The mine extracts lignite coal from the Jackson formation, seams A, B, C and D ranging in thickness from 1.0 to 4.5 feet with a combined average total lignite thickness of approximately 9 feet. The seams are separated by relatively thin partings which vary in thickness from 0.5 to 1.5 feet.
The deposit consists of four or more thin (generally < 1 m or 3.3 ft thick) lignite benches that are separated by claystone and mudstone partings (fig. 2). The partings are composed of altered volcanic air-fall ash that has been reworked by tidal or channel processes associated with a back-barrier depositional environment (Snedden and Kersey, 1981; Gowan, 1985; Ayers, 1986; Senkayi and others, 1987; and Warwick and others, 1996).
In the mine area, the Jackson strata dip to the southeast about 17 m/km (90 ft/mi) (Ayers, 1986). Gowan (1985) and Ayers (1986) suggest that the mud-dominated sediments the comprise the floor, partings, and roof of the San Miguel lignite deposit originated in lagoonal mires landward (west) of the sandstone-dominated barrier island complex. The lignite-bearing interval is laterally persistent along strike for more than 40 kilometers (25 mi) (Ayers, 1986, 1989; Tewalt and others 1983).
The San Miguel lignite interval consists of four (or more) thin (generally <1 m or 3.3 ft thick) lignite benches that are separated by claystone and mudstone partings. The upper bed is designated as the A bed and is underlain by the B, C, and D beds ........
