No abstract

The Eocene to Oligocene deposits of the Alpine Foreland Basin of Southeast France include a series of partially preserved, originally interconnected turbidite-filled subbasins. Within these remnants, well-constrained interpretations of sediment architectures have been developed by many geologists of the last five decades which are used to resolve both intrabasinal and interbasinal filling histories (e.g., the Annot, Grand Coyer, Col de la Cayolle, and Trois É vêchés subbasins). However, little is known about the depositional system in more distal areas. The Eastern Grès du Champsaur (Champsaur Sandstones), which has not been subject to detailed evaluation, is thought to represent the down-dip continuation of the Grès d'Annot Turbidite System. Presented here is an outcrop case study from the Eastern Champsaur Basin that describes how intrabasinal bathymetry affected gravity-current behavior, in turn driving quantifiable variations in sediment distribution patterns.The initial paleobathymetric template of the Eastern Champsaur Basin prior to turbidite deposition is reconstructed here through the generation of isopach maps constructed from field studies of the basin fill. The extent to which successive onlapping turbidity-current deposits were controlled by this bathymetry is determined through the analysis of paleocurrent, facies proportion, and isopach data from measured sections. Quantifiable variations in both facies (particularly the occurrence of clast-rich intervals) and sand/shale ratio are observed in association with an intrabasinal high, which is interpreted to have contained flows within this depositional system, allowing only a relatively finer fraction of the flow to move down dip in the distal parts of the basin. This study has direct application in aiding the understanding of sand emplacement processes at the fringes of turbidite basins, in defining the genesis of stratigraphic trap geometries in confined deep-water settings, and in better understanding connectivity issues within deep-water reservoirs.

Early foredeep successions can yield insight into tectonic processes operating adjacent to and ahead of fledgling orogenic belts but are commonly deformed by the same orogens. We develop a workflow towards stratigraphic understanding of these deformed basins, applied to the Eastern Champsaur Basin of the French Alps. This contains a down-system correlative of the southern-sourced (Eocene–Oligocene) Annot turbidites. These strata are deformed by arrays of west-facing folds that developed beneath the Embrunais–Ubaye tectonic allochthon. The folds vary in geometry through the stratigraphic multilayer. Total shortening in the basin is around 4 km and the restored (un-decompacted) stratal thickness exceeds 980 m. The turbidites are generally sand-rich and bed-sets can be correlated through the entire fold train. The succession shows onlap and differential thickening indicating deposition across palaeobathymetry that evolved during active basement deformation, before being overridden by the allochthon. The sand system originally continued over what is now the Ecrins basement massif that, although contributing to basin floor structure, served only to confine and potentially focus further sediment transport to the north. Deformation ahead of the main Alpine orogen appears to have continued progressively, and the past definition of distinct ‘phases’ (‘pre-’ and ‘post-Nummulitic’) is an artefact of the stratigraphic record.