Topographic Effects on Slumped Craters in the Lunar Highlands

TOPOGRAPHIC EFFECTS ON SLUMPED CRATERS IN THE LUNAR HIGHLANDS. Ann W. Gifford and Ted A. Maxwell, National Air and Space Museum,Smithsonian Institution, Washington, D.C. 20560

Detailed mapping of morphologically distinct units in Necho crater has demonstrated the importance of both topography and substrate in the modification stage of highland crater formation (1). The effect of substrate layer- ing, and in particular a megaregolith, has been suggested in studies of mor- phologic changes with crater size (2,3). However, the present study of 30 slumped highland craters similar to Necho can be used to further document the effects of pre-existing topography on crater morphology. These craters range in age from Imbrian to Copernican, and most are located in pre-Nectarian or Nectarian highlands (Fig. 1). They range in diameter from 15 to 40 km, but the majority cluster at about 27 - 30 km.

Using Necho crater as the type example, craters with similar patterns of terracing were studied. This group of craters is characterised by: 1) Pre- ferential terracing on one side of the crater; 2) a distinct uppermost terrace that is much wider than the lower terraces within the crater; and 3) Re- stricted occurrence on the rims of larger, older craters. Necho (Fig. 2a) is located at the intersection of three large degraded craters, which indicates both topographic and possibly structural control in a direction concentric to Necho on the western side of the crater. The prominent uppermost terrace of Necho coincides with the intersection of the rims of these craters. 75% of the craters studied are characterised by a wider uppermost terrace, much in the manner of Necho. These ledges vary from < 1 to a 10 km wide, corresponding to 1 to 40% of the rimcrest diameter.

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- ,#-\ w 0. \ D I -*0 0' @ NECHO PRE-NEC TARIAN HIGHLA NDS KEELER0.0 15.5 0 ,/-\ TSIOLKOVSKIY I '\* 30.5 / MlLNE '\ / I b-# 37.S 80.E 100°E 120°E 140.E 16 0.E - Fig.1. Location of slumped highland craters and associated underlying rims.

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Gifford, A.W. et al.

Figure 2. (a) Necho crater (30 km diam; ~OS,123O~), the type example of the terracing as discussed in text. (b) 38-km crater located in Keeler (lo's, 162'~). A portion of the crater's rim is superposed on the terraced wall of Keeler, forming a prominent terrace ledge with a striking similarity to Ne- cho's uppermost terrace. The head scarp of this terrace is also concentric to Keeler, and is fresher than any other terrace scarps associated with the older crater. This could indicate reactivation of a concentric fracture by the sub- sequent impact. (c) 27-km crater (16'~~107'~) located on the rim of Hilbert. (d) 28-km crater (16O~,115'~) located on the rim of Kondratyuk. (c) and (d) are more degraded than (a) and (b), but both display an upper terrace ledge which is concentric to and coincident with the underlying crater rims. (e), a 30-km crater at llos, 158'~and (f) , a 30-km crater at ll0NY 134OE, are not situated on a specific older crater rim but rather on degraded rim segments and uneven terrain. The slumping in these examples is not confined to one side of the crater, but the scallop-like portions of the rims are directly re- lated to underlying irregularities in the topography.

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Gifford, A. W. et al.

These craters are also characterized by the relative lack of central peaks. Although 70% of fresh highland craters 30 km in diameter display central peaks (3), only 20% in this study have central peaks or possible remnants. Those central peaks that are present are generally composed of clusters of conical blocks or low mounds. Occasionally a low ridge near the center cf the crater may be a remnant of a central peak, however, additional slumped material is also a possible explanation. A probable "central" peak in Necho is offset from the center of the crater by - 5 km in the direction of maximum slumping.

