ABSTRACT

Traditional ichnofabric-descriptive (semiquantitative) schemes rely upon subjective estimates of the degree to which original sedimentary fabrics have been disrupted. This paper presents an objective method of quantifying bioturbation in hemipelagic sediments based on a thin-section image-analysis technique. In order to perform these analyses, optical or backscattered electron micrographs are processed in order to obtain black and white images. In these images black pixels are silt-size, larger grains and the clay-size sedimentary matrix are white pixels. Once the initial processing has been accomplished, an index, H, is defined to quantify the horizontal orientation of the larger grain size fraction of the sediment. H is low for bioturbated fabric and high for laminated sediments. In hemipelagic suspension deposits, H can be used to quantify bioturbation, assuming that no other sedimentary processes have disrupted the original depositional fabric. The quantitative bioturbation scheme described here is consistent with existing semiquantitative bioturbation schemes; in addition, it is objective and provides higher-resolution descriptions of biological processes that disrupted the sediment after deposition. Applying this quantitative scheme should refine future paleoecological reconstruction derived from lacustrine and marine successions.