Fluvial Stratigraphy of Valley Fills at Aeolis Dorsa, Mars: Evidence for Base-Level Fluctuations Controlled by a Downstream Water Body

Home , Green Valley (Mars), Lakes on Mars, Water on Mars

Cardenas et al. Fluvial stratigraphy of valley fills at Aeolis Dorsa, Mars: Evidence for base-level fluctuations controlled by a downstream water body

Benjamin T. Cardenas†, David Mohrig, and Timothy A. Goudge Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA

ABSTRACT level cycles are consistent with the presence able transport of solids by liquid water across of an ancient large lake, sea, or ocean and ancient surfaces of the planet. In addition, there Aeolis Dorsa, a large sedimentary basin its fluctuating water-surface elevation. Addi- is evidence that many impact craters contained on Mars, contains an array of fluvially domi- tionally, channel bend asymmetry preserved lakes (Goldspiel and Squyres, 1991; Cabrol and nated sedimentary deposits. These deposits in channel-belt deposits indicates a south- Grin, 1999, 2001; Irwin et al., 2005; Fassett and preserve a record of fluvial erosion and de- eastern flow direction. Head, 2008b; Goudge et al., 2012), including position during early Martian history. We >200 craters that have both inflow and outflow present evidence that some of these fluvial INTRODUCTION valleys (Cabrol and Grin, 1999, 2001; Fassett deposits represent incised valleys carved and Head, 2008b; Goudge et al., 2012). The era and filled during falls and rises in base level, Sedimentary rocks and their stratigraphy of widespread surface runoff is thought to have which were likely controlled by changes in have a long history of being interpreted to place ceased by ca. 3.6–3.8 Ga, during the transition water-surface elevation of a large lake or sea. quantitative constraints on paleoenvironmental from the Noachian to the Hesperian time period The valley stratigraphy consists of three low- conditions on Earth (e.g., Rigby and Hamblin, (Howard et al., 2005; Irwin et al., 2005; Fassett albedo, channelized corridors, each several 1972), but they have only recently been used and Head, 2008a; Hoke et al., 2011; Hynek et tens of kilometers long in the streamwise di- to constrain paleoenvironmental conditions al., 2010; Mangold et al., 2012). rection. Deposits composing the basal valley on other planetary bodies. In particular, high- Some of the open-basin lakes identified by fills are characterized by laterally amalgam- resolution satellite images and topographic Fassett and Head (2008b) have volumes exceed- ated point-bar strata confined between valley maps now facilitate the study of exposed sedi- ing 200,000 km3, and it has even been hypothe- walls that preserve scoop-shaped segments mentary strata on Mars (e.g., Grotzinger and sized that the approximately hemispheric north- cut by the erosive outer banks of meander- Milliken, 2012). In spite of these products, ern lowlands basin once contained an ancient ing river bends. Both the point-bar deposits most of the remotely sensed data cannot resolve ocean (e.g., Baker, 2009; Parker et al., 1989; and valley walls were produced by a net- sedimentary packaging at the submeter scale, Head et al., 1999; Perron et al., 2007; Di Achille erosional river system. Subsequent valley- a scale that yields a particularly rich record of and Hynek, 2010; Moscardelli et al., 2012; Di filling deposits are defined by both channels interpretable sedimentary structures on Earth Biase et al., 2013; Moscardelli, 2014). However, and associated overbank strata. The stacked (e.g., Allen, 1982). Instead, satellite images de- not all observations are consistent with the oc- channel-filling deposits are sinuous in form, fine larger-scale stratigraphic relationships such currence of an ancient northern ocean (e.g., Ma- but unlike the basal strata, they preserve no as channelized depositional patterns. lin and Edgett, 1999; Carr and Head, 2003), and evidence of river migration. Within each val- Modern surface conditions on Mars are ex- the northern ocean hypothesis, with its implica- ley, there are multiple sinuous ridges ranging tremely cold and dry (Owen, 1992). Under tions for the planet’s climatic, biologic, and geo- from a few meters to several tens of meters these conditions, water ice is stable, and liquid logic history, remains subject to ongoing debate. thick, which we interpret as channel-belt water is not. Presently, water ice is primarily The work presented here provides new inter- deposits that have been topographically stored within two polar ice caps (Plaut et al., pretations of Martian channelized stratigraphy inverted via differential erosion. Evidence 2007; Phillips et al., 2008), glaciers (Holt et al., using recently developed ideas and methods for channel avulsions and reoccupations, 2008; Levy et al., 2014), and shallow subsurface from riverine geomorphology on Earth. We ex- the overall cutting and filling patterns, and reservoirs (Boynton et al., 2002). In contrast to amine a portion of the sedimentary rock record consistent up-section decreases in recorded the present-day conditions, various erosional in Aeolis Dorsa, an equatorial region of Mars channel migration support the interpretation landforms (e.g., Pieri, 1980; Aharonson et al., located ~200 km north of the crustal dichotomy of the low-albedo corridors as valley stratig- 2002; Howard et al., 2005; Irwin et al., 2005; (Fig. 1). Recognizing that these exposed strata raphy cut and filled in the presence of a mi- Barnhart et al., 2009; Hynek et al., 2010; Hoke represent weathered, time-integrated, poten- grating backwater zone. Crosscutting valleys et al., 2011) and sedimentary deposits (e.g., tially vertically and laterally amalgamated require at least two episodes of base-level fall Goldspiel and Squyres, 1991; Howard et al., channel-filling deposits, care has been taken to and rise at >50 m per episode. These base- 2005; Irwin et al., 2005; Burr et al., 2009, 2010; extract appropriate dimensional and kinematic DiBiase et al., 2013; Grotzinger et al., 2015; Ir- data on channels from their connected deposits. win et al., 2015) exposed in the southern high- We assembled stratigraphic evidence to sup- †[email protected] lands of Mars preserve evidence for consider- port the hypothesis that the studied sedimentary

GSA Bulletin; March/April 2018; v. 130; no. 3/4; p. 484–498; https://doi.org/10.1130/B31567.1; 12 figures; published online 14 September 2017.

