Posted by Goro Komatsu, International Research School of Planetary Sciences, Pescara, Italy.
Subaqueous bedforms produced by catastrophic floods are often represented by gravel dunes (also known as “giant current ripples”). While they may resemble aeolian dunes in remote sensing data, field observation reveals that they are composed of coarse-grained sediment including up to meter-scale boulders. Such examples are widely known in the Channeled Scabland (Baker, 1982) in North America, in Altai (Carling et al., 2002) and Sayan (Komatsu et al., 2009) mountain provinces of Siberia (e.g., Images 1 and 2).
Since gravel dunes are strongly linked with flow hydraulics (e.g., Carling et al., 2002), they provide information on parameters such as shear stresses, mean velocities, and stream powers even though their exact formation mechanism is poorly understood due to a lack of experimental data. Their existence along flood reaches depends primarily on sediment supply and appropriate hydraulic conditions (e.g., Komatsu et al., 2009).
There are few examples of flood-formed dunes on Mars despite the fact that outflow channels, cited as the evidence for catastrophic flooding, dominate a large fraction of its planetary landscape. One notable example is located in Athabasca Valles, Mars (Burr, 2004) (Image 3). Confirmation of their presence requires further studies including in-situ field observation. The absence of subaqueous dunes on Mars may be explained by the fact that that their formation was limited in Martian flood conditions (inadequate sediment supply and/or hydraulic conditions). Alternatively, it may be due to poor preservation and post-flood modifications in the 2-3 billion years since deposition.
Baker, V.R., 1982. The Channels of Mars. University of Texas Press, Austin, Texas, 198 pp.
Burr, D.M., Carling, P.A., Beyer, R.A., Lancaster, N., 2004. Flood-formed dunes in Athabasca Valles, Mars: morphology, modeling, and implications. Icarus 171, 68-83. [Abstract]
Carling, P.A., Kirkbride, A.D., Parnachov, S., Borodavko, P.S., Berger, G.W., 2002. Late Quaternary catastrophic flooding in the Altai Mountains of south-central Siberia: a synoptic overview and introduction to flood deposit sedimentology. In: Martini, I.P., Baker, V.R., Garzon, G. (Eds.), Flood and Megaflood Processes and Deposits: Recent and Ancient Examples. International Association of Sedimentologists Special Publication 32, Blackwell Science, London, pp. 17-35.
Komatsu, G., Arzhannikov, S.G., Gillespie, A.R., Burke, R.M., Miyamoto, H., Baker, V.R., 2009. Quaternary paleolake formation and cataclysmic flooding along the upper Yenisei River. Geomorphology 104, doi:10.1016/j.geomorph.2008.08.009, 143-164. [Abstract]