West Candor Chasma in central Valles Marineris contains some of the thickest of the fine-grained layered deposits on Mars.
We can't see the grain sizes with HiRISE, but as the material erodes in the wind it disappears--apparently carried away by the wind--so the grains must be small. The layers may have been deposited from windblown materials, fall of volcanic sediments, or carried in by water, or all of the above.
Subsequently the layers may have been altered by groundwater, producing hydrated minerals such as sulfates. The enhanced colors in the sub image are related to the minerals or to overlying dust or sand. The dark blue sharp-crested ridges are sand dunes.
Photo credit: NASA/JPL/University of Arizona