Evaluation of subsurface data in a part 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 6-county area in southwestern Virginia and adjacent Kentucky and West Virginia. The study area covers approximately 2,100 sq mi.

Results obtained from petrographic analyses, grain-size determinations, studies of cross-sectional configurations, and inferred relation to regional paleogeography indicate that there are 3 essentially contemporaneous clastic facies in the study area. These are: siltstone and sandy siltstone (delta front); clayey siltstone (prodelta); and pyritic, carbonaceous black clay shale (offshore marine). Each of the lithofacies represents a different subaqueous environment of a north-northwest-trending progradational deltaic complex.

Siltstones and shales of the study area were deposited in a shallow, euxinic sea probably no deeper than 100 ft. A low-lying drainage area, stable source, and restricted-marine circulation explain the occurrence of predominantly fine-grained deposits. Abundant carbonaceous matter and pyrite imply anaerobic, reducing conditions.

Inadequate physical and paleontologic 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 appears unlikely that an exact Devonian-Mississippian boundary can be established in this part of the southeastern Appalachian basin.

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