Grain size characteristics of the sand silt layers in the ancient delta of the dried Lop Nur lake (east Tarim Basin) and their environmental implications

Grain size characteristics of sediments are an effective index for the depositional environment. However, whether grain size is an accurate indicator for the formation environment is still obscure. In this study, two main types of lithology can be found in the yardang sediments from the Loulan area (Lop Nur): sand-silt layers and clay-silt layers. These two types are common in arid northwestern China. The clay-silt layers were primarily composed of lacustrine-swamp deposits, whereas the sand silt layers were formed under fluvial conditions based on geomorphic and stratigraphic lithologic evidence. However, grain size characteristics of the sand-silt layers, such as grain size distribution, C-M diagram, and grain size parameters, were consistent with that of the aeolian deposits. The study of the field deposition environment indicated that the sand-silt layers were fluvial sediments of materials originating from the aeolian sediments transported over a short distance and deposited by floods. As a result, the fluvial sediments exhibited the grain size characteristics of aeolian sediments. This indicates that some sediments may retain different depositional environment information, having undergone transportation and deposition processes. Therefore, when using grain size characteristics to determine the depositional environment, field deposition environment investigation should be combined with other environmental proxy indicators