2014

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Bedrock Geologic and Structural Map Through the 

Western Candor Colles Region of Mars

By Chris H. Okubo

Pamphlet to accompany

Scientific Investigations Map 3309

View of the map area looking toward the northeast, with the Candor Colles (low hills) in the foreground. HiRISE (High 

Resolution Imaging Science Experiment) observation PSP_001641_1735 overlain on the HiRISE DEM (digital elevation 

model).

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Contents

Introduction.....................................................................................................................................................1

Map Base and Data.......................................................................................................................................1

Methodology...................................................................................................................................................2

Styles of Deformation....................................................................................................................................4

Age Determinations.......................................................................................................................................4

Geologic History.............................................................................................................................................5

References Cited............................................................................................................................................7

Figures

1. Location of the map area and other geographic features in west Candor Chasma............ sheet 

2. Overview of the western Candor Colles region. .....................................................................sheet 

3. Stereonet plots showing layer and fault orientations within map area................................sheet 

4. Image data (A) and mapped structure and topographic contours (B) of an isolated

 collis within the map area ...............................................................................................sheet

5. Contact between units CeMk, CaC, and Fault Zone IV.............................................................. sheet

6. Cross section A–A’ showing the interpreted subsurface structure based on

 measured orientations of bedding, unit contacts, and faults. ........................................ sheet

7. Image data (A) and annotated features (B) of a group of colles along fault zone I. ...........sheet

Tables

1. HiRISE observations used during mapping, Candor Colles region of Mars. ..................................2 

2. CTX observations used during mapping, Candor Colles region of Mars......................................2

3. Unit stratigraphic relations, Candor Colles region of Mars. ..........................................................3
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1

Introduction

The Candor Colles are a population of low, conical hills 

along the southeast flank of Ceti Mensa, in west Candor 

Chasma, within the Valles Marineris system of Mars (fig. 1, on 

sheet). Ceti Mensa and the adjacent Candor Mensa are mounds 

of layered sedimentary deposits and are the most prominent 

landforms within west Candor Chasma. Prior to the arrival of 

the Mars Reconnaissance Orbiter (MRO) in orbit around Mars 

in 2006 (Zurek and Smrekar, 2007), geologic maps of the area 

utilized the relatively low resolution Viking Orbiter photomosaics (20–150 m/pixel). Geologic maps covering west Candor 

Chasma were created at scales of 1:15,000,000 for the western 

equatorial region of Mars (Scott and Tanaka, 1986), 1:2,000,000 

for the Valles Marineris region (Witbeck and others, 1991), and 

1:500,000 for the far eastern part of west Candor Chasma (Mars 

Transverse Mercator quadrangle–05072; Lucchitta, 1999).

Previous structural mapping in west Candor Chasma at 

scales of less than 1:24,000 (Okubo and others, 2008; Okubo, 

2010) employed digital terrain models (DTMs), with 1-m post 

spacings, derived from stereo MRO High Resolution Imaging 

Science Experiment (HiRISE) imagery (McEwen and others, 

2010) and focused on examining the relative timing between 

deposition of the youngest unit of the layered deposits in 

this area (unit Avme of Witbeck and others, 1991) relative to 

regional faulting related to chasma formation. These previous 

mapping efforts on the southwest flank of Ceti Mensa demonstrated that unit Avme is not deformed by faults attributed to 

formation of the chasma. Studies of other layered deposits (primarily unit Hvl, but also including units Avme, Avsl, Avsd, and 

Avfs; Witbeck and others, 1991) exposed along the southeast 

flank of Ceti Mensa using a High-Resolution Stereo Camera 

(HRSC) digital terrain model (50 m/pixel) refined the local 

stratigraphy and revealed evidence for syntectonic deposition 

of these deposits (Fueten and others, 2006, 2008; Jaumann and 

others, 2007; Birnie and others, 2012).

Layered deposits such as those that constitute Ceti Mensa 

are widespread throughout the interior regions of Valles 

Marineris (Witbeck and others, 1991). These sedimentary 

deposits have been variously interpreted as eolian sediments 

(Nedell and others, 1987), hyaloclastic debris (Chapman and 

Tanaka, 2001; Komatsu and others, 2004), lacustrine or fluvial 

sediment (Dromart and others, 2007; Mangold and others, 2008; 

Metz and others, 2009), pyroclastic deposits (Hynek and others, 

2003), evaporites (Mangold and others, 2008; Andrews-Hanna 

and others, 2010), or various combinations thereof.

