-
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.
0 Lunar and Planetary Institute Provided by the NASA
Astrophysics Data System
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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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MICROTEKTITES AND METEORITE IMPACT
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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Astrophysics Data System
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0 Lunar and Planetary Institute Provided by the NASA
Astrophysics Data System