PROCEEDINGS OF THE SEVENTEENTH LUNAR AND PLANETARY SCIENCE CONFERENCE, PART 1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B13, PAGES E139-E158, NOVEMBER 30, 1986
The Stratigraphyof Mars
KENNETH L. TANAKA
U. $. GeologicalSurvey, Flagstaff
A detailedplanetwide stratigraphy for Mars hasbeen developed from global mapping based on Viking imagesand cratercounting of geologicunits. The originalNoachian, Hesperian, and AmazonianSystems are divided into eight seriescorresponding to stratigraphicreferents. Characteristic crater densitiesand materialreferents of eachseries are (1) Lower NoaehianIN(16)] (numberof eraten > 16 km in diameter perl06 km 2) > 200]basement material; (2) MiddleNoachian IN(16) = 100-200]cratered terrain material; (3) Upper Noaehian[N(16)= 25-100; N(5) = 200-400]intercrater plains material; (4) Lower Hesperian IN(5) = 125-200]ridged plains material; (5) Upper Hesperian[N(5) = 67-125;N(2) = 400-750]complex plainsmaterial; (6) LowerAmazonian IN(2) = 150-400]smooth plains material in southernAcidalia Planitia; (7) Middle AmazonianIN(2) = 40-150] lava flowsin AmazonisPlanitia; and (8) UpperAmazonian IN(2) < 40] flood-plainmaterial in southernElysium Planitia. Correlations between various crater size-frequency distributionsof highland materials on the moon and Mars suggestthat rocks of the Middle Noachian Seriesare about 3.92-3.85b.y. old. Stratigraphicages eompi!ed for unitsand featuresof variousorigins show that volcanism,tectonism, and meteoritebombardment have generallydecreased through Mars' geologichistory. In recenttime, surficialprocesses have dominatedthe formationand modificationof rockunits. The overallstratigraphy of Mars is complex,however, because of temporaland spatial variations in geologicactivity.
INTRODUCTION appearance,to determinethe relativeages of map units. These units were classifiedand placedin three stratigraphicsystems: Anewglobal geologic map series at 1:15,000,000scale derived the N oachianSystem, characterized by rugged,heavily cratered fromViking images has expanded and improved our knowledge material;the HesperianSystem, whose base was defined as the ofthe geologic and stratigraphic framework of Mars.This series baseof the ridgedplains material; and the AmazonianSystem, consistsof threemaps covering the following regions: the western which included relatively smooth, moderatelycratered plains equatorialregion Oat + 57ø, long0 ø to 180ø [Scottand Tanaka, materialsand polar deposits.The namesfor thesesystems were 1986]),the easternequatorial region (lat ñ 57ø, long !80ø to selectedfrom regionshaving representativeand widespread 360ø;R. Greeleyand J. E. Guest,unpublished data, !986), and exposuresof the materialsused as referents. the north and south polar regions(lat > + 55ø [Tanaka and A new generationof local geologicmaps of Mars at different Scott,1987]. This mapping, combined with previousstudies and scaleswere based on improvedimagery acquired from the Viking newcrater counts, forms the basisfor the most detailed formal orbiters[Scott et al., 1981;Dial, 1984;Scott and Tanaka,1984; representationof Martian geologichistory to date.In this paper, Witbeckand Underwoo& 1984].All of theseworkers followed I proposenew stratigraphic series and epochs that form discrete the time-stratigraphicsystems developed by Scott and Carr stagesinto which the evolution of the planet'ssurface can be [1978]. The stratigraphic-systemboundaries were defined by divided. usingdensities of cratershaving diameters of 4-!0 km [Greeley In an early study of the Martian surface, Soderblom et al. and Spudis, 1981] and 2, 5, and 16 km [Scott and Tanaka, [1974]recognized four stratigraphicdivisions: ancient eroded !984, 1986].In addition,Gurnis [1981] obtained crater densities uplands,cratered (ridged) plains interpreted as volcanic, Elysium for broad terrain units, and many workers focusedon local volcanicrocks, and Tharsis volcanic rocks. Formal geologic or regionalgeologic histories that providecrater-density data mappingof Marsbegan with a 1:5,000,000-scalemap series based and stratigraphicrelations. mainlyon Mariner9 images.Because of the variedqu. ality and The presentstudy sets forth a new,Viking-based stratigraphy resolutionof the imagesand the differencein mappingstyle of Mars in which I (!) establisha more detailedchronostra- amongthe workers,the stratigraphicclassification systems that tigraphicclassification system, (2) ascertainthe craterdensities theydeveloped were commonly inconsistent. Most authorsused andpossible absolute ages of the chronostratigraphicunits, (3) a qualitativeapproach to stratigraphicage by definingthree, deducerelative ages of major geologicunits and features,(4) four,or fiveimpact-crater degradation classes [e.g., Masursky presentstratigraphic maps of the entire surface,and (5) et al., 1978; Wise, 1979; Moore, 1980; respectively];some summarizethe planet'sgeologic history in orderof epochs. [Milton,1974; Masursky et al., 1978;Wise, 1979] used crater- densitydata to determinetheoretical absolute ages from the SUBDIVISION OF MARTIAN CHRONOSTRATIGRAPHICUNITS cratetinghistory models of Soderblomet al. [1974]and Neukurn andWise [ 1976]. The three-period,time-stratigraphic classification established A formalizedstratigraphy was first presentedin the by Scott and Carr [1978] is useful,widely recognized, and in l:25,000,000-scalegeologic map of Marsby Scottand Carr no need of fundamental revision. Subdivision of the chrono- [1978].Condit [1978] counted 4- to 10-km-diametercraters and stratigraphicsystems into seriesis nowpossible because of recent usedthem, together with overlap relations and degradational and ongoingwork, particularlyon the westernregion of Mars [Scott and Tanaka, 1986], and is necessaryfor detailed stratigraphicwork. For simplicityand utility, seriesnames are Thispaper is notsubject to U.S. copyright.Published in 1986by theAmerican Geophysical Union. adapted from systemnames and qualified with "Upper," "Middle," and "Lower." Referentswere picked on the basisof Papernumber 6B7240. prior usageand recognition,established stratigraphic position,
E139 E140 TANAKA: THE STRATIGRAPHYOF MARS
TABLE 1. ChronostratigraphicSeries and Referentsfor Mars of about3 to 5 km.Generally, rims of superposedimpact craters are degradedand ejectablankets are not recognized.Because Series Referent of thisdegradation, crater densities are not reliablefor relative. Upper Amazonian Flood-plainmaterial, southern Elysium agecomparison between materials of theLower Noachian Se• Planitia and unitsof otherseries; however, they appearto be useful Middle Amazonian Lava flows, Amazonis Planitia in determiningrelative ages of the rockunits within the series Lower Amazonian Smoothplains material, Addalia at differentlocalities. One areaof basementmaterial at lat• Planitia Upper Hesperian Complexplains material, Vastitas S, long101 o hasrelatively well preserved craters [N(16) (number Borealis ofcraters > 16 km in diameterper 106 km 2) --294 :t: 81]. Lower Hesperian Ridgedplains material, Hesperia The lowermostpart of thisbasement material is not exposed Planurn and its top is embayedby Middle Noachiancratered terrain Upper Noachian Intercraterplains material, east of material. ArgyrePlanitia Middle Noaehian Crateredterrain material, west of Hellas Other outcropsof materialembayed by this crateredterrain Planitia material form basin rims that surroundHellas, Argyre,and Lower Noachian Basementmaterial, Charitum and IsidisP!anitiae and form Promethei Rupes (south polar basin), Nereidurn Montes as well as isolatedmassifs and ridges,which are mostlyin the westernpart of Mars [Scottand King, 1984].Material of similar areal extent, degreeof preservation,and, aboveall, represen- positionmay alsobe exposedat the baseof deepscarps, such as those within Valles Marineris. Possibly the material of the tation of a distinctivegeologic episode. In contrastto terrestrial Lower NoachianSeries represents upper parts of the primitive rock units,the Martian unitsare largely defined by their surficial crustof Mars formed by solidificationof a primordial,molten characteristics,and rock typesare inferred.Also, recognition surfaceand bombardedby large planetesimal-sizeobjects. of vertical changesin the units and descriptionof their bases generallyare not possible.The newlyproposed series and their referentsand crater densitiesare formally describedbelow (and Middle Noachian Series summarizedin Tables1 and 2). Possibleabsolute ages for these The Middle Noachian Series consists of cratered terrain seriesare discussedin a followingsection. materialthat characterizesmost of the ruggedhighland terrain Lower Noachian Series and scatteredcratered terrain remnantsin the northernplain• of Mars. Much of this material was mapped as "hilly and Recent mapping [Scott and Tanaka, 1986] distinguished cratered" or "cratered plateau" materials by Scott and Cart stratigraphicallylower "basement material" from the widespread [1978] and was recentlyremapped as the "crateredunit of the crateredterrain material in many areason Mars. Both of these plateausequence" [Scott and Tanaka, 1986]. This rock unitis materials are part of the Noachian System[Scott and Cart, the most widespreadof the Noaehian System.The designation 1978]. The basementmaterial includes units previously mapped of the Noaehian System was originally based on materialin as "basin rim material" and "mountain material" belongingto the Noachisarea west of the Hellas impact structure.This area the Noachianand HesperianSystems [Scott and Carr, 1978]. (lat 40ø S, long 320ø to 345ø) still appearsto be the mosttypical The type area selectedfor the Lower N oachianSeries consists of crateredterrain, and it is selectedas the type area of the of Nereidum and Charitum Montes, which form the uplifted Middle Noachian Series.Most other highlandregions are partly rim material surroundingArgyre Planitia (Figure 1). The rim buffedor morehighly degraded. The regionwest of SyrtisMajor has an inside diameter of about 700 km and an outside diameter Planurn,although highly degraded,displays one of the highest of about 1400km, and at mostplaces it rises1 to 2 km above densitiesof largeimpact craters and double-tingbasins on Main the interior plain but lessthan 1 km abovethe surrounding In general,the cratered terrain has a ruggedsurface of moderately plateau[ Wuet al., 1986].The material is rugged, heavily cratered, high relief, marked by secondarycraters, channels, wrinkle and faulted, and it forms hills, ridges, and massifsthat are ridges,and erosionalscarps. The rock unit embaysmaterial generallyaligned concentrically with Argyre Planitia. Some of of the Lower NoachianSeries in many areasand in turnis the massifsand ridges of northern Charitum Montes appear overlainby intercraterplains material of the UpperNoachian similar in size to those within Valles Marineris that have relief Series.
