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 δ

18

O than most marine cements

indicating itsmeteoric phreatic origin. Its δ

13

C is heavier than δ

13

C of most marine cements through the influence of original components in the rocks. The δ

18

O and δ

13

C 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

18

O and

13

C. 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.