Devonian Fauna Of Xainza Area, Tibet And Their Palaeozoogeographic Affinities

Xainza area is located in back land of north Tibet, and its average altitude is above4500m. It is underpopulated while it has a formidable natural conditions. Thepalaeozoic strata are very developed in Xaiza area. Xainza area is so ideal for studyDevonian and its fauna in north Tibet because of good outcropping, continuoussediments, simple structure, various categories fossiles of Devonian section. The distribution of Devonian are wide-ranging, and mainly outcropped inMendeoyao, Daredong and Chagoroma, and they are also outcropped in Jiabunong,northward Mujiu Co and near Tarema village, southeast Xainza county. Devoniansystem in Xianza area can be divided into Lower Devonian Daredong Formation andMid.-Upp.Devonian Chagoroma Formation. In Tarema area, Daredong Fm. is mainly composed of a set of carbonatite whichlaminate few fine clastic rock, and it belongs to stable shelf shallow sea sediment.Daredong Fm. can be divided into lower part and mid-upp. part distinctly according tocharacters of lithologic and lithofacies. Lower part is composed by deep colormedium-thin bedded micrite, bioclastic limestone interbedding medium-thin beddedsiltstone and medium-fine arkosite. The limestones of lower part bear conodonts,tentaculitids, cephalopods and abundant corals, brachipods. Mid-upp. part are composedby light color, mainly greyish-white and light grey, medium-thin to medium-thickbedded bioclastic limestone which laminate partially microbioreef limestones, marliteand calcirudite, and interbedding developped horizontal bedding medium-thin beddedsiltstones, dirty siltstones and medium-fine arkosite. The mid-upp. part developabundant benthonic organism, such as corals and brachipods, and a few pelagicorganism, such as conodonts, tentaculitids, cephalopods etc. In Yonzhu area, the rockassociation of Daredong Fm. are mainly composed of grey-deep grey medium-thinbedded bioclastic limestone and micrite, light grey-deep grey medium-bedded brecciola,khaki-violet red medium-thin bedded sandy limestone, bioclastic limestone, laminatinga few quartzy sandstone. Daredong Fm. contact with underlying dolomitic limestone ofMid-Upp. Silurian conformably, so does with overlying arkosite of Chagoroma Fm. The time of Daredong Fm. is Early Devonian, containing mostly Lochkovian andwholly Pragian, Emsian stage. Chagoroma Fm. is a set of carbonatite besides containing a layer of 4m-thickthin-bedded medium-fine sandstone and calcareous siltstone in the bottom. They aremainly composed of light grey, grey medium-thin bedded to medium-thick beddedmicrite, dolomitic limestone and limestone bearing flint core and laminating bioclasticlimestone. Chagoroma Fm. develop crinoids stems, fragments of brachipod shell androgusa. In Yonzhu area, the low part of Chagoroma Fm. are thick-bedded dolomiticlimestone; middle part are light grey thick-bedded or massive oolite limestonecontaining sandy oolite limestone, calcirudite and microlite; upper part are greythick-bedded limestone bearing bioclastic. And Chagoroma Fm. develop edgewise-likelimestone in near Derionma. It shows that the sea was located in high energy sediment IV吉林大学硕士学位论文circumstande of intertidal zone at that time. Chagoroma Fm. is conformable withunderlying Daredong Fm., overlying Yongzhu Fm. respectively. The time of Chagoroma Fm. is Mid-Late Devonian, may be from Efielian toFamennian. There are very abundant fossils in Daredong Formation. Rugosa list as follow: Lochkovian:Pseudamplexus cf. ligeriensis (Barrois), Acanthophyllum sp.,?Hemiaulacophyllum sp., Palaeocyathus cf. gansuensis Cao, Axocricophyllum cf.minbugouensis Cao, Lyrielasma cf. tenuiseptatum Cao, Crassophrentis cf. obesus Cao,Ellesmerelasma sp. Pragian : Gurievskiella cylindrica Zheltonogova, Cyathophyllum sp.,Neokyphyophyllum sp., Embolophyllum aeguiseptatum , E. alengchuenseYü et Liao ,Asterobillingsia sp., Loy...