Ages and stratigraphy of lunar mare basalts in Mare Frigoris

and other nearside maria based on crater size-frequency

distribution measurements

H. Hiesinger,1,2 J. W. Head III,2 U. Wolf,3 R. Jaumann,3 and G. Neukum4

Received 15 March 2009; revised 11 August 2009; accepted 5 October 2009; published 20 March 2010.

[1] We report on ages derived from impact crater counts for exposed mare basalt unitsin the northern part of the lunar nearside hemisphere (Mare Frigoris), the eastern andnortheastern part of the nearside hemisphere (Lacus Temporis, Joliot, Hubble, Goddard,Mare Marginis, and Mare Smythii), the central part of the nearside hemisphere (PalusPutredinis, Mare Vaporum, and Sinus Medii), and the southwestern part of the nearsidehemisphere (Grimaldi, Cruger, Rocca A, Lacus Aestatis, and Schickard). In Mare Frigoris,we dated 37 basalt units, showing ages from 2.61 to 3.77 Gyr, with most units beingformed in the late Imbrian period between 3.4 and 3.8 Gyr ago. In Mare Vaporum wedated six spectrally homogeneous units that show model ages of 3.10 to 3.61 Gyr. Ourmodel ages of basalts in Mare Marginis range from 3.38 to 3.88 Gyr and are mostly olderthan basalts in Mare Smythii (3.14–3.48 Gyr). The model ages of four units in SinusMedii indicate that the basalts in this region formed 3.63 to 3.79 Gyr ago. We find anexcellent agreement of our crater size-frequency model ages of the Palus Putredinis area,which contains the Apollo 15 landing site, with the radiometric ages of Apollo 15 samples.According to our crater counts, basalts in Palus Putredinis are 3.34 Gyr old and thiscompares favorably with the radiometric ages of 3.30–3.35 Gyr of the olivine-normativeand quartz-normative basalts of the Apollo 15 landing site. Lacus Aestatis is a smallirregular-shaped mare patch in the southwestern nearside and shows an Imbrian age of3.50 Gyr; basalts in Lacus Temporis in the northeastern nearside formed between 3.62 and3.74 Gyr ago and are, therefore, older than the basalts in Lacus Aestatis. We foundthat basalts in craters of the southwestern nearside (Schickard, Grimaldi, Cruger, andRocca A) are also mostly younger than basalts in craters of the northeastern nearside(Hubble, Joliot, and Goddard). While basalt ages vary between 3.16 and 3.75 Gyr in thesouthwest, basalts in the northeast are 3.60–3.79 Gyr old. These results confirm andextend the general distribution of ages of mare basalt volcanism and further underline thepredominance of older mare basalt ages in the eastern and southern nearside and in patchesof mare peripheral to the larger maria, in contrast to the younger basalt ages on thewestern nearside (Oceanus Procellarum).

Citation: Hiesinger, H., J. W. Head III, U. Wolf, R. Jaumann, and G. Neukum (2010), Ages and stratigraphy of lunar mare basalts in

Mare Frigoris and other nearside maria based on crater size-frequency distribution measurements, J. Geophys. Res., 115, E03003,

doi:10.1029/2009JE003380.

1. Introduction

[2] Compared to Earth we only have a small number ofsamples of the Moon that can help us to decipher itsgeologic history and evolution. For example, accurate

radiometric ages for lunar mare basalts, which cover about17% of the lunar surface [Head, 1976; Head and Wilson,1992] are available only for the spatially very limited areasaround the Apollo and Luna landing sites [e.g., BasalticVolcanism Study Project (BVSP), 1981; Stoffler and Ryder,2001, and references therein]. Because most lunar marebasalts are still unsampled [e.g., Pieters, 1978;Giguere et al.,2000], absolute radiometric age data for the majority ofbasalts are still lacking. Fortunately, by making use of remotesensing techniques, we can derive relative and absolutemodel ages for these unsampled regions. For example,inspection and interpretation of superposition of geologicunits onto each other, embayment and crosscutting relation-ships within high-resolution Apollo and Lunar Orbiter

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1Institut fur Planetologie, Westfalische Wilhelms-Universitat, Munster,Germany.

2Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

3DLR Institute of Planetary Exploration, Berlin, Germany.4Institut fur Geologie, Geophysik und Geoinformatik, Freie Universitat

Berlin, Berlin, Germany.

Copyright 2010 by the American Geophysical Union.0148-0227/10/2009JE003380

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images were used to obtain relative ages for lunar surfaceunits [e.g., Shoemaker and Hackman, 1962]. In addition ithas been shown that crater degradation stages and cratersize-frequency distribution measurements, calibrated to thelanding sites, are useful to derive relative and absolutemodel ages [e.g., Hartmann, 1966; Greeley and Gault,1970; Neukum et al., 1975; Neukum and Horn, 1976;Boyce, 1976; Boyce and Johnson, 1978; Wilhelms, 1987;Neukum and Ivanov, 1994; Hiesinger et al., 2000, 2001,2002, 2003; Morota et al., 2008; Haruyama et al., 2009].[3] The internal thermal history and evolution of a

planetary body is reflected in the timing and extent ofvolcanism on its surface [Head and Wilson, 1992]. Thus,investigations of ages and compositions of volcanic productson the surface provide clues to the geologic and thermalevolution of a planet. Despite the enormous scientific valueof the returned samples from six Apollo and three Lunalanding sites, these data are insufficient to completelyexplain the thermal evolution of the Moon. For example,the onset and extent of mare volcanism are not very wellunderstood (summarized by Nyquist et al. [2001]). Thereturned samples revealed that mare volcanism was activeat least between �3.9 and 3.1 Ga [Head, 1976; Nyquist andShih, 1992]. Ages of some basaltic clasts in older brecciaspoint to an onset of mare volcanism prior to 3.9 Gyr [Ryderand Spudis, 1980], perhaps as early as 4.2–4.3 Gyr in theApollo 14 region [Taylor et al., 1983; Dasch et al., 1987;Nyquist et al., 2001]. Early volcanism is also supported byremote sensing data. For example, Schultz and Spudis[1979], Hawke and Bell [1981], Bell and Hawke [1984],Antonenko et al. [1995], and Antonenko and Yingst [2002]interpreted dark halo craters as impacts into basaltic deposits(cryptomare) that are now buried underneath a veneer ofbasin or crater ejecta. These underlying basalts might beamong the oldest basalts on the Moon, implying that volca-nism was active prior to �3.9 Gyr ago. Support for such anearly onset of lunar mare volcanism comes from U-Pb datingof phosphate grains associated with basaltic clasts in the lunarmeteorite Kalahari 009 [Terada et al., 2007]. On the basis ofthese ages, Terada et al. [2007] proposed that volcanism onthe Moon started as early as 4.35 Gyr ago, relatively soonafter its formation and differentiation. They argued thatKalahari 009might represent a very lowTi cryptomare basalt.[4] Remote sensing data also suggest that the returned

samples represent only a small number of basalt types froma few limited locations and that the majority has still notbeen sampled [Pieters, 1978]. On the basis of craterdegradation stages, Boyce [1976] and Boyce and Johnson[1978] derived absolute model ages that indicate volcanismmight have lasted from 3.85 ± 0.05 Gyr until 2.5 ± 0.5 Gyrago. Support for such young basalt ages comes from arecently collected lunar meteorite, Northwest Africa 032,which shows an Ar-Ar whole rock age of �2.8 Gyr [Faganet al., 2002]. Schultz and Spudis [1983] made crater size-frequency distribution measurements for basalts embayingthe presumably Copernican crater Lichtenberg and concludedthat these basalts might be less than 1 Gyr old. However, onthe basis of crater counts on Lunar Orbiter IV images,Hiesinger et al. [2003] derived older ages for the Lichtenbergbasalts of 1.68 Gyr and even older ages of 2.20 Gyr resultedfrom crater counts on SELENE Terrain Camera images[Morota et al., 2008].

[5] On the basis of our earlier studies [Hiesinger et al.,2000, 2003], we find that that the period of active volcanismon the lunar nearside and in some farside regions lastedabout 2.8 Gyr, from �4.0 to �1.2 Gyr. Most of theinvestigated basalts on the lunar nearside erupted duringthe late Imbrian period, fewer basalts erupted during theEratosthenian period, and even fewer basalts are of Coper-nican age [Hiesinger et al., 2000, 2003]. This is consistentwith crater size-frequency ages of Haruyama et al. [2009]for mare basalts on the lunar farside. Haruyama et al.[2009] concluded, that the majority of mare volcanism onthe lunar farside ceased at �3.0 Gyr ago. However, theyalso identified mare deposits in several locations on thelunar farside (i.e., Antoniadi, Apollo N, Apollo S, Nishina,and Mare Moscoviense) that show much younger ages,clustering at �2.5 Gyr ago. Haruyama et al. [2009] arguedthat these young ages indicate that mare volcanism on thelunar farside lasted longer than was previously considered[Stuart-Alexander, 1978; Wilhelms et al., 1979; Wilhelms,1987] and may have occurred episodically.[6] In previous papers we reported on ages of several

lunar nearside mare basalts [Hiesinger et al., 2000, 2001,2002, 2003] and here we present model ages of lunar marebasalts in Mare Frigoris and other nearside maria (Figure 1)that are based on remote sensing techniques, that is, cratersize-frequency distribution measurements or crater counts.Compared to previous studies [e.g., Neukum et al., 1975;Greeley and Gault, 1970; Hartmann, 1966], we performedcrater size-frequency distribution measurements for basaltunits that are, to a first order, spectrally homogeneous. Amajor goal of this study is to provide absolute model agesfor these basalts in order to investigate their stratigraphy andto better understand the nature and evolution of lunar marebasalt volcanism.[7] On the basis of our new age data we address the

following questions: (1) What was the time period of activevolcanism in the investigated area; that is, when did volca-nism start and when did it end? (2) Was lunar volcanismcontinuously active or are there distinctive periods of volca-nic activity? Finally, (3) is there a trend in the spatialdistribution of basalt ages on the lunar surface?[8] We present results on the spatial and temporal distri-

bution of basalt ages and discuss our findings in the contextof previously published geologic and spectral maps as wellas age data [e.g., Wilhelms and McCauley, 1971; Boyce,1976; Boyce and Johnson, 1978; Pieters, 1978; Whitford-Stark and Head, 1980; Wilhelms, 1987].

