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

Abstract

Abstract—: The Early Chokrakian basin (Bolivina tarchanensis Beds) largely inherited the bathymetry and facies of the previously existing Tarkhanian Sea, differing only in the depletion of the most polyhaline elements of the fauna and microplankton, and in the appearance and early dominance of the characteristic Chokrakian benthic foraminiferal species. A regional tectonic reorganization, shallowing of the basin, accompanied by an improvement in the gas regime, by slope landslides, uplifts in the adjoining areas and a sharp increase in terrigenous runoff, occurred later, at the end of the Early–early Middle Chokrakian. The Lazarev-Gobustan deep-sea trough, which existed at the site of the Greater Caucasus, was mostly closed by the Middle Miocene. By the mid-Chokrakian, the axial part of the trough had approximately halved its width due to compression of the deep-water zones of the Greater Caucasus and due to the filling with sediment. The land that probably arose in the terminal Early Chokrakian at the central intersection of the Greater Caucasus and that was attached to the uplift of the Dziruli Massif, opened a pathway for migration of mammals of both African and Central Asian origin from the south to the Caucasian Island. By the Middle Chokrakian (Tschokrakella caucasica Beds), extensive transgression took place in the marginal parts of the basin. In the continental surroundings, the topography became more dissected resulting in a greatly increased supply of coarse terrigenous material. The rapid filling of the land depressions with sediment resulted in the flattening of topography and in the water flooding to the basin margins. On the northern shelf, where the Tarkhanian and Early Chokrakian basins did not reach, the Middle Chokrakian unconformably overlies different Upper and Middle Maikopian horizons. The shallow shelf was framed by specific reef buildups, the bryozoan-algal bioherm ridges. The base of the Upper Chokrakian is also commonly marked by unconformities, erosion, and transgressive sequences. At the beginning of the Late Chokrakian the hydrology of the basin changed dramatically, the diverse marine fauna became extinct, the number of bivalves decreased to six species, dwarf and aberrant forms occur in the sharply impoverished microfauna. However, these stressed conditions were favorable for stromatolites. The eastern and central parts of the Chokrakian Sea in the Ciscaucasia received mainly mature material from the Russian Plate, the sorting of which was further improved by the activity of powerful currents and repeated erosion and redeposition. The western part of the paleosea received sedimentary material from both the Russian Plate and the Caucasus Island, which differed sharply in the degree of chemical weathering. In the Chokrakian the Caucasian Island represented a large island with a low-mountainous topography (up to 400‒500 m) and, possibly, with an archipelago of low islands in the eastern Caucasus. Large shallow bays occurred on the Turan Plate – Ustyurt, South-Mangyshlak and Fore-Kopetdagh – with unstable continental-marine sedimentation. The major communications with open seas occurred in Transcaucasia via the Middle-Araks and South-Van straits.