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the Aeolis Dorsa region lasted for a minimum of 1–20 m.y. during a period of extended surface runoff on the planet (Kite et al., 2013). We focused on three low-albedo corridors containing stacked channel-belt deposits (Fig. 2), which make up only a small portion of the overall fluvial stratigraphy at Aeolis Dorsa. We propose that these corridors are the exhumed remnants of incised valley fills. To test this hypothesis, and then analyze the results further, five specific questions will be addressed. (1) What was the paleoflow direction associated with the preserved channelized deposits? (2) Do the low-albedo channelized corridors meet the geometric criteria for incised valleys? (3) Is the fluvial stratigraphy contained within the low-albedo corridors similar to valley-filling stratigraphy on Earth? (4) How many falls and rises in base level are recorded in the exposed stratigraphy? (5) What was the possible magnitude of change in the water-surface elevation of the large lake, sea, or ocean driving these base-level adjustments?

Geometry of Incised Valleys Figure 1. (A) Mars Orbiting Laser Altimeter (MOLA) digital elevation model (DEM) showing the location of Aeolis Dorsa. The sharp increase in elevation (black line) marks the boundary Incised valleys are defined as topographic between the southern highlands and northern lowlands, known as the crustal dichotomy of lows produced by riverine erosion (Van Wagoner Mars (Tanaka et al., 2014). The Martian datum is defined by a constant atmospheric pres- et al., 1990; Dalrymple et al., 2006). Valley for- sure. (B) Polygons defining the spatial extent of the studied deposits (Fig. 2), and the loca- mation through river-channel incision without tions of figures 3, 4, 7, 8A, and 12. (C) Polygon circumscribing a region of branching fluvial tectonic uplift is most commonly connected to deposits located southeast of the study area described by Hughes et al. (2016). These deposits a change in climate that leads to either (1) an in- appear similar to other deltaic deposits at Aeolis Dorsa (DiBiase et al., 2013) and are shown crease in the water-to-sediment discharge, which in detail in Figure 10. lowers the bottom slope of a river, or (2) a drop in water-surface elevation of a terminal lake, sea, or ocean, which lowers the base level for the river system. The spatial patterns of valley incision deposits represent channel-belt strata filling Head, 2010), with current surfaces exposed via tied to these two mechanisms are different, where incised valleys that were cut and filled in re- erosion during the Hesperian through the early erosion tied to changes in water and sediment sponse to fluctuations in a nearby body of water Amazonian (Zimbelman and Scheidt, 2012). discharges is focused in an upstream segment of (Figs. 1–3). Some of the best exposures of sedimentary the river system, and where erosion tied to base- deposits on Mars are located in the Medusae level control is focused further downstream in a BACKGROUND Fossae Formation at Aeolis Dorsa. Particularly broad coastal zone (Heller and Paola, 1996; Sun spectacular exposures include the sinuous ridges et al., 2002). The vast majority of preserved sedi- Aeolis Dorsa produced by differential erosion of sedimentary mentary deposits associated with incised valleys units (Figs. 3 and 4; Burr et al., 2009, 2010; Le- are connected to patterns of erosion and deposi- Aeolis Dorsa is an open basin bounded to fort et al., 2012; Williams et al., 2013; Kite et tion driven by changes in water-surface elevation the east and west by two erosionally produced al., 2015). These ridges have been hypothesized of a downstream water body (Boyd et al., 2006; lobes of the Medusae Fossae Formation (Kerber to represent eskers (Nussbaumer et al., 2003), Dalrymple et al., 1994, 2006). and Head, 2010) and to the south by the crustal inverted lava flows (Tanaka and Scott, 1987), The processes driving valley development via dichotomy, which separates the high-elevation and inverted river-channel-filling deposits (Burr river incision following a drop in base level have southern highlands from the low-elevation et al., 2009, 2010; Lefort et al., 2012; Williams been studied both experimentally (Cantelli et northern lowlands of the planet (Fig. 1). Similar- et al., 2013); however, their arrangement into al., 2004; Martin et al., 2011) and theoretically ities in erosional morphologies, such as yardang sinuous ridge networks and the occurrence of (Cantelli et al., 2007; Martin et al., 2011). Lateral aspect ratios and jointing, between the Medu- internal stratigraphy that appears to record the migration of a net-erosional sinuous channel pro- sae Fossae Formation and terrestrial ignimbrites lateral migration of channels support an inter- duces composite, diachronous valley walls (Mar- favor a pyroclastic flow origin for the Medusae pretation that a vast majority of these are river- tin et al., 2011; Blum et al., 2013). These valley Fossae Formation (Mandt et al., 2008). Deposi- channel deposits (Burr et al., 2009). Crater-river walls are characterized by a scalloped structure tion occurred during the Hesperian (Kerber and interactions suggest that fluvial sedimentation at (Figs. 2–4) recording a time-integrated history

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asymmetry in the shape of meandering river bends. They attributed the development of this asymmetry to relatively large values for outer- bank erosion being located at sites positioned downstream from the apexes of channel bends, a pattern connected to the secondary flow in meandering rivers (Dietrich and Smith, 1984; Sun et al., 1996; Parker et al., 2011; Schwenk et al., 2015). In order to measure bend asymmetry, Carson and Lapointe (1983) first defined a river-bend traverse as the river path between two points of maximum curvature (Fig. 6). Each traverse contains one inflection point, which they observed is most often located closer to the downstream point of maximum curvature and which is called a delayed inflection point. The greater the delay, the higher is the degree of downstream bend asymmetry for that particular traverse. Inflections may also be premature, i.e., closer to the upstream point of maximum curvature, but premature inflections were shown to be much less abundant than delayed inflections (Carson and Lapointe, 1983; Fig. 6). If the inflection-point delay is minor or nonexistent, the traverse is called symmetric. The asymmetry index, z, is a quantified measurement of this asymmetry, and it was defined by Carson and Lapointe (1983) as

z = 100 * u/(u + d), (1)

