JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 95, NO. Bll, PAGES 17,289-17,308, OCTOBER 10, 1990
TheOriginOf FluvialValleysAnd EarlyGeologicHistory,AeolisQuadrangle,
Mars
G. ROBERT BRAKENRIDGE
Su•ficialProcesses
Laboratory,
Department
ofGeography,
Dartmouth
College,
Hanover,
NewHampshire
In southern
AeolisQuadrangle
in easternMars,parallelslopevalleys,flat-floored
branching
valleys,Vshapedbranching
valleys,andflat-flooredstraightcanyons
dissectthe heavilycrateredplateausequence.
Associated
knife-likeridgesareinterpreted
asfissureeruptionvents,andthin,dark,stratiform
outcrops
are
interpreted
asexhumedigneoussillsor lavaflows. Ridgedlavaplainsarealsocommonbutarenot themselves
modifiedby fluvialprocesses.
I mapped56 asymmetric
scarps
or ridgesthatareprobable
thrustfaults.These
faultsexhibitanorientation
vectormeanof N63ow + 11o (95%confidence
interval),andtheytransect
thelava
plainsandthe olderplateausequence
units. By comparison,
thevectormeanfor the 264 valleysmappedis
N48ow + 12o, witha largerdispersion
aboutthemean.Thesimilarorientations
displayed
by thrustfaultand
valley axessuggestthat valleylocationsarepartlycontrolledby preexisting
thrustfaultsandrelatedfracture
systems.Most valleysare alsoarrangedorthogonally
to, and alongthe perimeterof, the ridgedplains. A
possiblemodelfor valleydevelopment
is: (1) freshlyoutgassed
waterbecameentombed
asfrost,snow,andice
within the crateredterrainsduringheavybombardmentandthe accompanying
deposition
of impactejecta,
volcanicash,andeolianmaterials,(2) effusivevolcanismandlava sill emplacement
heatedsubsurface
ice in the
vicinity of the ridgedplains,and faultsand fracturesprovidedzonesof increased
permeabilityfor water
transportto the surface,and (3) headwardsappingat thermalsprings,thermokarstsubsidence,and limited
downvalleyfluid flows thencarvedand modifiedthe valleys.
by conductionthroughmodeledice-coveredrivers is slow, and
latent heat is added to the system by water freezing (for a
INTRODUCTION
Ever sincetheir discoveryduringthe 1972 Mariner 9 planetary
mission, the ancient fluvial valley networks of Mars have been
describedas relict from an earlier warmer and denseratmosphere
[Sharp and Malin, 1975; Masursky et al., 1977; Chapman and
Jones, 1977; Pollack, 1979; Cess et al., 1980; Pollack and Yung,
1980; Mars Channel Working Group, 1983; Kahn, 1985; Pollack
et al., 1987]. If this inference is true, then these dry valleys
constitute spectacular evidence for planetary-scale climatic
change. Liquid water is not presently stable anywhere on the
planer'ssurface,and the operationof an Earth-like hydrological
cycle on Mars requiresa much warmer atmosphereand one very
much denserthanthe 7-mbar atmosphereof today [Pollack et al.,
1987]. Also, the valleys are nearly restrictedto heavily cratered
landscapes
datingfrom the Heavy Bombardmentperiod of early
solarsystemhistory[Pieri, 1976; Cart and Clow, 1981]. During
this period,the Sun'sluminositymay have beenonly 70% of its
presentvalue [Gough, 1981]. It is therefore unlikely that the
valleys are the direct result of an earlier, more favorable climate
associated
with solarevolution.Largeamountsof climaticchange
are most easily explained by depletion of an early dense
terrestrialexample,seeCorbinandBenson[1983]). The limiting
factorfor fluvial activityon Mars is waterrelease,andnot water
persistenceas an erosivefluid once discharged[Wallace and
Sagan, 1978; Cart, 1983]. Water releasescould be related to a
hydrologicalcycle anda denseatmosphere,
but otheralternatives
are (1) solar heating and melting of dust-rich snow and ice
depositedunderhigh obliquityorbital conditions[Jakoskyand
Cart, 1987; Clow, 1987], or (2) geothermallyheated waters
reachingthe surfacethroughfracturesandfaults[Brakem'idge
et
al., 1985; Wilhelms, 1986;Brakenridge,1987, 1988; Gulick et
aI., 1988; Wilhelmsand BaMwin, 1989]. Given thesealternatives,
theinferenceof an earlydensepaleoatmosphere
may be in error.
Do the ancientvalley networkscompelinferenceof a large
amountof climaticchange,or may othergeneticmodels,without
climatic change,suffice?The presentreportdemonstrates
that
Martian valleys in Aeolis Quadrangleexhibit spatial patterns,
stratigraphic
relationships,
andmorphologies
that are compatible
with genesisthroughvolcanism-induced
hot springdischarges.
The followingsections(1) summarizeknowngeologiceventsthat
occurredduringthe time periodof valley evolution(2) document
andspatialrelationships
of Aeolisvalleysto
atmosphere
richin CO2, H20, orsomeothergreenhouse
gas[e.g. detailedstratigraphic
tectonic
features
and
volcanic
landforms;
and
(3) describelocal
Cart, 1987]. Thisdenseatmosphere
mighthavetemporarilykept
evidence
for
one
valley
network's
episodic
growth
by subsidence,
theplanetwarm,despitea faintersun[Pollacket aI., 1987].
headward
sapping,
and
downvalley
fluid
flows.
Calculationsfor ice-coveredriver flows on Mars [Cart, 1983]
suggestthat fluvial features could form at present if sustained
waterdischargeat the surfacewereto somehowoccur. Heat loss
Copyright1990by the AmericanGeophysical
Union.
Papernumber90JB00540.
0148-0227/90/90JB-00540505.00
GEOLOGIC
UNITSIN AEOLISQUADRANGLE
Two disparate landscapesexist on Mars. One is heavily
crateredand is dissectedby relict valleys,andthe otheris lightly
crateredor uncrateredand is undissected.Aeolis Quadrangleis
astride the planet-wideboundarybetween thesetwo landscapes
(Figure 1). The southern,heavily crateredlandscapewas created
17,289
17,290
BRAKENRIDGE,
AEOLIS
QUADRANGLE,
MARS
indicatesthatmostpreserved
ridgedplainsformedneartheendo!
heavybombardment
[Barlow,1988].
North
I
I
,]
I
•
I
•
•
!.
]
.[
,,I
]
_
- 60
Elysium
Planitia
I•
, ,•,•
;•:• •
z:•• •
50
chronology
is inferredby 'Fanaka[1986]in hisglobalsummary:
(1) Noachiandepositionof plateausequence
strata,(,2) late
40
Nochianfluvial dissection,and (3) early Hesperianembayment
.Qlympus
30
lvlons
•
10
20
...................
:.......
:e'.•e•
andpartialburialof theplateausequence
materials
by extruded
lavas. This places a discreteinterval of plains volcanism
subsequent
to extensivefluvial valley development
[Tanaka,
1986] and also impliesthat any climatefavorableto valley
development
hadendedby Hesperian
time: mostlandforms
of
thisandyounger
agearenotdissected.
However,anychronology
Amazonis
Planitia
:'-:•
'.:-:.
• ..:.:':-:':':':':
'. •P;:•2>;'
••:;• ;,
An earlyHesperian
agefor mostundissected
ridgedplainshas
been usedto constrainvalley genesisin time. The following
• • ':
'.•m• :•:• ß'-x•S••••P• •/•.-•
--[
mustbe reconstructed
from the preservedgeologicrecord,and
preservation
factors
should
alsobeconsidered.
