Icarus 171 (2004) 68–83 www.elsevier.com/locate/icarus Flood-formed dunes in Athabasca Valles, Mars: morphology, modeling, and implications Devon M. Burr a,∗ , Paul A. Carling b , Ross A. Beyer c , Nicholas Lancaster d,1 a USGS Astrogeology Branch, 2255 N. Gemini Dr., Flagstaff, AZ 86001, USA b School of Geography, University of Southampton, Highfield, Southampton SO17 1BJ, UK c Department of Planetary Sciences, The University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA d Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA Received 2 December 2003; revised 6 April 2004 Available online 11 June 2004 Abstract Estimates of discharge for martian outflow channels have spanned orders of magnitude due in part to uncertainties in floodwater height. A methodology of estimating discharge based on bedforms would reduce some of this uncertainty. Such a methodology based on the mor- phology and granulometry of flood-formed (‘diluvial’) dunes has been developed by Carling (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165–179) and applied to Pleistocene flood-formed dunes in Siberia. Transverse periodic dune-like bedforms in Athabasca Valles, Mars, have previously been classified both as flood-formed dunes and as antidunes. Either interpretation is important, as they both imply substantial quantities of water, but each has different hydraulic implications. We undertook photoclinometric measurements of these forms, and com- pared them with data from flood-formed dunes in Siberia. Our analysis of those data shows their morphology to be more consistent with dunes than antidunes, thus providing the first documentation of flood-formed dunes on Mars. Other reasoning based on context and likely hydraulics also supports the bedforms’ classification as dunes. Evidence does not support the dunes being aeolian, although a conclusive de- termination cannot be made with present data. Given the preponderance of evidence that the features are flood-formed instead of aeolian, we applied Carling’s (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165–179) dune-flow model to derive the peak discharge of the flood flow that formed them. The resultant estimate is approximately 2 × 10 6 m 3 /s, similar to previous estimates. The size of the Athabascan dunes’ in comparison with that of terrestrial dunes suggests that these martian dunes took at least 1–2 days to grow. Their flattened morphology implies that they were formed at high subcritical flow and that the flood flow that formed them receded very quickly.  2004 Elsevier Inc. All rights reserved. Keywords: Mars, surface; Geological processes; Surfaces, planets; Terrestrial planets 1. Introduction Athabasca Valles are a distributary outflow channel sys- tem at the western end of the Cerberus plains (Fig. 1). The distribution of flood-formed (‘diluvial’) forms in scat- tered MOC images supports the evidence of the MOLA data that the floodwater that carved the channels emerged * Corresponding author. Fax: (982)-556-7014. E-mail address: dmburr@usgs.gov (D.M. Burr). 1 Currently at: Earth Surface Dynamics Program, U.S. Geological Sur- vey, MS 906, National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, USA. from one of the Cerberus Fossae (Burr et al., 2002a, 2002b). Athabasca Valles appear very young. The most recent resur- facing event down the main channel has been dated by crater counting of the channel floor as a few Ma old (Berman and Hartmann, 2002; Burr et al., 2002b; Werner et al., 2003). These ages are uncertain because many of the craters have been determined to be secondaries (McEwen et al., 2003), and the significance of age dating from counting small craters remains a subject of debate (e.g., Barlow, 2003; Bierhaus et al., 2001; McEwen et al., 2004). In addition, arguments have been presented (Malin and Edgett, 2001; Edgett and Malin, 2003) for exhumed terrain on Mars in this region, so that crater density would reflect not formation 0019-1035/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2004.04.013