Stratigraphy of Ejecta from the Lunar Crater Aristarchus

J. E. GUEST University of London Observatory, Mill Hill Park, London NW7 2QS England

ABSTRACT associated with the hummocky rim unit are interbedded, arguing that the smooth flows Ejecta from lunar crater Aristarchus con- are part of the sequence of ejected rocks and sist of mappable units that have different sur- are not younger volcanic flows. face characteristics, lithologies, and geneses. Photogeologic mapping demonstrates that INTRODUCTION these units can be ordered into a stratigraphic succession representing stages in the emplace- The crater Aristarchus lies on the nearside ment of ejecta during a single impact event. hemisphere of the Moon at lat 23°40' N., long Four main ejecta units are recognized: (a) a 47°20 W. Its diameter is 42 km, and its depth highly fractured rim unit consisting of an over- is more than 3 km. The markedly uneroded turned flap of country rock, (b) a continuous aspect of the crater and the extensive, well- ejecta blanket, (c) a zone of bright discontin- defined, bright rays surrounding it (Figs. 1, 2) ous ejecta outside the continuous ejecta suggest that Aristarchus belongs to the upper blanket associated with numerous secondary part of the Copernican System (Wilhelms and impact craters, and (d) a group of ejecta McCauley, 1971). Apparently, Aristarchus deposits on the rim which are genetically and Tycho are two of the youngest of the large related to each other and include hummocky rayed craters on the nearside of the Moon. The material (the hummocky rim unit), blocky aim of this paper is to describe the stratigraphic lobes, and smooth flows. Other units, such as relations of the Aristarchus ejecta units and to the ridged and leveed flows and the dark explain these in terms of the sequence of events "lakes" or "playas," are relatively younger occurring during crater excavation. The than the ejecta and are only briefly discussed. information is derived from detailed photo- The continuous ejecta blanket lies strati- geologic mapping of Aristarchus. Interpreta- graphically above the rim unit. To the east tions are based on studies of cratering mech- and south, the ejecta blanket has been stripped anisms. off the overturned rim unit by outward Rayed craters are now generally accepted as flowage from the crater during ejecta produc- being of impact origin, produced by the col- tion, leaving parts of the rim eroded bare. lision with the Moon of either asteroid-sized Fall of large missiles to form the secondary meteoroids, or cometary nuclei. Shoemaker craters and bright ejecta preceded the emplace- (1962) presents a convincing case that Coper- ment of the continuous ejecta. The general nicus was formed in this way: his main argu- asymmetry of the continuous ejecta-blanket ment is based on the presence of large, second- distribution appears to be related to the ary impact craters clustered in a broad annulus Aristarchus Plateau boundary fault, which encircling Copernicus about one crater diam- may have controlled the way material was eter away from the rim. The similarity is excavated. The hummocky rim unit only striking between this array of secondary occurs on the northern and western rim and craters and those developed around shock- may be an overturned flap of premare Aristar- wave craters produced by nuclear devices, for chus Plateau bedrock not present near the example Sedan (Shoemaker, 1965). Rim surface on the southern and eastern rim of the morphology (Shoemaker, 1962; Baldwin, 1963; crater. Blocky lobes and smooth flows mainly Guest and Murray, 1969), circularity (Murray

Geological Society of America Bulletin, v. 84, p. 2873-2894, 13 figs., September 1973 2873

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and Guest, 1970), and morphology of the preserved; also, there is very good coverage of ejecta blanket also support a single, high- this crater by high-resolution Orbiter V energy excavating event. An origin by vol- photography. canic explosion of the magnitude required The validity of the mapping approach to to form craters that may be as large as 200 km lunar problems has been emphasized by in diameter and have the other characteristics McCauley (1967), Mutch (1970), and Wil- of lunar rayed craters is not consistent with helms (1970). In the case of Aristarchus, the evidence, whereas the present under- individual ejecta units are recognized by sur- standing of impact mechanisms does explain face characteristics; these are quite different the observed features of these craters. for units of different composition, texture, or The formation of a large impact crater has mode of emplacement. Age relations are not been observed. Many large impact determined oy accepted photogeologic tech- craters have now been recognized on Earth, niques. Although stratigraphic studies nor- and some of these are in a state of preservation mally apply to a sequence of rock laid down sufficient to glean information on the detailed over a long period of time, the techniques mechanics of their formation. Much of our may be applied equally well to the ejecta of knowledge on this subject is also based on an impact crater where indi vidual units were theory and on observation of nuclear and emplaced to form a thick sequence during the chemical explosions. In the study of impact- relatively short time period of just a few cratering phenomena, the large scale and the minutes. By analyzing the results of the fresh, uneroded nature of many lunar craters photogeologic mapping, it is possible to compensate for their inaccessibility. Aristarchus reconstruct the events that occurred during the was chosen for study because it is so fresh and excavation of the crater. most of the original features apparently are Photographs used in this study were

Figure 1. Telescopic view of Aristarchus and the raphy. (Photograph from Consolidated Lunar Atlas). Aristarchus Plateau at low sun angle to show topog-

