JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. Bll, PAGES 17,289-17,308, OCTOBER 10, 1990

The OriginOf FluvialValleys And EarlyGeologic History, Aeolis Quadrangle, Mars

G. ROBERT BRAKENRIDGE

Su•ficialProcesses Laboratory, Department of Geography,Dartmouth College, Hanover, New Hampshire

In southernAeolis Quadrangle in easternMars, parallel slope valleys, flat-floored branching valleys, V- shapedbranching valleys, and flat-floored straight canyons dissect the heavilycratered plateau sequence. Associatedknife-like ridges are interpretedas fissure eruption vents, and thin, dark, stratiform outcrops are interpretedas exhumed igneous sills or lavaflows. Ridgedlava plains are also common but are not themselves modifiedby fluvialprocesses. I mapped 56 asymmetricscarps or ridgesthat are probable thrust faults. These faultsexhibit an orientationvector mean of N63ow + 11o (95% confidenceinterval), and they transect the lava plainsand the olderplateau sequence units. By comparison,the vectormean for the 264 valleysmapped is N48ow + 12o, witha largerdispersion about the mean. The similar orientations displayed by thrustfault and valley axessuggest that valley locationsare partlycontrolled by preexistingthrust faults and related fracture systems.Most valleysare alsoarranged orthogonally to, and alongthe perimeterof, the ridgedplains. A possiblemodel for valley developmentis: (1) freshlyoutgassed water became entombed as frost, snow, and ice within the crateredterrains during heavy bombardment and the accompanyingdeposition of impactejecta, volcanicash, and eolian materials, (2) effusivevolcanism and lava sill emplacementheated subsurface ice in the vicinity of the ridgedplains, and faults and fracturesprovided zones of increasedpermeability for water transportto the surface,and (3) headwardsapping at thermal springs,thermokarst subsidence, and limited downvalleyfluid flows thencarved and modifiedthe valleys.

INTRODUCTION by conductionthrough modeled ice-covered rivers is slow, and latent heat is added to the system by water freezing (for a Ever sincetheir discoveryduring the 1972 Mariner 9 planetary terrestrialexample, see Corbin and Benson[1983]). The limiting mission, the ancient fluvial valley networks of Mars have been factorfor fluvial activityon Mars is waterrelease, and not water describedas relict from an earlier warmer and denseratmosphere persistenceas an erosive fluid once discharged[Wallace and [Sharp and Malin, 1975; Masursky et al., 1977; Chapman and Sagan, 1978; Cart, 1983]. Water releasescould be related to a Jones, 1977; Pollack, 1979; Cess et al., 1980; Pollack and Yung, hydrologicalcycle anda denseatmosphere, but otheralternatives 1980; Mars Channel Working Group, 1983; Kahn, 1985; Pollack are (1) solar heating and melting of dust-rich snow and ice et al., 1987]. If this inference is true, then these dry valleys depositedunder high obliquityorbital conditions[Jakosky and constitute spectacular evidence for planetary-scale climatic Cart, 1987; Clow, 1987], or (2) geothermallyheated waters change. Liquid water is not presently stable anywhere on the reachingthe surfacethrough fractures and faults [Brakem'idge et planer'ssurface, and the operationof an Earth-like hydrological al., 1985; Wilhelms, 1986;Brakenridge, 1987, 1988; Gulick et cycle on Mars requiresa much warmer atmosphereand one very aI., 1988; Wilhelmsand BaMwin, 1989]. Given thesealternatives, much denserthan the 7-mbar atmosphereof today [Pollack et al., the inferenceof an earlydense paleoatmosphere may be in error. 1987]. Also, the valleys are nearly restrictedto heavily cratered Do the ancientvalley networkscompel inference of a large landscapesdating from the Heavy Bombardmentperiod of early amountof climaticchange, or may othergenetic models, without solarsystem history [Pieri, 1976; Cart and Clow, 1981]. During climatic change,suffice? The presentreport demonstratesthat this period,the Sun'sluminosity may have beenonly 70% of its Martian valleys in Aeolis Quadrangleexhibit spatial patterns, presentvalue [Gough, 1981]. It is therefore unlikely that the stratigraphicrelationships, and morphologiesthat are compatible valleys are the direct result of an earlier, more favorable climate with genesisthrough volcanism-induced hot springdischarges. associatedwith solarevolution. Large amountsof climaticchange The followingsections (1) summarizeknown geologic events that are most easily explained by depletion of an early dense occurredduring the time periodof valley evolution(2) document atmosphererich in CO2, H20, or someother greenhouse gas [e.g. detailedstratigraphic and spatialrelationships of Aeolisvalleys to Cart, 1987]. This denseatmosphere might have temporarily kept tectonic features and volcanic landforms; and (3) describe local theplanet warm, despite a faintersun [Pollack et aI., 1987]. evidencefor onevalley network's episodic growth by subsidence, Calculationsfor ice-coveredriver flows on Mars [Cart, 1983] headwardsapping, and downvalleyfluid flows. suggestthat fluvial features could form at present if sustained GEOLOGICUNITS IN AEOLISQUADRANGLE waterdischarge at the surfacewere to somehowoccur. Heat loss Two disparate landscapesexist on Mars. One is heavily Copyright1990 by the AmericanGeophysical Union. crateredand is dissectedby relict valleys,and the otheris lightly crateredor uncrateredand is undissected.Aeolis Quadrangleis Papernumber 90JB00540. astride the planet-wide boundarybetween thesetwo landscapes 0148-0227/90/90JB-00540505.00 (Figure 1). The southern,heavily crateredlandscape was created

