Sedimentological, geochemical and geomorphological factors in formation of coastal dunes and nebkha fields in Miankaleh coastal barrier system (Southeast of Caspian Sea, North Iran)

Abstract

Miankaleh coastal barrier system has about 71 km length with an average of 3 km width in southeast of Caspian Sea. Forty sandy samples, collected from 5 coastal dune zones, in Miankaleh were analyzed compositionally, texturally and chemically and 31 samples from 4 nebkha field zones were analyzed for textural and geomorphological interpretation. Samples from stoss and lee sides of coastal dunes and nebkha dune field are mainly composed of medium to fine grains of sand that are well sorted, fine skewed and leptokurtic. Modal analyses of coastal dune sands show that they comprise 61% quartz, 14.74% feldspar and 24.26% rock fragments. The chemical index of alteration (CIA) value in coastal dune samples is high because of relatively warm and humid climatic condition with high amount of feldspar and rock fragments. Sedimentary and plutonic rock fragments are more than the other lithic fragments in sand samples. Nekaroud is the main source of sediments supply and as move to the east, the amount of Fe2O3, TiO2, MgO and heavy minerals in coastal dune sands decreases. The changes of the ratios of Al2O3/Na2O and Rb/Sr can be used to evaluate the CIA value in the study area. High Zr value is another evidence of high zircon content and recycled sedimentary rocks in the source area. From west to the east, sand moving power index (M) decreases and active coastal dunes change to dormant coastal dunes. There is no coastal dune in the eastern and southern parts of the study area. Based on concentration, stabilization, height and biological cover, nebkha dune fields are classified into 4 zones. There is a meaningful relationship between different morphometrical parameters.