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Orbiter IV-150-H3 and Orbiter V 194 (Fig. 1). On the basis of surface appearance through 201 (medium and high resolution). and the results of the Apollo 11, 12, and 15 The Orbiter V medium-resolution frames have missions, the dark mare material is interpreted sufficient overlap to allow relative heights to as extensive lava flows of "basaltic" composi- be determined qualitatively. Detailed mapping tion (Lunar Sample Preliminary Examination on the high-resolution frames was carried out Team, 1969, 1970, 1972). The Aristarchus at a scale of 1:32,000. Mapping was accom- Plateau, on the other hand, may have varied plished by overlaying the photographs with compositions and origins; the northern part clear acetate film, which served as a base for of the plateau appears to consist of ejecta from plotting the geology. The Consolidated Lunar the Imbrium basin; Herodotus is a crater of Atlas (Kuiper and others, 1967) was used for middle Imbrium age and is overlain by a group comparison of the appearance of the area of volcanic rocks at least in part related to under different lighting conditions. Schroter's Valley (the Vallis Schroteri and Cobra Head Formations of Moore, 1965, GENERAL GEOLOGY 1967). The thickness of the mare material below Geologic Setting Aristarchus is unknown, but because Aristar- Aristarchus lies in Oceanus Procellarum on chus has been emplaced against a fault scarp the southeast margin of the Aristarchus bordering the plateau, rather than on a Plateau, and is the youngest major feature in gently inclined slope, the mare material may the area; the ejecta blanket from Aristarchus be thick, even close to the edge of the plateau. overlies dark mare material on the south- Thus, although the crater Aristarchus prob- eastern half and premare material of the ably cuts quite deeply into premare material on Aristarchus Plateau on the northwestern half the northwest, a considerable volume of mare

Figure 2. Same area as Figure 1 at high sun angle bright rays around Aristarchus. (Photograph from to show variations in albedo. Note the dark halo and Consolidated Lunar Atlas).

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material must have been excavated on the as the most likely cause. Certainly, study of southeastern side. As will be suggested later, volcanic processes indicates that such a large the position of Aristarchus on this major crater could not have been caused by volcanic- geologic boundary may have affected the gas explosion (Roddy, 1968). One piece of character of the ejected materials on either positive evidence that tends to confirm an side of the crater. impact rather than a volcanic-explosion origin is the presence of central peaks in many Geology of Aristarchus craters, including Aristarchus. These are Examination of Orbiter and telescopic characteristic of the larger terrestrial impact photographs shows that Aristarchus is made up craters (Roddy, 1968; Dence, 1968), which of a number of geologic units (Fig. 3), each have central peaks consisting of highly de- with different surface characteristics. There formed, often shock-metamorphosea rocks are also variations in albedo, but these do not (Howard and others, 1972). They are also always appear to correspond with geologic formed in artificial shock-wave craters pro- boundaries. duced by a surface charge and thus closely simulating an impact. Seme authors have Origin. Craters of the type represented by claimed a volcanic origin for peaks in the Aristarchus are generally considered to have floors of large lunar craters. Although this may been formed by impact for the following rea- be true in some cases, there are examples in sons: lunar rayed craters of central peaks with 1. The presence of an ejecta blanket indi- steeply dipping strata, as would be expected cates that the crater was excavated and not for a central uplift; one wsll-known example formed by collapse alone. Evidence that this is in a peak on the floor of Copernicus. Further- was one large event rather than a number of more, the nature and geometry of some fea- smaller ones is given by the presence of tures on the peaks in Aristarchus would be secondary impact craters, often several kilom- taken to indicate steeply dipping strata on any eters across; these must have been caused by terrestrial aerial photograph. large missiles that could have been thrown out only by an event of great magnitude, Some lunar workers dispute an impact 2. Rays extending several hundred kilom- origin for rayed craters in general, including eters from the crater also support one large, Aristarchus. Fielder (1965) has argued that crater-excavating event. such craters are the final stage in a long 3. The morphology of the crater and its rim development by extrusion of lava from ring is consistent with an origin by shock-wave fractures, combined with caldera collapse; cratering. The high degree of circularity is Green (1965, 1971) also considers the craters comparable with meteorite, nuclear, and TNT to be of caldera origin and explains the rays as craters on Earth. The nature of the rim is ignimbrites (ash flows). Evidence from Orbiter explained by shock-wave mechanics but is photographs does not in my opinion support difficult to attribute to any known form of these hypotheses. Rayed craters are so different volcanic activity. In addition, most volcanic from terrestrial volcanoes that any theory calderas and craters have their floors at a level which rests only on morphological similarities above that of the surrounding terrain, but the between rayed craters and terrestrial volcanoes floors of rayed craters are always far below the is difficult to support. If rayed craters are adjacent ground level; the floor of Aristarchus volcanic, the present evidence demands that is some 1,000 m below the mare plain into they were each formed by a single crater- which it is excavated. excavating process of the magnitude expected for an impact event. 4. Faulting associated with the crater has been used as evidence in favor of an endogenic The Crater. The inner walls of the crater (volcanic) origin. However, the patterns of are characterized by a series of well-marked faulting observed to form around artificial steps or terraces (Fig. 4), formed by fault- shock-wave craters are similar to those of lunar slumping of the inner rim of the crater by rayed craters (Guest, 1971). release of pressure during and immediately All the foregoing evidence supports an after the final stages of excavation; minor origin by one massive crater-excavating event. landslips may have occurred at a much later This may not have been generated by impact, time. Younger talus sheets cover the terraces, but based on our understanding of processes and the well-known dark bands (streaks of operating in the solar system, impact appears low albedo) of Aristrachus appear to correspond

