Insights into the age and genesis of the clay dunes in the Suhongtu Basin, Alashan Plateau, China

Abstract

Clay dunes are characterized by high content of silt and clay and they are both a unique aeolian landform and an important paleoenvironmental archive attesting to changing climatic and hydrological conditions. Although clay dunes have been identified on several continents, they have attracted relatively little research attention. Numerous clay dunes are developed in the Suhongtu (SHT) Basin, Alashan Plateau, China. However, their genesis, age, and process of evolution are unknown. In this study we measured the grain size of various clay dune samples using dynamic image analysis (DIA) and laser diffraction analysis (LDA). Comparison of the results obtained by the two methods revealed significant differences between them: the grain size of clay dune samples obtained by DIA have a unimodal distribution (with a mode at ~200–400 μm), and the grain-size composition is dominated by medium sand; while the results obtained using LDA show a bimodal distribution (with modes at ~4 and 200–400 μm), and the grain-size composition consists mainly of clay and silt. This indicates that the clay dunes in the SHT Basin are formed by wind transport and accumulation in the form of the mixing of sand-sized aggregates of clay-silt with quartz sand grains. The morphology, mobility, and sedimentary structures of the clay dunes in the SHT Basin differ substantially from those of the well-studied clay dunes in Australia and North America, which can be mainly attributed to the relatively low clay content, abundant sand source and extremely arid conditions. During field investigations we studied seven stratigraphic sections, comprising multiple aeolian clay dune and lacustrine units, with age control provided by optically stimulated luminescence (OSL) and AMS 14C dating. Lacustrine sediments in the basal parts of sections yielded OSL age between 7.84 ± 0.71 ka and 6.33 ± 0.84 ka, suggesting that the SHT Basin was occupied by a lake during 7.8–6.3 ka when the precipitation was relatively high and the climate was humid. The oldest clay dune deposit, with an AMS 14C age of 1.27 cal kyr B.P, probably represents the onset of the modern clay dune landscape in the SHT Basin. Most of the clay dune deposits yielded OSL ages between 0.50 ± 0.04 ka and 0.18 ± 0.05 ka, indicating aeolian reactivation in the past ~500 years.