Sporopollen Assemblages And Chronostratigraphy From The Late Jurassic Qigu Formation In Southern Junggar Basin, Xinjiang

In spite of considerable efforts, the boundary between Jurassic-Cretaceous has notbeen well defined. The Junggar Basin, a huge continental basin in northern China, isstratigraphically characterized by well-exposed Jurassic to Cretaceous sequences. Thebiostratigraphic study on those sequences will help our understanding of theJurassic-Cretaceous boundary. The Honggou Section along the Manasi River at thesouthern margin of the basin is a standard section of the Middle-Upper Jurassic. Thispaper reports the sporopollen assemblages of the Qigu Formation from the HonggouSection, which is predominately composed of red mudstone and pelitic siltstoneinterbedded with a small amount of greyish-green sandstone, gray tuff and tuffaceoussandstone. The pteridophytic spores, roughly occupy the22%of the total sporopollenassemblages, and are dominated by Cyathidites, Deltoidospora, Osmundacidites andConcavisporites. Among the gymnosperm pollen, the nonstriate bisaccate pollen(35%), including Pinuspollenites, Piceaepollenites, Cedripites, Quadraeculina, et al.,rank the first in content. Besides, Classopollis occupy nearly10%of the total content.Based on the sporopollen contents and characteristics, the Cyathidites-Pinuspollenites-Classopollis-Chasmatosporites Assemblage has been proposed. Thesporopollen characteristics of the Qigu Formation of the Honggou section imply thatit is of the Late Jurassic age. Five U-Pb ages of160Ma,159.3±1.3Ma,167.1±1.9Mand156.9±1.9Ma have been obtained by this work. Generally, the sporopollenassemblages and isotopic ages illustrate that the uppermost sequences of Jurassic areabsent within the unconformity between the Upper Jurassic Kalazha Formation andLower Cretaceous Qingshuihe Formation. The Qingshuihe Formation, overlying theunconformity, is believed to form in the Early Cretacesou. Besides, we suggest that,during the Late Jurassic, the paleobotany of the Junggar Basin was mainly composedof Cyatheaceae, Pinaceae and Coniferopsida, and the paleovegetation was likelycharacterized by mingled coniferous and broad-leaved forests. We therefore infer aSemi-arid-to humid-type paleoclimate for the region.