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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E10002, doi:10.1029/2009JE003365, 2009 Click Here for Full Article

Sublacustrine depositional fans in southwest Melas Joannah M. Metz,1 John P. Grotzinger,1 David Mohrig,2 Ralph Milliken,3 Bradford Prather,4 Carlos Pirmez,4 Alfred S. McEwen,5 and Catherine M. Weitz6 Received 16 February 2009; revised 1 June 2009; accepted 24 June 2009; published 14 October 2009.

[1] Two depositional fan complexes have been identified on the floor of southwest . The western fan complex is located near the center of an enclosed basin in southwest Melas Chasma and is composed of multiple lobes with dendritic finger-like terminations. These fans are flat and have a morphology unlike any other fan that has been previously identified on . On the basis of the morphologic similarity of the western fan complex to the Mississippi submarine fan complex, we suggest that it may be a deep subaqueous fan depositional system. There are numerous channels on the surface of the western fan complex, and measurements of channel length, width, and sinuosity are consistent with channels observed on terrestrial submarine fans. The eastern Melas depositional fans are less well preserved and may be of deltaic or sublacustrine origin. Recognition of the fans supports earlier suggestions for the presence of a former in Melas Chasma and indicates that a significant body of water was present and stable at the surface of Mars for at least 102 to 104 years. Citation: Metz, J. M., J. P. Grotzinger, D. Mohrig, R. Milliken, B. Prather, C. Pirmez, A. S. McEwen, and C. M. Weitz (2009), Sublacustrine depositional fans in southwest Melas Chasma, J. Geophys. Res., 114, E10002, doi:10.1029/2009JE003365.

1. Introduction of wall rock ridgelines, they have been interpreted to have been fed by precipitation [Mangold et al., 2004; Quantin et [2] Evidence for ancient on Mars is based on al., 2005]. These valleys drain into a subcircular closed multiple indirect criteria including constant-elevation depression within Southern Melas Basin, and this depres- contour lines inferred to be shorelines, the presence of sion follows the 1800 m elevation contour [Quantin et al., flat-lying finely layered deposits inferred to be lake-bottom 2005]. Quantin et al. [2005] suggest there may have been a sediments, terraced alluvial fans inferred to have formed in body of standing water in the basin. The basin contains response to lake-level rise, and deltas with aggradational numerous light-toned flat-lying layers that can be traced stratal geometries [Quantin et al., 2005; Lewis and Aharonson, over several kilometers. A set of strata that show clinoform 2006; Grantetal., 2008; Kraal et al., 2008b]. Direct geometries occurs in the northwest part of Southern Melas evidence of a lacustrine environment requires observation Basin, and it has been suggested that this feature represents of sedimentary deposits that could have formed only in fully either a channel-levee complex or a delta complex [Dromart submerged settings. The former presence of lakes, as well as et al., 2007]. the amount of time that they were stable on the surface, has [4] Channel-levee complexes are important components important implications for the history and role of water on of deepwater depositional systems. Submarine fans are fed Mars [Baker, 2001]. by turbidity currents that often flow down sinuous deepwater [3] One area on Mars with possible evidence of a channels through laterally extensive channel-levee systems paleolake is Melas Chasma in (Figure 1). [Wynn et al., 2007]. The channels are thought to avulse Dense, highly organized valley networks have been identi- frequently with only one channel active at any given time fied in the topographic highs surrounding a basin in [Damuth et al., 1983; Damuth and Flood, 1984; Wynn et al., southwest Melas Chasma, hereafter referred to as Southern 2007]. Many studies have found similarities in the planform Melas Basin [Mangold et al., 2004]. Since the heads of the geometry of channels on upper and middle submarine fans valleys occur at different elevations, including near the tops and river channels [Flood and Damuth, 1987; et al., 1992], but with a lower occurrence of meander loop cutoffs 1Division of Geological and Planetary Sciences, California Institute of and frequent avulsions in submarine channels as compared Technology, Pasadena, California, USA. to subaerial channels [Kane et al., 2008]. As the turbidity 2Department of Geological Sciences, University of Texas at Austin, Austin, Texas, USA. currents wane during flow events, deposition of lobes of 3Jet Propulsion Laboratory, Pasadena, California, USA. sediment is common [Nelson et al., 1992]. Cores through 4Shell Exploration and Production Company, Houston, Texas, USA. submarine fan deposits show clays with interbedded graded 5Lunar and Planetary Laboratory, University of Arizona, Tucson, sand and silt beds with distorted layers arranged in discon- Arizona, USA. tinuous lenses [Nelson et al., 1992]. These lenses of coarser 6Planetary Science Institute, Tucson, Arizona, USA. material occur where channelized flows ‘‘freeze’’ into Copyright 2009 by the American Geophysical Union. dendritic patterns, depositing their sediment near the edges 0148-0227/09/2009JE003365$09.00 of depositional lobes [Nelson et al., 1992]. ‘‘Freezing’’ in