with debris runs. There are also thin, leveed demonstrates that it was emplaced by fall flows of unknown origin on the crater walls. from above. Three facies, developed at different Some of these flows may be debris flows or distances from the crater, have been recognized impact melts, but it is also possible that some for large lunar craters in general (Mutch, are lava flows produced by impact-triggered 1970): nearest to the crater is a hummocky volcanic activity (Shoemaker and others, 1968, facies which grades out to a smoother surfaced Strom and Fielder, 1970). facies characterized by radial ridging, and The floor of the crater is covered by a layer beyond this is the third facies, including a zone of rock with a ridged surface. Three char- of secondary impact craters and a ray system. acteristics of this floor material may be con- For Aristarchus, the outer boundary of the sidered in relation to its mode of formation: radially ridged facies marks the approximate (a) ridging resembles that developed on limit of continuous ejecta; beyond this area, viscous flows; (b) large-scale, polygonal fissure the ejecta sheet is thin to discontinuous. The patterns suggest contraction, possibly caused thicker parts of the ejecta blanket tend to have by cooling; and (c) peripheral fractures on an a low albedo, gradually becoming brighter as inward sloping bench at the edge of the the ejecta thins. The zone of secondary impact crater floor suggest that the top was initially craters is bright with an albedo similar to that at a higher level and later subsided. From these of the rays. Patches of higher albedo do occur observations, it may be concluded that the within the area of dark ejecta, but it is difficult floor material was emplaced as a hot, very to correlate these bright areas with specific viscous layer, which, under lithostatic load, features. compacted in its lower parts causing the sur- Overlying the outer rim of the crater are a face to subside. Strom and Fielder (1970) number of flowlike units. Some have smooth believe that the floor represents a large lava surfaces and are probably related to the ejecta flow. The possibility that it is allochthonous blanket, and others have surface features such fallback breccia must be considered, however. as levees and flow ridges resembling those Studies of terrestrial impact craters have shown developed on flows of viscous material. These that extreme shock metamorphism leads to flowlike units lie above the ejecta blanket, but melting of country rock, which may then be it is not clear whether they formed at the time ejected as fragments of semimolten material. of crater excavation or are considerably Such material, on falling back into the crater, younger. Strom and Fielder (1970) supported would be capable of sintering to form a com- the latter situation on the basis of crater pact glassy rock. It could also flow, become counts. Shoemaker and others (1968) con- compacted (in much the same way as a thick sidered three possible origins for flows: (a) ash flow), and would develop contraction they are volcanic flows caused by the forces cracks during cooling. which formed the crater, (b) they were formed Rising above the ridged floor within the as relatively cold debris flows, or (c) they result crater, there are a number of hills; the largest from flows of hot debris produced by impact- hill forms the central peak. On the basis of melting of rock ejected from the crater. crater counts, Strom and Fielder (1970) argue Other materials about which there is some that the hills are younger extrusive domes of discussion are the "lakes" or "playas," con- volcanic origin. The way that the floor material sisting of smooth-surfaced, dark fillings in is draped around the edges of the hills suggests, depressions on the crater rim and inner walls. however, that the hills are composed of the Like the viscous flows, these are young relative higher parts of an irregular base below the to the ejecta blanket; they also occur on Tycho ridged floor of the crater. This exposure of the and Copernicus. Apparently, they were base is common in terrestrial impact craters deposited in a hot, fluid state but, as Strom and also in surface-generated, artificial shock- and Fielder (1970) have noted, they were wave craters. viscous enough to develop flow fronts in some Deposits outside the Crater. Most asso- cases. Shoemaker and others (1968) thought ciated rock units outside the crater Aristarchus that similar units on Tycho had been formed consist of excavated material. Evidence from by a thin, uniform layer of fairly fluid material the depth of burial of older craters mantled which drained into closed depressions, possibly by the ejecta blanket shows that this unit as a form of fluidized bed originating from the thins away from Aristarchus, and the way that crater. On the other hand, Strom and Fielder it is draped over the underlying topography (1970) and Greeley and Gault (1971) suggested

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Figure 3. Geologic map of the ejecta blanket of smooth flows; heavy dashed line = southeastern Aristarchus. The northern part of the map is shown in boundary of Aristarchus Plateau; lighter dashed lines Figure 4. Locations of Figures 6,7, and 11 are indicated. = buried topography. Secondary craters and V-shaped Key: R = rim unit; wide-spaced dots = ejecta blanket; features are indicated. close-spaced dots = hummocky rim, blocky lobes, and

that they are lava lakes analogous to those of to form the crater. These observations support Hawaiian volcanoes. an impact origin for lunar rayed craters, sug- Structure. As pointed out by Baldwin gesting that they also have rims consisting of (1963) and confirmed with more data by Guest highly folded and faulted strata. Calculations and Murray (1969), the raised rims of many by Roddy (1968) show that gas-generated types of lunar craters are narrow by compar- volcanic explosions would not give rims of this ison with volcanic craters and calderas; the type- rim width tends to be directly proportional Examination of the rim (Figs. 3, 5) shows to the diameter of the crater. Artificial shock- that it is marked by closely spaced, concentric wave craters and impact craters have similarly ridges and valleys: these features are best narrow rims. It appears likely that this peculiar- explained as surface expressions of faults and ity of shock-wave craters is related to the fact fractures developed in the rim as its response that their rims were formed essentially by to the release from outward pressure im- folding and overturning of the rocks displaced mediately after the bulk of material had been

Figure 4. Orbiter V photograph of Aristarchus (NASA V 200-M). For geology, see Figure 3. (F - from National Aeronautics and Space Administration floor material; CP = central peak).

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Figure 5. Southeast rim of crater (part of has been stripped off the rim. R = exposed surface of NASA V-197-Hi). This photograph represents the the rim; ER = partially stripped rim with remnants of upper part of Figure 7 and shows evidence that ejecta ejecta; E = the main ejecta blanket.

excavated. First-generation faulting was prob- by their surface morphologies. In this section, ably started by the initial shock wave and was each of these units is described, and the followed by later movement. On the northern relations between units discussed. and southern sides of the crater, the concentric fractures veer away following the directions of Rim Unit the grid pattern (NW.-SE. and NE.-SW.; see The rim unit forms the part of the rim that Fig. 3). is characterized by a strong fracture pattern; Strong fractures, again tending to follow the it lies stratigraphically below the continuous grid pattern, have been formed radially to the ejecta blanket forming the surface outside the crater. These are well marked on the inner rim and, with the hummocky rim unit, walls and extend out to give deep valleys on represents the oldest exposed rocks of the the outer rim. Features on the ejecta blanket ejecta sequence. The rim unit crops out con- outside the rim tend to follow directions of tinuously around the rim to the east and south fractures observed near the crater and may be of the crater; elsewhere it is overlain by the related to fracturing at depth. Some of these hummocky rim unit (Fig. 3). It consists of lineaments may also be primary depositional strongly jointed material, and the surface is features controlled by structures in the littered with partially buried, large angular excavated bedrock. blocks as much as 30 or 40 m across (F:.g. 5). The topographically higher parts appear to EJECTED MATERIAL be less marked by concentric features and tend The ejecta from Aristarchus may be divided to have different surface characteristics than up into a series of units (Table 1), recognizable the lower parts. It is possible that the higher