17,289 17,290 BRAKENRIDGE,AEOLIS QUADRANGLE, MARS

North indicatesthat most preserved ridged plains formed near the endo! I I ,] I • I • • !. ] . [ ,,I ] heavybombardment [Barlow, 1988]. An earlyHesperian age for mostundissected ridged plains has _ - 60 been usedto constrainvalley genesisin time. The following chronologyis inferredby 'Fanaka[1986] in his globalsummary: 50 (1) Noachiandeposition of plateausequence strata, (,2) late 40 Nochian fluvial dissection,and (3) early Hesperianembayment ElysiumPlanitia I• .Qlympus30 andpartial burial of theplateau sequence materials by extruded lvlons lavas. This places a discreteinterval of plains volcanism subsequentto extensivefluvial valley development[Tanaka, 1986] and also impliesthat any climatefavorable to valley developmenthad ended by Hesperiantime: mostlandforms of thisand younger age are not dissected. However, any chronology ;•:• • z:•• • :'-:• • 10 , ,•,• AmazonisPlanitiamust be reconstructedfrom the preservedgeologic record, and 20 '.:-:.• ..:.:':-:':':':':'. •P;:•2>;' ••:;• ;, • • ': preservationfactors should also be considered. Wilheims [1987, p. 279]models lava plains on the Moon as the visible results of ...... :...... :e'.•e• '.•m• :•:• ß '-x•S••••P• •/•.-• --[ increasedpreservation, not increasedextrusion, as large-impact rates declined at the end of heavy bombardment. A similar geneticmodel for ridgedlava plain preservation may apply to •... . • •,•.•:•. Mars. In thisrespect, the mapsof Scottand Tanaka [1986] and Greeleyand Guest [1987]also include widely scattered, older ridgedlava plains of Noachianage. These authors infer, as well, 240 210 180 150 120 that interbeddedflow volcanicsare a common internal component Fig. 1. Mapof majorlandscapes near Aeolis Quadrangle and their of the plateausequence. Extensive plains volcanism may, inte•edages, as redrawn from Barlow [1988]. The dark shading indicates therefore,have beenunderway during Noachian time, but such surfacesFormed during h•avy bombardment, the lighter shading indicates volcanismwas not widelypreserved before the earlyHesperian. surfacesformed n•ar the endot heavybombardmere (similar crater size TableI givesthis alternative process history reconstruction. The &equencydistribution, but lower crater densities), and the white areas are reconstructionagrees with the preserved stratigraphy described by lighfiycraigred or uncrat•redsurlhccs Formed after the end ot th• h•avy bombardmereflux, approximately3.2 Ga. The agesot OlympusMons Tanaka[1986] and with the craterstatistic results of Gurnis and•hree volcanos in ElysiumPlanitia are alsoshowfl. [1981] andBarlow [1988]. It impliesthat fluvial valley developmentand ridged plain volcanism overlapped in time. Directcrater dates on valley networksby Baker and Partridge duringthe final stage of planetaryaccretion (the late heavy [1986]also indicate that valley networks range from Noachian bombardment),and local examplesoccur of denselycratered throughearly Hesperian in ageand thus independently support surfacesexhibiting lunar-like preservation of small craters [e.g., plainsvolcanism and valley development ascoeval processes. Cart, 1981,p. 69]. In contrast,the northern plains landscape is

post-heavybombardment in age, and may consist of sedimentary VALLEY CLASSIFICATION plainsand/or lava flows. The cause of theplanetary dichotomy representedby thesetwo landscapesremains controversial Valleys developed on plateau sequenceunits exhibit [Wilhelmsand Squyres 1984; Wise et al., 1979]. semicirculartheater-shaped headwalls and steep valley sides, Martiantime stratigraphy is divided into the Noachian System, relativelyfew andshort tributaries, and aligned straight segments theHesperian System, and the Amazonian System [Tanaka, 1986] suggestiveof faultor fracturecontrol [Sharp and Malin, 1975]. In (seealso Table 1). Eachsystem is furthersubdivided into series, contrastto runoff-createdvalleys on the Earth,drainage densities eachwith mapped reference units. The heavily cratered landscape are very low, andtributary junction angles do not progressively in Aeolisis, mainly,of Noachianage and includes the dissected increasein the downstreamdirection. Apparently, structural unit andthe cratered unit (bothmiddle Noachian) and the subdued controlson Martianvalley location"randomize the junction angle unit (upperNoachian) of the plateausequence [Greeley and systematics,while regularizingthe link orientations"(D. Pieri, Guest,1987]. Scatteredwithin the plateau sequence landscape are written communication,1989). Such morphologicobservations ridgedplain units of inferredLower Hesperian age (Figure 2). suggestheadward erosion from initial spring locations, instead of The plateausequence may includeinterstratified impact surficial runoff and progressivedownstream integration into brecciaand ejecta, reworked aeolian debris, fluvial or lacustrineincreasingly larger, topography following channels [Pieri, 1976, deposits,lava flows or sills,and, possibly, ice [Tanaka, 1986; 1980]. Wilhelms and Baldwin, 1989]. These depositsare locally An earlier classification of Martian valleys is based on dissectedby valleysof probablefluvial origin (,Figure 2). In planimetricand cross-section morphology [Brakenridge eta!., contrast,the lowerHesperian ridged plains are mapped as lava 1985]. Figure3 is a modifiedversion of thatclassification and flowplains [e.g. Tanaka, 1986; Greeley and Guest, 1987], and also incorporatescommonly observed geologic contexts. The theyare not commonly dissected by fluvialvalleys [Tanaka, numberof valleyclasses has been increased from five to six, and 1986]. The sizedistribution of the superposedimpact craters class numbers are revised to establish a size trend from smaller BRAKENRIDGE, AEOLlS QUADRANGLE, MARS 17,291

TABLE 1. GeologicalContext of Valley Developmentin AeolisQuadrangle

Systema Series a Age Preserved Processes t' Gab Geologic Units a

Hesperian Lower 3.2 ridged plains; smoothunit heavy bombardmentends, of the plateau sequence ejectaand ice depositionslow, effusive volcanism continues, lava plainsare widelypreserved, widespreadthrust faulting

Noachian Upper 3.5 subduedunit of the continuedheavy bombardment, plateau sequence ejectaand ice deposition,effusive volcanism,formation of Ai-qahira and Ma'adim Vailes

Middle 3.85 hilly, dissected,and solidification of the crust, Noachian and cratered units of the heavy bombardment, Lower 3.92 plateausequence ejectaand ice deposition, effusive volcanism

aModified from Tanaka [ 19861,Greeley and Gleest[ 19871,and Barlow [ 1988]. bSpeculative(crater statistics-based) agesrefer to upper boundary ofseries. cValleydevelopment n-my have occurred during all threetime intervals.

North I...• II200km I mp12/,• Ap'ollinariseatera I • 6 • I ?"::::!::i•...... -10 ---:- ;-......

-2O

-30 220 210 200 190 180 Fig.2. Geologicmap of AeolisQuadrangle, illustrating the older geologic units of Greeleya,d G,est[ 1987]and the fluvial valleysof Cart a,d Ciow [19811.Dark shading illustrates lower Hesperian ridged plains units, "Npld" is thedissected unit, "Npll"the cratered unit, "Np12" is thesubdued unit, and "Hp13" is the smooth unit of the(middle Noachian to early Hesperian) plateausequence. The dashed border separating the dissected unit from the cratered unit indicates the gradational contact between thesetwo map units. The valleys are shown as thin solid lines, and the wrinkle ridges are shown as thick dashed lines; thick solid linesare large chasms or scarps.At feature1, "r"marks smooth plains material, and "e" marks ejecta associated with this crater. All numbered features are described in the text. 17,292 BRAKENRIDGE,AEOLIS QUADRANGLE, MARS

PARALLEL V-SHAPED SLOPE VALLEYS BRANCHING VALLEYS

II (Upstream) 5 km

(Downstream) .3-5 km 2 km

FLAT-FLOORED STRAIGHT FLAT-FLOORED BRANCHING CANYONS VALLEYS III IV I 'I i I

5 km

1 km

, -1okm.7S km

V-SHAPED TRIBUTARY CHASMS FRETTEDCHANNELS

V VI

10 km 5-30 km

Fig. 3. Combinedplanimetric/cross-sectional classification of valley networkson Mars, revisedfrom B,'aken,'idgeet al. [ 1985]. Shown for each classare representativemap views (on the left) and valley cross sections(on the right); dimensionsare approximate.Typical relationships to surfitcegeology are alsoshown, "RP" symbolizesridged plains flow volcanics,"PS" units areplateau sequence materials, "Cf' represent.,/.'modified crater floor deposits,and "Avf" indicateslarge chasm interior deposits.

