ages and stratigraphy of mare basalts in oceanus ... · younger basalts are generally exposed in...

Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare

Nubium, Mare Cognitum, and Mare Insularum

H. Hiesinger and J. W. Head IIIDepartment of Geological Sciences, Brown University, Providence, Rhode Island, USA

U. Wolf and R. JaumannDLR-Institute of Space Sensor Technology and Planetary Exploration, Berlin, Germany

G. NeukumInstitut fur Geologie, Geophysik und Geoinformatik, Freie Universitat Berlin, Germany

Received 27 September 2002; revised 3 March 2003; accepted 14 April 2003; published 2 July 2003.

[1] Accurate estimates of mare basalt ages are necessary to place constraints on theduration and the flux of lunar volcanism as well as on the petrogenesis of lunar marebasalts and their relationship to the thermal evolution of the Moon. We performed newcrater size-frequency distribution measurements in order to investigate the stratigraphy ofmare basalts in Oceanus Procellarum and related regions such as Mare Nubium, MareCognitum, and Mare Insularum. We used high-resolution Clementine color data to define86 spectrally homogeneous units within these basins, which were then dated with cratercounts on Lunar Orbiter IV images. Our crater size-frequency distribution measurementsdefine mineralogical and spectral surface units and offer significant improvements inaccuracy over previous analyses. Our data show that volcanism in the investigated regionwas active over a long period of time from �3.93 to 1.2 b.y., a total of �2.7 b.y.Volumetrically, most of the basalts erupted in the Late Imbrian Period between �3.3 and3.7 b.y., and we see evidence that numerous units have been resurfaced. During theEratosthenian Period, significantly less basalt was erupted. Depending on the absolutemodel ages that one can assign to the lunar chronostratigraphic systems, five units mightbe of Copernican age. Younger basalts are generally exposed in the center of theinvestigated area, that is, closer to the volcanic centers of the Aristarchus Plateau andMarius Hills. Older basalts occur preferentially along the northwestern margin of OceanusProcellarum and in the southeastern regions of the studied area, i.e., in Mare Cognitumand Mare Nubium. Combining the new data with our previously measured ages for basaltsin Mare Imbrium, Serenitatis, Tranquillitatis, Humorum, Australe, and Humboldtianum,we find that the period of active volcanism on the Moon lasted �2.8 b.y., from �4 b.y. to�1.2 b.y. On the basis of the basalts dated so far, which do not yet include the potentiallyyoung basalts of Mare Smythii [e.g., Schultz and Spudis, 1983], we conclude that OceanusProcellarum not only exhibits the widest range of ages of all investigated basins butprobably also is the location of some of the youngest basalts on the lunar surface. INDEX

TERMS: 6250 Planetology: Solar System Objects: Moon (1221); 5420 Planetology: Solid Surface Planets:

Impact phenomena (includes cratering); 5464 Planetology: Solid Surface Planets: Remote sensing; 5480

Planetology: Solid Surface Planets: Volcanism (8450); KEYWORDS: Moon, mare basalts, ages, Oceanus

Procellarum, crater size-frequency distributions, Clementine

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

Procellarum, Mare Nubium, Mare Cognitum, and Mare Insularum, J. Geophys. Res., 108(E7), 5065, doi:10.1029/2002JE001985,

2003.

1. Introduction

[2] Lunar mare basalts cover about 17% of the lunarsurface [Head, 1976], but radiometric ages for lunar basalts

are available only for spatially very limited areas, i.e., theApollo and Luna landing sites [e.g., Basaltic VolcanismStudy Project (BVSP), 1981; Stoffler and Ryder, 2001;Taylor, 1982, and references therein]. A significant portionof lunar mare basalts is exposed within Oceanus Procella-rum and associated regions for which absolute radiometricage data are still lacking. Fortunately, remote sensing

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E7, 5065, doi:10.1029/2002JE001985, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JE001985$09.00

1 - 1

techniques allow one to derive ages not only for the Apolloand Luna landing sites but also for unsampled regions.Superposition of geologic units onto each other, craterdegradation stages, and crater size-frequency distributionmeasurements have been used in order to obtain relative andabsolute model ages for lunar surface units from remotesensing data [e.g., Shoemaker and Hackman, 1962; Boyce,1976;Wilhelms, 1987; Neukum and Ivanov, 1994; Hiesingeret al., 2000].[3] Here we present model ages of lunar mare basalts in

Oceanus Procellarum, Mare Nubium, Mare Cognitum, andMare Insularum (Figure 1) that are based on one of theseremote sensing techniques, that is, crater counts. Comparedto previous crater counts [e.g., Neukum et al., 1975; Greeleyand Gault, 1970; Hartmann, 1966], we applied a newapproach in that we performed crater size-frequency distri-bution measurements for spectrally homogeneous basaltunits. A major goal of this study is to provide absolutemodel ages for these basalts in order to investigate theirstratigraphy and to understand better the nature and evolu-tion of lunar mare basalt volcanism.[4] On the basis of our new age data we address the

following questions: (1) What was the time period of active

volcanism in the investigated area, i.e., when did volcanismstart and when did it end? (2) Was lunar volcanismcontinuously active or are there distinctive periods ofvolcanic activity? (3) Is there a trend in the spatial distri-bution of basalt ages on the lunar surface? Finally, (4) Whatis the flux of lunar mare basalts, i.e., what volumes ofbasalts were erupted within a certain period of time?[5] We present results on the spatial and temporal distri-

bution of basalt ages and will discuss our findings in thecontext of previously published geologic and spectral mapsas well as 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

[6] 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., Neukum andIvanov, 1994; Hiesinger et al., 2000; Stoffler and Ryder,2001; Neukum et al., 2001; Ivanov, 2001; Hartmann and

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. Latitude, longitude grid is 30� �30� wide; simple-cylindrical projection.

1 - 2 HIESINGER ET AL.: AGES AND STRATIGRAPHY OF MARE BASALTS

Neukum, 2001; and references therein] we will only brieflyoutline how crater size-frequency distribution measure-ments can be used to date surfaces. The technique of cratersize-frequency distribution measurements on spectrally ho-mogeneous regions, including a discussion of modelassumptions, strengths and shortcomings, and an erroranalysis, has been described in detail by Hiesinger et al.[2000]. In short, in order to obtain the age of a photogeo-logical unit one has to (1) measure the surface area of theunit, and (2) measure the diameters of each primary impactcrater within this unit. It has been shown that lunar craterdistributions measured on geologic units of different agesand in overlapping crater diameter ranges can be alignedalong a complex continuous curve, the lunar productionfunction [e.g., Neukum and Ivanov, 1994]. The lunar pro-duction function is given by

log Ncumð Þ ¼ a0 þX11k¼1

ak log Dð Þð Þk ð1Þ

where a0 represents the amount of time during which theunit has been exposed to the meteorite bombardment[Neukum and Ivanov, 1994]. The cumulative crater densityof a geologic unit taken at a fixed reference diameter(usually 1 or 10 km) is directly related to the time the unithas been exposed to the meteorite flux and therefore gives arelative age of this unit. To obtain absolute model ages fromcrater size-frequency distribution measurements, one has tolink the radiometric ages from the returned Apollo and Lunasamples with crater counts of these landing sites in order toestablish the lunar cratering chronology. This is not a trivialtask and has led to several more or less differentchronologies [e.g., BVSP, 1981; Neukum, 1983, Neukumand Ivanov, 1994; Stoffler and Ryder, 2001; and referencestherein]. The empirically derived chronology of Neukumand Ivanov [1994], which we use for this study is given by

Ncum D � 1 kmð Þ ¼ 5:44 � 10�14 exp 6:93� tð Þ � 1½

þ 8:38� 10�4t: ð2Þ

[7] Once the lunar chronology is established we canderive absolute model ages for the entire lunar surface fromcrater size-frequency distribution measurements by solving(2) for time t for Ncum(D � 1 km) measured on the geologicunit to be dated.[8] The level of uncertainty of the crater retention age of a

given count is given by the following formula:

�sN ¼ logN 1ð Þ �

ffiffiffiffiffiffiffiffiffiffiN 1ð Þ

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. The±sN value gives the upper and lower limits of the error bar ofthe crater retention age, which are used for estimating theuncertainty of the absolute cratermodel age from the crateringchronology. We principally assume that the crateringchronology is free of errors. Therefore errors in our absolutemodel ages are only caused by errors in the determination of

crater frequencies [Neukum, 1983]. Neukum et al. [1975]estimated the systematic uncertainty of the standarddistribution curve or the measurement to be <10% for0.8 km�D� 3 km (this is the diameter range of most of ourcrater counts) and up to 25% for 0.8 km � D � 10 km.[9] 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]. A detailed discussion of this issue is givenelsewhere [Hiesinger et al., 2000]. Figure 2 is a comparisonof stratigraphies based on work by Wilhelms [1987], Neu-kum and Ivanov [1994], and Stoffler and Ryder [2001].While there is general agreement on the definition of thebase of the Eratosthenian system (i.e., 3.2 b.y.), the stratig-raphies vary substantially for the beginning of the Coperni-can system (i.e., 0.8–1.5 b.y.). Wilhelms [1987] pointed out,that there is no formal definition of the Copernican systembecause no extensive stratigraphic-datum horizons exist nearthe lower system boundary. Crater Copernicus is a goodearly Copernican marker, but does not mark the base of thesystem [Wilhelms, 1987]. The lack of a clear stratigraphic-datum horizon resulted in a vague definition of the Coper-nican system, a fact which is also reflected in the variationsof absolute ages assigned to the base of this system.[10] In this paper we adopt the system of Neukum and

Ivanov [1994], with Nectaris being 4.1 ± 0.1 b.y. (N(D =1 km) = (1.2 ± 0.4) � 10�1), and the Imbrium basin being3.91 ± 0.1 b.y. old (N(D = 1 km) = (3.5 ± 0.5) � 10�2).According to Neukum [1983], the Eratosthenian Systemstarted 3.2 b.y. ago (N(D = 1 km) = 3.0 � 10�3) and theCopernican System began 1.5 b.y. ago (N(D = 1 km) = (1.3± 0.3) � 10�3), while radiometric dating of samples, whichare thought to represent Copernicus ejecta, indicates an ageof 0.85 ± 0.1 b.y. [Silver, 1971] (Figure 2). As mentionedabove, the linkage of lunar sample ages to discreet basinforming events is still subject to discussion [e.g., Wilhelms,1987; Spudis, 1993; Neukum and Ivanov, 1994; Stoffler andRyder, 2001]. For example, on the basis of their reevalua-tion of lunar samples, Stoffler and Ryder [2001] concludedthat the Nectaris basin is 3.92 ± 0.03 b.y. old, hence youngerthan in the Neukum and Ivanov [1994] chronology. Thisinterpretation of Stoffler and Ryder [2001] is consistent withthe conclusions of Spudis [1993]. For the Imbrium basinStoffler and Ryder [2001] discuss two ages, ranging from3.85 ± 0.02 to 3.77 ± 0.02 b.y., the later age beinginconsistent with previously published ages of 3.80–3.85b.y. for the Orientale basin [e.g., Wilhelms, 1987; Schaeferand Husain, 1974]. Application of the Neukum and Ivanov[1994] chronostratigraphic system yielded 39 Imbrian units,42 Eratosthenian units, and 5 Copernican units in theinvestigated area. Using the chronostratigraphic system ofWilhelms [1987], the five Copernican units would be ofEratosthenian age.[11] A crucial prerequisite for reliable age determinations

with crater size-frequency distribution measurements isthe mapping of homogeneous count areas. The OceanusProcellarum region of the Moon and associated regions(i.e., Mare Nubium, Mare Cognitum, and Mare Insularum;Figure 1) were previously geologically mapped by severalauthors [e.g., Wilhelms and McCauley, 1971; Wilhelms,

