Geology of the Reull Vallis Region, Mars

Icarus 153, 89–110 (2001) doi:10.1006/icar. 2001.6655, available online at http://www.idealibrary.com on

Geology of the Reull Vallis Region, Mars Scott C. Mest and David A. Crown Department of Geology and Planetary Science, University of Pittsburgh, 321 Engineering Hall, Pittsburgh, Pennsylvania 15260 E-mail: [email protected] Received September 7, 2000; revised April 30, 2001

ing landforms and deposits (Schultz 1984, Wichman and Schultz 1988, Schultz and Frey 1990, Tanaka and Leonard 1995). Water is believed to have played a major role in modifying the martian highlands. Features interpreted to have been produced by water range from small valley networks (5–1000 km long, 1–10 km wide) to large outflow channels (1000–3000 km long, 20–220 km wide) (Milton 1973, Baker and Milton 1974, Pieri 1976, 1980, Baker 1978a, Baker and Kochel 1978, Carr 1979, 1995, 1996, Carr and Clow 1981, Mars Channel Working Group 1983). Most valley networks (∼99%) are found in the cratered highlands and are believed to be old (Noachian to Hesperian) features formed by a combination of surface runoff and groundwater sapping (Pieri 1976, 1980, Carr and Clow 1981, Mars Channel Working Group 1983, Baker and Partridge 1986, Grant and Schultz 1990, 1993, 1994, Craddock and Maxwell 1993, Carr 1995, 1996, Maxwell and Craddock 1995, Grant 1997, 1998, 2000, Gulick 1998). Martian outflow channels are large troughs formed by catastrophic outbursts of either subsurface or surface waters (Baker and Milton 1974, Baker 1978a,b, 1979, 1982, Carr 1979, Mars Channel Working Group 1983). Most outflow channels have source regions characterized by (a) large areas of chaotic terrain (Sharp 1973) or (b) large basins from which ponded water was released. The largest and most well known outflow channels (Ares, Kasei, and Tiu Valles) occur in the circum-Chryse region of Mars; however, four large outflow systems (Reull, Dao, Niger, and Harmakhis Valles) are also found along the eastern rim of the Hellas basin. Reull Vallis, which extends for ∼1500 km through Noachian, Hesperian, and Amazonian geologic materials east of Hellas basin, is a morphologically complex outflow channel system (Crown et al. 1992, Tanaka and Leonard 1995). Evidence for multiple flow events and extensive channel modification suggests it may have been long-lived and had a significant impact on the history of the region (Crown and Mest 1997, Crown et al. 1997). The presence of numerous highland valley networks and channels, as well as large debris aprons, provides further evidence for the influence of water, either as a fluid or as ice, on the evolution of the highlands in this area. The purpose of this study is to determine the sequence of events that formed the geologic features and units observed in the highlands surrounding Reull Vallis and by doing so provide

The geology and stratigraphy of the Reull Vallis region (27.5– 47.5◦ S, 245–270◦ W) of the southern highlands of Mars are documented through geomorphic analyses and geologic mapping at 1:2,000,000 scale. Crater size–frequency distributions are used to constrain relative ages of geologic units and determine the timing and duration of the various geologic events. The origin and evolution of the Reull Vallis outflow system and the extent to which Reull Vallis and other fluvial features modified the surrounding highlands are also evaluated. Rugged highland terrains were formed by repeated impact events and were extensively degraded and embayed by a series of plains. Highland terrains incised with well developed valley networks record some of the earliest fluvial activity in the region. Reull Vallis originates in Hesperia Planum, and dissects units of various types and ages. Truncated and bisected wrinkle ridges in Hesperia Planum indicate that the formation of Reull Vallis postdates ridge formation. The morphology of Reull Vallis suggests it had a long and complex history, including subsurface and surface movement of fluids followed by extensive modification by mass wasting. Volcanic and sedimentary plains units in the region record evidence of erosion by surface flow possibly related to development of Reull Vallis. Features produced by mass wasting, such as debris aprons and crater fill material, and aeolian processes appear to form the youngest deposits exposed in the region. °c 2001 Academic Press Key Words: geological processes; Mars; Mars, surface; surfaces, planets.

INTRODUCTION

The martian highlands cover more than 60% of the planet’s surface and consist mostly of rugged, densely cratered terrains believed to record the final phase of heavy bombardment (Murray et al. 1971, Schubert et al. 1992, Tanaka et al. 1992). These terrains are geologically and stratigraphically complex and likely contain some of the oldest rocks on Mars, emplaced during the Noachian Period (4.6–3.5 billion years ago) (Tanaka 1986, Tanaka et al. 1988), as well as show evidence of more recent, extensive degradation by various processes. Researchers attribute the geologic complexity of the highlands to the presence of large impact basins that may have influenced subsequent activity (i.e., volcanism, tectonism, fluvial erosion and deposition, and mass wasting) and controlled the distribution of the result89

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further understanding of highland evolution. This study also focuses on the fluvial history of the region and addresses the following questions: (1) What was the origin of the Reull Vallis outflow system and how did it evolve with respect to the surrounding terrain? and (2) What role did Reull Vallis and valley networks play in modifying the martian highlands in the region? These questions are addressed by a combination of geomorphic analysis, interpretation of Viking Orbiter images, and geologic mapping at the ∼1:2M scale for derivation of stratigraphic relations. Crater size–frequency distributions were compiled to constrain the relative ages of geologic units and determine the timing and duration of the observed geologic processes.

(Wilhelms 1973, King 1978, Peterson 1978, Schultz 1984, Wichman and Schultz 1988, 1989, Schultz and Frey 1990, Crown et al. 1992, Tanaka and Leonard 1995, Mest 1998, Mest and Crown 2001a,b). The northern and eastern rims of the Hellas basin exhibit rugged peaks and isolated massifs, which have been eroded and are embayed by volcanic and sedimentary plains. The study area consists of heavily cratered ancient highland crustal materials of moderate to high relief; ancient paterae; extensive tracts of smooth, channeled, ridged, and pitted plains; various surficial deposits; and the floor materials of Reull, Dao, Niger, and Harmakhis Valles (Greeley and Guest 1987, Crown et al. 1992). Previous Work

Physiographic Setting The Reull Vallis region (27.5–47.5◦ S, 245–270◦ W; Figs. 1 and 2) is located east of the Hellas impact basin, one of the largest impact structures (∼2000 km diameter; centered at 43◦ S, 291◦ W) identified on Mars (Wilhelms 1973, Schultz and Frey 1990). Hellas-related structures along with intersecting structures from other impact basins were suggested to have had a significant effect on the orientations of tectonically produced ridges and the movement of volcanic and fluvial materials in the region

The 1:15M-scale geologic map of the eastern equatorial region of Mars (Greeley and Guest 1987) provides a general geologic framework for the highlands adjacent to Reull Vallis. Noachian aged materials are shown to be the principal components of the highlands surrounding the Hellas basin. Highland volcanism, associated with the formation of Hadriaca and Tyrrhena Paterae and the emplacement of various plains units including Hesperia Planum, occurred during the Early Hesperian (Scott and Carr 1978, Scott and Tanaka 1986, Tanaka 1986,

FIG. 1. Sketch map showing physiographic provinces and major features of the highlands east of the Hellas basin of Mars. The box represents the location of the Reull Vallis region shown in Fig. 2.

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FIG. 2a. Viking Orbiter photomosaic of the Reull Vallis region showing the locations of Figs. 3–10. Reull Vallis begins in Hesperia Planum (along the eastern edge of the mosaic), dissects the highlands of Promethei Terra, and terminates near the source basin of Harmakhis Vallis. The three segments of Reull Vallis are labeled and separated by heavy black lines. Portions of Dao, Niger, and Harmakhis Valles (oriented NE–SW) are seen along the western part of the mosaic. Hadriaca Patera occurs to the northwest. Mars Digital Image Mosaic of latitude −27.5◦ to −47.5◦ , longitude 245◦ to 270◦ ; projection is sinusoidal.

