U.S. Department of the Interior Prepared for the Scientific Investigations Map 2953 U.S. Geological Survey National Aeronautics and Space Administration Atlas Of Mars: Western Hellas Planitia 311° 310° 309° 313° 312° 308° 307° 314° 306° 315° Npl 305° CORRELATION OF MAP UNITS and Wilhelms, 2001). As that report explains more fully, we maintain that the geologic relations and charac- Hc 304° –4000 teristics of most mapped rock units are the result of sedimentation from ice-covered bodies of water. As? Nc 303° Nb Npl Hp 302° –37.5° AHs BASIN-RIM CRATER STRATIGRAPHY HNp HNp 0 –700 301° PERIOD BASIN-FILL MATERIAL MATERIAL MATERIAL Nc Npl 0 The rim and wall of the Hellas impact basin rise above Hellas Planitia in the west quarter of the map area Npl 00 Hr Nb –7 300° 0 0 Hpl2 and culminate along the map edge as the classical feature Hellespontus Montes. The rock composing the –700 AHh 00 7 As Hr? – As more rugged, massif-like topography probably shaped by the impact is mapped as basin material (unit Nb) 00 AMAZONIAN Nb AHc –70 AHc and the intervening less-rugged terrain of uncertain origin is mapped as older plateau material (unit Npl). –37.5° –7000 Hc –4000 Nc AHs AHs AHh Both units appear smooth at fine scale on available images. This mountainous terrain is pocked by so many Nb –6000 0 large Noachian craters (unit Nc) that the basin must have been created very early in Martian history and be Nc 00 –4000 HNp As –7 –1000 As Hw –38° Hp Noachian in age. Similar terrain must extend beneath the exposed rocks of Hellas Planitia. Hpl2 Hk As Hpl2 Hellas Planitia may be underlain by postbasin materials deposited in the Noachian Period, but the As Hp Npl –3000 –7 Npl 000 exposed units are probably Hesperian or perhaps early Amazonian in age (Greeley and Guest, 1987; Tanaka 0 Hr 0 Npl Nc 0 and Leonard, 1995; Leonard and Tanaka, 2001). Age assignments of individual map units are based on crater 7 Hc – Hpl3 – HESPERIAN Hc 70 Hpl Hc 0 3 statistics given by Leonard and Tanaka (2001) for units identifiable as roughly conterminous with our units 0 –38° Hsr Hr Nb Hw Hpl (although differently named). Extensive and differential erosion precludes more precise dating from crater 0 2 Npl 700 Hpl3 HELLAS – statistics in the map area. Relative ages of adjacent units are determined from superposition and cross-cutting –2000 –3000 Nc Hc Hpl A AHs 1 relations. – –4000 5000 0 Hpl 0 The oldest postbasin geologic map unit is older plains material (unit HNp), which is equivalent to the 0 3 00 –7000 40 As 7 – 00 – HNp –70 Hpl2 mantling material of Moore and Wilhelms (2001, unit m). Its presence is revealed by faint textural elements Npl Hr As and a partial blanketing of craters and the basin wall as high as –3.1 km elevation (fig. 1; Moore and Wilhelms, HNp NOACHIAN Nc Nc Nc 2001). Because it reached so high, we assume that this unit also filled low elevations and interpret it on the Hpl 000 HNp Hc 3 Npl cross section as the oldest postbasin unit in Hellas Planitia. It presumably would level out the irregular basin –2 Nb –400 0 floor. However, other units not exposed in this map area very likely underlie the older plains material. AHc –39° A plateau, studded by the hills called Alpheus Colles, dominates the east half of the map area and is the Nc Npl western part of a broad plateau in Hellas Planitia’s interior (Leonard and Tanaka, 2001). The beds composing –4000 Hp DESCRIPTION OF MAP UNITS Nb Nb it are seen in MGS images and in the best Viking images to be finely stratified (fig. 2) but can be divided –7 Hc 000 BASIN-FILL MATERIAL HNp Npl consistently into only three map units, lower, intermediate, and upper Alpheus plateau materials (Hpl1, Hpl2, Nc –6000 As Younger smooth material (Amazonian)—Smooth, featureless, almost uncratered. On elevated Hp –39° Hpl3, respectively). The many strata form a bench-and-scarp topography that implies moderate coherence AHh 000 terrain in western map area. Interpretation: Aeolian deposits of sand and indurated dust, as –7 that holds up the scarps alternating with moderate erodability that creates the benches. All three map units As Nc Hw deduced from variable, high elevations where less subject to erosion than in Hellas Planitia and the entire plateau are bounded by irregular scarps that demonstrate postdepositional erosion. Small hills –7000 AHs HNp AHs Older smooth material (Amazonian or Hesperian)—Smooth, wavy, or lobate. Marginal to unit beyond the scarps are probably erosional outliers of once-continuous beds. Plateau materials are protected Nb HNp Hp Nc Hw AHh. Interpretation: Apparently related to unit AHh, therefore probably additional lacus- from erosion by the ejecta of some craters, for example the large (23 km) Hesperian-age Beloha crater 000 trine deposit, modified by flowage –4 intersected by the cross section, that may