HIESINGER ET AL.: AGES AND STRATIGRAPHY OF MARE BASALTS 1 - 3

1987; Holt, 1974; McCauley, 1967, 1973; Howard andMasursky, 1968; Schmitt et al., 1967; Eggleton, 1965; Traskand Titley, 1966; Hackman, 1962; Marshall, 1963; Moore,1965, 1967; Wilshire, 1973; Scott et al., 1977; Titley, 1967;Schaber, 1969; Ulrich, 1969; Scott and Eggleton, 1973;Lucchitta, 1978]. However, because unit definition wasbased mainly on brightness differences, morphology andqualitative crater densities on telescopic and Lunar Orbiterimages, these maps are not detailed enough to ensurehomogeneity of the investigated basalts. On the basis ofmorphology and spectral characteristics, Whitford-Stark andHead [1980] defined 21 distinctive basalt types in the

investigated part of the lunar nearside (Figure 3). It isknown that regional spectral differences mapped usingspectral ratios approximate relatively homogeneous surfacemare units [e.g., Whitaker, 1972; McCord et al., 1976;Johnson et al., 1977a, 1977b; Pieters, 1978; Head et al.,1993]. Thus we used a multispectral high-resolution Clem-entine color ratio composite (e.g., 750 � 400/750 + 400ratio as red, 750/990 ratio as green, and 400/750 ratio asblue) in order to remap the distribution of distinctive basaltsand found that the map of Whitford-Stark and Head [1980]well discriminates the major basalt types. However, on thebasis of the new high-resolution color data several of their

Figure 2. Comparison of stratigraphies ofWilhelms [1987], Neukum and Ivanov [1994], and Stoffler andRyder [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. InStoffler and Ryder [2001], two formation ages for the Imbrium basin have been proposed, that is, 3.85 b.y.and 3.77 b.y. (dashed line).


units can be further subdivided into spectrally differentbasalt sub-types (Figure 4). The purpose of remapping thebasalts in Oceanus Procellarum was to define spectrallyhomogeneous units. We assume that these units wereformed within a relatively short period of time and are toa first order similar in mineralogy. We further assume thatbecause of the spectral homogeneity, each of our spectrallydefined units represents a single eruptive phase.[12] As mentioned earlier it is very important to define

homogeneous units in order to obtain reliable age determi-nations with crater size-frequency distribution measure-ments. Having defined such units with Clementineimages, we transferred the unit boundaries to high-resolu-tion Lunar Orbiter IV images in order to measure the cratersize-frequency distribution. This was necessary becauseClementine images are not very well suited for crater counts

due to their high sun angles. Figure 6 is a comparison of aLunar Orbiter image and a Clementine image that illustratesthe differences in the ability to detect small craters in thesetwo data sets. The detailed views show that some cratersthat are clearly visible in the Lunar Orbiter image are barelydetectable and certainly not countable in the Clementineimage. Because miscounting certain craters has profoundeffects on the crater size-frequency distribution and hencethe age of an investigated unit, we chose not to performcrater counts on Clementine data. Despite the excellentquality, we also chose not to count on Apollo metric cameraor panoramic images because only Lunar Orbiter imagesprovide a systematic coverage of all investigated mareareas. Compared to previous age determinations, our datafit spectral and lithological units and represent a majorimprovement in accuracy. In contrast, data from Boyce

Figure 3. Map of the distribution of basalt types in Oceanus Procellarum based on detailed mapping ofmorphology and spectral differences [Whitford-Stark and Head, 1980]. Map coverage: 0�W–80�W,30�S–60�N.


Figure

4.

Colorratiocomposite

based

onthreespectral

ratiosofClementine

imagingdata(756�

409/756+409onred,756/409ongreen,409/562onblue).

Whitelines

definespectral

unitsin

OceanusProcellarum,MareNubium,Mare

Cognitum,andMareInsularum.Formostoftheseunitsweperform

edcrater

size-

frequency

distributionmeasurements.Map

coverage:

�25�W

–80�W

,�30�S

–58�N

;latitude,

longitudegridis15��

15�wide.

Figure

5.

Ages

of

lunar

nearside

surface

units

asderived

from

crater

degradation[Boyce,

1976;BoyceandJohnson,1978].

Ages

inbillionyears.

Whitelines

outlinespectrally

homogeneousunitsthat

wehavedefined

for

OceanusProcellarum,MareNubium,MareCognitum,andMareInsularum

(also

seeFigure

3).Map

coverage:

�25�W

–80�W

,�30�S

–58�N

;latitude,

longitude

gridis15��

15�wide.


[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 5).[13] In this paper we report on ages of 86 basalt units in

Oceanus Procellarum, Mare Nubium, Mare Cognitum, andMare Insularum. In order to facilitate the discussion of ageswe did not assign type locality names to each unit. Instead,we use a simple letter/number system. The letter indicatesthe basin (P = Oceanus Procellarum, N = Mare Nubium, C =Mare Cognitum, IN = Mare Insularum) and the numberdescribes the unit within a basin. The numbering is consis-tent with the geologic maps of the Moon with oldest unitshaving lower numbers and younger units having highernumbers.

3. Results

3.1. Oceanus Procellarum

3.1.1. Geologic Setting[14] Located on the western lunar nearside, Oceanus

Procellarum is the largest expanse of exposed mare basaltsand is characterized by a non-circular shape (Figure 1).Despite the irregular shape it has been proposed thatOceanus Procellarum is located within a very ancient,3200 km large impact structure, the Procellarum basin[e.g., Wilhelms and McCauley, 1971; Whitaker, 1981;Wilhelms, 1987]. The existence of this Procellarum basinis not completely accepted because the geochemical argu-ments for such a basin as well as the identification of ringstructures of this basin are subject to alternative interpreta-tion [e.g., Spudis, 1993; Spudis and Schultz, 1985;Wilhelms, 1987].[15] However, Feldman et al. [2002] interpreted new

Lunar Prospector data to be consistent with a Procellarumbasin. Investigating the spatial distribution and concentra-tion indices of FeO, thorium, epi/thermal and fast neutronsusing spherical harmonic expansion analyses, Feldman etal. [2002] found that the dipole vectors and eigenvectorscluster closely to the center of the putative Procellarumbasin. On the basis of their work, the center of the dipoleaxes is at 14.1� latitude and 16.4� west longitude, the centerof the quadrupole axes is located at 24.6� latitude and 25.1�west longitude, both being close to the center of theProcellarum basin given by Whitaker [1981] at 23� latitudeand 15�west longitude. In addition, according to Feldman etal. [2002] the existence of a Procellarum basin is consistentwith a sharp decrease in intensities of FeO, thorium, andepi/thermal and fast neutrons at �50� from the center whichis close to the boundary of Whitaker’s [1981] Procellarumbasin. In summary, Feldman et al. [2002] concluded that agiant impact, i.e., the Procellarum impact event, is a goodcandidate mechanism for producing the observed asymme-tries in crustal thickness, center of mass/center of figureoffset, mare basalt distribution and surface mineralogy.Haskin [1998] argued that a giant Procellarum impact couldhave affected the development of the Th-rich ProcellarumKREEP Terrane (PKT) by removal of overlying crust ordeep injection of heat. On the basis of ejecta depositmodeling he found that the global distribution of high-Thmaterial is consistent with the distribution expected forImbrium ejecta deposits. Haskin et al. [2000] argued that

the Imbrium event did not form the high-Th ProcellarumKREEP Terrane, but rather distributed the material globally.[16] On the basis of the exposure of highland islands and

crater rims such as Flamsteed, Wilhelms and McCauley[1971] concluded that the thickness of mare basalts withinOceanus Procellarum is relatively thin. Estimated thicknessof mare basalts in Oceanus Procellarum are on the order of0–500 m for most regions, with some areas having basaltsup to >1500 m thick [DeHon, 1979; DeHon and Waskom,1976]. Recent studies based on new Clementine data areconsistent with these estimates [Dunkin et al., 2000; Bor-oughs and Spudis, 2001; Heather and Dunkin, 2002].[17] Basalts of Oceanus Procellarum exhibit a wide range

of spectral characteristics [e.g., Pieters, 1978; McCord etal., 1976; Whitaker, 1972; Heather and Dunkin, 2002],compositions [e.g., Lawrence et al., 2000; Lucey et al.,2000; Elphic et al., 2000, 2002], and ages [e.g., Boyce,1976; Boyce and Johnson, 1978; Schultz and Spudis, 1983;Wilhelms, 1987; Hiesinger et al., 2001]. On the basis of thesparseness of impact craters and detailed mapping on high-resolution image material, several authors [e.g., Schultz andSpudis, 1983; Wilhelms, 1987] concluded that very youngbasalts, probably of Copernican age, are exposed in thevicinity of the crater Lichtenberg, the Flamsteed ring, andseveral other areas in Oceanus Procellarum and MareSmythii. Hence it has been speculated that the basalts thatterminate the period of active lunar volcanism occur withinOceanus Procellarum or the young mare on the easternnearside, i.e., within Mare Smythii [Schultz and Spudis,1983].3.1.2. Discussion of Units[18] We dated 60 spectrally distinctive basalt units in

Oceanus Procellarum (Figure 7). The outlines of our spec-tral units are generally in good agreement with the spectralunits of Pieters [1978] but our map also reveals additionaldetail as it subdivides some of the Pieters classes. Pieters[1978] used the spectral information of the UV/VIS-ratio,the albedo, and the depths of the 1-mm and 2-mm absorptionbands in order to establish a system of lunar basalt types(Table 1). Several units (P3, P25, P27, P29, P33, P34, P36,P37, P42, P54) were classified as ‘‘undivided’’ by Pieters[1978] and 11 additional units (P11, P14, P15, P16, P20,P21, P44, P48, P50, P51, P60) were at least partly mappedas ‘‘undivided’’. From these units, P11 and P50 were alsocharacterized as hDSP basalt and units P14, P15, P16, P20,and P44 were also classified as mISP basalts. Unit P21shows characteristics of ‘‘undivided’’, mISP, and LBG-basalts, and unit P48 is mapped as ‘‘undivided’’ andLBG- basalts. Finally ‘‘undivided’’ and hDSA basalts arepresent in unit P51, and also in unit P60, which has anadditional component of LBG- basalts. Two units (P1, P28)exhibit hDWA characteristics, five units (P32, P47, P52,P57, P58) are classified as hDSA basalts, and four units(P17, P38, P45, P46) are mapped as hDSP basalts. Units P6,P35, P43, and P49 are described as HDSA basalts in themap of Pieters [1978], nine units (P2, P12, P13, P18, P19,P22, P23, P30, P41) are mapped as mISP basalts, four units(P4, P10, P31, P40) are shown as LBG- basalts, and unitsP5 and P7 are LBSP basalts. The spectral map of lunarbasalt types [Pieters, 1978] indicates that unit P8 consists ofmIG-, and unit P56 of LISP basalts. According to this map,several units can be characterized by two or more spectral


basalt types. In unit P9 mISP and LBG- basalts are exposed,in unit P24 we found hDSA, hDSP, and mISP basalts, andin unit P26, P39, and P53 HDSA and hDSA basalts occur.Finally, units P55 and P59 are described as LBG- and hDSAbasalts. A comparison of our units with the spectral units ofPieters [1978] and several geologic maps is provided byTable 2.[19] The geologic map of the nearside of the Moon

[Wilhelms and McCauley, 1971] covers all basalts in Oce-anus Procellarum, except units P1, P21, P22, P28, P29, P37,P55, and P57. In this map several units are mapped asImbrian (Im) mare material (P2, P3, P4, P5, P6, P7, P10,P13, P16, P26, P30, P31, P32, P33, P36, P40, P41, P44,P46, P47, P48, P59, P60), and several units are mapped asdarker Eratosthenian (Em) mare material (P8, P12, P17,P18, P23, P34, P42, P54). In this map 21 units are mappedas Im and Em basalts (P9, P11, P14, P15, P19, P20, P24,P25, P27, P35, P38, P39, P43, P45, P49, P50, P51, P52,P53, P56, P58). The geologic map of Scott et al. [1977]covers 29 basalt units of western Oceanus Procellarum (P3,P9, P10, P12, P14, P15, P16, P18, P19, P20, P21, P22, P23,P25, P26, P29, P30, P32, P33, P34, P36, P37, P39, P44,P49, P51, P53, P58, P60) and indicates that all basaltsexposed in this region are of Imbrian and/or Eratosthenianage (EIm), consistent with the map of Wilhelms and

McCauley [1971]. Scott et al. [1977] interpret albedo andcolor differences of these basalts to be related to differencesin age and composition. Northern units in Oceanus Procel-larum are covered by the geologic map of Lucchitta [1978]and this maps shows that units P1, P10, P13, and P28 are ofImbrian age (Im) and that units P6 and P56 are of Eratos-thenian age (Em). In this map, Eratosthenian basalts arecharacterized as dark, smooth, flat surfaces with bluish colorand a lower albedo than Imbrian basalts. In the geologicmap of McCauley [1973] units P3, P15, P24, P33, P36, andP44 are classified as extensive featureless Imbrian mareplains material (Im) with albedos of 0.08–0.10. Units P18,P23, and P25 consist of Imbrian mare material (Im) anddark Eratosthenian mare material (Em) and units P34 andP54 are mapped as Eratosthenian in age (Em). According toMcCauley [1973], the albedo of Eratosthenian basalts is<0.08, and on the basis of Earth-based full-Moon imagestheir boundaries are difficult to delineate. An Eratosthenianage (Em) was also found for unit P42 in the map of Wilshire[1973]. Wilshire [1973] described this geologic unit Em assmooth, level plains with low albedo (0.08–0.09) andinterpreted it as basaltic lava flows formed by eruptionfrom fissures.[20] Our spectrally defined units (P12, P15, P16, P18,

P19, P20, P24, P44) in the Hevelius quadrangle are mostly

Figure 6. Comparison between Lunar Orbiter IV 109H2 (a) and Clementine (b) image of the RimaBode area for the purpose of crater size-frequency distribution measurements. Two examples of cratersclearly measurable in the lower-sun Lunar Orbiter image are either barely visible or saturated in the high-sun Clementine image. This would yield incorrect crater statistics and less reliable ages. The LunarOrbiter image has been subjected to a fast Fourier transformation in order to remove the stripes and hasbeen map-projected. A detailed description of the image processing of the Lunar Orbiter data is given byGaddis et al. [2001].


