Pro( LUI/ar Pial/e!. Sci. Con/. 10th (J979), p. 26..s1 -2(1()~L

PrirHed in Ihe UnI[ed Slales of AmeriG\

Ejecta emplacement of the martian impact crater Bamburg

Peter J. Mougjnis-Mark

Department of G eological Sciences, Brown University. Providence, Rhode Island 02912

Abstract- Six exterior deposits su rround the martian impact crater Bamburg (55 km in d iameter). T he sequence of ejecta emplacement , although more complex. conforms to the same depositiona l history that has prod uced the ejecta deposits a round martia n rampart craters smaller than 30 km in diameter. D uring ejecta e mplacement, secondary crater fo rmation preceeded the deposition of highly mobile su rface flows, which in tum we re over run by more viscous flows that are characlerized by longi tudinal grooves and transverse ridges. Numerous a reas of flat terrain upon the ejec ta deposits, and the identification of I veed channels on the wall s of Ba mburg , may indicate that either late in the cra tering event, or after fin al ejecta emplacement , sedi ment- laden melt water percolated out of the vol atile- rich ejec ta and the c rater ri m.

The number of secondary c raters associated wilh Bamburg is less tban one third the commensurate valu e for lunar and mercu rian craters of equ ivalent size . T he maximum areal density of these marli,m seconda ry cra ters is observed at less than hajJ the range of those associated with the comparable me rcu rian c rater Marc h. The defic iency of Bamburg secondary craters is attributed ei ther to pref­erent ial d struction of ejecta blocks sufficiently large to form secondary craters or the subseq uen t burial of such craters o nce formed .

INTRODUCTION

Bamburg crater, approximately 55 km in diameter, lies to the ea t of Acidalia Planitia and is centered at 40oN, 3°W. From Mariner 9 images, the crater was interpreted to lie On lhe boundary between plains material to the north and lower plateau material to the south (U nderwood and Trask, 1978) . Viking photograph y (Fig. I) ill ustrates that this plains material can be subdivided into remnant smooth plains material to the east and fractured plains in the west (Guest e! al ., 1977) . Bamburg lies approximately 80 km north of the martian highland boundary de­scribed by Scott (1978).

Because of the high resolution (40 meters per picture element) images acquired by the Viking orbiters of the crater and its surroundings, Bamburg affords an excellent opportu nity for the analysis of the depositional processes and resultant morphologicalfeatures associa ted with the formation ora complex martian impact crater. Secondary craters and a varie ty of ejecta units can be identified that are absent from martian craters smaller than 35 km in diameter (Carr et a! ., 1977 ; Mouginis-Mark and Head, 1979) . These ejecta materials are described here in deta il in an attempt to identify similarities between Bamburg and smaller martian

2651

2652 P . .J. Mouginis-Mark

Fig. I. Regional sell ing of Bamburg Crater. To the nOrth and west lies the fractured plains ma le rial of Guest e f £II. (1977). Smooth plains material occurs in the eastern part of the area illustrated . The highland boundary described by Scott (1978) approximately corresponds to the southeastern third of the image . Rectangle shows the location of Fig. 2. Viking orbiter frames 673/855-64 .

crat rs. The secondary crater distribution is compared to the lunar and mercurian examples cited by Gault et al . (1975) to contrast cratering events in the martian environment with those in the vacuum conditions of M rcury and the moon.

DESCRIPTION OF THE MORPHOLOGICAL UNITS

The type of exterior deposits surrounding fresh impact craters on Mars is gra­dational with crater size (Mouginis-Mark, 1979a). Single continuous ejecta facies, apparently emplaced by a . urface-ftow process (Carr et al ., 1977), typically are seen around craters smaller than 15 km in diameter. Craters 5- 30 km diameter may have two concentric deposit. . For diameters larger than 30 km, multiple, fluidized. lobate flows or complex ejecta blankets with large azimuthal variations for a given range predominate. Bamburg conforms to this "complex ejecta" clas­sification (Type 5 craters, Mouginis-Mark, 1979a) and possesses several mor­phological units analogous to deposits seen around martian rampart craters smaller than Bamburg, or fresh impact craters on the moon (Howard, 1974) and Mercury (Gault et £II., 1975; Cintala el al. , 1977) .

