Petrologic interpretations of thermal emission spectra from Mars orbiting spacecraft indicate the widespread occurrence of surfaces having basaltic and either andesitic or partly altered basalt compositions. Global concentration of ice-rich mantle deposits and near-surface ice at middle to high latitudes and their spatial correlation with andesitic or partly altered basalt materials favor the alteration hypothesis. We propose the formation of these units through limited chemical weathering from basalt interactions with icy mantles deposited during periods of high obliquity. Alteration of sediments in the northern lowlands depocenter may have been enhanced by temporary standing bodies of water and ice. ?? 2004 Geological Society of America.