A NEW FORMATION PROCESS FOR PATTERNED GROUND, ATACAMA DESERT, CHILE

Patterned ground on Mars has been interpreted to have formed through frost heave processes. On Earth, patterned ground and sediment-filled vertical cracks are common features of pariglacial environments. However, these features are also ubiquitious in the warm and hyper-arid environment of the Atacama Desert, Chile, and cannot be explained by frost heave. Geomorphic and soil data were collected from three sites in the Central Depression, ~ 80 km NE of Antofagasta Chile. Soils are characterized by 15-25 cm of unconsolidated horizons containing an average of 36 wt% salt minerals that overlie gravels up to 3 m thick indurated primarily with nitratine and halite. Vertical cracks (1-5 cm and 10-60+ cm wide) crosscut all soil horizons, intersect at the surface to form polygonal shapes 10-30 cm and 1-8.5 m in diameter, and are filled with alternating salt and sediment laminae. Surficial clasts are preferentially sorted into these vertical cracks forming patterned ground. These features are interpreted to have formed through salt heave, which occurs when salt minerals cement soil grains creating the cohesion necessary for tensional stresses (caused by desiccation, and/or thermal contraction of salt minerals) to form contraction cracks. The contraction cracks are filled with eolian dust (salt/sediment), preventing their closure during periods of expansion caused by salt mineral precipitation and/or thermal expansion. Expansion and contraction caused by repeated salt mineral dissolution/precipitation and thermal expansion/contraction over extended time periods (106 years) result in the upward and outward distortion of soil horizons forming salt wedges and patterned ground. Different salt wedge and patterned ground morphologies reflect differences in the tensile strength of soil materials and variability in salt heave controlled by salt mineralogy, salt content, soil texture, slope, age, and other factors. The results of this study indicate that paleosols containing sand wedges and associated features (polygonal networks, upturning of sediment adjacent to wedges, vertical lamination in wedges) can no longer be attributed solely to paraglacial conditions. Some of the similar polygonal patterns described on the hyperarid and sulfate-cemented surface of Mars may also be the result of salt heave processes.