Multidisciplinary study on the stratigraphy of the upper Cretaceous-Paleogene successions in the western Tajik Basin, Uzbekistan

Abstract

The Tajik-Afghan Basin is a large basin in Central Asia, whose sediments document the area's long tectonic and climatic history from the Paleozoic to the present. However, the sedimentary characteristics and stratigraphy of these successions remain poorly understood. The study of this basin is critical for understanding the depositional conditions that prevailed during the transformation of this area from a part of the Western Oceanic Tethys to a more restricted Para-Tethys. In the southwestern corner of the Tajik Depression, a well-exposed sedimentary sequence associated with a large anticlinal limb was sampled. The authors proposed a new age model based on a combination of biostratigraphic data from planktic foraminifera, calcareous nannofossils, and strontium chemostratigraphy (87Sr/86Sr). The lower part of the studied section is of the upper Cenomanian age (95.5 Ma), while the upper part is assigned to the early Oligocene age (34.05 Ma). The carbon isotope profile shows several excursions linked to major paleoenvironmental events including the Oceanic Anoxic Event 2 (OAE2) and the Paleocene-Eocene Thermal Maximum (PETM). The latter is revealed in the study area for the first time. New sedimentological observations and interpretations of the depositional environments and sea-level cycles are provided. The deposition in the Aktash area occurred between upper coastal lagoonal and transitional environments before fully transitioning to continental environments in the Neogene. The stratigraphic record in Aktash is divided into large-scale progradation/ retrogradation cycles that can be tentatively linked to global sea-level signals.