TABLE 2. Crater-densityBoundaries for Martian Series
Series Crater Density (N = no.craters > ( x ) kmdiam./10 6km 2) 14(1) N(2) N(5) N(16) N(4-10)
Upper Amazonian <160 <40 Middle Amazonian 160400 40-150 <25 <33 Lower Amazonian 600-1600 150-400 25-67 33-88 Upper Hesperian 1600-3000 400-750 67-125 88-165 Lower Hesperian 3000-4800 750-1200 125-200 <25 ! 65-260 Upper Noachian 200-400 25-100 >260 Middle Noachian >400 100-200 Lower Noachian >200
Craterdensities for N(1) andN(4-10) arederived from N(2) andN(5) valuesbased on the assumption of a-2 power-lawsize-frequency distribution. TANAKA:THE STRATIGRAPHY OFMARS El41
180'
57'5
Fig. 1. Shaded-reliefmap of Mars showinggeographic nomenclature used in thisstudy; names followed by an .asterisk areprovisional [ U.S. GeologicalSurvey, 1985].
Densitiesof craters no more than a few kilometers in diameter that are part of underlyingcratered terrain material are partly on the cratered terrain material are about the same as those buried by intercrater plains material. Smaller craters are onthe overlying intercrater plains material [Tanaka, 1985a] and commonlyrimless and largercraters have low rims. Crater ejecta thusaxe not helpfulin relative-agedeterminations. However, and floors are mostly buried. The intercrater plains surfaceis largecraters are morelikely to be preservedand recognizedsmoother than crateredterrain, but it is dotted by many small andto be usefulfor correlation.As a standard,cumulative cratersand locally cut by channelsor marked by wrinkle ridges. densitiesof craterslarger in diameterthan 16 km are usedfor Statisticsfor craters larger than about 5 km in diameter are correlationof materials of thisseries [Scott and Tanaka,1986]. fairly reliablefor relative-agedeterminations (Table 3). Althoughthe relative stratigraphic positions among the Upper, Intercrater plains material overlies and embays Middle Middle,and Lower Noachian are generally evident, the materials Noachian cratered terrain material throughout the Martian overlapincrater density from place to place. highlands.In turn, it is locally overlainby other plainsmaterial that is smoother, less cratered, and commonly ridged. UpperNoachian Series Preliminarywork with Viking images[Scott and Tanaka, 1984] did not show these relations;however, new, more complete The"cratered plateau material" was described by Scottand mapping[Scott and Tanaka, 1986]does show this stratigraphic Cart[1978]asdensely cratered but flat and smooth in intercrater distinction. The intercraterplains unit has few featuresthat can areas.This rock unit was placed in theuppermost part of the be usedto identify its origin;it may have multiple origins,such NoachianSystem. Recent mapping by Scott and Tanaka [ 1986] as lava flows, flood-plaindeposits, and eolian fill. showsthe smooth intercrater areas as a separateunit (subdued crateredunit of theplateau sequence) that overlies the cratered Lower Hesperian Series terrainmaterial. This distinction permits the designation of the UpperNoachian Series asrepresented bysuch intercrater plains The Hesperian System was named for Hesperia Planum material.The type area selected is east of theArgyre impact (Figure 1). The baseof the systemwas delineatedby the base basininthe vicinity oflat 45 ø S,long 15 ø. Here, many craters of the "ridged plains material" [Scott and Carr, 1978]. This E 142 TANAKA:THE STRATIGRAPHYOF MARS
TABLE 3. CraterDensities of SomeMartian GeologicUnits and Features
Unit or Feature CraterDensity (N= no.craters > ( x ) kmdiam./106 km2) N(1) N(2) N(5) N(16) N(32)
Smoothplains material southern Arcadia <50 southern Acidalia 3OO-450 55 Vastitas Borealis Formation north of lat 55 ø N 150-250 65-85 south of lat 55 ø N 250-375 65-75 Lava flows, Alba Patera 240-500 55-170 Noctis Labyrinthus 85 Lava flows, Ceraunius Fossae I00 Ridgedplains material SW ElysiumPlanitia 150 Lava flows, Dorsa Argentea 150-170 Ridgedplains material Synis Major Planurn 165 Acheron Fossae 240 60 Intercraterplains material Lower Hesperian 700 160 Upper Noachian 950-1000 300-400 30-100 <30 Cratered terrain material 650-1200 300-550 120-240 30-100 Impact basins Argyre 50 Hellas 8O Isidis 130 map unit is extensiveand readily identifiable,and becauseit After formationof the Lower Hesperianridged plains material still providesan excellentboundary for the baseof the Hesperian and before depositionof the lowermostAmazonian smooth System,I considerit the referentfor the LowerHesperian Series. plainsmaterial, extensive volcanic, fluvial, and complexplains The type area for the unit remainsthe same,at lat 20ø S, long unitswere emplaced [Scott and Tanaka,1986]. Therefore I have 245ø in HesperiaPlanurn. Here, the ridgedplains material forms selectedcomplex plains material (VastitasBorealis Formation) a smoothsurface marked by a pattern of wrinkleridges similar to representthe Upper HesperianSeries. This unit is composed to those of the lunar maria. The ridgescommonly exceed 100 of mottled, knobby, and patternedplains materialsin Vastitas km in lengthand 10 km in width, and they overlapone another. Borealis(Figure 1), which forms most of the lowland region Some wrinkle ridgesare found on stratigraphicallyhigher and of Mars north of lat 40 ø N. The Vastitas Borealis Formation lower rocks;however, those ridges are generallyless common wasformerly mapped as a combinationof smooth(Amazonian), and smaller. cratered (Amazonian), mottled (Noachian), and rolling Crater densitiesof the ridged plains material generallyare (Hesperian)plains materials [Scott and Carr, 1978]. in a restrictedrange [N(5)= 125-200] that can be used for Within the formation, densitiesof craters >2 km in diameter stratigraphiccorrelations with other geologicunits. The ridged tend to be lower north of about lat 55ø N (Table 3), where plains materialoverlies or erabaysrocks of the Upper N oachian smallercraters have probably been erased. At >5 km diameter, Seriesin areassouth of ChrysePlanitia, surroundingProtonilus however,crater densitiesare uniform and have valuesconsistent 'Mensac,and elsewhere.Wise et al. [1979] postulatedthat the with their stratigraphicposition. material is composedof lava flows; Greeleyand Spudis[!981] The members of the Vastitas Borealis Formation are suggestedthat the flowswere of low viscosityand wereerupted characterizedby complex surface features that includethe at high effusionrates. following:(1) a mottledalbedo pattern of light cratermaterial superposedon dark intercrater material, (2) smallknobs of varied Upper HesperianSeries density,(3) troughsthat form polygons5 to 20 km in diameter, and (4) ridgesthat form both polygonaland concentricpatterns. The Hesperian-Amazoniansystem boundary was formerly The compositionof the materialsis unknownbut may include definedby Scottand Carr [1978] asthe top of the ridgedplains lava flows,sediments, and pyroelasticmaterials [Greeley and material and the base of the "crateredplains material." The Spudis,1981; McGill, 1985].Origins of the knobs,ridges, and type area for the Amazonian System was defined as lat 18ø grooveshave attributed to masswasting, periglacial deformation, N, long 165ø and mappedto includeboth crateredand smooth volcanism,and tectonism [Carr and Schaber, 1977; Scott, 1979; plains•.material[Scott and Cart, 1978]. However,this area has Pechmann,1980]. sincebeen found to consistof smoothand ridgedplains material [Scott and Tanaka, 1986]. Moreover, areasformerly mapped Lower Amazonian Series as cra•teredplains material have now been shown by recent geologicmapping to consistof manyunits of thewestern volcanic As discussedabove, I redefinedthe baseof the Amazonian assemblageand the Vastitas Borealis, Arcadia, and Elysium Systembecause of complexitiesnewly discovered by geologic Formations[Scott and Tanaka, 1986; Tanakaand Scott, 1987; mappingof units at the baseof the system.The new referent R. Greeleyand J. E. Guest,unpublished data, 1986]. Scott and is the smoothplains material that makesup the lowermost Tanaka [1986] consideredthe base of the Amazonian System memberof theArcadia Formation [Scott and Tanaka,1986], .to be the base of the Arcadia Formation, which includes the whichis exposedprincipally in southernAmazonis and southern smoothplains material within the Amazonisquadrangle. AcidaliaPlanitiae. This rock unit is moderatelycratered and TA•/AKA: THr• STRATIGRAPHYOF MARS E143 smooth,and it surroundssmall knobs and high areas of older material, indicatingthat the depositsare perhapsonly tens of materials.The type area of the smoothplains material is at meters thick. lat33 ø N, long30 ø. The unit overliesridged plains material insouthern Amazonis Planitia; the Vastitas Borealis Formation CRATER-DENSITY BOUNDARIES FOR MARTIAN SERIES inAcidalia Planitia; northern, lower flows of AlbaPatera; and Relative-agelimits defined by craterdensities for the Martian UpperHesperian flood-plain deposits. Lobate flow scarps and series were derived from crater counts of referents and from distributarychannels in thislowermost member of theArcadia Formationin AmazonisPlanitia indicate that it is madeup other urdts of like statigraphicposition. Table 3 summarizes craterdensities determined in thisstudy. partlyof lavaflows and fluvial deposits; it may also