关键词: 砂山, 石英颗粒表面形态, 风成环境

Abstract: The origin of Quaternary sand deposition in the middle and lower reaches of the Yangtze River is still controversial. Preliminary study was made on the surface morphology of quartz grains from the subtropical“sand dune”section at Henggang，Nanchang County，Jiangxi Province，south China. Scanning electron microscopy（SEM） analysis demonstrations that of the quartz grains are mostly subangular or subrounded，with dish-shaped pits，crescent-
shaped pits and pockmarked pits that are characteristics of aeolian origin. Conchoidal fracture，V-shaped pits and underwater polished surface，which indicate aqueous origin，are observed. Dissolution and deposition of SiO2 is notable. Some aqueous features were masked or reworked by later aeolian superimposition. Chemical overprint on aqueous and aeolian features was also traced. this reveals the underwater features were formed originally，then the
aeolian process contributed，and finally the reworking of post-depositional chemical action occurred. Comprehensive analysis suggests that the sand deposit in Henggang section was formed by aeolian short range transport and accumulation of riverbed sands，being part of the Quaternary "sand dunes" in middle and lower reaches of the Yangtze River. The alternation of yellow sands and maroon sands in the section reflects the wet-dry variation in the depositional environment. The quartz grains in the yellow sands are often angular or subangular，with clean surface imprinted by mechanical action and displaying clear dish-shaped pits，conchoidal fracture and V-shaped pits，implying a relatively arid and wind strength dominated depositional environment. The quartz grains in the maroon sands are more rounded，with many directed dissolution pits，irregular dissolution gouges，siliceous spherules and siliceous scales. The mechanical action was masked by the later deposition of SiO2. The surfaces of the quartz grains are uneven，or even covered by siliceous films. It gives an indication of post-deposition pedogenesis in the maroon sands. Changes in the color and surface morphology of the quartz grains records the cyclical variations of the wet-dry environment. It was previously reported that ancient aeolian sand was widely distributed in north China，later researchers put forward that ancient aeolian sand could also found in the middle and lower reaches of the Yangtze River. Comprehensive study on the surface morphology of quartz grains from the“sand dune”section in Henggang confirms that the development of ancient aeolian sand was not only limited to north China and aeolian sand deposition could also happen along river banks and lakeshore areas in south China in a favored climate，with fluvial or lacustrine alluvial deposit sources.

Key words: sand dune, surface morphology of quartz grains, aeolian environment