GENERAL48106

• Permit No: C1981038
• IBM Index Class Name: General Documents
• Doc Date: 7/30/1993
• Doc Name: PROPOSED DECISION & FINDINGS OF COMPLIANCE FOR PR3
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

dips 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 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 have contributed to numerous landslides, mud
flows and rock falls. ihese 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 stratigraphic 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
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 300-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.