The discovery of Silurian Yuanbaoshan Formation graptolite in Wulanshangde area of Alxa Left Banner, Inner Mongolia, and its geological significance
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摘要:
阿拉善盟阿拉善左旗杭乌拉山以北哈日奥布日格断凹陷盆地两侧，北东向出露770km2的哑地层，前人将其划为二叠系双堡塘组，但该套地层的岩性和岩石组合及层序特征，与内蒙古岩石地层所述的双堡塘组的现在定义和地质特征及区域变化完全相悖。采用现代沉积学理论和层序地层学方法，开展岩石组合、基本层序、沉积构造与沉积环境研究，划分出3个段级填图单元。根据发现的笔石化石和被晚泥盆世中粗粒黑云母花岗闪长岩侵入的关系，将地层时代确定为早志留世，改划为志留系圆包山组。泥盆纪区域变质变形作用使其发生浅变质和片理化变形，并形成轴向北东的复式向斜构造。后期叠加构造变形，形成紧闭倒转褶皱。志留系圆包山组的确定对本区地层划分和区域构造演化研究有重要意义。
Abstract:A puzzling stratum is identified as Permian Shuangbaotang Formation, which is outcropped in NE direction for 770km2 on both sides of Hariaoribuge sag in northern Hangwula Mountain of Alxa Left Banner. However, the summarized lithology, rock association and sequence characteristics of strata is in contradiction with the definition, geological characteristics and regional change of Shuangbaotang Formation named by Inner Mongolian lithostratigraphic researchers. In this study, 1:50, 000 regional mineral geological survey adopted modern sedimentology theory and sequence stratigraphy working method to study rock association, basic sequence, sedimentary structure and sedimentary environment. The authors divided the strata into three lithologic section-level geological units. According to the first discovery of graptolite and the invading contact of biotite granodiorites in the Late Devonian (U-Pb/379±2.9Ma), the strata age should be Early Silurian and it was changed into Silurian Yuanbaoshan Formation.The Devonian regional deformation and metamorphic process produced low-grade metamorphism and schistositilization deformation and formed NE-trending compound syncline structure. Then it experienced superimposition ofstructural deformation and formed closed-reverse fold. The establishment of Silurian Yuanbaoshan Formation plays an important role in stratigraphic subdivision and regional tectonic evolution study.
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Key words:
- stratigraphic subdivision /
- rock association /
- graptolite /
- Yuanbaoshan Formation /
- Alxa League
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