THE CARBONIFEROUS AND PERMIAN BRACHIOPOD ASSEMBLAGE SEQUENCES FROM THE SOUTHWESTERN MARGIN OF THE TARIM BASIN, XINJIANG, NW CHINA

Marine Carboniferous Permian strata are widely distributed on the southwestern margin of the Tarim Basin. Those rocks are well exposed in simpler structures and bear rich and varied fossils including fusulinids, foraminifers, conodonts,corals and brachiopods etc. In recent years, thousands of brachiopod fossils were collected from the Damusi and Qipan sections of the Yecheng county, and the Keziliqiman, Tahaqi and Duwa sections of the Pishan county(Text fig.1). Those brachiopods are assigned roughly to 10 brachiopod assemblages. The correlation of brachiopod assemblages and other biozones of the researched areas is shown in table I(in Chinese). The marine Carboniferous Permian may be divided into 7 formations as follows: The Lower Carboniferous Series contains the Kelitag Formation below and the Heshilafu Formation above. The Upper Carboniferous Series comprises the Kalawuyi, Azigan and Tahaqi formations in ascending order. The marine Permian is divided into the Kziliqiman Formation below and the Qipan Formation above. The rocks of those formations spread over the studied areas. The brachiopod assemblages and ages of the above mentioned formations are introduced in ascending order as follows: The Kelitag Formation, distributed mainly in the Shache district, overlies conformably the Tiznap Formation of Upper Devonian. The brachiopod Cyrtospirifer Tenticospirifer assemblage, assigned to the Famennian (Late Devonian) in age, is yielded at the basal part of this formation, and the Martiniella chinglungensis assemblage, thought as the Late Tournaisian (Early Carbonife rous) in age, is recorded at its upper part, this assemblage is considered as the representative of the Tournaisian deposits of this area in this paper. The Heshilafu Formation contains three brachiopod assemblages in ascending order as follows: Marginatia voughai Rugosochonetes hardrensis ass., Pugilis hunanensis Vitiliproductus dewuensis ass. and Gigantoproductus edelburgensis Stratifera stratifer ass. The first two assemblages are referred to the Visean (Lower Carboniferous), and the third assemblage is considered as the Serpukhovian (equivalent to Early Namurian) in age. The Kalawuyi Formation includes the Productus productus Choristites gobicus assemblage below and Choristites Neospirifer assemblage above. The lower assemblage, associating with Pseudostaffella zone, is assigned to the Lower Bashkirian, and the upper assemblage, intergrowing with Profusulinella zone, is attributed to the Upper Bashikirian and the lower Substage of Moscovian. The Azgan Formation, marked by the Fusulina Fusulinella zone, containes few brachiopods, and its characters of brachiopod assemblage are vague. The Tahaqi Formation is divided into two parts: the lower part, characterized by the Triticites zone, records the brachiopod Meekella Enteletes assemblage, which is referred to the Kasimovian and Gzhelian in age; the upper part, containing Pesudoschwagerina zone assigned to Asselian and Sakmarian, yields few brachiopods. But no brachiopod assemblage can be established clearly. The Keziliqiman Formation yields the Chaoiella tenuireticulatus Juresania nebrascensis assemblage below and Orthotetina curata Liraplecta paojiangensis assemblage above. The lower assemblage, intergrowing with the topmost fusulinid Eoparafusulina zone of the southwestern margin of the Tarim Basin, is considered as Artinskian in age. This assemblage is also associated with conodont Neostreptognathodus spequopensis Sweetognathodus whitei Lonchodina festina zone. Owing to intergrowing with the same conodonts mentioned above, the upper brachiopod assemblage of the Keziliqiman Formation is also assigned to Artinskian in age. The Qipan Formation, the uppermost marine deposits of Carboniferous Permian in the researched areas, records abundant brachiopods, which are referred to the Acosarian indica Liraplecta aspera assemblage. Based on the brachiopod correlation of the Qipan Formation and that of other areas, this assemblage is considered as the Chi