The formation of the Vredefort Dome, a structure in excess of 100 km in diameter and located in the approximate center of the Witwatersrand basin, is still the subject of lively geological controversy. It is widely accepted that its formation seems to have taken place in a single sudden event, herein referred to as the Vredefort event, accompanied by the release of gigantic amounts of energy. It is debated, however, whether this central event was an internal one, i.e., a cryptoexplosion triggered by volcanic or tectonic processes, or the impact of an extraterrestrial body. The results of this debate are presented

Boer, R. H.

Jackson, M. C.

Jessberger, E. K.

Kunz, J.

Reimold, W. U.

Trieloff, M.

English

NASA Technical Reports Server

LPI Contribution No. 790 75
diffusion parameters of a typical anorthite. with an age of 500 Ma
constituting 0.8% of the samples' potassium. The degassing peak of
the relict phase coincides by and large with the second reservoir of
S Al -302 but the degassing feature is broader (Fig. 3). The resulting
calculated age pattern shows a low-temperature plateau of 65 Ma
and an irregular shape in the high-temperature steps. That means
that the measured age pattern can only be interpreted as a result of
65 -Ma age and a relict phase if the relict phase has exactly the same
gas release pattern—but if this is the case, the relict phase should
have been reset by the cratering event as well as the sample unless
it would have been incorporated afterward, e.g., as contamination.
A further condition that must be fulfilled to fit this scenario is that
the K fraction of 500-Ma old relict phases in the different samples
must be nearly the same (1 ± 0.2%), or another proper combination
of age and K content. We regard this as improbable.
Although the age spectra show well-defined plateaus, the plateau
fractions still exhibit fine-scale structures that deviate from a
theoretical calculated plateau (Fig. 2, bottom) as expected for a
maximum precise measurement of an undisturbed sample. Nor-
mally the mass-dependent diffusion difference of **Ar and "Ar is
considered as negligible, but if we induce it in our calculations in the
case of SA1-302 we get an age spectra as shown in Fig. 2 with
slightly increasing ages within each reservoir. This strengthens the
assumption that these deviations are not of statistical, but of
systematic nature, an artifact induced by the wAr-"Ar stepheating
technique. In this sense, the spectrum of SA1 -302 seems to represent
the "ideal" spectrum of an undisturbed sample. For the fine-scale
deviations of the other three impact melts " AT recoil redistribution
seems to play a major role, but this has to be investigated by further
petrographic studies on grain size and potassium distribution.
We conclude an age of 69—71 Ma for the Kara impact structure.
Hydrogen iso topic measurements by Nazarov et al. [6] show that the
impact occurred on dry land and the authors concluded a maximum
age of 69-70 Ma, the time of the end of the last regression within the
crater's region before the end of the Cretaceous. Our data are
consistent with an impact a short time after the regression.
Figure 4 shows the K/Ca and the age spectra of two impact
metamorphic anorthite samples (10BD5 and 10BD3C) of the
Manicouagan Crater, Canada. As visible in the K/Ca spectra (only
1 OB D5 is shown), the samples consists of two different phases, one
degassing at low temperatures having an age plateau indistinguish-
able from the cratering event of 212 Ma (7), the second one showing
the signature of a partially degassed phase, having ages increasing
up to 950 Ma (10BD5). the age of the target rocks [8]. 10BD3C
suffered a more complete degassing, having ages ranging only up to
300 Ma. The low-temperature plateaus are in agreement with the
crater age of 212 Ma and do not improve the age of the impact
structure. Anyway, while the crater age is quite accurate, the ages of
the adjacent geologic boundaries seem not to be. The last revision
[9] of the Triassic-Jurassic boundary in 1982 delivered an age of