TABLE 1. STRATIGRAPHY OF UNITS OUTSIDE ARISTARCHUS CRATER

Unit Symbol Characteristics Interpretation

(a) Ridged and (a) Thick flow units of limited extent; flow ] leveed flows ridging and levees; some seen to / Rock melts generated by shock V originate in craters } melting or volcanic flows induced (b) Dark flows (b) Dark, thin, smooth-surfaced flows filling I by impact and "lakes" depressions ) or "playas"

Mainly associated with hummocky rim unit \ (a) Blocky lobes (a) Flowlike units of blocky debris; lobe- 1 fronts 16 to 25 m high; 10 percent of / Clastic flows formed from surface covered by blocks greater than 3 m I avalanching on rim or other type F across / of radial clastic flow (b) Smooth flows (b) Smooth surfaces with longitudinal ridges; i lobate fronts 1 to 2 m high; interbedded j with blocky lobes /

Ejecta blanket Generally low albedo unit surrounding Fragmental layer of material from Aristarchus up to one Aristarchus crater overturned flap and ejecta with diameter from lip; thins away from crater; high-angle trajectory; outward surface characterized by broad hummocks and clastic flow contributed to upper E radial, linear ridging; surface blocks parts generally less than a few meters across; boulder spreads locally

Bright, Forms bright annulus outside ejecta blanket, Secondary impact craters and discontinuous grading out into rays; contains high density associated ejecta; many secondary ejecta of satellitic craters, many of which have craters may have been formed by S long axes radial to Aristarchus; on inner low-angle missiles from edge of margin, satellitic craters overlain by ejecta overturning flap before emplace- blanket; braided pattern of V-shaped ment of ejecta blanket. Probably features often developed emplaced simultaneously with R and H

Hummocky rim Occurs on North and West rims; individual Overturned flap of bedrock hummocks average about 450 * 600 * 80 m; long representing material from the H axes tend to be radial to Aristarchus; ranges Aristarchus Plateau from smooth to blocky (blocks up to 50 m across becoming smaller away from Aristarchus); weak, concentric pattern of ridge-groove features

Rim Forms outer rim on East and South; undulating Overturned flap of bedrock R surface cut by strong concentric features representing marial rocks cut by concentric faults

and lower parts of the rim are lithologically It is not known how much of the now-exposed different, allowing the rim to be mapped as two areas of the rim were initially covered by the units (Figs. 6, 7; see Fig. 3 for location). later ejecta; examination of the eastern rim As stated earlier, the broad morphological surface suggests that the ejecta blanket was evidence indicates that the whole rim of stripped off the rim during and after its forma- Aristarchus is an overturned flap, consisting tion. The surface of the rim is apparently of a mass of brecciated rock. In this case, the scoured, leaving radial grooves (which may two subunits could represent layers of litholog- initially have been fractures) cut in its surface. ically different bedrock, the upper unit There are also outliers of ejecta resembling originating from a relatively lower stratigraphic the form of roches moutonnees (Fig. 5). These level below the crater (Fig. 8.). become larger as the frontal scarp of the The nature of the upper surface of this unit ejecta blanket is approached, giving the and the position of its boundary with the impression that high velocity surges of debris continuous ejecta blanket are important in the swept over the rim, carrying the ejecta away understanding of the formation of this crater. and leaving the rim unit almost bare. This It is clear from mapping (Figs. 7, 8A) that the erosion, possibly during and directly after continuous ejecta blanket overlies the rim overturning, carried ejecta off the rim and unit and does not always continue right up to redeposited the debris in flowlike units to form the crater lip but leaves the rim bare. Where part of the ejecta blanket. These flowlike the rim is not covered by ejecta blanket on the units have flow markings, and fine fracture eastern side of the crater, the boundary of the patterns reminiscent of contraction cracks. ejecta blanket at the foot of the rim is marked The mapping also suggests that the upper by a shallow scarp rising above the surface of surface of the rim unit was undulating with the rim unit and facing Aristarchus (Fig. 5). broad, radial valleys and spurs over which the

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overturned flap of mare material {it possibly premare strata) ejecta blanket

overturned flap of prcmare strata from Aristarchus Plateau

block* lob«»