Greeley and Guest 1987). Erosion of highland materials began during the Late Noachian and continued throughout the Hesperian Period, resulting in deposition of extensive plains in and around the basin (Greeley and Guest 1987). Also during the Hesperian Period, Reull, Dao, Niger, and Harmakhis Valles formed, with deposition along their banks and in Hellas Planitia. Erosion of plains materials northeast of Hellas occurred in the Upper Hesperian/Lower Amazonian (Greeley and Guest 1987). Recent detailed geologic mapping studies of the east rim of the Hellas basin include portions of the region described herein. The western portion of the Reull Vallis region (27.5–42.5◦ S, 260–270◦ W) was previously mapped by Crown et al. (1992) and Crown and Greeley (2001) as part of a study of Hadriaca Patera. The northern portion of the region (27.5–30◦ S, 247.5– 265◦ W) was previously mapped by Greeley and Crown (1990) as part of a study of Tyrrhena Patera. These and other geologic and geomorphologic studies of the Hellas region (Crown and Greeley 1993, Tanaka and Leonard 1995, Gregg et al. 1998,

Price 1998) show that the northeast rim experienced a complex geologic history following the Hellas impact. The region mapped in the current study includes portions of these prior study areas in order to provide the full geologic context of Reull Vallis as well as integrate this study with previous analyses of the highlands east of the Hellas basin. GEOLOGY OF THE REULL VALLIS REGION

The following is a summary of the geology of the Reull Vallis region determined by geomorphic analysis, interpretation of Viking Orbiter images, and geologic mapping at ∼1:2M scale (Fig. 2). Many of the geologic units discussed were first identified in previous mapping studies (e.g., Greeley and Guest 1987 and Crown et al. 1992), though unit boundary locations have been refined, and features characteristic of those units appropriate for the mapping scale used in the current study are described. In addition, four newly identified units are included: crater fill material (unit AHcf), mantled highlands material (unit AHhm),

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FIG. 2b.

Simplified geologic map of the Reull Vallis region showing geologic units identified by Viking Orbiter image analyses.

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FIG. 2c. Correlation chart of geologic units shown in Fig. 2b. Stratigraphic positions were determined by cross-cutting relationships and crater sizefrequency distributions. The geologic units are as follows: Ada = debris aprons, AHcf = crater fill material, AHhm = mantled highlands material, AHv = vallis floor material, AHpp = pitted plains material, AHf = Tyrrhena Patera flank flow unit, AHh5 = channeled plains rim unit, Hcf = Hadriaca Patera calderafilling materials, Hvf = Hadriaca Patera volcanic flank materials, Hps = smooth plains material, Hpl3 = smooth plateau unit, Hr = ridged plains material, HNpd = dissected plains material, Nm = mountainous material, and Nh1 = basin-rim unit. See text for detailed descriptions of each unit.

smooth plains material (unit Hps), and dissected plains material (unit HNpd). Cratered Highlands Heavily cratered highlands are the oldest deposits in the study area and are interpreted to consist of impact breccias and volcanic materials uplifted during the Hellas and other local impact events (Scott and Tanaka 1986, Greeley and Guest 1987). The basin–rim unit (unit Nh1 ) (Fig. 3) (Greeley and Guest 1987) forms the rugged, mountainous terrains of the Reull Vallis region. Exposures of the basin–rim unit tend to be incised with small channels and appear more eroded than other highland surfaces in the area. Crown et al. (1992) noted that intermontane regions within the basin–rim unit exhibited smooth surfaces. Although difficult to map at the 1:2M scale, it was observed in 1:500K-scale geologic mapping studies by Mest and Crown (2001a,b) that intermontane regions were filled with smooth materials, called intermontane basin fill, subsequently eroded by valley networks. These smooth tracts within the basin–rim unit

provide evidence of both deposition and subsequent erosion by fluvial processes in the oldest parts of the region. Mountainous material (unit Nm) is also found in the region as rugged, isolated, or clustered massifs within the basin–rim unit and younger plains materials, often surrounded by debris aprons. Recent topographic data from the Mars Orbiter Laser Altimeter (MOLA track numbers 01684, 10090, 10178, 10725) show that individual massifs of mountainous material reach heights of up to 4 km above their surroundings and exhibit significantly more relief than the basin–rim unit. The smooth plateau unit (unit Hpl3 ) is the uppermost member of the Plateau Sequence of the Plateau and High-Plains Assemblage defined by Greeley and Guest (1987). The smooth plateau unit occurs throughout the southern highlands and is interpreted to consist of interbedded lava flows and sedimentary deposits that embay other units in the highlands (Greeley and Guest 1987). Exposures of this unit are found in the northwest corner of the study area (Fig. 4), apparently overlain by Tyrrhena Patera volcanic deposits and eroded by Dao Vallis (Crown et al. 1992). Most of these deposits are smooth and relatively

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FIG. 3. Deposits that fill low-lying areas within the basin-rim unit (unit Nh1 ) and mountainous material (unit Nm) are dissected by valley networks (v). Some valleys (c) have subdued morphologies as they extend onto smooth plains (unit Hps), which has lobate terminations where the unit embays the highlands (arrows). Viking Orbiter image 411S18; resolution is 99 m/pixel; centered at 39◦ S, 254◦ W.

featureless; some ridges and scarps are visible, but no distinct flow fronts are apparent. Pitted plains material (unit AHpp) is located south of the Tyrrhena Patera flank flow unit and adjacent to the source basin of Harmakhis Vallis (Fig. 4). Pitted plains fill low-lying areas among peaks of the basin–rim unit and mountainous material (Crown et al. 1992). Pitted plains are interpreted to be wateror ice-rich volcanic or sedimentary deposits, possibly emplaced as coalescing volatile-rich debris aprons. Pits may have resulted from collapse as volatiles were removed from these deposits (Crown et al. 1992).

Mantled highlands material (unit AHhm) occurs in the southeast corner of the study area (Figs. 1 and 2) and consists of relatively smooth deposits that mantle most highland craters and massifs, as well as low-lying areas. Its surface is relatively featureless, and apart from a few knobs and scarps, most landforms have subdued morphologies. The observed surface morphology may in part be due to the relatively low resolution (200 m/pixel or more) of the Viking Orbiter image coverage. Mantled highlands material is interpreted to consist of widespread, continuous deposits of wind-blown and mass-wasted materials covering older highland and plains units. Although no small-scale aeolian

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FIG. 4. Hadriaca Patera displays highly eroded flanks (unit Hvf); numerous valleys radiate from the central caldera (unit Hcf), some of which are truncated by Dao Vallis. The source areas for Dao (D), Niger (N), and Harmakhis (H) Valles are connected to their main canyons by areas of collapsed plains (p). The collapsed areas also show some evidence of scouring (s) by surface flow. Wrinkle ridges (r) in the smooth plateau unit (unit Hpl3 ) are observed to cross portions of the collapsed plains. Some channels (c) in the channeled plains rim unit (unit AHh5 ) originate within areas of collapsed plains (cp). Crosscutting relationships between the valles and several channels (arrows) indicate that outflow channel formation postdates at least some small channel formation in the channeled plains rim unit. Pitted plains material (unit AHpp) fills lowlying areas of the highlands. Mars Digital Image Mosaic of latitude −29.5◦ to −40◦ , longitude 261.5◦ to 270◦ ; centered at 34.8◦ S, 265.5◦ W; north is to the top of the mosaic.

features have been observed in Viking Orbiter images, Mars Orbiter Camera (MOC) images (M01-00368 and M02-01066) show an abundance of dune-forms in this part of the highlands, supporting the interpretation that aeolian processes are (or were) active east of the Hellas basin. Highland Volcanism The study area includes parts of the volcanoes Hadriaca and Tyrrhena Paterae as well as Hesperia Planum. The oldest evidence of volcanism is found in the main shield of Hadriaca