The morphology of the craters in this study suggests effects of both topography and substrate. The placement of the craters on large older crater rims means that the impacts have occurred on variably sloping ground. In the case of Necho, the impact occurred at a break in slope between the older crater's rim wall and floor. The coincidence of the most terraced side of the superposed crater with the steepest slope emphasizes the effect of topography. However, the fact that all of the craters in this group are located in the older (pre-Nectarian and Nectarian) highlands suggests that the blocky nature of the regolith reinforces the tendency to slump more than a cohesive substrate (e.g. mare). The west side highlands have been significantly altered by the Orientale event, which explains the absence of this class of crater near the Orientale Basin. The unusually low percentage of central peaks in these craters may also result from either substrate or topographic effects. The difference in the onset diameters of central peaks in mare and highland craters has been attributed to the more coherent nature of the mare (3); therefore, the difference between this group of craters and other highland craters may be a further subdivision of variable highland substrate. Alternatively, the pervasive nature of the slumping in these craters may have acted to reduce or destroy the central peaks. It is difficult to differentiate topographic and substrate effects. However, on the basis of the underlying crater rim associations the slumping observed in this groun of craters most likely represents the effect of pre- impact topography on the excavation and modification stages of impact. In the absence of a good understanding of energy partitioning in large impacts, fitting small groups of morphologically distinct craters such as this into crater statistics may be a clue to documenting finer-scale controls on crater formation.

References 1. Gifford, A.W., Maxwell, T.A., and El-Baz, F., 1979, submitted to The EIoon and The Planets. 2. Head, J.W., 1976, Proc. Lunar Sci. Conf. 7, 2913-2929. 3. Cintala, M.J., Wood, C.A., and Head, J.W., 1977, Proc. Lunar Sci. Conf. 8, 3409-3425.

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LIZEP-SEA IKICRL TEKTITES : C CRREUTIGF WITH C!THER EARTH EVET;TS AND IMPLICATIONS CONCERNING THE MAGNITUDE OF TEKTITE- PRODUCING EVENTS, B. P. Glass, M. B. Swinki, and P. A. Zwart, Geology Department, University of Delaware, Newark, DE. 19711. Most investigators now believe that tektites were formed by meteorite impact on the Earth (1). Microtektites ((1 mm diameter tektites) belonging to the Australasian, Ivory Coast and Tlorth American tektite strewnfields have been recovered from deep-sea sediments (2, 3, 4). These microtektites occur in layers that appear to be associated with other events in the Earth's history. The geographic distribution of microtektite- bearing cores indicates that the strewnfields are much larger than previously thought, Likewise the size of the strewnfields plus the calculated mass of microtektites in each strewnfield, indicate that the tektite-producing events are of greater magnitude than previously thought. Australasian microtektites have now been found in a total of thirty-three cores. Recently Australasian microtektites were found in two cores from the northwest Indian Ocean (Somali Basin) and two cores from the eastern equatorial Pacific. This increases the known size of the strewnfield and changes its shape. Calculations, based on the number of microtektites found at each core site, indicate that the Australasian strewnfield contains 100 million metric tons of tektite glass which is spread over nearly 1% of the Earth's surface. In eighteen of the Australasian microtektite-bearing cores there is a fairly well-defined peak in microtektite abundance. In ten of these cores the peak in abundance occurs no more than about 20 cm above the Brunhes/~atu~amageomagnetic reversal boundary. In another core it appears to be -45 cm above the boundary. In four cores the peak abundance seems to be right on the ~runhes/~atu~amareversal boundary and in three cores the peak is apparently below the ~runhes/nlatuyama boundary. On the average, the peak in abundance of Australasian microtektites is within about 6 cm of the ~runhes/~atu~amareversal boundary. Thus it appears that the Australasian tektite fall coincided with the last reversal of the Earth's magnetic field approximately 0.69 may. ago. K-Ar ages of Australasian tektites (5) and fission-track ages of Australasian tektites (6) and microtektites (7) support this conclusi~n. Several authors have pointed out that there is a major climatic change associated with the ~runhes/~atuyamareversal (e.g. 8, 9). Furthermore, Keany and Kennett (8) point out that the most conspicuous horizon of biostratigraphic change within the last 2.43 m.y. is at the Brunhes/~atuyama boundary where at least two radiolarian and one foraminiferal species disap- pear and two foraminifera1 species appear. Microtektites belonging to the Ivory Coast tektite strewnfield have now been found in five cores from the Atlantic Ccean, Recent discoveries of Ivory Coast microtektites in two cores from the North Atlantic and one from the South Atlantic show that the Ivory Coast strewnfield extends farther north (.-80 north latitude) and farther south (,-8O south latitude), and is about four times larger than previously thought.

