A lopingian (late permian) brachiopod fauna from the qubuerga formation at Shemgmi in the Mount Qomolangma region of southern Xizang (Tibet), China

Journal of Paleontology, Mar 2001 by Shen, Shuzhong, Archbold, N W, Shi, G R

ABSTRACT-A small fauna of 11 species belonging to 10 genera of Permian Brachiopoda from the lower part of the Qubuerga Formation outcropping near Shengmi village in the Qomolangma region of southern Xizang (Tibet) is figured and new taxa are described. New taxa are Quinquenella semiglobosa and Costatumulus shengmiensis. The fauna is most likely of Wuchiapingian (Djhulfian) age as indicated by the majority of the brachiopod species.

INTRODUCTION

THE INACCESSIBILITY of the area near Mt. Qomolangma (Mt. Everest) of Xizang (Tibet) has resulted in relatively little geological work being undertaken in the area prior to this study. Muir-Wood and Oakley (1941) first reported a small collection of poorly preserved fossils from the Mt. Qomolangma region, and confirmed the existence of the Permian in this region. In 1952, Li Pu completed a field survey over most areas of eastern Xizang and collected some Permian brachiopods near Mt. Qomolangma that were later described by Ding (1962) as Neospirifer kubeiensis Ding, N. tibetensis Ding, N. moosakhailensis (Davidson), Costiferina indica (Waagen), and Spiriferella salteri Tschernyschew.

In 1964, a scientific investigation team of the Chinese Academy of Sciences completed a field expedition in the Qomolangma region. Brachiopods collected by this team were described by Zhang and Jin (1976) and subsequently summarized by Jin (1985).

The brachiopod fauna described in the present paper was collected by the senior author in 1997 during a field expedition to the Mt. Qomolangma region about 430 km west from Lhasa, the capital of Xizang (Tibet). Tectonically, this region belongs to the Himalayan Terrane. The studied section is located on the northwestern slope of Mt. Jinong (5469.9 m above sea level), about 40 km north of Mt. Qomolangma. A small village (87'09'E, 28'34'N) called Shengmi is at the foot of Mt. Jinong (Fig. 1). Brachiopods have never been described before from this locality, but four brachiopod species, identified as Spiriferella rajah (Salter), Marginifera himalayensis (Diener), Martinia sp., and Cancrinella sp., associated with the late Changhsingian conodonts Clarkina changxingensis Wang and Wang, C. deflecta Wang and Wang and C. subcarinata (Sweet) in the Permian-Triassic boundary beds, were mentioned by Yu and Dai (1985) and Rao and Zhang (1985).

STRATIGRAPHY The Permian section in the Himalayan region of Xizang (Tibet) is commonly divided into two parts. The lower part of the section in the study area, named the Jilong Formation, consists of diamictites, sandstone, and siltstone (Yin and Guo, 1976) with "Stepanoviella" brachiopod fauna (Jin, 1979) of probable Sakmarian age. The upper part of the section overlying the Jilong Formation by fault contact has been subdivided into the Qubu and Qubuerga Formations in ascending order (Yin and Guo, 1976). The Qubu Formation is mainly composed of quartzose sandstone interbedded with black shale and silty shale, and contains a Glossopteris flora (Hsu, 1976) at the Qubu section about 30 km north of Mt. Qomolangma. This flora was regarded as early Late Permian (Wuchiapingian in the sense of Jin et al., 1997) in age, correlative with the Raniganj Beds in India (Hsii, 1976).

The Qubuerga Formation, lying above the Qubu Formation without obvious discordance in the Mt. Qomolangma region, is a sequence of siltstone interbedded with shale and bioclastic limestone containing abundant brachiopods, bryozoans, and corals. According to Yin and Guo (1976) and Yin (1997), three members have been recognized. The lower member is 118.3 m thick, and composed of brown sandstone, muddy siltstone, and shale. The middle member (120.5 m thick) is dominated by gray silty shale grading upwards to bioclastic siltstone interbedded with bioclastic limestone. The upper member, conformably overlain by the dolomitic limestone of the Lower Triassic Tulung Formation, is 86.3 m thick, and consists of broadly monotonous dark gray shale with bivalves, plant stems, and unidentifiable ammonoids.

Rao and Zhang (1985) named the Permian strata at Shengmi as the Baga Formation (the topmost 7 m of the Selong Formation) and the Menduo Formation (the remaining part of the Selong Formation) in descending order. These two formations are not significantly different in lithology from the previously-defined Qubuerga Formation, hence this nomenclature is not used herein.

The brachiopods described in this paper come from the lower member of the Qubuerga Formation outcropping about 1 km northeast of Shengmi village (Fig. 2). The strata are deformed. Most of the brachiopods are represented by isolated valves, and are poorly preserved with worn surfaces indicating that the shells were abraded during transportation prior to deposition.

DISCUSSION OF AGE

The Qubuerga Formation in the Qomolangma region is most likely of Wuchiapingian (Djhulfian) to Changhsingian in age. An apparently Wuchiapingian flora in the immediately underlying Qubu Formation (HsO, 1976) and the late Changhsingian conodont Clarkina changxingensis in the topmost beds of the Qubuerga Formation (Yu and Dai, 1985) constrain the age. Eleven brachiopod species, belonging to 10 genera, have been identified from the present collection (Table 1). Although of low diversity the fauna most likely indicates a correlation with the Wuchiapingian (Djhulfian) Stage. However, the possibility of an early Changhsingian age cannot be ruled out.