Tectonic Control on Paleovalley Incision at the Distal Margin of the Early Tertiary Alpine Foreland Basin, Southeastern France

Abstract

ABSTRACT Paleovalley systems preserved in the stratigraphic record provide useful information on the evolution of ancient landscapes. I investigate the controls on erosion of a set of bedrock-confined paleovalleys at the distal margin of the early Tertiary Alpine foreland basin in southeastern France. The paleovalleys are incised into pre-Tertiary subcrop along the basal unconformity of the foreland basin. They have been identified on the basis of: (i) they form incised paleotopographic features that erosionally truncate bedrock; (ii) they comprise a basal erosion surface that is locally characterized by paleosols along valley margins; (iii) valley-fill strata onlap the margins of the paleo-erosional features; (iv) they show abrupt lateral thickness and facies variations. Paleosol remnants and erosional scour indicate subaerial exposure and erosion of the foreland plate bedrock. The paleovalleys are located adjacent to or on the hanging wall of a set of pre-late Eocene reverse faults that bound a major basement massif. Provenance studies indicate that paleovalley-fill conglomerates were locally derived from erosion of paleo-fault scarps. Incision of the paleovalleys is linked to fluvial erosion of bedrock during surface uplift and tilting related to growth of basement-cored fault-propagation anticlines. The pre-Priabonian paleodrainage drained hanging-wall uplands created by uplift of these structures. Catchment basins were small and stream lengths relatively short. Paleovalley incision in response to growth of intraforeland uplifts contrasts with flexural forebulge uplift and eustatic sea-level fall as an explanation for development of paleodrainage at the d stal margin of foreland basins.