2. Technique, Approach, and the Definitionof Units

[9] Crater size-frequency distribution measurements are apowerful remote sensing technique to derive relative andabsolute model ages for unsampled planetary surfaces. Asthis technique is described elsewhere [e.g., Crater AnalysisTechniques Working Group, 1979; Neukum and Ivanov,1994; Hiesinger et al., 2000; Stoffler and Ryder, 2001;Neukum et al., 2001; Ivanov, 2001; Hartmann and Neukum,2001; Stoffler et al., 2006, and references therein], we willonly briefly outline how crater size-frequency distributionmeasurements can be used to date surfaces. The techniqueof crater size-frequency distribution measurements on spec-

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trally homogeneous regions, including a discussion ofmodel assumptions, strengths and shortcomings, and anerror analysis, has been described in detail by Hiesinger etal. [2000]. In short, in order to obtain the age of aphotogeological unit one has to (1) measure the surfacearea of the unit and (2) measure the diameters of eachprimary impact crater within this unit.[10] It has been shown that lunar crater distributions

measured on geologic units of different ages and in over-lapping crater diameter ranges can be aligned along acomplex continuous curve, the lunar production function[e.g., Neukum, 1983; Neukum and Ivanov, 1994; Neukum etal., 2001]. The lunar production function is given by

logðNcumÞ ¼ a0 þX11k¼1

akðlogðDÞÞk ð1Þ

where a0 represents the amount of time during which theunit has been exposed to the meteorite bombardment[Neukum, 1983; Neukum and Ivanov, 1994; Neukum etal., 2001]. Compared to the production function of Neukum[1983] that we used for our previous age determinations

[Hiesinger et al., 2000, 2001, 2002, 2003], Neukum et al.[2001] slightly reworked their production function for thelarger craters. This resulted in a new set of coefficients forequation (1), which are given in Table 1. For this paper wemade use of the coefficients of Neukum et al. [2001]. Thecumulative crater density of a geologic unit taken at a fixedreference diameter (usually 1 or 10 km) is directly related tothe time the unit has been exposed to the meteorite flux andtherefore gives a relative age of this unit. To obtain absolutemodel ages from crater size-frequency distribution measure-ments one has to link the radiometric ages from the returnedApollo and Luna samples with crater counts of these landingsites in order to establish the lunar cratering chronology. Thisis not a trivial task and has led to several more or less differentchronologies [e.g., BVSP, 1981; Neukum, 1983; Neukum andIvanov, 1994; Stoffler and Ryder, 2001; Stoffler et al., 2006,and references therein]. The empirically derived chronologyof Neukum and Ivanov [1994] and Neukum et al. [2001],which we use for this study is given by

NcumðD� 1 kmÞ ¼ 5:44�10�14½expð6:93 � tÞ�1�þ8:38�10�4t

ð2Þ

Figure 1. Map of the lunar surface showing the location of the investigated basins, the Apollo and Lunalanding sites, and the location of selected features mentioned in the text. MV, Mare Vaporum; SM, SinusMedii; PP, Palus Putredinis. Latitude, longitude grid is 30� � 30� wide; simple cylindrical projection.

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Once the lunar chronology is established, we can deriveabsolute model ages for the entire lunar surface from cratersize-frequency distribution measurements by solving (2) fortime t for Ncum(D � 1 km) measured on the geologic unit tobe dated.[11] The level of uncertainty of the crater retention age of a

given count is given by the following equation:

�sN ¼ logNð1Þ �

ffiffiffiffiffiffiffiffiffiffiNð1Þ

pA

" #ð3Þ

in which N(1) is the crater retention age calculated for cratersof 1 km diameter and A is the size of the counted area (CraterAnalysis Techniques Working Group, 1979). The ±sN valuegives the upper and lower limits of the error bar of the craterretention age, which are used for estimating the uncertainty ofthe absolute crater model age from the cratering chronology.We principally assume that the cratering chronology is free oferrors. Therefore, errors in our absolute model ages are onlycaused by errors in the determination of crater frequencies[Neukum, 1983]. Neukum et al. [1975] estimated thesystematic uncertainty of the standard distribution curve orthe measurement to be <10% for 0.8 km � D � 3 km (this isthe diameter range of most of our crater counts) and up to25% for 0.8 km � D � 10 km.[12] While crater size-frequency ages are based on count-

ing primary impact craters, it is important to understand theinfluence of secondary craters on the crater counts. McEwenet al. [2005] and Preblich et al. [2007] studied the 10 km indiameter Martian crater Zunil and found that this craterproduced �107 secondary craters in the size range of 10–200 m that extend up to 1600 km away from crater Zunil.McEwen et al. [2005] concluded that the production func-tions of Hartmann and Neukum [2001] overpredict primarycraters smaller than a few hundred meters in diameter by afactor of 2000, similar to the conclusions of McEwen andBierhaus [2006]. However, Malin et al. [2006] reported ontwenty Martian impacts 2–150 m in diameter, createdbetween May 1999 and March 2006. They concluded thatthe values predicted by models that scale the lunar crateringrate to Mars are close to the observed rate [Malin et al.,2006]. Therefore, Hartmann [2007a] argued that the newobservations, if correct, are inconsistent with suggestions byMcEwen [2003], McEwen et al. [2005], Bierhaus et al.

[2005], and McEwen and Bierhaus [2006] that small cratersmust be dominated by secondary craters by factors of 102–103. While the effect of secondary impact cratering on theproduction function of small craters is a matter of ongoingdebate [e.g., Hartmann, 2007a, 2007b], it is of lesserimportance for our crater counts because the spatial resolu-tion of the Lunar Orbiter IV images of 60–100 m [Wilhelms,1987] only allowed us to count craters larger than �300–400 m and these smallest craters are not used for thedetermination of our ages.[13] For the discussion of absolute model ages of basalt

units and the application of terms like ‘‘Eratosthenian’’ or‘‘Imbrian,’’ one must be aware that various authors definedthese chronostratigraphic systems in different ways [e.g.,Wilhelms, 1987; Neukum and Ivanov, 1994; Stoffler andRyder, 2001; Stoffler et al., 2006]. A detailed discussion ofthis issue is given elsewhere [e.g., Hiesinger et al., 2000].Figure 2 is a comparison of stratigraphies based on work byWilhelms [1987], Neukum and Ivanov [1994], and Stofflerand Ryder [2001]. While there is general agreement on thedefinition of the base of the Eratosthenian system (i.e.,3.2 Gyr), the stratigraphies vary substantially in their defini-tion of the base of the Copernican system (i.e., 0.8–1.5 Gyr).[14] In this paper we adopt the system of Neukum and

Ivanov [1994], with Nectaris being 4.1 ± 0.1 Gyr (N(D =1 km) = (1.2 ± 0.4) � 10�1), and the Imbrium basin being3.91 ± 0.1 Gyr old (N(D = 1 km) = (3.5 ± 0.5) � 10�2).According to Neukum [1983], the Eratosthenian Systemstarted 3.2 Gyr ago (N(D = 1 km) = 3.0 � 10�3) and theCopernican System began 1.5 Gyr ago (N(D = 1 km) = (1.3 ±0.3)� 10�3), while radiometric dating of samples, which arethought to represent Copernicus ejecta, indicates an age of0.85 ± 0.1 Gyr. [Silver, 1971] (Figure 2). Application of thischronostratigraphic system yielded 77 Imbrian ages, eightEratosthenian ages, and one Nectarian age for the investigatedunits, including resurfacing ages.[15] A crucial prerequisite for reliable age determinations

with crater size-frequency distribution measurements is themapping of homogeneous count areas. The Mare Frigorisregion of the Moon and other studied regions (i.e., MareSmythii, Mare Marginis, Mare Vaporum, Sinus Medii, PalusPutredinis, Lacus Aestatis, Lacus Temporis, Schickard,Grimaldi, Cruger, Rocca A, Hubble, Joliot, and Goddard;Figure 1) were previously geologically mapped by severalauthors [e.g.,Hackman, 1966;Wilhelms, 1968, 1987;Howardand Masursky, 1968; Ulrich, 1969; Wilhelms and McCauley,1971; M’Gonigle and Schleicher, 1972; Lucchitta, 1972,1978; McCauley, 1973; Wilshire, 1973; Karlstrom, 1974;Wilhelms and El-Baz, 1977; Scott et al., 1977]. However,because the unit definition was based mainly on brightnessdifferences, morphology and qualitative crater densities ontelescopic and Lunar Orbiter images, these maps are notdetailed enough to ensure homogeneity of the investigatedbasalts. It is known that regional spectral differencesmapped using spectral ratios approximate relatively homo-geneous surface mare units [e.g., Whitaker, 1972; McCordet al., 1976; Johnson et al., 1977a, 1977b; Pieters, 1978;Head et al., 1993]. Thus, we used a multispectral high-resolution Clementine color ratio composite (e.g., 750/415as red, 750/950 as green, 415/750 as blue) in order to mapthe distribution of distinctive basalt units (Figure 3a). Thepurpose of mapping the basalts in the investigated areas was

Table 1. Coefficients in Equation (1)a

Coefficient ‘‘Old’’ N (D)b ‘‘New’’ N (D)c

a0 �3.0768 �3.0876a1 �3.6269 �3.557528a2 +0.4366 +0.781027a3 +0.7935 +1.021521a4 +0.0865 �0.156012a5 �0.2649 �0.444058a6 �0.0664 +0.019977a7 +0.0379 +0.086850a8 +0.0106 �0.005874a9 �0.0022 �0.006809a10 �5.18 � 10�4 +8.25 � 10�4

a11 +3.97 � 10�5 +5.54 � 10�5

aNeukum et al. [2001].bNeukum [1983].cIvanov et al. [2001] and Neukum et al. [2001].

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to define spectrally homogeneous units.We assume that theseunits were formed within a relatively short period of time andare to a first order similar in mineralogy. We further assumethat because of the spectral homogeneity, each of our spec-trally defined units represents a single eruptive phase.[16] Having defined such homogeneous units with Clem-

entine images, we transferred the unit boundaries to high-

resolution Lunar Orbiter IV images in order to measure thecrater size-frequency distribution. This was necessary be-cause Clementine images are not well suited for cratercounts due to their high sun angles [e.g., Hiesinger et al.,2003]. We performed our counts on slide positive filmmaterial (approximately letter size format) professionallytaken from the large-scale prints of the Lunar Orbiter (LO)

Figure 2. Comparison of stratigraphies of Wilhelms [1987], Neukum and Ivanov [1994], and Stofflerand Ryder [2001]. Dashed lines in the stratigraphies of Wilhelms [1987] and Neukum and Ivanov [1994]indicate radiometric ages, which these authors attribute to the formation of the crater Copernicus. In thework of Stoffler and Ryder [2001], two formation ages for the Imbrium basin have been proposed, i.e.,3.85 Gyr and 3.77 Gyr (dashed line).

