Sinoceras

Abstract: The Ordovician rocks in the Changyang-Itu area,western Hupeh,may begrouped into two series,the Ichangian series of the Lower Ordovician and theNeichiashanian series of the Middle Ordovician,exclusive of the Wufeng shaleof Ashgillian.The sections dealt with in the present paper were made by thewriters during their geological excursion in 1951.The Ichangian and the Nei-chiashanian series in this region are very fossiliferous.Brachiopods,graptolites,cephalopods and trilobites have long been known in these two series.Besidesthese fossils,a large amount of ostracods which are unknown to former fieldgeologists age also present in the collection.In this paper four sections of theIchangian series and one section of the Neichiashanian series are discussed andseveral faunal zones are assigned.1.The Ichangian seriesThe name Ichang limestone was proposed by Prof.J.S.Lee thirty yearsago;since then the formation has been repeatedly studied by many geologists,especially Y.S.Chi,T.Y.Hsu and Y.Wang and S.C.Hsu and C.T.Ma.Since the creation of the term there have been considerable differences of opinionregarding the age and the classification of this formation.Many of them believ-ed that this formation belongs to the Tremadocian,while others considered itto be of the Cambrian and the Tremadocian age.A few geologists are of theopinion that it may contain some constituents of Arenigian.The formation maybe summarized in the following table (see next page):Lithologically the Ichang series may be divided into four divisions approx-imately corresponding to the four fossil zones respectivelly:(1) the zone ofDactylocephalus dactyloides,characterized by the yellowish green shalesintercalated with limestone lensis;(2) the zone of Asaphopsis immanis,agrey massive limestone with chert nodules;(3) the zone of Acanthograptus (?)sinensis,it contains the green shales interbedded with grey limestones;and (4)the zone of Cameroceras,the rock is a dark grey to blackthick-bedded limestone wi-th chert and abundance in Cameroceras.The first two zones are here confinedto the Ichang formation,and according to the table given by the writers these twozones may be correlated with the lower part of the Tremadocian.The zone ofAcanthograptus sinensis may be corresponding to the Fenhsiang shale of Mr.Chibelonging probably to the upper part of Tremadocian.The Cameroceras zonewhich the writers correlate with the Hunghuayuan limestone of northern Kuei-chow as shown in the table,belongs to the Arenigian age.All these four zonesare now called the Ichangian series.The total thickness of this series in the regionsurveyed ranges from 260-300 m.This series is comformable with both thesiliceous limestone below and the Neichiashanian series above.It may be cor-related with the Panho series,including the Meitan shale at the top of south-eastern Szechuan as defined by Y.Wang and may also be correlated with theYehli formation together with the Liangchiashan limestone of N.China.2.The Neichianshanian seriesThe name Neichianshan has long been known to the Chinese geologists,since it was established by Prof.J.S.Lee in 1924.According to Lee thisseries consists of two parts,the upper Pagoda limestone and the lower Triplecia (Yangtzeella) beds.In the Changyang district,the Neichiashanian series maybe divided,based on the detailed section measured by the writers,into five dif-ferent fossil zones.In descending order they are:5.Encrinurus-Cyphaspis zone4.Sinoceras chinense zone3.Glyptograptus teretiusculus zone2.Sinoceras ruduni zone1.Yangtzeella poloi zoneThe lower three zones as listed above (1,2 and 3) are mainly grey orpinkish limestones and dirty green shales corresponding to the Triplecia beds ofProf.Lee.The 4th zone containing Sinoceras chinense is the sun-cracked lime-stone corresponding to Lee's Pagoda limestone.The 5th zone or the zone ofEncrinurus-Cyphaspis is mainly greenish grey nodular limestone,and may be cor-related with the Linhsiang limestone of the Hunan province.The name Lin-hsiang limestone was given by A.T.Mu and J.C.Sheng during their geologicalexcursion in 1947.The fossils collected from the type locality of Linhsianglimestone,Wulipai,Linhsiang district,northern Hunan was determined by Y.H.Lu containing Encrinurus,Cyphaspis,Shaerexochus,Dalmanites?,Agnostids etc.This fossil assemblage indicates a late Middle Ordovician or early UpperOrdovician age.The