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Patera (unit Hvf) (Carr 1973, Peterson 1978, Pike 1978, Plescia and Saunders 1979, Crown et al. 1992), a broad, low-relief volcano with a summit caldera and radial channels and ridges on its flanks. Geologic and geomorphic studies of the paterae (e.g., Greeley and Crown 1990, Crown et al. 1992, Crown and Greeley 1993, Gregg et al. 1998) have shown that the distribution and morphology of volcanic units composing the paterae flanks are consistent with a pyroclastic origin. Hydromagmatic eruptions could have resulted as magma and water migrated through fractures and interacted to produce explosive volcanic eruptions (Greeley and Spudis 1981, Greeley and Crown 1990, Crown and Greeley 1993). The smooth surface of Hadriaca Patera’s caldera filling materials (unit Hcf) and a series of lava flows associated with the Tyrrhena Patera flank flow unit (unit AHf) indicate effusive activity in later stages of patera development. Hesperia Planum, one of many areally extensive exposures of ridged plains material (unit Hr) found on Mars, extends into the northern and eastern parts of the study area (Fig. 5). Hesperia Planum, characterized by high concentrations of orthogonal sets of sinuous mare-type wrinkle ridges and ridge rings (Scott and Carr 1978, Chicarro et al. 1985, Watters and Chadwick 1989), is believed to have been emplaced as flood lavas that filled in low-lying regions of the highlands (Potter 1976, King 1978, Greeley and Spudis 1981, Scott and Tanaka 1986, Greeley and Guest 1987). No flow fronts are visible in ridged plains material within the study area, and more recent sedimentary activity may have significantly modified at least the upper portions of the ridged plains. In Viking Orbiter images the surfaces of interridge areas are relatively smooth and featureless except for the presence of low-relief scarps and small sinuous channels, interpreted to be fluvial in origin, which surround the source basin of Reull Vallis (Crown and Mest 1997). Recent Mars Orbiter Camera (MOC) images (M02-03008 and M03-04480) show that inter-ridge areas contain an abundance of dune features indicating aeolian deposits have been deposited and redistributed within the plains. Tectonism The only obvious evidence for tectonism in the study area consists of contractional features such as wrinkle ridges and ridge rings. Wrinkle ridges typically consist of a broad arching rise (up to 20 km wide and several hundred meters high) topped with a narrow (1–5 km wide) crenulated ridge, generally attributed to folding and/or thrust faulting resulting from compressional stresses either within the lithosphere or confined within the deforming unit (Lucchitta 1976, 1977, Chicarro et al. 1985, Plescia and Golombek 1986, Sharpton and Head 1988, Watters 1988, 1991, 1993, Golombek et al. 1991). Ridge rings are circular wrinkle ridge-like structures believed to delineate buried craters; it has been suggested that the overlying unit underwent subsidence resulting in compressional stresses over the buried crater rim (Bryan 1973, Chicarro et al. 1985). Variations in ridge morphology are observed throughout the Reull Vallis region and

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FIG. 5. The source area of Reull Vallis consists of channels and scarps converging at a large irregular basin (b). One channel (c) extends for ∼120 km through ridged plains material (unit Hr), dissects several wrinkle ridges, and breaches a crater (A) before entering the basin. This channel is located within a wide (∼8 km) trough (t), possibly representing a floodplain. Scarps (arrows) may represent the locations of old channels or places where Reull Vallis flooded its banks and eroded the ridged plains. Reull Vallis contains features indicative of both collapse and surface flow of fluids, including clusters of blocks on the canyon floor, and streamlined islands (i) and scour marks (s). Viking Orbiter images 415S41-46; resolution is 92 m/pixel; centered at 33.5◦ S, 246.2◦ W; white bar in center is an image gap.

may indicate specific styles of deformation and/or strengths of materials, as well as states of preservation. The highest concentrations of wrinkle ridges in the study area occur within ridged plains material (Fig. 5) and the Tyrrhena Patera flank flow unit (Fig. 4). Two dominant ridge trends are observed—NE–SW (Hellas radial) and NW–SE (Hellas concentric)—indicating either multiple stress regimes were active concurrently or the stress regime shifted over time (King 1978, Watters and Chadwick 1989, Porter et al. 1991). According to Watters and Chadwick (1989), wrinkle ridges in Hesperia Planum most likely resulted from a combination of local subsidence producing ridges with one trend followed by a superimposed regional tectonic event forming the orthogonal set of ridges. Wrinkle ridge formation in ridged plains in the study area occurred after plains emplacement and prior to formation of Reull Vallis (Early to mid-Hesperian Period) because (1) a channel associated with Reull Vallis bisects several ridges and

(2) ridges are truncated by the source depression of Reull Vallis. In the Tyrrhena Patera flank flow, NE–SW trending ridges were found to be older than or contemporaneous with the unit’s lobate flows; some ridges appear to have flow lobes emanating from them, suggesting a partially volcanic origin (Porter et al. 1991). Porter et al. (1991) found NW–SE trending ridges to be younger than lava flows within the flank flow unit as these ridges tend to cut NE–SW trending ridges and lava flows; Hellas concentric ridges were interpreted to be tectonic in origin. In the region as a whole, ridge-forming deformation may have begun as early as the Early Hesperian Epoch and continued into Early Amazonian time, though ridge formation near the source area of Reull Vallis may have ceased in the Hesperian. Ridges also occur in the smooth plateau and channeled plains rim units and in dissected plains, Hadriaca Patera flank, and smooth plains materials, and they exhibit various trends and states of preservation. Many ridges in dissected plains

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and Hadriaca Patera flank materials, and the channeled plains rim unit, have degraded morphologies and appear to have been modified by fluvial processes. The occurrence of ridges with eroded morphologies indicates ridge formation predates erosion of these units. Ridges in the smooth plateau unit show pristine morphologies, whereas ridges in smooth plains material have subdued morphologies. Some ridges within the smooth plateau unit cross areas of collapsed plains connecting the source basins to the main channels of Dao and Niger Valles; other ridges are truncated by the outflow channels, indicating ridge formation in the smooth plateau unit predates outflow channel formation. The subdued nature of ridges in smooth plains material suggests burial of these ridges by the plains. Fluvial Geology The Reull Vallis region contains a series of features interpreted to have formed by surface flow of water. Parts of four large highland outflow systems—Reull, Dao, Niger, and Harmakhis Valles—are located in the study area. Highland terrains, the flanks of Hadriaca Patera, and many large craters contain numerous well-developed valley forms. Also, several plains units— including ridged plains, the channeled plains rim unit, and dissected plains material—contain small sinuous channels and scarps providing further evidence for erosion of surface materials by overland flow of fluids. Outflow channels. Dao, Niger, and Harmakhis Valles are three large (∼1200, 230, and 800 km long, respectively) outflow channel systems that extend to the southwest toward Hellas basin. They have source areas consisting of large steep-walled depressions (22–50 km across) formed by collapse of apparently volatile-rich plains and they exhibit knobs and hills surrounded by flat, relatively smooth materials on their floors (Crown et al. 1992, Crown and Mest 1997, Price 1998, Price et al. 1998). The source basins of the valles are connected to their main canyons by areas of collapsed plains (Fig. 4). Some collapsed areas contain undisturbed structures, such as wrinkle ridges, which indicate that the plains were at one time continuous (Squyres et al. 1987, Crown et al. 1992). The valles (as well as several smaller adjacent channels) most likely formed by a combination of subsurface flow that caused the plains to collapse and surface flow that downcut and eroded the plains (Baker 1982, Squyres et al. 1987, Crown et al. 1992). Heat associated with nearby volcanic centers could have mobilized subsurface volatiles causing erosion as fluids emerged at the surface (Squyres et al. 1987, Crown et al. 1992). Portions of the collapsed areas contain scour marks in channel floor materials that are indicative of surface flow. The main canyons of Dao, Niger, and Harmakhis Valles are morphologically similar, consisting of steep-walled, relatively flat-floored troughs with little sinuosity (Fig. 4). The valles cut into the channeled plains rim unit as they extend toward Hellas Planitia (Greeley and Guest 1987, Crown et al. 1992), breach the basin rim, and terminate on the basin floor. Vallis floor material

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(unit AHv) in Dao, Niger, and Harmakhis Valles consists of remnants of the collapsed plains, debris deposited by fluvial processes, and contributions from wall collapse (Crown et al. 1992). The Reull Vallis outflow system can be separated into three morphologically distinct segments. Segment 1, oriented roughly N–S, contains the source area and part of the main canyon, and is located in the ridged plains of Hesperia Planum. From here, Reull Vallis (segment 2—oriented NE–SW) winds through the highlands of Promethei Terra. The remainder of the canyon (segment 3) extends to the northwest through the channeled plains rim unit and terminates near the source basin of Harmakhis Vallis. Segment 1 (∼240 km long, 8–47 km wide, and 110–600 m deep; Mest et al. 1998) contains the source area for at least some of the fluids that carved Reull Vallis and consists of erosional scarps, scarp-bounded troughs, and small theater-headed channels that converge at a large (∼50 km across) depression within the ridged plains (Crown and Mest 1997; Fig. 5). The depression contains inliers of the ridged plains, of which some are streamlined, as well as scour marks on its floor. Beyond the depression, Reull Vallis narrows and opens into a series of irregular scarp-bounded basins, which also contain inliers of the ridged plains as clusters of blocks on their floors. Ridges visible on the basin floors have subdued morphologies and may be buried by vallis floor material. At some locations, scarps adjacent to the canyon suggest that the canyon flooded its banks and eroded portions of the ridged plains (Fig. 5); alternatively, these scarps could be faults or degraded ridges. Vallis floor material fills two large craters at the end of segment 1, surrounding blocks of ridged plains material. Vallis floor material is generally smooth along the length of segment 1 and includes some component of infilling by materials deposited as fluids receded and/or by debris contributed by collapse of the vallis walls. The morphology of segment 1 suggests it formed by a combination of subsurface and surface flow and collapse of ridged plains material. Theater-headed channels converging at the source depression suggest that sapping within the ridged plains contributed some fluids to Reull Vallis, as water could have utilized faults and fractures until emerging at the surface. Clusters of blocks within Reull Vallis, similar to those in chaotic terrain, also suggest parts of segment 1 underwent collapse. Scour marks on the canyon floor, streamlined inliers of ridged plains material, and lateral expansion of the canyon indicate erosion by surface flow. An obvious surface connection between segments 1 and 2 is not apparent and may be covered by younger plains materials, but it is inferred that Reull Vallis was at one time continuous, as there are no other obvious source regions in the area for segment 2. Segment 2 can be separated into morphologically distinct upper and lower parts. The upper part is sinuous and extends for ∼240 km through degraded cratered highland materials and smooth plains material. Photoclinometric measurements show that segment 2 ranges from 6 to 13 km wide and from 110 to 650 m deep (Mest et al. 1998) and exhibits various features

