Elemental geochemistry of the early Jurassic black shales in the Qiangtang Basin, eastern Tethys: constraints for palaeoenvironment conditions

Abstract

The early Jurassic black shales are the most important source rocks in the Qiangtang Basin, Tibet. These black shales may provide a useful example for understanding the sedimentary conditions during the early Jurassic in the eastern Tethys. Thirty‐two black‐shale samples were collected from the Qiangtang Basin to reconstruct the depositional environments during the early Jurassic in the eastern Tethys. The palaeoclimate index Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na) varies between 0.58 and 1.14, together with low Sr/Ba (0.20–0.60) and Sr/Cu (1.68–7.88) ratios, indicating a warm humid to semi‐humid climatic condition during black‐shale deposition. The early Jurassic black shales exhibit slight Ce anomalies (0.89–0.94) and Mn enrichment (EF = 1.2), suggesting a slightly oxic depositional environment. The conditions of the sedimentary water system during black‐shale deposition were apparently brackish as evidenced by middle B/Ga (3.96–5.73) and Th/U (3.96–6.51) ratios. Copyright © 2015 John Wiley & Sons, Ltd.