The Aeolis Dorsa region of Mars preserves many ancient and topographically inverted fluvial deposits, some of which represent analogs to pre-vegetated meandering fluvial deposits on Earth. The regional stratigraphy of the Aeolis Dorsa preserves a tradition from deposits of meandering fluvial channels to alluvial fans. On Earth, fluvial channel and alluvial fan environments have different hydrologic regimes, sedimentary conditions, and depositional slopes, implying that the Aeolis Dorsa region experienced significant changes in hydrology, sedimentology, and topography. Here, we map deposits and derive stratigraphic columns of four local areas—two in southeast Aeolis Dorsa and two in the northwest—to elucidate the local hydrologic processes and sedimentary conditions coincident with regional change. Formative processes and conditions are inferred from shared morphologic attributes between Martian and terrestrial analog deposits. Results suggest mutually consistent local histories; all areas show a transition from fluvial deposits to alluvial fan deposits. However, the hydrologic processes and sedimentary conditions were non-uniform. Specifically, local deposits form two dichotomies: (1) southeast Aeolis Dorsa preserves meandering fluvial deposits, whereas the northwest preserves only wide channel fills; (2) southeast Aeolis Dorsa includes alluvial fans with debris-flow deposits, but northwest fans include only sheetflood or channelized deposits. A relative abundance of cohesive, weathered sediment in southeast Aeolis Dorsa explains both the fluvial meandering channels and debris flows on alluvial fans in those localities. Greater weathering in southeast Aeolis Dorsa is consistent with previous theories of enhanced snowmelt or orographic precipitation in southeast Aeolis Dorsa.
The Aeolis Dorsa region of Mars (Fig. 1) is characterized by numerous sinuous ridges interpreted to be topographically inverted fluvial and alluvial fan deposits (e.g., Pain et al., 2007; Burr et al., 2009). These well-preserved and well-exposed deposits are products of interleaved hydrologic activity and concomitant sedimentation, burial, and exhumation by aeolian abrasion, with subsequent modification by tectonic and/or collapse processes (Fig. 2) (e.g., Burr et al., 2009, 2010; Lefort et al., 2012, 2015; Kite et al., 2015a). This interpretation of the Aeolis Dorsa deposits has relied on analyses of plan-view satellite images and topography of Mars and comparisons to analogs on Earth (e.g., Pain et al., 2007; Williams et al., 2007, 2013; Burr et al., 2009, 2010; Lefort et al., 2012; Matsubara et al., 2014). Analyses of meandering fluvial deposits in the Aeolis Dorsa region (Matsubara et al., 2014) have yielded analogs for newly recognized sandy meander belts in continental settings and pre-vegetated (i.e., pre-Devonian) meandering channels on Earth (Hartley et al., 2015; Ielpi et al., 2016). Thus, analyses of the Aeolis Dorsa deposits augment the record of Martian hydrologic activity and increase the number of planetary analogs for comparisons with ancient fluvial systems on Earth.
Analyses of geomorphology and stratigraphy indicate that the Aeolis Dorsa region transitioned from fluvial deposits to alluvial fan deposits (Burr et al., 2009; Kite et al., 2015a). On Earth, fluvial channels and alluvial fans have distinct depositional slopes, frequencies of formative discharges, and sedimentary conditions (Blair and McPherson, 1994). By analogy, the transition from fluvial deposits to alluvial fans deposits indicates that the Aeolis Dorsa region experienced significant changes in topography, hydrology, and sedimentology (Kite et al., 2015a).
