Marine to brackish depositional environments of the Jurassic-Cretaceous Suowa Formation, Qiangtang Basin (Tibet), China

The Late Jurassic to Early Cretaceous depositional environments of the Qiangtang Basin in Tibet have the potential to provide significant insight into mechanisms of black shale deposition, organic matter accumulation, and the timing of closure of the Mesotethys Ocean. However, the depositional setting has not been well constrained. Here we apply multiple geochemical proxies and petrologic analyses to representative samples of black shale, marl, and micrite collected from a section in the region. These indicate a transitional marine-continental environment, with brackish to saline water. Redox conditions were weakly oxic to suboxic. Environments represented by the studied section varied over time. The lowermost marls were deposited in a low-salinity, weakly oxic shore to shallow lake environment, under a warm and humid climate regime. The micrites in the middle part of the section were deposited in a lagoonal environment, with intense evaporation, high salinity, and water column stratification. The uppermost shales were deposited in a reducing, semi-enclosed lagoon environment during a marine transgression. These results suggest that the Late Jurassic-Early Cretaceous succession was deposited in a tidal-flat or lagoonal environment.