Palynological Assemblages from the Jurassic Qingtujing Formation in the Northwestern Margin of the Chaoshui Basin: Implications for Geological Chronology and Paleoclimate

The Qingtujing Formation of the Chaoshui Basin is an important Jurassic coal-bearing unit in the Hexi Corridor. Palynological data on this formation are crucial for regional stratigraphic correlation and paleoclimate reconstruction. In this study, we systematically identified and analyzed the palynological fossils in the Qingtujing Formation in Well Gao 4 and Well 801 on the northwest margin of the Chaoshui Basin and identified two characteristic palynological assemblages. The lower part is the Disacciatrileti–Cycadopites–Osmundacidites assemblage, dominated by gymnosperm pollen, followed by fern spores, with very low Classopollis content. The upper part is the Classopollis–Disaccites assemblage, dominated by gymnosperm pollen, among which Classopollis pollen accounts for a portion as high as 75.5%, and fern spores only appear sporadically. Based on a comparison of palynological assemblages and the distribution pattern of fossils, the sedimentary age of the Qingtujing Formation was determined to be early to late Middle Jurassic. The paleoclimate was reconstructed based on the ecological habits of the pollen/spore-producing plants, indicating that the study area had a warm and humid climate in the early Middle Jurassic and that this climate changed to a hot and dry one in the late Middle Jurassic. This evolution pattern is consistent with the Jurassic climate evolution trend in the northwest of China. These results provide key palynological evidence for the stratigraphic chronology and paleoclimate reconstruction of the Jurassic strata in the Chaoshui Basin.