FUNDAY

Shock-Metamorphosed Rocks (Impactites) in Impact Structures 61
61
5.1. ROCK TYPES IN THE FINAL
IMPACT STRUCTURE
A wide variety of distinctive rock types — breccias, melts,
and shock-metamorphosed target rocks — are produced
during formation of impact structures. The classification
of these complex and diverse rocks is an active and muchdebated
activity (see below). However, the general term
impactite is used here as a convenient overall designation
for all rocks affected by, or produced by, the shock waves
and other processes generated by hypervelocity meteorite
impact events.
Different varieties of impactites are produced at different
times during the impact process, and they occur in different
locations beneath, within, and around the final impact structure.
The diverse features of impactites reflect, in varying
ways, different aspects of the impact event itself: (1) the
initial shock-wave distribution around the impact point;
(2) the subsequent excavation flow, formation of the transient
crater, and ejection of material from it; (3) the crater
modification processes. The general model described below
will be modified, in actual impact structures, by such individual
factors as the target lithology, stratigraphy, and the
nature and impact angle of the projectile, but the model provides
a general basis for the identification and classification
of impactites (see also Dence, 1968; Grieve, 1991; Stöffler et
al., 1988).
The basic distribution of shock-wave pressures around
the impact point is largely established by the end of the contact/
compression stage. The expanding shock waves deposit
energy continuously in the target rocks through which they
pass, and both their peak pressures and the resulting postshock
temperatures drop rapidly with distance from the impact
point. As the contact/compression stage ends, and the
transient crater begins to form, the zones of shock pressure
form a series of approximately hemispherical shells around
the impact point, with the peak shock pressure decreasing
rapidly outward (Fig. 3.2).
During the subsequent excavation stage and formation
of the transient crater, virtually all the target rock exposed to
shock pressures of >25–30 GPa, which now consists of a
mixture of vapor, superheated rock melt, and coherent but
highly shocked target rock, is broken up and accelerated
outward (Dence, 1968; Dence et al., 1977; Grieve and Cintala,
1981). Because the excavation flow lines cut across the originally
hemispherical shock-pressure zones (Fig. 3.4), the excavated
material will consist of a mixture of target rocks
subjected to widely differing shock pressures and showing a
wide range of shock effects. A melt-rich portion flows downward
and outward from the center to form a coating along
the floor and walls of the growing crater (Grieve et al., 1977).
The remainder, a mixture of rock fragments and smaller bodies
of melt, is impelled outward from the center of the cavity.
Much of this material may be entirely ejected from the transient
crater; some may remain within the crater as a unit of
mixed rubble and melt above the fractured crater floor.
The subcrater rocks beneath the zone of excavation are
subjected to lower shock pressures ( >30 GPa), and the dominant
effects produced are shatter cones, brecciation, and inplace
fracturing. As the upper part of the target rocks are
excavated from the transient crater, these rocks are displaced
downward, more or less coherently, to form the floor of the
transient crater and the zone of parautochthonous rocks
beneath it.
The final modification of the transient crater into a simple
or complex impact structure involves several distinct gravity-
related processes that influence the distribution of
impactite units: (1) rapid relative movements of large blocks
of subcrater target rocks downward, inward, and upward
along relatively narrow faults; (2) collapse of oversteepened
Shock-Metamorphosed Rocks (Impactites)
in Impact Structures
62 Traces of Catastrophe
crater walls into the crater cavity; (3) deposition of a minor
amount of ejected material within the crater. The first process
may create additional breccias and related rock types
beneath the crater. The other two processes produce a large
portion of the crater-fill deposits, which are characterized
by a generally fragmental character and the presence of shockmetamorphic
effects that range from simple fracturing to
complete melting.
