Lithofacies palaeogeography in Late Ordovician Wufeng Age in southwestern Sichua

Abstract: According to the lithological characteristics, filling sequences, sedimentary structures and lab analysis of Late Ordovician Wufeng Formation, combined with the structural features, the authors divided the Wufeng Age in southwestern Sichuan into two periods, and the southwestern Sichuan can be divided into tidal-flat facies and deep shelf and shallow shelf facies. In the Late Ordovician early Wufeng Age, affected by the Caledonian Movement and the continuous extrusion and collision between Yangtze and Cathaysia blocks, the Central Sichuan and Guizhou were increasingly uplifted, and the Upper Yangtze region was submerged and transformed into carbonate ramp, and finally developed into the back-bulge basin confined by the marginal uplifts. Tidal flat facies were developed around Central Suchuan and Western Sichuan-Central Yunnan uplifts with dolomitic shales, muddy limestones and dolomitic limestones. Large areas of western Sichuan developed deep shelf facies which deposited carbonaceous shales, siliceous and carbonaceous-silty shales. In the late Wufeng Age of Late Ordovician, with the emergence of the worldwide glacial event, large-scale regression started in the study area and even in the whole Upper Yangtze area, shallow shelf facies replaced deep shelf facies with silty shales, muddy limestones, calcareous-carbonaceous shales and siliceous limestones, and they had more calcium in comparison with those in former periods.