Distributions Of Surface Deposits In The Gonghe Basin And Their Relationships With Environmental Processes

The Gonghe basin is a faulted intermontane basin surrounded by the Qilian Mountains, the Kunlun Mountains and the Qinlin Mountains on the northeastern Qinghai-Tibetan Plateau; As it is in the marginal regions dominated by the Asian monsoon and hence is sensitive to the climatic and environmental changes, studies of the various surface sediment in the Gonghe Basin has the benefit of understanding the the modern sedimentary environments in this basin.The study can be helpful to reveal the relationships between various surface sediments and relevant environmental processes. It also has the significant meaning to figure out potential dust source areas in northeastern China. The surface sediments in the Gonghe Basin is the object of the study. The spatial distributions of average annual precipitation, average annual temperature, wind regimes and vegetation coverage were analyzed. The characteristics of the surface sediments were analyzed by using spatial analysis. We discussed the material links of various surface sediment and possible environmental agents and weathering. The correlation analysis were used to infer the environmental factors which may affect the distribution patterns of the surface sediments. The primary conclusions were reached as follows:1. The average annual precipitation exhibits an increasing gradient from the northwest to the southeast in the Gonghe basin. The average annual temperature was increases from the surrounding mountains to the interior of the basin and the highest values exist around the Longyang Gorge. The average annual wind directions are vertically stable, basically exhibit western to northwestern. The difference of the spatial distribution of vegetation coverage is obvious. The normalized differential vegetation index(NDVI) was high in the surrounding mountains and the piedmont areas.In contrast, NDVI at the Longyang Gorge reservoir and dune fileds is lower.2. The characteristics of the grain size frequency curves from different surface sediments in the Gonghe Basin show evident difference, so do their grain size parameters; the descending order of the average values of common composition in the surface sediment are listed:SiO2>A12O3> CaO>Fe2O3> K2O>Na2O> MgO. Different surface sediments have varying proportions of common elements and trace elements, as well as variability coefficients. The magnetic susceptibility of the surface sediment is generally low. Small amounts of the ferrimagnetism mineral particles were found in the surface sediment. There are few superparamagnetic mineral particles in the sediment. The discrepancy of the ferrimagnetism mineral contents exists in different surface sediment.3. The regional difference of the percentage of the sand-sized sediment is evident in the Gonghe basin. The high values occur in the Shazhuyu River valley, the surrounding areas of Tanggemu, the dried-up Dalian lake and the Mangla River valley. The low values occur in the Qinghai Nan Mountains and the front of the Heka Mountains. The spatial distribution of silt-sized sediments resembles that of the clay-sized sediments. These two types of sediments have an strongly negative correction with the sand-sized sediments. The spatial distribution of mean grain size of the surface sediemts is evident in the study area. The various elements in the surface sediment have different spatial distributions. The similar phenomenon also exits between the sand fractions and the elements, probably making these spatial distributions. The correlation between the low frequency magnetic susceptibility and the elements is not evident in the terms of their spatial distributions, which contrasts with organic matter.4. By analyzing the ratios of different elements in the different surface sediments, it was revealed that the surface sediments have similar distribution patterns of trace elements, suggesting their uniform provenance. The A-CN-K graph supports such an observation. The chemical weathering of the sediments is overall weak, and most of the sediments belong to the initial stage of chemical weathering. A negative correction was found between the CIA and the ratio of Na/K. The high values of CIA occur in the front of the Qinghai Nan Mountain,the north part of the Longyang Gorge reservoir,the front of Heka Mountain,the mountains around the Mangla River, and the northwest of the basin; the low values mainly occur in the center of the basin and the Yitala terrace. The U-shaped distribution of CIA was observed from north to south,across this basin.5. The correlation analyes were applied to show the relationships between proxies of the surfface sediments and the environmental factors. Some correlations exist between the independent proxies. The correlations betwennt the proxies and the environmental factors are sophisticated. The spatial distributions of the surface sediments are affected by various environmental processes.6. The surface sediments in the Gonghe Basin are mainly derived from the surrounding mountains.The debris were produced by various weathering precesses and then transported to the interior of the basin by multiple surface agents.