DRAINAGE EVOLUTION IN MARGARITIFER SINUS, MARS

Geologic mapping in Margaritifer Sinus, Mars, defines a complex history of water transport, storage, and release that began in the late Noachian and persisted into at least the mid-Hesperian. Collection, transport, and discharge of the water from widely dispersed surfaces were accomplished by systems of differing character flanking opposite sides of the Chryse Trough. Drainage on the western side of the trough was accommodated by the segmented Uzboi-Ladon-Margaritifer mesoscale outflow system that heads in Argyre basin, drains approximately 9% of the Martian surface, and alternately incises and fills as it crosses ancient multi-ringed impact basins.

By contrast, Samara and Parana-Loire Valles, two of the largest and best-integrated valley systems on Mars, cover just under 0.5% of the surface and dominated drainage on the eastern side of the trough. These networks are characterized by a number of characteristics consistent with formation by groundwater sapping including: low drainage densities and ruggedness numbers, generally uniform valley width and cross-section, frequent scale dependence, and largely undissected interfluves. The valleys also possess morphometry consistent with formation by runoff including: a basin-wide distribution of tributaries, integrated sub-basins drained by high order trunk valleys, a tendency to head near basin divides, occasional scale independence, and an excess volume of alluvial sediment in sinks relative to the incised volume of upstream valleys. Although none of these morphometric parameters provide conclusive evidence for valley formation by either sapping versus runoff, as a suite they are consistent with formation by precipitation recharged groundwater sapping.

All of the systems debouched into Margaritifer basin along the trough axis and caused extensive ponding that persisted into the early Hesperian. Drainage evolution was coincident with widespread geomorphic activity elsewhere on Mars. As the formation of the channels and valleys ended, water in Margaritifer basin infiltrated and was stored in the subsurface. Subsequent collapse and release of water began shortly thereafter and persisted into mid Hesperian times, thereby leading to evolution of the Margaritifer and Iani chaos and incising the northward draining Ares Vallis outflow channel.