Untangling the Annot sand fairway: structure and stratigraphy of the Eastern Champsaur Basin (Eocene–Oligocene), French Alps

Abstract

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.