In cooperation between the Ontario Geological Survey and the Institute of Geology and Institute of Planetology, geological, petrological, and geochemical studies were carried out on impact-related phenomena of the Sudbury structure during the last decade. The main results of the field studies are briefly reviewed. Footwall rocks, sublayer, and lower sections of the Sudbury Igneous Complex (SIC) were mainly mapped and sampled in the northern (Levack Township) and western (Trillabelle and Sultana Properties) parts of the north range. Within these mapping areas Sudbury Breccias (SB) and Footwall Breccias (FB) were studied; SB were also investigated along extended profiles beyond the north and south ranges up to 55 km from the SIC. The Onaping Formation (OF) and the upper section of the SIC were studied both in the north range (Morgan and Dowling Townships) and in the southern east range (Capreol and McLennan Townships)

Avermann, M. E.

Bischoff, L.

Brockmeyer, P.

Dressler, B. O.

Lakomy, R.

Mueller-Mohr, V.

English

NASA Technical Reports Server

LPl Contribution No. 790 1
The granite, which is shocked to stage I. often shows in its upper
part in situ melting, which gradually increases toward the lop. This
partly molten granite is not part of the allochthonous coherent melt
sheet [3] that covered the granite and induced its melting. Except for
some relics this layer of molten target rock was eroded like most of
the ground-surge and fall-back breccias.
The original bedding of the Paleozoic formations is deformed by
impact-induced fracturing and folding. The concentric arrangement
with increasing younger formations from the center to the margin of
the structure give the structure the apparent domelike appearence.
This is due to the combination of a central uplift and the peripheral
graben structure. But, in fact, most contacts between the different
formations are of tectonic origin and each ring consists of a
multitude of fault blocks. Bedding in these blocks has a steep
dipping in the central ring, but has moderate to gentle dipping in the
outer parts of the structure. Folds are best developed in the sand-
stones of the Devonian Fumas Formation of the interior hills. The
axis of the steeply outward plunging open folds scatter remarkably
in different directions. They are sometimes arranged in a complex
fanlike pattern. From the analysis of the fault pattern and the
geometry of the folds, the principal stress components can roughly
be estimated with o, radial to the center.
Pebbles of some target rocks are heavily sheared. Shear plane
analysis may be used to reconstruct stress distribution during the
cratering process if the original position of the pebble-containing
beds is known.
References: [1] Deutsch A. et al. (1992) Tectonophysics, in
press. [2] Theilen-WilHge B. (1982) Geol. Rdsch., 71, 318-327.
[3] Engelhardt W. v. et al. (1992) Meteoritics, in ]
?-?
SUDBURY PROJECT (UNIVERSITY OF MONSTER-
ONTARIO GEOLOGICAL SURVEY): (2) FIELD STUDIES
1984-1989—SUMMARY OF RESULTS. L. Bischoff. B. O.
Dressier1. M. E Avermann1-3. P. Brockmeyerw. R. Lakomy'J, and
V. Miiller-Mohr'•J, 'Geologisch - Palaontologisches Insti tut. Univer-
sity of Munstcr.Corrensstr. 24. W-4400 Munster, Germany, HDntario
Geological Survey, 77 Grenville Street, Toronto, Ontario, M7A
1W4, Canada, Institut fur Planetologie. University of Munster,
WilhelmKlcmm-Str. 10, W-4400 Munster. Germany.
In cooperation between the Ontario Geological Survey (B. O.
Dressier) and the Institute of Geology and Institute of Planetology,
both of Westfilische Wilhelms-Universitit, Munster. geological,
petrological, and geochemical studies were carried out on impact-
related phenomena of the Sudbury structure during the last decade.
The main results of the field studies are briefly reviewed (see also
[1-5]). As shown in Fig. 1, Footwall rocks, sublayer, and lower
sections of the Sudbury Igneous Complex (SIC) were mainly
mapped and sampled in the northern (Levack Township) and
western (Trillabelle and Sultana Properties) parts of the north range.