Recent analysis of data from the MRO Compact 

Reconnaissance Imaging Spectrometer for Mars (CRISM) 

shows that these sediments consist primarily of hydrated 

sulfates (Murchie and others, 2009a,b). Further, hydrologic 

modeling indicates that spring-fed lakes likely occurred within 

the chasma (Andrews-Hanna and others, 2010). These recent 

findings point to a scenario in which the layered deposits accumulated as sequences of evaporites precipitating in hypersaline lakes, with contemporaneous trapping of eolian dust and 

sand, diagenesis, and iron-cycling, interspersed with periods of 

eolian and fluvial erosion (Murchie and others, 2009a). Water 

vapor released from these lakes may have also driven localized 

precipitation of snow and accumulation of layered deposits on 

the adjacent plateaus (Kite and others, 2011a,b). This scenario 

is in contrast to recent alternative interpretations that the layered 

deposits formed within the chasma through weathering of dustrich ice deposits (Niles and Michalski, 2009; Michalski and 

Niles, 2012).

The structure and geology of the layered deposits in the 

Candor Colles region corresponding to units Avfs, Avme, 

and Hvl of Witbeck and others (1991) are reevaluated in this 

1:18,000-scale map. The objectives herein are to gather highresolution structural measurements to (1) refine the previous 

unit boundaries in this area established by Witbeck and others 

(1991), (2) revise the local stratigraphy where necessary, (3) 

characterize bed forms to help constrain depositional processes, 

and (4) determine the styles and extent of deformation to better 

inform reconstructions of the local post-depositional geologic 

history.

Map Base and Data

The map was created using the Transverse Mercator projection with a central meridian of 285° E. Longitude increases 

to the east and latitude is planetocentric. The primary dataset 

used in mapping was a pair of stereo HiRISE observations, 

PSP_001641_1735 and PSP_002063_1735 (HiRISE stereopair 

H1; table 1). Both observations were acquired during the late 

southern winter, with incidence angles near 55°. A DTM with 

1-m post spacing was extracted from the red bandpasses (550–

850 nm) of these two observations. Both images were orthorectified to the DTM, exported at 1 m/pixel and 0.25 m/pixel, and 

compressed using the JPEG2000 image compression standard. 

The DTM production and image orthorectification follows the 

methodology outlined by Kirk and others (2008). Mapping was 

conducted exclusively within the areal extent of the DTM, thus 

the region of stereo overlap in HiRISE stereopair H1 effectively 

defines the map area. The 1 m/pixel orthorectified HiRISE 

images served as the mapping base. Map contours are extracted 

from the HiRISE DTM (HiRISE stereopair H1; table 1) using 

the Contour tool in ArcMap 10.1.

The 1 m/pixel orthorectified HiRISE images were most 

useful for identifying the mapped features (for example, bedding, faults, unconformities), and the 0.25 m/pixel images 

were used only to verify interpretations of meter-scale 

features such as bed forms and fault offsets initially made 

using the 1 m/pixel images. The 1 m/pixel images also have 

the advantage of being only 8 percent of the file size of the 

0.25 m/pixel images (more efficient file compression and lessuncompressed data to start with), and thus require significantly 

less system resources to display on the screen. Both halves of 

the HiRISE stereopair were used for mapping, because their 

slightly different illumination geometry aided with distinguishing between meter-scale albedo and topographic variability. Additionally, map-projected HiRISE color products, where 

available, were the most valuable for distinguishing between 

surface albedo and topography.
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Table 1. HiRISE observations used during mapping, Candor Colles 

region of Mars. Image locations shown in fig. 1. All data are 

available through the NASA Planetary Data System.

HiRISE observation ID HiRISE stereopair

PSP_001641_1735 H1

PSP_002063_1735 H1

ESP_022343_1735 H2

ESP_022699_1735 H2

ESP_027011_1735 H3

ESP_027156_1735 H3

ESP_029266_1735 H4

ESP_029899_1735 H4

ESP_028211_1735 H5

ESP_028277_1735 H5

ESP_026378_1730 H6

ESP_028422_1730 H6

ESP_011372_1730 H7

ESP_012295_1730 H7

Measurements of structural attitude were aided and verified using anaglyphs constructed from the HiRISE imagery, as 

well as overlays of the orthorectified images onto the DTM. 

The anaglyphs provided a quick map-view visualization of dip 

directions, while the image overlays were most useful for providing perspective views. These visualizations helped in locating the outcrops that were best suited for measuring structural 

orientations (that is, those outcrops that most clearly exposed 

the structures’ three-dimensions) and in verifying the general 

magnitude of dip and dip direction of the resulting measurement. Shaded-relief images were constructed from the DTM 

with the intent of using them to verify general structural trends 

in the map area, but these images were not helpful, because the 

topographic edges of the features of interest could not be clearly 

discerned from the surrounding outcrop.