5.2. CLASSIFICATION OF IMPACTITES
The definition and classification of impact-produced
materials, both individual rock fragments and large formations,
is a complex, longstanding, and difficult subject (for
details, see Stöffler, 1971; Stöffler et al., 1979; Taylor et al.,
1991; Stöffler and Grieve, 1994, 1996; Reimold, 1995). No
attempt will be made here to develop a complete and unanimously
acceptable system. The simplified system presented
here emphasizes field and petrologic characteristics and is
based, as far as possible, on objective features that are observable
in outcrop, hand specimen, and thin section. This
classification also uses, as much as possible, traditional terms
already applied to equivalent rocks (e.g., breccias, melt rocks)
formed by common geological processes. Although this system
is generally consistent with more detailed classifications
(e.g., Stöffler and Grieve, 1994), it is restricted to terrestrial
rock types produced in single impact events and does not
consider the special complexities of cratering on other planets,
including the effects of multiple impacts or the absence
of an atmosphere (see Taylor et al., 1991; Stöffler and Grieve,
1994, 1996).
The term impactite is used here to designate all rocks
produced during an impact event, including shock-metamorphosed
(but still recognizable) target rocks (both in place
and as fragments in breccias), breccias, and impact melts.
Under this umbrella, the classification and terminology of
impactite formations are based on a few key features: location
with respect to the crater, source(s) of component materials,
and lithologic characteristics (Table 5.1).
More detailed discriminators, used in other classifications,
include (1) particle sizes and size ranges; (2) relative percentages
of components in breccias, e.g., ratios of fragments/
matrix, and lithic/glassy fragments; (3) shock-metamorphic
effects in individual breccia fragments (both the shock level
in individual fragments and the range of shock effects in
multiple fragments); and (4) textures and crystallinity of
melt rocks.
In earlier discussions of impactites and the cratering process
(Dence, 1965, 1968; Grieve, 1991), a fundamental and
useful distinction has been made between the parautochthonous
rocks beneath the crater floor and the allogenic (or
allochthonous) units (breccias and melt rocks) that fill the
crater (crater-fill units) and form the units of ejecta outside
it (Figs. 3.7 and 3.13). The observed characteristics of these
different rock types are frequently distinctive enough that
they can be distinguished, even in isolated hand specimens
or outcrops.
The parautochthonous rocks beneath the crater have
remained relatively coherent during crater formation, although
they have been deformed and displaced. These rocks,
which correspond to the lower displaced zone of the transient
crater, are subjected to relatively lower shock pressures,
and observed shock-deformation effects are generally
limited to fracturing, brecciation, and the formation of shatter
cones, although higher-pressure mineral-deformation
features may be developed in a relatively small volume beneath
the crater floor. The allogenic rocks, chiefly breccias
and melts, that fill the crater and make up the ejecta beyond
the crater rim, are characterized by a more diverse lithology,
a fragmental or melted character, and a wide range
of observed shock effects. In particular, the crater-fill breccias
are a complex mixture of materials with different histories
of shock pressures and transport: unshocked rocks
derived from the distant parts of the crater rim and walls,
more highly shocked and melted fragments excavated from
the transient crater and redeposited, and large and small
bodies of impact-generated melt.
The following sections discuss impactites on the basis of
location with respect to the impact structure: (1) subcrater:
parautochthonous rocks, cross-cutting allogenic
units, and pseudotachylite; (2) crater interior: allogenic
crater-fill deposits (lithic breccias, suevite breccias, and impact
melt breccias); (3) crater rim region: proximal ejecta
deposits; (4) distant from crater: distal ejecta. A detailed
discussion of impact melt rocks in these different environments
is provided in Chapter 7.
5.3. SUBCRATER ROCKS
5.3.1. Formation Conditions
During formation of the transient crater, the rocks located
in the displaced zone below the zone of excavation are
driven downward and outward, more or less coherently
(Fig. 3.4), but they are not completely broken up or excavated.
Instead, they are deformed, thinned, and moved downward
and outward as the transient crater forms, and then (in
the central parts of larger structures) rapidly elevated as the
central uplift forms (Dence, 1968; Dence et al., 1977; Kieffer
and Simonds, 1980; Grieve and Cintala, 1981; Grieve et al.,
1981; Stöffler et al., 1988).
During these movements, the subcrater rocks are generally
displaced as large individual blocks typically tens to hundreds
of meters (or even larger) in size. However, adjacent
regions within this zone may display little displacement relative
to each other, and original stratigraphy and structural
features may be well preserved within individual blocks. The
term parautochthonous has therefore been applied to these
rocks to indicate their general relative coherence.