Wilheims[1987,
p. 279]models
lavaplainsontheMoonasthevisibleresults
of
increased
preservation,
not increased
extrusion,
as large-impact
rates declined at the end of heavy bombardment. A similar
geneticmodelfor ridgedlavaplainpreservation
mayapplyto
•... . •
240
•,•.•:•.
210
Mars.
180
150
120
In thisrespect,
themapsof Scottand Tanaka [1986]and
GreeleyandGuest [1987]alsoincludewidelyscattered,
older
ridged
lavaplainsof Noachian
age.These
authors
infer,aswell,
that interbeddedflow volcanicsare a common internal component
Fig. 1. Mapof majorlandscapes
nearAeolisQuadrangle
andtheir of the plateausequence.Extensiveplainsvolcanismmay,
inte•edages,
asredrawn
fromBarlow[1988].Thedarkshading
indicates therefore,havebeenunderwayduringNoachiantime, but such
surfaces
Formed
duringh•avybombardment,
thelightershading
indicates volcanism
wasnot widelypreserved
beforetheearlyHesperian.
surfacesformedn•ar the endot heavybombardmere
(similarcratersize
TableI givesthisalternative
process
history
reconstruction.
The
&equency
distribution,
butlowercraterdensities),
andthewhiteareas
are reconstruction
agrees
withthepreserved
stratigraphy
described
by
lighfiycraigred
oruncrat•red
surlhccs
Formed
aftertheendot th• h•avy
bombardmere
flux, approximately
3.2 Ga. The agesot OlympusMons
and•hreevolcanosin ElysiumPlanitiaarealsoshowfl.
Tanaka[1986] and with the craterstatisticresultsof Gurnis
[1981] andBarlow [1988]. It impliesthat fluvial valley
development
andridgedplainvolcanism
overlapped
in time.
Directcraterdateson valleynetworksby Bakerand Partridge
duringthefinalstageof planetary
accretion
(thelateheavy [1986]alsoindicatethatvalleynetworks
rangefromNoachian
bombardment),
and localexamplesoccurof denselycratered
through
earlyHesperian
in ageandthusindependently
support
surfaces
exhibiting
lunar-like
preservation
of smallcraters
[e.g.,
plains
volcanism
andvalleydevelopment
ascoeval
processes.
Cart, 1981,p. 69]. In contrast,
thenorthern
plains
landscape
is
post-heavy
bombardment
inage,andmayconsist
of sedimentary
VALLEY CLASSIFICATION
plainsand/or
lavaflows.Thecause
of theplanetary
dichotomy
represented
by thesetwo landscapes
remainscontroversial Valleys developed on plateau sequenceunits exhibit
semicirculartheater-shapedheadwallsand steep valley sides,
[Wilhelms
andSquyres
1984;Wiseet al., 1979].
relatively
few andshorttributaries,andalignedstraightsegments
Martiantimestratigraphy
isdividedintotheNoachian
System,
suggestive
of faultorfracture
control[SharpandMalin, 1975].In
theHesperian
System,
andtheAmazonian
System
[Tanaka,
1986]
contrast
to
runoff-created
valleys
on the Earth,drainagedensities
(seealsoTable1). Eachsystem
isfurthersubdivided
intoseries,
are
very
low,
and
tributary
junction
anglesdo notprogressively
eachwithmapped
reference
units.Theheavilycratered
landscape
increase
in
the
downstream
direction.
Apparently, structural
in Aeolisis,mainly,of Noachian
ageandincludes
thedissected
thejunctionangle
unitandthecrateredunit(bothmiddleNoachian)andthesubdued controlson Martianvalley location"randomize
while regularizing
the link orientations"
(D. Pieri,
unit (upperNoachian)
of theplateausequence
[Greeleyand systematics,
Guest,1987].Scattered
withintheplateausequence
landscape
are written communication,1989). Such morphologicobservations
suggest
headward
erosion
frominitialspringlocations,
instead
of
ridged
plainunits
ofinferred
LowerHesperian
age(Figure
2).
The plateausequence
may includeinterstratified
impact surficial runoff and progressivedownstreamintegrationinto
larger,topography
followingchannels
[Pieri,1976,
breccia
andejecta,reworked
aeolian
debris,
fluvialor lacustrineincreasingly
deposits,
lavaflowsorsills,and,possibly,
ice[Tanaka,
1986;
Wilhelms and Baldwin, 1989]. These depositsare locally
dissected
by valleysof probable
fluvialorigin(,Figure
2). In
contrast,
thelowerHesperian
ridgedplainsaremappedas lava
flowplains[e.g.Tanaka,1986;Greeley
andGuest,1987],and
theyarenotcommonly
dissected
by fluvialvalleys[Tanaka,
1986]. The sizedistribution
of the superposed
impactcraters
1980].
An earlier classification of Martian valleys is based on
planimetric
andcross-section
morphology
[Brakenridge
eta!.,
1985]. Figure3 is a modifiedversionof thatclassification
and
alsoincorporates
commonlyobserved
geologiccontexts.The
numberof valleyclasses
hasbeenincreased
fromfive to six,and
class numbers are revised to establish a size trend from smaller
BRAKENRIDGE, AEOLlS QUADRANGLE, MARS
17,291
TABLE 1. GeologicalContextof Valley Developmentin AeolisQuadrangle
Series a
System
a
Age
Preserved
Gab
Geologic
Processes t'
Units a
Hesperian
Lower
3.2
ridgedplains; smoothunit
of the plateau sequence
heavy bombardmentends,
ejectaand ice depositionslow,
effusive volcanism continues,
lavaplainsare widelypreserved,
widespreadthrustfaulting
Noachian
Upper
3.5
subduedunit of the
continuedheavy bombardment,
ejectaandicedeposition,
effusive
plateau sequence
volcanism,formationof Ai-qahira
and Ma'adim
Middle
Noachian
3.85
hilly, dissected,and
and
solidification of the crust,
heavy bombardment,
ejectaand ice deposition,
cratered units of the
Lower
3.92
Vailes
plateausequence
effusive
volcanism
aModified fromTanaka[ 19861,Greeleyand Gleest[ 19871,andBarlow [ 1988].
bSpeculative
(crater
statistics-based)
agesrefertoupper
boundary
ofseries.
cValleydevelopment
n-my
haveoccurred
duringall threetimeintervals.
North
I...•
II200km
II mp12/,
•
• 6
•
Ap'ollinariseatera
II ?"::::!::i•.
.............
-10
---:-
;-.......
-2O
-30
220
210
200
190
180
Fig.2. Geologic
mapof AeolisQuadrangle,
illustrating
theoldergeologic
unitsof Greeley
a,d G,est[ 1987]andthefluvial
valleysof Cart a,d Ciow [19811.Darkshading
illustrates
lowerHesperian
ridgedplainsunits,"Npld"is thedissected
unit,
"Npll"thecratered
unit,"Np12"
isthesubdued
unit,and"Hp13"
isthesmooth
unitof the(middle
Noachian
toearlyHesperian)
plateau
sequence.
Thedashed
border
separating
thedissected
unitfromthecratered
unitindicates
thegradational
contact
between
these
twomapunits.Thevalleys
areshown
asthinsolidlines,andthewrinkle
ridges
areshown
asthickdashed
lines;
thicksolid
linesarelargechasms
or scarps.
At feature1,"r"markssmooth
plainsmaterial,
and"e"marks
ejectaassociated
withthiscrater.
All numbered features are described in the text.
