James R. Zimbelman, Andrew K. Johnston, and Crystal G. Lovett
Center for Earth and Planetary Studies, National Air and Space Museum
MRC 315, Smithsonian Institution, Washington, D.C. 20560
This work was presented as a poster at the 1996 meeting of the Geological Society of America in Denver. For more information, please e-mail James Zimbelman at email@example.com
Click on the image below to download a copy (400K jpeg) of the preliminary map.
Preliminary geologic mapping at 1:500,000 scale has been carried out for the Mars Transverse Mercator (MTM) quadrangle 05142, which contains abundant exposures of the enigmatic Medusae Fossae Formation (MFF). The quadrangle extends over N2.5 to 7.5 lat. and W140 to 145 long., located west of the Tharsis Montes shield volcanoes, south of Olympus Mons volcano, and north of the boundary of the southern cratered highlands. Geologic analysis was greatly aided by the excellent Viking Orbiter images in this area, covering most of the quadrangle at a spatial resolution of 22 to 33 m/pixel. Significant findings include the identification of three distinct layers within materials previously mapped as the middle member of MFF, evidence of significant wind shifts recorded by yardangs on the tops of these three layers, abundant geomorphic evidence of aeolian erosion that stripped away the friable MFF materials, pristine-appearing volcanic and impact terrains exposed beneath the MFF materials, and possible folds in the upper member of MFF west of the Gordii Dorsum escarpment. These results are very encouraging for the eventual goal of assessing published hypotheses of origin for MFF, which range from ignimbrites to aeolian deposits to carbonate platform materials. The mapped geologic contacts and structural symbols were digitized in ARC/Info and digitally compiled using Adobe Illustrator. The final map (to be published by the U.S.G.S. following technical review) should also include geology for MTM 00142, which would complete coverage of the Gordii Dorsum escarpment.
Massive deposits in the Amazonis Planitia region of Mars, distributed within 12 S to 18 N latitude and 125 to 220 W longitude, have been mapped regionally as the Medusae Fossae Formation (MFF) . These materials have been subdivided locally into from three  to seven  individual units. Recently three Mars Transverse Mercator (MTM) sheets at 1:500,000 scale were approved by NASA for systematic geologic mapping, with the goal of obtaining constraints on numerous hypotheses of formation of these enigmatic materials. Several hypotheses have been proposed for the formation of the MFF materials, summarized in . These include the interpretation of MFF as ignimbrites [1,2,4], massive accumulations of variably indurated aeolian materials [1,5,6], paleopolar layered deposits emplaced when Mars rotation axis was at a different position than at present , exposed carbonate platforms deposited in a Martian ocean , and exhumed transcurrent faulting along the Gordii Dorsum escarpment [9, 10]. The current mapping effort eventually will provide new stratigraphic constraints that should prove helpful in evaluating some of the proposed hypotheses of formation.
Excellent Viking Orbiter images of MFF were obtained late in the extended mission of the long-lived Viking Orbiter 1 (revolutions 462 to 473 of the Survey Mission, equivalent to orbits 1462 to 1473). These images were obtained when the periapsis altitude of the orbiter had been lowered to around 300 km, resulting in an image resolution of 22-33 m/pixel. Digital processing and mosaicing of selected images (see figures below) was carried out in support of the geologic mapping undertaken for MTM quadrangle 05142 (where the numbers correspond to the center of the quadrangle at 5 N latitude, 142.5 W longitude). The images were combined in a photobase at 1:500,000 scale at the U.S. Geological Survey in Flagstaff. Geologic features were interpreted from the photobase and drawn by hand. The results were digitized and georeferenced using Arc/Info software. Illustrator was used for output of the preliminary map shown here.
