Middle–Late Miocene Paleogeography of the Eastern Paratethys. Part II. Tarkhanian Basin: Stratigraphy, Facies, Paleogeography and Biota

The rich benthic fauna and the composition of phyto- and zooplankton, indicate that the Tarkhanian basin had open connections with the World Ocean. These connections existed both in the west of the region–with the Early Badenian Sea, and in Transcaucasia–with the Central Iranian and Anatolian basins. A gradual transition from the anoxic regime of the Maikopian basin to active vertical circulation continued throughout the entire Tarkhanian and took place not simultaneously in different parts of the basin and at different depths. According to lithological and biotic data of most sections, the Tarkhanian has a distinct three-member structure: (1) a stage of transition to carbonate sedimentation and the formation of rich marine benthic fauna and phytoplankton, (2) a stage of its flourishing, and then (3) depletion due to the gas regime deterioration and some desalination. Based on these biota and facies changes, the Tarkhanian regional stage is subdivided into three substages, or the regionally recognizable Kuvinian, Terek and Argun Beds are distinguished. Magnetostratigraphic data show that the boundaries of these units are markedly asynchronous, facially and bathymetrically determined, and vary in different parts of the basin. Therefore, we believe it incorrect to rank them as substages but consider them as beds. The main source of terrigenous material on the entire northern shelf of the Tarkhanian Sea was the Russian Plate. Whereas the eastern and central parts of the basin received material that had undergone intensive chemical weathering, well sorted both during the transportation and as a result of the hydrodynamic activity of the sea, the less mature matter was accumulated in the western part of the basin. The clay minerals composition indicates a humid subtropical climate in the denudation areas on the Russian Plain, and the findings of marine phytoplankton and benthos, insects, and spore-pollen data suggest a subtropical frost-free climate with smooth seasonality. The assignment of the Tarkhanian to the planktonic foraminiferal Orbulina suturalis Zone (M6) and to nannoplankton Sphenolithus heteromorphus Zone (NN5), as well as magnetostratigraphic data, suggest a correlation of the Tarkhanian regional stage with the second half of the Early Badenian of the Central Paratethys and with the Late Langhian of the Mediterranean; and the age of the Tarkhanian regional stage ranges from 14.9 to 14.8 Ma.