African meteorite impact craters: characteristics and geological importance
CHRISTIAN KOEBERL t~
1Institute of Geochemistry, University of V i e n n a , Dr.-Karl-Lueger-Ring 1, A-1010 V i e n n a , Austria. 2Economic Geology Research Unit, Depa~ i.ment of Geology, University of the Witwatemmnd, Johannesburg 2050,
South Africa.
(Received 26 October 1993 : accepted 12 May 1994)
Abslzact - Geologists have realized that impact cratering is the single most important surface-forming and modifying process for the other terrestrial planets and the satellites of all planets. The recognition of impact cratering as an important geological process on earth has been rather slow. However, geologists are now realizing that giant impacts have had a determining influence on the geological and biological evolution of our planet. The study of impact craters allows important conclusions, not only about the origin mid history of our solar system and its planets, but also about a fundamentally important geological process. In addition, impact craters may have a definite economic importance as some craters have been shown to contain important mineral or oil deposits. F'dtesn meteorite impact craters have so far been identified on the African continent:. Amguid (Algeria), Aomunga (Chad), Aouelloul (Mauritania), B.P. (Libya), Bosumtwi (Ghana), Highbury (Zimbabwe), Kalkkop (South Africa), Oasis (Libya), Ouarkziz (Algeria), Roter Kamm (Namibia), Saitpan (South Africa), Talemzane (Algeria), Tenoumer (Mauritania), T'm Bider (Algeria), and V~:lefort (South Africa). This paper presents an overview of these craters, as well as a discussion of impact processes, the recognition of impact cTaters, and the geological arid economic importance of impact craters.
R&nml~ - Lea g~ioguea ont maintenant r6alis~ que les crat~rea d'impact constituent le processus majeur de formation et de modification des surfaces des autres plan~,tes telluriques ainsi que de lenrs satellites. La reconnaissance des crat~res d'impact en rant que processus g~ologique important sur Terre est relativement r&'ent. Les g6alogues ont cependant maintenant accept~ clue lea impacts g~ants ont eu une influence d~terminante sur l'~'volution g~logique et biologique de notre plan~e. L'~,tude des crat~es d'impact m~ne/~ des conclusions importantes non seulement sur l'origine et l'histoire de notre syst~me solaire et ses plan~tes rnais c~galement sur un processus g~ologique qui eat fondamental. De plus, les crat~ms d'impact petrvent avoir une importance &:onomique, en contenant parfois des mm,-',ra~i~ tions ou des hydrocarbums. Quinze impacts m@~oritiques ont ~ identifi~ jusqu'A pr~ent sur le continent africain: Amguid (Alg~'ie), Aomunga (Tchad), Aouelloul (Mauritanie), B.P. (Libye), Bosumtwi (Ghana), Highbury (Zimbabwe), Kalkkop (Afrique du Sud), Oasis (Libye), Ouarkziz (Alg~ie), Roter Kamm (Namibie), Saltpan (Afrique du Sud), Talemzane (Alg~rie), Tenoumer (Mauritanie), Tm Bider (Alg~rie) et Vredefort (Afrique du Sud). Cet article p~C, sente une rue d'e~semble de ces crat~rea ainai qu 'une discussion du processus d'impact, de la reconnaissance des crat~res d'impact et de l'importance g~'ologique et ~-onomique de ces c r a t~s .
INTRODUCTION
The recognition of the importance of impact cratering on earth has been slow in coming. The traditional thesis of geology calls upon uniformitarianism as postulated by James Hutton (1726-1797) and Charles LyeU (1797- 1875), who laid the foundation to the view that slow, endogenic processes lead to gradual changes in our geological record. Impact is an exogenic, relatively rare, violent, and unpredictable event and initially was thought to violate every tenet of uniformitarianism. The impact origin of craters on the earth (and the moon) has, therefore, been opposed by traditional geologists over much of our century. The history of impact studies is, in some ways, similar to the history of accepting plate tectonics (Mark 1987 and Marvin 1990 give a historical account of impact cratering, ~
The planetary exploration program and extensive lunar research led to the recognition of the fact that practically all craters visible on the moon are of impact origin. From there, it is a logical step (that still many geologists were not willing to take) to accept that, over its histor~ the earth has to have been subjected to an even larger number of impacts than the moon because of its larger gravi ta t iona l cross-section. From observations of bodies crossing the earth's orbit, astronomers have by now a fairly good understanding of the rate with which asteroids and comets strike the earth (Shoemaker et al., 1990; Weissman 1990). For example, bodies with diameters >1 km, creating craters >10 km in diameter, collide with the earth at a frequency of about 4.3xl0~/year (Shoemaker et al., 1990). Our current understanding of other planets and satellites with solid surfaces (i.e. Mercur~ Venus, Mars and the
263
264 C. KOEBERL
satellites of the outer planets) in the solar system shows that impact is either the most important, or one of the most important, surface-forming or -modifying factors.
This leads to the question, if there is such a large number of impacts on earth, where are all the impact craters? In attempting to answer this question, one must consider several factors. On the one hand, the earth is rather unique among the terrestrial planets as its surface is actively reshaped by volcanism and tectonics (rifting, s u b d u c t i o n , faul t ing , etc.) and it has an act ive atmosphere and hydrosphere. These processes lead to a rapid, in geological terms, obliteration of the impact record on earth, at best leaving either deeply eroded structures, or craters that are covered by later sediments. On the other hand, impact craters have not been of main research interest and therefore many structures have not yet been discovered. Nevertheless, an improved u n d e r s t a n d i n g of impac t craters has led to the recognition of many structures in recent times. While in 1972 only about 50 confirmed impact craters were listed, the number had increased to more than 130 by 1991 (Grieve 1991), and currently (1994) stands at around 150.
The currently known impact craters in Africa are shown in Fig. 1. and their basic characteristics are listed in Table 1. However, the discovery rate of impact craters in Africa lags behind that of most of the rest of the world. Dietz (1965) listed 8 structures as probable young impact craters, all of which are now confirmed. Four of those were in Africa. Today, Africa has far from a similar share of established young impact craters, even though, considering cratering rate estimates (Grieve 1984,1987; Trefil and Raup 1990), there must be numerous craters in Africa still waiting to be discovered. Considering the substantial importance of impact craters for geology, but also for a possible economic interest and influence on the evolution of life on our planet, impact craters in general (and in Africa in particular) deserve more extensive study. In this paper I will summarize criteria for the recognition of impact craters, give a description of the known African impact craters and conclude with a discussion of their geological importance.
RECOGNITION OF IMPACT CRATERS
The formation of an impact crater is an almost instantaneous process. Space limitations do not permit me to describe the full basics of cratering mechanics. Contrary to some opinions, this process is fairly well u n d e r s t o o d f rom theore t ica l and expe r imen ta l considerations (Gault et al., 1968; Roddy et al., 1977; Melosh 1989). Some important concepts have to be ment ioned though. It is necessary to consider the enormous energy released upon the impact of a large meteorite, which hits the earth with a velocity between about 11 and 72 km s-'. Most of the characteristics of an impact crater are the consequence of the enormous impact energy, which is instantaneously released, and, in particular, the resulting shock waves that penetrate
Ao,~l_~u/ • W Oan,
v.nga
X~v.~m'ntw£
Kamm
Figure 1. Distribution of currently known (1994) meteorite impact craters in Africa (see Table 1 for details).
the target area and attenuate in its environs. It may be interesting to compare the energy released by typical meteor i te impacts to that of "normal" terrestrial processes, such as earthquakes and volcanic eruptions. Events forming small impact craters (5-10 km diameter) release about 102"25 ergs, while formation of larger craters (50-200 km diameter) releases about 10 ~'3° ergs (French 1968; Kring 1993; B. French pers.comm. 1994). This compares with about 6x1023 ergs for the 1980 eruption of Mount St. Helens (which is comparable to the energy released by the largest U.S. nuclear device - Bravo), about 102. ergs for the big 1906 San Francisco earthquake, or the total annual energy release from the earth, including heat flow (which is by far the largest component), volcanism, and earthquakes ofabout 10 ~ ergs (French 1968; Kring 1993; B. French pers.comm. 1994).
A n u m b e r of cri teria for the recogni t ion and confirmation of impact structures have been developed over the past decades. These criteria include:
i) crater morphology ii) geophysical anomalies iii) evidence for shock metamorphism iv) presence of meteorites or traces thereof. These points will be briefly discussed below. It
should be noted that the impact origin of a structure usually cannot be confirmed using a single criterion, unless diagnostic shock metamorphic effects are found. Even then, a combination of several criteria, including morphological observations, should be used. The interested reader is urged to consul t some of the following works for more details on various aspects of impact cratering: geological importance of impacts - Silver and Schultz (1982), Sharpton and Ward (1990);
African meteorite impact craters: characteristics and geological importance 265
Table 1. The Known African Meteorite Impact Craters
Name Country Latitude Longitude Diameter Age (Ma) Ref. (km)
Amguid Algeria 26005 ' N 04023 ' E 0.45 <0.1 [1]
Aorounga Chad 19006 ' N 19015 , E 12.6 0.01 [6]
AoueUoul Mauritania 20015 ' N 12041 . W 0.36 3.1+0.3 [1]
B.P. Stucture Libya 25019 ' N 24020 ' E 2.8 <120 [1]
Bostumtwi Ghana 06032 ' N 01025 , W 10.5 1.1+0.2 [1]
Highbury 71mbabwe 17005 ' S 30°09' E 15-25 <1800 [7]
Kalkkop South Africa 32°43 ' S 24°26 ' E 0.64 <5 [2]
Oasis Libya 24035 ' N 24024 ' E 11.5 <120 [1]
Ouarkzi7 Algeria 29000 ' N 07033' W 3.5 <70 [1]
Pretoria Salt-pan South Africa 25°24 ' S 28005 , E 1.13 0.2 [3]
Roter Kamm Namibia 27046 ' S 16018 ' E 2.5 3.7+0.3 [4]
Talemzane Algeria 33°19' N 04°02' E 1.75 <3 [1]
Tenoumer Mauritania 22°55' N 10°24' W 1.9 2.5+0.5 [1]
Tin Bider Algeria 27°36' N 05007 ' E 6 <70 [1]
Vredefort South Africa 27000' S 27030' E 180-300 2002.+_52 [5]
Data from: [1 ] Grieve (1991); [2] Reimold eta/. (1993a); [3] Koeberl et ai. (1994b); [4] Koeberl et ai. (1993a); [5] Therriault et al. (1993), Walraven et al (1990); [6] Becq-Girdaudon et al. (1992); [7] Master et ai. (1994).
shock metamorphism- French and Short (1968), St6ffler (1972, 1974); cratering mechanics - Gault et al. (1968), Roddy et al. (1977), Melosh (1989); impact melts and glasses - Dence (1971), St6ffler (1984), Koeberl (1986), Bou~ka (1993). Some aspects are briefly described in the next sections.