Figure

7.

Spatialdistributionofmodelages

forspectrallydefined

unitsinOceanusProcellarum.a:U

SGSshaded

reliefmap,

simplecylindricalmap

projection.Spectralunitsareoutlined

inblack.b:Sketch

map

ofOceanusProcellarum

showingunit

numbersandmodelages

inbillionyears(alsoseeTable3).Cratersize-frequency

distributionmeasurementswereperform

edfortheareashighlightedin

darkgray.Black

areasarenon-m

arematerialsorhavebeenexcluded

from

thisinvestigation.


Imbrian mare materials of different albedo (Ipm, Ipmd)[McCauley, 1967]. The albedo of the geologic unit Ipmd is<0.06 and is lower than that of Ipm. Parts of units P12,P18, and P19 also are mapped as Copernican CavaleriusFormation (Cca). The Reiner Gamma Formation which isexposed within unit P16 was mapped as Copernican in age(Cre) and has an intermediate albedo of 0.08–0.09[McCauley, 1967]. Ulrich [1969] mapped the J. Herschelquadrangle of the Moon and in this map unit P56 consistsof Eratosthenian/Upper Imbrian mare material with verylow albedo (EIm). Units P9, P10, P13, P31, P39, P40,P53, P58, and P59 are covered by the geologic map ofScott and Eggleton [1973], which indicates an Imbrian(Im) age for these mare units. Parts of several units (P9,P39, P53, P58) also consist of Eratosthenian mare material(Em). Compared to Imbrian mare material (Im), Eratos-thenian mare material (Em) is darker (<0.08–0.085) andbluer. The geologic map of Moore [1965] which is basedon telescopic observations also shows characteristics ofImbrian mare material (Ipm) for numerous basalt units (P4,P5, P7, P40, P43, P48, P51, P59, P60). Similarly, unitsP30, P39, P59 and P60 are shown as Imbrian in age and arange of albedos in the map of Moore [1967]. In this mapmost of the units (P14, P22, P26, P32, P49, P53) consistof Imbrian mare material (Ipm) and dark Imbrian marematerial (Ipmd). Regions south and east of Lichtenberg(P53) are mapped as low albedo Copernican mare material(Cmd). Titley [1967] mapped unit P17 as Imbrian (Ipm3,Ipm4) in age, with Ipm4 being the very darkest memberand Ipm3 being the dark member of the ProcellarumGroup. In an older map by Marshall [1963] unit P17was mapped as Pm, hence being part of the so-calledProcellarian System. Several other units (P8, P11, P27,P35, P38, P45, P46, P47, P54) are also attributed to theProcellarian System. This stratigraphic system dates backto early work of Shoemaker and Hackman [1962] whodefined the Imbrian System as equivalent to the immensesheet of material around the Imbrium basin and theProcellarian System to consist of mare material that isyounger than the Imbrium ejecta sheet. The same strati-graphic system is also used in the map of Hackman[1962], which shows that all covered basalts (i.e., unitsP2, P5, P6, P35, P41, P47, P50, P52) were erupted duringthe Procellarian System. However, for a variety of reasonsthis stratigraphic system was quickly abandoned [Wil-helms, 1987] and was not used in later maps. As the

1:1,000,000 USGS geologic maps of the Moon havenever been updated since the end of the initial mappingprogram, the available maps of significant parts of thelunar nearside, i.e., the Kepler and the Letronne quad-rangles of the Moon show stratigraphic systems that arelong out of date.3.1.3. Ages[21] On the basis of our crater size-frequency distribu-

tions we conclude that basalt model ages in OceanusProcellarum range from �1.2 to �3.93 b.y. (Figure 7;Table 3). Sixteen units, that is �25% of all dated units,show characteristic kinks in their crater size-frequencydistributions which have been interpreted by Neukum andHorn [1976] to indicate resurfacing, that is flooding withsubsequent lavas. These kinks in the crater size-frequencydistribution form when a new flow unit preferentially coverssmaller craters of an older surface while larger, uncoveredcraters of the older surface are still detectable after the flowunit has been emplaced [Neukum and Horn, 1976]. It hasbeen shown that the diameters at which these kinks occurcan be used to estimate the thickness of the resurfacing flowunit [e.g., Neukum and Horn, 1976; Hiesinger et al., 2002].On the basis of their study of 58 units in Oceanus Procella-rum, Cognitum, Nubium, Insularum, Imbrium, Tranquilli-tatis, and Humorum, Hiesinger et al. [2002] found that theaverage thickness of late-stage flows in these basins is�30–60 m.[22] In Oceanus Procellarum we dated 60 units. Using the

chronostratigraphic system of Neukum and Ivanov [1994],basalts of 19 units are Imbrian in age. For 3 of these 19units, we were able to detect late-stage flooding eventsduring the Imbrian Period. Thirty-six units in OceanusProcellarum are Eratosthenian in age, with 13 of them alsoshowing older Imbrian ages. Finally, if we apply thechronostratigraphic system of Neukum and Ivanov [1994],5 basalt units are of Copernican age. As discussed earlier,these basalts would be of Eratosthenian age if one uses thechronostratigraphic system of Wilhelms [1987] or Stofflerand Ryder [2001] (Figure 2).[23] Earlier attempts to measure the ages of surface units

relied on crater degradation processes and rates [e.g.,Boyce, 1976; Boyce and Johnson, 1978]. Crater degrada-tion ages of Boyce [1976] and Boyce and Johnson [1978]were performed for 1/4� squares (�8 km), were interpo-lated by spatial filtering into a continuous image, and donot necessarily fit lithological or spectral units (Figure 5).These estimates were very useful when detailed spectralunits were not available. The new data, however, usespecifically defined units and do not require spatial filter-ing. Units P1, P18, P20, P21, P22, P23, P29, P30, P31,P32, P36, P38, P41, P44, P48, P50, and P59 exhibit asingle degradation age; all other units show at least two,and some units (P10, P13) up to four different ages in themap of Boyce and Johnson [1978]. The implication is thatages derived from the map of Boyce and Johnson [1978]can vary up to 1.25 b.y. for a single spectral unit. Innumerous cases (P2, P3, P4, P8, P9, P11, P13, P14, P17,P24, P26, P27, P34, P35) the ages of Boyce [1976] andBoyce and Johnson [1978] give us an upper and lowerboundary with our ages either right between or close toone or the other boundary (Table 3). In other cases (P1,P5, P7, P10, P15, P16, P19, P33), the most abundant age

Table 1. Lunar Mare Basalt Typesa

UnitUV/VISRatio Albedo

1 mmAbsorption

2 mmAbsorption

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].


of Boyce [1976] and Boyce and Johnson [1978] is similarto our age of a particular unit. Generally we find a goodagreement of our ages with ages of Boyce [1976] andBoyce and Johnson [1978] for units of Imbrian age. Onlyunits P6 and P12 appear to be older than in the maps ofBoyce [1976] and Boyce and Johnson [1978]. Ages ofunits that are young according to our data, are systemat-ically overestimated in age (older) in the Boyce map (P25,P28, P39, P43, P46, P47, P49, P51, P55, P56, P57, P58,P60), and we find less agreement for younger units, i.e.,late Eratosthenian and Copernican units. Units P40, P45,P52, and P53 show evidence for resurfacing with bothages bracketing or being similar to the Boyce ages.Finally, units P37, P42, and P54 are not covered in theBoyce map.[24] Units P1, P2, P3, P4, P5, and P7 were mapped as

Imbrian in age in the geologic maps of Wilhelms andMcCauley [1971], Moore [1965], McCauley [1967], andLucchitta [1978]. Our data confirm an Imbrian age for theseunits and also confirm Wilhelms and McCauley’s Imbrianages for units P10, P14, and P16. Lucchitta [1978] mappedunit P6 as Eratosthenian and Scott et al. [1977] mappedseveral units (P3, P9, P10, P12, P14, P15, P16, P18, P19) asImbrian and/or Eratosthenian in age. Our data only showImbrian ages and no Eratosthenian ages for these units. Forunit P8, P9, P11, P12, P14, P15, P17, P18, and P19Eratosthenian or Eratosthenian/Imbrian ages are shown inthe map of Wilhelms and McCauley [1971]. However, ourdata indicate that these units are Imbrian in age, consistentwith the mapping ofMcCauley [1967, 1973],Moore [1967],Titley [1967], Scott and Eggleton [1973], and Lucchitta[1978]. Our crater counts do not confirm an Eratosthenianage for parts of unit P9 as shown in the map of Scott andEggleton [1973]. Our data do not agree with the map of

Table 2. Comparison of Spectral Units Defined in This Study

With the Spectral Map of Pieters [1978] and Geologic Maps

Unit Pieters [1978] Geological Map Referencesa

CognitumC5 mIG- Ipm; Im; Em 3; 12C4 mIG- Ipm; Im 3; 12C3 mIG- Ipm; Im 3; 12C2 mIG-, u Ipm; Em; Im 3; 12C1 mIG- Ipm; Im; Em 3; 12

InsularumIN4 mIG- Im; Ipm 8; 12IN3 u Im; Ipm; Pm 1; 8; 12IN2 mIG- Im; Em; Ipm; Pm 1; 3; 8; 12IN1 DM Im; Ipm 8; 12

NubiumN17 mIG- Im; Em; Ipm; Ipm1; Ipm2 3; 5; 12N16 mIG- Im; Em; Ipm; Ipmd; Im2 10; 12; 16N15 mIG- Im; Em; Ipm1; Ipm2; Ipm3 5; 12N14 mIG-, u Em; Ipm 3; 12N13 u, mIG-, hDG- Em; Im; Ipm; Im1; Im2; Em 3; 12; 16N12 u Im; Ipm 10; 12N11 u, mIG-, hDG- Im; Ipm; Ipmd; Em 10; 12; 16N10 mIG- Em; Im; Ipm; Ipm2 5; 12N9 u Em; Im2 12; 16N8 LBSP, LBG- Im; Ipm1; Ipm2; Ipm3 5; 12N7 mIG- Im; Em; Ipm2 5; 12N6 mIG- Em; Ipm; Ipm3 5; 12N5 u Em; Ipm 5; 12N4 u Im; Em; Ipm 5; 12N3 u Im; Em; Ip; Im2; Im1 12; 16N2 u Em; Ipm2; Ipm4 5; 12N1 LBG- Im; Ipm; Ipm1; Ipm2 5; 12