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MICROTEKTITES AND METEORITE IMPACT Glass, B.P. et al.

Correla.tion between the microtektite layer and the paleo- magnetic stratigraphy for the Ivory Coast microtektite-bearing cores shows that this microtektite layer is associated with the Jaramillo geomagnetic event and may in fact be associated with the beginning of that event 0.95 may. ago. Again, K-Ar dating of Ivory Coast tektites and fission-track dating of the Ivory Coast microtektites is consistent with this interpretation. As with the ~runhes/~atuyamareversal boundary, the extinction of several species of marine microfossils apparently correlates with the Jaramillo event (e,g. 8). North American microtektites have been reported from one piston core taken in the Caribbean Sea and from cores from two Deep Sea Drilling Project (DSDP) sites (one in the Caribbean and one in the Gulf of Mexico). North American microtektites have now been found in cores from three DSDP sites across the equatorial Pacific and one from the Indian Ocean. This indicates that the North American strewnfield extends at least half-way around the Earth. Calculations indicate that there is over one billion metric tons of glass in this strewnfield. Although the North American microtektite layer may not be associated with a geomagnetic reversal, it is associated with the extinction of several species of Radiolaria (10). In ad- dition, there is evidence for a sharp drop in temperature at the end of the Eocene (11, 12) which may correlate both with the North American tektite event and radiolarian extinctions. If it is correct that tektites were formed by terrestrial impact, then it seems that these impacts were responsible for spreading from 10 million to one billion metric tons of glass at least half-way around the Earth and may have triggered reversals of the Earth's magnetic field while producing climatic changes that caused extinctions and evolutionary appear- ances of various marine micro-organisms. REFERENCES: (1) King, E.A. (1977) Am, Scientist, 65, 212-218. (2) Glass, B.P. (1969) Geochim. Cosmochim. Acta, 3, 1135-1147. (3) Glass, B.P. (1972) Antarctic Res. Ser., Q, 335-348. (4) Glass, B.P., Baker, R.N., Storzer, D. and Wagner, G.A. (1973) Earth Planet. Sci. Letters, Q, 184-192. (5) ~ghringer,J. (1963) K-Ar measurements of tektites. In Radioactive Datin . p. 289-305. International Atomic Energy Agency, Vienna. (2) Gentner, W., Storzer, D. and Wagner, G.A. (1969) Geochim. Cos- mochim. Acta, 3, 1075-1081. (7) Gentner, W. Glass, B.P., Storzer, D. and Wagner, G.A. (1970) Science, l6J, 359-361. (8) Keany, J. and Kennett, J.P. (1972) Deep-Sea Research, 19, 529- 548. (9) Hays, J.D. and Donahue, J.G. (1972) Antarctic Quater- nary climatic record and radiolarian and diatom extinctions. In Adie, R.J. (ed) Antarctic Geology and Geophysics. p. 733-738. Universitetsforlaget, Oslo. (10) Glass, B.P. and Zwart, P.A. (1975) North American microtektites, radiolarian extinctions and the age of the Eocene-Gligocene boundary. In Swain, F.M. (ed) Stratigraphic Micropaleontology of Atlantic Basin and Borderlands. p. 553-568. Elsevier, N.Y. (11) Margolis, S.V., Kroopnick, P.M., Goodnew, D.E., Dudley, W.C. and Mahoney, M.E. (1975) Science, 189, 555-557. (12) Shackleton, N.J. and Kennett, J.P. (1975)~altem~eraturehistory of the Cenozoic and the

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MICROTEKTITES AND METEORITE IMPACT Glass, B.P. et al. initiation of Antarctic glaciation: Oxygen and carbon isotope analyses in DSDP sites 277, 279, and 281. In Kennett, J.P., Houtz, R.E. et al. (ed) Initial Reports of the Deep Sea Drilling Project, vol. 29, p. 743-755. U.S. Government Printing Cffice, Washington, D.C.

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