Study On Sediment Characteristics And Depositional Environment In Kumtagh Desert

Kumtag Desert is the 6th biggest desert in China.The specialized research for the desert had not been conducted until the beginning of 20th century due to the extremely arid climate and harsh natural condition.It is the last desert in China where studies were conducted.Till now,there's no systematic study on the characteristics of the sediments in the desert.Especially,studies on the formation and development of deserts as well as the ancient climate are scarce.The initial study found the natural stratigraphic section is widely distributed in Kumtag Desert,there are various sedimentation types,and there is much ancient wind-blown sand in the stratum.So the desert is an ideal place for the research on the formation and evolution of the desert in arid areas.Kumtag is located at the northern fringe of Tibet plateau,so its formation and development is closely related to the uplift of Tibet plateau.This study will provide the evidence for the uplift of Tibet plateau and the formation and evolution of the droughty environment in northwest China.Field study and observation were conducted.Samples of deposits on the surface of Kumtag Desert were collected,and samples of sediment strata on the on the typical stratigraphic section ZH on the up-reaches of Suosuo gully were also collected.Through the analysis of the sediments granularity,geochemistry,mineral composition and the climate,the particle size,geochemistry composition and distribution characteristics of the wind-blown sand in Kumtag Desert were illustrated.The analysis of sedimentation provenance further clarified the wind-blown sand provenance of Kumtagh Desert.Based on the sediment characteristics on the stratigraphic section,the ancient climate recording sequence,such as the sedimentation granularity,different substitute index of climate and geochemistry,were set up.The characteristics of ancient environment and ancient climate during the formation and development of Kumtagh Desert were disclosed.Following are the results of this study:1.The particle size of the aeolian sand in Kumtagh Desert varied between-1¹0?,but mostly between 1³?,that means the sand is mainly composed of medium-sized and fine sand,with average grain size of 2.25??0.21 mm?,It is coarser than that in Takelamagan Desert whereas finer than those in Badajara and Tenggeli Deserts.Pneumatic classification is not good?average?=0.65?.The sand becomes coarser from the southwest to northeast,which is the result of the long-term wind erosion and pneumatic classification of the prevailing northeastern and northerly winds.The provenance analysis showed the wind-blown sand comes through wind-carrying,alluvial-diluvial sediments and lacustrine sediments.So it is a short-distance carrying.2.The chemical composition of the sediments on the surface of Kumtagh desert is characterized by terrigenous intermediate rock,with Si and Al as the dominates.SiO₂ content is enriching at various degree besides the area in the east.CaO is enriching in the areas of southeast and south.Other dominant elements,such as Al,Fe,Mg,K and Na are deficient.The spatial distribution of the dominant elements is different.Si and K take a significant positive relation with latitude,whereas Fe,Ca and Mg take a significant negative relation with latitude.Most of the micro-elements are enriching,among which,W,Sn,Zn,Cu,Y,Ni,Pb,Ga,Co,As and U are enriching at a high degree.The spatial difference is the result of the different provenance environment.The analysis of SiO₂/Al₂O₃ shows the maturity degree of the sediments is increasing from the south to the north.3.The provenance evidence analysis based on sedimentology,geochemistry and mineral composition proved the sand in Kumtagh Desert is from local area,mainly from the rock weathering debris in Aerjin Mout.The eigenvalue of provenance identification Sr/Ba estimated the sediment provenance in the north is different from that in the south.The sand in the south is mainly from the underlying rushed fluvial sediments.It has experienced the erosion and pneumatic classification process of fluvial argillaceous fine sand?sandy silt?silty sand?fine sand?aeolian fine sand.The sand in the area to the north of 40°12?11?N is mainly from the sediments of fluvial and lacustrine facies along Aqike valley to Yumen Pass.The north part of the desert is low in terrian and is dominated by wind erosion.The alluvial sediments from Aerjin Mountain and the wind-carried fluvial-lacustrine sediments continuously invade here.These sediments are carried by wind southward to the south part of the desert.4.Taking into consideration of the analysis of OSL stratigraphic chronology on ZH section and the comparison of global climate change,we think the stratum of ZH section was formed since the end of Quaternary,and it had a good reaction recording to the ancient climate since the end of the ice age,the Yonger Dryas Event and Holocene Thermal Maximum.The position of 3845cm is the end line of the last sub-ice age,which indicated the beginning of the interglacial period,and the climate change from cold to mild.Based on the sediment environment and the climate recording sequence reflected by the sedimentation granularity,we classified the ancient climate in Kumtagh desert since the ZH section sedimentation into 6stages:cold and humid?cold and drought?mild and drought?mild and humild?warm and humid?mild and drought.5.According to the structure of sediment facies of the stratal section in Holocene in Kumtagh Desert and the distribution characters of aeolian sand in the section,we found the evolution model of desert is different spatially.The foot of the mountain in the south is characterized by the interbedding ofaeolian sand and alluvial gravel or flood alluvial sand,the middle is by the interbedding of aeolian sand and alluvial gravel and fine sand,the north is by the interbedding of aeolian sand and fluvial and lacustrine sand and clay.The desert initially formed from the southwest,then,it gradually developed.