where u and d refer to the upstream concave and downstream concave lengths of a traverse, respectively (Fig. 6). By definition, z is greater than 55 for traverses with delayed inflection points, z equals 45–55 for symmetric traverses, Figure 2. (A) Context Camera (CTX) mosaic of the studied low-albedo corridors (marked and z is less than 45 for premature inflection by arrows). The three branching corridors are several tens of kilometers long. Each corridor points. The majority of river bends measured by contains several channel-belt deposits preserved as inverted sinuous ridges. (B) Interpretation Carson and Lapointe (1983) had bend asymme- of the image above with the three distinct corridors outlined in dotted lines colored red, green, try values greater than 55. Carson and Lapointe and purple. The inverted sinuous ridges present in each corridor are also displayed, but at (1983) also noted that goosenecks, which are exaggerated widths to increase visibility. Four discrete ridges are mapped in the red corridor, meander bends with extremely curved central four ridges in the green corridor, and three ridges in the purple corridor. Each ridge repre- axes and which are essentially overdeveloped sents a minimum of one generation of channel belt, since channel-belt stacking is observed examples of downstream meander asymmetry, here and is best observed only where there is High Resolution Imaging Science Experiment have central axes that tend to point in the overall (HiRISE) image and digital elevation model (DEM) coverage (Fig. 4). Paleoflow direction is upstream direction (Fig. 7). interpreted to be from left to right on the image (toward the southeast). From CTX images B11_014080_1740 and B20_017548_1736. Systematic Variation in Rates of Lateral Migration for River-Channel Bends with Distance from Coastline of wall erosion and valley widening by outer and possibly estuarine strata tied to valley filling banks of many meandering river bends (Mar- promoted by a rise in base level (Van Wagoner et It has long been recognized that rates of lat- tin et al., 2011; Blum et al., 2013; Limaye and al., 1990; Allen and Posamentier, 1993; Simms eral migration for channel bends decrease to- Lamb, 2016). The same channel meandering that et al., 2006; Fig. 5). ward the coastline (Russell et al., 1936; Kolb, sculpts the valley walls also produces a laterally 1963; Hudson and Kesel, 2000). The reasons for amalgamated set of point-bar deposits that com- Connecting Channel-Bend Asymmetry to this decrease constitute an active and ongoing monly make up the basal valley-filling deposits, Flow Direction research avenue, but there is now consensus that positioned directly on top of the valley-bottom this change in meandering is tied to a change in erosional surface (Blum and Aslan, 2006). These Carson and Lapointe (1983) inspected several sediment transport associated with the zone of basal deposits are later buried beneath riverine active rivers on Earth and identified an inherent backwater flow (Nittrouer et al., 2012; Smith,

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2012; Blum et al., 2013), the river segment characterized by channel-bottom elevations less than the water-surface elevation for the downstream body of standing water. Rates of bend migration are systematically high upstream of this shoreline-influence zone, and they are significantly smaller, by at least an order of magnitude, in the backwater zone nearest the coastline (Nittrouer et al., 2012; Smith, 2012). This large difference in rate of lateral migration translates into even greater differences in the widths of channel-belt deposits. Moving downstream, a channel belt can transition from being many times the average river width to only frac- tionally greater than the average river width (e.g., Blum et al., 2013). Everything else being equal, the relative width of channel-belt deposits can serve as a proxy for relative distance to the associated shoreline (e.g., Petter, 2010; Armstrong, 2012). For example, during a highstand in sea Figure 3. Clusters of sinuous ridges, channel bends, and scoop- level, when the shoreline is shifted toward land, shaped corridor boundaries associated with cutting by the erosive a river segment might lie within the backwater outer banks of these bends. The solid black arrows point to the strati- zone and construct channel-filling sediments graphically highest sinuous ridge in the green corridor (Fig. 2, yellow only marginally wider than the channel itself. channel belt), which rarely reoccupied the locations of lower channel However, during a lowstand in sea level, when belts and has a relatively low sinuosity. The sinuous ridge has a rela- the shoreline is shifted basinward, this same river tively constant width across bends, which is indicative of minimal segment might lie outside of the backwater zone lateral migration at those bends. The solid white arrow points to a and via migration produce channel-filling sedi- relatively smooth area of the dark corridor, compared to the rough ments many times wider than the channel itself. surrounding terrain, which can be used to distinguish the dark corri- By tracking these differences, there is an op- dors from the surrounding terrain. The dashed white arrow points to portunity to use channel-belt stratigraphy to es- an example of the curved amalgamated strata. From High Resolution timate relative changes in basinal water-surface Imaging Science Experiment (HiRISE) image PSP_010322_1740. elevation and shoreline position.

Figure 4. An example of fluvial stratigraphy observed within one of the dark corridors. Corridor boundaries are mapped as the solid red lines. Flow is interpreted to be toward the right (southeast). Lateral accretion ridges (point-bar remnants) are preserved at the lowest elevations (black arrows). Above the basal deposits are at least three generations of channel-belt deposits preserved as inverted sinuous ridges. These deposits are mapped as dotted lines. Colors are consistent from Figure 2. The black channel is younger than the orange channel and stacked on top of it. At points 1 and 3, the orange channel failed to topographically attract the black channel. Note that the black channel runs adjacent to the corridor boundary for several hundred meters, as if the active channel had been redirected along a topographic boundary. The black channel takes a less sinuous path down the corridor, and at points 2 and 4 it reoccupies the orange channel. An increase in inverted channel elevation near point 4 indicates the appearance of the brown channel, which is stacked on top of the black channel. There are ~40 m of relief between the basal and upper deposits. The white arrow points to an inverted sinuous ridge that is positioned directly against the corridor boundary for a distance greater than 250 m. This digital elevation model (DEM) is derived from High Resolution Imaging Science Experiment (HiRISE) images PSP_010322_1740 and ESP_019882_1740.

