江西武宁宁国组上部牙形刺兼论黑色页岩的形成环境和牙形刺的生态

LLANVIRNIAN CONODONTS FROM THE UPPER NINGGUO FORMATION OF WUNING, JIANGXI, WITH DISCUSSION ON SEDIMENTARY ENVIRONMENT OF BLACK SHALES AND CONODONT ECOLOGY

本文研究了江西武宁奥陶系宁国组上部黑色页岩中的牙形刺。共发现牙形刺11属15种,其中Eoplacognathus suecicus Bergstrom,Eoplacognathus foliaceus(Fahraeus)为世界性分布的牙形刺带化石,为宁国组上部黑色页岩的划分与对比提供了可靠的依据。根据牙形刺的保存状态,笔者推论当时本区水底层为偏酸性还原环境,并认为牙形刺动物是浮游生活在不同深度的水层中。

Although over the last two decades Ordovician conodonts have been a subject of manyreports,most of the reports dealt with faunas from the limestone in the Yangtze and NorthChina Platforms. The conodonts studied in this paper were collected from the black shales ofthe Llanvirnian Upper Ningguo Formation. The section under study is located at the village Tangpan ca. 4.5km NW of Wuning,Jiangxi (Text-fig. 1). The Black Shales,ca. 6.84m in thickness (Text-fig.2),yield extremelyabundant graptolites and conodonts. The gratoplites mainly include Pterograptus elegansHolm, P. flexuosus Ni, Didymograptus murchisoni (Beck), D. Jiangxiensis Ni, Glossograptushincksii (Hopkinson), Climacograptus forticaudatus Hsu, C. uniformis Hsu, Glyptograptusdentatus (Brongniart), G. tricornis Carruthers, Amplexograptus differtus Harris et Thomas,and A. modicellus Harris et Thomas; the conodonts including 484 elements, of about 15species, which were discovered on the bedding-planes of the black shales, mostly belongingto Periodon acletus Hadding,with 265 ramiform elements, 52 prioniodiform elements, 57 ois-todiform elements, and 32 tortiliform elements (Text-fig. 3). The 15 conodont species havebeen identified as Baltoniodus prevariabilis (Fahraeus),B. variabilis (Bergstrom),Complexo-dus sp.,Drepanodus arcuatus (Pander), Drepanoistodus basiovalis (Sergeeva), D. ? venustus (Sauffer), Eoplacognathus foliaceus (Fahraeus), Eo. suecicus Bergstrom, Periodon aculeatus(Hadding),Protopanderodus cooperi (Sweet et Bergstrom),P. varicostatus (Sweet et Bergst-rom), Paltodus? jemtlandicus Lofgren, Pygodus serrus (Hadding), Walliserrodus ethingtoni(Fahraeus),and Spinodus spinatus (Hadding). Among the conodonts,Eoplacognathus suecicus Bergstrom and Eo. foliaceus (Fahraeus)are the stratigraphically important multi--element species,with worldwide distribution,whichare the nominal species of their respective zones in West Europe and the Yangtze Platform,the former being found in the lower part of the graptolite Didymograptus murchisoni zone,while the latter in the Upper part of the same zone. The Llanvirnian conodont faunas of thissection are poorly preserved, but they are similar to the coeval faunas of the Yangtze Plat-form. The conodont elements on the bedding-planes of the Black Shales in the section wereweathered to a powdery state with a light yellowish colour (Pl. Ⅲ,figs. 1,8a) or to the formof moulds (Pl. Ⅰ,fig. 1; Pl.Ⅲ,fig. 6). The weathering to which the conodont elements weresubjected,in my opinion, took place before the diagenesis of the sediment or during its earlystage, because the Black Shales of the Upper Ningguo Formation were deposited in acidic re-ducing environments. Since the conodont animals could not live in such acidic reducing bot-tom condition, they are here regarded as living a pelagic mode of life and in different depthlayers in the column of water.