40th Lunar and Planetary Science Conference (2009) 2235.pdf THE AGE OF THE MEDUSAE FOSSAE FORMATION: REASSESSMENT USING LAVA FLOW CAST AND MOLD CONTACTS. L. Kerber1 and J. W. Head1, 1Brown University Department of Geological Sciences, Box 1846 Providence RI 02912, USA [email protected].

Introduction. The Medusae Fossae Formation (MFF) is an enigmatic formation located between a 130°-230°E and 12°S-12°N, in the southern parts of Lava the Elysium and Amazonis quadrangles and the northern parts of the Aeolis and Memnonia quadrangles. It covers an area of approximately 2.1 x 106 km2 and has an estimated volume of 1.4 x 106 km3 [1]. The formation is composed of highly eroded, fine- grained, friable deposits. Theories for the origin of this deposit include aeolian debris, paleo-polar deposits, MFF pyroclastic flows, and volcanic ash [2,3,4,5]. It is interpreted to have been deposited during the Amazonian period [1,4,6,7]. Some modified and inverted fluvial channels have been found within the deposit [8,9,10], indicating that b there was some fluvial activity during or after the emplacement of the MFF. If the MFF is among the youngest surficial MFF deposits on Mars [1], this implies that meandering, Lava 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 Martian climate history, we reassess the evidence for the Amazonian age of the MFF. The initial conclusion of a young age comes from two main arguments: the superposition of the MFF on young lowland lava c deposits, and the relatively few superposed craters on Lava the unit [1,11,12]. Using a combination of MOC, CTX, and HRSC data, we re-examine 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 [13], but later works Fig. 1 From top to bottom, (a) the embayment of a field of MFF yardangs by lava flows (filling the mold), (b) the erosion of the have placed the MFF at the top of the stratigraphic yardangs through time (removal of the mold), (c) the resulting column because it overlies lowland lavas [11]. In negative yardang lava morphology (cast). many places direct contacts are lacking, but ancient contacts may be recognized where lava units embayed lava. The recognition of lava flow yardang casts the MFF yardangs (filling up the negative space provides evidence of the former presence of the MFF. between and around them), and then the MFF We have examined several type-areas that illustrate subsequently eroded away (Figs. 1, 2). The serrated ambiguity in the stratigraphic record which can be yardang edges of the MFF form a “mold”, and when partially resolved by recognizing ancient contacts, and lavas flood and embay this topography, they cool to which emphasize the highly mobile nature of the MFF: form “casts” (Fig. 1a). The friable MFF “molds” are 1) Northeastern Aeolis Planum. Cerberus lavas (unit then removed by erosion (Fig. 1b), leaving “casts” of AEc3, [14]) embay the formation in Northeast Aeolis the former yardangs at the edge of the flow fronts that Planum. Additionally, yardang cast morphology (Fig. are easily distinguished from the lobate flow fronts 1c) indicates embayment of the MFF by the Hesperian commonly observed in plains lavas (Fig. 1c). While unit Hbu2 (defined by [14]), suggesting an older age normal yardangs are three-dimensional and faceted, than previously recognized. 2) Northern Zephyria lava flow “yardang casts” have relatively flat tops Planum. A large pedestal crater composed of MFF consistent with the equipotential surface of a flood material is embayed by Cerberus lavas (AEc3),

MFF

Lava 40th Lunar and Planetary Science Conference (2009) 2235.pdf

Fig. 2. Schematic drawing of ongoing Medusae Fossae processes.

but eroded MFF material, once contiguous with the Discussion. The Medusae Fossae Formation is a crater’s pedestal, has been blown out onto the younger mobile unit which erases craters on its own surface lava unit where it collects as dunes and streaks (Fig. 3). and shelters adjacent units from incoming projectiles, 3) Southeastern Eumenides Dorsum near Amazonis resulting in a misleadingly young age. The MFF Mensa. Lavas from Arsia (AHt3) show yardang cast appears to continuously erode into yardangs and morphology in close proximity to retreating MFF reaccumulate as dunes and massive deposits. yardangs, indicating the presence of the MFF during the Unconsolidated dune material gradually becomes emplacement of these lavas. 4) Northwestern Gordii indurated (perhaps through overburden loading or Dorsum. Lava flows, previously identified as part of the cementation through interaction with the atmosphere) MFF (unit Amu [4]), end in yardang cast morphology, until it eventually erodes into yardangs again. Lava indicating embayment of the MFF in the past (Fig. 1b). flows, which remain relatively stable through time, The presence of these ancient and modified contacts make useful chronological markers for determining places chronological constraints on the emplacement of the MFF. the current and past location of the MFF. Recognition Cratering Record. Craters formed in the MFF and of cast and mold morphologies is helpful in other easily eroded units are often erased, buried, or unravelling the stratigraphic relationships between the degraded beyond recognition. Throughout the MFF, MFF and regional lava units where direct contacts are pedestal craters, inverted craters, and remnant knobs no longer available. Lava unit cast contacts strongly can be found which trace the progression of crater suggest that the MFF is at least in part Hesperian in erosion (Fig. 2) and suggest that a large number of age, and may be related to Hesperian-aged explosive craters originally superposed on the unit have been volcanic sources [17], a conclusion which is also removed, making crater count ages unreliable more consistent with the presence of fluvial channels. [2,3,15,16]. References. [1] Bradley, B.A. and Sakimoto, S.E.H. (2002) JGR, 107, E8. [2] Tanaka, K.L. (2000) Icarus 144(2), 254-266. [3] Schultz, P.H. and Lutz, A.B. (1988) Icarus 73, 91-141 [4] Scott, D.H. and Tanaka, K.L. (1986) USGS Misc. Invest. Ser. Map I-1802-A. [5] Hynek, B.M. et al. (2003) JGR 108 E9. [6] Greeley, R. and Guest, J. (1987) USGS Misc. Inv. Series Map I-1802-B. [7] Werner, S.C., (2009) Icarus, in press. [8] Zimbelman, J.R. (2000) GSA Abs. Prog., 32(7), A303. [9] Edgett K.S. and Williams R.M.E. (2006) LPSC XXXVI, Abs. 1099. [10] Burr, D.M. et al. (2008) Icarus, in press. [11] Bradley, B.A. and Sakimoto, S.E.H. (2001) LPSC XXXII, Abs. 1335. [12] Zimbelman, J.R. (1999) LPSC XXX, Abs. 1652. [13] Scott, D.H. and Tanaka, K.L. (1982) JGR 87 B2 [14] Tanaka, K.L. et al.

Fig. 3. This large pedestal crater, preserving an outlying section of (2005) Scientific Investigations Map 2888. [15] Schultz, the MFF in northern Zephyria Planum, is embayed by the young P.H. (2006) Plan. Chron. Workshop, Abs. 6024. [16] Cerberus lavas (AEc3) at (A). Outlying MFF sheds unconsolidated Schultz, P.H. (2007) Science 318, 1080-1081. [17] Kerber, fine particles on top of the Cerberus lavas after their emplacement (B). The unconsolidated material is transported by the wind into L. et al. (2009) LPSC XXXX, Abs. 2176. adjacent terrain, where it forms blankets and wind streaks that lie on top of young units (C).