include coljanmaterial. Densities of craterslarger than 2 km in diameter Increasingdiameters are used for crater-densityranges of series for theunit differfrom placeto place,indicating differences of increasingage for severalreasons. First, imagesof most areas generallyallow identificationof cratersno smallerthan 1-2 km inrelative ages and perhaps in craterdegradation. in diameter. Second, a large proportion of craters several kilometers in diameter or less are obliterated on older surfaces. MiddleAmazonian Series Third, the density of large cratersis generallytoo sparseon younger surfacesfor determining precise crater densities. Smoothplains material in AmazonisPlanitia, which Another effectthat is not yet well studiedis differencein crater characterizesthe AmazonianSystem [Scott and Carr, 1978], size causedby differenttarget-material properties [Boyce and wasshown to be composedof five overlappingmembers of Witbeck, 1985]. the ArcadiaFormation [Scott and Tanaka, 1986]. Members Generally,densities of craterslarger than 16 km in diameter 2and 3, which are lava flows, are similar in stratigraphicposition were determined for the Noachian referents. This diameter was to someof the extensivevolcanic rocks of the Tharsis Montes usedin countsfor severalterrain types [Gurnis, 1981], and for Formationand to thick, degradedmaterials that composethe recentlymapped units on Mars [Scottand Tanaka,1986]. At lowerand middlemembers of the MedusaeFossae Formation. this diameterand greater,the effect of crater obliterationis A Middle AmazonianSeries is proposedto distinguishthe negligible[Woronow, 1977]. On Upper Noachianintercrater positionofthese two members ofthe Arcadia Formation. Their plainsmaterial and some older resistant terrains, however, craters typearea is at lat 30ø N, long160 ø. Here,member 3 is smooth, as small as 5 km in diameter are sufficientlywell preserved sparselycratered, and marked by a few scarpsthat appearto to yield reliablerelative ages from cratercounts. bethe edges of lava flows.It overliesthe lowermostmember For moderatelyresurfaced Hesperian and Lower Amazonian of the Arcadia Formation in Amazonis Planitia and erabays surfaces,5-kin-crater densities (N(5)) weremostly used in order theOlympus Mons aureoles, and it isoverlain by upper members to avoid obliterationeffects and to maintain a sufficientsample of the Arcadiain southernArcadia Planitia and apparentlyby size.On manyTharsis region and northern plains units, crater theupper member of the Medusae Fossae Formation in southern densitieswere determinedat this 5-km size [e.g., Scott and AmazonisPlanitia. Densities of craters >2 km in diameter are Tanaka,1981b; Plescia and Saunders, 1982; Dial, 1984;i44'tbeck usefulin correlatingrock units of MiddleAmazonian age. and Underwoo&1984; Table 3]. However, authorsof many publishedcrater countscite densitiesfor Hesperianto UPperAmazonian Series Amazonianunits at othersizes as well [e.g.,N(4-!0), Condit, 1978;N(1), Neukumand Hiller, 1981]. Newmapping of Marshas i-evealed many young units that For Middle and UpperAmazonian surface, the 2-kin-crater havefew superposedkilometer-size craters. These include densities(N(2)) are optimal for general application. Frequencies uppermostmembers of the Arcadia, OlympusMons, Medusae of smallercraters of similarlyaged surfaces range widely due Fossae,and Tharsis Montes Formations and polar, eolian, to secondarycraters, nonimpact craters, and resurfacing landslide,debris-apron, and channelmaterials [Scott and [Soderblomet al., 1974;Neukum and Wise,1976; Hartmann Tanaka,!986; Tanaka and Scott, 1987]. Upper Amazonian et al., 1981].For theyoungest surfaces, however, the statistical matehalsare noteworthybecause of their relativeyouth and sampleof craters2 kmin diameteris small,and crater-density theirconsequent indication of currentand potentialMartian data have little value. geologicactivity. Recent mapping in southernElysium Planitia Crater size-frequencydistributions of Amazonianand [Tanakaand Scott, 1986; R. Greeleyand J. E. Guest,unpublished HesperianTharsis plains units follow approximately a-2 power data,1986] has delineated vast, smooth flood-plain material that law for cumulative numbers of craters larger than 1 km appearsto have been depositedby floods emanatingfrom [Hartmann,1977; Tanaka, 1985c]. On thebasis of thispower westernCerberus Rupes. This unit is chosenas the referent law, densitiesof N(1) and N(4-!0) craterswere converted to for theUpper Amazonian Series; the type areais at lat 6ø N, N(2) and N(5) counts.Thus crater-density ranges that were long208 ø. Thearea of theflood plain exceeds 100,000 km 2 determinedfor Hesperianand Amazonianseries at 2- and5- andit extendsfrom long 168b to 222ø (nearly3000 km); it km diameterswere converted to N(!) and N(4-10)densities for ismore than 750 km wideat long 195ø. The unit consistsof easycross-reference (Table 2). TheN(1) values should be used low-albedomaterial marked by light,wispy streaks. Channel cautiouslybecause of theirvariation, as described above. scarpsand teardrop-shapedbars are observednear the source In mostcases, crater densities for the referentsspan the range and south and east of Oreus Patera. The east channel of the givenin Table 2. However, small areas of some units have crater floodplain cuts into materialsthat includethe MedusaeFossae frequenciesthat suggest overlap into adjacent series. and Arcadia Formations. Cited crater densities of the channel floorsare N(1) <600 [Carr and Clow, 1981] and N(1) <50 [Scott ABSOLUTE AGES OF MARTIAN EPOCHS andTanaka, 1982]. Sparse cratering of theentire regional flood plainsupports the smaller crater count. Most craters larger than Several models have been made to determine absolute ages about1 km in diameterappear embayed by theflood-plain of surfaceson Mars;they remain a subjectof controversy.The E 144 TANAKA: THE STRATIGRAPHYOF MARS
Age Lunar Martian Epochs calibrationcurve. Similarly, heavily crateredterrain on Mars (b.y.) Period• Model I Model 2 wasassigned a !-km-crater density on the basisof frequencies 0- of largercraters in TempeTerra (quadrangle MC-3). Assuming ... that the highlandsterrains are all of aboutthe sameage and ., accountingfor velocityeffects, Neukum and Wise deduced that ., 0.70 thecrater-production rate on Mars is 4.5 timeslower than on .. the moon.They alsoindicated that Phobosis about4.5 to 4.6 1-- .lO . b.y. old, and that a comparisonof craterdensities between Phobosand the Martian highlandssuggests a 4.4 b.y. agefor .. thehighlands. Considering crater-scaling effects, they suggested that the meteorold flux at Mars and the moon was the same ., within a factor of 2. On the other hand, reviewingthe crater- 2-
.. productionrates on Marsdetermined by variousauthors and 2.30 .. consideringthe astronomicaleffects of known asteroidand
. cometpopulations, impact velocities, gravity, and energy scaling, Hartmannet al. [1981,p. 1080]suggested a Martian craterflux 3-- twicethat of the moonto be the mostlikely, and a rangebetween 3.10 •3.20 a factor of 1 and 4 to be possible. The differenceof a factor of 2 in relative crater-production 3.50 3.55 rates betweenthe Neukum and Hartmann proposalscreates 3.85 3.70 3.80 quite differentchronologies for the Martian epochs.A major uncertaintyin the Neukum and Wise model is the age of the 4-' ,3.92 Martian highlands.Can this be ascertainedby usingonly the 4.40 assumptionthat the meteoroidpopulations have the samesize- 4.6- frequencydistribution for both planets?(This assumptionis common to both chronologymodels.) An attempt is madein Fig 2. Chart showingmodel chronologies for the moon and Mars. the followingdiscussion. Absoluteages for lunar periodsfrom g/ilhelms[1980, 1986].Inferred The distributionof craterslarger than 10 km in diameter agesfor Martian Hesperianand Amazonian epochs are based on crater- densitiesof series(Table 2) and correlationto modelehronologies of on Noachiansurfaces deviates from a simplepower-law function Neulcumand Wise[1976] (Model 1) and Hartmannet aL [1981](Model and is more preciselyexpressed by a lognormal distribution 2). Becauseof obliterationof smallerNoachian craters, these models [Woronow, 1977].Specifically, fewer smallercraters are found could not be useddirectly for the Noachianboundaries. Neukurn and than expectedfor a power-law distribution.The lognormal Wise[1976] assigned a 4.4 b.y. ageto crateredterrain material (Middle Noachianrocks) but gaveno age range;thus the boundariesof this distributionalso describesthe populationof ancientcraters on epochare dashed.Noaehian ages derived in thispaper are compatible the moon [Wilhelms et al., 1978]. On Mars, this distribution with the Hartmann model and are combined with the Hesperianand may either (1) reflect the actual ancientdistribution of impact Amazonianages of that model. cratersthat, in turn, may representa populationof planetesimals that bombardedthe planet[Strom and Whitaker, 1976;Gumis, modelsallow predictionof the relation betweencrater density !981] or (2) result from intenseobliteration of smallercraters and absoluteage of Martian surfacesbased on (1) the same that wereoriginally part of a power-lawdistribution [Hartmann, relation derivedfor lunar rocksfrom sampledata, and (2) the 1973; Chapmanand Jones,1977]. Woronow[1977] indicated ratio of the cratering fluxes of Mars and the moon. Because from severallines of evidenceand theory that a simple power- thesefactors are not preciselydetermined, estimates of crater- law function doesnot explain the large-craterdistribution on productionrates for Mars differ considerably.The resultsof Mars, and that craterobliteration was mainly confinedto craters two dorrdnantcrater-chronology methods, one by Neukum and <15 km in diameter. Therefore I assume that the Martian Wise[1976] andanother by Hartmannez al. [1981],demonstrate population of craterslarger than 15 km in diameter closely the range in estimates.They are given in Figure 2, and the reflectsthe initial distributionfor the Middle Noachian Epoch, methodsand assumptionsare briefly reviewedhere [see also and that craters having smaller diametersare more fully the reviewby Cart, 1981,p. 61]. preservedon surfacesof higherstratigraphic positions. Neukumand Wise[1976] first developeda correlationbetween Highland surfaceson the moon and on Mars have similar N(I) and radiometricages for lunar sitesbased on a review large-crater distributions[Strom, 1979, Figure 20]. More of existingmodels. They then determineda "productionsize- specifically,the distribution of lunarNectarian craters [ Wilhelms frequencydistribution" (or calibrationcurve) for lunar craters et aL, 1978] is statisticallysimilar to that of Martian craters between0.3 and 20 km in diameterin the age range of 3 b.y. (scaledin sizeon the basis of relativegravity and velocity effects) to older than 4 b.y. [Neukum et al., 1975] and comparedit on Middle Noachianhighland surfaces [ Tanaka, 1984]. In order to a similar distribution derived for Mars based on Mariner to compare more accurately lunar and Martian crater 9 images.The Mars distributionwas found to be somewhat distributions, I modified the Martian distributions of craters flatter. Neukum and Wise[1976] suggestedthat this difference largerthan 16 km in diameterso that onlythe crater distributions was causedby the productionof cratershaving diameters 1.5 of particularepochs are represented,such as in the lunar counts times larger on the moon than on Mars becauseof the greater by Wilhelmset al. [1978].Furthermore, I increasedthe diameters averageimpact velocity of meteoroldsbombarding the lunar of Martian cratersby a factor of 1.3 to accountfor gravity- surface.They assigneda frequencyof 1-km-diametercraters and velocity-scaling effects [Tanaka, !984]. The lunar to lunar highlandsthat have an age of 4.4 b.y. by projection distributionsare statistically distinct from eachother according from the 100-kin-diameter crater population, using their to a X2 testof binnedcrater-size frequencies with a 99% TANAKA: THE STRATIGRAPHYOF MARS E145
acceptancecutoff (Table 4). I used countsby Gurnis [19.81] to representthe crater distributionsof time-stratigraphicunits on Mars, and I subtractedthe frequencyof craterssuperposed on intercraterplains material(Upper Noachian)from their frequency on cratered terrain material (Middle N oachian), leavinga remainderof Middle Noachiancraters. I did the same for intercrater plains material and the Vastitas Borealis Formation (Upper Hesperian)to derive a crater distributi6n for the interval betweenthe Late Noachianand the Early Hesperian.Binned counts for cratersof Middle Noachianage >16 km in diameter correlate with the lunar Nectarian crater distribution(Table 4). For craters>8 km in diameter,the Late Noachian to Early Hesperiandistribution correlates well with that for the lunar Irabrian System[ Wilhelmset al., 1978] but only fairly we!! with the lunar mare distribution[Hartmann, 1978], and the Late Hesperianto Amazonian distribution matchesthe average lunar mare and Copernican-Eratostheni .a:n distributions[ Wilhelmset al., 1978]. The similarityin crater distributionsof the Middle N oachian Seriesand the Nectarian Systemsuggests that they may have about the same averageage. This suggestioncan be accom- modatedby the Hartmann model,which indicatesan age range of 3.8-4.2 b.y. (4.0 mostlikely) for the heavilycratered terrain, but not be the Neukum model, which requiresthat the age of the Martian crateredterrain (Middle Noachian)be abo•.U;t 4.4 b.y. The crater-densityratio between Martian Middle Noachian and lunar Nectarian surfacesis about 2.0, which• suggeststhat the periodsare similarin duration,based on a crater-productionratioof 2.0. Within these guidelines', I .have': set the limits of the Middle NoachianEpoch at 3.92 and 3.85 b.y.mthesame as the lunarNectarian Period (Figure 2), The Lower Noachian Series, then, is older than 3.92 b.y. The baseof theLower Hesperian Series, according to thecrater; densityboundary (Table 2) and the Hartmannchronology, is• 3.5 b.y. old. The Late NoachianEpoch thus is assignedthe periodfrom 3.85 to 3.5b.y. The crater distribution and inferred age(3.85-3.10 b.y.) of the intervalbetween the LateNoachian and Early HesperianEpochs on Mars nearly corresponds.to. that of the Imbrian Period on the moon [3.85-3.20 b.y.; Table 4; Wilhelms,1980, 1986]. Cumulative crater distributions for surfacesyounger than about 3.5 b.y. on both the moon and Mars generallyfollow a -2 powerlaw. Thesesurfaces include. those of lunar Eratosthenianand Copernicanrocks [<3.2 b.y.; [Viihelmset aL, 1978, Figure 6; W'ilhelms,1980, 1986]; !unar• mare(3.8 to 1.0b.y.; D. E. Wilhelms,personal commurdcation; 1986);and Martian post-Noachian (<3.5 b.y.)rocks. The Upper Hesperianto UpperAmazonian crater distribution on Mars. correlateswith the distributionsof these youngerlunar rock•s (Table 4).
STRATIGRAPHIC-AGEDETERMINATIONS OF GEOLOGIC UNITS AND FEATURES Thereferents (Table 1) and crater-density boundaries (Tabl•, 2) for the seriesdefined above form the basis for stratigraphic-. agedeterminations ofgeologic units and features on Mars. Many.• cratercounts for suchunits and featuresare alreadypublished;, additionalcounts performed in order to completethis study aregiven in Table3. (More completedocumentation of many of thesenew counts is intendedfor futurepublication.). ,: Crater countsare generallypublished in graph or tab,ula•r form, and craterdensities can be read directlyor adjustedto one or more of the sizeranges shown in Table 2. Exceptions, E 146 TANAKA:THE STRATIGRAPHYOF MARS are the data in Hartmannet al. [1981,Table 8.6.1], which are regions),surface properties, and atmosphericeffects. Collec- ratiosof craterdensities (mostly at 4 km diameter)relative to tively,they help to unravelthe evolutionof the planet. a typicallunar-mare crater distribution [in whichN(4) = 188]. Assumingthat the countsfollow a ~2 power4awdistribution Early NoachianEpoch (which is approximate),I convertedthe ratios to N(5) by multiplyingthem by a factor of 125 and to N(2) by usinga The oldestexposed materials on Mars are: (1) basin-rim factor of 750. materialsurrounding Isidis, Hellas, Argyre,south polar, and Martian geologicunits and featuresin this study(Table 5) other basins,and (2) isolatedhilly or mountainousmatedfl. are classified as follows: These materialscharacteristically have bold relief and are (1) Plains and highland units. Deposits that have embayedby crateredplains material. The bold relief indicates miscellaneous(e.g.. volcanic, eolian, mass-wasting, impact) or that the Martian lithospherewas sufficientlythick, cool,and uncertain origins. elasticduring this early period to preservelarge craterforms. (2) Volcanoesand associated lavario ws. Featuresthat have Craterdensities are generally in the rangeN(16) = 120to 300; definitivevolcanic morphologies. however,because of substantialdegradation and fill withinlow (3) Channels. Includesoutflow, fretted, and runoff types. areas, the crater-densitydata commonly are not representative (4) Fracture systems. Most are related to Tharsis of the age of the material. From superpositionrelations with tectonism. crateredterrain material, the LowerNoachian series is assigned (5) Impact basins. Circular, multiringedstructures larger N(16) > 200. If the crater populationshave been degradedin than 200 km in diameter. a consistentfashion, crater counts (Table 3) indicate that the Generally, crater-densityages and stratigraphicrelations oldestof the three largestbasins is Isidis, followedby Hellas closely complement each other. Where crater densities andthen Argyre. This sequence is alsosupported by comparisons determinedby variousauthors are assignedto series,the relative among the basinsof the relief and degreeof preservationof agesof mostfeatures are in fair agreement.Where discrepancies mountain-ringstructures. Erosion probably was not the main exist, stratigraphicrelations are used to determinethe relative causeof the lower reliefof the olderbasin rims, as suggested age as preciselyas possible.However, someof the featuresare by the moderateamount of obliterationof superposedcraters. volcanicprovinces or volcanogroups that havehad long periods More likely,the greater diameter of the olderbasins and probable of activity; the inferred entire durationsare given in Table 5. higher crustal temperaturescaused greater viscousrelaxation The relativeages of channels,fractures, and impactbasins were