SANDSTONE COMPOSITIONS AND PALEOGEOGRAPHIC EVOLUTION OF THE UPPER TRIASSIC XUJIAHE FORMATION IN THE WESTERN SICHUAN BASIN, CHINA
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摘要: 以四川盆地西部上三叠统须家河组的砂岩组分为基础,详细分析该期研究区古地理的演化。在现有分层的基础上,按照Gazzi-Dickinson计点法,对研究区上三叠统须家河组的砂岩薄片进行统计、分析,并根据Dickinson等人的研究思路,对各段砂岩作以QmFLt, QpLvLs为端元成分的砂岩三角投影图。结果表明:在QmFLt图上,砂岩投点从须二段开始落于过渡再旋回和岩屑再旋回分区上,须三、须四段投点结果更加明显;在QpLvLs图上,砂岩投点也是从须二段开始落于褶皱—逆掩带分区上的,到须三、须四段投点结果也更加明显。结合研究区的沉积相特征,分析表明:龙门山逆冲带在须二期开始了逆冲推覆,并开始为研究区提供了沉积物质,研究区也从须二期开始演化形成川西前陆盆地;须三期,龙门山逆冲推覆体进一步强烈的运动,并开始成为研究区的主要物源区;须四期,受安县运动的影响,龙门山逆冲推覆体褶皱成山,研究区真正进入了以陆相沉积环境为主的演化期。Abstract: 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.
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