ANCIENT MARTIAN DRAINAGE BASIN RE-INTEGRATION BY SEDIMENT INFILLING AND BASIN OVERFLOW

As Noachian cratering created a multibasinal landscape in the martian highlands, the development of lengthy valley networks depended primarily on two drainage basin re-integration processes. To provide a flow path across dividing ridges, sediment or water commonly filled some closed drainage basins originally created by impacts, such that overflow of surface water could then downcut the ridge. Contemporary erosion of the ridges appears to have expedited the re-integration process independently of through-flowing water, as evidenced by widespread gullies originating near ridge crests. Headward growth of channels, either through runoff channel downcutting or headward extension of groundwater-fed valleys, may have resulted in capture of some closed drainage basins. Sediment infilling appears to have been the most common means for re-integrating drainage basins, particularly at higher elevations, as most impact crater rims and closed intercrater basin divides were not cut by single outflowing channels. Channels like Ma’adim or Parana Valles that outflowed from formerly closed (possibly lacustrine) basins, in contrast, exhibit a suite of morphologic characteristics that are inconsistent with the more typical Noachian small valley networks. These valleys originate full-born at water gaps cut into older drainage divides, which could be impact basin rims or other ridges. These outflowing channels can have sparse tributary networks, with tributaries poorly graded to the outflowing channel thalweg. The valley courses were controlled by original surface topography rather than any apparent structurally-controlled groundwater flow path. The base-level control on these channels can be a distant basin rather than local plains, which can be deeply cut by these flows. For channels outflowing from lacustrine basins, downcutting of the rim could allow rapid discharges of water, much larger than the volume provided by inflowing channels during the time of the overflow. These outflows could overcome other drainage divides downstream of the source basin, re-integrating formerly closed basins. Some channel source basins exhibit morphologic evidence for sedimentary deposits. At Ma’adim Vallis, a clear spillway and a knickpoint along the channel floor allow a characterization of the outflow that carved the valley.