The layered sediments at Sinus Meridiani, Mars, ~ 1 km thick and covering 300,000 km2, have been probed by the rover Opportunity. Numerous observations on these rocks are reevaluated through the poorly-known model of the fluvial megafan. We conclude that at least some sections of the Meridiani stack are vestiges of large fluvial megafans. Our reasons include the following: the southern uplands of Mars are a feasible sediment source; sediment was likely delivered via rivers that cut the extensive valley network that drain the upland toward Meridiani; the units cover large areas commensurate with terrestrial megafan landscapes, and display the same very low slopes; megafan landscapes lie directly adjacent to upland sediment sources, as seen at Meridiani; megafans require neither closed basins nor waterbodies for sedimentation to occur; numerous examples of fluvial channels appear in some units; and morphologies of the widespread raised ridges of the ridge-forming unit (RFU) are suggestive of indurated channel networks seen on megafans in Oman. Features of vast aggradational landscapes as encapsulated in the novel megafan analogue thus provide answers to several key observations, whereas existing fluvial analyses usually apply the classic attributes of erosional landscapes, leading to significant difficulties in interpretation of the Meridiani units.