Global documentation of overlapping lobate deposits in Martian gullies

Abstract

Martian gullies are landforms comprising an alcove, channel and debris apron. In some gullies lobate deposits have been observed, implying a debris-flow-like process, but uncertainty remains as to the importance of this process in the gully-population and whether lobate deposits are related to a specific context. We document evidence of overlapping lobate deposits on gully-fan surfaces within craters emplaced between 30�-75� in both hemispheres. We have identified 26 craters in which lobate deposits occur in gullies, of which 6 were previously reported. This corresponds to 3.39% of the 765 gullied craters studied using 1004 HiRISE images. We show that gullies with lobate deposits (1) do not show any location preference, (2) are poleward-facing, (3) are found in craters with and without latitude dependent mantle (LDM) and/or glacier-like-forms, (4) are emplaced at average slopes of 22� and 15� at the foot of fans in the northern and southern hemispheres, respectively, and (5) form in craters of all ages. We infer that preservation of lobate deposits in gullies is the main factor leading to their relative paucity within the gully-population. Additionally, we find that the size of the clasts embedded in the lobes does not seem to vary with host-crater age and the presence of LDM and/or glacial landforms. Further, we observe morphological features associated with terrestrial wet-debris flows including: overlapping tongue-shaped terminal lobes, levees, channel backfilling, plug formation and avulsion. We have not observed any significant present-day changes in the morphology and topography of gullies and/or lobes. We conclude that a debris-flow-like process is likely responsible for the majority of sediment transport in gully-landforms, but whether the fluidising agent is or was liquid water or CO2 sublimation remains unknown.