The relationship of Late Jurassic paleoenvironment and paleoclimate with geochemical elements in Amdo Country of northern Tibet

Abstract: Amdo Country of northern Tibet is close to south boundary of Qiangtang Basin and belongs to Qiangtang stratigraphic area. The late Jurassic Shamuluo Formation of the study area is mainly composed of a series of neritic clastic rock and carbonate deposits, which are in turn weathering crust sediments, tidal flat facies sediments and mixed platform facies sediments, characterized especially by a lot of organic reefs. Based on analyzing 18 samples from Shamuluo Formation, combined with the sedimentary facies analysis, the authors investigated the ancient sea-level eustacy, oxidation-reduction conditions and paleoclimate of late Jurassic period in the study area. Specifically, the changes of Ni, Sr, Cu, V, Cr, Ni/Co and carbon isotopes were used to detect the changes of paleoenvironment, and the fluctuation of Mn, Na, Sr/Cu, P, Ti, oxygen isotopes and Z values were employed to deduce paleoclimate. The results show that paleoenvironment evolutions experienced three stages: (1) strong oxidation environment of weathering crust; (2) semi-oxidation-reduction environment of tidal flat; (3) three times of changes between oxidation and reduction environment in platform, corresponding to the three times of reef formation; paleoclimate evolutions also experienced three stages: (1) arid climate of weathering crust; (2) semi-arid and semi-humid climate of tidal flat; (3) three times of changes between arid and humid climate in platform, respectively corresponding to the three reef-forming periods.