Study Of Yellow River Terraces At Mijiashan Area Using Joint Methods Of LiDAR And Dating

The yellow river terraces at Mijiashan area are located in Jingtai county of Gansu province, as an important tectonic region where Yellow River and the Haiyuan fault intersected. Mijiashan area also is also one of the regions with many terrace levels along the Yellow River. It’s important to study the characteristics of the terraces and their ages systematacially, for understanding the relationship between the evolution of terraces and tectonic structure、climate as well as the relationship with the activity of Haiyuan fault. In this thesis, I studied the terraces by field survey, scanning with terrestrial LiDAR, cosmogenic dating of gravels and Optically Stimulated Luminescence(OSL) dating of loess samples, to get the high-resolution topophology and ages of the terraces.The terraces at Mijiashan are classified into 14 levels, and we measured the height of terraces above the yellow river. LiDAR with high precision and efficiency are deployed to scan the topography of the terraces, and we got millions of points data.Points cloud are filtered, classified and combined to extract more accurate terrace elevation data.We sampled the gravel on the surfaces of terraces where fluvial sediments are preserved respectively, and also collected depth profile samples on 3 representative terraces, for cosmogenic nuclides dating. OSL samples are taken in two terraces covered by thick loess. Meanwhile, loess samples are collected in profiles to measure the particle size and magnetisability. Pretreatment and testing work are done for all the samples.So far we have got the OSL ages and parameters of particles from the loess, but the cosmogenic nuclides dating is still in progress. The analyses of the changes of loess parameters show the boundary between eolian loess and river loess, where the ages of terrace can be predicted. Combining the elevation data with the ages,we getthe tectonic rate preliminarily: 150ka-70 ka, T8 terrace loess deposition rate is about5.16 cm/ka; 205ka-150 ka, terrace rised at the rate of 2.48 m/ka; Since 150 ka, terrace rose at the rate of 2.48 m/ka.Due to different survey lines, terrace elevation measurement and classification have great difference each other in previous studies. So it is necessary to carry out high-resolution LiDAR scanning. Our LiDAR data is of the highest resolution in Mijiashan area. Most of previous dating of terraces were indirect or even just qualitative, rather than direct dating. More comprehensive understanding of Mijiashan yellow river terraces will be achieved after all the cosmogenic dating results are obtained.