A new type of polygonal terrain formed by sulfate weathering in arid regions

Abstract

Polygonal terrain (or simply, polygons) is a common geomorphological feature in arid regions. Polygons with diverse shapes and different sizes ranging from several meters to more than one hundred meters are widespread in playas. Here, we report a new geometric type of polygons in the playas in the western Qaidam Basin on the northern Tibetan Plateau. These polygons are characterized by raised rims that are interlocked with each other, forming irregular boundaries resembling jigsaw puzzle piece-shapes. The average size of these polygons is around 80 m, which is larger than most of the reported polygons in playas elsewhere. While most polygonal terrain in playa environments is bounded by cracks generated by desiccation or thermal contraction, these unusual features are delimited by raised rims likely caused by a chemical weathering process. Thenardite, a dense anhydrous sodium sulfate mineral originally formed in a saline playa, has been weathered and hydrated to form widespread gypsum. As gypsum leads to a larger volume than thenardite during precipitation, we propose that the volume expansion develops the raised rims of polygons as pressure ridges.