CRATER M O R P H O L O G Y
Evidence of an appropriate crater form is a key criterion for the initial identification of an impact structure. A fundamental distinction between craters of meteoritic and volcanic origin is that impact is a surface process that produces circular, shallow, and rootiess structures. Small structures (on eerth < 4 km [< 2 lan in sedimentary targets] in diameter) have a bowl- shaped cross section and are called simple craters (Fig. 2a). Structures >4 km in diameter show a complex morphology (Fig. 2b). All craters have an outer rim and show some crater infill (i.e. brecciated a n d / o r fractured rocks, impact melt rocks; Fig. 2a, b). In complex craters, a central structural uplift, consisting either of a central peak or of one or more central peak ring(s), exposes basement rocks uplifted from considerable depth (up to several kin).
During theinitial phases of crater formation a deep cavity, called the "transient crater", is developed. The depth of the transient cavity is the sum of the excavation depth (which is approximately one-third of the transient
crater depth , or abou t one- tenth of the t ransient diameter) plus the amount of downward displacement of the target rocks (Gault et al., 1968; Roddy et al., 1977; Grieve 1987; Melosh 1989). The transient cavity is unstable leading the crater walls to collapse. Impact breccia fills the crater leading to the morphology shown schematically in Fig. 2a. In larger crater structures the cavity floor is unstable and rises rapidly to form a central uplift. Slumping of the rim may lead to terracing and a generally flatter morphology (i.e. a lower dep th / diameter ratio) than for simple craters (Fig. 2b). The central peak or peak ring structure contains severely shocked material and is often more resistant to erosion than the rest of the crater. In old eroded structures it may, thus, be the only remnant of the crater (see the section on the Oasis structure). Later modification of the crater may result not only in the erosion of the s t ruc tu re , b u t m a y a lso be caused by tec tonic deformat ion , y i e ld ing t runca ted or non-ci rcular structures (e.g. the Sudbury impact structure in Canada; P y e et al., 1984).
GEOPHYSICAL ANOMALIES
Geophysical studies are essential in the identification and s tudy of impact craters (Pilkington and Grieve 1992). A number of crater structures were only located because of the discovery of geophysical anomalies. This
266 C. KOEBERL
a
, D r
Simple crater
b
I D r I
Complex crater " " "
• Shocked breccia A Unshocked breccia W Impact m e l t
[] Imp~ em-ta F-tattered bedrock
Figure 2. Schematic cross section of (a) simple and (b) complex meteorite impact craters. Note that complex craters are shallower and may contain a continuous impact melt sheet. They also show a structural uplift which exposes basement rocks from greater depth. (Koebefl and Sharpton 1992.)
holds true especially for craters that are deeply eroded or are covered by later sediments and that, therefore, do not have a direct surface expression. Important examples of sediment-covered craters include the 65 Ma Chicxulub impact structure in Yucatan, Mexico (-300 km in diameter, Sharpton et al., 1993), the 36 km in diameter Manson crater in Iowa, USA (Hartung et a/., 1990), several craters in Russia, and the economically important Ames s tructure in Oklahoma, USA (Carpenter and Carlson 1992; see section on economic significance).
Geophysical anomalies in craters include gravity anomalies, which are usnally negative for simple craters due to the presence of a breccia lens and fractured bedrocks with reduced density in comparison to the target rocks. For complex craters, the signature may be more complicated and varied, often forming a gravity high over the central uplift surrounded by an annular gravity low. Seismic studies, especially reflection seismic surveys, provide important details on the subsurface structure of craters. Indeed, the initial discovery of the only confirmed underwater impact crater, the 45 km-diameter Montagnais crater on the continental shelf about 200 km south of Halifax, Nova Scotia (Canada), which is covered by 510 m of marine sediments, was due to seismic studies done during oil exploration. In addition, the magnetic anomalies
associated with impact structures can be highly variable. Such magnetic patterns may not be unambiguous. Some larger structures may show high- amplitude anomalies due to remanently magnetized target rocks. Ground penetrating radar is a relatively new technique for the study of (covered) ejecta in or around smaller impact craters (Grant and Schultz 1993). For a review of the geophysics of impact craters, see Pilkington and Grieve (1992).
SHOCK METAMORPHISM
During impact, a supersonic shock wave is generated and propagates into the target rock. If rocks are compressed at pressures above their Hugoniot elastic limit, irreversible (structural) changes occur in the minerals and rocks. The Hugoniot elastic limit (HEL) is, broadly speaking, the maximum stress above which plastic, or irreversible, distortions occur in the solid medium through which the compressive wave travels; see compilations by Roddy et al., 1977; Melosh 1989. The HEL is on the order of about 5-10 GPa for most minerals and rocks. The only known ~atural process leading to such high shock pressures that exceed the HELs is impact cratering. In stark contrast, endogenlc metamorphism of crustal rocks rarely exceeds temperatures of 1200°C and pressures of 2 GPa. Shock pressures and temperatures during impact may reach, depending on the magnitude of the event, many 100 GPa and several 1000°C, which may lead to the superheating of matter without vaporization. It is important to realize that shock compression is not a the rmodynamica l ly reversible process and the Hugoniot equations conserve mass, momentum, and energy, but not entropy (see review by Melosh 1989). Some of the structural changes in minerals and rocks due to shock metamorphism are uniquely characteristic of the high pressures and extreme strain rates associated with impact. Static compression yields different products. The formation conditions for shock metamorphic products are relatively well understood (H6rz 1968; French and Short 1968; Gratz et al., 1992; H u f f m a n et al., 1993). The shock pressure regime, up to about 100 GPa, has been experimentally calibrated with laboratory shock experiments with most rock-forming minerals (see references listed above, and St6ffler 1972, 1974).
A number of these diagnostic shock features are listed in Table 2. Planar deformation features (PDFs) are usually best developed in quartz, but occur also in feldspar minerals or olivine (for examples see Figures in French and Short 1968; Alexopoulos et al., 1988; or Fig. 17c). They are generally accepted to be diagnostic for shock (French and Short 1968; St6fxqer 1972, 1974; Alexopoulos et al., 1988; Sharpton and Grieve 1990). PDFs are only one of many features characteristic for shock. Others include diaplectic glass (an amorphous, isotropic phase preserving the crystal habit and sometimes planar features, but formed without melting;
Tab
le 2.
Fea
ture
s Dia
gnos
tic of
Pm
gm-~
ive S
hock
Met
amor
phis
m
Pres
sure
Ran
ge (G
Pa)
Feat
ures
T
arge
t Cha
ract
eris
tics
Feat
ure
Cha
ract
eris
tics
2-30
Sh
atte
r co
nes
Bes
t dev
elop
ed in
hom
ogen
eous
, fi
ne-g
rain
ed,
mas
sive
rock
s C
onic
al f
ract
ure
surf
aces
wit
h su
bord
inat
e st
riat
ions
rad
iati
ng f
rom
a f
ocal
poi
nt
5-4.
5 Pl
anar
fra
ctur
es a
nd
Plan
ar d
efor
mat
ion
feat
ures
(PD
Fs)
30-4
0 D
iapl
ectic
gla
ss
15-5
0 H
igh-
pres
sure
pol
ymor
phs
45->
70
Min
eral
mel
ts
>60
Roc
k m
elt
70-1
40
Impa
ct d
iam
ond
Hig
hest
abu
ndan
ce in
cry
stal
line
rock
s; F
ound
in
man
y ro
ck-f
orm
ing
min
eral
s; e
.g.,
quar
tz,
Feld
spar
, ol
ivin
e, a
nd z
icro
n.
Mos
t im
port
ant
in q
uart
z an
d fe
ldsp
ar (
e.g.
, m
aske
lyni
te fr
om p
lagi
ocla
se).
Qua
rtz
poly
mor
phs
mos
t co
mm
on:
Coe
site
, St
isho
vite
; bu
t als
o R
ingw
oodi
te fr
om o
livin
e,
and
othe
rs.
Roc
k-fo
rmin
g m
iner
als
(e.g
., le
chat
elie
rite
fro
m
quar
tz)
Bes
t dev
elop
ed in
mas
sive
silic
ate
rock
s O
ccur
as
indi
vidu
al m
elt b
odie
s (m
oon t
o m
siz
e) o
r as
a co
here
nt m
elt s
heet
s, u
p to
100
0km
3.
From
car
bon
pres
ent i
n ta
rget
rock
s; ra
re.
Set
s of
ext
rem
ely
stra
ight
, sh
arpl
y de
fine
d pa
rall
el l
amel
lae;
oc
cur o
ften
in m
ultp
le a
sets
w
ith
spec
ific
cry
stal
logr
apic
ori
enta
tions
.
Isot
ropi
zatio
n th
roug
h so
lid-s
tate
tm
mfo
nnat
ion
unde
r pr
eser
vatio
n of
cry
stal
hab
it a
s w
ell
as
prim
ary
defe
cts a
nd s
omet
imes
pla
nar
feat
ures
. In
dex
of r
efra
ctio
n lo
wer
tha
n in
cry
stal
but
hi
gher
than
in f
usio
n gl
ass.
Rec
ogni
zabl
e by
its
cry
stal
pa
ram
eter
s,
conf
irm
ed
usua
lly
wit
h X
RD
or
NM
R;
abun
danc
e in
flue
nced
by
po
st-s
hock
te
mpe
ratu
re a
nd s
hock
dur
atio
n; S
tish
ovit
e is
te
mpe
ratu
re-l
abile
.
Con
trar
y to
di
aple
ctic
gl
ass,
co
mpl
ete
tran
sfor
mat
ion
into
gla
ss b
y ~
ion
of a
min
eral
.
Impa
ct m
elts
are
eit
her
glam
y (f
usio
n gl
asse
s)
or c
ryst
alli
ne;
of m
acro
scop
ical
ly h
omo-
ge
neou
s,
but
mic
rosc
opoi
call
y of
ten
hete
roge
neou
s co
mpo
siti
on.
Hex
agon
al f
orm
; us
uall
y ve
ry s
mal
l bu
t oc
casi
onal
ly u
p to
nun
-siz
e.
Q D.
o
Data from: Alexopoulos et
al. (1
988), French an
d Short (1968), S
harpton an
d Grieve (1
990), St6ffler (1972,1974).