of their rims. Most other Martian impact basinsidentified or mostly determinedby overlaprelations or crater countsof the postulated[Schultz et al., 1982]appear older than or equivalent surfacesthat they cut or overlie; only a few of the agesrefer in ageto the Middle Noachiancratered plains, and their positions to the features themselves. are designatedas Lower to Middle Noachian (Table 5). Other isolated hilly material is exposedin relatively small patches STRATIGRAPHIC MAPS OF MARS surroundingthe Tharsis region and in patchesin the eastern part of Mars; they may also be old basin-rimremnants of old To summarizethe vast amount of stratigraphicdata in a volcanicor tectonicmountains [Scott and King, !984]. simple, consistent,and legibleformat, the time-stratigraphic Knobby remnants of heavily cratered terrain occur in the series describedabove are portrayed in map form. Formal northernplains as far north as ScandiaColles (Figure 1) and geologicmapping of Mars at 1:!5,000,000scale from Viking other localitiesnorth of the highland-lowlandboundary scarp. imagesis nearly complete[Scott and Tanaka, 1986; Tanaka Theiroccurrence suggests that the northern, topographically low and Scott, 1987;R. Greeleyand J. E. Guest,unpublished data, region (several kilometers below the highland surface)was 1986]and washeavily relied on to completea stratigraphicmap formed duringthe Early NoachianEpoch. This low regionis of the entire surface of the planet. The spatial and temporal approximatelycircular and has a diameterof about 130ø of resolutionof the stratigraphicmapping surpasses previous work. latitude.Faults radial and concentricto the Argyre, Hellas,and The resultingmaps are shownon Plate la-c. Isidisimpact basinsformed during the Early Noachianand on A few exposuresconsist of materialsthat wereeraplaced over into the Middle Noachian,according to cratercounts [ Wichman longtime spans, such as canyon-w• materialin VailesMarineils, and Schultz,1986] and transectionrelations with plateaurocks. etchedterrain near the southpole, and knobbyplateau remnants in the northern plains. These materialsare mostly Lower to Middle NoachianEpoch Upper Noachian,but for simplicityall are shownas Middle Noachianon the maps. Likewise, the activities of manyvolcanoes The MiddleNoachian Epoch represents the final periodof and fracture setscover wide age ranges,but only the latest or heavy bombardmenton Mars, when crateting rates werevery averagesurface ages are presentedon the maps.Detailed age high and crater-production distribution may have been ranges,stratigraphic relations, and crater-count sources of these lognormal[ Woronow, 1977]. Consequently, many of thesmaller units and featuresare compiledin Table 5. Overall, the maps impactbasins are of this age (Table 5). Crater countsof the providean easyreference for the stratigraphicpositions of all Middle NoachianSeries referent, the crateredterrain material, major units on Mars. wereobtained from the followingMars Charts:3, 10 [Neukum and Wise,1976]; 20, 22, 23, 26 [Gurnis,1981]; 20, 23 (A. L. GEOLOGIC HISTORY OF MARS Dial, Jr., personalcommunication, 1984); and 10, 11, 18,19, 24, 25, 26 [Tanaka, 1985a].Crater densitiesmostly fall in the In the following sections,the surfacehistory of Mars is range of N(16)-- 120 to 240, concentratedwithin the range reviewed,adding a geologicperspective to thestratigraphic maps of N(16) = ! 60 to 200. Resurfacingand erosionaleffects were on Plate 1. The individualepochs are characterizedby events minimizedfor the countsin this studyby avoidingintercrater and activitiesthat are governedby planetesima!bombardment, deposits(mapped as younger units) and dissected, knobby, and the deepmantle and lithosphere (manifested by volcanotectonicetched exposures [Scott and Tanaka,1984]. Even so, in some TANAKA:THE STRATIGRAPHYOF MARS E147
t
.... J E 148 TANAKA:THE STRATIGRAPHYOF MARS TANAKA:THE STRATIGRAPHYOFMARS E149
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z
z
'T' I--'
I::] Z El50 TANAKA:THE STRATIGRAPHY OFMARS
TABLE 5. Stratigraphyof Martian GeologicUnits and Features
Unit or Feature Stratigraphic Cited Crater Ages Key GeologicRelations Position
A. Plains and Highland Units Polar dunes,mantle, ice, and UA (andlower?) MAm, UA {2) layered deposits Landslides Olympus Mons UA none =Olympus flows (UA) Vailes Madneris MA-UA MA {3)
B. Volcanoes and Associated Lava Flows OlympusMons (including LA(orlower)-UA MAm, UH-UA (?),
TABLE 5. (continued)
Unit or Feature Stratigraphic Cited Crater Ages Key GeologicRelations Position
Channels CerberusRupes channel UA UA(6), MA=UA (•6) D. Fracture Systems CerberusRupes none =western Amazonis channel (UA) Northeast of Ascraeus Mons MA-UA MA-UA(•4), MA- =Tharsis flows (MA-UA) UA ½•7) South of Ceraunius Tholus LA-MA LA-MA{•4), LA- =Thatsis flows (LA-MA) MA (•7) Elysium Fossae UH-LA none =Elysiumflows (UH-LA) Eastern Memnonia Fossae UH-LA LA(m, UH_LA (•*) =Tharsis flows (UH-LA) Daedalia Planurn UH-LA LH-LA(m, UH- =Thatsis flows (UH-LA) LA ('7) East of Ceraunius Tholus UH-LA LH_LA(•4),LA {•7) =Tharsis flows (UH-LA) Labeatis Fossae UH UH04),UH (•7) =Tharsis flows (UH) Fortuna Fossae UH >Thatsis flows (MA) Alba and Tantalus Fossae LH-LA LH(2•, LH.LA 18• ' TABLE 5. (continued) Unit or Feature Stratigraphic Cited Crater Ages Key GeologicRelations Position E. Impact Basins Lyot LA none "Stratigraphicposition" is determinedby crater countsand geologicrelations; N = Noachian, H = Hesperian,A = Amazonian,L = Lower, M = Middle, U = Upper. "Cited crater age" is mostly deducedfrom crater counts;includes geologic relations from some studies;counts may be for only part of the unit or feature;footnote is for referencebelow. "Key geologicrelations" are those most helpful in establishingage; includes ages more detailedthan in table (e.g., Tharsisflows are subdivided); < meansyounger than, = meanscontemporaneous with, and > meansolder than. References:(1) Hartmann et aL [1981]; (2) Dial [1984];(3) Lucchitta [1979]; (4) Masurskyet aL [1977]; (5) Squyres[1978]; (6) Scott and Tanaka[1982]; (7) Hiller et aL [1982]; (8) Table 3; (9) Condit [1978]; (10) Greeley and Spudis [1981]; (11) Witbeck and Underwood [1984]; (12) Soderblomet aL [1974];(13) Plesciaand Saunders[1979]; (14) Plesciaand Saunders[1982]; (15) Gurnis[1981]; (16) Carr and Clow [1981]; (17) Scott and Tanaka [1981b]; (18) Neukum and Hiller [1981];(19) Masurskyeta!. [1986];(20) Wichmanand Schultz[1986]. placesmore than half the cratersare embayedby local deposits. of distributions of fresh to degraded craters in the 5- to 15- Densities of fresh, unembayedcraters greater than about 4 km km-diametersize range. The obliterationof smallcraters may in diameteron Upper Noachianintercrater plains are consistently be a resultof easilyeroded surface materials,, mantling by e01ian lower than densities of similarly sized craters on Middle and volcanicdeposits, and wind and water erosionproduced Noachian terrain [Tanaka, 1985a]. by a thickeratmosphere and warmer surfacetemperature. Also during the Middle Noachian (and possibly earlier), High-resolutionimages of the crateredand intercraterplains Acheron, Claritas, Coracis, Nectaris, and Melas Fossaeand the showthat significanterosion or burial occurredin mostplaces broad, arcuate ridge along the south edge of Softs and Sinai sufficientto obliteratepreexisting kilometer-size craters. Small Plana (Figure 1) were formed. but densely arrayed channels dissect cratered terrain and intercraterplains material and predate the ridgedplains material; Late Noachian Epoch they appear to have formed by water runoff [th'eri, 1976;Cart and Clow, 1981]. Larger runoff channelssuch as Ma'adim,A1- During this epochan earlyintercrater unit wasemplaced that Qahira, and Nirgal Vailes(Figuie 1) alsocut the crateredplains overlies much of the older Martian highland material. Only and were formed at about the same time as the smaller channels. larger exposuresare mapped(Figure 3); many smallerpatches Faulting was extensivein the Tharsisregion during the Late are not shown. These materials are relatively smooth but have NoachianEpoch, continuingat Claritas, Coracis,and Nectaris channelsand wrinkle ridgesin places.Crater densitiesfor this Fossaeand commencingat Noctis and CerauniusFossae. Most unit range from N(16)= 30 to 100 and averageabout 90 in faults are oriented radially to Syria Planurn [Plesciaand the westernpart of Mars. Saunders,1982]. More than 30 mountains(not shown)of The intercraterdeposits are thicker in someareas than others, possiblevolcanic origin that may haveformed during this epoch and crater densitiesare highly variable; many exposedcraters occur from Claritas and Thau•asia Fossae west to Sirenurn are embayed.The distributionof cratersgreater than 8 km in Fossae[Scott and Tanaka,1981a], at NectarisFossae [Roth diameter is intermediate in form between that of earlier et aL, 1980],and at otherhighland areas (including Apollinaris (lognormal)and later (power-law)distributions. The size- Patera).Fissure volcanism along Acheron Fossae produced lava frequencydistributions of cratersabout 2 to 4 km in diameter ffil within the arcuatestructure [Scott and Tanaka,1986]. are about the same for Middle Noachian to Lower Hesperian Althoughtheir