(classesI and II) to larger(classes III-VI) valleys. Valley widths 10 km). The straightcanyons are relativelyshort and commonly givenin thefigure are typicalvalues; depths are estimated and are debouchat ridged plain/plateausequence boundaries. Most not well established. Note that the enormous, hundreds of exhibitsmooth floors that appear to be continuouswith theplains kilometer-wide, "outflow channels" [Mars Channel Working (Figure3). Theheadward terminations of thesecanyons are steep, Group, 1983] are not includedin Figure 3. As determinedfrom andthe straightnessof canyonwall orientationssuggest fault or cratercounting, the outflowchannels are of a widevariety of ages fracture controls. In contrastto the canyons, the flat-floored and have, therefore,never been usedas evidencefor or againstan branchingvalleys traverse hundreds of kilometersof complex early denseMars atmosphere. plateausequence landscapes (Figure 3). Large-scalecircular ClassI valleys(parallel slope valleys) and class II valleys(V- patternsmay suggest that their locations are partly controlled by shapedbranching valleys) includethe finest scalevalleys visible impact-relatedfaults or fractures[Brakenridge et al., 1985; on Viking imagery.The parallelslope valleys occur on the flanks Gulick,1986; Schultz et al., 1982]. Gapsbetween segments of of large modified crater landformsor on other steep,relatively thesedry valleys may be locallycaused by post-valleyfaulting or uniformslopes that are adjacentto low-albedoridged plains. The by piecemealvalley genesis through headward sapping (Figure 3; V-shaped branching valleys are of similar dimensionsand seealso following discussion). geologicsetting, but branchupstream, and their upstream reaches The last two classesoccur only at a few locationswithin exhibit narrow, V-shaped cross sections. Both valley classes Aeolis.Many classV (V-shapedtributary chasm) and class VI commonly dissectthe plateau sequence,and terminate at the (frettedchannel) valleys are young uncratered landforms and are, marginsof adjacentridged plains (Figure 3). therefore,not evidencefor changedconditions. The V-shaped Class III (flat-floored straight canyons) and class IV (flat- chasmsare characterizedby steepgradients, and by largewidths flooredbranching valleys) exhibit flat valley floors,and widths anddepths compared to lengths.They occur as tributaries to the comparableto terrestrialriver valleys of moderateto largesize (5- Valles Marineris and other large chasmsand to some large BRAKENRIDGE,AEOLIS QUADRANGLE, MARS 17,293 outflowchannels. The V shapemay resultfrom intersectingonly; and Figure 5c, undifferentiatedfaults and fractures only. debrisslopes that are still active.The frettedchannels exhibit The measuredorientations are illustratedby rosediagrams in locallysinuous valley reaches, suggesting that downstream fluid thesemaps, and the relevant descriptive statistics are summarized flowsmay have occupied the entire valley widths (see discussion in Table2. In computingthe statistics,no weightingis usedfor byBaker [1982]. The floorsof somefretted channels may consist featurelengths (short linear features are included on an equalbasis of debrismantles whose movement is facilitatedby interstitialice with longones) and, for gentlycurvilinear features, the two end~ [Squyres,1979]. This reportis concernedwith the originsof pointsof thefeature define the orientation. valleyclasses I-IV: thosevalleys that are relict from early Mars The 264 measuredvalleys (Figure 5a) in Aeolis commonly historyand thus indicative of changedconditions. occur in north to northwestorientations; the vector mean is N48ow + 12o (95% confidenceinterval). The 56 inferred thrust GEOMORPHOLOGICALMAPPING faults exhibitsimilar orientations (Figure 5b), with a vectormean Heavilycratered southern andcentral Aeolis isa landarea of of N63ow+ 1lO (95%confidence interval). The uniform 4.3x 106km2, or approximately thesize of theUnited States east distributionhypothesis for both valleys and thrustfaults can be rejectedat the 0.05 significancelevel, andthe confidenceintervals of the Rocky Mountains. The abundantMariner 9 and Viking orbiter imagery availableranges in scale from single frames aboutboth meansoverlap. This, as well as visualcomparison of includingall of this land area to frameswith a resolutionof 33 the rosediagrams (Figures 5a and5b), suggestthat valley growth may have beenaffected by preexisting,northwest oriented faults m/pixeland land area "footprints" ofapproximately 70km 2. The and/or fractures. Aeolis portions of two previously published maps (the The strengthsof the vectormeans (R/n; Table 2) measurethe 1:15,000,000geologic map of easternMars [Greeleyand Guest, amountof dispersionin eachdata set. Thesestatistics can 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 dispersedthan the apparentassociation of valleyswith Noachianplateau sequence thrust faults (R/n = 0.75). This difference suggeststhat other terrains,and a lack of valleys on the Hesperianridged plains. variablesalso influenced valley orientations. Valleys appearto be most commonin the generalvicinity of the In regard to the other photolineations,the calculatedvector ridgedplains, and also near the largechasms or 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 Quadrangleprepared using, as basemaps, the 1:2,000,000Viking distributionand two meanorientations, at approximatelyN10ow photomosaics[U.S. GeologicalSurvey, 1979a, b, c, 1982]. This map emphasizes (1) fluvial valley features, (2) other linear and N65OE (Figure5c). No confidenceintervals can be calculated featuresof possibletectonic origin, and (3) ridged plains and without first subdividing these data, and the number of individualvolcanic constructs. Impact cratersare not illustrated observationsis not sufficientfor a reliable subdivision. Despite these limitations, the undifferentiated faults and fractures exhibit exceptwhere they form the originationor terminusof a mapped valley. For geomorphicmapping purposes, "fluvial valleys"are differentdominant orientations, as well asdiffering morphologies, narrow linear or curvilinear troughswith dimensionssimilar to than the thrustfaults. Valley genesismay have been affectedby thosegiven in Figure 3. "Undifferentiatedfaults or fractures"are thesefaults and fractures,also, as some overlapof orientationsis linearor slightlycurvilinear ridges or (ill-defined)lineations. On evident(compare Figures 5a and5c). high-resolutionimagery, several of theselineations appear to be GEOLOGICAL CONTEXTS OF AEOLIS VALLEYS highly elongatedstrips of knobbyor hilly topography.Finally, "thrust faults or wrinkle ridges" are linear or curvilinear The orientationanalysis suggests that structuralfeatures did asymmetricridges that exhibit steepscarps and relatively gently influencethe locationof valley developmentin Aeolis. However, slopingland on opposingsides. Thesefeatures are the probable other geologicalfactors should be importantas well [e.g.,Kochel complex surfaceexpressions of deep-seatedfaulting within a and Phillips, 1987]. Preexistingscarps of non-tectonicorigin may compressionalstress field [Plescia and Golombek,1986; AubeIe, also be favorable sites for spring sappingto be initiated. If 1988]. They commonlyform complexwrinkle ridges in the lower geothermalheating is important,spatial controls may be exerted Hesperianridged plains, but relatively simple scarpsin the by locally high geothermal gradients related to volcanic or surrounding,apparently weaker, plateau sequence deposits. tectonic activity. Additional information concerning the geologicalcontexts of Aeolis valley developmentis presentin the 1:2,000,000Viking photomosaics.The photomosaicsare widely VALLEY,FAULT, ANDFP,.ACTURE ORIENTATIONS available [U.S. Geological Survey, 1979a, b, c, 1982] and are Most sappingmodels for valleyorigin predict that faults and not reproducedhere. The following large-scalefeatures are fractures,where present,should be importantin localizing importantto the questionof Aeolis valley morphogenesis,are groundwaterdischarge at the surface.However, preferredvalley visiblein thephotomosaics, and are locatedin Figures2 and4. erosionalong faults and fracturesmay obscurethe underlying Feature Descriptions structural features, or such features may be covered by undisturbeddeposits. In orderto testfor the presenceof structural Feature1. Superimposedon the flat floor of this largecrater is controlson valleyorientations, Figure 4 is abstractedinto three a preservedinterior remnant ("r" in Figures2 and4) of smooth componentmaps: Figure 5a, valleysonly; Figure 5b, thrust faults plainsmaterial similar to thatmapped immediately to thenorth 17,294 BRAKENRIDGE,AEOLIS QUADRANGLE, MARS

