Neogene Magnetostratigraphy Of The Erdaoquan Section In The Danghe Basin(Gansu) And Uplift Of Western Qilian Mt.

A college of Cenozoic sedimentary basins developled in the Qilian mountains and its surrounding areas and filled with extremely thick fluvial and lacustrine sediments,which records important information of geomorphology and drainage systems evolution of surrounding mountains and basins in the Qilian mountain.It is an ideal areas for studying regional tectonics,geomorphological evolution,and understanding the effects of tectonic uplift and climate environment after India-Asia continental collision.Most of the existing works are concentrated in the Hexi Corridor,Qaidam basin and other basins surrounding of the Qilian Mountain.The data of intermountain basin is still lacking,which is mainly manifested in the lack of high precision stratigraphic chronological sequences and sedimentary environment evolution data.In this study,combined with the stratigraphic and depositional environment data,a systematic magnetostratigraphy study is carried out on the Erdaoquan section of the Danghe Basin in the hinterland of the Qilian Mountains.The magnetic stratigraphic age of the Neogene Shulehe Formation of the section is determined,and the stratigraphic deposition and rock magnetic data are obtained,which provides basic data for understanding the tectonic evolution and mountain uplift of the Qilian Intermountain Basin.The following results are obtained in this paper:1)The Erdaoquan section in Danghe Basin is composed of the top of Baiyanghe Formation,Shulehe Formation and the bottom of Yumen Formation,which formed from two cycles of thinning to thickening from the bottom up.Shallow lacustrine,alluvial fan,fluvial and diluvial fan facies are developed from bottom to top.Gravel measurements and rock magnetic experiments show that the profile sediments originated from the Yemanan Mountain in the north.The stratigraphic-sedimentologic data indicate that there was a river with increasing magnitude and hydrodynamic energy,flowing from north to south in the Danghe Basin during the deposition of the Shulehe Formation.The modern Danghe was formed after the middle period of the Yumen Conglomerate deposit,and the current direction has changed from north-south to north-west.2)The rocks magnetic experiments and thermal demagnetization experiments show that magnetite and hematite are the main magnetic minerals in sediments of Erdaoquan section and they are the characteristic remanent carrying minerals.There is also a small amount of goethite in the samples.The relative content of hematite is higher than that of magnetite and it is a detrital material transported in the source region.The hematite content in the yellow-toned coarse-grained sandstone is significantly higher than that in the red-toned mudstone.3)In Erdaoquan section,we distinguished 21 normal magnetic chrons and 20 reverse magnetic chrons,which marked as N1～N21 and R1～R20.The sedimentary age of Shulehe Formation spans from 15.7 Ma to 4.6 Ma.The age of the bottom of the Yumen Conglomerate Formation spans from 4.6 Ma to 4.2 Ma.4)The comparisons of the intermountain and piedmont of the Qilian Mountains based on sedimentary characteristics show that the Qilian Mountain had the characteristics of stage overall uplift during the Miocene.Due to the intense tectonic activity,complex topography,and changing climatic environment in the Qilian Mountains,there are varying degrees of variation between the upper and lower limits of strata depositional ages in the intermountain and piedmont basins in the same period.The depositional bottom age of the Shulehe Formation in the southern Danghe Basin is 15.63 Ma,which is approximately 2 Ma later than that in the northern Hexi Corridor(18–17 Ma).The depositional age of the Yumen Formation is basically the same,ranging from 5～4.5 Ma.Generally,there are four relatively consistent periods of strong uplift activity,resulting in the overall strong uplift of the western Qilian Mountains around 16 Ma,14 Ma,8 Ma and 5 Ma.