Oceanus ProcellarumP60 hDSA, u, LBG- Im; Ipm; EIm 4; 9; 12; 17P59 hDSA, LBG- Im; Ipm 4; 9; 12; 15P58 hDSA Em; Im; EIm 12; 15; 17P57 hDSAP56 LISP Em; Im; EIm 11; 12; 18P55 LBG-, hDSAP54 u Em; Pm 2; 12; 13P53 hDSA, HDSA Em; Im; Cmd; Ipm; Ipmd; EIm 7; 9; 12; 17P52 hDSA Em; Im; Pm 1; 12P51 hDSA, u Im; Em; Ipm; EIm 4; 12; 17P50 hDSP, u Em; Im; Pm 1; 12P49 HDSA Im; Em; Ipmd; Ipm; EIm 9; 12; 17P48 LBG-, u Im; Ipm 4; 12P47 hDSA Im; Pm 1; 2; 12P46 hDSP Im; Pm 2; 12P45 hDSP Im; Em; Pm 2; 12P44 mISP, u Im; Ipm; Ipmd; EIm 6; 12; 13; 17P43 HDSA Im; Em; Ipm 4; 12P42 u Em 12; 14P41 mISP Im; Pm 1; 12P40 LBG- Im; Ipm 4; 12; 15P39 HDSA, hDSA Em; Im; Ipm; EIm 9; 12; 15; 17P38 hDSP Im; Em; Pm 2; 12P37 u EIm 17P36 u Im; EIm 12; 13; 17P35 HDSA Im; Em; Pm 1; 2; 12P34 u Em; EIm 12; 13; 17P33 u Im; EIm 12; 13; 17P32 hDSA Im; Ipm; Ipmd; EIm 9; 12; 17P31 LBG- Im 12; 15P30 mISP Im; Ipm; EIm 9; 12; 17P29 u EIm 17P28 hDWA Im 18P27 u Im; Em; Pm 2; 12P26 HDSA, hDSA Im; Ipm; Ipmd; EIm 9; 12; 17P25 u Im; Em; EIm 12; 13; 17P24 hDSA, mISP, hDSP Im; Em; Ipm; Ipmd 6; 12; 13P23 mISP Em; Im; EIm 12; 13; 17P22 mISP Ipmd; Ipm; EIm 9; 17

Table 2. (continued)

Unit Pieters [1978] Geological Map Referencesa

P21 LBG-, mISP, u EIm 17P20 mISP, u Em; Im; Ipm; Ipmd; EIm 6; 12; 17P19 mISP Em; Im; Ipmd; Ipm; Cca; EIm 6; 12; 17P18 mISP Em; Ipmd; Cca; Im; EIm 6; 12; 13; 17P17 hDSP Em; Pm; Ipm3; Ipm4 2; 7; 12P16 mISP, u Im; Ipm; Cre; EIm 6; 12; 17P15 mISP, u Em; Im; Ipm; Ipmd; EIm 6; 12; 13; 17P14 mISP, u Em; Im; Ipmd; Ipm; EIm 9; 12; 17P13 mISP Im 9; 12; 18P12 mISP Em; Ipmd; Ipm; Cca; EIm 6; 12; 17P11 hDSP, u Im; Em; Pm 2; 12P10 LBG- Im; EIm 12; 15; 17; 18P9 mISP, LBG- Em; Im; EIm 12; 15; 17P8 mIG- Em; Pm 2; 12P7 LBSP Im; Ipm 4; 12P6 HDSA Im; Pm; Em 1; 12; 18P5 LBSP Im; Ipm; Pm 1; 4; 12P4 LBG- Im; Ipm 4; 12P3 u Im; EIm 12; 13; 17P2 mISP Im; Pm 1; 12P1 hDWA Im 18

a1, Hackman [1962] (I-355); 2, Marshall [1963] (I-385); 3, Eggleton[1965] (I-458); 4, Moore [1965] (I-465); 5, Trask and Titley [1966] (I-485);6,McCauley [1967] (I-491); 7, Titley [1967] (I-495); 8, Schmitt et al. [1967](I-515); 9,Moore [1967] (I-527); 10, Howard and Masursky [1968] (I-566);11, Ulrich [1969] (I-604); 12, Wilhelms and McCauley [1971] (I-703); 13,McCauley [1973] (I-740); 14, Wilshire [1973] (I-755); 15, Scott andEggleton [1973] (I-805); 16 Holt [1974] (I-822); 17, Scott et al. [1977] (I-1034); 18, Lucchitta [1978] (I-1062).


Table

3.ComparisonofAges

forBasaltsin

OceanusProcellarum

a

Unit

Lunar

Orbiter

Image

Area,

km

2

Crater

Retention

AgeN(1)

Error

Model

Age,

b.y.

Error,

b.y.

Boyce

[1976]

Wilhelms

and

McC

auley

[1971]

Moore

[1965]

Moore

[1967]

Marshall

[1963]

Hackman

[1962]

McC

auley

[1967]

McC

auley

[1973]

Wilshire

[1973]

Titley

[1967]

Scott

and

Eggleton

[1973]

Ulrich

[1969]

Scott

etal.

[1977]

Lucchitta

[1978]

P60

IV157H3

1429

1.01E-03

+0.26E-03/

�0.30E-03

1.2

+0.32/

�0.35

3.2;

2.5;

3.5

ImIpm

Ipm

EIm

P59

IV158H1

789

1.01E-03

+0.34E-03/

�0.35E-03

1.21

+0.40/

�0.42

3.2

ImIpm

Ipm

Im

P58

IV158H2

2551

1.11E-03

+0.17E-03/

�0.21E-03

1.33

+0.19/

�0.25

3.2;

3.5;

3.65

Em;Im

Em,Im

EIm

P57

IV150H1

2631

1.12E-03

+0.41E-03/

�0.08E-03

1.33

+0.49/

�0.08

2.5;

3.2;

3.5

P56

IV170H3

1765

1.25E-03

+0.41E-03/

�0.46E-03

1.49

+0.49/

�0.55

2.5;

3.2

Em;Im

EIm

Em

P55

IV170H3

1057

1.40E-03

+0.44E-03/

�0.46E-03

1.67

+0.52/

�0.55

3.2;

3.5

P54

IV149H2

1118

1.40E-03

+0.49E-03/

�0.50E-03

1.67

+0.58/

�0.59

no

age

Em

Pm

Em

P53

IV170H1

5886

1.40E-03/

2.88E-03

+0.26E-03/

�0.09E-03

+0.20E-03/

�0.20E-03

1.68/

3.18

+0.30/

�0.12

+0.08/

�0.10

2.5;

3.2

Em;Im

Cmd,

Ipm,

Ipmd

Em,Im

EIm

P52

IV150H2

4075

1.45E-03/

1.17E-02

+0.24E-03/

�0.26E-03

+0.67E-03/

�2.57E-03

1.73/

3.72

+0.29/

�0.31

+0.08/

�0.05

2.5;

3.2

Em;Im

Pm

P51

IV144H2

1357

1.55E-03

+0.31E-03/

�0.29E-03

1.85

+0.37/

�0.34

3.2;

2.5

ImEm

Ipm

EIm

P50

IV133H1

2000

1.56E-03

+0.48E-03/

�0.20E-03

1.87

+0.56/

�0.25

2.5

Em;Im

Pm

P49

IV157H2

4822

1.63E-03

+0.31E-03/

�0.36E-03

2.01

+0.37/

�0.43

2.5;

3.2;

3.5

Im;Em

Ipmd;

Ipm

EIm

P48

IV144H3

1091

1.71E-03

+0.39E-03/

�0.45E-03

2.04

+0.46/

�0.54

3.5

ImIpm

P47

IV144H1

1665

1.74E-03

+0.55E-03/

�0.32E-03

2.08

+0.65/

�0.39

2.5;

3.2

ImPm

Pm

P46

IV132H3

1076

1.74E-03

+0.50E-03/

�0.56E-03

2.08

+0.58/

�0.67

3.2;

2.5

ImPm

P45

IV137H3

5666

1.75E-03/

1.03E-02

+0.24E-03/

�0.11E-03

+0.54E-02/

�0.20E-02

2.09/

3.70

+0.28/

�0.14

+0.08/

�0.05

2.5;

3.2

Im;Em

Pm

P44

IV156H3

3245

1.77E-03

+0.39E-03/

�0.39E-03

2.11

+0.47/

�0.47

3.2

ImIpm;

Ipmd

ImEIm

P43

IV138H2

1426

1.78E-03

+0.72E-03/

�0.77E-03

2.12

+0.82/

�0.91

2.5;

3.2;

3.5

Im;Em

Ipm


Table

3.(continued)

Unit

Lunar

Orbiter

Image

Area,

km

2

Crater

Retention

AgeN(1)

Error

Model

Age,

b.y.

Error,

b.y.

Boyce

[1976]

Wilhelms

and

McC

auley

[1971]

Moore

[1965]

Moore

[1967]

Marshall

[1963]

Hackman

[1962]

McC

auley

[1967]

McC

auley

[1973]

Wilshire

[1973]

Titley

[1967]

Scott

and

Eggleton

[1973]

Ulrich

[1969]

Scott

etal.

[1977]

Lucchitta

[1978]

P42

IV156H2

698

1.78E-03

+0.49E-03/

�0.53E-03

2.12

+0.58/

�0.63

no

age

Em

Em

P41

IV138H1

1786

1.79E-03

+0.64E-03/

�0.72E-03

2.13

+0.75/

�0.85

2.5

ImPm

P40

IV158H1

3266

1.79E-03/

3.80E-03

+0.48E-03/

�0.50E-03

+0.56E-03/

�0.66E-03

2.14/

3.40

+0.56/

�0.60

+0.07/

�0.12

3.2;

2.5;

3.5

ImIpm

Im

P39

IV163H2

2298

1.83E-03

+0.46E-03/

�0.52E-03

2.19

+0.53/

�0.62

2.5;

3.2;

3.5

Em;Im

Ipm

Im;

Em

EIm

P38

IV143H2

1127

1.93E-03

+0.47E-03/

�0.49E-03

2.31

+0.53/

�0.60

3.2

Im;Em

Pm

P37

IV183H1

1091

1.99E-03/

5.82E-03

+0.30E-03/

�0.25E-03

+0.86E-03/

�0.75E-03

2.38/

3.56

+0.34/

�0.31

+0.04/

�0.04

no

age

EIm

P36

IV156H3

1082

2.02E-03/

2.54E-02

+0.38E-03/

�0.39E-03

+1.94E-02/

�0.47E-02

2.41/

3.86

+0.43/

�0.46

+0.09/

�0.04

3.2

ImIm

EIm

P35

IV143H3

3441

2.13E-03

+0.26E-03/

�0.14E-03

2.54

+0.29/

�0.17

3.2;

2.5

Im;Em

Pm

Pm

P34

IV156H3

1118

2.17E-03

+0.69E-03/

�0.71E-03

2.59

+0.59/

�0.85

3.2;

2.5

Em

Em

EIm

P33

IV156H3

1855

2.18E-03/

7.51E-03

+0.15E-03/

�0.15E-03

+1.73E-03/

�1.40E-03

2.59/

3.63

+0.18/

�0.17

+0.04/

�0.06

2.5;

3.2

ImIm

EIm

P32

IV157H3

IV162H3

6475

2.32E-03

+0.33E-03/

�0.15E-03

2.76

+0.30/

�0.18

2.5

ImIpm;

Ipmd

EIm

P31

IV158H1

1719

2.44E-03/

1.17E-02

+0.69E-03/

�0.16E-03

+0.58E-02/

�0.22E-02

2.88/

3.72

+0.39/

�0.17

+0.08/

�0.04

3.2

ImIm

P30

IV169H3

583

2.46E-03

+0.36E-03/

�0.32E-03

2.9

+0.26/

�0.35

2.5

ImIpm

EIm

P29

IV183H2

1546

2.49E-03

+0.37E-03/

�0.32E-03

2.93

+0.25/

�0.34

3.2

EIm

P28

IV183H3

840

2.51E-03/

1.14E-02

+0.66E-03/

�0.33E-03

+0.66E-02/

�0.21E-02

2.94/

3.72

+0.34/

�0.34

+0.08/

�0.05

3.65;

3.5

Im

P27

IV132H3

2135

2.52E-03

+0.88E-03/

�0.17E-03

2.96

+0.38/

�0.17

3.2;

2.5;

3.5

Im;Em

Pm


Table

3.(continued)

Unit

Lunar

Orbiter

Image

Area,

km

2

Crater

Retention

AgeN(1)

Error

Model

Age,

b.y.

Error,

b.y.

