Sedimentary facies and palaeogeography of northwestern Sichuan Basin during the Leikoupoan (Middle Triassic)

Three sedimentary subfacies are recognized in the Middle Triassic Leikoupo Formation in northwestern Sichuan Basin, including the evaporate platform, restricted platform and open platform subfacies, of which the restricted platform subfacies is accentuated. The sedimentary environments during Stages 1 -5 of the Leikoupoan spanned the evolutionary processes from deepening to shallowing to deepening again.During Stage 1, the evaporate platform deposits were developed over a vast area of the Guangyuan-Wangcang and Zitong-Santai regions, and the gypsum lake deposits occurred extensively in the Zitong region. During Stage 2, the evaporate platform deposits were restricted only in the Wangcang and Zitong regions, and the restricted platform deposits appeared in other regions. With the deepening of water body during Stage 3, the Wangcang region was occupied completely by the open platform deposits, and the gypsum lake deposits disappeared in the Zitong region. During Stage 4, the restricted platform deposits were organized in much of the study area and the gypsum lake deposits became into existence once again in the Zitong region. During Stage 5, the open platform deposits covered the study area once again. The sea-level fluctuations are believed to be the important factors controlling the development and spatial distribution of sedimentary facies.