ejecta blanket

Figure 8. Hypothetical cross sections through southeastern (A) and northern (B) rims of Aris- tarchus. Not to scale. upper part of the ejecta blanket was emplaced. of closely spaced hummocks on the Orbiter V Stereoscopic viewing of the northeast rim of the medium-resolution photographs. It extends crater reveals that the ejecta blanket cuts out to about 6 km from the crater lip (see across a spur of upper rim-unit material extend- Fig. 3). ing out from the crater rim (Fig. 6). Prior to The surfaces of the hummocks show a weak the emplacement of the ejecta, the surface of fault pattern that is concentric to the lip of the the rim unit apparently was either strongly crater and similar to that on the ejecta blanket deformed into radial valleys and ridges at the where it directly overlies the rim units. The same time that it was cut by the concentric fault pattern is generally much less well faults, or parts of the upper rim unit of the developed on the hummocky rim unit, how- overturned flap were stripped off differentially ever, perhaps because of the greater thickness by radial surges from the crater. The boundary of the rim or lithologic differences in the between the ejecta blanket and rim unit may overturned flap. The question of why this therefore be uncomformable in places. hummocky rim unit is not present on the Although the fracturing is regular around eastern side of Aristarchus must be considered. most of the exposed rim, in some places there Some form of asymmetry in crater excavating is are local "kinks" in the pattern and even indicated with differences in the bedrock as angular foldlike shapes. Examples of these the most likely cause. It has already been noted are seen on Figures 6 and 7. These structures that a large fault bounding the Aristarchus may be irregularities in the fault pattern, or Plateau runs underneath tne crater (Fig. 3). the fractures themselves may have become As a result, the nature of the bedrock on either folded by movements in the rim. Another side of the fault is different; therefore, the possibility is that some of the features on the material excavated from the southeastern side rim represent bedding in the country rock and of the crater was basaltic mare lava, but on that the kinks are fold closures. This last the northwestern side, the material excavated situation would provide further support for an was older rock from beneath the Aristarchus overturned-flap origin for the rim unit. Plateau. Figure 3 shows that the hummocky ejecta unit lies only on the northwestern side Hummocky Rim Unit of the fault and, in view of this, probably When present, the hummocky rim unit represents bedrock from below the Aristarchus gives the crater rim a characteristic appearance Plateau which has been folded back on itself

Figure 9. Hypothetical cross sections through Aristarchus before (above) and after (below) crater excavation. Letters indicate units of Table 1. C = central peak; EC = floor material.Not Aristarchus Plateau to scale.

pre-mare strata

H R

to form the upper surface of the overturned Murray (1971). These features consist of two flap (Figs. 8, 9). ridges meeting in a V and pointing to the nearest principal crater, in this case Aristarchus. Bright, Discontinuous Ejecta Unit Most of the V-shaped features near Copernicus General. This unit is characterized by enclose a secondary crater at the apex. This is numerous small craters, which have a much generally true for those of Aristarchus (Fig. 3), less regular shape, tend to be elongated toward although in certain areas, V-shaped features Aristarchus, and are thus distinguished from form a braided pattern on otherwise featureless normal, primary impact craters. This area has ground. Guest and Murray (1971) note that, a thin veneer of continuous ejecta blanket and for the V-shaped features around Copernicus, patches of thicker ejecta. the included angle tends to increase with The distribution and shape of satellitic distance from Copernicus. Guest and Murray craters indicate that they were formed by suggested two possible origins to explain the impacts of debris thrown out from Aristarchus. observed characteristics: A direct analogy is found in the secondary 1. The V-shaped features were produced by cratering developed around nuclear and TNT deposition of debris under bow waves devel- craters (Shoemaker, 1965). The secondary oped in a base-surge cloud or some form of cratering in this zone was probably accom- radial flow away from Aristarchus. Secondary panied by fall of finer grade ejecta on the inner craters formed obstacles, causing bow waves in parts; farther away in the ray zones, the higher the radial flow as it poured over them, and albedo probably relates to ejecta thrown out also provided debris to build ridges. By this from the secondary craters. mechanism, the included angle would change Many of these secondary craters predate the with velocity, the angle becoming larger as the ejecta blanket; the blocks that formed the flow slowed down away from its source. secondary craters must have landed prior to 2. The second explanation derives from the material forming the bulk of the ejecta laboratory cratering experiments by D. Gault units. These secondary missiles probably were (in Guest and Murray, 1971) who had ob- material that broke away from the leading served that clots of ejecta were thrown out to edge of the flap while it was overturning. The give V-shaped ridges, the included angles of position of the crater straddling the mare- which increased with distance from the parent plateau boundary (Fig. 1) may have caused crater in the same way as for lunar craters. In the asymmetrical distribution of the bright this case, the included angle depends on the ejecta facies around Aristarchus (Fig. 2) be- angle of trajectory of the ejected clot. cause the difference in bedrock and structure Oberbeck and others (1972) have since could modify the trajectories of the ejecta. suggested that those V-shaped features in the Interpretation of V-shaped Features. The rays may be ridges of ejecta built up between presence of V-shaped features developed in adjacent craters formed by simultaneous association with secondary craters around impact. This idea is supported by convincing large rayed craters was discussed by O'Keefe evidence. and others (1969), and a further analysis for The V-shaped features are well developed those of Copernicus was given by Guest and up to about 2Yl crater diameters from the