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FIG. 6. The upper part of segment 2 of Reull Vallis shows numerous gullies (g) dissecting floor materials. Small-scale layering (arrows) is seen along the canyon walls; these layers may be depositional terraces or erosional benches. Two small tributaries (t) entering Reull Vallis are apparent. Crater A is breached and contains crater fill material with concentric rings on its surface. Viking Orbiter image 414S48; resolution is 91 m/pixel; centered at 238◦ S, 248◦ W; crater A is ∼11.5 km in diameter.

indicative of surface flow. Small-scale layering (tens to hundreds of meters thick) is observed along both walls in segment 2 (Fig. 6); the layers have relatively flat surfaces and irregular, often fluted scarps. These layers could be erosional benches, representing exposed strata, or depositional terraces emplaced at various stages of flow within Reull Vallis. The canyon also contains a series of braided gullies incised in vallis floor material. The presence of these gullies suggests vallis floor material in this part of Reull Vallis may consist of unconsolidated materials remobilized in the final stages of vallis formation. The lower part of segment 2 begins where a narrow (1–2 km wide), shallow (∼100 m deep; Mest et al. 1998) gully downcuts into the canyon floor. This part of segment 2 extends for roughly 70 km through degraded highlands and smooth plains material before opening into a large basin (Fig. 7), after which segment 2 continues for ∼300 km where it joins with segment 3.

Morphologically, the lower part of segment 2 is steep-walled and has a relatively flat floor (Crown and Mest 1997). The portions of segment 2 adjacent to the basin are narrower (6–18 km) and shallower (140–350 m) than the remainder of this segment (8–24 km wide, 150–800 m deep) (Mest et al. 1998). The planimetric shape of the lower part of segment 2 indicates preexisting craters may have influenced formation of this part of the canyon; no features on its floor indicative of erosion by surface flow are observed, though the canyon contains some small-scale layering (tens to hundreds of meters thick; Mest et al. 1998) along its walls near the junction with segment 3. Vallis floor material consists of debris infilling the canyon from fluvial deposition, wall collapse, and tributary canyons, and exhibits pits and lineations that parallel the vallis walls (Fig. 7). These lineations are similar to those seen on lineated valley fill in the fretted terrain and are interpreted to have formed as interstitial ice/water caused the

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FIG. 7. Vallis floor material (unit AHv) within the lower part of segment 2 shows lineations (a) parallel to canyon walls and pitting (p) within a large basin. Smooth plains material (unit Hps) adjacent to Reull Vallis exhibits undulations on its surface attributed to some combination of fluvial and aeolian processes. Long linear scarps (arrows) south of the basin either represent faults or provide evidence of lateral deposition from Reull Vallis as it flooded its banks. Crater fill material (unit AHcf) displays pitting similar to Reull Vallis, suggesting similar materials and/or processes are affecting the area in and adjacent to this part of the canyon. A debris apron (unit Ada) contains lineations on its surface similar to those on vallis floor material. Viking Orbiter images 411S22 and 413S52; resolutions are 99 and 94 m/pixel, respectively; centered at 41◦ S, 250◦ W.

infilling debris to undergo shear during motion (Squyres 1978, 1979, Lucchitta, 1984, Squyres and Carr 1986), similar to terrestrial rock glaciers (Wahrhaftig and Cox 1959). Debris entering Reull Vallis from a side canyon (42◦ S, 254◦ W) shows similar lineations on its surface that have been deflected in the direction of flow of debris within the main canyon. Also, several collapse features/slump blocks occur along the southern wall in this area. The morphology of segment 2 suggests formation by fluvial processes. Several channels enter the upper part of segment 2 and near the junction with segment 3, suggesting contributions of fluids to Reull Vallis may have come from external sources. These tributaries begin within and cut through various units including the basin–rim unit, dissected plains and smooth materials, and the channeled plains rim unit. The channels tend to be <2 km wide, tens to hundreds of kilometers long, and are steepwalled, flat floored, and sometimes braided. The morphology of the lower part of segment 2 indicates it has been extensively modified by wall collapse. Segment 3 of Reull Vallis (∼600 km long) begins at the junction of segment 2 and a large side canyon (Fig. 8) and dissects the channeled plains rim unit, smooth plains material, and the basin–rim unit. The theater-headed side canyon, located in the channeled plains rim unit, enters the main canyon from the south; the side canyon displays fluted layers along its walls and contains relatively smooth floor materials. Several smaller tributaries enter this side canyon near its junction with segments 2 and 3. Segment 3 is wider (13–55 km) and deeper (200–2800 m) (Mest et al. 1998) than segments 1 and 2 and appears more

heavily eroded along its walls than the other segments. The side canyon may have been a secondary source region for fluids and may have caused enlargement of segment 3. Changes in the course of segment 3 are angular, suggesting formation of this part of the canyon may have been structurally controlled. Previous researchers have suggested ring structures of several ancient impact basins may be located in this part of the highlands (Wichman and Schultz 1988, 1989, Schultz and Frey 1990), and these may have influenced the location and orientation of portions of the Reull Vallis system. Reull Vallis terminates close to the source basin of Harmakhis Vallis, and it has been suggested that a subsurface connection between Reull and Harmakhis Valles may have existed. However, a debris apron covers the area between the two valles, and any expression of a connection (either surface or subsurface) may be buried (Crown et al. 1992, Tanaka and Leonard 1995). Vallis floor material in segment 3 is hummocky and appears to consist of debris infilling the canyon by fluvial deposition and mass wasting, as several debris aprons are seen along the walls of Reull Vallis near its terminus. Segment 3 shallows as it nears its terminus, and unlike Dao and Harmakhis Valles, Reull Vallis has no obvious terminal depositional area. Fluids may have backed up in the canyon resulting in deposition of its bedload near the terminus and overflow of its banks. The occurrence of numerous channels and scour marks in the channeled plains rim unit, southwest of segment 3, suggests large volumes of water flowed over the surface toward Hellas basin (Crown et al. 1992, Leth and Treiman 1997, Price 1998).

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FIG. 8. Segment 3 of Reull Vallis begins at the junction of segment 2 and a large side canyon (sc). Changes in the course of segment 3 are highly angular, suggesting its formation may be structurally controlled. A large slump block (sb) along the southern wall of Reull Vallis shows enlargement of the canyon by collapse. Numerous large debris aprons (unit Ada) surround highland massifs; other debris aprons (d) extend from the canyon walls, contributing debris to vallis floor material (unit AHv). The channeled plains rim unit (unit AHh5 ) exhibits channels (c) indicating erosion by surface flow and evidence for widespread deposition by fluvial and/or aeolian processes in the form of crater fill material (unit AHcf). Flat-topped mesas (m) have smooth surfaces and fluted scarp boundaries and resemble smooth plains material. Mars Digital Image Mosaic of latitude −38.5◦ to −45.5◦ , longitude 254◦ to 265◦ ; centered at 42.5◦ S, 259.5◦ W; north is to the top of the mosaic.

