Palynofacies classification of the depositional elements of confined turbidite systems: Examples from the Gres d'Annot, SE France

Abstract

Turbidites deposited in confined mini-basins may demonstrate extremely complex stratigraphic architectures. We hypothesize that particulate organic matter preserved in turbidites will not be randomly distributed and may be used to assist the identification of architectural elements. An integrated sedimentological and palynological study was conducted on confined turbidites in the Peïra Cava sub-basin of the Eocene to Oligocene Grès d'Annot system, SE France; this provides a natural laboratory, where certainty in the stratigraphy allows confidence in sampling of sub-environments. Elements reflect deposits from high and low density flows including: thin-beds that onlap the basin margin and heterolithics spread across the basin; base of slope megabeds; and thick-beds that have a proximal to distal expression from south to north across the basin. One hundred samples were collected from logged sections across the basin, with 10 g of mudstone per sample being processed for a count of three hundred pieces of organic matter. Both allochthonous terrestrial and relatively autochthonous marine matter were recovered, with results showing a progressive fining of material from proximal to distal areas. Base of slope megabeds and proximal ponded thick-beds are dominated by dense humic matter, medial areas become dominated by light plant material, and distal samples are dominated by amorphous matter, interpreted as a result of density sorting of organic matter in turbidity currents. Exploratory ordination analysis and fuzzy cluster analysis were used to examine these results. Based on this study, we provide evidence of density fractionation of organics in turbidity currents, which is implied to be a major control on the distribution of particles in deep-marine depositional systems. This allows a palynofacies classification scheme to be developed to recognise architectural elements, which may be applied to sub-surface samples to assist the characterization of deep-water mini-basin architecture, understanding of which is crucial for hydrocarbon exploration and development.