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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 eolian processes, we interpret these deposits and their associated morphologies as remnants of depositional features and propose that 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.

Plain Language Summary Light-toned layered deposits rich in sulfates are widespread on Mars in several locations. Understanding how these deposits formed is crucial to infer the climatic conditions during Martian history and, in light of the fact that potentially similar deposits on Earth have been shown to be conducive to bacterial life, is critical in the search for habitable environments on Mars. We focused on Kotido crater because it shows a remarkable infill and excellent data coverage. We drew a detailed geological map, recognizing subunits interbedded within the layered deposits: light-toned and darker-toned layers, subconical mounds, and associated fractures. We interpret these materials to result from episodes of water upwelling sourced from the subsurface: a mixture of fluids, gases, and sediments emerged in Kotido leading to the deposition of the different subunits depending on the relative ratio of the upwelling materials in the different phases. The mounds and fractures would represent the morphological expression of the upwelling process, while the light- and darker-toned material would represent the resulting deposits. The proposed mechanism is identical to that of terrestrial environments such as playa and spring deposition typical of arid settings where bacterial life is well adapted.