Groundwater Control and Process Variability on the Equatorial Layered Deposits of Kotido Crater, Mars

Understanding the origin of the Hesperian-aged sulfate-bearing Equatorial Layered Deposits (ELDs) is crucial to infer Mars’ climatic conditions during their formation and to assess their habitability potential. We investigated well-exposed ELDs in Kotido crater (Arabia Terra and produced a detailed geological map of the crater infill, distinguishing different units within the ELDs based upon their morphological and sedimentological characteristics. The ELDs consist of interbedded light-toned, darker-toned deposits and mounds, associated with possible fissure ridges. Although heavily eroded by younger aeolian processes, we interpret these deposits and their associated morphologies as remnants of depositional features, and propose they are the result of fluid, gas and sediment expulsion processes sourced from the groundwater. The textural characteristics, their depositional geometry, the associated morphologies, and the inferred composition of the light-toned deposits suggest an evaporitic origin, whereas the darker-toned deposits might reflect clastic sedimentary processes, related or not to fluid expulsion and/or residual deposition following dissolution of the evaporites. The relative ratio of fluids, salts and clasts controlled the depositional process, analogous to what happens in terrestrial playas. The controls on fluid expulsion is interpreted to depend on groundwater emplacement and fluctuations, possibly related to climatic changes, and to the interactions with the fractures related to the crater formation which allowed the actual upwelling from a pressurized aquifer.