213 Ma. while a later determination [10] gives a lower age of
208 Ma. We think so far as ages are concerned it is not possible to
conclude or exclude an association of the two events until the age of
the boundary is determined more precisely.
Our measurements enable us to estimate the intensity of the
therm al event induced by the cratering event for the two Manicouagan
samples. Our results are consistent with a time-temperature combi-
nation of 1 Ma at 337 °C or 12 hr at 1100°C for 1 OB D5 and 1 Ma at
361°C or 50 hr at 1100°C for 10BD3C. Future investigations may
allow us to infer a cooling model for the Manicouagan impact melt
sheet.
References: [1] Alvarez L. W. et al. (1980) Science, 208.
1095. [2] Hartung J. B. and Anderson R. R. (1988) LPl Tech. Rept
88-08. [3] Alvarez L. W. and Asaro F. (1990) Sci. Am.. 362.
[4] Kolesnikov E. M. et al. (1988) In LPl Contrib. No. 673.
[5] Koeberl C. et al. (1990) Otology. 18. 50. [6] Nazarov M. A.
et al. (1991) LPSCXXI1. 961. [7] Grieve R. A. F. (1991) Meteorit-
ics. 26. 175. [8] Wolfe S. H. (1971) JGR, 76, 5424. [9] Harland
W. B. et al. (1982) A Geologic Time Scale. Cambridge Univ.
[10] Palmer A. R. (1983) Geology. 11. 103.
N 9/8 9^0 1 8
-Ar-»Ar DATING OF PSEUDOTACHYLITES FROM THE
WITWATERSRAND BASIN, SOUTH AFRICA, WITH
IMPLICATIONS FOR THE FORMATION OF THE
VREDEFORT DOME. M.Trieloff>.J.Kunz'.E.K.Jessbergeri.
W. U. ReimokP. R. H. Boer*, and M. C. Jackson*. 'Max-Planck
Instirut fur Kcrnphysik, P.O. Box 103980, W-6900 Heidelberg.
Germany, 'Economic Geology Research Unit. University of the ^  .. ^
Witwatersrand, P.O. Wits 2050. Johannesburg. RS A. ft A/ 0$ 7 /#*
The formation of the Vredefort dome, a structure in excess
100 km in diameter and located in the approximate center of the
Witwatersrand basin, is still the subject of lively geological contro-
versy. It is widely accepted that its formation seems to have taken
place in a single sudden event, herein referred to as the Vredefort
event, accompanied by the release of gigantic amounts of energy . It
is debated, however, whether this central event was an internal one,
i.e., a cryptocxplosion triggered by volcanic or tectonic processes,
or the impact of an extraterrestrial body.
Ages obtained on rocks from the Vredefort structure cluster
largely around 2.0 Ga (e.g., review by [1 ]). Granophyre, an unusual
melt rock forming dykes in the Vredefort Dome and thought to be
related to the Vredefort event, yielded a Pb-Pb zircon age of 2002 ±
52 Ma [2]. Pseudotachylite, a melt breccia first discovered and
•«:
O
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o
— 1
<
0
UJ
o
z
UJ
OH
Q-
Cu
LS
0.5
-O.S
-1.S
U
O A2.0
1.8
1.6
1.4
U-
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rr^~ ~" — i
^1 ' (P Ga
J
!
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o so no
Fractional 39Ar- Release CXD
Fig. 1. Age spectrum for EL-28B.
https://ntrs.nasa.gov/search.jsp?R=19930001001 2020-03-17T10:04:03+00:00Z
76 1992 Sudbury Conference
extremely abundant in the Vrcdefort structure, is, like the grano
phyre, widely regarded to be the direct result of the dome-forming
process.
We dated eight pseudotachy lite samples from a bedding-parallel
fault zone across the north-central Witwatersrand basin with the
"Ar^Ar step heating technique. All age results (Table 1) are
indistinguishable from 2.00 Ga within 1 o errors, except for those
obtained for two samples: 10D05 (2.10 Ga) seems to be severely
disturbed in the low-temperature fractions; the other one, EL- 30 ha*
an extremely low K content of 20 ppm—compared to K contents of
several percent in the other pseudotachylite samples. The relative
fraction of excess argon is significant, which results in an unrealistic
K-Ar age of 4.7 Ga.
Figure 1 shows the exemplary spectrum of pseudotachylite
sample EL28B with a plateau over 92% of the MAr release. In light
of the appearance of the age spectra and the large number of samples
analyzed to identical results, we do not believe that the 2-Ga ages are
reset ages caused by thermal overprint. Our ages are consistent with
the «> Ar-" Ar age of a Vredefort pseudotachylite (USA29,2.00 Ga
