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 modifica- tion 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 ter- race 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 upper- most 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.

Fig.1. Location of slumped highland craters and associated underlying rims.

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SLUMPED HIGHLAND CRATERS

G i f f o r d , A.W. e t a l .

F igu re 2. (a ) Necho c r a t e r (30 km diam; ~ O S , 1 2 3 O ~ ) , t h e type example of t h e t e r r a c i n g a s d i scussed i n t e x t . (b) 38-km c r a t e r l o c a t e d i n Keeler (lo's, 1 6 2 ' ~ ) . A p o r t i o n of t h e c r a t e r ' s r i m i s superposed on t h e t e r r a c e d w a l l of Keeler , forming a prominent t e r r a c e ledge wi th a s t r i k i n g s i m i l a r i t y t o Ne- c h o ' s uppermost t e r r a c e . The head s c a r p of t h i s t e r r a c e i s a l s o c o n c e n t r i c t o Kee le r , and i s f r e s h e r t han any o t h e r t e r r a c e s c a r p s a s s o c i a t e d w i t h t h e o l d e r c r a t e r . This could i n d i c a t e r e a c t i v a t i o n of a c o n c e n t r i c f r a c t u r e by t h e sub- sequent impact. (c ) 27-km c r a t e r ( 1 6 ' ~ ~ 107 '~) l o c a t e d on t h e r i m of H i l b e r t . (d) 28-km c r a t e r ( 1 6 O ~ , 1 1 5 ' ~ ) l o c a t e d on t h e r i m of Kondratyuk. ( c ) and (d) a r e more degraded t h a n ( a ) and (b) , b u t bo th d i s p l a y an upper t e r r a c e l edge which i s c o n c e n t r i c t o and c o i n c i d e n t w i t h t h e unde r ly ing c r a t e r r ims. ( e ) , a 30-km c r a t e r a t l los, 1 5 8 ' ~ and ( f ) , a 30-km c r a t e r a t l l 0 N Y 134OE, a r e no t s i t u a t e d on a s p e c i f i c o l d e r c r a t e r r i m b u t r a t h e r on degraded r i m segments and uneven t e r r a i n . The slumping i n t h e s e examples is n o t conf ined t o one s i d e of t h e c r a t e r , b u t t h e s c a l l o p - l i k e p o r t i o n s of t h e r i m s a r e d i r e c t l y re - l a t e d t o under ly ing i r r e g u l a r i t i e s i n t h e topography.

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SLUMPED HIGHLAND CRATERS

G i f f o r d , A . W . e t a l .

These c r a t e r s a r e a l s o c h a r a c t e r i z e d by t h e r e l a t i v e l a c k of c e n t r a l peaks . Al though 70% of f r e s h h i g h l a n d c r a t e r s 30 km i n d i a m e t e r d i s p l a y c e n t r a l p e a k s ( 3 ) , o n l y 20% i n t h i s s t u d y h a v e c e n t r a l peaks o r p o s s i b l e remnants . Those c e n t r a l peaks t h a t a r e p r e s e n t a r e g e n e r a l l y composed of c l u s t e r s o f c o n i c a l b l o c k s o r low mounds. O c c a s i o n a l l y a low r i d g e n e a r t h e c e n t e r c f t h e c r a t e r may b e a remnant of a c e n t r a l p e a k , however , a d d i t i o n a l slumped m a t e r i a l is a l s o a p o s s i b l e e x p l a n a t i o n . A p r o b a b l e " c e n t r a l " peak i n Necho is o f f s e t from t h e c e n t e r of t h e c r a t e r by - 5 km i n t h e d i r e c t i o n of maximum slumping.

