GENERAL49235

• Permit No: C1981038
• IBM Index Class Name: General Documents
• Doc Date: 7/23/1993
• Doc Name: PROPOSED DECISION & FINDINGS OF COMPLIANCE FOR RN2
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

Cyprus Orchard Valley mines the D seam of the Upper Coal member of the'
Mesaverde Formation. The seam ranges in thickness from 2 to 21 feet in the
permit area, with one split ranging in thickness from 15 1/2 feet to several
inches. The coal dips 4° to 7° to the northeast. Two high-angle, normal
fault zones are found in the area with displacement ranging between 1 and
50 feet (Map 2-B). The earliest fault system trends N65°E and dips 70° to 80°
with displacements of 2 to 6 feet. The other fault system trends N35°W and
dip 70° to 80° to the northeast. The geology is further modified in the
northeast corner of Lease C-37210 where coked coal gives evidence of igneous
activity associated with the Miocene Iron Point pluton.
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° to the north and northeast, and range in age from late
Cretaceous to early Tertiary.
Coal is produced from the Mesaverde Formation, a 2500-foot-thick sequence of
sandstone, shale and coals overlain by the Ohio Creek conglomerate and
underlain by the Mancos Shale (Figure 2). The Mesaverde Formation is composed
of four members, which are, in order of decreasing age, the Rollins Sandstone,
the Lower and Upper Coal members and the-Barren member.
Minor faulting of limited vertical displacement has been observed in other
nearby mines. However, in the Orchard Valley Mine, a fault with a
displacement of fifty <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. The faults which have been encountered in these mines
tend to he high-angle, normal faults.
The steep slopes of the stream. valleys and the instability of the rock strata
in the North Fork drainage basin have 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. The units of
the Late Cretaceous in the general area are described below in ascending
order. A stratigraphlc column representing the geology of the coal member of
the Mesaverde Formation in the permit area can be found on Map 2-10, Volume 2,
and is found in this document as Figure 3.
The Mancos Shale is the oldest formation exposed in the region. 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 over-
steepening of slopes in this formation produce the numerous rock falls and
landslides observed in the lower North fork drainage basin.
The Mesaverde Formation 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