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center of Aristarchus. From that distance of topographic control, imply that at least one outward, they occur only sparsely and are of :he mechanisms responsible for this unit poorly developed compared with the pattern was outward (radial) flow. Most of this unit, around the much larger crater Copernicus. however, was emplaced by falling on the surface The variation of the size of the included angle as indicated by the way in which it mantles the (i3) with distance (D) is given in Figure 10. As underlying terrain. with Copernicus, this angle tends to increase Outer Boundary. This boundary is not with distance up to the edge of the ejecta easy to define precisely, as the ejecta blanket blanket. The distribution of V-shaped features thins to a wafer edge until it becomes dis- is closely related to the distribution of the continuous. Small craters in the inner part of ejecta blanket, which for Aristarchus is asym- the zone of discontinuous ejecta tend to be metrical. To the northwest and south, the V- covered by the ejecta blanket, showing it to shaped features only occur close to the crater be younger than these satellitic craters. On and increase in angle more rapidly with distance the basis of the depth to which these craters (Fig. 10). This asymmetry tends to obscure have been buried by the ejecta blanket, an the more regular trend, as seen at Copernicus outer boundary may therefore be chosen that where there are many more of these features. corresponds to the same thickness of ejecta. If the V-shaped features occurring in a north- The limit taken in the present mapping was easterly direction are considered alone (Fig. that at which the satellitic craters begin to 10), the increase with distance is more clearly become subdued in form. It is evident that seen. A change in ¡3 with distance occurs even much of the ejecta exposed at the surface was on a single crater chain; one chain south of emplaced after both the uplift of the rim and Aristarchus has five successive craters with the fall of large missiles, resulting in secondary related V-shaped features increasing regularly craters in the satellitic zone. in angle ¡3 from 79° to 117° over a distance of 9 km. This is somewhat higher than the aver- Blocky Lobes and Smooth Flows age rate of increase for V-shaped features in this Blocky Lobes. On the northern outer edge area. of the hummocky rim unit, and extending for The interpretation of these features is about 3 km in front of it, are lobes of blocky important in understanding rayed craters: if, debris (Fig. 11). The lack of high-resolution for example, the ballistic explanation or that of photographic coverage of hummocky ejecta Oberbeck is correct, then angle /3 could be on the western side of the crater prevents used to define the trajectories of the causative detailed observations, and it is not known if the missiles. blocky lobes occur there also. The blocky lobes are flowlike in character, Ejecta Blanket consist of poorly graded material, and Description. The albedo of the ejecta have very irregular surface:;, suggesting that blanket is lower in the thicker parts, becoming the lobes accumulated as debris from the higher away from the crater until the ejecta hummocky rim unit which moved as an blanket grades into the bright, discontinuous avalanche, mixing material of all sizes to give ejecta unit. The darker facies of the blanket an unsorted deposit. This implies that the rim tends to follow the underlying topographic was unstable during the last stages of over- relief. Thus, to the northwest, a darker lobe turning and afterward (possibly because of the extends through the gap in the Aristarchus nature of the material depcsited), and masses Plateau leading to Schroter's Valley; to the of debris slid down the slope as this part of the north, the darker facies is bounded by the edge rim was emplaced. Instability may also have of the plateau; to the northeast, it extends resulted from outward flowage over the hum- along the eastern boundary of the plateau mocky rim of debris from the crater, as part of beyond the edge of the brighter parts of the the same process which eroded the upper sur- ejecta blanket (Fig. 2). face of the rim elsewhere. Developed on the ejecta blanket in some Spreads of boulders found on the eastern parts, especially on the northwestern and north- side of the crater (Fig. 7) bear a slight resem- ern sides (Fig. 11), there are low, lobate scarps; blance to the blocky lobes:, these spreads lie these have the form of flow fronts and, to- directly above the rim uni: but are possibly gether with the above-mentioned evidence younger than the continuous ejecta blanket.

Boulder counts show, however, that these ridging also occurs on landslips such as the spreads are less sorted and have a higher pro- Sherman Glacier landslide (Shreve, 1966), portion of much larger boulders than do the where the ridges are formed by vertical shear blocky lobes. A different origin is implied for planes in the moving flow. the spreads on the eastern side; they were There has been great controversy about the probably formed from clots or streams of origin of flows of this type on both Aristarchus blocky ejecta thrown out of the crater at a and Tycho. Shoemaker and others (1968) very late stage of the excavation. appear to consider it most likely that many Smooth Flows. Seven well-developed flows flows on Tycho were produced from nueelike with smooth surfaces may be identified on the flows generated by the impact. high-resolution Orbiter photographs (Fig. 11) On the other hand, Strom and Fielder (1970) in the same areas as the blocky lobes. Another argue that these flows (also the younger ridged flow of this type can be distinguished on the and leveed flows and the dark flows and lakes) medium-resolution photographs to the north- are volcanic (see Fig. 11). In this paper, the west of the hummocky facies, and to the east author distinguishes between the smooth (Fig. 6), there is one not related to a hummocky flows and the other relatively younger ones on rim. In all cases, these overlie the ejecta the basis that they were clearly produced by blanket. different mechanisms; although it could be The characteristics of these flows indicate argued that the leveed and dark flows are that they consist of fragmentai material, the volcanic, the same evidence is not forthcoming maximum grain size of which is less than 1 m. for the smooth-surfaced flows. The flowlike form of the units suggests that Evidence from mapping shows that smooth- this fragmentai material was emplaced from surfaced flows are interbedded with blocky density flows similar in character to the dense lobes, and that blocky lobes overlie them in basal phase of nuées ardentes. These flows places (Fig. 11). If this is correct, then the mantle low hills and, thus, are not lava flows smooth flows must be related in time to the which would have filled up depressions in the blocky lobes. Photographic evidence for this is topography to give a level surface with no given in Figures 12 and 13. The nature of the indication of the shape of the terrain below. blocky lobes suggests that they are ejecta, and The longitudinal ridging is characteristic of the way in which the blocky lobes grade back flows of fragmentai material. In a discussion of into the hummocky rim unit strongly sup- the geology of Tsiolkovsky, Erlich and others ports the view that both the blocky lobes and (1970) show an aerial photograph of pyroclastic the smooth flows are part of the ejecta se- flow deposits in Kamchatka having a pattern quence. of longitudinal ridges similar to those of the If the smooth flows are taken as part of this smooth flows described here. Longitudinal sequence, they can, like the blocky lobes, be

140 •

I20 •

• • I 2 3 4 5 6 D Figure 10. Plot of included angle (/3) of V-shaped superimposed curve is a best fit for Copernicus features features with distance from Aristarchus (D, in Aris- (scaled to Aristarchus diameters); the envelope for Co- tarchus crater diameters). Dots indicate features to the pernicus features is also given (after Guest and Murray, northeast; circles, those to the northwest. For com- 1971). parison with V-shaped features for Copernicus, the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/9/2873/3443274/i0016-7606-84-9-2873.pdf by guest on 26 September 2021 Figure 11. Geologic map of part of the northern rim. Mapped from NASA V-201-Hs and Ha. For key, see also Figure 6.

explained as deposits formed by outward move- depends on the numerical densities of craters ment of debris, and in this case, as flows of on these units. These have been used by Strom finer material surging downslope as clastic and Fielder (1970) to argue that the smooth flows. flows are much younger than the cratering The argument against this interpretation event.

