MANCINI, ERNEST A., and BERRY H. TEW, Geological Survey of Alabama, and the University of Alabama, Tuscaloosa, AL, RICHARD E. CARROLL, Geologic Survey of Alabama, Tuscaloosa, AL
ABSTRACT: Paleocene-Eocene Lignite Beds of Southwest Alabama: Parasequence Beds In Highstand Systems Tracts
In southwest Alabama, lignite beds are present in at least four stratigraphic intervals that span approximately 8 m.y. of geologic time. Lignite is found in the Paleocene Oak Hill Member and Coal Bluff Member of the Naheola Formation of the Midway Group and the Paleocene Tuscahoma Sand and the Eocene Hatchetigbee Formation of the Wilcox Group. Lignite beds range in thickness from 0.5 to 11 ft and consist of 32-53% moisture, 13-39% volatile matter, 4-36% fixed carbon, and 5-51% ash. The organic matter includes structured plant material characteristic of nonmarine, brackish water coastal marsh and freshwater swamp environments. Pollen of herbaceous angiosperms, bissacate gymnosperms, and fungal elements dominates marsh-derived lignite assemblages, and pollen of arborescent angiosperms a d fern spores dominates swamp-derived lignite assemblages.
These Paleocene and Eocene lignite beds occur as parasequence deposits in highstand systems tracts of four distinct third-order depositional sequences. These sequences consist of lowstand or shelf-margin systems tract sand, transgressive systems tract glauconitic sand and marl, and highstand systems tract fluvial-deltaic sand, carbonaceous clay, and lignite. Generally, lowstand-systems-tract incised valley 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 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 obs rved within a single highstand systems tract. Lignite beds were deposited in coastal marsh and low-lying swamp environments as part of deltaic systems that prograded into southwestern Alabama from the west. 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, sediment yield, and subsidence, led to cyclical flooding of the low-lying coastal marshes and swamps, followed by periods of progradational and regression, as recorded by the superimposition of upward-shallowing, lignite-capped, mud- ominated 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 subaerial erosion characterized landward areas; incised valleys were depositional loci of lowstand sands of the overlying depositional sequence. Following sea level fall and the subsequent accumulation of the lowstand deposits, significant relative sea level rise resulted in the marine inundation of the area previously
occupied by coastal marshes and swamps, and deposition of the transgressive systems tract of the overlying sequence.
AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.