EVIDENCE FOR TECTONIC FACTORS IN THE PRESERVATION OF AUTOCHTHONOUS CARBONIFEROUS FORESTS, BLACK WARRIOR BASIN

Autochthonous Carboniferous fossil forests are common in Langsettian (Early Pennsylvanian) rocks of the Appalachian Basin, preserved both above coals (rooted in histosols) and within siliciclastic sequences (rooted in entisols). Standing tree bases up to 4 meters have been reported, and often the remainder of the aerial trunk of each tree is preserved prostrate on the bedding surface that represents the molding event. There has been speculation as to both the sedimentological regime(s) and the duration of the casting event responsible for preserving such forests. A detailed sedimentological analysis associated with the the preservational event of a standing forest above the Blue Creek coal, Mary Lee Coal Zone, of the Black Warrior Basin, Alabama, has provided insights into the type of regime responsible for this assemblage, as well allowing for an estimate of the event's duration. The sediments responsible for burial of the ultimate forest floor litter are heterolithic pinstripe laminae of alternating fine-to-very-fine siltstone and sandstone in which the discontinuous parallel lamination infill hollow voids within plant axes. Gradational and suprajacent to the plant litter are continuous parallel, pinstripe lamination in which thicker sandstone laminae often are draped with silt- and sand-sized comminuted plant detritus. There is a distinct thin-thick variation in lamination thickness of the with a pronounced rhythmicity with periodicities between 10 and 20 laminations per cycle. Additionally, sediments casting the erect lycopsid, calamitean, and pteridophyllous trees display the same sedimentological signature which has been interpreted as tidalite deposition. Burial of the forest floor litter was extremely rapid as indicated by its excellent preservational state. Plant detritus in the upper litter shows no signs of decay, limiting its residence time on the forest floor to no more than 6 months (based on decay rates of Recent tropical leaf litters). Burial of trees up to 4 m in height is indicative of at least this amount of accommodation space, and estimates for deposition of these tidalites is less than a decade. Hence, it is proposed that tectonic rather than eustatic mechanisms are the primary extrinsic factor responsible for the genesis of such assemblages.