Figure 2 shows the high resolution Viking photography from which the mor­phological map (Fig. 3) was produced. Four materials con titute the interior

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Fi.g. 2. Photomosaic of Bamburg C rater, showing the area for which the morphological map (Fig. 3) has been compiled . Also displayed are the locations of lhc type localities of the five ejecta deposits illustrated in Fig. 4. Viking orbi ter frames 70A21-32 e '" and 72A 19-32. <..v

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Fig. 3. (a) Morphological map of the inte rio r and exterior deposits associated with Bamburg . See key (Fig. 3b) and tex t for descrip tio ns .

Ejecta emplacement of the martian impact crater Bamburg 2655

depos its and six materials were identified beyond the rim crest. A brief descrip­tion of each unit is given b low, together with the type localities for the ejecta depos its (F ig. 4).

INTERIOR DEPOSITS

Central peak material (Cp)

T he diameter of the central pe k material is approximately 10 km. The summit portion of this peak comprises a near-circular pit 6 km in diameter, breached on its northern wall , with a fla t floor 1 x 2 km in extent. Such features have been identified by Smith and Hartnell (1977) and Hodge (1978) and are attributed by Wood et al. ( 1978) to explosive decompression of subsulface volatiles within the target duri ng crater formation . No . {ratification i evident within the peak ma­terial, but a linear ridge can be extrapolated from the southern pit rim to outcrops on the northern pit rim and may represent a structural trend orientated 10° west of north.

Floor material (Fm)

Much of Bamburg's floor appears to be covered by eolian or other sedimentary material. The surface of this material is relatively flat but domed slightly toward the cent ral peak . Gentle slopes extend from the walls of the peak to the crater floor whereas smooth deposi ts are interdigitated between the floor and the in­nermost wall ma terial.

INTERIOR DEPOSITS EXTERIOR DEPOSITS

WALL FLOW SMOOTH TE R R AIN ~

'_ I .~,I'" M ATERIAL (Wf) MA T ERIAL (SI)Cd ~ WA LL M AS S FLO W[I] D2JMAT ERI AL (Wm) MAT ERIA L (Mf)

FLOO R ROUGH RADIAL - .0 MATERIAL (Fm) D.", MATE RIAL (Rr)

[II CENTRAL PE AK SMOOT H RADIALDMATERIAL (Cp) >, " MATE RIAL (Sr)

• PITTED TE RRA I N MATER IAL (PI)

RIM Fig. 3. (b) ~ MA TERIAL (Rm)

2656 P. 1. M Ollginis -Murk

WaJl material (Wm)

Multiple occurrences of ridged material characterize the wall unit of Bamburg. In places, this wall material may be [5 km wide, corresponding to 0.55 crater rad ii. As many as eight discrete ridges can be identified in any radial direction from the central peak, but few ridges conti nue for more than lOG of arc. Each ridge may re present the edge of a tilted terrace block (hat has been partially buried by subsequent material, Ilt there are no direct counterparts to the terraces and scallop of lunar and mercurian craters (Cin tala ef a l. , 1977). In between these wall ridges , deposition by creep and slumping ha produced flat inliners which occasionally extend to, and merge with . the floor material.

Wall How material (Wf)

A series of leveed channel · 2 - 4 km in length extends from the lim crest to the lower wall ridges of the southern wall of Bamburg. In detail, these channels are similar to the channels observed on the ails of the lunar craters Tycho and Aristarchus (Strom and Fielder, 1971; Hulme and Fielder, 1977). No obvious source areas (pits or ponded material) are evident for an y of these martian chan­nels, however, so that an impact melt origin such as described by Hawke and Head (1977) for the lunar examples appears inapplicable in the case of Bamburg. Impact melts associated with martian craters remain unidentified. and extrapo­lation of terrestrial field data and theore tical models (Kieffer and Simonds, 1979) predict that melt sheets would be preferentially assimilated during cratering events in volatile-rich targets on Mars. If this is the case. then the best alternate explanation for the formation of the e channels appears to be that they are of fluvial origin. possibly associated wi th the outward percolation of melt water incorporated within the rim unit of the crater. The duration of this release of melt water is not clear, but the superpo ition ing of these channels upon the wall ridges indicates that they post-date the wall-fai lure stage of crater formation .

