Icarus 206 (2010) 669–684

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Icarus

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The age of the Medusae Fossae Formation: Evidence of Hesperianemplacement from crater morphology, stratigraphy, and ancient lava contacts

Laura Kerber *, James W. HeadDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 June 2009Revised 29 September 2009Accepted 2 October 2009Available online 9 October 2009

Keywords:MarsMars, SurfaceGeological processesVolcanism

0019-1035/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.icarus.2009.10.001

* Corresponding author. Address: Geological ScieBrook St., Box 1846, Providence, RI 02912, USA. Fax:

E-mail address: Laura_Kerber@brown.edu (L. Kerb

The Medusae Fossae Formation (MFF), covering about 2.1 � 106 km2 (with an estimated volume of1.4 � 106 km3) and straddling the equatorial region of Mars east of Tharsis, has historically been mappedand dated as Amazonian in age. Analysis of the MFF using a range of new observations from recent mis-sion data at multiple resolutions reveals evidence that the formation is older than previously hypothe-sized, with parts of the MFF having formed in the Hesperian and parts having been reworked andreformed throughout the Amazonian, up to the present. Ancient outcroppings of the MFF, edged with jag-ged yardangs, became a ‘‘mold” for embaying Hesperian-aged lavas. The erosion of the MFF left solidifiedlava ‘‘casts” in the embaying lava unit. This lava edge morphology permits the identification of ancientcontacts between the MFF and Hesperian-aged lava terrain. Additionally, the flanking fan of the Hespe-rian-aged Apollinaris Patera volcano embays the formation at its foot, indicating that parts of the MFFwere formed in the Hesperian. Erosion has erased and inverted many of the superposed craters in theregion, showing that very young Amazonian ages derived from impact crater size–frequency distribu-tions are resurfacing ages, and not emplacement ages. We find abundant evidence that the formationis extremely mobile and continuously reworked. We conclude that a significant part of the MFF may haveoriginally been emplaced in the Hesperian. These observations place new constraints on the mode oforigin of the MFF.

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1. Introduction

The Medusae Fossae Formation (MFF) is a complicated and dis-continuous formation located in southern Elysium and AmazonisPlanitias and northern Memnonia and Aeolis Planitias(130–230�E and 12�S–12�N), covering an area of approximately2.1 � 106 km2 with an estimated volume of 1.4 � 106 km3 (Bradleyet al., 2002) (Fig. 1). The formation is characterized by large accu-mulations of fine-grained, friable deposits (e.g., Scott and Tanaka,1986; Greeley and Guest, 1987; Zimbelman et al., 1996). Theboundaries of the deposit correlate with some parts of the ‘‘stealth”and ‘‘greater stealth” regions defined by Muhleman et al. (1991)and Butler (1994) on the basis of their unusual radar properties(extremely low return when probed with 3.5-cm wavelengthradar, indicating a very low-density material with very few rocks),though whether the units are related is unresolved (Edgett et al.,1997). Later radar analysis conducted with the Mars AdvancedRadar for Subsurface and Ionospheric Sounding (MARSIS) at longer

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nces, Brown University, 324+1 401 863 3978.er).

wavelengths confirmed that the MFF had dielectric properties con-sistent with either relatively clean water ice or dry, low-densitymaterials (Watters et al., 2007).

The age of the MFF has been determined mainly using cratercounting techniques and stratigraphic relationships. Scott andTanaka (1982) divided the formation into seven separate unitsand counted the number of >1 km diameter craters on each unit,yielding cumulative crater counts between 729 and 63.6 > 1 kmcraters/106 km2. The majority of the formation was thus placedin the mid-Amazonian, with a few units in the lower Amazonianand one unit in the upper Amazonian. These units were latercombined into three main geologic units based on their colorand states of erosion: an upper member (Amu) characterizedby smooth and rolling light-colored plains, a middle member(Amm) characterized by more progressed erosion, and a lowermember (Aml) characterized by extensive erosion and a darkercolor (Scott and Tanaka, 1986; Greeley and Guest, 1987). Theseunits were placed in the middle to late Amazonian based oncumulative superposed craters (0–200 > 2 km craters/106 km2).Later crater counts done by Werner (2005) on areas near GusevCrater and southwest Amazonis Mensae were interpreted to sug-gest a global formation age for the MFF of 1.6 Ga (early to mid-Amazonian according to Hartmann and Neukum (2001)).

Fig. 1. A regional MOLA hillshade of the Medusae Fossae Formation, stretched to show detail within the formation. Bottom panel shows the locations of the figures. Blackoutline indicates the boundary of the Medusae Fossae Formation as mapped by Scott and Tanaka (1986) and Greeley and Guest (1987). Lat.: 27�S–24�N, Lon.: 140 to �130�E.

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There is evidence of significant wind-related erosion; yardangsare abundant in the region (Ward, 1979), and their frequent shiftsin orientation (depending on stratigraphic level exposed) has beencited as evidence for changes in wind direction in the past(Zimbelman et al., 1997; Zimbelman and Griffin, 2009), or changesin structurally controlled jointing in the material itself (Scott andTanaka, 1982; Bradley et al., 2002), though any joints would haveto be subparallel to the prevailing wind in order to initiate yardangformation (Livingstone and Warren, 1996; Inbar and Risso, 2001).Evidence for aeolian modification of the unit suggests that muchof the impact cratering record may have been erased as friableunits were eroded and long-buried terrains were exhumed(Schultz and Lutz, 1988; Schultz, 2006, 2007). For this reason, itis important to attempt to distinguish between the original forma-tion age of the MFF and the modification or resurfacing age(s) ofthe MFF.

