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Cite This Paper:.2013.[J].Acta Geologica Sinica(),87(2):540-554
Jurassic Black Shales Facies from Qiangtang Basin (Northern Tibet): Rare Earth and Trace Elements for Paleoceanographic Implications    Download PDF
CHEN Lan1  2  3  *  YI Haisheng4  5  TSAI Louis Loung-Yie2  XU Guiwen1  DA Xuejuan1  LIN Andrew Tien-Shun3
1 College of Petroleum Engineering, Chongqing University of Science and Technology, Chongqing 401331, China;2 Institute of Applied Geology, National Central University, Jhongli 32001;3 Institute of Geophysics, National Central University, Jhongli 32001;4 Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China;5 State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
this work was supported by Natural Science Foundation Project of CQ CSTC (Grant No. 2009BB7383), National Natural Science Foundation of China (Grant No. 41102066, 40972084) and Opening Foundation of the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.
      The Biluo Co and Amdo 114 station, northern Tibet, cropping out the Early Toarcian and Middle-Late Tithonian (Jurassic) organic-rich black shales, have been a focus to petroleum geologists in discussing their oil-producing potential. This paper first reports the trace elements and rare earth elements to discuss the paleoenvironments, redox conditions and sedimentary mechanisms of those black shales. Both sections exhibit variation in trace element abundances with concentrations <0.1 ppm to 760 ppm, mostly enriched in V, Cr, Ni, Cu, Zn, Mo, Ba and U. Element ratios of Ni/Co, V/Cr, U/Th and V/(V+Ni) plus U were used to identify redox conditions. The shale-normalized rare earth element (REE) patterns are characterized by the flat-shale type with instable Ce anomalies and very weekly positive Eu anomalies. Positive Ceanom values are significant with values varying between –0.064 and 0.029 in Biluo Co, which may be interpreted as release of REE and input of riverine terrestrial matter with rich Ce (resulting in pH change) during the anoxic conditions. In the middle parts of Amdo 114 station, distinct negative Ceanom values are observed (?0.238 to ?0.111) and associated surface water warming were interpreted as being related to a major sea level rise. In contrast, the formation of the black shales in the lower and upper part of the studied succession took place during a cooler (Ceanom values >–0.10), lower surface water productivity, and lower sea-level stage. Thus, we emphasize the role of different factors that control the formation of local and regional black shales. The most important factors are sea-level fluctuations and increasing productivity.
Key words:Jurassic, black shale facies, rare earth and trace elements, paleoceanography, northern Tibet
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