The Mars Exploration Rover Opportunity has examined sedimentary structures in the Burns formation at Meridiani Planum. The materials in this formation reflect, in part, subaqueous deposition of reworked, sulfate-rich, clastic sediments that likely formed in a playa-interdune setting. The chemistry and mineralogy of the sedimentary rocks record an origin by evaporation of sulfate- and chloride-rich brines mixed with a fine, altered, basaltic mud or dust component, prior to reworking. Cementation and postdepositional reactions to form hematite-rich concretions and crystal-mold porosity reflect diagenesis in a groundwater-saturated subsurface. More recent dehydration events are evidenced by polygonal textures in rocks within craters and exposed on the plains. The timing of formation of fracture fillings that cut across bedding is not well constrained and may be early postdiagenetic or later. The fracture fillings may have formed by solutions remobilized along zones of weakness. Alteration rinds may reflect more recent interactions between rock and atmospheric water vapor.

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