MODELING HOLOCENE STRATIGRAPHY AND DEPOCENTRE MIGRATION OF THE VOLGA DELTA DUE TO CASPIAN SEA-LEVEL CHANGE

The Volga system is sensitive to allogenic control due to its low on and offshore gradient (~5 cm/km). In sequence stratigraphy little attention has been paid to the effects of sea-level change in these ramp-margin fluvio-deltaic settings. The high-frequency sea-level changes of the Caspian basin have considerable amplitude (upto 18 m) over the Holocene time-span, which is usually considered as the lower boundary of 5th-order cycles. Fieldwork in the delta front area shows the progradation of the delta distributaries under influence of the last-century 3m sea level cycle. Augerings upto 7m show rapid lateral facies changes over the Holocene of; (a) lagoonal clays deposited between paleodunes,(b) channel sands,(c) levee sands and silts,(d) laminated overbank and interdistributary bay deposits,(e) mouthbar deposits and (f) prodelta clays. A 2-D process-response model, AQUATELLUS, has been used to investigate fluvio-deltaic response to sea-level fluctuation. Calibration of the model with measured data over the last century, comprising a full 3m sea-level cycle, showed plausible progradation and sedimentation rates. The numerical modelling showed that sea-level changes forced the Holocene Volga delta to migrate ~200 km over the Caspian plain, leaving only thin laterally extensive deposits. The frequent depocentre shifts add a whole new perspective to the ongoing discussion about the impact of sea-level changes along the longitudinal profile. The periods during which significant deposition occurred coincide with the times that migration distances were relatively low. Thicker progradational wedges have been deposited at these time intervals, at ~ 9000-8000, ~7400-6700,~ 5200-3700 and ~2400-900 years BP. 14C dated deposits in the study area corroborate the model output. Remarkably, this is both in highstand and lowstand conditions. The low gradient makes wave effects insignificant and tides are non-existent, so that the fluvial deposits are hardly reworked and no shoreface facies or maximum flooding surface develop. The Volga delta response indicates that sequence stratigraphic concepts are not scale-independent in low-gradient settings and that short-term high-frequency sea-level changes have a far-reaching impact on the stratigraphy.