湘中佘田桥剖面上泥盆统弗拉斯阶地层研究新进展

NEW ADVANCES IN THE STUDY OF THE UPPER DEVONIAN FRASNIAN STRATA OF THE SHETIANQIAO SECTION, CENTRAL HUNAN

湘中佘田桥剖面的佘田桥阶大致与弗拉斯阶范围一致 ,其下为吉维阶泥质灰岩 ,含腕足类 Emanuella tak-wanensis等及竹节石 Viriatellina kienelensis等。佘田桥阶自下而上分别为榴江组、蒸水河组、老江冲组。榴江组主要是紫红色粉砂质泥岩 /粉砂岩互层、底部为黑色薄层硅质岩。蒸水河组下部以灰黑色及暗灰色钙质泥页岩为主 ,含介形虫 Bertillonella erecta、B.cicatricosa?、Franklinella calcarata、Entomozoe(N ehdentomis) tenera及菊石 Meso-beloceras?sp.;上部泥质灰岩、似瘤状灰岩、粉砂质泥岩含牙形石 Palmatolepis cf.semichatovae、大型菊石 Mantico-ceras sp.及介形虫 Entomozoe(N ehdentomis) pseudorichterina。老江冲组以泥页岩为主 ,顶部附近则出现一些泥质灰岩、似瘤状泥灰岩层 ,产丰富的小嘴贝类、无洞贝类及弓石燕类等腕足动物 ,少量海绵 (?)及珊瑚等。佘田桥阶的顶界与弗拉斯阶的顶界一致 ,即以无洞贝类和大量四射珊瑚的绝灭为标志 ,而 Yannanellina腕足动物群的出现开始于锡矿山阶 (或法门阶 )。综合各类化石资料 ,佘田桥剖面的榴江组大致相当于牙形石 falsiovalis带— punctata带 ,蒸水河组相当于 L ower hassi带— L ower rhenana带 ,老江冲组相当于 U pper rhenana带—

The Shetianqiaoan Stage is approximately equal to the Frasnian. The underlying Middle Devonian Givetian (upper part) yielding brachiopod Emanuella takwanensis, and tentaculoid Viriatellina kienelensis, etc., consists mainly of argillaceous limestones. Lithologically the Shetianqiaoan Stage consists of three units in ascending order: Liujiang Fm, Zhengshuihe Fm, and Laojiangchong Fm. The Liujiang Fm is predominated by alternating purplish silty mudstones and siltstones, with black thin-bedded cherts in the lower part. The Zhengshuihe Fm may actually be divided into two parts: the lower part characterized by grayish black and dark gray calcareous argillutites, yielding ostracodes Bertillonella erecta, B. cicatricosa?, Franklinella calcarata, Entomozoe (Nehdentomis) tenera and ammonoids Mesobeloceras? sp.; and the upper part characterized by argillaceous limestones, nodule-bearing limestones, and silty mudstones, yielding conodonts Palmatolepis cf. semichatovae, large-sized Manticoceras ammonoids and ostracodes Entomozoe (Nehdentomis) pseudorichterina. The Laojiangchong Fm consists mainly of argillutites, with argillaceous limestones and nodule-bearing marls predominated at the top, yielding abundant rhynchonellid, atrypid, and cyrtospiriferid brachiopods, minor sponges and rugose corals. The top of the Shetianqiaoan Stage is the same as that of the Frasnian, i.e., the boundary between the disappearance of atrypid brachiopods and rugose corals and the emergence of the Yunnanellina fauna. Analyses of conodont, ammonoid, and pelagic ostracod data show that in the Shetianqiao section, the Liujiang Fm should be approximately correlated with the conodont falsiovalis to punctata Zones, the Zhengshuihe Fm with the Lower hassi to Lower rhenana Zones, and the Laojiangchong Fm with the Upper rhenana to linguiformis Zones.