Summary:
The Project Area lies within a region that is deformed by several major folds, indicating that various seams have folded over and split. The Project Area occurs on the southern and northern limbs of the generally southeast-trending asymmetrical Collom Syncline and extends east toward the north-northeast-trending Elkhorn Syncline (KEC 2005, USGS 2008). The complex structures seen in the Project Area are overlain by younger sedimentary sequences that reflect upward-diminishing deformation. Periodic movements along the ancestral Axial Fault located north of the Danforth Hills coal field are believed to have been the source of the major deformation seen presently in the Project Area. The latest movement along the fault was during the Laramide Uplift, a Tertiary orogenic event (35-70 million years ago), which led to the uplift of the modern Rocky Mountains. This episode of uplift was a compressional event that eventually formed faults and major folds, such as the Collom and Elkhorn Synclines, and the prominent Axial Basin Anticline, the axis of which occurs in the basin north of the Project Area (BLM 2006).
The coal seams in the Project Area are contained within the Upper Cretaceous Williams Fork Formation of the Mesaverde Group (BLM 2006, USGS 2008). The Mesaverde Group generally consists of a thinly to thickly interbedded succession of shale, siltstone, and sandstone that was deposited largely in a terrestrial environment. The Mesaverde Group is categorized into two formations: the overlying Williams Fork Formation, and the underlying Iles Formation (USGS 2008).
The Williams Fork Formation has been subdivided into five stratigraphic units. In ascending order, these are the Fairfield coal group, barren interval, Goff coal group, Lion Canyon Sandstone, and Lion Canyon coal group. The Iles Formation has been subdivided into three stratigraphic units: in ascending order, these are the Lower coal group, the Black Diamond coal group, and the Trout Creek Sandstone Member (USGS 2008). The Williams Fork and Iles Formations comprise a sedimentary rock sequence that originated from a deltaic and marginal marine depositional environment. The Trout Creek Sandstone Member consists of thick marine sandstone that represents the marine facies (beach) of the delta front. The high-quality, low-sulfur coal seams present in the Project Area occur within the Fairfield coal group of the Williams Fork Formation, which conformably overlies the Trout Creek Sandstone Member of the Iles Formation. Local occurrences of Quaternary alluvium, colluvium, alluvial fan deposits, and landslide deposits unconformably overlie the Williams Fork Formation, particularly in stream valleys within the Project Area (BLM 2006).
A total of 13 coal seams occur within the Project Area. In descending order (the order in which they would be mined) they are: X3, X4, D1, D2, D12, FA, FB, G7, G8, G9, GA, and GB. These coal seams have been categorized into five composite units: X34, D12, FAB, G789, and GAB. The X34 unit occurs within the top portions of the ridges at the Collom Pit and averages approximately nine feet thick. The D12 unit averages approximately 11 feet thick. The FAB and G789 units are thick seams that constitute a large percentage of the coal in the Project Area. The GAB unit resides at the base of the proposed mining sequence, with variable thickness (KEC 2005).
Geologic structure and composition in the vicinity of the proposed Project are responsible for the location and presence of groundwater and as noted above, groundwater is present in or near the surface within the Project Area at a few locations (e.g., gaining reaches of streams, seep and springs). The beds on the northern limb of the syncline dip toward the south at up to 40°, whereas the beds on the southern limb dip from 2° to 8° to the north. Although faults are not prevalent in the area, there are two joint sets that were determined to contribute to directionally-dependent permeabilities.
Overlying the Iles Formation is the Williams Fork Formation. It is up to 1,200-feet thick and consists of interbedded coal, shale, sandstone, siltstone, and mudstones. Some of these beds contain localized groundwater (notably the coal seams) and others serve as confining units (notably the KM Layer). The KM Layer (also known as the Yampa Bed) is a laterallycontinuous, low-permeability clay bed that was formed from altered volcanic ash. It is present about 200 feet above the base of Williams Fork Formation. Of the coal seams that would be mined, the lowermost coal seam is located about 200 feet above the KM Layer. With a bed thickness ranging from about 0.5 foot to 5 feet, it serves as an aquitard separating the beds within the coal sequence to be mined and the underlying rocks including the lowest part of the Williams Fork Formation and the Trout Creek Sandstone. The valley fill found along area streams also generally contains and transmits groundwater.