Distribution and composition of loess sediments in the Ili Basin, Central Asia

摘要

Loess deposits in Central Asia provide an important record of regional climate and environmental change. However, in contrast to the intensively investigated loess deposits on the Chinese Loess Plateau (CLP), loess sediments in the Ili Basin of eastern Central Asia are poorly understood. Based on field investigation and existing literature, this paper presents a preliminary study of the distribution, strata and composition of the Ili loess. The distribution of the Hi loess is clearly controlled by topographic and geomorphic conditions, mainly in the river terraces, low uplands, and slopes of the Tianshan Mountains. The thickness of the loess varies from several meters to over two hundred meters. To characterize the Hi loess composition, the authors analyzed its grain size, geochemistry, X-ray diffraction pattern and heavy mineral assemblage. Grain size analyses reveal that the Ili loess consists predominantly of silt (4-63 μm) with a minor proportion of sand, which is coarser than the loess of the CLP and suggests a nearby provenance. The bulk mineral components of the Ili loess are dominated by quartz and feldspar with minor amounts of calcite, chlorite, mica, dolomite and hornblende. More than 20 types of heavy minerals were observed with major components of amphibole, magnetite and epidote. The major elements of the Ili loess are characterized by high abundance of SiO_2, Al_2O_3 and CaO and minor amounts of Fe_2O_3, MgO, Na_2O and K_2O. Compared to the CLP loess, the Hi loess is relatively rich in Na_2O and displays higher Na_2O/Al_2O_3 ratios, suggesting weak weathering. The distribution patterns of REEs indicate that both the Ili loess and CLP loess are of typical upper crustal composition, with enrichment in LREE, negative Eu anomalies, and depletion of HREE. Both the heavy mineral assemblage and geochemistry indicate that the local bedrock of the Ili Basin may have contributed only a little to the loess sediments.