The Biocommunity Evolution and Depositional Environment Transition of the Lower Ordovician Fenxiang Formation in the Western Hubei Aera

Key words: 
• Fenxiang Formation /
• ecologic assemblage /
• biocommunity /
• community evolution /
• de-positional environment.

Abstract: The fossils in Fenxiang Formation of the Lower Ordovician could be subdivided into three ecologicassemblages and ten communities: (1), shelly facies assemblage: Tritoechia-Pelmatozoan community, Tri-toechia-Pomatotrema community;(2) Reef facies assemblage: Archaeoscyphia community, Archaeoscyphia-Calathium community, Batostoma- pelmatozoan community,Calathium-Bluegreen algae community, and(3), stll water facies assemblage: Acanthograptus- Dendrograptus community, Nanorthis-Psilocephalinacryptocoenosis, Yichangopora? community.Among these assemblages, evolutionary successions could beobserved in the transition cycle of (1) to (2) to (3)and then back to (1) and (2).Similarly, certain succes-sion events and faunal replacements took place between the reef-building communities.The communityevolution indicated that a transgression and regression cycle led to progressive increase and decrease inwater depth, and such a process might have extended over the middle Yangtze platform and be consistentto the eustatic change of sea level during the late Tremadocian to the Early Arenigian time.