Permian And Fusulinids Fauna Of Xainza Area, Tibet

Xainza area of Tibet lies to the north of Yalungzangbu suture and south of Ban-gonghu-Nujiang suture, which belongs to Cuoqing-Xainza stratigraphic subregion of Gangdisi-Nianqintangula stratigraphic region in stratigraphic regionalization. This thesis deals mainly with the strata and fusulinids fauna of Mujiu-lake Permian including description of deposit sequence, classification and correlation of strata, description of fusulinids, discussion on its age, zoology and biostratigraphy. Moreover, the paleogeographic transition of Xainza area can be deduced from its correlation of fusulinids, coral, brachiopod and foraminifer with adjacent regions. Permian sequence of Xainza area can be diveded into four Formations lithostra-tigraphically after analysis, respectively Laga Formation, Anji Formation, Xiala Formation and Mujiuco Formation in ascending order. Four Formations interact conformly. Laga Formation, with thickness of 1177.41m, is composed mainly of coarse-grained sandstone in lower part and sandstone with limestone interlayers in upper part. It is characteristic of coarse-grained deposits with many interlayers of gravel-laden sandstone and conglomerate. Anji Formation is 163.09m in thickness, which composed mainly of fine-grained sandstone, siltstone and shale with bioclastic limestone interlays. Xiala Formation is 1515.95m in thickness. It is composed of all kinds of limestone including muddy limestone, bioclastic limestone, sandy limestone, cherty limestone and chert-strip limestone with dolomitic limesto-ne interlays in upper horizon. Mujiuco Formation situated in the core of Mujiu-lake syncline. It is very thick, about 24445.9m in thickness, which composed mainly of dolomite and dolomitic limestone. Fossils collected from Laga Formation are chiefly brachiopods and corals. But unfortunately, the fossils can range from Carboniferous to Permian in time, contribu-ting little to derterming the accurate age of the strata. So, the Laga Formation may stand for the deposits ranging from Late Carboniferous to Early Permian. One brachiopod species Neospirifer kubeiensis Ting emend. Chang was found in Anji Formation, which is Early Chihsian (Kungurian) in age. The small solitary corals and small brachiopods attributed to cold-water faunas are collected from Lower Xiala Formation, which indicate the time of Late Chihsian (Roadian). Middle and Upper of Xiala Formation are abundant with fusulinids such as Nankinella, Schwagerina, Chusenella and a foraminifer genus Shanita. They indicate the time of Maokou age (Wordian-Capitanian). So the whole Xiala Format-ion is corresponding to Guadalupian in age. There are few fossils from Mujiuco Formation due to its dolomite. But the coral assemblage Waagenophyllum-Liangshanophyllum-Lobatophyllum , indicating Wuchiapingian of Late Permian in age, is collected from Lower occurrence of Mujiuco Formation. Based on fossils, We arrange Laga Formation into Late Carboniferous to Early Permian, Anji Formation into Kungurian, Xiala Formation into Guadalupian and Mujiuco Formation to Wuchiapingian at least. Laga Formation, a sequence of coarse-grained sandstone with interlays of gravel and conglomerate, can be correlated with Laigu Formation of Lhasa-Chayu stratigraphic subregion, Zhanjin and Qudi Formation of South Qiangtang stratigraphic region and Jilong Formation of Himalaya stratigraphic subregion. Anji Formation and Xiala Formation contain two different types of fauna. Lower Xiala Formation is characteristic of cold-water faunas such as some brachiopod and small solitary coral assemblage Cyathaxonia-Lophophyllidium, Faunas of Anji Formation is composed of compound corals and cold-water brachiopods, while Middle and Upper Xiala Formation is characteristic of mixed fauna composed