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Figure 3. (a) Color ratio composite based on three spectral ratios of Clementine imaging data (750/415on red, 750/950 on green, 415/750 on blue). Black lines define spectral units. We have performed cratersize-frequency distribution measurements for most of these units. Map coverage is �90�W–120�E,�75�S–75�N; latitude, longitude grid is 15� � 15� wide. (b) Crater degradation ages of lunar nearsidesurface units [Boyce, 1976; Boyce and Johnson, 1978]. Ages in Gyr. Black lines outline spectrallyhomogeneous units (also see Figure 3a). Map coverage is �90�W–120�E, �75�S–75�N; latitude,longitude grid is 15� � 15� wide.

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images available at the Berlin Regional Planetary Imagefacility (RPIF). The counts were done on a Zeiss stereocomparator located at the Free University of Berlin. Com-pared to previous age determinations, our data fit spectraland lithological units and presumably represent a majorimprovement in accuracy. In contrast, data from Boyce[1976] and Boyce and Johnson [1978] do not fit such unitsand the outline of their ages may be controlled or at leastinfluenced by the applied filtering technique rather than theactual geologic diversity (Figure 3b).[17] In this paper we report on ages of 66 basalt units in

Mare Frigoris, Mare Smythii, Mare Marginis, MareVaporum, Sinus Medii, Palus Putredinis, Schickard, Grimaldi,Cruger, Rocca A, Goddard, Joliot, Hubble, and an area southof Endymion. In order to facilitate the discussion of ages wedid not assign type locality names to each unit. Instead, weuse a simple letter/number system. The letter indicates thebasin (F, Mare Frigoris; V, Mare Vaporum; Ma, MareMarginis; Sy, Mare Smythii; SM, Sinus Medii; LT, LacusTemporis; LA, Lacus Aestatis; PP, Palus Putredinis; CGr,Grimaldi; CCr, Cruger; CRo, Rocca A; CSc, Schickard; CHu,Hubble; CJo, Joliot; CGo, Goddard) and the numberdescribes the unit within a basin. The numbering is consistentwith the geologic maps of the Moon with oldest units havinglower numbers and younger units having higher numbers.

3. Results

3.1. Mare Frigoris

3.1.1. Geologic Setting[18] Mare basalts occur preferentially on the lunar near-

side, and often fill the low-lying inner depressions of largeimpact basins and craters such as Imbrium or Orientale.Basalts in Mare Frigoris are special in that they occur in anarea that is not clearly related to any unambiguouslyaccepted impact structure (Figure 1). Mare Frigoris maybe part of the large and very old Procellarum basin, but theexistence of this basin is still disputed [e.g., Whitaker, 1981;Spudis and Schultz, 1985; Spudis, 1993; Wilhelms, 1987].Wilhelms [1987] found the basalts of eastern Mare Frigoristo be of Imbrian age and the basalts of central and westernFrigoris (west of �10�E) to be younger and of Eratosthenianage. He suggested that the concentration of Eratosthenian andImbrian eruptions in Mare Frigoris, Mare Imbrium andOceanus Procellarum is due to a thin lithosphere beneaththe putative Procellarum basin. In the spectral basalt clas-sification scheme of Pieters [1978] (Table 2), the majority

of mare basalts in Mare Frigoris is characterized by rela-tively homogeneous low-titanium abundances, high albedo,strong 1 mm and prominent 2 mm absorption bands (LBSP)[Pieters, 1978; Belton et al., 1994]. Besides these LBSPbasalts, Pieters [1978] also mapped ‘‘undivided’’ basaltsand LISP basalts within Mare Frigoris. These LISP basaltsshow a low UV/VIS ratio, an intermediate albedo (0.03–0.09), a strong 1 mm and a prominent 2 mm absorption band[Pieters, 1978] and were interpreted as very low Ti, high Febasalts, similar to the Luna 24 samples [BVSP, 1981;Wilhelms, 1987]. Based on the morphology of premarecraters, the thickness of the basalts in Mare Frigoris hasbeen estimated to be less than 500 m [DeHon and Waskom,1976; DeHon, 1979]. Reviewing the assumptions thatunderlie the thickness estimates of DeHon and Waskom[1976], Horz [1978] concluded that the thicknesses ofDeHon [1979] are a factor of 2 too high. Therefore, MareFrigoris might be filled with only 200–250 m of marebasalts. However, in his comprehensive study of MareFrigoris, Whitford-Stark [1990] argued that the basalt fillwithin Mare Frigoris is more than 400 m thick and wasemplaced in at least three major volcanic episodes, rangingin composition from titanium-poor to titanium-rich.Whitford-Stark [1990] proposed that the basalts in Mare Frigoris wereemplaced by flood-style eruptions 3.2–3.6 Gyr ago.3.1.2. Discussion of Units[19] Basalts in Mare Frigoris have been mapped by

Ulrich [1969], Wilhelms and McCauley [1971], M’Gonigleand Schleicher [1972], and Lucchitta [1972, 1978]. Basedon Lunar Orbiter IV images, we performed new crater sizefrequency distribution measurements for 37 spectrally de-fined units in order to determine surface model agesof basalts exposed within Mare Frigoris (Figure 4 andTable 3). Among these 37 units, we found three units thatare younger than 3.2 Gyr, hence being Eratosthenian in age,and one late Imbrian unit that has been resurfaced during theEratosthenian. According to our crater counts, most of theinvestigated basalts are of late Imbrian age. The geologicmaps also indicate Eratosthenian and Imbrian ages for thebasalts in Mare Frigoris. However, because ages of basaltsin the geologic maps were mainly assigned based on surfacebrightness [e.g., Wilhelms, 1987], the location of thesebasalts and their ages do not necessarily correlate with ourspectrally defined units and their crater size-frequency ages.For example, units F3, F4, F10, F11, F18, F19, F25, F29,F31, and F33 have been mapped at least partially asEratosthenian or Imbrian in age (EIm), but our crater countsyielded Imbrian model ages between 3.72 and 3.22 Gyr[M’Gonigle and Schleicher, 1972; Ulrich, 1969]. Unit F37has been partially mapped as Eratosthenian to Imbrian(EIm) and Imbrian (Im) in age [M’Gonigle and Schleicher,1972] and we found a crater model age of 2.61 Gyr, hencebeing Eratosthenian in age. Lucchitta [1978] mapped thesame unit partly as Imbrian and partly as Eratosthenian inage, while the map of Wilhelms and McCauley [1971]shows an Imbrian age. According to the map of Lucchitta[1978], unit F37 consists of Eratosthenian and Imbrianbasalts (Em, Im) and Imbrian light plains (Ip1). Two of ouryoungest units (F35, F36) were mapped as Imbrian (Im) inage by M’Gonigle and Schleicher [1972], Wilhelms andMcCauley [1971], and Lucchitta [1978], while we deter-mined Eratosthenian ages. Similarly, our Eratosthenian unit

Table 2. Lunar Mare Basalt Typesa

Unit UV/VIS Ratio Albedo 1 mm Absorption 2 mm Absorption

HDWA high dark weak attenuatedHDSA high dark strong attenuatedhDSA high dark strong attenuatedhDSP high dark strong prominenthDG high dark general average not observedmISP medium intermediate strong prominentmIG medium intermediate general average not observedmBG medium bright general average not observedLISP low intermediate strong prominentLIG low intermediate general average not observedLBG low bright general average not observedLBSP low bright strong prominent

aPieters [1978].

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Figure

4.

Spatialdistributionofmodelages

forspectrally

defined

unitsin

MareFrigorisandLacusTem

poris.(a)USGS

shaded

relief

map,simplecylindricalmap

projection.Spectralunitsareoutlined

inblack.(b)Sketch

map

ofMareFrigoris

showingunitnumbersandmodelages

inGyr(alsoseeTables3and5).F,MareFrigoris;LT,LacusTem

poris.Cratersize-

frequency

distributionmeasurementswereperform

edfortheareashighlightedin

gray.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

8 of 22

E03003

F34 has been mapped as Imbrian in age by Wilhelms andMcCauley [1971] and Lucchitta [1972, 1978]. The geologicmap of Wilhelms and McCauley [1971] indicates anEratosthenian age for unit F33 and we determined a modelage of 3.22 Gyr, hence being very close to the Eratosthenian-Imbrian boundary at 3.20 Gyr. In the geologic map ofLucchitta [1978], this unit is of Imbrian age. In her map,unit F32 is partly mapped as Imbrian and Eratosthenian inage, but other maps indicate an Imbrian age [Wilhelms andMcCauley, 1971; Ulrich, 1969], consistent with our craterage. While units F6, F10, F18, F24, F25, F29, F30, and F31are mapped as Eratosthenian in age [Wilhelms andMcCauley,1971; Lucchitta, 1978], our crater counts indicate Imbrianages for these units. According to the geological map ofWilhelms and McCauley [1971], units F6 and F29 are atleast in parts Eratosthenian and Imbrian in age, whereasunits F3, F4, F5, F7, F8, F9, F11, F12, F13, F15, F16, F17,F19, F21, F22, F23, F24, F26, F27, F28, F30, F32, F34,F35, F36, and F37 are only Imbrian in age. In this map,units F1, F2, and F14 are at least partly covered by Imbrianlight plains material. While typically not interpreted as marebasalts but impact ejecta (summarized by Wilhelms [1987]),we dated these units because the age might help to constraintheir origin. For example, if the ages of the light plains arenot correlated with the Orientale or the Imbrium impact, thiswould be an argument that at least some light plains mightbe volcanic in origin. Unit F1 and F2 were mapped byLucchitta [1972] as smooth, intermediate albedo, heavilycratered Imbrian plains (Ip) that are embayed by marematerials. Similarly, unit F5 has been mapped in parts asImbrian mare materials (Im) and in parts as light plains (Ip1)[M’Gonigle and Schleicher, 1972]. In the Lucchitta [1972]map, units F13, F14, and F34 are of Imbrian age. Unit F20is not covered by any 1:1M geologic maps, but is mapped asyounger light plains of Imbrian age (Ip2) in the geologicmap of the north side of the Moon [Lucchitta, 1978]. In thismap, units F1 und F2 are also shown as Imbrian light plains(Ip2) and unit F5 is mapped partially as Imbrian light plains(Ip2) and partially as Imbrian mare materials (Im). Most ofthe basalts in Mare Frigoris show an Imbrian age in the mapof Lucchitta [1978]. In particular, units F3, F4, F7, F8, F9,F10, F11, F12, F13, F14, F16, F17, F18, F19, F21, F23,F25, F26, F33, F34, F35, and F36 are all Imbrian in age andunits F15, F22, F27, F28, F30, and F32 are Imbrian (Im) orEratosthenian (Em) in age. The geologic map of Lucchitta[1978] indicates an Eratosthenian age for our units F6, F24,F29, and F31. The geologic map of Ulrich [1969] showsImbrian ages for units F6, F15, F17, F21, F22, F24, F28,and F32. Finally, in the geologic map of M’Gonigle andSchleicher [1972], units F7, F8, F9, F12, F16F22, F23, F26,F27, F30, F35, and F36 are Imbrian in age and units F4,F11, F19, F25, F33, and F37 are of Imbrian or Eratosthenianin age (EIm).[20] Based on our crater counts, we determined Eratosthe-

nian ages only for four units with the youngest (2.61 Gyr)having relatively large errors of +0.5/�0.34 Gyr. Three ofthese units are located in the central parts of Mare Frigorisnorth of the crater Plato, and one Eratosthenian unit islocated close to the eastern edge of Mare Frigoris northwestof crater Baily. This concentrated distribution of Eratosthe-nian basalts north of Plato is consistent with the map ofWilhelms [1987] and M’Gonigle and Schleicher [1972].

