ANCIENT AQUEOUS SEDIMENTATION ON MARS

Quasi-dendritic valley systems produced by fluvial erosion are common in the ancient cratered terrain of Mars. Examination of Viking orbiter images shows that the valley networks in some instances debouch into closed depressions which can act as local ponding basins for flow. These topographic lows therefore are expected to be locations of past aqueous sedimentation. A global survey reveals 36 such sites, concentrated between the equator and 30°S lattitude in the ancient cratered terrain. Because they are low- lying, closed depressions, they are also sites favorable to accumulation of low-viscosity lavas, and a number of them appear to have undergone volcanic filling subsequent to the cessation of fluvial activity. Others appear to be free of volcanic fill. Using Viking orbiter images, approximate volumes for the inflow valleys are calculated. A sediment transport model is then used to estimate conservatively the amount of water necessary to remove this volume of debris from the valleys. The results for four basins having especially well-developed inflow networks suggest sediment thicknesses in the basins of tens to hundreds of meters, and indicate that the water involved in the transport process would have been enough to fill the basins to a depth of several kilometers had it all been present simultaneously. This analysis does not provide information on the actual water depth at any given time, but in a few instances the water appears to have overtopped the basin rim, attesting to the attainment of significant water depths for at least a brief period. The total water volume for each basin is a strong function of the mean size of transported sediment, but when compared to the megaregolith porosity, the calculations suggest that the quantity of water required to transport the sediment is greater than that which could be produced by a single discharge of the associated aquifer unless the material of the Martian highlands was very fine-grained and noncohesive to a depth of hundreds of meters. The water volume calculations also indicate that the maximum mass of carbonates that could reasonably have been precipated in these basins is equivalent to only a few millibars of atmospheric CO2. © 1991.