MC–19

MC–3

MC–4 MC–5

MC–6

MC–10 MC–11 MC–12 MC–13

MC–1

MC–18 MC–20 MC–21

MC–25

MC–26 MC–27

MC–28

MC–30

300°

300°

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0°

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270° 315° 0° 45° 90°

–30°

–65°

0°

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–30°

–65°

Prepared for the National Aeronautics and Space Administration

Geologic Map of MTM –20012 and –25012 Quadrangles, Margaritifer 

Terra Region of Mars

By J.A. Grant, S.A. Wilson, C.M. Fortezzo, and D.A. Clark

Pamphlet to accompany

Scientific Investigations Map 3041

2009

U.S. Department of the Interior

U.S. Geological Survey

1

Introduction

Mars Transverse Mercator (MTM) –20012 and –25012 

quadrangles (lat 17.5°–27.5° S., long 345°–350° E.) cover a 

portion of Margaritifer Terra near the east end of Valles Marineris (fig. 1). The map area consists of a diverse assemblage of 

geologic surfaces including isolated knobs of rugged mountainous material, heavily cratered and dissected ancient highland 

material, a variety of plains materials, chaotic terrain materials, 

and one of the highest densities of preserved valleys and their 

associated deposits on the planet (Saunders, 1979; Baker, 1982; 

Phillips and others, 2000, 2001). The map area is centered on 

a degraded, partially filled, ~200-km-diameter impact structure 

(lat 22° S., long 347.5° E.), informally referred to as Paraná 

basin, located between Paraná Valles to the east and Loire 

Valles to the west (figs. 1, 2). Paraná Valles is a network of 

multidigitate, mostly east-west-oriented valleys that flowed 

west and discharged into Paraná basin (Grant, 1987, 2000; 

Grant and Parker, 2002). Loire Valles, broadly comparable 

in length to the Grand Canyon on Earth, has a deeply incised 

channel within the map area that originates at the west-northwest edge of Erythraeum Chaos within Paraná basin (Grant, 

1987, 2000; Grant and Parker, 2002; Strom and others, 2000). 

Paraná and Loire Valles, combined with Samara Valles to the 

west (fig. 1), form one of the most laterally extensive, wellintegrated valley networks on Mars (Grant, 2000) and record 

a long history of modification by fluvial processes. The origin 

and morphology of the valley networks, therefore, provide 

insight into past environmental conditions, whereas their relation with other landforms helps constrain the timing and role 

of fluvial processes in the evolution and modification of the 

Margaritifer Terra region.

MTM –20012 and –25012 are two quadrangles in a series 

of 1:500,000-scale study areas on Mars sponsored by the 

National Aeronautics and Space Administration (NASA) Planetary Geology and Geophysics Program grants (NAG5-4157 

and NAG5-10390). Photogeologic mapping utilizes a variety of 

image datasets including the Martian Digital Image Mosaic 2.1 

(MDIM 2.1) with a resolution of ~231 m/pixel as the base map 

(fig. 2), the Thermal Emission Imaging System (THEMIS) daytime (fig. 3) and nighttime (fig. 4) thermal infrared (IR) mosaics 

with resolutions of 230 m/pixel, THEMIS visible wavelength 

(VIS) images at ~20 m/pixel (fig. 5; table 1), and narrow angle 

(NA) images from the Mars Orbiter Camera (MOC) with resolutions of <10 m/pixel (fig. 5; table 1), as well as topographic 

data from the Mars Orbiter Laser Altimeter (MOLA) at 1/128 

degree/pixel or ~463 m/pixel (fig. 2). Crater size-frequency 

distributions of the map area (Grant, 1987) are used to constrain 

the relative ages of geologic units and the timing and duration 

of the inferred geologic events (fig. 6).

Regional Geology

Margaritifer Terra is approximately located within lat 

15°–25° S., long 325°–350° E. (Saunders, 1979). The oldest 

recognizable materials are the heavily cratered Noachian 

surfaces (Saunders, 1979; Scott and Tanaka, 1986) that include 

relief associated with the ancient, degraded Holden, Ladon, and 

Noachis multi-ringed impact basins (Saunders, 1979; Schultz 

and others, 1982; Grant, 2000; Frey and others, 2001; Grant 

and Parker, 2002). Multiple geomorphic events subsequently 

resurfaced portions of the region, creating a range of landforms 

and emplacing relatively competent materials inferred by the 

preservation of structural features such as wrinkle ridges, 

scarps, and faults. These materials date from the Early Noachian 

to the middle Hesperian and into the Amazonian and bury most 

of the oldest Noachian highland materials (Grant, 1987; Grant, 

2000; Grant and Parker, 2002). The first two resurfacing events 

affecting the map area were regional in extent and occurred 

during the Noachian, creating widespread surfaces of diverse 

morphology. The third geomorphic event occurred near the Late 

Noachian-Early Hesperian transition and may have continued 

into the Hesperian (Grant, 1987; Grant, 2000; Grant and Parker, 

2002), coincident with waning highland volcanism (Wise and 

others, 1979; Scott and Tanaka, 1986). During the Late Noachian and continuing into at least the Early Hesperian (Grant 

and Parker, 2002) or middle Hesperian (Rotto and Tanaka, 

1995), most valley networks and associated depositional sinks 

in the map area formed (fig. 1), creating one of the highest 

densities of valley networks preserved on Mars (Carr, 1981, 

1996; Baker, 1982). Finally, a more localized geomorphic event 

took place between the middle Hesperian and the Early Amazonian. This final geomorphic event emplaced materials that often 

partially fill topographic lows and typically embay the valley 

networks but may be locally incised in a few isolated locations 

(Grant, 2000; Grant and Parker, 2002).