age is not preciselyestablished, Vailes Marineris surfaces[Tanaka, 1985a].Furthermore, a densitymap of 0.6 mayhave been initiated by faulting at thistime. East of Elysium to 1.2 km cratersfor most of Mars by Soderblomet al. [1974, Planitia, faults that bound wide grabenscut the knobby, Figure5] indicates.densities that correspond'to Amazonian and degradedcratered terrain and the grabens are embayed by Lower Hesperianages. The lack of regionshaving high densitiesof Hesperianridged plains material; these fauks may have formed smallcraters indicates that nearlycomplete obliteration of these duringthe Late N oachian. cratersoccurred during the Late Noachianand possiblyearlier as well. However, by Early Hesperiantime, surfaceswere Early HesperianEpoch producedthat show no indication of obliterationof craterslarger than 1 km in diameter.Chapman and Jones[1977, p. 525] have The LowerHesperian Series referent is the ridgedplains also indicatedthat an episodeof crater obliterationoccurred material,which is characterizedby smooth surfaces marked by duringan intermediatestage of Martian history,on the basis long,sinuous wrinkles ridges. This unit is extensiveand covers TANAKA: THE STRATIGRAPHYOF MARS E153 thefollowing areas: (1) part of the southern edge of the northern large sheetflows and were contemporaneouswith a late stage plains;(2)plains between Elysium and Amazonis Planitiae; (3) of wrinkle-ridgeformation [Schaber, 1982]; probable calderas, HellasPlanitia; and (4) many high plains such as Lunae, Sinai, volcanoes,and tectonicdepressions in Tempe Fossae[Scott, SyrtisMajor, Hesperia, and Malea Plana (Figure 1, Plate 1). 1982];and Alba Patera,which produced its northernand eastern Ridgedplains also occur in smallpatches in cratersand in distal flows duringthis time [Dial, 1984]. Flows of this epoch intercraterhighland areas. In places,the VastitasBorealis also may have occurredaround TharsisMontes, but they have Formationappears to erabayand encroach on the ridged plains beenobscured by youngerflows and their morphologic character material;the formation perhaps results from modificationof destroyedby fractures. ridgedplains material. For example, insouthern Utopia Planitia, The NoctisLabyrinthus-Valles Marineris rift systemdeveloped theridged plains material is locallyhummocky and knobby. to a large extent during the Early Hesperian Epoch. Also, Hencemuch of the VastitasBorealis Formation may have extensivefractures formed on highlandsurfaces surrounding originallybeen ridged plains material emplaced byvoluminous most of the Tharsisregion at Alba, Tantalus,Mareotis, Tempe, eruptionsoflow-viscosity, mafic lava [Greeley and Spudis, 1981]. Ulysses, Uranius, western Memnonia, Sirenurn, Icaria, and Possiblelocal sources of the ridgedplains materialinclude ThaumasiaFossae. Many of thesefractures are radial to Syria Hadriaca,Tyrrhena, Amphittites, and PeneusPaterae (Figure Planum, and the youngestsurfaces that they cut have counts i). of N(5) -- 219 to 115[Plescia and Saunders,1982]. This episode ByEarly Hesperian time, the cratering rate was much reduced of rifting and faultingwas perhaps the mostintense in Martian fromthat of the Noachianheavy bombardment [Hartmann et history. al.,1981, Figure 8.6.3]. Stratigraphic relations (Table 5) suggest thatthe double-tingimpact basinLowell is Early Hesperian Late HesperianEpoch inage. Cumulative size-frequency distributions of craters more than I km in diameter on most pristine Martian surfaces The Upper Hesperian referent, the Vastitas Borealis generallyfollow a powerlaw, similar to the distribution ofcraters Formation[Scott and Tanaka,1986], covers most of thenorthern onthe lunar mafia [Hartmann, 1978]. Densities of craterslarger lowlandsin Acidalia,Isidis, Elysium, and Utopia Planitiaeand than 5 km in diameter for materials of this epoch are most all lowland areasnorth of lat 55ø N (Figures1 and 2). Densities reliableat 5 km diameterand generallyrange from 125 to 200 of craterslarger than 5 km in diameterfor the formation are forthe ridged plains referent. On MaleaPlanum, ridged plains remarkably uniform, ranging from N(5)--65 to 110 and materialhas a slightlyhigher crater density [Hartmann et al., averagingabout N(5) = 75 [Dial, 1984; Witbeck and 1981];however, some of the craterswithin the unit appear Underwoo&1984; Table 3], suggestingthat mostof its surface embayedby theridged plains material. Counts of craterslarger formedin a relativelyshort time. Counts for craters larger th•n than4 km in diameteron Lunae Planum suggestthat the ridges 2 km in diameter(N(2)), however,are erratic. They are generally areabout the sameage as the plainsmaterial surrounding the lowerthan expectedfrom a -2 power-lawprojection from N(5) ridges[Tanaka, 1982], but the ridgeshave a higherdensity of counts and are latitude dependent.North of lat 55ø N, craters2 to 4 km in diameterthan the plains.Burial of interridge N(2) -- 150-250and south of lat 55ø N, N(2) -- 250-375(Table areasis evidenced in easternSinai Planurn,where high-resolution 3).This trend suggests that the craters farther north are composed imagesshow lava flows originatingfrom the Syria Planurn of easilyerodible material or weresubjected to greatererosive region. processes,or both.Although the VastitasBorealis Formation Intenseerosion of cratered terrain and intercrater plains in is confinedto the northernplains, knobby material resembling thenorthern lowlands and alongthe highland-lowlandboundary theknobby member of theformation was produced in thecenter scarppreceded emplacement of ridged plains material. of HellasPlanitia at nearlythe sametime [N(5) = about60]. Deuteronilusand Protonflus Mensae (large polygonal mesas Althoughformation of the intercraterplains material had formedby developmentof fretted channelsin highlandsalong nearlyceased, activity at manylarge volcanic regions on Mars thehighland4owland scarp) were formed after depositionof commencedduring this epoch,according to cratercounts of UpperNoachian intercrater plains material (some of whichhas the vent areas and lava flows that emanate from these vents. wrinkleridges), but they are embayedby ridgedplains. This Theseflows have lobate scarps and leveedchannels and closely stratigraphicrelation is also found in other areas along the resemblelava flows on Earth and the moon [Schaberet al., highland-lowlandscarp and around highlandremnants in 1978].Volcanoes and lava flows were emplaced at thesouthwest Utopia,Elysium, and westernAmazonis Planitiae. The fretted and northeastends of the Tharsisswell; in plainswest of Alba channelswithin the mensac appear to be structurallycontrolled Patera;and at SyriaP!anum, Tempe Terra, Elysium Planitia, [Sharpand Malin, 1975]and relatedto ancientimpact basins CerauniusFossae, and Dorsa Argentea.Possible cinder cones inplaces [Schultz et al., 1982].The rim of Hellasimpact basin in thenorthern plains [Frey et al., 1979]are probably also Upper generallyhas high relief but is absentwhere coveredby ridged Hesperian.The complex, overlapping, lobate lava flows common plainsmaterial along the northeast and south edges. Burial alone to manyof theseareas overlie ridged plains material. Most of cannotexplain this absence,because the ridged plains surface these surfacesretain craterslarger than 1 km in diameter isfound well below adjacent basin-rim material. In manyother [Neukurnand Wise,1976; Scott and Tanaka,1981b] except localities,embayments of highly degraded remnants of highland for the Hesperianflows north of Alba Patera,whose crater terrainbyridged plaips material show that intense degradation counts (Table 3) indicateobliteration of small(<5 kmdiameter) precededthe formation of ridgedplains. craters. Severallarge volcanic loci that eitherwere coeval with or Fracturingwas intense at Albaand Tantalus Fossae, but weak immediatelypostdated ridged plains material produced extensive or absentelsewhere. The faultsproduced are dominantlyradial complexeslate in the Early HesperianEpoch. These centers to Pavonis Mons and have crater densitiesof N(5)- 115 to includeapparent calderas onSyrtis Major Planurn that produced 70 [Plesciaand Saunders, 1982]. They occur primarily on the E154 TANAKA:THœ STRATIGRAPHY OF MARS lowerflanks of theTharsis swell at Labeatis,Tempe, Mareoffs, reportedN(5) = 100and N(2) = 420.In northeasternArcadia and eastern Memnonia Fossae and on southern Daedalia Planitia,N(2) is about 150 to 200.Variations in thecrater counts Planurn.Rifting and collapse continued at NocffsLabyrinthus mayin partbe attributableto inclusionof otherunits in the andValles Marineris, cutting into Syria Planurn flows and ridged areascounted and to obliterationof smallercraters. The unit plainsmaterial [Scott and Tanaka,1986]. Arcuate faults partly locallyconsists of lavaflows [Scott and Tanaka,1986] but is buriedby Tharsisflows at FortunaFossae may form part of mantiedin mostareas. Some of theunit, particularly in Acidalia a largevolcanotectonic feature of aboutLate Hesperian age. Planitiaand north of TempeFossae, may be composed ofeolian Deep erosionin the southpolar highlandsformed the cavi or alluvialsediments [ Witbeck and Underwood,1984]. terrain(plateaus marked by deep,irregular pits), cutting into