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

Although plate flexure exerts a first-order control on facies patterns in f oreland basins, the influence of other factors can overprint flexural effec ts on basin stratigraphy, We examine the influence of preexisting foreland structural elements on paleotopography and sedimentation in the distal Alpi ne foreland basin in southeastern France. The Eocene Nummulitic Limestone F ormation records progressive backstepping of a shallow-marine carbonate ram p at the distal margin during flexurally induced transgression of the Europ ean foreland plate. Analysis of paleogeomorphic features preserved along th e basal foreland basin unconformity permits reconstruction of paleotopograp hy during early foreland-basin development, The Nummulitic Limestone unconf ormably 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 lan d surface comprised a set of structurally controlled basement highs with an estimated relief of up to 500 m at the time of the Nummulitic transgressio n. Locally, bedrock-confined paleovalleys filled with fluvial conglomerates are preserved along the unconformity surface and indicate that the pre-Ter tiary subcrop was exposed subaerially, Subsequent marine transgression took place across a foreland substrate that showed considerable variation in se afloor topography, Erosional and depositional processes adjacent to structu ral highs created a complex facies mosaic, which overprinted the simple bac kstepping carbonate-ramp facies succession that is typically associated wit h distal forelands, Although flexure determines large-scale facies trends, preflexural relief related to foreland structures can exert significant con trol 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 patt erns at the basal unconformity show that there were abrupt variations in th e depth of erosion into the foreland plate. Field- and basin-scale stratigr aphic 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 las depicted by a migrating flexural forebulge) or b y eustatic sea-level fall as has been proposed for other basal foreland-bas in unconformities.