ABSTRACT

In southwest Alabama, lignite or lignitic beds are present in at least five stratigraphic intervals that span approximately 8 million years of geologic time. Lignite is found in the Paleocene Oak Hill and Coal Bluff members of the Naheola Formation, the Paleocene Grampian Hills Member of the Nanafalia Formation, the Paleocene Tuscahoma Sand, and the Eocene Hatchetigbee Formation. Lignite beds range in thickness from 0.5 to 11 feet and consist of 30 to 53 percent moisture, 13 to 39 percent volatile matter, 4 to 36 percent fixed carbon, and 5 to 51 percent ash. The organic matter includes structured plant material characteristic of brackish water coastal marsh and freshwater swamp environments. Pollen of herbaceous angiosperms, bisaccate gymnosperms, and fungal elements dominate marsh-derived lignite assemblages, whereas pollen of arborescent angiosperms and fern and moss spores dominate swamp-derived lignite assemblages.

These Paleocene and Eocene lignite beds occur as parasequence deposits in highstand systems tracts of five distinct third-order, unconformity-bounded depositional sequences. These sequences consist of lowstand systems tract sand or shelf margin systems tract sand, transgressive systems tract glauconitic sand and marl, and highstand systems tract fluvial-deltaic sand, silt, carbonaceous clay, and lignite. Generally, lowstand systems tract incised valley fill sand deposits overlie the lignite beds; however, where these sand beds are absent, a distinct transgressive surface (merged with sequence boundary) marked by clasts and marine fossils is developed. The lignite beds are interpreted as strata within highstand systems tract parasequences that occur in mud-dominated regressive intervals. As many as six individual lignite beds have been observed within a single highstand systems tract.

Lignite beds were deposited in interdistributary coastal marsh and swamp environments as part of the delta plain of fluvial-deltaic systems, which prograded into southwest Alabama from the northwest. As sediment was progressively delivered into the basin from these deltas, the effects of relative sea level rise during an individual cycle were overwhelmed, producing a net loss of accommodation and concomitant overall basinward progradation of the shoreline (regression). Small-scale fluctuations in water depth, resulting from the interaction of eustasy, subsidence, and sediment influx and distribution, led to cyclical flooding of the coastal marshes and swamps, followed by periods of progradation and regression, as recorded by the superimposition of upward-shallowing, lignite-capped, mud-dominated parasequences on the overall regressive stratigraphic succession.

Highstand systems tract deposition within a particular depositional sequence culminated with a relative sea level fall that resulted in a lowering of base level and an abrupt basinward shift in coastal onlap. During this lowstand, fluvial incision and erosion characterized landward areas. Following sea level fall, significant relative sea level rise resulted in marine inundation of the area previously occupied by coastal marshes and swamps.