[3]). This indicates that the Vredefort event and the Witwatersrand
pseudotachylite are related. Taking all our results into account, die
resulting age for the Vredefort event is calculated to 1997 ± S Ma;
a larger error of 19 Ma would be realistic if the additional error
induced by our NL-25 hornblende standard is considered
A circumstance in favor of an origin by internal processes is die
apparent coincidence of the Vredefort dome formation and other
regional magmatotectonic processes. These are the emplacements
of several alkaline granites and ayenites, namely the Schurwedraal
complex (Rb-Sr crrorchron of 2016 ± 61 Ma [4]), the Lindequesdrift
intrusion (2163131 Ma «Ar-»Ar plateau age of hornblende [S]),
the Rietfontein complex (2004 ±16 Ma "Ar-MAr plateau ages of
biotite and hornblende [6]), and—in an even wider regional con-
text—theBushveldcomplex, especially because Bushveldsynchro-
nous intrusives, such as the Losberg complex, occur close to the
Vredefort structure. The age of the Losberg complex (Rb-Sr iso-
chron of 2041 ± 41 Ma [7]), is in agreement with ages thought to
represent the main Bushveld activity (2050-2060 Ma [1]), but, in
general, ages have been determined ranging from 1900 to 2100 Ma,
e.g., an "Ar-^Ar age of Bushveld magnetite gabbro of 20% ±
12 Ma [8] and an * Ar-5'Ar laser probe age of Merensky reef biotite
of 2010 Ma [9].
In agreement with an extended rather than short period of
Bushveld activity are our own "Ar-"Ar step heating results ob-
tained for a Bushveld gabbro (whole rock, anorthite, and pyroxene),
where two partial plateau can be identified in the age spectra: one
corresponding to a mean age of 2.00 Ga and the other to a mean age
of 2.10 Ga. This could indicate a complex cooling or reheating
history of the sample. Investigations are currently in progress to
determine whether excess Ar could be the cause of this age di-
chotomy or whether the spectra are the result of a true two-stage
geological history. No matter what the result will be, an event close
to 2.00 Ga will remain indicated.
If high-resolution geochronology could prove that the ages of the
rocks related to the dynamic Vredefort event (granophyre,
pseudotachylite) and the ages of the rocks formed in magmatotectonic
processes are different, this would be an argument favoring Vredefort
as an impact site, but considering the ages of the geologic activity
around the Vredefort structure, we do not think that this is possible
because of the extended duration of the tectonic processes.
Previous *° Ar- * Ar investigations of pseudotachy lit« from other
localities in and around the dome resulted in a number of ages
significantly tower than 2 Ga for four out of six dated specimens [3].
Such recent formation ages would even question the single-stage
formation of the Vredefort dome and its associated pseudotachy li tcs
[3]. Ages lower than 2 Ga are not restricted to pseudotachylite
either: a plagioclase separate of the lamprophyre from the
Lindequesdrift intrusion dated at 2163 Ma yielded a 1248 ± 22 Ma
age [S]. Several mineral separates from Rietfontein complex rocks
gave partial plateaus at about 2004 Ma, but also showed low ages in
the low-temperature release steps comprising 20 to 50% of the M Ar
release, suggestive of disturbances at 1170 ± 14 and 725 Ma [6]. A
pseudotachylite from the Riefontein complex (USA35) showed an
irregular age pattern suggesting disturbances at 1400 Ma and
800 Ma [3]. In the outer (OGG) core of the Vredefort structure,
pseudotachylite USA30A yielded an age of 1.39 Ga [3], whereas a
Rb-Sr mineral isochron for the host granite resulted in an age of
2002 ± 8 Ma and Ar-dated mineral separates yielded ages ranging
from 1.39 to 1.76 Ga, all significantly lower than 2 Ga[ 10]. Biotite
separates from a granophyre sample (WG) from a site less than
3 km from the USA30 locality yielded a possible formation age of
2006 ± 9 Ma, but with evidence in the age spectrum for significant
overprinting at 700 and/or 1200 Ma [6]. Biotite from OGG sample
KK55 from the northwestern sector of the core yielded a 1935 ±
