COMPOSITIONAL GROUPS AND COAL-BENCH ARCHITECTURE ANALYSES TO RECONSTRUCT PENNSYLVANIAN WETLAND ENVIRONMENTS

Coal beds often exhibit lateral and vertical variability in composition. Clastic splits, partings, and surfaces of oxidation (fusain layers) can be used as intra-seam bounding units to subdivide a seam into benches. Bench-scale analyses can be used to interpret original mire types, as well as vertical and lateral transitions in mire types. Compositional group analysis, using palynologic, petrographic, and geochemical data, can be used to interpret the original mires represented by individual coal benches or as increments in benches.

Analyses of coal underclays shows that many Middle Pennsylvanian coalbeds were preceded by clastic, non-peat forming swamps dominated by spores of tree ferns and sphenopsids, as well as cordaite pollen. Localized coals and basal benches of more extensive coals tend to be dominated by Lycospora and vitrinite. Palynoflora often contain abundant Lycospora orbicula and L. micropapillata, the spores of the colonizing lycopod, Paralycopodites. Basal layers of thick coal benches are generally high in vitrinite but variable in ash and sulfur contents and record initial peat accumulation in pioneering rheotrophic forest mires. They may be succeeded by, or are laterally transitional with, mixed palynoflora-vitrinite dominant, or mixed palynoflora-high ash increments. Mixed palynoflora increments contain abundant small lycopod and fern, tree fern, and sphenopsid spores, and cordaite pollen. In the upper part of thick coal benches, local development of mixed palynoflora-low vitrinite increments is indicative of domed ombrogenous mire development. These low-vitrinite (higher inertinite and liptinite) increments are dominated by “densospore” producing small lycopods and tree ferns. Coal that resulted from the domed, ombrogenous parts of mires is characteristically very low in ash and sulfur. The change from planar-rheotrophic to domed peat formation records a transition from mainly groundwater-fed to predominantly rainwater-fed mires. Rider coals, where they occur, are variable in composition, laterally restricted in area, and likely represent ephemeral rheotrophic peat formation. In most cases, the cycle of mire successions is not temporally or spatially symmetric.