There have been many hypotheses regarding the origin of theMFF, including ash flow tuffs or ignimbrites (Scott and Tanaka,1982, 1986), pyroclastic or aeolian materials (Scott and Tanaka,1986; Greeley and Guest, 1987; Tanaka, 2000), paleopolar depositsor loess (Schultz and Lutz, 1988; Schultz, 2002), carbonate plat-forms (Parker, 1991), ice-rich dusty mantling deposited duringhigh obliquity (Head and Kreslavsky, 2004), or ash fall (Tanaka,2000; Bradley et al., 2002; Hynek et al., 2003). As image resolutionhas improved, the carbonate platform hypothesis and the raftedpumice deposit hypothesis have been deemed less likely (Mandtet al., 2007, 2008). Paleopolar deposits have been rejected basedon the young age of the formation: Grimm and Solomon (1986)have suggested that any significant true polar wander would leave

a tectonic signature, which is not found. This result would suggestthat no spin axis reorientation has taken place since at least theend of the heavy bombardment. Additionally, the locations andages of the south-pole pitted terrains suggest that no significantpolar wandering has taken place since the early Hesperian (Tanaka,2000). Mantling during high obliquity could result in deposits thatresemble polar deposits without the need for true polar wander(Head and Kreslavsky, 2004); however, shallow radar (SHARAD)analysis of the MFF has failed to detect the fine-scale layering thatis characteristic of the polar deposits (Carter et al., 2009). Datafrom SHARAD suggest that unlike the polar deposits, MFF layershave low permittivity contrasts or the layers are discontinuous(Carter et al., 2009). Several analyses of these hypotheses havebeen done, reaching the conclusion that the formation is mostlikely composed of volcanic ash, ignimbrites, or aeolian dust(Zimbelman et al., 1997; Mandt et al., 2007, 2008).

The composition and mode of emplacement of the depositshave broad implications for the evolution of the surface and atmo-sphere of Mars in the Amazonian. A volcanic hypothesis would re-quire the eruption of large amounts of pyroclastic material froma nearby volcanic center. Both the Tharsis Montes and ElysiumMontes have been active in the Amazonian and could potentiallyprovide a source for the needed pyroclastics for the deposit. Thesurfaces of these volcanoes are currently dominated by effusivelava flows; however, some evidence of recent ash has been notedat the summit of Arsia Mons (Mouginis-Mark, 2002). Explosive vol-canic eruptions would be able to emplace blankets of material rel-atively instantaneously over large areas (Wilson and Head, 1994).If the deposits were poorly lithified, they could be steadily eroded

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over long periods of time in the Mars environment. Different yard-ang orientations and bidirectional yardang patterns would form asa result of changing wind directions (Zimbelman et al., 1997;Zimbelman and Griffin, 2009).

Emplacement as atmospheric dust would require a periodic cli-mate change sufficient to transform the region alternately from adust sink into an area of intense aeolian erosion, and for a suffi-ciently long time to accumulate up to 3 km of airborne dust. Peri-ods of accumulation would have to be interrupted by periods oferosion to form internal layers of yardangs with different orienta-tions (Sakimoto, 1999; Zimbelman and Griffin, 2009). This processmay require a longer time period than that of volcanic emplace-ment, as volcanic emplacement does not require long periods ofaccumulation.

Some modified and inverted fluvial channels have also beenfound within the deposit (Zimbelman et al., 2000; Burr et al.,

Fig. 2. Evidence of fluvial reworking of the MFF. (A) A channel near Arsia Mons (3�N, 140�the larger channel in (A). (C) Inverted channels in southeast Aeolis Planum described by BP03_002279_1737, respectively).

2009; Griffin and Zimbelman, 2009; Zimbelman and Griffin,2009), indicating that there was some fluvial activity during theemplacement or modification of the Medusae Fossae Formation(Fig. 2). If the MFF is among the youngest surficial deposits onMars, it is implied that meandering, channelized flow of watermust have extended into the Amazonian (Burr et al., 2006). If theage is correct, this is a significant observation in that such fluvialactivity in the Amazonian is very rare (e.g., Dickson et al., 2009;Fassett et al., 2009).

Because of the significant implications that these findings havefor the evolution of Mars and the martian atmosphere and climate,it is important to re-examine the evidence for the Amazonian ageof the Medusae Fossae Formation. The current conclusion thatthe MFF is of Amazonian age comes from two main lines of argu-ment, as discussed earlier. First, the relatively few impact craterssuperposed on the unit led to an Amazonian age assignment for

W) described by Zimbelman et al. (2000). (B) Small, dendritic channels flow towardsurr et al. (2009) (HSRC images h2146, h2135, h2124; HSRC image h2146; CTX image

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the MFF in global mapping (e.g., Scott and Tanaka, 1986; Greeleyand Guest, 1987; less than 200 craters greater than 2 km in diam-eter in 106 km2). In addition, impact crater size–frequency distri-bution analyses provide a similar age (1.6 Ga; Werner, 2005). Thesecond line of argument for the Amazonian age comes fromstratigraphic relations. The MFF is superposed on Amazonian-aged

Fig. 3. A comparison of MFF and mid-latitude pedestal craters. Mid-latitude pedestal craasymmetric with zigzagging pedestal margins (see Kadish et al., 2009a,b). From top righ

Fig. 4. Sea-urchin-like pedestal craters in CTX and HRSC images. The indurated portions othe sinuosity of the pedestal outline is anomalously high. From top right: P06_003544_

lowland terrain and directly overlies Hesperian-aged outflowchannels such as Labou Vallis (Bradley and Sakimoto, 2001; Bradleyet al., 2002). Using high resolution data from the Mars Global SurveyorMars Orbiter Camera (MOC) and Mars Orbiter Laser Altimeter(MOLA), the Mars Odyssey Thermal Emission Imaging System(THEMIS), the Mars Express High Resolution Stereo Camera (HRSC),

ters are very symmetrical with subdued edges. MFF pedestal craters are often quitet: V18359003, V19230010, V09954010, V13935010.

f the crater ejecta deposits extend much further than is typical for such craters, and1703, h0998, P02_001843_1716 (bottom two panels).

