Study On The Formation Mechanism Of Old Red Sand In Pingtan Qingfeng

The old red sand widely exposes in Qingfeng,Pingtan,Fujian Province which has large thickness,four clear,stable and natural layer.Qingfeng old red sand profile is a complete preservation of typical profile which records the response of the temperatures,sea,monsoon variability during last interglacial in coastal sand.In this paper,OSL dating(Optically Stimulated Luminescence)will be used to determine the formation age of old red sand;grain size test will be used to analysis the winter monsoon,summer monsoon components in old red sand and rare earth elements will be used to detecte the material sources of the old red sand.Some conclusions from a number of experiments and tests as follow:(1)The data of the Optically Stimulated Luminescence dating Qingfeng profile QF four layer display:the formation age from down to up were 122.5±6.7 Ka.B.P.,97.7±6.1 Ka.B.P.,83.1±4.2Ka.B.P.,61.9±3.4Ka.B.P.,which mainly from in the last interglacial.(2)The grain size of Qingfeng old red sand presents log normal distribution,the peak value is about 300?m;the grain size of the winter wind sand and summer wind sand also presents log normal distribution,its peak value is respectively about 400?m and 280?m.The experimental results of the mixture which from the different proportion winter monsoon sand and summer monsoon sand show that:the winter monsoon and the summer monsoon sand sand mixed with 3:7 can fit the frequency distribution of the old red sand.This result explains that the duration or intensity of summer monsoon is far greater than the winter monsoon when the old red sand deposition,which at the climate warming period.(3)The determination of rare earth elements of Qingfeng old red sand,Minjiang estuary sand,Tannanwan beach sand in Pingtan show that:the Content value of various rare earth elements and the chondrite distribution curve of Qingfeng old red sand are very close to Minjiang estuary sand,their element ratio is less than 0.5;is very different from the Tannanwan beach sand,Qingfeng old red sand mainly from the Minjiang estuary sand.(4)The comprehensive analysis of many experiments and test results show that Qingfeng old red sand formed in high temperature,high sea level during the last interglacial.The summer monsoon is dominant and high rainfall at that time,large quantity of sediment was brought into the sea by Minjiang,froming a strong southward coastal current,then enter the Haitan strait.The sediments was transported to the Qingfeng coast by monsoon and deposited at there which containing iron mineral of river erosion virgin rock.The iron-bearing minerals occurred strong oxidation with oxygen and water under the condition of high temperature,generating red iron oxide and dyeing the deposited sand red,finally forming the old red sand.The study of the formation mechanism of the old red sand in Qingfeng Pingtan is a supplement and correct for a formation of the old red sand that predecessors thought that old red sand in Minyue mainly formed in the last glacial period,the low sea continental shelf sand and beach sand were carried by strong winter monsoon then depositied in the coastal formed the old red sand.The study of this mechanism helps to understand the environment and climate conditions of the formation of the late Quaternary special red sand in South China,reconstruct the change process of Asian monsoon during the last interglacial.