Interpretations of local stratigraphic relations and structure 

within the map area were checked for consistency with other 

outcrops in the Candor Colles and south Ceti Mensa region 

using adjacent HiRISE observations. Interpretations checked 

for regional consistency include the extent and continuity of 

individual units, the persistence of regional unconformities at 

unit boundaries, the relative ages of units, and the crosscutting 

relations between units and structures such as faults and folds. 

Map-projected MRO Context Camera (CTX) images (Malin 

and others, 2007) and anaglyphs were used to link interpretations made in adjacent, but nonoverlapping, HiRISE images. 

Tables 1 and 2 list the HiRISE and CTX observations that were 

employed during mapping. CRISM multispectral summary 

products (Pelkey and others, 2007) of the area were assessed for 

their potential in correlating units on a regional scale but were 

ultimately not used due to their limited coverage and low spatial 

resolution. Other datasets such as those from the Thermal 

Emission Imaging System (Christensen and others, 2004), 

HRSC, and the Mars Orbiter Camera (MOC; Malin and others, 

1992) provided scant new information relevant to this study 

that could improve upon the HiRISE and CTX observations. At 

~6 m/pixel, CTX is the lowest resolution dataset that was useful 

for this mapping effort. HiRISE- and CTX-coordinated observations (Zurek and Smrekar, 2007) were used when possible to 

maintain consistent lighting and viewing geometries between 

observations.

Table 2. CTX observations used during mapping, Candor Colles 

region of Mars. Image locations shown in fig. 1. All data are 

available through the NASA Planetary Data System.

CTX observation ID CTX stereopair

P02_001641_1734_XI_06S075W X1

P03_002063_1733_XI_06S075W X1

G09_021631_1739_XI_06S076W X2

G10_022066_1739_XI_06S076W X2

P05_002841_1744_XI_05S076W X3

P07_003896_1743_XI_05S076W X3

B01_009882_1743_XN_05S075W X4

B02_010515_1742_XN_05S075W X4

Methodology

All mapping was conducted using the ArcMap 10.0 and 

10.1 geographic information system (GIS) software package 

with custom plug-ins. The HiRISE DTM and both associated 

orthorectified images, at 1 m/pixel and 0.25 m/pixel, were 

imported into ArcMap. Because the orthorectified images 

were processed using the DTM, no further coregistration 

was required in ArcMap. The DTM was imported as a 32-bit 

floating point TIFF image, and the orthorectified images were 

imported in 8-bit unsigned integer JPEG2000 format. Features 

were point digitized at 1:4,000 to 1:6,000 scale. Linear features 

with a length of <50 m and area features <20 m in maximum 

extent were not mapped.

The majority of structural orientation measurements were 

calculated using the Orion 2.4 software package. The DTM 

and 1 m/pixel orthoimages were imported into Orion and used 

to calculate the orientations of discontinuities (bedding planes, 

unconformities, faults). Orientations were defined by first 

measuring northing, easting, and elevation triplets at five points 

along the trace of each discontinuity from the DTM. These five 

points were roughly evenly spaced along a 50–100 m chord 

length along the discontinuity. A best-fit plane was calculated 

for the five points, and the orientation of the resulting plane 

was used to define the strike and dip of the discontinuity. Errors 

associated with these measurements are stored attributes in the 

supplementary GIS database. This technique is similar in concept to a classic three-point problem but is less susceptible to 

noise in the DTM and the discontinuities’ actual deviation from 

planarity at length-scales of 10 to 100 m. This was the same 
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process adopted by Okubo and others (2008) and Okubo (2010) 

to perform similar structural measurements elsewhere in west 

Candor Chasma.

Although the Orion software provides reliable measurements of structural orientations, these data must be manually 

exported to a text file and then imported into ArcMap. To 

follow a more efficient workflow, the Layer Tools plug-in for 

ArcMap (Kneissl and others, 2010) was adopted during the final 

stages of mapping. Measurements of discontinuity orientations in Layer Tools follow the same methodology as in Orion. 

To ensure continuity between measurements made with Orion 

and measurements made with Layer Tools, a random subset of 

orientations of bedding, fractures, and unconformities that were 

previously measured with Orion were remeasured using Layer 

Tools and following the aforementioned measurement procedure used with Orion. Comparison of the measurements made 

by Orion and Layer Tools yielded consistent measurements for 

all feature types. Accordingly, Layer Tools and this measurement procedure were used exclusively for the remainder of this 

map.

Previous geologic maps of west Candor Chasma defined 

units largely on the basis of geomorphology (for example, 

Scott and Tanaka, 1986; Witbeck and others, 1991; Lucchitta, 

1999). This approach was not strictly adopted here, because 

the geomorphology of the layered deposits observed today is 

largely a product of erosion, and, at the scale of this map, erosional morphology cannot be expected to be invariably distinct 

between units of similar composition. In this map, all units are 

allostratigraphic; that is, they are defined by unconformities at 

their upper and lower contacts. Allostratigraphy was selected 

because, at HiRISE scale, bounding unconformities can be 

directly observed, are the most prominent stratigraphic divisions 

observed, and are mappable in a systematic way.