However, our Eratosthenian units are much smaller in sizeand while located in the general area mapped by Wilhelms[1987] as Eratosthenian, are not connected to each other.With respect to the size and distribution of our Eratosthe-nian units, we find a much better agreement with the map ofM’Gonigle and Schleicher [1972] compared to the map ofWilhelms [1987]. The geologic map of Wilhelms andMcCauley [1971] does not show Eratosthenian basalts inthe area north of Plato. In the geologic map of Lucchitta[1978], most of the basalts in Mare Frigoris are of Imbrianage (Im) with only a small area southwest of the craterFontenelle and a larger area around the crater Harpalusbeing Eratosthenian in age. In summary, our new cratercounts confirm Imbrian ages for most of the Frigoris basaltsand we also observe Imbrian ages for some areas that weremapped as Eratosthenian in age by Lucchitta [1978].However, our crater counts confirm Eratosthenian ages forareas north of Plato as indicated in the geologic map ofM’Gonigle and Schleicher [1972].3.1.3. Ages[21] We performed crater counts for 37 spectrally homo-

geneous basalt units within Mare Frigoris. Our data indicatethat surface ages of Frigoris mare basalts range from 2.61 to3.77 Gyr (Figures 4 and 8 and Table 3). Most units wereformed in the late Imbrian period between 3.4 and 3.8 Gyr.This is consistent with observations made for other lunarnearside basalts inOceanus Procellarum, Imbrium, Serenitatis,Tranquillitatis, Cognitum, Nubium, Insularum, Humorum,Humboldtianum, and Australe [Hiesinger et al., 2000, 2002,2003]. In detail, of our 37 units, 6 units (�16%) are youngerthan 3.4 Gyr, 12 units (�32%) were formed between 3.4 and3.5 Gyr ago, 9 units (�24%) were formed between 3.5 and3.6 Gyr ago, 5 units (�14%) were formed between 3.6and 3.7 Gyr ago, and 5 units (�14%) are older than 3.7 Gyr(Figure 4 and Table 3).[22] Among the 37 units dated in this study, there are

12 units that show evidence for resurfacing (F8, F12,F13, F15, F17, F19, F20, F22, F24, F31, F32, F35).These resurfacing events occurred between 3.62 (F8) and3.11 Gyr (F35) and affected older surfaces of 4.0 (F32) to3.62 Gyr (F31).[23] If we look at the spatial distribution of basalt ages in

Mare Frigoris, it appears that older basalts (3.5–3.8 Gyr)occur east of 0� longitude and younger basalts (3.4–3.6 Gyr)are exposed between 0� and 40�W. We also found evidencethat basalts west of 40�W are older (3.3.5–3.8 Gyr) andsimilar in age to the basalts of eastern Mare Frigoris.[24] Crater degradation ages are available only for western

and central Mare Frigoris [Boyce, 1976; Boyce and Johnson,1978]. Units F1, F2, F3, F10, F13, F14, F20, F21, F25, andF34 are not covered in the maps of Boyce [1976] and Boyceand Johnson [1978]. We found the best agreement betweenthe two techniques for unit F33, for which the crater degra-dation age (3.20 Gyr) and our crater model age (3.22 Gyr) arebasically identical and the worst agreement for unit F6(2.50 versus 3.64 Gyr), for which the ages differ by 1.14 Gyr.The crater size-frequency model age of unit F28 (3.45 Gyr) isconsistent wit the crater degradation ages (3.20/3.50 Gyr) ofBoyce [1976] and Boyce and Johnson [1978]. Similarly, forour unit F35 we derived an old age of 3.68 Gyr and aresurfacing age of 3.11 Gyr, which are in the range of thecrater degradation age of 3.20 Gyr [Boyce, 1976; Boyce and

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

9 of 22

E03003

Table

3.ComparisonofAges

forBasaltsin

MareFrigorisa

Unit

Lunar

Orbiter

Image

Area

(km

2)

CraterRetention

AgeN(1)

Error

Model

Age

(Gyr)

Error

(Gyr)

Boyce[1976];

BoyceandJohnson

[1978]

Wilhelmsand

McC

auley[1971]

Lucchitta

[1978]

Ulrich

[1969]

M’G

onigle

andSchleicher

[1972]

Lucchitta

[1972]

F37

IV140H1

6728

2.19E-03

+0.54E-03/�

0.29E-03

2.61

+0.50/�

0.34

2.50/3.20

ImIm

,Em,Ip1

Im,EIm

F36

IV116H1

1455

2.60E-03

+0.18E-03/�

0.34E-03

3.02

+0.12/�

0.33

3.20/3.50

ImIm

ImF35

IV140H1;

IV128H1

2520

2.74E-03/

9.41E-03

+0.40E-03/�

0.18E-03;

+6.29E-03/�

1.76E-03

3.11/3.68

+0.17/�

0.12;

+0.10/�

0.05

3.20

ImIm

Im

F34

IV92H1

1875

2.85E-03

+0.80E-03/�

0.19E-03

3.17

+0.21/�

0.11

Noage

ImIm

ImF33

IV128H1

741

2.98E-03

+1.03E-03/�

0.39E-03

3.22

+0.21/�

0.20

3.20

Em

ImEIm

,Im

F32

IV152H1

3301

3.70E-03/

6.22E-02

+0.92E-03/�

0.69E-03;

+3.88E-02/�

1.50E-02

3.39/4.00

+0.10/�

0.16;

+0.07/�

0.04

2.50

ImIm

,Em

Im

F31

IV164H1;

IV152H1

8642

3.90E-03/

7.32E-03

+0.60E-03/�

0.50E-03;

+3.38E-03/�

1.76E-03

3.42/3.62

+0.06/�

0.08;

+0.08/�

0.07

2.50/3.20

Em

Em

EIm

,Im

F30

IV140H1

3654

4.05E-03

+1.11E-03/�

0.76E-03

3.43

+0.09/�

0.12

3.20

ImIm

,Em

ImF29

IV164H1

4047

3.98E-03

+1.07E-03/�

0.51E-03

3.43

+0.09/�

0.08

2.50/3.20

Im,Em

Em

Im,EIm

F28

IV152H1;

IV140H1

6763

4.21E-03

+0.94E-03/�

0.78E-03

3.45

+0.07/�

0.11

3.20/3.50

ImEm,Im

Im

F27

IV140H1

1520

4.29E-03

+0.89E-03/�

0.81E-03

3.46

+0.07/�

0.11

3.20

ImIm

,Em

ImF26

IV140H1

350

4.30E-03

+1.78E-03/�

0.80E-03

3.46

+0.11/�

0.10

3.20

ImIm

ImF25

IV116H1

1668

4.41E-03

+1.39E-03/�

0.82E-03

3.47

+0.09/�

0.10

Noage

Em

ImEIm

,Im

F24

IV152H1

4475

4.38E-03/

1.04E-02

+0.90E-03/�

0.81E-03;

+4.30E-03/�

1.97E-03

3.47/3.70

+0.06/�

0.10;

+0.06/�

0.05

3.20

ImEm

Im

F23

IV140H1;

IV128H1

5279

4.49E-03

+1.04E-03/�

0.84E-03

3.48

+0.07/�

0.10

3.20

ImIm

Im

F22

IV140H1;

IV152H1

21713

4.51E-03/

7.72E-03

+0.80E-03/�

0.89E-03;

+2.95E-02/�

1.86E-03

3.48/3.63

+0.05/�

0.10;

+0.09/�

0.07

2.50/3.20

ImIm

,Em

ImIm

F21

IV140H1

1149

4.69E-03

+1.30E-03/�

0.88E-03

3.49

+0.08/�

0.09

Noage

ImIm

ImF20

IV79H3

159

4.68E-03/

1.78E-02

+1.39E-03/�

0.88E-03;

+1.04E-02/�

0.43E-03

3.49/3.80

+0.08/�

0.09;

+0.07/�

0.05

Noage

Ip2

F19

IV140H1

5305

4.89E-03/

1.69E-02

+1.07E-03/�

0.92E-03;

+1.01E-02/�

4.10E-03

3.51/3.79

+0.06/�

0.09;

+0.08/�

0.05

2.50/3.20

ImIm

Im,EIm

F18

IV164H1

2306

5.32E-03

+1.43E-03/�

1.28E-03

3.53

+0.07/�

0.10

2.50

Em

ImEIm

F17

IV152H1

6558

5.25E-03/

1.67E-02

+1.10E-03/�

0.98E-03;

+1.06E-02/�

3.10E-03

3.53/3.79

+0.05/�

0.07;

+0.08/�

0.04

2.50/3.20

ImIm

Im

F16

IV128H1

1298

5.35E-03

+1.49E-03/�

1.00E-03

3.54

+0.06/�

0.08

3.20

ImIm

ImF15

IV164H1;

IV152H1

8329

5.48E-03/

1.31E-02

+1.16E-03/�

0.78E-03;

+1.47E-02/�

4.35E-03

3.54/3.74

+0.06/�

0.05;

+0.13/�

0.08

2.50

ImIm

,Em

Im

F14

IV92H1;

IV80H1;

IV79H3

11046

5.85E-03

+1.00E-03/�

1.09E-03

3.56

+0.04/�

0.06

Noage

Im,Ip

ImIm

F13

IV104H1;

IV92H1

42044

5.76E-03/

7.94E-03

+0.61E-03/�

0.66E-03;

+4.76E-03/�

1.48E-03

3.56/3.64

+0.03/�

0.04;

+0.10/�

0.05

Noage

ImIm

Im

F12

IV128H1;

IV116H1

17652

5.77E-03/

1.12E-02

+0.75E-03/�

0.78E-03;

+5.10E-03/�

2.73E-03

3.56/3.71

+0.03/�

0.05;

+0.07/�

0.06

3.20/2.50

ImIm

Im

F11

IV116H1

1272

6.18E-03

+1.67E-03/�

1.15E-03

3.58

+0.06/�

0.06

3.20

ImIm

Im,EIm

F10

IV140H1

649

6.79E-03

+2.31E-03/�

2.38E-03

3.60

+0.07/�

0.13

Noage

Em

ImEIm

F9

IV104H1;

IV116H1

12037

7.35E-03

+1.32E-03/�

1.55E-03

3.62

+0.04/�

0.06

3.50/3.20

ImIm

Im

F8

IV116H1

2226

7.33E-03/

1.42E-02

+1.20E-03/�

0.95E-03;

+5.80E-03/�

2.60E-03

3.62/3.76

+0.04/�

0.03;

+0.06/�

0.04

3.50/3.20

ImIm

Im

F7

IV128H1;

IV116H1

7340

7.78E-03

+1.00E-03/�

1.04E-03

3.63

+0.03/�

0.03

3.50/2.50

ImIm

Im

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

10 of 22

E03003

Johnson, 1978]. The same is true for unit F37, for which thecrater size frequency model age of 2.61 Gyr lies in betweenthe crater degradation ages (2.50/3.20 Gyr) of Boyce [1976]and Boyce and Johnson [1978]. A comparison of the agesderived by the two techniques reveals that crater size-frequency ages of 22 units (F4, F5, F6, F7, F8, F9, F11,F12, F15, F16, F17, F18, F19, F22, F23, F24, F26, F27, F29,F30, F31, F32) are systematically older compared to craterdegradation ages. Only for unit F36 is the crater size-frequencymodel age younger than the crater degradation age (Table 3).

