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.
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