Highland degradation Highland materials in the region exhibit well developed valley networks and channels, and the interior walls of large craters and the flanks of Hadriaca Patera are incised with parallel gullies, providing evidence for surface flow of fluids. Most valley networks occur in the low-lying areas of the basin–rim unit (Fig. 3), whereas valley forms on steeper massifs and interior crater walls occur as parallel channels. Recent mapping of portions of the Reull Vallis region (Mest and Crown 2001a,b) shows that valley networks erode a sedimentary unit that fills intermontane areas among highland massifs. These networks consist of narrow (<1 km) valleys up to several tens of kilometers in length and exhibit dendritic to rectilinear patterns with well-developed tributary systems similar to other large martian valley networks and terrestrial drainage basins (e.g., Mars Channel Working Group 1983, Carr 1995, 1996, Grant 1997, 1998, 2000). Most networks in the region significantly erode the deposit in which they occur. Some channels erode headward into surrounding highland materials toward basin divides; others contain theater-headed terminations located within the sedimentary deposits. Drainage basins containing networks

are 80–240 km wide and 80–640 km long; the largest drainage systems typically consist of two or more basins connected by individual valleys that breached their divides. Several valleys in the central part of the map area are subdued where they emerge from the highlands onto smooth plains (Fig. 3). These valleys could be partially buried by smooth plains material, indicating that at least some plains emplacement postdates valley network formation, or the erosive ability of these valleys decreased as fluids flowed from steeper highland terrains onto relatively flat plains surfaces. These highland valley networks could have formed by various combinations of runoff and sapping processes, as shown by variations in drainage pattern among the networks, the extent to which the networks have eroded the intermontane deposits, the presence of some short theater-headed valleys, and the fact that some channels erode to the basin divides whereas others terminate within the deposits. Several large craters (∼26–70 km in diameter) in the study area have highly degraded rims, parallel interior gullies that head near crater rims and terminate on crater floors, dissected or a complete lack of ejecta blankets (Fig. 9), and smooth floors.

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FIG. 9. Dissected plains material (unit HNpd) is characterized by a smooth surface exhibiting narrow channels (c), scarps (s), and ridges with degraded morphologies (r). A channel (arrow) breaches highland terrain, suggesting fluids ponded in this area until eroding through the highlands. Crater A displays a heavily channeled interior wall; its crater fill material most likely consists of debris eroded from the crater walls. Crater B contains crater fill material with lobate margins and concentric rings on its surface, possibly resulting from coalescing debris aprons. Debris aprons (unit Ada) in craters suggest that mass wasting is an important process of crater degradation in the region. Viking Orbiter images 411S07-15; resolution is 100 m/pixel; centered at −35◦ S, 258◦ W.

Other researchers (Craddock and Maxwell 1993, Grant and Schultz 1993) have observed that many highland craters similarly record various stages of degradation. The morphologies of the incised channels and the range of crater preservation suggest that a combination of fluvial processes and mass wasting is responsible for erosion and degradation of highland craters (Craddock and Maxwell 1993, Grant and Schultz 1993). Most craters in the study area are partially filled by smooth or hummocky deposits, consisting of debris eroded from the crater rim and wall. Many craters in the Reull Vallis region also contain debris aprons that extend from the craters’ interior walls onto their floors, indicating that mass wasting contributes significantly to crater degradation. The flank materials of Hadriaca Patera contain numerous valleys that radiate from the volcano’s summit (Fig. 4). Most of the valleys are trough-shaped, lack tributaries, and are theaterheaded near the summit (Gulick and Baker 1990, Crown et al. 1992). V-shaped channels are observed in some valley interiors, suggesting a contribution by surface runoff; however, the overall morphology indicates that sapping has modified the channels (Gulick and Baker 1990, Crown et al. 1992). Dao Vallis truncates several of these channels, indicating most of the flank erosion occurred prior to formation of Dao Vallis. Highland resurfacing Plains units in the study area, including dissected and smooth plains materials and the channeled

plains rim unit, embay highland terrains and record the effects of a series of erosional and depositional events (Crown and Mest 1997, Crown et al. 1997). Dissected plains material (unit HNpd, Fig. 9), previously mapped as the southwestern-most extension of Hesperia Planum (Greeley and Guest 1987), is characterized by a smooth surface dissected by many narrow sinuous channels and numerous lowrelief scarps, and is interpreted to consist of volcanic and/or sedimentary materials eroded by fluvial processes. Dissected plains material fills low-lying areas of the highlands, isolating knobs and hills of mountainous material. The contact between the ridged plains and dissected plains materials is not well defined and the two units may grade into one another. However, the mottled texture and the lack of high concentrations of pristine cross-cutting wrinkle ridges in dissected plains material allow it to be distinguished from ridged plains material. Two possibilities exist for the origin of dissected plains material: (1) it is a discrete unit (volcanic or sedimentary) emplaced prior to the ridged plains and was subjected to tectonic deformation and fluvial erosion, and (2) it is an extension of the ridged plains that underwent a period of fluvial erosion. Channels breach the basin–rim unit at two locations along the dissected plains/highland boundary (34◦ S, 258◦ W (Fig. 9) and 36◦ S, 259◦ W), eroding the basin– rim unit and depositing material where the channels emerge from the highlands. The large impact crater in the northern part of the map area and/or heat from emplacement of ridged plains

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material could have released water from the subsurface. Based on local topography (U.S. Geol. Surv. 1989) and channel and scarp orientations, water flowed southwest until it reached the highland massifs of the basin–rim unit. Ponding and deposition of sediments occurred along this boundary, shown by several large infilled craters, until fluids were able to erode westward through the highlands. Most exposures of smooth plains material (unit Hps), originally mapped as part of the smooth plateau unit by Greeley and Guest (1987), are found adjacent to Reull Vallis (Fig. 2). Smooth plains are interpreted to be sedimentary materials deposited from overflow of Reull Vallis and/or deposited following erosion of the highlands by valley networks. At moderate resolutions, smooth plains display relatively featureless surfaces except for a few wrinkle ridges with subdued morphologies, low-relief scarps, and small channels. Smooth plains appear to embay Noachian highland units and exhibit lobate terminations at some locations where the plains meet the highlands (Fig. 3). Low-relief scarps and small channels suggest fluids eroded some of the plains after emplacement (Fig. 7). At higher resolutions (∼47 m/pixel—Viking Orbiter images 584B22 and 23), smooth plains material adjacent to Reull Vallis contains pits or small-scale undulations, possibly resulting from scouring and/or aeolian processes. Smooth plains material appears to change in character where exposed along Reull Vallis. Along the upper part of segment 2, smooth plains material shows a fluted scarp boundary (Fig. 6); along most of the lower part of segment 2 smooth plains material extends to the canyon wall (Fig. 7). As segment 2 approaches segment 3, the edge of the plains again appears as a fluted scarp. Smooth plains material north of segment 3, and mesas within the channeled plains rim unit, also have a fluted appearance. Early flooding from Reull Vallis may have resulted in emplacement of smooth plains material adjacent to the canyon; as fluids receded, portions of the plains were eroded. Ponding may have also occurred near the lower part of segment 2 resulting in deposition of a thicker, laterally extensive sequence of smooth plains material. Late-stage outbursts of water, which formed the side canyon and enlarged segment 3, may have also removed portions of the smooth plains adjacent to segment 3 and scoured the channeled plains rim unit south of the canyon. Isolated exposures of smooth plains material, within the basin–rim unit (i.e., 38◦ S, 253◦ W), may consist of sedimentary materials eroded from the highlands via valley networks. A component of eroded highland material may also be included in smooth plains material adjacent to Reull Vallis. The channeled plains rim unit (unit AHh5 , previously mapped by Greeley and Guest 1987) occurs adjacent to the Hellas basin and extends eastward into the highlands of Promethei Terra (Fig. 2). The channeled plains rim unit is interpreted to consist of interbedded volcanic and sedimentary plains heavily dissected by fluvial channels and scarps (Greeley and Guest 1987, Crown et al. 1992). Channels are sinuous in nature and are oriented roughly radial to the Hellas basin, indicating fluids flowed