Stratigraphicrelations and crater counts[Plescia and extensivelobate and sheetlikedeposits of probablelava-flow Saunders,1979; Scott and Tanaka,1981b] indicate that lava offgin(Dorsa Argentea flows) of Late Hesperianage and older flowswere emplaced at thistime, mostlyfrom the largeshield [Tanakaand Scott,1987; Tables 3, 5]. volcanoesand fissures around Tharsis Montes, Alba Patera, Crater countsmade by Masurskyet al. [1977]and Neukum and ElysiumMons. A few of the smallerTharsis volcanoes and Hiller [1981] of the outflow-channelfloors indicate a wide werealso active. Some of thetharsis flows entered northwestern rangeof ages.However, Carr and Clow[1981] noted that only KaseiValles [Scott and Tanaka,1986]. Fault activitywas crater countsof deeplyscoured channel surfaces may be valid; localized around Tharsis volcanoesin southern Daedalia smooth or lightly erodedsurfaces may yield crater agesthat Planurn, eastern Memnonia Fossae, and south of Uranius reflect resurfacingperiods rather than channelingepisodes. Patera;around Alba Pateraat Alba andTantalus Fossae; and Countsfor thedeeply scoured surfaces and gradational contacts on the flanks of Elysium Mons, forming Elysium Fossae of the flood-plainmaterial with the VastitasBorealis Formation [Mouginis-Mark et al., 1984]. in Chryse Planitia suggestthat outflow-channelactivity was The OlympusMons aureoleswere emplaced about this time. mostly restrictedto the Late HesperianEpoch. The channels Oneof the aureolesoverlies Lower Hesperian terrain cut by originatedfrom chasmataand chaotichighland terrain. If the UlyssesFossae, and all of the aureolesunderlie Upper chaotic terrain was formed by collapsedue to withdrawal of Amazonian lava flows of Olympus Mons. The lowermost subsurfacevolatiles, as is commonlybelieved, it is the same aureoleswest of Olympus Mons are embayedby Middle age as the channels.Juventae and Ganges Chasmata contain Amazonianlava flows and plains material. Crater counts by layered depositsthat now appearas streamlined,moderate-size Hiller et al. [1982] indicatean Early Amazonianage for the (tens of kilometersacross), rounded hills risingabove the valley aureoles;however, the cratercounts may be unreliablebecame floors, showingthat, during Late Hesperiantime, deposition of thedegraded nature of theaureoles' surfaces [Morris, 1982]. of layered material followed channel formation. In channels Widely varying explanationshave been postulatedfor the east of Vailes Marineris and in Kasei Vailes,multiple terrace aureoles,but most workersascribe origins related to volcanism levelsin the channelsand chaoticterrain suggest multiple stages [McCauleyet al., 1972;Hodges and Moore, 1979; Morris, 1982] of channelformation. Flood-plain materialsfrom the channels or to gravityslides on the flanks of OlympusMons [Lopes fan out into Chryse Planitia and grade into the lowermost et al., 1980;Francis and Wadge,1983; Tanaka, 1985b]. In either member of the Arcadia Formation in western Acidalia Planitia case, the aureole material must somehowbe a productof [Scott and Tanaka, 1986]. The smooth plains here appear to volcanismin the Olympus Mons area. The relativeage of bury remnantsof sinuouschannels. The MangalaVailes outflow- formationof the volcano'sbasal scarp is looselydefined and channel system,which largely originatesfrom a fracture in the maybe aboutthe sameas that of the aureoles.Thus development highlands west of Daedalia Planurn, debouchesinto southern of OlympusMons beganduring this epochor earlier,and the Amazonis Planitia, cuts ridged plains material,and appears volcanohad probablyattained most of its presentsize by the contemporaneouswith Upper Hesperian-Lower Amazonian end of the epoch. Tharsis flows [Masursky et al., 1986; Scott and Tanaka, 1986]. Some of the floor materialsin Vailes Marineris are younger Thus the Late Hesperian Epoch marks the major period of than middle Hesperianlayered deposits and older than Middle outflow-channelactivity on Mars. to Upper Amazonian slide deposits.The floor materialsare Remnantsof smoothplains deposits are superposedon the moderatelycut by the faultsof smallgrabens and arerelatively ridged plains material of Malea Planurn and appear to occur uneroded.During the HesperianPeriod, major faulting,filling, in low areas(tinged by wrinkle ridges)or belowejecta blankets and erosion occurredwithin the Noctis Labyrinthus-Valles of large impact craters.The depositspostdate the ridgedplains Marinerissystem, indicating that geologicactivity was greatly and may in part be Late Hesperianin age, as indicatedby reducedduring the Lower Amazonian Epoch. Local side canyons the apparentdensity of superposedcraters. in westernVailes Marineris were formed by headwardsapping of plateaumaterials that includethe UpperHesperian Syria Early Amazonian Epoch PlanurnFormation. Many of theseside canyons appear to be controlledby buffed faults. At Mangala Vailes,crater counts The Lower Amazonianreferent is moderatelycratered smooth and stratigraphicrelations indicate that narrow channels plains material in southernAcidalia Planitia (Figures 1 and dissectedthe floors of earlier,broad outflowchannels during 2). This unit also occursin patchesin Amazonis,Arcadia, and the EarlyAmazonian [Masursky et al., 1986].Also during the southernElysium Planitiae and extendsas far north aslat 68ø N, EarlyAmazonian, large floods (producing Hrad, Tinjar, and north of Alba and Tantalus Fossae. A similar unit occurs in GranicusVailes), debris or pyroelasticflows [Christiansen and the lowlands north of Protonflus Mensac (R. Greeley and J. Greeley,1981; Tanaka and Scott, 1986], and lava flows emanated E. Guest, unpublisheddata, !986). Crater countsin southern from northwesternElysium Fossae into UtopiaPlanitia. Acidalia Planitia yield about N(5) = 55 and N(2) = 300 to 450. Eventhough the earlier cratering rate was now greatly reduced, For easternAcidalia Planitia, Witbeckand Underwood[1984] the peak-ring basin Lyot north of DeuteronilusMensac was TANAKA: T•/r. STRATIGRAPHYOF MARS E155 formedin therocks of theVastitas Borealis Formation. Lyot's in centralVailes Marineris [Blasius et aL, 1977].Minor faulting extensivesecondary craters occur on surrounding highland and also occurredin southernElysium Planitia, forming long, lowlandsurfaces and are buried by EarlyAmazonian smooth arcuate, west-northwest-trendingfractures (Cerberus Rupes) plainsmaterial and debris aprons [Ferguson andLucchitta, 1985; that appearto cut UpperAmazonian plains material in places. R.Greeley and J. E. Guest,unpublished data, 1986]. Therefore The channelmaterial that is thereferent for thisseries appears Lyotimpact basin is aboutEarly Amazonian in age--theto haveissued from these fractures [ Tanaka and Scott,1986]. youngestimpact basin on Mars. The layered depositsin the north polar region, forming PlanumBoreum, are uncrateredat availableimage resolutions MiddleAmazonian Epoch [Dial, 1984].On the southpolar layered deposits making up The referentfor the Middle Amazonian Epoch consistsof PlanurnAustrale, one fresh-appearing20-kin-diameter crater lavaflows in Amazonisand southernArcadia Planitiae (Figures is found;other craters are subdued because they either underli• 1 and3), whichwere recentlymapped as members3 and 4 the layereddeposits or are superposedon layereddeposits that ofthe Arcadia Formation [Scott and Tanaka,1986]. The flows were subsequentlybuffed. Interpretationsof the origin of the generallyform smooth plains and have subdued scarps that layereddeposits and of the arcuatetroughs that transectthem appearlightly mantled [Scott and Tanaka, 1986]. Similar plains areimportant in determiningthe ageof the deposits.Although occurin southernElysium Planitia. Crater countsfor eastern the paucityof cratersindicates that the deposits'surfaces are AmazonisPlanitia by Hiller et al. [1982] yielded N(2)= 70; lessthan a fewmillion years old, the material is possibly reworked Tharsisflows of this epoch have N(2) = 90 to 120 [Scott and periodicallyand may have accumulatedover a much longer Tan&a,1981 b; Plesciaand Saunders,1982]. During the Middle time [Carr, 1982]. AmazonianEpoch, volcanism was largely restrictedto the At both poles,the layereddeposits are superposed by residual TharsisMontes, Olympus Mons, the northern plains, and ice caps.Extents, compositions, and depositionalrates of the perhapsto the highland-lowland boundary between the Tharsis ice caps probablychange gradually because of oscillationsof andElysium regions. the polar heatbudget and dust-storm activity. These oscillations A sequenceof thick, mostly light-coloredmaterial was are governedby Mars' rotationalobliquity (50,000-year period) depositedalong the highland-lowlandboundary scarp in and orbitaleccentricity (2,000,000-year period). Due to seasonal southernAmazonis Planitia between Ulysses Fossae and dust-stormactivity, perennialwater ice is presentlyretained at ApollinarisPatera and in southernElysium Planitia. Scott and the north pole [Kieffer et al., 1976]. Water ice may also be Tanaka[1982] subdivided these deposits into sevenunits having present at the south pole, but it cannot be detectedbecause cratercounts ranging from N(1) -- 65 to 730, which indicate the CO2 seasonalcap doesnot completelydissipate. Also, dust