BOUNDARIES AND STRATIGRAPHY OF THE MEDUSAE FOSSAE FORMATION AND ELYSIUM BASIN MATERIALS USING MARS ORBITER LASER ALTIMETER. Bethany A. Bradley¹, Eric B. Grosfils¹, and Susan E.H. Sakimoto², ¹Department of Geology, Pomona College, Claremont, CA 91711, ²USRA at NASA's Goddard Space Flight Center, Geodynamics Branch, Code 921, Greenbelt, MD 20771.

Introduction: The Medusae Fossae Formation (MFF) is an extensive unit covering the southern edge of the Elysium Basin on Mars. The unit is broken into three members, upper, middle, and lower, which range from 125° -225° W, 15° S-15° N. The unit is unlike any other in the area in that it forms massive, lobate deposits that are actively eroding, evidenced by parallel yardangs and exhumed craters (1). MFF is one of the youngest units in the area (2,3) so its mode of origin characterizes Mars’ most recent geologic history. Because of this there have been many hypotheses for the formation of MFF, including ignimbrites or ash flows (4,5), carbonate platforms (6), paleopolar deposits (7), and rafted pumice deposits (8). Recent studies using Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) data have shown that some unit boundaries do not agree with geologic maps, and often MOLA topography does not support previously hypothesized stratigraphic relationships between MFF and surrounding units (9). We used MOLA data to test the accuracy of previously mapped boundaries and stratigraphy based on unit elevation, roughness, and the slope of the westernmost outcrop of MFF in an area ranging from 202° -222° W, 8° S-5° N.

Table 1: Heights and slopes of Npl₂

*refers to lowland outcrops of the unit.

Elysium Basin Plains Unit: The Elysium Basin Plains Unit (Aps) has recently been interpreted as flood-like lava flows (10), a hypothesis supported by the flat MOLA signature across the area. Aps has an elevation of —2600 ± 100 m and a slope of 0° ± 0.1° . The unit underlies MFF, and in one case is surrounded by MFF, indicating that it was deposited earlier. The western outcrop of Aps in this area is knobby due to a high concentration of Npl₂ peaks poking through. The presence of Aps between these peaks indicates that the plains unit was able to flow around obstacles to evenly coat the basin. While Aps does extend to the dichotomy boundary, no fanlike structures are evident at the boundary where it remains flat and even. Thus outwash from the local Noachian Plains Unit’s channels is probably not the source of Aps.

Medusae Fossae Formation: The western outcrop of MFF can be broken down into two roughly parallel lobate masses stretching southeast to northwest (Figure 1). MFF is easily distinguishable from the surrounding Elysium Basin Plains Unit by a marked increase in roughness and elevation. Using only Viking photomosaic images the western outcrop of MFF appears dark and rough, while the east seems smooth. However, MOLA passes reveal that both lobes contain large-scale parallel dunes, which give the pass a jagged texture. Mars Orbiter Camera (MOC) image 02907 (3.87 m/pixel) gives a clear picture of this dune field. The western dunes average 100 m in height while the eastern are <50 m. The difference in dune size may be a result of extreme wind patterns at the dichotomy boundary (which forms the southern border of the western lobe), and probably accounts for the differing appearances in Viking images. Aside from the surficial roughness caused by the yardangs, MFF exhibits uniform, gradual slopes, which supports the hypothesis that the formation is unlayered.

The elevations of the two outcrops are similar. In the west the formation ranges from —2600 m to —1400 m, with a maximum thickness of 1200 m assuming Aps retains an average elevation of —2600 m beneath the outcrop. In the east the range is —2600 m to —1800 m, a thickness of 800 m (Table 2). The western outcrop is highest in the center and becomes thinner toward the edges, while in the east the unit forms a series of four rolling hills from north to south. The southern slopes of the hills tend to contain yardangs that run parallel to slope direction and are large enough to be visible in Viking images. While MFF in this area was originally mapped as containing both the middle and lower members (2), based on the consistency of the elevations we believe the area contains only one member, probably the lower.

Relative elevations indicate that MFF was deposited on top of the smooth Elysium Basin Plains Unit (Aps), making it the youngest material in the area. While MFF appears more heavily cratered than the surrounding Aps, of the four craters larger than 25 km in diameter found in MFF in this area, two show the characteristic double ring of exhumed craters (1). Thus, while MFF might at first appear older given crater counts, it is important to note that some of those craters were probably present before the formation was deposited.

Table 2: Medusae Fossae Formation Elevations

Conclusions: We find that the boundaries mapped for this locality of MFF in 1987 (2) are not accurate. MFF is more extensive than originally mapped, and probably only one of the unit’s members is present. Npl₂ can also be seen surrounded by MFF in several more areas, ranging 350 km from the present dichotomy boundary. The uniform elevation and flat signature of the plains underlying MFF indicate that they are a single unit that was present prior to the deposition of MFF. The stratigraphy of the area can be simplified into three main units. Npl₂, the oldest, was heavily channeled to form a series of resistant peaks in the lowlands that were later surrounded by Aps, which was able to flow evenly around the peaks to coat the basin to a uniform —2600 m elevation. Finally MFF was deposited on top of both Npl₂ and Aps and has subsequently been eroded, forming sets of parallel yardangs and exhumed craters.

References: (1) Malin, M.C., et. al., Science, 279, 1681-1685, 1998 (2) Greeley, R., and J.E. Guest, Misc. Invest. Map I-1802-B, U.S. Geol. Surv., 1987 (3) Scott, D.H., and M.G. Chapman, Misc. Invest. Map I-2397, U.S. Geol. Surv., 1995 (4) Scott, D.H., and K.L. Tanaka, J. Geophys. Res., 87(B2), 1179-1190, 1982 (5) Zimbelman, J.R. et. al., Lunar Planet. Sci. XXVIII, 1623-1624, 1997 (6) Parker, T.J., Lunar Planet. Sci. XXII, 1029-1030, 1991 (7) Schultz, P.H. and A.B. Lutz, Icarus, 73, 91-141, 1988 (8) Mouginis-Mark, P., Lunar Planet. Sci. XXIV, 1021-1022, 1993 (9) Sakimoto, S.E.H., et. al., J. Geophys. Res., 104(E10), 24,141-24,154, 1999 (10) McEwen, A.S., et. al., Geol. Soc. Amer., 30, 402, 1998