Longmenshan foreland basin formed in the Late Triassic, but the sedimentary environment of that period has long been controversial. This study based on analysis of the geochemistry, and combined with paleontology, petrology and sedimentary characteristics of Xujiahe Formation in Longmenshan foreland basin, discuss its sedimentary environment and the pattern of organic matter enrichment. The study shows that the Xujiahe Formation belongs to warm and humid tropical-subtropical climate during the sedimentary period. Xujiahe 1st member is marine facies, and the shale is mainly formed in the anoxic condition. There is a small amount of pyrite can be seen in the shale, and the enrichment of organic matter is controlled by the REDOX conditions. The study found that the geochemical characteristics, paleontological characteristics and organic characteristics of the shale of Xujiahe 2nd～5th member have similarities, and there is a significant difference with the marine sediments of Xujiahe 1st member. After comparison with the Caspian Sea, it is believed that the Xujiahe Formation gradually turned into continental sedimentation from Xujiahe 2nd member period. And Xujiahe 2nd～5th member should be deposit in the brackish water-fresh water. The pyrite in the shale of Xujiahe 2nd～5th member disappeared, but the siderite appeared, according to the geochemical indicators such as V/Cr, U/Th and dU indicate that the shale should be deposited in weak oxidation condition. There is a good correlation between the TOC and paleoproductivity and sedimentary rate of shale in the Xujiahe 2nd～5th member, paleoproductivity and sedimentary rate are the major controlling factors for the enrichment of organic matter. The organic-rich shale deposit in the foreland basin, which is fast and continuous sinking, has a fast sedimentary rate, and the organic matter could be well preserved in the weak oxidation condition. This study suggests that when the sedimentary rate is faster than 10 cm/kyr, and have sufficient productivity, the enrichment and preservation of organic matter in the shale will not be controlled by REDOX conditions.
Chen Bin Li Yong Deng Tao Dong Shunli Zhao Shaoze Hu Wenchao. The sedimentary environment and organic matter enrichment pattern of Xujiahe Formation shale in the Late Triassic Longmenshan foreland basin, SW China[J]. Chinese Journal of Geology, 2019, 54(2): 434-451.