268 c. KOEBERL
a stage intermediate between crystalline and normal glassy phases), high-pressure polymorphs (e.g. coesite, stishovite), or mineral or rock melts (impact glass) formed by fusion of minerals or of the complete target rock. PDFs are <1-3 mm thick, extremely straight (planar) zones spaced at about 2-10 mm and are often filled with glass. They are usually arranged in the form of one or multiple sets per host grain and composed of numerous, strictly parallel features that may extend through the whole grain. PDFs are best studied using a universal stage (or spindle stage), or by transmission electron microscopy (TEM; Gratz et al., 1992; Leroux et al., 1994). They often occur in multiple sets parallel to cer ta in c rys t a l log raph ic or ienta t ions , of w h i c h especially the {0001} or c (basal), {1013} or ca, and {1012} or 7r orientations of quartz-hosted PDFs are impact-diagnostic and their relative abundances are used to calibrate shock pressure regimes (Robertson et al., 1968; H6rz 1968). The occurrence of diagnostic shock features is by far the most important criterion for evaluating the impact origin of a crater structure, part icular ly when several aspects of the range of progressive shock metamorphic effects (Table 2) can be identified.
On a macroscopic scale, the occurrence of shatter cones is widely believed (but not universally accepted) to be characteristic of impact (Dietz 1968; Milton 1977). The formation of these features, which have been d u p l i c a t e d in explos ion cra ter exper iments , is dependent on the target rock and the shock regime. A good lithological indicator for impact may be a layer of fragmental breccia found as crater fill or overlying a possibly raised, part ia l ly brecciated, and up- or overturned rim. This ejecta breccia may display the inverted stratigraphic sequence of the target area, as the youngest target rock on top of the target sequence will be ejected and deposited first, followed by older target rocks. A suite of various breccia types can be generated by impact (St6ffier and Grieve 1994): monomict or polymict breccias consisting of :
i) cataclastic (fragmental) ii) suevitic (fragmental with a minor component of
melt fragments) iii) impact melt (melt breccia with a minor dastic
component) breccias. Whether all these breccia types can be identified at a
particular impact site depends on factors such as the size of the crater, the composition of the target area (Kieffer and Simonds 1980) and the level of erosion (Roddy et al., 1977; H6rz 1982; Grieve 1987).
Another good indicator for shock metamorphic processes is the occurrence of impact glass. At pressures in excess of about 60 GPa, rocks melt to form impact melts. The high temperature of the melts is produced by shock waves , which generate temperatures far beyond those common in the earth's crust (or in volcanic eruptions), as evidenced by the presence of high- temperature minerals such as lechaterlierite (a melt glass which forms from pure quartz at temperatures
>1700°C) or baddeleyite (the thermal decomposition product of zircon forming at a temperature of about 1900°C). Some impac t mel t s m a y have been superheated, without being vaporized, to temperatures of 10 000°C or higher. D e p e n d i n g on the initial conditions of formation, and the cooling history, impact melts may either be quenched to form impact glasses, or, if cooled slower, usually form very fine-grained impact melt rocks. Due to their metastable nature, impac t glasses m a y dev i t r i fy af ter some t ime, depending also on the post-impact conditions. Impact glasses are found at numerous, usually younger, impact craters, including several African craters. Such glasses have a chemical composition that is indistinguishable from that of the target rocks (i.e. they may have a sedimentary composition). For example, their distinct Rb-Sr isotopic composition, which is identical to that of the target rocks, and distinct from that of any intrusive or volcanic rocks, can be used to infer an impact origin (e.g. at the Tenoumer crater, French et al., 1970).
Impact glasses have very low water contents (about 0.001 - 0.05 wt%), often show rather inhomogeneous chemical compositions, sometimes preserve shocked minera l s f rom the ta rge t rocks, conta in h igh- t e m p e r a t u r e d e c o m p o s i t i o n phases (such as baddeleyi te) , and m a y show indicat ions for the admixture of a small meteorit ic component. More details on various aspects of impact melts and glasses are discussed by E1 Goresy et al. (1968), Dence (1971), St6ffler (1984), Koeberl (1986, 1992) as well as in the sections on the Aouelloul, Bosumtwi, and Saltpan craters.
REMNANTS OF THE METEORITIC PROJECTILE
Although the presence of meteorites at a crater is, of course, definitive proof for origin by meteorite impact, impact physics and erosion prevent the wider use of this criterion. First of all, the meteorit ic impactor (projectile, or bolide) is also subjected to a very high pressure shock wave, which leads to almost instant vaporization of most or all of the projectile. Only during the impact of smaller objects, with lower impact velocity due to atmospheric retardation, does a small percentage of projectile material survive. The cut-off is somewhere around a I - 1.5 km crater diameter, but even at craters around I km diameter (e.g. Meteor Crater, Arizona) only a few percent of the original projectile mass are not vapor ized due to the part ial b reak ing-up of the projectile prior to impact. The other complication is the low resistance of meteorites towards erosion. Under normal terrestrial conditions, stony meteorites survive erosion for only a few thousand years, while iron meteorites may resist maybe ten times longer. Thus, only a minor fraction of all meteorite craters, namely the very young and small ones, can be expected to contain meteoritic fragments.
African meteorite impact craters: characteristics and geological importance 269
METEORITIC COMPONENT IN IMPACT C R A T E R S : R E - O S I S O T O P E S
The mass balance of impact melting and vaporization shows that, in addition to the meteorite being essentially completely vaporized upon impact, a large volume of target rocks (several orders of magnitude larger than that of the projectile) is mel ted and even par t ly vaporized. Thus, all impact-derived rocks, such as breccias, impact melts, and impact glasses, have a composition that is predominantly derived from the target rocks. A possible minor meteoritic component is extremely difficult to differentiate from the terrestrial composition. Only a few characteristic elements, which are much more a b u n d a n t in meteor i tes than in terrestrial crustal rocks, may be used as tracers for a meteoritic component. The platinum group elements (PGEs, e.g. Ir, Os, Pt, Pd) are several orders of magnitude more abundant in most meteorites (i.e. chondrites and iron meteorites) than in crustal rocks. Chondritic meteorites contain, for example, on the order of 400- 800 ppb Ir or Os, while the average crustal Ir and Os abundances are on the order of 0.02 ppb (Taylor and McLennan 1985). The addition of only 0.1 wt% of a chondritic component to a terrestrial crustal rocks would result in an addit ion of ~0.4 ppb Ir to the background value. An enrichment of PGEs (usually Ir) in impact melts or breccias may, thus, provide good evidence for a meteoritic component (Morgan et al., 1975; Paime et al., 1978; Palme 1982). Figure 3a shows how the concentrations of Au and Ir in impact melts or breccias can be used to distinguish between a cosmic and a terrestrial signature. However, Au is a rather mobile (and volatile) element, which can easily be enriched in various rocks. In addition, some particular target rocks (e.g. mafic or ultramafic rocks, or target areas containing mineralized zones) may also yield elevated PGE abundances (see section on the Bosumtwi crater). Thus, while elevated PGE abundances may be good evidence for a meteoritic contribution, they are not u n a m b i g u o u s and need to be eva lua ted in comparison to the indigenous content of PGEs in the target rocks.
A new tool, which al lows a more def in i te identification of a cosmic component, is the study of Re and Os isotopes (Koeberl and Shirey, 1993). The Re- Os isotopic system is based on the [5-decay of ~tRe to ~STOs (half-life = 42.3-3:1.3 Ga). Re and Os have a different geochemical behavior during the formation of crustal rocks which are formed by the partial melting of mantle rocks. Os is highly compatible and remains in the residue, but Re is moderately incompatible and is, therefore, enriched in the melt. Thus, crustal rocks have high Re and very low Os concentrations. Due to the high Re concentrations, the abundance of 1~Os in crustal rocks increases significantly with time. The growth of 187Os from the decay of 187Re can be described by normalizing to a non-radiogenic Os isotope:
lS7Os/1~Os = (lSTOspSSOs) i + [18rRe/l~)s](e~ - 1)
where 187Os/I~Os and ~Re/ l~Os are the present day ratios, (187Os/1~:)s)1 is the initial ratio, ~ is the decay constant and t is the age of the rock.
f:h
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Figure 3(a). Diagram of Au versus Ir contents of terrestrial a n d
extraterrestrial materials, after Palme (1982). The Ir and Au contents of some impact melt rocks and breccias from several impact craters are shown, plotting near the meteoritic side of the diagram. The I r / Au ratio chosen for the meteoritic line is for ordinary chondrites (type H) and represents an extreme value; most chondritic meteorites have ratios closer to the terrestrial line. However, because of the mobility of gold, the classification of rocks as cosmic vs. terrestr ial i s
ambiguous; this is shown by the value for a breccia from the Kalkkop crater, which plots in between the two fields, while, in fact, it does contain a cosmic component (see Fig. 3b).
Figure 3(b). l~Os/lasOs vs. leTRe/leeOs diagram for rocks from the Kalkkop crater, South Africa. The data array for meteorites is shown in the lower left part of the plot. The field defined by target rock samples (sandstone and shale) is in the upper right, showing values that are typical for the upper continental crust. The four samples of suevitic breccia (from different depths in the drfllcore) plot in a mixing field between a meteoritic component and target rocks (from which the breccias are derived). Breccia-3 (112.7 m) contains about 10-times higher Os abundance than the target rocks and has a near-meteorific 187Os/1~Os ratio, clearly indicating the presence of a meteoritic component. Such mixing relations can be used to help constrain the origin of a crater structure (after Koeberl et al., 1994a.).
270 C. KOEBERL
Rocks from the terrestrial mantle have a low present- day '87Os/'~Ds ratio of about 0.13, and meteorites also have low '~ZOs/'~)s ratios of about 0.11 to 0.18 ('"7Os/ '~Ds=0.95-1.5) (Walker and Morgan 1989; Horan et al., 1992). As Os is much more abundan t than Re in meteorites (and in the mantle), the '~ZOs/'~Ds ratio in such rocks changes only very slowly with time. The crustal ' a O s / ' ~ ) s ratio, on the other hand, increases rapidly with time, because Re abundances are several orders of magni tude higher than Os abundances. Isotopic ratios of crustal rocks depend on age and elemental abundances. '~ZOs/'~)s ratios of about 0.67 to 1.61 (with an average of about 1.2) are thought to be representative of currently eroding upper continental crust (Esser and Turekian 1993).
The absolute abundances of Os as well as the 'r/Re/ '~Os and '~ZOs/'=Os ratios in meteori tes are thus distinctly different from those in old crustal target rocks. Impac t mel t rocks, glasses , or b recc ias consis t predominantly of terrestrial target rocks, mixed with a usua l ly very small (<1%) meteor i t ic component . However, the difference in absolute Os abundances between meteorites and crustal target rocks favors detection of small amounts of a meteoritic component. This concept has been used for the study of Cretaceous- Tertiary (K-T) boundary clays (Luck and Turekian 1983). The first application of the Os isotopic system to impact craters was attempted by Fehn et al. (1986), but low precision and the complicated laboratory method prevented its wider use.
