A Mud-Filled Submarine Canyon Cutting Through a Sand-Prone Turbidite Succession: A Lower Oligocene Field Analogue of Stratigraphic-Structural Trap in the Western Alpine Foreland Basin, France

Abstract

Submarine canyons are key elements of the evolution of convergent margins. Their infilling can be mud-prone and thus represents a good seal if they cut through reservoir series, which can be enhanced by tectonic activity. The present study focuses on the Western Alpine Foreland Basin where we use the outcrops of the Schistes à Blocs Formations (SaB Fm) to study the syn-tectonic evolution of a Lower Oligocene submarine canyon that cuts through the Annot Sandstones Formation (GA Fm). The Grès d’Annot Upper Erosion Surface (GAUES), the Intra Schistes à Blocs Erosion Surface (ISaBES) and the N80°E faults have been mapped in the field. Airborne and drone pictures were also used. Moreover, the deposition model and the stratigraphic architecture of the canyon-fill were reconstructed from the analysis of six sedimentary logs. Finally, the sedimentary system of the SaB Fm has been constrained by the measurement of paleocurrents, and the provenance analysis of detrital zircons from four samples. The SaB Fm is composed of two units: a lower one, dominated by fine- to very-fine- grained turbidites (Schistes Bruns Member; SB Mbr), and the upper one, only composed of olistostromes (Schistes à Blocs Exotiques Member; SaBEx Mbr). The paleocurrent measurements on the unidirectional ripples deposited within the SB Mbr is witness of a turbulent flow rebound against the steep northern flank of the canyon, during the construction of internal levees which often experimented gravity collapse. Slumping of internal levees improves the seal properties of the canyon-fill, while the canyon thalweg was dominated by by-pass processes. Detrital zircons reveal that the provenance of the SB Mbr is sourced by the same sedimentary system of the GA Fm. Thus, we interpreted that both the canyon excavation and the sediment by-pass within its thalweg during the deposition of the SB Mbr, were trigerred by an increase in basin slope related to the tectonic evolution of the accretionary prism. Finally, the ISaBES is a composite and diachronous surface resulting both from the morphology of the internal levees, and from their reworking during the deposition of the olistostromes, that predates the emplacement of the Alpine nappes within the foreland basin.

Keywords: Submarine canyon, mud-filled canyon, Schistes à Blocs Fm., Annot Sandstones Fm., turbidites, debrites, combination trap, Lower Oligocene