Paleoclimate Transition Recorded by Palynological and Clay Mineral Evidence in the Santanghu Basin during the Middle Jurassic

The arid event that began in the Middle Jurassic Bathonian stage marks the second major drought event in northern China during the Jurassic period and had a profound impact on the development of modern terrestrial ecosystems. This study reconstructs the paleoclimate and paleoecosystem of the Middle Jurassic Santanghu Basin by analyzing palynological assemblages and clay minerals from the Xishanyao and Toutunhe formations. Two distinct palynological assemblages were identified: the Cyathidites-Deltoidospora-Osmundacidites-Cycadopites (CDOC) assemblage (Aalenian–Bajocian) and the Cyathidites-Classopollis-Quadraeculina (CCQ) assemblage (Bathonian). In terms of vegetation types, during the Aalenian–Bajocian stage, the Santanghu Basin was dominated by ground cover vegetation primarily consisting of ferns, with a midstory composed mainly of cycads/ginkgophytes, and a sparse canopy dominated by conifers. By the Bathonian stage, this shifted to a vegetation structure dominated by coniferous canopy vegetation, a subordinate midstory of cycads/ginkgophytes, and a severely degraded ground cover dominated by ferns. Sporomorph EcoGroup (SEG) analysis indicates that the CDOC assemblage is characterized by high Lowland SEG wetter/drier and warmer/cooler ratios, reflecting a warm and humid climate. In contrast, the CCQ assemblage also shows high Lowland SEG wetter/drier and warmer/cooler ratios but indicates a warm and arid climate. Clay mineral data reveal that the kaolinite content in the CCQ assemblage is significantly lower than in the CDOC assemblage. Evidence from both palynology and clay minerals suggests that the Santanghu Basin experienced a transition from a humid to a semi-arid climate near the Bajocian–Bathonian boundary in the Middle Jurassic.