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内蒙古锡林郭勒草原沙质荒漠化的沙源及其地质学成因分析

Analysis on Sand Source and Its Geological Origin of Desertification in Xilinguole Grassland, Inner Mongolia

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【Author in Chinese】 吴昕

【Supervisor】 李江风

【Author's Information】 中国地质大学, 土地资源管理, 2018, 博士

【Subtitle】以吉尔嘎郎图凹陷小草原为例

【Abstract in Chinese】 锡林郭勒草原是经济欠发达的“资源型地区”,长期以来依靠矿产资源开发来实现资金积累、经济发展和民生改善。近年来由于矿产资源的开发、利用过度,以及全球气候变化、天气渐趋干旱,这里的草原很快出现了严重的环境破坏和生态失衡状况,甚至引起了生态环境灾难——土地荒沙质荒漠化或沙化。为了查明锡林郭勒草原中、北部沙质荒漠化的潜在危险和沙源,提供其标本兼治的科学依据,本学位文结合国家专项研究基金资助项目,借鉴了中国土地荒漠化和草原沙化大量研究的丰硕成果,针对开展了相关研究。研究中确立了从地质视角出发,把野外宏观沉积学分析、沉积物颗粒粒度分析与盆地构造-充填演化历史分析结合起来的技术路线,完成了如下研究工作量:(1)分析、评述了国内外荒漠化的发展趋势和研究现状,特别是着重分析、评述了从地质学角度揭示土地荒漠化内在原因的研究现状、存在问题和发展趋势;(2)建立了一种以“风蚀坑-沙丘组合”为标志的草原沙化程度可视化简易宏观评估方法,并对浑善达克沙地和锡林郭勒草原南部9.6万km2范围的沙化程度进行了快速评估;(3)结合遥感信息,在典型凹陷中开展了草原退化状况的野外实地考察,并对32个点位的第四系全新统剖面、更新统剖面和下白垩统剖面,开展了露头宏观沉积学分析;(4)根据露天煤矿、第四系露头剖面和钻孔岩心资料,对吉尔嘎郎图凹陷赛汉塔拉组顶部沉积相进行分析和编图,研究赛汉塔拉组上段沙体的分布;(5)进行第四系全新统栗钙土微相的精准取样和粒度分析,以及微相的粒度参数模型构建,并开展了有初始点的动态聚类分析;(6)利用大量的煤炭地质勘探和油气地质勘探的钻孔岩心资料,以及地震勘探资料,进行赛汉塔拉组的构造地层格架分析、沉积相和沉积环境分析;(7)开展了研究区第四系更新统沙层(Qp3)与下伏赛汉塔拉组上段沙体(K1bs)的接触关系、盆地构造-沉积演化对草原沙化的约束与控制,以及草原沙化的内在根源和地质背景分析。所取得的基本成果可归纳为5点:(1)确认在吉尔嘎郎图凹陷内,草原极重度退化和重度退化现象出现锡林河两岸河滩和平地草原,中度退化的区域是凹陷内的斜坡小草地;草原沙化的潜在沙源是草原之下第四系和赛汉塔拉组顶部的巨厚松散沙层。(2)提出了一种以“风蚀坑-沙丘组合”形态和空间结构为标志的草原沙化程度可视化简易宏观评估方法,并利用该评估模型和标准图谱,实现对浑善达克沙地和锡林郭勒草原南部的沙化程度及沙化阶段的快速评估。(3)确认全新统栗钙土形成于现代河流环境中,并建立了栗钙土的6种微相的粒度参数模型(包括:河床滞留、河床边滩、天然堤、决口扇、河漫滩和洪泛洼地等),采用聚类法快速而高效地实现了未知微相归类。(4)查明赛汉塔拉组顶部巨厚松散沙层形成于早白垩世晚期河流-三角洲环境,是二连盆地基底减速沉降晚期的平原化阶段产物,之所以呈现未固结状态是因为区域构造反转和持续隆升,盆地各凹陷生命周期完整。(5)论证了二连盆地各凹陷表层的巨量松散沙层,是锡林郭勒草原沙漠化的潜在沙源,进而指出草原沙漠化的本质是一种地质灾害,其内在原因是地质环境的脆弱性,而地质环境的脆弱性决定了生态环境的脆弱性。通过研究还获知,由于二连盆地演化的生命周期完整,导致湖退体系域的河流-三角洲沙体在各凹陷中广泛分布,而随后的构造反转和缓慢隆升,则使这些沙体出露地表而未能固结成岩。在这种情况下,如果不对草原退化进行有效遏制和合理治理,则二连盆地群的50多个凹陷所蕴藏的巨量松散砂质碎屑,足以使整个锡林郭勒大草原大规模沙漠化。因此,从某种意义上说,锡林郭勒草原的沙化是一种地质灾害。这种类型的沙化在中国华北、西北和东北乃至世界其他地区,具有具有普遍性。本项研究成果,可以为深入认识这种类型的草原沙化的地质成因,以及寻找标本兼治的途径提供参考科学依据显然,为了巩固现有草原沙化治理效果,并且从根本上解决问题,一方面应当更加珍惜和保护这些平地小草原,另一方面需要在坚持并强化各种治理行动的同时,从改善地质环境脆弱性的角度出发,把临时性治理与永久性治理结合起来,对症下药以求标本兼治。为了便于对比研究,本文把经历完整构造-沉积演化生命周期的盆地上发育的草原,即完整地经历过初始沉降、快速沉降、减慢沉降和反转沉降阶段的中新生代盆地之上发育的草原,称为“锡林格勒型草原”,而把这种类型的草原沙漠化称为“锡林格勒型沙漠化”。综上所述,本文的主要创新点可归纳为:(1)以典型凹陷为例,从沉积地质学视角出发,把野外宏观沉积学分析、沉积物粒度分析与盆地构造-沉积演化历史分析结合起来,论证了锡林郭勒草原沙漠化的沙源,是下白垩统上部赛汉塔拉组的巨厚松散沙体。(2)在宏观沉积学研究基础上,进行第四系微相精准取样和粒度分析,建立了6种微相的粒度参数模型并实现动态聚类;利用“风蚀坑-沙丘组合”遥感形态及空间格局图谱,建立了沙漠化程度宏观标志评估模型。(3)提出了“锡林郭勒型草原”和“锡林郭勒草原型沙漠化”的概念,论证了该类型沙漠化本质上是地质灾害,凹陷地质环境的脆弱性决定了草原生态环境的脆弱性,而巨量沙源是盆地沉积作用对构造演化的响应。

【Abstract】 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.

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