17,292
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
PARALLEL
V-SHAPED
SLOPE VALLEYS
BRANCHING
VALLEYS
(Upstream)
II
5 km
(Downstream)
.3-5
FLAT-FLOORED
2 km
km
FLAT-FLOORED
STRAIGHT
BRANCHING
VALLEYS
CANYONS
IV
III
I
'
I
i
I
5 km
1 km
,
V-SHAPED
-1o
km
.7S
km
TRIBUTARY
CHASMS
FRETTEDCHANNELS
V
VI
10 km
5-30
km
Fig. 3. Combinedplanimetric/cross-sectional
classification
of valleynetworkson Mars, revisedfrom B,'aken,'idge
et al. [ 1985].
Shown for each classare representativemap views (on the left) and valley crosssections(on the right); dimensionsare
approximate.Typicalrelationships
to surfitcegeologyarealsoshown, "RP"symbolizesridgedplainsflow volcanics,"PS"units
areplateausequence
materials,"Cf' represent.,/.'
modifiedcraterfloor deposits,
and"Avf" indicateslargechasminteriordeposits.
arerelativelyshortandcommonly
(classesI andII) to larger(classesIII-VI) valleys. Valley widths 10 km). The straightcanyons
debouch
at
ridged
plain/plateau
sequenceboundaries. Most
givenin thefigurearetypicalvalues;depthsareestimated
andare
not well established.
Note that the enormous, hundreds of
exhibitsmoothfloorsthatappearto be continuous
with theplains
(Figure3). Theheadward
terminations
of thesecanyons
aresteep,
and
the
straightness
of
canyon
wall
orientations
suggest
faultor
Group, 1983] are not includedin Figure 3. As determinedfrom
kilometer-wide, "outflow channels" [Mars Channel Working
cratercounting,theoutflowchannels
areof a widevarietyof ages fracture controls. In contrastto the canyons,the flat-floored
and have, therefore,neverbeenusedas evidencefor or againstan
early denseMars atmosphere.
branching
valleystraversehundreds
of kilometersof complex
plateausequence
landscapes
(Figure3). Large-scale
circular
ClassI valleys
(parallel
slope
valleys)
andclass
II valleys
(V- patterns
maysuggest
thattheirlocations
arepartlycontrolled
by
impact-related
faultsor fractures[Brakenridge
et al., 1985;
shapedbranchingvalleys)includethe finestscalevalleysvisible
on Viking imagery.The parallelslopevalleysoccuron theflanks
of large modifiedcrater landformsor on other steep,relatively
uniformslopesthatare adjacentto low-albedoridgedplains. The
V-shaped branching valleys are of similar dimensionsand
geologicsetting,but branchupstream,
andtheirupstreamreaches
exhibit narrow, V-shaped cross sections. Both valley classes
commonly dissectthe plateau sequence,and terminate at the
marginsof adjacentridgedplains(Figure3).
Class III (flat-floored straight canyons) and class IV (flatflooredbranchingvalleys)exhibitflat valley floors,and widths
comparable
to terrestrialrivervalleysof moderate
to largesize(5-
Gulick,1986;Schultzet al., 1982]. Gapsbetweensegments
of
thesedryvalleysmaybe locallycaused
by post-valley
faultingor
bypiecemeal
valleygenesis
through
headward
sapping
(Figure3;
seealsofollowing discussion).
The last two classesoccur only at a few locationswithin
Aeolis.ManyclassV (V-shaped
tributarychasm)andclassVI
(frettedchannel)valleysareyounguncratered
landforms
andare,
therefore,not evidencefor changedconditions.The V-shaped
chasmsarecharacterized
by steepgradients,
andby largewidths
anddepths
compared
to lengths.Theyoccurastributaries
to the
Valles Marineris and other large chasmsand to some large
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
17,293
outflowchannels.The V shapemay resultfrom intersectingonly;andFigure5c, undifferentiated
faultsandfractures
only.
debrisslopesthat are still active.The frettedchannels
exhibit The measured
orientations
are illustratedby rosediagrams
in
locallysinuous
valleyreaches,
suggesting
thatdownstream
fluid thesemaps,andtherelevantdescriptive
statistics
aresummarized
flowsmayhaveoccupied
theentirevalleywidths(seediscussionin Table2. In computing
thestatistics,
no weightingis usedfor
byBaker[1982]. Thefloorsof somefrettedchannels
mayconsist featurelengths
(shortlinearfeatures
areincluded
onanequalbasis
of debrismantleswhosemovement
is facilitatedby interstitial
ice with longones)and,for gentlycurvilinearfeatures,
thetwo end~
[Squyres,1979]. This reportis concerned
with the originsof pointsof thefeaturedefinetheorientation.
valleyclassesI-IV: thosevalleysthat arerelictfrom earlyMars
The 264 measuredvalleys(Figure 5a) in Aeolis commonly
historyandthusindicativeof changedconditions.
occur in north to northwestorientations;the vector mean is
N48ow + 12o (95% confidenceinterval). The 56 inferred thrust
GEOMORPHOLOGICAL
MAPPING
faultsexhibitsimilarorientations
(Figure 5b), with a vectormean
Heavily
cratered
southern
andcentral
Aeolis
isaland
area
of of N63ow+ 1lO (95%confidence
interval).
Theuniform
4.3x 106km2,orapproximately
thesizeof theUnitedStates
east distributionhypothesisfor both valleys and thrustfaults can be
rejectedat the 0.05 significancelevel, andthe confidenceintervals
of the RockyMountains. The abundantMariner9 and Viking
aboutbothmeansoverlap. This, as well as visualcomparisonof
orbiter imageryavailablerangesin scalefrom single frames
the
rosediagrams(Figures5a and5b), suggest
thatvalley growth
includingall of this land area to frameswith a resolutionof 33
m/pixel
andlandarea"footprints"
ofapproximately
70km2. The
Aeolis portions of two previously published maps (the
1:15,000,000geologicmap of easternMars [Greeleyand Guest,
may have beenaffectedby preexisting,northwestorientedfaults
and/or fractures.
The strengthsof the vectormeans(R/n; Table 2) measurethe
amountof dispersionin eachdata set. Thesestatisticscan range
1987] and the global map of valleys [Cart and Clow, 1981] are
from zero for very high orientationdispersions
to 1 for very low
superimposedin Figure 2. The figure thereby illustratesan
dispersions.The valleys (R/n = 0.39) are more dispersedthanthe
apparentassociation
of valleyswith Noachianplateausequence
thrust faults (R/n = 0.75). This difference suggeststhat other
terrains,and a lack of valleys on the Hesperianridgedplains.
variablesalsoinfluencedvalley orientations.
Valleys appearto be most commonin the generalvicinity of the
In regard to the other photolineations,the calculatedvector
ridgedplains,andalsonearthe largechasmsor scarps.
mean for the 83 undifferentiated faults and fractures is N49OE, but
Figure 4 illustratesa new geomorphologicalmap of Aeolis
visual inspection of the rose diagram suggests a bimodal
Quadranglepreparedusing,asbasemaps,the 1:2,000,000Viking
distributionandtwo meanorientations,at approximatelyN10ow
photomosaics[U.S. GeologicalSurvey, 1979a, b, c, 1982]. This
map emphasizes (1) fluvial valley features, (2) other linear andN65OE (Figure5c). No confidenceintervalscanbe calculated
featuresof possibletectonic origin, and (3) ridged plains and without first subdividing these data, and the number of
individualvolcanicconstructs.Impact cratersare not illustrated observationsis not sufficientfor a reliable subdivision.Despite
these limitations, the undifferentiated faults and fractures exhibit
exceptwherethey form the originationor terminusof a mapped
aswell asdifferingmorphologies,
valley. For geomorphicmappingpurposes,"fluvial valleys"are differentdominantorientations,
than
the
thrust
faults.