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and the Mars Reconnaissance Orbiter Context Imager (CTX) andHigh Resolution Imaging Science Experiment (HiRISE), we re-examined the relationships both within the MFF and with respectto adjacent units. In this contribution, we examine first the state ofcrater preservation in the MFF, and thus the basis for impact cratersize–frequency distribution ages, second, the nature of the MFF it-self and its mode of emplacement and modification, and third, thestratigraphic relationships between the MFF and other unitsformed prior to, subsequent to, and contemporaneously with theMFF. On the basis of this analysis, we conclude that the MFF is adynamic unit, constantly being eroded, re-deposited, and ex-humed, making impact crater size–frequency distribution datingdifficult, and making most stratigraphic relationships not definitiveof a formation age. We conclude first that a significant part of theMFF is Hesperian in age, much older than previously hypothesized,and second, that it has been eroded, re-deposited and exhumedthroughout the Amazonian.

2. Observations

2.1. Crater preservation

The method of age-dating by crater counting requires severalpreconditions (e.g., Hartmann and Neukum, 2001). Each unit beingdated must be emplaced during one, geologically short event. After

Fig. 5. Twenty-kilometer diameter impact crater showing contrasting ejectadeposit morphologies. The left half of the crater displays a double-layered lobateejecta deposit. The perched, jagged edges of the right half of the ejecta deposit aretypical of MFF pedestals, indicating that the projectile struck the edge of a previousMFF deposit. HRSC images h2965_0000.nd2.03.04 and h2976_0000.nd2.04.04,supplemented by the THEMIS global mosaic.

emplacement, the unit must remain unmodified except for cratersformed by a well-known population of incoming projectiles. Lastly,the population of craters must not become so great as to reach sat-uration. If these conditions are met, the crater count age of a unitmay be said to be synonymous with the emplacement age, anduncertainties are limited to the character of the incoming projectilepopulation.

Of these conditions, the Medusae Fossae Formation meets nei-ther the condition of a geologically short emplacement nor thecondition of a subsequently unmodified exposure surface. In par-ticular, the MFF has been drastically modified by aeolian processes,which have continuously eroded the exposure surface and maskedthe cratering record through erasure, modification, and burial ofcraters. Because of these modifications, the crater record preservedon the MFF (i.e., the crater retention age) is not likely to representits emplacement age. The relative paucity of craters on its surfaceyields far more information about its level of activity through timethan it does about when the formation was emplaced. This is trueof the MFF and any other terrain which has been subject to signif-icant erosion, an issue that has been noted by others in regard tothe MFF as well as the polar regions and Arabia Terra (Schultzand Lutz, 1988; Tanaka, 2000; Greeley et al., 2001). By studyingthe degradation and selective preservation of the MFF cratering

Fig. 6. Progressive crater modification in the MFF. (A) Craters are formed in a fine-grained unit. (B) The heavily cratered unit is eroded by the wind, leaving armoredremnant ejecta deposits. (C) The ejecta deposit is finally eroded away, leaving onlythe strongly welded crater floor. (D) In time, the wind erodes the remaining craterfloor into a knob. HRSC images h0987 (A and C), h0998 (B and D).

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record, information may be extracted about the material propertiesof the unit and the qualities that may (or may not) be used to char-acterize its age.

Pedestal craters (Fig. 3) are observed both within the MFF andaround its margins (Schultz and Lutz, 1988). Pedestal craters aredistinct from other craters in that their ejecta deposits are perchedabove the surrounding terrain. They are believed to originally format the same level as the surrounding plains and are often associ-ated with terrain which is interpreted to be volatile rich (e.g.,Schultz and Lutz, 1988; Wrobel et al., 2006; Kadish et al., 2008).Over time, areas around the crater are deflated but the crater bowland pedestal remain at their original elevations, because they areindurated (perhaps during the impact process as a result of atmo-spheric interactions, substrate gardening, or impact melt genera-tion, e.g., Wrobel et al., 2006; Barlow, 2006). While this type ofcrater is common poleward of 40� (Mouginis-Mark, 1979), it is rareat low latitudes (Kadish and Barlow, 2006; Kadish et al., 2008,2009a,b), and the pedestal craters that appear in the MedusaeFossae Formation differ from those found at higher latitudes (e.g.,Kadish et al., 2009a) in several ways (Fig. 3). Pedestal craters foundat higher latitudes have both perched ejecta deposits and perchedcrater bowls, meaning that the bottom of the crater bowl is alsoperched above the surrounding terrain (Kadish et al., 2009a). TheMFF pedestal crater-bowls extend lower than the surrounding ter-rain, despite evidence for extensive erosion. Barlow (1993) hasnoted that craters formed in this deposit have unusual depth todiameter ratios (approximately 79% deeper than similar-sized sim-ple craters elsewhere), suggesting that projectiles may penetrate

Fig. 7. Crater degradation in the MFF. THEMIS IR mosaic of Memnonia Sulci (southwesterindicates a crater that is still embedded in its original layer. Letter B indicates pedestal chas been completely inverted and now stands as a mesa. The exclusion of these craters

deeper into this unit compared to other units on Mars (perhapsdue to more efficient compression of the low-density material).One of the most striking differences between the MFF pedestal cra-ters and those in higher latitudes is that their pedestals usuallyhave a crisp and asymmetric morphology when compared to polarpedestal craters, which exhibit a far more subdued, circular shape(Fig. 3) (Kadish et al., 2009a). The character of the MFF pedestals isinterpreted to be an indication that unlike polar pedestal craters,the surrounding terrain is most likely being removed by the windrather than by sublimation.

MFF pedestal craters have sinuous pedestal margins and long,thin rays. Like other pedestal craters, MFF pedestals have a largerextent than that expected for the ejecta blanket given the craterbowl diameter (Kadish and Barlow, 2006). These distinctive pedes-tal morphologies may be a function of either the erosion of thepedestal craters or their substrate and are very different than theirhigher latitude counterparts (compare Fig. 3, top, to Fig. 3, bottom).This morphology is found both in large MFF pedestal craters (suchas those shown in Fig. 3) and in a class of small pedestal craters(�100 m in diameter) which exhibit an irregular sea-urchin-likemorphology (Fig. 4). The effect of the substrate on crater morphol-ogy can be seen best in Fig. 5, where a crater formed on the edge ofan MFF deposit. The western half of the crater displays a lobate cra-ter morphology commonly seen on Mars, while the eastern half haslong, thin, perched rays of material characteristic of MFF pedestals.The unique morphologies of MFF craters may thus be used as adiagnostic tool to determine whether the MFF was present at thetime that a crater was formed.

n Lucus Planum) showing craters in various stages of the inversion process. Letter Araters whose original substrate has been removed. Letter C indicates a crater whichin a crater-derived age would produce an erroneously young age for the deposit.

