PLEASE NOTE:
*
CCNet-ESSAY, 14 April 2000
--------------------------
DISCOVERY OF THE WORLD'S FOURTH LARGEST IMPACT, AND THE TALE
OF TWO CRATERS
By Andrew Glikson <andrew.glikson@anu.edu.au>
Research School of Earth Science,
Australian National University
Canberra, ACT 0200
As Eugene and Carolyn Shoemaker knew so well during their 13
years-long
(1984-1997) exploration in the Australian outback, for every
eventually-proven extraterrestrial impact crater there are
numerous
false alarms. The question is often raised whether any particular
circular feature, be it a lake, a geological dome, or a
geophysical
anomaly, may be of impact origin, even though positive
identification
depends on the occurrence of diagnostic criteria for shock
metamorphism, i.e. planar deformation features (in quartz,
feldspar or
zircon), shatter cones, impact melt, contribution by meteoritic
components including siderophile elements (Ni, Co, Cr, V) and
platinum
group elements (PGE) anomalies, and other criteria. Compounding
the
enthusiasm of crater hunters is the geo-centric philosophy which
lingers among many geologists, denying the reality and the
essential
role of asteroid and comet impacts in Earth history.
This is why, when a new bona-fide impact crater is found, adding
to the
170 or so known terrestrial craters, it makes news, all the more
when
the impact structure is 120 km in diameter - the 4th largest
known on
Earth. The present article summarises aspects of the discovery,
as well
as current progress in the study, of Woodleigh impact structure
(Mory
et al., 2000, Earth Planet Science Letters, 177, 119-128).
Remarkable parallels can be drawn between Woodleigh and Chicxulub
- the
K-T boundary "dinosaur killer" of Yucatan. Both are
buried under flat
lying sedimentary cover, were detected through their Bouguer
anomaly
signatures as a series of concentric rings which sharply
intersect the
regional structure. Both have surface expressions of the outer
ring
fault, i.e. cenote water holes at Chicxulub and circular surface
drainage at Woodleigh. However, unlike Chicxulub, where the
central
uplift collapsed, a central basement uplift is well preserved at
Woodleigh. In so many ways the discovery of Woodleigh mimics that
of
Chicxulub, now well documented (see for example
"Impact" by G.L.
Verschuur, Oxford University Press, 1996, p. 17-31). For over 20
years,
first Robert Baltosser, and the Glen Penfield, both oil
exploration
contractors, battled company restrictions, an indifferent
geological
community, the Yucatan jungle, burnt core sheds and drill holes
filled
with pig manure, to prove the impact origin of the bulls-eye
multi-ring
crater which Chicxulub is.
As for Chicxulub, it has taken over twenty years to establish the
impact origin of Woodleigh. In 1981 oil exploration drilling by
Layton
and Associates (acting on behalf of the tenement holder, Eagle
Corporation) near Woodleigh Station, southern Carnarvon Basin,
east of
Shark Bay, Western Australia, uncovered quartzo-feldspathic rocks
with
deformation lamella in quartz at a depth of about 200 metre. At
the
time the features were attributed to mechanical drilling effects,
a
natural oversight for anyone not familiar with shock-produced
planar
deformation features (PDF). Gravity surveys by Wapet and Conoco
and 11
km-line interval regional survey by the Australian Geological
Survey
Organisation (AGSO), further processed by GSWA, delineated a
near-perfect multi-ring Bouguer anomaly structure centred on
Woodleigh
Station, accentuated by 1st vertical derivative analysis. From
1997,
studies of the Carnarvon Basin by the Geological Survey of
Western
Australia re-focused attention on this structure. Credit goes to
Robert
Iasky (geophysics) and Arthur Mory (basin stratigraphy and
sedimentology) for pointing out the potential impact origin of
the
structure. The structure was brought to my attention by John
Gorter,
who knew of Robert and Arthur's discovery. John is
Australia's
foremost hunter of buried craters, with several discoveries to
his
credit (Tookoonooka - Cooper Basin; Fohn impact structure and
Carolyn
Shoemaker Strewn Crater Field - Timor Sea; Ashmore - North west
Shelf).