have formed between the intermediate and upper Alpheus plateau Hsr Hpl3 Hpl2 Beloha Hpl (Hc) AHh Honeycomb material (Amazonian or Hesperian)—Subplanar overall, with distinctive materials. The cross section derived from MOLA data confirms the superposition relations among the three –4000 3 AHs plateau units but shows that parts of the intermediate unit (Hpl ) rise higher than parts of the upper unit. This 0 cell-like, closely packed, subequal elliptical or polygonal rimmed pits a few kilometers 2 0 0 00 70 7 –4000 Hc – – across producing honeycomblike appearance; more elongate, less regular near unit edges irregular surface of the intermediate unit and the sandwiching of Beloha and its associated ejecta suggest that Hc Nb Hc (fig. 6). In depression in north-central map area. Dated as Amazonian by Leonard and Tanaka erosion cut into the intermediate layer before the upper layer was deposited; other alternations between depo- –40° Nb As Hc Npl Hpl3 (2001). Interpretation: Collection in depression indicates lacustrine origin, probably sition and erosion are expected within the plateau stack. Thicknesses are estimated from the elevations at – 1000 Hpl deformed while still wet by grounding of ice blocks as lake level dropped, resulting in which the mapped units intersect the surface of the cross section: approximately 350 m each for the lower and Hsr Hpl 3 –7 3 000 soft-sediment deformation textures and morphology (figs. 2, 7) intermediate plateau units and less than 150 m for the preserved outcrops of the upper unit. Future imaging Hp will undoubtedly reveal new details of depositional and erosional history. Hr Hpl Hk Younger plains material (Hesperian)—Expansive, sparsely cratered, smooth to wavy. Locally Npl Hc 1 Hpl3 Lying upon and extending beyond the lower plateau units are sinuous ridges (fig. 3) believed to consist 000 –40° incised, forming scarp-bounded enclosures. Interpretation: Scarps and smoothness indicate –2 Hsr of a material deposit, sinuous ridge material (unit Hsr), rather than a structural modification of other units. Npl –7000 thickness sufficient to bury or mantle pre-existing topography over extensive area; therefore, 0 The ridges seem to have formed during or after the growth of the plateau and were later exhumed, suggesting 0 probably water-laid, not aeolian. Possibly volcanic in part Npl 7000 0 – 7 Hpl2 – an esker-like origin. 00 Hw Wavy material (Hesperian)—Smooth, flat to undulatory, sparsely dotted with small (~1 km) Hc Hr –60 Hr –6000 Reticulate material (unit Hr) is more regularly textured than the plateau materials and forms north-south –6000 knobs; flow structure visible at high resolution. Gradational with knobby material (Hk) and 0 bands (and one questionable circular patch) in the western map area. The unit name is derived from roughly 0 Nb 0 Hpl Hpl Hpl Interpretation 7 A' Alpheus plateau materials ( 3, 2, 1). : Debris flows and plateau Hp – rectangular or elliptical depressions separated by interconnected bounding ridges that are generally tangential Hpl2 remnants, probably created by chaotic ground collapse caused by ice loss or normal to the basin wall (figs. 4, 5). The MOLA altimetry reveals that this interesting unit varies in eleva- Hpl3 Hk Knobby material (Hesperian)—Closely spaced, roughly equidimensional knob-like hills Hsr Hw tion. Where intersected by the cross section, the main exposure lies at –5.8 km elevation. Farther south, this 000 typically a few kilometers across and possibly a few hundred meters high; some knobs –3 Hc unit is exposed below –6.0 km elevation. On the map, a branch of unit Hr extends northward from the main –41° Nc 7000 HNp – Hpl elongate, some trend northeast. Occupies deep depression in southeastern map area. Inter- 0 2 exposure between long 309.5° and 310° W. and continues in the subsurface—revealed by subdued traces of Hellespontus Montes Hellespontus Hp Nb Hr –2000 pretation: Broken from plateau materials during erosional retreat of plateau, probably –400 HNp its characteristic texture—beneath a trough filled as high as –6.9 km elevation by younger plains material Hr Hpl2 Alpheus through chaotic ground collapse caused by loss of ice matrix or substrate; elongate knobs Hc (unit Hp). The exposures at –5.8 km are higher than any plateau surface, so the reticulate material is probably –1000 –2000 Hr 0 Hpl possibly aeolian yardangs 0 1 PLANITIA at least partly younger than the preserved plateau beds. Less erosion also suggests this relative youth. –50 000 Hpl2 –7 Hr 000 Reticulate material (Hesperian)—Rough overall appearance from many short, sharp intercon- As 7– –41° However, the lower exposures and buried extensions of reticulate material are level with the lower units of –4000 nected ridges connected into a net-like pattern of cells typically 1–2 km across (figs.