a Middleto Late Amazonianage span.They were interpreted depositionis greaterin the north polar region [Pollack et al., tobe either palcopolar deposits [Schultz and Lutz-Garihan, 1981] 1979]. A dust mantle as much as 200 m thick [Squyres,1979] or interbeddedignimbrite and lava-flow deposits[Scott and apparentlyexists on the north circumpolarplains but is absent Tanaka,1982]. Scott and Tanaka [1986] remappedthem into or very thin in the southpolar region. three members of the Medusae Fossae Formation of which the Extensivedunes surround the north pole [Tsoar et al., 1979; lower and middle members were considered to have been Dial, 1984]but are sparsein equatorialancl. south polar regions, eraplacedduring the Middle AmazonianEpoch. where they generallyoccur within impact cratersand valleys Craterdensities indicate that some of the large landslides and along the basesof scarps.Although roost of the dunes withinVailes Marineris [Lucchitta, 1979] were crop!aced during may be active, a large, relativelystable set of dunesadjacent thisepoch and in the Late AmazOnian.A few normal faults to north polar layereddeposits in Olympia Planitia(Figure 1) orientedparallel to the valleywalls cut the landslides[Blasius mayb• at leaseseveral million years old [Breedet al., 1979], etal., 1977] and are obviouslyrecent. andthus may haveendured through the periodic climatic changes Debrisaprons in ProtonilusMensac and debris flows in fretted of Mars causedby orbital and rotational dynamics.The sand channelshave crater densities within the rangeof N(1) = 200 (or sand-sizeaggregates [Greeley et al., 1982]) probably was derived from sources such as the Vastitas Borealis Formation to 500 [Masurskyet al., 1977; Squyres,1978], which are consistentwith the age of this series.However, the available [Tsoar et al., 1979], the polar layereddeposits [Breed et al., craterstatistics are highly uncertainand somewhatolder or 1979],or fluvial depositsformed by channelingin the northern youngerages are possible.Some of the debris apronsmay still lowlands[McCauley et al., 1980]. beactive [Squyres, 1978]. Landslidematerials along the basalscarp of OlympusMons are superposedon UpperAmazonian lava flows.Many of the LateAmazonian Epoch landslidesin Vailes Marinefts lack craters and therefore may alsobe UpperAmazonian. Concentrically ridged debris blankets Thereferent is a flood-plaindeposit in southernElysium on the western flanks of the Tharsis Montes are about the same Planitiathat apparently originated from Cerberus Rupes; it ageas the volcanoes'youngest flows [Scott and Tanaka,1981b]. extendsinto a large,smooth-floored outflow channel in western The modeof originof the blanketsis unknown.Lucchitta [ 1981] AmazonisPlanitia [Tanaka and Scott, 1986]. The unit is marked proposedthat theymay be recessionalmoraines of formerice bywidely scattered craters generally <1 kmin diameter. caps.Also, local debris aprons and flows in thefretted channels Smalllava-flow units on the flanks of the Tharsis Montes and mensac,in Vailes Marineris, and along other high-latitude andOlympus Mons and smooth plains material in southernscarps are sparselycratered and were probablyactive during ArcadiaPlanitia have crater densities of N(1) <160 (Tables 3 the Late Amazonian. and5). Other uncratered, possibly volcanic, deposits occur along faultsand overlie landslides inValles Marinefts [Lucchitta, 1985]. SUMMARY AND CONCLUSIONS Minorstructures associated with Tharsis volcanoes that formed duringthis epoch include caldera and circumferential flank faults A detailed analysisof the global stratigraphyof Mars, onthe Tharsis Montes and Olympus Mons and recent faults supportedby new and revisedgeologic maps based on the E156 TANAKA: THE STRATIGRAPHYOF MARS recentlycompleted Viking l:2,000,000~scalephotomosaic series constructsoccurred mostly in areaswhere the lithospherewas of the planet,has beenpresented. Published crater counts and thin[Comer et al.,1985]. Gravimetry ofMars [Sjogren, 1979] stratigraphicanalyses were used in part, but new work was indicatesthat gravity anomalies occur over younger mass features centeredon geologicunits and featureswhose relative ages were suchas TharsisMontes, Olympus Mons, Alba Patera,and not well known. The geologic history of Mars has been ElysiumMons. Such features were possibly supported by an summarizedon geologicmaps showing eight time-stratigraphic increasedflexural rigidity (or thickness)of the lithosphere divisions that include all surface materials. [Comeret al., 1985]. The three major time-stratigraphicperiods of Marsrathe TheMartian climatic and volatile histories are oiher important Noachian, Hesperian,and Amazonian Systems--developedby factorsin the planet'sgeologic evolution. Fluvial activity was Scott and Carr [1978] from Mariner 9 mapping have been widespreadduring Noachian time, perhaps aided by a thicker, subdividedinto eight epochs,based on the eightcorresponding warmeratmosphere and a warmersurface that allowedsurficial time-stratigraphicseries of deposits.Their inferred eventsare runningwater and perhapseven rain. In Hesperiantime, summarized as follows: however,runoff channels ceased to form.Instead, voluminous, (1) Early NoachianEpoch--Major impact-basinformation water-richaquifers in the highlandregolith were tapped, (Isidis, Hellas, Argyre, and south polar); formation of the producingthe huge outflow channelsthat drainedinto the northern lowlands. northern lowlands. Also during the Hesperian, and in the (2) Middle Noachian Epoch--Cratered terrain formation; Amazonian, ground ice was apparently responsiblefor lognormalcratering distribution; faulting in southTharsis. producing landslides, debris flows, channels, and other (3) Late Noachian Epoch--Intercrater plains resurfacing; landforms.At present,subfreezing temperatures envelop the reduced cratering flux, transition to power-law cratering entiresurface of the planet,and resurfacingis dominatedby distribution; intense erosion and runoff-channeldevelopment; eolian processes.These processes are observedprincipally in beginning of intense faulting centered at Syria Planum; the polar regions,where dust-stormand seasonalfrost activities formation of highlandvolcanoes and ridges. are strongest. (4) Early HesperianEpoch--Ridged plains emplacement, The absoluteages of the Martian epochboundaries are model particularlyat Lunae,Syrtis Major, Hesperia,and Malea Plana, dependentand are largelyuncertain. According to two models and in northern lowlands;burial and degradationof lowland discussedin this paper, most geologic activity (withinthe cratered terrain; extensivefaulting centeredat Syria Planurn; Noachianand HesperianPeriods) occurred either duringthe major rifting at Noctis Labyrinthus-VallesMarineris; Alba first one billion years of surfacehistory [Neukum and IVac, Patera andTempe Terra volcanism; fretted channel development; 1976; Neukum and Hiller, 1981] or during the first two billion formation of Lowell impactbasin. years[Hartmann et al., 1981, Figure 8.6.3; this paper].Other (5) Late HesperianEpoch--Resuffacing of northernplains absolute-agemodels [e.g., Soderblomet al., 1974] generally (by complex of lava flows, eolian deposits,and alluvial suggestcrater-flux historiesintermediate between the Neukum sediments),then erosionof theseplains; major volcanismat and Wise and the Hartmann et al. models. A similar evolution Tharsis Montes, Alba Patera, Terra Tempe, Syria Planurn, of crater size-frequency distributions demonstratesthe Elysium Planitia, Ceraunius Fossae, and Dorsa Argentea; plausibilitythat impactson the moon and Mars originatedfrom outflow-channeldevelopment by water flooding;deposition of the same population of planetesimals.As in the caseof the Vailes Marineris layered deposits;waning Tharsistectonism; moon, however,we will firmly resolvethe absolutechronology depositionof southpolar unconsolidated material. for the Martian surfaceonly after future missionsreturn rock (6) Early Amazonian EpochmLavaflows form northern samplesfrom selectedareas for radioisotopic-ageanalysis. smooth plains;local volcanicflows at TharsisMontes, Alba Acknowledgments. I thank D. H. Scott,R. Greeley,and J. E. Guest Patera, and Elysium Mons; formation of Olympus Mons for permissionto useunpublished stratigraphic data and geologicmaps aureoles;formation of Lyot impactbasin. on which parts of this paper, particularlythe stratigraphicmaps, are (7) MiddleAmazonian Epoch--Continued accumulation of based.M. H. Can:, L. A. Rossbather,D. H. Scott, D. E. Wilhelms, lava flowsin northernplains, particularly in AmazonisPlanitia; and J. R. Zimbelman reviewedthe manuscriptand providedmany volcanismat TharsisMontes and OlympusMons; emplacement helpfulsuggestions. M. F. Tuesinkperformed some of the cratercounts presented,and N. K. Robertsonand A. Wassermandigitized the of thick, poorly consolidatedmaterial of Medusae Fossae stratigraphicmaps. The research in thispaper was supported by NASA Formation;development of debrisflows and aprons surrounding Work Order W-15,814. Deuteronilus and Protonilus Mensac; landslides in Valles REFERENCES Marineris. (8) Late AmazonianEpoch--Formation of broad flood Blasius,K. 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