Middle Ordovician-Early Silurian Lithofacies Paleogeography Of Middle-Upper Yangtze And Its Southeastern Margin

The middle-upper Yangtze and its southeastern margin in South China are key areas for Palaeozoic tectonic-basin evolution and marine-facies oil-gas exploration. There are still many questions surrounding the basin evolution, sedimentary response, and distribution of source, reservoir, and cap rocks. The paper focus on lithofacies palaeogeography of these areas during middle Ordovician to early Silurian. Following the thinking of’tectonic movement controls the basin, the basin controls sedimentary facies, and sedimentary facies controls oil-gas orignal conditions’, the tectonic-basin evolution and distribution of source, reservoir, and cap rocks are discussed by lithofacies palaeogeography of the study area.The palaeogeographic framework of Late Palaeozoic was principally controlled by inherited palaeotectonics, basin-mountain coupled evolution, and global geological events. Based on recent biostratigraphical studies, a lithologic-stratigraphic correlation scheme is established for discussing palaeogeographies on various stages. As principal evidences for palaeogeographical reconstructing, the sedimentary facies in the areas are detailedly classified into five sedimentary systems, eight sedimentary facies and various subfacies. Then, the palaeogeographies in six successive periods, including Dapingian-Darriwilian, Sandbian-early Katian, late Katian-Hirnantian, Rhuddanian, Aeronian and Telychian, are reconstructed. Detailed descriptions on land-sea location and distribution of lithofacies zones are summarizing as the basis for probing into basin evolution and related geologic issues. The results show that:1) sedimentary and palaeogeographical evidences support the possible pattern of one huge basin into South China Plate during Palaeozoic,2) the tectonic-basin evolution processes of middle-upper Yangtze can be divided into two stages called’intraplate adjustment’and’Plate lifting’, which respectively have characteristic sedimentary responses,3) the distribution of Lungamachi formation source rock mainly developed in deep-water shelf facies areas during the postglacial transgression in early Rhuddanian,4)’carbonate ramp’model controls the chemical basis and spatial distribution of reservoir growth in Aeronian,5) clastic reservoirs concentrate in the Xiaoheba Formation of shore facies and delta facies areas in Telychian.