ARTICLE IN PRESS EPSL-09953; No of Pages 10 Earth and Planetary Science Letters xxx (2009) xxx–xxx Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Identification of gully debris flow deposits in Protonilus Mensae, Mars: Characterization of a water-bearing, energetic gully-forming process J.S. Levy ⁎, J.W. Head, J.L. Dickson, C.I. Fassett, G.A. Morgan, S.C. Schon Department of Geological Sciences, Brown University, Providence, RI 02912, United States article info abstract Article history: Gullies are a class of geologically recent landform on Mars that show evidence of downslope transport of Accepted 4 August 2009 sedimentary material from recessed alcoves, through incised channels, to distributary fans or aprons. The Available online xxxx mechanisms invoked to account for the formation of gullies on Mars range from completely dry, granular flows or landslides, to debris flows that incorporate some component of liquid water, to fluvial erosion and Editor: T. Spohn alluvial-fan-like deposition. Each of these processes requires different amounts of liquid water, and produces different characteristic morphologies. We report on the identification of unusual lobate structures present in Keywords: Mars proximity to gullies in Protonilus Mensae. The lobes are up to ~3 m thick and terminate in rounded snouts. gully These lobate structures are present mostly downslope of gullies, and can be traced upslope through pasted-on terrain channels, to gully fan termini, and in places, onto gully fan surfaces. Crater dating indicates that the deposits permafrost formed recently—potentially within the past ~500 ka. We use HRSC digital elevation models to constrain debris flow mechanical properties of the lobate deposits, and to compare their formation environment to that of typical climate martian gullies. The Protonilus Mensae lobate deposits are interpreted to indicate local dominance of wet debris flows in the formation of the observed gullies and lobes. These observations are consistent with 1) top-down melting of the ice component of the latitude-dependent mantle terrain in which the gullies form and 2) initiation of debris flows by mobilization of the dusty lithic component of the mantle. The suite of morphological observations diagnostic of wet debris flow processes suggests the identification of an unusual environment in which the wet debris-flow formational end-member is locally the dominant gully forming process; elsewhere on Mars gully morphology may be more consistent with a range of other water- related sediment transport mechanisms including fluvial erosion, hyperconcentrated flow, and low-strength mudflows (that deposit as fans rather than lobes). These results suggest that 1) gully-forming processes involve liquid water, 2) that the water source is associated with the martian latitude-dependent mantle, and 3) that a range of water-related sediment transport processes is involved in gully formation. © 2009 Elsevier B.V. All rights reserved. 1. Introduction formation. Previous studies favoring a debris flow origin for gullies have been based on indirect evidence from the morphology of gully Martian gullies are characterized by evidence for the geologically channels (Costard et al., 2002; Mangold et al., 2008a,b) or gully recent downslope movement of sediment from topographically high depositional fans (Malin and Edgett, 2000; Malin et al., 2006; Pelletier alcoves, through incised channels, to lower-lying depositional fans et al., 2008). Here, we present HiRISE observations of gully-related (Malin and Edgett, 2000)(Fig. 1). Leading hypotheses for the origin of deposits in Protonilus Mensae (44°N, 51°E) (Fig. 2) that meet many of martian gullies range from entirely dry sediment flows (e.g., Treiman, the morphological criteria for identifying terrestrial debris flow 2003; Shinbrot et al., 2004; Pelletier et al., 2008), to debris flows with deposits (Johnson and Rodine, 1984; Coussot and Meunier, 1996). To variable water contents (Malin and Edgett, 2000; Costard et al., 2002; our knowledge, such deposits have not been detected in HiRISE images Hartmann et al., 2003; Mangold et al., 2008a,b; Pelletier et al., 2008), to of gullied terrain elsewhere on Mars (McEwen et al., 2007; Dickson fluvial (water-rich) erosion and alluvial deposition (Heldmann and and Head, 2008), suggesting that the Protonilus Mensae fretted terrain Mellon, 2004; Heldmann et al., 2005; Dickson et al., 2007a; Dickson (Sharp, 1973; Lucchitta, 1984) may be a unique environment in which and Head, 2008; Head et al., 2008; Parsons et al., 2008). Determining debris flows were the dominant gully formation mechanism. gully formation mechanisms is critical for assessing the amount, origin, timing, and climatological significance of liquid water involved in gully 2. Morphological observations ⁎ Corresponding author. Tel.: +1 401 863 3485; fax: +1 401 863 3978. The study site is located on an isolated mesa surrounded by E-mail address: [email