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) corresponing 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 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 1930-1995 and the records showed that it included a lot of short-term (c. 4-5 yrs) 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 c. 30-100 m. Overall, a step-like morphology, 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.