Application of CT-Scan Data to the Study of Bioturbation Features in Cores of the New Albany Shale in Indiana
Riese, David; Schieber, Juergen
CT scans of core from the Selmier and Camp Run Member of the New Albany Shale were used to better visualize and document the type and amount of bioturbation in these two units. Of particular interest was cryptobioturbation, a form of bioturbation difficult to observe owing to small size of burrows and lack of contrast from surrounding media. In previous visual descriptions of these cores, the most abundant trace fossils recognized were biodeformational structures, mantle and swirl structures, Zoophycus, Planolites, Teichichnus, and Chondrites. Cryptobioturbation, often manifested as fuzzy laminae was present in every core. General difficulties with recognition of trace fossils are due to (1) oblique sections of traces in slabbed cores; (2) lack of contrast between trace and host rock; and (3) severe compaction of traces due to high initial water contents. The latter can significantly distort original trace geometry and makes identification of small traces very difficult. CT scans solved these problems through easier visualization that utilizes density differences between host rock and burrow fill, and through the ability to view traces in three dimensions. Twelve core intervals of 55.4 ft total length were analyzed. CT scans show many high density contrast traces that reflect an elevated abundance of diagenetic pyrite in the burrow fill. Trace fossils such as Chondrites and Planolites appear to be more abundant than previously described, suggesting they are not as readily identified in cross-section alone. In CT-scans, laminations that appear fuzzy in visual inspection also contain small sinuous traces 1 mm or less in diameter that appear lighter than the surrounding rock due to elevated pyrite contents. Shale that appears unbioturbated lacks significant density contrast due to a high degree of dissemination of pyrite and is free of cryptic traces. The trace fossil category Agrichnia, not recognized in initial core descriptions is visible in CT scans of Selmier Member core as polygonal horizontal networks. Agrichnia are interpreted as agriculture traces found near the sediment-water interface and have low preservation potential due to subsequent bioturbation. Preservation of these traces thus suggests that other burrowers were excluded because of very shallow location of the redox boundary within the sediment, or due to very slow sedimentation rates only refractory organic matter was buried and thus the media was of no interest to other organisms.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013