Preserved valley networks and associated landforms in 

Margaritifer Terra record a complex history involving water 

transport, storage, and release, suggesting a paleoclimate that 

was capable of supporting water-driven erosional processes on 

the surface of Mars (Carr 1981, 1996; Mars Channel Working Group, 1983; Baker and others, 1992; Grant, 2000; Grant 

and Parker, 2002). Regionally, valleys and channels drain a 

north-sloping topographic depression, informally known as the 

Chryse trough. The axis of the Chryse trough extends northward 

from the Argyre impact basin and passes through Margaritifer 

Terra (fig. 1) into the northern lowlands of Chryse and Acidalia 

Planitiae north of the map area (Saunders, 1979; Baker, 1982; 

Phillips and others, 2000, 2001). Drainage on the west side of 

the Chryse trough is dominated by the informally named Uzboi, 

Ladon, and Morava (ULM) Valles, whose mesoscale outflow 

system (fig. 1) covers more than ~11 x 106 km2 (Banerdt, 2000; 

Phillips and others, 2001) and extends from the Argyre impact 

basin south of the map area to Ares Valles northwest of the map 

area (Parker, 1985; Grant and Parker, 2002). The ULM outflow 

system is characterized by incised trunk sections separated by 

depositional plains that fill ancient multi-ring impact basins 

(Saunders, 1979; Schultz and others, 1982) and is interrupted 

near the north end of Uzboi Valles by the ~150-km-diameter 

Holden crater. By contrast, the Samara and Himera Valles (SH) 

and the Paraná and Loire Valles (PL; fig. 7) networks, whose 

drainage basins cover roughly 540,000 km2 (Grant and Parker, 

2002), dominated drainage on the east side of the Chryse trough 

(including the map area, see fig. 1). The area covered by these 
2

watersheds is equivalent to ~85 percent of Earth’s Colorado 

River drainage basin. Drainage basins encompassing Paraná and 

Loire Valles in the map area are mostly defined by topography 

associated with impact craters and basins. As a result, they vary 

considerably in relative steepness, exhibiting relief ratios (basin 

relief divided by the length of the trunk valley within the basin) 

that range between 0.0001 and 0.13 (Grant, 2000). The ULM, 

SH, and PL outflow systems converge into the Margaritifer 

basin along the axis of the Chryse trough, immediately south of 

the Margaritifer Chaos and Ares Valles (Grant and Parker, 2002) 

to the northwest of the map area (fig. 1).

Paraná Valles drains the flank of the ~350-km-diameter 

Noachis basin along the east (Schultz and others, 1982; Grant, 

1987, 2000) and debouches into Paraná basin to the west (fig. 

1). The heads of the valley segments forming Paraná Valles 

generally occur at elevations of 1–2 km relative to the MOLA 

datum (Smith and others, 1999); are typically subparallel, variably flat-floored, and relatively invariant in width along their 

lengths; and yield drainage densities (total length of valleys 

divided by basin area) of 0.03–0.11 km/km2 (Grant, 2000; 

Grant and Parker, 2002). Although interior incised channels 

are generally not detected at map scale, a few channels, often 

muted in appearance or only apparent in thermal IR datasets, are 

preserved within the map area (fig. 8).

The process(es) that is responsible for the formation of the 

valley networks in Margaritifer Terra remains uncertain (Grant, 

2000; Grant and Parker, 2002; Luo, 2002). Some geomorphic 

characteristics of the valleys, such as low drainage densities, 

generally uniform valley widths and U-shaped cross sections, 

scale dependence, ruggedness numbers, and paucity of incised 

interfluves are consistent with a groundwater sapping mechanism (Grant and Parker, 2002; Luo, 2002). Other geomorphic 

characteristics of the valleys, however, such as their integrated 

dendritic form and tendency to head near basin divides, are 

more consistent with terrestrial-like fluvial activity driven by 

surface runoff (Grant and Parker, 2002; Luo, 2002). Previous 

studies and modeling of valleys in Margaritifer Terra indicate that their morphology and overall distribution are most 

consistent with precipitation-recharged groundwater sapping 

(Grant, 2000; Grant and Parker, 2002; Luo, 2002). This model 

for valley formation involves precipitation onto surfaces with 

relatively high infiltration capacities, thereby leading to limited 

runoff and initiation of valley formation through the recharge of 

aquifers that enable groundwater discharge. As groundwater is 

discharged, basal support along valley walls is reduced, resulting in sidewall collapse and the headward extension of valleys 

(for example, Pieri, 1976; Sharp and Malin, 1975; Howard, 

1988; Baker and Kochel, 1979; Baker, 1990; Malin and Edgett, 

2000; Gulick, 2001). Valley incision, primarily by groundwater 

sapping with variable contributions from runoff, imparts characteristic sapping-related morphometry to networks that often 

extend across encompassing basins and head near local divides. 

