Sedimentary characteristics and internal architecture of a river-dominated delta controlled by autogenic process:implications from a flume tank experiment

Autogenic processes are widely found in various sedimentary systems and they play an important role in the depositional evolution and corresponding sedimentary architecture.However,autogenic processes are often affected by changing allogenic factors and are difficult to be identified and analyzed from modern and ancient records.Through the flume tank experiment under constant boundary conditions,the depositional process,evolution principles,and the sedimentary architecture of a river-dominated delta was presented,and a corresponding sedimentary architecture model was constructed.The evolution of river-dominated delta controlled only by autogenic process is obviously periodic,and each autogenic cycle can be divided into an initial progradational stage,a middle retrogratational stage,and a late aggradational–progradational stage.In the initial progradational stage,one feeder channel incised into the delta plain,mouth bar(s)was formed in front of the channel mouth,and small-scale crevasse splays were formed on the delta plain.In the middle retrogradational stage,the feeder channel was blocked by the mouth bar(s)which grew out of water at the end of the initial stage,and a set of large-scale distributary splay complexes were formed on the delta plain.These distributary splay complexes were retrogradationally overlapped due to the continuous migration of the bifurcation point of the feeder channel.In the late aggradational–progradational stage,the feeder channel branched into several radial distributary channels,overlapped distributary channels were formed on the delta plain,and terminal lobe complexes were formed at the end of distributary channels.The three sedimentary layers formed in the three stages constituted an autogenic succession.The experimental delta consisted of six autogenic depositional successions.Dynamic allocation of accommodation space and the following adaptive sediments filling were the two main driving factors of the autogenic evolution of deltas.

VEGETATION CHANGES OF THE TAIHANG MOUNTAINS SINCE THE LAST GLACIAL

The vegetation of the Taihang Mountains was dominated by grass with less than 10％ forested area during the last pleniglacial. During the period of 17 000－ 10 000 a B.P. the forested area reached to 20％ － 30％ , and reached to 20％ － 40％ during the Early Holocene in the period of 10 000－ 8 000 a B.P. followed by the temperature increasing rapidly. But the northern part had more forested area than that of the southern part. During the period of 8000－ 5000 a B.P. forests developed further to 50％ or more. It was the highest period of the forest cover degree. After then, the forests began to be destroyed first in the southern part of the Taihang Mountains by people. Since 2500 a B.P. the forests were destroyed deadly. The forest cover decreased to less than 5％ before 1949. Since 1949 , although the government hold the policy of close hillsides to facilitate afforestation, the forest cover of Taihang Mountains could not exceed 11.6％ . So afforestation is an important work to keep economic and ecological sustainable development.

Application of remote sensing in the description of fluvial system in dryland:A case study of Golmud distributive fluvial system,Qaidam Basin,NW China

Distributive fluvial systems(DFS)are widespread in drylands in the northwestern China.Analyzing differences in fluvial morphology in drylands is beneficial for studying ancient rock records and the extraterrestrial surface environment.The remote sensing image,characterized by real-time and possibility of repeated observations,is a vital tool for recording and comparing fluvial systems in drylands.Satellite remote sensing technology is a method of investigating fluvial morphologies.Due to the limited accuracy of satellite imagery,there are few reports on the detailed description of the fluvial system in drylands of NW China.We analyze the pattern of fluvial morphology changes in the Golmud distributive fluvial system(DFS)in the Qaidam Basin,northwestern China,using satellite remote sensing and unmanned aerial vehicles(UAV).Firstly,we use Google Earth real-time image data,historical image data,and radar digital elevation data to extract geomorphological information;then the UAV remote sensing image data were used to interpret fluvial network information;finally,we use the gray-scale differential vector method to describe the fluvial morphologies.Three zones have been identified in the Golmud DFS:the proximal,the medial,and the distal,by comparing the differences in topographic and geomorphic characteristics,fluvial morphologies,and sedimentary characteristics of the Golmud DFS.The proximal slope is higher than the other two zones,and the geomorphic features are mainly gravel gobi.The proximal fluvial morphologies are mainly large braided rivers,and sediments are more gravelly and less sandy.The medial slope is relatively small,and the geomorphic features are mostly oasis plains.The medial fluvial morphologies are mainly meandering rivers associating with braided rivers,and sediments are more sandy and less gravelly.The distal slope is the lowest,and the geomorphic features are mostly oasis plains,lakes,and marsh plains.The distal fluvial morphologies are mainly meandering rivers,and sediments are sandy and muddy.Comparison of the DFS from proximal to medial to distal in Golmud confirmed the potential of remote sensing image technology in identifying the fluvial morphologies and sedimentary facies distribution in dryland.