Analysis on Sand Source and Its Geological Origin of Desertification in Xilinguole Grassland, Inner Mongolia--以吉尔嘎郎图凹陷小草原为例
The Xilinguole Grasslands is the less developed "resource-based regions",which have long relied on exploitation of mineral resources to implement the capital accumulation,economic development and people’s livelihood improvement.In recent years due to the development and utilization of mineral resources and global climate change,the grasslands in this area soon experienced serious environmental damage and ecological imbalances,and even caused ecological disasters—sandy desertifieation.In order to identify the potential hazards and the sand sources of desertification in the northern and central Xilinguole Grassland,and provide scientific basis for its treatment,this dissertation has carried out related research based on the national special research fund projects and referring to the results of a large number of studies on land desertification and grassland desertification in China.In this research,a technical route that combines the field macroscopic sedimentology analysis,grain size analysis of sediments,and historical analysis of basin structure-filling evolution from a geological perspective has been established to complete following research works:(1)Both.domestic and foreign research status and development trends of desertification were analyzed and reviewed,with particular emphasis on analysis and review of the current status,existing problems,and development trends of the intrinsic causes of desertification from a geological perspective.(2)A simple visualized macroscopic assessment method to evaluate the degree of grassland desertification marked by "wind erosion pit-dune combination" was established,and a rapid assessment of the desertification degrees in the 96,000 km2 area in the southern Hunshandake and Xilinguole grasslands has been conducted.(3)Based on remote sensing information,field observations of grassland degradation were conducted in typical depressions,and macroscopic sedimentary analysis of outcrop was carried out on 32 sites located in Quaternary Holocene section,Pleistocene section and Lower Cretaceous section.(4)Based on data collected from open-pit coal mines,Quaternary outcrop profiles and borehole cores,the sedimentary facies at the top of Saihantala Formation in Jiergalangtu Depression were analyzed and mapped to study the distribution of the sand bodies of Upper Saihantala Formation.(5)Accurate sampling and particle size analysis on the microfacies of the Quaternary Holocene chestnut soil were conducted,the particle size parameter model of the microfacies was constructed,and dynamic clustering analysis with initial points was carried out.(6)By utilizing a large number of drilling core data from coal geological exploration,petroleum and gas exploration and seismic exploration data,the analyses of the structural stratigraphic framework,sedimentary facies,and sedimentary environment of the Saihantala Formation were conducted.(7)The contact relationship between the Quaternary Pleistocene sand layer(Qp3)and the upper Saihantala sandstone body(K1bs)in the study area,the constrains and controls of basin tectonic-sedimentary evolution to grassland desertification were studied,and the intrinsic causes and geological background of grassland desertification were analyzed.The research results can be summarized as the following 5 points:(1)It was confirmed that in Jiergalangtu depression,extremely severe and severe grassland degradation appeared on the river beach grassland and the plain-grassland among the banks of the Xilin River,and moderate degradation appeared on sloped grassland;The potential sand source comes from the thick and loose sand layer at the top of the Quaternary and Saihantala groups layed under the grasslands.(2)A simple macroscopic assessment method for visualizing the degree of grassland desertification marked by the form and spatial structure of ‘wind erosion pit-dune combination’ is established.By using this assessment method and the standard atlas,a rapid assessment on the degree and stage of desertification in the southern Hunshandake and Xilinguole grasslands was conducted.(3)The Holocene chestnutite was proved to be formed in a modern river environment.,and a particle size parameter model of six kinds of microfacies of chestnut soil was established(including: riverbed retention,point bar,natural levee,crevasse splay,flood-plain and backswamp).The unknown microfacies can be classified quickly and efficiently throuth the clustering method.(4)The ultra-thick and loose sand layer at the top of the Saihantala Formation was proved to be formed in the river-delta environment in the late Early Cretaceous.It was the product of the base decelerating subsidence of the Erlian Basin.Its unconsolidated state was due to regional tectonic inversion and continuing uplift.The life cycle of each depressions in the Erlian Basin is complete.(5)The massive loose sand layer on the top of each depressions in the Erlian Basin was demonstrated to be a potential sand source for the desertification of the Xilinguole grassland.Further more,it was proved that the nature of grassland desertification is a kind of geological disaster.The internal reason is the fragility of the geological environment,and the geological environment vulnerability determines the vulnerability of the ecological environment.It is also known from this study that due to the complete life cycle of the evolution of the Erlian Basin,the river-delta sandstone bodies in the Retreat System are widely distributed in the depressions,and subsequent tectonic inversions and slow uplifts made these sandstone bodies exposed to the surface and could not be consolidated into diagenetic bodies.Under such circumstances,if the grassland degradation can not be contained effectively and managed properly,the huge amount of loose sandy debris spreaded over more than 50 depressions in the Erlian Basin is enough to cause a largescale desertification of the entire Xilinguole grassland.Therefore,in a sense,the desertification of the Xilinguole grassland is a geological disaster.This type of desertification is universal in North,Northwest and Northeast China and even some other regions in the world.The results of this study can provide a reference for deep understanding of the geological origin of this type of grassland desertification,as well as methoes to tackle the problems.Obviously,in order to consolidate the results of grassland desertification control and fundamentally solve the problems,it is necessary to cherish and protect these small plains on one hand.On the other hand,from the perspective of improving the fragile geological environment,it is necessary to combine temporary treatment with permanent treatment and solve the problem by treating both root causes and symptoms.In order to facilitate the comparative study,the grasslands that developed in basins that have undergone a complete tectonic-sedimentary evolution life cycle(ie,the Mesozoic and Cenozoic basins that have undergone initial sedimentation,rapid sedimentation,slowed sedimentation,and inversion sedimentation),are named as ‘Xilingole-type grassland’,and desertification of this type of grassland is called ‘Xilingole-type desertification’ in this dissatation.In summary,the main innovations of this dissertation can be summarized as below:(1)Taking typical depression as an example,from the perspective of sedimentary geology,this dissertation combined the macro sedimentological study,micro grain size parameter analysis,and basin tectonic-sedimentary evolution history analysis,demonstrated that the massive loose sand bodies of the Saihantala Formation of the upper Lower Cretaceous are the sandy sources of Xilinguole grassland desertification(2)On the basis of macro-sedimentology research,accurate sampling and particle size analysis on the Quaternary microfacies were carried out,and six kinds of microphase particle size parameters models were generated and dynamic clustering was established.An evaluation model for the degree of desertification macro marks was established by utilizing the remote sensing morphological and spatial pattern maps of ‘wind erosion pit-dune combination’.(3)The concepts of ‘Xilinguole-type grassland’ and ‘Xilinguole-type desertification’ were proposed,and the nature of this type of desertification was demonstrated to be geological disasters.The fragility of the geological environment of the depression determines the fragility of the grassland ecological environment,and the massive sand source is the response of basin sedimentation to tectonic evolution.