Dale M. Schneeberger

COASTAL GEOMORPHOLOGY OF THE MARTIAN NORTHERN PLAINS

The paper considers the question of the formation of the outflow channels and valley networks discovered on the Martian northern plains during the Mariner 9 mission. Parker and Saunders (1987) and Parker et al. (1987, 1989) data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains. It is suggested, that of the geological processes capable of producing similar morphologies on earth, lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on Mars.

Transitional morphology in West Deuteronilus Mensae, Mars: Implications for modification of the lowland/upland boundary

Abstract West Deuteronilus Mensae, which lies along the lowland/upland boundary of Mars at 45° latitude, 345° longitude, exhibits both gradational ( L. A. Rossbacher, 1985 ,in Models in Geomorphology (Woldenberg, Ed), pp. 343–372, Allen & Unwin, Boston) and fretted terrain ( R.P. Sharp, 1973 , J. Geophys. Res. 78, 4073–4083) boundary types. To the west, the boundary is gradational and to the east it is defined by fretted terrain. The two types of boundaries do not merge with one another within the west Deuteronilus Mensae region, however. The gradational boundary materials appear to overlap the fretted terrain by about 500 km. Contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap. The fretted terrain can be identified as much as 300 km north of the gradational boundary in this region. Lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion. Changes in the fretted terrain across the gradational boundary (from uplands to lowlands) include a reduction of canyon wall slopes and depths, such that the fretted terrain north of the gradational boundary appears mantled but not obscured. There are at least two major classes of processes which might explain the lateral overlap: (1) erosion of stratified upland terrain and (2) deposition of plains materials onto the sloping upland margin and fretted terrain. Erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places. Of the possible plains emplacement mechanisms, eolian deposition would not produce the sharp, apparently topographically conformal gradational unit contacts. Volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain. Sediment deposition in either a liquid or ice-covered sea could produce the draped appearance of the fretted terrain. Outflow channels along the lowland/upland boundary, particularly those of the circum-Chryse and west Elysium regions, may have flooded the northern lowlands to depths of tens to hundreds of meters. The gradational unit contacts may represent the shorelines of such a sea. The scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine-style wave erosion and redistribution of material along the contacts. Our limited understanding of the Martian paleoclimate and H 2 O inventories allows the possibility of clement periods in the past, and other geological evidence (e.g., small valley networks and outflow channels) strongly suggests an extensive role of liquid water.