The sequence of the Lower Ordovician in Yichang, Hubei is dominated by carbonates. The carbonates consist of allochems, such as bioclast, intraclast, pellet and ooid,abundant micrite and dolomite. The petrological characters indicate that the sequence was deposited in the carbonate platform-epeiric sea. The Lower part of the sequence mainly developed in shallow areas of higher energy, while the upper part in shallow shelf areas of lower energy, indicating the process of transgression. According to the conventional and cathodoluminescent petrography and stuble isotopes of carbon and oxygen, diagenesis, which includes the cementation, compaction, micritizatiom, neomorphism, silicification, dissolution and dolomitization, has been identified.The cementation and dolomitization are most common. The equant cement contains lighter δ18O than most marine cements, indicating itsmeteoric phreatic origin. Its δ13C is heavier than δ13C of most marine cements through the influence of original components in the rocks. The δ18O and δ13C values of most micrites are within the range of marine calcites, suggesting that they formed in marine phreatic environments. The dolomites are mostly euhedral with partly subhedral and enriched relative to calcites in 18O and 13C. The dolomitization occurred in the mixing environment. Both equant cements and dolomites have mostly bright cathodoluminescence and sometimes may contain interlayers of dull luminescence. The bright luminescence is the characteristic of slightly reducing environments. The dull luminescent interlayers may have developed in temporal oxidizing conditions. A large proportion of the diagenesis can be ascribed to the early diagenesis.
