A magnetostratigraphic time frame for Plio-Pleistocene transgressions in the South Caspian Basin, Azerbaijan

Abstract

The isolation of the Caspian Sea took place in the latest Miocene coinciding with a significant lowering of sea level and the deposition of a massive lowstand deltaic system. This so-called Productive Series is the main reservoir unit of the SouthCaspian oil-province. The Productive Series is overlain by marine clays from the Akchagylian and Apsheronian regional stages. During the Plio-Pleistocene, the Caspianbasin experienced several short periods of intermittent connectivity with other marine basins. This work aims to create integrated, high-resolution, bio-magnetostratigraphic dating of these regional transgressions in key sections of the SouthCaspianBasin in Azerbaijan. We sample two long sections, the Lokbatan section in the Palaeo-Volga delta and the Xocashen section in the Kura Basin. Palaeoenvironmental reconstructions are derived from characteristic ostracod species. Rock magnetic analyses combined with thermal demagnetisation data indicate that the magnetic signal is carried dominantly by the iron oxide magnetite in the Productive Series of Lokbatan section and in the Xocashen section. The marine Akchagylian and Apsheronian of Lokbatan are characterised by the iron sulphide greigite, which appears to be of (near-) primary origin. The most logical correlation of the magnetic polarity patterns to the Geomagnetic Polarity Time Scale dates the Akchagylian transgression at ~ 3.2 Ma, a major transgression during the Apsheronian at ~ 2.0 Ma and the Bakunian transgression at 0.85–0.89 Ma. Ostracod assemblages indicate increasing salinities during these transgressions, from fresh water lacustrine to brackish-marine species. This implies that marine connections have been created with an adjacent basin that has a higher salinity, most likely the Black Sea