ATLAS OF MARS 1:1,000,000 GEOLOGIC SERIES DAO, HARMAKHIS, AND REULL VALLES—MARS U.S. DEPARTMENT OF THE INTERIOR Prepared for the M 1M –40/262, –40/267, –40/272 G, 1998 U.S. GEOLOGICAL SURVEY NATIONAL AERONAUTICS AND SPACE ADMINISTRATION I–2557 272 271° 270° 269° 268 CORRELATION OF MAP UNITS or, more likely, to thermokarst modification of the surface material that overlies an ice-rich 273° ° ° North 267° 274° 266° substrate. Thermokarst topography is generated from softening and partial collapse of ice- 275° 265° MOUNTAIN 264 SURFACE CHANNEL AND PLAINS CRATER rich surface materials, generating a hummocky, irregular surface. These surface materials ° SYSTEM AND UPLAND Nm 263° MATERIALS VALLIS MATERIALS MATERIALS MATERIALS may consist of sediments shed from nearby Noachian highlands, or they may be eolian c MATERIALS HNh 3 AHvi 262 −37.5° ° deposits derived from a combination of local and more distant terrain. Alternatively, the sur- cf Ada Nm Nm 261° face materials may be lavas that were erupted from local fissures or from Hadriaca or Tyr- AHch Nm rhena Paterae to the north (Crown and Greeley, 1993). Whatever the nature of the surface AHch 260° Ada Hps Nm Nh1 Nm materials, partial melting of subsurface ice probably generated the observed thermokarst appearance of the hummocky plains material. The thermokarst topography may be inter- AHch AHvi c −37.5 preted as a relict landform because it seems to be most prominently displayed on the oldest 3 Nm Nm ° c3 plains unit (unit HNh). c2 AHpp c2 As Smooth plains material (unit Hps), which may be either interbedded volcanics or a sedi- −38° AHch mentary mantle or a combination of both, locally overlies the hummocky plains material. Nm AHpp Most likely the smooth plains material is sedimentary because it lacks visible lava flow struc- c 2 Nm tures, such as lobate flow margins. At three places near the center of the map area, in-facing AHch Nm AHv scarps of the younger smooth plains material form windows through which the underlying Hps AHv hummocky plains material is exposed (fig. 3). Sinuous features, having positive relief and c −38° AHk AHpp 2 lobate margins in places, are also seen in the smooth plains unit (fig. 3). These features are Hps Nm Nm cf Nm c cf Hps 2 interpreted as former channels that were filled with material which is now more resistant to AHch AHvi erosion than the surrounding plains materials, and they are topographically inverted. These AHch Ada AHv AHm AHvi c AHv resistant materials may have originated as lava that issued from fissures, though sources for 3 As 2 AHpp AHch the lava flows are not evident in the near vicinity. A sedimentary origin is more likely, as Nm AHv1 cf Hps mudflows or sediment-laden water from the release of ground water under pressure might AHk AHvi c 2 Nm Hesperian–Amazonian Amazonian have filled the preexisting valleys. In order for the resulting sedimentary materials to be more HNh AHvi c2 AHk Nm resistant than surrounding materials, they must have been cemented or laden with large Nh clasts. Proposed catastrophic release of ground water (Carr, 1979; Baker and others, 1992; c 1 2 AHk As HNh c1 Clifford, 1993) would have had the capacity and competence to transport such a sediment −39° load. AHk As In places along the margins of the valleys and at the head of Harmakhis Vallis, both S Nm Nh1 I Nm smooth and hummocky plains materials (units Hps and HNh) collapsed during vallis forma- c2 L Hps L AHv tion and were modified to form large, smooth, irregularly shaped blocks. The collapse of A AHpp Noachian V cf Nm these materials—mapped as irregular vallis floor material (unit AHvi; figs. 4, 5)—probably AHvi AHch −39 c ° occurred following saturation of the plains materials due to melting ground ice at the time of O HNh 2 Ada AHv2 A vallis formation. D c2 Ada of figure 5 AHch c3 Nm Along the walls of Reull Vallis, the layered nature of at least two plains units (probably of figure 4 cf units HNh and Hps) can be observed (fig. 5). The layered appearance is probably a result of HNh c3 DESCRIPTION OF MAP UNITS differential weathering and erosion, which is due to the units possessing slightly different c2 I S Boundary L c1 resistances to weathering and erosion processes and which may or may not follow forma- L AHk Boundary SURFACE MATERIALS AHv1 A HNh tional contacts. Hps V AHch Ada Debris apron material—Smooth or pitted lobate deposits form slopes at base North of Dao Vallis, the smooth plains material (unit Hps) has a ridged appearance (fig. cf of mountains. Interpretation: Weathered debris from mountainous terrain 6). The ridges are large-scale, gently arcuate, parallel features that trend nearly east-west and AHch As that occupy more than 2,000 square km. The rhythmic spacing between ridge crests ranges AHk AHv moved downslope