UNUSUAL RHOMBIFERAN-BEARING SHELL BED IN UPPER ORDOVICIAN, REEDSVILLE SHALE OF CENTRAL PENNSYLVANIA

The lower portion of the Reedsville Shale of central Pennsylvania contains predominantly laminated gray to black shale and unfossiliferous, turbiditic siltstone and litharenite. However, at Martinsburg, Pennsylvania, the lower Reedsville contains one fine-grained sandstone bed that is rich in benthic fossils. This bed is both under- and overlain by strata completely lacking benthic fauna. Shale from below the fossil-bearing bed contains graptolites belonging to the G. pygmaeus graptolite zone, indicating that the fossiliferous bed is Edenian in age.

Fossils include predominantly external molds of echinoderm plates and small articulate brachiopods (primarily Sowerbyella). The fossils are towards the bottom of a weekly graded bed and are relatively uniform in size (generally less than 1 cm long). In many respects, taphonomy indicates that the fossils represent the coarsest distal material transported to this part of the Appalachian basin by a particularly strong turbidity or gradient current. The lack of benthic fossils above and below this bed may indicate only the absolutely strongest currents had enough energy to carry shell material to this distal setting. However, the fossiliferous bed presents taphonomic and paleoecologic enigmas in that it contains large clusters of rhombiferan cystoid plates. Typically large clusters of rhombiferan plates indicate in situ or nearly in situ disarticulation. Yet, no complete rhombiferans were identified, although the fossiliferous bed seems to record ideal conditions for echinoderm preservation: rapid sedimentation and low oxygen. The paleoenvironmental setting recorded by this bed was certainly less than ideal for benthic fauna. For these reasons, it seems most likely that the rhombiferan bed in the Reedsville records the distal tail of a high density turbidity flow during which disarticulated or partly disarticulated rhombiferans were transported from the west downslope. Due to the high viscosity of the flow, clusters of plates were not separated. Although extremely rare in eastern North America, Edenian rhombiferans are present in the Fulton Shale of the Cincinnati Arch area. The rhombiferans from the Fulton Shale however, are distinctly older than the Reedsville rhombiferans. Thus, this occurrence of Edenian rhombiferans may also be of evolutionary importance.