XRCT AS A NON-DESTRUCTIVE WAY TO STUDY SIDERITE NODULES: AN EXAMPLE FROM PENNSYLVANIAN DELTAIC SYSTEMS IN ALABAMA

Deltaic to marine invertebrate paleoecology of coal-bearing strata in North Alabama (Upper Cliff Coal Interval in the Plateau Coal Field and Mary Lee Coal Interval in the Black Warrior Coal Field) were characterized in the 1960’s-1990’s using typical methods of the day that utilized field relationships, hand sample surface mapping, polished slabs, and thin-sections. Plant and animal macrofossil identification, including traces, and organism-sediment relationship studies were limited by taphonomic & diagenetic factors such as tectonic compaction, groundwater dissolution, and fracturing during weathering exposure. Authigenic siderite nodule formation offered potential for preserving non-compacted organisms, organism-sediment relationships, and preservation of organisms that would have normally left no trace; however, studying such processes in nodules has been difficult due to their hardness. Typical methods of study included physical cracking, freeze-thaw fracking, and serial thin-sections, all of which are destructive to the nodule. We applied x-ray computed tomography (XRCT Scanning) in order to non-destructively penetrate more than forty nodules. XRCT uses transmission of x-rays along a central rotating axis to emphasize chemical composition contrasts within nodules, which appear as a series of image slices that can then be manipulated into a three-dimensional image of internal and external structures. XRCT scanning provided non-destructive 3-D data on body and trace fossils allowing us to: (1) trace burrows with more accurate shape and directionality, (2) obtain more precise morphology for taxonomic identification, (3) increase known taxonomic diversity, (4) image skeletal shell material not visible on nodule surfaces, (5) image nodule nuclei and paragenesis, and (6) see micro-scale bedding features.