Various types of unconformities are recognized. Angular 

unconformities are observed where the underlying layers are 

truncated by a common irregular surface, and, in turn, layers 

with a significantly different orientation overlie this surface. 

These unconformities are interpreted as surfaces formed by erosion of existing layered deposits, which are subsequently buried 

by sediments in a recognizably different layer orientation. 

Similar to angular unconformities, disconformities are observed 

where the underlying layers are truncated by a common irregular surface, but, in this case, the subsequent layers are subparallel with the older layers. Paraconformities are inferred where 

an unconformity is observed to become parallel with a distinct 

layer surface that is bounded by adjacent layers with similar 

attitudes. Paraconformities are interpreted as surfaces that result 

from erosion, or nondeposition, that parallel existing layers and 

that are buried by later sediments in layers that parallel the older 

strata.

While there are numerous unconformities in the mapped 

layered deposits, not all unconformities define unique geologic 

units. Instead, geologic unit contacts are defined by the most 

prominent regional unconformities observed within the map 

area and in adjacent areas. Secondarily, each geologic unit is 

further described based on characteristic surface expressions 

(albedo, erosional texture, bed forms). Each unit may contain 

numerous subordinate unconformities that do not define unit 

boundaries. The intent of this approach is to define broad units 

that are mappable at regional scales (in other words, these units 

could be sensibly included within a smaller-scale geologic map 

of west Candor Chasma in its entirety). This map is the first in a 

series of large-scale geologic maps in west Candor Chasma, and 

an important objective of this broader effort is to define units 

that can be correlated between maps. Further, only bedrock 

units were defined. Surficial materials such as eolian sand dunes 

were not mapped.

Unit names are based on their morphology and reference their geographic occurrence where appropriate (table 3). 

Unit symbols are abbreviations of the unit names. Geographic 

features are abbreviated using upper-case letters, followed by 

any lower-case letters that are required for uniqueness among 

regional geographic features. Morphologic descriptions are 

abbreviated in lower-case letters and appended as a subscript 

to these geographic abbreviations where appropriate. Thus the 

Ceti Mensa knobby unit is abbreviated CeMk, with “CeM” 

being an abbreviation for Ceti Mensa (the “e” being necessary 

to distinguish it from a possible future abbreviation of Candor 

Mensa), and with the subscript “k” being an abbreviation for 

its knobby morphology (future mapping may reveal additional 

units that would be logically grouped with “CeM” but have 

different characteristic morphologies). Units that have no 

apparently meaningful geographic designation are given unit 

symbols with only morphologic descriptions abbreviated in 

lower-case letters (for example, materials similar in morphology 

to the chasma footwall unit occur throughout Valles Marineris, 

thus this unit is given a symbol, “cf”, which is not specific to 

any one geographic feature). While previous mapping efforts 

assigned chronologic designations to these deposits (Scott and 

Tanaka, 1986; Witbeck and others, 1991), these designations 

have been temporarily set aside until more of this region can be 

mapped at higher resolution and the regional stratigraphy can be 

reassessed.

Table 3. Unit stratigraphic relations, Candor Colles region of Mars.

Unit label Areal extent 

(km2

)

Relative age

cf 3.65 Contemporaneous with all other 

units and most recently active

ld 0.03 Younger than unit CeL

CeMk 32.15 Younger than units CaC, CeL

CaC 62.53 Younger than unit CeL

CeL 9.65 Older than all other units

Map symbology is taken from the Federal Geographic 

Data Committee’s (FGDC) digital geologic map standard 

(http://ngmdb.usgs.gov/fgdc_gds/geolsymstd.php) in compliance 

with U.S. Geological Survey cartographic standards. The FGDC 

standards developed for terrestrial mapping applications are 

used here instead of the traditional planetary map standards. The 

planetary map standards do not sufficiently capture the level of 
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classification and certainty that is obtained at this novel scale 

of mapping. This map represents the first application of solely 

FGDC terrestrial map standards for describing another planetary 

surface.

The level of confidence in locating the surface trace of 

unconformities and faults is classified as accurate, approximate, 

inferred, and concealed. A surface trace that is accurate is one 

where the feature is clearly exposed at the surface (not partially 

or completely buried by surficial materials). This represents the 

highest level of confidence in the location of a mapped feature. 

An approximate trace is one where the feature is known to exist 

due to well-defined exposures elsewhere, but the feature cannot 

be precisely located due to incomplete exposure or equivocal 

observations. An inferred trace is assigned where a feature is 

known to exist at a certain stratigraphic interval or along a fault, 

based on observations elsewhere, but there is no clear evidence 

of that feature along that particular segment. A concealed trace 

is one where the feature is known to exist, based on ancillary 

observations, and its general location is surmised, but it cannot 

be directly observed due to an obvious cover of surficial deposits. Concealed features have the lowest level of confidence in 

their assigned locations.