Neichiashanian series,as a whole,may be correlated with the Shihtzepuformation together with the Orthoceras limestone in the Szechuan-Kueichowborder and may also be correlated with the Machiakou limestone of N.China.The relation of the geological formations and their faunal zones between theYangtze Gorge district and N.China may be clearly shown in the followingtable (see next page):3.The facies problem between S.China and N.ChinaIt is known to Chinese geologists that the faunal elements are quite dif-ferent between S.China and N.China,especially in the Middle Ordovician.In N.China the Ordovician rocks are characterized by Actinoceroids,while inS.China they are characterized by Orthoceroids,hence the Ordovician rocks of N.China is usually called the Actinoceras limestone and in S.China it is common-ly named the Orthoceras limestone.The rocks in N.China are mainly massivelimestones representing a deep sea condition.Sun-crack or nodular limestones, (?)shales and sometimes even sandstones are deposited in the Ordovician of S.China.The presence of these rocks indicates a comparatively shallow watercondition of this area during the Ordovician period.The present writers areof the opinion that different geological facies may develop different faunal ele-ments.In other words,different faunas may be effected by different environ-ments.It is probable that the Actinoceroids are in favor of living in deep sea,and the Orthoceroids are in favor of shallow sea while Endoceroids may live inboth the deep and the shallow waters.The preserved condition of the Endoceras shells between N.China and S.China is quite different.Well preserved or complete specimens of Endoceroidsare commonly found in the nodular or sun-crack limestones of S.China andin the massive limestones of N.China only the specimens of siphuncle had beenfound.Orthoceroids and complete specimens of Endoceroids in association withthe conchs have been found in Ordos,eastern Kansu and northern Shensi,theN.W.border of the N.China sea,where the Ordovician sea was comparativelyshallower and the lithological characters are more or less similar to those of S. China.In addition,the Middle Ordovician graptolites which occurred commonlyin S.China are also presented in these regions.During the Arenigian time,rock characters of both N.China and S.Chinaare similar,and the fossils Cameroceras,Piloceras,Ophileta and Archaeoscyphiaare found in the Liangchiashan limestone of N.China as well as in the Hung-huayuan limestone of S.China.So the writers are led to the conclusion thatthe difference of faunal elements between N.China and S.China at Ordoviciantime is not only due to the different faunal provinces but also chiefly due tothe environmental conditions.

Abstract: Lithostratigraphy, biostratigraphy of the Ordovician Xainza area, northern division and correlation divided of approximates of the early as of Ordovician of Tetragraptus (Paratetragraptus)-Didymograptus (Corymbograptus) deflexus combination of Middle Ordovician Michelinoceras elongatum- combination with Sinoceras chinese and the Late Ordovician Michelinoceras huangnigang-Columenoceras priscum combination band. Early Ordovician strata author from 2000 to 2001 making Xainza County Sheet 1:250000 regional geological survey newly discovered, is the oldest strata of reliable fossils based in northern Tibet. Especially Tetragraptus (Paratetragraptus) approximatus discovery, not only to expand the distribution of it in our geographic area Early Ordovician strata the similar stratigraphic correlation international biological basis. The paper deals with the classification and correlation of lithostratigraphy and biostratigraphy of the Ordovician System in the Xainza region, North Tibet. Based on fossils, the Ordovician System is divided into several biozones as follows: Tetragraptus (Paratetragraptus) approximatus Didymograptus (Corymbograptus) deflexus graptolite assemblage of Early Ordovician, The sediments of Early Ordovician is discovered recently by authors during 2000~2001 when making the geological surveying 1:250000 scale of Xainza county, Which has been the oldest stratum based on fossils up to now in northern Tibet. What is more, the discovery of Tetragraptus (Paratetragraptus) approximatus enlarges its geographical distribution in our nation as well as provides biology evidence to international correlation of the Ordovician in Xainza county.