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The presence of pedestal craters in the MFF suggests a history ofaeolian erosion and the removal and redistribution of a thicknessof material at least up to the level of the pedestal heights (whichaverage �115 m in height (Kadish et al., 2009a), with maximumheights up to 2 km (Schultz, 2007)) in and around the formation(Schultz and Lutz, 1988). Any smaller craters whose imprint intothe substrate is shallower than this layer of removed materialwould be erased completely as the surrounding layer was removed(e.g. Schultz, 2007). For example, secondary craters from 10.1 kmZunil crater, while abundant on adjacent terrain, were absent inMFF, leading Preblich et al. (2007) to estimate that some parts ofthe MFF are eroding by at least 0.08 m per year. This erosion wouldhave two effects: it would make counts of smaller craters unreli-able for age-dating and it would skew the crater size–frequencydata towards larger crater sizes.

Over time, the relatively resistant inner deposits of pedestal cra-ters are also eroded. This often occurs from the distal edges of thecraters, where unarmored materials become exposed to the windas the plains are deflated. This erosional process tends to leavethe resistant bowl of the crater perched above the surroundingplain. In time, this most resistant part of the crater is also degradedand eroded, leaving only a remnant knob. This progression was de-scribed by Schultz (2002, 2006) and is seen throughout the MFF(Fig. 6). While small craters erode to form remnant knobs, largercraters erode to leave pedestal craters and finally remnant mesas(Fig. 7). The resulting knob-like features are distinct from other

Fig. 8. Stratigraphic relationships. Medusae Fossae Formation yardangs bothoverlie (CTX image P02_001791_1852) and are embayed by (CTX imageP07_003756_1822) Late Amazonian Cerberus Aec3 lavas. Top, the northwesterntip of Zephyria Planum. Bottom, northeastern Aeolis Planum.

knobby terrains, rootless cones, pingos, and mounds in that theyare generally circular (or slightly elongated with wind direction),free-standing, non-overlapping, and lacking moats. Confident iden-tification of the most eroded inverted craters is difficult in the ab-sence of other nearby pedestal craters as context.

If many of the knobs and mesas in the MFF are indeed modifiedand inverted impact craters (Schultz and Lutz, 1988), the numberof craters superposed on the deposit would increase dramatically,

Fig. 9. Complex stratigraphic relationships in Gordii Dorsum. MFF yardangs areboth covered by and superposed on lavas from Arsia Mons. These stratigraphicrelationships make the MFF difficult to date through traditional means. THEMIS VISimage (V12699008).

Fig. 10. Redistribution of MFF. This large pedestal crater, preserving an outlyingsection of the MFF in northern Zephyria Planum below its ejecta deposit, isembayed by the young Cerberus lavas at (A). Outlying MFF sheds unconsolidatedfine particles on top of the Cerberus lavas after their emplacement (B). Theunconsolidated material is transported by the wind into adjacent terrain, where itforms blankets, wind streaks, and new yardangs that lie on top of young units (C).

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suggesting that the deposit is older than the dates obtained bycounting recognizable superposed craters only. Furthermore, evenif crater counts included these knobs and mesas (inverted craters),this would still yield a minimum age, because the crater countswould be biased toward a young age by the final erasure of knobsand mesas (Schultz and Lutz, 1988). The observed processes ofcrater modification, burial, exhumation and destruction willproduce not a formation age, but a cumulative exposure age (orcrater retention age) due to repeated burial and shielding of units

Fig. 11. A roughness map created by Kreslavsky and Head (2000). Darker areasdenote smoother surfaces; brighter areas denote rougher surface. The youngCerberus lavas (A, black) comprise the smoothest unit in the study region. There is atrough (B) between the MFF of Aeolis Planum and the adjacent Hesperian lavas (C).The smooth Cerberus lavas embay the Hesperian lavas and fill in the trough, flowingin through its southeast end (D) (see Fig. 12, extent outlined in dotted line). Theimage was created as an RGB composite. Each channel corresponds to a roughnesslength scale, in this case 9.6, 2.4, and 0.6 km. Lat.: 12�S–21�N, Lon.: 137� to �176�E.

Fig. 12. The progression of erosion and embayment in northeast Aeolis Planum. TheMFF boundary originally lay at the northeast margin of the trough. The MFFwas embayed by Hesperian lavas. The MFF then eroded towards the southwest.An impact at B armored the MFF. Further erosion caused continued recession ofthe MFF toward the southwest. In the Late Amazonian Cerberus lavas floodedand embayed the area from the northeast. A channel (A) is present where theAmazonian Cerberus lavas enter the trough. The Amazonian lavas embay thepedestal crater and yardangs at B (Fig. 13). Serrated lava margins characterizethe northeast wall of the trough at C (Fig. 16D). The MFF can be seen eroding fromthe Hesperian lava contact at D (Fig. 15). THEMIS IR mosaic of greater northeasternAeolis Planum overlaid with MOLA gridded data (red = high, blue = low). (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

interrupted by periods of exhumation (Schultz and Lutz, 1988;Schultz, 2002). Both of these factors will result in the underestimationof the formation age of the Medusae Fossae Formation (e.g., Greeleyet al., 2001; Schultz, 2006).

The cratering record of the MFF thus has three signatures: (1)superposed unmodified fresh craters, (2) remnant craters (knobs,mesas, and pedestal craters) remaining after the removal of mate-rial by aeolian processes, and (3) preserved impact craters ex-humed from older surfaces below. As a result, crater counts ofeither the MFF or the underlying terrain will yield ages that areanomalously young, since craters in the MFF were removed andthe underlying terrain was shielded by the MFF during a significantpart of its existence (Schultz, 2002). These observations and rela-tionships suggest that any age for the MFF derived from cratercounts will be a minimum age for its formation and that it will rep-resent a modification age, not a formation age, as pointed out bySchultz and Lutz (1988).

