A comparison of late Paleocene and late Eocene lignite depositional systems using palynology, upper Wilcox and upper Jackson Groups, east-central Texas
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Lignites of the Wilcox and Jackson Groups in east Texas were deposited in marginal marine depositional complexes during times of cyclic sediment deposition. Thick upper Wilcox lignites occur within cycles of estuarine strata. Thin upper Jackson lignites occur within strandplain/shoreface deposits. Palynology of the lignites and enclosing sediments reveal two distinct climatic regimes: warm and equable during Wilcox deposition versus variable warm-cool during Jackson deposition. Four palynologic assemblages have been recovered from lignite-bearing upper Wilcox strata, and six palynologic assemblages have been recovered from upper Jackson non- marine and marine strata. Wilcox lignites contain assemblages indicating change from closed-canopy freshwater swamps populated by a community dominated by chestnut and walnut family trees, to open-canopy swamps that add ferns to the community, to a community of palms and ferns that extends into the overlying marine- influenced mudstones, and capped by marine siliciclastics containing an assemblage of dinoflagellates and transported cypress pollen and fern spores. The Jackson assemblages indicate a transition from a palm-dominated community in the sands and silts to a fern marsh community in the silty mudstones and base of the lignites, to closed- canopy freshwater communities in the lignite populated by a tropical walnut and swamp tupelo, to an open-canopy community populated where ferns replace the tupelo, capped by a swamp community dominated by a chestnutlike tree and leather- wood, especially in lignites overlain by marine sediments; marine sediments contain an assemblage of dinoflagellates and transported pollen. The dominant tree in Wilcox swamp communities is chestnut, whereas Jackson swamps are dominated by a tropical walnut; ferns are common in both settings. The dominance of cypress in the estuarine-marine transition sediments of the Wilcox suggests an open-water transition between peat swamp and marginal marine environments. The dominance of the chestnutlike tree in the swamp-marine transition of the Jackson indicates a sharp boundary between peat swamp and marine environments. 2005 Geological Society of America.