These regional changes recorded in the Aeolis Dorsa stratigraphy motivate us to examine local deposits for the purpose of specifying the formative processes and conditions that accompanied regional change. Increased specificity augments the regional history with finer-scale information, and provides additional evidence with which to constrain paleoclimate and tectonic cause(s) of regional adjustments. Elucidating the formative processes and conditions involves three tasks. First, we delineate the various fluvial and alluvial fan deposits in the Aeolis Dorsa region to show their distinct geospatial distributions and to identify local areas for mapping and stratigraphic analysis. We scope our mapping and stratigraphy to four local areas, two in the southeast and two in the northwest, based on their well-preserved and stacked deposits. Second, we use shared geomorphic attributes with deposits on Earth to interpret the mapped deposits in the Aeolis Dorsa. Third, we derive and correlate stratigraphies among the four local areas to compare records of formative processes and conditions. Fluvial and alluvial fan deposits in the two southeast areas differ from those in the two northwest areas and imply a contrast in formative processes and conditions. We attribute this contrast to have been caused by regional differences in weathered sediment. Regional differences of deposits, formative hydrologic processes, and sediment conditions are consistent with previous theories of enhanced runoff, produced by snowmelt or orographic precipitation, in southeast Aeolis Dorsa (Burr et al., 2009; Kite et al., 2013).
The Aeolis Dorsa: Formation and Geologic Context
The formation model of the Aeolis Dorsa deposits involves four processes: hydrologic activity, associated sedimentation, preservation by burial and cementation, and subaerial exposure (Fig. 2) (cf. Pain et al., 2007; Williams et al., 2007; Burr et al., 2009). This formation model requires a climate favorable for hydrologic activity. The Aeolis Dorsa region is near the Mars equator and in the transitional zone (Tanaka et al., 2014) between the southern highlands and northern lowlands (Fig. 1). The topography in this area has been suggested (Burr et al., 2009) and modeled (Kite et al., 2013) as being conducive to orographic precipitation, snowmelt, and subsequent fluvial runoff sourced from either snowmelt or orographic precipitation, as has been modeled elsewhere on Mars (Scanlon et al., 2013).
The Aeolis Dorsa region coincides with mapped units of the western Medusae Fossae Formation, which is an extensive, multi-lobed, layered deposit that is hypothesized to be an aeolian and/or pyroclastic deposit (e.g., Ward, 1979; Scott and Tanaka, 1982; Schultz and Lutz, 1988; Hynek et al., 2003; Mandt et al., 2008, 2009; Burr et al., 2009; Kerber and Head, 2010; de Silva et al., 2010; Kerber et al., 2011; Kite et al., 2015a). The pervasively layered and eroded nature of the Medusae Fossae Formation suggests numerous episodes of widespread deposition yielding substantial quantities of sediment (Bradley et al., 2002; Mandt et al., 2008; Burr et al., 2009; Kerber et al., 2011). Therefore, the geologic context for the Aeolis Dorsa deposits includes a ready source of sediment for their formation.
Preservation through burial is indicated by Medusae Fossae Formation deposits observed around and on top of fluvial features and alluvial fans (e.g., Burr et al., 2009, 2010). Geochemical cementation of the deposits is indicated by preserved sedimentary textures and enhanced thermal inertia of fluvial features, although the delivery mechanism, timing, and composition of the cement(s) are unknown (Burr et al., 2010).
Exposure of the Aeolis Dorsa deposits likely resulted from aeolian erosion and deflation (e.g., Pain et al., 2007; Burr et al., 2009, 2010) as evidenced by yardangs throughout the region (Ward, 1979; Scott and Tanaka, 1982; Mandt et al., 2009; Zimbelman and Griffin, 2010). Many hundreds of deposits in the Aeolis Dorsa region appear as sinuous ridges (i.e., in inverted relief) because differential erosion has removed the material surrounding the deposits (Pain et al., 2007; Burr et al., 2009).