as viscous or partly fluid mass. Surfaces pitted by col- −40° Hps As lapse of water- or ice-rich deposits from 5 to 15 km (fig. 6). The ridged topography is crosscut by Dao Vallis, though south of West c c 3 2 Hps AHk Dao Vallis these features are less aerially extensive. The large ridges may have formed as AHv HNh Nh1 R As Slide material—Irregular, lobate deposits; some at base of curvilinear slopes. AHk E U L glacial moraines; periglacial features associated with permafrost; giant eolian, fluvial, or c3 Boundary of figure 3 L V A L L I S cf Interpretation: Plains materials wetted during outflow events moved down- Hps c2 lacustrian ripple marks; or volcanic or tectonic features. The magnitude of the rhythmic fea- slope as viscous mass into channels c2 tures suggests that the ridges probably formed by solifluction—the slow downslope move- East Hps Nh cf −40° CHANNEL AND VALLIS MATERIALS ment of permafrost terrain. If so, a regional paleoslope that was nearly due south (about 45 1 AHm As AHch Ada AHv Vallis floor material—Smooth surfaces on floors of steep-walled outflow chan- degrees from the slope radial to Hellas along which Dao Vallis flowed) is suggested by the HNh figure 6 nels. Interpretation: Sediments deposited and smoothed by water during nearly east-west ridge trend. The existence of a more southerly paleoslope is also supported As Nm times of outflow in valles by the presence of a channel network near Harmakhis Vallis (discussed below with other AHch As channel features). Just south of the large-scale ridges is a line of similarly sized cuspate c3 AHk Knobby vallis floor material—Hummocky surfaces on steep-walled vallis Boundary of c1 As ridges, with cusps ~50 km long trending generally north-northeast to south-southwest across As AHch floors. Interpretation: Plains materials modified by fluvial processes. Knobs Hps the west-central part of the map area (fig. 6). This line of ridges does not have the same AHch cf c1 c1 probably partially eroded remnants that collapsed into vallis AHvi c morphology as the large-scale ridges and so is likely of a different origin—probably either c3 2 c2 AHvi Irregular vallis floor material—Large, smooth, irregularly shaped blocks of low Nh glacial moraines or old, degraded wrinkle ridges. AHm AHv 1 relief dissected by small channels and small to large fractures. Located at c2 Two other plains units are mapped in the east half of the map area. Mesa material (unit heads and margins of valles. Interpretation: Plains materials modified by −41° AHm) forms smooth, raised, flat-topped features with irregular escarpment margins in the cf fluvial erosion in initial stages of collapse southeastern part of the map area. The larger isolated mesas extend over areas of ~300 AHm AHch Channel floor material—Smooth surfaces in elongate, sinuous channels; linear km2. Shadow measurements (fig. 7) show the mesa escarpments to be ~350 meters high. HNh HNh and curvilinear features parallel channel margins. Interpretation: Channel The escarpments appear to be erosional remnants of a former wide-spread mantle, most AHch deposits containing fluvial erosional and depositional features likely eolian material, which rests on the older smooth plains material (unit Hps). Pitted c1 c AHm AHv2 Younger cut-off vallis floor material—Smooth materials in elongate, sinuous plains material (unit AHpp) is associated with mountainous terrain and debris aprons and is cf 3 −41° cf channels; crosscut by unit AHv. Interpretation: Channel deposits in situated north of Reull Vallis (fig. 5). Pitted plains are interpreted to be partially collapsed AHm HNh AHch younger cut-off channel segments of Dao Vallis water- or ice-rich materials—either lava flows or, more likely, sedimentary deposits (Squyres, Boundary of figure 8 Hps AHv 1979; Squyres and Carr, 1986; Crown and others, 1992; Squyres and others, 1992). c3 1 Older cut-off vallis floor material—Smooth surfaces in elongate, sinuous S channels; similar to materials in unit AHv ; crosscut by younger channels HNh H I Hps 2 HESPERIAN AND AMAZONIAN SYSTEMS Hps K Hps (units AHv and AHv2). Interpretation: Channel deposits in older cut-off A channel segments of Dao Vallis Channels within the map area show two distinct morphologies. Younger outflow chan- Hps M c3 R AHch Hps A nels are predominant and form features called valles—somewhat degraded trough-shaped H AHm PLAINS MATERIALS valleys that head in large semicircular depressions and that have a fairly consistent down- AHm AHpp Pitted plains material—Smooth material with regularly spaced, shallow, circu- stream width of ~10 km. Shadow measurements show that the vallis walls stand between AHv lar depressions. Interpretation: Partially collapsed water- or ice-rich depos- 0.75 and 3.5 km above the channel floor.