EXTERIOR DEPOSITS

Rim material (Rm)

Bamburg possesses hummocky rim mate Iial (hat resembles the equivalent unit around lunar craters (Howard . 1974) . In many places, th is hummocky facies is composed of a series of small ridges, aki n to the ridges of the wall material. On the northern rim. there is evidence of radial scouring that is morphologically similar to triations o n the southeastern ri m of the lunar crater Aristarchus (Guest, 1973). Guest believes that the scouring on the rim of Aristarchus is associated with the stripping of the initial ejecta deposits by high velocity debris

Ej ecta emplacemel1t of the martian impact cra ter Bamburg 2657

su rges, po sibly during or di rectly after the overturning of the crater rim. Such an explanation may also be applicable to Bamburg, because the scour marks fade rapidly with increasing distance from the rim rest and blend with the surrounding flo w deposits that overlie the dista l portions of the rim mate rial.

Pitted terrain material (Pt)

Widely distributed around Bamburg to radial distances greater than fou r crater radii from the pri mary center, this u nit comprises a series of closely packed, small-scale depre sions and mounds, each several hundred meter in diameter (F ig. 4a). Because these features are close to the re olution of the Viking images, their de tailed morphology and mode of emplacement cannot be identified, but they appear to represent a discrete component of the ejecta inasm uch as similar topography is not observed at greater distances from Bamburg, Only faint bound­aries of the pined terrain material can be seen as a consequence of subsequent ejecta emplacement. Th is is indicated by a fainl pattern of ridges and grooves orientated radial to Bamburg that have been superimposed upon the pitted terrain material by the overriding smooth radial material.

Smooth radial material (Sr)

Extending to more than 4.7 crater radi i from the center of Bamburg, the mooth radial material (Fig. 4b) is the mo< t extensive of the ejecta deposits. The presence of longitudinal grooves and subdued, but readily identifiable, distal ridges on much of the smooth radial material present close similarities to the continuous ejecta facies around rampart craters mailer than 35 km in d iameter (Carr et at ., 1977).

The maximum ejecta range (ER) for the Bamburg smooth rad ial material is comparable to (bUl slightly larger than) the ER values obtained for the continuous ejecta deposits surrounding rampart craters 2- 35 km in diameter formed in similar environments, suggesting a common mode of formation. ER value s for martian rampart raters have been shown to be a function of c rate r altitude, latitude, target material and c rater size (Mouginis-Mark. 1979b). Comparing the range of the Bamburg smooth radial material to the data of Mouginis-Mark (I 979b), this maximum range is 6.8% larger than the mean ER value (4 .4 crater radii) obtained for a < ample oC 105 rampart craters 2- 35 km in diameter excavated in the martian fractured plains material. A mean ER value of 4. 1 crater radi i for 92 rampart craters formed at an altitude of O- l km above Mars datum (Bamburg lies at an e levation of 500 meter ) between 40° - 50o N, indicates that the smooth radial ma­terial of Bamburg i likely to be an equivalent ejecta deposit to the continuous ejecta flows surrounding rampart craters smaller than 35 km in diameter. The slightly larger ER value for Bamburg than for craters snmlJer than 35 km is

2658 P . J. M ougillis-Mark

Fig. 4. f ype localities for the five ejec ta deposits surrou nding Bamburg: A) Pitted terrai n material; B) Smooth rad ial material; C) Rough radial material ; 0 ) Ma~s Row material; E) Smooth terrain material. Location of each area is jllustrated in Fig. 2.

attributed to the difference in crater size ; Mouginis-Mark (1979b) demonstrated that craters larger than 30 km in diameter commonly have ejecta materials that travelled a proportionally greater distance than ejecta from 15- 30 km diameter craters excavated in the ame target material. In tum, ejecta materials around 15-30 km diameter craters are proportionally more extensi ve than ejecta sur­rounding craters smaller than 15 km in diameter.