The Aeolis Dorsa: Classifications and Interpretations
The numerous sinuous ridges of the Aeolis Dorsa have a variety of geomorphic attributes indicating deposits formed by disparate hydrologic processes and sedimentary conditions (Table 1). Here, we summarize four primary classifications for the Aeolis Dorsa deposits that are derived from previous classification schemes and mapping (Burr et al., 2009; Zimbelman and Griffin, 2010; Lefort et al., 2012; Williams et al., 2013; Kite et al., 2015a). Classifications are based on results of previous analyses and comparisons with terrestrial analogs (Pain et al., 2007; Williams et al., 2007, 2013; Burr et al., 2009, 2010; Lefort et al., 2012; Matsubara et al., 2014), but the formative processes and conditions for alluvial fans have not been interpreted. In this case, we follow previous investigators by deriving interpretations from geomorphic comparisons with previously examined deposits on Earth. Classifications and interpretations presented in Table 1 support our analyses of local deposits to elucidate the hydrologic processes and sedimentary conditions in the Aeolis Dorsa region. The four classifications are flat features, which include type 1 and type 2 subclasses, thin features, Aeolis Serpens, and fan features (Table 1).
Flat features are the widest and some of the most common features in the Aeolis Dorsa region (Pain et al., 2007; Burr et al., 2009). In general, flat features have widths of up to a few kilometers and form branched or subparallel networks (Fig. 3A) (e.g., Burr et al., 2009). Networks of flat features appear along the margins of the central depression between Aeolis and Zephyria Plana (Fig. 1B) (Burr et al., 2009; Cardenas and Mohrig, 2015).
We distinguish two subclasses of flat features (Table 1). Type 1 flat features have semi-concentric ridges on their upper surfaces (Fig. 3B), and type 2 features have textureless upper surfaces (e.g., Fig. 3C). Type 1 features have been interpreted as different units of a meandering fluvial environment (Table 1). Specifically, some type 1 features have been interpreted as channels that aggraded as they migrated laterally (Matsubara et al., 2014) or as stacked fluvial deposits, consisting of lower point bars and upper sinuous channel fills (Cardenas and Mohrig, 2015).
Type 2 flat features are straighter and narrower than type 1 features (e.g., Fig. 3C). Type 2 features are interpreted as fluvial channel fills (cf. Gibling, 2006) based on their morphological similarity to inverted fluvial deposits previously analyzed as terrestrial analogs (Harris, 1980; Williams et al., 2007). In plan-view images, terrestrial channel fills have relatively flat upper surfaces and steep sides (Fig. 4) similar to the textureless upper surfaces and steep sides of type 2 flat features (Fig. 3C).
Thin features have widths on the order of tens of meters, and may have sharp medial crests (Burr et al., 2009; Zimbelman and Griffin, 2010) or flat upper surfaces (Fig. 5). Thin features are the most sinuous ridges, with sinuosities (i.e., ratios of landform length to straight end-to-end length) up to ∼2 (Burr et al., 2010), although they may be much straighter (e.g., Fig. 5A). Some thin features appear stratigraphically stacked above flat features (e.g., Fig. 5B) (Burr et al., 2009; Zimbelman and Griffin, 2010). The textureless upper surfaces of thin features (Fig. 5) are similar to the upper surfaces of channel fills on Earth (Fig. 4). Therefore, thin features with sharp medial crests may be explained as channel fills that have been eroded along their margins (cf. Burr et al., 2010).
This sinuous feature, centrally located in the central depression between Aeolis Planum and Zephyria Planum, is ∼600 km long and a couple hundred meters wide. Aeolis Serpens has multiple branches and exhibits variable cross-sectional shapes, including medial troughs and twin lateral ridges (Fig. 6) (Burr et al., 2009; Lefort et al., 2012; Williams et al., 2013). These cross-sectional characteristics of Aeolis Serpens are also observed on inverted fluvial deposits in the Atacama Desert (Chile) (Lefort et al., 2012). Aeolis Serpens has been interpreted as resulting from variable cementation along the lateral margins (Williams et al., 2013). The moderate sinuosity of Aeolis Serpens was interpreted as evidence of meandering, but evidence of lateral migration was not observed (Williams et al., 2013).