In locations where groundwater discharge is limited, such as 

along elevated drainage divides, the runoff component of incision may be more important. Finally, it is possible that some 

aspects of the sapping-related morphometry were imprinted 

during the final drawdown of an aquifer after cessation of precipitation and runoff.

Stratigraphy

The methodology used for mapping is consistent with the 

planetary mapping guidelines established by Ford and others 

(1993) and Tanaka and others (1994). In general, the geologic 

units in MTM –20012 and –25012 quadrangles are described 

as materials and are defined by differences in relative elevation, 

morphology, surface texture, relief, and relative ages. The relative ages of the geologic units were determined using stratigraphic relations, as well as numerous crater size-frequency distributions compiled from a regional study of Margaritifer Terra 

(Grant, 1987) that included counts from within the map area 

(fig. 6). The crater statistics include craters >1–2 km in diameter 

and were compiled from Viking Orbiter image mosaics possessing resolutions of ~200 m/pixel. The consideration of surfaces 

beyond the limits of the map area provides a regional context 

for the interpretation of local mapped surfaces (Grant, 1987), 

and the similarity between derived relative ages for surfaces 

within and outside of the map area supports the contention that 

geologic relations observed at the local map scale are reflected 

regionally.

The relative ages of the surfaces studied using crater 

statistics are based on N5 ages (number of craters >5 km in 

diameter per 1,000,000 km2) derived by comparison with the 

standardized crater curve from Neukum and Hiller (1981). 

Comparison with other standard crater curves (Hartmann and 

Neukum, 2001) yield broadly similar results. These crater sizefrequency distributions typically required minimal (~1–2 km) or 

no extrapolation to yield relative N5 ages for mapped surfaces. 

Derived relative ages are consistent with those from previous 

studies that include the map area (Saunders, 1979; Grant, 1987) 

and provide confidence that comparisons with units defined 

elsewhere on Mars (Scott and Tanaka, 1986; Greeley and Guest, 

1987) are valid, despite slightly different approaches (Tanaka, 

1986). A more detailed discussion of the mapping techniques 

and resulting mapped units can be found in Grant (1987, 2000) 

and Grant and Parker (2002).

The oldest units exposed in the map area are mountainous material (unit Nm) and the dissected unit of the plateau 

sequence (unit Npld), which include surfaces eroded during 

the first two resurfacing events associated with increased 

geomorphic activity (Grant, 1987, 2000; Grant and Parker, 

2002). Mountainous material is characterized by large, rugged, 

raised-relief blocks interpreted to be ancient crustal materials 

uplifted during multi-ring, impact-basin formation (Greeley 

and Guest, 1987). The few occurrences of mountainous material (~lat 19.5° S., ~long 349° E.) are topographically isolated 

knobs, typically surrounded by the younger dissected unit. The 

dissected unit, previously mapped by Scott and Tanaka (1986) 

and Greeley and Guest (1987), is prevalent throughout the map 

area. This moderate-relief surface has undergone extensive 

impact gardening, or eolian reworking, and is often dissected by 

valley networks. The dissected unit likely consists of a variety 

of materials including volcanic flows, pyroclastic materials, and 

impact breccia that formed during the period of heavy bombardment (Scott and Tanaka, 1986; Greeley and Guest, 1987). The 

modified nature of these ancient materials, likely produced by 
3

multiple geomorphic processes, often precludes assignment of a 

more definitive origin.

The bright plains materials (unit ANb) are confined to the 

floors of some of the larger craters in the map area. Materials 

forming these surfaces are rough to locally knobby on scales of 

meters to tens of meters (fig. 9) and appear anomalously bright 

in THEMIS nighttime data (fig. 4). Crater statistics cannot be 

used to accurately determine an age of the bright plains due 

to small surface areas and the presence of only a few, small, 

degraded craters. The degraded state of most craters that contain 

bright plains, however, coupled with their relation to adjacent 

units, suggests that the true emplacement age of the bright 

plains spans a considerable portion of Martian history from the 

Noachian to the Amazonian.

There are relatively few valleys incising the walls of 

craters containing bright plains materials, thereby making an 

alluvial origin unlikely. Similarly, the lack of nearby volcanic 

constructs argues against a volcanic origin. The knobby and 

stripped appearance of the surface coupled with the bright 

nighttime temperatures is inconsistent with mantling by eolian 

materials, and impact ejecta superposing the bright plains (for 

example, in Novara crater) displays a lower relative brightness. 

Hence, the bright plains materials are likely indurated and may 

be exposures of bedrock rather than mantled or blocky surfaces 

produced by a range of geomorphic processes. Any process 

invoked to explain the bright plains must be relatively independent of time, because the deposits occur in craters covering a 

range of sizes and could have formed episodically over much 

of Martian history. An origin as bedrock impact melt on crater 

floors that has been more recently exposed by eolian stripping 

is consistent with the bright plains occurrence. Deflation and 

stripping of any fines to expose the impact melts would have 

proceeded efficiently in the absence of any coarser-grained 

mantling deposit associated with extensive ejecta deposits or 

alluvium, as seen in other craters.

