APPLICATIONS OF COMPUTERIZED TOMOGRAPHY TECHNOLOGY IN THE ANALYSIS OF FOSSILIFEROUS SIDERITE CONCRETIONS FROM THE FAYETTEVILLE SHALE (LATE MISSISSIPPIAN), SOUTHERN MIDCONTINENT

Increased resolution and software enhancements of traditional Computerized Tomography (CT ) scanning techniques have provided a potential new tool for non-invasive analysis of fossiliferous concretions from the Fayetteville Shale (Late Mississippian) of the southern midcontinent. Concretions are abundant throughout the Fayetteville Shale and range in size from very large (> 1 meter in the longest dimension), unfossiliferous micritic concretions in the basal Fayetteville, to smaller (average length 8 cm) sideritic bodies in the upper part of the lower and the upper Fayetteville Shale. Almost all of the siderite concretions from the Fayetteville contain ammonoids. Previous taphonomic investigations of the Fayetteville cephalopods suggest variable rates of sedimentation and sea floor induration. Variability in seawater chemistry is demonstrated by compositional differences in Fayetteville concretions. The concretions have also provided evidence for sedimentation rates and the depositional dynamics of this unit. CT scans were performed on ammonoid-bearing siderite concretions from the Fayetteville Shale. Portions of the conchs of the ammonoids were observed on the exterior surfaces of the concretions. CT analysis allowed viewing of the interior regions of the concretions. CT imaging manipulation allowed rotation of the image and "slicing" of the concretion. The CT's document a two stage development in all the concretions. The interior region of the concretion is dense and the CT penetrates this area poorly. The CT can "see" into the outer portion of the concretions easily, where ammonoid conchs and septa can be observed. Prior to the use of the CT, concretions were either broken with a hammer or slabbed with a rock saw to allow investigation of their interiors. Geochemical data in conjunction with CT analysis of these concretions will provide important new information concerning black shale deposition, the mineralogy, timing and formation of the concretions, and changes in water column chemistry that are not reflected in the shale itself. Better understanding of concretionary development through CT scanning and geochemical analysis adds significant data to provide insight into the history of black shale deposition.