Sequence stratigraphy of Lower Ordovician reef-bearing strata in the Middle Yangtze platform

Abstract：The Lower Ordovician reef-bearing strata in the middle Yangtze platform may be divided into four II-type depositional sequences based on the principle and method of outcrop sequence stratigraphy. According to the characteristics of the systems tracts and depositional dynamic mechanism of these sequences and the controlling effects of sea level changes on the sedimentation, organism migration, bioreefs and environment, it is held that there occurred four sea level change cycles during the formation of the reef-bearing sequences in the study area： the first one occurred from the early to middle Lianghekouan age, the second one from the late Lianghekouan age to early Daobaowanian age, the third one from the early to late Daobaowanian age, and the last one from the early to middle Dawanian age. In the second cycle three subcycles are distinguished： the first subcycle occurred in the Second Member (Oolite Member) of the Fenxiang Formation, the second one in the Third Member (Alternatation Member) of the Fenxiang Formation, and the third one in the Fourth Member (Intercalation Member) of the Fenxiang Formation. Three keep-up carbonate systems and one lag carbonate depositional system are recognized. The reefs of the Fenxiang and Honghuayuan Formations formed in the highstand phase. Because of the occurrence of multiple subcycles in the process of sea level change, the reefs in the study area are characterized by the small thickness, small scale, large number and wide distribution.