Middle-Upper Devonian (Givetian-Famennian) bone/conodont beds from central Kentucky, USA: Reworking and event condensation in the distal Acadian foreland basin

Exposures of the Devonian Ohio Shale (Chattanooga Shale), southeast of Clay City, central Kentucky, show a complexly condensed interval, about 1 m thick, spanning the late Givetian to early middle Famennian stages and encompassing, perhaps, as much as 10 million years. This section features three thin, but distinctive, conodont and bone-rich sandstone beds which overlie disconformities. The lowest unit, black shale equivalent to the Blocher Member of the Olentangy Shale (∼70 cm), yields conodonts diagnostic of the latest Givetian disparilis Zone. It is crosscut by neptunian dikes filled with phosphatic sandstone continuous with a 0.5-1.0 cm thick bone-bed (A), which yields fragmentary early Frasnian conodonts, probably equivalent to CONKIN's (1985) Bone Bed 15. A second, thin, sandy bone-bed (B), about 40 cm higher also yields fragmentary Frasnian conodonts. A complex, lenticular bone-bed (C) (0.5-10 cm thick) occurs at the base of the Huron Shale Member, above a 16 cm thick interval of burrowed black shale. This bed, equivalent to CONKIN's Bone Bed 16, yields early Famennian conodonts commingled with reworked Frasnian elements. This highly condensed bed is compound. The lower portion is composed of amalgamated calcareous sandstone and gray shale, containing abundant coalified/calcified wood and well preserved conodonts. The upper 1-5 cm is a sharply based, crudely graded, Zoophycos-bioturbated bed with abundant clasts of reworked, phosphatized bone, coalified, calcified, and pyritized wood, reworked pyrite nodules and rare crinoid ossicles and quartz pebbles. The upper surface displays well preserved, though disarticulated, arthrodire (Eastmanosteus?) plates, acanthodian spines, cladoselachid teeth and scales of paleonisciform actinopterygians. Coalified logs show a vaguely preferred, nearly N-S orientation; bones are concentrated on the western (leeward?) sides of flow-perpendicular logjams. The complex history of the bone-beds includes long-term condensation, erosional concentration, and bioturbation, and, finally, rapid burial associated with physical reworking by storm-generated currents. These condensed bone-beds are interpreted as transgressive systems tracts that developed following periods of prolonged submarine erosion.