Calcium sulphate-wedge formation in deposits from the Aroma alluvial fan as indicator for haloturbation in the Atacama Desert

Polygonal patterned grounds on the Earth’s surface are typically associated with subsurface-wedge structures in periglacial environments. The presence of such wedges is usually taken as an indicator for cryogenic processes in the subsurface, which form a characteristic vertically laminated sequence. However, similar structures can be found in the subsurface of the Aroma fan in the Central Depression of the Atacama Desert in northern Chile. Within the salt-bearing deposits of the alluvial fan, the calcium-sulphate wedges appear to be preliminary formed by haloturbation and may represent the hyperarid equivalent to periglacial wedge structures. The characteristic vertical lamination of the wedges contains calcium-sulphate phases accompanied by clastic minerals, as found by X-ray diffraction and X-ray fluorescence analyses. Hence, the calcium-sulphate phases in the wedges are assumed to be potential drivers for salt dynamics causing subsurface wedge-growth and surface polygonal patterned ground formation. Due to varying water availability in a generally extremely water-limited environment, these salt dynamics possibly led to significant volumetric changes in the deposits induced by dissolution and (re)precipitation of salts from infiltrating solutions and phase transitions of calcium-sulphate phases.

The subsurface-wedge network of the Aroma-fan outcrop is covered by a ~ 20 cm thick calcium sulphate-bearing surface crust, which potentially covered a polygonal patterned ground. The formation and preservation of the surface crust might indicate an amplification of arid conditions leading to the inhibition of wedge growth in the subsurface. To unravel the mechanisms and governing environmental conditions of calcium-sulphate wedge and crust formation at the Aroma site, we present various mineralogical, geochemical, and sedimentological data of wedge and crust material.

Furthermore, we applied geochronological methods to resolve wedge-growth phases and episodes of local moisture supply. We tested meteoric ¹⁰Be dating and post-infrared infrared stimulated luminescence (post-IR IRSL) dating on wedge material to gain information on the evolution and activity of wedge growth under arid to hyperarid conditions. Such geochronological data is indispensable for using the wedges as terrestrial proxy record for the palaeoclimate in the northern Atacama Desert.