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interpolation of MOLA point-shot data, and it has a spatial resolution of ~460 m per pixel. For active channel point bars higher-resolution topographic analyses, HiRISE and CTX stereo-pair images were used to create digital elevation models (DEMs) with a spatial resolution of ~1 m per pixel and 18 m per pixel, A respectively, with submeter vertical resolution (Kim and Muller, 2009). DEMs were produced using open-source Ames Stereo Pipeline (ASP) stereogrammetry software from the National Aeronautics and Space Administration (NASA; Broxton and Edwards, 2008; Moratto et al., floodplain 2010; Beyer et al., 2014). As a part of our data processing procedure, active channel CTX DEMs were tied to MOLA point-shot data using the ASP pc_align function. This algo- rithm minimizes the error between the two point clouds via rotation and translation of the CTX B DEM in three-dimensional space (Beyer et al., 2014), and thus it minimizes errors in regional- scale topography (e.g., long-wavelength slopes) buried valley fill in the output DEMs. The smaller footprint of HiRISE images results in fewer covering MOLA point shots, and so the output HiRISE DEMs were instead tied to overlapping, cor- valley extent relief rected CTX DEMs. DEMs produced with the erosion of ASP have been shown to agree well with DEMs basal point surrounding bar created with other common software packages terrain (Watters et al., 2015), and so they provide reli- C channel stacking able, high-resolution topographic data. In addition to visible images and topography, nighttime thermal infrared data from the Ther- mal Emission Imaging System (THEMIS) were inverted channel-belt used for mapping (Christensen et al., 2004). All of the processed images, topography, and thermal infrared data were imported into a GIS to Figure 5. Interpreted history of erosion and deposition patterns leading to facilitate mapping and measurements on the formation of a dark corridor and inverted channel belts. (A) Valley inci- coregistered data. sion and lateral valley growth via an actively migrating, net-eroding river during base-level fall. (B) Valley filling by aggrading, minimally migrat- RESULTS ing rivers. (C) Erosion of the surrounding Medusae Fossae Formation and floodplain deposits, with preferential preservation of interpreted coarse- The low-albedo corridors are composed of grained, well-cemented channel-belt deposits. The maximum relief is an distinct sedimentary deposits that extend for approximation of the thickness of the valley fill and a minimum estimate several tens of kilometers along their primary of valley incision depth. axes (Fig. 2). In addition to their distinct surface roughness and form, the corridors are relatively bright in nighttime thermal infrared images ac- quired by THEMIS, suggesting a larger grain METHODS study area is complete, and we created a contin- size or a greater degree of induration (Chris- uous CTX mosaic of the region using the U.S. tensen et al., 2004), most likely cementation We analyzed surface exposures of sedimen- Geological Survey (USGS) Integrated Software (Pain et al., 2007; Burr et al., 2010). There are tary deposits and their stratigraphy at Aeolis for Imagers and Spectrometers (ISIS). This mo- three distinct deposit types exposed within each Dorsa using visible images from the Context saic was used as a base map and imported into corridor, which we mapped using a combina- Camera (CTX) and High Resolution Imaging a geographic information system (GIS). Twelve tion of HiRISE and CTX images and DEMs. Science Experiment (HiRISE) instruments on- HiRISE images of the study area were available. The first type is sinuous ridges (Figs. 3, 4, and board the Mars Reconnaissance Orbiter. CTX The Mars Orbiting Laser Altimeter (MOLA) 5C), 40–100 m wide, that can in some instances images have a spatial resolution of ~6 m per provides a regional look at topography with a be traced continuously over tens of kilometers pixel (Malin et al., 2007). HiRISE images have vertical accuracy of ~1 m (Smith et al., 2001). (Fig. 2). The sedimentary deposits making up a spatial resolution as high as ~25 cm per pixel The publicly available, gridded MOLA base individual sinuous ridges range from a few me- (McEwen et al., 2007). CTX coverage of the map used for this study was produced from ters to over 10 m thick. The second deposit type

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consists of laterally amalgamated strata defining local valley direction the meandering of the lowermost paleochan- NSnels. The local width of these deposits delin- eates the local width for the associated corridor (Figs. 3 and 5C). The final deposit type makes A 2 up all strata within corridors that is not a sinuous ridge or a direct product of sinuous ridge d1,2 migration. Based on patterns of surface roughness, these background deposits are easily dis- e 2,3 tinguishable from those located adjacent to but in u rl outside of corridors. Wind erosion has produced nte u1,2 ce a very rough surface that includes yardangs on nel 1 chan the terrain surrounding the corridors, while the rock within the corridors is smooth by compari- son (Figs. 3, 4, and 5C). d2,3 The individual sinuous ridges were interpreted by Burr et al. (2009) as topographically asymmetry value (z) = 100 * u/(u+d) 3 inverted, erosional remnants of channel-filling deposits. Although sinuous, these deposits preserve no evidence for significant lateral migration of channel bends, which would be indicated by more extreme variations in preserved deposit width such as that observed in the laterally Aeolis Dorsa, paleoflow direction assumed BC35 Carson and Lapointe (1983) 35 to be towards the southeast amalgamated strata (Fig. 3). Even if strata con- n = 277 30 30 n = 59 nected with channel migration were relatively minor, these deposits would still represent the 25 25 time-integrated product of an active channel

y y and as such represent channel-belt deposits 20 20 rather than the fill of a single channel geometry.

equenc equenc Ridges also integrate time vertically, as a single

fr 15 fr 15

% % ridge can contain a record of multiple stacked 10 10 episodes of flow (Fig. 4). This is best observed with HiRISE DEMs, which do not cover the 5 5 entirety of the study area, so it should be em- 0 0 phasized that our mapped ridges (Fig. 2) each represent a minimum of one channel-belt deposit. Even at HiRISE resolution, stratigraphic asymmetry value (z) = 100 · u/(u+d) asymmetry value (z) = 100 · u/(u+d) contacts between stacked channel-belts are most confidently identified at points where the path taken by the stacked deposits differs. Contacts Figure 6. (A) Traverse asymmetry analysis. Points 1, 2, and 3 mark local maximums in the were not identified within ridge walls. This fur- curvature on the drawn channel centerline. These points of maximum curvature are used to ther emphasizes our mapping as a minimum divide the channel into a set of successive segments (e.g., traverse , traverse ). The degree 1–2 2–3 estimate of channel-belt stacking. Significantly, of asymmetry associated with these two channel segments is estimated using the method of the vertical and lateral integration of time into Carson and Lapointe (1983; Eq. 1 in this paper), where two lengths (u, d) are measured from these features indicates they are inverted depos- the inflection point to the points of perpendicular intersections with lines that are parallel to its, and not an inverted landscape (DiBiase et the local valley direction and tangential to the adjacent points of maximum curvature. The al., 2013). lengths d and u refer to the portion of a traverse that is concave toward the downstream or Basal sinuous ridges are associated with sets to the upstream direction, respectively. Notice that inflection point of traverse does not 1–2 of curved strata that appear to be linked to the bisect the traverse, while the inflection point of traverse does. As a result, u is longer than 2–3 1,2 lateral migration of channel bends (Fig. 4). d , while u and d , are equal. (B) Histogram of bend asymmetry values from the Beat- 1,2 2,3 2,3 These curved strata, exposed on subhorizontal ton, Pembina, Big Sioux, Iowa, Neuse, Savannah, Sabine, Au Sable, Lumber, Rum, Rough, erosional surfaces, are interpreted as the lateral Blacks Fork, and Animas Rivers, which all show negative skewness (Carson and Lapointe, accretion deposits of point bars growing on the 1983). The gray bar represents symmetrical bends (45 < z < 55). A chi-squared test rejects inner banks of river bends (e.g., Edwards et al., the distribution as normally distributed at 5% confidence with p-value of 3 × 10−17. (C) Histo- 1983). Repeated episodes of point-bar accre- gram of bend asymmetry values calculated with an assumed southeastern flow direction for tion and outer-bank erosion are inferred to have channel-belt deposits at Aeolis Dorsa, which is negatively skewed. The gray bar represents driven channel-bend migration and deformation symmetrical bends (45 < z < 55). A chi-squared test rejects the hypothesis that the distribu- similar to that occurring on Earth (Ikeda et al., tion is normal at a 5% confidence level with a p-value of 3 × 10−5. 1981; Parker et al., 2011). The borders of each corridor are characterized by a series of convex,

