Ejecta Emplacement of the Martian Impact Crater Bamburg
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Pro( LUI/ar Pial/e!. Sci. Con/. 10th (J979), p. 2 6..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 Geological Sciences, Brown University. Providence, Rhode Island 02912 Abstract- Six exterior deposits surround the martian impact crater Bamburg (55 km in diameter). T he sequence of ejecta emplacement , although more complex. conforms to the same depositional history that has prod uced the ej ecta 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 were overrun by more viscous fl ows that are characlerized by longi tudinal grooves and transverse ridges. Numerous areas of flat terrain upon the ej ecta deposits, and the identification of I veed channels on th e wall s of Ba mburg, may indicate that either late in the cratering event, or after fin al ejecta emplacement, sedi ment-laden melt water percolated out of the vol atile- rich ejecta and the c rater ri m. The number of secondary craters associated wilh Bamburg is less tban one third the commensurate value for lunar and mercurian craters of equ iv alent size. T he maximum areal density of these marli,m seconda ry craters is observed at less than hajJ the range of those associated with the comparable me rcurian c rater Marc h. The deficiency of Bamburg secondary craters is attributed either to pref erential d structi on of ejecta blocks sufficiently large to form secondary craters or the subsequent 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 40o N, 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 photography (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 de scribed 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 opportunity for the analysis of the depositional processes and resultant morphologicalfeatures associated with the formation ora complex martian impact crater. Secondary craters and a variety 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 detail 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 li es the fractured plains male ri al 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 gra dational 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 sm aller 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" clas sification (Type 5 craters, Mouginis-Mark, 1979a) and possesses several mor phological 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 mor phological map (Fig. 3) was produced. Four materials con titute the interior ~",,' '"i:i ""~ '"S '" ""~ "";os <8, ;;:. "":: i5.., ~ . ~ ~:::, Q ~ ~.., tll :::, ~ 2' ;j Fi.g. 2. Photomosaic of Bamburg C rater, showing the area for wh ic h the morphological map (Fig. 3) has been compiled. Also di splayed 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 "" ~ "ll ' s:~ ri.2 . ~ z;;' ::,~ ~ 25 km (a) Fig. 3. (a) Morphological map of the interi or and exterior deposits associ ated with Bamburg . See key (Fig. 3b) and text for descriptio ns. Ejecta emplacement of the martian impact crater Bamburg 2655 deposits and six materials were identified beyond the rim crest. A brief descrip tion of each uni t is given b low, together with the type localities for the ejecta deposits (Fig. 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 ma terial, 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 central peak . Gentle slopes extend from the walls of the peak to the crater fl oor whereas smooth deposits are interdigitated between the floor and the in nermost wall ma terial. INTERIOR DEPOSITS EXTERIOR DEPOSITS WALL FLOW SMOOTH TE R R AIN ~ Cd'_ I .~,I'" M ATERIAL (Wf) ~ MA T ERIAL (SI) WA LL M AS S FLO W [I] MAT ERI AL (Wm) D2J M AT ERIA L (Mf) - . FLOO R ROUGH RADIAL 0 MATERIAL (Fm) D.", MATER IAL (Rr) CENTRAL PE AK SM OOT H RADIAL [II MATERIAL (Cp) D>, " MATERIAL (Sr) PITTED TE RRAI 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 continue 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 al., 1977). In between these wall ridges, deposition by creep and slumping ha produced flat inliners which occasionally extend to, and merge with . the fl oor 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; Hul me and Fielder, 1977). No obvious source areas (pits or ponded material) are evident for any of these martian chan nels, however, so that an impact melt origin such as described by Hawke and Head (1 977) for the lunar examples appears inapplicable in the case of Bamburg. Impact melts associated with martian craters remain unidentified. and extrapo lation of terrestrial field data and theoretical 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 flu vial 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 cl ear, 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 mateIi al (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, akin to the ridges of the wall material. On the northern rim. there is evidence of radial scouring that is morphologically similar to triations on the southeastern ri m of the lu nar 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 surges, po sibly during or di rectly after the overturning of the crater rim.