For faults, the level of confidence in identifying their sense 

of displacement is indicated by the specificity of their classification. Classifications of normal fault and thrust fault are given 

where the strike and dip of the fault plane are clear and the 

sense of displacement along the fault is apparent based on crosscutting relations. The generic “fracture” classification is given 

where a structural discontinuity is observed, but either the fault 

geometry, sense of displacement, or both cannot be determined.

The majority of faults are symbolized with lines because 

these faults are short enough in length that their surrounding 

damage zones cannot be resolved at the scale of HiRISE. There 

are three larger fault systems that are sufficiently long (greater 

than ~10 km length in map view) so that their damage zones 

are readily apparent (fig. 2, on sheet). These larger structures 

consist of numerous subparallel anastomosing ridges that are 

either mantled by rocks and sand or surrounded by intricately 

fractured bedrock. These ridges are generally 5–10 m wide 

and stand as much as ~5 m above the surrounding terrain. 

Additionally, fins of light-toned rock are exposed along the 

crests of many of these ridges. Observations of layer offsets 

across the lateral tips of these fault zones reveal a normal sense 

of displacement. These structures are mapped using stipple patterns that encompass the damage zones, including the areas of 

high-standing ridges and intricately fractured bedrock.

Styles of Deformation

Given the sedimentary nature of the layered deposits, 

these three large fault systems are interpreted as being largely 

composed of deformation bands and related faults (Okubo 

and others, 2009). By analogy with terrestrial observations of 

exhumed deformation-band fault systems (for example, Davis, 

1999), the fins of light-toned rock along the ridges are interpreted to correspond with fault traces. Therefore, these fins 

within the damage zone are mapped using the aforementioned 

line symbols for faults. For clarity, only representative faults are 

mapped within these damage zones. Orientation measurements 

show that these faults consistently dip southward. Faults bordering the damage zones have a gentler dip than faults within 

the interiors of these damage zones (fig. 3, on sheet). These 

geometries are consistent with the architecture of deformation 

band-type damage zones surrounding normal faults in sandstone 

on Earth (for example, Davis, 1999).

Within the Candor Colles geographic area, each collis 

(hill) contains a core of massive rock, which is generally 

exposed from its mid-elevation to its summit (fig. 4, on sheet). 

Exposures of layers within the Candor Colles unit (CaC) commonly skirt the lower elevations of the colles. In places, these 

layers can exhibit strikes that are roughly concentric about the 

collis’ summit and have outwardly sloping dip directions. The 

outwardly dipping layers can be subtle; in other places, the 

layers exhibit no apparent change in attitude in proximity to a 

collis.

The structure of the Candor Colles is consistent with that 

of eroded terrestrial injectite megapipes. Injectite megapipes 

are soft-sediment-deformation features formed through the 

liquefaction and upward injection of water-saturated sediments 

into overlying deposits and are formed in paleo-sabkha environments (Netoff and Shroba, 2001; Chan and others, 2007). 

Bedding of the host rock within which the injectite megapipes 

are intruded are observed to dip either away from or toward the 

core of the megapipe, or show no change in orientation with 

proximity to the megapipe. The cores of these injectite megapipes are commonly massive and are tens of meters in diameter. 

Based on their similarities in structure, the Candor Colles are 

interpreted to be injectite megapipes and are mapped as structural domes.

Sedimentary layers within the Candor Colles geographic 

area exhibit contorted bedding (Chan and others, 2007), another 

manifestation of soft-sediment deformation. These signs of 

soft-sediment deformation (injectite megapipes and contorted 

bedding) affect layers that overlie a regional unconformity 

above the Ceti Labes unit but are not apparent in layers above 

the regional unconformity at the base of the Ceti Mensa knobby 

unit. Thus the Candor Colles unit is recognized as a distinct 

stratigraphic interval characterized by bed forms resulting from 

soft-sediment deformation.

Age Determinations

Previous geologic maps of west Candor Chasma assigned 

chronostratigraphic designations (Noachian, Hesperian, 

Amazonian) to the various mapped units (Scott and Tanaka, 

1986; Witbeck and others, 1991; Lucchitta, 1999). These ages 

were not adopted here, because the new scale of mapping that 

is now possible with HiRISE and CTX data calls for a reassessment of the classic Viking-based units and relative ages. 

New crater counts of the newly defined units were not acquired 

because these layers likely consist of sulfate-rich sands similar to those found in Meridiani Planum (Murchie and others, 
5

2009a), and the standard techniques for relative age dating 

based on impact crater retention cannot be relied upon in these 

highly friable deposits. Thus the relative ages of units in this 

map are determined on the basis of superposition.

