Sedimentary characteristics of the Upper Triassic in western Sichuan foreland basin
Jiang Zaixing Tian Jijun Chen Guiju Li Xizhe Zhang Manlang
1 China University of Geosciences (Beijing) Beijing100083
2 Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou510640 Guangdong
3 Langfang Branch Research Institute of Exploration & Development PetroChina， Langfang065007, Hebei
The Upper Triassic in the western Sichuan foreland basin can be divided into 4 third order sequences and 12 system tracts. The palaeogeographical setting，provenance，sedimentary rock types and sedimentary facies distribution in different periods have been studied on the basis of the sequence stratigraphic framework. Four areas including Longmenshan oldland，MicangshanDabashan oldland，Kangdian oldland，Jiangnan oldland comprise the detrital provenance areas for the Upper Triassic in the western Sichuan foreland basin. Eight sedimentary facies are developed, ieestuarine，alluvial fan，meandering fluvial，braided fluvial，fan delta，braided fluvial delta，meandering fluvial delta，and lacustrine facies. During the deposition of sequence I, the basin was transferred from marinecontinental alternating facies to continental facies with a filling and reconstruction character. Estuarine facies and tidal flat facies were developed in this period. The unified inland basin was formed during the deposition of sequence Ⅱ，and its transgression was the most extensive during the deposition of Xujiahe Formation. Deltaic and lacustrine facies were developed during this period. During the deposition of sequence Ⅲ, abundant sediments were provided by the MicangshanDabashan mountains. Alluvial fan，braided fluvial，and braided fluvial delta were well developed along the MicangshanDabashan mountains thrust frontal zones. During the deposition of sequence Ⅳ, influenced by the Jiangnan oldland, deltaic and lacustrine facies were developed. The distribution of sedimentary facies in the western Sichuan foreland basin is mainly controlled by tectonics，and the facies are distributed paralleling to the structural belt. The coarsegrained alluvial fan, fan delta and lake floor fan were developed along the northwestern steep slope belt and the braided fluvial delta was developed along the southwestern steep slope belt, the northernnortheastern steep slope belt, while the meandering fluvial and meandering fluvial delta were developed in the southeastern ramp slope belt.
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