Ichnofabric characterization in cores : a method of digital image treatment

Ichnofabric analysis, as a relatively young ichnological approach, has witnessed rapid growth, showing its usefulness in basin analysis, with special attention to palaeoenvironmental interpretations. The ichnofabric approach has evolved from the description of trace composition and the intensity of bioturbation to integrate detailed information on numerous ichnofabric features, such as primary sedimentary structures, ichnological diversity, ichnological features, cross-cutting relation ships or tiering structures. This development has been associated with its application to the study of deep-sea sediments, especially in research on cores, which is not easy, owing to the particular features of cores. Here a method for improving ichnofabric characterization in modern marine cores is presented, on the basis of digital high-resolution image treatment, with special emphasis on the quantification of ichnofabric attributes. The proposed methodology is based on the modification of three image adjustments (image adjustment), the estimation of the percentage of the area occupied by bioturbation (digital estimation), the lateral and vertical quantification and comparison of pixel values for the infill of the trace fossils and the host sediment (pixel counting), and the integration of the information obtained in the visual representations of ichnofabrics (the ichnofabric representation). The sequential application of these proposed steps allow, 1) better identification of trace fossils, together with cross-cutting relation ships and the characterization of trace-fossil assemblages, 2) estimation of the percent age of bioturbation associated to each ichnotaxon, the whole ichnocoenosis, or a complete ichnofabric, 3) differentiation between biodeformational structures and trace fossils, discrimination between ichnotaxa, distinction between passively and actively infilled structures, and 4) evaluation of the depth of penetration by particular tracemakers.