The Ceti Labes unit (CeL) is unconformably overlain by 

the Candor Colles unit (CaC) and the Ceti Mensa knobby unit 

(CeMk). Along the northern periphery of the Ceti Labes unit, 

a prominent unconformity along the contact with the Candor 

Colles unit is observed in most areas, and the contact is buried 

by colluvium derived from the Ceti Labes unit in other areas. 

The Candor Colles unit mantles the southern extent of the 

Ceti Labes unit; ridges and troughs that are part of the intrinsic landslide morphology of the Ceti Labes unit can be traced 

below a thin layer of the Candor Colles unit in the southernmost areas of the map. This thin mantle of the Candor Colles 

unit possesses subdued layering and has a moderate to low 

albedo. Much of the southern extent of the Candor Colles unit is 

covered by dark sand, making the distinction between the Ceti 

Labes unit and the Candor Colles unit difficult in some areas. 

Careful observation shows that layering in this mantle of the 

Candor Colles unit follows the underlying topography of the 

Ceti Labes unit, and this layering is traceable northward into the 

more extensive and prominent outcrops of the Candor Colles 

unit. Stereo CTX observations of Ceti Labes outside of the 

map area (CTX stereopair C1) show that the Ceti Labes unit is 

entirely buried by a continuous sequence of the Candor Colles 

unit. This is especially well exposed along the northernmost and 

eastern extents of Ceti Labes. The Ceti Mensa knobby unit also 

appears to overlie the Ceti Labes unit within the topographic 

depression between the toe and headscarp complexes of the 

Ceti Labes landslide. Therefore, the Ceti Labes landslide is 

older than the local layered deposits of the Candor Colles unit 

and the Ceti Mensa knobby unit. The Candor Colles unit also 

appears to be buried by the lobate deposit unit and likely by 

the chasma footwall unit as well (assuming these materials are 

wall rock-derived colluvium and eolian sediments). Lucchitta 

(1990) showed that some of the youngest layered deposits in 

west Candor Chasma (the mottled deposits) bury some of the 

landslides derived from the chasma’s walls. This relation not 

only holds true for the Ceti Labes unit, but the Ceti Labes unit 

is found to predate all of the layered deposits in this part of west 

Candor Chasma.

The map shows three disparate occurrences of the Ceti 

Mensa knobby unit; one area on Ceti Labes, one near the northwest periphery of Ceti Labes, and one large area in the northern 

part of the map. CTX stereopair C1 shows that the outcrops of 

the Ceti Mensa knobby unit adjacent to Ceti Labes are contiguous outside of the map area. Continuity between these southern sections of the Ceti Mensa knobby unit and the northern, 

higher elevation sections of the Ceti Mensa knobby unit is 

best observed in CTX stereopair C3 and HiRISE stereopair 

H4, ~36 km northwest of the map area. There, layers from the 

northern outcrops of the Ceti Mensa knobby unit, on top of Ceti 

Mensa, are traceable into the southern exposures of the Ceti 

Mensa knobby unit, at the base of Ceti Mensa. Exposures of the 

Ceti Mensa knobby unit at lower elevations have a darker tone 

than comparable outcrops at higher elevations. In HiRISE color 

coverage, this appears to be due largely to accumulations of 

dark sand at lower elevations. Thus the Ceti Mensa knobby unit 

is a regionally extensive unit that is younger than the Candor 

Colles unit.

The relative ages of the lobate deposit unit (ld) and the 

chasma footwall unit (cf) are not well constrained. The lobate 

deposit unit overlies the mantle of the Candor Colles unit (CaC) 

on Ceti Labes, so it is older than the Candor Colles and Ceti 

Labes units. In CTX stereopair C1, the lobate deposit unit is 

observed to overlie the chasma footwall unit, thus the lobate 

deposit unit is either younger or coeval with the chasma footwall unit. Clear evidence of the chasma footwall unit burying 

any other units is not observed, although disparate boulders are 

observed along the periphery of adjacent units. If the chasma 

footwall unit consists of mainly wall rock-derived colluvium 

as interpreted here, then this unit has likely been active since 

Candor Labes formed and is probably active at present. The 

relative age between the Ceti Mensa knobby unit and the lobate 

deposit unit is not clear due to a lack of apparent superposition 

relations.

The relative ages between the large fault zones located 

within the map area have been determined based on crosscutting 

relations. The eastern tip of Fault Zone III terminates against 

Fault Zone II (outside of the map area; observed in HiRISE stereopair H5), indicating that Fault Zone III is younger than Fault 

Zone II. Fault Zone II is also crosscut by Fault Zone I (outside 

of the map area; observed in CTX stereopair C1; table 2), 

indicating that Fault Zone I is younger than Fault Zone II. The 

relative timing between Fault Zones I and III have not been 

determined due to a lack of pertinent crosscutting relations. 