'* I

z

z

.: z,.. :7' .- z

/'/,z BRAKENRIDGE,AEOLIS QUADRANGLE, 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 190 ø 180ø

Fig. 5. Thematicmaps obtained from Figure4, (a) Smallvalleys only. (b) Thrustfaults only. (c) Undifferentiatedfaults and fracturesonly. Also shownare rosediagrams of orientationdata for eachclass of 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 the boundary and closer to the crater . The ejecta and the southeasternmargins of the remnantindicates that this materialis secondarycraters thus appear to be partiallyburied by the subdued internally stratified. A V-shaped valley with four straight crateredunit. The actualchronology of eventsmay be (1) large segmentsbreaches the raisedsouthwest rim of the crater(Figures impactinto older plateausequence units, (2) partial burial of the 2 and4), andthe drainagedirection was towardthe craterfloor. craterand ejecta by the subduedcratered unit, and(3) subsequent How old is the valley? Greeley and Guest [1987] map the valleyerosion and removal of muchof the craterfill. Single,short craterand its ejectaas superposedon the upperNoachian subdued valleys radial to old, flat-floored modified craters are common in crateredunit of the plateausequence (Figure 2). However, the Aeolis (Figure4). Valley cuttingmay extendinto Hesperiantime, ejecta ("e" in Figure 4) and the associatedstrings of secondary andmay be coeval to cratermodification processes. cratersare prominent and sharply defined to the southof •he Feature 2. A strip of plateausequence terrain separatestwo subduedunit boundaryand becomeabruptly diffuse or absentat lower Hesperianridged plains at this location,and it is heavily

TABLE 2. Tectonic And Fluvial Landform Orientations

Landforms n Vector Strength Standard RaleighTest Meana of Vector Error for Meanb Uniformityc

Smallvalleys 264 N48øW+ 12.0ø 0.39 6.1 0.00

Undifferentiated 83 N49øE 0.19 23.1 0.05 faults and fractures Thrustfaults 56 N63øW + 11.2ø 0.75 5.7 0.00

a Vectormean is arctan [X/Y]; X = Y•cos Oi; Y = • sinOi; shown with 95% confidence intervals. bStrengthofvector mean isR/n, where R= [X2 + y2]1/2. CRaleighstatistic isexp -[R2/n] ßfor Raleigh values <0.05, the uniform vector distribution hypothesis isrejected atthe 0.05 level of significance. BRAKENRIDGE,AEOLIS QUADRANGLE, MARS 17,297 dissectedby V-shapedbranching valleys. Basedon thebranching 4). Two unusuallywide east and northeast oriented (adjoining) pattern,the most prominentvalleys shownon the Viking reachesexhibit dark floors and scallopedmargins and are photomosaicsdrain toward the northwestand terminate at the transitionalnortheastward into an irregularclosed depression with westernridged plain border. The locationof thisheavily dissected abundantknobs that are probable collapseblocks. Genetic landis typicalfor Aeolis. processescould include sill volcanismand overburdencollapse, Approximately70 km to the southwest,a NNE trendingflat- perhaps along a fault or fracture zone. However the chasms floored straight canyon extendsnearly to an intersectionwith a themselvesoriginated, numerouspost chasm valleys extend NW trendingscarp interpreted as a thrustfault. The SW-facing headwardfrom the chasmscarps into the surroundingplateau scarpis mappedas a valley by Cart and Clow [1981], but thereis sequencematerials. One such valley, at the chasm'ssouthern no opposingwall. Near to the scarp,the valleytums abruptly NW terminus,appears to follow a secondaryscarp mapped as a thrust and is located along it; the valley then becomesindistinct in a fault (Figure4). complexarea near the marginof the westernplain. A permissive Approximately 100 km to the east of feature 6 is an isolated inferenceis that valley cuttinghere was postthrust faulting. 40-km-longplateau or mesa(Figure 4). At least14 light anddark Feature3. At thislocation, dark-floored V-shaped branching strata crop out alongits hillslopesand are visiblein the valleysdissect lighter plateau sequence material. Drainage photomosaic [U.S. GeologicalSurvey, 1979c]. The banding can directioniswestward and toward a ridged plain that is mapped in betraced continuously along the perimeter ofthe plateau, and the Figu•re4 but not in the smaller scale Greeley and Guest [1987] concentric outcrop pattern suggests thatthe plateau isan erosional map(Figure 2). Immediatelytothe southwest ofthe valleys, an remnantformed by approximatelyhorizontal strata. Similar interdigitatecontact isvisible between a dark surficial unit (to the regularlybedded internal stratification isnoted also at feature 1 west)and a muchlighter unit (to the east). The simplestand has been previously described for the plateau sequence units explanationforthis contact isdifferential stripping ofthe lighter, [Malin, 1976]. The origin and nature of such stratification arean superposedstratum from an underlying darker stratum. Thus the unresolvedquestion: possibilities include interbedded sediment terrainimmediately surrounding the valleysappears to be andlava; interbedded sediments of varying clast lithology, mean

