# Lithological Mapping Research Based on Feature Selection Model of Relief-RF
## 2 Geological Background of Study Area  
The northern margin of the Qaidam Basin is located in the northeast of the Qinghai-Tibet Plateau, Northwest China. It consists of the following second-order structural units: (I) the Qaidam platform block, (II) the orogenic belt in South Qilian and (III) the north margin active belt [37]. A prominent UHP metamorphic belt can be observed in the northern margin of Qaidam, extending approximately 400 km in the northwestern-western direction and consisting of UHP metamorphic terranes, such as the Yuka, Lvliangshan, Xitieshan and Dulan terranes, from northwest to southeast [38; 39; 40]. The UHP metamorphic rocks in these terranes are mainly granitic and argillaceous gneiss, and some of them contain eclogite lenses and various terranes (Yukadi, Xitieshan and Dulan terranes) or garnet peridotite (Lvliangshan terrane). The study area is located in Xitieshan in the North Qaidam Basin (37\({}^{\circ}\)20\({}^{\prime}\)-37\({}^{\circ}\)40\({}^{\prime}\) N, 95\({}^{\circ}\)30\({}^{\prime}\)-95\({}^{\circ}\)45\({}^{\prime}\) E). It is bounded by the Ulan Yuka Fault in the northeast and the fault zone of the North Qaidam Basin. The study area is adjacent to Oulongbulak Mountain and connected to the Cenozoic depression of the Qaidam Basin (Figure 1). In this area, daytime and nighttime are characterised by high-intensity ultraviolet radiation and large temperature differences. The climate is dry in summer and cold in winter. The study area also has minimal vegetation and bare rock, hence its suitability for lithological mapping research. The study area includes 14 stratigraphic rock units (Figure 2 and Table 1). The rocks belong to the medium- and high-grade metamorphic rock series of the Daken Daban Group  
Figure 1: (a) Division of geotectonic units in China (modified after [PERSON] et al. [37]) and Sentinel-2 (4, 3, 2) true colour image of study area; (b) regional geotectonic map (modified by 1:1,000,000 geotectonic map of Qinghai province). with lithologies of migmatite, gneiss, quartz schist, granulite with marble and plagioclase amphibolite schist. The sedimentary rocks in this area belong to the following five formations: (1) Chengqianggou Formation of the Early Carboniferous period, with the lithology of grey dark grey limestone and dolomite; (2) Early-Middle Jurassic Dameigou Formation, which is a set of coal-bearing clastic rock formations (i.e., sedimentary rocks that are mainly composed of gravelly sandstone, fine sandstone, medium coarse quartz sandstone and siliceous slate with coal); (3) the lower members of the Ganchaigou Formation in the Oligocene to Miocene periods, which is mainly composed of gravelly coarse sandstone and a conglomerate with a moderately thin layer of fine sandstone, siltstone and mudstone, belonging to Piedmont accumulation facies, and upper members mainly composed of interbedded gravelly sandstone and fine sandstone, siltstone and mudstone belonging to fluvial facies; (4) the Miocene Youshashan Formation, which is mainly composed of interbedded mudstone, siltstone and thin-layer gravelly sandstone with parallel bedding (i.e., the rock colours are mainly grey, greyish yellow and brown yellow, indicating a lacustrine sedimentary environment); and (5) the Qigequan Formation in the lower Pleistocene, which is composed of grey and greyish purple conglomerate with medium thin sandstone. The five component is a Quaternary system that is mainly distributed in the piedmont plain and intermountain valley basin. Its genetic types mainly include alluvial and alluvial pvoluvial in the Late Pleistocene period and alluvial, pvoluvial-alluvial, aeolian, lacustrine and chemical deposits in the Holocene period.