Polygons with halite-crusted floors and gypsum-raised rims in western Qaidam Basin and implications for polygonal landforms on Mars

Polygons ranging in size from a few meters to kilometers have been observed in playa fields on Earth and Mars. The historically hyperarid climate of the Qaidam Basin has allowed the development of extensive polygonal landforms with diverse geometric and genetic types. Here we report a terrain of polygons with a pan-like structure, raised rims, and a size of about 60 to 120 m, and spatial variation in mineral composition and geometry from the Dalangtan area of the western Qaidam Basin on the Tibetan Plateau. Spatially, the polygons in the northeastern part of the study area have complete rims, while the polygons in the southwest have incomplete rims. These polygons consist of a halite crust in the subsurface and raised rims formed mainly of gypsum. In some areas, the polygonal rims are broadened and form boundary belts that are up to ~30 m wide and about 1.2 m high. We suggest that the formation of the halite crust in the subsurface redirects upward migration of evaporitic pore fluids that accumulate gypsum deposits to form the wide polygonal boundary belts. We argue that the similarly sized polygons with raised rims on Mars have similar lateral and vertical structures caused successively by the strong evaporation of lacustrine brines and subsurface pore fluids.