EJec ta emplacement of the martian impacr craieI' Bamburg 2659

Many circular depressions up to several kilometers in diameter occur within the smooth radial material. OnJy a few of these depressions have raised rims, but the majority are int rpreted to be partially infilled secondary craters . Small-scale, pre-existing topography also appears to have been affected by the passage of the material compri ing th is unit; obs tacles a few hundred meters high have been traversed by the outward movement of the ejecta flows .

Rough radial material (Rr)

T he material of this unit extends to radial distances of 2.5 - 3.5 crater radii from the center of Bamburg. It is characterized by hummocky terrain, with superposed radial scour marks and elongate depressions ( "ig. 4c). Where pre-existing topog­raphy was encountered by the outward moving ejecta (e.g. , northeast of Bam­burg), there is an increase in the size and frequency of depressions. Very few of these depres ions have raised rims, and there is ample evidence of partial infilling by subsequent deposits. This infi ll is best seen within the three primary craters to the south of Bamburg, which have had their proximal rim crests overridden by the rough radial ejecta so that their floo rs are partially buried. The rough rad ial material is interpreted to have a similar mode offormation as the inner continuous ejecta facies that surround many rampart craters 10- 30 km in diameter (Mouginis­Mark, 1979a), due to their similar positions relative to their parent crater and the convex distal edges that are a charac teristic feature of both materials.

Mass How material (Mf)

Mass flo w material is typified by hummocky topography with many low trans­verse ridges spaced approximately 0. 5- 2 .0 km apart (Fig. 4d). These ridges, which commonl y extend to the crater rim and blend with the slumped wall blocks of the rim material, may in part owe their origin to the failure and subsequent movement of the rim deposits. The distal edges of these flows, which may extend three crater radi i fro m the primary center, end abruptly in convex slopes where the flows have overridden the earlier ejecta deposits . Few secondary craters are preserved wi thin this unit, the only examples observ d are larger than 2 km in diameter and lack prominen t rims as the result of burial of their exterior depos its . Examples of wall pluc king also are apparent on the distal sides of several smaIl craters that occur within the mass flows. Whereas the proximal and side waIls of these craters have remained in tact , waIl material from the distal rim has been "rafted" along with the flows.

Smooth terrain material (St)

Abnormally smooth material (Fig . 4e) is superimposed upon the other ejecta units of Bamburg. A paucity of surface detail on this material can be identified, al­

2660 P . J. M OII&inis-M ark

though faint ridge radial to the primary crater can be observed at a few localities . The large area of sm oth terrain material east of Bamburg may be due to re ur­faci ng of that area by channel deposits originating from the highlands to the southeast Scott, 1978). Howe ver, other examples of smooth terrain material appear to have a differen t, impact-related origin, because of the ir intimate a ' so­cjation with the ejecta material of Bamburg. The ponding of sedi ment-laden ma­terials from ejecta excavated from water- or ice-rich ubstra te may have produced this material after final ejecta deposition by the upward percolation of entrapped melt water (Rehfuss et al., 1978; Mougin is-Mark , I 979c). However, no feeder tributaries or channels are observed around the perimeter of the smooth terrain material to suggest that it was formed by the collection of sediment in localized depressions. A primary. late-stage depositional process during the cratering event therefore appears to be more accep table as a mechanism to produce the smooth terrain material. although il. emplacement mechanism remains unresolved.

SECONDARY CRATER DISTRIBUTION

A total of 657 near-circular depressions larger than 1 km diameter were identified around Bamburg out to radial distances of 110 km from the primary crater center (Fig. 5). Although many of these depressions lack raised rims and the character­istic herringbone pattern of lunar secondary craters (Guest and Murray, 1971;

Fig. S. Areal distribution of 657 depressions larger than I km diameter interpreted to be secondary craters associated with the Bamburg impact. Al so shown are the primary crater' s rim crest and radial distance from the center of Bamburg.

