DELTAS AT RAPIDLY CHANGING SEA LEVEL: THE RECENT AND PLIOCENE VOLGA DELTAS IN THE CASPIAN SEA

For over 3 Ma after the Messinian crisis the Caspian was still an almost dry basin. The Volga river carved a canyon up to 600 m deep into the Northern and Middle Caspian, and deposited its load in a huge delta in the rapidly subsiding South Caspian Basin (SCB). This PaleoVolga delta is now the Lower Pliocene Productive Series (PS), the main reservoir of oil and gas in the SCB. Deposition in the PaleoVolga delta was governed by rapid fluctuations in the level of Caspian Sea unrelated to eustasy, and by climate-triggered fluctuations in sediment input. Four lithofacies associations have been distinguished in the PS which reflect a gradational basinward shift from proximal sheet sandstones, through medial, channelised sheetsands, unconfined sheetfloods and floodplain facies, to distal floodplain and lacustrine environments. Deltaic deposition is restricted to 4 to 8 m thick CU sequences which prograded into extremely shallow water at the system’s distal margin. Vertical lithofacies variation results from repeated proximal to distal translations of depositional environments along the same depositional profile. The recent Volga delta has an extremely gentle gradient (5cm/km) and is strongly bifurcated, with over 800 outlets along a delta front of 200 km. This is due to the absence of a gradient break at the delta front and the absence of any marine reworking. Progradation in the Volga delta is triggered by sea-level fall. Forced regression during the last three-metre sea-level cycle (1929-1977 AD) initiated deposition of a thin progradational CU cycle by seaward extension of small alongstream levees. During sea-level rise (1977-1995AD) aggradation on the levees and in the basins prevailed, while vegetation preserved the outlines of the delta front by keeping pace with sea-level rise. Holocene CU sequences up to 3 m thick, with freshening upwards mollusc assemblages record progradation during the 1000 BP Derbent regression. Both deltas are similar low-gradient river-dominated ramp deltas subject to rapid sea-level change. Yet their 3D architecture is radically different due to different basin dimensions: the Pliocene Volga focussed up to 7 km of sediment in the narrow, rapidly subsiding SCB, whereas the Quaternary Volga spread its sediment over a quarter-million sq.km on the stable border of the Eurasian plate