Implications of foreland paleotopography for stratigraphic development in the Eocene distal Alpine foreland basin

Abstract

Although plate flexure exerts a first-order control on facies patterns in foreland basins, the influence of other factors can overprint flexural effects on basin stratigraphy. We examine the influence of preexisting foreland structural elements on paleotopography and sedimentation in the distal Alpine foreland basin in southeastern France. The Eocene Nummulitic Limestone Formation records progressive backstepping of a shallow-marine carbonate ramp at the distal margin during flexurally induced transgression of the European foreland plate. Analysis of paleogeomorphic features preserved along the basal foreland basin unconformity permits reconstruction of paleotopography during early foreland-basin development. The Nummulitic Limestone unconformably onlaps basement in the hanging walls of a set of reverse faults in the foreland plate. The pattern of onlap indicates that the late Eocene land surface comprised a set of structurally controlled basement highs with an estimated relief of up to 500 m at the time of the Nummulitic transgression. Locally, bedrock-confined paleovalleys filled with fluvial conglomerates are preserved along the unconformity surface and indicate that the pre-Tertiary subcrop was exposed subaerially. Subsequent marine transgression took place across a foreland substrate that showed considerable variation in seafloor topography. Erosional and depositional processes adjacent to structural highs created a complex facies mosaic, which overprinted the simple backstepping carbonate-ramp facies succession that is typically associated with distal forelands. Although flexure determines large-scale facies trends, preflexural relief related to foreland structures can exert significant control on smaller scale depositional patterns. Our study has implications for the interpretation of the basal unconformity of the Alpine foreland basin in southeastern France. Regional subcrop patterns at the basal unconformity show that there were abrupt variations in the depth of erosion into the foreland plate. Field- and basin-scale stratigraphic relations demonstrate that the geometry of the basal unconformity is controlled by paleotopography related to relict basement structures. It is not exclusively compatible with an origin either by broad wavelength uplift of the foreland plate (as depicted by a migrating flexural forebulge) or by eustatic sea-level fall as has been proposed for other basal foreland-basin unconformities.