The recent development of the negative thermal ionization mass spectrometry technique (Creaser et al., 1991) made a broader application for impact crater studies possible. This method allows the determination of abundances and isotopic ratios of Os and Re at the low abundance levels found in target rocks and in impact-derived rocks, while using relatively small amounts of material. An example for the use of this method is given in Fig. 3b, which also shows its advantage over using only elemental abundances. Abundances and isotopic compositions of Re and Os were measured in target rock and impact breccia samples from the Kalkkop crater, South Africa (Koeberl et al., 1994a). On an Ir vs. Au diagram (Fig. 3a), most breccias would fall close to the terrestrial line, while only one (from 89.15 m depth, which is shown in the Figure) shows an indication of a cosmic component. Howeve r , us ing the Re-Os i so topic sys tem, the abundances and isotopic ratios of Re and Os in the target rocks are typical for old continental crust, while the breccia samples have Os contents up to 10 times higher than the target rocks, as well as low '~ZOs/'~Os ratios. Such va lues are i n c o m p a t i b l e wi th old continental crustal values. The higher Os content together with the lower 'SZOs/'~3s ratios demonstrate that the breccia conta ins up to 0.05% of an extraterrestrial component. The mixing relationship between the target rocks at Kalkkop and a meteoritic
component is evident and clearly shown in Fig. 3b. This method enables not only the discrimination
between a crustal and meteoritic source of any Os (PGE) enrichments, but also permits quantification of such a component (Koeberl and Shirey 1993; Koeberl et al., 1994a,c). The presence of a small meteoritic component in breccias or melt rocks is very good evidence for the impact origin of such rocks (and the structure where they are found). The study of Re-Os isotopes may be used as a new and important diagnostic tool for assessing the impact origin of crater structures (Reimold et al., 1993; Koeberl et al., 1994a), which could, in some aspects and for certain crater structures, rival the diagnostic capacity of shock metamorphic effects.
CONFIRMED METEORITE CRATERS IN AFRICA
Amguid, Algeria This crater was first recognized from an aircraft in
1954 and is described in detail by Lambert et al. (1980). The 450 m d iamete r crater is exposed in Lower Devonian sandstones. It has an elevated rim up to 50 m high and is filled with very bright and fine-grained compacted eolian silts (Fig. 4a). In cross-section the crater shows sandstone beds with a dip that becomes progressively steeper in the upper part of the wall, with overturning of the upper sandstone layers at the NN W and SSE parts of the rim (Fig. 4b). The crater is surrounded by a nearly continuous ejecta blanket up to about 100 m from the crater rim. The near-perfect preservation state of the crater led Lambert et al. (1980) to est imate an age of up to 0.1 Ma. Petrographic examination of samples from the crater wall showed the existence of up to three sets of PDFs in quartz grains, thus confirming the impact origin of the crater (Lambert et al., 1980).
Ouarkziz, Algeria This crater (earlier called '~I'mdouf") is a severely
eroded ring structure with a diameter of 3.5 km and is superimposed on a fault structure, which is part of the local geology (Fig. 5). The structure has rarely been visited, and only one geological study is available (Fabre et al., 1970). The crater is set in Carboniferous limestones and shales of the Upper Vis~en and Lower Namurian formations which are upturned at the crater rim. The rock formations also show f r a c t u ~ g and folding at the rim, and the central area is largely covered by alluvium. Fabre et al. (1970) report on the presence of breccias and "planar features" in quartz from the inner part of the rim. The crater consists of three concentric zones: an outer zone with outward dipping concentric faults, an inward dipping zone, and in the centre, brecciated vertical dipping beds of a concentric uplift. Its age is not well constrained. It occurred after the formation of the Mesozoic peneplain and is pre-Tertiary (Fabre et al., 1970). Clearly, this crater deserves a more thorough study.
African meteorite impact craters: characteristics and geological importance 271
a
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I Figure 4. Amguid crater, Algeria. (a) Schematic map of the crater, showing the infill with fine-grained eolian deposits; the dashed line shows the extent of the continuous ejecta blanket;, the structure on the right (and in the lower left comer) is a riverbed. (b) Schematic cross section through half of the crater, showing the original transient cavity ("true" crater floor) which is now filled with impact debris and later sediments (compare Fig. 2a); overturned strata are characteristic of impact craters. (Lambert et al., 1980).
Talemzane, Algeria This s imple bowl-shaped crater has a diameter of
1750 m and is locally also known under the geographic name "Daiet E1 Ma/idna". Following a first visit on the ground in 1951 and an investigation from the air in 1952,
a meteoritic origin of the crater was proposed (Karpoff 1953). A more detailed investigation of the structure by Lambert et al. (1980) yielded definitive evidence for an impact origin. The crater is emplaced in Senonian or Eocene limestones and has an elevated rim up to 70 m
272 C. KOEBERL
Figure 5. Ouarkziz crater, Algeria. The crater (as indicated by the arrows) is superimposed on a fold structure trending NW-SE. In this and all following Space Shuttle photographs, north is up. Space Shuttle photograph 41C-31-1032.
above the crater floor. In cross-section (Fig. 6), the crater shows s t rong ly f rac tured u p t u r n e d and local ly overturned limestone beds at the rim, with large blocks of ejected limestone scattered along the outer rim (Lambert et al., 1980). Breccia dikes occur at the crater wall, and detrital or reworked monomict breccia is found at the crater floor near the rim. Quartz is rare In the breccias, because limestone is the main target rock, but some quartz grains showing characteristic PDFs have been found (Lambert et al., 1980). The age estimate of <3 Ma is based on the level of erosion (Lambert et al., 1980).
T'm Bider, Algeria The structure (also known as "radema/t"), is about
6 km in diameter, appears as a series of at least three concent r ic a n n u l a r r idges at 2, 3.5, and 6 km, respectively, and is set in Lower to Upper Cretaceous
clay and limestone formations. Lower Cretaceous sandstones are exposed in the central part of the structure, about 500 m above their normal stratigraphic position, which indicates the presence of a structural central uplift. The s t ra t igraphy of the concentric limestone ridges is very complex and the strata are intensely folded. No definitive macroscopic evidence (e.g. shatter cones) for an impact origin was found. However, a petrographic study of the central sandstone showed its brecciated nature, and quartz grains with up to 8 sets of PDFs have been found (Lambert et al., 1981), p rov id ing pos i t ive ev idence for shock metamorphism. The unusual structure of the crater, the lack of brecciation and macroscopic evidence for shock metamorphism in a major part of the crater, as well as the presence of considerable ductile deformation, may be due to its formation in soft target rocks that are partly dominated by day. The age is not well known and is
/ Large blocks of e j e c t e d ~ ~ l i m e s t o n e --- , / Breccia veins
Figm'e 6. Schematic cross section of Talerazane crater, Algeria. The 1.75-kin-diameter crater has a raised rim with upturned target limestone (after Lambert et al., 1980).
African meteorite impact craters: characteristics and geological importance 273
given only as younger than the target rocks. The lack of al lochthonous crater fill materials may indicate erosion to the crater floor. A more detailed field study seems warranted.
B.P. Structure, Libya This structure was first described by Martin (1969)
and is named after the B.P. Exploration company. It consists of two eroded and discontinuous rings of hills. The inner ring is about 2 km in clian~eter with an average relief of 30 m, while the outer ring has a diameter of about 2.8 km and a maximum relief of about 20 m. The rocks at the centre of the structure show intense jointing. The rocks exposed are from the Early Cretaceous Nubian Sandstone Formation and include sandstone, siltstone, and quartz conglomerate. Medium- to coarse- grained orthoquartzite yielded highly shocked quartz grains with multiple sets of PDFs (French et al., 1974). The geology of the structure has been studied in some detail (Underwood 1975, 1976; Underwood and Fisk 1980), but no geochemical s tudy of the crater rocks is available so far. Such a s tudy is particularly desirable because of a possible association of the B.P. or the Oasis structure with the occurrence of Libyan Desert Glass (see below). Again, the age is poorly constrained, being given only as postdating the target rock age.
Oasis, Libya This e roded crater, named af ter the Oasis Oil
company, has a dlanaeter of about 11.5 km but the most prominent part is a central ring of bills, about 5.1 km in diameter and 100 m high, sun 'ounded by an annular depression (Fig. 7). The structure is exposed in the same rocks as the B.P. structure (about 85 km south of the
B.P. crater). Mult iple sets of planar elements were detected in quartz grains from orthoquartzite (French et al., 1974). A f ew samp l e s of a g l a s s -bea r ing microbreccia were found at the crater, containing fragments of brownish, partly devitrified glass with sandstone fragments and shocked quartz grains (French et al., 1974). As with the B.P. structure, its age is only constrained to post-date the target rocks, which are also Nubian sandstones.
Libyan Desert Glass Libyan Desert Glass (LDG) is an enigmatic natural
glass found in an area of about 6500 km z between sand dunes of the south-western corner of the Great Sand Sea in western Egypt, near the Libyan border. The first to report about LDG were Fresnel (1850) and Clayton and Spencer (1934). The glass occurs as centimetre to decimetre-sizecl irregular and strongly eroded blocks. Its fission track age has been determined at around 29 Ma (Storzer and Wagner 1977). LDG is very silica-rich at about 96.5-99 wt.% SiO 2, and shows a l imited variat ion in major and trace element abundances (Barnes and Underwood 1976; Fudali 1981; Weeks et al., 1984; Koeberl 1985). Although the origin of LDG is still debated by some workers, an origin by impact seems most likely. There are, however, some differences to "ela~ical" impact glasses. Evidence for an impact origin includes the presence of schlieren, partly digested mineral phases, lechatelierite (a high-temperature minera l mel t of quar tz) , b a d d e l e y i t e (a h igh temperature breakdown product of zircon, Kleinmann 1969; Storzer and Koeberl 1991), and the possible existence of a meteoritic component (Murali et al., 1989; C. Koeberl unpublished data). Although there is a
Figure 7. Oasis crater, Libya. The ring structure (composed of discontinuous hills) on the left side of the image has a diameter of about 5.1 km and constitutes the central uplift, while the crater is about 11.5 km in diameter and is visible as a slightly darker area surrounding the central ring (as indicated by the arrows). Space Shuttle photograph 51B-S2-2577.
