A COMPLETE EROSIONAL-TO-DEPOSITIONAL SYSTEM IN MELAS CHASMA, MARS

Identification of complete source-to-sink systems on Mars is rare, because commonly parts of the system are not preserved, are eroded, or are covered by later deposition. Southern Melas Basin in Valles Marineris is an exception, as it could represent a complete erosional to depositional system, from the fluvially-incised source region in the surrounding highlands to the terminal sediment sink formed by the sublacustrine fans in the topographically lowest part of the basin. Fluvial incision of bedrock, interpreted as caused by runoff from precipitation, drains the ridges bordering the western and eastern parts of the basin. Sediments generated during erosion of the upland areas were transported by fluvial drainage systems to form a classic, cone-shaped alluvial fan at the western edge of the basin where confined channel flow emerges onto the fan surface. Sediments which pass through the alluvial fan were deposited as clinoforms, which may record a potential shoreline or upslope channel levee part of the submarine fan system. The clinoforms give way further down the topographic profile to a sublacustrine fan, very similar in morphology to the Mississippi submarine fan. This ultimate depositional low in the system provides the terminal sink for the sediments. MRO CRISM spectra show evidence for opaline silica and jarosite in layers near the fans; these hydrous minerals imply that water was present when they formed. The presence of sublacustrine fans in Melas Chasma indicates that a significant body of water was present and stable at the surface of Mars for at least 10² to 10⁴ years, which provides important constraints for past environmental conditions on Mars.