An analysis of Ordovician rock association and sedimentary environment in southern Hunan Province

The Ordovician strata in southern Hunan Province consist mainly of sandstone and mudstone (slate)，with minor siliceous rocks and limestone. According to the rock components，they can be further divided into two sub-rock association types，i.e., rhythmically alternate sandstone and mudstone type，which is mainly developed in Jueshangou，Qiaotingzi and Tianmashan formations; and mudstone (slate)-siliceous rocks type，which is mainly developed in Yanxi Formation. According to a comprehensive analysis of lithology, palaeontology and sedimentary structures, it is held that the sedimentary environment should be deep-water slope to basin，where deep-water autochthonous (in situ) deposits and turbidite deposits were developed. On such a basis, the sedimentary model has been built and the sedimentary evolution has been analyzed. The sedimentary evolution indicates that the water became gradually deeper on the whole， and there existed an Ordovician sedimentary cycle of transgression-regression in this section. The first transgression began in Qiaotingzi stage of Early Ordovician, and the regression began in Tianmashan stage of Late Ordovician.