Ichnodiversity and ichnoabundance: Revealing depositional trends in a confined turbidite system

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Abstract

Ichnological studies are still in their infancy when it comes to the interpretation of deep-marine deposits. The Eocene–Oligocene turbidite system of the Grès d'Annot Formation in south-east France is well-studied sedimentologically, but its trace-fossil content is poorly known. Here, an integrated ichnological–sedimentological study is presented from the Annot sub-basin for the first time, which demonstrates its value for interpreting proximal to distal and axial to marginal trends in confined turbidite systems. A comprehensive trace-fossil data set was collected from seven outcrops situated in the southern part of the basin. These data are presented following a morphology-based classification scheme to allow easy recognition and characterization of ichnotaxa. Ichnodiversity and the abundance of ichnotaxa are regarded as important parameters in such interpretations. Instead of simply counting ichnotaxa per outcrop or stratigraphic unit, an equation has been developed in which the ‘ichnoabundance’ (new term) of each counted ichnotaxon is calculated. An exponential growth factor is applied to the increase of the frequency of trace fossils, and is assumed in this equation to better reflect the population dynamics of benthic organisms. A comparison of the solution for pre-turbidite and post-turbidite trace-fossil suites seems to be more suitable for revealing regional and stratigraphic trends compared with conventional approaches. Despite varying size and conditions of the studied outcrops, the results achieved from the Grès d'Annot Formation can help in the reconstruction of sedimentary processes acting in this confined turbidite basin.

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