Geochemical characteristics of the Upper Triassic Tanzhuang Formation in the Jiyuan sag, southern North China Block: Implications for the depositional environment and organic matter enrichment

The organic-rich black oil shales of the Upper Triassic Tanzhuang Formation represent important source rocks within the southern North China Block. However, the mechanism of organic matter enrichment remains unclear. Additionally, the climate evolution and driving factors of the Late Triassic in the southern North China Block are still unknown. This study explores the paleoclimate, paleohydrology, paleoenvironment, and organic matter accumulation mechanism of the section containing the Tanzhuang Formation in the Jiyuan area, utilizing its geochemical characteristics as a basis for analysis. The oil shales in the Tanzhuang Formation can be categorized into two types: organic-rich shales (with total organic carbon (TOC) contents varying between 1.61 % and 4.17 %) and organic-lean shales (with TOC contents varying between 0.29 % and 1.44 %). The paleoclimatic indices, the chemical alteration index (CIA) and the C value, suggest that the organic-lean shales were deposited under warm and semihumid paleoclimatic conditions, whereas the organic-rich black shales formed under warm and humid climate conditions. The positive correlation between the Hg/Al ratios and C values suggests that volcanic activity was the cause of paleoclimatic changes, which may have been associated with global climatic events that occurred during the Late Triassic Carnian period. The oxidation–reduction indices, V, U, and Mo contents and the Mo–U correlation diagram indicate that dysoxic conditions prevailed in the Jiyuan Basin during the deposition of the oil shales in the Tanzhuang Formation. The primary productivity indices (e.g., Cu_EF and Zn_EF) suggest that higher primary productivity was observed during the deposition of the organic-rich shales. The positive correlations between the TOC contents and the Cu_EF and C values indicate that the enrichment of organic matter in the oil shales of the Tanzhuang Formation was influenced primarily by primary productivity and a warm and humid climate. The sedimentary model indicates that the elevated primary productivity may have originated from a warm and humid climate coupled with intense weathering caused by volcanic activity during the Late Triassic period, which formed a productivity model characterized by the accumulation of organic matter in the oil shales of the Tanzhuang Formation.