£jecla empia cemenl of Ihe mar/ian impaCl crilier B amburg 2661

Oberbeck and Morri on, 1973). they are inferred to be Ba mbu rg secondaries because of their spatial distribution ce ntered arou nd Bamburg. These morpho­logical differences are consequent ly att ributed to the overriding of the martian secondary craters by the surface flow of ejecta duri ng the latter stages of the cratering even t.

Schultz and Gault (L979) commented that the total vis ible secondary crater population around Bamburg is notably deficient in comparison to lunar and mer­curian examples at equivalen t distance s from the primary. T hi s analys is supports the ir observation, and compares the areal distribution of martian secondaries with the data of Gault el aL. (1975) for satellite craters around Ari tarchus (40 krn in diameter) on the moon and the 77 km diameter mercurian crater March CHoN. 176°W). Gault et aL. ( 1975) included in the ir counts secondary craters larger than 0.014 of the primary rim diameter. Thi arbitrary cut-off poin t conesponds to Aristarchus secondaries 0.56 km in diameter and 1.08 km diameter craters around March . In this analy, is. the de nsity of all Bamburg secondary craters larger than 1 km in diameter was measured . The areal density of Bamburg secondary craters. normalized to the diameter of the parent crater, is presented in Fig. 6 with the equivalent distributions for Aristarchus and March.

120 N E -'" MARCH_"b 10 0 (77km) "­>­~ 80

AR IS TARCHUS lJ.J :J ~ (40km)o w 60 cr u...

>- 4 0 cr BAMBURG <f - (55km)o 5 2 0 u w CJ')

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DISTA NCE FRO M CE NTE R OF PRI MA RY (CRATER RADII)

Fig. 6. Distribution of secondary c ra ters around Bamburg compared to the data of Gault el al. (1975) for the luna r crater A ristarchus and March on Mercury. Data demonstrate that many more seconda ry craters are observed around the lunar and merc urian c raters than Bamburg. T he maximu m conce ntration of secondary craters is observed muc h closer to the primary rim c rest for B.~mburg than for March, despite similar surface gravities for the two planets , suggesting that atmospheric ueceleration may have influ­enced the distribution of the martian ejecta .

2

2662 P . J. M ouginis-M ark

Fig. 7 . . tereo pair () f the c rater chain north of Bamburg, illustrating the area discussed in lex t and mapped in Fig. 8. Viking orbiter frames 37A45 and 37A46 (left), 70A27 (righ t).

The distribution of Bamburg secondary craters differs from that of March and Aristarchus in two respect :

I. The maximum density (44.7 craters/1 03 km2) of preserved secondary craters

for Bamburg is approximately one third the number of secondaries around the other two craters. This may be an observational effect , however, since many of the observed craters di play evidence of subsequent overriding by ejecta flows, suggesting that additional craters may have been totally buried and remain undetected. Alternatively, fewer secondary craters may actually have been created, perhaps as a consequence of pre-impact target charac­teristics preventing the production of sufficiently large ejecta blocks to form satellite craters (Head , 1976a; Schultz and Mendell, 1978).

2. Maximum areal density for Bamburg secondary craters occurs at a distance of 1.3- 1.7 primary radii from the center of the parent crater, much closer to the rim crest than for March (2 .5 -3.0) and Aristarchus (3.0- 3.5) . Previous attempts to model ejecta trajectories on Mars (Blackburn , 1977; Tauber et al ., 1978; Schultz and Gau lt, 1979) and the role of atmospheric drag on

Ejecta Cl I1pi llCefl]C I11 of rhe //I onian impact crarer Bam bll rg 2663

ejecta (Sheerwood, 1967: Wilson, 1972; Settle, 1979) all qualitively predict the bserved reduction in range of ejecta particle' and the ob erved con­centration of martian secondary craters closer to the primary than compa­rable features produced under the similar gravitational field but vacuum environment of Mercury. The extent of this atmospheric deceleration of ejecta may be related to the ejecta particle size distribution (Seebaugh , 1976; Schultz and Gault, 1979), which in turn should be influenced by the target lithology.