Relative ages for the smaller faults have not been determined 

because exposures of their crosscutting relations generally provide inconclusive evidence of timing.

The relative ages of the major fault zones and the layered deposits in the map area have been determined based on 

superposition and crosscutting relations. The eastern tip of Fault 

Zone III is crosscut by another fault zone, Fault Zone IV. Fault 

Zone IV is not present within the map area, but this crosscutting 

relation is observed in HiRISE stereopair H3. The Ceti Mensa 

knobby unit unconformably overlies Fault Zone IV (fig. 5, on 

sheet). Fractures from Fault Zone IV do not continue beyond 

the unconformity at the base of the Ceti Mensa knobby unit, 

and the lower-most layers of the Ceti Mensa knobby unit show 

onlap against the erosional, ridge-like morphology of Fault 

Zone IV, thus Fault Zone IV was eroded and exposed at the 

surface previously, before deposition of the Ceti Mensa knobby 

unit. Because Fault Zone IV is younger than Fault Zones II 

and III, the Ceti Mensa knobby unit is younger than these fault 

zones. Thus deposition of the Ceti Mensa knobby unit occurred 

after these fault zones (and the Candor Colles unit host rock) 

were exhumed through erosion.

Geologic History

The oldest material in the map area is the Ceti Labes unit 

(CeL). Ceti Labes consists of landslide deposits derived from 
6

the volcanic rock that constitutes the south wall of west Candor 

Chasma (fig. 6, on sheet). Accordingly, this and all subsequent 

units within the map area were deposited after west Candor 

Chasma had formed. The wall rock composing the south wall 

of west Candor Chasma is exposed just outside of the map area 

and was not mapped in this study.

After emplacement of the Ceti Labes unit (CeL), the 

Candor Colles unit (CaC) began to accumulate. Accumulation 

of the Candor Colles unit occurred within a depositional environment that was at a sufficiently low energy that the primary, 

~2- to 3-m-scale landslide textures of the Ceti Labes landslide 

were preserved. Erosional remnants of the Candor Colles unit 

can be seen on top of the Ceti Labes unit as either a thin mantle 

of layered materials or as small isolated hills. A facies change 

between the initial sediments of the Candor Colles unit and later 

sediments within this unit is not apparent at HiRISE resolution. 

Deposition was accompanied by occasional erosional events 

and the formation of numerous local unconformities, but this 

erosion did not cause extensive modification of the Ceti Labes 

unit. The Candor Colles materials were deposited in layers that 

lap onto pre-existing topography. Away from unconformities, 

faults, and other deformation, the layers generally dip toward 

the south at ~10° or less. The youngest layers of the Candor 

Colles unit were deposited in extensive successive layers that 

can be traced well outside of the map area and show no apparent unconformities. Fluvial and eolian bed forms at scales of 

1 m or more are not observed in these layers. These findings are 

consistent with the mechanism of formation recently advocated 

by Murchie and others (2009a), in which these sediments accumulated and underwent diagenesis in a sabkha environment, 

similar to some of the layered deposits exposed in Meridiani 

Planum (Arvidson and others, 2011).

Once accumulated, the Candor Colles unit (CaC) experienced soft-sediment deformation, leading to formation of 

contorted bedding and injectite megapipes. The lack of breccia, 

faults, or other occurrences of brittle failure associated with this 

soft-sediment deformation suggests that the Candor Colles unit 

was poorly lithified at the time of deformation. These features also point to the sediments being water-saturated during 

deformation.

The Candor Colles unit experienced brittle deformation 

along major fault zones. Several colles of Candor Colles appear 

to disrupt Fault Zone I near its west tip (HiRISE stereopair H5), 

and the west tip of Fault Zone III overprints a different colles 

(HiRISE stereopair H3) outside of the map area (fig. 7, on 

sheet). Accordingly, slip along these major fault zones appears 

to have been contemporaneous with the formation of Candor 

Colles landforms. Thus, these fault zones are the most likely 

source of the seismicity that drove the soft sediment deformation within the Candor Colles unit.

Within the Candor Colles unit (CaC), the geometries and 

offsets along the major fault zones are consistent with a landslide origin. These fault zones generally have an arcuate trace in 

map view (fig. 2), with the concave side of the arc facing southward. The faults show a normal sense of offset, which is most 

apparent in displaced layers traceable around the ends of the 

fault zones, and displacement is down to the south. The faults 

are classic structural features of large circular failures (Cruden 

and Varnes, 1996). Accordingly, these fault zones are interpreted 

as the failure surfaces of a landslide complex that displaced 

material of the Candor Colles unit down toward the south. 