Looking at the Bouguer anomaly image and a microphotograph of
lamellar
quartz at the GSWA office in May 1998, I was impressed by the
intersection of the regional North-South structural trends by the
outer
ring fault of the multi-ring feature. As distinct from tectonic
domes,
impact craters such as Chicxulub and Shoemaker (formerly Teague
Ring,
Western Australia) are sharply discordant in relation to the
surrounding geology, as if, so to speak, they fell from the
sky...! At
the time I was still getting over an unsuccessful attempt of
testing a
potential Archaean impact structure in the west Pilbara. I told
myself
- if Woodleigh does not turn out to be an impact structure,
nothing
else will ... I thought Arthur and Robert should secure their
credit
for this discovery through a GSWA report and an early
announcement in
the international literature.
The report was published in 1999 (GSWA Report 69). Between us we
were
working on a short manuscript for an international journal.
Unfortunately, the drill cutting shown in the microphotograph
could not
be located and, in their absence, since our case was primarily
based on
geophysical data and circumstantial evidence, acceptance of the
paper
by the reviewers was doubtful. As sure as the Moon follows the
sun, the
manuscript was returned with questions regarding shock
metamorphism and
the original diameter of the structure. By the time a revised
paper was
finally accepted, in January 2000, no fewer than 21 updates were
exchanged between us.
To its credit, the GSWA management supported re-drilling of
Woodleigh,
with the aim of testing the impact hypothesis. In March, 1999,
Arthur
E-mailed me from the drill site, impressed by black-veined
granitoid
cores turning out at shallow depth of 170 metres, where no
basement
rocks were supposed to exist. It turned out to be the central
granitoid
rebound uplift, complete with PDF, solid-state fusion of
feldspars
(diaplectic glasses), and penetrative pseudotachylite vein
systems
(comminuted microbreccia/glass veins). A second drill hole 13 km
west
from the centre penetrated a ring syncline containing a 380
metre-thick
section of lower Jurassic sediments overlying a basal diamictite
(matrix-supported conglomerate) zone containing clasts of shocked
granitoid and of thermally deformed shale and marl fragments, in
turn
overlying Silurian sub-crater formation. The drilling results
provided
a beautiful confirmation of Robert Iasky's gravity modelling of a
central granitoid uplift.
We now had material to work on. Microscopic work by Franco
Pirajno
(GSWA), Scanning Electron Microscopy coupled with Energy
Dispersive
Spectrometry by myself, and laser Raman spectroscopy by Terry
Mernagh
(AGSO), yielded new surprises. Pseudotachylites formed by
friction-melting along fractures are often depleted in
low-melting-point (volatile) elements (K, Na, Si) and enriched in
high-melting-point (refractory) elements (Al, Ca, Mg). This was
the
case with the Woodleigh pseudotachylites, however Mg and Fe were
locally enriched by factors of 2 to 4, even within diaplectic
feldspars
which originally contain little or no Mg or Fe. This hinted at
large-scale introduction of ferromagnesian components, confirmed
by
anomalously high Ni, Co, Cr and V. If these were introduced into
sub-cater levels from the exploding projectile, possibly by
condensation of a vapour component, this would be the first time
such
phenomenon is observed, with the exception of Ni-Cr-Fe metallic
veins
under the Ries crater, south Germany.
Now came the second big hurdle. Once an impact origin was proven,
the
main question everyone was asking was the age of Woodleigh. Due
to the
instantaneous melting/quenching sequence inherent in impacts,
isotopic
systems may not be reset, which precludes age determination. True
to
this rule - the Rb-Sr and K-Ar ages of biotites and U-Pb ages of
zircons from the shocked granitoids yielded percussor Precambrian
ages
unrelated to the impact, as shown by my RSES colleagues Richard
Armstrong and Julie Smith, and by Simon Kelley (Open University).
During that time two working hypotheses were considered: (1) a
Permian-Triassic boundary age, which I favoured in view of
regional
apatite fission track cooling ages of 280-250 Ma and a possible
correlation between the size of Woodleigh and the P-T boundary
extinction, or (2) late Triassic age, which Arthur favoured, in
view of
the absence of Triassic crater fill and the Early Jurassic age of
the
overlying lacustrine sediments of the Woodleigh Formation.