The older plains material (unit HNp) occurs as widespread, relatively low-relief, and lightly cratered plains surfaces. Mapped older plains surfaces are usually topographically 

lower than the dissected unit of the plateau sequence (unit 

Npld) and were previously described as being comprised of 

deposits emplaced during a resurfacing event of unspecified 

origin (Grant 1987, 2000; Grant and Parker, 2002). Older plains 

material comprises the youngest extensively dissected surfaces 

in the map area, and its emplacement predates incision of the 

vast majority of the numerous preserved valley networks (units 

HNvn, AHvn) and evolution of the ULM outflow system to the 

west (fig. 1). The older plains material was emplaced during 

a regional event that began at N5≈320 and ended at N5≈170 

(Grant, 1987; fig. 6). This corresponds with the Noachian to 

Hesperian transition and is roughly coincident with waning 

highland volcanism (Scott and Tanaka, 1986). Mapped occurrences of the older plains material appears competent and 

preserves wrinkle ridges (typically displaying a north-south orientation), faults, and scarps. The older plains are often modified 

by incision, partially buried by younger plains material (unit 

AHp) and other materials (impact crater ejecta and accumulations of eolian drift), and preserve a paucity of landforms diagnostic of a particular emplacement process(es). Nevertheless, 

the association with valleys and the dearth of nearby volcanic 

constructs suggest that the materials represent a mixture of fluvial/alluvial deposits and colluvium, possibly including contributions from volcanic and (or) impact materials.

Drainage in the map area is characterized by multidigitate, 

parallel to dendritic valley networks (unit HNvn) associated 

with the PL valley system. The proximal reaches of Loire Valles 

outside of Paraná basin deeply incise the older plains material (unit HNp), and the dense, well-integrated, small valleynetwork tributaries near the outlet to Paraná basin impart a 

crenulated expression to the landscape in plan view (fig. 7). The 

deeply incised assemblage of tributary valleys in Loire Valles’ 

proximal reach likely evolved in response to local downcutting 

as a steeper gradient along the medial reach translated eastward 

into Paraná basin due to headward extension of the trunk valley. 

Local changes in slope associated with downcutting around the 

margin of Paraná basin could have resulted in piracy of some 

originally eastward-draining tributaries to Paraná basin, and 

the diversion of these segments may have contributed to the 

observed crenulated surface morphology. The head of Loire 

Valles continues well into Paraná basin but becomes poorly 

defined where it encounters younger chaotic terrain material 

(unit AHct). The floors of the highly degraded valleys within the 

map area are typically covered by light-toned transverse eolian 

ridges (fig. 8; Wilson and Zimbelman, 2004) that correlate to 

low-brightness temperatures in THEMIS nighttime IR data 

(Mellon and others, 2000; Christensen and others, 2003). Rare 

occurrences of channels, however, are preserved in some older 

valley-network materials in Paraná Valles (fig. 8) and the surrounding area.

The formation of the valley network material (unit HNvn) 

associated with Paraná and Loire Valles began in the Late 

Noachian (N5≈230), likely extended well into the Hesperian 

(N5≈100; fig. 6), and was approximately contemporaneous with 

widespread valley formation and degradation elsewhere on Mars 

(Grant and Schultz, 1990, 1993; Grant, 2000). The timing of the 

vast majority of valley and channel formation in the map area is 

constrained by stratigraphic relations with the bulk of the plains 

materials associated with the last two resurfacing events: the 

valleys incise the older plains material (unit HNp) but are almost 

always embayed by the younger plains material (unit AHp). 

A set of crater statistics from the chaotic terrain material (unit 

AHct), which partially fills Paraná basin (fig. 6), and the partial 

burial of nearby portions of Loire and Samara Valles by ejecta 

deposits associated with Jones crater, located approximately 300 

km west of the map area (lat 18.9° S., long 340.1° E.; fig. 1), 

support the contention that widespread valley formation occurred 

from the Late Noachian into the middle Hesperian.

Five isolated valleys in the map area may postdate the 

major interval of valley formation and are mapped as younger 

valley network material (unit AHvn). Three of these valleys are 

north of Erythraeum Chaos in Paraná basin and may cut into 

younger plains material (unit AHp; lat 20.57° S., long 347.32° 

E.; lat 20.85° S., long 348.32° E.; lat 20.91° S., long 348.34° 

E.). The other two valleys occur as potentially rejuvenated 

segments of older inlet and outlet valleys on the rim of a highly 

eroded, 30-km-diameter crater (fig. 10) south of Paraná basin 

(for example, lat 23.84° S., long 347.93° E.). All of these val-
4

leys lack well-developed tributaries and the three north of the 

chaos in Paraná basin display a muted appearance suggesting 

that they may be the partially buried remnants of older valleys. 

The potentially younger valley segments flowing into and out of 

the unnamed crater (fig. 10) display sharply defined walls and 

prominent theater-head morphology in contrast to the apparently older valleys that they incise.