EJECTA EMPLACEMENT

Contrasting emplacement sequences have been inferred for lunar and martian impact crater ejecta . On the moon, the ballistic transport and deposition of ejecta appears to be the dominant process for sub-kilometer sized craters (Oberbeck, 1975). For craters larger than 1 km diameter, the ejecta have sufficient velocity to crater pre-existing terrain and generate a mixture of primary and locally derived material that moves a short distance laterally away from the crater (Oberbeck, 1975; Oberbeck ef nl ., 1975). The generation of ground surges and the surface flow of lunar ejecta have been deduced from detailed mapping of large craters and basins (Howard, 1972; Guest. 1973; Head. 1976b).

On Mars. ejecta emplacement is dominated by apparently similar surface flow processes (Carr el (II., 1977), probably as a con equence of ejecta fluidization by volatiles originally within the target material (Mouginis-Mark, 1979a). Mart ian ejecta mobility has been predicted to be related to ejecta viscosity (Gault and Greeley, 1978), and is influenced by the target material, the latitude and altitude of the crater. and the size of the cratering event (Mouginis-Mark, 1979a). Multiple phases of deposition have been proposed for the emplacement of ejecta around martian craters larger than approximately 30 k.m in diameter (Mouginis-Mark and Head, 1979).

Image resolution of much of Bamburg's ejecta blanket is insufficiently clear to identify this emplacement sequence, but a ke y locality for thi s purpose exists around the crater chain extending northward from the crater. A stereo photo-pair of this area is illustrated in ig. 7 and a morphological sketch map is provided in Fig. 8. Contained within the chain are approximately 40 individual craters, sep­arated from a sequence of flow lobes (fla-fld and fza-f2c in Fig. 8) within the smooth radial material by longitudinal ridges along tbe side of the chain. In total the chain is 60 km long and extends 100 km from the northern rim of Bamburg.

Very few secondary crater ' within the chain, or beneath the exterior flows, posse s raised rims. The majority of these craters have been partially overridden and buried by late arriving ejecta flows. The few examples of secondary crater rims that exist are on the ioward facing walls of the craters close to the center of the chain (Rr in Fig. 8), and these appear to have been preserved by the deflection of the flows by the other chain members. Several longitudinal ridges

2664 P . J . !vIougini.I-Mark

parallel much of the length of the chain. T hese are interpreted to be the remnants of secondary ejecta deposits, the majority of which have been removed by the passage of the ejecta flows,

Furth r eviden e for the production of the crate r ha in as a discrete feature prior to the arrival of the firs t ejecta flows can also be identi fied at several local ­ities:

1. The st ring of craters to the east of the chain (Be in F ig. 8) have een uniformerly blanketed by the f2c flow lobe.

2. The chain has effectively spli t the sequence of fl ows comprising the smooth radial material into two unrelated groups, with li tHe correlation between the bou ndaries of the f l lobe seque n e to the west and the eastern, f2 , series .

3. At the points Br in Fig. 8, the proximal walls of large secondary crate rs have been breached by the ejecta flows, resulting in partial infi lli ng of the crater floor. At the poin t Sf, the distal wall of the secondary crater has also been removed, allowing a small lobate flo w to emerge fro m thi s area and cover part of the already e mplaced Cb depos it.

Pitted terrain mat rial is difficult to place in the em placement chronology of th is area, primarily because the small-scale topography cannot be adequately resol ved in the available Viki ng image . Northwest of Bamburg, the superposition relationshi ps are easier to identify. Long it ud inal ridges parallel to the flow lines wi thin the smooth radial ejecta flow exist upon tbe pitted terrain, impl ying a pene ontemporaneous emplaceme nt ti me for the pitted terrain and secondary craters. Pitted terrain material may be a close analogue to the format ion of sec­ondary craters, and might be the product of mall-scale secondary cratering by the fragmented "clumps" of fine ejecta post ulated by Schultz and Mendenhall ( 1979) .

