The Onaping Formation fills the structural basin at Sudbury, Ontario, Canada. This formation is composed of three members: a basal, coarse, mainly quartzitic breccia (Basal Member); a light-colored, heavily included, polymict middle unit (Gray Member); and a similar but dark-colored upper unit (Black Member). Two different origins were proposed for the Onaping: (1) volcanic ash-flow sheet; and (2) impact fall-back ejecta. These origins are critically discussed in a review paper coauthored by proponents of each view

Betterton, W. J.

Bohor, B. F.

English

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LP1 Contribution No. 790 9
LARGE METEOROID IMPACT
HOT
K VAPOR
("Fireball")
X
Target Materia
{"melt ejecta'? (Modified from Hfh*h. 19991
SMALL METEOROID IMPACT
A t m o s p h e r e
scale height
AIR AIR
AIRBLAST
Target Material Hnele ejeclal (Modifirdfrom Mrlo*ri. I 9891
Fig. 1. Vapor plume cloud (fireball) exceeds the scale height of
atmosphere and expands globally above it.
Fig. 2. Vapor plume cloud (fireball) contained and dispersed
within scale height of the atmosphere.
transported both ballistically and in a detached ejecta curtain within
the atmosphere [11], forming an areally limited, continuous ejecta
blanket. This dual-phase impact model applies only on planets with
atmospheres when the size of the stabilized vapor plume exceeds the
scale height of the atmosphere (Fig. 1) [12]. In this case, the cloud
of vaporized bolide penetrates above the atmosphere and is dis-
persed globally, resulting in two-layer deposition within the limits
of the continuous ejecta blanket. Only a single layer (fireball) is
present elsewhere beyond the bounds of the basal melt ejecta
blanket. This pattern of ejecta dispersal explains many of the early
misconceptions about the nature of the K/T target rocks and impact
site that were based on geochemical analyses of far-field ejecta
comprised solely of the fireball layer.
Distal ejecta from other large terrestrial impacts should re-
semble this dual-phase K/T model. Thus, if distal ejecta from the
Sudbury structure can be located, it would be expected to be
composed of two layers within the extent of the continuous ejecta
blanket and single-layered beyond this limit. However, analyses of
ejecta deposits from Archean impacts in South Africa and Australia
[ 13] indicate that most of these thick spherulitic beds are comprised
of both fireball (N i - spinels and high Ir) and melt ejecta (microtektites)
components. This com ingling of components suggests either lack of
a significant atmosphere during the late bombardment period, or
small-scale impacts where the fireball is contained within the
atmosphere (Fig. 2) [12]. Other planets with atmospheres, such as
/ Venus and Mars, may also show a dual-phase distribution of ejecta
for large impacts. ^^
\ Acknowledgments: \This work was partially supported by
\ NASA Grant T5715P. W. 1. Betterton provided valuable technical
support.
\ References: {l] Bohor B. F. (1990) In GSA Spec. Pap. 247,
2 J2J^B6hor B. F and Betterton W. J. (1992) LPSC XXIII,
135-136. [3] Izett G. A. and Bohor B. F. (1986) GSA Abst. with
Prog.. 19. 644. [4] Bohor B. F. and Betterton W. J. (1991) Meteor-
itics. 26, 321. [5] Bohor B. F. and Izett G. A. (1986) LPSC XVIII,
6869. [6] Pollastro R. M. and Pillmore C. L. (1987) J. Sed. Petrol.,
54, 456^66. [7] Bohor B. F. (1988) Meteoritics. 23, 258-259.
[8] Bohor B. F. and Betterton W. J. (1991) Meteoritics, 26, 320.
[9] Bohor B. F. et al. (1989) Meteoritics. 24, 253. [10] Pollastro
R. M. and Bohor B. F. (1991) Clay Min. Soc. Prog, and Abst., 129,
LPI Contrib. No. 773 [11] Bamouin O. and Schultz P. H. (1992)
LPSC XXIII, 65-66. [12] Melosh H. J. (1989) Impact Cratering,
Oxford, New York. [13] Lowe D. R. et al. (1989) Science. 245,
959-962. 115155
SHOCKED ZIRCONS IN THE ONAPING FORMATION:
FURTHER PROOF OF IMPACT ORIGIN. B. F. Bohor and
W. J. Betterton, U.S. Geological Survey, Box 25046, MS 972,
Denver CO 80225, USA.
The Onaping Formation fills the structural basin at Sudbury,
Ontario, Canada. This formation is composed of three members: a
basal, coarse, mainly quartzitic breccia (Basal Member), a light-
colored, heavily included, polymict middle unit (Gray Member),
and a similar but dark-colored upper unit (Black Member). Two
different origins have been proposed for the Onaping: (1) volcanic
ash-flow sheet and (2) impact fall-back ejecta. These origins are
critically discussed in a review paper coauthored by proponents of
each view [1].
French [2] identified multiple sets of shock lamellae in quartz
and feldspar grains from the Onaping Formation at Sudbury. We
have also identified sets of shock lamellae (called planar deforma-
tion features, or PDF) in a single quartz grain from a thin section of
the Black Member. These PDF usually consist of "decorated"
lamellae that are much less distinct than those in younger impacted
rocks and ejecta, such as the K/T, because of annealing by subse-
quent metamorphic events.
Because it is more refractory than quartz and feldspar, zircon
should resist annealing by thermal metamorphism. We have already
shown that some zircons from K/T distal ejecta display PDF when