274 c. Ko~n~L
similarity of LDG trace element composit ion with Nubian sandstone in general (Murali et al., 1988), no specific comparison has yet been made with material from either the B.P. or Oasis structures.
Aouelloul, Mauritania The AoueUoul impact crater has a diameter of about
390 m and is situated in the Adrar region of the western Sahara Deser t in Maur i t an ia (Fig. 8a,b). It was discovered from the air in 1938 and first visited on the ground in 1950 (Monod and Pourquie 1951). The crater is exposed mainly in Ordovician Zli sandstone, with some Oujeft sandstone present. Some parts of the rim, which rises 15 to 25 m above the surroundings (53 m above the crater floor), show an over turned rock
sequence. No unequivocal mineralogical evidence of shock metamorphism has yet been described for the crater rocks. However, planar fluid inclusion trails (presumably remnants of PDFs) in the form of distinct sets in quar tz grains were recently found in Zli sandstone from the crater rim (Koeberl and Reimold in prep.). The crater is filled with a poorly sorted sandy silt which is overlain by well-sorted windblown sand. A gravity s tudy indicated a maximum sedimentary fill thickness of about 23 m, underlain by a breccia lens extending to a maximum depth of 130 m (Fudali and Cassidy 1972). Slightly different dimensions based on gravity data (i.e. about 100 m for the depth of the breccia lens) are given in Grieve et al. (1989).
Numerous fragments of a dark glass, which, in view
F'~gure 8. Aouelloul crater, Mauritania. (a) Vertical aerial photograph (west is up); (b) oblique view. Photographs courtesy Prof. Th. Monod, Paris.
African meteorite impact craters: characteristics and geological importance 275
of absence of any indication of volcanic rocks or activity, was taken as key evidence for an origin by impact, have been found mainly outside the crater rim, and, to a lesser degree, on the inner slope (Campbell-Smith and Hey 1952; Chao et al., 1966b). Some of the glasses were found to contain microscopic Ni-rich (9 wt%) iron spherules which may be related to the meteori t ic projectile (Chao et al., 1966a). The glass is somewhat i n h o m o g e n e o u s , showing a b u n d a n t schl ieren of different chemical composition (Fig. 9a) and partly digested quartz and feldspar grains (Fig. 9b). The composition of the glass is similar to that of the Zli sandstone, but a few, mainly siderophile, elements show
some enrichments in the glass (Chao et al., 1966b; Koebefl and Auer 1991), which was used to suggest that the crater was formed by an iron meteorite projectile (Morgan et al., 1975). The high-temperature impact or igin of the glass is s h o w n by the presence of lechatelierite and baddeleyite, which are diagnostic of an impact origin (El Goresy 1965; E1Goresy et al., 1968). The age of the crater was determined by K / A r and fission track dating of impact glass to be 3.1+0.3 Ma (Storzer and Wagner 1977; Fudali and Cressy 1976).
Tenoume~; Mauritania This almost perfectly circular crater has a diameter
Figure 9. Impact glass (sample AOL-21) from Aouelloul crater, Mauritania. (a) Microphotograph of schlieren and partly melted mineral grains (predominantly quartz); dark areas are of brownish color and enriched in Fe (plane polarized light; width of image: 2.2 ram); (b) Back-scattered electron (BSE) microphotograph showing the inhomogeneity of the glass and partly digested quartz crystals (darker gray); scale bar = 1 mm.
276 C. KOEBERL
of about 1.9 km and is located in the Western Sahara in Mauritania, almost 400 km to the NW of the Aouelloul crater. The crater is excavated in a peneplained surface of Precambr ian gneisses and granites, which are covered by a thin veneer of young (probably Pliocene) sediments. The crater is filled with unconsolidated sediments and has a present depth of about 100 m. The depth to the base of the post- impact sediments is est imated to be 200 - 300 m based on geophysical measurements (Fudali and Cassidy 1972; Grieve et al., 1989). A swarm of small "dikes" supposedly containing "rhyodacitic lava", which are intrusive into concentric fractures of the crater and outcrop just outside the crater rim, were originally taken as evidence for a possible volcanic origin of the structure (Monod and Pomerol 1966).
However , French et al. (1970) found up to eight different sets of PDFs in quartz grains from obviously strongly shocked inclusions of granitic basement rocks in the "lava", showing that the crater is of impact origin. The "lava" is in fact a rapidly quenched impact melt rock, consist ing of a f ine-grained in tergrowth of plagioclase laths and pyroxene crystals up to 50 mm in length in a matrix of brownish glass (French et al., 1970). The melt rock also contains lechatelierite (diaplectic quartz glass) inclusions. The composition of the melt
rock is not identical to that of the gneissic and granitic basement rocks and seems to incorporate a component derived from rare amphibol i te veins found in the gneissic terrain (Fudali 1974). Some Rb-Sr isotope data show that the glasses are indeed melted basement rocks (French et al., 1970). However, no detailed trace element or isotope studies have been made to date. The age of the crater was determined to be 2.55:0.5 Ma from K/Ar measurements of the melt rock (French et al., 1970).
Aorounga, Chad The circular depression of Aorounga, which appears
on some geological maps, has a diameter of 12.6 km and is s i tuated in Nor the rn Chad, about 110 km southeast of the Emi Koussi volcano in the Tibesti Massif. It has first been studied in a photogeological investigation of Gemini, Apollo, Landsat, and aerial photographs by Roland (1976), who suggested an origin either as a granite diapir or an impact crater, but concluded that a diapir is the more likely explanation. The structure was mentioned by Grieve et al. (1988) as a possible impact crater. A recent French expedition to the area succeeded in collecting a few samples from the structure, which show evidence of multiple sets of PDFs (Becq-Giraudon et al., 1992). The host rock of the crater is a fine-grained, well-sorted, slightly carbonate-
Figure 10. Aoroun~a, Chad. This structure has a diameter of 12.6 km and is situated about 110 km southeast of the Emi Koussi volcano in northern Chad. It comprises a central uplift, surrounded by two concentric ring s[n'uctures with a topographic relief of about 100 m. Northeast is up; Space Shuttle photograph STS43-75-~.
African meteorite impact craters: characteristics and geological importance 277
bearing sandstone of Upper Devonian age. The structure has an outer and an inner ring wall,
both rising about 100 m above the mean level of the s u r r o u n d i n g p l a i u , and which are separated from each other by a depression of uniform width (see Fig. 10). A somewhat eccentric central uplift is located in the central depression. The ring wall.q consist of steeply outward dipping strata, with dips of 40 ° - 50 ° in the outer, and 80 ° in the inner wall (Becq-Giraudon et al., 1992). Some breccia, containing some fine-grained clasts with a fluidal texture, was found on top of the inner rim wall (Becq-Giraudon et al., 1992). Becq-Giraudon et al. (1992) estimated the age as young as 12 000 to 3 500 years old, which seems too young, considering the obvious erosional state of the s t ructure (Fig. 10). Unfortunately, the availability of samples is extremely limited, and a civil war currently (1994) raging in northern Chad prevents more detailed field studies (J.- F. Becq-Giraudon pers. comm. 1994).
Bosumtwi, Ghana The almost circular Bosumtwi crater in Ghana has a
rim to rim diameter of 10.5 km (Fig. 11a) and is exposed in 2 Ga old lower greenschist facies metasefliments of the Lower Bh'imian Group (Fig. 11b; Schnetzler et al.,
1966; Kolbe et al., 1967). The crater is almost completely filled by Lake Bosumtwi, which has a maximum depth of about 80 m, with the crater rim rising about 250 - 300 m above the lake level (Fig. 11c). The structure has been known since late last century, but its origin was the subject of a controversy (Jones 1985b). While Mar'laren (1931) though t the crater to be of impact origin, Rohleder (1936) preferred an endogenic explanation. However, outcrops of suevitic breccia were found around the crater (Jones et al., 1981), and the high- pressure quartz modification coesite (Littler eta/., 1961) as weU as Ni-rich iron spherules and baddeleyite, the high-temperature decomposition product of zircon, discovered in vesicular glass from the crater rim (El Goresy 1966; El Goresy et al., 1968), are supporting an impact origin for the structure. The composition of melt in the suevite is shnilar to that of the basement rocks (Jones 1985a). Siderophile element contents of some melt rocks do not show any obvious extraterrestrial c o m p o n e n t (Pa lme et al. , 1981). Some gene ra l geophysical studies of the area are available (Jones et al., 1981), but no detailed geophysical measurements of the crater itself have been made. An assessment of the structure of the crater (Fig. 11c) is, thus, largely hypothetical (Jones et al., 1981).
Figure 11. Bosumtwi crater, Ghana. (a) The 10.5-kin-diameter crater is filled with a lake and is clearly vis~le on this Landsat TM photograph.
278 C. KOEB~:Rt.
i/ !
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Figure 11 Bosumtwi crater, Ghana Co) General geological map of the crater area (after Jones et al., 1981); (c) Inferred schematic cross section of the crater, based on the interpretation of geological and geophysical data (after Jones et al., 1981).
African meteorite impact craters: characteristics and geological importance 279
Ivory Coast tek t i tes Tektites are natural glasses occurring on earth in four
strewn fields: Australasian, Ivory Coast, Central European, and North American. Tektites occur in three different forms (Muong Nong-type, splash-form, and ablation shaped) on land (e.g. O'Keefe 1963), and as nu'crotektites predominantly in deep sea cores (Cassidy et al., 1969; Glass et al., 1979). Geochemical arguments have shown that tektites have been derived by hypervelocity impact melting from terrestrial upper crustal rocks (Taylor 1973; Koeber11986,1992). Tektites from the Ivory Coast (Cote d'Ivoire) have been known since 1934 (Lacroix 1934), while microtektites, which are related to the tektites found on land by having very similar chemical composition and age, have been found in deep sea sediments off the coast of western Africa (Glass 1968, 1969). The geographical distribution of microtektite-bearing deep sea sediments can be used to define the extent of the strewn field (Glass and Zwart 1979; Glass et al., 1979, 1991).
