Trends And Controls Of Late Quaternary Sedimentation Rates And Channel Change In The Hetao Plain

The Inner Mongolian reaches of the Yellow River are characterized by desert environments which extend along either side of the river channel,as well as by high sedimentary loads,by severe sedimentary siltation,and by extensive river channel migration,forming a typical wide-valley desert and alluvial river channel.In the past50 years,the relationship between water discharge and sedimentary loads in this section has been changed significantly,and the riverbed has risen markedly,forming a?hanging river?.Potential flooding disasters have threatened the safety of people‘s life and property in the region seriously,and caused substantial social,economical,and environmental problems.The researches of the sediment composition,sedimentary characteristics and the process of the river channel change in the Hetao Plain,not only provide the data support and historical background for the integrated study of the erosion and siltation changes of the wide-valley desert reach and regulation measures,but also play an important role in the implementation of regional hydrographic engineering and reasonable development of water resources in the Yellow River.In this study,the physical and chemical characteristics of sediment and sedimentary facies were analyzed based on the grain size parameters and the core structure characteristics of fourteen borehole cores along the cross section of the Yellow River in Dengkou and twelve borehole cores along the longitudinal section of the river channel.The chronological framework of boreholes was established by OSL and ¹⁴C dating methods.Furthermore,based on sedimentological analysis and chronological framework,we reconstructed the temporal and spatial characteristics of Late Quaternary sedimentary rates in the Hetao Basin and the change of the river channel.Finally,the response mechanism of the river system to external environmental factors was discussed combined with regional tectonics,climate and human activity data.Some important findings and conclusions are draw as follows:(1)The sedimentary sequence of the twenty-six cores is dominated by gray-green,taupe fine sand,medium and coarse sand,with loose structure and high water content,and some of the sand layers contain clay blocks and rounded gravel.The brownish yellow,reddish brown clay and silty clay layers are only distributed in the six(DKZ01-06)boreholes in the northern margin of the basin near the Langshan Mountain,and the thickness of each layer is not more than 7 m.Distribution characteristics of borehole sediments indicate that the area is dominated by fluvial sedimentary environments.(2)Sediment grain size parameters can provide important information concerning depositional processes and environments because the size-range,mixing,and sorting of sediment populations vary systematically in relation to sedimentary processes,dynamics and provenance.Thus,the analysis of textural parameters is the basis for determining the process-response characteristics of individual sediment units and their environment.The grain-size analysis shows that the surface sedimentary environment in Hetao Plain can be distinguished by characteristic of grain-size distribution.The desert and fluvial channel sediments are dominated by sand components.The sand component of the former accounts for more than 95%,which consists of obvious coarse-grained jumping components,containing a small amount of rolling components,and the sorting is better.The channel sediments that the sand accounts for more than 70%and the silt accounts for 30%,are mainly carried by gradient suspension,and the sorting is medium.The sediments of the floodplain are mainly occupied by silt(more than 50%),and the suspended components account for50%,which is mainly suspended and transported,and the sorting is poor.Lake sediments are mainly silt and clay,and silt sand accounts for 50⁷5%,clay accounts for 25⁵0%,suspended components account for more than 80%,main hydrostatic sedimentation have a poor sorting and negative skewness.In addition to the significant differences of particle size composition,sorting,and handling methods,although the particle size characteristics of different environmental sediments overlap,the boundaries of point groups are significantly different.Furthermore,it is applied to the division of the core sedimentary facies in the area.It is found that the sedimetary facies is mainly dominated by fluvial channel facies,and several of the thicker channel deposits indicate that the river has experienced many sedimentary cycles and the river channel had frequent migration.(3)The partial bleaching of the luminescence signal prior to deposition results in age overestimation,and can be a problem in delineating fluvial evolution within an OSL chronological framework.To test the incomplete bleaching occurring in this type of environment,residual doses and the luminescence signal characteristics of different particle size fractions from 14 modern fluvial sediment samples were analyzed.Besides,26 OSL ages derived from drilling cores were compared with 11 radiocarbon ages.Our results show that the equivalent residual doses principally range between0.16 and 0.49 Gy for silt grains,and between 0.35 and 3.72 Gy for sand grains of modern samples.This suggests that medium-grained quartz has been well bleached prior to deposition,and is preferable to coarse-grained quartz when dating fluvial sediments in this region.The results also show that the De values of coarse-grained fractions display a better correlation with distance downstream.In addition,the comparison with OSL and radiocarbon ages from drilling cores establishes further confidence that any initial bleaching of these sediments was sufficient.As a result,we believe that the fluvial samples were well bleached prior to deposition.Thus,a total of75 OSL and 11 ¹⁴C results were obtained,with an age distribution between 0.6 and156 ka.According to the sedimentation rate,the bottom boundary of the borehole show a obvious difference and the distribution range is between 2.1 and 160 ka,which reflects the complex river channel change process and frequent erosion and siltation changes in this region.(4)Estimation of sedimentation rates is useful in the reconstruction of sedimentary environments as well as the study of fluvial dynamics under the alluvial plain.The sedimentation rates at different periods from the sedimentary drilling of the Dengkou section show that the sedimentation rates tend to be stable during the period of 80¹8 ka,with very low values,among which is 0.11 mm/yr during 80³0 ka,0.16mm/yr during 30²0 ka and 0.32 mm/yr during 20¹8 ka.The sedimentation rates are higher overall and large variation range since 18 ka,among which is within the range of 0.67¹.35 mm/yr during 18⁸ ka,and the sedimentation rates(0.52⁰.68 mm/yr)are stable from 8 ka.Combined with regional climate data,it is found that the change of the sedimentation rate since 20 ka is controlled by discharge and sediment supply from the upper reaches of the river which is affected by the climate change.In cold and dry climates,the discharge is small and the river schlep capacity is weak,it is impossible to effectively transport the sediments of the upstream mountain section to the downstream plain area,therefore,affected by the decreasing discharge and sediment supply,the rate is lower accordingly.Under relatively warm and humid climatic conditions,the river discharge increases,and a large amount of weathering materials accumulated in the river channel in the upstream mountainous area are transported to the plain downstream area for deposition,so the sedimentation rate is large.The average sedimentation rate of the Hetao Plain obtained from age-depth of the borehole is exponentially along with the decrease of the age,reflecting the acceleration process of the plain.And the sedimentation rates have increased significantly in the past 2 ka years,and they may be more controlled by the increase of human activities that lead to a significant increase in desertification.Under warm and humid conditions,the discharge increases,the weathered materials accumulated in the mountainous areas during the glacial period entered the river and were carried out of the mountain,which causes the sedimentation rate of the plain increased.(5)Based on the temporal and spatial distribution of the sedimentary facies of the borehole core,the river process of the Dengkou section since the late Pleistocene was reconstructed.It was found that,during the period of¹60⁸0 ka,the rivers wandered in the form of multiple rivers and migrated frequently.During the period of 80¹8 ka,the river activity was weakened,the rivers were relatively stable,and the sedimentation rates were low.During the period of 18⁸ ka,the river migrated to the relatively low-lying Langshan Mountain,and in the western plain near the Langshan Mountain,it migrated and received sediments in the form of multiple rivers,and the sedimentation rates increased and the river activity was strong.Since 8 ka,the main river channel has changed from the northwestern part of the plain to the current river channel,which may be related to the migration of the center of gravity of the Langshan piedmont fault zone to the interior of the basin and the expansion of the Ulan Buh desert,but there are still rivers in the western part of the plain.Due to the migration of the river,the rivers and lakes gradually formed,and after about 2 ka,the northern Ulan Buh desert landscapes appeared.