The sense of displacement across these failure surfaces 

indicates that the paleo-slope of the Candor Colles unit (CaC) 

dipped toward the south at the time of landsliding. Therefore, 

a paleo-moat between the layered deposits and the south 

wall of west Candor Chasma, analogous to the present-day 

moat, existed here when the landslides were active. Similar, 

southward-directed landslides of the layered deposits have 

been mapped ~65 km to the west of the map area by Okubo 

and others (2008) and Okubo (2010), and another landslide 

is currently being mapped ~30 km to the east of the map area 

(HiRISE stereopair H7; Okubo, 2013).

Deposition of the Candor Colles unit (CaC) eventually 

ceased, and erosion of this unit ensued, resulting in the development of a bounding unconformity. Fault Zone IV is known 

to have been exhumed at this time because the lower boundary of the overlying Ceti Mensa knobby unit (CeMk) exhibits 

onlap relations on either side of this fault zone ridge in HiRISE 

stereopair H2—thus this fault zone was previously exposed at 

the surface and had a ridge-like erosional morphology similar 

to its current form. The degree of exhumation and exposure of 

Fault Zones I, II, and III is not apparent in observed superposition relations. The Ceti Labes (CeL) unit appears to have been 

re-exposed at this time as well, because the Ceti Mensa knobby 

unit also unconformably overlies it. The primary landslide 

textures of the Ceti Labes unit were preserved throughout this 

erosional episode. Evidence of either fluvial erosion or marine 

regression is not apparent along this unconformity.

The eolian-dominated environment within which the 

Candor Colles unit (CaC) is inferred to have accumulated may 

have also fostered development of the aforementioned paleomoat, landsliding, and erosion. Topographically forced wind 

circulation and the resulting eolian erosion is proposed to be the 

cause of the moats surrounding the mounds of layered deposits 

in impact craters elsewhere on Mars (Kite and others, 2013). 

Thus, an analogous process of eolian erosion within the basin of 

west Candor Chasma is a plausible mechanism for widespread 

erosion of the Candor Colles unit and development of the paleomoat. Further, eolian excavation of this moat into the Candor 

Colles unit is a credible process by which regionally extensive, 

over-steepened slopes could have developed and resulted in the 

observed landslides. This scenario implies that deposition of the 

Candor Colles unit largely ceased once the paleo-moat began to 

form. Therefore, the transition from deposition of the Candor 

Colles unit to development of the paleo-moat, subsequent 

landsliding, and the ensuing regional erosion, can be elegantly 

ascribed to either, or a combination of, a change in wind regime, 

a decrease in sediment supply, or diminished sediment trapping 

due to lowering of the water table.

The Ceti Mensa knobby unit was then deposited on top of 

the regional unconformity at the upper boundary of the Candor 

Colles and Ceti Labes units. The Ceti Mensa knobby unit buried 

the remnant mound of the Candor Colles unit, the exhumed 

landslide faults along its flanks, and the paleo-moat. The onset 
7

of deposition occurred in an environment where the sediments 

accumulated in layers that lap onto pre-existing topography. 

This depositional environment was at a sufficiently low energy 

that the primary landslide textures of Ceti Labes were again 

preserved. A change in facies between the initial sediments of 

the Ceti Mensa knobby unit and later sediments within this unit 

is not apparent at HiRISE resolution. Deposition was accompanied by occasional erosional events and the formation of numerous local unconformities, and subsequent layers lap on to these 

unconformities. Away from unconformities, faults, and other 

deformation, the layers generally dip to the south at ~20° or 

less. Fluvial and eolian bed forms at scales of 1 m or more are 

not observed in these layers. These key similarities between the 

Ceti Mensa knobby unit and the Candor Colles unit suggest that 

both units were deposited in comparable sabkha environments. 

Accordingly, the transition from the erosional environment following deposition of the Candor Colles unit to deposition of the 

Ceti Mensa knobby unit may have been triggered by a change 

in wind regime, an increase in sediment supply, a raising of the 

water table and enhanced sediment trapping, or some combination thereof. Unlike the Candor Colles unit, major fault zones 

and soft sediment deformation is not observed within the Ceti 

Mensa knobby unit.

Deposition of the Ceti Mensa knobby unit eventually 

ceased, and a predominantly erosional environment reemerged. 

Similar to the erosional event following deposition of the 

Candor Colles unit, this event was widespread, with no evidence of a fluvial or marine component, and was at a sufficiently low energy to preserve the primary landslide textures of 

Ceti Labes. Again, eolian erosion is most consistent with these 

observations and may have been triggered by changes in sediment supply, wind regime, or water table depth. This environment presumably persisted into the present, given the apparent 

lack of subsequent layered deposits in the area.

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