The Bosumtwi crater was suggested to be the source crater of the Ivory Coast tektites, because the tektites and the crater lithologies have the same Rb-Sr and Sta- Nd model ages (Schnetzler et al., 1966; Shaw and Wasserburg 1982), a similar chemical composition (Schnetzler et al., 1967; Jones 1985a), as well as similar isotopic characteristics (e.g. Schnetzler et al., 1966; Shaw and Wasserburg 1982). ALso, tektites and Bosumtwi impact glasses have the same age (Koeberl et aL, 1989b). The Ivory Coast tektites have large negative end ValUes of about -20, which are typical for old continental crust, yielding mantle extraction Sm-Nd model ages of about 1.9 Ga. This is in agreement with the whole rock Rb-Sr ages of the rocks around the Bosumtwi crater which range from 1.9 to 2.1 Ga (Schnetzler et al., 1966; Kolbe et al., 1967). Figure 12 shows that the oxygen isotopic characteristics of the tektites are almost indistinguishable from those of the sedimentary
80
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Figure 12. Correlation between silica content and the oxygen isotopic composition (81eO) for Bosumtwi crater target rocks (open squares- granodiorites, asterisks-metasediments, filled cirde-microgranite), Ivory Coast tektites (triangles), and Bosumtwi impact glass (stars) (after Chamberlain et al., 1993).
country rocks around Bosumtwi (Chamberlain et al., 1993).
Glass from the Bosumtwi crater as well as Ivory Coast tektites have been dated with a variety of methods (Gentner et al., 1964; Lippolt and Wasserburg, 1966; Storzer and Wagner 1977). The preferred ages are 1.03i0.11 Ma for Bosumtwi glass (fission track age), and 1.05+0.11 Ma and 1.10~0.10 Ma (fission track and 4°Ar- 39Ar ages, respectively) for the Ivory Coast tektites (Koeberl et al., 1989b). A recent Re-Os isotopic study has provided evidence for the presence of up to 0.6% of an extraterrestrial component in Ivory Coast tektites and demonst ra ted that the Re-Os isotopic characteristics of the tektites and Bosumtwi impact glasses can be derived by mixing of metasedimentary target rocks from Bosumtwi and a meteorit ic component (Koeberl and Shirey 1993). This study also revealed that some target rocks contain higher abundances of siderophile elements (e.g. Os), which would make it difficult to uniquely identify a meteoritic component in the Bosumtwi impact glasses and Ivory Coast tektites, as already suspected by Jones (1985a). However, the study of Re and Os isotopes allows to distinguish between Os having a terrestrial crustal signature, and Os having an extraterrestrial (meteoritic) signature. While there is little doubt about the impact origin of the crater and the connection to the Ivory Coast tektites, the geophysical characteristics of the crater and the detailed petrography of rocks from the crater remain poorly known and should be investigated in the future.
Highbury, Zimbabwe The Highbury structure is a near-circular feature of
about 15 km in diameter, which has first been noted in 1985 on Landsat imagery by German geologists; however, no follow-up study or publications resulted. In 1993, an expedition (including the author) was mounted to investigate several crater-like structures in Zimbabwe, including Highbury. This study resulted in the recognition of the Highbury structure as an impact crater (Master et al., 1994). The country rocks in the area are arkoses and metadolomites of the Deweras Group, which are flanked to the east and west by the Striped Slates and Mountain Sandstone members of the Nyagari Formation of the Lomagundi Group (Master et al., 1994). The ring structure can be seen on Landsat imagery (Fig. 13), showing some contrast with the highly vegetated Striped Slates of the Lomagundi Group to the east and west of the structure. The feature has a diameter of about 15 km. However, the abrupt termination of the Mountain Sandstones to the northwest and west, and some hints from the space images, suggest that the structure might be as large as 25 km in diameter.
Some brec~as were found in the central area, which may represent the remnant of a central uplift. Quartz crystals from samples from the central uplift area show planar fluid inclusion trails similar to those found at the Vredefort structure in South Africa. However, some
280 C. KOEBERL
Figure 13. The Highbury impact structure in Zimbabwe. The diameter of the structure, as indicated on the photograph (arrows), is about 15 kin. However, there is some evidence that the crater may be 25 km in diameter. This structure is deeply eroded. The linear feature in the lower right comer of the image is the Great Dike, Zimbabwe. Space Shuttle infrared photograph STS36-92-28.
quartz grains also show bona fide PDFs, and pockets of almost unaltered glassy material were found as well (Master et al., 1994). These samples will be of great value to determine the age of the structure, which is, so far, not well constrained. The crater postdates the north- south trending thrust faults of Magondi (1.8 Ga) age, but seems to be offset in the southeastern sector by dextral wrench faults, which are thought to be of late Irumide (ca 1 Ga) age (Master et al., 1994).
Roter Kamm, Namibia The 2.5 km diameter, near-circular Roter Kamm
crater in southern Namibia, which has an obvious 150 m high rim, is located in the Namib Desert, about 80 km due north of Oranjemund (Fig. 14). An impact origin for this crater structure was first proposed by Dietz (1965) and FUdali (1973) and was confirmed in the late 1980s by a detailed geological study of the crater and the discovery of clasts in impact melt breccias with characteristic shock deformation features indicative of shock pressures up to 30 GPa (Reimold et al., 1988; Reimold and Miller 1989, Koeberl et al., 1990a). Rock deformation in basement rocks exposed along the crater rim include severe jointing and cataclastic, mylonitic and pseudotachylitic breccias. [Pseudotachylites are very fine-grained or glassy rocks containing evidence
of melting; clasts therein are generally devoid of shock metamorphic features; these melt rocks are usually assumed to be locally derived by friction melting during impact or tectonic processes (Reimold 1991; Magloughlin and Spray 1992).] The crater was formed in a two-layer target: an upper layer of Gariep metased iments (schist, marble, + quar tz i te and sandstone), overlying the crystalline basement of the Namaqualand Metamorphic Complex. The basement was also heavily intruded by coarse-grained quartz veins and quartz- and quartz-feldspar pegmatites.
Melt breccias found at the crater were derived mainly from metased imenta ry target rocks, wi th rarely detected contributions from pegmatite and granite/ granodiorite. Three varieties of melt breccias can be defined as:
i) "schistose" ii) quartzitic melt breccias iii) "true" impact melt breccias. All these melt breccia types are petrologically and
chemically he terogeneous (Reimold et al., 1994; Degenhardt et al., 1995). Most of the breccias, including the " t rue" impact melt breccias, may have been produced in situ and not from a coherent impact melt body (Reimold et al., 1994). A few of the impact melt breccias contain basement rock clasts displaying shock
African meteorite impact craters: characteristics and geological importance 281
Figure 14. Roter Kamm crater, Namibia. The excellent preservation state of the crater is clearly visible. Northeast is up. Space Shuttle photograph 61C-40-001.
effects, such as planar deformation features in quartz or diaplectic quartz glass (Reimold and Miller 1989).
The presence of unusual quartz pebbles at the crater rim, which have been formed after the impact event, was interpreted to be evidence for a post-unpact phase of hydrothermal activity (Koeberl et al., 1989a). The presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact-induced hydrothermal event in the crater area (Koeberl et al., 1990b; Reimold et al., 1994). The age of the Roter Kamm crater has recently been shown to be 3.7+0.3 Ma by ~°Ar-~Ar dating of glass from an impact melt rock (Koeberl et al., 1993a). The limited outcrop, the heterogeneity of some target rocks, the scarcity of true impact melt breccias (containing glass and shock metamorphosed clasts) and the extensive sand cover of the crater area make surface studies of the Roter Kamm crater very difficult. This crater is very well preserved and one of the few craters on earth which will allow the determination of important morphometric data (i.e. the exact internal structure; Grieve 1993). Although there are some
geophysical data (Fudali 1973), which give some depth estimates, a future drillirtg project at the crater, as well as detailed geophysical investigations, including ground probing radar and possibly a reflection seismic survey, will be of extreme importance.
Vredefort, South Africa The Vredefort structure (or Vredefort Dome) is
situated about 120 km SW of Johannesburg (Fig. 15a,b). Its origin has, for a long time, been the subject of a controversy. Daly (1947) first proposed that it was formed by large-scale meteorite impact. This proposal was later renewed by Dietz (1961) and Hargraves (1961). The structure consists of a central core of mainly granitic gneiss terrane with a diameter of about 45 km. About three fourths of the core are surrounded by a collar, which is formed by a series of upturned and overturned strata including 3.07 Ga Dominion Group metavolcanics, 2.7-3.0 Ga Witwatersrand metasedlments and Ventersdorp lavas, and 2.25-2.5 Ga rocks from the Transvaal Sequence (Reimold 1993). Vredefort is the type locality for pseudotachylite, which is commonly developed in many Vredefort lithologies. Geophysical studies show a semi-annular magnetic anomaly in the inner part of the structure, and a positive concentric Bouguer gravity anomaly in the central area of the Vredefort Dome, indicating the presence of denser material below (Antoine et al., 1990; Corner et al., 1990).
The controversy regarding the origin of the Vredefort structure has recently been reviewed by Reimold (1993). The study of Vredefort is complicated by the fact that it is a very old (2.0 Ga, Walraven et al., 1990), large, and deeply eroded structure, which cannot be directly compared to younger and smaller craters. Despite some open problems (Reimold 1993) the evidence for impact is compelling. Shatter cones, which record a shock environment (Dietz 1968; Milton 1977), are abundant at Vredefort (Hargraves 1961; Dietz 1961). Planar microdeformation features were found in quartz grains from Vredefort and have been interpreted as being produced by impact shock (Carter 1965,1968; Grieve et al., 1990; Leroux et al., 1994). Their origin has been somewhat controversial (Lilly 1981). The features, however, do not have the complete set of crystallographic orientations normally associated with shock-produced planar deformation features in quartz. Grieve et al. (1990) admitted that the PDFs at Vredefort are abnormal, but at tr ibuted this to post-shock recrystani~ation of quartz. The presence of the high- pressure quartz polymorphs coesite and stishovite have been documented (Martini 1978, 1991; McHone and Nieman 1988).
A very fine-grained and homogeneous rock, occurring as dikes at the contact between core and collar and within the basement gneisses, is called the Vredefort Granophyre. It was interpreted as an impact melt rock that was injected into the basement (French et al., 1989; French and Nielsen 1990), although this view was not universally accepted (Reimold et al., 1990). If there was
282 C. KOEBERL
Figure 15. Vredefort structure, South Africa. (a) large-scale view: the Vredefort Dome is in the centre of the image, with the Vaal river winding from the lower left part of the image to the upper right. This view shows the whole Witwatersrand Basin. Space Shuttle photograph S08- 35-1294.
any doubt as to an impact origin of Vredefort, recently obtained results should settle this question. A new study of the mid-amphibolite facies metamorphism in the collar of the Vredefort Dome, which has been thought to be supportive of an endogenic model, indicates that the metamorphic event was unrelated to the crater-forming event, supporting the impact model (Gibson et al., 1994). Rhenium-osmium isotopic studies show very good evidence for the presence of a small meteoritic component in some Vredefort Granophyre samples (Koeberl et al., in prep.).
