Eocene to Miocene Charophytes from the Qaidam Basin on the northern Tibetan Plateau and its calibration to the Geomagnetic Polarity Time Scale

Abstract

Charophyte assemblages from the Qaidam Basin on the northern edge of the Tibetan Plateau were studied. The taxonomy of charophytes is based on a study of intraspecific gyrogonite polymorphism and biometric data. A total of 6 species belonging to 5 genera were identified, including Sphaerochara inconspicua Feist-Castel, 1977, Psilochara sp., Chara media Grambast, 1958, Lychnothamnus barbatus var. antiquus Soulié-Märsche, 1989, Lychnothamnus pinguis forma major (Grambast and Paul, 1965) Soulié-Märsche, 1989, Nitellopsis (Tectochara) merianii (Braun ex Unger, 1852) Grambast and Soulié-Märsche, 1972. Lamprothamnium ganchaigouensis (Tang and Di, 1991) Li et al., 2020 and Lychnothamnus vectensis (Groves et al., 1926) Soulié-Märsche, 1989 have already been reported from the Xiaganchaigou Formation by Li et al. (2020). These Eurasian species allow direct correlation between Chinese and European basins. The distribution of these species in the Qaidam Basin is correlated to the high-resolution magnetostratigraphy of the Paleogene–Neogene sequence, ranging from >52 Ma to ∼7 Ma at Dahonggou in the northern Qaidam Basin. N. (T.) merianii, with living representatives Nitellopsis obtusa, was reported to be expanded eastwards to Asia during the late Oligocene and Miocene. On the contrary, L. barbatus var. antiquus was found in the Oligocene (Chattian) of the Tibetan Plateau, representing the oldest record of this still living species. This may indicate that L. barbatus originated in Asia and expanded to other parts of Eurasia during the Miocene.