Mineralogy and geochemistry of the Middle Permian Pingdiquan Formation black shales on the eastern margin of the Junggar Basin, north‐west China: Implications for palaeoenvironmental and organic matter accumulation analyses. Issue 5 (12th January 2022)

• Record Type: Journal Article
• Authors: Bai, Jianke; Zhang, Shaohua; Liu, Chiyang; Jia, Langbo; Luo, Keyong; Jiang, Ting; Peng, Heng
• Is Part Of: Geological journal. Volume 57:Issue 5(2022)
• Page Start: 1989
• Page End: 2006
• Publication Date: 2022-01-12
• Subjects: black shale -- Junggar Basin -- organic matter accumulation -- palaeoenvironmental condition -- Pingdiquan Formation; Geology -- Periodicals; 551
• DOI: 10.1002/gj.4392
• Languages: English
• ISSNs: 0072-1050

Abstract

Abstract : The Middle Permian black shales from the eastern Junggar Basin of northern Xinjiang, north‐west China are one of the richest and thickest lacustrine hydrocarbon source rocks in China. The palaeoenvironmental settings and controlling factors of organic matter accumulation along the margins of the Middle Permian lacustrine basin remain unclear, although the same works have been undertaken in the centre of the Middle Permian lacustrine basins. Here, we performed a combination of mineralogical and geochemical analyses on the black shales of the Middle Permian Pingdiquan Formation from the eastern margin areas of the Junggar Basin. The results show that the Pingdiquan Formation black shales in the study areas are organic‐rich, with total organic carbon (TOC) contents of 0.74–17.61 wt% (avg. 5.81 wt%). Salinity proxies including Sr/Ba and Rb/K ratios, combined with traces of gypsum and halite in a few samples, suggest brackish to saline water conditions. Palaeoclimate proxies indicate a warm‐humid environment during the deposition of the Pingdiquan Formation black shales. The redox proxies V/(V + Ni), V/Cr and U/Th ratios, and Eu and Ce anomalies together indicate dysoxic‐anoxic conditions of lake water. Strong positive correlations between palaeoproductivity proxies (P/Al, Ni/Al, Cu/Al, and Zn/Al) and TOC contents imply that palaeoproductivity may have played an essential role in organic matter accumulation during the deposition of the black shales. In contrast, negative correlations between terrestrial detrital input proxies (Al and Ti) and TOC contents indicate that terrestrial detrital input may have acted as a diluent to organic matter accumulation. Additionally, although the correlation between redox proxies [V/(V + Ni) and U/Th] and TOC contents is not straightforward, dysoxic‐anoxic conditions of lake water are an essential prerequisite for organic matter accumulation in the Pingdiquan Formation black shales. Abstract : The positive correlations between palaeoproductivity proxies (P/Al, Ni/Al, Cu/Al, and Zn/Al) and total organic carbon (TOC) contents suggest that palaeoproductivity played a dominant role in organic matter accumulation of black shales, while the negative correlations between terrestrial detrital inputs proxies (Al and Ti) and TOC contents imply that terrestrial detrital inputs acted as a diluent.