Subsurface Volume Loss and Collapse due to Surface Infiltration of Osuga Valles' Catastrophic Floods, Mars

Osuga Valles (OV) form a catastrophic system in which multiple flood events can be tracked from their origin within chaotic terrain down to their sink at terminal depressions. We are analyzing OV's terminal depressions to understand their possible formation mechanism(s). The depressions' association with outflow channels indicates that there may be a link between inflowing floodwaters and the formation of depressions, hence raising the possibility of surface collapse following subsurface dissolution.

Our mapping suggests that preliminary volume loss and collapse resulted from lower discharges of infiltrating floodwaters prior to the onset of collapse. Subsequent collapse was likely due to additional higher discharge floods that drained directly into the forming depressions. A terrestrial analog to OV's depressions may be the previously documented Ze'elim Fan Sinkholes along the Dead Sea shores, in Israel, which are located at the termini of flood channels. A suggested model for their formation is that ground-water dissolution of subsurface salt layers formed the initial surface collapse, followed by subsequent draining of floodwater through the collapsed region, which became the dominant subsurface salt dissolution agent and induced further surface collapse. For the OV's depressions on Mars, it is not clear whether the infiltrating floodwaters removed the substrate volume by dissolving subsurface salt deposits, or by excavating rocks and melting ground ice. However, the region includes older valley networks, which terminate in this potential sedimentary basin, and implies that an evaporite-rich sedimentary basin may have previously existed at this site. We have estimated flood discharges using sediment transport equations and assessed the total subsurface volume loss according to the volumes of the depressions, channels, and chaotic sources. These calculations helped us constrain the possible subsurface stratigraphy and the volume-changing processes that may have formed OV's depressions and possibly other Martian depressions.