The discovery of Silurian Yuanbaoshan Formation graptolite in Wulanshangde area of Alxa Left Banner, Inner Mongolia, and its geological significance

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.