Fossil shore platforms and drowned gravel beaches; evidence for high-frequency sea-level fluctuations in the distal Alpine foreland basin

Abstract

ABSTRACT The Eocene Nummulitic Limestone Formation records progressive backstepping of a shallow marine carbonate ramp at the distal margin of the Early Tertiary Alpine foreland basin during flexurally induced transgression of the European foreland plate. Analysis of paleoshoreline features preserved along the basal unconformity of the basin in southeastern France enables identification of rapid fluctuations in the rate of relative sea-level rise. In the Champsaur district of Les Hautes Alpes, the Priabonian Nummulitic Limestone Formation unconformably overlies pre-Tertiary bedrock across a stepped transgressive ravinement surface. This surface consists of a succession of subplanar bedrock terraces that are separated by high-angle risers. The terraces display an overall staircase geometry, stepping upwards in the direction of shoreline transgression. Progradational gravelly beachface deposits overlie the terraces, and are themselves abruptly overlain either by transgressive shoreface facies or by younger beachface units. The prograded beaches are arranged in an overall landward-stepping stacking pattern. The terraces are interpreted as a flight of fossil wavecut shore platforms that record episodic landward migration of the paleoshoreline onto a paleotopographic high. Mapping of the relict paleoshorelines indicates successive episodes of terrace cutting, beach progradation, drowning, and shoreline backstepping on a late Eocene rocky coastline. The formation of the platforms and preservation of drowned gravel beaches is interpreted as a response to abrupt variations in the rate of relative sea-level rise during transgression of the region by the distal shoreline of the Alpine foreland basin. Periods of slow rise or stillstand enabled platform cutting and subsequent beach progradation, whereas pulses of rapid sea-level rise caused abrupt drowning and landward shift of the shoreline. We speculate that these high-frequency episodic fluctuations in relative sea level are a consequence of glacioeustatic sea-level oscillations of < 0.5 m.y. duration superimposed upon a steady relative sea-level rise resulting from the background flexural subsidence.