e145 © 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. COMMENT: doi.10.1130/G24181C.1 Joanne Bourgeois Department of Earth and Space Sciences and Sedimentology Laboratory, University of Washington, Box 351310, Seattle, Washington 98195, USA Sanjoy Som Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Box 351310, Seattle, Washington 98195, USA Dromart et al. (2007) describe a spectacu- lar stratigraphic complex within southern Melas Chasma, Vallis Marineris, Mars. Following a rigorous stratigraphic description of the complex, they proceed to interpret the responsible depo- sitional processes as analogous to subaqueous channel-levee processes on Earth. The observed stratigraphy, however, can be explained as large- scale cross-bedding typical of eolian bed forms. Large-scale cross-bedding in the Jurassic Navajo Sandstone of the Colorado Plateau re- gion, United States (Rubin, 1987), has also been the subject of debate regarding its subaerial versus subaqueous origin (Picard, 1977, and subsequent discussions). Traditionally interpreted as eolian, a subaqueous tidal bedform interpretation for cross-bedding in the Navajo Sandstone was sug- gested (Freeman and Visher, 1975) based partly on the discovery via seismic sounding of large- scale tidal bedforms in estuaries and in the North Sea (Houbolt, 1968). However, these large sub- aqueous bedforms did not have angle-of-repose cross-bedding, although in vertically exaggerated images, it appeared they did. In addition, Freeman and Visher (1975) invoked deformed bedding in the Navajo Sandstone as indicative of a subaqueous environment. However, an eolian interpretation for the Navajo and similar formations is now very well accepted (Kocurek, 1991; Rubin, 1987). Dromart et al. (2007, p. 364) propose a “channel-levee system” (CLS) as the most likely explanation for the stratigraphic complex seen in Melas Chasma. They correctly point out that their CLS interpretation is challenged by the observed Martian “levee” slope of 25°–30°, versus a maxi- mum of 9° observed in the subaqueous Rhône delta in Lake Leman. Furthermore, the slope of the buried Oligocene CLS they present, revealed from seismic stratigraphy, has a levee slope of 2°, an order of magnitude less than observed on Mars. Coarseness of the Mars Orbiter Laser Altimeter data at the scale of the complex makes it diffi- cult to measure slopes of the bedding accurately, but 25°–30° is certainly more consistent with the angle of repose of sand for Mars (~34°) (Matijevic et al., 1997), than with a subaqueous CLS. Dromart et al. dismiss the eolian hypothesis based on scale, but whereas the height of sub- aqueous bedforms is depth-limited, eolian bed- forms are only limited by sediment supply. Indeed, the largest-scale dunes and associated cross- bedding on Earth are eolian in origin. Very large cross-sets can attend Gilbert deltas, which have flat subaerial tops and subaqueous avalanche foresets, but the geometry of the Martian case does not fit this model. As such, we feel it is premature to dis- miss the subaerial hypothesis in favor of the sub- aqueous one based solely on a scale argument. Indeed, the morphology of the levee and channel bed can be obtained from bedform migration alone. We modeled bed morphology and internal structure (Rubin, 1987; Rubin and Carter, 2005) of a deposit caused by spurs oscil- lating back and forth but with a net migration direction, normal to a migrating bedform, and produced a similar morphology (Fig. 1). While we do not claim this result as being the correct one, we feel it is sufficiently compelling to stress the importance of not dismissing the eolian bed- form hypothesis prematurely. Another important point to address is the relationship of the stratigraphic complex with the history of Valles Marineris. Dromart et al. suggest that the subaqueous environment occured follow- ing the formation of Valles Marineris under a “thick ice sheet” (Dromart et al., 2007, p. 365), thus allowing significant water discharge to form the complex fairly recently in Martian history. We find little evidence to support the ice-sheet claim. In contrast, a more likely hypothesis is that the complex was deposited prior to the opening of Valles Marineris, and was exposed during forma- tion of the canyon. Indeed, other layered outcrops in and around Valles Marineris have been stud- ied by different workers and found to have been exhumed, rather than deposited (Catling et al., 2006; Malin and Edgett, 2000; Montgomery and Gillespie, 2005). In this interpretation, the strati- graphic complex is evidence that the early periods of sedimentation (whether aqueous or eolian) on Mars are now buried under several kilometers of volcanic rock at the Tharsis locale (Clifford and Parker, 2001), except where exhumed by the for- mation of Valles Marineris. ACKNOWLEDGMENTS Discussions with David R. Montgomery were particularly helpful during the writing of this comment. REFERENCES CITED Catling, D.C., Wood, S.E., Leovy, C., Montgomery, D.R., Greenberg, H.M., Glein, C.R., and Moore, J.M., 2006, Light-toned layered deposits in Juventae Chasma: Mars: Icarus, v. 181, p. 26– 51, doi: 10.1016/j.icarus.2005.10.020. Clifford, S.M., and Parker, T.J., 2001, The evolution of the Martian hydrosphere: Implications for the fate of a primordial ocean and the current state of the northern plains: Icarus, v. 154, p. 40–79, doi: 10.1006/icar.2001.6671. Dromart, G., Quantin, C., and Broucke, O., 2007, Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars: Geology, v. 35, p. 363–366, doi: 10.1130/G23350A.1. Freeman, W.E., and Visher, G.S., 1975, Stratigraphic analysis of Navajo Sandstone: Journal of Sedimentary Petrology, v. 45, p. 651–668. Houbolt, J.J.H.C., 1968, Recent sediments in the southern bight of the North Sea: Geologie en Mijnbouw, v. 47, p. 245–273. Kocurek, G., 1991, Interpretation of ancient eolian sand dunes: Annual Review of Earth and Planetary Sciences, v. 19, p. 43–75, doi: 10.1146/annurev.ea.19.050191.000355. Malin, M.C., and Edgett, K.S., 2000, Sedimentary rocks of early Mars: Science, v. 290, p. 1927– 1937, doi: 10.1126/science.290.5498.1927. Matijevic, J.R., and 29 others (Rover Team), 1997, Characterization of the Martian surface deposits by the Mars Pathfinder rover, Sojourner: Science, v. 278, p. 1765–1768, doi: 10.1126/ science.278.5344.1765. Montgomery, D.R., and Gillespie, A., 2005, Formation of Martian outflow channels by catastrophic dewatering of evaporite deposits: Geology, v. 33, p. 625–628, doi: 10.1130/G21270.1. Picard, M.D., 1977, Stratigraphic analysis of Navajo Sandstone; a discussion: Journal of Sedimentary Petrology, v. 47, p. 475–483. Rubin, D.M., 1987, Cross-Bedding, Bedforms, and Paleocurrents: Tulsa, Oklahoma: Society of Eco- nomic Paleontologists and Mineralogists, 187 p. Rubin, D.M., and Carter, C., 2005, Bedforms 4.0: MATLAB Code for Simulating Bedforms and Cross-Bedding., U.S. Geological Survey Open- File Report 2005–1272, 13 p. Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars: COMMENT Figure 1. Bedform representation obtained using the model of Rubin and Carter (2005) of the “channel-levee system” of Dromart et al. (2007). The cusps have a net migration direction to the right. Top right: Dromart et al.’s Figure 3a. The dotted lines represent the location of the unconformable contact mapped by Dromart et al. (2007). Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/35/1/e145/3534560/i0091-7613-35-1-e145.pdf by guest on 19 May 2019