Paleoecological Analysis of Two Early Pennsylvanian Mineral-Substrate Wetlands

The biomass within two Early Pennsylvanian (Langsettian [Westphalian A] equivalent) penecontemporaneous swamp communities was sampled quantitatively to obtain an estimate of the taxonomic contribution to each assemblage. Blocks of organic-rich shale were removed from a clastic parting within the Black Creek Coal, and 0.5-m2 siltstone quadrats were chain-sawed from a clastic swamp community directly above the Bear Creek Coal. Bedding planes were exposed, and the surface areas for each taxon per bedding surface were measured and used as proxies for biomass contribution in each locality. Biomass over a combined area of 5.47 m2 was assessed for the Black Creek Coal parting; biomass covering an area of 9.70 m2 was evaluated for the assemblage preserved above the Bear Creek coal. In addition to calculating standard diversity indices, this data set was analyzed using cluster analyses and non-metric multidimensional scaling (NMDS) to differentiate variations within the general flora. Low species diversity characterizes both floras. Diversity indices for both assemblages are very similar, indicating essentially no difference in assemblage composition, despite the difference in edaphic conditions. The Bear Creek Coal wetlands show greater variation in species content, while the mineral-enhanced peat of the Black Creek Coal overlaps this species diversity within a slightly more restricted range of variation. Cluster analysis produced 5 stable clusters, whereas three dimensions of the NMDS analysis provided the best fit to explain the variation among the samples. The dimensions are interpreted as representing abundance of (1) arborescent lycopsids, (2) arborescent and climbing sphenopsids, and (3) pteridosperms. The plant community preserved within the clastic parting of the Black Creek Coal is comparable to that of the community found above the Bear Creek Coal. Hence, vegetation that colonized mineral-substrate soils in Early Pennsylvanian coastal lowlands, whether in peat or non-peat accumulating settings, are very similar. The dominance of pteridosperms in these depositional regimes appears to remain stable throughout the Early and Middle Pennsylvanian and portends community replacements in the Late Westphalian D some 8–10 million years later.