2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 128-11
Presentation Time: 9:00 AM-6:30 PM


CAO, Yumeng Melody1, CURRAN, H. Allen2 and GLUMAC, Bosiljka1, (1)Department of Geosciences, Smith College, Clark Science Center, 44 College Lane, Northampton, MA 01063, (2)Department of Geosciences, Smith College, Northampton, MA 01063, mcao@smith.edu

Ichnologists commonly use the semi-quantitative, visual-analysis indices of Droser and Bottjer (1989) or Taylor and Goldring (1993) to evaluate ichnofabrics in sedimentary rocks deposited in continental and marine settings. While both indices have proved highly useful, the range of bioturbation represented by a given index number can be large, e.g. Ichnofabric Index 3 on the Droser-Bottjer scale represents 10 to 40% bioturbation of original bedding. Thus, a methodology that facilitates determination of actual bioturbation values generated by specific ichnotaxa should be an effective tool. We used Photoshop and ImageJ, a freeware analysis program, to evaluate ichnofabrics in field photos of cliff-face and bedding-plane exposures. In brief, step one is use of Photoshop to “paint” all trace fossils present in the image. It is saved as a high-resolution .jpeg or .png file. Next step is to open the file in ImageJ and use the Wand tool to select “painted” areas (hold “shift” for multiple ichno-shapes), which will be outlined in yellow. Go to Analyze then Measure, and read and record area shown in Results window. Then use Rectangle Tool to select the full image, determine its area using Measure, and calculate ichno-shape percent coverage. As an initial methodology test, we used the Ophiomorpha schematic of Droser and Bottjer. Next we measured stellate-burrow ichnofabric in Holocene carbonate eolianites in cliff-face outcrops on Moriah Harbour Cay and bedding-plane exposures on San Salvador, Bahamas. Stellate burrows, likely formed by insects, have a central shaft of 3-7 cm diameter with upward, unlined branches of ~1 cm diameter that form gentle arcs toward their distal ends. There is no secondary branching, but crossovers are common. The informal name “stellate burrow” comes from horizontal bedding planes where central shafts are cross cut and branches form a radiate pattern. Bioturbation generated by several ichnotaxa also was evaluated in Pleistocene shallow subtidal grainstones of vertical surfaces on Harry Cay, Little Exuma, Bahamas. The method worked well for identifying the specific percentage of bioturbation contributed by each ichnotaxon, but degree of coincident bioturbation could not be easily determined and requires further investigation, as does potential for evaluation of bioturbation in core slices.