The Age of the Medusae Fossae Formation: Reassessment Using Lava Flow Cast and Mold Contacts

Introduction. The Medusae Fossae Formation (MFF) is an enigmatic formation located between 130°-230°E and 12°S-12°N, in the southern parts of the Elysium and Amazonis quadrangles and the northern parts of the Aeolis and Memnonia quadrangles. It covers an area of approximately 2.1 x 10 km and has an estimated volume of 1.4 x 10 km [1]. The formation is composed of highly eroded, finegrained, friable deposits. Theories for the origin of this deposit include aeolian debris, paleo-polar deposits, 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 there was some fluvial activity during or after the emplacement of the MFF. If the MFF is among the youngest surficial deposits on Mars [1], this implies 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 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 deposits, and the relatively few superposed craters on 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 have placed the MFF at the top of the stratigraphic column because it overlies lowland lavas [11]. In many places direct contacts are lacking, but ancient contacts may be recognized where lava units embayed the MFF yardangs (filling up the negative space between and around them), and then the MFF subsequently eroded away (Figs. 1, 2). The serrated yardang edges of the MFF form a “mold”, and when lavas flood and embay this topography, they cool to form “casts” (Fig. 1a). The friable MFF “molds” are then removed by erosion (Fig. 1b), leaving “casts” of the former yardangs at the edge of the flow fronts that are easily distinguished from the lobate flow fronts commonly observed in plains lavas (Fig. 1c). While normal yardangs are three-dimensional and faceted, lava flow “yardang casts” have relatively flat tops consistent with the equipotential surface of a flood 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 yardangs through time (removal of the mold), (c) the resulting negative yardang lava morphology (cast).