An Appalachian-sourced deltaic sequence, northeastern Alabama, U.S.A.: biofacies-lithofacies relationships and interpreted community patterns

Abstract

The thin sequence of Lowe Pennsylvania rocks along Sand Mountain, Plateau coal field, northeastern Alabama, U.S.A., records the deposition in a deltaic coastal-plain paleoenvironment along the ancient Appalachian seaway. The section is laterally continuous, well exposed, and preserves a rich macrobiota. Identified coexisting paleodepositional environments contain distinctive biofacies. Biofacies in deltaic sites are characterized by the presence of various macrofloral assemblages. Alluvial-plain swamps, identified lithologically by homogeneous mudstone and siltstone, preserve bedded-plant litter as coalified compressions and impressions. Deep-swamp biofacies are comprised either of monotypic lycophyte canopy assemblages ( Lepidophloios) or their subterranean axial systems ( Stigmaria). Alluvial swamps and proximal levee sites contain canopy detritus of a mixed flora. This is reflected in the reduced domination of Lepidophloios, the increased importance of the lycophytes Sigillaria and Lepidodendron, and an abundance of gymnosperms, pteridosperms, pteridophytes, and Calamites. Macro-invertebrates occur almost exclusively as behavioral trackways of xiphosurid arthropods and epifauna attached to fragmentary plant parts. The peat-accumulating swamp biofacies is identified from palynological preparations. Palynofloras parallel macrofloral clastic swamp diversity and contain an abundance of palynomorphs with affinities to ferns and lyginopterid pteridosperms. Channel-form sandstone represent distributary and crevasse channel deposits in the lower part of the section, and uncomformable bedload-dominated, laterally migrating, braided-river channel deposits at the top of the sequence. Oriented sandstone cast and compressed logs (lycophytes and Calamites) occur with bedload features in distributary and braided channels. Crevasse sanstones preserve a higher proportion of calamitean axes, as well as trunks and rachises of medullosan pteridosperms. Macro-invertebrates and ichnofaunas have not been identified in these paleoenvironments. Bayfill sequences contain several in situ macro-invertebrate communities in addition to allochthonous plant detritus. This plant biofacies is characterized by calamitean and pteridospermous vegetation, that originated from levee sites. The macrofaunal biofacies is characterized by a molluscan assemblage, with community replacement relative to physical parameters of the water. The initial bayfill phase contains an inarticulate brachiopod community of Orbiculoidea and Lingula. The transition to the molluscan-dominated biofacies is signaled by infaunal colonization by Pteronites and Planolites (burrows). Biotic changes are reflected by the increasing abundance of brachiopods and other invertebrates usually considered to represent more open-marine conditions. Insights into Late Carboniferous open-marine communities can be discerned from lag accumulations of marine epifauna in storm-generated sandstones.