Development of the Southern Coastal Area of the Caspian Sea during the Pliocene–Quaternary According to Biostratigraphic and Magnetostratigraphic Data

The paper considers geological structure of the coastal region of the South Caspian, including paleontological and magnetostratigraphic dating of the Neogene–Quaternary deposits. Western and eastern segments of the region between the South Caspian Basin and the Alborz Ridge developed differently in the Late Cenozoic. In the west, marine sediments did not penetrate beyond the coastal plain into the neighboring lowered part of the Alborz Ridge during the Pliocene–Quaternary. This indicates the stability of the marine basin boundary or its expansion due to the abrasion of the Alborz slopes. In the east, a piedmont step emerged, bounded by thrust faults. The marine deposition at the piedmont step occurred in the Miocene. At the end of the Miocene, the marine sediments were folded and eroded. The Akchagylian (Piacenzian–Gelasian) marine sediments accumulated at the northern edge of the piedmont step. The Khazar fault raised these sediments up to 120–150 m and isolated the piedmont step. Thus, the expansion of the Alborz mountain building and the reduction of the South Caspian Basin occurred in the eastern part of the coastal area from the Late Miocene. The differences between the western and eastern segments of the coastal area are related to the development of the South Caspian Basin. Until the Late Miocene, it remained a residual trough of the Paratethys. During the Pliocene–Quaternary, the eastern part was filled with sediments up to 6 km thick and retained the features of a thinned continental crust 30–37 km thick with sedimentary cover up to 16 km thick. The western part of the basin was filled with sediments about 10 km thick and acquired the features of suboceanic crust with the Mohorovichich surface at a depth of 28–30 km with a thickness of the sedimentary cover exceeding 20 km.