Mapping Medusae Fossae Formation materials in the southern highlands of Mars

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.016 Byline: S.K. Harrison (a)(b), M.R. Balme (b), A. Hagermann (a), J.B. Murray (b), J-P. Muller (c) Keywords: Mars; Mars, Surface; Geological processes Abstract: The Medusae Fossae Formation (MFF) is an extensive deposit (2.2x10.sup.6 km.sup.2, Bradley, B.A., Sakimoto, S.E.H., Frey, H., Zimbelman, J.R. [2002]. J. Geophys. Res. 107, 5058) of wind-eroded material of widely debated origin, which unconformably overlies a considerable area of the crustal dichotomy boundary on Mars. The MFF shows a variety of layering patterns, erosional styles and channel-like forms and has been mapped into five main outcrops and three geological members according to exposure and stratigraphy (Scott, D.H., Tanaka, K.L., 1986. USGS Map I-1802-A; Greeley, R., Guest, J.E., 1987. Map I-1802-B; Zimbelman, J.R., Crown, D., Jenson, D., 1996. Lunar Planet. Sci. XXVII. Abstract #1748.). Away from the three main lobes are numerous outliers of MFF materials. These have mainly been reported in the northern lowlands regions (Keszthelyi, L., Jaeger, W.L., and HiRISE team, 2008. Lunar Planet. Sci. XXXIX. Abstract #2420.) but few studies have examined the possibility of MFF outliers on high ground south of the dichotomy boundary. We have searched Mars Orbiter Camera Narrow Angle (MOC NA) images for outliers in this region. Our observations show that there are many MFF outliers on the southern highlands. The characteristics of the outliers indicate materials which overlie the underlying terrain for they appear widely in dips, craters and topographic lows. The surfaces are typified by yardang fields and have a similar patchy and discontinuous nature to materials of the upper member of the MFF. Most have consistent lineation orientations across the wider area which match the dominant orientation of yardangs in the main MFF outcrops. Furthermore, elevation data shows that the maximum, minimum and mean elevations of these newly discovered outliers are closest to those of the upper member of the MFF. We therefore conclude that these deposits are MFF outliers and that they probably represent remnant upper member material. We suggest that there might be two possible explanations for these outliers: (1) the MFF had a much greater pre-erosional extent than previously estimated, or (2) materials from the main outcrops were eroded and then blown south to accrue in the highland areas, where they were subsequently reworked. We suggest that the topography of the region favors the first option. We outline an 'overflowing' layer-cake deposition model, in which layers of sediment stacked up against the dichotomy boundary until they reached the topographic level of the highlands. Further materials (that went onto become upper-member MFF material and outliers) were then deposited across a wider area, including south of the dichotomy boundary. Severe erosion subsequently removed much of this material. Author Affiliation: (a) Planetary and Space Science Research Institute, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK (b) Dept. of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK (c) University College London, Department of Space & Climate Physics, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking RH5 6NT, UK Article History: Received 18 January 2010; Revised 21 April 2010; Accepted 21 April 2010