In view of the unsuccessful attempts at dating the central
granitoid
core, my attention was focused on thermally modified diamictite
sediments at the base of a ring syncline drilled at Woodleigh 2A,
which
showed signs of hydrothermal activity such as can result from
impact-heating of ground water and recrystallisation of clay
minerals.
Subsequently, both illite and smectite were identified by X-ray
diffraction by T. Uyssal, University of Queensland, and K-Ar ages
of
two samples indicated a late Devonian (Frasnian-Fammenian
boundary) age
of c. 365 Ma - a period during which Earth was hit by an impact
cluster, including Charlevoix (Quebec) (D=54 km; 367+/-15);
Siljan
(Sweden) (D=52 km ;368+/-1.1 Ma); Ternovka (Ukraine) (D=15 km;
350 Ma);
Kaluga (Russia) (D=15 km; 380+/-10); Elbow (Saskatchewan) (D=8
km;
395+/-25 Ma). Iridium anomalies related to the Frasnian-Famennian
events are known from the north Canning Basin, Western Australia,
microtektites are found along this boundary in south China, and
marked
13C/12C excursions occur in Alberta. The extinction saw the
elimination
of rugose coral reefs, trilobites, ammonoids, brachiopods and
conodont
species. However, as pointed out by Arthur, the occurrence of
Permian
palynomorphs in clasts within the diamictite raises a question
regarding the impact age. Possibly, repeated isostatic uplift of
the
central granitoid core in post-impact times and its recurrent
denudation resulted in sedimentary mixing. At present we are
trying to
resolve this problem by K-Ar analysis of hydrothermal alteration
products from the central granitoid core.
The other essential question is the diameter of the Woodleigh
impact
structure. The literature on impact structures and the Geological
Survey of Canada crater listing (Crater@gsc.nrcan.gc.ca)
indicate that
the criteria for size definitions of impact structures vary
between
authors. For example, Vredefort (Orange Free State; 300 km) and
Acraman
(South Australia; 90 km) include wide outer zones showing little
or no
impact deformation. By contrast, the diameter of Morokweng (south
Kalahari; 70 km) is defined on the basis of an inner crater-fill,
although it is surrounded by circular geophysical anomalies some
340 km
in diameter. For Woodleigh, the same criterion has been applied
as for
Chicxulub, namely the intersection of regional structure by the
outermost geophysical ring zone. Three different observations
combine
to indicate a diameter of 120 km, including (1) Bouguer anomaly
intersections at the north and south ends of the structure; (2)
airborne magnetic data, defining the eastern rim, and (3)
sub-circular
drainage features, suggestive of sub-recent isostatic
reactivation.
Several lessons arise from the discovery and ongoing study of
Woodleigh. The criteria for recognition of buried underground
craters
are becoming increasingly well defined, including sharp
intersections
of regional tectonic trends by multi-ring Bouguer and magnetic
structures and anomalous central regions representing uplifted
cores,
combined with reduced sesmic response within brecciated and
block-faulted crater aureoles. Unfortunately, no one can continue
the
Australian crater search with the enormous expertise and
enthusiasm of
Eugene and Carolyn Shoemaker. However, with the help of the
Australian
National University (ANU), the Australian Geological Survey
Organisation (AGSO), and the Geological Survey of Western
Australia
(GSWA), we hope to continue the search, using geophysical,
multispectral imagery and field investigations methods.
I thank John Gorter, Robert Iasky and Arthur Mory for their
comments on
the article.
Andrew Glikson
Research School of Earth Science,
Australian National University,
Canberra, A.C.T. 0200
(W) andrew.glikson@anu..edu.au:
ph 61 2 6249 4076
(H) geospectral@spirit.com.au
; ph/fax 61 2 6296 3853
(H) P.O. Box 3698, Weston, ACT 2611
Copyright 2000, Andrew Glikson
-----------------
CCNet-ESSAY is part of the Cambridge Conference Network. It
includes
interesting and thought-provoking essays about our place in space
and
the prospects of a planetary civilisation that is in control of
our
terrestrial and extraterrestrial environment. Contributions to
this
ongoing debate are welcome. To subscribe or unsubscribe from
CCNet,
please contact Benny J Peiser at <b.j.peiser@livjm.ac.uk>.
The fully
indexed archive of the CCNet, from February 1997 on, can be found
at
http://abob.libs.uga.edu/bobk/cccmenu.html