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Late Devonian — Early Mississippian Subaqueous Deltaic Facies in a Portion of the Southeastern Appalachian Basin
Evaluation of subsurface data in a portion of the southeastern Appalachian basin permits paleoenvironmental reconstruction of a Late Devonian and/or Early Mississippian deltaic complex.
Electric logs and samples from 114 oil and gas wells were used to construct 21 stratigraphic cross sections through a six county area in southwestern Virginia and adjacent Kentucky and West Virginia. The study area covers approximately 2100 square miles.
Results obtained from petrographic analyses, grain size determinations, studies of cross sectional configurations and inferred relationships to regional paleogeography indicate that there are three essentially contemporaneous clastic facies in the study area. These are: (1) siltstone and sandy siltstone (delta front); (2) clayey siltstone (prodelta); (3) pyritic, carbonaceous black clay shale (offshore marine). Each of the three lithofacies represents a different subaqueous environment of a north-northwestward trending progradational deltaic complex.
Siltstones and shales of the study area were deposited in a shallow, euxinic sea probably no deeper than a few hundred feet. A low-lying drainage area, stable source, and restricted marine circulation explain the occurence of predominantly fine grained deposits. Abundant carbonaceous matter and pyrite imply anaerobic, reducing basinal conditions.
Inadequate physical and paleontological control and the fact that these facies intertongue laterally raise questions concerning the validity of traditional time rock units in this area. Considering these facts, it seems unlikely that an exact Devonian-Mississippian boundary can be established in this portion of the southeastern Appalachian basin.
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