Fan features have ridges that radiate and descend from apices, some of which connect to branching networks of ridges (Fig. 7) (Burr et al., 2009). Most fan apices border the interior margins of Aeolis and Zephyria Plana (Burr et al., 2009; Kite et al., 2015a). Based on their morphologic and topographic characteristics, fan features have been interpreted as deposits of alluvial fans (Burr et al., 2009; Kite et al., 2015a).
Geomorphic attributes of alluvial fans on Earth are present on fans in the Aeolis Dorsa region (Table 1). On Earth, alluvial fan catchments with appreciable mud and weathered sediments produce debris flows, which form debris-flow levees and lobes (Figs. 8A and 8B) (Blair and McPherson, 1998; Blair, 1999). Fan catchments with lower mud content produce sheetflood and channelized deposits, which when topographically inverted form low-sinuosity ridges (Fig. 8C) (Blair, 2000; Maizels, 1987). Fans in the Aeolis Dorsa region include parallel ridges (Fig. 7B) analogous to smaller debris-flow levees on Earth (Fig. 8B). Lobes at channel termini in Aeolis Dorsa (Fig. 7C) are similar to terrestrial debris-flow lobes (Fig. 8B). Fans in Aeolis Dorsa also consist of low-sinuosity ridges (Fig. 7B) similar to inverted channelized deposits on Earth (Fig. 8C). These similarities between terrestrial deposits and fans in the Aeolis Dorsa region suggest that debris flows, sheetfloods, and/or channelized flows contributed to fan formation in the region (Table 1).
Stratigraphic History of the Aeolis Dorsa
The classifications and interpretations of the Aeolis Dorsa deposits suggest environments with disparate depositional slopes, sedimentary conditions, and hydrologic processes. Regional stratigraphy of the Aeolis Dorsa suggests a transition from meandering fluvial deposits to alluvial fan deposits (Kite et al., 2015a). On Earth, fluvial meandering requires channels with perennial flows, low slopes, and banks that are stabilized, e.g., by cohesive silts and clays or floodplains with permafrost or geochemical cements (Matsubara et al., 2014). Therefore, ancient meander deposits imply frequent supplies of water and sediment, and stable banks to maintain constructional hydraulics and sedimentation. Unlike meandering fluvial channels, alluvial fans form on high slopes, typically at escarpments, and are characterized by intermittent or “flashy” discharge events (e.g., Blair and McPherson, 1994). An analysis of deposits in local areas of the Aeolis Dorsa region allows us to elucidate the formative processes and conditions that accompanied regional changes in hydrology, sedimentology, and topography.
The previously derived history of regionally consistent stratigraphic transitions establishes a null hypothesis that the hydrologic processes and sedimentary conditions were uniform across the Aeolis Dorsa region (e.g., uniform fluvial processes followed by uniform alluvial fan processes). We test this null hypothesis by deriving the local conditions and processes during the formation of the Aeolis Dorsa deposits. The null hypothesis would be supported with similar fluvial and alluvial fan deposits in multiple local areas, whereas any variation among local deposits would suggest differences in the regional formative processes and conditions. Discerning variations among local deposits requires (1) delineating the distribution of fluvial deposits and alluvial fans to identify areas for local analyses, (2) mapping deposits in local areas and interpreting their hydrologic processes and sedimentary conditions, and (3) deriving stratigraphic columns to compare local histories.
Geospatial Distribution of Fluvial and Alluvial Fan Deposits
Identifying the most appropriate areas for local mapping and stratigraphy requires the geospatial distributions of preserved deposits in order to select areas with good exposure and stacking of deposits. We use a mosaic of 83 images from the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX, 6 m/pixel) (Malin et al., 2007) largely covering the area adjacent to and between Aeolis and Zephyria Plana (Fig. 1B) to identify fluvial and alluvial deposits. We use ArcMap software (ESRI; http://desktop.arcgis.com/en/arcmap/) to trace shapefile polylines through the centers of features. Features are classified as flat, thin, or fan features, or Aeolis Serpens, based on their morphologic attributes as presented in Table 1.