toward the basin, consistent with local topography (U.S. Geol. Surv. 1989). The channeled plains rim unit between and immediately adjacent to Dao and Harmakhis Valles (Fig. 4) contains channels that parallel and in some cases are truncated by the canyons, indicating that at least some erosion of the plains in this area preceded outflow channel formation (Crown et al. 1992). Some channels appear to originate at sites of collapsed plains, suggesting fluids were released as lavas were emplaced over volatile-rich materials (Fig. 4) (Squyres et al. 1987, Crown et al. 1992). Crown et al. (1992) suggested that channeling on the plains near Dao and Harmakhis Valles could represent the initial stages of vallis formation in this area. The channeled plains rim unit south of Harmakhis and Reull Valles contains narrow sinuous channels with erosional scarps, scour marks, and flat-topped mesas, but no identifiable source regions (such as collapse areas or theatre-headed canyons) (Fig. 8). Flat-topped, scarp-bounded mesas south of Reull Vallis (Crown et al. 1992) appear to have the same morphologic characteristics as smooth plains material north of Reull Vallis. Also, several craters within the channeled plains rim unit have highly degraded rims, lack ejecta blankets, and are filled with smooth materials (Crown et al. 1992). These craters may be filled with deposits similar to smooth plains material and have been exhumed by fluvial erosion. The occurrence of mesas of smooth plains and of smooth materials infilling craters suggests that smooth plains extended over part of the channeled plains rim unit prior to formation of segment 3 of Reull Vallis. The nature of the channeled plains rim unit south and west of Reull Vallis suggests that large volumes of water (possibly from Reull Vallis) flowed over the surface toward the Hellas basin, leaving mesas of smooth plains, exposing buried craters, and scouring the plains. Surface Deposits Aeolian, mass wasting, and possibly fluvial activity appears to have formed some of the youngest deposits in the Reull Vallis region. Many craters contain deposits of crater fill material (unit AHcf); this material varies morphologically, suggesting crater fill material may have multiple origins. Crater fill material observed in craters with channeled interior walls appears to consist of sediments eroded from the crater walls and deposited on the crater floor (Fig. 9). Crater fill materials in other craters have pitted surfaces and lobate edges and may be composed of coalescing debris aprons mass wasted from crater rims (Figs. 7 and 9). Other occurrences of crater fill material, such as those in the channeled plains rim unit, may consist of material deposited from external sources. Several craters in the map area, especially those within the basin–rim and channeled plains rim units, appear to have been breached by channels allowing deposition of debris in the craters. Some deposits are pitted or contain ring features concentric to the crater walls (Fig. 6), similar to concentric crater fill (Squyres and Carr 1986, Carr 1996) or resulting from deposition of layered sediments within the crater (Zimbelman et al. 1988, 1989). Some crater fill materials in

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(Wahrhaftig and Cox 1959). Debris aprons also occur in craters and along vallis walls; these are relatively small and display mottled albedoes, relatively featureless surfaces, and arcuate to lobate fronts (Figs. 8 and 9) (Crown and Stewart 1995, Stewart and Crown 1997). Pits are seen on the surfaces of several massif-, crater-, and vallis-related debris aprons, especially close to Reull Vallis. A large (∼40 km long) massif-related debris apron (46◦ S, 250◦ W; Fig. 10) displays many of the features mentioned above as well as compressional ridges (oriented perpendicular to the direction of flow) (Stewart and Crown 1997). Portions of the apron also contain a depression bounded by the parent massif and a massif-facing scarp and are interpreted to have formed by deflation of volatile-rich debris during or after the apron’s emplacement (Stewart and Crown 1997); pits on the surfaces of debris aprons may have a similar origin. Small gullies are incised in the surface of the debris apron. Some gullies appear to originate at the depression; other gullies appear to have utilized junctions between coalescing lobes of the apron (Stewart and Crown 1997). RELATIVE AGE RELATIONSHIPS

FIG. 10. A large debris apron (Ada) south of Reull Vallis surrounds a massif of mountainous material (unit Nm). The apron is composed of multiple flow lobes and displays a variety of features on its surface, including longitudinal and compressional ridges (r), pits (p), a depression (d) adjacent to the massif, and channels (c). The contact between smooth plains material (unit Hps) and the channeled plains rim unit (unit AHh5 ) is represented here by a scarp (s). In this region, the channeled plains rim unit also contains a small tributary canyon (t) of Reull Vallis, which has several of its own tributary channels (arrows). Viking Orbiter images 584B13–17; resolution is 47 m/pixel; centered at 45◦ S, 255.2◦ W.

the study area may also consist of or be blanketed by aeolian deposits. Debris aprons (unit Ada) occur throughout the region and are interpreted to consist of debris mass-wasted from highland materials (Crown et al. 1992, Crown and Stewart 1995, Stewart and Crown 1997). Most of the debris aprons are associated with mountainous material and often completely surround their parent massifs, spreading laterally and extending for distances up to 40 km (Figs. 2, 7, 8, and 10). Massif-related debris aprons typically have uniform or mottled albedoes, lobate frontal morphologies, and appear to be composed of multiple coalescing flows. Lineations parallel to the direction of flow occur in large massif-related aprons and indicate shearing of materials facilitated by interstitial water and/or ice (Squyres and Carr 1986, Zimbelman et al. 1989, Crown et al. 1992) similar to lineated valley fill (Squyres 1978, 1979) and terrestrial rock glaciers

Tanaka (1986) subdivided the three martian time-stratigraphic systems into eight series using crater densities for craters greater than 2, 5, and 16 km in diameter. Crater counts and superposition relations are used in this investigation to place the geologic units of the Reull Vallis region into the stratigraphic framework developed for Mars (Table I). In general, the relative ages of geologic units mapped in this study, as determined by crater counts (Fig. 11; see the Appendix), are consistent with superposition relations observed via image analysis, as well as with the results of other studies of the eastern Hellas region (e.g., Greeley and Guest 1987, Crown et al. 1992, Tanaka and Leonard 1995). Inconsistencies in relative ages between the current and previous studies were resolved by analyzing cross-cutting and superposition relationships to accurately place units in the appropriate time-stratigraphic series. Uncertainties of series designations from crater statistics are indicated in Table I (“Series range”). Crater size–frequency distributions for highland materials demonstrate that they are the oldest materials exposed in the Reull Vallis region, but show slightly younger ages than indicated in other studies (e.g., Greeley and Guest 1987). Crater size–frequency distributions show a range from Middle Noachian to Lower Hesperian for the basin–rim unit and mountainous material. The degradational history of these units and their limited exposures may have influenced the ability of these units to accumulate and preserve large craters. Also, the occurrence of intermontane basin fill within the basin–rim unit would have buried most small- and medium-sized craters, giving the basin–rim unit a younger age than expected. N(5) data indicate that the basin–rim unit and mountainous material are Middle to Upper Noachian in age. N(2) and N(5) ages for the smooth plateau unit are consistent with those of Greeley and Guest

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TABLE I Crater Size–Frequency Data and Time-Stratigraphic Determinations

Unit

Total craters (>0.72 km)

Area (km2 )

N(2)

N(5)

N(16)

Series range

Series designation

Ada AHcf AHhm AHv AHpp AHh5 AHf Hcf Hvf Hps Hpl3 Hr HNpd Nm Nh1

52 67 23 65 24 341 300 8 188 196 82 437 380 132 513

35,282 17,974 35,253 68,463 15,275 315,493 112,544 5,213 76,725 83,276 48,479 214,775 156,207 63,169 192,513

227 ± 80 445 ± 157 539 ± 124 307 ± 67 589 ± 196 406 ± 36 951 ± 92 768 ± 384 899 ± 108 901 ± 104 825 ± 131 745 ± 59 845 ± 74 1235 ± 140 1070 ± 75

57 ± 40 0 170 ± 70 132 ± 44 393 ± 160 127 ± 20 142 ± 36 0 235 ± 55 144 ± 42 248 ± 72 168 ± 28 218 ± 37 427 ± 82 436 ± 48

0 0 0 0 131 ± 93 16 ± 7 9±9 0 26 ± 18 36 ± 21 41 ± 29 42 ± 14 83 ± 23 63 ± 32 156 ± 28

UH–MA UH–LA UN–UH LH–LA MN–LA LH–LA LH–UH LH–LA UN–LH UN–UH UN–UH UN–UH LN–LH MN–LH MN–LH

LA or above UH or above UH or above UH–LA UH–LA UH–LA LH or above LH or above LH LH LH LH LH MN–UN MN–UN

Note. N(2), N(5), and N(16) represent the cumulative number of craters >2, 5, and 16 km in diameter/106 km2 . Error = ±((N1/2 )/A) × 106 km2 , where A = area. Series ranges are based upon crater counts using the crater-density boundaries determined by Tanaka (1986); MA = Middle Amazonian, LA = Lower Amazonian, UH = Upper Hesperian, LH = Lower Hesperian, UN = Upper Noachian, MN = Middle Noachian, and LN = Lower Noachian. Series designations are based upon superposition relationships and crater counts.

