Mineral Magnetic Characteristics Of 2018 Flood Sediments In The Upper Reaches Of The Yellow River

As an abnormal mutation event,the alluvial sediments formed by flood are usually significantly different from other sediments in terms of sedimentary characteristics and physical and chemical indexes.To fully understand the sedimentary characteristics of modern flood is an important prerequisite and basis for identifying palaeoflood sediments and carrying out palaeoflood reconstruction.The Yellow River basin has always been one of the most flooded rivers because of its unique geology and landform.At present,the research on flood deposition characteristics in the upper reaches of the Yellow River is relatively weak.Based on this,this paper selects the natural dike sediments formed by the 2018 Yellow River flood near Lanzhou Sports Park,and reveals the response characteristics and mechanism of magnetic parameters to the flood process through systematic rock magnetic study and X-ray diffraction method.(1)The magnetic minerals in the flood season sediments of Lanzhou section of the Yellow River in 2018 include both low coercoerity magnetite and high coercoerity hematite/goethite;(2)Parameters such as magnetic susceptibility,saturated isothermal remanence,non-hysteretic remanence,S-ratio and L-ratio have no significant changes on the natural dike-section,indicating that the source area of flood sediment and the types and contents of magnetic minerals in Lanzhou section in 2018 have no significant changes.(3)The maximum axial deviation Angle of AMS ellipsoid in the lower part of the natural dike section(66-110cm)is concentrated(K1-Dec=22.8°±10.3°),while the maximum axial deviation Angle of AMS in the upper part of the dike section(0-64cm)is randomly distributed in the upper half plane.Magnetic mineral particle size index of the bottom sediment parameter of ?ARM/?,?ARM/SIRM and NRM strength is lower than the upper;The lower and upper indicative flood sediments were formed during the Yellow River No.2 and No.3 floods in 2018,respectively.Due to the abrupt rise and fall of water level and discharge in the No.2 flood process with short duration,the rise and fall of water level in the No.3 flood process was relatively slow and lasted for a long time.Cause the AMS characteristics of the two flood sediment,?ARM/?,?ARM/SIRM and NRM intensity difference;(4)The research in this paper reveals that the magnetic parameters of flood sediments can respond sensitively to flood periods with different hydrodynamic conditions and durations,thus having the potential to distinguish flood events with high resolution.