The wandering of the Volga delta: a response to rapid Caspian sea-level change

Abstract

Due to its very low gradient and absence of tide and surf, the Volga delta is an even more extreme example of the fluvially dominated type than the Mississippi delta. However, it differs from all other large delta 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 oscillations of at least 5 orders of magnitudes have been distinguished, which seem grossly out of phase with eustatic sea-level. Between the Weichselian Early Khvalyn highstand of +50 m and the Early Holocene Mangyshlak 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 deposit but probably represents a minor transgression in a major regressional stage. The delta does not show a coarsening-upwards sedimentary sequence, but consists of a Weichselian transgressional fining-upwards sequence topped by eolian deposits, in which the delta distributary channels have been incised. Present-day sedimentation is limited to a narrow fringe along the delta front, and to deeper waters over 200 km offshore. 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 cycles of five orders of magnitude are superimposed, and because there is not enough sediment loading or tectonic subsidence to create sufficient accomodation space.