In summary, the characteristics of the subunits of the MFF areunsuited to be age dated, using the crater size–frequency distribu-tion method, and other methods of dating must be considered. Analternative technique is to use stratigraphic relationships betweenthe formation and surrounding units, such as lava flows, whichmay themselves be better suited for deriving crater age dates

Fig. 13. The contact between the Medusae Fossae Formation and the youngCerberus lavas in Aeolis Planum (142.8�E, 2.8�N). The lavas embay the formationand bury the yardangs, which can be seen faintly beneath the thin veneer. Severalinverted craters can be seen in the frame; two smaller craters have beentransformed into remnant knobs and the larger crater in the lower left has beeneroded to leave a circular mesa. These relationships indicate that the MFF predatesthe embaying Late Amazonian Cerberus lavas (AEc3, from Tanaka et al. (2005)). CTXimage (P05_002833_1819).

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due to their more coherent and less erodible nature. We nowinvestigate regional and local stratigraphic relationships withinand adjacent to the MFF.

2.2. Stratigraphy

2.2.1. Relationships with lava unitsBecause individual lava units are commonly emplaced in a geo-

logically short period of time, cease to evolve after emplacement,and are relatively resistant to erosion, they are useful age markersin relation to adjacent overlying and underlying units, such as theMedusae Fossae Formation, which are more difficult to date. How-ever, close examination of the MFF by previous workers yieldsambiguous and sometimes contradictory stratigraphic relation-ships with nearby lava units. For example, early descriptions notedthat lowland Amazonian lava flows embay the MFF in places (Scottand Tanaka, 1982). Indeed, the Amazonian-aged Cerberus plainslavas in Elysium Planum embay the MFF at Aeolis Planum(Fig. 8). Later works have placed the MFF at the top of the

Fig. 14. Yardang-molds and lava flow casts. Top: the MFF is eroded into yardangsby wind coming from the northwest, creating ‘‘molds”. Middle: the yardangs areembayed by lavas flowing from the northwest, filling the molds. Bottom:subsequent aeolian erosion of the MFF from the contact leaves serrated lavamargins (casts).

stratigraphic column because it overlies lowland Amazonian lavas(Bradley and Sakimoto, 2001). This is also correct; for example, thesame Amazonian-aged Cerberus lavas are overlain by the MFF innorthern Eumenides Dorsum and Zephyria Planum (Fig. 8). Inone particularly striking example, a lava unit near southwesternGordii Dorsum (Fig. 9), both superposes and is superposed byMFF yardangs in close proximity.

These ambiguous stratigraphic relationships lead to two possi-ble conclusions: first, that the entire MFF was not formed at thesame time, leading to different ages for different parts of the for-mation, and second, that the location and margins of the MFF havenot remained static or constant over time. According to analysis ofthe data surveyed in this study, both of these conclusions are likelyto be true. In favor of the first (the emplacement of the MFF at dif-ferent times), is the observation that there are many sequentiallayers of yardangs with different orientations (Bradley et al.,2002; Zimbelman and Griffin, 2009), implying successive periodsof emplacement and erosion. In support of the second idea (thatthe deposits and its margins have not remained constant overtime), is the erosional state of the MFF, and the evidence that itseroded material is often deposited on top of adjacent units. Forexample, in northern Zephyria Planum, there are seemingly contra-dictory juxtaposed stratigraphic relationships (Fig. 10). Here,Amazonian Cerberus lavas embay a large pedestal crater thatoriginally formed in MFF material (using the criteria discussed inFig. 5), but MFF slightly to the east of this crater is superposed

Fig. 15. MFF eroding from lava cast boundary. The MFF continues to erode awayfrom the margin of Hesperian unit HBu2 (Tanaka et al., 2005). Light colored fine-grained material collects as dunes on top of the younger embaying Cerberus lavas.This eroded material is now stratigraphically above the Cerberus lavas. Because ofthis cycle of erosion and deposition, the MFF often appears younger than adjacentunits. HRSC image h4136.

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on the young lava surface. In context, it may be seen that thematerial eroded from the ejecta deposit of the crater has likelybeen transported to the east in the form of loose material, creatingwind streaks and dunes. This loose, unconsolidated material restson top of the same Cerberus lavas that embay the large pedestalcrater. This is not because the original emplacement of the MFFpostdates the plains, but rather because the MFF has been eroded,modified, and redistributed subsequent to the emplacement of thelavas. Much of the MFF that lies on top of the lavas at this contactconsists of dunes, suggesting the lateral aeolian transport ofpreviously emplaced MFF, rather than primary formationalemplacement. Even the local presence of superposed yardangsdoes not require that this part of the deposit was formed byprimary deposition, as a large percentage of the yardangs on Earthare composed of indurated sediments and could thus be secondary,not primary (Ward, 1979). The part of the MFF that today rests ontop of the Cerberus flows was likely transported there by the windand indurated, later eroding to form yardangs. Further to the south,below the region of unconsolidated wind-streak material, theCerberus lavas once again embay the MFF.

We thus conclude that the boundaries of the Medusae FossaeFormation as they appear today are a combination of originalboundaries defining where the MFF was initially emplaced andnew boundaries where the formation was eroded back or re-em-placed as a result of aeolian processes. We conclude on the basisof our analyses (Figs. 2–10) that the MFF is very mobile, initiallyforming and then eroding from one place to be re-deposited in an-other within the region. In order to constrain the history of the for-mation and modification of the MFF, new tools must be developedto glean more information from the obvious contacts and to ex-plore and understand hidden or ambiguous contacts. To this end,we have studied lava flow-front morphology (likely to be morewell preserved than the friable MFF) near unit contacts to establishwhether the lava flows interacted with the Medusae Fossae Forma-tion at the time of lava emplacement. This technique can be used toestablish stratigraphic relationships in places where there is no di-rect contact between the MFF and nearby lava flows due to theMFF’s erosion.

