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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 Previous HitSystemsNext Hit Previous HitTractsNext Hit

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 Previous HitsystemsNext Hit Previous HittractsNext Hit of four distinct third-order depositional sequences. These sequences consist of lowstand or shelf-margin Previous HitsystemsNext Hit tract sand, transgressive Previous HitsystemsNext Hit tract glauconitic sand and marl, and highstand Previous HitsystemsNext Hit tract fluvial-deltaic sand, carbonaceous clay, and lignite. Generally, lowstand-Previous HitsystemsNext Hit-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 Previous HitsystemsNext Hit tract parasequences that occur in mud-dominated regressive intervals. As many as six individual lignite beds have been obs rved within a single highstand Previous HitsystemsNext Hit tract. Lignite beds were deposited in coastal marsh and low-lying swamp environments as part of deltaic Previous HitsystemsNext Hit that prograded into southwestern Alabama from the west. As sediment was progressively delivered into the basin from these deltas, the effects of relative Previous HitseaNext Hit Previous HitlevelNext Hit rise during an individual Previous HitcycleNext Hit 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 Previous HitsystemsNext Hit tract deposition within a particular depositional sequence culminated with a relative Previous HitseaNext Hit Previous HitlevelNext Hit fall that resulted in a lowering of base Previous HitlevelNext Hit 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 Previous HitseaNext Hit Previous HitlevelNext Hit fall and the subsequent accumulation of the lowstand deposits, significant relative Previous HitseaNext Hit Previous HitlevelNext Hit rise resulted in the marine inundation of the area previously

occupied by coastal marshes and swamps, and deposition of the transgressive Previous HitsystemsTop 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.