https://ntrs.nasa.gov/search.jsp?R=19930000933 2020-03-17T10:07:02+00:00Z
10 7992 Sudbury Coherence
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Fig. 1. PDF (4 sets). Fig. 2. Granular texture.
subjected to an alkaline etch [3]. We dissolved samples of both the
Gray and Black Members of the On aping with acids to free the
contained zircons, which were then etched in alkaline solutions to
reveal any PDF. These samples were collected from outcrops of the
Onaping along Highway 144 south of Levack and west of the High
Falls on the Onaping River. In the process of separating the zircons,
we also recovered other resistant trace minerals indicative of target
rock lithologies. These include tourmaline, garnet, kyanite, rutile,
staurolite, chromite, pyrite, and pyrrhotite.
Many of the etched zircons from both the Gray and Black
Members display PDF when viewed in an SEM. Figure 1 shows an
area of a shocked zircon from the Black Member that displays at
least four sets of PDF. These shock lamellae in zircon are much
narrower than those in quartz and do not etch as deeply, probably
because they contain less glass within them. Precession X-ray
photos of zircons from K/T ejecta with PDF show extreme broaden-
ing and streaking (asterism) of diffraction maxima, confirming that
they have been highly shocked [3].
Krogh et al. [4] reported zircons from the Onaping Formation at
Sudbury that show linear, crystallographically oriented fracturing.
They ascribed these features to shock caused by impact, and this
conclusion is supported by U-Pb data. The discordance of these
zircons is crudely proportional to the amount of fracturing they
display, caused by the Sudbury event dated from a lower intercept
age of -1836 Ma. We have also observed both linear and irregular
fractures in our Onaping zircons, but the finer-scale PDF probably
indicate a significantly higher level of shock than do these coarse,
open features.
In addition to fractures and PDF, Onaping zircons also show
another type of textural feature that indicates exposure to a high
level of shock. We have called this texture, first noted in zircons
from K/T ejecta [3], "granular" or polycrystalline (Fig. 2). Often,
zircons displaying this texture are idiomorphic with some of the
original crystal surfaces still visible. This indicates that the zircons
have not melted, but instead have been recrystallized due to shock.
Thus, these granular zircon grains can be considered to bediaplectic—
that is, shock-converted by solid-state transformation into polycrys-
talline zircon below their fusion temperature. On the other hand,
zircons also can be melted by impacts, as shown by fused grains
partially or completely converted to baddeleyite in high-tempera-
ture impact glasses [5]. The granular zircons in K/T ejecta and from
the Onaping show no phase or compositional changes by X-ray
analyses, in either diffraction or energy-dispersive modes. This is
another indication that their texture is due to solid-state transforma-
tion induced by shock, and not thermal melting.
It is instructive that these granular-textured zircons have been
found only in ejected material, and not in shocked, in situ target
rocks around craters. However, zircons bearing impact-generated
PDF have been identified from three types of sites and materials:
K/T distal ejecta, Sudbury fall-back ejecta (Onaping), and
Manicouagan target rocks. More importantly, PDF and granular
textures have never been seen in volcanic zircons. Therefore,
zircons can provide corroborating evidence of impact-generated
shock; this discovery could prove very useful in evaluating the
metamorphic histories of quartz-poor lunar rocks and meteorites.
Acknowledgments: We thank B. Dressier for guiding one of
us (BFB) to the "type" locality of the Onaping. T. Krogh provided
the Manicouagan zircons, and E. Foordperformed the X-ray analyses.
References: [1] MuirT. L. and Peredery W. V. (1984) In Ont.
Geol. Sur. Spec. Vol. I (E. G. Pye et al.. eds.), 139-210. [2] French
B. M. (1967) Science, 156,1094-1098. [3] BohorB. F. et al. (1990)
Meteoritics, 25,350. [4] Krogh T. E. et al. (1984) In Ont. Geol. Sur.
Spec. Vol. 1 (E.G. Pye etal.,eds.),431-446. [5] BGoresy A. (1968)
In Shock Metamorphism of Natural Materials (B. M. French and
N. M. Short, eds.), 531-553. Mono, Baltimore.
$/c? — y& /.
SUEVITE SUPERPOSITION ON THE BUNTE BRECCIA IN
NORDLINGER RIES/GERMANY: NEW FINDINGS ON
THE TRANSPORT MECHANISM OF IMPACTITES. D.
Bringemeier. Marie-Hedwig-Str. 15, W-3392 Clausthal-Z., Ger-
many.
475/5^
Research undertaken in the last decades in Nord linger Ries,
Germany, has repeatedly emphasized the sharp contact between
Bunte breccia and suevite. However, extensive investigations into
this layer boundary have not yet been possible due to insufficient
outcrop ratios.
New outcrops enabled an in-depth investigation into the
superposition of suevite on the Bunte breccia, which is assigned a
key role in interpreting the transport mechanisms of ejecta of large
impact.


Shocked zircons in the Onaping Formation: Further proof of impact origin

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