1000-0569/2006/022（09）-2405-12       Acta Petrologica Sinica   岩石学报

Sea-level drops in the Permian-Triassic boundary section at Laolongdong, Chongqing, Sichuan Province

WU YaSheng¹, JIANG HongXia¹, LIAO TaiPing²

1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

2. Chongqing University of Science and Technology, Chongqing 400042, China

(Wu YS:   wys@mail.igcas.ac.cn)

Abstract     Restudy of the Permian-Triassic boundary section at Laolongdong, Chongqing, Sichuan Province, south China shows evidence for sea-level drops. This section comprises a basal Changhsingian calcisponge limestone of normal shallow-water condition. It is overlain by 10.5 m thick packstone of platform facies. The latter is covered by 1.4 m thick lime mudstone and packstone of restricted platform facies, which is characterized by speckled appearance. The top of this interval is an undulated ancient karstic surface. This surface is coated by a 1 mm thick layer of terrestrial clasts of quartz, feldspar, and clay minerals. The overlying 0.8 m thick packstone and lime mudstone has a characteristic dendroid appearance. This unit has a more remarkable erosional top surface. The height difference of the undulated surface at least reaches 0.3 m. The lentiform deposits in the lower areas of the surface are bedded grainstone composed of microgastropods cemented by fibrous calcite. The horizontal beddings of the grainstone are intersected with the erosional surface. The grainstone is overlain by a thin layer of packstone of small bivalves. The above rock is caesious tabular marly lime mudstone containing abundant small syndepositional pebbles 0.3~3 mm wide, which are internally recrystallined.

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Keywords: Permian-Triassic boundary; end-Permian; sea-level change; erosional surface; Sichuan

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