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accretion deposits, we mapped the centerline of the lowermost continuous sinuous ridge (Fig. 9), assuming that this sinuosity was inherited via reoccupation of the final underfilled basal channel, a property of channel stacking commonly observed on Earth (e.g., Mohrig et al., 2000; Jones and Hajek, 2007; Chamberlin and Hajek, 2015). Over the entire study area, 59 traverses for reconstructed channel bends were measured. The results of our meander asymmetry analysis (Fig. 6) show that bend asymmetry values calculated with an assumed southeastern flow are negatively skewed (Fig. 6C), which is consistent with observations of modern meandering rivers (Fig. 6B; Carson and Lapointe, 1983). Asymmetry values calculated with an assumed northwestern flow are positively skewed, which is inconsistent with modern meandering rivers (Carson and Lapointe, 1983). Additionally, the central axes of preserved gooseneck morphologies curve toward the northwest (Fig. 7), further suggesting that paleoflow of the studied deposits was toward the southeast. An interesting side note from our bend asymmetry analysis is that a large percentage (>30%) of the measured traverses are symmetric Figure 7. (A) Goosenecks are defined as meander bends with very curved central axes, and (Fig. 6), indicating that a significant component they are essentially examples of extreme meander asymmetry. Goosenecks (black arrows) of of the preserved migration record was lateral, the Juruá River, Brazil, feature axes (white dotted lines) that point upstream, as predicted rather than downstream. Sun et al. (1996) dem- by Carson and Lapointe (1983). (B–C) Goosenecks preserved in deposits of the green and onstrated that a flatter floodplain will encour- purple corridors at Aeolis Dorsa point to the north-northwest. Dotted white lines mark age lateral meander migration over downstream the axes of the goosenecks, which curve toward the northwest. Arrows indicate the known migration. It is possible that the lower gravita- (A) and inferred (B, C) flow directions. Panels B and C both show Context Camera (CTX) tional acceleration on Mars might have had a image B11_014080_1740. similar effect on meander migration as does a flatter floodplain. An abundance of primarily symmetric channel bends should therefore not scooped-shaped features that mirror the form of INTERPRETATION be surprising for Mars. Systematic application interpreted channel bends (Fig. 3, dashed lines). of the Carson and Lapointe (1983) method is Corridor stratigraphy consists of a laterally Paleoflow Direction thus necessary to detect the subtle signature for amalgamated basal deposit overlain by one to the transport direction preserved in deposits of three vertically stacked sinuous ridges (Figs. 3 It has generally been thought that paleoflow Martian paleochannels (Fig. 6). and 4). At many locations, ridges are clearly direction for the channelized deposits exposed In addition to our meander asymmetry analy- stacked one directly on top of the other (Fig. 4), at Aeolis Dorsa was toward the north (Williams sis, two lines of valley-scale evidence point to- and no case was observed where these sinuous et al., 2013; Matsubara et al., 2015), based on ward a transport direction and a coastline to the ridges crossed over corridor borders (Figs. 2–4). the regional dip of modern topography (e.g., southeast. First, the spatial arrangement of the Total thicknesses for corridor deposits were Fig. 1). The uncertainty in the number of red, green, and purple valleys defines a classic estimated using HiRISE and CTX DEMs to stacked channel belts within a ridge, the natu- distributary pattern if the coastline is located construct topographic profiles oriented perpen- ral variation in channel-bed scouring, and the to the southeast. Such a distributary pattern is dicular to the long axes of the channelized corri- low gradients of net-depositional rivers make commonly observed in coastal-zone valley sys- dor at half-kilometer intervals. The topographic the interpretation of paleoflow direction from tems on Earth, and its associated deposits are relief for each profile is shown in Figure 8, and changes in basal ridge topography extremely commonly preserved in the stratigraphic record it represents a minimum estimate of thickness, uncertain. Here, we applied an independent (Greene et al., 2007). Second, ~30 km to the since erosion may have removed some upper- technique for determining the paleoflow direc- southeast of the study area, there is a large region most portion of the deposit and not yet exposed tion of ancient fluvial systems using the pre- of stacked, branching fluvial deposits (Figs. 1 the very base of the deposit. At the largest scale, served channel forms themselves. We focused and 10), which appear to be qualitatively simi- ridges within the red corridor appear to be trun- on estimating the channel-bend asymmetry as- lar to the delta deposit identified by DiBiase cated by strata filling the green corridor, while sociated with laterally migrating basal channels et al. (2013). The ~30 km zone separating our no truncation is observed at the contact between found in each corridor. Rather than attempting study area and this branching deposit is covered the green and purple corridors (Fig. 4). to reconstruct channel pattern from the lateral by postfluvial deposits, so definitive correlation

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Figure 8. (A) Image shows the location of an example profile A-A′ used to take a measurement of deposit thickness in the red valley at Aeolis Dorsa (location shown in Fig. 1B) from a Context Camera (CTX)–derived digital elevation model (DEM). A portion of the valley wall appears to have been preserved at this location. Taken from CTX image B20_017548_1739. (B) Distribution of local deposit thickness measurements within the red, purple, and green valleys at Aeolis Dorsa. (C) Difference map of the modern bathymetry of the Gulf of Mexico minus a DEM of the buried surface associated with the stage II sequence boundary and the last glacial eustatic lowstand in sea level throughout the northwestern Gulf of Mexico (Simms et al., 2007). Valleys incised during the associated 60 m drop in eustatic sea level are clearly seen on the difference map as topographic lows that underwent focused deposition. (D) Distribution of local relief values for valley topography assembled from the Gulf of Mexico DEM. Notice that all of these values are less than the sea-level fall that drove valley formation.

