西藏南部定日、聂拉木地区早-中侏罗世有孔虫

EARLY AND MIDDLE JURASSIC FORAMINIFERA FROM TINGRI AND NYALAM REGIONS OF SOUTHERN TIBET,CHINA

记述早、中侏罗世有孔虫13属17种,标本采自西藏南部定日县的卧龙剖面及聂拉木县中尼公路5264km路标处剖面。早侏罗世普普嘎组以产大有孔虫为特征,在卧龙剖面仅有下侏罗统的中、上部,缺失下部地层;中侏罗世聂聂雄拉组以产小有孔虫为特征,基本没有大有孔虫。

Summary The Lower and Middle Jurassic strata in southern Tibet, China were named the Pupuga and Niehnieh Hsiungla formations respectively. The stratotypes are located on the north of Derirong Bridge, the west side of the road from China to Nepal （Text-fig. 1）. The fossils found in the Pupuga and Niehnieh Hsiungla formations include ammonites, bivalves, corals and foraminifera. The samples of Jurassic foraminifera studied in this paper were collected from two localities of southern Tibet. One is Wǒlong section in Tingri County, and the other is in Nyalam County, which is beside the road sign 5264th km of the road from China to Nepal. It was called 5264th section for short. Ammonites were not found in the Wǒlong section. But abundant foraminifera were collected. Especially some larger foraminifera were found in the lower part of the section. They are Nautiloculina oolitheca Mohle （K1-6, 12, 18）. Ammobaculites tibeticus sp. nov. （K1-6, 21）, Lituolipora termieri （Hottinger） （K1- 12）, Labyrinthina compressa （Hottinger） （ K1-14）, Orbitopsella praecursor （ Gtimbel） （K1-3, 6）, Amijiella amiji （Henson） （K1-24）, Haurania deserta Henson （Kl-17） and Pseudocyclammina liasica Hottinger （K1-18, 21）. The fossils Amijiella amiji and Hanrania deserta might range from Late Sinemurian to Bajocian/Bathonian in age, but they appear frequently in the Middle Liassic. It is well-known that Orbitopsella praecursor , Lituolipora termieri , Pseudocyclammina liasica and Labyrinthina compressa are all typical fossils of Middle Liassic stage. Thus, the age of the above larger foraminifera undoubtedly belongs to the Middle Liassic stage. In the 5264th section the larger foraminifera are found only in the lowermost part of the section. They are Amijiella amiji （Henson）, Haurania deserta Henson and Socotraina serpentina Banner et al. in the sample N-2, and Ammobaculites tibeticus sp. nov. and Amijiella amiji （Henson） in the sample N-4. No typical Middle Liassic foraminifera were found in the sample N-2 and N-4. In Yeman and the United Arab Emirates Socotraina serpentina was found in Upper Liassic （Toarcian） and associated with Amijiella amiji and Haurania deserta. The larger foraminifera from 5264th section are identical to those of Yemen and United Arab Emirates. The age of them also should belong to Late Liassic stage （Toarcian）. Abundant smaller foraminifera were also found in the W61ong setion. They are Glomospira tingriensis sp. nov. , G. sinensis Ho, G. wǒlongensis sp. nov. , Glomospirella minuscula sp. nov. , Vidalina martana Farinacci, V. tingriensis sp. nov., Ophthalmidium elegantum sp. nov. and O. rotula Lalicher. They appear in the both lower and upper parts of the section except O. rotula, which appears only in the upper part of the section. G. sinensis was known in the Triassic Jialinjiang Limestone of Sichuan, China. V. martana appeared in the Middle and Upper Triassic and Lower Jurassic from Italia, Carpathians, Caucasus and Turkey. Only O. rotula appeared in the Middle Jurassic of Montana, U. S. A. The smaller foraminifera listed above, as a whole, can not indicate a precise age. On the basis of the larger foraminifera in the lower part, the age of upper part of the Wǒlong section may be refered to Middle Jurassic. In the 5264th section of Nyalam County abundant smaller foraminifera were found as well. They are Glomospira tingriensis sp. nov., G. sinensis Ho, G. wǒlongensis sp. nov., Glomospirella minuscula sp. nov. and Vidalina tingriensis sp. nov. The smaller foraminifera are identical to those of the Wǒlong section. The age of upper part of the section thus may also belong to Middle Jurassic. Jadoul et al. （1998） called the strata of Wǒlong section the Kioto Limestone, which is a Jurassic unit of Nepal. They divided the strata into the lower and upper parts as K1 and K2 respectively. In the K1, larger foraminifers Orbitopsella praecursor （ Gtimbel） , Cyclorbitopsella tibetica Cherchi, Schroeder et Zhang, Haurania deserta Henson, Mesoenthyra sp. and Pseudocyclarnrnina cf. liasica Hottinger were found. The age of K1 was dated as Middle Liassic. The appearance of above fossils is identical to that in our section. The foraminifera from the lowermost part of stratotype of Pupuga Formation were described by Ho et al. （1976） as Orbitopsella praecursor （JSB 63 ）. Cherchi etal. （1983, 1984） collected the samples from the same stratum of stratotype and believed that the foraminifera should represent a new genus and species. Therefore, Cherchi et al. established Cyclorbitopsella tibetica Cherchi, Schroeder et Zhang gen. et sp. nov. and refered the most parts of Orbitopsella praecursor described by Ho et al. （1976） to the new taxon. At present we have examined all samples of O. praecursor described by Ho et al. and found that all should be refered to Cyclorbitopsella tibetica. It means that Orbitopsella praecursor does not exist in the lowermost part of Pupuga Formation. Originally the age of Cyclorbitopsella tibetica was dated as probably Pliensbachian by Cherchi et al. Now our opinion is that Cyclorbitopsella tibetica found in the lowermost part of Pupuga Formation should be refered to Hettangian in age. Our reason is that above Cyclorbitopsella （JSB 63） there are ammonites Schlo- theirnia sp. （JSB 68）, Gleviceras sp. and Sulciferites sp. （JSB 69）. Whose ages are Hettangian and Sinemurian respectively. The C. tibetica associated with O. praecursor in the W61ong section should belong to Middle Liassic. Based on Banner et al. （1997）, C. tibetica associated with Socotraina serpentina in Yemen and United Arab Emirates is Late Liassic in age. It is clear that C. tibetica can exist in Lower, Middle and Upper stages ofLiassic. This means the Wǒlong section is lacking of lower part of Liassic series, because the Wǒlong section directly overlies the sandstone of Triassic. As for 5264th section the upper part of Liassic was found. It is not clear whether the middle and lower parts of Liassic exist or not, because no samples were collected below the section.