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Figure 1. White box shows location of southern Melas Basin within central Valles Mariners. sediment deposition occurs because of loss of pore fluid pattern with an age of 3 Gyr followed by a period of pressure or because of frictional grain resistance and cohe- resurfacing until 10 Myr [Quantin et al., 2005]. The sive grain interactions [Dasgupta, 2003]. crater-size distribution of the valley networks indicates that [5] A fan was previously identified near the western they are younger than 3.5 Gyr [Quantin et al., 2005]. Thus margin of Southern Melas Basin, and it was suggested that it is possible that the fluvial and lacustrine features in it may be of deltaic origin [Quantin et al., 2005]. We Southern Melas Basin date back to the beginning of the identify four fans in Southern Melas Basin, two in the Valles Marineris system. western part of the basin and two in the eastern part of the [8] There is some debate about the ages of materials on basin. One of the western fans was considered part of the floor of Valles Marineris, such as the interior layered the delta in the Quantin et al. [2005] analysis. We evaluate deposits, which are distinct finely layered materials located the potential origins for the two western fans which include on the floors of troughs within Valles Mariners. There are submarine fan, delta, alluvial fan, or gully deposit. We many hypotheses suggested for their formation, many of compare the detailed morphology, basin position, and slope which suggest they were deposited after the formation of of each of the possibilities to those of the western Melas Valles Marineris [Lucchitta, 1990; Komatsu et al., 2004; fans. Although many of the features of the fans are Fueten et al., 2006; Okubo et al., 2008], but some suggest consistent with a deltaic origin, we propose that a sublacus- they are ancient materials exhumed from below the trine origin is also consistent with the morphology and material forming the trough walls [Malin and Edgett, properties of the two western fans. 2000; Montgomery et al., 2008]. It is possible that both of these hypotheses are true; for example, there could be large expanses of wall rock that predate the opening of Valles 2. Geological Setting Marineris, but more local topographically enclosed basins [6] The area of study in Southern Melas Basin is located may contain deposits that formed after the depression was within central Valles Marineris (Figure 1). Surface features formed. The age of the materials on the floor of Southern and materials in Melas Chasma have been studied using Melas Basin may in fact be older than their crater-counting data [Pelkey and Jakosky, 2002] and ages suggest if they were recently uncovered by erosion Mars Odyssey Thermal Emission Imaging System (THEMIS) [Malin and Edgett, 2001; Hartmann, 2005]. data [Pelkey et al., 2003]. These studies conclude that the recent surface in Melas Chasma has been dominated by eolian processes. 3. Methods [7] There is still much debate about how Valles Marineris [9] Structural attitudes were obtained using planar fits to formed, but mechanisms that have been suggested include bedding seen in a Digital Elevation Model (DEM) con- structural collapse [Spencer and Fanale, 1990; Schultz, structed using stereo Mars Reconnaissance Orbiter (MRO) 1998; Rodriguez et al., 2006], tectonic rifting [Mege and High-Resolution Imaging Science Experiment (HiRISE) Masson, 1996; Schultz, 1998; Peulvast et al., 2001], and images (PSP_007087_1700, PSP_007667_1700). The gravity spreading due to loading of aerially widespread salt DEM was constructed using the methods of Kirk et al. deposits [Montgomery et al., 2008]. The Valles Marineris [2008], and the vertical precision of the DEM is 0.2 m canyon system is thought to have formed over several (resolution 1 m). Linear segments were traced out along periods of tectonic activity spanning the late to well-exposed layers. Only layers with some natural late or early , and the material filling curvature in the horizontal direction were used in order to southwestern Melas Chasma is thought to be Hesperian to provide accurate constraints on the three-dimensional Amazonian in age on the basis of counting geometry of the layer. We employed the method presented [Scott and Tanaka, 1986; Pelkey and Jakosky, 2002]. Crater by Squyres et al. [2007] and Lewis et al. [2008], which uses counting of the material thought to compose the paleolake principal component analysis to ensure the layers used are surface of Southern Melas Basin yields a complicated well fit by a plane.

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Table 1. Characteristics of the Submarine Fans and Deltas Compared in This Studya Fan Location Fan Type Fan Area (km2) Water Depth (m) Drainage Basin Areab (km2) Grain Size Tectonic State BT Basin IV 27°200N94°240W ponded 128 1500 1.2 105 mixed passive Makassar 0°230S118°370E seafloor 2500 2400 7.5 104 mud active Mississippi 26°300N85°300W seafloor 300,000 3200 4.76 106 mud passive Pochnoi 54°350N 173°560W seafloor 20,000 ? 1.2 105 ? active Rhone Neofan 41°450N4°540E seafloor 1430 2400 9.0 104 sand passive Speculobe 35°300N7°300W seafloor 48 1500 5.7 104 sand active Wax Lake Delta 29°300N91°260W delta 40 0 ? - passive Lena Delta 72°130N 126°90E delta 30,000 0 2.5 106 - passive Melas North 9°490S76°250W lacustrine 2.3 ? 500 ? ? Melas 9°490S76°250W lacustrine 4.3 ? 500 ? ? aGrain size refers to the predominant grain size on the fan and the tectonic state refers to the margin on which the fan is developed. bDrainage basin area is based on the size of the drainage basin of the river that ultimately feeds the fan.

[10] Channels on the fans were identified using both the bathymetry (J. V. Gardner, Law of the Sea Mapping images and the DEM. There are several ways to define a Program, 2007, available at http://www.ccom-jhc.unh.edu/ channel, but for this study two new channels are defined index.php?p=39j46&page=law_of_the_sea.php; hereinafter after each channel branch point. We define the main trunk referred to as Gardner, 2007). The slope was found by channel as a first-order channel. When a first-order channel fitting a plane to a small area of the surface, similar to the branches, the resulting channels are second-order channels. methods used by Lewis et al. [2008], and repeating this When a second-order channel branches, the resulting chan- measurement for several areas on the fan. The values found nels are third-order channels. Thus, the main trunk channel were between 0.04° and 0.4° with an average of 0.08°, but is a first-order channel and the more distal channels are there is an uncertainty of at least 0.04° in the measured higher order. The width of each channel was measured at values due to artifacts in the bathymetry. Despite the the beginning of each channel reach, and each width uncertainty, these values are close to the average seafloor measurement has an uncertainty of about 1 m. The sinuosity slope of 0.06° reported for the Mississippi fan [Nelson et al., of the channels was found by dividing the channel length, 1992; Schwab et al., 1996]. which is the length traced along the path of the channel, by [14] Fan lobes on the western Melas fans were identified the valley length, which is the straight-line distance from the and their thicknesses measured using the DEM. The lobe beginning to the end of the channel [Schumm, 1963]. areas were measured, and their minimum volume calculated [11] To compare the channel patterns seen on the Melas by multiplying lobe area by the thickness. These are all fans against terrestrial depositional fans, we need to account minimum estimates since some parts of the fans are for the different size of the systems. This was accomplished obscured by overlying strata. The lobe areas and volumes using nondimensional parameters that included dimension- were added to find the total area and volume of each fan. less width and length and sinuosity. The width of each channel was divided by the width of the main trunk channel, and length of each channel was divided by the length of the 4. Description of Fans Used for Comparison main trunk channel to make these parameters nondimen- sional. The depth of each channel was measured from the [15] Channels identified and mapped on the Melas fans DEM and has an uncertainty of 1 m. The reported channel were compared to six terrestrial deepwater submarine fans depths for the Melas fans are minimum estimates, since (Brazos-Trinity Basin IV, Makassar, Mississippi, Pochnoi, many of the channels have been at least partially filled in by Rhone, and Speculobe), two terrestrial deltas (Wax Lake eolian material and the channel levees may have been and Lena), and one Martian delta (). These six partially eroded. A minimum estimate of the channel terrestrial submarine fans were chosen because they were gradient was calculated by finding the difference in eleva- imaged at sufficient resolution to make a comparison, and tion between the beginning and end of each channel and we were able to obtain the data. The Brazos-Trinity Basin dividing by the length of the channel. IV ponded apron is located in the northwestern Gulf of [12] Ideally, the channels on the Melas fans would be Mexico and is formed in the terminal basin of four linked compared to channels on terrestrial sublacustrine fans; intraslope basins (Table 1). The seafloor fan in Makassar however, images of the plan view morphology of terrestrial Straits is a very low relief mud-rich fan located between the sublacustrine fans either have not been acquired or are not islands of Borneo and Sulawesi (Table 1). The Mississippi publicly available. Thus, the channel patterns on terrestrial submarine fan is a large, mud-rich seafloor fan located in sublacustrine fans could not be mapped and compared using the Gulf of Mexico (Table 1) that was largely constructed the methods of this study. Bathymetric and seismic reflec- during the Plio-Pleistocene [Wynn et al., 2007]. The Pochnoi tion profiles across sublacustrine fans have been acquired fan is located in the Aleutian Basin in the Bering Sea and show that channels are present on sublacustrine fans (Table 1), and it is thought to be a very young feature, [Normark and Dickson, 1976b; Scholz and Rosendahl, possibly of late Pliocene-Quaternary age [Herman et al., 1990; Back et al., 1998; Nelson et al., 1999]. 1996]. The Rhone Neofan is part of the much larger Petit- [13] The surface slope of the Mississippi submarine fan Rhone Fan and formed from the most recent avulsion of the was found by superimposing the side-scan sonar image over Rhone channel in the Gulf of Lion (Table 1). The Speculobe fan is a small sand-rich seafloor fan located in the Gulf of