The three young valleys (unit AHvn) north of Erythraeum 

Chaos in Paraná basin head at elevations of -1,130 m, -1,067 m, 

and -1,086 m from west to east, whereas the young valley segment flowing into the unnamed crater heads at -348 m relative to 

the MOLA datum. The stubby appearance of the valleys north 

of the chaos materials combined with their lack of well-defined 

tributaries and the fairly uniform elevation at which they begin 

may be consistent with a groundwater sapping origin, perhaps 

in response to final drawdown of a regional aquifer or limited 

release of groundwater in response to local tectonic or poorly 

recorded volcanic activity. Similar attributes of the young valleys 

associated with the unnamed crater coupled with their theaterhead morphology also point to possible incisement in response to 

groundwater discharge. Determination of the age of these valleys 

is inhibited by their small area and the fact that their bounding 

surfaces do not appear to represent distinct units that could be 

dated. Regardless of whether these isolated valleys are younger 

than most in the map region, their limited number and extent 

argues against a late, widespread period of valley formation.

Crater floors located near the west edge of the map area 

(lat 24.8° S., long 345.1° E.) and on the south margin of Paraná 

basin (lat 23.5° S., long 347.75° E.) contain deposits of fan 

materials (units AHf and (or) HNf). The fan material in the 

crater on the west edge of the map area originates at the termini 

of numerous valleys that incise the crater walls. Most of the fans 

exhibit steep fronts, suggesting that shallow water occasionally ponded on the crater floor during fan emplacement, which 

enabled the transport and deposition of fine-grained sediment 

onto the crater floor. The absence of valleys heading very far 

outside the rim of this crater suggests that the fans mostly 

consist of crater rim and wall material. Although the small size 

of the fans precludes accurate age dating by crater statistics, 

the degraded morphology of their source valleys is consistent 

with valleys mapped as HNvn, thereby yielding a HesperianNoachian age designation for the deposits (unit HNf).

By contrast, the large (165 km2), multilevel fan in the 

crater on the south margin of Paraná basin is more complex 

and likely records two periods of deposition (fig. 10). Older fan 

material (unit HNf) is associated with degraded valley networks 

(unit HNvn) that head well beyond the rim of the crater. These 

valleys likely transported and deposited material onto the crater 

floor from the surrounding plains, as well as from the crater 

walls and rim. The older fan materials, which cover most of the 

crater floor, were predominantly deposited at the terminus of 

the inlet valley on the east rim and had fewer contributions from 

valleys dissecting the west rim. As water ponded in the crater, 

relatively high-standing fans were deposited near the main 

entrance breach. During periods of ponded water, distal deposition of fine-grained sediments likely occurred. Water levels in 

the crater decreased and deposition waned when the north rim 

was breached and water flowed north into Paraná basin. Lobate 

margins and sinuous channels from this earlier depositional 

period are preserved on the eastern crater floor.

Later discharge into the crater via a potentially younger, 

rejuvenated, theater-headed valley segment (unit AHvn) within 

the older inlet valley (unit HNvn) appears to be responsible for a 

limited volume of younger fan deposits (unit AHf). The younger, 

medium- to light-toned fan materials likely incorporated material 

from the surrounding plains, as well as from the crater wall and 

rim, and locally incise or bury the older fan materials (unit HNf). 

Depositional features such as channels and multiple small lobes 

are preserved (fig. 10). No materials can be confidently defined 

as either period of discharge outside of the crater, but association 

with the etched plains material (AHe) along the south margin of 

Paraná basin cannot be definitively ruled out.

The setting associated with the fan materials in both craters 

and their association with valleys, channels, and preserved 

evidence for multiple depositional events (as seen in the crater 

south of Paraná basin) implies an origin by alluvial processes 

similar to those responsible for deltaic deposits observed 

elsewhere on Mars (Malin and Edgett, 2003; Moore and others, 

2003), thereby providing support for ancient sustained fluvial 

activity in the map area.

Portions of the map area are mapped as chaotic terrain 

material (unit AHct), which is characterized by closely spaced, 

rounded to flat elongate plateaus, buttes, knobs, and linear 

ridges (fig. 11). The largest exposure of these materials occurs 

as Erythraeum Chaos in Paraná basin, which extends west to the 

head of Loire Valles and east to Paraná Valles, covering an area 

of ~7000 km2 (fig. 11). Sets of somewhat parallel mesas and 

plateaus are typically separated by troughs as much as ~100-

200 m deep in Paraná basin but only tens of meters deep farther 

to the north and east (for example, lat 18.7° S., long 349.2° E.) 

and as much as 275 m deep elsewhere (for example, lat 23.7° 

S., long 346.7° E.). Troughs between plateaus often approximate a roughly east-west orientation (varies by ~45 degrees 

from occurrence to occurrence) but are occasionally interrupted 

by troughs with a more north-south orientation. The floors of 

the troughs are often covered with light-toned eolian ridges, 

which result in a low- to intermediate-brightness temperature 

in THEMIS nighttime data (fig. 12). Some plateau surfaces are 

smooth at scales of tens to hundreds of meters, but most appear 

stripped and expose apparently more resistant material. The 

plateaus exhibit intermediate- to high-brightness temperatures 

in THEMIS nighttime infrared IR data and are often crossed by 

low, narrow, intersecting ridges (fig. 12). A few wrinkle ridges 

possessing roughly north-south orientations traverse the unit. 