mainly of Tethyan warm-water fauna such as fusulinids and forami-nifers and some cold-water faunas. Xiala Formation corresponds to Luobadui Formation of Lhasa-Chayu stratigraphic region, Upper Quburiga Formation of North Himalaya stratigraphic subregion, Baidingpu Formation of Laguigangri stratigraphic subregion and Lower Jiangyema Formation of Zhongba-Qusong stratigraphic subre-gion. Upper Permian Mujiuco Formation can be correlated with Upper Jiangyema Formation of Zhongba-Qusong stratigraphic subregion, Jipuria Formation of South Qiangtang stratigraphic region, which have Waagenophyllum fauna in common. The Fusulinids fauna in this section is mainly abundant in Middle Permian Xiala Formation, composed mainly of Nankinella, Ozawainella, Schwagerina and Chusenella.Fusulinids described in this paper can be attributed to 8 genera , 23 species, 2 variety species , 6 similar species and 2 undetermined species including 3 new species. The list is as follows: Nankinella xainzaensis Chu, N.hunanensis (Chen), N.cf.hunanensis (Chen), N.quasihunanensis Sheng, N.nanjiangensis Chang et Wang, N.pseudolata Chu, N.hunanica(Chen), N.longgensis Nie et Song, Chusenella schwagerinaeformis Sheng, C.douvillei (Colani), C.xizangensis Chu, C.sinensis Sheng, C.cf.sinensis Sheng, Chusenella sp., Schwagerina chihsiaensis var. brevis Chen, S. cf.xainzaensis Chu, S.xainzaensis Chu, S.xizangica sp.nov., S.pseudocompacta Sheng, S.quasiregularis Sheng, S.cf.quasiregularis Sheng, S. parabicornis Chen, S.cf.parabicornis Chen, S.chihsiaensis var.regularis Chen, Staffella sphaerica (Abich), S. xizangica sp.nov., Ozawainella xianzaensis Chu, O. nankinellaeformis Yang,O.cf.xainzaensis Chu, Sphaerulina crassispira Lee, Sph.hunanica Lee, Sph.cf. hunanica Lee, Pisolina subspherica Sheng.Pisolina sp. Mufushanella nankinellaformis sp.nov. Cheng, Zhang et Wu. Three new species are Schwagerina xizangica sp.nov., Staffella xizangicasp.nov., and Mufushanella nankinellaformis sp.nov. Cheng, Zhang et Wu. The biostratigraphy of fusulinids is determined as Nankinella hunanensis-Schwagerina xainzaensis-Chusenella schwagerinaeformis assemblage zone. The fusulinids of Xiala Formation have some characters as follows: (1) High abundance and low diversity, especially Nankinella is most abundant.(2) Xainza area is different from Linzhou area in Fusulinids at the same time. The former is abundant with Nankinella and Schwagerina, while the latter is abundant with Neoschwagerina and Verbeekina.(3) The fusulinids appear suddently. Fusulinids of Lower occurrence is not contained. Northern Tibet have fusulinids of lower occurrence, while southern Tibet only have fusulinids of high occurrences. The first appearance of fusulinids gradually rises from Northern Tibet to Southern Tibet. During Permian, Xainza terrane, along with Duoma area of South Qiangtang, Baoshan terrane of West Yunnan Province, is part of Cimmerian continent. They share same history as derivement from Gondwana during Late Palaeozoic, drift northward and collision with North Continent. The features of Permian faunas are, from early to late, from South to North, the rise of the first appearance of fusulinids from north to south in Tibet and the alteration from cold-water brachiopod→cold-water solitary coral and Monodiex-odina fauna→warm faunas, which are evidences of northward movement of Cimmerian Contiment. It can be infered from features of corals, fusulinids and brachiopods that the solitary corals of Xiala Formation are the result of temperature-fall and that Xainza area separated from Gondwana and drifted northward during Early and Middle Permian. To Late Permian, it mixed into Tethyan low-latitude region and developed Cathyan fauna. Descriptions of New Species Super Family Fusulinidea Moeller, 1878 Family Ozawainellidae Thompson et Foster, 1937 Subfamily Staffellinae A.M.