古地形对黄土区岩土侵蚀趋势的控制作用

Control of ancient landform on rock-soil erosion in loess area

利用高精度的GPS(卫星全球定位系统)结合RS(遥感)与GIS(地理信息系统)重建了陕北府谷黄土区一个小流域的侏罗纪(J)、第三纪(N)时期古地形数字模型(DTM),与该区现代小流域地形和土壤侵蚀的现状进行了分析对比,发现府谷黄土区经历了古准平面-古湖泊洼地-高原沟谷三大地貌发育过程,而且该小流域现代地形与侏罗纪、第三纪古侵蚀面之间具有很高的继承性,可见古地形在陕北黄土地区的岩土侵蚀过程中有着重要的控制作用,研究结果可以更好地为水土保持提供科学依据。

The soil erosion in the loess area of China is very serious.To study the developing tendency of rock-soil erosion in the loess area helps the control and treatment.The digital model of ancient landform in Jurassic and Tertiary in a small river basin in Fugu County of Shaanxi Province,China,has been established with the application of the high-precision GPS,RS and GIS.The elevation of the top of Jurassic and Tertiary strata as well as the elevation of modern landform has been measured with the grid density of 1 000 m × 1 000 m,the analysis and comparison have been made for the soil erosion evolution with the remote sensing images processed by the TM satellite data.It has been found that the loess area in Fugu County has gone through three geomorphologic developing processes,namely,ancient peneplane-ancient lake lowland-plateau gully.Each geomorphologic process developed based on the previous geomorphologic process.The rising of Jurassic strata led to the missing of Cretaceous strata.In the late Cretaceous,the Jurassic strata in the study area were submerged with lake water because of the tectonic movement.The Tertiary deposited with a considerable thickness and the sediments were lacustrine.However,the Tertiary argillaceous sediments were not in the same lacustrine environment,they deposited in the lowest ancient land and gradually upwards in the formation process of mudstone sedimentation,which can be verified by the densely-developed,lateral,continuously-distributed calcareous concrete beds.After the Tertiary sedimentation,the large faults occurred in the late Tertiary Himalaya movement.Some faults followed the faults in the late Yanshan movement and the basalt eruption occurred before the laterite accumulation.The graben of the Yellow River was formed by the influence of the Himalaya movement.Influenced by the tectonic movement,the whole area rose to form the smooth ground surface,which was composed of purplish-red mudstone with densely-developed Tertiary calcareous concrete beds.The deposit thickness was uneven,the maximum occurred in the lowest ancient land and the minimum occurred in the highest land.Due to the influence of the tectonic movement,the relatively ＂smooth land＂ was changed,the easily-changed section should be ＂tectonic fragile belt＂.It would consequentially change the overlying Tertiary mudstone,which was most influenced by the tectonic movement and the weathering of the mudstone was relatively high,so it was easily eroded to form small striae-small gullies-big gullies,resulting in the change of the level ground surface.The rock-soil erosion occurred because of various external forces,the Tertiary mudstone was also denuded.The mudstone in the gradually-formed landform was almost eroded out and washed away by the water flow because of large water catchment and strong water hydrodynamics,while part of mudstone remained in the relatively-high land because of the poor hydrodynamics and low rock-soil erosion.That was why the mudstone was rarely seen in the low land and easily seen in the high land.It was found from the results that the modern landform in the river basin had high inheritance of Jurassic and Tertiary erosion surface.Therefore,the ancient landform had important control on the rock-soil erosion in the loess area of northern Shaanxi area.The study results could provide the solid basis for the water-and-soil conservation.