Delta-Fed Turbidites Infilling Topographically Complex Basins: A New Depositional Model for the Annot Sandstones, SE France

Abstract

The Eocene-Oligocene Annot Sandstones of southeast France record the predominantly deep-water, siliciclastic infill of the early French Alpine foreland basin. They accumulated in a topographically complex basin as recorded by variable onlap relationships with the underlying Globigerina Marls. Onlap configurations and paleo-bathymetric data from the western outcrops of the Annot Sandstones enables a contour map of the pre-Annot Sandstone basin-floor topography to be reconstructed. The basin comprised a northern and a southern sub-basin, which were separated by a ridge, through which a trough was cut (the Coyer Trough) linking the two sub-basins during the latter stages of infilling. Data on provenance and paleocurrents suggest that the sandstones were sourced from two main feeder systems: an eastern source from the Alps fed the northern sub-basin, and a southern source fed the southern sub-basin. There is evidence of mixing of provenance in the lower parts of the southern sub-basin. Integration of the reconstructed basin-floor topography, sediment-dispersal data, and facies and stratal architectures suggests a depositional model comprising sand-rich, delta-fed submarine ramps and/or aprons whose development was strongly influenced by the basin-floor topography. The southern sub-basin initially had a steeper delta front, characterized by debris flows; this contrasted with the lower-gradient northern sub-basin. The southern sub-basin comprises approximately 800 m of thick-bedded sandstones interpreted as the deposits of sustained and possibly ponded turbidity currents. The upper part of the southern sub-basin records increased channeling and scour related to bypass of material after the basin was filled; these channels transported material northwards through the Coyer Trough, and capped the northern sub-basin. This development of the southern sub-basin is comparable to the fill and spill development of perched, intra-slope basins in the Gulf of Mexico.