The Paleocene to Eocene calcareous nannofossil assemblage from Kurgan-Tyube West section in the Tajik Basin

The Tajik Basin, located in southern Tajikistan, is a Cenozoic foreland basin preserving relatively complete marine sedimentary sequences. This study conducts a quantitative biostratigraphic analysis of calcareous nannofossils from a Paleocene to Eocene section West to the Kurgan-Tyube (Qurghonteppa) of the northeastern Tajik Basin. The investigated interval comprises mudstone, marl, sandstone, silty mudstone, gypsum, and clay interbeds. Calcareous nannofossil assemblages, including 31 genera and 75 species, were identified. With the analysis of calcareous nannofossil data, the section was constrained to the latest Paleocene through the end of the Eocene. Within the bottom unit of the section (the Ganjina Unit), index fossils for the Paleocene-Eocene boundary, including Discoaster backmanii, Tribrachiatus orthostylus, and Discoaster diastypus, were identified. Alongside these, characteristic taxa of the Paleocene-Eocene Thermal Maximum (PETM), namely Rhomboaster bramlettei and Discoaster araneus (collectively referred to as the R-D assemblage), were also recorded. The fossil assemblage within this interval is predominantly composed of long-ranging species such as Prinsius martinii and Coccolithus pelagicus, indicating a warm, shallow marine environment with high productivity. The early Eocene in the study section was characterized by a significant increase in the diversity and abundance of Discoaster and the thriving of Coccolithus pelagicus, during which fossil abundance and diversity reached a peak, reflecting a comprehensive recovery of the marine ecosystem during this period. The middle to late Eocene was marked by the continued prosperity of the genus Discoaster, albeit with changes in species composition, and the emergence of Reticulofenestra as the dominant taxon. During this period, fossil abundance declined, and the community structure underwent significant turnover, directly responding to global temperature changes and nutrient fluctuations. These characteristics of biotic succession show consistency with the manifestations of the biostratigraphic patterns from the late Paleocene to the end of the Eocene in shallow marine deposits.

Acknowledgements: This research was supported by the National Natural Science Foundation of China (Nos. 42072001, 41930218), National Key R&D Program of China (Grant No. 2023YFF0804000).