between the two sites using exhumed stratig- incised valleys discussed here, which supports in the most upstream portion of a drainage basin raphy is not possible. However, the location our interpretation of paleoflow direction. that flowed toward the northwest, which is the and arrangement of these deposits, as well as Our interpreted distributary pattern differs downslope direction of the modern topography an analysis of stratigraphic contacts contained from those of previous workers, who have in- (Lefort et al., 2012; Williams et al., 2013; Mat- within them (Hughes et al., 2016), suggest that stead interpreted the branching deposits in Aeo- subara et al., 2015). Additionally, present-day they may be the remnants of a deltaic coastline lis Dorsa as remnants of a tributary system. In erosional valleys in the southern highlands in- deposit positioned at the downstream end of the this scenario, the studied deposits were created dicate global-scale paleotransport toward the

Figure 9. Within each valley, there is a decrease in channel-belt sinuosity up section. The white lines represent the paths of maximum sinuosity in each valley. The bold lines represent the path of least sinuosity, which follows the youngest channel belts. Sinuosity is defined as the path length divided by the length of the valley axis. The purple valley contains only a single sinuous ridge for most of its path. Mosaic of Context Camera (CTX) images B11_014080_1740. B20_017548_1739, and P08_004270_1746.

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north (Hynek et al., 2010). As such, the null hypothesis has generally been that paleotransport at Aeolis Dorsa was toward the north, but several of our observations suggest it was instead toward the south-southeast. From our observations, we note that (1) channels in upstream portions of a drainage basin are net erosional and should not be expected to be filled and preserved with several generations of stacked fluvial deposits, and (2) the modern surface has obviously been shaped by erosional processes, as indicated by the exhumed channel- filling deposits, so it should not be assumed that the modern erosional surface should follow the regional surface slope of ancient fluvial systems. DiBiase et al. (2013) came to similar conclusions from observations of fluvial deposits elsewhere at Aeolis Dorsa. Local increases or decreases in elevation along the surfaces of sinuous ridges hypothesized to be the result of tec- tonism deforming a geomorphic surface (Lefort et al., 2012; Williams et al., 2013) can be linked to differential cutting through stacked channel- belt deposits (Figs. 4 and 10; Hughes et al., 2016; DiBiase et al., 2013). This was hypothesized by Lefort et al. (2012), but it is best observed with HiRISE DEMs (e.g., Fig. 4), which were not available for that study. Stratigraphic contacts, rather than surface topography, would preserve a clear record of deformation that could not be modified by erosion. Studies that have measured the strike and dip of stratigraphic contacts at Aeolis Dorsa have found no evidence for postdepositional deformation (DiBiase et al., 2013; Hughes et al., 2016). Furthermore, and importantly, we observed a thickening in the branching deposits to the southeast (Fig. 10). This overall thickening is not consistent with a tributary network, but it is consistent with fluvial deposits approaching a coastline (e.g., Sydow and Roberts, 1994; Heller et al., 2001). Paleoflow toward the north would instead require an unknown mechanism to consistently thicken these deposits away from the coastline. Therefore, although the modern, regional-scale slope for Aeolis Dorsa is toward the north, we conclude that there is clear stratigraphic evidence for an east-southeast paleoflow direction for the studied fluvial deposits. We interpret the modern northward slope to be the result of postdepositional deflation of the Figure 10. Context Camera (CTX)–derived digital elevation model (DEM) of part of the Medusae Fossae Formation and the exposure stacked, branching fluvial deposits in the interpreted downstream direction from the valleys of older stratigraphic layers toward the north, (Fig. 1; Hughes et al., 2016). The deposits in this image are tens of meters thicker than the rather than representing a preserved geomorphic upstream valley-filling deposits (Fig. 4). The modern topographic dip toward the north shown surface that existed during the period of fluvial here is the result of erosion exposing progressively older, lower generations of fluvial strata to- sedimentation. It must be noted that there is no ward the north and does not necessarily reflect the paleotopography of the region (Hughes et requirement for paleoflow direction across Aeo- al., 2016). This DEM is derived from CTX images B17_016203_1744 and B22_018260_1744. lis Dorsa to have been constant through time, and the interpretations are only applied to the studied deposits.

