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Lignite occurrence was related to sandbody geometry in two subsurface studies: a 12-county regional study and a local study of the Trawick gas field area, north-central Nacogdoches County. For both studies, the Wilcox Group was informally divided into lower progradational (deltaic) and upper aggradational (fluvial) units. The local study utilized closely spaced data to investigate a more detailed Wilcox stratigraphy.
The most continuous lignite-bearing zone lies at the transition between lower and upper Wilcox strata. Mapping of lignite occurrence in both studies shows this zone to be coincident with distributary channels indicative of delta-plain settings. Lignites and laterally equivalent muds rest on platforms of sandy sediments. Initiation of peat accumulation in interdistributary basins, with upward and subsequent lateral development as blanket peat, is inferred from the local study. Thickest and most laterally extensive seams occur in Shelby and Panola Counties on the flanks of major delta lobes.
Thick upper Wilcox lignites (> 5 ft, 1.5 m) occur regionally between major fluvial channel sand belts and cap 30 to 40-ft (9 to 12-m) upward-coarsening sequences (crevasse splays?). These lignites are surface-mined in Panola and Harrison Counties at Martin Lake and Darco. Westward, in northern Cherokee County, our drilling shows thick lignites (up to 11 ft, 3.4 m) have limited lateral extent in channel sand belt areas. Similarly, the local study lies within a major sand belt; small interchannel basins limit lateral continuity of lignites.
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