coal was to be produced from the Mesaverde Formation, a 2,500- foot-thick sequence of sedimentary strata overlain by the Ohio Creek Conglomerate and underlain by the Mancos Shale. Beds exposed in the mine area dip 3 to 5 degrees north-northeast. The E seam overburden ranges from less than 20 feet near the western edge of the Blue Ribbon mine permit area to over 1,400 feet in eastern portions of the permit area. The Mancos Shale is the oldest stratum 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. The Mesaverde 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 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 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 _lg_