3.2. Mare Vaporum3.2.1. Geologic Setting

[25] The Vaporum basin is 220 km in diameter and islocated at the central nearside at 23.3�E and 3.6�N [Wilhelms,1987] (Figure 1). Basalts in Mare Vaporum are characterizedby a medium high UV/VIS ratio, an albedo of less than0.08, a weak 1 mm and an attenuated 2 mm absorption band(hDWA) [Pieters, 1978]. Compared to basalt flows in MareImbrium, which are 10–35 m thick and up to 1200 km long,Schaber [1973], Schaber et al. [1976], and Wilhelms [1987]argued that flows in Mare Vaporum are shorter and thinner.3.2.2. Discussion of Units[26] We mapped six spectrally homogeneous units within

Mare Vaporum that we can compare to the geologic maps ofWilhelms [1968], Hackman [1966], and Wilhelms andMcCauley [1971] (Figure 5 and Table 4). On the basis ofour crater counts, we identified one unit (V6) of Eratosthe-nian age, two units (V4, V5) very close to the boundary ofthe Eratosthenian to the Imbrian period, and three Imbrianunits (V1, V2, V3). The geologic map of Wilhelms andMcCauley [1971] shows that units V2, V4, and V5 areImbrian in age, units V3 and V6 are Imbrian or Eratosthenianin age, and unit V1 is Eratosthenian in age. Curiously,according to our crater counts, this unit is the oldest unitwithin Mare Vaporum. In the map of Wilhelms [1968] thereare several geologic units including two Imbrian marematerials of the so-called Procellarum Group (Ipm1, Ipm2)and two materials interpreted to be of pyroclastic origin(CEm1, CEm2). According to this map, our unit V2 consistsof Imbrian mare materials (Ipm1, Ipm2), units V3, V4, andV5 contain Imbrian mare materials (Ipm1, Ipm2), but alsoCopernican and/or Eratosthenian pyroclastic material(CEm1, CEm2), and units V1 and V6 are characterized byCEm1 and CEm2, respectively. In the past, pyroclastic darkmantle units have been problematic in terms of deriving theirages because of their unusual low albedo that makes itdifficult to see craters, the physical properties of the mantle,crater degradation, or all of the above, which often led to anunderestimation of their ages.[27] While they were considered to be Eratosthenian/

Copernican in age and much younger than the Imbrian marebasalts [Wilhelms, 1968], our crater counts reveal that onlyone unit mapped as CEm2 (V6) is indeed younger than thesebasalts. Finally,Hackman [1966] mapped units V4 and V5 asImbrian low-albedo smooth volcanic material.3.2.3. Ages[28] On the basis of our crater counts we found that our

unit V6 is 3.10 Gyr old, hence Eratosthenian in age. Twounits, V4 and V5, are 3.22 and 3.23 Gyr old, all other units(V1, V2, and V3) are clearly Imbrian in age, with agesranging from 3.44 to 3.61 Gyr. Only unit V3 showsT

able

3.(continued)

Unit

Lunar

Orbiter

Image

Area

(km

2)

CraterRetention

AgeN(1)

Error

Model

Age

(Gyr)

Error

(Gyr)

Boyce[1976];

BoyceandJohnson

[1978]

Wilhelmsand

McC

auley[1971]

Lucchitta

[1978]

Ulrich

[1969]

M’G

onigle

andSchleicher

[1972]

Lucchitta

[1972]

F6

IV164H1

4015

7.81E-03

+1.72E-03/�

1.46E-03

3.64

+0.04/�

0.06

2.50

Em,Im

Em

ImF5

IV116H1

1591

1.12E-02

+3.00E-03/�

3.36E-03

3.71

+0.05/�

0.07

3.20

ImIp2,Im

Ip1,Im

F4

IV128H1

550

1.15E-02

+3.80E-03/�

4.24E-03

3.72

+0.05/�

0.10

3.20

ImIm

EIm

,Im

F3

IV164H1

4170

1.16E-02

+2.60E-03/�

2.88E-03

3.72

+0.04/�

0.06

Noage

ImIm

Im,EIm

F2

IV79H3

721

1.29E-02

+2.50E-03/�

3.12E-03

3.74

+0.03/�

0.06

Noage

IpIp2

IpF1

IV79H3

2741

1.54E-02

+2.70E-03/�

3.20E-03

3.77

+0.03/�

0.04

Noage

IpIp2

IpaSee

textfordetails.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

11 of 22

E03003

evidence of resurfacing of a 3.84 Gyr old surface at 3.44 Gyr(Figures 5 and 8 and Table 4). A comparison between craterdegradation ages of Boyce [1976] and Boyce and Johnson[1978] and our model ages reveals an excellent agreement forsome units and substantial differences for other units.We findconsistent ages for units V4, V5, and V6. For units V4, ourmodel age of 3.23 Gyr compares favorably to the craterdegradation ages of 3.20–2.50 Gyr [Boyce, 1976; Boyceand Johnson, 1978]. Similarly, the model age for unit V5(3.22 Gyr) is in the same range as the crater degradation ageof 3.20–2.50 Gyr [Boyce, 1976; Boyce and Johnson, 1978].For unit V6, our model age is 3.10 Gyr; the Boyce age is3.20–2.50 Gyr [Boyce, 1976;Boyce and Johnson, 1978]. Forunit V3, several crater degradation ages can be extracted fromthe map of Boyce and Johnson [1978], ranging from 3.20to 3.75 Gyr. For this unit, we were able to constrain the ageto 3.44 Gyr. The largest differences were detected for unitsV1 and V2. For unit V1, the ages differ by 0.41 Gyr (craterdegradation age of 3.20 – crater countmodel age of 3.61Gyr)and for unit V2 there is a difference of 0.25 Gyr (crater deg-radation age of 3.20 – crater count model age of 3.45 Gyr).

3.3. Mare Smythii and Mare Marginis

3.3.1. Geologic Setting[29] The Smythii basin is located at the eastern limb at

2�S and 87�E and is of pre-Nectarian age [Wilhelms, 1987;Spudis, 1993] (Figure 1). The main ring of the basin is

approximately 840 km in diameter, which is defined bypartial arcs bounded by a steep inner scarp and a gentleouter flank. Wilhelms [1987] identified a 360 km diameterring within the interior of the basin, which is continuous butlow. Citing work by Pike and Spudis [1987], Spudis [1993]reported 5 ring structures of the Smythii basin with diam-eters of 260, 370, 540, 740, and 1130 km. The main ring intheir interpretation has a diameter of 740 km. Pieters [1978]mapped only one spectral type of basalt (undivided) in MareSmythii. At the southwestern margin of Mare Smythii,Wilhelms [1987] mapped a high concentration of floor-fractured impact craters, similar to the concentration alongthe western edge of Oceanus Procellarum. The formation offloor-fractured craters could indicate that magma has stallednear the surface and formed shallow sills or laccoliths[Schultz, 1976]. This interpretation is supported by studiesof Wichman and Schultz [1995, 1996] and by modeling offloor-fractured craters, which led Dombard and Gillis[2001] to conclude that, compared to topographic relaxa-tion, laccolith emplacement is the more viable formationprocess for floor-fractured craters.[30] North of Smythii lies the 580 km large Marginis

basin, which is centered at 20�N and 84�E (Figure 1).According to Wilhelms [1987], the pre-Nectarian Marginisbasin predates the Smythii basin. Moore et al. [1974]suggested that Orientale ejecta might have been concentratednear the Marginis basin, which is antipodal to the Orientale

Figure 5. Spatial distribution of model ages for spectrally defined units in Mare Vaporum, Sinus Medii,and Palus Putredinis. (a) USGS shaded relief map, simple cylindrical map projection. Spectral units areoutlined in black. (b) Sketch map of Mare Vaporum, Sinus Medii, and Palus Putredinis showing unitnumbers and model ages in Gyr (also see Tables 4 and 5). MV, Mare Vaporum; SM, Sinus Medii; PP,Palus Putredinis. Crater size-frequency distribution measurements were performed for the areashighlighted in gray. For unit SM3 and SM4, the count area is represented by the dark gray area.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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basin, to form an unusual furrowed texture [Wilhelms andEl-Baz, 1977]. Alternatively, Schultz and Gault [1975]proposed a concentration of seismic waves antipodal tothe Orientale basin in order to explain the furrowed texture.As in the case of Smythii, the basin is filled with ‘‘undivided’’basalts in the spectral map of Pieters [1978].3.3.2. Discussion of Units[31] Mare Smythii is covered by the geologic map of

Wilhelms and El-Baz [1977]. In this map, our unit Sy1,which in our crater count shows an old, underlying age of3.48 Gyr and a resurfacing event at 3.14 Gyr, has beenmapped as Imbrian in age (Im2) (Figure 6 and Table 5).[32] Basalt units in Mare Marginis were mapped by

Wilhelms and El-Baz [1977]. Three of our four units(Ma2, Ma3, Ma4) were mapped as younger Imbrian marematerials (Im2) and one unit (Ma1) was mapped as olderImbrian mare materials (Im1) [Wilhelms and El-Baz, 1977].These ages are consistent with our ages derived from cratersize-frequency measurements, which indicate Imbrian agesfor all these units. Furthermore, our data confirm thestratigraphic relationships of Wilhelms and El-Baz [1977]because our unitsMa2,Ma3,Ma4 are younger than unitMa1,which is consistent with the relative ages in the geologic mapof Wilhelms and El-Baz [1977].3.3.3. Ages[33] Our results indicate that basalts in Mare Smythii are

significantly younger than basalts in Mare Marginis and thatthey might be of Eratosthenian age. As mentioned above,there is evidence for resurfacing within Mare Smythii at3.14 Gyr ago, affecting an underlying 3.48 Gyr old surface.For Mare Marginis basalts we found late Imbrian ages thatvary from 3.38 to 3.88 Gyr. Again, we see evidence forresurfacing of two units (Ma1, Ma2) at 3.80 and 3.62 Gyr,which affected older surfaces of 3.88 and 3.81 Gyr in age,respectively (Figures 6 and 8 and Table 5). Compared tocrater degradation ages of Boyce [1976] and Boyce andJohnson [1978], we find a reasonably good agreement.According to our age determinations, the surface age ofunit Ma1 is 3.80 Gyr; in the Boyce data it is 3.65 Gyr old[Boyce, 1976; Boyce and Johnson, 1978]. Crater degra-dation ages for unit Ma2 vary between 3.20 and 3.65 Gyr[Boyce, 1976; Boyce and Johnson, 1978] and we deter-mined a surface age of 3.62 Gyr. Similarly, crater degrada-tion ages of 3.65 Gyr for unit Ma3 are in excellent agreementwith our ages (3.60 Gyr). Finally, unit Ma4 exhibits a craterdegradation age of 3.20 Gyr [Boyce, 1976; Boyce andJohnson, 1978] compared to crater size-frequency ages of3.38 Gyr.