In addi t ion , a de ta i led TEM s t u d y of the characterist ics of the p lanar microdeformat ions revealed the presence of mechanical Brazil twin lamellae in the basal plane (Leroux et al., 1994). Such lamellae have only been found as quartz growth features and as deformation effects in impactites, or in statically deformed rocks that were subjected to pressures of more than about 2 GPa. In addition, other characteristic PDF orientations were identified in the TEM study, which should finally put the discussion about anomalous PDFs to rest. A recent interpretation of the geological structure of the Vredefort Dome suggested a diameter of 180-300 km (Therriault et al., 1993), which means that the structure is larger than
previously assumed. This would make Vredefort one of the largest impact structures recognized on earth, on a par only with the 200-250 km Sudbury structure in Canada (Grieve et al., 1991) or the ~300 km diameter Chicxulub multi-ring impact structure in Mexico (Sharpton et al., 1993). More work is necessary to understand the details of the remnant of this huge impact structure.
S a l t p a n , S o u t h A f r i c a
The Salt-pan (also termed Pretoria Saltpan, or Zoutpan; also: Tswaing - Place of Salt) crater has a
diameter of 1.13 km and is located about 40 km NNW of Pretoria. The interior of the structure consists of a fiat crater floor that is partially covered by a highly saline lake (Fig. 10a). The maximum elevation of the crater rim over the present crater floor is 119 m, and the rim rises up to 60 m above the surrounding plains. The crater was formed about 220,000 years ago (based on fission track dating; Storzer et al., 1993; Koeberl et al., 1994b) in crystalline basement of 2.05 Ga (Walraven et al., 1990) Nebo granite of the Bushveld Complex (Fig. 16b). Some l amprophyre , t rachyte , and minor carbonatite occur in sparse outcrops along the inner rim wall and in the region surrounding the structure (Brandt and Reimold 1993).
African meteorite impact craters: characteristics and geological importance 283
Figure 15 Vredefort structure, South Africa (b) The centre of the picture shows the brighter core, in the northwestern part surrounded by the darker collar, which are made up by the rugged hills of the Vredefort Mountain Land. Space Shuttle photograph STS27-33-56Z.
The origin of the Saltpan crater has been debated for a long time (for a historical account of reports on the Pretoria Saltpan see Levin 1991). Wagner (1920, 1922) discussed the findings of a dolomitic breccia, thought to be of volcanic origin, which led to wide acceptance of the hypothesis of a volcanic origin. Results of a gravity survey were interpreted also to support a volcanic origin (Fudali et al., 1973). However, it has been recently shown that the occurrence of volcanic rocks is not restricted to the crater area and is part of a regional volcanic event that took place at about 1.2 - 1.4 Ga (Brandt and Reimold 1993). Apparent ly , the well-preserved crater and the Late Proterozoic regional volcanism are unrelated.
A meteori te impact origin for the Saltpan, first suggested in 1933, was largely based on morphological observations (Rohleder 1933), and was supported by later structural and fission-track work (Milton and Naeser 1971). In 1988, a borehole was drilled to finally settle the question regarding the origin of the crater, and to study the uninterrupted paleoenvironmental record preserved in the crater lake sediments (Partridge et al., 1993). The drillcore revealed an internal crater stratigraphy (Reimold et al., 1992b) comprising about 90 m of crater sediments that are underlain by at least 53 m unconsolidated granitic breccia (Fig. 16b, c). This
breccia, classified as unconsolidated suevitic impact breccia, contains numerous shock-metamorphosed quartz, feldspar and biotite grains, besides glass and melt breccia fragments (Fig. 17a-c), confirming the impact origin of the Saltpan crater (Reimold et al., 1991, 1992b). The breccia layer is underlain by strongly fractured and locally brecciated (monomict cataclastic breccia) granite, which is occasionally intercalated with narrow layers of sandy breccia to a depth of 161 m. The amount of solid granite increases gradually with depth, until drilling was stopped at a depth of 200 m in undeformed granitic crater floor (Fig. 16c).
The chemical composition of the bulk breccia and impact glass fragments is very similar to that of average basement granite, but the impact glass fragments show considerable enrichments of Mg, Cr, Fe, Co, Ni, and Ir (Koeberl et al., 1994c). This can be explained by admix tu re of <10% of a chondr i t i c me teor i t e component. High Ir concentrations (-100 ppb) found in sulf ide spheru le samples c o m p l e m e n t the Ir abundances in the impact glasses (which are lower than those of the spherules, but still elevated compared to the contents in the target granite) and indicate some fractionation during impact. Re-Os isotopic studies of the target granite show very low Os abundances of about 7 ppt and high 'STOs/ISSOs ratios of about 0.72,
F igure 16. Salt 'pan crater, South Africa. (a) Obl ique aerial v iew from the west (courtesy D. Brandt); (b) Cross section of the crater as deduced from the analysis of drill core samples and geophysical measurements (gravity data after Fudal i et al., 1973; geology after Reimold et al., 199"2b).
which are typical for old continental crust (compare Fig. 3b). In contrast, the breccia samples have much higher Os abundances (-80 ppt) and lower '87Os/l~Ds ratios of about 0.205, which dearly indicates the presence of a meteoritic component (Koeberl et al., 1994c).
Kalkkop, South Africa The Kalkkop structure has a diameter of 640 m and
is located in the Eastern Cape Province of South Africa. First studies of the geological setting and subsurface geology (Blignaudt et al., 1948; Haughton et al., 1953) did not reach any conclusions regarding the origin of the structure. The possibility of an origin by meteorite impact was mentioned by Dietz (1965) and Reimold et al. (1992a), who, however, noted that not enough information was available to decide this question. Morphologically the structure is a circular feature with a light interior limestone fill that is conspicuous in aerial photographs (Fig. 18a). The basement of the structure
consists of sandstone, some mudstones, shales, and rare chert lenses collectively belonging to the Beaufort Group of the Karoo Sequence. The drill-core from the 1940s (Blignault et al., 1948; Haughton et al., 1953) is unfortunately no longer available.
In mid-1992, a new vertical bore-hole was sunk into the centre of the crater. The core has reached a depth of 151.8 m, where drilling was temporarily suspended (Reimold et al., 1993). The top 89.3 m consist of crater sediment, namely a finely laminated limestone. The limestone is underlain by breccia (Fig. 18b) which is very similar to glass-bearing, suevitic, breccia known from other impact craters. The breccia contains abundant day-mineral inclusions, presumably resulting from alteration of glass fragments, as well as some small glass particles. The dast population varies considerably with depth over a scale of meters and consists mostly of sandstone and mudstone or shale. Shale clasts are often highly fractured and break easily. A large number
African meteorite impact craters: characteristics and geological importance 285
Figure 16 Saltpan crater, South Africa (c) Borehole stratigraphy (after Reimold et al., 1992b).
of quartz crystals (as well as some feldspar grains) from the breccia show diagnost ic PDFs wi th up to 6 orientat ions per grain. As ment ioned before, the abundances and isotopic compositions of Re and Os demonst ra te that the suevitic breccia contains an extraterrestrial component (cf Fig. 3b), and confirm the impact origin of the Kalkkop crater (Reimold et al., 1993; Koeberl et al., 1994a).
Proposed, Doubtful, And Discounted Structures A n u m b e r of crater s t ructures on the African
continent have been proposed to be of impact origin, but, for various reasons, have not, or not yet, been accepted as confirmed impact craters, while others have since been discounted. For example, it was long thought that the Richat and Semisiyat structures in Mauritania, which are composed of a s tr iking succession of concentric rings of bills, may be of impact origin. A detailed field and petrographic stud)~ however, showed no evidence for meteorite impact, and it was concluded that they are eroded domes formed by uplift (Dietz et al., 1969). Lambert et al. (1980) investigated the E1 Mouilah (33o51 ' N, 02o03 ' E, 4.5 km diameter) and Aflou (34o00 . N, 02o03 . E, 3x5 km) structures in Algeria and
concluded that they were not of impact origin. The same conclusion was obtained by Lambert et al. (1981) for Foum Teguentour (26o15 ' N, 02025 ' E, 8 km diameter) and Mazoula (28024 ' N, 07o49 ' E, 0.8 km diameter).
A meteorite impact origin has been proposed for a number of other structures, but, mainly due to the lack of detailed (or, sometimes, any) studies, no conclusions have yet been reached. One of the best candidates is Temimichat Ghallaman in Mauritania (24 ° 15'N, 09 ° 39'W), about 150 km NE of Tenoumer. The crater has a diameter of about 750 m and is slightly hexagonal, similar to the Saltpan or Meteor craters. A gravity study indicates that the crater is rather shallow compared to normal impact structures (Fudali and Cassidy 1972). No definitive evidence for shock metamorphism has yet been found, but future studies are dearly necessary. Denaeyer and G6rards (1973) considered the possibility that some crater-like structures in Rwanda might be of impact origin, but did not find any supportive evidence. Some breccias, which were claimed to contain shocked quartz, were reported from the Lukanga Swamp, a depression in central Zambia with a diameter of about 52 km (Vrana 1985), but the structure has not been studied any further. Also in Zambia, the Bangweulu Basin was proposed as a 150 km diameter multiring impact basin (Master 1993). In addi t ion , it was suggested that the 800-km-fliameter Bangui magnetic anomaly in Central Africa may be the remnant of a huge and very old impact structure (Girdler et al., 1992). This, however, is highly speculative. Another hotly contested suggest ion is the interpretat ion of the Bushveld Complex as being of impact origin (Hamilton 1970; Rhodes 1975; Elston 1992); however, so far no evidence for shock metamorphism has been found (French 1990).
Approximately 3.2 Ga spherule beds in the Barberton Greenstone Belt, South Africa, were suggested to be of impact origin, largely based on their petrographic structure and their considerable PGE enrichments (Lowe and Byerly 1986; Lowe et al., 1989; Kyte et al., 1992). Such spherule beds are, however, unknown from any confirmed impact structure on earth, and are distinctly different from any spherule deposits at the Cretaceous-Tertiary boundary, or from (usually very rare) glassy spherules found within the breccias of a few impac t craters. In addi t ion , the absolu te concentrations of PGEs found to be associated with some of the Barberton spherule layers are far too high to be accounted for by a meteoritic source (up to, and even above, the equivalent of 100% of a chondritic meteorite component!). Recent detailed mineralogical and geochemical studies have shown that the PGEs are always associated with sulphide minerMiTations, and that all spherules are composed of secondary minerals, which exhibit a growth pattern that might be mistaken for chondrule-like quench textures (Koeberl et al., 1993b). In addition, Ni-rich Cr-spinels, which were thought to be supportive of an impact origin (Byerly and Lowe 1992), have compositions and iron oxidation states unlike any known meteoritic (or impact-related)
286 C. KOEBERL
Figure 17. Saltpan crater, South Africa. (a) Photomicrograph of quartz clasts (white), mottled sericitized feldspar, and impact glass particles of various shapes (spherules, ropes, droplets, fragments) from a suevitic bn~ccia (90.7 rn depth; 2.2 mm wide, Koeberl et al., 1994b); (b) PDFs in quartz grains from breccia at 102.0- 103.5 m depth; width: 355 mm, parallel nicols (courtesy W.U. Reimold).
spinels (Koeberl and Reimold 1994). These spherules are, therefore, most likely not associated with any impact event and the source of the enrichments of various elements must be sought for in hydrothermal alteration processes.