492 Geological Society of America Bulletin, v. 130, no. 3/4

Low-Albedo Corridors as Incised Valleys

Boyd et al. (2006) proposed a set of criteria for identifying an incised valley system in the terrestrial rock record that highlighted four particular elements: (1) The valley and its filling deposits must be regional in extent; (2) the valley must truncate existing stratigraphy; (3) the valley must be an erosional topographic feature; and (4) valley-fill deposits must onlap the valley walls. If the deposits making up the low-albedo corridors at Aeolis Dorsa are indeed the erosional remnants of incised valley systems, corridor stratigraphy should meet these necessary conditions within the limits of resolution set by the satellite-procured data sets. The first condition is clearly satisfied by the stratigraphy of the corridors. The second condition of erosion by valley walls cannot be directly observed from orbit, though the scooped-shaped patterning on the corridor borders provides a com- pelling line of indirect evidence. These boundary scallops (Figs. 3 and 4) clearly preserve the planform of the erosional outer banks of laterally migrating channel bends. On Earth, these characteristic scoop-shaped valley walls are observed on modern valleys (Fig. 11), as well as ancient valley successions preserved in the subsurface (Armstrong, 2012). An additional line of evidence for a significant erosional phase is the total thickness of corridor deposits, which are 2–4 times the thickness of component channel-belt deposits, indicating that the topographic low containing these deposits was at least as deep as 2–4 individual channel belts. Necessary valley conditions 3 and 4 will be assessed in the following paragraph. Direct evidence of erosional valley topography at Aeolis Dorsa, such as preserved valley walls, has been removed via subsequent erosion of the Martian surface. Without this topography, the vertical incision and lateral erosion that defined the formation of the river-confining valleys must be inferred from the preserved Figure 11. (A) A Shuttle Radar Topography Mission digital elevation model (DEM) of the Bra- fluvial stratigraphy (Fig. 5). Having only or- zos River valley, ~100 km upstream from the Gulf of Mexico coastline, Texas, USA. Many of bital images, the most convincing evidence for the topographic features inferred from deposits preserved in the Martian dark corridors are the past occurrence of valley topography is the seen here. Even though the valley contains roughly a 30-m-thick sedimentary deposit that has observation that all of the channel-belt deposits accumulated since the last glacial lowstand in eustatic sea level (Taha and Anderson, 2008), are always confined within the boundaries of it remains underfilled, with active channels confined within its walls. Notice that both active the channelized corridors (Fig. 2). Furthermore, channels abut the valley walls at multiple locations. From this map, it is clear how the shape there are several locations at Aeolis Dorsa where of channel bends can become etched into valley walls. The topographic profile from C to C′ is channel-belt deposits run parallel to the corridor shown in panel B. (B) Elevation profile from C to C′ across the modern valley. Notice that most boundaries for significant distances (Fig. 4), of the surface relief is associated with the channels themselves. suggesting that a paleochannel came into contact with and was redirected along a valley wall, as is often observed in terrestrial incised val- albedo corridors is therefore interpreted to DISCUSSION leys (e.g., Sylvia and Galloway, 2006; Fig. 11). satisfy all of the criteria of Boyd et al. (2006) These observations hold true for a distance of for incised-valley systems. Additionally, the Origin of the Aeolis Dorsa Deposits as an at least 40 km of stacked fluvial stratigraphy presence of deltaic deposits in the downstream Incised Valley-Fill System spread over multiple channel-belt deposits, a direction is consistent with this interpretation, pattern consistent with onlapping, valley-filling as incised valleys and deltas can be associated The fluvial stratigraphy observed within deposits. The stratigraphy making up the low- coastal features (Hughes et al., 2016). the corridors is consistent with stratigraphic

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successions found in incised valleys of coastal by sea-level change. Since the two changes are the water level in a large standing body of wa- rivers on Earth. Meander belts (e.g., Fig. 4) are typically correlated, the stratigraphy of incised ter, such as a large lake or ocean, offshore of the common basal deposits in incised valleys (Syl- valley-fill systems can provide a record of the coastal river system at Aeolis Dorsa. via and Galloway, 2006; Blum and Aslan, 2006; amount of sea-level change via the total amount Miall, 2006), and the link between valley inci- of valley incision. To assess the amount of sea- How Many Times Did Base Level Change? sion and channel migration of a net-erosional level change that may be represented by valley river has been demonstrated experimentally incision, we analyzed a DEM of incised valleys Three incised valley fills have been identi- (Koss et al., 1994) and through numerical mod- in the northwest Gulf of Mexico cut during the fied at Aeolis Dorsa (red, green, and purple; els (Martin et al., 2011). Valley cutting, both ver- eustatic sea-level fall between oxygen isotope Fig. 12). Examination of exposed strata at the tical and lateral, during base-level fall and low- stage 3 and oxygen isotope stage 2 (the last sea- junctions between these individual valleys was stand increases the available coarse sediment level lowstand at ca. 18 ka; Simms et al., 2007; used to evaluate their relative timing and his- within the river system, adding to the sediment Fig. 8). These valleys were cut in response to a tories. Crosscutting relations and particularly necessary to maintain the point-bar deposition 60 m drop in eustatic sea level (Simms et al., truncation of otherwise continuous channel-belt that helps to drive river-bend migration (Di- 2007). Profiles across every Gulf of Mexico val- deposits indicate that the entire red valley fill is etrich and Smith, 1984; Miall, 1985). As base ley were taken at 10 km intervals, and incision older than the green valley (Fig. 12), as the latter level begins to rise, valley-bottom deposition depths are shown in Figure 8. Valleys rarely truncates the former. Red valley strata are now and landward shifting of the backwater zone are incise as deeply as the total sea-level fall, and only found downstream of the junction with the promoted. Sites along the river that transition they never exceed it. The most frequently mea- green valley. We interpret this junction point as a from being located upstream of the backwater sured incision depths are approximately half of location where the river avulsed during the early zone to within the backwater zone experience the total sea-level fall (26–30 m). Valley incision stages of incision of the green valley. The chan- a change in the characteristic style of sand and depths that are less than the driving reductions in nel then shifted eastward and continued cut- gravel transport through these reaches, which downstream water-surface elevation have previ- ting the green valley. No truncated channel-belt in turn affects the cross-section geometry and ously been observed by Anderson et al. (1996), deposits are found at the junction between the bend-migration rate of the river channel (Hud- Paola et al. (2001), Sylvia and Galloway (2006), green and purple valleys (Fig. 12). This absence son and Kesel, 2000; Petter, 2010; Nittrouer et Mattheus et al. (2007), and Greene et al. (2007). of any crosscutting relationships is consistent al., 2012; Armstrong, 2012; Smith, 2012). In Valley incision depths record less than the to- with coeval evolution of the green and purple incised valleys of the northern Gulf of Mexico tal sea-level fall due to coincident progradation valleys. The stratigraphy therefore records two coast, for example, such an upstream shift in of the fluvial system into the downstream basin, unambiguous cycles of base-level fall and rise position of the backwater zone can be tied to a which decreases the amount of incision required that we interpret as being driven by fluctuating change from laterally amalgamated meander- for the river to reach equilibrium at the new, lower lake or sea levels through time. ing river deposits to avulsive, channel-belt and coastline elevation. This emphasizes the fact that floodplain deposits with no substantial record valley incision depths can only serve as minimum Reconstruction of Valley Histories from for lateral migration of bends (Blum and Aslan, estimates for the amount of change in the water- Interpreted Stratigraphy 2006; Sylvia and Galloway, 2006). surface elevation of the terminal body of standing Except for the basal strata, the topographi- water. Although incision depth cannot be directly Based on the observed stratigraphy, we propose cally inverted channel-belt deposits at Aeolis measured at Aeolis Dorsa, the total thickness of the following sequence of valley-forming and Dorsa (Fig. 5) record evidence for considerable exhumed valley-filling deposits (Fig. 8) provides valley-filling events at the Aeolis Dorsa study area, channel sinuosity without significant lateral mi- a measure for the minimum incision depth (Fig. 5) arranged here from oldest (1) to youngest (4): gration (Fig. 4). The stacking patterns of these because (1) there is evidence that the channel-belt (1) incision of the red valley, which extended exhumed channel belts (Figs. 3, 4) suggest that deposits did not fill any valley above its rims, upstream into the position of the current green a great deal of this sinuosity was inherited, with (2) later surface erosion may have removed upper valley, during an episode of base-level lowstand; younger channels reoccupying the pathways of portions of the valley fill, and (3) the basal me- (2) near complete filling of a downstream older channels, most likely because these older andering deposits, which could constitute tens of reach of the red valley during an episode of channels left behind slight topographic lows meters of stratigraphy (e.g., Miall 2002), may not coastline transgression and base-level rise; that preferentially routed flow during avulsion be completely exhumed. (3) incision of the green and purple valleys, (Mohrig et al., 2000; Reitz et al., 2010). Chan- The greatest thickness of exhumed Martian including the reworking of unfilled, upstream nels situated directly above the basal mean- valley deposits is 50 m in the green valley. The portions of the red valley, during an episode dering river(s) appear to have reoccupied and most frequently measured thicknesses within of coastline regression and base-level fall of at therefore inherited a high sinuosity. Younger each of the three valleys are in the 11–20 m least 50 m; and channels located at stratigraphically higher po- range. Considering that (1) the most frequent in- (4) filling of the green and purple valleys dur- sitions would have been much less likely to be cision depths in the northwestern Gulf of Mex- ing an episode of coastline transgression and guided by the older, fully buried channels, de- ico are about half of the sea-level drop (Fig. 8), base-level rise of at least 50 m. veloping a new and typically less sinuous path- (2) the maximum incision depths are less than Given the Hesperian age of the Medusae Fos- way (Figs. 2–4 and 8). the total sea-level drop, and (3) the thickness of sae Formation (Kerber and Head, 2010; Zimbel- valley fill places only a minimum bound on val- man and Scheidt, 2012), the base-level change How Much Did Base Level Change? ley depths, the frequent thicknesses of 11–20 m that drove valley incision and fill occurred dur- and upper limits of 50 m at Aeolis Dorsa could ing the early Hesperian (no older than 3.5 Ga). On Earth’s surface and in laboratory experi- reasonably be attributed to a sea- or lake-level The formation of these valleys requires a ments, a major control on valley incision depth change of >50 m. We interpret these major large standing body of water toward the south- is the overall amount of base-level change driven changes in base level to have been controlled by east of the valleys that controlled at least the