The chaotic terrain materials sometimes occur in depressions 

that are bound along their margins by discontinuous troughs 

(generally 50–100 m deep and 3–7 km wide, as seen in Paraná 

basin; fig. 11), whereas in other locations the materials rise 

above the level of the surrounding plains (for example, lat 23.7° 

S., long 346.7° E.). Although this chaotic terrain material was 

previously mapped by Grant (1987) as a positive-relief chaos 

unit, new orbital data sets permit a more detailed and somewhat 

different description and interpretation.

Although the volume of the chaos materials is difficult to 

determine, the apparent thickness of nearby units and partially 

buried craters within the chaos provides loose constraints on the 
5

thickness of the materials. Based on these parameters, Erythraeum Chaos in Paraná basin alone may represent over 20,000 

km3 of material (Grant and Parker, 2002). Alternate methods 

may yield different estimates of thickness and (or) volume, 

but like the method employed, are fraught with uncertainty. 

Therefore, the approximate volume, which should be viewed as 

an order of magnitude estimate, implies there is a considerable 

amount of chaotic terrain material.

In some locations, the chaotic materials appear cut by adjacent younger, etched, and bright plains materials (units AHp, 

AHe, and ANb, respectively) but, in other locations, may embay 

them. A single set of crater statistics from Erythraeum Chaos 

yields an N5 age of just over 100, which is consistent with the 

initial formation beginning around the end of the major interval 

of valley formation (fig. 6). Hence, the present expression of the 

chaotic materials likely postdates emplacement of the bulk of 

the valley network materials (unit HNvn) and may be younger 

than the younger plains materials (units AHp, AHe, ANb).

A variety of hypotheses have been proposed to explain the 

development of chaotic terrains on Mars. Many involve removal 

of material and (or) water at varying rates, resulting in subsequent collapse (Carr, 1979; Rodriguez and others, 2003, 2004; 

Grant and Parker, 2002; Harrison and Grimm, 2008), whereas 

others involve sedimentary diapirism and mud volcanism (Skinner and Tanaka, 2007). Although some occurrences of chaotic 

materials in the map area appear morphologically similar to 

chaos elsewhere on Mars, others are unique and may involve a 

combination of processes in their evolution.

The location of many of these chaos materials in basins, 

the occurrence of blocks in some locales that rise above the 

surrounding plains, and a generally stripped appearance provide 

insight into the processes responsible for the formation of this 

material. Because the chaotic-terrain materials often occur near 

the distal margins of valleys (for example, Paraná basin), they 

likely contain substantial fluvial and alluvial sediments and 

may have incorporated significant amounts of ice. One possible 

formation scenario that may be consistent with the range in 

attributes involves initial concentration of volatiles and lifting 

of the surface due to expansion of ice and (or) possible diapirism (Skinner and Tanaka, 2007). This process accounts for 

the blocky nature of the deposits, occurrence of some blocks 

that rise above the surrounding plains, and presence of bounding troughs in some locations. Concurrent and (or) subsequent 

eolian stripping of such a surface would preferentially remove 

fines and materials not welded by ice, thereby producing the 

stripped appearance and erosion of troughs between blocks. 

This implies that the blocks consist of resistant materials, 

perhaps due to the incorporation of coarser grained sediments 

and (or) welding by ice. The presence of eolian dunes or ripples 

in some troughs between blocks within the chaos indicates that 

eolian processes were active in the past, which is consistent 

with this model (fig. 12). Moreover, small dikes of apparently 

resistant material throughout the deposits could mark locations 

where limited water moving through fractures in the material 

resulted in alteration and local diagenesis (fig. 12).

The etched plains material (unit AHe), mapped around 

the margins of Paraná basin, express relatively high local relief 

(over distances of tens to hundreds of meters) and are sometimes bounded by lobate margins (fig. 13). The etched plains 

are interpreted as consisting of alluvial materials, similar to 

those comprising the chaos materials (unit AHct), but are either 

coarser grained or have greater or lesser ice content relative to 

the chaos. Hence, the emplacement age of the etched plains 

materials may be similar to that of the adjacent chaos and probably occurred during the Hesperian or even Late Noachian. The 

exposure age of the current surface, however, is much younger 

as evidenced by the small number of craters preserved on local 

surfaces. Although outcrops of the etched plains material cover 

an insufficient area for reliable relative age determination, the 

exposure age of the materials is estimated to be Hesperian to 

Amazonian based on the inferred relations to adjacent materials 

and the small number of craters.

If the etched plains material were more ice rich than the 

adjacent chaotic terrain material, then blocks comparable to 

those in the chaos might be expected. If there were sufficient ice 

present to effectively lithify the material, but not lead to concentration of ice and (or) diapirism (as described for the chaos 

materials), then the material may be more uniform in properties and eolian stripping could yield the observed etched, rather 

than blocky, appearance. The lobate margin around some etched 

plains may reflect the limit of the alluvial deposits or, perhaps, 

a transition to less ice-rich materials. Because much of the 

etched-plains materials are equally distant from or more distal 

to major valleys in the region (for example, Paraná Valles) than 

the chaos materials, it is unlikely that the etched plains would be 

coarser-grained than the chaos materials. The complex patterns 

and associated relief exposed by differential stripping of the 

surface appears consistent with a fluvial/alluvial depositional 

environment (fig. 13).

