THE WANDERING OF THE VOLGA DELTA - A RESPONSE TO RAPID CASPIAN SEA-LEVEL CHANGE

Due to its very low gradient and absence of tide and surf, the Volga d elta is an even more extreme example of the fluvially dominated type t han the Mississippi delta. However, it differs from all other large de lta systems in that it borders a closed basin, the Caspian Sea, now at -26 m below global sea-level. Caspian sea-level is much more dynamic than that of the world oceans, and rises at present about 15 cm/yr, a hundred time the eustatic rate. Within the Quaternary, sea-level oscil lations of at least 5 orders of magnitudes have been distinguished, wh ich seem grossly out of phase with eustatic sea-level. Between the Wei chselian Early Khvalyn highstand of +50 m and the Early Holocene Mangy shlak lowstand at -80 m the apex of the Volga delta has wandered over 700 km alongstream. The present-day Volga delta is not a highstand dep osit but probably represents a minor transgression in a major regressi onal stage. The delta does not show a coarsening-upwards sedimentary s equence, but consists of a Weichselian transgressional fining-upwards sequence topped by eolian deposits, in which the delta distributary ch annels have been incised. Present-day sedimentation is limited to a na rrow fringe along the delta front, and to deeper waters over 200 km of fshore. Sea-level changes outpaced aggradation to such an extent, that Volga sediment is spread over the whole North Caspian Plain. Sequence -stratigraphical principles are difficult to apply because sea-level c ycles of five orders of magnitude are superimposed, and because there is not enough sediment loading or tectonic subsidence to create suffic ient accommodation space.