The morphology of t h e c r a t e r s i n t h i s s t u d y s u g g e s t s e f f e c t s of b o t h topography and s u b s t r a t e . The p lacement of t h e c r a t e r s on l a r g e o l d e r c r a t e r r ims means t h a t t h e i m p a c t s h a v e o c c u r r e d on v a r i a b l y s l o p i n g ground . I n t h e c a s e o f Necho, t h e impac t o c c u r r e d a t a b r e a k i n s l o p e between t h e o l d e r c r a t e r ' s r im w a l l and f l o o r . The c o i n c i d e n c e o f t h e most t e r r a c e d s i d e of t h e s u p e r p o s e d c r a t e r w i t h t h e s t e e p e s t s l o p e emphas izes t h e e f f e c t of topo- g raphy . However, t h e f a c t t h a t a l l of t h e c r a t e r s i n t h i s g roup a r e l o c a t e d i n t h e o l d e r ( p r e - N e c t a r i a n and N e c t a r i a n ) h i g h l a n d s s u g g e s t s t h a t t h e b l o c k y n a t u r e of t h e r e g o l i t h r e i n f o r c e s t h e t endency t o slump more t h a n a c o h e s i v e s u b s t r a t e ( e . g . mare) . The w e s t s i d e h i g h l a n d s have been s i g n i f i c a n t l y a l t e r e d b y t h e O r i e n t a l e e v e n t , which e x p l a i n s t h e a b s e n c e of t h i s c l a s s of c r a t e r n e a r t h e O r i e n t a l e B a s i n . The u n u s u a l l y low p e r c e n t a g e of c e n t r a l peaks i n t h e s e c r a t e r s may a l s o r e s u l t f rom e i t h e r s u b s t r a t e o r t o p o g r a p h i c e f f e c t s . The d i f f e r e n c e i n t h e o n s e t d i a m e t e r s of c e n t r a l p e a k s i n mare and h i g h l a n d c r a t e r s h a s been a t t r i b u t e d t o t h e more c o h e r e n t n a t u r e of t h e mare ( 3 ) ; t h e r e f o r e , t h e d i f f e r e n c e between t h i s g roup of c r a t e r s and o t h e r h igh- l a n d c r a t e r s may b e a f u r t h e r s u b d i v i s i o n of v a r i a b l e h i g h l a n d s u b s t r a t e . A l t e r n a t i v e l y , t h e p e r v a s i v e n a t u r e of t h e s lumping i n t h e s e c r a t e r s may h a v e a c t e d t o r e d u c e o r d e s t r o y t h e c e n t r a l p e a k s .

I t is d i f f i c u l t t o d i f f e r e n t i a t e t o p o g r a p h i c and s u b s t r a t e e f f e c t s . However, on t h e b a s i s of t h e u n d e r l y i n g c r a t e r r i m a s s o c i a t i o n s t h e s lumping o b s e r v e d i n t h i s groun of c r a t e r s most l i k e l y r e p r e s e n t s t h e e f f e c t of p r e - impac t topography on t h e e x c a v a t i o n and m o d i f i c a t i o n s t a g e s of i m p a c t . I n t h e a b s e n c e of a good u n d e r s t a n d i n g of e n e r g y p a r t i t i o n i n g i n l a r g e i m p a c t s , f i t t i n g s m a l l g r o u p s o f m o r p h o l o g i c a l l y d i s t i n c t c r a t e r s s u c h a s t h i s i n t o c r a t e r s t a t i s t i c s may b e a c l u e t o documenting f i n e r - s c a l e c o n t r o l s on c r a t e r f o r m a t i o n .

R e f e r e n c e s 1. G i f f o r d , A.W. , Maxwell , T . A . , and El-Baz, F . , 1979 , s u b m i t t e d t o The EIoon

and The P l a n e t s . 2. Head, J . W . , 1976, P r o c . Lunar S c i . Conf. 7 , 2913-2929. 3. C i n t a l a , M . J . , Wood, C . A . , and Head, J.W., 1977, P r o c . Lunar S c i . Conf.

8, 3409-3425.

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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 . Swink i , and P. A . Z w a r t , Geology Depar tment , U n i v e r s i t y of Delaware , Newark, DE. 19711.

Most i n v e s t i g a t o r s now b e l i e v e t h a t t e k t i t e s were formed by m e t e o r i t e impact on t h e E a r t h (1) . M i c r o t e k t i t e s ((1 mm d i a m e t e r t e k t i t e s ) be long ing t o t h e A u s t r a l a s i a n , I v o r y Coas t and Tlorth American t e k t i t e s t r e w n f i e l d s have been r e c o v e r e d from deep-sea s ed imen t s ( 2 , 3 , 4 ) . These m i c r o t e k t i t e s occu r i n l a y e r s t h a t a p p e a r t o b e a s s o c i a t e d w i t h o t h e r e v e n t s i n t h e E a r t h ' s h i s t o r y . The geog raph i c d i s t r i b u t i o n of m i c r o t e k t i t e - b e a r i n g c o r e s i n d i c a t e s t h a t t h e s t r e w n f i e l d s a r e much l a r g e r t h a n p r e v i o u s l y t h o u g h t , L ikewise t h e s i z e of t h e s t r e w n f i e l d s p l u s t h e c a l c u l a t e d mass of m i c r o t e k t i t e s i n each s t r e w n f i e l d , i n d i c a t e t h a t t h e t e k t i t e - p r o d u c i n g e v e n t s a r e of g r e a t e r mag- n i t u d e t h a n p r e v i o u s l y t h o u g h t .

