Carnian cherts in the northwestern Sichuan Basin: a potential causal link with the Carnian Pluvial Episode (Late Triassic)

Chert is widespread in the Carnian (Late Triassic) shallow-water carbonate platform sedimentary successions of the northwestern Sichuan Basin, an unusual setting compared with its more typical deep-marine occurrence. It is found in strata recording the sedimentary evidence of a Late Triassic hyperthermal Event, the Carnian Pluvial Episode (CPE). Besides in the Sichuan basin, chert at the CPE is also reported in shallow-water successions of the Western Tethys. The coincidence between the CPE and chert occurrence in shallow-water carbonate rocks suggest a potential link with the CPE environmental perturbation that has not been explored yet. We investigated cherts from the shallow-water carbonates of the Upper Triassic Ma'antang Formation in the northwestern Sichuan Basin using petrography and geochemistry. Cherts in the Ma'antang Formation are mainly composed of microcrystalline quartz, with remnants of calcite. Geochemical (e.g., REE + Y pattern and the Ge/Si ratio) and petrological characteristics indicate a primary biogenic origin. The elevated input of silica and nutrients triggered by the CPE, together with potential upwelling created favorable conditions for the proliferation of siliceous organisms. These organisms were not outcompeted by carbonate secreting organisms due to clastic influx and changes in the chemical properties of the seawater. During early burial process, siliceous sponges released silica into the sediments. When these became saturated, silica precipitated and evolved into chert. Our findings suggest that saturation with respect to silica was achieved in shallower settings. This was an unusual event as cherts are usually formed in deep water starved basins or under hydrothermal influence, where silica saturation is more easily reached.