Fig. 16. Observed formation progression for yardang cast formation, from top tobottom: (A) a field of yardangs is formed, (B) the yardangs are embayed by lava (themold is filled), (C) the yardangs erode away from the boundary through time, (D)the resulting lava-cast morphology (casts). (A) P22_00909_1674, (B and D)P05_002833_1819, (C) h2965.

2.3. Three type locations: Aeolis Planum, Amazonis Planitia, andApollinaris Patera

2.3.1. Aeolis PlanumFirst, we study the relationship between the MFF and the

Cerberus lavas at Aeolis Planum. The roughness map compiled byKreslavsky and Head (2000) (Fig. 11) provides evidence to recon-struct the history of this area. The young lava flows from Cerberusconstitute some of the smoothest units in the region, a combina-tion of their low viscosity (Jaeger et al., 2007) and young age (LateAmazonian according to Tanaka et al., 2005). The roughness mapreveals the location where the Cerberus lavas flows fill a trough.This trough lies between the current MFF boundary and the adja-cent Hesperian-aged lavas (mapped as unit HBu2 in Tanaka et al.(2005)).

With the addition of MOLA gridded altimetry data (Fig. 12), achannel can be seen where the new lava entered the trough atits southeast margin and filled it. In this area, the Cerberus flowsembayed and buried MFF yardangs, as evidenced by their subduedtopography still visible below the veneer of lava. Fig. 13 shows indetail the relationship between the Amazonian lavas and theMFF. Inverted craters and mesas are also embayed by the lavas,indicating that the MFF previously extended further to the eastthan it does at the present. This evidence suggests that the MFFis older than the Late Amazonian Cerberus lavas, which is consis-tent with recent mapping (Tanaka et al., 2005).

However, there is an unusual lava-flow margin that composesthe northeast wall of the trough into which the young Cerberus la-vas poured in northwest Aeolis Planum (Fig. 12C). While normally a

Fig. 18. Northwest Gordii Dorsum. A region in northwest Gordii Dorsum demon-strates a multi-staged history of emplacement and erosion. First a crater is formedin older plains (A). Second, the MFF is emplaced (B), shielding the old plains fromfurther impacts. Third, newer lavas (C) embay the Medusae Fossae Formation andthe old crater (A), surrounding outlying outcrops of MFF and embaying yardangs atthe edge of the deposit. Fourth, the MFF is eroded away, leaving pedestal craters,inverted craters, yardangs, and lava casts (D). The MFF can still be seen retreatingfrom the contact (E). The cross-hatched pattern in the eroding MFF suggest thatmore recent winds have been bidirectional, both aligned with yardang casts and(perhaps more recently) oblique to them. Indurated parts of the formation remain(F), and troughs surround these outliers, indicating the extent of the MFF at the timeof lava emplacement (G). Secondaries from crater (A) are visible where old volcanicterrain is exhumed (H). HRSC image h2965.

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lava flow forms a lobate pattern and thins at its margins, the lava atthe edge of the trough forms a cliff face punctuated by serrated, zig-zagging ridges. Given the proximity of the MFF and the similarity toyardang morphology, we interpret the serrated edge of the olderlava flows as indicating that the lava flows embayed the MFF yard-angs during their emplacement (see Fig. 14). The jagged MFF yard-angs served as a ‘‘mold” into which the embaying lavas were‘‘poured”. Subsequent to the emplacement of the lavas, the MFFwas further eroded towards the southwest, leaving the steep-sided,serrated ‘‘lava casts”. In this scenario, the cliff that forms the edge ofthe trough was created by the abutment of lava against the side ofthe MFF deposits. This shape would remain after the erosion of theadjacent MFF, as shown diagrammatically in Fig. 14. Indeed, a close-up of the trough wall (Fig. 15) shows a section of the lava marginwhere the Medusae Fossae material continues to be eroded fromthe lava contact, even while being embayed by the younger,trough-filling lavas. During the emplacement of the Hesperian lavaflows (HBu2), the lavas were deflected around the former edges ofthe Aeolis Planum MFF. This critical relationship shows that theMFF was present and undergoing erosion and yardang formationin the Hesperian, prior to the emplacement of HBu2.

The serrated edges of MFF yardangs provide a unique boundarythat is diagnostic of the MFF throughout the region where the MFFoccurs. As fluid lavas embay the MFF, they fill the spaces betweenadjacent yardangs in a manner similar to the filling of a mold.When the MFF is later eroded away by the wind, the solidifiedand durable lavas remain, just as a cast remains after its moldhas been removed. The result is a lava flow cast whose edge mim-ics the serrated pattern of the former yardangs. Yardang lava castsand true yardangs can be distinguished in that the upper surface oflava flow casts is a smooth flat plane. In contrast, the top surface ofa yardang is shaped like the overturned hull of a ship and facetedby the wind. The temporal progression of yardang-mold and lava-cast morphology is shown in Fig. 16.

2.3.2. Amazonis Planitia and northern Memnonia PlanitiaWe describe two more examples of cast-and-mold relationships

in the context of the general geological unit boundaries in the area

Fig. 17. Geological context for Amazonis region. Each color represents a separate unit. Unare Amazonian lavas; Aoa1 is the Olympus Mons aureole; AHt3 is an Amazonian–HesperiNoachian terrains. Box A (Fig. 18) shows the boundary between the Amm unit of Gordii Dof Eumenides Dorsum with the Amazonian–Hesperian lavas from Arsia. Geological map fr�160� to �140�E.