3.4. Sinus Medii3.4.1. Geologic Setting

[34] Sinus Medii is located south of Mare Vaporum at4�W–9�E and 2�S–9�N (Figure 1). North and east of craterTriesnecker, Sinus Medii is characterized by a complexsystem of tectonic graben. These graben are oriented mainlynorth-south, but also show other directions, with mutualtransections indicating a contemporaneous formation[Wilhelms, 1968]. Wilhelms [1987] proposed that theTriesnecker graben system might be related to isostaticuplift of Sinus Medii due to viscous relaxation of a weaklithosphere. Pieters [1978] spectrally classified the basalts ofSinus Medii as mIG basalts, indicating a medium UV/VIST

able

4.ComparisonofAges

forBasaltsin

MareVaporum

a

Unit

Lunar

Orbiter

Image

Area,

km

2

Crater

Retention

AgeN(1)

Error

Model

Age

(Gyr)

Error

(Gyr)

Boyce[1976];

BoyceandJohnson[1978]

WilhelmsandMcC

auley

[1971]

Hackman

[1966]

Wilhelms

[1968]

V6

IV102H2

463

2.72E-03

+1.90E-04/�

3.60E-04

3.10

+0.10/�

0.30

3.20/2.50

Em,Im

CEm2

V5

IV102H2

10846

2.96E-03

+6.20E-04/�

3.80E-04

3.22

+0.15/�

0.21

3.20/2.50

ImIpm

Ipm1,Ipm2,CEm1,CEm2

V4

IV102H2

12814

2.99E-03

+4.50E-04/�

2.00E-04

3.23

+0.11/�

0.09

3.20/2.50

ImIpm

Ipm1,Ipm2,CEm1

V3

IV102H2

3102

4.13E-03

+1.07E-03/�

7.70E-04

3.44

+0.09/�

0.11

3.75/3.65/

Em,Im

CEm1,Ipm2,Ipm1

V2

IV102H2

6324

4.23E-03

+1.11E-03/�

7.90E-04

3.45

+0.09/�

0.11

3.20

ImIpm1,Ipm2,CEm1

V1

IV102H2

1044

6.89E-03

+2.24E-03/�

1.66E-03

3.61

+0.06/�

0.08

3.20

Em

aSee

textfordetails.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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ratio, an intermediate albedo, a gentle 1 mm and an ‘‘unde-termined’’ 2 mm absorption band. The mIG basalts arewidespread, occur in the Apollo 12 samples, and wereinterpreted as low-to-medium Ti basalts [BVSP, 1981;Wilhelms, 1987]. Like for Mare Vaporum, lava flows inSinus Medii are shorter and thinner than lava flows in MareImbrium [Schaber, 1973; Schaber et al., 1976; Wilhelms,1987].3.4.2. Discussion of Units[35] The geologic maps of Wilhelms [1968], Howard and

Masursky [1968], andWilhelms and McCauley [1971] coverthe Sinus Medii region. The map of Wilhelms [1968] indi-cates two different late Imbrian basalts in this area (Ipm1,Ipm2), two older units of the Cayley Formation (Ica, Icah),and a Copernican/Eratosthenian dark mantle unit. In thismap, our unit SM1 is described as smooth, flat mare-likematerial of intermediate to high albedo (Ica), unit SM3consists of a mixture of units of the Caley Formation (Ica,Icah), and unit SM4 is characterized as relatively brightImbrian mare material (Ipm1). Howard and Masursky[1968] only mapped unit SM2. In their map, this unit isshown as near level, mare-like plains of intermediate albedo,which constitute the Imbrian Calyey Formation (Ica). In thegeologic map of the lunar nearside [Wilhelms and McCauley,1971], unit SM4 is shown as Imbrian mare basalts. UnitsSM1, SM2, and SM3 in Sinus Medii are mapped as smooth,flat Imbrian Cayley plains with intermediate albedo, inter-

preted to be of possible volcanic origin [Wilhelms andMcCauley, 1971]. In summary, the geological maps[Wilhelms, 1968; Howard and Masursky, 1968; Wilhelmsand McCauley, 1971] indicate that all investigated units inSinus Medii are Imbrian in age. Our crater counts confirmImbrian ages for these units. According to our age deter-minations, light plains (Cayley Formation) in this regionare of late Imbrian age and are younger than the Imbrium orOrientale event. Kohler et al. [2000] made similar obser-vations for northern nearside light plains. They found thatlight plains vary in age over more than 350 million yearsand concluded that the ejecta distribution from the Imbriumand/or Orientale impact event cannot exclusively be respon-sible for the formation of the light plains.3.4.3. Ages[36] Our crater size-frequency measurements revealed

model ages of 3.63 (SM4), 3.74 (SM3), 3.76 (SM2), and3.79 Gyr (SM1) (Figures 5 and 8 and Table 5). Craterdegradation ages of Sinus Medii correspond very well withour model ages. For example, according to Boyce [1976]and Boyce and Johnson [1978], unit SM1 is 3.85 Gyr old,hence within 0.06 Gyr of our derived model age. For unitSM3, both techniques basically yield identical ages of 3.74and 3.75 Gyr. A somewhat larger discrepancy was found forunit SM4. The crater degradation age (3.50 Gyr) is signifi-cantly younger than our crater size-frequency model age(3.63 Gyr). Despite the relatively large area of Sinus Medii

Figure 6. Spatial distribution of model ages for spectrally defined units in Mare Smythii, MareMarginis, and the craters Hubble, Joliot, and Goddard. (a) USGS shaded relief map, simple cylindricalmap projection. Spectral units are outlined in black. (b) Sketch map of Mare Smythii, Mare Marginis, andthe craters Hubble, Joliot, and Goddard showing unit numbers and model ages in Gyr (also see Table 5and 6). Sy, Mare Smythii; Ma, Mare Marginis; CHu, Hubble; CJo, Joliot; CGo, Goddard. Crater size-frequency distribution measurements were performed for the areas highlighted in gray. For unit Ma2, thecount area is represented by the dark gray area.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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Table

5.ComparisonofAges

forBasaltsa

Unitb

Lunar

Orbiter

Image

Area

(km

2)

Crater

Retention

AgeN(1)

Error

Model

Age

(Gyr)

Error

(Gyr)

Boyce[1976];

Boyceand

Johnson

[1978]

Wilhelms

andMcC

auley

[1971]

Wilhelms

[1968]

Wilhelms

andEl-Baz

[1977]

McC

auley

[1973]

Hackman

[1966]

Howard

andMasursky

[1968]

Lucchitta

[1978]

Scottet

al.

[1977]

Sy1

IV17H1

26363

2.79E-03/

4.53E-03

+2.00E-04/�

1.90E-04;

+1.04E-03/�

5.90E-04

3.14/3.48

+0.09/�

0.12;

+0.07/�

0.06

2.50/3.20

Im2

Ma4

IV17H2

824

3.65E-03

+5.40E-04/�

4.70E-04

3.38

+0.07/�

0.09

3.20

Im2

Ma3

IV17H2

1935

6.81E-03

+1.70E-03/�

1.78E-03

3.60

+0.06/�

0.08

3.65

Im2

Ma2

IV17H2

14599

7.35E-03/

1.88E-02

+1.01E-03/�

1.07E-03;

+1.01E-02/�

3.50E-03

3.62/3.81

+0.03/�

0.04;

+0.07/�

0.04

3.20/3.50/3.65

Im2

Ma1

IV17H2

2133

1.77E-02/

2.99E-02

+4.20E-03/�

4.20E-03;

+1.47E-02/�

7.20E-03

3.80/3.88

+0.03/�

0.05;

+0.07/�

0.04

3.65

Im1

SM4

IV102H1

8999

7.76E-03

+1.31E-03/�

1.42E-03

3.63

+0.04/�

0.05

3.50

ImIpm1,Ipm2

SM3

IV102H;

IV102H2

8876

1.26E-02

+1.33E-02/�

1.19E-02

3.74

+0.01/�

0.01

3.75

IpIca,

Icah

SM2

IV102H1

7260

1.41E-02

+2.20E-03/�

2.50E-03

3.76

+0.02/�

0.04

Noage

IpIca

SM1

IV102H1

3515

1.70E-02

+3.50E-03/�

4.00E-03

3.79

+0.03/�

0.05

3.85

IpIca

PP2

IV109H3

492

2.67E-03

+3.06E-03/�

2.32E-03

3.07

+0.18/�

0.31

3.50

Em

Ipm

PP1

IV102H3;

IV109H3

9196

3.64E-03

+4.09E-03/�

3.19E-03

3.35

+0.07/�

0.11

3.50

Im,Em

Ipm

LA1

IV168H2

400

4.84E-03

+1.20E-03/�

9.00E-04

3.50

+0.07/�

0.08

Noage

Em

Em

EIm

LT2

IV74H3

1007

7.31E-03

+2.44E-03/�

1.76E-03

3.62

+0.06/�

0.07

Noage

ImLT1

IV67H2

13328

9.37E-03/

+1.01E-02/�

8.65E-03

3.68/3.76

+0.01/�

0.02

Noage

ImIm

IV67H3;

IV74H3

1.45E-02

+1.78E-02/�

1.12E-02

+0.04/�

0.05

aSee

textfordetails.

bSy,

MareSmythii;Ma,

MareMarginis;SM,SinusMedii;PP,

PalusPutredinis;LA,LacusAestatis;LT,LacusTem

poris.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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(�300 � 150 km), our model ages are extremely similar,which is surprising considering the variety of geologic unitsexposed within this lunar region. On the basis of our cratercounts, mare basalts and Cayley plains were emplaced moreor less contemporaneous. Together with the fact that ourmodel ages do not correspond to either the Imbrium or theOrientale event, we interpret this to be more consistent with avolcanic origin of the studied light plains in Sinus Medii.

