Abstract

Lignites occur in the Gulf Coastal Plain of Mississippi and Alabama in a number of formations ranging in age from Upper Cretaceous to Miocene. However, deposits of potential economic significance are found only in the Naheola Formation of the Midway Group (Paleocene) and in the Nanafalia, Tuscahoma, and Hatchetigbee Formations of the Wilcox Group (lower Eocene), and in undifferentiated Wilcox sediments.

At the outcrop, the Midway and Wilcox Groups attain a maximum thickness of about 1,200 feet (366 m) and are composed of terrigenous sediments deposited in fluvial, deltaic, and shallow marine environments. The sequence in Mississippi is predominantly fluvial and deltaic, while the equivalent units in Alabama accumulated in deltaic and shallow marine environments. The regressive nonmarine intervals are characterized by thick sequences of relatively nonfossiliferous cross-bedded sands; laminated to thin-bedded sands, silts and clays, and lignite. The shallow-marine transgressive units are generally thin and consist of fossiliferous, glauconitic, argillaceous sands and limestones.

Lignite in the Midway Group occurs as seams up to 14 ft (4.3 m) in thickness at or near the top of the Oak Hill Member of the Naheola Formation. In Mississippi the Oak Hill Member is unconformably overlain by the nonmarine Fearn Springs Member of the Nanafalia Formation, while the glauconitic sands of the Coal Bluff Marl Member unconformably overlie the Oak Hill Member in Alabama. The Oak Hill lignite generally occurs as a single uninterrupted seam extending across western Alabama and east-central Mississippi.

In the Wilcox Group, lignite is present in the Gravel Creek Sand Member of the Nanafalia Formation in eastern Alabama, in the upper nanafalia of east-central Mississippi, in the upper Tuscahoma and the Hatchetigbee Formations in east-central Mississippi and western Alabama, and in undifferentiated non-marine sediments of north-central and northern Mississippi.

Lignite in the Gravel Creek Sand Member occurs as highly lenticular seams (up to 40 ft/12.2 m thick) which rarely exceed one mile in width. Deposition of these lignites apparently occurred in solutional or erosional channels developed in the upper Clayton limestone (Midway). The lignites are conformably overlain by the "Ostrea thirsae beds" of the Nanafalia Formation.

The lignites of the upper Nanafalia and the Tuscahoma and Hatchetigbee Formations occur as multiple seams within fluvial and deltaic sequences. These seams rarely exceed 10 ft (3.0 m) in thickness and are intermediate in areal extent between the Oak Hill seam and lignites of the Gravel Creek Sand Member.