Sedimentology And Geochemistry Of The Black Rock Series In Early Cambrian Niutitang Formation In Zhangjiajie, Hunan

Black rock series has been well known as"Multi-element Concentration Stratum". Geologists who come from all over the world have always been paying attention to it. And it has been being researched in multiple disciplines. Focusing on the sedimentary petrology and geochemistry, especially the organic geochemistry, the present study discusses the sedimentary environment of black rock series, the genesis of polymetallic sulfide deposit and the input /abundance /type /maturity of the organic matter of the black rock series. The main conclusions are as following:(1) On the basis of the characteristic of the sedimentary petrology and fossil assemblage, the sedimentary environment of black rock series in Hunan province is the reducing environment in neritic shelf. The genesis of black rock series in the Early Cambrian Niutitang Formation connects with oceanic anoxic event which corresponds to Tommotian oceanic anoxic event in Siberian.(2) Based on the research of the element geochemistry, we found that there is a positive correlation among the curves of content variation of Ni,Mo and V. The ratio of U/Th is 12.37-19.96 >1, indicating the genesis of polymetallic sulfide deposit may be affected by hydrothermal fluid and be hydrothermal exhalative sedimentary type. The relation among Ni,Mo and V is positive correlation, the ratio of U/Th is 12.37-19.96 >1.25, the ratio of V/Cr is 6.38-7.78 >4.25 and the ratio of Ni/Co is 11.59-12.08 >7.00. All of these records indicate that the sedimentary environment of black rock series of Zhangjiajie in Hunan is an anoxic reducing environment.(3) In this article, we test the content of organic carbon of black rock series of Niutitang Formation in this area. With adjusting the content of organic carbon by a correction coefficient, we draw a conclusion that the black rock series are full of organic carbon with the high content and are good source rocks. It also indicates that the sedimentary environment is an anoxic reducing environment.(4) Based on the studies of geochemistry, we found that abundant biomarkers, including n-alkanes, isoprenoid, steranes and terpanes are in our samples. According to the research of parameters of biomarkers, we discussed the input /type /maturity /environment of the organic matter of the black rock series.Based on the above-mentioned research, we considered that rock types of black rock series in the Early Cambrian Niutitang Formation of Zhangjiajie in Hunan belong to clastic rock. The black rock series also contains phosphorites, stone coal, baritic rock and the mineral of Ni, Mo and V, etc. They are generated under the anoxic reducing environment, could be an occlusive deep water environment in shallow water. The rock facies probably are deep water continental shelf. And they have a closely connection with oceanic anoxic event, which corresponds to Tommotian oceanic anoxic event in Siberian. In this period, the genesis of polymetallic sulfide deposit may be affected by hydrothermal fluid and be hydrothermal exhalative sedimentary type. The organic matter derives from bacteria and algal phytoplankton like blue green algae and dinophyceae, and the type of organic matter is kerogen typeⅠ——sapropelic type. The black rock series are full of organic carbon with the high content and are good source rocks. It is indicated that the balck rock series are of high thermal evolution, but the evolutional degree is still not reached the ultimate stage.