GEOLOGY OF THE ERIDANIA QUADRANGLE--MARS R. A. DeHon, University of Arkansas at Monticello, Monticello, Arkansas 71655 The Eridania Quadrangle (Fig. 1) is located within the density cratered terrain of the southern hemisphere of Mars, east of the Hellas Basin. The area contains three distinct physiographic provinces which divide the quadrangle into latitudinal belts. The northern belt is dominated by a cratered upland plateau, the central belt by plains, and the southern belt is mottled rugged terrain. Generally, the oldest geologic units occur in the upland terrains, and the youngest units occur in the central lowlands. The largest crater, Kepler, (unit c) is 215 km in diameter and has an internal raised ring 100 km in diameter. Relief forming materials, units that are higher and rougher than plains forming materials, occupy upland regions of high crater density. The oldest materials are probably the mottled rough terrain (map unit mr) in the south which is characterized by dark mottling with sharp albedo boundaries and high density of large degraded craters. These materials may represent the late stages of planetary accretion. Dark mottling and the lack of young appearing craters suggest that these materials may be extensively altered by erosion and redeposition within the winter polar cap. The hilly and cratered terrain (unit hc) in the northern part of the quadrangle consist of densely to moderately cratered uplands associated with the near equatorial dark albedo band on Mars. Craters within the hilly and cratered terrain range from degraded to fresh, but there is a deficiency of small craters. Sinuous furrows or channels are locally abundant in areas of moderate regional slopes. Knobby terrains (unit k) are present in limited areas of locally rugged relief which are probably formed at the expense of other terrains by faulting and erosional break-up of subjacent materials. Knobby terrain on the western edge of the quadrangle is peripheral to the Hellas basin and is probably related to post-basin faulting. Plains forming materials exhibit flat to rolling surfaces which are most extensive in the broad low area in the central part of the quadrangle. Cratered plains (unit pc) material is the most wide spread plains unit and the most cratered. Cratered plains material forms undulatory planar surface with moderate crater density and abundant lobate scarps. It is embayed by other plains units and, therefore, is probably the oldest. Mottled plains material (unit pm) in the south forms level to rolling plains with low crater density and varying albedo. It mantles and subdues subjacent topography. Mottled plains appears to fill a topographic low in the southern rugged terrain belt. Smooth plains (unit ps), occurring in topographically

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low areas, is relatively level and nearly featureless. Due to the presence of atmosphere the surface of Mars is more actively weathered and eroded than that of the moon. A discontinuous surficial cover of weathered and transported materials is present throughout much of the region. Distinctive albedo mottling in the southern part of the quadrangle is, in part, related to compositional and thickness variations of the surficial cover. Some intercrater dark patches are dune fields, others may be lag gravels. Light and dark wind streaks are common downwind from some craters. Light streaks, in the northern part of the quadrangle, exhibit southeasterly trends. Dark streaks, in the north central part of the quadrangle, trend west to southwest along an apparent topogrpahic trough. The early history of the region was dominated by a high flux of meteorids and impact cratering. The impact responsible for the Hellas Basin occurred during this late stage of planetary accretion. As the cratering rate decreased, volcanic flows were deposited across the central belt filling a regional depression with rolling plains material. As cratering by impact waned further, other plains materials of volcanic or eolian origin were deposited in lowlying basins. Erosion and redeposition have been continuous, though of variable intensity, in this region throughout Martian history. Acknowledgement Geologic mapping of the Eridania Quadrangle of Mars is supported by Grant NGR 04 008 001 as part of the Mars Geologic Mapping Program of the Planetology Programs Office, Office of Space Sciences.

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Eridania Quadrangle--Mars R. A. DeHon

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