underlainby severalstratiform units of contrastingalbedo... The size,or matrix; or interbeddedsediment and ice similar to that darkvalley floorshave tappedan underlyingdarker stratum. occurringin the polar layeredterrains. Feature 4. Valley classesI-III densely dissectthis plateau Feature 7. Faultingor fracturingwas probablyinvolved in the sequenceterrain, and many of the valleys exhibit dark floors. origin of this 180-km-long,north northeastoriented chasm, but Thereis disagreementregarding the natureof two adjacentplains: wall slumping,floor collapse,or fluid flowsalong the chasmmay (1) The plainto the westof the valleysis not illustratedin Greeley also have played importantroles. Headward erosionthen carved andGuest's smaller-scale map (compareFigures 2 and4). Valley the smallertributary valleys that debouch into the canyon along its branchingdirections, however, indicate that the surface of this southeasternmargin. relatively smoothplain is lower than the surroundingheavily Feature8. Two en echelon,northeast trending ridges are here dissectedand higher-albedolandscape. (2) Greeley and Guest mappedas undifferentiatedfaults or fractures(Figure 4). The [1987] map the easternplain as a superposedsmooth unit of the westernridge, approximately60 km in length,forms a straight plateausequence, instead of as a ridgedplain (compareFigures 2 segmentalong the northernrim of the large craterMolesworth and 4). I could find no evidencefor such superposition:the (not shown), and at least sevensmall classI valleys originate plain'swestern margin is belowthe adjacent surface of theplateau alongthis ridge and debouch southward at the smoothcrater floor sequence,and numerousdark-floored valleys debouch from the (also not shown).The easternridge, approximately20 km in plateausequence materials at the margin of the plain. These length, is lesswell-marked but alsois coincidentwith the heads valleys are arranged othogonally to the boundary, and the of four similar valleysthat exit northwardonto a ridgedplain. complex,interdigitate nature of the boundaryitself is similarto Suggestiveevidence of furthertectonic complexity is presentin that southwestof Feature 3. the form of a 25-km-diameter crater bisected by the eastern As noted,the simplestinterpretation of the outcroppattern at terminusof the westernfault (Figure4) andshowing an apparent this and otherinterdigitate boundaries is that a dark, stratiform offset of approximately10 km in the right lateral sense(examine unit, entombedwithin the plateau sequence,extends from U.S. Geological Survey [1979b]). Whatever the exact nature of relatively interior positions(where it is exposedby the deep these faults, the associatedvalleys appear to be syntectonicor valleyfloors) to the subaerialp!ains surface itself. The dark post-tectonicfeatures: they are not transectedby the faults. valleyfloors may represent lava sills or buried lava flows that are Feature 9. This 40-km-wide, 150-km-long belt of dissected now exhumedby erosion(see discussionof suchstratigraphy by plateausequence is adjacentto an unusuallywell definedridged Wilhelmsand Baldwin [ 1989]). plain. The valleys debouchonto this plain. At least 20 classI and Feature5. At this typicallocation, southeastward draining II valleyscan be countedon the photomosaic;10 of the more subparallelslope valleys and branching V-shaped valleys dissect prominent ones are shownon Figure 4. the plateausequence, and are locatedabout the periphery.of an Feature 10. Al-qahira Vallis, a 600 km long, 25 km wide unambiguousridged plain (compare Figures 2 and4). chasm,is discussedby Sharp and Malin [1975] as an an outflow Feature6. Immediatelyto the southof a northeastoriented channel(Figure 4). However, unlike many outflow channels,no chasmalso mappedby Greeleyand Guest[1987] is a complex channel bedforms are visible (see discussionof such criteria by andinterconnecting system of broad,flat-floored chasms (Figure Baker, 1982). The exceptionallystraight wall segmentssuggest 17,298 BRAKENRIDGE,AEOLIS QUADRANGLE, MARS

structural control over sappingor, perhaps, an active tectonic plateau sequencedeposits to be stratified and, perhaps, (rift?) origin. In support of such inferences,other faults or composi.tionallyheterogeneous. All of the mapped valleys fracturesare abundantin thisvicinity, and manyare parallelto the transectthe plateausequence, and mostare arrangedorthogonal NNE and WNW trendsof Al-qahira'swalls (Figure 4). In detail, to, and alongthe perimeterof, ridgedvolcanic plains. However, the chasm walls are scalloped,and a variety of much smaller some valleys occur as tributaries to large chasms that are tributary valleys are eroded into the surrounding, mostly surroundedby otherwiselightly dissectedor undissectedplateau undissected,plateau sequence. If faulting was involved in the sequencelandscapes. Four specificexamples are also noted of origin of Al-qahira, then thesesmall valleys alsopostdate, or are inferredsyn-faulting or post-faultingvalley development,and coevalto, thisperiod of faulting. theseexamples support the orientationstatistics-based conclusion No ridgedplains are mappedin this area, but a 60-km-wide, that faults and fracturesare importantcontrols over valley approximatelycircular, positive topographicfeature is present locations. along the chasm'swest side (Figure 4). Associatedradial reentrantsseparating sloping, plateau-like surfaces and a visible TABLE 3. Summaryof Viking Photomosaic-BasedObservations centralcrater suggestthat this landformis a "hydromagmatic" (Tyrrhena Patera-like)volcanic complex. Suchvolcanic piles on

Mars are inferred to result from basaltic eruptionsin ice-rich Observation Feature Numbers terrains [Greeley and Spudis, 1987; Crown et al., 1988]; the growth of this one may alsobe coupledto tectonismalong A1- qahira. Valleystransecting plateau sequence deposits ! ,2,3,4,5,6,7,8,9,1 O, 12 Feature 11. This north and northwesttrending chasm may be Valleys orthogonalto ridgedplains 2,3,4,5,8,9 the erosionallyand/or slumped surface expression of a low-angle Valleys tributaryto chasmwalls 6,7,10, 12 thrust fault. Thus the easternportion of a 20-km-diametercrater Low albedo valley floors 3,4,6,7 on the northeasternblock has, apparently,been thrust over the Syn-tectonicor post-tectonicvalleys 2,6,8,10, 12 southwesternblock, whereno visible counterpartcrater fragment Evidencefor down-valleyflows 7,11,12 Interstratificationof plateausequence deposits 1,6 exists(Figure 4; see also U.S. GeologicalSurvey [1979a]). An alternative suggestedby one reviewer (that the miss!ng southwesterncrater half hasbeen eroded) is possible,but thereis no accompanyingmechanism for erosionalremoval of onecrater half and not of the other. If thrusting indeed initiated this Complexinterdigitate boundaries occur between the relatively landform'sgenesis, then the implied compressionalstresses are smooth,low-lying, darker,ridged plains and the adjacenthigher, congruentwith thosethat producedthe many other northwest dissected,lighter, plateau sequence terrains. If the ridgedplains orientedthrust faults in this region. arefloored by extrudedvolcanic units., then two alternatives could A 22-km-wide volcanois presentalong the extensionof this explain such boundaries: (1) Surface lava flows, originating chasmto the southand within the inferred over-ridingblock, east within the plains,partially fill the downstreamreaches of valleys of the surfacetrace of the fault (Figure4). This volcanoexhibits cut into the surroundingterrain but do not cover the interfluves. In this case,the valleys are embayedby, and are older than, the clear diagnostictopography, such as an apica! caldera, steep , symmetricalflanks, and radial reentrants (see also Figure 7.15 by lavas (as inferredby Tanaka [1986]). (2) Stratiformsill lavas, Greeley[1987, p. 165]).The plateau sequence here is not heavily injectedinto the plate•tusequence from below, exit onto the dissected,but the chasm itself may have been modified by surfac'eat thebases of slopesbounding th,e plains.In thisevent,

