Sefidrud delta and Quaternary evolution of the southern Caspian lowland, Iran

The southern Caspian Sea lowland in Iran forms a thick sedimentary prism produced by interactions of marine, deltaic, and fluvial processes during the Late Neogene and Quaternary. Deltaic and fluvial deposits have been developed mainly by the Sefidrud, which is the largest river of the country. Its drainage area (ca. 61,600 km2) includes nearly the entire Alborz mountain range and forms deep valleys, gorges, and a large delta at the southern coast of the Caspian Sea. The Sefidrud delta is so large that it covers one third of the large Guilan-Mazanderan plain, both from an aerial and volumetric point of view. Consequently, the drainage network of the Sefidrud represents the Quaternary evolution of northern Iran and can be used to test the hypothesized evolution of the region. The Sefidrud delta has been investigated using many short and long cores, satellite images, and lithofacies maps, and the results show that this delta complex (1680 km3) consists of a covered lower part and exposed upper deltaic sequences. The lower and upper parts of the complex were formed during the Early Pleistocene and Late Pleistocene, respectively. The upper exposed alluvial and deltaic sediments represent a total progradation of approximately 50 km forming three separate delta subsequences (D1-3) of which the last one (D3) is the modern Sefidrud delta. The modern delta is a wave-dominated, blanket-like sediment cover (ca. 30 m thick and 1550 km2) produced during the last three millennia. It was formed by 30 km progradation of six juxtaposed lobes; each lobe corresponds to a long-term sea-level fluctuation of the Caspian Sea. The active lobe (ca. 50 km2) has been forming since 1550 AD. Late Pleistocene deltaic sequences (D1-3), particularly the modern one (D3), were produced by relatively fast deposition due to high volume sediment load of the Sefidrud. It is suggested that they were produced after the opening of the Rudbar gorge by river capture that in turn six times enlarged the drainage area. Previous deltaic progradation and marine transgressions were relatively slow and the development of the whole lowland at the southern Caspian Sea lasted ca. 1-1.2 Ma BP.