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

The Eocene Nummulitic Limestone Formation records progressive backstepping of a shallow marine carbonate ramp at the distal margin of the Early Tertia ry Alpine foreland basin during flexurally induced transgression of the Eur opean foreland plate, Analysis of paleoshoreline features preserved along t he basal unconformity of the basin in southeastern France enables identific ation of rapid fluctuations in the rate of relative sea-level rise, In the Champsaur district of Les Hautes Alpes, the Priabonian Nummulitic Li mestone Formation unconformably overlies pre Tertiary bedrock across a step ped transgressive ravinement surface, This sur face consists of a successio n of subplanar bedrock terraces that are separated by high-angle risers. Th e terraces display an overall staircase geometry, stepping upwards in the d irection of shoreline transgression. Progradational gravelly beachface depo sits overlie the terraces, and are themselves abruptly overlain either by t ransgressive 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 paleotopog raphic high. Mapping of the relict paleoshorelines indicates successive epi sodes of terrace cutting, beach progradation, drowning, and shoreline backs tepping on a late Eocene rocky coastline. The formation of the platforms and preservation of drowned gravel beaches i s interpreted as a response to abrupt variations in the rate of relative se a-level rise during transgression of the region by the distal shoreline of the Alpine foreland basin. Periods of slow rise or stillstand enabled platf orm cutting and subsequent beach progradation, whereas pulses of rapid sea- level rise caused abrupt drowning and landward shift of the shoreline. We s peculate that these high-frequency episodic fluctuations in relative sea le vel are a consequence of glacioeustatic sealevel oscillations of < 0.5 m.y. duration superimposed upon a steady relative sea-level rise resulting from the background flexural subsidence.