Controls on the occurrence of beach‐bar sandstone in a Neogene saline lake basin, southwestern Qaidam Basin, China

Abstract

To identify the developmental characteristics and controlling factors of the beach-bar sandstone in saline lake basins in the Qaidam Basin, China, the lithology and bedding structure of some representative outcrops of the Neogene saline lake basins were comprehensively investigated by means of experimental test data of local drilling cores, casting thin sections, and grain size, together with field outcrops in the basin and the modern salt lake deposition record of Qinghai Lake. Two types of sand bodies in the saline lake basins, that is, delta sand bodies and shore-shallow lake beach-bar facies were revealed. Neogene saline lake basins were found to host widespread beach-bar sand bodies. Lithologically, the sand bodies were composed of fine-grained sandstone and siltstone, with wavy bedding, low-angle cross-bedding, and bimodal cross-bedding. The beach-bar sandstone was characterized by multi-layer superimposition, lateral migration, and NW–SE wide-coverage continuation. Drilling data showed that beach-bar sand bodies extended from 10 to 35 km in width and most of them occurred 3–15 km away from braided river delta front. It has been predicted that the total area of beach-bar sand bodies of 14 sand groups in the Neogene spread across 5,000 km2. Four factors, that is, lake water salinity, terrigenous input, palaeogeomorphology, and monsoons, controlled the construction of the beach-bar sand bodies developed in the Neogene saline lake of the Qaidam Basin. We can conclude that the settling velocity and transport distance of the sand bodies were affected by the water body salinity in such saline lake basins. Also, the distribution range of the beach-bar sand bodies was affected by the clastic input from a terrigenous provenance. Furthermore, the wide-coverage distribution and local enrichment of the beach-bar sandstone were affected by palaeogeomorphology, and the distribution direction of the beach-bar sandstone was affected by the driving effect of NW monsoons. We propose a deposition model of the beach-bar sandstone in saline lake basins that can predict favourable beach-bar sand body regions, to support the major discoveries of lithologic traps in southwestern Qaidam, China.