Boyce

[1976]

Wilhelms

and

McC

auley

[1971]

Moore

[1965]

Moore

[1967]

Marshall

[1963]

Hackman

[1962]

McC

auley

[1967]

McC

auley

[1973]

Wilshire

[1973]

Titley

[1967]

Scott

and

Eggleton

[1973]

Ulrich

[1969]

Scott

etal.

[1977]

Lucchitta

[1978]

P26

IV158H1

2309

2.53E-03/

4.62E-03

+0.61E-03/

�0.33E-03

+1.06E-03/

�0.69E-03

2.96/

3.49

+0.32/

�0.34

+0.06/

�0.07

2.5;

3.2

ImIpm;

Ipmd

EIm

P25

IV156H3

2176

2.52E-03/

7.46E-03

+0.77E-03/

�0.16E-03

+1.11E-03/

�1.39E-03

2.96/

3.62

+0.35/

�0.17

+0.04/

�0.05

3.5;

3.2

Im;Em

Im;

Em

EIm

P24

IV150H1

6330

2.58E-03/

1.27E-02

+0.58E-03/

�0.17E-03

+0.84E-02/

�0.18E-02

3.00/

3.74

+0.28/

�0.15

+0.09/

�0.04

3.2;

2.5;

3.5

Im;Em

Ipm;

Ipmd

Im

P23

IV157H1

513

2.67E-03

+0.19E-03/

�0.18E-03

3.07

+0.11/

�0.14

2.5

Em

Em;

ImEIm

P22

IV169H3

925

2.68E-03

+0.40E-03/

�0.18E-03

3.08

+0.18/

�0.14

2.5

Ipmd;

Ipm

EIm

P21

IV183H2

2725

2.75E-03

+0.40E-03/

�0.19E-03

3.12

+0.16/

�0.13

3.5

EIm

P20

IV157H1

1702

2.74E-03/

3.94E-02

+0.50E-03/

�0.50E-03

+2.61E-02/

�0.74E-02

3.12/

3.93

+0.18/

�0.45

+0.08/

�0.03

3.2

Em;Im

Ipm;

Ipmd

EIm

P19

IV162H2

IV169H2

8051

3.27E-03

+0.23E-03/

�0.22E-03

3.31

+0.05/

�0.06

3.2;

3.5;

2.5

Em;Im

Ipmd;

Ipm;

Cca

EIm

P18

IV162H1

1874

3.31E-03

+0.49E-03/

�0.23E-03

3.32

+0.08/

�0.06

2.5

Em

Ipmd;

Cca

Em;

ImEIm

P17

IV132H2

2909

3.30E-03

+0.79E-03/

�0.42E-03

3.32

+0.08/

�0.14

3.5;

3.2

Em

Pm

Ipm3;

Ipm4

P16

IV157H1

1108

3.36E-03

+0.60E-03/

�0.13E-03

3.33

+0.08/

�0.05

3.2;

3.5;

2.5

ImIpm;

Cre

EIm

P15

IV156H3

1662

3.43E-03/

2.75E-02

+0.51E-03/

�0.44E-03

+0.41E-02/

�0.51E-02

3.34/

3.87

+0.08/

�0.11

+0.02/

�0.03

3.2;

3.5

Em;Im

Ipm;

Ipmd

ImEIm

P14

IV162H3

IV169H2

IV169H3

7939

3.50E-03/

7.35E-03

+0.37E-03/

�0.39E-03

+2.85E-03/

�1.38E-03

3.36/

3.62

+0.05/

�0.09

+0.07/

�0.05

2.5;

3.2;

3.5

Em;Im

Ipmd;

Ipm

EIm

P13

IV163H2

1120

3.76E-03

+1.19E-03/

�0.70E-03

3.4

+0.11/

�0.15

3.65;

3.5;

3.2;

3.75

ImIm

Im

P12

IV157H1

1807

3.97E-03

+1.09E-03/

�0.51E-03

3.42

+0.10/

�0.07

2.5;

3.2

Em

Ipmd;

Ipm;

Cca

EIm


Unit

Lunar

Orbiter

Image

Area,

km

2

Crater

Retention

AgeN(1)

Error

Model

Age,

b.y.

Error,

b.y.

Boyce

[1976]

Wilhelms

and

McC

auley

[1971]

Moore

[1965]

Moore

[1967]

Marshall

[1963]

Hackman

[1962]

McC

auley

[1967]

McC

auley

[1973]

Wilshire

[1973]

Titley

[1967]

Scott

and

Eggleton

[1973]

Ulrich

[1969]

Scott

etal.

[1977]

Lucchitta

[1978]

P11

IV143H2

2383

4.00E-03

+1.12E-03/

�0.52E-03

3.43

+0.09/

�0.08

3.2;

3.5

Im;Em

Pm

P10

IV175H3

8419

4.08E-03

+0.83E-03/

�0.52E-03

3.44

+0.07/

�0.07

3.5;

3.2;

3.65;

2.5

ImIm

EIm

Im

P9

IV170H2

3808

4.45E-03

+0.71E-03/

�0.56E-03

3.47

+0.08/

�0.06

3.2;

3.5

Em;Im

Em;

ImEIm

P8

IV132H3

2525

4.44E-03

+1.32E-03/

�0.83E-03

3.47

+0.08/

�0.09

2.5;

3.5;

3.2

Em

Pm

P7

IV144H3

1774

4.48E-03

+1.20E-03/

�1.08E-03

3.48

+0.07/

�0.14

3.5;

3.2

ImIpm

P6

IV144H1

884

4.52E-03

+1.68E-03/

�1.09E-03

3.48

+0.10/

�0.14

3.2;

2.5

ImPm

Em

P5

IV138H2

1792

4.57E-03

+1.30E-03/

�0.67E-03

3.48

+0.08/

�0.06

3.5;

3.2;

2.5

ImIpm

Pm

P4

IV151H1

1327

4.49E-03/

1.29E-02

+1.19E-03/

�0.84E-03

+0.67E-02/

�0.39E-02

3.48/

3.74

+0.07/

�0.10

+0.07/

�0.07

3.65;

3.5;

3.2

ImIpm

P3

IV149H3

718

5.26E-03

+1.97E-03/

�0.98E-03

3.53

+0.09/

�0.07

3.2;

3.5;

2.5

ImIm

EIm

P2

IV144H1

862

6.10E-03

+2.10E-03/

�1.47E-03

3.57

+0.08/

�0.08

3.2;

3.5;

2.5

ImPm

P1

IV183H3

2269

6.47E-03

+1.53E-03/

�1.62E-03

3.59

+0.05/

�0.09

3.5

Im

aSee

textfordetails.

Table

3.(continued)


McCauley [1967] that units P12, P16, P18, and P19 arepartially of Copernican age, nor with the map of McCauley[1973] that unit P18 is partially of Eratosthenian age.[25] Several units for which we determined an Eratos-

thenian age (P23, P34, P42, P54) are also mapped asEratosthenian in age in the map of Wilhelms and McCau-ley [1971]. Numerous units exhibit Eratosthenian andImbrian ages in this map, and with our crater size-frequency distribution measurements we obtained Eratos-thenian ages for all these units (P20, P24, P25, P27, P35,P38, P39, P43, P45, P49, P50, P51, P52, P53). Wilhelmsand McCauley [1971] mapped 12 units as Imbrian in age,but our data indicate an Eratosthenian age for these units(P26, P30, P31, P32, P33, P36, P40, P41, P44, P46, P47,P48). The geologic map of Scott et al. [1977] attributesEratosthenian and/or Imbrian ages to units that we foundto be of Eratosthenian age (P20, P21, P22, P23, P25,P26, P29, P30, P32, P33, P34, P36, P37, P39, P44, P49,P51, P53). Several other geologic maps show Imbrianages for the units that, according to our crater counts, areEratosthenian in age, i.e., units P20–P55. For example,Moore [1965] found Imbrian ages for units P40, P43,P48, and P51 and in the map of Moore [1967] 6 units(P22, P26, P30, P32, P39, P49) show Imbrian ages andone unit (P53) is partially Imbrian and Copernican in age.Imbrian ages are also indicated for units P20, P24, andP44 in the map of McCauley [1967], and for unit P28[Lucchitta, 1978], as well as for units P31, P39, P40, andP53 [Scott and Eggleton, 1973]. However, significantparts of units P39 and P53 were also mapped as Eratos-thenian in age, consistent with our dating. Finally, in themap of McCauley [1973], units P24, P33, P36, and P44are Imbrian and units P23 and P25 are Imbrian andEratosthenian in age. Our crater counts confirm anEratosthenian age for unit P34 and unit P54. The mapof Wilshire [1973] indicates an Eratosthenian age for unitP42, consistent with our age for this unit. A summary ofthis discussion of our units in the context of the geolog-ical maps of the U. S. Geological Survey and the Boyceages is given in Table 3.[26] As mentioned earlier, for the application of terms like

‘‘Copernican’’ or ‘‘Eratosthenian’’ in an absolute sense, thereader must be aware that there is no formal definition of theCopernican system [Wilhelms, 1987] and that the chrono-stratigraphic systems of different authors vary in the begin-ning of the Copernican system [e.g., Wilhelms, 1987;Neukum and Ivanov,1994; Stoffler and Ryder, 2001]. Thesedifferences are reviewed elsewhere [e.g., Hiesinger et al.,2000; Stoffler and Ryder, 2001] and for this study weadopted the chronostratigraphic system of Neukum andIvanov [1994]. According to this chronostratigraphic sys-tem, our crater counts revealed Copernican ages for 5 units(P56, P57, P58, P59, P60). Application of the Wilhelms[1987] and the Stoffler and Ryder [2001] models wouldindicate Eratosthenian ages for these units. These units havebeen mapped partially as Eratosthenian and Imbrian (P56,P58) or as Imbrian (P59, P60) by Wilhelms and McCauley[1971]. Imbrian ages for units P59 and P60 are also shownin the maps of Moore [1965, 1967] and for units P58 andP59 in the map of Scott and Eggleton [1973]. This map alsoshows an Eratosthenian age for unit P58. Ulrich [1969] andLucchitta [1978] found Eratosthenian ages for unit P56 and

Scott et al. [1977] mapped units P58 and P60 as Eratosthe-nian and/or Imbrian in age.

3.2. Mare Nubium

3.2.1. Geologic Setting[27] The Nubium basin is centered at 21�S and 15�W, has

a diameter of �690 km, and is older than the Imbrium andHumorum impacts [Wilhelms, 1987] (Figure 1). Locatedwithin the disputed Procellarum basin, basalts of MareNubium show a thickness of <1 km [DeHon, 1979; DeHonand Waskom, 1976; Rose and Spudis, 2000]. Remotesensing data reveal subtle color differences of the basaltswithin Mare Nubium [e.g., Pieters, 1978; McCord et al.,1976; Whitaker, 1972], as well as compositional differences[e.g., Lawrence et al., 2000; Lucey et al., 2000; Elphic etal., 2000, 2002]. Bullialdus, an Eratosthenian impact craterthat is located in the northwestern quadrangle of MareNubium, penetrated the mare basalts and excavated under-lying crustal rocks. The spectral signature of its central peakindicates two types of gabbroic-noritic-troctolitic anortho-sites (with 80–85% plagioclase and 85–90% plagioclase,respectively), an anorthositic norite and a norite [Tompkinsand Pieters, 1999].3.2.2. Discussion of Units[28] We defined and dated 17 spectrally distinctive units

within Mare Nubium (Figure 8). We found the outlines ofour spectral units to correlate well with previously spectrallydefined lunar basalt types [Pieters, 1978]. However, 35% ofall units within Mare Nubium (N2, N3, N4, N5, N9, N12)were mapped as ‘‘undivided’’ in the map of Pieters [1978]and three more units (N11, N13, N14) were at least partiallymapped as ‘‘undivided’’ basalts. Two of these units (N11,N13) exhibit additional characteristics of mIG- and hDG-basalts, and unit N14 partially consists of mIG- basalts. Themost frequently occurring basalt type in Mare Nubium is amIG- basalt, which is exposed in eight units (N6, N7, N10,N11, N13, N15, N16, N17). Finally, we identified LBG-basalts within unit N1 and LBSP and LBG- basalts withinunit N8 (Table 2).[29] This region of the Moon was geologically mapped

by Wilhelms and McCauley [1971], Holt [1974], Howardand Masursky [1968], Trask and Titley [1966], and Eggle-ton [1965] (Table 4). In the geologic map of Wilhelms andMcCauley [1971], units N1, N8, N11, and N12 consist ofImbrian mare materials (Im). The majority of units (N3, N4,N7, N10, N13, N15, N16, N17) is mapped as Imbrian andEratosthenian mare materials (Im, Em), and several units(N2, N5, N6, N9, N14) are Eratosthenian mare materials(Em). Imbrian ages for unit N11 (Ipm, Ipmd), N12 (Ipm),and N16 (Ipm, Ipmd) can be derived from the geologic mapof Howard and Masursky [1968]. In this map, the geologicunits Ipm and Ipmd are part of the Procellarum Group withIpmd having a lower albedo than Ipm. Eggleton [1965]mapped units N13, N14, and N17 as Imbrian mare materials(Ipm). In the geologic map of Trask and Titley [1966] marematerial is classified according to its albedo in five classes(Ipm = undifferentiated, Ipm1 = light, Ipm2 = intermediate,Ipm3 = dark, Ipm4 = very dark). All these volcanicmaterials form the Imbrian age Procellarum Group whichis not to be confused with the Procellarian System; the firstone being a rock-stratigraphic unit, the later one being atime-stratigraphic unit. Units N1 and N17 are mapped as


Figure

8.

