Pliocene deposits of the Cheleken and Akchagyl Formations constitute the most important hydrocarbon reservoirs in the Caspian Basin of Iran and other adjacent countries. In this study, facies characteristics and depositional setting of these sequences were examined using detailed field observations, petrographical studies, wireline-log and 2D seismic interpretations. Three outcrop sections were measured as the oldest and thickest Paleogene–Neogene units in the southeastern part of the Caspian Sea. Based on macroscopic and microscopic investigations, eleven sedimentary facies were identified in the studied sequence. These facies indicate deposition in alluvial fan, mud-flat, foreshore, shoreface and offshore of a wavedominated lacustrine environment. The Akchagyl Formation is mainly composed of foreshore and shoreface facies with low gamma-ray values, which grades into offshore facies towards southwestern parts of the basin. The presence of marine biota such as Globigerinoides fistulosus, Ammonia becccarii, Pullenia praecursor, Cardium dombra and nanoplanktons (Discoaster brouweri and Discoaster pentaradiatus) demonstrate that this lacustrine setting was periodically connected to the global oceans, particularly in the Late Pliocene–Pleistocene. The stratigraphic correlation of the studied sections shows significant changes in lithology, facies and thickness of the Pliocene deposits. These changes were controlled by tectonic subsidence and clastic sediment supply during the Early-to-Middle Pliocene along with sea-level fluctuation as a consequence of the Akchagylian transgression episode in the Late Pliocene. Lateral changes in the geological parameters of the Akchagyl deposits indicate a more stable condition within the most part of the study area, especially towards the South Caspian Basin.

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