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The South Caspian Basin is believed to contain more than 20 km of Mesozoic and Tertiary sediments deposited on oceanic or thinned continental crust. Mesozoic, Palaeogene and Oligo-Miocene sediments have not been penetrated within the South Caspian Basin itself but are exposed onshore in the basin margins. The Pliocene–Recent sequence has been mapped on a regionally extensive grid of two-dimensional (2D) seismic data and penetrated by recently drilled exploration wells, and is over 7 km thick. Most of this sequence (6 km) is formed of fluvial–lacustrine deltaic sediments of the Pliocene Productive Series that are deposited unconformably above a marine Miocene shale sequence and form the principal hydrocarbon reservoirs in the basin. The Productive Series is overlain by about 1 km of Late Pliocene–Recent marine sedimentsThe thickness of the Pliocene sedimentary sequence implies that relatively rapid, late Tertiary subsidence occurred in the South Caspian Basin; however, there is no geological evidence of a tectonic event capable of generating a major thermal subsidence event at this time. Modelling presented in this paper suggests that it is possible to account for the observed pattern of subsidence and sedimentation in the South Caspian Basin by a process of sediment loading and compaction on a thermally subsiding, late Mesozoic crust without the need for additional Tertiary subsidence mechanisms. Crucially, this model interprets the Pliocene Productive Series to have been deposited in a topographic depression, isolated from the global oceanic system, in which base level was controlled by local factors rather than by global sea level.

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