Extensive Noachian-aged intercrater planation surfaces comprise much of the southern highlands of Mars. We mapped aggradational and stable to degradational surfaces in three study areas with diverse relief elements and ages: the high and rugged relief of Libya Montes, the well-preserved intercrater plains of Noachis Terra, and the rolling relief with more drainage development in Terra Cimmeria. Here we describe four major geomorphic features that formed in these regions: debris-mantled escarpments, regolith pediments, sloping aggradational surfaces, and depositional plains. We interpret that with tectonic stability and an arid paleoclimate, these features supported slow pedogenesis, sediment transport, and diagenesis over hundreds of millions of years during heavy impact bombardment. Slow aqueous weathering generated primarily fine- or medium-grained particles from basaltic surfaces of impact ejecta and megabreccia. These sediments collected in local lows, reducing surface roughness, permeability, and populations of small craters. Larger crater walls and structural escarpments retreated radially or linearly as 5–20° slopes, indicating efficient removal of fine- or medium-grained debris but little downslope transport of coarse material by fluvial erosion or creep. Gently to moderately sloping, composite intercrater planation surfaces evolved as regolith pediments with tectonic stability and little fluvial dissection. Noachian impact craters degraded in place on pediments and became embayed or buried on basin floors. The concentration of aggradational surfaces in low-lying areas, lack of coarse-grained alluvial fans in most locations, and resistance to later aeolian deflation suggest intermittent low-magnitude (hypo-) fluvial erosion with aqueous cementation or development of a lag in basins.
