GENERAL40636

• Permit No: C1982057
• IBM Index Class Name: General Documents
• Doc Date: 8/8/1985
• Doc Name: PROPOSED DECISION AND FINDINGS OF COMPLIANCE
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

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 No. 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 (No. 3) coals of the lower group situated about 900 to 1,000 feet above
the base; and 4) the Trout Creek sandstone member which caps the formation
(Figure No. 1). 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 No. 1). The formation is conformable at its base and top, and it is
divided into three units, the lower, middle and upper units.
The lower unit is about 1,000 feet thick, consisting of sequences of shale,
thin sandstone beds, sandy shale and several coal beds. The middle coal group
of this unit contains the coal seams of economic importance for most of the
mining in this area. In ascending order they are the Wolf Creek, Wadge and
Lennox seams. All the mines considered in this CHIS surface water area mine
one or more of these coals except the Grassy Gap Mine and the Meadows No. 1
Mine. The Seneca II-W Mine will mine the Wadge coal seam. The Lennox seam
has been eroded throughout much of the mine site and the Wolf Creek seam lies
too deep for recovery by surface mining methods. 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 Member
(Figure No. 1). 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 Mesaverde Group is the Lewis Shale (Figure No. 1), 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 30 feet thick. The
maximum thickness of these unconsolidated deposits occurs in the Yampa River
alluvirmi which is estimated to be less than 100 feet thick (Brogden and Giles,
1981). These deposits are found most extensively along the Yampa River,
Trout, Middle, Foidel, Fish, Sage and Dry Creeks and consist predominately of
clay, sand and lenticular, discontinuous gravel layers.
The alluvium is thin or absent in areas where streams cross the resistant
sandstones of the Mesaverde Group. The alluvial aquifers are wider where the
streams cross less-resistant rock units of the Lewis and Mancos Shales.
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