Within these mapping areas Sudbury Breccias (SB) and Footwall
Breccias (FB) have been studied; SB have also been investigated
along extended profiles beyond the north and south ranges up to
55 km from the SIC [5]. The On aping Formation (OF) and the upper
section of the SIC were studied both in the north range (Morgan and
Dowling Townships) and in the southern east range (Capreol and
McLennan Townships) (see Fig. 1).
The Footwall rocks [6-12] exhibit shock metamorphic phenom-
ena of shock stage I, which is characteristic of an impact crater floor.
Shock metamorphism, including especially planar, elements in
quartz and shatter cones, can be observed up to a radial distance of
9 km from the lower SIC contact or up to 17 km (shatter cones). The
Foot w al 1 rocks were subjected to a thermal overprint by the SIC up
to a distance of 1 km [8,12].
The FB [3.6,10.12.13] is the remnant of a clastic matrix crater
floor breccia. It occurs as irregularly or lenticular!y shaped bodies
at the contact between the Footwall and the SIC. Further on FB dikes
intrude into the brecciated crystalline basement rocks up to 250 m
away from the FB layer [ 10,12,13]. The contacts between FB and the
underlying Footwall rocks are always sharp, even on a microscale.
The fragment content reflects a local origin. Fragments of FB occur
enclosed in the sublayer and norite. The crystalline matrix was
formed after localized melting of the originally clastic matrix due
to the thermal overprint by the SIC. Locally megaclastt (>2S m)
adjacent to the country rock are embedded in huge intercalations of
FB matrix.
The SB [5,7,9,11,12,141 occur as various types of clastic or
crystalline matrix breccia forming irregular dikes whose orienta-
tions appear not to be controlled by the geometry of the Sudbury
structure. In the north range dikes have been observed up to a
distance of at least 55 km from the lower contact of the SIC, but are
abundant within a zone of 10 km adjacent to the SIC and a second
zone located approximately between 20 and 33 km north of the SIC.
The thickness of the dikes ranges from centimeters to several
hundred meters. Most of the dikes exhibit sharp contacts to the wall
rock. Several phases of brecciatkm can be distinguished by breccia
fragments in SB, whereas crosscut ting of different breccia types is
extremely rare. Most of the fragments of the SB are directly derived
from the country rock, but in large dikes relative movements of
clasts over several hundred meters can be found. The SB were
formed by shearing and friction of Footwall rocks.
The breccias of the OF [4,15-17] can be divided from bottom to
top into Basal Member, Gray Member, Green Member, and lower
and upper Black Member according to their lithological composi-
tion, matrix, clast population, contacts, and texture. The term "Basal
Member" comprises a clast-rich melt breccia on top of the SIC and
the "melt bodies" [18], as they were formerly called, whereas the
term "Green Member" implies the former "chlorite shard horizon"
[18]. The Black Member was subdivided due to obvious changes
upward in fragment content, size, matrix texture, grade of foliation,
and occurrence of both carbonaceous material and sedimentary
features. The lower part of the polymict allochthonous breccias of
the OF are interpreted as impact-melt breccias, the overlying
breccias as ground-surge breccias, and the uppermost as original or
resedimented fall-back breccias [see 1,2,4].
References: [ 1 ] Stdffler D. et al., this volume. [2] Avermann
M. et al., this volume. [3] Deutsch A. et al., this volume. [4] Aver-
mannM., this volume. [5] Miiller-Mohr V., this volume. [6] Lakomy
R. (1986) Unpublished diploma thesis, Munster, 135 pp. [7] Brock-
meyer P. (1986) Unpublished diploma thesis, Munster, 101 pp.
[8] Avermann M. E (1988) Unpublished diploma thesis, Munster,
113 pp. [9] Miiller-Mohr V. (1988) Unpublished diploma thesis,
MOnster. 106 pp. [ 10] Lakomy R. (1989) Ph.D. thesis. Munster, 165
pp. [11] Muller-Mohr V. (1992) Ph.D. thesis. Munster. 139 pp.