Valley
genesis
may have been affectedby
narrow linear or curvilinear troughswith dimensionssimilar to
these
faults
and
fractures,
also,
as
some
overlapof orientationsis
thosegiven in Figure 3. "Undifferentiatedfaultsor fractures"are
evident
(compare
Figures
5a
and
5c).
linearor slightlycurvilinearridgesor (ill-defined)lineations.On
high-resolution
imagery,severalof theselineationsappearto be
GEOLOGICAL CONTEXTS OF AEOLIS VALLEYS
highly elongatedstripsof knobbyor hilly topography.Finally,
The orientationanalysissuggeststhat structuralfeaturesdid
"thrust faults or wrinkle ridges" are linear or curvilinear
influence
the locationof valley developmentin Aeolis. However,
asymmetricridgesthat exhibit steepscarpsand relatively gently
other
geological
factorsshouldbe importantaswell [e.g.,Kochel
slopingland on opposingsides. Thesefeaturesare the probable
and
Phillips,
1987].
Preexistingscarpsof non-tectonicoriginmay
complex surfaceexpressionsof deep-seatedfaulting within a
also
be
favorable
sites
for spring sappingto be initiated. If
compressionalstressfield [Plescia and Golombek,1986; AubeIe,
geothermal
heating
is
important,
spatialcontrolsmay be exerted
1988]. Theycommonlyformcomplexwrinkleridgesin thelower
Hesperianridged plains, but relatively simple scarpsin the by locally high geothermal gradients related to volcanic or
tectonic activity. Additional information concerning the
surrounding,
apparentlyweaker,plateausequence
deposits.
geologicalcontextsof Aeolis valley developmentis presentin the
1:2,000,000Viking photomosaics.The photomosaics
are widely
VALLEY,FAULT,ANDFP,.ACTURE
ORIENTATIONS
available [U.S. Geological Survey, 1979a, b, c, 1982] and are
Mostsapping
modelsfor valleyoriginpredictthatfaultsand not reproducedhere. The following large-scalefeaturesare
are
fractures,where present,shouldbe importantin localizing importantto the questionof Aeolis valley morphogenesis,
andare locatedin Figures2 and4.
groundwater
discharge
at thesurface.However,preferredvalley visiblein thephotomosaics,
erosionalongfaultsandfracturesmay obscurethe underlying
structural features, or such features may be covered by
undisturbed
deposits.In orderto testfor thepresence
of structural
Feature Descriptions
Feature1. Superimposed
on the flat floorof thislargecrateris
controls
onvalleyorientations,
Figure4 is abstracted
intothree a preserved
interiorremnant
("r" in Figures
2 and4) of smooth
component
maps:Figure5a, valleysonly;Figure5b,thrustfaults plainsmaterialsimilarto thatmappedimmediately
to thenorth
17,294
BRAKENRIDGE,
AEOLIS
QUADRANGLE,
MARS
'*
I
z
z
.:
.-
z,..
z
:7'
/'/,z
BRAKENRIDGE,AEOLISQUADRANGLE,
MARS
17,295
I
I
I
I
i
I
I
220 o
2oo ø
190 •
18(P
B
N=56
......
'-•
,
220 ø
210 ø
200 ø
$
190ø
180ø
Fig. 5. ThematicmapsobtainedfromFigure4, (a) Smallvalleysonly. (b) Thrustfaultsonly. (c) Undifferentiated
faultsand
fracturesonly.Also shownarerosediagramsof orientationdatafor eachclassof features.
17,296
BRAKENRIDGE, AEOLIS QUADRANGLE,MARS
l
i
220ø
210ø
200ø
190ø
180ø
Fig. 5. (continued)
[Greeley and Guest, 1987]. Light-dark banding along the
southeastern
marginsof the remnantindicatesthat this materialis
internally stratified. A V-shaped valley with four straight
segmentsbreachesthe raisedsouthwestrim of the crater(Figures
2 and4), andthe drainagedirectionwastowardthe craterfloor.
How old is the valley? Greeley and Guest [1987] map the
craterand its ejectaas superposed
on the upperNoachiansubdued
crateredunit of the plateausequence(Figure 2). However, the
ejecta ("e" in Figure 4) and the associatedstringsof secondary
the boundaryand closer to the crater . The ejecta and the
secondary
cratersthusappearto be partiallyburiedby thesubdued
crateredunit. The actualchronology
of eventsmay be (1) large
impactinto older plateausequenceunits,(2) partialburial of the
craterandejectaby thesubduedcrateredunit, and(3) subsequent
valleyerosionandremovalof muchof thecraterfill. Single,short
craters
areprominent
andsharply
definedto thesouthof •he
Feature 2. A strip of plateausequenceterrain separatestwo
lower Hesperianridgedplainsat this location,and it is heavily
subduedunit boundaryand becomeabruptly diffuse or absentat
TABLE
Landforms
n
valleys radial to old, flat-floored modified cratersare common in
Aeolis(Figure4). Valley cuttingmay extendinto Hesperiantime,
andmaybecoevalto cratermodification
processes.
2. Tectonic And Fluvial Landform Orientations
Vector
Strength
Standard
RaleighTest
Meana
of Vector
Error
for
Mean
b
Uniformity
c
Smallvalleys
264
N48øW+ 12.0ø
0.39
6.1
0.00
Undifferentiated
83
N49øE
0.19
23.1
0.05
56
N63øW + 11.2ø
0.75
5.7
0.00
faults and fractures
Thrustfaults
a Vector
mean
isarctan
[X/Y];
X = Y•cosOi;Y= • sinOi;shown
with95%confidence
intervals.
bStrength
ofvector
mean
isR/n,where
R= [X2+ y2]1/2.
CRaleigh
statistic
isexp-[R2/n]
ßforRaleigh
values
<0.05,
theuniform
vector
distribution
hypothesis
isrejected
atthe0.05level
ofsignificance.
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
17,297
dissected
by V-shapedbranching
valleys.Basedon thebranching 4). Two unusuallywideeastandnortheast
oriented(adjoining)
pattern,the most prominentvalleys shownon the Viking reachesexhibit dark floors and scallopedmarginsand are
photomosaics
draintowardthe northwest
andterminateat the transitionalnortheastward
intoan irregularcloseddepression
with
westernridgedplainborder. The locationof thisheavilydissected
landis typicalfor Aeolis.
Approximately70 km to the southwest,
a NNE trendingflatfloored straightcanyon extendsnearly to an intersectionwith a
abundantknobs that are probable collapseblocks. Genetic
processescouldincludesill volcanismand overburdencollapse,
perhapsalong a fault or fracture zone. However the chasms
themselvesoriginated, numerouspost chasm valleys extend
NW trendingscarpinterpreted
asa thrustfault. The SW-facing headwardfrom the chasmscarpsinto the surrounding
plateau
scarpis mappedasa valleyby Cart and Clow [1981], but thereis sequencematerials. One such valley, at the chasm'ssouthern
no opposingwall. Near to the scarp,the valleytumsabruptlyNW terminus,appearsto follow a secondaryscarpmappedasa thrust
and is located along it; the valley then becomesindistinct in a fault (Figure4).
complexareanearthe marginof thewesternplain. A permissive
Approximately 100 km to the east of feature 6 is an isolated
inferenceis that valley cuttingherewaspostthrustfaulting.
40-km-longplateauor mesa(Figure4). At least14 lightanddark
Feature
3. At thislocation,
dark-floored
V-shaped
branchingstratacropout alongits hillslopesand are visiblein the
valleysdissect
lighterplateau
sequence
material.Drainagephotomosaic
[U.S.GeologicalSurvey,
1979c].
