An Appalachian-sourced deltaic sequence, northeastern Alabama, USA: biofacies-lithofacies relationships and interpreted community patterns

Abstract

The thin sequence of Lower 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. Specific paleoenvironments of deposition contain unique biofacies in this Late Carboniferous (Westphalian A) sequence. The criteria established for the recognition of these biofacies can be utilized to assist in refined interpretations of deltaic sites in Carboniferous coastal paleoenvironments. Identifiable biofacies include those preserved under a variety of forested wetland (swamp) conditions, distributary and crevasse-splay channels, coastal bays (“interdistributary” and lagoonal), barrier sands, and distal storm deposits (Fig. 1; Gastaldo et al., 1989). Vegetation in forested wetlands grew either in clastic substrates or peat substrates. The principal biofacies preserved in clastic substrate swamps were lycophyte-dominated, and can be recognized by either a predominance of canopy litter or subterranean stigmarian appendages (Gastaldo, 1986). The canopy litter that has accumulated on the forest floor was preserved under unique sedimentological conditions, and reflects the ecological gradient associated with the distribution of lycophyte genera in the swamp (Gastaldo, 1987). A monotypic assemblage of lycophytes characterized edaphically stressed sites. In sites proximal to the levee, a mixed assemblage of lycophytes, calamiteans, pteridophytes and pteridosperms is common. In the absence of compressed canopy macrodetritus, subterranean axes with helically arranged appendages (“rootlets”) crosscutting the bedding may be preserved. Macro-invertebrates are restricted to traces and trails, reflecting behavioral traits when conditions were conducive for their movement into these sites. Peat-colonizing vegetation parallels that of the clastic swamp. Deep distributary channels contain sandstone-cast and compressed aerial trunks of lycophytes and spenophytes. These occur in bedload deposits along with quartz and quartzose pebbles and cobble-size phyllite clasts. Degradation of external morphology usually precludes assignment of logs to a systematic position lower than order. In shallower, en-echelon stacked crevasse sands occur a mixture of lycophyte, calamitean and pteridosperm “woody” parts. Additionally, scoriaceous fern-like foliage may be found. Little evidence exists for macro-invertebrate communities in these unstable settings. Coastal bays preserve in situ macro-invertebrate communities, as well as allochthonous macrodetritus that was derived principally from levee vegetation. Four phases of biofacies development can be delineated (Gibson and Gastaldo, 1987). Stress-tolerant inarticulate brachiopods dominate the initial transgressive phase. Individuals are found isolated in the siltstone, commonly preserved by authigenic cementation in siderite concretions. Rarely are patches or clusters of individual encountered. Where clustering does occur it is associated with the colonization of “woody” plant parts. Transition to the molluscan-dominated phase is accompanied by the establishment of a rich ichnofauna. Continued transgression and the development of more normal salinities under lagoonal conditions are paralleled by an increase in species richness and abundance. The third biofacies phase remains molluscan dominated, but the assemblages at any particular point in time are represented by monospecific genera. Plant macrodetritus was utilized by the macro-invertebrate communities, and that which is preserved is restricted to highly fragmentary, unindentifiable remains. The fourth biofacies phase reflects the development of lagoonal conditions. This change can be recognized by macro-invertebrate body and ichnofossil fauna (Seilacher's Cruziana ichnofacies) diversification. An increase in the abundance of brachiopods and other invertebrates is considered to represent more open-marine conditions. Barrier sandstone and distal storm deposits contain disarticulated epifaunal brachiopods, bivalves, gastropods, and trilobites. These stenohaline forms are concentrated in a shell lag at the base of migrating barrier-sand bodies. The overlying sandstone is devoid of macro-invertebrate body fossils and plant macrodetritus, but is characterized by the colonization of Zoophycos traces.