Origin and Formation Mechanism of the Late Permian Black Siliceous Rocks in the Lower Yangtze Region

The Late Permian witnessed a Permian Chert Event (PCE) and distinctive oceanic geochemical fluctuations, such as an ocean acidification event, large-scale volcanic eruption, and rapid global warming. However, the links between siliceous rock formation mechanism, ocean, and climate changes are rarely discussed. In this article, two well-preserved deeper-water sections of the Dalong Formation from the Lower Yangtze region in southeast China are selected for analysis. To document the coeval oceanic changes, we present thin section authentication, scanning electron microscope (SEM) observation, and multiple geochemical proxies, including total organic carbon contents (TOC), major element contents, trace element contents, and rare earth element contents (REEs). The results show that siliceous rocks are mainly of biological origin in this region. The low content of MgO (0.10-0.94%, mean = 0.36%) in the Fantiansi section of Tongling area indicates that it is affected by regional hydrothermal fluids. The correlation between Al₂O₃/TiO₂, Al₂O₃ versus SiO₂ /Al₂O₃, and TiO₂/ΣREEs indicates that the Late Permian was greatly influenced by the continuous input of terrigenous materials. The correlations of MnO/TiO₂, La_N/Ce_N, Ce/Ce*, and LREE/HREE imply that the Dalong Formation siliceous rocks were deposited in a continental margin setting. Redox geochemical data (EF_U, EF_Mo, and EF_V) imply the water column experienced widespread anoxic/euxinic during the Late Permian, which aided in the preservation of organic matter following biological decay. The accumulation of siliceous rocks is related to South China experiencing a hot tropical climate, coastal upwelling and continental weathering-enriched marine nutrients, fostering high primary productivity, and benefiting abundant siliceous zooplankton. Volcanic and regional hydrothermal activity further enhanced nutrient flux, and the simultaneous ocean acidification event provided favorable chemical conditions for the preservation of silica, leading to the formation of siliceous rocks in the Dalong Formation.