Spatial and Stratigraphic Relationships Among the Clay-Bearing Outcrops in the Plains Above Mawrth Vallis.

Details Spatial and Stratigraphic Relationships Among the Clay-Bearing Outcrops in the Plains Above Mawrth Vallis. Spatial and Stratigraphic Relationships Among the Clay-Bearing Outcrops in the Plains Above Mawrth Vallis.

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p23d0091n&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #P23D-0091

Mathematics

Logic

5410 Composition (1060, 3672), 5464 Remote Sensing, 5470 Surface Materials And Properties, 5494 Instruments And Techniques, 6225 Mars

Scientific paper

Given the potential geological significance of the recent identification of a large (~100's km) region of aluminous and ferruginous clay-bearing outcrops on the plains above Mawrth Vallis [1], a detailed evaluation of the mineralogy and distribution of alteration minerals is due. In this work, we perform an independent assessment of OMEGA data, and use MOC, and THEMIS, and MOLA data to understand the mineralogical, morphological, and stratigraphic relationships between these two clay types as well as their relationship to the surrounding geological units. Our correction for atmospheric effects in OMEGA data uses a different method than the one described by [2]: it uses MOLA altimetry data of the scene to derive a functional relationship between atmospheric absorption and elevation throughout the scene for every channel, and then uses this relation to perform a correction for each pixel in the scene. The advantages of this method is that a) it does not rely on an atmospheric absorption template derived from observations taken at other times and in different regions, and b) it also corrects for the absorptive effects of aerosols that are uniformly distributed throughout the scene. Our results agree with the observations presented by [1]: we find that the brunt of the clay-bearing outcrop displays a spectroscopic signature similar to that of nontronite and it is distributed primarily along the plains on the western side of Mawrth Vallis, although there are regions of greater "areal concentration" (enhanced band-depth) occurring in localized exposures on both sides of the valley. Montmorillonite exposures are also identified. Although only one small (~ km's) zone of pure montmorillonite is identified in the southern bank of the valley, this clay is also found to be distributed throughout the scene, usually in combination with nontronite. Preliminary models of spectra obtained from the regions where the two spectral types occur are consistent with a linear mixing regime between the two clay types, suggesting checkerboard mixing of multiple surface lithologies rather than intimate mixing of minerals within a single surface unit. The presence of segregated clay mineralogies suggests a change in either the alteration regime or layering of primary mineral composition during the period of aqueous alteration. MOC-NA observations of some of the outcrops show an observable difference in morphology and tone between the two clay-bearing units, where the montmorillonite unit appears darker in tone. Comparisons of their distribution with MOLA MEGDR maps indicate that most of the nontronite outcrops occur at elevation ranges of -3600 to -1650 meters, whereas most montmorillonite outcrops occur at elevation ranges of -3600 to -2450 meters. Overall, the outcrop units are found at the highest elevations in the southern portion of the outcrop, and at the lowest elevations at the northern part of the outcrop, where they intersect the dichotomy boundary. We interpret these observations by the presence of nearly-flat lying units with a gentle (~1 degree) regional dip to the north. The presence of a dip in such close proximity to the dichotomy boundary is not unreasonable, and additional observations may provide clues regarding the formation and relative age of the dichotomy boundary. Further studies of these units may also enable us to understand the geologic context in which this alteration occurred. [1] Poulet et al. (2005) Nature, 438 [2] Langevin et al. (2005) Science, 307

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