T he juxta position of the mass flow material and the rough radial material illus­trated in F ig . 8 makes their times of e mplacement qu ite s imilar. South and east of the crater chain, the mass flows appear to override the rough radial mate rial, but in many places one uni t blends into the other with no obv io us break in continu ity. Both ejecta materials override the smooth rad ial material. with well defined convex slopes mark ing their distal edges .

No good local it ies ex ist close to the crater chain that offer the evidence needed to con fi den tly place the smooth terrain material into a un ique position within the deposi tional ,.equence. EJ ewhere on the ejecta material of Bamburg, the absence of secondary crater and the ridges characterist ic of the smooth and rough radial materials would indicate that the deposition of the smooth terrain material is a late-stage phenomenon. T he in terfingering of the unit edges at the low boundaries of the smooth and rough radial materials argues against a surface-flow emplace ­men t mechanism for the smooth terrai n materia l. Nor is the infilling of local depre sions by the percolation of sediment-laden melt water a n acceptable mode of formation because of the absence of feeder channels on the perimeter of each de posit. Consequently , it is postulated that the smooth terrain material represents

J::.jecta emplacement of th e martian ill7pact Cra IeI' B amburg 2665

the accumulati n of fine ejecta tha t is directly emplaced in the observed localities after the emplacement of the surface flow materials.

CONCLUSIONS

High resolution Viking images of Bamburg permit the identification of ejecta fac ies not previo usly recognized arou ntl martia n impact c raters. Several conclu­sions can b drawn that relate to the emplacement seque nce of rhe eject a and the gross characterist ics of the target material :

I. Ejecta emplacement was a mult i-phased process, initiated by the formation of secondary craters and the pitted terrain material. Successively less ex­

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Fig. 8. Morphological sketch map of the crater chain north of Bamburg. irection of ejecta flow was from south to north. Mapped from Viking orbiter frames 70A25-30.

2666 P . .!. M oItRin i.I -M ({ I'k.

Len ive deposit. of mooth radial. rough radial and mas flow materials were then emplaced. The smooth. discontinuolls deposits of the smooth terrain material represent the final ejecta material to be deposited

2. Surface-flow emplacement of the ~mooLh radial and rough radial materials were similar to the depositional prOl.eS<i inferred by Carr el al . (1977) for rampart craters smaller than approximatel y 30 km in diameter .

3. The presence of mUltiple wall ridges extending an abnormall y large di tane toward the center of the crater and the blending of the mass flow material with the rim deposits indicate that significant post-cratering modifications of crater geometry took place .

4. The large summit Pit in the central peak. leveed channel on the wall ma ­terial that cannot be related to impact melts , and the fluidiLeti haracter of the ejecta materials all suggest that volatiles existed within the target at the time of crater formation .

5. Many fewer secontlary craters larger than J km diameter were identified around Bamburg (Fig. 5) than the number observed around lunar and mer­curian craters of comparable size. Either the production of secondary crater forming blocks has been drastically reduced for Bamburg or the emplace­ment sequence for the martian ejecta caused the total burial of many small atellite crater, he maximum density of Bamburg secondary crater ' is

observed much closer to the primary rim than is tme for the mercurian crater March (Fig . 6). Thjs is attributed to deceleration of the martian ejecta either by the atmosphere or a transient local increase in atmospheric density as a consequence of the cratering event.

Acknowledgments- J . W. Heau and 1. L Whitfo rd-Slll rk providcd ll ' cful commen t during the prep ­ara tion of the manuscript. Helpful reviews were given by G. E McGill and an anonymous reviewer. T he photographic ab il it ies of David Haas were also most helpfUl for the produc tion of the figures . This work was supported by NASA grant\ NGR 40-002-088 an d GR 40-002- 11 6.

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Blac kburn E. A. ( 1977) Some aspects of explos ive volcanism o n (he planets . Ph .D . t.hesis, UIll V.

Lancas ter . U.K. 2 10 pp. Carr M. H , Crumpler L S . • Cutts J. A. , Greeley R., Gllest J. E., and Masursky H . (1977) Martian

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