Spatialdistributionofmodel

ages

forspectrally

defined

unitsin

MareNubium,MareCognitum,andMare

Insularum.a:

USGSshaded

relief

map,simple

cylindricalmap

projection.Spectral

unitsareoutlined

inblack.b:Sketch

map

ofMareNubium,MareCognitum,andMareInsularum

showingunitnumbersandmodelages

inbillionyears(alsosee

Tables4,5,and6).Cratersize-frequency

distributionmeasurementswereperform

edfortheareashighlightedin

darkgray.

Black

areasarenon-m

arematerialsorhavebeenexcluded

from

thisinvestigation.


Ipm, Ipm1 and Ipm2, unit N2 is mapped as Ipm2 and Ipm4,and units N4 and N5 are mapped as Ipm. Unit N6 isclassified as Ipm and Ipm3, unit N7 is intermediate inalbedo (Ipm2), and units N8 and N15 are characterized bya wide range in albedo (Ipm1, Ipm2, Ipm3). Finally, unitN10 consists of Ipm and Ipm2 material. Holt [1974]mapped the Purbach quadrangle of the Moon. Accordingto this map, unit N3 consists of Eratosthenian (Em) andImbrian (Im1, Im2) mare basalts with distinctive albedo andan Imbrian smooth plains unit (Ip). Unit N9 is a darkImbrian basalt (Im2) and unit N11 is an Eratosthenian basalt(Em). Unit N13 is characterized by a variety of basalt types(Im1, Im2, Em), similar to unit N16 (Im2, Em).3.2.3. Ages[30] On the basis of our new crater size-frequency distri-

bution measurements we identified 14 Imbrian and 3Eratosthenian units in Mare Nubium (Figure 8; Table 4).Compared to the crater degradation ages of Boyce [1976]and Boyce and Johnson [1978] we find a good agreementfor several units (Table 4). We measured an Imbrian age forunits N1–N14, and these ages are generally similar to thedegradation ages. However, there are some discrepancies inthe details. According to the map of Boyce [1976] andBoyce and Johnson [1978], unit N2 is 2.5 ± 0.5 b.y. old, butour data indicate that this unit is significantly older, i.e.,3.63/3.85 b.y. Similarly, very young ages (2.5 b.y.) havebeen shown for parts of units N6, N10, and N11. In all thesecases our ages are older than the degradation ages. Veryoften the most abundant degradation age or at least one of thedegradation ages that can be assigned to a particular unit issimilar to our age for this unit (N1, N4, N5, N6, N7, N8,N10, N11, N12, N13, N16, N17). For unit N14 we derivedan age of 3.76 b.y., with a resurfacing event taking place at3.25 b.y. Crater degradation ages of Boyce [1976] and Boyceand Johnson [1978] do not resolve this resurfacing event andindicate ages of 3.75, 3.65, and 3.85 b.y., with 3.75 b.y.being the most abundant age of this unit. Unit N9 issignificantly older in the Boyce data (3.85 b.y.) comparedto our data (3.48 b.y.), as is unit N15 (3.50 b.y.) compared toour age of 3.16 b.y. Unit N3 is considerably younger(3.2 b.y.) than our age (3.63 b.y.). Eratosthenian ages of unitN16 and probably N17 in the maps of Boyce [1976] andBoyce and Johnson [1978] are consistent with our findings.[31] The geologic map of the nearside of the Moon

indicates that units N1, N8, N11, and N12 are Imbrian inage, consistent with our ages [Wilhelms and McCauley,1971]. Several units (N3, N4, N7, N10, N13) were mappedas Imbrian and Eratosthenian in age, but we derived Imbrianages on the basis of our crater counts. Eratosthenian ages forunits N2, N5, N6, and N9 are not consistent with our data.Unit N14 is shown as Eratosthenian in the geologic map andwe obtained an age of 3.25 b.y., very close to the border ofthe Eratosthenian and Imbrian System. Finally, units N15,N16, and N17 were mapped as Imbrian and Eratosthenian inage [Wilhelms and McCauley, 1971; Holt, 1974] and ourdata confirm an Eratosthenian age of these units. In addi-tion, our data are consistent with Imbrian ages of units N11and N12 [Howard and Masursky, 1968], N1, N2, N4, N5,N6, N7, N8, and N10 [Trask and Titley, 1966], N13 andN14 [Eggleton, 1965], and N3, N9, and N13 [Holt, 1974].However, we cannot confirm Eratosthenian ages of unitsN3, N11, and N13 as shown in the map of Holt [1974]. And

our data are not consistent with Imbrian ages of units N15,N16, N17 as indicated by various maps [Eggleton, 1965;Trask and Titley, 1966; Howard and Masursky, 1968; Holt,1974]. On the basis of our crater counts we see that theseunits are Eratosthenian in age. A summary of our new dataand a comparison between geological maps and the Boyceages is shown in Table 4.

3.3. Mare Cognitum

3.3.1. Geologic Setting[32] Mare Cognitum is located northwest of the Nubium

basin (Figure 1). The rim of the pre-Nectarian small basin orlarge crater that contains Mare Cognitum has been mostlydestroyed or obliterated since its formation by subsequentprocesses. It was interpreted to be superposed, hence to beyounger than the postulated Procellarum basin [Wilhelms,1987], but the existence of the Procellarum basin is notuniversally accepted [e.g., Spudis, 1993]. Thicknesses ofbasalts within Mare Cognitum are <500 m [DeHon, 1979;DeHon and Waskom, 1976], and spectral data of Pieters[1978] indicate a rather homogeneous composition of thesebasalts.3.3.2. Discussion of Units[33] Compared to the spectral map of Pieters [1978] we

see that 80% of our basalt units (C1, C3, C4, C5) consist ofmIG- basalts. Unit C2 was mapped as mIG- and ‘‘undivided’’basalts (Table 2).[34] Mare Cognitum is covered by the geologic maps of

Wilhelms and McCauley [1971] and Eggleton [1965]. Wedated five spectral units in Mare Cognitum, of whichWilhelms and McCauley [1971] mapped unit C1, C2, andC5 as Imbrian (Im) and Eratosthenian (Em) mare materials.Units C3 and C4 were mapped as Imbrian (Im). The map ofEggleton [1965] does not differentiate these units. In thismap all basalts are Imbrian mare materials (Ipm) of lowreflectivity (Table 5).3.3.3. Ages[35] On the basis of our crater counts we see that all units

in Mare Cognitum are of Imbrian age (3.32–3.49 b.y.;Figure 8; Table 5). Boyce [1976] and Boyce and Johnson[1978] found similar ages (3.2–3.5 b.y.) and there is anexcellent agreement between the two data sets for theseunits. Only unit C2 appears a little younger in the Boyce[1976] data (3.2 ± 0.2 b.y.) compared to our age (3.45 b.y.).For unit C5 we found evidence for a resurfacing event(3.32/3.65 b.y.) but this is not resolved in the Boyce data(3.2 ± 0.2 b.y.).[36] Imbrian ages for the units in Mare Cognitum are

consistent with the mapping of Eggleton [1965] and Wil-helms and McCauley [1971]. However, Wilhelms andMcCauley [1971] also mapped units C1, C2, and C5 toconsist, at least partially, of Eratosthenian basalts. TheseEratosthenian ages are not consistent with our ages, but arewithin the error bars of the degradation ages (3.2 ± 0.2 b.y.).

3.4. Mare Insularum

3.4.1. Geologic Setting[37] The geologic map of Wilhelms and McCauley [1971]

shows two partially preserved ring structures that outline theInsularum basin. Insularum is a pre-Nectarian impact basinwith rings of 600 km and 1000 km in diameter, and iscentered at 9�N and 18�W [Spudis, 1993] (Figure 1).


Table

4.ComparisonofAges

forBasaltsin

MareNubium

a

Unit

Lunar

Orbiter

Image

Area,

km

2

Crater

Retention

AgeN(1)

Error

Model

Age,

b.y.

Error,b.y.

Boyce[1976]

Wilhelmsand

McC

auley[1971]

Howard

and

Masursky

[1968]

Eggleton

[1965]

Trask

and

Titley[1966]

Holt[1974]

N17

IV120H2

IV125H2

3540

2.34E-03

+0.34E-03/�

0.16E-03

2.77

+0.31/�

0.17

3.2;2.5;3.5

Im;Em

Ipm

Ipm;Ipm1;Ipm2

N16

IV113H2

865

2.70E-03

+0.19E-03/�

0.18E-03

3.09

+0.10/�

0.14

3.2

Im;Em

Ipm;Ipmd

Im2;Em

N15

IV120H2

1696

2.83E-03

+0.20E-03/�

0.19E-03

3.16

+0.08/�

0.11

3.5

Im;Em

Ipm1;Ipm2;Ipm3

N14

IV113H2

963

3.06E-03/

1.46E�02

+0.46E-03/�

0.39E-03

+0.34E-02/�

0.66E-02

3.25/3.76

+0.11/�

0.18

+0.04/�

0.09

3.75;3.65;3.85

Em

Ipm

N13

IV113H2

IV113H3

3855

3.33E-03

+0.49E-03/�

0.43E-03

3.32

+0.09/�

0.13

3.2;3.5

Em;Im

Ipm

Im1;Im

2;Em

N12

IV113H2

522

3.60E-03

+0.54E-03/�

0.46E-03

3.37

+0.07/�

0.09

3.5;3.2

ImIpm

N11

IV113H2

1299

3.62E-03

+0.54E-03/�

0.46E-03

3.38

+0.07/�

0.10

2.5;3.2

ImIpm;Ipmd

Em

N10

IV120H1

IV125H1

6992

4.49E-03

+0.91E-03/�

0.58E-03

3.48

+0.06/�

0.06

3.2;2.5

Em;Im

Ipm;Ipm2

N9

IV113H1

1048

4.54E-03

+1.09E-03/�

0.85E-03

3.48

+0.07/�

0.09

3.85

Em

Im2

N8

IV120H2

3016

4.62E-03

+1.26E-03/�

0.86E-03

3.49

+0.07/�

0.09

3.2;3.5

ImIpm1;Ipm2;Ipm3

N7

IV120H2

695

4.83E-03

+1.82E-03/�

1.16E-03

3.5

+0.10/�

0.12

3.5

Im;Em

Ipm2

N6

IV125H2

1166

5.20E-03

+1.67E-03/�

0.97E-03

3.53

+0.07/�

0.08

3.2;2.5

Em

Ipm;Ipm3

N5

IV125H1

863

6.85E-03

+1.57E-03/�

0.89E-03

3.6

+0.05/�

0.03

3.5

Em

Ipm

N4

IV125H1

686

7.75E-03

+1.78E-03/�

1.45E-03

3.63

+0.05/�

0.05

3.5;3.2

Im;Em

Ipm

N3

IV113H1

1639

7.58E-03

+2.09E-03/�

2.34E-03

3.63

+0.05/�

0.10

3.2

Im;Em

Em;Ip;

Im2;Im

1N2

IV132H1

2332

7.50E-03/

2.49E�02

+1.30E-03/�

1.31E-03

+1.00E-02/�

0.73E-02

3.63/3.85

+0.03/�

0.05

+0.06/�

0.05

2.5

Em

Ipm2;Ipm4

N1

IV120H1

1528

9.26E-03

+2.89E-03/�

2.09E-03

3.67

+0.05/�

0.06

3.5

ImIpm;Ipm1;Ipm2

aSee

textfordetails.