A u s t r a l a s i a n m i c r o t e k t i t e s have now been found i n a t o t a l of t h i r t y - t h r e e c o r e s . R e c e n t l y A u s t r a l a s i a n m i c r o t e k t i t e s were found i n two c o r e s from t h e no r thwes t I n d i a n Ocean (Somal i B a s i n ) and two c o r e s from t h e e a s t e r n e q u a t o r i a l P a c i f i c . T h i s i n c r e a s e s t h e known s i z e of t h e s t r e w n f i e l d and changes i t s shape . C a l c u l a t i o n s , based on t h e number of m i c r o t e k t i t e s found a t each c o r e s i t e , i n d i c a t e t h a t t h e A u s t r a l a s i a n s t r ewn- f i e l d c o n t a i n s 100 m i l l i o n m e t r i c t o n s of t e k t i t e g l a s s which i s s p r e a d over n e a r l y 1% of t h e E a r t h ' s s u r f a c e .

I n e i g h t e e n of t h e A u s t r a l a s i a n m i c r o t e k t i t e - b e a r i n g c o r e s t h e r e i s a f a i r l y w e l l - d e f i n e d peak i n m i c r o t e k t i t e abundance. I n t e n of t h e s e c o r e s t h e peak i n abundance occu r s no more t h a n abou t 20 cm above t h e B r u n h e s / ~ a t u ~ a m a geomagnet ic r e v e r s a l boundary. I n a n o t h e r c o r e i t a p p e a r s t o be -45 cm above t h e boundary. I n f o u r c o r e s t h e peak abundance seems t o b e r i g h t on t h e ~ r u n h e s / ~ a t u ~ a m a r e v e r s a l boundary and i n t h r e e c o r e s t h e peak i s a p p a r e n t l y below t h e ~runhes/nla tuyama boundary. On t h e a v e r a g e , t h e peak i n abundance of A u s t r a l a s i a n mic ro t ek - t i t e s i s w i t h i n a b o u t 6 cm of t h e ~ r u n h e s / ~ a t u ~ a m a r e v e r s a l boundary. Thus i t a p p e a r s t h a t t h e A u s t r a l a s i a n t e k t i t e f a l l c o i n c i d e d w i t h t h e l a s t r e v e r s a l of t h e E a r t h ' s magnet ic f i e l d app rox ima te ly 0 . 6 9 may . ago . K - A r a g e s of A u s t r a l a s i a n t e k - t i t e s (5 ) and f i s s i o n - t r a c k a g e s of A u s t r a l a s i a n t e k t i t e s ( 6 ) and m i c r o t e k t i t e s ( 7 ) s u p p o r t t h i s c o n c l u s i ~ n .

S e v e r a l a u t h o r s have p o i n t e d out t h a t t h e r e i s a major c l i m a t i c change a s s o c i a t e d w i t h t h e ~ r u n h e s / ~ a t u y a m a r e v e r s a l ( e . g . 8 , 9 ) . Fu r the rmore , Keany and Kenne t t ( 8 ) p o i n t ou t t h a t t h e most consp icuous h o r i z o n of b i o s t r a t i g r a p h i c change w i t h i n t h e l a s t 2.43 m.y. i s a t t h e Brunhes /~a tuyama boundary where a t l e a s t two r a d i o l a r i a n and one f o r a m i n i f e r a l s p e c i e s d i s a p - p e a r and two f o r a m i n i f e r a 1 s p e c i e s appea r .

M i c r o t e k t i t e s be long ing t o t h e I v o r y Coas t t e k t i t e s t r ewn- f i e l d have now been found i n f i v e c o r e s from t h e A t l a n t i c Ccean, Recent d i s c o v e r i e s of I v o r y Coast m i c r o t e k t i t e s i n two c o r e s from t h e North A t l a n t i c and one from t h e Sou th A t l a n t i c show t h a t t h e I v o r y Coas t s t r e w n f i e l d ex t ends f a r t h e r n o r t h (.-80 n o r t h l a t i t u d e ) and f a r t h e r s o u t h (,-8O s o u t h l a t i t u d e ) , and i s abou t f o u r t i m e s l a r g e r t h a n p r e v i o u s l y t h o u g h t .

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G l a s s , B.P. e t a l .

Cor re l a . t i on between t h e m i c r o t e k t i t e l a y e r and t h e pa l eo - magnetic s t r a t i g r a p h y f o r t h e I v o r y Coast m i c r o t e k t i t e - b e a r i n g c o r e s shows t h a t t h i s m i c r o t e k t i t e l a y e r i s a s s o c i a t e d w i t h t h e J a r a m i l l o geomagnetic event and may i n f a c t be a s s o c i a t e d w i t h t h e beginning of t h a t event 0 .95 may. ago. Again, K - A r d a t i n g of I v o r y Coast t e k t i t e s and f i s s i o n - t r a c k d a t i n g of t h e I v o r y Coas t m i c r o t e k t i t e s i s c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n . A s w i t h t h e ~ r u n h e s / ~ a t u y a m a r e v e r s a l boundary, t h e e x t i n c t i o n of s e v e r a l s p e c i e s of marine m i c r o f o s s i l s a p p a r e n t l y c o r r e l a t e s w i th t h e J a r a m i l l o event ( e , g . 8 ) .

