Paleoecology of an Estuarine Sequence in the Breathitt Formation (Pennsylvanian), Central Appalachian Basin

A sequence of strata containing 24 or more ichnogenera was investigated along the western outcrop margin of the Breathitt Formation in eastern Kentucky. Four facies were delineated within the study interval. Dark shales of the Carbonaceous Shale Facies (CSF) exhibit a low-diversity/low-abundance ichnofossil assemblage dominated by small Conostichus, Planolites, and Thalassinoides. Some interbedded siltstones exhibit greater diversity with Lockeia, Paleophycus, Monocraterion, and Rhabdoglyphus. The CSF is truncated by the Bioturbated Heterolithic Facies (BHF), consisting of bioturbated sandstones with robust Asterosoma, Conostichus, and Rosselia, siltstones and gray shales with abundant Planolites and Teichichnus, and non-bioturbated sideritic claystones. The BHF is truncated by the Interbedded Sandstone and Shale Facies (ISF), which occurs as two sheet-form tongues of ripple-bedded sandstone and shale with variable-diversity and variable-abundance trace fossils, including Asterosoma, Curvolithus, Helminthoida, Helminthopsis, Lockeia, Monocraterion, Planolites, Olivellites, Rosselia, and Scolicia. The ISF is truncated by the Ripple Bedded Sandstone Facies (RSF), which consists of two ripple-dominated channel fills. Bedding within parts of the fills is crudely rhythmic; some repetitive form sets consist of 10 to 12 laminae couplets. Trace fossils are rare in the channel fills, but Beaconichnus, arthropod resting traces, and arthropod rake marks occur. Flaser, wavy, and lenticular bedding, bi- to polymodal current orientations, local rhythmic sedimentation, and bioturbation indicate that the facies were deposited in a shallow marine- to brackish-water tidal setting with minimal wave reworking. Changes in trace fossil abundance, diversity, and size were caused by changes in current velocity and possibly salinity. Scour-filling tidal facies with indications of salinity changes and a mixed Cruziana-Skolithos Ichnofacies are interpreted to represent estuarine infilling. The fill sequence was controlled by the accomodation space of the estuary and lateral shifting channels. Limited accomodation space caused repetitive scouring of older fill facies and incisement of multiple channels.