SANDSTONE COMPOSITIONS AND PALEOGEOGRAPHIC EVOLUTION OF THE UPPER TRIASSIC XUJIAHE FORMATION IN THE WESTERN SICHUAN BASIN, CHINA

Based on the sandstone compositions analysis of the Upper Triassic Xujiahe Formation in Western Sichuan Basin,the authors tried to discuss the paleogeographic evolution of the study area.According to the current stratigraphic division,sandstone-chips were counted and analyzed with the Gazzi-Dickinson point-counting method.And the QmFLt & the QpLvLs triangular diagrams were established from Dickinson's research ideas.The triangular QmFLt plot show that sandstone compositions began locating Transitional Recycled Provenance and Lithic Recycled Provenance in the second section of the Xujiahe Formation (T₃x²),and the results were more obvious in the third and fourth sections of the Xujiahe Formation (T₃x³ and T₃x⁴).The triangular QpLvLs plot show that sandstone compositions began locating Fold-Thrust Provenance in the second section (T₃x²),and the results were also more obvious in the third and fourth sections (T₃x³ and T₃x⁴).Combined with sedimentary facies of the Xujiahe Formation in the study area,it was indicated that Longmenshan thrust belt began uplifting and providing the sediment in the second section (T₃x²),and West Sichuan Foreland Basin began developing in the second section (T₃x²);In the third section (T₃x³),the Longmenshan thrust belt took place strongly uplifted and became the primary provenance area.Because of influence by the An'xian movement in the forth section (T₃x⁴),the Longmenshan thrust belt uplifted and became a mountain which really changed the study area into terrestrial sedimentary environment.