Inverted channels, polygonal fractures and layered mounds from the Makgadikgadi Pan of Botswana: possible analogues for Martian morphologies

Inverted channels with polygonal fractures and layered mounds from the Ntwetwe pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Plio-Pleistocene lake and is currently part of the world’s largest evaporitic system.

The mounds in the Ntwetwe pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion.

The inverted channels, identified in the southern part of the Ntwetwe pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 m wide) polygonal fractures localized at the front of the fan, disappear at the transition with the present-day pan floor.

We consider that these particularmounds, withinthe Ntwetwe pan, are remnants of the strandline of the paleo-Makgadikgadi Lake, and that the inverted channels representdistributary channels of a relict fan delta,formed by an ephemeral river, most likelythe paleo-Boteti River, during a Lake Paleo-Makgadikgadihighstand stage. We consider that largescale (up to 100 m wide) polygonal fractures, located on thechannel-mouth lobes, representlarge-scale desiccation cracks formed byrapid water evaporation from deltadeposits.

The results of this investigation highlight the importance of the paleo-drainage system and itsinteractions with thewater table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories.