Characteristics Of Surface Sediments Of The Alluvial Fan Gobi In Arid Area

Gobi is a kind of desert landscape of flat ground covered by gravel in the arid zone.The gobi in China was divided into two different types according to its origin,that is,the erosion gobi and the accumulation gobi.The accumulation gobi on the alluvial fans are lagre,more extensive,which is about 70%of total area.The alluvial fan is a common geomorphologic landscape and a buffer between mountains and plains.In addition,the alluvial fan contains rich resources,and alluvial plain connected by many alluvial fans is an important place for human activities.At present,the research foundation of China gobi was weak,the definition and classification are still in dispute,the formation process and evolution rule are still not clear,and the surface sediments are not well studied.At the same time,there are lack of related research data in some gobi.In this study,the alluvial fan gobi in the northern and sourthern pidemonts of Eastern Tianshan Mountain,the northern pidemont of Altun Mountain and the eastern pidemont of Langshan Mountain were studied.The physical geography and sedimentology was used to carry out ground investigation,sampling and analysis.The characteristic of gobi surficial gravel,grain-size of sediments,abundance of geochemical elements,and chemical weathering in different area were analyzed.The grain-size composition of sediments and distrubution on gobi surface of alluvial fan were revealed and the results can enrich and comply the basic data for gobi research,which can provide scientific basis for ecological protection and developmental utilization.The main results and conclusions indicated that:?1?A set of extraction methods on gravel characteristics based on the digital image processing technology was established.The process of extracting gravel characteristics included image preprocessing?format conversion,cropping,noise reduction,and radiation correction?,image classification?design decision tree,training samples statistics,and ensure segmentation threshold?,classification post-processing?clump,eliminate,and vectorization?,and information extraction of gravel particle,respectively.This technology has advantages of minor error?5.33%?and high accuracy?94.67%?,which can satisfy the requirement of experimental accuracy,and test results are reliable.?2?The average particle size of gravels ranged from 2.21 to 17.44 mm,the roundness of gravels ranged from 0.75 to 0.8,and the axial ratio of gravels ranged from 1.5 to 1.7,which showed that most of gravel shapes were sub-rounded,nearly elliptic or long strip.The correlation among the gravel size and roundness and axial ratio was not significant,while there was a negative relationship between roundness and axial ratio.The gravel coverage ranged from 10.90%-84.51%,which were mainly composed of middle coverage?40%-70%?,and accouted for about 71%.Due to the differences of sedimentary environment,climate and lithology of parent rock,there were differences in the roundness,axial ratio and coverage of gravels.The differences in size composition,roundness,axial ratio and coverage of gravels on the alluvial fan gobi mainly depended on its developing topgraphy position.Due to the differences of material source and dynamic condition of transportation and accumulation,the dominant particle size of gravel size composition from alluvial fan apex to alluvial fan margin has changed from extremely coarse particles or coarse particles to middle particles or fine particles.The roundness of gravel increased 0.002-0.006,the axial ratio decreased 0.012-0.017,and the coverage decreased 2-6 percentage points with the altitude decreased for 100 m.Because of the formation time,sedimentary environment,climatic environment,and gravel lithology varied,so there are some differences in topography characteristic parameters and coverage of gravels in three study areas.?3?The grain-size composition of sediments in different geomorphologic parts of the alluvial fan was obviously different due to the spatial difference in the transport dynamic conditions.The grain-size distribution curves of the surface sediments for alluvial fan gobi was unimodal,the peak grain-size was extreme coarse gravel or coarse gravel,and the grain-size frequency accumulation curve had two sections.The grain-size distribution curves of the underlying layers sediments were mostly bimodal or trimodal,the grain-size of the main peak was medium sand or fine sand,and the grain-size frequency accumulation curve had three sections.The provenance of sediments for alluvial fan gobi was relatively simple,the erosion material of the bedrock had been transported and accumulated under the action of gravity and hydraulic power.The extremely coarse particle and coarse particle in surface sediment decreased significantly and the content of middle gravel and fine gravel increased significantly,while there were significant change in silt clay content changed not significantly from apex to margin of alluvial fan.There were visible differences in the grain-size composition of surface sediments of alluvial fan gobi because of the difference of parent rocks lithology,length and gradient of alluvial fan.?4?The major oxide of surface sediments on the alluvial fan gobi was mainly Si O₂,then A1₂O₃.The content of Si O₂was nearly to UCC value without obvious enrichment or leaching.The content of Al₂O₃?Fe₂O₃?Na₂O?K₂O?P₂O₅was lower than that of UCC and showed different levels of leaching.The content of trace elements?Cl,Sr,Ba and Zr?was high,and the trace elements showed different degrees of leaching or enrichment except for Ba and Sr.The content of chemical elements in the sediments with different grain-size was different,and the trace elements mainly existed in silt and clay.The coarse particle?d>0.063 mm?of surface sediments were in-situ weahering production on the alluvial fan gobi,then the fine particle?d<0.063 mm?include the dust fall and in-situ weahering,but the dust fall existed only in local range and there were no particles though long distance and large scale transportation.The chemical weathering tendency of surface sediment on the alluvial fan gobi was uniform as UCC weathering tendency.They were no or weak continental weathering in the early stage,and lack of effective chemical weathering process.While due to the differences of sediment environment,climate environment and lithology of mother rock,the degree of elements loss and the chemical weathering degree of sufrace sediment on alluvial fan gobi in eastern side of Langshan was higher than that of another two research regions.