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2015). Our results emphasize the importance of examining the stratigraphy of sedimentary deposits on Mars to fully reconstruct their depositional history. The fluvial stratigraphy at Aeolis Dorsa also displays clear vertical changes that we interpret to be the result of a migrating backwater zone tied to fluctuations in the water-surface elevation of a large lake, sea, or ocean. Our results independently corroborate work by others who have interpreted Aeolis Dorsa as a paleo- coastal region. Parker et al. (1989) mapped the hemisphere-spanning “Arabia” coastline near Aeolis Dorsa, and the water-surface elevation of the northern ocean suggested by Di Achille and Hynek (2010) implies that Aeolis Dorsa was a coastal region to a northern hemisphere-spanning ocean. DiBiase et al. (2013) and Hughes et al. (2016) identified deltaic deposits at Aeolis Dorsa, as well as fluvial deposits that were influenced by a large standing body of water. Most importantly, the multiple episodes of significant coastline transgression and regression captured in the sedimentary rock record here indicate that this body of water was dynamic and fluctuated in volume and surface ele- Figure 12. (A–B) Colored arrows point to inverted channel belts mapped in the same color. vation slowly enough for sedimentation styles to The green corridor truncates deposits from the red corridor, indicating that the red corridor adjust. This is akin to eustatic changes in ocean formed first, and upstream portions were later reworked by flow through the green corri- levels on Earth, which are driven primarily by dor. (C–D) Deposits at the contact between the green and purple corridors have not been orbitally forced climate changes, unlike a brief, truncated. Panel A shows High Resolution Imaging Science Experiment (HiRISE) image catastrophic episode of hydrologic activity. A PSP_010322_1740 and Context Camera (CTX) image B11_014080_1740. Panel C shows large, dynamic body of water on Mars has im- HiRISE image ESP_022084_1740 and CTX image B11_014080_1740. portant implications for the planet’s paleohydro- logic cycle and paleoclimate, which should be considered in future work. regional base level. Although the paleocoastline at this scale, we have interpreted fluvial stratig- would have been mobile, it would have been raphy at Aeolis Dorsa to be the exhumed and ACKNOWLEDGMENTS positioned in the vicinity of the downstream inverted remnants of incised valley-fill systems, This work benefited from the constructive sug- deltaic deposits (Figs. 1and 10). Although the which are common in terrestrial coastal environ- gestions of Editors Aaron J. Cavosie and J. Bruce H. location of this body of water is consistent with ments. The Medusae Fossae Formation, which Shyu, two anonymous reviewers, and R. Wayne Wag- a hemisphere-spanning ocean in the north, it is contains the Aeolis Dorsa deposits, dates to the ner, and from discussions with Cory Hughes, Travis not diagnostic. The number of discrete episodes early Hesperian (Kerber and Head, 2010; Zim- Swanson, Kelsi Ustipak, Michael Lamb, Gary Kocu- rek, and the David Mohrig Research Group. Funding of base-level fall and rise required to form the belman and Scheidt, 2012), indicating that our for this project was provided by the University of valley-filling deposits indicates that this body interpreted episodes of sea- or lake-level change Texas Jackson School of Geosciences and the RioMar of water was dynamic and underwent multiple occurred during the early Hesperian, consistent Industry Consortium. episodes of fall and rise in the elevation of its with other planet-wide observations of an active water surface. hydrologic cycle near the time of the Noachian- REFERENCES CITED Hesperian transition (Howard et al., 2005; Irwin Aharonson, O., Zuber, M.T., Rothman, D.H., Schorghofer, CONCLUSIONS et al., 2005; Fassett and Head, 2008a; Man- N., and Whipple, K.X., 2002, Drainage basins and gold et al., 2012; Hoke et al., 2011; Hynek et channel incision on Mars: Proceedings of the Na- tional Academy of Sciences of the United States of On Earth, stratigraphic sequences at the al., 2010). 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