Previous workers [e.g., Whitford-Stark, 1981] have shownthe important influence of the Insularum basin on thetopography and morphology of the younger Imbrium basin.[38] Spectrally, large parts of Mare Insularum are influ-

enced by superposed ejecta of crater Copernicus and Era-tosthenes. In the map of spectrally defined lunar basalttypes, Mare Insularum consists mainly of two basalt types[Pieters, 1978] and new lunar Prospector data indicate highconcentrations of thorium and samarium in the southwest-ern regions of this lunar mare [Lawrence et al., 2000; Elphicet al., 2000].3.4.2. Discussion of Units[39] We identified only four basalt units within Mare

Insularum. Two of these units (IN2, IN4) exhibit character-istics of mIG- basalts in the spectral map of Pieters [1978],one unit (IN3) was mapped as ‘‘undivided’’, and one unit(IN1) is covered with dark mantling material (Table 2).[40] The geologic map of Wilhelms and McCauley [1971]

shows that most basalts are Imbrian mare materials (Im)with unit IN2 also showing characteristics of Eratosthenianmare materials (Em) (Table 6). Hackman [1962] mappedunits IN2 and IN3 as Pm, and Eggleton [1965] describedunit IN2 as Imbrian mare material (Ipm) with low reflec-tivity and small local contrast. This is consistent with themap of Schmitt et al. [1967] in which all basalts in MareInsularum are of Imbrian mare material (Ipm) with analbedo of 0.086–0.102.3.4.3. Ages[41] Ages based on crater counts vary from 2.93 to

3.54 b.y. (Figure 8; Table 6). Three of our four units areEratosthenian in age, one unit is of Imbrian age and twounits show signs of resurfacing at about the same time(2.93/3.50; 2.96/3.53 b.y.). Ages based on crater degrada-tion stages [Boyce, 1976; Boyce and Johnson, 1978] show awide range with up to four (IN3: 2.5–3.65 b.y) or five ages

assigned to a single unit (IN4: 2.5–3.75 b.y.). The geologicmaps generally show Imbrian ages [Eggleton, 1965; Schmittet al., 1967; Wilhelms and McCauley, 1971] and only unitIN2 was partially mapped as Eratosthenian in age in themap of Wilhelms and McCauley [1971].

3.5. Synoptic View of the Investigated Basins

[42] On the basis of the new age data, Figures 7 and8 show the spatial distribution of basalt ages in the inves-tigated mare regions in Oceanus Procellarum, MareNubium, Mare Cognitum, and Mare Insularum. Figure 9shows the temporal distribution of basalt ages found in theinvestigated areas.[43] In these regions the largest number of basalt units per

time bin were formed in the Late Imbrian Period at �3.3–3.5 b.y. The general distribution can be described asasymmetrical with the peak toward the older ages. Begin-ning at about 4 b.y., the frequency rapidly increases to apeak at �3.3–3.5 b.y. and then declines generally, butperhaps episodically, to �1.2 b.y. Our new crater size-frequency distribution data of the remapped basalt unitsindicate that the ages of basalts in Oceanus Procellarumand the other investigated regions range from �1.20 to�3.93 b.y., a total duration of �2.7 b.y. There are 22 unitswhich show evidence for resurfacing by late-stage flows, 16of which occur in Oceanus Procellarum. Ages in MareCognitum vary from �3.32 to �3.65 b.y. We dated 5 unitsin Mare Cognitum but only one unit showed evidence forlate-stage flooding events. In Mare Nubium we dated 17mare basalt units. Ages inMareNubium are generally similarto ages obtained for basalts in Mare Cognitum but show awider range of ages of �2.77–3.85 b.y., for a total durationof �1.08 b.y. At least two basalt units in Mare Nubiumshow two clearly distinguishable ages, indicating that late-stage flooding affected these units. In Mare Insularum we

Table 5. Comparison of Ages for Basalts in Mare Cognituma

Unit

LunarOrbiterImage

Area,km2

CraterRetentionAge N(1) Error

ModelAge, b.y. Error, b.y.

Boyce[1976]

Wilhelms andMcCauley [1971]

Eggleton[1965]

C5 IV125H3 1597 3.30E-03/8.38E-03

+0.65E-03/�0.43E-03+4.22E-03/�2.45E-03

3.32/3.65 +0.10/�0.14+0.08/�0.08

3.2 Im; Em Ipm

C4 IV125H2 1359 3.50E-03 +0.83E-03/�0.45E-03 3.36 +0.10/�0.11 3.5; 3.2 Im IpmC3 IV120H2 1621 3.88E-03 +0.79E-03/�0.50E-03 3.41 +0.08/�0.08 3.5 Im IpmC2 IV125H2 1325 4.23E-03 +1.24E-03/�0.55E-03 3.45 +0.09/�0.06 3.2 Em; Im IpmC1 IV125H2,

IV125H36051 4.64E-03 +1.45E-03/�0.92E-03 3.49 +0.08/�0.10 3.5; 3.2 Im; Em Ipm

aSee text for details.

Table 6. Comparison of Ages for Basalts in Mare Insularuma

Unit

LunarOrbiterImage

Area,km2

CraterRetentionAge N(1) Error

ModelAge, b.y. Error, b.y. Boyce [1976]

Wilhelmsand

McCauley[1971]

Hackman[1962]

Schmittet al. [1967]

Eggleton[1965]

IN4 IV120H3 1614 2.40E-03/4.71E-03

+0.18E-03/�0.17E-03+1.11E-03/�0.90E-03

2.93/3.50 +0.14/�0.17+0.07/�0.08

3.2; 3.5;3.65; 2.5;

3.75

Im Ipm

IN3 IV133H1 1885 2.53E-03/5.18E-03

+0.18E-03/�0.17E-03+1.20E-03/�0.66E-03

2.96/3.53 +0.13/�0.16+0.06/�0.05

3.2; 2.5;3.5; 3.65

Im Pm Ipm

IN2 IV120H3IV133H1

2861 2.79E-03 +0.41E-03/�0.19E-03 3.14 +0.15/�0.12 3.2; 3.5; 3.65 Im; Em Pm Ipm Ipm

IN1 IV120H3 712 5.34E-03 +1.78E-03/�1.35E-03 3.54 +0.07/�0.11 3.2; 3.5 Im IpmaSee text for details.


performed crater counts for four units. We find ages from�2.93 to �3.54 b.y. and two units are influenced by late-stage lava flooding at�2.95 b.y. In summary, our new cratercounts indicate that active mare volcanism in OceanusProcellarum and adjacent regions ranges over a long periodof time from about 1.20 b.y. to about 3.93 b.y., a total of�2.7 b.y.

3.6. Link to Ages of Mare Basalts in Other Basins onthe Lunar Nearside

[44] In a previous paper we presented model ages for�139 basalt units in several lunar nearside impact basins[Hiesinger et al., 2000]. The basalts for which we per-formed crater counts were exposed in Mare Imbrium,Serenitatis, Tranquillitatis, Humorum, Humboldtianum,and Australe. In this paper we expanded our age determi-nations to the Oceanus Procellarum, Mare Cognitum, MareNubium, and Mare Insularum regions. In these areas wedated �86 basalt units with crater size-frequency distribu-tion measurements. Figure 10 shows the distribution ofmodel ages of �225 basalt units of the investigated regions.The data indicate that lunar volcanism in the investigatedlarge nearside mare started at �4 b.y. ago and ended at �1.2b.y. Most of the investigated basalts on the lunar nearsideerupted during the Late Imbrian Period, fewer basaltserupted during the Eratosthenian Period, and even fewerbasalts are of Copernican age (Figure 11).[45] In the past extensive work has been done on crypto-

maria, i.e., maria that were subsequently buried by theejecta blankets of large craters or impact basins [e.g.,Schultz and Spudis, 1979, 1983; Hawke and Bell, 1981;Bell and Hawke, 1984; Head and Wilson, 1992; Antonenkoet al., 1995; Antonenko and Yingst, 2002]. Due to the nature

of cryptomare one cannot date the emplacement of the mareunits with crater counts. However, on the basis of strati-graphic relationships, previous studies interpreted crypto-mare as evidence for early mare volcanism, which wasalready active before the formation of the large lunar impactbasins and the volcanism that filled these basins [e.g.,Schultz and Spudis, 1979, 1983; Hawke and Bell, 1981;Bell and Hawke, 1984; Head and Wilson, 1992; Antonenkoet al., 1995; Antonenko and Yingst, 2002]. The existence ofthese cryptomaria implies that mare volcanism likely startedprior to the emplacement of the oldest dated basalts at�4 b.y., possibly expanding the total duration of activevolcanism on the Moon to more than �3 b.y.[46] The spatial distribution of model ages indicates that

younger (Eratosthenian/Copernican) basalts occur preferen-tially in the Oceanus Procellarum region and in the vicinityof volcanic centers such as the Aristarchus Plateau.

3.7. Flux of Lunar Mare Basalts

[47] Establishing the volume of mare basalts emplaced onthe surface as a function of time (the flux) is important inorder to place constraints on the petrogenesis of lunar marebasalts and their relation to the thermal evolution of theMoon [Head and Wilson, 1992; Zhong et al., 1999; Wiec-zorek and Phillips, 2000; Parmentier et al., 2000; Wilsonand Head, 2001]. Head and Wilson [1992] reviewed andsummarized different approaches to determine the extentand flux of mare basalts and found that on the basis ofreturned lunar samples only one would obtain a more or lessGaussian flux curve with a peak flux prior to �3.5 b.y.(Figure 12a). Looking at the surface exposure of units, onewould derived a similar flux curve, which is somewhatwider and shifted toward younger ages (�3–3.5 b.y.)

Figure 9. Histogram showing the temporal distribution of model ages for all investigated basalts inOceanus Procellarum, Mare Nubium, Mare Cognitum, and Mare Insularum.


Figure

10.

Color-coded

map

ofthespatialdistributionofmodelages

oflunar

marebasaltssuperposedonaUSGSshaded

relief

map.Ages

shownherearefrom

thisstudyandfrom

Hiesinger

etal.[2000].Modelages

arein

billionyears,bin

size

is100m.y.Map

coverage:120�W

–150�E

,90�S

–90�N

;latitude,longitudegridis30��

30�wide;sinusoidalmap

projection.


(Figure 12b). Investigation of the stratigraphy of mare basaltvolumes would yield an asymmetric flux curve with thepeak at older ages at �3.8 b.y. (Figure 12c). Figure 12dshows a flux curve that accounts for probable volcanism inthe period of early impact bombardment which is nowobscured. Finally, a flux curve based on the combinationof approaches is shown in Figure 12e. On the basis of theirinvestigation, Head and Wilson [1992] argued that the fluxin the last half of lunar history is characterized by episodicrather than continuous eruptions.[48] In order to calculate the flux we need accurate

information on (1) the age, (2) the surface extent and(3) the thickness of a basalt flow. Early work mostlyfocused on estimating the total thickness of mare basaltfill in the mare basins [e.g., DeHon, 1974, 1979; DeHonand Waskom, 1976; Horz, 1978]. These studies used avariety of techniques, which are summarized elsewhere[e.g., Head, 1982; Budney and Lucey, 1998]. Results fromcrater geometry techniques using pristine crater morpho-metric relationships and the diameter of partially to almostwholly flooded craters showed that lunar impact basins arefilled with up to 2 km of basalts, with 200–400 m onaverage [DeHon and Waskom, 1976]. Horz [1978]reviewed the assumptions that underlie the thickness esti-mates of DeHon and Waskom [1976] and concluded thatsuch values were overestimates. Similarly, Budney andLucey [1998] concluded that basalts in Mare Humorumare generally less thick than estimated by DeHon [1979].Using craters that excavated highland material from be-neath the mare basalts in Oceanus Procellarum, Heatherand Dunkin [2002] estimates that the basalt are 160–625 mthick, with thicknesses ranging from tens to hundreds of

meters near the mare/highland boundaries and severalhundreds of meters closer to the center of the mare. Allthese estimates, which are based on flooded impact cratersor impact craters that penetrated the mare basalts andexcavated underlying highland material, provide the totalthickness of basalts in a particular basin, but not thethickness of individual basalt flow units. Head [1982]pointed out that two stages of filling a basin can bedistinguished: Stage 1 is characterized by flooding theinterior and thick deposits (�6 km) of small to intermediatevolumes covering small areas; stage 2 is characterized byflooding outward to the basin-defining scarp and thindeposits (�2 km) of large volume covering large areas.[49] To place better constraints on the flux of lunar mare

basalt volcanism we need to know the thicknesses ofindividual mare flow units. In the past extensive work hasbeen done to estimate the thicknesses of individual flowunits [e.g., Howard et al., 1972; Schaber, 1973; Brett, 1975;Schaber et al., 1976; Gifford and El-Baz, 1978, 1981].Gifford and El-Baz [1978, 1981] noted that despite the wideacceptance of the idea of multiple flow units filling thebasins, morphometric characteristics of individual flowunits have not been extensively studied. Measurements offlow unit thicknesses are complicated by (1) the limitedavailability of suitable data necessary for the recognitionof flow fronts, i.e., high-resolution topography and near-terminator images, (2) regolith formation processes, i.e.,impact cratering, which can obliterate flow fronts of up to15 m [Head, 1976], and (3) the composition and theeruption style of lunar lavas which are thought to beresponsible for the sparseness of mare flow features [Schultzet al., 1976; Head, 1976].