Mapping and Stratigraphy of Local Areas
To augment the geospatial distributions with information about the hydrologic processes and sedimentary conditions (Table 1), we map the four local areas shown in Figures 1 and 9. Alphanumeric identifiers of local areas correspond to Burr et al. (2009, their table 1). Interpretations of mapped units are based on their morphologic attributes as described in Table 1. Surfaces are mapped as units based on similarities and differences in texture, albedo, and the presence of attributes described in Table 1. Using ArcMap software, we map fluvial and alluvial fan deposits as shapefiles over the CTX mosaic. Where available, stereo-pair digital terrain models (DTMs) from the MRO High Resolution Imaging Science Experiment (HiRISE, 0.3 m/pixel) (McEwen et al., 2007) are analyzed in order to better describe and interpret the deposits.
We analyze the stratigraphy of the four local areas to examine changes in the formative processes and conditions. We derive stratigraphic columns from a combination of plan-view observations and topographic analyses. Plan-view observations of CTX and HiRISE images give evidence of superposition, crosscutting, and embayment relationships of mapped units that indicate the relative order of units in stratigraphic columns. Analyses of topographic profiles are used to infer thicknesses of units in stratigraphic columns. Topographic data include point and gridded data from the Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA) (Smith et al., 2001), three DTMs derived from CTX stereo-image pairs using Ames Stereo Pipeline software (Moratto et al., 2010), and two DTMs derived from HiRISE stereo-image pairs using BAE SOCET SET software (Kirk et al., 2008).
OBSERVATIONS AND INTERPRETATIONS
Geospatial Distributions of the Aeolis Dorsa Deposits
Approximately 1600 individual features were delineated in the Aeolis Dorsa region (Fig. 9). Flat features are the most numerous, accounting for nearly two-thirds of all features, and are the most broadly distributed. Individual features combine to form networks around the central depression (Fig. 1), interior to the Aeolis and Zephyria Plana. Thin features are the second-most abundant feature type (Fig. 9). They are commonly found near or generally cluster in the same locations as networks of flat features, and may be superposed on individual flat features (e.g., Fig. 5B). Thin features also form networks, but they commonly exhibit braid-like patterns. Fan features have the most limited geospatial distribution, and are clustered in the northwest and the southeast (Fig. 9). Aeolis Serpens makes a southeast-northwest traverse through the central depression between the plana (Fig. 9). In summary, the four classifications of fluvial deposits and alluvial fan deposits have distinctive geospatial distributions. These deposits are the products of fluvial and alluvial fan processes, which are interpreted through mapping, and the resulting stratigraphic history is presented below.
Mapping and Stratigraphy of Local Areas
Mapping and stratigraphy of four local areas—areas 35 and 40 in southeast Aeolis Dorsa and areas 45N and 45S in northwest Aeolis Dorsa—are consistent with transitions from older, broad networks of fluvial deposits, to intermediate thin channel fills, and finally to alluvial fan deposits. Maps with detailed descriptions of mapped units and topographic profiles used to derive stratigraphic columns are included in the Supplemental File1. Here, we summarize the mapping (Table 2) and stratigraphy of the local areas (Fig. 10). Results of mapping and stratigraphy suggest differences in the hydrologic processes and sedimentary conditions between northwest and southeast Aeolis Dorsa.
