Major and trace element geochemistry of the lacustrine organic-rich shales from the Upper Triassic Chang 7 Member in the southwestern Ordos Basin, China: Implications for paleoenvironment and organic matter accumulation

Abstract

Abstract Lacustrine organic-rich shales are widespread in the Upper Triassic Yanchang Formation Chang 7 Member of the Ordos Basin. However, information on the short-term climate evolution within the Chang 7 Member is still lacking, and the mechanism of organic accumulation in the shales remains controversial. To address these issues, this study analyzes the palaeoenvironment and depositional setting of different intervals in the Chang 7 Member of the southwestern Ordos Basin and constrains the main control factors on accumulation and preservation of organic matter based on organic, major, and trace element data. The lower Chang 73 shale, which contains a high abundance of thermally mature Type I and II kerogen, has excellent original hydrocarbon generative potential. The lower Chang 73 interval has the most humid climatic condition, the strongest weathering condition, a euxinic bottom water condition, a high paleo-productivity, and a low terrigenous input. During the period of upper Chang 73 and Chang 72, the climate became somewhat drier and hotter, which was marked by relatively weak weathering conditions, suboxic to anoxic conditions, low paleoproductivity, and high terrigenous input. The Chang 71 was characterized by a moderately humid climate, moderate weathering conditions, paleoproductivity, and terrigenous input. The relationship among the primary productivity, redox conditions, clastic influx indicators and TOC content suggests that primary productivity was the most critical factor controlling the enrichment of organic matter. The high primary productivity during the early Chang 73 interval was probably driven by the deposition of volcanic ash and was influenced by marine incursion events. A warm and humid paleoclimate, combined with input of volcanic ash and seawater, resulted in eutrophic, nutrient-rich, and stratified surface water during the early Chang 73 period, which led to algal blooms with high primary productivity. A stratified water column with high primary productivity, together with oxygen-deficit bottom water conditions and low terrigenous input, resulted in the greatest enrichment of organic matter in the lower part of Chang 73 shale.