(Scott and Tanaka, 1986) (Fig. 17). Near the northwestern tip ofGordii Dorsum (Fig. 17 Box A; Fig. 18), the full erosional progres-sion of the MFF from embayed yardangs to free-standing lava casts

its Amm and Amu represent the middle and upper members of the MFF; Aa1 and Aa3

an lava unit from Arsia Mons; Hch are Hesperian channels; Npl1 and Nplf are ancientorsum with Amazonian lavas. Box B (Fig. 19) shows the boundary between the Ammom Scott and Tanaka (1986), overlaid on the THEMIS IR mosaic. Lat.: 6�S–10�N, Lon.:

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can be found. Initially, a 10-km crater was formed (Fig. 18A). Thecrater is characterized by a common, lobate crater morphology(as described in Fig. 5) and does not show the unique MFF cratermorphology described earlier (Figs. 3–5). This indicates that thecrater did not impact into MFF material. Second, the MedusaeFossae Formation was emplaced across the terrain, also filling thecrater. Third, the MFF eroded away, its edge moving southeast(bottom right of Fig. 18), leaving some of the deposit stranded inthe crater where it was sheltered from the wind. Fourth, flood lavas(Fig. 18C) embayed the edge of MFF (Fig. 18D and G), which wasmade up of yardangs, pedestal craters, and outlying mesas(Fig. 18E and F). Fifth, the MFF continued to erode away from thecontact, leaving lava casts at lava flow-front edges, moats aroundpedestal craters (which are eroding radially inwards), and hollowswhere mesas once stood. During the latest activity of the MFF, itunderwent further erosion, leaving inverted craters to the eastand stripping away the last of the yardangs that still remained atthe contact (Fig. 18E). The embaying lava unit (Fig. 18C) appearsto be part of long lava lobes that are flowing from south to north,perhaps from the Tharsis region (Fuller and Head, 2002). These la-vas are thought to be middle to late Amazonian in age. Thus, the

Fig. 19. Yardang-molds and lava casts between Eumenides Dorsum and AmazonisMensae. Lavas coming from Arsia Mons (from the bottom right of the image, LavaFlow 2, unit AHt3 from Scott and Tanaka, 1986) stop short of an outcropping of theMFF. Lobate lava flow-fronts are indicated with white arrows (blue boundaries).This flow-front morphology indicates natural, unimpeded lava flow. This morphol-ogy sharply contrasts with the serrated lava casts indicated by black arrows (andred boundaries in lower panel). The presence of these casts indicates that the MFFhas retreated from this unit boundary to its present position to the northwest.THEMIS Daytime IR mosaic. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

embayment of the MFF by these lavas does not impose stringenttime constraints for the initial formation of the MFF. It does, how-ever, indicate that the MFF has continued to be actively eroded be-fore, during, and after the emplacement of the middle to lateAmazonian lava flows (Fuller and Head, 2002; Tanaka et al., 2005).

Further to the southeast, lobes of lava, originating from theArsia Mons region as part of the Amazonian–Hesperian AHt3 unit(Scott and Tanaka, 1986), flow between the ancient Noachian ter-rain to the south and Amazonis Planum, coming to a stop nearan outcropping of the MFF that makes up part of Eumenides Dor-sum (Fig. 17 Box B; Fig. 19). Most of the flows have lobate marginsand flow-front patterns, typical of lava flows and different from thelava-yardang casts and molds. However, where the flows approachthe outcropping of MFF, the flow edges are characterized by theserrated, yardang-like edges seen in Aeolis Planum and Gordii Dor-sum (Figs. 16D and 18). The MFF is exposed on the opposite side ofa small depression. This example shows the distinctive differencein flow-front morphology displayed by adjacent flows: serratednear the outcrop of MFF, and lobate away from the MFF. It is likelytherefore that the MFF formerly abutted the lava flows, causing thelava to flow between the MFF yardangs. The yardangs were theneroded away, leaving the interfingering lava ‘‘cast” pattern that isdistinct from normal flow-fronts. This relationship suggests thatthe MFF was present and being actively eroded to produce yard-angs at the time when the Arsia lavas were emplaced, placing thiseroded part of the MFF (the middle member according to Scott andTanaka (1986)) in the early Amazonian, late Hesperian, or earlier.

2.3.3. Apollinaris PateraApollinaris Patera is a volcano approximately 200 km in diame-

ter, which shows evidence of explosive eruptions (Robinson et al.,1993) (Fig. 20). Scott et al. (1993) classified Apollinaris asHesperian in their map of the volcano and the surrounding region.Its earliest flows were extruded in the early Hesperian or lateNoachian and the latest flows (associated with the southeasternfan) in the mid-Hesperian. Crater size–frequency distribution agesby Werner (2009) suggest that the active period of Apollinarisceased around 3.71 Ga. The Apollinaris edifice is associated witha magnetic anomaly, indicating that its construction preceded

Fig. 20. A regional view of Apollinaris Patera. The volcano (early Hesperian unitHa1) is embayed by the MFF (unit Amm) at A (Fig. 21). In the southeast (B), thevolcano’s younger fan material (mid-Hesperian unit Ha4) embays the lower MFFmember (unit Aml) (Fig. 22). This suggests an emplacement of the lower member ofthe MFF in the mid-Hesperian or before and an emplacement of the middle memberof the MFF in the early Hesperian or later. Map units from Scott et al. (1993). HRSCimages h1009, h0998, h0987, h0024, h0335.

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the end of the martian dynamo (Langlais and Purucker, 2007). Thepaleopole responsible for the magnetic anomaly at ApollinarisPatera appears to be close to the current rotational pole, suggestingthat polar wander subsequent to the construction of Apollinariswas minimal (Langlais and Purucker, 2007).

There are two main contacts between the MFF and the Apollin-aris edifice. One contact is on the northern side of the edifice,where the unit Amm (the middle member of the MFF) comes intocontact with the main Apollinaris edifice. The other is on thesoutheastern flank of Apollinaris, where the large fan of the vol-cano comes into contact with the unit Aml (the lower member ofthe MFF). Scott and Tanaka (1982) observed that while the volcanois embayed by MFF unit Amm on the northern flank, the youngerApollinaris fan flows overlap MFF unit Aml in the southwest. How-ever, later maps (e.g. Scott et al., 1993) placed the MFF unit Amlstratigraphically above the southwestern Apollinaris fan. Although

Fig. 21. Northern contact between the MFF and Apollinaris (A). In this case, the MFF(unit Amm) superposes the Apollinaris edifice (early Hesperian unit Ha1) (A, B). Anolder MFF unit with degraded yardangs with a different orientation is also visible,indicating that there may have been multiple episodes of MFF emplacement (C).CTX images (P02_001645_1726, P04_002634_1707). Map units from Scott et al.(1993).