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Figure 2. (a) Portion of High-Resolution Imaging Science Experiment (HiRISE) image PSP_007667_1700 with white arrows showing the location of the two putative western sublacustrine fans intercalated within the layers in southern Melas Basin. (b) Mosaic of CTX images showing southern Melas Basin where the putative sublacustrine fans occur. White line is actual topographic transect for schematic in Figure 12.

Cadiz offshore Spain (Table 1). As shown in Table 1, the six [Wood, 2006]. It is an erosional remnant of a larger submarine fans chosen for comparison comprise the range paleodeltaic deposit and has numerous bifurcating distribu- of diversity seen on submarine fans (e.g., mud-rich, sand- taries preserved in positive relief [Malin and Edgett, 2003]. rich, and mixed sediment; active and passive continental margin, basin floor and ponded fans). [16] Wax Lake Delta is a modern river-dominated bay 5. Results head fan delta located at the mouth of the Wax Lake outlet, 5.1. Morphology which is a manmade channel excavated in 1941 [Wellner et [17] The western fan complex is located near the center of al., 2005]. The formation of this delta was tracked through Southern Melas Basin and has two fans, each of which is time by images taken over the last 30 years. The Wax Lake composed of multiple lobes with dendritic finger-like Delta is a relatively young delta that has not had much terminations (Figure 2). The lobes are elongated and the human interference and thus appears in a natural state. The ‘‘fingers’’ often branch off at high angles in the downstream Lena delta is a river-dominated delta located at the Laptev direction (up to 90° to the overall transport direction). The Sea coast in northeast Siberia [Olariu and Bhattacharya, mean branching angles for the fingers are 75 degrees while 2006]. The Eberswalde delta is located in Eberswalde crater the median is 80 degrees. The surface of the fans in near the western margin of the 65-km diameter crater Southern Melas Basin is marked by numerous mildly

4of17 E10002 METZ ET AL.: SUBLACUSTRINE DEPOSITIONAL FANS E10002 sinuous channels, and its average surface slope is 1° al., 2008; Weitz et al., 2009]. The opaline silica was (Figure 3). The dips of the beds in the fans are also about detected in a small hill just to the east of the northern Melas 1° (Figure 3). The fans have few craters, because they were fan and in bedded outcrops to the northeast of the fan probably buried beneath later sediments, which prevented (Figure 8b). These latter exposures occur in strata that lie small craters from forming on their surfaces. Subsequent stratigraphically above the fans. The hydrated silica was exhumation has now exposed the surfaces of the fans. There identified by the presence of absorption bands centered near are layered beds surrounding the fans, so the beds must have 2.21–2.26 mm due to the presence of Si-OH groups. The been joined in the past and then eroded back to expose the position and shape of a doublet band at 2.23 and 2.26 mmis fans. The preservation of the channels, including a topo- most consistent with laboratory spectra of opal- A/CT graphic signature, suggests the exhumed paleofan surface (Figure 8c). Hydrated silica can occur as primary sedimen- corresponds closely to the original fan surface. tary deposits [McLennan, 2003], altered ash deposits, pre- [18] The fans occupy different stratigraphic levels with cipitation as pore-filling cements, or precipitation from the southern fan located stratigraphically above the northern shallow evaporating bodies of water [Milliken et al., 2008]. fan, but both are located in the topographically lowest part [23] Mineral spectra that show a good spectral match of the basin (Figure 4). The fans are surrounded by other to jarosite were detected in the clinoforms (Figure 8b). layered material, and these beds are also relatively flat-lying The jarosite was identified on the basis of bands at 1.85 with dips of between 0.6 and 1.4° (Figure 3). The fans and 1.93 mm and an OH-feature near 2.26 mm and is most appear to be visible through an erosional window through consistent with Fe-deficient H3O-bearing jarosite (Figure 8d). the other layers. H3O-bearing jarosite is consistent with precipitation under [19] Table 2 lists the characteristics of the western Melas low-temperature acidic conditions [Milliken et al., 2008]. fans, including slope, minimum area and volume, minimum Jarosite is not as soluble as other sulfates, such as Mg- length, mean and maximum channel depths, mean channel sulfates, and could be transported by fluids if they were gradients, and channel sinuosity. Six lobes were identified already saturated with S and Fe. It would depend on the on the northern fan and ten lobes on the southern fan composition of the transporting fluid and the amount of (Figure 5). The numbering of the lobes indicates the relative time the mineral was in contact with the fluid. The jarosite order that they formed in; parts of earlier formed lobes can could also be diagenetic, and this cannot be ruled out since be seen to be covered by later formed lobes. Some of the the CRISM data are too coarse to determine if the jarosite is later formed lobes appear to have filled in the low areas cutting across bedding contacts. However, cementation is surrounding earlier formed lobes. The channels identified likely the most probable origin for the jarosite. The defin- on each fan are shown in Figure 6. itive sedimentary origin for the clinoforms implies that if [20] There are two additional fans in the eastern part of this is a jarosite cement it would be of low-temperature Southern Melas Basin (Figure 7a), but they are not as well- origin since the clinoforms are not a volcanic deposit. This preserved as the western fans. They have an elongate provides a definitive constraint on paragenesis. branching morphology and are preserved in positive relief. [24] The presence of these hydrous minerals implies that The bulk of the fans appear to be composed of inverted water was present when they formed, possibly as a lake or channels (Figure 7b), although there are a few channels on as pore fluids in preexisting sediments [Milliken et al., the surface of the southern fan which form topographic 2008]. depressions. The channels composing the fans can be seen to migrate and crosscut. The northern fan appears to have formed first (labeled 1 in Figure 7b); the main feeder 6. Discussion channel switched south and built up a new channel system [25] Because the western Melas fans are a newly identi- (labeled 2 in Figure 7b). The main channel appears to have fied class of fan-like features on Mars, we compare them to ultimately switched to the north as evidenced by the common depositional fans found on both Earth and Mars crosscutting channel relationships (labeled 3 in Figure 7b). to evaluate the most likely process responsible for their There are several large and small impact craters on the formation. surfaces of the fans. Since these fans are not as well- preserved as the western fans, this paper will focus on the 6.1. Depositional Fan Comparisons western fans. 6.1.1. Alluvial Fans [21] The combination of low slope, channel branching [26] Alluvial fans, both on Mars and on Earth, are cone- geometry, presence of distinctive small-scale lobes, and shaped deposits radiating from a dominant source channel. position near the center rather than near the margin of a They typically develop near the base of topographic high- topographic basin show that the western fans are distinct lands, thus fringing basin margins. The Melas fans, from previously identified fluvial features on Mars. however, occur near the center of a basin. Alluvial fans typically have relatively steep slopes; the average surface 5.2. Mineralogy slope of alluvial fans on Mars based on our compilation of [22] The surfaces of the fans are covered in dust, which existing data is 2.5° (see Table 4) [Moore and Howard, makes mineralogical detections difficult (Figure 8a). How- 2005; Kraal et al., 2008a]. Alluvial fans on Earth have ever, MRO Compact Reconnaissance Imaging Spectrometer average slopes of 2–12° [Hashimoto et al., 2008; Blair and for Mars (CRISM) spectra do show evidence for opaline McPherson, 1994b]. Some authors have suggested that in silica (Figures 8b and 8c) and jarosite (Figures 8b and 8d) in humid areas alluvial fan formation is dominated by braided layers near the fans, similar to the mineral assemblages seen fluvial river processes and that these alluvial fans have more on the plains directly south of Melas Chasma [Milliken et gentle slopes of 1° or less [Stanistreet and McCarthy, 1993;