(1987) and Crown et al. (1992), indicating a Lower Hesperian age (range Upper Noachian to Upper Hesperian). Crater size–frequency data for volcanic materials in the area are consistent with other studies. The N(5) value for ridged plains material (168 ± 28) is in agreement with those of Tanaka (1986) and Greeley and Guest (1987), indicating a Lower Hesperian age. The N(2) and N(5) values for Hadriaca Patera flank materials are consistent with those determined by Crown et al. (1992), indicating a Lower Hesperian age. Stratigraphic relations indicate the caldera-filling materials are younger than the flank materials. N(2) and N(5) values for the part of the Tyrrhena Patera flank flow within the map area are Lower to Upper Hesperian. Earlier studies by Gregg et al. (1998) of the upper part of the flank flow (north of 27.5◦ S) and Crown et al. (1992) of the lower part (south of 30◦ S) indicate Amazonian to Hesperian ages. We have retained AHf as a unit designation because none of these studies covers the entire unit and it is possible that the flank flow unit developed over a long time period and preserves regions of different ages. The N(2) and N(5) values for dissected plains material indicate an Upper Noachian to Lower Hesperian age; this is slightly older than or contemporaneous with ridged plains material, and older than the Tyrrhena Patera flank flow unit. Superposition relationships show flank flow materials are emplaced on dissected plains material, supporting this interpretation. The gradational nature of the contact between dissected plains and ridged plains materials does not provide definitive superposition relations. N(2) and N(5) values suggest smooth plains material may range from Lower to Upper Hesperian in age. Cross-cutting relationships between smooth plains material and Reull Vallis, as

well as superposition relations between smooth plains material and highland valley networks, suggest smooth plains material is Lower Hesperian in age. Mesas composed of smooth plains material occurring within the channeled plains rim unit suggest that smooth plains material is older and that it formed as Reull Vallis flooded its banks and eroded the smooth plains south and west of the canyon. Superposition relations between smooth and dissected plains materials show smooth plains material is younger. The stratigraphic position of the channeled plains rim unit is based on the N(2) and N(5) values and superposition relationships. The channeled plains rim unit is interpreted to be a sedimentary unit deposited in conjunction with formation of Reull, Dao, Niger, and Harmakhis Valles, and erosion and redeposition of smooth plains material. The current surface contains large (commonly >10 km in diameter) craters that were exhumed as smooth plains material was removed. The N(2) and N(5) values for the channeled plains rim unit suggest formation during the Late Hesperian to Early Amazonian, consistent with Greeley and Guest (1987) and Crown et al. (1992). Crown et al. (1992) suggested the channeled plains rim unit had an extensive history, possibly beginning in the Early Hesperian, with activity continuing until the Early Amazonian. However, formation of the erosional surface of the channeled plains rim unit near Reull Vallis should postdate emplacement of smooth plains material as mesas of smooth plains material occur within the channeled plains rim unit. The N(2) and N(5) values for pitted plains material show a wide range of ages (Middle Noachian to Lower Amazonian), similar to those determined by Crown et al. (1992). According to Crown et al. (1992) this range most likely reflects modification

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FIG. 11. Crater size–frequency data for the geologic units of the Reull Vallis region summarized as N(2), N(5), and N(16) relative ages (normalized to 106 km2 ; see Table I for data). Error = ±((N1/2 )/A) × 106 km2 . The geologic units are as follows: Ada = debris aprons, AHcf = crater fill material, AHhm = mantled highlands material, AHv = vallis floor material, AHpp = pitted plains material, AHf = Tyrrhena Patera flank flow unit, AHh5 = channeled plains rim unit, Hcf = Hadriaca Patera caldera-filling materials, Hvf = Hadriaca Patera volcanic flank materials, Hps = smooth plains material, HNpd = dissected plains material, Hpl3 = smooth plateau unit, Hr = ridged plains material, Nm = mountainous material, Nh1 = basin–rim unit. Crater density boundaries for the martian time-stratigraphic series determined by Tanaka (1986) are shown as horizontal lines; UA = Upper Amazonian, MA = Middle Amazonian, LA = Lower Amazonian, UH = Upper Hesperian, LH = Lower Hesperian, UN = Upper Noachian, MN = Middle Noachian, and LN = Lower Noachian.

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of the basin–rim unit, from which the pitted plains appear to have originated. Large craters preserved in the pitted plains indicate a Noachian age, whereas smaller craters reflect the younger ages (Upper Hesperian to Lower Amazonian) of materials which resurfaced this part of the highlands (Crown et al. 1992). Mantled highlands material shows very few fresh craters. N(2) and N(5) values for this unit indicate it is Lower to Upper Hesperian in age, possibly extending into the Lower Amazonian. The fact that several large craters with subdued morphologies are visible within mantled highlands material is consistent with the interpretation that this unit includes deposits that cover older craters. Resurfacing of the highlands may have occurred during the Hesperian and Amazonian and continued to the present, burying small craters. It should be noted that this part of the study area is covered by low-resolution images making identification of fresh craters difficult. Few craters are observed within vallis floor material; previous studies have shown these deposits to be Upper Hesperian (Greeley and Guest 1987) to Lower Amazonian (Crown et al. 1992). The N(2) and N(5) crater statistics determined in the current study for vallis floor material for Reull, Dao, Niger, and Harmakhis Valles are in agreement with Crown et al. (1992) and range from Lower Hesperian to Lower Amazonian. The fact that the canyons cut into several Lower to mid-Hesperian units (including ridged plains, Hadriaca Patera flank, and smooth plains materials, and the smooth plateau and the Tyrrhena Patera flank flow units) and truncate ridges and channels indicates outflow channel formation must postdate formation of these units and features. The undissected appearance of much of the vallis floor material indicates it represents the final stages of outflow activity in which deposition occurred. Lower Hesperian ages for these deposits are most likely due to inclusion of the entire channel of Reull Vallis; the study by Crown et al. (1992) only measured a portion of segment 3 of Reull Vallis near its terminus. If individual segments of Reull Vallis are considered separately, vallis floor material in segment 1 and the upper part of segment 2 exhibits more craters than the lower part of segment 2 (which contains lineated vallis floor material) and segment 3, as well as Dao, Niger, and Harmakhis Valles. This suggests that segment 1 and the upper part of segment 2 of Reull Vallis have vallis floor material older (Lower Hesperian) than the other parts; or these segments appear to have been affected by infilling to a lesser degree than the lower part of segment 2 and segment 3. If lineated vallis floor material is assumed to be young relative to unlineated floor materials then this suggests that the lineated material may have been mobile in the geologically ‘recent’ times. Based on N(2) crater statistics, formation of Reull Vallis would have begun in the Lower Hesperian and may have preceded formation of Dao, Niger, and Harmakhis Valles. Other surficial materials, such as debris aprons and crater fill material, are not areally extensive and show low crater densities. The N(2) and N(5) values of these materials suggest that they are some of the youngest deposits in the study area, ranging from Upper Hesperian to Middle Amazonian for debris aprons

and from Upper Hesperian to Lower Amazonian for crater fill material. Crater fill material is found within several units that range in age and are interpreted to have various origins. It was observed that crater fill material resulting from erosion of crater walls and rims contained more craters on its surfaces than crater fill material formed by coalescing debris aprons or aeolian deposits; thus a wider range of ages for crater fill materials was expected. GEOLOGIC HISTORY

The basin–rim unit and mountainous material form highland terrains and are the oldest materials exposed in the map area. They are Noachian in age and formed as a result of repeated impacts during and following the period of heavy bombardment. A combination of fluvial, mass wasting, and aeolian processes have extensively degraded these units, depositing materials within low-lying intermontane areas. Fluvial activity, presumably during the Noachian Period or Early Hesperian Epoch, eroded highland materials and intermontane regions forming well-developed valley networks within small drainage basins. Valley networks and channels in the highlands and along the interior walls of large highland craters record a combination of sapping and runoff processes. Further degradation of the highlands occurred in the Late Hesperian and into the Early Amazonian with formation of mantled highlands and emplacement of pitted plains, with particular styles of modification concentrated geographically. The subdued appearance of craters and highland massifs and the relative lack of small craters in the highlands in the southeast part of the map area suggest that aeolian and mass wasting activity were long-lasting. Pitted plains has a surface texture similar to debris aprons, suggesting the influence of volatiles, possibly as ice-driven creep. The mantled highlands and pitted plains indicate that the highlands have been affected by processes that obscure small craters but preserve large, old craters. Mass wasting of debris from highland massifs and interior crater walls in the Amazonian Period formed debris aprons and some crater fill throughout the region. The effects of erosion and degradation on the highlands in the Reull Vallis region have greatly reduced their relief, redistributing materials in a variety of units east of the Hellas basin. The oldest preserved volcanic deposits in the Reull Vallis region are associated with Hadriaca and Tyrrhena Paterae, which formed in the Late Noachian to Early Hesperian Epochs. Channels dissecting the flanks of Hadriaca Patera indicate fluids eroded friable materials most likely by a combination of groundwater sapping and surface runoff. Crater statistics determined in the current study show that emplacement of the ridged plains of Hesperia Planum occurred in the Early Hesperian Epoch. Regional volcanic activity apparently ended in the Late Hesperian/ Early Amazonian with filling of Hadriaca Patera’s caldera and emplacement of the Tyrrhena Patera flank flow. The occurrence of the filled caldera and the flank flow supports the finding of