Fig. 22. Southeastern contact between the MFF and Apollinaris. The southeasterncontact between the Apollinaris Patera fan (mid-Hesperian unit Ha4) and theMedusae Fossae Formation (unit Aml). The fan material embays and buries theyardangs of the MFF (arrows). MOC image (m2100205). Map units from Scott et al.(1993).

the contacts between Apollinaris Patera edifice and the MFF aresomewhat complex, with evidence for multiple episodes ofemplacement, the edifice clearly underlies MFF yardangs (unitAmm) along its northern margin (Fig. 21). At the contact betweenthe MFF and the southeastern Apollinaris fan, the fan material ap-pears to bury and embay yardangs associated with MFF unit Aml(Fig. 22). This embayment relationship supports the original con-clusions of Scott and Tanaka (1982) and suggests that the MFF(Aml according to Scott and Tanaka, 1986) was already presentand being eroded during the construction of the Apollinaris fan,thus placing emplacement of the formation in the Hesperian orearlier.

A summary of the stratigraphic constraints provided by lavaunits is shown in Fig. 23. The relationships that we have docu-mented and discussed are consistent with initial emplacement ofthe MFF beginning in the mid-Hesperian (Kerber et al., 2009) orperhaps even earlier. Recognition of yardang-mold and lava-castmorphology in areas where the MFF is present has shown thatthe MFF is an active and dynamic unit that has undergone large-scale erosion, causing migration of its borders and re-depositionof eroded material in adjacent areas. A diagram of the major pro-cesses at work within the MFF is shown in Fig. 24. Analysis ofthe cast-and-mold structures can lead to improved knowledge ofstratigraphic relationships between the MFF and adjacent unitswhich are otherwise ambiguous. The relationships that we havedocumented and discussed are consistent with initial emplace-ment of the MFF beginning in the mid-Hesperian or perhaps evenearlier. It is plausible that the bulk of the MFF was deposited at thistime and continuously modified since then. However, because ofthe ambiguous stratigraphic relations caused by erosion and re-deposition of the MFF, we cannot rule out the possibility that someparts of the MFF may have been deposited episodically over an ex-tended period of time, with perhaps some primary emplacementtaking place into the Amazonian. More detailed analysis of strati-graphic relationships, combined with assessment of the ages of ex-humed craters, may help to resolve this outstanding question.

3. Conclusions

Previous studies have proposed that the Medusae Fossae For-mation is Amazonian in age on the basis of impact crater size–fre-quency distribution data and stratigraphy (e.g., Scott and Tanaka,

Fig. 23. A synthesis of present and ancient stratigraphic relationships with adjacent lava units as discussed in the text. The oldest stratigraphic relationship in each MFFregion indicates an upper limit for the primary deposition of that part of the formation. Supplementary images are included to illustrate stratigraphic relationships notincluded in other figures (Zephyria Planum: h2176; Lucus Planum: h2980). Aec3: Amazonian Cerberus lavas; HBu2: Late Hesperian lava plains; Aha: Apollinaris Patera; AHt3:Amazonian/Hesperian lava flows from Arsia Mons; AAa2s: late to middle Amazonian lavas from Amazonis Planitia. Map units from Scott and Tanaka (1986), Greeley andGuest (1987), and Tanaka et al. (2005).

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1982, 1986; Greeley and Guest, 1987; Werner, 2005). After exam-ining the characteristics of the Medusae Fossae Formation, wecome to several conclusions. First, we conclude that the unit is fri-able and easily eroded (confirming and supporting previous inter-pretations). Second, we find that there are three classes of impactcraters associated with the MFF: relatively fresh, heavily modified(pedestal craters, knobs, and mesas), and exhumed. The nature ofthis crater population suggests that any attempt to date the surfacewith impact crater size–frequency distributions (particularly withsmaller crater class sizes) will produce minimum (and incorrect)formation ages. Ages derived by this method will be modificationages, not unit formation ages, as discussed by Schultz and Lutz(1988). Third, examination of stratigraphic relationships with asso-ciated and adjacent stratigraphic units, particularly interlayeredlava flows, shows that the MFF is very mobile and has constantlybeen eroded to produce yardangs and transported laterally ontoyounger stratigraphic units. Lava flows have flooded and embayedMFF yardangs, creating yardang-cast and lava-mold structures.

Continued erosion and retreat of yardangs have left isolated lavacasts that are further testament to the dynamic nature of theMFF as a geological unit.

In summary, on the basis of our analyses, we conclude that theMedusae Fossae Formation is an evolving geological unit, undergo-ing seemingly continuous erosion and re-deposition since its initialemplacement. On the basis of stratigraphic relationships with lavaflows and Apollinaris Patera, the initial formation age of the MFF ismost likely to have been in the Hesperian. Amazonian ages as-signed to the MFF appear to be modification ages, not formationages. Currently unknown is whether the MFF has undergone con-stant and continuous modification since its initial formation inthe Hesperian, or whether erosion and modification processes havebeen episodic. Also unknown is whether there have been someadditional primary contributions to the MFF since its initialemplacement in the Hesperian. The source and mode of emplace-ment for the MFF remain unknown, but initial deposition in theHesperian allows for the possibility that explosive eruptions from

Fig. 24. A schematic drawing of ongoing Medusae Fossae processes.

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Hesperian sources (such as centrally located Apollinaris Patera)may have contributed to the MFF (Kerber et al., 2009).

Acknowledgments

We gratefully acknowledge the financial support from theNational Aeronautics and Space Administration from grants fromthe Mars Data Analysis Program (MDAP) (NNG04GJ99G), theApplied Information Systems Research (AISR) Program(NNX08AC63G), and the Mars Express High Resolution StereoCamera Guest Investigator Program (JPL 1237163). We also thankNadine Barlow and Rebecca Williams for helpful reviews, andC.I. Fassett and L.M. Garber for fruitful discussions.

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