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Figure 3. (a) Shaded relief perspective view of the western fans. The southern fan is topographically and stratigraphically above the northern fan. (b) Image of the fans showing the dips of the fan surfaces and layers. The main feeder channel of the southern fan is 50 m above the basin plain.

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6.1.3. Deltas [29] Deltas are partially subaerial masses of sediment deposited near where a river enters a standing body of water. The Eberswalde delta on Mars has a low surface slope that averages 2° [Lewis and Aharonson, 2006], and many meandering and sinuous channels, scroll bars and branching terminations [Wood, 2006]. Terrestrial deltas also commonly have low surface slopes (<1°) and can show lobate stacking patterns from lobe buildup and switching [Coleman and , 1975]. Delta channels show a range of sinuosities and can be meandering and sinuous [Wood, 2006] or mildly sinuous (Figure 9). The western Melas fans Figure 4. Stratigraphic column illustrating that the southern show low surface slopes and mildly sinuous channels fan is located stratigraphically above the northern fan. (sinuosity of 1.02), which both fall within the range of values common for deltas (Tables 3 and 4, and Figure 9). The Eberswalde delta is located at the margin of its crater, Hashimoto et al., 2008]. Other authors argue that these similar to terrestrial deltas which are located near the should not be classified as alluvial fans and are actually margins of basins. crater delta, southeast of Nili braided stream systems [Blair and McPherson, 1994b]. The Fossae on Mars, is also located at the margin of its crater slope of the Melas fans (1°) does, however, overlap with [Ehlmann et al., 2008]. However, the western Melas fans the slopes of humid alluvial fans. This would indicate that if are found near the center of the basin. The delta channels the fan was not of sublacustrine origin that it would have also have low branching angles, and its lobes do not lead formed by alluvial processes that required continuous rather into the high-angle fingers seen in the Melas fans. than flashy or sporadic discharge, consistent with a wetter 6.1.4. Submarine Fans climate. [30] The morphology of the Melas fans compares favor- [27] The processes observed to act on alluvial fans are ably with depositional features formed on the Mississippi debris flows, sheet floods, and shallow braided streams submarine fan (Figures 2 and 10). The distal Mississippi [Schumm, 1977; Blair and McPherson, 1994b; Harvey et submarine fan also has a lobate morphology with finger-like al., 2005]. The channels on the Melas fans are distinct and dendritic terminations, elongated lobes, and splays that moderately sinuous, not braided. The Melas fans also have branch off of the main deposit at high angles (up to 90°). distinct depositional lobes from individual flows. Debris The distal Mississippi submarine fan also has low relief flow-dominated alluvial fans could be expected to have mildly sinuous channels and is very flat, with a surface lobate deposits, but these typically have boulder-laced slope of 0.08° based on our analysis of existing bathy- snouts [Whipple and Dunne, 1992]. The Melas fans do metric data (Gardner, 2007). Although the slope of the not show evidence for large boulders at the scale of HiRISE Melas fans (1°) is somewhat higher than that of the images (25 cm/pixel). Also, debris flow-dominated fans Mississippi fan, it is possible that slight differential erosion typically have slopes above 5° while the Melas fans have of the surface of the Melas fans could cause an increase in much lower slopes [Harvey, 1984; and Harvey, 1987; the slope. If the surfaces of the Melas fans were not an exact Blair and McPherson, 1994a]. Furthermore, the high bedding plane, measured surface slopes would be higher branching angles of the lobes in the Melas fans are not than the true slope. seen in subaerial debris-flow-dominated fans. [31] Submarine fans are densely channelized distributary 6.1.2. Gully Deposits systems and the distal Mississippi fan is composed of [28] Gully deposits that have been identified on Mars depositional lobes whose beds consist of graded sand and have lobe-shaped terminations but are developed on steep silt [Nelson et al., 1992]. The detailed structure of these rather than gentle slopes. The average slope on which lobes shows a ‘‘finger-like’’ or high-branching-angle den- gullies are developed is 27° based on over 200 measure- dritic backscatter pattern in side scan sonar which correlates ments from recent studies [Malin et al., 2006; Dickson et to the sand-silt beds formed by abrupt deposition of chan- al., 2007; Heldmann et al., 2007]. These elongate deposits nelized flows to form small lobes at their distal reaches are thought to form by dry mass wasting on steep slopes or (Figure 10) [Nelson et al., 1992; Klaucke et al., 2004]. perhaps by liquid water flows [Pelletier et al., 2008]. The These finger-like lobes are commonly oriented at high flows typically originated from a small region, and the angles to the channels which delivered the sediment to the flows did not diverge widely to create complex branching lobes (except for the cluster of termination lobes). Similar networks as seen in the Melas fans. finger-shaped deposits have been observed on the Monterey

Table 2. Characteristics of the Western Melas Fans

Channel Depth (m) Mean Channel Fan Slope Area (km2) Volume (km3) Length (km) Mean Maximum Gradient (m/km) Sinuosity Northern 1° 2.3 0.05 2.1 1 4 23 1.02 Southern 1° 4.3 0.10 1.3 1 2 32 1.02

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Figure 5. Outlines of the six lobes composing the northern fan and the ten lobes composing the southern fan are outlined in white.