The younger plains material (unit AHp), previously interpreted as having been formed by a localized resurfacing event 

by Grant (1987, 2000) and Grant and Parker (2002), partially 

fills topographic lows and usually embays valleys (for example, 

25.75° S., long 347.71° E.). The geomorphic event responsible for these materials began in the Early to Late Hesperian 

(N5≈130) and may have extended into the Early Amazonian 

(N5≈45). The surface is relatively low relief over distances 

of tens of kilometers but is punctuated by wrinkle ridges and 

sometimes exhibits moderate to high THEMIS brightness temperatures (fig. 4). The thickness of this unit is largely unknown, 

but partial burial of valleys in some places (for example, lat 

25.5° S., long 348° E.) implies thinning towards the margins. 

Such relationships imply minimum thicknesses of roughly hundreds of meters to perhaps a kilometer or more to account for 

burial of relief associated with craters on underlying surfaces.

Much like the older plains material (unit HNp), the 

younger plains material may have been emplaced by a range of 

processes. Volcanic edifices in the immediate proximity are not 

recognized, and the nearest possible volcano (~lat 14° S., ~long 

338° E.) is Amazonian (Williams and others, 2005) and located 

roughly 700 km northwest of the map area (fig. 1). As a result, 

it appears unlikely that the younger plains material is volcanic, 

although this possibility cannot be ruled out (Milam and others, 

2003). The limited number and extent of valley materials incising the younger plains argues against a purely alluvial origin. 

Perhaps most likely, the younger plains reflect reworking of 
6

older alluvial materials (emplaced as HNvn) by a variety of 

processes that may include eolian, colluvial, and limited alluvial 

contributions. Any of these processes could account for the 

ambiguous appearance of the younger plains material, and many 

could result in a variable volatile content. The occurrence of 

wrinkle ridges on the unit implies a degree of competence.

Impact Craters and Structural Features

All impact craters greater than 2 km in diameter were 

considered in mapping and display a range of morphologies. 

Pristine craters (unit c3) generally have ejecta blankets and continuous, elevated rims and are not filled by the plains materials 

(units AHp, ANb). One 8-km-diameter pristine crater located at 

lat 19.3° S., long 348.73° E. has preserved ejecta rays that are 

visible in THEMIS nighttime IR data (fig. 4). The properties 

of this crater indicate that it is likely the youngest crater larger 

than 5 km in diameter in the map area. Most craters larger than 

15 km in diameter in the map area display flat floors, terraced 

walls, and sometimes central peaks/pits and are classified as 

complex craters (Melosh, 1989). Most of these craters are moderately degraded (unit c2), displaying subdued crater rims, flat, 

filled floors, and occasionally partially buried or eroded ejecta 

blankets. The few examples of highly degraded craters (unit 

c1) within the map area have a discontinuous rim that has little 

relief relative to the surrounding materials, featureless crater 

floors, and no mapped ejecta.

Structural features in the map area include relief associated 

with ancient, multi-ring impact basins and numerous mare-type 

wrinkle ridges, scarps, and faults (Grant, 1987). The wrinkle 

ridges are generally oriented north-south and occur across most 

units in the map area, including the mountainous material, the 

dissected unit of the plateau sequence, and the older and younger 

plains materials (units Nm, Npld, HNp, and AHp, respectively). 

These broad arching rises topped with narrow, crenulated ridges 

are interpreted to be folded and (or) faulted strata that likely 

formed as a result of compressional stresses related to the formation of the Tharsis rise to the west (Grant, 1987). The absence of 

significant numbers of ridges with alternate orientations implies 

little local deformation due to subsidence or filling of basins 

within the map area. Deformation of units as young as the Amazonion implies that uplift and deformation caused by Tharsis was 

ongoing until at least that time.

Mass-wasting Materials

Although mass wasting has undoubtedly contributed to 

ongoing modification of some of the higher gradient crater walls 

and other similarly sloped surfaces, discrete outcrops of material 

associated with this activity are rare. One example of mass-wasting material (unit AHmw) in the map area occurs as a landslide 

deposit near the base of a crater wall (lat 24.5° S., long 345.18° 

E.) that superposes bright plains material (unit ANb). The surface of the deposit is somewhat degraded and primary landforms 

are difficult to resolve. Given the lack of information regarding 

the nature of the constituent material, it is impossible to evaluate when it formed or how quickly it may have been modified. 

The small size, however, relative to other modified landforms of 

comparable size that have better constrained ages (for example, 

plateaus in unit AHct), implies that the mass-wasting materials 

were emplaced during the Hesperian or Amazonian.

Conclusions

The map area covering MTM –20012 and –25012 records 

a long history of geomorphic activity that is punctuated by a 

period of widespread valley formation and emplacement of 

associated alluvial materials. In contrast to companion quadrangles MTM –10022 and –15022 mapped to the northwest 

(Williams and others, 2005), where volcanic and water-related 

processes may have continued into the Early Amazonian, the 

majority of geomorphic activity within the map area came to 

a gradual end during the middle Hesperian. Independently, 

preserved valleys appear to have formed by the early to middle 

Hesperian. Five small valleys are exposed on younger surfaces, 

but it is unclear whether they are the modified (partially buried) 

remains of older valley segments or whether they represent a 

very minor period of late activity, perhaps associated with drawdown of local/regional aquifers.

