| The formation of the deserts in the Asian inland is considered as the direct results of the extremely drought climate,and the history of the formation and evolution of the deserts could provide some evidences for understanding the process of the arid climate development in the Asian inland.Among these deserts,the proximal deserts of the Loess Plateau are important sources of the aeolian materials supplied to the Loess Plateau,and thus play important roles in the formation of the Loess Plateau.As one of important proximal deserts,the Hobq(Kubuqi)Desert is located on the south of the Hetao Basin.Due to its special geographical,geomorphologic and topographic conditions,the formation of the Hobq Desert is not only controlled by the enhanced aridification of the Asian inland,but also affected by the paleoenvironmental variation of the Hetao Basin as well as the activity of the Yellow River within the Hetao Basin.Reconstructing the paleoenvironmental history of Hetao Basin is important not only for revealing the evolution history of the Yellow River,but also for understanding the formation of the Hobq Desert.In this thesis,we present the stratigraphic framework of drill core DR01,and carry out a systematic study on the sedimentology and chronology of drill core WEDP05,form central of the Hetao Basin.The chronological framework of the core WEDP05 was established by the methods of ESR and magnetostratigraphy,and its sedimentary facies were determined on the basis of visual observation of the lithologic characteristics,paleontological evidence,grain morphology analysis(SEM)and grain-size analysis.Furthermore,the paleoenvironmental and paleoclimatic evolution history of the Hetao Basin was reconstructed by multi-environmental proxy indicators,including color index,magnetic susceptibility,grain size,carbonate carbon and oxygen isotopes,organic carbon and nitrogen isotopes,the total organic matter content and total nitrogen content.Meanwhile,the lithological and chronological results of core DR01 and the other cores in the basin were also employed to reconstruct the Cenozoic paleoenvironment of the basin.Finally,the formation age and the development model of the Hobq Desert were discussed;the activity of the Yellow River within the Hetao Basin and its important role in evolution of the paleoenvieonment of the Hetao Basin and formation of the Hobq Desert were also discussed.The conclusions were obtained as follows:1.Stratigraphic sequence of core DR01,from bottom to top,consists of following parts:the lower Cretaceous strata(2503.18-2125.45 m)are composed of reddish-brown clay interbedded with grey clay in the lower part;and reddish-brown clay and silty clay interbedded with yellowish sand and violet clay layers in the upper part;the late Neogene strata(2125.45-1834.55 m)consist of light-brown clay,silty clay and pink,yellowish sand interbedded with occasional layers of violet clay and silty clay,and which unconformably overlay the lower Cretaceous strata;the Quaternary strata(1834.55-0 m)consist of yellowish sand interbedded with reddish and brown clay and silty clay layers,some occasional greyish and green-greyish clay and silty clay layers in the strata are typical lacustrine sediments.2.Sedimentary facies of the core WEDP05 were determined as following:From 0-11.3 m,the sediments are fluvial and aeolian deposits,coarse fluvial sands dominate the uppermost 5.5 m of the strata;From 11.3-87.51 m,the sediments were dominated by fine lacustrine clay and silty clay;From 87.51-141.63 m,the sediments contain aeolian sands and lacustrine deposits;From 141.63-183.35 m,the sediments consist primarily of lacustrine deposits interbedded with aeolian sand layer;From 183.35-268.14 m,the sediments are composed of aeolian sands interbedded with a lacustrine silty clay layer with the thickness of 30 m;From 268.14-274.60 m,the sediments are mainly lacustrine deposits.3.The B/M boundary of the strata in core WEDP05 is at the depth of 143.41 m,and the age of the bottom of the core is estimated to be approximately 1.68 Ma.4.Hetao area was uplifted and eroded during the early Cenozoic,and then subsided to form a sedimentary basin at least since the Pliocene.During the Pliocene,a fluvio-lacustrine environment formed within the basin.During the Quaternary,the basin was experienced alternating desert and fluvio-lacustrine conditions.The Hetao Basin was subjected to several alternately process of lake and desert environment expansion.A fluvio-lacustrine system developed in the basin during the following intervals:?1.47-?1.30 Ma,?1.17-?1.07 Ma,?0.68-?0.60 Ma and from?0.47 Ma to the last interglacial.In these periods,the expansion of lake in the basin was accompanied by the contraction of the Hobq Desert,and the climate of the basin was relative humid;A desert landscape expanded within the basin during the stages of lake regression:?1.60-?1.47 Ma,?1.30-?1.17 Ma,?1.07-?0.68 Ma,?0.60-?0.47 Ma and the last glacial,and the climate was relative arid.The significant expansion of a lake environment within the basin may be occurred at?0.47 Ma,?0.3 Ma,and the last interglacial,when the Hobq Desert regressed remarkably.During the last glacial and Holocene,the lake shrinked and died out,the Hobq Desert significant expanded and the large sand dunes developed in the desert to form modern landscape pattern.5.The origin of the Hobq Desert can be traced back to the early Pleistocene and formed by at least?1.65 Ma.The sediment materials of the desert are derived from the erosion and transportation of exposed fluvio-lacustrine deposits within the Hetao Basin by near-surface winds associated with the Asian winter monsoon.The suff-icient sediment supply by the activity of the Yellow River within the basin is one of the great causes of desert development.On the other hand,the vanishing of the paleolake within the basin and regional arid climate may also have made important contributions.6.The channel of the Yellow River within the Hetao Basin may have formed since the Pliocene,which was occupied by the Yellow River when its upper reaches formed by at least?1.6 Ma.Subsequently,at least since?1.2 Ma,the Yellow River flowed through the Jinshan Gorge,and connected the Hetao Basin with the Fenwei Basin,the Sanmen Gorge and the Kouma area to completely establish its current drainage pattern.The remarkable uplift of the basin outlet caused by the strong differential tectonic movement is responsible for the development of lake within the basin after the formation of the Yellow River,and a relative humid climate may also have made an important contribution.In addition,the downcutting of the Yellow River is the main reason for the shrinking and desiccation of the paleolake within the Hetao Basin. |
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