Fan and in the distal areas of Permian submarine fans (<1.3), and the channels on the Melas fans show similar preserved in the Tanqua Karoo Basin, South Africa [van sinuosities (Figure 9). These measurements show the der Werff and Johnson, 2003; Klaucke et al., 2004]. The channels on the Melas fans have properties that are consis- relatively abrupt terminations associated with the finger-like tent with channels on submarine fans. lobes are characteristic of sediment-gravity flow deposits. [32] The Melas fans are located near the center of the Full preservation of these lobes on subaerial fans is unlikely basin in the current topographically lowest area, which because of subsequent reworking and sediment transport by could be the result of subsequent erosion of the basin or flowing water [Whipple and Dunne, 1992]. The unmodified the original depositional setting. Many submarine fans also preservation of sediment-gravity flow deposits such as those occur on basin plains in the topographic lows of the basin, observed in Southern Melas Basin is consistent with accu- but this may not necessarily correspond to the center of the mulation in the distal reaches of a subaqueous fan. The basin. For example, the Mississippi submarine fan is located channels present on distal submarine fans are mildly sinuous at 3300 m water depth and is over 500 km away from the

Figure 6. Channels on the Melas fans are outlined by the solid white lines. Dashed lines show inferred positions of channels.

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interpreted as sublacustrine fans. Sublacustrine fans commonly occur in deep lakes on Earth and form important depositional systems that commonly are fed by density underflows originating from regional drainage systems [Nelson et al., 1999]. Turbidity current experiments in the laboratory have produced subaqueous fan deposits that are channelized with mildly sinuous, low-relief channels [Yu et al., 2006]. Thickness maps of these deposits show finger- shaped deposits that resemble the distal stretches of subma- rine fans. [34] In contrast to the small size of the Melas fans, the overall area and volume of the Mississippi submarine fan is 3 105 km2 and 2.9 105 km3, and the fan is 600 km long [Wynn et al., 2007]. The much larger size of the Mississippi fan is expected since the size of a submarine fan is as much a function of sediment flux as its duration, and the Mis- sissippi drains a large part of the North American continent. For Melas Chasma, the sediment flux must have been much smaller because of the small size of the catchment area which would have yielded the sediments which formed the fan. Terrestrial submarine fans comparable in size to the Melas fans do exist, such as the upper fan in Brazos-Trinity Basin IV and the Pochnoi fan [Kenyon and Millington, 1995; Beaubouef et al., 2003], but they have not been imaged in as much detail as the Mississippi fan so their small-scale morphology cannot be compared. [35] The properties of the channels on the Melas fans (i.e., channel length, width, and sinuosity) are also consis- tent with the values measured for terrestrial submarine fans and deltas (Figures 9 and 11). The mean sinuosity of the channels on the six distal terrestrial submarine fans is low overall. Sinuosity ranges from 1.0 to 1.3 but the majority of the submarine fans have channels with a sinuosity less than 1.1. The Melas fan channels also have low sinuosity with most values below 1.05. The relationship between mean channel length and mean channel width for submarine fans and deltas roughly follows a power law distribution (Figure 11). The channels on the Melas fans follow a similar trend. The steepness of the power law trend does not appear Figure 7. (a) Portion of HiRISE image PSP_007377_1700 to correlate with the dominant grain size present on the fan, with the white arrows showing the location of the eastern the water depth of the fan, fan size, margin type or drainage Melas fans. (b) White lines show outlines of channels. basin size. It is possible that a combination of these factors Black lines show an older generation of channels. The determines how quickly or slowly the channel width numbers refer to the relative ages of the channels based on decrease for a given decrease in channel length. We have crosscutting relationships. not yet identified a good quantitative measure of channel properties that is able to distinguish between the submarine Mississippi Canyon on a very low gradient surface [Schwab channels formed on submarine fans and the subaerial et al., 1996]. It is bounded to the east by the Florida channels formed on deltas. escarpment [Schwab et al., 1996]. Although the Mississippi [36] Terrestrial submarine fans typically have only one submarine fan is not located in the center of the Gulf of active main channel at a time [Damuth et al., 1983; Wynn et Mexico, it is located on the basin plain in a depositional al., 2007]. Each channel is eventually abandoned, probably low. Sublacustrine fans are also found in the deepest areas by avulsion, and a new channel is formed nearby. This may of lakes [Normark and Dickson, 1976a]. They typically have been the case for the Melas fans as well. The northern occur on the basin plains near the centers of their lakes, such fan was deposited first since it is located stratigraphically as in Lake Baikal, Siberia [Nelson et al., 1999] and Lakes and topographically below the southern fan. Eventually, its Malawi and Tanganyika, east Africa [Scholz and Rosendahl, feeder channel was abandoned, and the channel may have 1990]. Thus, the current basin position of the Melas fans is avulsed to a new course further south. This could have similar to that of terrestrial sublacustrine fans. resulted in the deposition of the southern fan. [37] Many features of the Melas fans are also shared with 6.2. Formation of the Melas Western Fans deltas. However, on the basis of detailed morphologic [33] On the basis of their morphological similarity to the comparisons of the lobes as well as basin location, the Mississippi submarine fan, the western Melas fans could be

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Figure 8. (a) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) targeted image overlain on a CTX mosaic of southern Melas Basin. White box shows location of Figure 8b. Light brown tones, such as on the eastern and western Melas fans, are indicative of dusty areas. (b) Close-up view of the area outlined in a. The white arrow shows the location of turquoise outcrops showing a good spectral match to opaline silica. Red arrow shows location of jarosite. (c) CRISM spectra (in black) of turquoise outcrops compared with library spectra of opaline silica A/CT (in red). Note good match of the double bands between 2.2 and 2.3 mm. (d) CRISM spectra (in black) of the clinoforms compared with library spectra of K-, Na-, and H30-bearing jarosite.

