Many of the chasmata (the plural of “chasma”) in Valles Marineris contain light-toned mounds that are distinct from the darker rocks that define the walls. The light-toned mounds, like this one in Juventae Chasma, generally contain sulfates, which are salts of sulfuric acid that form when water is evaporating.
The darker material that we see is collecting near the base of the light-toned mound, and likely represents wind-blown debris. [More at link]