protected] (J.S. Levy). lineated valley fill and lobate debris aprons (Carr, 2001; Head et al., 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.08.002 Please cite this article as: Levy, J.S., et al., Identification of gully debris flow deposits in Protonilus Mensae, Mars: Characterization of a water- bearing, energetic gully-forming process, Earth Planet. Sci. Lett. (2009), doi:10.1016/j.epsl.2009.08.002 ARTICLE IN PRESS 2 J.S. Levy et al. / Earth and Planetary Science Letters xxx (2009) xxx–xxx 2006a,b, submitted for publication). The mesa itself is incised by gullies, mostly on the southern (equatorwards) slope face (Figs. 3–5). High Resolution Stereo Camera (HRSC) topography data provides context and geometry in this region (Neukum et al., 2004). Gullies (Malin and Edgett, 2000) are present at the margins of the latitude-dependent mantle (Head et al., 2003)in“pasted-on terrain” (Christensen, 2003) occurring along the south flank of the mesa (Figs. 3a and 5). The gullies have small alcoves or lack them entirely (Malin and Edgett, 2000)(Figs. 3a and 5). Over 300 gully channels are present along the ~5 km mesa scarp (Fig. 3). Gully channel widths are typically 5–20 m, and are narrower high on the mesa (Figs. 3 and 5a). Shadow measurements indicate gully channel depths of ~5–6m(Fig. 5a, b). Gully channel lengths are typically ~200 m, ranging from ~100–500 m. Gully channels initiate high on the mesa slope as fine, linear depressions Fig. 1. A typical martian gully located at 52.1°S, 246.8°E. Note the presence of a large alcove with multiple emanating channels. Channels coalesce from tributaries into a main trunk channel that grades into a fan. The fan is dissected in places by distributary channels. Typical gullies form in thick deposits of latitude-dependent mantle material (Head et al., 2003) (polygonally patterned in this image) and erode through to underlying rocky surfaces. Portion of PSP_002368_1275. North is to image top, indicating that the gully is pole-facing, typical of gullies at this latitude (Dickson et al., 2007a). Illumination is from the lower left. Fig. 3. The Protonilus Mensae study site. a) Context view of mesas, lobate debris aprons, and lineated valley fill. Box indicates area shown in part b. Portion of HiRISE image PSP_007148_2445 overlain on CTX image P03_002401_2246. North is to image top and illumination is from the lower left. b) View of the Protonilus Mensae mesa scarp. Gullies with well-formed channels and fans dissect the southern flank of the mesa. The locations of subsequent images are marked. c) Sketch map showing the locations of gully channels and the furthest down-slope extents of lobate flow fronts. Basemap is Fig. 2. Regional view of Protonilus Mensae fretted terrain and the martian dichotomy local slope derived from HRSC topography data. The occurrence of gully channels on boundary. The study region is indicated by the white box. The map is composed of steep slopes and lobate flows on shallow slopes is illustrated. Note, however, that MOLA shaded relief overlying MOLA topography—lighter tones are higher in elevation channels and lobes are sub-grid features and that background slopes are measured. than darker tones. HRSC topography from image h1523_000. Please cite this article as: Levy, J.S., et al., Identification of gully debris flow deposits in Protonilus Mensae, Mars: Characterization of a water- bearing, energetic gully-forming process, Earth Planet. Sci. Lett. (2009), doi:10.1016/j.epsl.2009.08.002 ARTICLE IN PRESS J.S. Levy et al. / Earth and Planetary Science Letters xxx (2009) xxx–xxx 3 Fig. 4. Lobate deposits in Protonilus Mensae. a) Digitate lobes with steep flow fronts (arrows). b) Lobate flow fronts deflected around underlying topography (arrows). c) Narrow (left) and broad (right) lobe fronts. d) Complexly stacked lobe fronts are highlighted in the white box. All images excerpted from PSP_007148_2245, with north towards image top, down-slope towards image bottom, and illumination from the lower left. and broaden, either gradually or abruptly, downslope and as the slope aprons (Malin and Edgett, 2000; Schon et al., 2009)(Fig. 5c, d). Gully fan steepens (Fig. 5a). Typically, deeper and wider gully channels are surfaces immediately beneath Protonilus Mensae gully channels have a present in areas of thick pasted-on terrain, while finer channels occur in rubbly texture with scattered boulders present—the source of these thin pasted-on deposits (Figs. 5a and 6). Pasted-on terrain appears large clasts can be traced up-channel to eroding outcrops of bouldery thickest towards the base of the mesa, and thins towards the top of the mesa material present beneath the pasted-on terrain. gullied portion of the mesa, disappearing completely in some instances The most striking geomorphological element in the study site is a (Figs. 5a and 6). Some gully channels have minor sinuosity on meter to collection of lobate features located down-slope of the gully fans (Figs.