downstream fluid flows. the volcanicstrata forming the plains extend, in.to the surrounding Feature 12. Finally, Ma'adimVailis is a 700-km-long,15-km- plateausequence units, and are locally exposed at thesurface by wide, gentlywinding chasm about 1 km deep[Sharp and Malin, valley incisionthere (as inferredby Wilhelmsand Baldwin 1975]. Numerousflat-floored straight canyons and other valleys [1989]). If this is the actual stratigraphy,then valley ei'osion form scatteredtributaries to it. Fluid flows may have occurred postdatesor is coevalto phi.nsvolcanism. I ,favorthe second

alongthis chasm: an inner channelexists near its downstream alternative for the examples cited, because, the relatively dark terminus where the chasm transects a modified crater (location valley floors extend continuouslyto the valley headwalls (see shownin Figure4), and medial,stream-lined ridges occur along classIII valley in Figure 3 for an exampleof the resultingoutcrop several reaches of the flat chasm floor. However, tectonic pattem). processesmayalso be involved inchasm genesis: (1) the north Therealso exist in Aeolis small, commonly unmapped, ridged andnorthwest orientations oftwo major segments arecongruent plains confined within flat-floored craters. One larger than normal withthe preferred orientation ofthe regional thrust faults, and (2) example'isthe rid. ged plain within a modified crater immediately a 30-km-diameterflat-floored crater transected by Ma'adimwest of feature11 in Figure4. A hypot.hesis to explainsuch (southofthe "12" symb. olin Figure 4)appears tobe left-laterally associations isthat the deep-seated ringfractures associated with displacedapproximately 8km. impactstructures provided conduits forcrater-interior lavaflows and/or for lava sill injections into remnants.of the stratified, Summaw possiblyice-rich post-crater deposits [Costard and Dollfus, '1987]. Table3 liststhe qualitativephotogeological observations as Cratermodification processes may, in thisevent, be genetically theyrelate to valleygenesis. Local banded outcrops indicate the relatedto valleydevelopment. BRAKENRIDGE,AEOLIS QUADRANGLE, MARS 17,299

FLUVIAL VALLEYS AT HIGHER RESOLUTION constructedfrom a mosaicof Viking orbiter images;see Figure4 for the location within Aeolis. Illustrated are ridged plain and Valleysin PlateauSequence/Ridged Plain Borderlands plateau sequenceterrains, inferred thrustfaults (wrinkle ridges),a Additionalobservations of valley geologicalcontexts at higher prominent lava flow front within one of the ridged plains, other surfaceresolution are useful in analyzingvalley origins. Figure 6 faults or fractures,flat-floored straight canyons,and small V- is a geomorphologicalmap of a portionof southernAeolis shaped branching valleys. In agreement with the observations

Ejecta

.:...... •Unmodified,Superposed Craters

ModifiedCraters

i i t '-'- • Buried i I I i , , Craters

Npld Frames A-D, i Figure 7 ii$ ,

Npld

k

RidgesInterpreted as Thrust Faults

ß Possible Volcanic Constructs

Ridged Plains (arrows indicateflow margin)