The Medusae Fossae materials are extremely friable and easily eroded by the wind, which apparently has blown across the surface in significantly different directions during the emplacement of three discreet layers (Figure 1 and Figure 2) which we have identified within the previously mapped single unit of the middle member of MFF. Photoclinometry of partially exhumed impact craters and erosional scarps indicate minimum thicknesses of 200-300 m for portions of all three layers of the middle member of MFF . Several exhumed surfaces, exposed as MFF materials have been partially to completely stripped away, show pristine-appearing morphology of lava flow fronts and impact craters (Figure 3 , Figure 4 , and Figure 5). The preservation of these morphologies after both burial by and exhumation from beneath MFF place a useful constraint on the physical strength of these materials; namely, the strength of the MFF materials is substantially less than the strength of the underlying volcanic rocks preserved in the visible flow fronts. Also, the deposition and erosion associated with identifiable units of MFF was insufficiently energetic to disrupt the underlying surface detail. No internal layering has been observed within the individual units mapped here; some hypotheses (e.g., the ignimbrite and paleopolar origins) might be expected to result in layering of some form that could be resolvable in the Viking images. The Gordii Dorsum escarpment cuts diagonally across the map area, providing the main boundary between the upper member of MFF (southwest of the escarpment) and the partially to completely exhumed middle member units, along with exposed patches of even older units. The upper member of MFF (unit Amu) also appears to have some layering comparable in thickness to the three layers documented here for the middle member of MFF, as in an anticline within the Amu materials at the southern edge of the map. We hope that future mapping, such as in MTM quadrangle 00142 (directly south of the present map), will allow us to further subdivide the Amu materials. Efforts are also underway to compare the MFF materials to the aeolian-eroded Los Frailes ignimbrite in Bolivia [11, 12], as a specific test of the ignimbrite hypothesis.
1) D.H. Scott and K.L. Tanaka (1986) Geologic map of the western equatorial region of
Mars, U. S. Geological Survey Misc. Invest. Series Map I-1802-A, scale 1:15,000,000.
2) D.H. Scott and K.L. Tanaka (1982) Ignimbrites of Amazonis Planitia region of Mars, J. Geophys. Res. 87, 1179-1190.
3) J.R. Zimbelman (1995) An examination of hypotheses of formation for the enigmatic massive deposits in Amazonis Planitia, Mars, Lunar and Planetary Science XXII, pp. 1559-1560, Lunar and Planetary Institute, Houston, Texas.
4) M.C. Malin (1979) Evidence of indurated deposits of fine materials on Mars, NASA Conf. Pub. 2072, 54.
5) W. Ward (1979) Yardangs on Mars: Evidence of recent wind erosion, J. Geophys. Res. 79, 8147-8166.
6) G.L. Wells and J.R. Zimbelman (1989) Extraterrestrial arid surface processes, in Arid Zone Geomorphology (D. Thomas, ed.), pp. 335-358, Belhaven Press, London.
7) P.H. Schultz and A.B. Lutz (1988) Polar wandering on Mars, Icarus 73, 91-141.
8) T.J. Parker (1991) A comparison of the Martian Medusae Fossae Formation with terrestrial carbonate platforms, Lunar and Planetary Science XXII, pp. 1029-1030, Lunar and Planetary Institute, Houston.
9) R.D. Forsythe and J.R. Zimbelman (1988) Is the Gordii Dorsum escarpment an exhumed transcurrent fault?, Nature 336, 143-146.
10) R.D. Forsythe, J.R. Zimbelman, and N.G. Barlow (1991) Comparison of data sets (topographic, geology, geophysics, cratering) for Mars western equatorial transition zone, Lunar and Planetary Science XXII, pp. 403-404, Lunar and Planetary Institute, Houston, Texas.
11) J. Zimbelman, D. Crown, and D. Jenson (1996) Initial investigation of the enigmatic massive deposits in Amazonis Planitia, Mars, Lunar and Planetary Science XXVII, pp. 1495-1496, Lunar and Planetary Institute, Houston, Texas.
12) S.L. de Silva and P.W. Francis (1991) Volcanoes of the Central Andes, p. 160-163, Springer-Verlag, London.
April 2, 2001 (jh)
©1994-2001 National Air and Space Museum