GEOLOGIC AND GEOMORPHOLOGIC MAP OF A PORTION OF NOACHIS AND ARABIA TERRAE, MARS

The Martian highland/midland transition zone (HMTZ) is a subtle SW-to-NE-trending topographic break between the densely cratered, rugged southern highlands and the smoother, moderately cratered midlands of Arabia Terra. The midlands constitute a narrow transition between the highlands and the vast, sub-horizontal, relatively sparsely cratered northern lowlands. The mapping area, 0 to -10E and -17.5 to -27.5S, encompasses ~337,000 km² along a portion of the HMTZ between Noachis and Arabia Terrae. This region slopes from SSE to NNW, with ~4.4 km of relief. Noachis basin, a 350 km diameter, flat-floored, ancient impact crater dominates the central map area, and the western rim and floor of the 250 km diameter Newcomb crater dominates the eastern edge of the map. Paraná Valles, a network of ancient fluvial valleys, originated in the western part of the map area along a local drainage divide and drained west. Highland surfaces are pervasively dissected in the southern part of the map area. Bashkaus Valles, a well-integrated system of valleys, channels, and tributaries, stretches ~245 km from its head at the southern margin of the map area to a breach in the southern rim of Noachis basin. Although many channels debouched into Noachis basin along its southern and eastern boundaries, only a single outlet drains the crater at the northwest rim. The northeast and northwest regions are typical of the HMTZ and define the upper reaches of basins that stretch farther into the midlands. Geologic mapping identifies 13 geologic units divided between highland (2), basin (6), and crater materials (5). Mapped linear features include contacts, crater rims, buried/exhumed craters, wrinkle ridges, and channels. During the Noachian, large impact basins formed, as did the megaregolith, a thick, disorganized deposit comprised of ejecta, sediment and fractured basement materials. Impact gardening and crustal cooling extended into the Late Noachian. Widespread fluvial activity resulted in intensive dissection of regional surfaces from the Late Noachian to the Early Hesperian. Widespread fluviation appears to have ceased during the Early Hesperian, though local, episodic activity may have continued into the Late Hesperian. The prevailing geologic processes from the Late Hesperian to the present include impact gardening and eolian processes.