ESR Dating Of Channel Sand Deposited On The Er Long Hill By Ancient Fen River,Taiyuan,China

The elevation of the Ordovician limestone gorge export in Shanglan village part of Fen River is 820m,the River bank of this part is Erlong hill,the elevation of this hill is 980m,and there are sediments of paleo Fen river riverbed remained on the top of it,the thickness and width of the sediments is 40m and 210m respectively;the extraction of the temporal information of the paleo riverbed sediments is very useful for the quantitative description of the fractionation of Taiyuan west mountain and Jinzhong rift basin ridge.Recently,the main fluvial sediments dating methods include:FT,K-Ar,40Ar-399Ar,14C,TL,U-series,OSL,Paleomagnetism,Universe nuclide and ESR dating.But FT,K-Ar,40Ar-39Ar dating has relationship with thermal history of samples.Upper limit of 14C dating is only fifty thousands year.the upper age limit of both U-series dating and OSL dating are in five hundred thousands.Paleomagnetism dating needs continuous section to sample.ESR dating fill gaps in Quaternary dating from several hundreds thousand to several million years,it is applicable to quaternary sediments dating especially to river terrace deposits dating.Through field surveying and analyzing of ancient Fen River deposition section at Erlong hill,we confirmed the undercut rate of the Fen river gorge,and got conclusions as follow:1.Section characteristic:South part of the section consists of gravel layer with poor sorting in which 13 sand lens are intercalaried;the middle part is a gravel seam of 13-14m thick of poor sorting;the north part are gravel layer with poor sorting?silty clay layer?thick sand layer and sand-gravel layer.Main component of gravels is limestone?Huo mountain sandstone?purple-red porphyry with plagioclase phenocryst?gray-black porphyry with plagioclase phenocryst?quartz sandstone?diabase?gabrro?gray-white sandstone?felsic dikes?yellow-green sandstone?granite which is severe weathering?apricot yellow siltstone?monaonite which is severe weathering?granitic gneiss which is severe weathering?feldspar dikes and schist.Grain seize analysis show that distribution of grain seize of river sands are concentrating at ranges of 125?450 ?m or 105?300 ?m.Granularity distribution probability accumulation curve shows that the transportion of the analyzed sands are mainly saltation transportation,and the saltation parts are above 85 percent,and the saltation component and suspention component are above 95 percent.2.The results of ESR dating show that,the time when sediments deposited at Erlong hill ancient Fen river terrace is near 198.1 ±29.7ka to 225.2 ±42ka.3.The tectonic activity rate of Erlong hill block is 2.634×10-4(m/a)?3.263×10-4(m/a).4.The average trenching rate of Taiyuan west mountain Fen river gorge is 7.254×10-4(m/a)-9.813×10-4(m/a).