ENIGMATIC FOSSILS FROM THE MARTINSBURG FORMATION (UPPER ORDOVICIAN), NORTHEASTERN TENNESSEE

This study documents the occurrence of rare, enigmatic fossils in the strata of the upper Martinsburg Formation (Sherman Stage, Caradocian Epoch) at the Thorn Hill locality in northeastern Tennessee. These strata consist of skeletal limestone interbedded with gray siltstone and shale, interpreted as tempestites or storm deposits from a shallow marine shelf environment. Black to dark gray carbonized tubes, oriented parallel to bedding and commonly subparallel to each other, are found in the upper parts of tempestite successions. The tubes are up to 60 mm long and 0.5-2 mm wide. They are unbranched, straight or slightly bent, occasionally twisted, and never tapered. Their shape is circular (noncompressed) to elliptical and flattened (compressed). The noncompressed tubes are filled with sediment and scattered organic material. Many tubes are fractured along straight lines, parallel to each other, and perpendicular or at an angle to tube elongation. All tube terminations formed by breaking along these and other irregular fractures. SEM examination revealed bumpy, irregular tube surfaces, and simple tube walls, 3-80 µm thick, with fine laminations parallel to the wall surface.

These fossils are very different from trace fossils of benthic deposit feeders and from the remains of common Lower Paleozoic marine organisms (brachiopods, trilobites, bryozoa, graptolites), which are abundant in the Martinsburg Formation. The carbonized tubes are pieces of an organism that were broken, transported, and deposited by storm waves and currents. They may represent non-calcified thalli of marine algae. The fossils, however, superficially resemble simple unbranched sporophytes of early land plants, but there is no evidence of spore-bearing organs or a vascular system. Thus, these enigmatic fossils are, for the moment, of uncertain affinity.