Thebanding
can
direction
iswestward
andtoward
a ridged
plainthatismapped
in betraced
continuously
along
theperimeter
oftheplateau,
andthe
Figu•re
4 butnotinthesmaller
scale
Greeley
andGuest
[1987]concentric
outcrop
pattern
suggests
that
theplateau
isanerosional
map(Figure
2). Immediately
tothesouthwest
ofthevalleys,
an remnant
formed
by approximately
horizontal
strata.Similar
interdigitate
contact
isvisible
between
a darksurficial
unit(tothe regularly
bedded
internal
stratification
isnoted
alsoatfeature
1
west)anda muchlighterunit(to the east). The simplestandhasbeen
previously
described
fortheplateau
sequence
units
explanation
forthiscontact
isdifferential
stripping
ofthelighter,[Malin,1976].Theorigin
andnature
ofsuch
stratification
arean
superposed
stratum
fromanunderlying
darker
stratum.
Thus
the unresolved
question:
possibilities
include
interbedded
sediment
terrainimmediately
surrounding
the valleysappears
to be andlava;interbedded
sediments
ofvarying
clast
lithology,
mean
underlain
byseveral
stratiform
unitsofcontrasting
albedo.
The size,ormatrix;
orinterbedded
sediment
andicesimilar
tothat
..
darkvalley floorshavetappedan underlyingdarkerstratum.
occurringin the polar layeredterrains.
Feature 4. Valley classesI-III densely dissectthis plateau
sequenceterrain, and many of the valleys exhibit dark floors.
Thereis disagreement
regardingthe natureof two adjacentplains:
(1) The plainto thewestof thevalleysis not illustratedin Greeley
andGuest'ssmaller-scale
map (compareFigures2 and4). Valley
Feature 7. Faultingor fracturingwas probablyinvolvedin the
origin of this 180-km-long,north northeastorientedchasm,but
wall slumping,floor collapse,or fluid flowsalongthe chasmmay
also have played importantroles. Headward erosionthen carved
thesmallertributaryvalleysthatdebouchintothecanyonalongits
branching
directions,
however,
indicate
thatthesurface
of this southeastern
margin.
relatively smoothplain is lower than the surroundingheavily
dissectedand higher-albedolandscape.(2) Greeley and Guest
[1987] mapthe easternplain as a superposed
smoothunit of the
plateausequence,
insteadof asa ridgedplain (compareFigures2
and 4). I could find no evidencefor such superposition:the
plain'swesternmarginis belowtheadjacentsurfaceof theplateau
sequence,
andnumerousdark-flooredvalleysdebouchfrom the
plateausequencematerialsat the margin of the plain. These
valleys are arranged othogonally to the boundary, and the
complex,interdigitatenatureof the boundaryitself is similarto
Feature8. Two en echelon,northeasttrendingridgesarehere
mappedas undifferentiated
faults or fractures(Figure 4). The
westernridge,approximately
60 km in length,formsa straight
segmentalongthe northernrim of the largecraterMolesworth
(not shown),and at least sevensmall classI valleys originate
alongthisridgeanddebouch
southward
at thesmoothcraterfloor
(also not shown).The easternridge, approximately20 km in
length, is lesswell-markedbut alsois coincidentwith the heads
of four similarvalleysthat exit northwardonto a ridgedplain.
Suggestive
evidenceof furthertectoniccomplexityis presentin
that southwestof Feature3.
the form of a 25-km-diameter crater bisected by the eastern
As noted,the simplestinterpretation
of the outcroppatternat
this andotherinterdigitateboundaries
is that a dark, stratiform
unit, entombedwithin the plateau sequence,extendsfrom
relatively interior positions(where it is exposedby the deep
terminusof the westernfault (Figure4) andshowingan apparent
offset of approximately10 km in the right lateralsense(examine
U.S. Geological Survey [1979b]). Whatever the exact natureof
these faults, the associatedvalleys appear to be syntectonicor
valleyfloors)to the subaerial
p!ainssurfaceitself. The dark post-tectonicfeatures: they are not transectedby the faults.
valley
floors
mayrepresent
lavasillsorburied
lavaflows
thatare
Feature 9. This 40-km-wide, 150-km-long belt of dissected
now exhumedby erosion(see discussionof suchstratigraphyby plateausequenceis adjacentto an unusuallywell definedridged
WilhelmsandBaldwin [1989]).
plain. The valleys debouchonto this plain. At least20 classI and
Feature5. At thistypicallocation,southeastward
draining II valleyscanbe countedon the photomosaic;
10 of the more
subparallel
slope
valleys
andbranching
V-shaped
valleys
dissectprominentonesare shownon Figure 4.
the plateausequence,
andare locatedaboutthe periphery.
of an
unambiguous
ridgedplain(compareFigures2 and4).
Feature6. Immediatelyto the southof a northeastoriented
chasmalsomappedby Greeleyand Guest[1987] is a complex
andinterconnecting
systemof broad,flat-flooredchasms(Figure
Feature 10. Al-qahira Vallis, a 600 km long, 25 km wide
chasm,is discussedby Sharp and Malin [1975] as an an outflow
channel(Figure4). However,unlike many outflow channels,no
channel bedforms are visible (see discussionof such criteria by
Baker, 1982). The exceptionallystraightwall segmentssuggest
17,298
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
structural control over sappingor, perhaps, an active tectonic
(rift?) origin. In support of such inferences,other faults or
fracturesare abundantin thisvicinity,andmanyareparallelto the
NNE and WNW trendsof Al-qahira'swalls (Figure 4). In detail,
the chasm walls are scalloped,and a variety of much smaller
tributary valleys are eroded into the surrounding, mostly
undissected,plateau sequence. If faulting was involved in the
origin of Al-qahira, then thesesmall valleys alsopostdate,or are
coevalto, thisperiodof faulting.
No ridgedplainsare mappedin this area,but a 60-km-wide,
approximatelycircular, positive topographicfeature is present
along the chasm'swest side (Figure 4). Associatedradial
reentrantsseparatingsloping,plateau-likesurfacesand a visible
plateau sequencedeposits to be stratified and, perhaps,
composi.tionallyheterogeneous. All of the mapped valleys
transectthe plateausequence,
andmostare arrangedorthogonal
to, and alongthe perimeterof, ridgedvolcanicplains. However,
some valleys occur as tributaries to large chasms that are
surrounded
by otherwiselightly dissectedor undissected
plateau
sequencelandscapes.Four specificexamplesare also noted of
inferredsyn-faultingor post-faultingvalley development,and
theseexamplessupportthe orientationstatistics-based
conclusion
that faults and fracturesare importantcontrolsover valley
centralcrater suggestthat this landformis a "hydromagmatic"
(TyrrhenaPatera-like)volcaniccomplex. Suchvolcanicpiles on
Mars are inferred to result from basaltic eruptionsin ice-rich
terrains [Greeley and Spudis, 1987; Crown et al., 1988]; the
growthof this one may alsobe coupledto tectonismalongA1qahira.
Feature 11. This north and northwesttrendingchasmmay be
the erosionallyand/orslumpedsurfaceexpression
of a low-angle
thrustfault. Thus the easternportionof a 20-km-diametercrater
on the northeasternblock has, apparently,been thrust over the
southwestern
block, whereno visiblecounterpartcraterfragment
exists(Figure4; seealso U.S. GeologicalSurvey[1979a]). An
locations.