Geologic activity persisting into the Late Hesperian or 

even the Early Amazonian is mostly associated with the evolution of possibly volatile-rich chaotic terrain material (unit 

AHct), deflation and evolution of nearby etched plains (unit 

AHe), exhumation of the bright plains within some large craters 

(unit ANb), and emplacement of the younger plains material 

(unit AHp). The younger plains material is of unknown origin 

but may be the result of a mix of alluvial, volcanic, eolian, and 

other processes.

The source of water responsible for widespread valley incision and related alluvial activity in Margaritifer Terra remains 

uncertain; associated morphometry suggests precipitationrecharged groundwater sapping is a likely candidate (Grant, 

2000; Grant and Parker, 2002). The timing and distribution of 

map units indicates valley-forming activity was approximately 

concurrent with enhanced geomorphic activity occurring elsewhere on Mars (Grant and Schultz, 1990), and the widespread 

occurrence (laterally and with elevation) of map units suggests 

formation was most likely related to both regionally distinct 

intervals of runoff and groundwater discharge caused by local 

endogenic processes. Exceptions may include the five, small, 

possibly younger valleys mapped in and along the south margin 

of Paraná basin. Formation of the vast majority of the valleys 

likely required some type of precipitation over broad surfaces, 

either as rainfall or snowfall, enabling recharge of subsurface 

aquifers contributing to valley formation.

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9

Table 1. Released THEMIS visible (VIS) images and narrow-angle (NA) images from the

Mars Orbiter Camera (MOC) as of June, 2007. Numbers correspond to footprints in Figure 5.

No. Image ID No. Image ID No. Image ID No. Image ID No. Image ID

THEMIS VIS IMAGES

1 V18199002 26 V15416005 51 V02298003 76 V09563004 101 V18099002 

2 V15104003 27 V18174004 52 V15391001 77 V06792001 102 V17787004 

3 V14480003 28 V16327005 53 V17837005 78 V17887002 103 V09800004 

4 V17837004 29 V17238003 54 V17525004 79 V17575005 104 V14405005 

5 V13831003 30 V10786002 55 V06767001 80 V18174005 105 V14505004 

6 V14218001 31 V15079003 56 V00850003 81 V18199004 106 V04957003 

7 V08702003 32 V18748003 57 V16614006 82 V14817005 107 V15391002 

8 V09563003 33 V10449003 58 V16302005 83 V19110003 108 V17837006 

9 V16614005 34 V06742003 59 V19060005 84 V16040002 109 V18149004 

10 V06405001 35 V15653003 60 V02273004 85 V17862003 110 V18748005 

11 V17288001 36 V10474003 61 V16589003 86 V19085003 111 V17812004 

12 V11148002 37 V15990001 62 V14742004 87 V16926003 112 V17500004 

13 V09301003 38 V15054005 63 V15965004 88 V15079004 113 V06405002 

14 V16976002 39 V09825003 64 V04495003 89 V14767002 

15 V15129003 40 V07853004 65 V18411003 90 V09538003 

16 V06480002 41 V05993003 66 V15029003 91 V17213005 

17 V18199003 42 V15341003 67 V10449004 92 V16901004 

18 V10212003 43 V10811003 68 V16277004 93 V15990002 

19 V17575004 44 V14792002 69 V00825005 94 V15678002 

20 V10836002 45 V08964003 70 V07104003 95 V16589004 

21 V01986004 46 V04570004 71 V16876003 96 V09825004 

22 V14505003 47 V18461004 72 V06717003 97 V15965005 

23 V19110002 48 V16639003 73 V17263001 98 V17188005 

24 V06455003 49 V14480004 74 V16951005 99 V15341004 

25 V16040001 50 V08265003 75 V15104004 100 V14717005 

25 V16040001 50 V08265003 75 V15104004 100 V14717005 

MOC NA IMAGES

1 R1302697 16 M0202900 31 R0903303 46 M0100022 61 E0200627 

2 E0202137 17 E1000049 32 M0900897 47 R1203258 62 R0100031 

3 M1900530 18 M0703016 33 E0202847 48 R1104600 63 M0902686 

4 S1902225 19 M0804918 34 E1302176 49 M2100948 64 M1302167 

5 R1601195 20 M2200445 35 E1001475 50 M2101437 65 E1401984 

6 M1300695 21 S0703153 36 E1101913 51 S1902063 66 E1000329 

7 M0101050 22 M1900918 37 E1601354 52 S1802174 67 S1303059 

8 R1002690 23 E2001127 38 S1300129 53 S0703151 68 E2300171 

9 E1002687 24 E1103854 39 M0302255 54 M0402189 69 R0101145 

10 S0601978 25 R0601939 40 M0401148 55 E0400485 70 S0701488 

11 S1800364 26 E1500848 41 M0101441 56 R1201040 71 M0806740 

12 M0802792 27 M0706141 42 M0203370 57 E1102972 72 E1400604 

13 S0501623 28 M0002197 43 M1700616 58 E2301077 73 E0503227 

14 E2300068 29 M0200803 44 R1103831 59 E1601908 74 R0200723 

15 FHA00475 30 E0503229 45 FHA00619 60 M0801905 75 S1200033