3.5. Palus Putredinis

3.5.1. Geologic Setting[37] Located at approximately 3�W–4�E and 24–30�N

(Figure 1), Palus Putredinis is one of the few locations whereLIG basalts are exposed [Pieters, 1978]. These basalts arecharacterized by a low UV/VIS ratio, an intermediate albedoof 0.03–0.09, a gentle 1 mm and an ‘‘undetermined’’ 2 mmabsorption band [Pieters, 1978]. Our unit PP2, which is southof Palus Putredinis proper, has been mapped as ‘‘undivided’’in the spectral classification map of Pieters [1978]. Basedon work performed for the Basaltic Volcanism StudyProject [BVSP, 1981], Wilhelms [1987] interpreted thePalus Putredinis basalts as being low in titanium and richin radioactive elements. In its southeastern parts, PalusPutredinis contains Hadley Rille and has been sampled byApollo 15.Mostmare basalt samples of theApollo 15 landingsite are dominated by two low-titanium varieties, the olivine-normative basalts (3.30 Gyr) and the quartz-normativebasalts, which are slightly older (3.35 Gyr) [Stoffler andRyder, 2001]. According to Stoffler and Ryder [2001], theolivine-normative basalts dominate the surface and are super-posed on the quartz-normative basalts. A few exotic compo-nents such as volcanic glasses exhibit ages of 3.30–3.60 Gyr[Stoffler and Ryder, 2001].3.5.2. Discussion of Units[38] The Palus Putredinis area has been mapped, for

example by Hackman [1966], Wilhelms and McCauley[1971], Swann et al. [1972], and Howard et al. [1972].While the maps of Hackman [1966] and Howard et al.[1972] only show one type of mare basalt exposed inPalus Putredinis, the other two maps show two units. Inthe 1:1M geologic map of Hackman [1966], both of ourunits are contiguously mapped as low-albedo volcanicmaterial of Imbrian age (Ipm). Our crater counts are con-sistent with Imbrian ages of basalts in Palus Putredinisproper as indicated in the map of Hackman [1966]. How-ever, on the basis of our crater size-frequency distributionmeasurements, we derived an Eratosthenian age for thesouthern count area PP2. The Swann et al. [1972] maponly differentiates between ‘‘young mare basalt’’ aroundthe Apollo 15 landing site and the eastern regions of PalusPutredinis and ‘‘old mare basalt’’ in the western parts ofPalus Putredinis. The geologic map of the nearside of theMoon shows an Imbrian age for the western parts and anEratosthenian age for the eastern parts of Palus Putredinis[Wilhelms and McCauley, 1971]. In this map, our secondunit (PP2) is separated from the basalts in Palus Putredinisand of Eratosthenian age, hence being consistent with ourmodel age of 3.07 Gyr.3.5.3. Ages[39] For Palus Putredinis unit PP1, which contains the

Apollo 15 landing site, we determined an Imbrian basalt

model age of 3.35 Gyr (Figures 5 and 8 and Table 5). This isin excellent agreement with radiometric ages of basalt sam-ples returned from the Apollo 15 mission, which indicateages from 3.30 to 3.35 Gyr [Stoffler and Ryder, 2001] or3.20–3.40 Gyr [Nyquist and Shih, 1992]. Our second unitPP2 is slightly younger (3.07 Gyr) and of Eratosthenian age.Crater degradation ages of Palus Putredinis indicate olderages of 3.50 Gyr for both of our count areas [Boyce, 1976;Boyce and Johnson, 1978] and are less consistent with theages in the geologic maps [Hackman, 1966; Wilhelms andMcCauley, 1971] and the radiometric ages of the Apollo 15samples [Stoffler and Ryder, 2001; Nyquist and Shih, 1992].

3.6. Lacus Aestatis

3.6.1. Geologic Setting[40] Lacus Aestatis lies northwest of crater Cruger and

south of crater Grimaldi within the southwestern highlandsof the Moon; Oceanus Procellarum is approximately 350 kmto the northeast (Figure 1). Mare Aestatis is an irregular-shaped, �400 km2 large, smooth mare deposit within thewesternmost, distal parts of the Hevelius Formation, which ispart of the Orientale ejecta [McCauley, 1973; Scott et al.,1977]. In the map of Scott et al. [1977], Lacus Aestatis islocated right at the boundary between the inner and outerfacies of the Hevelius Formation. The inner facies (Iohi) hasbeen interpreted as continuous part of the Orientale ejectadeposits emplaced during the outward flow of turbulent,mobile material [Scott et al., 1977]. The outer facies (Ioho)is the thin, discontinuous part of the Orientale ejecta deposit[Scott et al., 1977]. In the spectral classification map of lunarmare basalts, Lacus Aestatis is mapped as ‘‘undivided’’basalts [Pieters, 1978].3.6.2. Discussion of Units[41] We dated the basalts in Lacus Aestatis, which are

located at approximately 68–69�W and 13.5–16�S. Threegeologic maps cover the Lacus Aestatis region. In the 1:1Mgeologic map of McCauley [1973], there is only one basaltunit in Lacus Aestatis, which is of Eratosthenian age. The1:5M geologic maps of the nearside [Wilhelms and McCauley,1971] and the west side of the Moon [Scott et al., 1977] alsoshow an Eratosthenian age of the Lacus Aestatis basalts. Wecannot confirm such an Eratosthenian age because our cratercounts yielded an Imbrian age for Lacus Aestatis basalts.3.6.3. Ages[42] Our crater size-frequency measurements revealed a

model age of 3.50 Gyr for the Lacus Aestatis basalt, hencebeing Imbrian in age. Crater degradation ages for this partof the Moon are not available [Boyce, 1976; Boyce andJohnson, 1978] (Figures 7 and 8 and Table 5).

3.7. Lacus Temporis

3.7.1. Geologic Setting[43] Lacus Temporis is located at 43.5–49�N, 51–62�E,

south of the Nectarian crater Endymion, west of the NectariancraterMercurius and east of the Imbrian floor-fractured craterAtlas (Figures 1 and 4). The mare basalts occur within theNectarian or Imbrian highlands [Lucchitta, 1978] as localpatches of volcanic material that mostly fill old craters andalso occur as irregular shaped deposits. In the spectralclassificationmap ofPieters [1978], these basalts are mappedas ‘‘undivided.’’

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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E03003

3.7.2. Discussion of Units[44] Lava ponds in the Lacus Temporis south and south-

west of the crater Endymion were mapped by Lucchitta[1978] as Imbrian in age. Wilhelms and McCauley [1971]also mapped our unit LT1 as Imbrian in age. Our crater size-frequency distribution measurements confirm Imbrian agesfor all basalts in Lacus Temporis (LT1, LT2).3.7.3. Ages[45] Based on our new crater counts we determined late

Imbrian ages for the basalts in the Lacus Temporis region.According to our crater size-frequency distribution mea-surements, unit LT1 shows an old model age of 3.76 Gyrand a resurfacing model age of 3.68 Gyr (Figures 4 and8 and Table 5). Our unit LT2, located southwest of craterEndymion, has a model age of 3.62 Gyr. There are no craterdegradation ages available for these basalts [Boyce, 1976;Boyce and Johnson, 1978].

3.8. Basalts in Schickard, Grimaldi, Cruger, Rocca A,Goddard, Joliot, and Hubble

3.8.1. Geologic Setting[46] We investigated two groups of craters that are filled

with mare basalts. In the southwestern hemisphere, westudied craters Schickard, Grimaldi, Cruger, and Rocca A,and in the northeastern hemisphere we dated basalts in thecraters Goddard, Joliot, and Hubble. All of the craters ofeach group are within �350–400 km of each other. Thepre-Imbrian Schickard crater is located at 44.3�S and55.3�W within the southwestern highlands (Figure 1). Theregion is characterized by rugged and complex topography.Schickard is filled with two younger mare basalt depositsin the north and south, separated by older Imbrian plainsmaterial (Ip) [Karlstrom, 1974]. In a manner similar toLacus Aestatis, Cruger, Rocca A, and Grimaldi, the craterSchickard has been incompletely covered with the discon-

Figure 7. Spatial distribution of model ages for spectrally defined units in the craters Schickard,Grimaldi, Cruger, Rocca A, and Lacus Aestatis. (a) USGS shaded relief map, simple cylindrical mapprojection. Spectral units are outlined in black. (b) Sketch map of Schickard, Grimaldi, Cruger, Rocca A,and Lacus Aestatis showing unit numbers and model ages in Gyr (also see Tables 5 and 6). CSc,Schickard; CGr, Grimaldi; CCr, Cruger; Cro, Rocca A; LA, Lacus Aestatis. Crater size-frequencydistribution measurements were performed for the areas highlighted in gray.

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Figure

8.

Color-coded

map

ofthespatialdistributionofmodelages

oflunar

marebasaltssuperposedonaUSGSshaded

reliefmap.A

ges

shownarefromthisstudyandfromHiesingeretal.[2000,2003].Modelages

areinGyr;binsize

is100Myr.

