Abstract
The Medusae Fossae Formation (MFF) is a geological formation comprising three geological units (members) spread across five principal lobes. It dominates a quarter of the longitudinal extent of the equatorial region of Mars. Positive relief features referred to as ‘sinuous ridges’ (commonly interpreted as inverted paleoflow channel or valley fills) have been observed in the lowest member of the western MFF, but have not been identified within the central and eastern portions of the formation, in the middle and upper members. This paper presents the identification and analysis of a branching, positive relief system which occurs in the central lobe of the MFF in what appears to be an exposure of the middle member. A simple geomorphological map of the system is presented, from which we have adopted the working hypothesis that this is an inverted fill of a branching fluvial channel or valley system. A suite of morphological and topographic evidence supporting this hypothesis is presented, including analysis of the network using a ∼15m/pixel digital terrain model derived from a Context Imager (CTX) stereo image pair. The evidence supporting this hypothesis includes: (1) the local slope and topography of the upper surface of the network are consistent with a contributory network; (2) the braided, fan-like form at the termination of the branching network is consistent in morphology with it being a depositional fan at the end of a fluvial system; (3) the terminal fan and surrounding deposits show layering and polygonization; and (4) there is strong association between the lower order branches and amphitheater shaped scarps in the depression walls. We evaluate the possible origins of this fluvial system and suggest that seepage sapping is the most probable. Two possible models for the evolution of the network and related features are presented; both require melt of ice within the MFF to form liquid water. We conclude that at least some portions of the Medusae Fossae Formation, if not the entire formation, were once volatile-rich. Finally, we note that our observations do not rule out the case that this network formed before MFF emplacement, and has since been exhumed. However, this conclusion would suggest that much of the surrounding terrain, currently mapped as middle-member MFF, is not in fact MFF material at all.
Highlights
The Medusae Fossae Formation (MFF) is a geological formation comprising three geological units spread across five principal lobes
The presence of such a feature in the central lobe, within an apparent exposure of the middle member, may have important implications regarding the volatile content of the MFF
Layering or planes of horizontal weakness are not inconsistent with an erosional origin for the ridge network, but we note that multiple layers are not seen to crop out at the margins of the southern depression, nor in the northern crater walls, as might be expected if this were an erosional landform within terrain of horizontal, “layer-cake” stratigraphy
Summary
The MFF is typified by a surface with a discontinuous, highly eroded appearance at kilometer to meter scales It occurs in five primary outcrops or lobes which have been further mapped into three separate geological members: lower, middle and upper (Greeley and Guest, 1987; Scott and Tanaka, 1986). The origins of the MFF remain uncertain, despite numerous studies aiming to better constrain the age, distribution and material properties Such studies have resulted in varied hypotheses being proposed to account for the MFF's existence, including volcanic deposits (Scott and Tanaka, 1982), palaeo-polar deposits (Head and Kreslavsky, 2001; Schultz and Lutz, 1988) and lacustrine/oceanic origins (Parker, 1991,1994). The possibility that the MFF is ice-rich is just one of at least two working hypotheses, and the available radar data are not able to discriminate between ice-rich or ice-poor conditions
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