Sedimentary and environmental characteristics of the Gilan–Mazenderan plain, northern Iran: influence of long- and short-term Caspian water level fluctuations on geomorphology

Abstract

The south–southwestern Caspian coastal lowland in Iran, or the Gilan–Mazenderan plain, is a relatively narrow but long, composite depositional area of late Quaternary age. The Sefid Rud delta, the Anzali Lake (connected to the Caspian Sea by a meandering outlet 3.5 km long) and storm-dominated beaches are its prominent sedimentary features. They are controlled by the present water level of the Caspian Sea at −26.36 m in 2000. The late Pleistocene–Holocene deposits of the lowland, which are covered commonly by a modern loess-origin soil, mainly consist of alternations of marine and non-marine sediments. The marine units form coastal terraces at 19–20 m (I), 2–0 m (II) and −6–−8 m (III) corresponding to the Late Khvalinian and Neocaspian transgressions. The deposits of the youngest terrace (terrace III) that represents a prograding beach–ridge complex are a consequence of several medium-term, cyclic water level oscillations in late Holocene. Just after the initiation of the beach–ridge complex, Lake Anzali that formed by damming of rivers and then by progradation of the complex in time forced to form the outlet of the lake. The nearest medium-term cycle lasted c. 65 years between 1930 and 1995, and the records showed that it included a lot of short-term (c. 4–5 years) and very short-term (week to months) water level oscillations. During the last erratic rise of sea level (1977–1996), the area of Lake Anzali doubled; the delta and the coastal sands including modern beaches were eroded on a width of around 30–100 m. Overall, a steplike morphology, through the repetition of marine and non-marine facies and also water level records of the last 75 years, indicate that the ancient and recent deposition on the coastal lowland has been controlled by long-, medium- and short-term fluctuations of the Caspian Sea level.