Abdallah Zaki, Colin Frederick Pain, Kenneth S. Edgett, Sebastien Castelltort 

Data from orbiting and landed spacecraft have provided vast amounts of information regarding fluvial and fluvial-related landforms and sediments on Mars. One variant of these landforms are sinuous ridges that have been interpreted to be remnant evidence for ancient fluvial activity, observed at hundreds of martian locales. In order to further understanding of these martian landforms, this paper inventories the 107 known and unknown inverted channel sites on Earth; these offer 114 different examples that consist of materials ranging in age from Upper Ordovician to late Holocene. These examples record several climatic events from the Upper Ordovician glaciation to late Quaternary climate oscillation. These Earth examples include inverted channels in deltaic and alluvial fan sediment, providing new analogs to their martian counterparts. This global dataset provides environmental context regarding the formation mechanisms and conditions that accompanied channel formation and inversion. There are five documented processes by which channel sediment and valley fill become consolidated and inverted after adjacent floodplain sediments are eroded away: (1) channel fill cementation during near-surface, early diagenesis; (2) channel fill lithification during burial diagenesis in the subsurface; (3) filling of a channel or valley by extrusive volcanism (lavas, tuffs); (4) channel surface armouring of coarse clasts by aeolian and fluvial processes; (5) compaction of bank-forming peat, which can lead to early inversion without removal of floodplain material. On Earth, early diagenesis (shallow surficial cementation) dominates among inverted fluvial channels (59%), volcanism is an important contributor (23%), and deep burial diagenesis, surface armouring, compaction of bank-forming peat are comparatively minor (11%, 6%, and 1% respectively). Water erosion, wind erosion, and exhumation due to tectonic activity play an important role in removing the surrounding terrain, leaving the channel deposits standing as a ridge. Wind erosion rates involved in causing topographic inversion range from 10 mm/year in the Bodélé Depression, Chad, to 0.21 mm/year in the Kumtagh Desert, China. These observations have important implications for understanding the formation and paleoclimate associated with similar landforms on Mars. Shallow, near-subsurface cementation of channel sediment could be prevalent on Mars due to a favorable climate like that of where they occur on Earth (tropical and subtropical climates), particularly because most Martian examples are not capped by volcanic materials and because tectonism, as compared to Earth, plays a negligible role in returning buried materials to the surface. Studying inverted channels on Mars at the global scale will bring new information on the early climate of Mars, particularly transition from wet to dry conditions. Finally, we propose that inverted deltaic deposits, which occur around the margins of the Pleistocene Tushka paleolake, in Egypt, are an excellent terrestrial analogue for fan-shaped deposits in relief inversion on Mars, particularly the deltaic deposits in Jezero crater, the landing site for NASA’s Mars 2020 rover.