Melas fans appear more similar to terrestrial submarine fans 2005; Metz et al., 2009] and at crater by [Lewis than to either terrestrial or Martian deltas. et al., 2008]. [40] Distinctive features that could be observed in a deltaic environment, but would not be expected in a 6.3. Discriminating Between Deltas and Submarine submarine fan environment include point bar deposits, Fans floodplain deposits, paleosols and mudcracks [MacNaughton [38] Analysis of plan view morphology and slopes provides et al., 1997]. Submarine fans are largely composed of the broad constraints on interpretations of depositional fans, but deposits of turbidity currents. However, turbidity currents does not uniquely constrain their origin. Morphology alone can also occur in the distal delta front and in prodeltas, and so points to a sublacustrine origin, however, a deltaic origin the presence of these deposits alone is not diagnostic [Mutti et cannot be excluded. The observed facies, as well as their al., 2003; Pattison, 2005]. Distinguishing a prodelta from a stacking pattern, allows us to discriminate conclusively submarine fan may require examination of the fan architec- whether deposits were formed as part of a delta or a ture. The study of detailed facies transitions and stacking submarine fan. patterns may be necessary to distinguish between submarine [39] Since most of the information necessary to distin- fans and distal deltaic facies. guish different sedimentary facies is small-scale, orbital images do not provide the necessary resolution. Views of 6.4. Timescales the deposit in cross section that expose details at the [41] In an effort to obtain the most conservative estimate centimeter to decimeter scale would be required to provide of fan formation time, we have calculated minimum dura- definitive evidence of sublacustrine sediment transport via tions. This calculation assumes a continuous sediment turbidity currents. Obtaining this kind of data would require accumulation rate, but it is known from Earth that sedimen- rover images, similar to those taken at by tation is characteristically discontinuous in time, and that the Mars Exploration Rover Opportunity [Grotzinger et al., discontinuities are the rule not the exception [Sadler, 1981].

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Figure 9. Plot showing how mean channel sinuosity changes downfan (i.e., for increasing order) for six terrestrial submarine fans, two terrestrial deltas, and the two western Melas fans. Sinuosities were measured for channels on the distal portions of the terrestrial submarine fans.

Minimum formation time can be constrained using several distal subaqueous fans are viewed as efficient sediment approaches; analyses of formation times for subaerial traps. deposits of roughly comparable volume on Mars [Jerolmack [44] Formation times for the Melas fans were also esti- et al., 2004; Kraal et al., 2008b], estimates of subaqueous mated by calculating how long subaqueous turbidity fan formation on Earth provided by age dating fan deposits, currents would need to flow in order to deposit the volume and quantitative estimates of how long turbidity currents of material observed to compose the visible part of the fans. would have taken to deposit the fans. To estimate how long each lobe takes to form, we need to [42] The well-studied terrestrial deposit of the Brazos- know the rate of sediment transport (Qs). Fluid discharge Trinity fan in Basin IV is somewhat larger in size than the through the channels is given by Melas fan and formed in no more than 35,000 years [Behrmann et al., 2006; Mallarino et al., 2006]. This Qw ¼ uBH ð1Þ measure is similar to 17,000 years of sediment deposition leading to development of several submarine fans in the California Borderland [Covault et al., 2007]. Together these where u is the mean flow velocity, B is the channel width examples constrain a millennial-scale estimate for the and H is the channel depth [Jerolmack et al., 2004]. minimum time that water must have been present in Channel width and minimum depth (see discussion in the Southern Melas Basin. This estimate includes within it section 3) can be measured from the Melas DEM. The flow intervals of nondeposition. Deposits which span longer time velocity can be found by the Chezy flow resistance relation intervals have the opportunity to incorporate more and ÀÁ longer hiatuses [Sadler, 1981]. Removing this inactivity 2 1=2 u ¼ ta*RgDar ð2Þ would shorten the formation time and duration for standing water in Melas Basin. where t*a is the dimensionless formative shear stress, R is [43] Theoretical analyses of primarily subaerial fan depo- the submerged specific density of grains, g is Martian sition on Mars assuming no intermittency in construction gravity (g = 3.7 m/s2), D is the grain diameter, and a is the place minimum formation times at the decadal to century r resistance coefficient for flow in a channel. In order to scale [Jerolmack et al., 2004; Kraal et al., 2008b]. determine the values of these parameters, a grain size and Comparison of formation times between these relatively composition must be assumed. We assumed sand sized proximal depositional systems and the distal fan of Southern quartz grains (D = 0.3 mm) which leads to t* = 1.8, R = Melas Basin is reasonable because of differences in a 1.65, and a =15[Parker et al., 1998]. This yields a flow the efficiency with which these systems trap sediment. r velocity of 0.8 m/s which is reasonable for turbidity Proximal subaerial systems have more sediment moving currents. The characteristic velocity of turbidity currents through them but also have relatively low trapping efficien- can be related to the buoyancy flux by cies for this detritus [Trimble, 1981; Allison et al., 1998; Goodbred and Kuehl, 1998; Walling et al., 1999], while Bf ¼ RguhB ð3Þ

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Table 3. Depositional Fan Characteristicsa Feature Basin Position Slope Shape Channels Alluvial Fan Margin 2.5° Cone-shaped Braided Gully Deposit Wall 27° Elongate lobes - Delta Margin 2.1°*<1°** Fan-shaped, lobes Sinuous, meandering, mildly sinuous Submarine Fan Center 1° Elongate branching Mildly sinuous lobes, fingers Melas Western Fans Center 1° Elongate branching Mildly sinuous lobes, fingers aOne asterisk, average slope reported for the Eberswalde Delta [Lewis and Aharonson, 2006]. Two asterisks, average slope value for terrestrial deltas.

where Bf is the buoyancy flux and h is the flow thickness. are needed to form channel networks on the surfaces of Britter and Linden [1980] related the buoyancy flux per unit submarine fans [Yu et al., 2006]. Thus, what we recognize width to the velocity of the current by as a lobe in a plan view of the Melas fans likely took several flow events to form. This could be tested by a rover  B 1=3 u ¼ W f ð4Þ front B

