Pith casts, commonly found in Carboniferous rocks, have been investigated as to their mode of infilling through sedimentological studies conducted on ancient specimens and actualistic experiments conducted in a natural fluvial environment. This investigation documents the interrelationship of sedimentary environment, orientation of fluvially transported stem material and its effect on a depositional system, and the mode of hollow stem infill in this depositional environment.
A concentrated Pennsylvanian-aged xylocoenosis (dynamically ordered wood assemblage) preserved in a sandstone split of the Mary Lee coal seam, Mary Lee Coal Zone, Pottsville Formation, in northwestern Walker County was investigated in this study. The sandstone is interpreted as an abandoned channel capped by a log jam (xylocoenosis), based on an evaluation of the sandstone geometry and primary sedimentary structures, and log orientation with respect to the sandstone structures. Observations indicate the log jam restricted flow and likely caused eventual channel abandonment.
Cordaites pith casts collected from the log jam provided an excellent opportunity to examine the sedimentological parameters involved in hollow stem infill. In addition, an experiment conducted in a natural stream allowed modelling of hollow stem infill by bedload sedimentation for comparison. This approach provided a more realistic analog environment than that of a laboratory flume and the results are comparable with the fossil pith casts. Ripples, sinuous infills, grain-size sorting, and other structures observed in the ancient and experimental casts indicate stem infill is dependent on individual stem characteristics, disposition with respect to the sediment/water interface, and the environment of deposition.
The results of this study indicate that where large numbers of logs are preserved and oriented perpendicular to primary sedimentary structures it may indicate a log jam. Furthermore, pith cast infills may aid and support depositional environment interpretations. The upper Mary Lee coal zone of Lower Pennsylvanian "Pottsville" Formation is well exposed in the study area. This provides an ideal opportunity for an assessment of its provenance, environments of deposition, and transgressive character.