Figure 12. Flows on the northern rim: part of smooth flows (F), leveed flows (L), and dark flows NASA V-201-H3, showing hummocky rim unit (H), (DF). This is the western part of Figure 9.

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CRATER COUNTS to provide an area large enough to make the crater counts significant. Strom and Fielder It has been generally accepted that counts group together all the flows regardless of of impact craters on a lunar surface provide a morphology, but count the hummocky rim measure of the age of the surface being in- unit and ejecta blanket separately; Greeley vestigated, on the basis that if the surface is and Gault, on the other hand, group together being bombarded by meteoritic particles, the all the units near the rim for one set of counts older the surface, the more craters there will and compare these results with those for the be. Mutch (1970) has, however, listed some ejecta blanket. The conclusion of all these of the factors that may invalidate this method authors is that there are fewer craters per unit under certain circumstances. area near the rim than on the ejecta blanket. Strom and Fielder (1970) and Greeley and Strom and Fielder go fuither in suggesting Gault (1970) have measured crater densities that all the distinct flows on or near the rim on surfaces in the ejecta blanket area north are younger than both the ejecta blanket and of Aristarchus. For the purposes of counting, the rim by a considerable pe riod of time. When the authors of both these papers group the counts are grouped in this way, the results younger units of the rim area together so as appear to contradict the mapping evidence

Figure 13. Flows on the northern rim: an enlarge- flow (F) interbedded with block) lobes (B). ment of part of NASA V-201-M2 showing a smooth

which indicates that, although the ridged and during the impact event may not have retained leveed flows on the rim may be relatively many of the secondary impact craters formed young, the smooth flows are older and part on them and, on the basis of crater counts of the sequence of throwout produced during alone, appear younger than the relative ages the excavation of Aristarchus. assigned to them from geologic mapping. If In order to consider this further, I counted secondary impact craters are responsible for the craters for each mapped geologic unit the anomalous crater densities, then many of separately (Table 2). These figures are not the craters on these units are not primary presented here with the aim of using them to impact craters. Under these conditions, it is demonstrate a stratigraphic sequence or to difficult to use crater counts to derive relative interpret them, since the arguments that areas ages for units. used are too small may be valid. However, the new figures do demonstrate that if the CONCLUSIONS ridged and leveed flows and the smooth flow Present understanding of cratering mech- units are grouped together for counting, the anisms and the observed geology of Aristarchus sparsely cratered, ridged and leveed flows on from Orbiter photographs indicate that the rim will reduce the average for the smooth Aristarchus was formed by a shock-wave- flow units, implying that the smooth flows cratering event generated by impact. Geolog- are less cratered than they really are. Thus, the ical mapping of the crater suggests that the counts of Strom and Fielder give the misleading rim is an overturned flap of country rock con- impression that the smooth flows are much sisting of mare lava on one side and Aristarchus younger than Aristarchus. Therefore, the Plateau rocks on the other side of a major fault crater counts as expressed by Strom and below the crater (Fig. 9). Differences in Fielder do not necessarily argue against my morphology of the rim relate to these dif- interpretations. ferences in lithology. It does appear, nevertheless, that the surfaces As the flap was overturning, large blocks of units emplaced within a short time of one broke away from its leading edge; these be- another can have different crater densities came secondary missiles which landed about which do not necessarily correspond with the one crater diameter away from Aristarchus, stratigraphic relations; thus, some older units giving the characteristic bright halo of dis- may have lower crater densities on their sur- continuous ejecta and secondary craters. The faces than younger ones have. A similar prob- rays were probably also formed at this time lem was noted for Tycho (Shoemaker and by falling filaments of ejected material. The others, 1968). These authors concluded that continuous ejecta blanket was then deposited this anomalous situation was caused by second- by a number of mechanisms, including fall ary impact craters being formed (from missiles of material and outward flowage. Studies of in high-angle trajectories) on the units as they artificial shock-wave craters predict that much were being emplaced: flows that were moving of the ejecta blanket was formed as part of the overturning process but was modified into its TABLE 2. CRATER DENSITIES ON UNITS NEAR present form, which is distinct from the rim RIM OF ARISTARCHUS units, by continued outward movement after

Unit Number of craters emplacement. The rim was then denuded of (per 10 km2) ejecta-blanket material by continued outward (radial) flowage. Instability of the hummocky Rim (north) 68 rim unit was induced by the radial flowage; Rim (southeast) 48 avalanching produced lobes of blocky ejecta Ejecta blanket (north) 89 and smooth flows overriding the ejecta blanket.

Ejecta blanket (southeast) 51 Instability of the hummocky rim was likely due to the nature of the Aristarchus Plateau Hummocky material 46 rocks overturned here or the nature of the Smooth flows 62 overturning itself. Dark flows 71 Finally, flows of viscous material and dark Ridged and leveed flows 19 flows were formed locally on some parts of the rim. The origin of these materials is not clear, Note: Only craters larger than 30 m included. and, although they may have been formed by