TABLE 3. Summaryof Viking Photomosaic-Based
Observations
Observation
Feature Numbers
Valleystransecting
plateausequence
deposits
Valleys orthogonalto ridgedplains
Valleys tributaryto chasmwalls
Low albedovalley floors
Syn-tectonicor post-tectonic
valleys
Evidencefor down-valleyflows
Interstratification
of plateausequence
deposits
! ,2,3,4,5,6,7,8,9,1 O, 12
2,3,4,5,8,9
6,7,10, 12
3,4,6,7
2,6,8,10, 12
7,11,12
1,6
alternative suggestedby one reviewer (that the miss!ng
southwestern
craterhalf hasbeeneroded)is possible,but thereis
no accompanying
mechanism
for erosionalremovalof onecrater
Complexinterdigitateboundariesoccurbetweenthe relatively
half and not of the other. If thrusting indeed initiated this
smooth,
low-lying, darker,ridgedplainsand the adjacenthigher,
landform'sgenesis,then the implied compressional
stressesare
lighter,plateausequence
terrains.If theridgedplains
congruentwith thosethat producedthe many other northwest dissected,
orientedthrustfaultsin this region.
A 22-km-wide volcanois presentalong the extensionof this
chasmto the southand within the inferred over-ridingblock, east
of the surfacetraceof the fault (Figure4). This volcanoexhibits
areflooredby extruded
volcanic
units.,
thentwoalternatives
could
explain such boundaries: (1) Surface lava flows, originating
within the plains,partially fill the downstreamreachesof valleys
cut into the surroundingterrain but do not cover the interfluves.
clear diagnostictopography,such as an apica! caldera, steep In this case,the valleys are embayedby, and are older than, the
symmetrical
flanks,andradialreentrants
(seealsoFigure7.15 by lavas (as inferredby Tanaka [1986]). (2) Stratiformsill lavas,
,
Greeley
[1987,
p. 165]).Theplateau
sequence
hereisnotheavilyinjectedinto the plate•tusequencefrom below, exit ontothe
dissected,but the chasm itself may have been modified by
surfac'e
atthebases
of slopes
bounding
th,
e plains.In thisevent,
downstream
the volcanicstrataforming the plainsextendin.tothe surrounding
fluid flows.
,
plateau
sequence
units,andarelocallyexposed
at thesurface
by
valley
incision
there
(as
inferred
by
Wilhelms
and
Baldwin
wide, gentlywindingchasmabout1 km deep[SharpandMalin,
Feature 12. Finally, Ma'adimVailis is a 700-km-long,15-km-
1975]. Numerousflat-flooredstraightcanyonsandothervalleys [1989]). If this is the actual stratigraphy,then valley ei'osion
or is coevalto phi.nsvolcanism.I ,favorthe second
form scatteredtributariesto it. Fluid flows may have occurred postdates
alternative
for the examples cited, becausethe relatively dark
alongthis chasm: an innerchannelexistsnearits downstream
terminus where the chasm transects a modified crater (location valley floors extend continuouslyto the valley headwalls (see
shownin Figure4), andmedial,stream-lined
ridgesoccuralong classIII valley in Figure3 for an exampleof the resultingoutcrop
several reaches of the flat chasm floor. However, tectonic pattem).
,
processes
mayalso
beinvolved
inchasm
genesis:
(1)thenorth There
also
exist
inAeolis
small,
commonly
unmapped,
ridged
andnorthwest
orientations
oftwomajor
segments
arecongruent
plains
confined
within
flat-floored
craters.
One
larger
than
normal
withthepreferred
orientation
oftheregional
thrust
faults,
and(2) example
'istherid.
gedplain
within
amodified
crater
immediately
a 30-km-diameter
flat-floored
cratertransected
by Ma'adimwestof feature
11in Figure4. A hypot.
hesisto explain
such
(south
ofthe"12"
symb.
olinFigure
4)appears
tobeleft-laterally
associations
isthat
thedeep-seated
ring
fractures
associated
with
displaced
approximately
8km.
impact
structures
provided
conduits
forcrater-interior
lavaflows
Summaw
possibly
ice-rich
post-crater
deposits
[Costard
andDollfus,
'1987].
and/or for lava sill injections into remnants.of the stratified,
Table3 liststhequalitative
photogeological
observations
as Cratermodification
processes
may,in thisevent,be genetically
theyrelateto valleygenesis.Localbandedoutcrops
indicatethe relatedtovalleydevelopment.
BRAKENRIDGE,
AEOLIS
QUADRANGLE,
MARS
FLUVIAL VALLEYS AT HIGHER RESOLUTION
Valleys
in PlateauSequence/Ridged
PlainBorderlands
Additionalobservationsof valley geologicalcontextsat higher
surfaceresolutionare useful in analyzingvalley origins. Figure 6
is a geomorphological
map of a portionof southernAeolis
17,299
constructedfrom a mosaicof Viking orbiter images;seeFigure4
for the location within Aeolis. Illustrated are ridged plain and
plateausequenceterrains,inferredthrustfaults (wrinkle ridges),a
prominent lava flow front within one of the ridged plains, other
faults or fractures,flat-floored straight canyons,and small Vshapedbranching valleys. In agreement with the observations
Ejecta
.:........•
Unmodified,
SuperposedCraters
Modified
Craters
t'-'- •
I
I
,
,
i
i
i
Buried
Craters
i
Frames A-D,
i
Npld
Figure 7
ii$
,
Npld
k
RidgesInterpretedas ThrustFaults
ß
Possible Volcanic
Constructs
Ridged Plains
(arrowsindicateflow margin)
Modified
20 km
Crater Floor Sediments
Valleys or Large CollapseDepressions
Undifferentiated
Faults or Fractures
DissectedUnit of the PlateauSequence
s: Smoothto rough,largelyintact
JNpld
,]
r: Isolated smooth remnants
d' Intricatelydissected
k: Karst-like
Fig.6. Geomorphological
mapofaportion
ofAeolis
Quadrangle,
illustrating
valley
development
along
plateau
sequence/ridged
plainborderlands.
Themapisbased
ona mosaic
of 11Vikingframes
(425S27-31'
426S26-31);
maplocation
isillustrated
in
Figure 4.
17,300
BRAKENRIDGE,
AEOLISQUADRANGLE,
MARS
madeabove,thefluviallandforms
(1) areincisedintotheplateau
sequencedeposits,(2) are orientedorthogonal!y
to the ridged
plain,and(3) terminate
at thetheridgedplain. Figures7a, 7b,7c,
and7d arefourof theVikingframesusedin producing
themap.
In Figure7a, thesmallbranching
valleybelowandto theright
of the "A" symbolcould be interpreted,on lower-resolution
imagery,as embayedby the lava plains(to the left). However,
thisframedemonstrates
thatthematerialsincisedby thevalleyare
sequence
is modifiedby a complex,closelyspacednetworkof
closed
depressions:
theentiresurface
ispittedandalsois gouged
bytroughs
(the"karst-like"
plateau
sequence
ofFigure6). Several
flat-floored
straight
canyons
alsooccur,
butregional
collapse
here
dominated
overfluvialerosion.Clearembayment
relations
with
theplains
areabsent.
Instead,
theridged
plainmaterial
interfingers
in a verycomplex
manner
withthecollapsed
plateausequence.
Thisdetailismissing
onthe1:2,000,000
Vikingphotomosaics
[U.
at a considerable
altitudeabovethe ridgedplain (notethe scarp S. Geological
Survey 1979b], whichmisleadingly
showthe
near the widest portion of the valley). Instead of having boundary
toberelatively
sharp
andcongruent
withanembayment
undergone
embayment,
thevalleymusthavedeveloped
duringor interpretation.
afterplainsemplacement.
