EPSC Abstracts,
Vol. 3, EPSC2008-A-00402, 2008
European Planetary Science Congress, © Author(s) 2008
The Age of the Medusae Fossae Formation
L. Kerber, J.W. Head
(1)Brown University, Rhode Island, USA ([email protected] / Fax: 001-401-863-3978 )
Introduction
The Medusae Fossae Formation (MFF) is a
complicated and discontinuous formation located in
the southern parts of Elysium Planitia and Amazonis
Planitia (130°-230°E and 12°S-12°N), covering an
area of approximately 2.1 x 106 km2 and having an
estimated volume of 1.4 x 106 km3 [1]. It is thought to
have been deposited during the Amazonian period
[2,3]. However, much of the cratering record may
have been erased as friable units were eroded and
long-buried terrains exhumed [4-6]. The formation is
characterized by large accumulations of fine-grained,
friable deposits and evidence of large amounts of
erosion. There are many theories regarding the
emplacement of this formation; recently the literature
has focused on three possibilities: ignimbrites, ash fall,
and aeolian dust.
Some modified and inverted fluvial channels have
been found within the deposit [7,8], (Fig. 1),
indicating that there was some fluvial activity during
or after the emplacement of the MFF.
Cratering Record
The cratering record of the MFF and other easily
eroded units has often been deemed unreliable [4, 10,
12], but it continues to be cited as evidence for the
formation’s young age. Throughout the MFF, pedestal
craters, inverted craters, and remnant knobs can be
found which trace the progression of crater erosion
and suggest that a large number of craters originally
superposed on the unit have been erased or degraded
beyond recognition (Fig. 2).
Fig. 2 Progression of aeolian erosion of craters from pedestal craters
to remnant knobs, as described by Schultz [5,6].
Fig. 1 CTX image (P03_002279_1737) of inverted channels in
southwestern Aeolis Planum.
If the MFF is among the youngest surficial
deposits on Mars [9], it is implied that meandering,
channelized flow must have extended into the
Amazonian, a significant constraint when considering
the atmospheric evolution of the planet through time.
Because of the wide implications that these
findings have for the evolution of Mars and the
Martian atmosphere, it is instructive to re-examine the
evidence for the Amazonian age of the MFF. The
initial conclusion comes from two main arguments:
the relatively few superposed craters on the unit, and
the superposition of the MFF on young lowland lava
deposits [1, 9]. Using new high resolution data, we reexamine the relationships both within the MFF and
with respect to adjacent units.
Stratigraphy
Because lava units are emplaced in a geologically
short period of time, cease to evolve after
emplacement, and are relatively resistant to erosion,
they are useful age markers for adjacent units such as
the MFF which are more difficult to date. Early
descriptions noted that lowland lava flows embay the
formation in places [12], but later works have placed
the MFF at the top of the stratigraphic column because
it overlies lowland lavas [9]. We have observed that
Cerberus lavas embay the formation in Northern
Aeolis Planum and Zephyria Planum, though eroded
Medusae Fossae Formation has been blown out onto
the lava where it collects as sand dunes.
In other places direct contacts are lacking, but
ancient contacts may be recognized where lava units
embayed the MFF after which the MFF was eroded
from these areas, leaving irregular depressions and
negative yardang patterns in the embaying lava units
(Fig. 3).
Fig. 4 Hesperian volcano Apollinaris Patera embays the MFF,
suggesting an older age for the formation (MOC image m2100205).
Discussion
The Medusae Fossae Formation is a mobile unit
which both erases craters on its own surface as well as
sheltering nearby units from incoming projectiles,
resulting in a misleadingly young age. The MFF
appears to continuously erode into yardangs and
reaccumulate as dunes and massive deposits. If it
comes to a rest, the unconsolidated material gradually
becomes indurated (perhaps through compression or
interaction with the atmosphere) until it reaches a
point where it can begin eroding into yardangs again.
Lava flows, which remain relatively stable through
time, make useful chronological markers for where the
unit is and where it used to be. Recognition of
remnant contacts between MFF and lava units is
helpful in unravelling relationships between the MFF
and lava units where direct contacts are not available.
Lava unit contacts suggest that parts of the MFF may
be older than previously hypothesized, perhaps
Hesperian. This conclusion is consistent with the
presence of fluvial channels within the deposit and
relaxes the time constraint on its emplacement.
Fig. 3 From top to bottom, (1) the embayment of a field of MFF
yardangs, (2) the erosion of the yardangs through time, (3) the
resulting negative yardang lava morphology.
These boundaries are found in most places where
there is a contact with a lava unit, including
northwestern Aeolis Planum, north and east of
Eumenides Dorsum, and in north-eastern Gordii
Dorsum. In between Eumenides Dorsum and
Amazonis Planum, several lobes of lava from Arsia
Mons, mapped as Late Hesperian/Early Amazonian
(AHt3) by [2], end abruptly in a negative yardang
pattern, in close proximity to an eroding patch of MFF.
Additionally, the large fan on the southeastern flank of
Apollinaris Patera appears to embay the MFF
yardangs at its base (Fig. 4).
References
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107, E8. [2] Scott, D.H. and Tanaka, K.L. (1986)
USGS Misc. Invest. Ser. Map I-1802-A. [3] Greeley, R.
and Guest, J. (1987) USGS Misc. Inv. Series Map I1802-B. [4] Schultz, P.H. and Lutz, A.B. (1988)
Icarus 73, 91-141 [5] Schultz, P.H. (2006) Plan.
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Science 318, 1080-1081. [7] Zimbelman, J.R. (2000)
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