GENERAL35005

• Permit No: C1980003
• IBM Index Class Name: General Documents
• Doc Date: 1/23/1987
• Doc Name: Proposed Decision and Findings of Compliance For RN1
• Permit Index Doc Type: Findings
• Media Type: D
• Archive: No

The Mancos Shale is a thick (approximately 5,000 ft.) homogeneous light gray
to dark-gray fossiliferous marine shale with interbedded sandstone and
limestone beds and is the oldest unit exposed in the area (Figure 4). The
sandstones are generally thin bedded, fine grained, tan, and fossiliferous,
and form resistant ledges in the basal and upper parts of the formation. The
overall area occupied by the tAancos Shale is characterized by rolling,
hummocky topography.
The Mesaverde Group is approximately 3,000 feet thick and conformably overlies
the t4ancos Shale (Figure 4). It consists of the Iles Formation and lilliams
Fork Formation.
The Iles formation is the lower unit and is approximately 1,500 ft. thick. It
consists of interbedded light brown to white, massive, fine-grained,
ledge-forming sandstones, brown to black carbonaceous shale, sandy shale and
coal beds. The coal beds are assigned to the lower coal group (Figure 4) of
the Mesaverde, and are distributed throughout the middle and upper parts of
the formation. Four persistent sandstone beds occur within the Iles
Formation. They are: 1) the Tow Creek sandstone member at the base, 2) a
double ledge-forming sandstone sequence 400 feet or more above the base, 3) a
light gray sandstone sequence of variable composition associated with the
upper'(NA. 3) coals-bf-the lower:gr•oup situated about 900 to 1,000 feet above
the base, and 4) the Trout Creek sandstone member which caps the formation
GFigure 4)..-The-Trout=Creek-sandstone is a 50 to 100 foot thick, light brown
to light gray, fine-grained, massive sandstone.
The upper unit of the-Mesaverde Group is the Williams Fork Formation which is
approximately 1,000 to 2,000 feet thick and includes all beds between the top
of the Trout Creek Sandstone Member and the base of the overlying Lewis Shale
(Figure 4). The formation is conformable to its base and top, and it is
divided into three units: the lower, middle and upper units.
The lower unit as about 1,000 feet thick, consisting of shale, thin sandstone
beds, sandy shale and several coal beds of the middle coal group. The middle
coal group contains the coal seams of economic importance in this area. In
ascending order they are the Wolf Creek, 4tadge and Lennox coals. The middle
unit of the Williams Fork Formation includes a massive, white, cross-bedded,
cliff-forming sandstone about 100 to 200 feet thick, called the Twentymile
Sandstone Plember (Figure 4). It is used as a key marker bed throughout the
region. The upper units of the Williams Fork Formation consist of interbedded
sandstone, sandy shale, shale, sandstone and several thin coal beds of the
upper coal group.
Above the Desaverde Group is the Lewis Shale (Figure 4), which is a 1,500 to
2,000 foot thick sequence of dark-gray to bluish, homogenous marine shale with
several thin interbedded sandstones and calcareous concretions.
Unconsolidated alluvial deposits of Quaternary age constitute the youngest
geologic units in the area and are generally less than thirty (30) feet
thick. The maximum thickness of these unconsolidated deposits occurs in the
Yampa River alluvium which is estimated to be less than 100 feet thick
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