Summary:
The most prolific lignite-bearing stratum in the state is the Sentinel Butte Formation of the Fort Union Group. The most prominent characteristics of the Fort Union Group formations are the cyclical deposition and lateral persistence of the lithologic units, especially the lignite seams. The Sentinel Butte Formation is a continental deposit comprising interbedded calcareous clays, sandy clays, and lignite beds, with isolated lenses of fine-grained sands, silts, and rare limestones, ranging in color from light gray to dark brown and black. Except for the limestones and an upper sandstone unit, the sediments are essentially nonindurated. The claystone and sandstones are generally thinly bedded and not jointed. These sediments were deposited mainly in swamps or in the floodplains of very slow, meandering rivers. The Sentinel Butte Formation conformably overlies the Tongue River Formation (also known as Bullion Creek Formation). The Golden Valley Formation, which is stratigraphically above the Sentinel Butte Formation elsewhere, is absent in the Freedom Mine area, due to postdepositional erosion; therefore, the younger glacial deposits of the Coleharbor Group unconformably overlie the Sentinel Butte Formation in the study area. Outcrops of the Sentinel Butte Formation are sporadic; bedrock is largely masked by residual soils, slopewash, and/or glacial drift.
Where the entire Sentinel Butte Formation is present, as in western Mercer County, its thickness is approximately 500 feet. The upper part of the formation in eastern Mercer County, however, has been removed by erosion, thus it is only about 350 feet thick. The thickening of this formation westward is due also to the increasing thickness of the upper sandstone unit to the west. This upper member is a fine to medium grained, yellowish-brown sandstone. Generally, it is poorly cemented, but locally it is moderately to well cemented. The cemented sandstone is dark brown and forms conspicuous ledges 5 to 10 feet in height, but these are laterally discontinuous.
The Sentinel Butte Formation incorporates a varying number of lignite coal beds. The major lignite bed within the mine area is the Beulah-Zap (Beulah) bed, the only economical lignite deposit. This lignite horizon has a thickness of generally 15 to 22 feet, except near the edges of the glacial diversion channels, where some of the lignite bed has been removed by erosion. Also, along the glacial diversion channels are areas of soft lignite (leonardite). Soft lignite is a result of areas of the bed being exposed to oxidizing conditions via erosion caused by glacial meltwaters. The soft or weathered lignite is of very poor quality and not economically recoverable.
At the Freedom Mine, the Beulah bed can split into four components, an upper split (Upper Beulah), and a lower split (Lower Beulah). In some locations, the Upper Beulah splits into the Upper Beulah 1 and Upper Beulah 2. Also, in some locations, the Lower Beulah splits into the Lower Beulah 3 and Lower Beulah 4. The splits hold a combined thickness of approximately 15 to 20 feet and then they start to thin to a combined thickness of approximately 10 to 11 feet.
Located above the Beulah bed are several thin beds of lignite. These include, in the order that they occur, the Twin Buttes and Schoolhouse beds. These beds typically average 2 feet thick, although they range from 1 foot to 6 feet within short distances. Interburden between the Schoolhouse and Beulah beds ranges from 5 to 50 feet, and interburden between the Twin Buttes and Schoolhouse beds averages 65 feet.
Located below the Beulah bed are several other beds of lignite. The major beds, in the order below the Beulah bed that they occur, are the Spaer and Stanton beds. These range from 65 to 130 feet below the Beulah bed. The Spaer bed has an average thickness of 5 feet, and lies an average of 80 feet below the Beulah bed. The Stanton bed averages 5 feet thick, and lies an average of 35 feet below the Spaer bed. Other thin beds and splits of the two beds, typically less than 2 feet thick, are occasionally found above and below the Spaer and Stanton beds. Below the Stanton bed can be found the Hagel lignite zone, generally made up of two to three beds, and then below that the Tavis Creek bed. Below the Tavis Creek bed is generally a long sequence of sands and clays before the next lignite (150-300 feet). The Jim Creek bed is a localized lignite bed that appears between the Beulah and Spaer beds.