The palaeogeographic changes of the Chuxiong Basin in Yunnan during the Late Triassic

The Chuxiong Basin in Yunnan was a marine-continental sedimentary basin during the Late Triassic, grounded upon the Precambrian basement on the western margin of the Yangtze plate. The deep-water grey thin-bedded silty mudstone and muddy siltstone were laid down during the Ladinian to the Carnian. During the early Nornian, there occurred two depocenters in the basin separated by the Huaping uplift:the open sea shelf fine-grained clastic rocks and distal turbidites above the nappes west of the uplift, while the (fine)-(grained) clastic rocks intercalated with turbidites in the restricted environments east of the uplift. Till the late Nornian, the (large)-(sized) delta front sandstones constituted downlapping complexes. The palaeogeographic framework displays a symmetrical EW-trending zonation. The continental and (continental)-(marine) deposits occur in the western part of the basin, while the marine deposits in the central part of it. During the Rhetian, the fluvial-littoral deposits were formed in the Shezi Formation, and the alluvial-pluvial deposits in the Baitutian Formation above the nappes. Collectively, the palaeogeographic changes are characterized by the gradual (overlapping) from west to east and coarsening from bottom to top, and the transition from marine to continental deposits.