The sedimentology and development of a modern sandspit (Miankaleh Peninsula) and a lacustrine lagoon (Gorgan Bay), Caspian Sea, Iran

Gorgan Bay is a ~240 km² lacustrine lagoon on the southeastern margin of the Caspian Sea, from which it is separated by the 48 km long barrier beach/sandspit of the Miankaleh Peninsula to the north. The northern, windward side of the sandspit displays an eroded berm. The bulk of the sandspit is characterized by the presence of beach ridges and transverse, nebkha and parabolic dunes. The southern, leeward side of the sandspit faces the muddy low-energy lagoon of Gorgan Bay. The shore sediments are well sorted, negatively skewed, very fine to fine sands. The sand dunes are composed of well sorted, positively skewed fine sand. Grain types of the shore and dune sediments are very similar, with quartz grains and small superficial ooids being the major constituents. These similarities suggest the dune deposits represent windblown shore sediments. In contrast, the Gorgan Bay sediments are poorly sorted, positively skewed very fine to coarse sands with abundant bivalves, gastropods, ostracods, and charophyte oogonia. Clay minerals include chamosite, illite and chlorite, all of volcanic origin. The presence of pyroxenes and amphiboles may suggest a provenance from the Gorgan Metamorphic Complex. The study shows that the growth of the Miankaleh Peninsula sandspit is a result of predominantly eastward longshore transport of littoral fine sands, which has been thrown by storm waves above the mean spring tide waterline in the form of slightly curved beach ridges. This has allowed the development of dry beach conditions (backshore) and the growth of vegetation. The beach ridges have been successively accreted in an eastern direction (younging to the east). The formation of superficial ooids is mainly linked to low-energy conditions. However, occasional high-energy events existed, and long, hot summers may also have played also a role in ooid formation. The sand was reworked by the prevailing northwesterly winds (dune formation). Spit formation facilitated the infilling of the sheltered lagoon by the trapping of sediment derived from the Gorgan Metamorphic Complex to the south.