Magnetostratigraphy Of The Shulehe Formation In Jiuxi Basin And The Tectonic Uplift Of Western North-Qilian Mountain During Miocene

The uplift of Qinghai-Tibetan Plateau has a significant impact to East Asia and the global climate and environment,but there are still controversies about the uplift process and mechanisms.Located in the north margin of the plateau,North Qilian Mountain is an ideal area to understand the uplift process,particularly its piedmont basin-Hexi Corridor,which has thick sediments of Late Mesozoic to Cenozoic that records important information about the northern mountain uplift since India-Asia collision.In recent years,a growing number of studies suggest that the Neogene uplift of Qinghai-Ti-betan is an important period,but there is still no clear understanding about the details of Neogene tectonic uplift for the lack tecto-sedimentary record of high-precision age constrained.Aiming at this problem,this paper collected the magnetostratigraphic samples from the Neogene Sulehe formation on the Hongliuxia section in the Jiuxi basin,which is in the western end of Hexi Corridor and controlled by Altyn Tagh fault and North Qilian fault,and obtained the magnetostratigraphic dating.Tied with the analysis on the sedimentary environment,the uplift process was discussed.The main conclusions are as followings.1)The strata of Shulehe formation on the Hongliuxia section in Jiuxi basin is 284m thick,which is composed of the mudstone-siltstone-sandstone composition,and has a small amount of conglomerates and calcareous mudstone,and they are fine-grained.The magnetostratigraphy results manifest that the section contains 32 positive polarity events and 32 negative polarity events,corresponding to the CSAAr to C3An.ln of 2012 standard Geomagnetic Polarity Time Scale,and the age of the magnetic strata is 13.76?6.20 Ma.2)Rock magnetic measurements show that the magnetic minerals are magnetite,maghemite,hematite and goethite.Wherein the celadon mudstone and marl contains magnetite and goethite,and the latter also has a small amount of maghemite.Celadon sandstone,brown mudstone,light maroon mudstone,orange calcareous mudstone,brownish red silty mudstone and light taupe calcareous argillaceous siltstone contain magnetite and maghemite,the latter four lithologies also contain hematite.The characteristic remanence magnetization is carried by magnetite and hematite(red-hued rock).3)Shulehe formation of Hongliuxia section can be divided into three segments from the bottom up.The lower segment from 23m to 107m is celadon mudstone and tawny celadon mudstone,which mostly outputs by interbed and belongs to semi-deep lake face sediment.The middle segment from 107m to 246m is light tawny siltstone,fine sandstone,sandstone and maroon muddy sandstone,which belongs to lake delta facies and lakeside facies sediment.Upper to 170m the sandstone layer becomes thicker and more and oblique bedding develops,which belongs to fluvial facies deposition.The upper segment from 246m to 307m is interb of tawny thick shale with thin sandstone,and is shallow lake facies deposit.4)The magnetostratigraphic dating of the Hongliuxia section indicates that the Jiuxi basin began to develop in the mid-Miocene(13.76 Ma),and Qilian Mountain began to uplift at this time.Sedinentary facies and deposition rate manifest that the evolution of the Jiuxi basin can be divided into three stages:during 13.76 and 11.2 Ma the basin rapidly subsiding stage,the sedimentation rate is relatively large and the change amplitude is big,indicating that on the northwestern basin the Alkyn Tagh fault actives strongly and Qilian Mountain fast uplift.During 11.2 and 8.2 Ma the sedimentary basin stable phase,the sedimentation rate is relatively small,Qilian Mountain is uplifting in the stable phase.After 8.2 Ma the Jiuxi basin was rapidfy sinking,the sedimentation rate is gradually fast increased,indicating that Qilian Mountain was uplifting fiercely again.