In thelowerrightquarterof theimage,
The lava flow front mappedin Figure6 and illustratedin
abundant irregular closed depressionscause a scab-like Figure7b suggests
thatlavaventingoccurred
froma source
area
appearance
of thismarginalplateausequence
area. The boundary to thewest,butventsarenotvisible.It is possible
thatthevents
between the plains and the plateau sequenceis not sharp,as lie buriedwithinthe collapsed
plateausequence
thatformsthe
expectedfor embayment,
butis irregularandmarkedby apparent westernmarginof the plain. Movementof effusivelavasto the
collapseof theplateausequence
materialat somelocations.
eastcouldhaveoccurred
asoneormorelavasillslocalized
along
In Figure 7b, a visible lava flow front (arrows; also see subsurface
lithological
discontinuities.
Thesesillsmaythenhave
examplesfrom Theilig and Greeley [1986]) is approximately emerged
as subaeriallavaflowsin the areanowmappedas a
parallelto the ridgedplain/plateau
sequence
contact,but the ridgedplain. If plateausequence
strataincludeice-cemented
eastward
flowing
lavadidnotreachthatboundary.
Several
flat- clasticmaterial,
sill volcanism
couldexplainplateau
sequence
flooredstraight
canyons
andmodifiedcraters
areflooredby collapse
(seealsoWilhelms
andBaldwin
[1989]andSquyres
etal.
similarappearing,smoothmaterialthatis continuous
with the lava [1987]).
plain(center
of image).In thelowerrightcorner
of theframe, Analternative
stratigraphy
could
belocally
important
atridged
threeV-shaped
valleys
extend
headward
intotheplateau
sequence
plain/plateau
sequence
borders.Studies
of thestratigraphy
of
from scarpsproducedby local collapse,again at the ridged ColumbiaPlateau(U.S.)basaltsdocument"invasive"behaviorof
plains/plateau
sequenceboundary(seealsoFigure6). Valley thoselavaswhere they encountered
much less densemarine
erosion
washerepreceded
by scarpproduction,
andthecollapsesediments.
There,120m thicksurface
lavaflowsdeeply
intruded,
features
themselves
mayberelatedto thenearbyvolcanism.An at theirmargins,
siliclastic
sediment
piles[WellsandNiem,1987;
alternative
hypothesis
(thatplainsvolcanism
simplyembays
older, ByeflyandSwanson,
1987;PfaffandBeeson,1987]. If sediment
alreadydissected
andlocallycollapsed
terrain)lackstheneededdensities
of theplateausequence
strataarerelativelylow,then
supporting
evidenceof clearlavaflow frontsalongthe complex invasivelava behaviormay have occurred. This couldalso
andhighlyirregular
plains/plateau
sequence
contact.
explain
someinterfingering
of plateau
sequence
andtheridged
Given the presenceof subaeriallava flows,someexamples plainmaterials.
shouldexist of lava embaymentcontactswith older landforms. In
Figure 7c, the easternmarginof the plateausequencehighlandis
densely dissectedby numerous small valleys and ravines (the
"intricatelydissected"
unit of Figure6). The fluvial landformsare
rectilinear,parallel,or digitatenear the right centerof the frame,
and suchdetailedmodificationof the plateausequence
borderland
is accompaniedby two relatively large, flat-floored branching
canyons. The plains/plateau sequence contact here could
reasonablybe interpretedas one of embayment: it is relatively
sharpandregular,andthe intricatelydissected
hillslopesappearto
dip, at variousangles,into andbelowthe plainsmaterial.
Despitethepossibilityof embayment,it is notclearthatfluvial
ValleysWithinThePlateauSequence
Several
ancient
Aeolisflat-floored
branching
valleyscoalesce
intointegrated
systems
extending
hundreds
of kilometers;
they
followregional
topographic
gradients,
exhibitvalleywidthsof 510kmormore,andarenotproximal
tolargeridgedplainsunits.
A geological
mapandimageof a portion
of sucha valleysystem
compriseFigures8a and 8b; see Figure 4 for the locationin
Aeolis. This and similar integratedvalley systemsappearto
constitute
theclearestevidencefor greatlychangedatmospheric
conditions
onMars. However,Brakenridge
et al. [ 1985]propose
an alternativehypothesis:that the valley systemdevelopedin a
landscape
modification
herepredates
plainsvolcanism.The piecemeal
fashion,through
headward
sapping
andfluidflows
floorsof the relativelylargestraightcanyonsarecontinuous
with,
and not embayedby, the plains. Severalcone-shaped,
radially
rilled mountains occur on the plateau sequence near its
northwestern
border(markedby arrows,top of Figure7c; seealso
Figure 6). These mountainsappear to be volcanic constructs
(usingcriteriagiven in Greeleyand Spudis[1987]) and they are
accompaniedby extensivemodificationof the plateausequence.
causedby impactmeltsandthermalsprings.
Part of the Brakenridge et al. [1985] hypothesismay be
unnecessarily
restrictive. Thus,endogenetic
volcanismas well as
impactmeltcouldbe an important
heatsourcefor thermalsprings.
As illustrated
in theearlierreport, two branches
of theFigure8
systemforma 95-kin-widecircularpattern.However,theterrain
interiorto thesebranchesis complexanditself cratered:no direct
Volcanismandvalleydissection
in thisexampleare,at theleast, association
of a largecraterwiththevalleysis nowvisible.Better
spatiallyrelated;theymay alsobe causallyrelated(seeother candidate
sitesexistfor directimpactmeltheatingasa factorin
examplesin WilhelmsandBaldwin,1989).
valleygenesis(e.g. seeMouginis-Mark[1987,p. 282]. Although
In Figure7d, and to the westof the ridgedplain, the plateau preferentialexcavationof the Figure 8 flat-floored branching
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
17,301
17,302
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
17 303
A
N
Fig. 8. (a) Geomorphological
mapand (b) image(Viking frame596A26) of a portionof the extensiveflat-flooredbranching
valley systemlocatedin Figure4. See Figure6 for symbologykey to the map. Lettersmark locationsdiscussed
in the text,and
the positionof Figure 9 is also illustrated.
valleysmay indeedbe relatedto an old, buried,impact-associated"c"). The plains may representeither subaeriallava flows or
ringfracturesystem,valley developmentcouldhavegreatlypost- exhumedsills associatedwith fissureridge emplacement.The
datedcoolingof theimpactmelt.
combinedigneousactivitycertainlycouldhaveprovidedlocal
The internallycomplex nature of the plateausequenceis heatsources
for icemeltingandthermalsprings.
visible in Figure 8 becausesome strata have been partially
The entire valley networkis locally interruptedat numerous
removed.At locations"a"and"b"(Figure8a), straightknife-like locationsand especiallyin the headwaterregions{,seedetailed
ridgesexhibit much lower albedosthan surroundinglithologies, map in Brake/,'idge et al. [1985]). It is not certainwhether(1) a
andaresimilarto featuresinterpreted
asfissureeruptionridgesby continuous,
integratedvalleysystemonceexistedand wasthen
Wilhelms[1986]. Their crosssections
areexposedin the wall of a modifiedby post-valleyresurfacingprocesses
suchas cratering,
prominenterosionalscarp(compareFigures8a and8b to locate volcanism,or eoliandeposition;
or (2) the valleylinksdeveloped
this exposure;note clear outcropat letter "b"). Two plains independently,
at differenttimes. In the lattercase,the system
adjacentto the scarpare underlainby similar,dark material,and wasnevermoreintegrated
thanat present.
an igneousoriginis supported
by their physicalcontinuitywith
Someevidencesupports
the latterpossibility.Althoughpost-
the knife-likeridges. A thirdplainlies to the north(nearletter valley modifications
are obviousat somelocations(e.g., fresh
17,304
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
B
t
Fig. 8. (continued)
superposed
cratersinterrupta valleysegmentnear"c"), other continuousto an abrupt terminus approximately 150 km to the
valleyinterruptions
appearto beprimary.Forexample,northwest northeast,at an elevation approximately 500 m lower (see also
trendingridgesinterpreted
asthrustfaultsarecommonin Figure Brakenridge et al. [1985]). The valley is no wider at the terminus
8. Isolatedvalleylinksof thevalleysystemterminateat them(at
than it is far upstream, and its general morphology resembles
whichmay still be activetoday(see
"c" in Figure8a). In agreement
with the regionalorientation certainfrettedchannels
example
of
Cart
[
1981,
pp.
