湘西北奥陶系新认识

位于江南古陆西段北侧,扬子区南缘的湘西北慈利地区的奥陶系地层的研究,对全面认识扬子区与江南区奥陶系的过渡性质和阐明其古地理情况,关系重大。但以往研究程度较低,为此,笔者于近年选择慈利宜冲桥奥陶系进行较全面的工作,在该系中段大湾组至宝塔组的三叶虫获得丰富成果,并首次进行了牙形刺的研究。通过多门类化石面貌的分析,重新修正了若干“组”的厚度,确立了大田坝组的存在,并将本区该段地层与其毗邻的江南区同期地层进行了对比。对研究本区的区域地质特点具有重大意义。

ORDOViCiAN STRATIGRAPHY OF YICHONGQIAO, CILI, NW HUNAN

The Ordovician Strata of Yichongqiao, Cili, NW Hunan Province is subdivided in ascending order as the Dawan Formation (L.Ord.);Guniutan, Datianba and Baota Formations (M.Ord.). The area under studying is situated within the south marginal part of the Yangtze Plateform.The study of the Ordovician stratigraphy of this area will be of great significance for the recognition of the Palaeogeography and characteristics of the transitional belt between the Yangtze plateform and the Southeast China Geosyncline.Ordovician rocks exposed in this area are composed mainly of red carbonates with polygonal structures as sun-cracks. The lithological characters,thickness and fossils of these four formations mentioned above are tabulated in descending order below. Overlying formation:Linxiang Fm: greyish-green nodular limestone, intercaitead with shale in the basal part(2.5m) Conformity Baota Fm: red limestone with polygonal structures as sun-cracks (30.74m), yielding Nileus, Geragnostus, Calymeninae, Trinodus, Lonchodomas, Remopleurides, Telephina, Decoroproetus, Paratzuchiatocnemis(nov.gen. MS),Cekovia, Hammatocnemis, Paraceraurus, Cyclopyge, Paraphillipsinella, Thysanopeltidae,Corrugatagnostus(Trilobites); Protopanderodus,Spinodus,Scabbardella,Dapsidodus, Drepanodus(Conodonts); and Sinoceras (Cephalopoda)etc. Datianba Fm: compact limestone of light red colour (3.9m),containing Calymeninae, Trinodus, Lonchodomas, Remopleurides, Mieroparia, Telephina,Decoroproetus, Paratzuchiatocnemis(nov. gen. MS) (Trilobites);Protopanderodus,Periodon,Paltodus,Prioniodus,Dapsilodus, Pseudooneotodus, Scabbardella, Acontiodus.Pygodus Eoplacognathus (Conodonts);and Lituiles (Cephalopoda). Guniutan Fm:red massive limestone intercalated with nodular limestone (37.71m), containing Nilens, Agnoslus, Calymoninae,Trinodus, Lonchodomas, Remopleurides, Microparia (Trilobites); Drepanoistodus, Panderodus, Belodella, Periodon, Polonodas, Amorphognathus, Baltoniodus, Eoplacognathus and Ambalodus (Conodonts). Dawan Fm: red argillaceous nodular limestone (20.3m), base not yet exposed, containing Pseudobasilicus, Nileus, Agnostus, Calymeninae(Trilobites);Drepanoistodus, Scolopodus, Drepanodus and Periodon (Conodonts). Trilobites collected from the Ordovician rocks of Yichongqiae will be published in another paper.Conodonts are studied also. As a result of the study of the Ordovician rocks of this area, the thickness of each formation is measured and affirmation of the existance of the Datianba Formation is recognized.