Jurassic black shales facies from qiangtang basin (Northern Tibet): Rare Earth and trace elements for paleoceanographic implications

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 lirst 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, VICr, U/Th and Vl(V+Ni) pins U were used to identify redox conditions. The shale-normalized rare earth element (REE) patterns are characterized by the hat-shale type with instable Ce anomalies and very weekly positive Eu anomalies. Positive Ce_anom, 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 Ce_anom 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 (Ce_anom 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.