Lithofacies Palaeogeography of the Middle Ordovician in the eastern North China Platform

We subdivided the Middle Ordovician in the eastern North China Platform into five level 3 sequences using sequence stratigraphic classification and correlation in order to determine clear sedimentary patterns and evolution characteristics. Sequence palaeogeographic maps of sedimentary facies were drawn, and the coastal evolutions were described using detailed sequence isochronism. Persistent transgression occurred in the eastern North China Platform during the Middle Ordovician. Lithofacies palaeogeography at five different stages of the Middle Ordovician took the appearance of some inherited features coexisting with significant evolution. Paleotopography of five stages kept on pre-senting higher in the north and south, and in central North China there developed a quite complete sedimentary sequence. The paleogeographical environment was influenced by tectonics and sea level changes and the distribution pattern of sedimentary facies in each phase was different. With the growing marine transgression, restricted platform and open platform were prominent. The north-south symmetry of sedimentary facies manifested as a restricted platform located on both sides of the open platform. Overall, it was the sensitive feedback to sea level change of the epicontinental sea coupled with crustal uplift, that controlled the sedimentary cyclicity and rhythmicity of lithologic associations, thus developing favorable source-reservoir-cap assemblages. It is worth mentioning, with beneficial Meso-Cenozoic preservation conditions, the Bohai Bay Basin is the most advantageous exploration area.