Modified Crater Floor Sediments

20 km Valleys or Large CollapseDepressions

Undifferentiated Faults or Fractures

DissectedUnit of the PlateauSequence s: Smoothto rough, largelyintact r: Isolated smooth remnants JNpld , ] d' Intricately dissected k: Karst-like Fig.6. Geomorphologicalmapof a portionof AeolisQuadrangle, illustrating valley development along plateau sequence/ridged plainborderlands. The map is based on a mosaicof 11Viking frames (425S27-31' 426S26-31); map location isillustrated in Figure 4. 17,300 BRAKENRIDGE,AEOLIS QUADRANGLE, MARS madeabove, the fluvial landforms (1) areincised into the plateau sequenceis modifiedby a complex,closely spaced network of sequencedeposits, (2) are orientedorthogonal!y to the ridged closeddepressions: the entire surface is pittedand also is gouged plain,and (3) terminateat thethe ridged plain. Figures7a, 7b,7c, bytroughs (the "karst-like" plateau sequence of Figure6). Several and7d arefour of theViking frames used in producingthe map. flat-flooredstraight canyons also occur, but regional collapse here In Figure7a, the smallbranching valley below and to theright dominatedover fluvial erosion. Clear embayment relations with of the "A" symbol could be interpreted,on lower-resolution theplains are absent. Instead, the ridged plain material interfingers imagery,as embayedby the lava plains(to the left). However, in a verycomplex manner with the collapsed plateau sequence. this framedemonstrates that the materials incised by thevalley are Thisdetail is missing on the 1:2,000,000 Viking photomosaics [U. at a considerablealtitude above the ridgedplain (note the scarp S. GeologicalSurvey 1979b], whichmisleadingly show the near the widest portion of the valley). Instead of having boundaryto berelatively sharp and congruent with an embayment undergoneembayment, the valleymust have developed during or interpretation. afterplains emplacement. In thelower right quarter of the image, The lava flow front mappedin Figure6 and illustratedin abundant irregular closed depressions cause a scab-like Figure7b suggeststhat lava venting occurred from a sourcearea appearanceof thismarginal plateau sequence area. The boundary to thewest, but vents are not visible. It is possiblethat the vents between the plains and the plateau sequenceis not sharp,as lie buriedwithin the collapsedplateau sequence that formsthe expectedfor embayment,but is irregularand marked by apparent westernmargin of the plain. Movementof effusivelavas to the collapseof theplateau sequence material at somelocations. eastcould have occurred as one or morelava sills localized along In Figure 7b, a visible lava flow front (arrows; also see subsurfacelithological discontinuities. These sills may then have examplesfrom Theilig and Greeley [1986]) is approximately emergedas subaeriallava flowsin the areanow mappedas a parallelto the ridgedplain/plateau sequence contact, but the ridgedplain. If plateausequence strata include ice-cemented eastwardflowing lava did not reach that boundary. Several flat- clasticmaterial, sill volcanismcould explain plateau sequence flooredstraight canyons and modified craters are floored by collapse(see also Wilhelms and Baldwin [1989] and Squyres etal. similarappearing, smooth material that is continuouswith the lava [1987]). plain(center of image).In thelower right corner of theframe, Analternative stratigraphy could be locally important atridged threeV-shaped valleys extend headward into the plateau sequence plain/plateau sequence borders. Studies of thestratigraphy of from scarpsproduced by local collapse,again at the ridged ColumbiaPlateau (U.S.)basalts document "invasive" behavior of plains/plateausequence boundary (see alsoFigure 6). Valley thoselavas where they encounteredmuch less densemarine erosionwas here preceded by scarpproduction, and the collapse sediments. There, 120 m thicksurface lava flows deeply intruded, featuresthemselves may be relatedto thenearby volcanism. An at theirmargins, siliclastic sediment piles [Wells and Niem, 1987; alternativehypothesis (that plains volcanism simply embays older, Byeflyand Swanson, 1987; Pfaffand Beeson, 1987]. If sediment alreadydissected and locally collapsed terrain) lacks the needed densities of theplateau sequence strata are relatively low, then supportingevidence of clearlava flow frontsalong the complex invasivelava behaviormay have occurred. This could also andhighly irregular plains/plateau sequence contact. explainsome interfingering of plateau sequence and the ridged Given the presenceof subaeriallava flows,some examples plainmaterials. shouldexist of lava embaymentcontacts with older landforms. In Figure 7c, the easternmargin of the plateausequence highland is ValleysWithin The Plateau Sequence densely dissectedby numerous small valleys and ravines (the Severalancient Aeolis flat-floored branching valleys coalesce "intricatelydissected" unit of Figure6). The fluvial landformsare intointegrated systems extending hundreds of kilometers;they rectilinear,parallel, or digitatenear the right centerof the frame, followregional topographic gradients, exhibit valley widths of 5- and suchdetailed modification of the plateausequence borderland 10km or more,and are not proximal to largeridged plains units. is accompaniedby two relatively large, flat-floored branching A geologicalmap and image of a portionof sucha valleysystem canyons. The plains/plateau sequence contact here could compriseFigures 8a and 8b; see Figure 4 for the location in reasonablybe interpretedas one of embayment: it is relatively Aeolis. This and similar integratedvalley systemsappear to sharpand regular, and the intricatelydissected hillslopes appear to constitutethe clearest evidence for greatlychanged atmospheric dip, at variousangles, into andbelow the plainsmaterial. conditionson Mars. However,Brakenridge et al. [ 1985]propose Despitethe possibilityof embayment,it is not clearthat fluvial an alternativehypothesis: that the valley systemdeveloped in a landscapemodification here predates plains volcanism. The piecemealfashion, through headward sapping and fluid flows floorsof the relativelylarge straight canyons are continuouswith, causedby impactmelts and thermal springs. and not embayedby, the plains. Several cone-shaped,radially Part of the Brakenridge et al. [1985] hypothesismay be rilled mountains occur on the plateau sequence near its unnecessarilyrestrictive. Thus,endogenetic volcanism as well as northwesternborder (marked by arrows,top of Figure7c; seealso impactmelt could be an importantheat source for thermalsprings. Figure 6). These mountainsappear to be volcanic constructs As illustratedin the earlierreport, two branchesof the Figure8 (usingcriteria given in Greeleyand Spudis[1987]) and they are systemform a 95-kin-widecircular pattern. However,the terrain accompaniedby extensivemodification of the plateausequence. interiorto thesebranches is complexand itself cratered:no direct Volcanismand valley dissection in thisexample are, at theleast, associationof a largecrater with the valleys is nowvisible. Better spatiallyrelated; they may alsobe causallyrelated (see other candidatesites exist for directimpact melt heating as a factorin examplesin Wilhelmsand Baldwin, 1989). valley genesis(e.g. seeMouginis-Mark [1987, p. 282]. Although In Figure7d, and to the west of the ridgedplain, the plateau preferentialexcavation of the Figure 8 flat-floored branching BRAKENRIDGE,AEOLIS QUADRANGLE, MARS 17,301 17,302 BRAKENRIDGE,AEOLIS QUADRANGLE,MARS BRAKENRIDGE,AEOLIS QUADRANGLE, MARS 17 303

A

N

Fig. 8. (a) Geomorphologicalmap and (b) image(Viking frame596A26) of a portionof the extensiveflat-floored branching valley systemlocated in 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 ring fracturesystem, valley developmentcould have greatly post- exhumedsills associatedwith fissureridge emplacement.The datedcooling of theimpact melt. combinedigneous activity certainlycould haveprovided local The internally complex nature of the plateau sequenceis heatsources for ice meltingand thermal springs. 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 {,see detailed ridgesexhibit much lower albedosthan surroundinglithologies, map in Brake/,'idge et al. [1985]). It is not certainwhether (1) a andare similar to featuresinterpreted as fissureeruption ridges by continuous,integrated valley system once existed and wasthen Wilhelms[1986]. Their crosssections are exposedin the wall of a modifiedby post-valleyresurfacing processes such as cratering, prominenterosional scarp (compare Figures 8a and8b to locate volcanism,or eoliandeposition; or (2) the valleylinks developed 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 wasnever more integrated than at present. an igneousorigin is supportedby their physicalcontinuity with Someevidence supports the latterpossibility. Although post- the knife-likeridges. A thirdplain lies to the north(near letter valley modificationsare obviousat somelocations (e.g., fresh 17,304 BRAKENRIDGE, AEOLISQUADRANGLE, MARS

B

t

Fig. 8. (continued)

superposedcraters interrupt a valley segmentnear "c"), other continuousto an abrupt terminus approximately 150 km to the valleyinterruptions appear to beprimary. For example, northwest northeast,at an elevation approximately 500 m lower (see also trendingridges interpreted as thrust faults are common in Figure Brakenridge et al. [1985]). The valley is no wider at the terminus 8. Isolatedvalley linksof the valleysystem terminate at them(at than it is far upstream, and its general morphology resembles "c" in Figure8a). In agreementwith the regionalorientation certainfretted channels which may still be activetoday (see analysis,a major northwest oriented tributary valley (near location exampleof Cart [ 1981, pp. 154-155]; also, comparevalley classes "e",Figure 8a) isaligned along a thrustfault: thislocal episode of IV and VI, Figure 3). Carr considersthe young fretted channelsto valleydevelopment postdates the faulting. Also, a valleysegment result from some combination of valley-side mass wasting and breachesthe craterrim at "f", insteadof being transectedby it. downvalley debris flow, possiblyassisted by interstitial ice. Such Similarprimary drainage net gapsoccur along other branching processesmay also have been active in the ancient past, and large valley networks,and they are suggestiveevidence for valley environmentalchanges are not requiredfor their occurrence. erosion controlled mainly by local fault and fracture systems Other Evidencefor Valley Mo;phogenesis insteadof by topography. Individual, continuousdownstream reaches along this valley Figures9a (Viking frame) and 9b (map of the frame)provide systemdo reachconsiderable lengths. At "f" in Figure8a, the a closeview of the morphologyand stratigraphyof one reachof valley originatingat the breachedmodified-crater rim first theabove-discussed valley. In theframe, a flat-flooredmodified transectsa rounded, northwest oriented ridge. It is then craterseparates two valley segments,and a relativelydark, thin, BRAKENRIDGE,AEOLIS QUADRANGLE, MARS 17,305

