Re-Investigations of the Early–Middle Jurassic Flora from the Qaidam Basin, NW China: With Emphasis on the Biostratigraphic and Palaeoclimatic Implications

The Qaidam Basin is one of the major coal-bearing basins in northwestern China. The Jurassic strata mainly crop out in the northern margin of the basin. The Dameigou section represents the most complete Jurassic sediments in the basin, and yields abundant fossil plants. About 181 species of 57 genera of plant fossils have been documented, belonging to horsetails, ferns, ginkgophytes conifers, and, cycadophytes. Based on the previous and recent collections, we hereby make re-investigations on theses Early–Middle Jurassic floras, with emphasis on the biostratigraphy and palaeoclimate implications. Two fossil plant assemblages can be recognized, i.e., Neocalamites–Cladophlebis and Coniopteris–Phoenicopsis. The Neocalamites–Cladophlebis assemblage corresponds to the Early Jurassic, with Filicopsida predominant and high species diversity of Cladophlebis. The Coniopteris–Phoenicopsis assemblage corresponds to the Middle Jurassic, dominated by ginkgophytes and ferns, characterized by the high abundance and diversity of Dicksoniaceae and ginkgophytes, decrease of Neocalamites and increase of Equisetites, and the existence of Cheirolepidiaceae at the upper part. Fossil specimens from various strata were re-investigated in composition and turnover of flora, stomatal characteristics, reconstruction of atmospheric CO2. The results suggest a response in geological events during the Early–Middle Jurassic. The composition of the flora suggests a warm and humid temperate-subtropical climate in general, but a short interval with hot and arid conditions in late Early Jurassic, indicated by the increase of thermophilous plants. Besides, the analyses on maceral inertinite and reflectance reveal the wildfire type and temperature during the Early–Middle Jurassic in the Qaidam Basin, which show close coupling relationship with warming and aridity, probably linked to variations in Hg concentration caused by Karoo-Ferrar volcanism. The variation and turnover of the floral composition could be the response of the terrestrial ecosystem to the climate changes related to the Toarcian Oceanic Anoxic Event.