AGE RELATIONS OF MARTIAN HIGHLAND DRAINAGE BASINS

Dendritic valley patterns in the equatorial highlands of Mars show evi dence of internal drainage into restricted basins, which are interpret ed to be floored with sedimentary fill. Based on crater frequency char acteristics of six areas of enclosed basins, the origin of these inter crater plains fill units ranges from middle to late Noachian. In contr ast, the age of modification of the same plains units derived from the frequency of fresh craters occupies a relatively narrow range centere d on the Noachian/Hesperian boundary. In half the areas studied the ti ming of highlands and plains crater modification is consistent with a sedimentary origin for basin fill materials. The other plains units mo st likely consist of interlayered sedimentary and volcanic materials. Relations between the age of stability of these internally drained hig hland units and their elevation are not as distinct as prior studies s uggested; a trend of decreasing age with decreasing elevation for the plains materials is not matched by similarly derived ages of the disse cted highlands. Remapping and age dating of the dissected highlands an d associated basins suggest that volcanic plains may be more extensive than those used in past models for magma and Volatile evolution, and support local volcanism rather than a global-scale magmatic head model for highlands plains formation.