GEOLOGY OF THE REULL VALLIS REGION

Crown et al. (1992) that volcanic activity east of the Hellas basin underwent a transition from explosive to effusive during the Hesperian. Wrinkle ridges occur in many geologic units and exhibit various states of preservation. Ridges located in Hesperia Planum, the Tyrrhena Patera flank flow, and the smooth plateau unit are the most pristine, whereas ridges on the flanks of Hadriaca Patera, in the channeled plains rim unit, and in the dissected plains have a degraded appearance, and ridges in the smooth plains show subdued morphologies and may be buried. Crosscutting relationships with the source basin of Reull Vallis indicate that some ridge formation occurred in the Early Hesperian Epoch, prior to formation of Reull Vallis. Ridges in the Tyrrhena Patera flank flow may be significantly younger. Formation of ridges in the region may have been influenced by reactivation of Hellas-induced stresses. The fact that most ridges in the study area exhibit one of two main orthogonal trends and occur within units that have different ages suggests ridge formation occurred in at least two distinct events; however, multiple ridge-forming events may have occurred as numerous ridges that have no preferred orientation are observed within several other units. Modification and degradation of the highlands continued throughout the Hesperian Period. Emplacement of sedimentary materials, interbedded with volcanic deposits, formed the smooth plateau unit. The dissected plains formed as a result of extensive fluvial erosion of sedimentary and/or volcanic deposits. Superposition of the Tyrrhena Patera flank flow on dissected plains material and crater statistics indicates the dissected plains formed in the Late Noachian to Early Hesperian. Formation of the smooth plains appears to have begun in the Early Hesperian Epoch. Isolated exposures of the smooth plains could have formed as sediments were eroded from highland massifs and deposited in low-lying regions. The majority of the smooth plains, adjacent to Reull Vallis, is interpreted to have been emplaced by flooding of the canyon prior to formation of segment 3. Flat-topped, scarp-bounded mesas and filled craters within the channeled plains rim unit suggest that the smooth plains were more widespread than presently observed. Formation of the channeled plains rim unit appears to have begun in the Early Hesperian and continued into the Early Amazonian. Superposition relations show formation of the channeled plains rim unit predates emplacement of the Tyrrhena Patera flank flow. Some channels within the channeled plains rim unit originate at areas of collapsed plains, suggesting fluids were released from the subsurface, whereas other channels (south of Reull and Harmakhis Valles) have no definitive source and appear to have formed by surface flow, possibly by fluids from Reull Valles. Other channels in the channeled plains rim unit are truncated by Dao and Harmakhis Valles and appear to predate the outflow channels. Valles formation east of the Hellas basin appears to have begun in the Hesperian Period with Reull Vallis. Reull Vallis appears to have undergone a long and complex history, preserving little evidence of subsurface flow. Jumbled blocks in the source area

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of Reull Vallis suggest it may have been formed by collapse of ridged plains material and/or a volatile-rich unit. However, any further evidence of subsurface flow in Reull Vallis was removed and overprinted by features indicative of surface flow. Streamlined islands, scour marks, and braided gullies in the source area and the upper part of the main canyon show erosion of vallis floor material. Small-scale layering (tens to hundreds of meters thick) (Mest et al. 1998) in the upper part of the main canyon and in segment 3 suggests deposition of debris forming terraces, or erosion of vallis wall materials to form benches. Periodic flooding from Reull Vallis may have resulted in deposition of smooth plains material as overbank deposits. Flooding occurred as Reull Vallis became choked with debris, possibly by blockage of a connection with Harmakhis Vallis and/or infilling by mass wasting. The presence of a large side canyon at the junction of segments 2 and 3 may indicate a second source region for the fluids that formed segment 3. Release of fluids from the side canyon could have greatly enlarged segment 3, assuming it was originally similar in size to segment 2. Mesas, channels, and scour marks in the channeled plains rim unit indicate fluids from Reull Vallis could have modified the surrounding plains by eroding the smooth plains south of the canyon and flowing southwest towards the Hellas basin. The morphology of the lower part of segment 2 and segment 3 suggests that this part of the canyon also underwent modification by mass wasting processes, further enlarging the canyon. Formation of Dao, Niger, and Harmakhis Valles appears to have been less complex than Reull Vallis. Areas of collapsed and eroded plains materials indicate the canyons formed by a combination of subsurface and surface flow (Crown et al. 1992). The duration of vallis activity at Dao, Niger, and Harmakhis Valles may have been shorter than for Reull Vallis. Observation of collapse features in the channeled plains rim unit and the presence of large collapse basins at the heads of the highland outflow channels indicate subsurface flow and collapse of volatile-rich materials are common components in outflow channel formation. Numerous channels in the channeled plains rim unit are oriented parallel to Dao, Niger, and Harmakhis Valles; also small box-like tributary canyons are observed to extend from the highlands and join Reull Vallis. Some of these smaller channels may have been related to the initial stages of highland outflow channel formation, but changes in the distribution of volatiles in the highlands northeast of the Hellas basin or changes in the martian climate prevented their formation into outflow channels the size of Reull, Dao, Niger, and Harmakhis Valles. In this study, geologic mapping and geomorphic analyses were conducted to identify the geologic units and processes that operated in the Reull Vallis region. Crater statistics have provided limits on the timing and duration of these processes and constrained the geologic history of this region of the ancient cratered highlands. Volatiles have played a large role in the evolution of the Reull Vallis region, from formation of valley networks, small channels, and outflow channels by groundwater

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sapping and surface runoff, to emplacement of plains materials by fluvial deposition, and mass-wasted debris facilitated by interstitial water or ice. The recent observation of geologically “young” gullies in the walls of Dao Vallis and nearby highland craters by the Mars Orbiter Camera (MOC image numbers M0306266 and M03-07529) (Malin and Edgett 2000) supports the results of this study as well as others (e.g., Squyres et al.1987, Crown et al. 1992) that show groundwater has played a key role in the formation and modification of many of the features observed in the southern highlands. Future analysis and quantification of the morphometric properties of highland valley networks and the channeled flanks of Hadriaca Patera, with comparisons to terrestrial analogs, will better constrain the history of volatiles in the martian highlands. The results of this study can be supplemented and improved upon by incorporation of Mars Global Surveyor (MGS) data. Analysis of high-resolution images from the Mars Orbiter Camera (MOC) will allow geologic unit contacts to be refined and possibly lead to identification of new units within the Reull Vallis region. Mars Orbiter Laser Altimeter (MOLA) data will provide constraints on local and regional topography enabling identification of locations where fluids could have ponded and sediments accumulated. Thermal Emission Spectrometer (TES) data will provide information on the physical properties of surface materials to further refine knowledge of the types and distribution of geologic units in the region. APPENDIX: CRATER COUNTING METHODOLOGY Relative age information for the geologic units in the Reull Vallis region was derived by determining the number and size distribution of superposed impact craters for each geologic unit mapped (Tanaka 1986). Unit boundaries were digitized on a mosaic of twenty Mars Digital Image Mosaics of latitude −27.5◦ to −47.5◦ , longitude 245◦ to 270◦ , and unit areas were determined using NIH Image 1.6 software. Individual craters were counted on the mosaic MDIM base for each unit, and their diameters were measured and recorded in NIH Image 1.6 software. Craters <0.72 km diameter were not counted due to the limits of resolution of the MDIMs used. Table I summarizes the crater size–frequency distribution data for N(2), N(5), and N(16) for each geologic unit mapped in the Reull Vallis region; N(2), N(5), and N(16) represent the cumulative number of craters with diameters >2, 5, and 16 km/106 km2 , respectively (crater size–frequency errors = ±((N1/2 )/A) × (106 km2 )). The N(2), N(5), and N(16) data were then plotted for each unit; these data were used to determine the appropriate time-stratigraphic series.

ACKNOWLEDGMENTS The authors thank Robert Craddock and James Zimbelman for their support and assistance in obtaining topographic data of the outflow channels via photoclinometry while Scott C. Mest was a Smithsonian Graduate Fellow at the Center for Earth and Planetary Science. Thorough reviews by Baerbel Lucchitta and an anonymous reviewer significantly improved the manuscript. This research was supported by NASA Grant NAG5-3642.

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