North American m i c r o t e k t i t e s have been r e p o r t e d from one p i s t o n c o r e t a k e n i n t h e Caribbean Sea and from c o r e s from two Deep Sea D r i l l i n g P r o j e c t (DSDP) s i t e s (one i n t h e Caribbean and one i n t h e Gulf of Mexico). North American m i c r o t e k t i t e s have now been found i n c o r e s from t h r e e DSDP s i t e s a c r o s s t h e e q u a t o r i a l P a c i f i c and one from t h e I n d i a n Ocean. Th i s i n d i - c a t e s t h a t t h e North American s t r e w n f i e l d ex tends a t l e a s t half-way around t h e E a r t h . C a l c u l a t i o n s i n d i c a t e t h a t t h e r e i s over one b i l l i o n m e t r i c t o n s of g l a s s i n t h i s s t r e w n f i e l d .

Although t h e North American m i c r o t e k t i t e l a y e r may n o t b e a s s o c i a t e d wi th a geomagnetic r e v e r s a l , i t is a s s o c i a t e d w i t h t h e e x t i n c t i o n of s e v e r a l s p e c i e s of R a d i o l a r i a ( 1 0 ) . I n ad- d i t i o n , t h e r e i s ev idence f o r a s h a r p drop i n t empera tu re a t t h e end of t h e Eocene (11, 12 ) which may c o r r e l a t e bo th w i t h t h e North American t e k t i t e event and r a d i o l a r i a n e x t i n c t i o n s .

I f it is c o r r e c t t h a t t e k t i t e s were formed by t e r r e s t r i a l impac t , t h e n i t seems t h a t t h e s e impacts were r e s p o n s i b l e f o r sp read ing from 10 m i l l i o n t o one b i l l i o n m e t r i c t o n s of g l a s s a t l e a s t half-way around t h e E a r t h and may have t r i g g e r e d r e v e r s a l s of t h e E a r t h ' s magnetic f i e l d whi le producing c l i - mat ic changes t h a t caused e x t i n c t i o n s and e v o l u t i o n a r y appear - ances of v a r i o u s marine micro-organisms. REFERENCES: (1) King, E.A. (1977) Am, S c i e n t i s t , 65, 212-218. ( 2 ) G l a s s , B.P. (1969) Geochim. Cosmochim. Acta , 3, 1135-1147. ( 3 ) G l a s s , B.P. (1972) A n t a r c t i c Res. S e r . , Q, 335-348. (4) Glass, B .P . , Baker , R.N., S t o r z e r , D . and Wagner, G . A . (1973) Ea r th P l a n e t . S c i . L e t t e r s , Q, 184-192. ( 5 ) ~ g h r i n g e r , J . (1963) K - A r measurements of t e k t i t e s . I n Rad ioac t ive D a t i n . p. 289-305. I n t e r n a t i o n a l Atomic Energy Agency, Vienna. (2) Gentner , W . , S t o r z e r , D . and Wagner, G . A . (1969) Geochim. Cos- mochim. Acta , 3, 1075-1081. ( 7 ) Gentner , W . G l a s s , B.P. , S t o r z e r , D . and Wagner, G . A . (1970) S c i e n c e , l6J, 359-361. ( 8 ) Keany, J . and Kenne t t , J . P . (1972) Deep-Sea Research, 19, 529- 548. ( 9 ) Hays, J . D . and Donahue, J . G . (1972) A n t a r c t i c Q u a t e r - n a r y c l i m a t i c r e c o r d and r a d i o l a r i a n and diatom e x t i n c t i o n s . I n Adie , R . J . ( e d ) A n t a r c t i c Geology and Geophysics. p . 733-738. U n i v e r s i t e t s f o r l a g e t , Oslo. ( 1 0 ) G l a s s , B.P. and Zwart , P.A. (1975) North American m i c r o t e k t i t e s , r a d i o l a r i a n e x t i n c t i o n s and t h e age of t h e Eocene-Gligocene boundary. I n Swain, F.M. ( e d ) S t r a t i g r a p h i c Micropaleontology of A t l a n t i c Bas in and Borde r l ands . p . 553-568. E l s e v i e r , N . Y . (11) Margo l i s , S . V . , Kroopnick, P . M . , Goodnew, D.E. , Dudley, W . C . and Mahoney, M.E. (1975) S c i e n c e , 189, 555-557. ( 1 2 ) Shack le ton , N . J . and Kenne t t , J . P . ( 1 9 7 5 ) ~ a l t e m ~ e r a t u r e h i s t o r y of t h e Cenozoic and t h e

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Glass, B.P. et al.

initiation of Antarctic glaciation: Oxygen and carbon isotope analyses in DSDP sites 277, 2 7 9 , 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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