Paleoceanographic Environment Of The Mesoproterozoic Xiamaling Formation In The North China Platform: Evidence From Sedimentary Geochemistry

The origin of life and its earlier evolution in the Precanbrian is one of the majorconcerns in nature science, and the cutting-edge research is also a challengingproblem. It has been suggested that the Mesoproterozic ocean was probably in aperminant stratification state, with an oxygenated surface layer, but very low oxygenlevel and moderate sulfate concentration in deep part, and likely in a suboxic toanoxic state that exhibited transitional characteristics between the Archaen andPhanerozoic oceans. This study attempts to find some more new evidence from theMesoproterozoic sediments in the North China platform, and to further explore intothe interactions between early-life evolution and environments.In this study we take the Xiamaling Formation outcropped in ZhaoJiashan,Xiahuayuan, Hebei province as an example, which has a sediment succession of465.5m in thickness. In this section, black shale is predominant, with some sandstoneinterbeds. Two major facies can be distinguished in the Xiamaling Formation—theshallow-shelf facies and no-barrier coast facies with reduction to weak reductionconditions in anoxic environment. The Xiamaling Formation can be subdivided into7sequences, showing an overall tendency of deepening upwards. This paper mainlydeals with the third member of the Xiamaling formation, which is composed typicallyof black shale. In this study, various sedimentary geochemistry methods have beenused in order to elucidate the depositional environments and paleoceanal conditionsduring the depositional period of the Xiamaling Formation.According to the analysis of U, V, and Mo concentrations of the tested35samplesand their covariation relationships with Th and Al contents in the XiaMaling blackshale, we thought that the black shale samples are suitable for the study, and theobtained results can be used to reflect the ancient marine environment and the redoxsate. The enrichment factors show clearly enrichment of the redox sensitive elementsMo, V and U in this formation，especially Mo. A weak correlation of enrichmentfactors between the three elements and TOC suggests that the black shale in the third member of the Xiamaling Formation was possibly deposited in an anoxic or eveneuxinic environment.The test results of redox sensitive element ratios V/(V Ni)，V/Cr，Ni/Co，V/Sc，Mo/TOC, and δU content in the black shale may have reflected an anoxic depositionalenvironment, with strong reducing conditions, or even a euxinic environment.Hydrothermal activities were likely present in the basin.during the deposition.REE analysis shows that ω(LREE)/ω(HREE) is lower than the PAAS, suggestive ofenriched HREE. δCe shows a negative anomaly. These two indicators may suggestthat during the deposition of the XiaMaling black shale, the studied area was probablyunder an influence of hydrothermal activities.Both the sedimentary facies characteristics and geochemical analytic results showthat the Xiamaling Formation at the ZhaoJiashan was deposited in an anoxicenvironment. Hydrothermal activities might have been involved during the deposition,which were most likely related to the active tectonisms and their associated magmaticactivity of that time.