Sedimentary facies and Palaeogeography of the northern margin of the Yangzi block during the Hirnantian (Late Ordovician)

The northern margin of the Yangzi block has witnessed two stages of evolution including the Upper Yangtze craton basin(∈-O₂) and post-uplift basin(O₃-S) during the Early Palaeozoic, and stratigraphically, consist of the Upper Ordovician Guanyinqiao Formation and/or Nanzheng Formation strata. The sedimentary facies in the Guanyinqiao Formation may be classified into the littoral facies and shelf facies. Lithologically, the shelf facies may be subdivided into the epicontinental clastic shelf and mixed shelf. The littoral facies aligned along the Hannan uplift is dominantly made up of quartz sandstones, calcareous sandstones, and gravel-bearing sandstones in which brachiopods occur. The shelf facies in the Dalianghui-Qiaoting zone is significantly built up of silty mudstones, bioclastic limestones and marls intercalated with calcareous mudstones and Ca-bearing mudstones in which trilobites(Dalmanitina), brachiopods, and cold-water and shallow-water Hirnantia faunas are observed. The occurrence of the Hirnantian fossils recognized both in the littoral zones and shallow-sea zones has mirrored that due to the effects of the southern Gondwana glacial event, the ice water once invaded into the northern margin of the Yangtze Block. The sea-level falling caused by this glacial event led to the shallowing of the sea water, the transition of the deep-water shelf to the shallow-water shelf and planktonic graptolite to the shelly Hirnantia, and steady uplifting and stratigraphic hiatus in the study area.