Southeast Aeolis Dorsa: Areas 35 and 40
Local areas in southeast Aeolis Dorsa preserve self-consistent but non-uniform transitions from meandering fluvial deposits to alluvial fan deposits (Table 2; Fig. 10). Area 35 is adjacent to Zephyria Planum (Fig. 9) and preserves a unit of fluvial channel fills (Fig. 11A) that connect to broad, meandering fluvial deposits (see map of area 35, Supplemental File [footnote 1]). Area 40 is located further south than area 35 (Fig. 9) and preserves two separate units of fluvial channel fills connected to meandering fluvial deposits (Fig. 11B). Meandering fluvial deposits in both areas imply low regional slopes, paleoclimate conditions yielding frequent flows, and cohesive material to form stable channel banks. Together, they suggest the presence of cohesive sediment and significant hydrologic activity that increased to the south (Table 2).
In both areas, thin fluvial channel fills are superposed on meandering fluvial deposits (Figs. 11C and 11D). These channel fills indicate a later episode of hydrologic activity within a younger stratum of material. A unit of mounds covers the meandering fluvial deposits and channel fills (see map of area 40, Supplemental File [footnote 1]), suggesting that episodes of fluvial activity were followed by widespread deposition. The mounds are cut by alluvial fan deposits, which include debris-flow deposits. In area 35, large alluvial fans cut the mounds unit and have parallel ridges consistent with debris-flow levees (Fig. 11E). Area 40 preserves lobes that increase in size away from channel termini (Fig. 11F) and this morphology is similar to the morphology of terrestrial debris-flow lobes. Alluvial fans in areas 35 and 40 imply high depositional slopes, typically at escarpments (e.g., Zephyria Planum), and intermittent hydrologic activity. The presence of debris-flow levees and lobes in these local areas implies appreciable mud (Table 2), possibly sourced from erosion of the mounds unit.
Northwest Aeolis Dorsa: Areas 45N and 45S
Areas 45N and 45S preserve similar stratigraphies of fluvial and alluvial fan deposits, but these deposits are distinct from those in southeast Aeolis Dorsa (Table 2; Fig. 10). Areas in the northwest are next to Aeolis Planum and preserve networks of broad fluvial channel fills (Figs. 12A and 12B). However, these networks do not show evidence for meandering. Networks of broad channel fills imply regular hydrologic activity over low slopes (Table 2), but lack of meandering fluvial deposits suggests that these northwest areas lacked sufficient cohesive material to stabilize channel banks and maintain meander dynamics.
Both areas 45N and 45S preserve thin channel fills on top of the networks of broad channel fills (Figs. 12C and 12D). These superposed fluvial channel fills indicate a second episode of hydrologic activity. Local areas in the northwest lack a mounds unit, but do preserve alluvial fans (Figs. 12E and 12F). Fan deposits consist of thin and broad ridges that are consistent with sheetflood and channelized deposits. These alluvial fan deposits suggest development of relief subsequent to the formation of the channel fills, and infrequent, flashy hydrologic activity (Table 2). However, alluvial fans in areas 45N and 45S show no levees or lobes, suggesting an absence of cohesive sediment to initiate debris flows.
Previous analyses suggest that the Aeolis Dorsa region experienced significant change, specifically (1) a transition from fluvial channels to alluvial fans (Kite et al., 2015a) and (2) a decrease in hydrologic activity through time (Kite et al., 2015b), both of which imply changes in hydrology, sedimentology, and topography. In this study, we elucidate the local hydrologic processes and sedimentary conditions that occurred during these regional changes. Our work largely supports an interpretation of a region-wide decrease in hydrologic activity and at the same time augments the regional history with evidence for contrasting fluvial deposits and alluvial fan deposits between southeast and northwest Aeolis Dorsa (Table 2; Fig. 10). Based on this evidence, we reject the null hypothesis that formative processes and conditions in the Aeolis Dorsa region were uniform. Here, we discuss the history of local areas and the differences in processes and conditions among local areas, and propose causes of those differences.
