Study On Sequence Lithofacies Paleogeography Of Early Ordovician In Sichuan Basin And Its Adjacent Areas

Through the comprehensive study on sequence stratigraphy, sedimentary system and lithofacies paleogeography of Early Ordovician in Sichuan Basin and its adjacent areas, at the same time, the existing data and related research results, this article edits several sequence lithofacies paleogeographic map with the super sequence and the third order sequence system domain as a unit, revealing the lithofacies paleogeographic characteristics and evolution. Basically achieved the following understanding:The rock types of Early Ordovician in Sichuan Basin and its adjacent areas mainly include carbonate rocks and clastic rocks. Carbonate rocks include dolostone and limestone. According to the composition, texture and structure, limestone can further be divided into grain limestone, nodular limestone, etc. Dolostone can be further divided into grain dolostone, fine crystal dolomite, powder crystal dolomite and dolomicrite. Clastic rocks can be classified into conglomerate, sandstone,siltstone and mudstone and shale, etc.The sequence interface characteristics and genetic type of Lower Ordovician Series in Sichuan Basin its adjacent areas is studied in detail. This article has recognized the surface of unconformity, the maximum flooding surface, the lithofacies transition surface and the bottom flow scour erosion surface such as sequence interface types. On this basis, the study divides Ordovician into a complete super sequence, OSQ1- OSQ4 altogether four sequences. The features of sequence and system domain of the super sequence and the third order sequence are discussed in detail.In the study area, it identified carbonate platform facies, littoral facies and shelf facies, which further divided into shoal, tidal flat and reef beach. Super sequence corresponds to the Tongzi period and Honghuayuan period of Early Ordovician. Carbonate deposition is given priority to edge carbonate platform. Super sequence TST corresponds to early Ordovician Tongzi period. Lithofacies from west to east are coast, tidal flat and restricted platform; Super sequence HST corresponds to Early Ordovician Honghuayuan period. Carbonate deposition remains carbonateplatform. Lithofacies from west to east are coast, tidal flat, restricted platform and platform margin shoal. Sequence lithofacies distribution with super sequence lithofacies distribution are of the same. Shore-tidal flat distributes in north-south and north-east in the western Sichuan Basin, which is mainly deposited with dolomite,intercalated with shale. Restricted platform distributes nearly north-east in the central Sichuan Basin, which is mainly deposited dolomite, intercalated with limestone and shale. There is beach developed in the restricted platform.