Middle Jurassic climate transition in the Ordos Basin, China: evidence from palynology and clay minerals

Increasing evidence indicates significant climatic fluctuations during the Jurassic Period. This study aims to reconstruct the paleoclimate and paleoenvironment of the Middle Jurassic through palynological and clay mineral analyses of Well Nd17 (2024.0–1827.0 m) in the central Yanchi region of the central Ordos Basin. We identified 138 species across 49 genera of spores and pollen, alongside clay minerals such as illite, kaolinite, chlorite, montmorillonite and illite/smectite mixed layers from the upper Yan’an to lower Zhiluo formations. Based on variations in the abundance of key spore and pollen taxa and the composition of clay minerals, we established two palynological assemblages as the Cyathidites–Osmundacidites–Cycadopites (COC) assemblage and the Cycadopites–Classopollis–Bisaccates (CCB) assemblage, corresponding to the Aalenian to Bajocian and Bathonian, respectively. The clay mineral assemblages were classified into an illite–kaolinite assemblage (upper Yan’an Formation) and an illite assemblage (lower Zhiluo Formation). Our findings indicate a climatic transition in the Ordos Basin during the Middle Jurassic from humid to semi-arid/semi-humid conditions, accompanied by a temperature decline. This shift reflects vegetation changing from mixed coniferous and broad-leaved forests to predominantly coniferous forests, highlighting a Middle Jurassic climate transition that affected large areas of northern China and contributed to coal scarcity.