Flood-related surface collapse at Osuga Valles: Subsurface salt dissolution on Mars?

Constraining the formation mechanism of geologic depressions on the surface of Mars can shed light on the planet’s climatic, hydrological, and geochemical evolution. At Osuga Valles, surface morphologies indicative of catastrophic flood events can be tracked from their chaotic origins down to their sink at terminal depressions, or cavi. The channel-cavi morphological association suggests a genetic link between inflowing floodwater and cavi formation and evolution. The geologic settings are consistent with a pre-flood, evaporite-containing sedimentary basin. Estimates of the water volumes associated with the Osuga Valles floods suggest cavi formation within this sediment-filled basin by surface collapse into voids generated by dissolution of subsurface magnesium sulfate minerals, which are commonly detected in the region. Geologic mapping reveals that initial cavi collapse was associated with floodwaters from relatively low-discharge floods, which likely infiltrated the subsurface through fractures. Subsequent higher-discharge floods drained directly into the evolving cavi, causing further dissolution and collapse.