Study On Formation Mechanism And Vulnerability Of Qiaoba-Zhongdu Danxia Landform In Southwest Sichuan

The Danxia landform research has gone through initial stage,formation stage,and development stage since this concept was first proposed by Mr.Feng Jinglan.The related disciplinary research has become increasingly mature.As an important red basin in southwestern China,the Sichuan Basin is a very representative place for the study of Danxia landform.Numerous researches have been carried out in this area,including distribution of Danxia landform,morphological description of geomorphic landscape and evaluation of tourism resources,whereas the researches on rock foundation,lithofacies palaeogeography,landscape mechanism and evolution model of Danxia landform are lacking.Based on this,as supported by the projects of “Comprehensive Survey of Geology and Mineral Resources in Wumeng Mountain-1:50000,four Regional Geological Survey Projects of Muchuan County(H48E019008),Mabian County(H48E019007),Dianlanba(H48E020007)and Xinshi Town(H48E020008)and “Study on the Protection of Typical Geological Relics and the Enhancement of Function of Global Tourism Landscapes in the Western Sichuan Plateau”,this thesis selects the Qiaoba-zhongdu Danxia landform at the junction of Leshan and Yibin in southwest Sichuan as the target of investigation.The development types and spatial distribution of Danxia landscape were analyzed by using remote sensing image interpretation,rock ore identification,grain size analysis,geochemical analysis and seismic profile interpretation,in combination with the theories of sedimentology,paleoclimatology,tectonic dynamics and surface process dynamics.Then a five-stage landscape model of “Breeding scene-Deying scene-Landscaping design-Carving scene-Embellishing scene”of Danxia landform was put forward,and the vulnerability assessment of Danxia landscape was carried out in the study area.Finally,on the basis of vulnerability assessment and oriented by protection and development of the landscape,the protection and development strategies of Danxia landscape in the study area were put forward,the main results and conclusions are as follows:(1)Based on remote sensing data color,mountain shadow,terrain slope,terrain relief,and interpretation of slope shadow pattern,combined with field investigation and three-dimensional geomorphic model analysis,the Qiaoba-zhongdu Danxia landform is found to be rich in landscape types,which include ring-cliff Danxia,waterfall,pothole,stone wall,peristele and various cave landscapes.These landscapes are mainly exposed in areas with large slope and undulation,and distributed along water system,fault zone and near “right angle” geomorphic turning part.The Hypsometric Integral(abbreviated as HI)value of Danxia landform in the study area is 0.43,indicating that the Danxia landform is in the morphogenetic period of geomorphology evolution.(2)In accordance with rock ore identification,grain size analysis and field investigation on the Danxia red beds,the Cretaceous Woshantou formation deposited by meandering river and braided river is developed in the Qiaoba-zhongdu Danxia landform,the lithology of which is brick-red feldspathic quartz sandstone intercalated with argillaceous fine siltstone and sandy mudstone.The sandstones are medium-fine and fine-grained in structure,with grain size of 0.1-0.3 mm and poor to medium sortability and roundness.The cementation type of sandstone is primarily calcareous cementation,followed by iron and argillaceous cementation.The difference of lithology provides material foundation for the landscape formation of Danxia landform.(3)The sedimentary environment of the Wotoushan formation in the study area belongs to arid semi-arid climate according to the geochemical analysis of major,trace and rare earth elements,and the component analysis of clastic rocks.The climate was relatively warm in the early stage,and gradually became dry in the later stage,during which the extreme drought events were occurred.The sedimentary medium is fresh water sediment under oxidation environment,and the sediment source is from Songpan-Ganzi area.(4)The tectonic evolution process of the study area is studied based on structural interpretation and seismic profile interpretation.During the Yanshan period,an NNW-SSE syncline structure caused by SSW-NNE stress compression was formed in Dianlanba-Zhonghezhuang.Subsequently,it converted to NNW-SEE stress extrusion during the Himalayan period,and the strike of the eastern segment of the syncline changed to nearly NW.Thus,the syncline is of arc-shaped structure with nearly “V” shape in the north-south longitudinal section and nearly “right angle” in the plane,which establishing the landscape pattern of the Danxia landform.(5)Through field investigation and theoretical analysis,we found that weathering,water erosion,and collapse are important external forces for the formation of Danxia landscape,whereas human activities also have an important impact on Danxia landscape.In the whole process of landscape formation,the Danxia landform experiences a five-stage evolution process of “Breeding scene by sedimentation,Dyeing scene by palaeoenvironment,Landscaping design by tectonism,Carving scene by surface agents,and Embellishing scene by human activities”.(6)The macro presentations of vulnerability of the Danxia landscape are mainly cliff fracture,dangerous rock and unstable slope,collapse and landslide,debris flow disaster and so on.The results show that the main factors affecting landscape vulnerability include topography,vegetation coverage,rainfall,temperature,air quality,lithologic conditions,structural conditions,human activities,etc.The primary vulnerable model of the Danxia landform can be divided into five types as follows: gravity collapse type,water erosion type,weathering and spalling type,structure control type and lithology control type.Meanwhile,referring to previous studies,the vulnerability assessment model of is established and then the evaluation on Danxia landform is conducted.The evaluation result is 63 points,indicating this area is in moderate vulnerability.On this foundation,three development suggestions and six protection countermeasures of the Danxia landscape are put forward based on regional resource endowment and development status.