Map

coverageis�90�W

–120�E

,�75�S

–75�N

;latitude,longitudegridis15��

15�wide.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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tinuous, thin ejecta material of the Orientale basin, i.e., theouter facies of the Hevelius Formation before the emplace-ment of the mare deposits [Scott et al., 1977]. Cryptomaria,defined as mare deposits hidden beneath bright deposits ofcrater and basin ejecta [Head and Wilson, 1992] and oftenrevealed by postejecta impact dark halo craters [Schultzand Spudis, 1979; Hawke and Bell, 1981; Antonenko et al.,1995], exist below the younger maria and the Orientaleejecta deposits in the area southwest of Oceanus Procella-rum, particularly in the Schiller-Schickard region [Belton etal., 1992; Greeley et al., 1993; Mustard and Head, 1996].For example, Mustard and Head [1996] used Galileo multi-spectral images and mixture modeling to document evi-dence for significant incorporation of basalt into the distalbasin ejecta of Orientale; these results underlined theinterpretation that pre-Orientale volcanism and volcanicdeposits were widespread in the Procellarum region. Thisconclusion was further corroborated by the mare-rich ejectadeposit observed for the 100 km diameter impact craterLetronne at the edge of southern Oceanus Procllarum[Mustard and Head, 1996]; for this region of the Moon,volcanism was well established and widespread in earlyImbrian time but had become more focused to a few majorvolcanic regions (Humorum, Procellarum) by the lateImbrian. Grimaldi crater is approximately 172 km in diam-eter and is located at the western margin of OceanusProcellarum, at 5.5�S and 68.3�W (Figure 1). Accordingto the 1:1M geologic map of McCauley [1973], this craterformed in late pre-Imbrian times and was later filled withbasalts during the Eratosthenian period in a manner similarto the nearby craters Cruger and Rocca A. Cruger is 45 kmin diameter and is located at 16.7�S and 66.8�W; Rocca A issituated at 13.8�S and 70.0�W (Figure 1).[47] Goddard crater is located east of Mare Marginis at

about 14.8�N and 89�E (Figure 1). The crater is 89 km indiameter and of pre-Nectarian age [Wilhelms and El-Baz,1977]. Joliot crater, a 164 km large crater centered at 25.8�Nand 93.1�E is like Goddard, of pre-Nectarian age [Wilhelmsand El-Baz, 1977] (Figure 1). Somewhat younger, Hubblecrater is Nectarian in age [Wilhelms and El-Baz, 1977]. Thiscrater, located at 22.1 �N and 86.9�E and has a diameter of80 km (Figure 1). All three craters were flooded with marebasalts during the Imbrian period [Wilhelms and El-Baz,1977]. In the spectral classification map of Pieters [1978],the basalts in Schickard, Grimaldi, Cruger, Rocca A, Goddard,Joliot, and Hubble are all mapped as ‘‘undivided.’’3.8.2. Discussion of Units[48] The floor of the 206 km large Schickard crater

exhibits a bright central part and two darker parts in thenorthwest and southeast. Karlstrom [1974] mapped thebright part as Imbrian in age and the dark parts to beEratosthenian in age. We performed crater counts only forthe dark parts and found Imbrian ages for both areas. Thisis inconsistent with the geologic map of Karlstrom [1974],but is consistent with crater counts of Greeley et al. [1993].The mare basalt unit exposed within Grimaldi has beenmapped as Eratosthenian byWilhelms and McCauley [1971]andMcCauley [1973]. This is consistent with our new cratercounts and with crater counts of Greeley et al. [1993]. Lavaponds in the craters Cruger (16.7�S, 66.8�W, 45 km indiameter) and Rocca A (13–15.5�S, 69–71.5�W) are ofImbrian and/or Eratosthenian age (EIm) in the geologicT

able

6.ComparisonofAges

forBasaltsin

SelectedCratersa

Unitb

Lunar

Orbiter

Image

Area,

km

2CraterRetention

AgeN(1)

Error

Model

Age

(Gyr)

Error

(Gyr)

Boyce[1976];

BoyceandJohnson

[1978]

Wilhelmsand

McC

auley[1971]

Wilshire

[1973]

Wilhelms

andEl-Baz

[1977]

McC

auley

[1973]

Karlstrom

[1974]

Scottet

al.

[1977]

CSc2

IV160H2

1801

7.25E-03/1.34E-02

+8.74E-02/�

5.73E-03;

+1.93E-02/�

1.02E-02

3.62/3.75

+0.04/�

0.06;

+0.06/�

0.06

3.85

Em

Em

EIm

CSc1

IV160H2

6516

1.15E-02

+1.36E-02/�

9.36E-03

3.72

+0.03/�

0.04

3.85

Em

Em

EIm

CGr1

IV168H3

14201

2.81E-03/4.58E-03

+3.02E-03/�

2.63E-03;

+5.63E-03/�

3.72E-03

3.16/3.48

+0.08/�

0.12;

+0.07/�

0.09

Noage

Em

Em

EIm

CCr1

IV168H2

1271

3,62E-03/5,69E-03

+4.13E-03/�

3.15E-03;

+8.01E-03/�

4.63E-03

3.38/3.55

+0.06/�

0.10;

+0.09/�

0.06

Noage

Em

Em

Em

EIm

CRo1

IV168H2

392

3.51E-03

+4.32E-03/�

2.86E-03

3.36

+0.10/�

0.18

Noage

Em

Em

EIm

CHu1

IV17H3

2543

1.66E-02

+2.03E-02/�

1.24E-02

3.79

+0.03/�

0.06

Noage

Im1

CJo2

IV17H3

2081

6.65E-03

+8.99E-03/�

5.41E-03

3.60

+0.07/�

0.06

Noage

Im1

CJo1

IV17H3

1494

7.43E-03/1.47E-02

+9.08E-03/�

5.72E-03;

+2.37E-02/�

1.19E-02

3.62/3.76

+0.05/�

0.06;

+0.09/�

0.04

Noage

Im1

CGo1

IV17H2

3817

6.68E-03

+8.29E-03/�

4.92E-03

3.60

+0.05/�

0.09

3.5/3.65

Im2

aSee

textfordetails.

bCSc,

Schickard;CGr,Grimaldi;CCr,Crueger;CRo1,RoccaA;Chu,Hubble;CJo,Joliot;CGo,Goddard.

E03003 HIESINGER ET AL.: AGES OF MARE BASALTS IN MARE FRIGORIS

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map of Scott et al. [1977], but our data show evidence onlyfor an Imbrian age of these basalts.[49] The map of Lucchitta [1978] shows that the basalt

fill of crater Goddard is of Imbrian age and this was verifiedby our crater counts. The craters Hubble and Joliot are filledwith Imbrian basalts (Im1) in the geologic map [Wilhelmsand El-Baz, 1977] and crater counts also suggest an Imbrianage of these basalts. Their map shows that the basalt fill ofcrater Goddard (Im2) is somewhat younger than the fillof craters Joliot and Hubble (Im1) and is of Imbrian age.Imbrian ages are supported by our crater counts for all threecrater fills. However, our ages indicate that the basalts incrater Goddard are more or less of the same age as basaltsin crater Joliot, whereas the basalts in crater Hubble are older.3.8.3. Ages[50] For the larger northern mare basalt in Schickard crater

(CSc1), we derived an Imbrian model age of 3.72 Gyr. Thesmaller southern mare basalt (CSc2) shows evidence forresurfacing of an older 3.75 Gyr old surface at 3.62 Gyrago. Because the two model ages of 3.75 and 3.72 Gyrare extremely similar, this could imply that both regionswere flooded with mare basalts at the same time. However,while volcanism stopped in the northern part at 3.72 Gyr,it continued at least to 3.62 Gyr as demonstrated by theresurfacing event at this time (Figures 7 and 8 and Table 6).Crater degradation ages are older (3.85 Gyr) and show thesame age for both mare basalt deposits [Boyce, 1976; Boyceand Johnson, 1978]. For the basalt unit in crater Grimaldi(CGr1), our crater size-frequency distribution measure-ments yielded a model age of 3.48 Gyr with a resurfacingat 3.16 Gyr ago (Figures 7 and 8 and Table 6). For Grimaldicrater, no crater degradation ages are available [Boyce,1976; Boyce and Johnson, 1978]. According to our cratercounts, the basalts in Cruger crater (CCr1) also showevidence for resurfacing. In this crater, we found that a3.55 Gyr old surface has been resurfaced at 3.38 Gyr ago.We derived a similar age of 3.36 Gyr for the basalt in RoccaA crater (CRo1) (Figures 7 and 8 and Table 6). Like forCruger crater, there are no crater degradation ages available[Boyce, 1976; Boyce and Johnson, 1978].[51] In the northeastern hemisphere, we dated the basalts

in the craters Hubble, Joliot, and Goddard. We found amodel age of 3.79 Gyr for the basalts in Hubble crater(CHu), and a model age of 3.60 Gyr for the basalts inGoddard crater (CGo1) (Figures 6 and 8 and Table 6). InJoliot crater, we mapped and dated two basalt units. UnitCJo1 shows evidence of resurfacing of a 3.76 Gyr oldsurface at 3.62 Gyr ago. The second unit (CJo2) exhibitsonly one age of 3.60 Gyr (Figures 6 and 8 and Table 6).Crater degradation ages (3.50/3.65) are only available forGoddard crater and these ages are in good agreement withour crater size-frequency ages (3.60 Gyr) [Boyce, 1976;Boyce and Johnson, 1978]. Generally, we found that allbasalt ages within the studied craters in the northeasternhemisphere are substantially older than the basalts of thecraters in the southwestern hemisphere (Figure 8).

4. Conclusions

[52] Based on our new age determinations for basalts thatare exposed in Mare Frigoris, Mare Vaporum, Mare Smythii,Mare Marginis, Sinus Medii, Palus Putredinis, Lacus

Aestatis, Lacus Temporis, Schickard, Grimaldi, Cruger,Rocca A, Hubble, Joliot, and Goddard, we conclude that(1) Mare Frigoris is mostly filled with Imbrian basalts, butthere are a few areas that are covered with Eratosthenianbasalts; (2) these Eratosthenian basalts occur in severalsmall-sized areas north of the crater Plato, but are notconnected with each other as shown by Wilhelms [1987];(3) basalts in Mare Vaporum, Sinus Medii, and PalusPutredinis are Imbrian in age; (4) basalts in Mare Smythiiare younger than in Mare Marginis, contrary to the geo-logic map of Wilhelms and El-Baz [1977]; (5) basalticplains in the craters Goddard, Hubble, and Joliot are ofImbrian age; (6) there are no Eratosthenian basalts in thecrater Schickard, but basalts in the crater Grimaldi areEratosthenian in age as mapped by McCauley [1973];(7) lava ponds in the craters Cruger and Rocca A andLacus Aestatis were formed during the Imbrian period;(8) basalts in craters of the southwestern hemisphere(Schickard, Grimaldi, Cruger, Rocca A) are systematicallyyounger than the basalts exposed in the craters of thenortheastern hemisphere (Goddard, Hubble, Joliot); (9) lavaponds in Lacus Temporis south of the crater Endymion are oflate Imbrian age; (10) none of the areas mapped as Eratos-thenian dark mantle deposits is Eratosthenian but all areImbrian in age and often older than adjacent mare basalts.

[53] Acknowledgments. The authors would like to thank the NASAPlanetary Geology and Geophysics Program, which provided support forthis analysis to J.W.H. and H.H. while at Brown University.

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�����������������������J. W. Head III, Department of Geological Sciences, Brown University,

Providence, RI 02912, USA.H. Hiesinger, Institut fur Planetologie, WestfalischeWilhelms-Universitat,

D-48149 Munster, Germany. (Hiesinger@uni-muenster.de)R. Jaumann and U. Wolf, DLR Institute of Planetary Exploration,

Rutherfordstr. 2, D-12489 Berlin, Germany.G. Neukum, Institut fur Geologie, Geophysik und Geoinformatik, Freie

Universitat Berlin, D-14195 Berlin, Germany.

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