EVALUATION OF ICHNODIVERSITY BY IMAGE-RESAMPLING METHOD TO CORRECT OUTCROP EXPOSURE BIAS

Abstract

We propose a new method to evaluate the diversity of ichnofossils from outcrop. Ichnodiversity (defined here as the number of ichnotaxa) characterizes paleoenvironmental conditions. However, the apparent numbers of ichnotaxa observed in outcrops are significantly affected by differences in areas of exposed outcrops. This study proposes a new method to evaluate ichnodiversity, independent of outcrop exposure bias, by using an image-resampling technique combined with the shareholder quorum subsampling method. In this method, the relationship between observed and detected numbers of ichnotaxa is estimated by subsampling from existing outcrop images. The relative diversity of ichnotaxa is obtained at a given value of the estimated coverage parameter, representing the ratio of the observed number of ichnotaxa to the actual diversity. The method was verified by analyzing artificial images of ichnoassemblages, and the method successfully estimated reasonable values of relative diversity of ichnotaxa. It was also suggested that the spatial distribution patterns of ichnofossils on the bedding planes does not affect the estimated intensity of ichnodiversity when using this method. This method was also applied to field data pertaining to deposits of the submarine channel-levee complex in the Oligocene Izaki Olistolith of the Nichinan Group, southwest Japan. As a result, the ichnodiversity of the successions in the Izaki Olistolith was reconstructed to be relatively high in channel deposits and low in levee deposits.