鄂西地区早奥陶世分乡期生物群落演化与沉积环境变迁

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

鄂西地区下奥陶统分乡组生物群按其生态特征划分为３个生态组合，１０个化石群落或埋藏群：（１）壳相组合：Ｔｒｉｔｏｅｃｈｉａ－Ｐｅｌｍａｔｏｚａｎ群落、Ｔｒｉｔｏｅｃｈｉａ－Ｐｏｍａｔｏｔｒｅｍａ群落；（２）礁相组合：Ａｒｃｈａｅｏｓｃｙａｈｉａ群落、Ａｒ－ｃｈａｅｏｓｃｙｐｈｉａ－Ｃａｌａｔｈｉｕｍ群落、Ｂａｔｏｓｔｏｍａ－ｐｅｌｍａｔｏｘｏａｎ群落、Ｂａｔｏｓｔｏｍａ群落、Ｃａｌａｔｈｉｕｍ－蓝绿藻群落；（３）静水相组合：Ａｃａｎｔｈｏｇｒａｐｔｕｓ－Ｄｅｎｄｒｏｇｒａｐｔｕｓ群落、Ｎａｎｏｒｔｈｉｓ－Ｐｓｉｌｏｃｅｐｈａｌｉｎａ埋藏群、Ｙｉｃｈａｎｇｏｐｏｒａ？群落。它们之间随环境变化显示出由壳相组合→礁相组合→静水相组合→壳相、礁相组合的演化序列，造礁生物之间也存在着一定的演替和取代。群落演化特征反映了研究区分乡期至红花园期沉积环境演变过程是一个海水不断加深复又变浅的海侵—海退旋回。

おhe fossils in Fenxiang Formation of the Lower Ordovician could be subdivided into three ecologic assemblages and ten communities:(1),shelly facies assemblage:TritoechiaPelmatozoan community,Tritoechia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Psilocephalina cryptocoenosis,Yichangopora?community.Among these assemblages,evolutionary successions could be observed in the transition cycle of (1) to (2)to (3)and then back to (1)and (2).Similarly,certain succession events and faunal replacements took place between the reefbuilding communities.The community evolution indicated that a transgression and regression cycle led to progressive increase and decrease in water depth,and such a process might have extended over the middle Yangtze platform and be consistent to the eustatic change of sea level during the late Tremadocian to the Early Arenigian time.