8 Ma plateau age, still lower than 2 Ga [6]. Finally, pseudotachylite
USA31 provided an age pattern suggestive of partial degassing at
1.4 Ga [3], whereas its host rock. NW28, gave ages of 2070 Ma
(biotite) and 3030 Ma (hornblende). It should also be mentioned that
four of our Witwatersrand pseudotachylite samples show low-
temperature fractions consistent with disturbances at 1400 Ma.
Post-2-Ga events could thus have taken place at 1400,1200.1000.
and 700 Ma ago. There are two possible scenarios that could explain
this record of low ages for Vredefort samples. Firstly, the low
pseudotachylite ages are seen as formation ages that date the result
of post-2-Ga tectonic events. This would explain the different
resetting degrees observed for samples from different localities, as
well as observations of multiple geological hints for the existence
of several pseudotachylite generations in the Vredefort structure
Rietfontein
(| Lindequesdrift
Fig. 2. Sample locality overview at the Vredefort structure (after
[10]).
LPI Contribution No. 790 77
[10]. Then it should be clear why there are not pseudotachylites
older than 2 Ga. Secondly, the low ages could be due to post-2-Ga
thermal overprint. Host rocks were then more or less intensely
affected in accordance with the different Arretentivities of different
minerals, or different closure temperatures in the case of Rb-Sr
isotope systematics. A major problem is seen in the different
intensities of the resetting events at different localities within the
dome. This may be explained partly by additional hydrothcrmal
activity.
Future investigations are still needed to completely clarify the
nature and the duration of post-2-Ga processes that took place in the
Vredefort structure.
Acknowledgments: We gratefully acknowledge the generous
support from Anglo American Prospecting Services that permitted
the analysis of Witwatersrand pseudotachylites, and the permission
to report the results.
References: [1] Walraven F. et al. (1990) Tectonophysics,
1 71 . 23-48. [2] Compston W. and Nicolay sen L. O. in [ 1 ] . [3] Reimold
W. U. et al. (1990) Tectonophysics. 171. 139-152. [4] Nicolaysen
L. O. et al. (1963) Gen. Assem. Int. Union. Geol. Geophys. 13th,
Berkeley, Calif., abstract. [5] Margraves R. B. (1987) S. Afr. J.
Geol.. 90. 308-313. [6] Allsopp H. L. et al. (1991) Suid afrikaanse
Tydskrifvir wetenskap. 87, 431^442. [7] Coetzow H. and Kruger
F.J. (1989) S. Afr. J. Geol., 92, 37-41. [8] Burger A. J. and
Walraven F. (1977) Anna. Geol. Survey S. Africa. 11, 323-329.
[9] Onstott and Kruger in [1]. [10] Reimold W. U. et al. (1990)
Geol. Soc. Austr., 27, 82.
N ife 0
AL UMCHAIMIN DEPRESSION, WESTERN IRAQ: AN
IMPACT STRUCTURE? James R. Underwood Jr., Department
of Geology, Kansas State University, Manhattan KS 66506-3201,
Al Umchaimin, in Arabic "hiding place" or "place of ambush"
[1], is located at latitude 32° 35.5*N and longitude 39° 25'E. It lies
some43 km S36° of the H-3 pump station on the abandoned Kirkuk-
Haifa oil pipeline [2] and 60 km S49°W of the western desert town
of Rutba. The nearly circular depression averages 2.75 km in
diameter and is 33—42 m deep. It is floored with fine-grained, clay-
rich deposits, estimated to be 36 m thick [3], the surface of which
shows well-developed desiccation fissures or mudcracks when dry.
Because of its nearly circular planimetric shape and its apparent
isolation from other surface and subsurface features, it has been
considered by some to be a possible meteorite impact structure [4]
and by others [5] to be a surface collapse feature that originated
following removal of magma from the subsurface as the magma
extruded elsewhere. Al Umchaimin was listed in the U.S. Geologi-
cal Survey tabulation of 1 10 structures worldwide for which a
meteorite impact origin had been suggested [6]. It was placed in
Category VI Structures for which more data are required for
classification.
K. M. Al Naqib, Iraq Petroleum Company, reported [7] that the
petroleum geology community considered that Al Umchaimin had
