Abstract
The Caspian Sea, being a closed basin, shows much higher rates of sea-level change (up to 340 mm/yr) than the oceans. It experienced a full sea-level cycle with an amplitude of 3 m between 1929 and 1995. Its coasts therefore present an attractive physical model to test predicted changes along oceanic coasts as a result of expected eustatic sea-level rise, and to validate sequence-stratigraphical interpretations of ancient sediments. Along the wave-dominated coasts with intermediate shore-face slopes in Dagestan, low onshore bars were formed during the 1929–1977 sea-level fall. They represent minor short lived (1–2 years) transgressive oscillations within the major regressional trend. After the 1977 lowstand a prominent barrier–lagoon system was formed, which during the first phase of sea-level rise (1977–1988) encroached landwards about 18 m each year by washover during storms (continuous retreat). Between 1988 and 1995, however, barrier size diminished, lagoons widened and deepened, apparently due to lack of coarse-grained sediment supply (discontinuous retreat). The 1995 highstand prevented the barriers from being overstepped altogether. Sea-level fall after 1995 again produced low bars without lagoons. The Caspian Sea barrier coast demonstrates that for the formation of barrier–lagoon complexes the combination of transgression and storms is essential. Barriers may show either continuous or discontinuous development at constant rate of sea-level rise and constant offshore slope, depending essentially on the availability of coarse-grained sediments. The rate of sea-level rise as a factor in barrier behaviour cannot be viewed independently from barrier dimensions, which in turn depend on wave base depth and maximum storm wave height. Relations between (para)sequence thickness, and frequency and amplitude of sea-level change in closed basins with an independent sea-level regime may differ markedly from those in basins influenced by eustasy.
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