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Clay minerals detected spectroscopically in the Mawrth Vallis region of Mars correspond to a complex, layered, thick (>600 m) stratigraphic section of ancient bedrock. Because the lighttoned, clay-bearing rocks are lithologically diverse over a broad area (>80,000 km2), have significant internal layering and complexity, and contain buried impact craters within the section (demonstrating that they were deposited over a geologically significant duration of time), we interpret the host rocks as sedimentary or possibly pyroclastic. Crater counts date the claybearing rocks to early-middle Noachian time (estimated as 3.8-4.1 Ga). Geomorphic observations suggest that the rocks were lithified early and deeply eroded during the late Noachian-early Hesperian. The combination of a probable ancient sedimentary context of the clays and a moderate pH formation environment implied by the occurrence of smectites clearly places these rocks among the most important targets for future astrobiological exploration. [ABSTRACT FROM AUTHOR]