The four mapping areas suggest two dichotomies of sedimentary units. One dichotomy is of fluvial deposition: meandering fluvial deposits appear only in southeastern Aeolis Dorsa and are absent from northwest Aeolis Dorsa (Table 2). These multiple units of meandering fluvial deposits imply low slopes, regular supplies of fluid and sediment, and appreciable cohesive sediment to stabilize channel banks. We thus suggest that areas in southern Aeolis Dorsa were more favorable for the development of cohesive sediments. We propose that higher elevations in southern Aeolis Dorsa favored enhanced precipitation due to orographic lifting (Burr et al., 2009; cf. Scanlon et al., 2013), which led to enhanced weathering, and greater yield of cohesive sediment, which promoted bank stabilization (cf. Matsubara et al., 2014).
Aeolis Serpens connects with fluvial channel fills in area 45N, but does not appear to connect with fluvial units in southern Aeolis Dorsa (Fig. 10). It is unclear whether Aeolis Serpens is a separate deposit that formed after the meandering fluvial deposits or if Aeolis Serpens is part of the northernmost extension of the widespread fluvial deposits in southern Aeolis Dorsa (Kite et al., 2015a). Thin fluvial channel fills appear in all local areas and represent the last-preserved episode of fluvial activity and sedimentation (Fig. 10). Delineations throughout Aeolis Dorsa suggest that thin channel fills formed discontinuous deposits that were less widespread than the older, broad fluvial deposits (Fig. 9).
Alluvial fans appear in each local area and represent the final episodes of hydrologic activity in the Aeolis Dorsa region (Fig. 10). The stratigraphic change from widespread fluvial deposits to localized alluvial fans (Fig. 9) implies a change in the paleoclimate. Alluvial fan formation requires intermittent hydrologic activity to enable the accumulation of weathered sediment in the fan catchment (e.g., Blair and McPherson, 1998). Therefore, the formation of meandering fluvial deposits followed by alluvial fans implies a change from regular to intermittent episodes (e.g., flashy discharges) of hydrologic activity. The paleoclimate implications of this stratigraphic transition suggest a regional history of decreasing hydrologic activity over time.
Alluvial fans also imply a change in topography. Whereas older fluvial deposits imply hydrologic activity over low slopes, alluvial fans imply hydrologic activity in areas with enhanced relief. Tectonic activity, such as normal faulting along the interior margins of Aeolis and Zephyria Plana, is one viable cause of enhanced relief (cf. Lefort et al., 2012).
The results presented here suggest a second dichotomy in the alluvial fan deposits: only fans of southeast Aeolis Dorsa exhibit debris-flow deposits, whereas fans in northwest Aeolis Dorsa only exhibit sheetflood or channelized deposits. By analogy with deposits on earth (Table 1), the interpretation of debris-flow deposits implies that mud was present in southeastern Aeolis Dorsa (Table 2). The coincidence of debris-flow deposits and the mounds unit, which is missing in northwest Aeolis Dorsa (Table 2; Fig. 10), suggests that mud may have been sourced from the mounds unit. Involvement of mud in these alluvial fans processes is consistent with our interpretation of more cohesive material required for fluvial meandering in the southeast. As with the meandering fluvial deposits, we suggest that higher elevations in southeast Aeolis Dorsa were favorable for orographic precipitation, weathering, and involvement of mud in alluvial fan processes.
These results from local areas increase the specificity of conditions and processes in the Aeolis Dorsa region. There are some differences, however, between our interpretations of local areas and previous interpretations of regional history. In a previous analysis of regional stratigraphy, multiple units of meandering fluvial deposits, Aeolis Serpens, and the thin channel fills compose one unit (R-1 of Kite et al., 2015a), whereas in our stratigraphies, they comprise four units. Additional mapping of the mounds unit, beyond our local areas, suggests that the unit contains fluvial deposits (unit R-2 of Kite et al., 2015a). These fluvial deposits are younger and less widespread than the older deposits, suggesting that this additional information does not change the overall history of decreasing frequency and extent of hydrologic activity in Aeolis Dorsa.