Figure 11. Histogram showing the temporal distribution of model ages for all investigated basalts inOceanus Procellarum, Mare Nubium, Mare Cognitum, Mare Insularum, Mare Imbrium, Mare Serenitatis,Mare Tranquillitatis, Mare Humorum, Mare Humboldtianum, and Mare Australe. Ages shown here arefrom this study and from Hiesinger et al. [2000].


[50] A variety of techniques has been applied to estimatethe thicknesses of individual flow units. Such techniquesincluded: (1) shadow measurements in high-resolution low-sun images [e.g., Schaber, 1973; Schaber et al., 1976;Gifford and El-Baz, 1978, 1981]; (2) in situ observationsof flow units by the Apollo astronauts, e.g. within the wallsof Hadley Rille at the Apollo 15 landing site [Howard et al.,1972], and (3) studies of the chemical kinetic aspects of lavaemplacement and cooling [Brett, 1975].[51] On the basis of Apollo 14 and 15 near-terminator

images of the region southeast of the crater Kunowsky,Lloyd and Head [1972] determined a flow front height of3–5 m for a single flow unit. Reporting on a much largernumber of units, Gifford and El-Baz [1981] found flowheights of 1–96 m with an average thickness of �21 m.

These results are similar to thicknesses of 10–20 mobserved in the wall of Hadley Rille [Howard et al.,1972]. Gifford and El-Baz [1981] argued that a few flowunits might even have actual thicknesses in excess of 100 mbecause only the shadowed portions of the flow fronts weremeasured. Schaber [1973] found that the average thicknessof the Eratosthenian flows in Mare Imbrium is 30–35 m.However, they considered these Eratosthenian flows asatypical because their thickness (10–63 m) is much largercompared to other lunar flow fronts that could be identifiedin imaging data (5–10 m) [Schaber et al., 1976]. Finally,chemical kinetic considerations based on Apollo samplessuggest that lunar lava flow units are no thicker than �8–10 m at the Apollo 11, 12 and 15 sites [Brett, 1975].[52] A fourth technique was applied by Neukum and

Horn [1976]. They showed that endogenic lava flowprocesses could be identified by their characteristic effectson crater size-frequency distributions even if these indi-vidual flows were not visible in the images. This is animportant result because photogeologic and morphologicrecognition of individual flow fronts on the Moon isdifficult and is mostly restricted to thicker flows and areaswhere low-sun images or samples were obtained, asdiscussed above. On the basis of deflections (knees) incrater size-frequency distribution curves, Neukum andHorn [1976] estimated the thickness of Imbrian-aged flowunits in Mare Imbrium. They found that these flows areabout 200 m thick, and reported that Eratosthenian flowsare about 60 m thick. The later value is consistent withphotogeologic estimates of the same flow unit by Schaber[1973], Schaber et al. [1976], and Gifford and El-Baz[1981]. Hiesinger et al. [2002] applied this technique to amuch larger number of their crater counts and found thatthe average thickness of 58 of their investigated individualbasalt flow units is �30–60 m with a range of flow unitthickness of �20–200 m. A comparison with previouslypublished thickness estimates [e.g., Howard et al., 1972;Schaber, 1973; Brett, 1975; Schaber et al., 1976; Giffordand El-Baz, 1978, 1981] showed that crater size-frequencydistribution measurements yield thicknesses that are inexcellent agreement with results from these other techni-ques and allow one to obtain thicknesses for additionalflow units that have not been detected in low-sun images.On the basis of their thickness estimates, Hiesinger et al.[2002] also reported that the volumes of individual basaltflows range from 30–7700 km3, with an average of 590–940 km3.[53] For our flux estimates we will use ages and thick-

nesses derived from crater size-frequency distribution meas-urements and surface extents of mare basalt units measuredin imaging data. We have already demonstrated that cratersize-frequency distribution measurements are an adequatemethod in order to derive the surface age of the uppermostbasalt unit with sufficient accuracy [Hiesinger et al., 2000].We also demonstrated that crater size-frequency distributionmeasurements can be used to estimate the thickness of mareflow units [Hiesinger et al., 2002].[54] Using this approach we have to consider potential

caveats. First, because older units might have been coveredby younger flows, the measured surface area of older basaltflow units might be too small. Consequently, volumesestimated for these flow units must be considered as

Figure 12. Diagram of the flux of lunar mare basaltsbased on different approaches [Head and Wilson, 1992].


minimum estimates. Second, we also have to considervariations of flow unit thicknesses with time, that is, olderflow units might be thicker than younger flow units asindicated by the work of Head [1982]. Again, this impliesthat volumes estimates of early flow units are minimumestimates.[55] As discussed above, craters which penetrate the

entire stack of basalts have been used to estimate thethicknesses of basalts within individual basin structures[e.g., DeHon, 1979]. These studies indicate that basaltthicknesses are up to 2 km with 200–400 m on average.If we assume a thickness of 30–60 m for the uppermostbasalt flow unit [e.g., Hiesinger et al., 2002], this wouldimply that at least �150 m or up to �1900 m of basalts arecovered by the youngest flow units. Similarly, thicknessestimates by Heather and Dunkin [2002], would imply thatat least �100–550 m of basalts are buried beneath theyoungest flow units. Using the Orientale basin as anexample, Head [1982] argued that the basalt thickness inthe central parts of lunar basins can be up to 8 km. From thecomparison of this maximum total thickness of mare basaltsin a basin with the thickness of the uppermost flow units,we conclude that a large number of flow units must havefilled the lunar impact basins.[56] Figure 13 shows a cumulative plot of the flux of

basalts in all investigated areas, that is, Imbrium, Serenitatis,Tranquillitatis, Humorum, Humboldtianum, Australe, Oce-anus Procellarum, Cognitum, Nubium, and Insularum. Weplotted the flux curves of four scenarios, each of whichassumed a different average thickness of the flow units. Thethicknesses were chosen to be within reasonable limitsgiven by the discussion above, i.e., 10 m, 25 m, 50 m,and 100 m. For comparison we also plotted a flux curve forlinearly increasing thicknesses with increasing age. In thismodel we assumed a thickness of 10 m for flows of 1.0 b.y.age, which increases to 100 m for flows of 4.0 b.y. age. Iftrue that the thickness of early flow units was larger [Head,1982] and that the surface exposure of older units isunderestimated, the implication is that the flux curve shouldbe steeper at older ages, and this is what we observe for ourflux curve on the basis of increasing flow unit thicknesses.While the actual amount of increase in thickness is notknown to date, the model illustrates several effects on theflux curve. First, even with a 10-fold linear increase inthickness with increasing age, the overall shape of the fluxcurve remains similar and phases of higher and lowervolcanic activity are detectable. Second, by assuming alarger thickness of older flow units, the model indirectlyoffsets effects introduced by the fact that older units arecovered to a larger extent with subsequent units thanyounger units. While the models provide a qualitative basisfor discussions of the lunar volcanic flux, exact quantitativemeasurements are left for future studies.[57] Our data indicate that the flux of mare basalts was

highest in the Late Imbrian �3.3–3.7 b.y. ago. On the basisof our data we observe a decreased flux during the Eratos-thenian and Copernican Period. The decrease in flux wasnot continuous but was interrupted by brief phases ofrelatively higher volcanic activity as indicated by the steepersegments of our flux curve (Figure 13). This is consistentwith predictions of Head and Wilson [1992], who argued forepisodic eruptions during the later half of lunar history

(Figure 12e). Flattening of the curve at ages >3.7 b.y. mightbe a combination of covering older units by younger unitsand/or a probably lower flux.

4. Conclusions

[58] On the basis of our new age determinations forbasalts that are exposed in Oceanus Procellarum and adja-cent mare regions we conclude that (1) not consideringpossible cryptomaria, volcanism in this region was activeover a long period of time, starting at �3.93 b.y. and endingat �1.20 b.y., (2) the largest number of basalt units wereformed in the Late Imbrian Period at �3.3–3.5 b.y., (3) thetemporal distribution of erupted mare basalts is asymmetricwith the peak toward the older ages, and (4) the youngestbasalts were detected in the vicinity of volcanic centers suchas the Aristarchus Plateau.[59] On the basis of the combination of our new data with

our previous age data of several lunar nearside maria weconclude that (1) the investigated basalts on the lunarnearside erupted over a long period of time of at least�2.8 b.y., between �4 and �1.2 b.y., (2) if one takes intoaccount cryptomaria, which suggest that volcanism wasalready active prior to the formation of the large impactbasins, the period of active volcanism on the Moon mightbe as long as >3 b.y., (3) of all dated regions in our study,

Figure 13. Flux of mare basalts for four estimated flowunit thicknesses (10, 25, 50, 100 m). Solid line is the fluxcurve for linearly increasing thicknesses of basalt flow unitswith time, that is, older units are thicker than younger units.For this case it was assumed that the thickness of a 1.0 b.y.old flow is 10 m and that this thickness increases linearly to100 m for flows of 4.0 b.y. age.


Oceanus Procellarum is the location of the youngest basaltson the lunar nearside, (4) on the basis of the regionsinvestigated in our study, Oceanus Procellarum also exhibitsthe widest range in ages within a single mare region,(5) according to our crater counts there are basalt units,for example south of the Aristarchus Plateau that might beyounger than the Lichtenberg and Flamsteed basalts, whichwere previously thought to be among the youngest lunarbasalts [Schultz and Spudis, 1989], (6) there are significantdifferences in the flux of lunar nearside basalts, (7) the fluxof lunar basalts was largest during the Late Imbrian Period,especially between �3.3 and �3.7 b.y., and (8) the flux ofbasalts is significantly smaller during the Eratosthenian andCopernican Period.[60] We are currently expanding our efforts to date lunar

mare basalt surfaces with crater size-frequency distributionmeasurements to additional maria, for example Mare Fri-goris and Mare Nectaris, and are investigating changes inbasalt mineralogy as a function of time and changes ineruption conditions, styles and locations with time. Finally,we intend to combine our results and characterize marebasalt volcanism in space and time, which will result in anew detailed stratigraphy of lunar mare basalt.

[61] Acknowledgments. The authors would like to thank Anne Coteand Peter Neivert for their help with the manuscript preparation. Thanks areextended to the NASA Planetary Geology and Geophysics Program, whichprovided support for this analysis.

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��J.W.Head III andH.Hiesinger,Department ofGeological Sciences, Brown

University, Box 1846, Providence, RI 02912, USA. ( [email protected]; [email protected])R. Jaumann and U. Wolf, DLR-Institute of Space Sensor Technology and

Planetary Exploration, Berlin, Germany.G. Neukum, Institut fur Geologie, Geophysik und Geoinformatik, Freie

Universitat Berlin, Germany.


ages and stratigraphy of mare basalts in oceanus ... · younger basalts are generally exposed in...

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