Regional Factors Controlling the Type of Pliocene Deposits in the Southeastern Caspian Basin, NE Iran: Implication for Tectono-stratigraphic Analysis

Providing information about the role of the major elements controlling the type of Pliocene deposits in the south­eastern part of the Caspian Basin by assessing regional and global parameters is very important to demonstrate the relationship between the tectonosedimentary evolution and facies distribution of the area, and thereby discover the fields with the best stratigraphic and structural traps potential. Also, there is an indispensable query: which factors had controlled the the Type of Pliocene Deposits in the Southeastern Caspian Basin, NE Iran? In the present research, the Cheleken and Akchagyl formations with the Pliocene age, as the most potent constituents of reservoir intervals in the Caspian Basin, were investigated by considering facies types and the factors that controlled their variations. This research was done based on field observations, microscopy studies, and 2D seismic interpretations. Interpretation of facies and correlation along three surface stratigraphic sections and eleven exploration wells clearly justify the role of tectonosedimentary agents controlling the distribution of the Pliocene formations in the Gorgan and Gonbad plains. Furthermore, the thickness of the Cheleken Formation increases towards the South Caspian Basin (SCB), and lithologically, it is mainly composed of conglomerates and gravely sandstones at the marginal parts grading into the silty claystone to marl facies towards the SCB. In addition, deposition of the Cheleken was accompanied by sealevel falling, and rapid subsidence resulted from the uplift and subsequent erosion of the Alborz and Kopet Dagh moun­tains. In contrast, the Akchagyl Formation represents a relatively uniform thickness, and lithologically, it is consists of highenergy carbonate facies in the northeastern Gonbade Kavous area changing into the claystone and siltstone facies towards eastern parts of the South Caspian Basin. Ultimately, according to this study, it is found out that Akch­agyl Formation records a major regional transgression and reconnection of the study area to the global oceans, which was confirmed by the presence of marine biota and flat geometry pattern of the sedimentary basin.