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volcanism long after the Aristarchus event, it ing lunar defluidization i.nd volcanism: New is also possible that they consist of shock-melted York Acad. Sci. Annals, v. 123, p. 403-467. rocks that either fell back onto the rim area as • 1971, Copernicus as a lunar caldera: Jour. ejecta or were extruded from pseudovents on Geophys. Research, v. 76. p. 5719-5731. Guest, J. E., 1971, Geology of Tsiolkovsky, a lunar the crater rim. farside crater, in Fielder, G., ed., Geology and Other rayed craters such as Copernicus, physics of the Moon: Amsterdam, Elsevier, Tycho, and Kepler have units similar to those p. 93-103. of Aristarchus. There are, however, notable Guest, }. E., and Murray, J. B., 1969, Nature and differences in distribution and sequence of origin of Tsiolkovsky crater, lunar farside: units; this may indicate differences in lithology Planetary and Space Sci., v. 17, p. 121-141. and structure of the bedrock at the other sites 1971, A large-scale surface pattern associated or different types of impacting body (comets, with the ejecta blanket and rays of Copernicus: The Moon v. 3, p. 326-336. solid meteroids, or friable meteoroids). A Howard, K. A., Offield, T. W„ and Wilshire, M. C., careful study of other rayed craters could lead 1972, Structure of Sierra Madera, Texas, as a to a better understanding of impact mech- guide to central peaks of lunar craters: Geol. anisms and also of the geology of the under- Soc. America Bull., v. 83. p. 2795-2808. lying rocks. Kuiper, G., Strom, R. G„ Whitaker, E. A., Fountain, J. W., and Larson, S. M., 1967, ACKNOWLEDGMENTS Consolidated lunar atlas: Tucson, Ariz., Lunar This work forms part of a research project, and Planetary Laboratory, 228 plates. financed by the National Environmental Lunar Sample Preliminary Examination Team, Research Council (London), to study the 1969, Preliminary examination of lunar samples from Apollo 11: Science, v. 165, p. nature and origin of lunar surface features. 1211-1227. Discussions with R. G. Strom were particularly — 1970, Preliminary examination of lunar valuable during the final mapping stage of the samples from Apollo 12: Science, v. 167, p. work, as were general discussions with G. 1325-1339. Fielder, J. B. Murray, and R. Greeley. D. — 1972, Preliminary examination o£ lunar Roddy reviewed the manuscript with care and samples from Apollo 15: Science, v. 175, p. gave much constructive criticism. Orbiter 363-375. photographs were kindly provided by the McCauley, J., 1967, The nature of the lunar surface National Aeronautics and Space Administra- as determined by systematic geologic mapping, tion through the National Space Science Data in Runcorn, S. K., ed., Mantles of the Earth and terrestrial planets: New York and London, Center. John Wiley & Sons, Inc., p. 431-460. Moore, M. J., 1965, Geologic map of the Aristar- chus region of the Moon: U.S. Geol. Survey REFERENCES CITED Misc. Geol. Inv. Map 1-465, scale 1:1,000,000. Baldwin, R. B., 1963, The measure of the Moon: 1967, Geologic map of the Seleucus quad- Chicago, Univ. Chicago Press, 488 p. rangle of the Moon: U.S. Geol. Survey Misc. Dence, M. R., 1968, Shock zoning at Canadian Geol. Inv. Map 1-527, scale 1:1,000,000. craters: Petrography and structural implica- Murray, J. B., and Guest, J. E., 1970, Circularities tions, in French B., and Short, N., eds., of craters and related structures on Earth and Shock metamorphism of natural materials: Moon: Modern Geology, v. 1, p. 149-159. Baltimore, Mono Book Corp., p. 170-184. Mutch, T., 1970, Geology of the Moon: Princeton, Erlich, E. N„ Gorshkov, G. S„ Melehester, I. V., N.J., Princeton Univ. Press, 324 p. and Steinberg, G. S., 1970, The structure of Oberbeck, V. R., Morrison, R. H., and Wedekind, the lunar crater Tsiolkovsky: Modern Ge- J., 1972, Lunar secondary craters, in Apollo ology, v. 1, p. 197-201. 16 preliminary science report: U.S. Natl. Fielder, G., 1965, Lunar geology: London, Lutter- Aeronautics and Space Adm. Spec. Pub. SP- worth Press, 184 p. 315, p. 29-51-29-56. Greeley, R., and Gault, D. E., 1970, Precision O'Keefe, J., Cameron, W. S., and Masursky, H., size-frequency distributions of craters for 1969, Hypersonic gas flow, in Analysis of 12 selected areas of the lunar surface: The Apollo 8 photographs and visual observations: Moon, v. 2, p. 10-77. U.S. Natl. Aeronautics and Space Adm. Spec. • 1971, Endogenic craters interpreted from Pub. SP-201, p. 30-32. crater counts on the inner wall of Copernicus: Roddy, D., 1968, The Flynn Creek crater, Ten- Science, v. 171, p. 477-479. nessee, in French, B., aid Short, N., eds., Green, J., 1965, Tidal and gravity effects intensify- Shock metamorphism oi natural materials:

Baltimore, Mono Book Corp., p. 291-322. Strom, R. G., and Fielder, G., 1970, Multiphase Shoemaker, E. M., 1962, Interpretation of lunar eruptions associated with the lunar craters craters, in Kopal, Zednek, ed., Physics and Tycho and Aristarchus: Arizona Univ. Lunar astronomy of the Moon: New York, Academic and Planetary Lab. Commun., no. 150, p. Press, p. 283-380. 235-288. 1965, Preliminary analysis of the fine struc- Wilhelms, D., 1970, Summary of lunar stratigraphy ture of the lunar surface in Mare Cognitum, —Telescopic observations: U.S. Geol. Survey in Hess, W. N., Menzel, D. M., and O'Keefe, Prof. Paper 599-F, 47 p. J. A., eds., Nature of the lunar surface: Balti- Wilhelms, D„ and McCauley, J. F„ 1971. Geologic more, Johns Hopkins Univ. Press, chap. 2. map of the nearside of the Moon: U.S. Geol. Shoemaker, E. M., Batson, R. M., Holt, H. E„ Survey Map 1-703, scale 1:5,000,000. Morris, E. C., Rennilson, J. J., and Whitaker, E. A., 1968, Television observations from Surveyor VII, in Surveyor VII: A preliminary report: U.S. Natl. Aeronautics and Space MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER Adm. Spec. Pub. SP-173, p. 13-81. 8, 1971 Shreve, R. L., 1966, The Sherman Landslide, REVISED MANUSCRIPT RECEIVED OCTOBER 19, Alaska: Science, v. 154, p. 1639-1643. 1972

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