Characteristics And Evolution Processes Of Subsurface Erosional Landforms In Yuanmou Dry-hot Valley

The study area is located in subtropical climate zone, prevailing southwestmonsoon from Indian Ocean in rainy season and dominated by tropical continentalair mass in dry seasons. The enclosed topography leads to significant foehn effectwhich contributes to a famous hot dam with a hot dry climate. The investigation isconducted in rectangular Yuanmou dry-hot valley with a rift and deposit basinsurrounded by mountains and influenced by a north-south great fault. Stratumexposed in Yuanmou County are dominated by Mesozoic Red Stratum acrossMid-Yunnan Area and Cenozoic Stratum attributed to lacustrine, alluvium sediments.Cenozoic stratum is loose in structure and poor in adhesion with the feature of easydisintegration and erosion. Additionally, they have poor water capacity so that it isprone to cause large amount of water and soil loss. Gullies and earth forestlandforms are widely distributed in the study area, so the surface has been cut andfragmented accompanied by serious soil erosion and vulnerable eco-geologicalenvironment. Typical subsurface erosional landforms (SELs) in study area arestudied, and especially on sink hole, soil bridge, shaft and scoop hole. Depending ona variety of methods including geology, geomorphology, morphology and statistics.the tectonic background, topography, morphological characteristics, impact factorsof development and their evolution processes of the landforms were assessed andanalyzed. Furthermore, morphological database of typical SELs is established andthe role of SELs on the evolution of regional landforms as well as the damagemechanism on eco-geology environment is identified. The highlights andachievements of this dissertation are as follows:（1）Basin on the basis of previous research, the topography and tectonicbackground of Yuanmou are analyzed. The northward moving of Indian platecollided with Eurasian plate, which made Qinghai-Tibet Plateau uplifting, and whichlead Yunnan-Guizhou Plateau to the intermittently uplift. The formation andevolution of Yuanmou basin are exactly the responses to each tectonic movement inthe uplifting process for Qinghai-Tibet Plateau. As a result, these geological eventsformed the outline of basin today at the end of Yuanmou Movement and provided the basis of geological conditions for the evolution of SELs. The evolution processed ofthis basin are divided into four stages, namely uplift-erosion, formation anddeposition, lake water discharge, and modern geomorphology development.（2）According to the characteristics of Yuanmou SELs, they are categorized bythe dynamic system cause, evolution direction and spatial pattern, respectively.Taking into account of dynamic system cause, the SELs can be classified intowater-induced, gravity-induced and compound types. Among them, water-inducedtype includes shaft, falling cave and pipeline; sink hole is representative ofgravity-induced type; compound type involves earth cavity, soil bridge, soil pillarand soil wall. From the view of evolution direction, the SELs are summarized astransverse type (horizontal erosional pipeline, washing hole, earth cavity and soilbridge), vertical type (sink hole, vertical hole, dish-shaped cave, falling cave, soilpillar and soil wall) and oblique one (non-horizontal erosional pipelines). As forspatial pattern, they are divided into isolated type and aggregative type.（3）It comes to conclusion that the evolution of Yuanmou SELs should beattributed to common effect both internal and external factors. The properties of soilprofiles are the material basis of the landforms. Neotectonic movement is the forceof landform development. Meanwhile, local meteorological, hydrological andvegetation conditions change development process. In addition, local eco-geologyenvironment change caused by human activities indirectly intensifies the spreadingand development of SELs.（4）Based on continuous field investigation, distribution characteristics of SELsare summarized and we concluded that Yuanmou Formation of Quaternary isregarded as the main stratum of subsurface erosional development. Most SELs areclosely related to the distribution of gully and spread across the units of negativelandform. Physiognomy Morphological characteristics for four typical SELs unitsare quantitatively investigated and inherent mechanism of subsurface erosion isexpounded. Physiognomy Morphological characteristics database of subsurfaceerosion is established, multi-level researches on Shape-Quantitative-Theory arecomprehensively carried, and the development characteristics and mechanism oftypical SELs are understood.(5) Combined the traditional theory of landforms development with themorphology and mechanism of different development stages for typical SELs unitsin Yuanmou, the development process of SELs are divided into infant, young, mature,and old sequence. Correlation between SEL and SEL as well as between SEL and gullies is found. Depending on graphics expression, We vividly displayed theevolution processes of SELs, the mode of stratum damage and the impact on theformation of regional geomorphic caused by SELs. It is concluded that the evolutionof SELs is one of the most factors resulting in regional soil erosion andeco-geological environment deterioration.