Carbon and oxygen isotopic composition and palaeoenvironment analysis of lacustrine carbonate rocks in the upper member of Early Triassic Sunjiagou Formation, Yiyang area, western Henan Province
The Sunjiagou Formation records the sedimentary history of the Permian and Triassic transition period in the North China Block. Therefore, its study is of great significance to understand the momentous paleoenvironment changes of North China Block during this transition period. C and O analysis was conducted for the rhythmic thin carbonate rocks in the upper member of Sunjiagou Formation, which crop out in the Yiyang area of western Henan Province in the south of North China Block, to reveal the cause of formation of these carbonate rocks beds and their implication for paleoenvironment. The results show that the carbonate minerals in the upper member of Sunjiagou Formation are mainly authigenic carbonates, and there are no significant changes after diagenesis. Therefore,C and O isotope compositions and their changes can be used to reconstruct the paleoenvironment. According to the composition and controlled factors of C and O isotopes in the upper member of Sunjiagou Formation,and combined with the analysis of macroscopic sedimentary environment, it is considered that the carbonate rocks in the upper member of Sunjiagou Formation was formed in the inland lacustrine basin environment. When delta~(18) O>- 7. 9, delta~(13) C and delta~(18) O are positively correlated, indicating that the lake was close and carbonate minerals consisted mainly of dolomite with poor crystallization; when delta~(18) 〇 < - 7. 97, delta~(13) C and delta~(18) O are negatively correlated, suggesting that the lake was open and carbonate minerals consisted mainly of magnesium calcite with good crystallization. The vertical variation of C-O isotopes of the carbonate rocks from lower to upper parts in the upper member of Sunjiagou Formation reveals that the lake environment underwent the evolution from relatively open to relatively close,then to relatively open, suggesting an evolution trend of lower temperature,lower humidity and worse vegetation. The formation of lacustrine environment in the upper member of Sunjiagou Formation may be related to the uplift of the North Qinling Orogenic Belt, and the rhythmic layers consisting of thin carbonate rocks layers and overlying purple mudstone layer are mainly related to rainfall caused by climate changes. The paleoenvironment and its changes recorded in the upper member of the Sunjiagou Formation are consistent with the global changes of the terrestrial environment after the mass extinction at the end of Permian.