[ 12] Dressier B. O. (1984) In Ont. Geol. Sum. Spec. Vol. 1 (E. G. Pye
et al.. eds.). [13] Lakomy R. (1990) Meteoritics. 25, 195-207.
[ 14] Miiller-Mohr(1992)Tectonophysics,'mpre&s.[l5]Brockmeyer
P. (1990) Ph.D. thesis. Munster. 228 pp. [16] Avermann M. E.
(1992) Ph.D. thesis. Munster, 175 pp. [17] Avermann M. E and
BrockmeyerP. (1992) Tectonophysics, in press. [18] MuirT. L. and
Peredery W. V. (1984) In Ont. Geol. Surv. Spec. Vol. 1 (E G. Pyc
et al.. eds.).
https://ntrs.nasa.gov/search.jsp?R=19930000931 2020-03-17T10:06:45+00:00Z
8 7992 Sudbury Conference
Sudbury Structure
Superior Province
/ /
Southern Province
Legend
Chelmsford Formation
Onwatin Formation
Onaping Formation
Sudbury Igneous Complex
incl Basal Member
Footwall Breccia It Sublayer
Grenville Province
Areas of investigation
1 footwall rocks/SIC
2 Sudbury Breccia
3 Footwall Breccia
4 Onaping Formation
4-6151
Fig. 1. Sketch map of the Sudbury structure showing the areas of investigation.
I . °v /\ h Cd-7/M?>
LARGE METEORITE IMPACTS: THE K/T MODEL. B.F.
Bohor, U.S. Geological Survey, Box 25046, MS 972, Denver CO
80225, USA.
The Cretaceous/Tertiary (K/T) boundary event represents prob-
ably the largest meteorite impact known on Earth. It is the only
impact event conclusively linked to a worldwide mass extinction, a
reflection of its gigantic scale and global influence. Until recently,
the impact crater had not been definitively located and only the
distal ejecta of this impact was available for study. However,
detailed investigations of this ejecta's mineralogy, geochemistry,
micros tratigraphy, and textures have allowed its modes of ejection
and dispersal to be modeled without benefit of a source crater of
known size and location [1,2).
Initially, only K/T boundary sites in marine rocks with a single,
thin (-3-5 mm) layer of Ir-rich ejecta were known. Subsequently,
nonmarine sites were discovered in the western interior of North
America with thicker (-3 cm) boundary claystone units composed
of two distinct layers [3]. The uppermost of these two layers, which
I have called the "fireball" layer, contains a substantial Ir anomaly,
skeletal Ni-rich magnesioferrite crystals, and a large population of
shocked minerals in the nonclay fraction. The maximum grain size
of these shocked minerals decreases regularly away from the site of
the putative crater, both in the western interior [4] and on a global
basis [5]. Geochemically, the fireball layer has a basaltic signature
and is mainly composed of laminated smectitic clay. It immediately
overlies the lower layer with a sharp textural contact. This lower
layer, which I have named the "melt ejecta" layer, has a very
subdued Ir anomaly, contains only a small amount of shocked
minerals and no magnesioferrite. has a silicic geochemical signa-
ture, and displays a turbated texture of unsorted, altered, imbricate
sharcU[6],vitricclasts[7],aridrnicroiektitcs[8]inarnicTospherulilic
kaolinite matrix [6]. The melt ejecta layer thins radially away from
the putative crater location in the Caribbean region and cannot be
identified beyond -4000 km of the crater. Both the fireball and melt
ejecta layers contain a similar suite of trace minerals [9], which
argues for a mutual origin from a single impact
The partitioning of these impact components and signatures
between the two layers supports a dual-phase model of ejection and
dispersal from a single impact [2]. Moreover, the distinctive clay
minerals formed from the vitric components in each of these two
layers in the western interior lends further support to the dual-phase
model [10]. In this model, the fireball layer represents sedimenta-
tion from a radially expanding cloud of vaporized bolide and
entrained target material dispersed above the atmosphere. The melt
ejecta layer represents melted target rock ejected from the crater and


Sudbury project (University of Muenster-Ontario Geological Survey): Field studies 1984-1989 - summary of results

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