SIGNIFICANCE OF IMPACT CRATERS
Scientific Importance A very common question is, why should we study
impact craters? A number of different answers can be given to this question. A philosophical answer would
be that humanity engages in scientific research because it is curious and wants to increase its knowledge and derive satisfaction out of the increased knowledge. This is more or less the same reason why we engage in art or cultural activities. The history of science is full of examples that were of absolutely no practical use at the time (or even hundreds of years later), but without knowledge of which our human culture would be so much poorer. For those who think that pure knowledge is not enough, I have a few other answers.
Impact is probably the most impor t an t and fundamental process in the solar system (Shoemaker
African meteorite impact craters: characteristics and geological importance 287
Figure 17 Saltpan crater, South Africa (c) PDFs in quartz grain from granite breccia 23A, collected near the northern rim; widtl~ 0.7 mm, parallel nicols (courtesy W.U. Reimold).
1977). The earth and all other planets were formed by the accretion of small bodies, with a succession of impacts leading to the accumulation of larger planetary bodies. In the earlier history of the solar system, impacts were m u c h more c o m m o n than now and led to catastrophic resurfacing processes on most solid bodies, including the earth and the moon.As mentioned before, impacts continue to be the most important surface- forming and -modifying process for most terrestrial planets and satellites. By studying impact craters and processes , we can thus learn some th ing ve ry fundamental about the history and evolution of the bodies of our solar system.
Another aspect is the influence of impacts on the geological and biological evolution of our own planet. While geologists used to confine themselves to the preconceived limitations of internal processes to explain the evolu t ion of the earth, we are beg inning to appreciate that impacts played a larger role than we have real ized before. A concrete example is the discussion about the causes for major biological extinctions. After more than one decade of intense debating for and against impact, which also led to much fruitful research, it is now commonly accepted that an enormous impact event occurred at the Cretaceous- Tertiary (K-T) boundar~ 65 Ma (see papers in Silver and Schultz 1982; Sharpton and Ward 1990, which document the geological and biological aspects of this debate). Just very recently, the "culprit" has been identified in form of the Chicxulub multi-ring impact basin, which is almost 300 km diameter (Sharpton et al., 1992,1993). This 65 Ma crater is the largest currently known crater on earth and escaped detection for so long only because it is completely covered by about I km of
Tertiary sediments and thus has no surface expression. Numerous other buried craters of various sizes are
certainly still to be detected on earth. How many of them affected evolution as severely as Chicxulub dida should be subject of future research. In addition, it is important to realize that what happened in the past can happen at any time again. Impacts can have disastrous consequences for our civilization, as there is a I in 10 000 chance that a large asteroid or comet, with about 2 km diameter, may collide with the earth during the next century, disrupting the ecosphere and erasing a large percentage of humanity (Chapman and Morrison 1994). Even a relatively small impact can be devastating: a meteoritic projectile (iron or stony meteorite) of 250 m diameter (impact energy approximately equivalent to 1000 megatons) would produce a crater of about 5 km in diameter. Such events happen on earth about every 10 000 years, and would devastate about 104 km 2, locally disrupting civilization (Chapman and Morrison 1994).
Economic Significance It may be surprising to hear that impact craters can
have an economic importance. A number of workers have addressed this question (Sawatzky 1975; Donofrio 1981; Reimold and Dressler 1990; Masaitis 1992). In addition, they can be used for other types of scientific study. For example, a somewhat important aspect is the use of crater lake deposits to study the paleoc-limatic record (e.g. at Lake Bosumtwi, Talbot and Delibrias 1980; or at the Saltpan crater, Partridge et al., 1993).
The unique morphology of impact craters, as well as structural and lithological changes in the target rocks due to their origin, may lead to economically important accumulations of mineral resources. For example, a
288 C. KOEBERL
(b)
Figure 18. Kalkkop crater, South Africa. (a) Vertical aerial photograph, showing the bright circular limestone filling of the structure (courtesy W. U. Reimold); (b) Drillcore sample of suevitic breccia from 133.9 m depth, diameter 5 cm (courtesy W. U. Reimold).
number of craters are associated with hydrocarbon reservoirs. The Red Wing Creek crater in North Dakota, USA, is about 200 Ma, has a diameter of 9 km, and contains significant oil reserves trapped in the crater structure (Brenan et al., 1975; Sawatzky 1977; Donofrio 1981). A number of other probable or confirmed impact craters (e.g. Viewfield or Eagle Butte, both in Canada) contain either oil or gas. Another important oil- producing crater, which was recently identified to be of impact origin, is the 15 km diameter Ames crater in Oklahoma, USA (Hamm and Olsen 1992; Roberts and Sandridge 1992; Carpenter and Carlson 1992). This structure hold an estimated total of about 20 million barrels of crude oil. The newly recognized Avak impact
structure in Alaska is associated with important gas reserves (Kirschner et al., 1992). In Africa, the existence of a gas pool due to decomposing organic matter in the lake sediments has been proposed underneath the Bosumtwi crater in Ghana (Jones 1983).
Some crater structures are associated with significant mineralizations. For example, the Sudbury impact structure (Ontario, Canada) hosts the largest Ni-Cu mineralization in the world, as well as important PGE mineralizations, which are a direct result of the impact event (Grieve et al., 1991). It has also been speculated that the nature of some of the Witwatersrand gold mineralization is a consequence of the Vredefort impact event (Reimold 1994). The Ternovka crater (Ukraine)
African meteorite impact craters: characteristics and geological importance 289
hosts economic deposits of ferruginous quartzites, the Boltysh (Ukraine) impact crater hosts several million tons of oil shales, the Carswell Lake crater (Canada) hosts important u ran ium deposits , while impact- produced diamonds are extracted from rocks at the Popigai structure in Siberia (Masaitis 1992). Impact breccias have also been used as building materials. For example, numerous buildings in N6rdlingen, German~ and the Rochechouart Castle in France are constructed with suevite (a glass-bearing impact breccia) from the Ries and Rochechouar t craters, respectively. The exp lo ra t ion of impac t craters m a y wel l y ie ld economically interesting results.
CONCLUSIONS AND OUTLOOK
In this review of African impact craters, I tried to summarize our knowledge about the individual crater structures on this continent, as well as to put them into a scientific and economic context. It is dear from the discussion above that our knowledge of meteorite craters in Africa, in general, and that of most specific craters, in particular, is still sparse. Some craters, for example, the Salt-pan crater have been studied in great detail, while research on others has been, to say the least, meagre. Most craters have been studied in the field and petrological investigations on crater rocks led to their identification as impact craters. However, detailed structural, geological, petrological, geochemical, or geophysical studies are missing for a surprisingly large number of structures. Drill cores are available for only three (South) African impact craters. Clearly, further investigations of all these African craters should provide a wide field of s tudy for interested geologists.
However, not only the study of known impact craters is of importance. Table 1 and Fig. 1 demonstrate that there must be huge gaps in our current knowledge of meteorite craters present in Africa. First of all, there is no good physical reason why impact craters in Africa should occur mainly in southern Africa and in certain Nor th African deser t areas. Surely there are still unrecognized impact craters in most other African countries, particularly in the centre of the continent. The history of crater discoveries shows that it took quite some time to find some structures, or, if they were known, to correctly deduce their origin. Furthermore, the size distribution of the known African meteorite craters is highly suspicious. Almost all African craters, with only two exceptions, are smaller than about 15 km diameter (see Table 1).
If we take the size distribution of all craters known wor ldwide (Grieve 1991) and compare it with the number of the presently known African craters, we immediate see that (with one exception) n o impact craters more than about 15 km are known in Africa - which is physically implausible, and only reflects our poor knowledge of African craters. Such a calculation
indicates that, even taking only the presently known (low) number of small craters in Africa into account, there should be at least five craters in the 10-50 km diameter range in Africa. It may well be craters of this size that are of particular economic interest. In addition, considering recent estimates for current cratering rates on earth (Grieve 1984,1987; Trefil and Raup 1990), it is ve ry l ikely that we have, at p resen t t ime, only recognized a small percentage of all craters (no matter which size) on the earth. Such an estimate suggests that we may only have discovered about 10% of all impact craters present in Africa. Furthermore, considering the percentage of the area of Africa compared to the rest of the earth's continents, Africa is underrepresented regarding the number density of impact craters by a factor of 2-3 (considering the craters known per unit area in other continents).
In conclusion, I hope that by the time this paper is published, at least one or two new craters will have been added to the list of African meteorite craters. (This is exactly what has already happened in the interval between the time when the initial manuscript was written, and the time when the revision was completed.) Remote sensing will certainly play a major role for identifying impact craters in the future. Researchers should also be cautioned, though, not to overinterpret available data. I have pointed out that there are a number of criteria that have to be fulfilled before an impact origin can be accepted for a given crater. Circularity of a structure, or the presence of fractured rocks, is not enough. It is, therefore, important to consider the impor tance of impact craters in the education of geologists. The study of impact craters is not an esoteric pursuit for the few, but an integral part of our understanding of the geological evolution of our planet.
Acknowledgements I am grateful to the University of the Witwatersrand,
Johannesburg, for a visiting research fellowship during which this paper was written, and to C.R. Anhaeusser and T.S. McCarthy for the invitation to work at EGRU and the Department of Geology, Universi ty of the Witwatersrand. I furthermore thank the organizers of the 16th Colloquium on African Geology in Swaziland, especially R. Maphalala and M. Mabuza, for the invitation to present this paper as a keynote address. This study was supported by the Austrian Fonds zur F6rderung der wissenschaftlichen Forschung, Project P9026-GEO. I especially appreciate the efforts of my friend and colleague W.U. Reimold, who has reviewed this pape r and made n u m e r o u s sugges t ions for improvement, and with whom I have had the pleasure to collaborate on many aspects of the study of African impact craters. Furthermore, I am grateful to S. Master and D.J. Robertson for comments, and especially to B. French and R. Grieve for very helpful and detailed reviews and comments on this paper.
290 C. KOEBERL
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