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The Light-toned Sediments in and near lower Mawrth Vallis are a Drape Deposit Alan D. Howard Jeffrey M. Moore
Phyllosilicates in the Marwth Vallis Region • Interbedded dark- and light-toned deposits occur within and on the uplands surrounding the lower end of Marwth Vallis over an elevation range of from about –1800 to –3700 m • Parts of these deposits exhibit extensive and concentrated phyllosilicate spectra, but their origin and time of emplacement remain uncertain (Poulet et al., 2005; Bibring et al., 2006) • Previous studies have suggested that the deposits pre-date the formation of Mawrth Vallis (Edgett, 2005; Poulet et al., 2005; Bibring et al., 2006; Ferrand et al., 2006; Michalski et al., 2006) • We present observations that suggest that the deposits post-date excavation of Mawrth Vallis and occur as a drape deposit
Mawrth Vallis occurs near the highlands-lowlands transition in a portion of Arabia that exhibits few normal valley networks The arrows point to the source and termination of the vallis Mawrth Vallis
The red line shows the approximate distribution of strong exposures of light-toned sediments
Blue= Light deposits Bibring et al., 2006 phyllosilicates Red=high p. Orange=mid p. HRSC 1293-0 & MOLA • Phyllosilicates are present both on divides and within Mawrth Vallis, including the valley floor • Phyllosilicates correlate with light-toned deposits.
Discussionwill focus on two locations along lower Mawrth Vallis At location 1 the flows through the valley bifurcated and created two central benches Location 2 contains the highest concentration of phyllosilicates on the bed of the valley 1 2 Subset of HRSC 1293-0000 & MOLA-based topography
Arrows show the inferred flow paths through Mawrth Vallis The two central islands (e.g., at (g)) rise about 600 m above the valley floor The working hypothesis is that if the layered deposits predated formation of Mawrth Vallis, thick sequences of erosionally truncated layers should be seen on the steep sides of the islands THEMIS VIS mosaic & MOLA topography
MOC S0300725 THEMIS VIS V01546003 MOC E2001536 These exposures of layered deposits from Galle, Becquerel, and Terby craters are what one might expect on steep slopes eroded into layered deposits
MOC E2202974 Exposures of layered deposits in the interior walls of fresh craters demonstrate their well-layered habit. Scale bars are 500 m
MOC M0706145 At (b) the surface slopes at about 0.06 but layer outcrops appear to parallel the surface
MOC M0706145 At (e) the surface slopes at about 0.06 but outcrops, where found, appear to parallel the surface.
MOC M0706145 The surface here slopes at about 0.2 but exposed layering, if present, parallels the surface
MOC E2202974 In the middle portion of this image the surface slopes at about 0.2, and exposes a massive, medium-toned unit which we interpret to be exposure of the underlying bedrock.
Light-toned phyllosilicate-rich deposits occur both on the valley floor “&” and on the upland flat “@” • These are discontinuously capped by a dark-toned mantle in the valley bottom “&” and on the uplands and upper valley sidewalls (arrows) • Channels mantled with dark-toned deposits occur both within the valley bottom “C” and on the valley sidewalls “D” • A continuous ramp of partially dark-mantled light-toned deposits may extend from the uplands to the lowlands at “$” • Medium-toned, massive materials are exposed along the lower valley walls “#” Region 2 D THEMIS VIS mosaic & MOLA topography
Brown is inferred exposure of medium-toned, massive bedrock along the steeper valley sideslopes. Scattered channels occur within or below the layered deposits (violet). These channels conform to the modern topography HRSC 1293-0000 & MOLA-based topography
Summary • We suggest a simple stratigraphic sequence of terrain-conforming, light-toned, phyllosilicate-rich sedimentary layers overlain by a dark mantle that is also terrain-conforming • On steep slopes the layered deposits have been eroded (or were not deposited), exposing the underlying medium-toned, massive bedrock
Edgett (2005) concluded that crater “X” impacted after the bright-toned layered deposits were emplaced. Examination of layering in relationship to topography suggests, however, that the layers draped over a pre-existing crater X
This is a THEMIS VIS mosaic superimposed upon MOLA-derived topography We will look at layer relationships at the crater rim at locations 1 and 2 and at layering on the crater floor at 3. 1 2 3
Summary of Observations • Layers are well exposed on the inner rims of small craters superimposed on the main crater rim • Layers are also exposed near “#” on the steepest crater rim and appear to be absent on the steep, lower crater wall • Layer truncation is not obvious on the relatively steep slopes dipping toward and away from the rim highpoint • If ejecta from the crater once covered the layered deposits, it has been gently eroded without creation of erosional holes through the layered deposits on the outside rim of the crater
2 Note that layers show no obvious truncation on steeply-sloping outer (north) crater rim
3 Regularly bedded light- and dark-toned layering is found on the crater floor. If the layers were present before impact, they should be strongly distorted and fracture.
Implications: If the Phyllosilicates are a Drape Deposit • The drape was deposited over a vertical elevation range exceeding 1.6 km, from about -1800 m to below -3400 m • The geographical distribution suggests deposition in association with the highlands-lowlands boundary • The light-toned layers have a variable cover of a dark mantle that is also terrain-conforming • A reasonable suggestion is deposition from standing water, either a northern ocean or otherwise ponded water • Because of its geographical restriction to the vicinity of Mawrth Vallis, the sediment might have been introduced by flows down the valley
Age Implications • If a drape deposit, the phyllosilicates post-date the period of main excavation of Mawrth Vallis • The deposits, however, could be associated with late-stage flow activity within the valley • The age of the deposits thus hinges on the age and origin of Mawrth Vallis
Mawrth Vallis • Marwth Vallis is not your typical outflow channel • It lacks an obvious source from chaotic terrain or breaching of an enclosed basin, but rather appears to have a number of sources • The source region is a local topographic high • Mawrth lacks appreciable tributaries, has nearly constant width (~20 km), and is broadly curving (radius ~40 km) • A number of >10 km craters superimpose Marwth Vallis and the phyllosilicate deposits, possibly suggesting a Noachian age and an origin predating most other outflow channels