Characterization of an Oligocene submarine canyon system and its controlling factors: The La Bonette canyon of the Southern French Alpine Foreland Basin

Abstract

Submarine canyons are commonly controlled by tectonic structures and, therefore, are key elements of the evolution of convergent margins. The present study focuses on the Southern French Alpine Foreland Basin. Here we use the outcrops of Grès d’Annot Formation and Schistes à Blocs Formation of the Sanguinière-Restefond Sub-basin to study the formation of an ancient canyon in relation to extensive tectonics of the Colombart Structure. The Grès d’Annot Upper Erosion Surface (GAUES) and faults have been mapped in the field and using airborne and drone pictures. Moreover, the planform pattern of the canyon was reconstructed using GIS modelling. Finally, the deposition age of the Schistes à Blocs Formation, which is considered to belong to the Lower Rupelian, has been constrained by the analysis of calcareous nannofossils and benthic foraminifera coming from nine samples. The Colombart Structure is composed of two normal faults of southern vergence bordering a northward dipping rollover anticline, and axially controls the 700 m deep La Bonette Canyon cutting through the underlying Annot Sandstones. The La Bonette Canyon exhibits a succession of sharp erosive features, such as erosive walls, ramps and terraces. The cross-section profile of the La Bonette Canyon exhibits a tectonic control at several scales: it is asymmetric as well as the thalweg is, which is made of an erosive ramp exploiting the underlying structural dip, and an axial incision whose location is controlled by faults. Faults also commonly control smaller scale morphologies, but also the capture of tributaries at right angles with the canyon axis, testifying for a rectangular drainage pattern. The GAUES is interpreted to result from retrogressive erosion and erosion by gravity currents affecting partially lithified turbidites following three main triggering factors which are: i) a possible tilting of the basin towards the west or north-west, and ii) the 3rd order eustatic fall linked to the Oi1a δ18O event, and iii) the increasing erosive power of gravity currents, which were sourced from the remobilized front of the Autapie nappe during its advance in the basin. Tectonic activity and canyon excavation also triggered destabilization of the flanks during deposition of the Schistes à Blocs Formation.