Fluvial and alluvial fan deposits have distinct geospatial distributions in the Aeolis Dorsa region. Meandering fluvial deposits and broad fluvial channel fills are widespread, whereas thin channel fills are less widespread. Alluvial fans are most limited in their distribution and found interior to the margins of Aeolis and Zephyria Plana.
Local deposits form two dichotomies. First, meandering fluvial deposits only appear in areas of southeast Aeolis Dorsa, whereas areas of northwest Aeolis Dorsa only preserve networks of fluvial channel fills. Second, alluvial fans with debris-flow deposits formed only in southeast Aeolis Dorsa, whereas fans in northwest Aeolis Dorsa are consistent with sheetflood and/or channelized deposits. Cohesive sediment limited to southeast Aeolis Dorsa may explain both of these dichotomies as it would provide sufficient bank cohesion to maintain fluvial meandering (Matsubara et al., 2014) and the resultant of reduced porosity would have been necessary to initiate debris flows (cf. Blair, 1999). Enhanced orographic precipitation or snowmelt, subsequent runoff, and weathering proximal to the highland-lowland boundary is one plausible explanation for this inferred relative abundance of cohesive sediment (Burr et al., 2009; Kite et al., 2013).
Local areas show self-consistent but non-uniform stratigraphic histories. Local stratigraphies include transitions from networks of meandering fluvial deposits and broad fluvial channel fills, to intermediate thin fluvial channel fills, and finally to alluvial fan deposits of debris flows, sheetfloods, and/or channelized deposits. Consistent with results from regional analyses, local stratigraphies indicate that the Aeolis Dorsa region experienced a decrease in the frequency of hydrologic activity and a late increase in topographic relief required to form alluvial fans.
The history of the Aeolis Dorsa region—its transition from predominately fluvial to alluvial fan deposits—is broadly similar to the globally derived hydrologic timeline for Mars (Kite et al., 2015a). However, our examinations of local deposits elucidate formative hydrologic processes and sedimentary conditions that are not apparent at regional or global scales. Dichotomies in the Aeolis Dorsa deposits suggest heterogeneous paleoclimate conditions, specifically enhanced runoff in southern Aeolis Dorsa which lead to enhanced production of weathered sediment. Therefore, local analyses of geomorphology and stratigraphy, in conjunction with regional analyses and modeling, are promising and complementary means of inferring the history of ancient Mars. Finally, these well-exposed and well-preserved deposits on Mars provide interesting analogs for comparison with ancient fluvial systems on Earth (e.g., Ielpi et al., 2016).
Alluvial fan deposits in southeast and northwest Aeolis Dorsa imply late formation of relatively steep slopes, possibly by tectonic activity. The cause of tectonic activity in Aeolis Dorsa may be associated with regional compression, regional extension, or both. Analyses of inferred tectonic features, such as wrinkle ridges (Borden and Burr, 2017), within Aeolis Dorsa may suggest the type of relief-forming deformation.
Although preliminary spectral analyses have shown that regional dust coverage effectively obscures observations in near-infrared wavelengths, an in-depth analysis of high-resolution MRO Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) images has not been performed. Spectroscopic analyses that examine the Aeolis Dorsa region for altered materials could test the hypothesis that differences in the distribution of cohesive sediments resulted in the observed differences in fluvial meandering deposits and debris-flow deposits.
We thank the U.S. Geological Survey Astrogeology Branch for producing the CTX base map and Alexandra Lefort, Edwin Kite, and the HiRISE team for producing the DTMs analyzed here. Discussions with Chris Fedo, Ross Irwin, Edwin Kite, and Jim Zimbelman, reviews by Martin Gibling and an anonymous colleague, and editorial assistance by Drs. Shan de Silva and Lesli Wood greatly helped improve this work. This project was supported by grants from the NASA Mars Data Analysis Program (NNX09AM02G to Alan Howard at the University of Virginia and DMB, NNX14AM03G to DMB and REJ), and a University of Tennessee Knoxville Summer Research Fellowship to REJ.