A Possible Factor Responsible for the Recent Deepening of the Black and Caspian Seas and Caspian Sea Level Fluctuations

摘要

The residual intracontinental Paratethys basin that resulted from the closure of the Tethys Ocean after the collision between India and Eurasia underwent several developmental phases in the Oligocene-Quaternary. The first (Oligocene-Early Miocene) phase corresponds to the existence period of the spacious and deep H_2S-contaminated Maikopian basin, which extended from the northwestern Black Sea region nearly to the present-day Aral Sea area and was eventually filled with a thick sedimentary sequence. The second phase of the Paratethys development commenced in the Middle Miocene, when approximately the same region was occupied by a shallow marine basin with slightly lower (but variable) salinity. This basin was intermittently either connected with or isolated from the Eastern Mediterranean, which represented the other Tethyan relict. In the Late Miocene, the Mediterranean Sea lost its connection with the World Ocean, which triggered the Messinian salinity crisis. The drastic drop of sea level initiated the accumulation of Messinian evaporites. At this time, similar events were also occurring in the Pon-tian-Caspian domain. By the beginning of the Pliocene, the Caspian Sea was almost entirely drained: its level dropped by several hundreds of meters. The land that originated in the North and Middle Caspian basins was cut by river valleys hundreds of meters deep, of which the paleo-Volga valley was the most prominent. The paleo-Volga valley extended up to the Apsheron Peninsula and formed a spacious delta that accumulated the 5-km-thick productive petroliferous sequence. The analogue of this sequence is identified as redstones in Turkmenistan.