The Falkirk Mining Company (Falkirk), a wholly-owned subsidiary of North American Coal (NACoal), which is a wholly-owned subsidiary of NACCO, ownes and operates the Falkirk Mine in North Dakota.
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Summary:
The Falkirk Mine is located in the Sentinel Butte Formation of the Fort Union Group which is one of the most prolific lignite-bearing stratum in the state.
The economically minable coals in the reserve occur in the Sentinel Butte Formation (Paleocene) and the Bullion Creek Formation (Paleocene) and are unconformably overlain by the Coleharbor Formation (Quaternary). The Sentinel Butte Formation conformably overlies the Bullion Creek Formation. Paleocene deposition was initiated by an influx of coarse sediment dispersed eastward and southeastward, in deltaic fashion, across early Bullion Creek swamps. The paleoslope appears to have been variable, both in magnitude and in direction, and reflects Paleocene tectonism to the west and northwest. The elevation of western North Dakota, relative to base level, increased during Sentinel Butte time and probably reflects increased deposition to basinal subsidence.
The reserve area is situated on a glacially modified upland drainage divide of relatively low relief. The reserve is defined on the south and west by the present day Missouri River Valley. The northern and eastern extents of the field are defined by pre-glacial channels that have subsequently been modified by glacial and interglacial activity. The valley to the east is incised by a glacial meltwater channel (Coal Lake Coulee). The valley bisecting the reserve, whose surface is now occupied by the Weller Slough Complex, appears to have been a main tributary of the pre-glacial Knife River. This bisecting valley divides the reserve into two coalfields, locally referred to as the Underwood Coal Field to the north and the Riverdale Coal Field to the south.
The Bullion Creek Formation (Paleocene) underlies much of the reserve area. The Sentinel Butte Formation (Paleocene) conformably overlies the Bullion Creek Formation. Lithologically, the two formations are very similar. Interbedded silts and clays that occur in beds that range in thickness from less than one (1) foot to tens of feet make up about sixty) to eighty percent of the sediment of the Bullion Creek and Sentinel Butte Formations. From fifteen to thirty-five percent of the sediment making up these formations consists of silty, fine-grained to medium-grained sand in beds that range in thickness from one-half (½) to one hundred feet. Lignite is a minor constituent, generally comprising less than five percent of the formations. The lignite occurs in beds ranging in thickness from less than one-tenth) foot to about fifteen feet locally. The Coleharbor Formation (Pleistocene) unconformably overlies the Sentinel Butte and Bullion Creek Formations. It includes all of the unconsolidated sediments resulting from deposition during glacial and interglacial periods. Lithologic types include gravel, sand, silt, clay, and till. The Oahe Formation (Holocene) occurs as a thin veneer of eolian silt sized sediment that blankets upland surfaces in the area. The modified glacial channels that delineate the reserve limits are in-filled with sediments of the Coleharbor Formation. The channel fill systems contain a complex of interbedded glaciofluvial gravels, sands, silts, and clays overlain by till. The coarser gravel and sand beds are generally limited to near the bottom of the channel fill.
The general stratigraphic sequence in the upland portions of the reserve area (Sentinel Butte Formation) consists of till, silty sands and clayey silts, main Hagel (Hagel A) lignite bed, silty clay, lower lignite of the Hagel lignite interval (Hagel B), and silty clays. Both the Hagel A Bed and the Hagel B Bed are split by clay partings in portions of the reserve; although the two beds are not split in the same areas. Where the beds have partings, the splits are refered to as Hagel A1, Hagel A2, Hagel B1, and Hagel B2. There are thinner beds of lignite above the Hagel bed in some areas. These thin lignite beds are part of the Kinneman Creek seam. The Kinneman Creek in most areas is thin, very weathered, and very high in ash. In areas where it is mineable it can reach up to 3.5 feet thick and is lower in ash. Where the Kinneman Creek is parted, it is refered to as the Upper Kinneman Creek and Lower Kinneman Creek.
Near the contact of the Sentinel Butte and Bullion Creek Formations in a complex of sands and thin (less than one (1) foot ) lignite beds locally referred to as the C Sand and C Seam. As in the Sentinel Butte, the Bullion Creek Formation consists of till, silty sands and clayey silts, main Tavis Creek lignite bed, silty clay, and silty clays. The Tavis Creek Bed is split by a clay parting in portions of the reserve. Where the bed has a parting, the splits are referred to as Upper Tavis Creek and Lower Tavis Creek. There are thin lignite beds immediately above and/or below the Tavis Creek in many areas. The first of these thinner seams below the Tavis Creek bed is the Coal Lake Coulee seam. The Coal Lake Coulee seam parted in areas of the reserve and the splits are refered to as the Upper Coal Lake Coulee and the Lower Coal Lake Coulee. Below the Coal Lake Coulee seam, there is a repeating sequence of silty to sand clays with generally thin lignite beds.