A marine incursion during the onset of T-OAE in Sichuan Basin, China: Evidence from green clay minerals and carbonate concretions

A high-amplitude global sea level rose rapidly during the early Toarcian (Early Jurassic), making the transgression deposits potential stratigraphic correlation markers of marine and terrestrial successions. Green clay minerals (i.e., glauconite and chamosite), carbonate concretions, and geochemical database associated with the early Toarcian global transgression are archived in the Lower Jurassic Da'anzhai Member (the Early Toarcian age) in the Sichuan Basin of China, a mega lake along the Tethys Ocean. The fine-grained glauconites with high Al and low Si contents suggested that they were transported from offshore to the lake by marine incursion and were oxidized during the transport processes. The green authigenic chamosite cement and redeposited chamosite grains with high Mg/Fe and low Al/Si ratios were diagenetically transformed from berthierine formed in the brackish water caused by marine incursion. The chemically impure siderite concretions with low ⁸⁷Sr/⁸⁶Sr ratios and high δ¹³C values suggested that the lake water and sediment pore waters were mixed with seawater. Coeval calcareous concretions have low ⁸⁷Sr/⁸⁶Sr ratios and high δ¹³C values, indicating the mixing of lake water with seawater. Combining the stratigraphic low ⁸⁷Sr/⁸⁶Sr values, we suggested that there was a marine incursion into the Sichuan Basin. According to the organic carbon isotopic stratigraphy correlation, the marine incursion occurred during the onset of T-OAE. The rapid and high-amplitude global sea-level rise may have caused this marine incursion.