Chapter 5: Wilcox Group (Paleocene to Eocene) Coals of the Sabine Uplift Area, Texas and Louisiana

The Wilcox Group (Paleocene to Eocene) of the Sabine uplift, a structural arch in northeastern Texas and northwestern Louisiana (Figure 1), has lignite zones that approach subbituminous rank (see Chapter 4, this publication). These coals are among the highest quality resources known within the Gulf Coastal Plain because of their low ash yield and sulfur content. The surface expression of the Sabine uplift is defined by the contact between coal-bearing rocks of the Wilcox Group and overlying fluvial rocks of the Carrizo Sand, which is the basal unit of the Claiborne Group (Figures 2, 3). The Sabine uplift study area includes parts of Harrison, Marion, Nacogdoches, Panola, Rusk, Sabine, San Augustine, and Shelby Counties in Texas and Bossier, Caddo, De Soto, Natchitoches, Red River, and Sabine Parishes in Louisiana (Figure 1). Adjacent counties and parishes that include the subsurface Wilcox Group extend the regional Sabine uplift area. TheWilcox in the subsurface is underlain by the Midway Group (Figure 3), a mudstone-dominated marine sequence of Paleocene age. Quaternary alluvium and terrace deposits overlying the Wilcox Group at the surface are limited to areas of modern drainage. The total thickness of the Wilcox Group within the Sabine uplift area ranges from approximately 400 ft on outcrop to 2500 ft in subsurface (Kaiser, 1990). In a few places, the contact between the overlying Carrizo Sand andWilcox Group is erosional, but in other places, the contact is gradational. Where the Carrizo Sand directly overlies sandstones of the Wilcox, the contact is approximated because they cannot be differentiated. In the central Texas assessment area, the Wilcox Group consists of three formations: the Hooper, Simsboro, and Calvert Bluff (Figure 3). Separate formations are not differentiated in the Wilcox Group in the Sabine uplift area of Texas because the Simsboro, a predominantly fluvial sandstone unit, is not mappable. In west-central Louisiana, where marine beds interfinger within the Wilcox, numerous formations (Figure 3) have been described (Murray, 1948; Andersen, 1960, 1993). However, these subdivisions do not persist regionally beyond Louisiana. The Texas Bureau of Economic Geology has published estimates of coal resources in the Texas Sabine