| Tertiary is a significant period to global climate which changes from the cretaceous"greenhouse" into "icehouse" in Eocene.The research on the evolution of the Tertiary climate is of great significance to understanding the evolution of Earth's climate and environment.The research on the Tertiary climate change in Jiuxi basin is crucial for understanding the climate change of the past Tibet plateau northern and Asian inland.Hexi Corridor is located in the center of the Asian inland,where is an ideal region for researching the climate changing process of the East Asia continent after the India-Asia collision.This paper choose Shulehe Formation sediments(284m in thick,its magnetic stratigraphy is about 13.26-6.20Ma)of the Hongliuxia section which has been already strictly controlled by high resolution magnetostragraphy,based on observing sedimentary sequence and the measurement of magnetic susceptibility,combined with rock magnetic analysis,in order to explore the climate change in the Middle to Late Miocene.We obtain the following results:1.High resolution magnetic susceptibility curve is obtained.The susceptibility values are totally lower,?L is(1.37-28.8)×10-8m3/kg,its average is 11.94×10-8m3/kg,?H is(1.25?28.5)×10-8m3/kg,its average is 11.71×10-8gm3/kg,?(?)is(0?0.91)×10-8m3/kg,its average is 0.24×10-8m3/kg,mudstone(?L,the same below)is 12.88×10-8m3/kg,and is highest,the lowest is pebby sandstone which is 8.09×10-8m3/kg,the value of the sandstone is consistent with marl,respectively are 10.30×10-8m3/kg,10.37×10-8m3/kg.Red and brown is high that is 12.89×10-8m3/kg,green,yellow and gray is 10.72×10-8m3/kg which is lower.shallow lake deposition and lacustrine-delta facies are high,respectively are 14.92×10-8m3/kg,13.94×10-8m3/kg,semi-deep lake facies is 9.77×10-8m3/kg that is lowest,fluviatile facies and lakeshore facies are lower,respectively are 11.59×10-8m3/kg,10.06×10-8m3/kg.2.There are both magnetite in the lithology,marl and mudstone also contain a certain amount of hematite and maghematite,sandstone contains more hematite and smaller maghematite.All sedimentary facies contain different content of magnetite hematite and maghematite,semi-deep lake facies mainly on magnetite,lacustrine-delta facies primarly magnetite and maghematite,lakeshore facies,fluviatile facies and shallow lake deposition mainly on magnetite and hematite.in this statum,lower segment mianly magnetite-oriented,central and upper section both mainly on magnetite and hematite.3.It can be classified into three segments at 9.99Ma,7.48Ma:I(13.26-9.99Ma),? is complicated change and fluctuation,its average is 11.01×10-8m3/kg,mainly magnetite-oriented,green is its major color;corresponding to semi-deep lake facies,lacustrine-delta facies and lakeshore facies;?(9.99?7.48Ma),?increased slowly and fluctuated bigger,its average is 11.57×10-8m3/kg?mainly on magnetite and hematite,the red is its major color,corresponding to fluviatile facies;?(7.48-6.20Ma),? fluctuated smaller,its average is 14.92×10-8m3/kg,and the highest profile section,red is its lithology major color,corresponding to fluviatile facies and shallow lake deposition.4.Based on the comprehensive analysis,the Middle to Late Tertiary climate recorded in Shulehe group sediments is mainly humid and warm,it is still developing to arid.the evolution process could be divided into three stages:the relatively ws sub-humid stage(13.26?11.18Ma),the relatively warm and humid stage(11.18-8.64Ma),and the sub-drought and sub-humid climate occurs alternately(8.64-6.20Ma),the sub-drought stage during(8.64-8.49Ma)and(6.93-6.64Ma),the sub-humid stage during(8.49?6.93Ma)and(6.64-6.20Ma).Climate during(13.26?8.20Ma)is prominently correspond to the change of the global climate,it is affected smaller at 8.20?6.20Ma,especially during(8.00?7.13Ma)and(6.86?6.20Ma),it is concerned with the uplift of the western and middle part of North Qilian Mountain since 8.2Ma. |
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