GENERAL52614

• Permit No: C1981044
• IBM Index Class Name: General Documents
• Doc Date: 4/24/1987
• Doc Name: Proposed Decision & Findings of Compliance for PR1
• Permit Index Doc Type: Findings
• Media Type: D
• Archive: No

C. Description of the Existing Environment
Regional Geolooy
The Eagle Mines lie within the Sand Wash structural basin of northwest
Colorado. The Sand Wash basin is a product of the Laramide Orogeny, a period
of regional geologic structural development that began in late Cretaceous time
and continued into the Tertiary Eocene. The Sand Wash basin is bounded by
uplifts of similar age including the Uinta Mountains on the west and the Axial
Basin anticline, the structural extension of the Uinta uplift, on the south.
The South Park Range borders the Sand Wash basin on the east white the
subsurface Cherokee Ridge lies to the north. (See figure 6).
The influence of several folds of lesser magnitude within the Sand Wash basin
is seen in the permit area of the Eagle No. 5 and No. 6 Mines (See Figure 3).
All of these folds have general NW-SE axial trend. The surface Trapper Mine
and a large portion of the proposed future mining in the Eagle permit area lie
within the southwestern flank of one of these folds, the Big Bottom syncline.
Older developments in the western extremes of the Eagle No. 5 Mine, however,
extend across the axis of the adjacent Williams Fork anticline. Structural
dips in the area of the Eagle Mines range from 6 to l2 degrees north and
northeast toward the axis of the Big Bottom syncline and may exceed 20 degrees
southwest into the Round Bottom syncline.
The area of the Eagle Mines has also undergone faulting as a result of
regional and local structural development. Faults with several miles of
surface expression lie to the north and west of the Big Bottom syncline (See
Figure 3). In the immediate area of the mines, relatively minor normal faults
with less than 50 feet of displacement have been encountered. Some but not
all of these also have surface traces. Most of these faults conform to
regional structural grain and trend roughly NW-SE.
The geologic formations exposed in the
Cretaceous and Tertiary age (See Figure
formations are: the Mancos Shale, the
the Mesa Verde Group, the Lewis Shale,
Formation. Unconsolidated deposits of
the local streams and rivers.
general area are largely of Late
's 4 and 7). In ascending order, these
Iles and Williams Fork Formations of
the Lance Formation and the Browns Park
Quaternary age occupy the valleys of
The Mancos Shale is the oldest stratigraphic unit exposed in the area. It is
a thick (over 4,000 feet) homogenous, light to dark gray, fossiliferous,
marine shale with some intercalated sandstone and limestone beds. The
sandstones within the Mancos are generally thin bedded, fine grained, tan, and
fossiliferous and form resistant ledges in the basal and upper parts of the
formation. The outcrop areas of the Mancos Shale are characterized by
rolling, hummocky topography.
The Mesa Verde Group is approximately 2,500 feet thick and conformably
overlies the Mancos Shale. The Iles Formation and Williams Fork Formation
comprise the Mesa Verde Group.
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