A

Fig. 9. (a) Locationmap and (b) imageof a smallportion of the flat-flooredbranching valley system mapped in Figure8. The imageis croppedfrom Viking frame427S03 and is approximately32 km in width. Seetext for descriptionsat lettersA, B, andC. In Figure9a, the heavystippling is inferreddark igneousmaterial, and the whitearcs transecting the valley floorsare scarps discussed in the text.

and resistantstratiform unit occupiesthe centralportions of the "A" in Figure 9, and also to the left of the letter "C", are small, crater's interior (above letter "A"). This may be either an faintly lobate scarpssituated across the fiat valley floor. The exhumedlava sill or a lavaflow. Stratigraphicallybelow this unit scarpsmay mark the distaltermini of downvalley.freezing water, is a higheralbedo, slope-forming stratum that is muchthicker and ice, and/ordebris flows. Episodicvalley growthand modification couldrepresent impact ejecta, volcanic ash, or eolian-reworked would then be implied, and also valley erosion by wall-to-wall materials,perhaps once associated 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 seeminglysupported by the presence in Figure9). The entire sequenceis now boundedby a deep, of a long narrow interfiuvedownstream from the junctionof two trough-shapeddepression (immediately adjacent to "B"), whichis tributary valleys: imlnediately above "C" in Figure 9. Such developedat the approximateposition of the old craterrim. It is interfiuvesare typical of glaciatedpiedmont valley junctionson clearthat subsidencehas occurled in this area. It is possiblethat it Earth and also some fretted channeljunctions on Mars. These could be involved, also, in the initial genesisof nearby valley small-scale features are near the limit of frame resolution, and segmentsalong lhults or fractures. alternative genetic modelsexist. They do indicate,however, that Two other geomorphicfeatures preserved along the present new images from Mars Observer will be useful in constraining valleysmay alsorelate to geneticprocesses: (1) Below the letter valley origin modelsand associatedclimatic changeinferences. 17,306 BRAKENRIDGE,AEOLiS QUADRANGLE, MARS

B

o ©

c

A

Fig. 9. (continued)

CONCLUSIONS episodic)downvalley water, ice, anddebris flows, and headward erosionalong stratal discontinuities and individual conduit faults The valleys mapped in Aeolis exhibit strong preferred and fractures. alignmentsthat suggestthe past operationof structuralgeologic AlthoughAeolis valleys might have been carved by seepage controlsover valley location.This supportsthe generalconclusion flows without the interventionof hot springs[Pieri, 1980], this that headwardspring sapping is importantin valley genesis[Sharp alternativerequires that past mean temperaturesand pressures and Malin, 1975; ?ieri, 1980]. Also, evidence exists at a variety were muchhigher in orderto allow springconduits to remain of image scalesfor ancient volcanic activity near many Aeolis open. In theabsence of independent,non-ambiguous evidence •talleys. Probable fissure eruption ridges, small volcanos, for therequired large amount of climaticchange, the coldspring exhumed lava sills, and collapsed, (scabby) karst-like sappinghypothesis is morecomplex. Also, certain aspects of' morphologiesall occur within the fluvially dissectedplateau Martianfluvial morphologyare notexplained by climate-induced sequence,and thesefeatures suggest the occurrenceof extensive valleycutting but are by the thermalspring model. Theseaspects subsurfaceigneous activity. A possiblevalley genesismodel is are (1) the nearlycomplete restriction of valleysto the possibly that, in response to widespread effusive volcanism in ice-rich plateau sequenceunits, (2) the common spatial interstratified, ice-rich terrains, local subsidenceoccurred along associationof denselydissected plateau sequence materials with fracturesand faults and produced scarps that intersected local nearbyvolcanic plains, (3) theintermittent gaps along branching aquifers. Heatedspring discharges issuing from thesescarps may valley networksthat suggestactual lack of continuityduring then have carvedthe valleysthrough a combinationof (probably valleyformation, and (4) themaintenance of constanttrunk valley BRAKENRIDGE, AEOLIS QUADRANGLE, MARS 17,307 widths along hundredsof kilometer-longvalley reaches,into detailedcriticism and helpful comments on theoriginal manuscript. I whichdebouch numerous tributary valleys. It is unlikelythat the thankGeorge Brakenridge for photographic enlargements of many Viking branchingvalley networksever functioned, as terrestrial networks orbiterframes. The research was supported by NASAMars Data Analysis do, to collect and transportwater from headwatersto the mouth. Programgrant NAGW-1082 and is a contributionof theNASA-sponsored Theoretical models of early atmospheric evolution are "Mars:Evolution of Volcanism,Tectonism, and Volatiles" project. sometimescited incorrectly as independentevidence for the putativeancient dense and warm Mars atmosphere.All such REFERENCES models are uncertain, but several do allow Mars to evolve without Allen, C.C., Volcano-ice interactionson Mars, J. Geophys. Res. 84, ever developing an Earth-like atmosphere. For example, an 8048-8059, 1979. ancientwarm and denseatmosphere is not predictedfor Mars if Aubele, J.C., Morphologicalpatterns in lunar mare wrinkle ridgesand the later stagesof planetaryaccretion were slow: the planet's kinematic implications,in Abstracts,!9th Lunar and Planetary atmospherewould remaincold as H20, CO2 and other volatiles ScienceConfi'rence, Houston, Texas, pp. 19-20, Lunarand Planetary Science Institute, 1988. condensed on the planet's surface [Matsui and Abe, 1987]. Baker, V. R., The Channelsof Mars, 198 pp., Universityof TexasPress, Although enriched D/H ratios suggest atmospheredepletion Austin, 1982. [Owenet al., 1988], the oxygenisotope data do not, and a variety Baker, V. R., and J. B. Partridge,Small Martian valleys, pristine and of historiesand controllingprocesses are possible[Jakosl•y, 1988]. degradedmorphology, J. Geophys.Res., 91, 3561-3572,1986. Mars may indeedbe volatile-rich[Cart, 1986, 1987] withoutever Barlow, N.G., Crater-sizefrequency distributions and a revisedMartian havingexperienced a thick atmosphere. relativechronology, Icarus, 75, 285-305, 1988. Under modernclimatic conditions,most newly outgassedand Brakenridge,G.R., Small valley networksand the past and present condensedvolatiles would be deposited in high latitudes. 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