Table 4. Compilation of Martian Alluvial Fan Gradientsa where ufront is the velocity of the head of the turbidity current and W is a proportionality factor. The densimetric Apex Apex Fan Froude number is given by Latitude (N) Longitude (E) Gradient (°) Reference 1.4 58.3 4.75 Kraal et al. [2008a] u2 1.46 58.39 4.06 Kraal et al. [2008a] 2 front 7.27 356.22 2.64 Kraal et al. [2008a] Frdensimetric ¼ ð5Þ Rgh 18.09 322.89 2.66 Moore and Howard [2005] 18.32 340.11 2.21 Moore and Howard [2005] 18.41 323.35 5.67 Moore and Howard [2005] [45] Middleton [1993], and can be related to W by 20.1 123.2 6.41 Kraal et al. [2008a] 20.34 324.21 1.3 Moore and Howard [2005] 2 21.14 320.66 1.83 Moore and Howard [2005] 2 3 ufront Frdensimetric ffi W ¼ : ð6Þ 21.35 72.68 1.82 Moore and Howard [2005] Rgh 21.47 67.22 1.48 Moore and Howard [2005] 21.52 320.09 2.03 Moore and Howard [2005] For bed slopes between 0 and 10°, W3 is between 1 and 1.5 21.66 72.56 1.82 Moore and Howard [2005] [Britter and Linden, 1980]. Since the slope of the southern 21.68 66.41 2.25 Moore and Howard [2005] 3 22.36 66.53 3.24 Moore and Howard [2005] Melas fan is 1°, W and hence the densimetric Froude 22.73 74.46 2.23 Moore and Howard [2005] number should equal 1. In the case of small slopes, the 22.76 74.03 1.42 Moore and Howard [2005] front velocity of the turbidity current is the same as the 23.04 74.74 3.21 Moore and Howard [2005] 23.14 73.83 3.6 Moore and Howard [2005] characteristic velocity, so ufront = u [Britter and Linden, 1980]. We can calculate the rate of sediment transport by 23.31 73.99 3.59 Moore and Howard [2005] 23.32 27.07 2.58 Moore and Howard [2005] 23.37 74.58 2.33 Moore and Howard [2005] Qs ¼ concsedufronthB ð7Þ 23.45 74.35 1.96 Moore and Howard [2005] 23.56 27.44 2.08 Moore and Howard [2005] 23.62 27.18 2.56 Moore and Howard [2005] where concsed is the concentration of sediment in the 23.91 28.15 2.08 Moore and Howard [2005] turbidity current. Typical sediment concentrations in 24.31 28.29 1.67 Moore and Howard [2005] turbidity currents range from 1 to 10%, and we assume a 24.84 27.42 2.16 Moore and Howard [2005] sediment concentration of 1% [Middleton, 1993]. 24.96 325.71 1.52 Moore and Howard [2005] 25.88 324.85 2.66 Moore and Howard [2005] [46] The timescale of formation is given by 26.23 331.52 2.1 Moore and Howard [2005] 26.43 324.84 2.53 Moore and Howard [2005] teq ¼ ðÞ1 l V=Qs ð8Þ 27 332.99 2.33 Moore and Howard [2005] 27.58 332.85 2.36 Kraal et al. [2008a] 27.65 332.93 1.58 Kraal et al. [2008a] where V is the lobe volume and l is the porosity (assume 28.04 332.91 1.55 Kraal et al. [2008a] l = 0.35) [Jerolmack et al., 2004]. The time for the 28.09 332.77 2.26 Kraal et al. [2008a] formation of each of the ten lobes of the southern Melas fan 28.49 84.07 1.77 Moore and Howard [2005] was calculated, and these were added to get an estimated 28.54 84.51 3.21 Moore and Howard [2005] time of formation of 106 years. 33 84.26 1.32 Kraal et al. [2008a] 33.02 84.19 1.58 Kraal et al. [2008a] [47] This calculation assumes that each lobe was formed 49.6 113.6 2.33 Kraal et al. [2008a] by one turbidity flow event, which is conservative in that it 51 114.3 1.85 Kraal et al. [2008a] likely would have taken several flows to develop lobes with 51 113.5 2.33 Kraal et al. [2008a] levee-bounded channel networks [Yu et al., 2006]. Experi- Average 2.5° ments on turbidity currents show that multiple flow events aThe average gradient is 2.5°.

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Figure 10. SeaMARC IA side-scan sonar image of the Mississippi submarine fan showing the elongated nature of the lobes and the fingers that branch off at high angle (available from the USGS at http://coastalmap.marine.usgs.gov/regional/contusa/gomex/flplatform/msfan/data.html). Inset shows location of the side-scan sonar image. High backscatter areas are composed of clay facies with interbedded sand and silts, whereas low backscatter areas are composed of clay facies with no siliciclastic sands or silts [Nelson et al., 1992]. mission, which could examine each lobe in cross section to take place on the order of hours to days, the time between determine the number of flow events required to form each events is on the order of years to thousands of years lobe. [Rothman et al., 1994]. If there was a hiatus of 10 years [48] Our calculation also assumes there was no hiatus between each event, the southern fan would have taken between different turbidity flow events, and so the actual 200 years to form, but if the hiatus was 1000 years than it formation time is likely to be longer. Laboratory and field would have taken 10,000 years to form. The above experiments show that although turbidite events appear to timescale estimate does not take into account the formation

Figure 11. Plot showing power law fits to mean scaled channel length versus mean scaled channel width data. Fits for six terrestrial submarine fans, two terrestrial deltas, one Martian delta, and the two western Melas fans are plotted. This plot shows that the channel widths and lengths for the Melas fans fall within the typical range of values seen for terrestrial submarine fans and deltas.

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Figure 12. Ideal schematic arrangement of environments including sublacustrine fans, clinoforms, alluvial fan, and incised channels; actual topography represented by white line in Figure 2b. The inferred sublacustrine fans occur in the topographically lowest part of the basin.

