Discovery of sporopollen assemblages of Yan'an Formation in Yanchi, Ordos basin, and its paleoclimate significances

For rebuilding the paleoclimate and paleoenvironment during the early-middle Jurassic in the Yanchi area of the Ordos basin, -we analysis the palynological characteristics and clay minerals of the core Ningdong-2 , sporopollen assemblages and sporopollen spectra -were established. The lower part of the Yan'an Formation is the Inaperturopollenites-Psophosphaera-Protopinus assemblage, -whose age is the late Toarcian of Early Jurassic. The upper part of this formation is the Cyathidites-Deltoidospora-Cycadopitees assemblage, -whose age is early Middle Jurassic. The boundary of lower and middle Jurassic is located between 2144. 5 - 2153. 0 m of the core. The paleoclimate and paleovegetation established by pollen spectrum shows that the late Toarcian of early Jurassic period -was a -warm temperate semi-humid climate -whose vegetation type -was coniferous forest. The early Middle Jurassic -was generally subtropical humid climate, -whose vegetation appearance transformed from coniferous broadleaf mixed forest-grass type to the coniferous forest-dilute grass type. From the late Toarcian of Early Jurassic period to the early Middle Jurassic period, the humidity changed from semi-humid to wet while the temperature changed from low to high. This climate change trend was also found in clay minerals analysis. The climate shift has had a clear role in promoting coal gathering in the region. © 2024 Geological Society of China. All rights reserved.