originated by fracture-controlled dissolution in the subsurface and
eventual collapse into the resulting solution cavity. Al-Din and
others [3] made geological and geophysical surveys of the depres-
sion in 1969 and 1970 and found no evidence for an impact origin.
They concluded, as did Al Naqib, that a solution-collapse origin was
likely.
Abbas and others [8] conducted additional geophysical studies
and arrived at similar conclusions: (1) Al Umchaimin was not
formed by meteorite impact and (2) probably it represents a solu-
tion-collapse feature. Greeley and others [9], in preparation for the
Magellan mission to Venus, studied shuttle radar images of nine
maar volcanos, one volcanic caldera dome, one impact structure,
and one possible impact structure (Al Umchaimin). Concerning Al
Umchaimin, they wrote: "Although no definitive impact features
have been reported, the circularity and slightly uplifted rims suggest
an impact origin."
In 1 965 , the author and the late Randolph Chapman, both visiting
professors at the College of Science, University of Baghdad, spent
half a day at Al Umchaimin during which a section was measured
up the east wall, samples were collected for later thin sectioning, and
a search made for meteoritic debris, shatter cones, impact glass,
melt breccia, and so on. No evidence was found for an impact origin
of the depression, nor did study of the thin sections from the east wall
of the depression reveal any microscopic evidence of impact.
It is concluded that, on the basis of the studies that have been
made of Al Umchaimin and on the basis of the brief site visit made,
Al Umchaimin probably is not an impact structure but most likely
resulted from the enlargement and coalescence of sink holes and
eventual collapse of the roof material into the resulting cavity.
References: [1] Suleiman M. W., personal communication.
[2] Army Map Service (1952) Wadt Hauran, Iraq, Sheet NI 37-1 5 .
scale 1:250,000. [3] Al-Din T. S. et al. (1970)7. Geol. Soc. Iraq. 3.
73-88. [4] Merriam R. and Holwerda J. G. (1957) Geograph. J..
CXXIII. 231-233. [5] Mitchell R. C. (1958) Geograph. J.. CXXIV,
578-580. [6] Freeberg J. H. (1966) U. S. Geol. Surv. Bull. 1220,21.
[7] Al Naqib K. M. (1967) U. S. Geol.SurveyProf. Pap. 560-G, G7.
[8] Abbas M.J.etal. (1971) J.Geol.Soc. Iraq. 4,25^0. [9] Greeley
R. et al. (1987) Earth Moon Planets, 37, 89-111.
fa
A LATE DEVONIAN IMPACT EVENT AND ITS ASSO-
CIATION WITH A POSSIBLE EXTINCTION EVENT ON
EASTERN GONDWANA. K. Wang' and H. H. J. Geldsetzer',
'Department of Geology, University of Alberta, Edmonton, Alberta
T6G 2E3, Canada, Kjeological Survey of Canada, 3303-33rd Street,
N.W., Calgary, AlbertaT2L 2A7, Canada. ^
W n-]t?7 QrW 3
Evidence from South China and Western Australia for a 365-Ma
impact event in the Lower crepida conodont zone of the Famennian
stage of the Late Devonian (about 1. 5 Ma after the Frasnian/
Famennian extinction event) includes microtektitelike glassy
microspherulcs [I], geochcmical anomalies (including a weak Ir),
a probable impact crater (>70 km) at Taihu in South China [2], and
an Ir anomaly in Western Australia [3]. A brachiopod faunal
turnover in South China, and the "strangelove ocean"-like 6"C
excursions in both Chinese and Australian sections indicate that at
least a regional-scale extinction might have occurred at the time of
the impact. A paleoreconstruction shows that South China was very
close to and facing Western Australia in the Late Devonian [4].
South China: An Upper Devonian carbonate section exposed
at Qidong, Hunan, was studied for biostratigraphy, geochemistry,
and sedim ontology. A brachiopod faunal changeover from the
traditional Yunnanellinato Yunnanella faunas [5] was recognized in
the section. Abundant microspherules were found in a single
stratigraphic horizon immediately below a 3-cm clay with a geo-
chemical anomaly. The microspherule horizon occurs in the Lower


Ar-40 to Ar-39 dating of pseudotachylites from the Witwatersrand basin, South Africa, with implications for the formation of the Vredefort Dome

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