time of the northern Melas fan, but assuming it took a very similar in morphology to the Mississippi submarine comparable amount of time to form, this would double the fan (Figures 2b and 12). This ultimate topographic low in amount of time standing water would have been required in the system then provides the terminal sink for the sediments the basin. By comparing analogous formation times for [Leeder, 1999]. terrestrial submarine fans of comparable size, subaerial fan [50] The above interpretation assumes that the various deposits on Mars, and our own calculations, we estimate a elements of the geomorphic system are all the same age. If formation timescale of 102 to 104 years. these features have different ages, then the linkages between elements in the sediment transport system would not nec- 6.5. Sublacustrine Fans as Terminal Sediment Sinks essarily hold. However, even if not all of these elements are [49] We suggest that Southern Melas Basin could repre- the same age, they still represent pieces of the sediment sent a complete erosional-to-depositional system, from the transport system. For example, crosscutting relationships fluvially incised source region in the surrounding highlands suggest there are several generations of valley networks to the terminal sediment sink formed by the sublacustrine preserved in the ridges surrounding Southern Melas Basin fans in the topographically lowest part of the basin (Figures 2b [Quantin et al., 2005]. It is not possible to determine which and 12). Sediments are moved from their source in the generation of the drainages may have fed the depositional surrounding mountainous areas to their sink in depositional fans currently preserved in the bottom of the basin. Similar areas by the sediment transport system [Allen, 2008]. to the valley networks, perhaps the currently exposed clino- Fluvial incision of bedrock, interpreted to be caused by forms overlie a set of older buried clinoforms that are the runoff from precipitation, drains the ridges bordering the same age as the depositional fan. As sediments were trans- western and eastern parts of the basin [Mangold et al., ported into the basin, the depositional system could have 2004; Quantin et al., 2005]. Sediments generated during responded with retrogradation, aggradation, or progradation erosion of the upland areas were transported by fluvial of the sediment depocenter further into the basin; these drainage systems to form a classic, cone-shaped alluvial fan dynamic responses depend on how the incoming sediment at the western edge of the basin where confined channel flux balances with basin subsidence and lake level flow emerges onto the fan surface [Quantin et al., 2005]. [Flemings and Grotzinger, 1996; Hodgson et al., 2006]. The preservation of sediments in the alluvial fan depends on [51] We are assuming here that the sediments described in whether there is accommodation space available to store the Southern Melas Basin postdate canyon formation and can sediment over the long term. Accommodation space can be therefore be linked to the modern geomorphic surface. If generated when the graded profile of these streams moves these sediments are actually older than the basin itself and upward in response to a rise in base level or to uplift of the have been exposed through erosion of the basin floor, then source area [Viseras et al., 2003]. The amount of sediment the depositional fans would predate, and thus not be related permanently stored in the alluvial fans is likely small to the clinoforms, the alluvial fan, or the valley networks. compared to the total flux of sediment [Allen, 2008]. Sedi- Conclusive stratigraphic contacts that show that layered ments which pass through the alluvial fan could then be sediments underlie the volcanic material composing the deposited as the clinoforms, which could record a potential ridges in Melas Chasma have not been observed. Areas shoreline or the upslope channel levee part of the submarine where stratigraphic contacts are conclusive, such as to the fan system [Dromart et al., 2007]. The clinoforms give way east of Southern Melas Basin, show these layered materials further down the topographic profile to a sublacustrine fan, onlap the ridges. Thus, there is no evidence to suggest that

14 of 17 E10002 METZ ET AL.: SUBLACUSTRINE DEPOSITIONAL FANS E10002 the layered materials in Southern Melas Basin are older than classification of submarine fans, Geology, 20, 633– 636, doi:10.1130/ 0091-7613(1992)020<0633:QAOTGO>2.3.CO;2. the basin. Coleman, J. M., and L. D. Wright (1975), Modern river deltas: Variability of processes and sand bodies, in Deltas: Models for Exploration, edited by M. L. Broussard, pp. 99–149, Houston Geol. Soc., Houston, Tex. 7. Conclusions Covault, J. A., W. R. Normark, B. W. Romans, and S. A. Graham (2007), Highstand fans in the California borderland: The overlooked deep-water [52] The novel depositional fans in Southern Melas Basin depositional systems, Geology, 35(9), 783–786, doi:10.1130/ have a unique morphology which includes multiple lobes G23800A.1. with dendritic finger-like terminations that branch off at Damuth, J. E., and R. D. Flood (1984), Morphology, sedimentation processes, and growth pattern of the Amazon deep-sea fan, Geo Mar. high angles in the downstream direction. The southern fan is Lett., 3, 109–117, doi:10.1007/BF02462455. located at a higher stratigraphic level than the northern fan, Damuth, J. E., R. O. Kowsmann, R. D. Flood, R. H. Belderson, and M. A. and layers near the fans show evidence for hydrated Gorini (1983), Age relationships of distributary channels on Amazon minerals including opaline silica and jarosite. The Melas deep-sea fan: Implications for fan growth pattern, Geology, 11, 470– 473, doi:10.1130/0091-7613(1983)11<470:ARODCO>2.0.CO;2. fans appear morphologically similar to deltas and submarine Dasgupta, P. (2003), Sediment gravity flow: The conceptual problems, fans; however, the details of the morphology and their basin Earth Sci. Rev., 62, 265–281, doi:10.1016/S0012-8252(02)00160-5. position suggest that they are likely to be sublacustrine fans. Dickson, J. L., J. W. Head, and M. Kreslavsky (2007), Martian gullies in the southern mid-latitudes of Mars: Evidence for climate-controlled Quantitative comparisons of the channel patterns present on formation of young fluvial features based upon local and global topogra- the Melas fans show they are consistent with the channel phy, Icarus, 188, 315–323, doi:10.1016/j.icarus.2006.11.020. patterns observed on terrestrial submarine fans, although the Dromart, G., C. Quantin, and O. Broucke (2007), Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars, Geology, channel pattern on deltas is similar. A rover mission could 35(4), 363–366, doi:10.1130/G23350A.1. acquire the detailed facies information necessary to conclu- Ehlmann, B. L., J. F. Mustard, C. I. Fassett, S. C. Schon, J. W. Head III, sively discriminate between submarine fans and deltas. D. J. Des Marais, J. A. Grant, and S. L. Murchie (2008), Clay minerals in Estimates of minimum fan formation timescales suggest delta deposits and organic preservation potential on Mars, Nat. Geosci., 2 4 1, 355–358, doi:10.1038/ngeo207. the fans formed in 10 to 10 years, and thus a stable body Flemings, P. B., and J. P. Grotzinger (1996), STRATA: Freeware for of water must have been present for at least this long. The analyzing classic stratigraphic problems, GSA Today, 6(12), 1–7. preservation of valley networks in the topographic highs Flood, R. D., and J. E. Damuth (1987), Quantitative characteristics of sinuous distributary channels on the Amazon deep-sea fan, Geol. surrounding the basin, an alluvial fan where these valley Soc. Am. Bull., 98, 728–739, doi:10.1130/0016-7606(1987)98 networks drain into the basin, clinoforms, and depositional <728:QCOSDC>2.0.CO;2. fans in the lowest part of the basin, suggest that the entire Fueten, F., R. Stesky, P. MacKinnon, E. Hauber, K. Gwinner, F. Scholten, T. Zegers, and G. (2006), A structural study of an interior depositional system is preserved from the source area to the layered deposit in southwestern , Valles Marineris, Mars, sink. This suggests that Mars was capable of supporting using high resolution stereo camera data from Mars Express, Geophys. surface liquid bodies of water for a significant period of Res. Lett., 33, L07202, doi:10.1029/2005GL025035. time during the initial stages of Valles Marineris formation. Goodbred, S. L., and S. A. Kuehl (1998), Floodplain processes in the Bengal Basin and the storage of Ganges-Brahmaputra River sediment: An accretion study using 137Cs and 210Pb geochronology, Sediment. [53] Acknowledgments. We would like to thank Kevin Lewis and Geol., 121(3–4), 239–258, doi:10.1016/S0037-0738(98)00082-7. Terry Suer for making the digital elevation models of the Melas fans used in Grant, J. A., R. P. Irwin, J. P. Grotzinger, R. E. Milliken, L. L. Tornabene, this study. 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