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 overlaprelations 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 this basementmaterial 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 following section. 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