154-155]; also,comparevalley classes
analysis,
a majornorthwest
oriented
tributary
valley(nearlocation
IV
and
VI,
Figure
3).
Carr
considers
the young fretted channelsto
"e",Figure8a) isalignedalonga thrustfault: thislocalepisode
of
result
from
some
combination
of
valley-side
mass wasting and
valleydevelopment
postdates
thefaulting.Also,a valleysegment
downvalley debrisflow, possiblyassistedby interstitial ice. Such
Similarprimarydrainagenetgapsoccuralongotherbranching processesmay also have been active in the ancient past, and large
valley networks,andthey are suggestive
evidencefor valley environmentalchangesare not requiredfor their occurrence.
breachesthe craterrim at "f", insteadof being transectedby it.
erosion controlled mainly by local fault and fracture systems
Other Evidencefor Valley Mo;phogenesis
insteadof by topography.
Individual, continuousdownstreamreachesalong this valley
Figures9a (Viking frame) and 9b (mapof the frame)provide
of one reachof
systemdo reachconsiderable
lengths.At "f" in Figure8a, the a closeview of the morphologyand stratigraphy
valley originatingat the breachedmodified-crater
rim first theabove-discussed
valley. In theframe,a flat-floored
modified
transectsa rounded, northwestoriented ridge. It is then craterseparates
two valley segments,
anda relativelydark, thin,
BRAKENRIDGE,AEOLISQUADRANGLE,MARS
17,305
A
Fig. 9. (a) Locationmapand(b) imageof a smallportionof theflat-flooredbranchingvalleysystemmappedin Figure8. The
imageis croppedfrom Viking frame427S03andis approximately
32 km in width. Seetext for descriptions
at lettersA, B, andC.
In Figure9a, the heavystipplingis inferreddark igneousmaterial,and the whitearcstransecting
the valleyfloorsarescarps
discussed in the text.
"A" in Figure 9, and also to the left of the letter "C", are small,
faintly lobate scarpssituatedacrossthe fiat valley floor. The
exhumedlavasill or a lavaflow. Stratigraphically
belowthisunit scarpsmay mark the distaltermini of downvalley.freezingwater,
is a higheralbedo,slope-forming
stratumthatis muchthickerand ice, and/ordebrisflows. Episodicvalley growthandmodification
couldrepresentimpactejecta,volcanicash,or eolian-reworked would then be implied, and also valley erosionby wall-to-wall
materials,perhapsonceassociated
with interstitialice. The light fluid flow. This valley may actually be a relict channel. !,2)
stratumrests,in turn, on anotherthin, dark unit (below letter "B" Valley-filling fluid flows are seeminglysupportedby the presence
in Figure9). The entire sequenceis now boundedby a deep, of a long narrowinterfiuvedownstreamfrom thejunctionof two
trough-shaped
depression
(immediatelyadjacentto "B"), whichis tributary valleys: imlnediately above "C" in Figure 9. Such
developed
at the approximate
positionof the old craterrim. It is interfiuvesare typical of glaciatedpiedmontvalley junctionson
clearthatsubsidence
hasoccurledin this area. It is possiblethatit Earth and also some fretted channeljunctions on Mars. These
and resistantstratiformunit occupiesthe centralportionsof the
crater's interior (above letter "A"). This may be either an
could be involved,also, in the initial genesisof nearby valley
small-scale features are near the limit of frame resolution, and
alternative geneticmodelsexist. They do indicate,however, that
Two other geomorphicfeaturespreservedalong the present new imagesfrom Mars Observerwill be useful in constraining
valleysmayalsorelateto geneticprocesses:(1) Belowthe letter valley origin modelsand associatedclimatic changeinferences.
segmentsalong lhults or fractures.
17,306
BRAKENRIDGE,AEOLiSQUADRANGLE,MARS
B
o
©
c
A
Fig. 9. (continued)
CONCLUSIONS
episodic)downvalleywater,ice,anddebrisflows,andheadward
erosionalongstrataldiscontinuities
andindividualconduitfaults
The valleys mapped in Aeolis exhibit strong preferred and fractures.
AlthoughAeolisvalleysmighthavebeencarvedby seepage
alignmentsthat suggestthe pastoperationof structuralgeologic
controlsover valley location.This supportsthe generalconclusion flows withoutthe interventionof hot springs[Pieri, 1980], this
and pressures
thatheadwardspringsappingis importantin valley genesis[Sharp alternativerequiresthat past meantemperatures
and Malin, 1975; ?ieri, 1980]. Also, evidence exists at a variety
of image scalesfor ancient volcanic activity near many Aeolis
•talleys. Probable fissure eruption ridges, small volcanos,
exhumed lava sills, and collapsed, (scabby) karst-like
morphologiesall occur within the fluvially dissectedplateau
sequence,andthesefeaturessuggest
the occurrence
of extensive
subsurfaceigneousactivity. A possiblevalley genesismodel is
that, in response to widespread effusive volcanism in
interstratified, ice-rich terrains, local subsidenceoccurred along
weremuchhigherin orderto allow springconduitsto remain
open. In theabsence
of independent,
non-ambiguous
evidence
for therequired
largeamountof climaticchange,
thecoldspring
sapping
hypothesis
is morecomplex.Also,certainaspects
of'
Martianfluvial morphology
arenotexplainedby climate-induced
valleycuttingbutareby thethermalspringmodel.Theseaspects
are (1) the nearlycompleterestriction
of valleysto the possibly
ice-rich plateau sequenceunits, (2) the commonspatial
association
of denselydissected
plateausequence
materialswith
plains,(3) theintermittent
gapsalongbranching
fractures
andfaultsandproduced
scarps
thatintersected
local nearbyvolcanic
aquifers. Heatedspringdischarges
issuingfrom thesescarpsmay valley networksthat suggestactuallack of continuityduring
and(4) themaintenance
of constant
trunkvalley
thenhave carvedthe valleysthrougha combinationof (probably valleyformation,
BRAKENRIDGE, AEOLIS QUADRANGLE, MARS
17,307
widths along hundredsof kilometer-longvalley reaches,into detailedcriticismandhelpfulcomments
on theoriginalmanuscript.
I
whichdebouchnumerous
tributaryvalleys. It is unlikelythatthe thankGeorge
Brakenridge
forphotographic
enlargements
of manyViking
wassupported
byNASAMarsDataAnalysis
branchingvalleynetworkseverfunctioned,asterrestrialnetworks orbiterframes.Theresearch
Program
grantNAGW-1082andisa contribution
of theNASA-sponsored
do, to collect and transportwater from headwatersto the mouth.
"Mars:
Evolution
of
Volcanism,
Tectonism,
and
Volatiles"
project.
Theoretical models of early atmospheric evolution are
sometimescited incorrectly as independentevidence for the
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