-Maclay, 1949 Genus Staffella Ozawa, 1925 Species Staffella xizangica sp.nov. (Pl.Ⅱfigs.6-11, 13-15) Description: shell small, subspheroidal. Lenticular in inner 3-4 whorls with sharply periphery, outer whorls subspheroidal with round periphery, umbilicus convex. 6-9 whorls, 1.1-1.6mm in length, 1.5-2.12mm in width, the ratio of length to width is about 0.65-0.77:1. Holotype has 8 whorls about 1.375 in length and 2.05 in width, ratio of length to width is 0.67:1.The spirotheca is generally compos-ed of tectum, diaphanotheca and inner tectorium. Septa plane. Chomata developed,flat in outer whorls and extend to umbilicus. The aperture is broad and low. Prolocu-lum spheroidal, about 0.05-0.1mm in outer diameter. Comparison:The new species is similar to Staffella sphaerica (Abich) in shape, diameter and ratio, but the new species is lenticular in inner whorls and convex in outer whorls.The new species is similar to Staffella pseudosphaeroidea Dutkevich and Staffella sphaerulinaeformis Li in shape, but they are different in spirotheca for the former has three layers while the latter two have four layers in spirotheca. In addition to it , the former is low in ratio of length to width. When compared with Staffella paraumbilicaris Yang , it has fewer whorls and bigger ratio. It is similar to Staffella rabanalensis Van Ginkel,S.volieosis Chen et Wang and S.discoides Liu Dong et Xian, but the new species have more whorls. Occurrence: Xainza of Tibet, Xiala Formation of Middle Permian. Family Schwagerinidae Dunbar et Henbest, 1930 Genus Schwagerina Moeller 1877 Species Schwagerina xizangica sp.nov. (Pl.Ⅶfigs.1, 2, 4-7, 9, 10, 15, 16, 18, 20) Description: Medium in size, coarse-fusiform in shape, bluntly round at the poles and net-shaped. Whorls enlarge gradually, inner whorls slightly compact, the outest whorls sometimes tight. Alveoli of Keriotheca of outer two whorls are coarse. Spirotheca added 0.05-0.1mm in diameter when compared with inner whorls. Generally it has 5-7 whorls about 2.03-3.75mm(average 2.5mm) in length and 1.23-2mm (average 1.4) in width.The ratio of length to width is 1.88-2.3, mostly 2. Holotype has 6.5 whorls, 3.5mm in length and 1.625 in width, the ratio of length to width is 2.15:1. Spirotheca composed of tectum and keriotheca, outer whorls iscoars in keriotheca. Septa fluted strongly and regularly, the height of folds is above 2/3 of the chamber. Axial fillings developed in inner 3-4 volutions. Chomata developed throughout the whorls, aperture crack-shaped. Proloculum spheroidal, about 0.1-0.18mm in diameter, mostly 0.15. Comparision: The new species is different from S.xainzaensis Chu in that the new species is cross-fusiform in shape and its small shell and ratio and that its keriotheca become thick in outer volutions. It is different from S.pseudocompacta Sheng in that the former is small in ratio and bluntly round at the poles and same extent of speta flute in both inner and outer volutions.In a world, the new species is characteristic of coarse-fusiform shape, small shell, thickness of spirotheca rise in outer volutions, small ratio, little whorls. Occurrence: Xainza of Tibet, Xiala Formation of Middle Permian. Schwagerina xizangica sp.nov. Unit: mm (except: ratio) specimen whorl length width ratio ProluculumWidth of shell Superfamily Verbeekinidea Staff et Wedekind, 1910 Family Verbeekinidae Staff et Wedekind, 1910 Subfamily Verbeekininae Staff et Wedekind, 1910 Genus Mufushanella Chen, 1964 Species Mufushanella Nankinellaeformis sp.nov. Cheng, Zhang et Wu (Pl.Ⅵfigs.1-12) Description:Medium in size, shell lenticular with sharply rounded periphery , convex lateral and slightly-concave umbilicus. Inner volutions is sharply pointed periphery. It has 8-12 whorls about 2.3-3.3mm in length and 4.1-5.0mm in width,...