The shock pressures imposed on the parautochthonous
rocks vary widely because of the complex relationship beShock-
Metamorphosed Rocks (Impactites) in Impact Structures 63
tween the original shock-wave distribution and the subsequent
crater modification. Shock pressures in the parautochthonous
rocks are therefore highest near the center of
the structure and decrease rapidly outward toward the margin.
Along the floor of the transient cavity (approximately
the floor of the final crater), shock pressures may exceed 25–
30 GPa in the center, decreasing to >2 GPa at the rim, the
minimum pressure needed to excavate material from the transient
crater (Grieve and Robertson, 1976; Robertson and
Grieve, 1977; Kieffer and Simonds, 1980; Dressler et al., 1998).
Shock pressures also drop off rapidly with increasing depth
below the crater floor. In the center, pressures typically drop
from about 25–30 GPa to a few GPa over distances of less
than a few hundred meters in small structures (Dence et al.,
1977; Grieve et al., 1981) and over no more than a few kilometers
in larger ones (Stöffler et al., 1988).
5.3.2. In-Place Shock-Metamorphosed Rocks
The shock effects preserved in the parautochthonous
subcrater rocks therefore reflect a wide range of shock pressures.
In a small region immediately below the central part
of the crater floor (i.e., at the base of the excavation zone),
TABLE 5.1. Criteria for impactite classification.
1. Location with respect to crater (Rc = crater radius)
Crater Floor and Subcrater Within Crater Crater Rim and Near-Surface
Parauthochtonous rocks: Allogenic rocks: Allogenic rocks:
target rocks (coherent) Crater-fill deposits Ejecta:
lithic breccias (= crater-fill breccias) proximal (<5 Rc)
(= “breccia lens”) distal (>5 Rc)
Allogenic rocks (cross-cutting) lithic breccias
breccia dikes melt-bearing breccias
impact melt dikes suevites
impact melt breccias
Pseudotachylite (= melt-matrix breccias)
impact melt rocks
2. Sources of component materials
Parautochthonous rocks Allogenic rocks
Approximately in place (local). Original stratigraphy Derived from single or multiple sources elsewhere.
and structure (largely) preserved.
3. Breccia characteristics
a. Fragment character Lithic breccia Suevite (breccia)
Rock/mineral fragments only Melt/glass fragments present
Rock/mineral fragments
b. Fragment lithology Monomict (breccia) Polymict (breccia)
Single rock type Multiple rock types
c. Matrix character Clastic-matrix (breccia) Impact melt breccia (= melt-matrix breccia)
Discrete fragments Coherent melt (glassy or crystalline)
4. Melt rock character (standard geological terms)
Holohyaline (glassy) For grain size, texture, etc., use other standard igneous rock
Hypocrystalline (mixed glassy/crystalline) discriminators, e.g.:
Holocrystalline (completely crystalline) Microcrystalline
Porphyritic
Trachytic, etc.
64 Traces of Catastrophe
pressures of 10–30 GPa produce distinctive microscopic
deformation effects in quartz and feldspar, while creating
postshock temperatures of >300°C. In smaller impact structures,
this zone of identifiably high shock pressures is less
than a few hundred meters thick, partly because of the rapid
decay of the original shock wave with distance from the impact
point, and partly because of the subsequent compression,
thinning, and displacement of the subcrater rocks during
transient crater formation (Dence et al., 1977; Grieve and
Cintala, 1981). Beneath this zone, lower shock pressures
(possibly 2–6 GPa) produce distinctive megascopic deformation
features (shatter cones) in a deeper region near the
center of the crater.
Shock pressures over most of the zone of parautochthonous
rocks are too low ( >2 GPa) to produce distinctive
shock-deformation effects, but they are high enough to exceed
the yield strengths of near-surface crustal rocks (typically
<1–2 GPa; Kieffer and Simonds, 1980). As a result, large
volumes of rock beneath the crater floor are broken and
crushed during the early stages of crater formation, producing
units of in-place lithic breccia that generally lack distinctive
high-pressure shock-metamorphic effects. At the
same time, and subsequently, larger fractures that develop in
this zone may be intruded by allogenic materials (rock fragments
and/or melt) to form cross-cutting dike-like bodies
(e.g., Lambert, 1981; Bischoff and Oskierski, 1987; Dressler
and Sharpton, 1997).
The parautochthonous rocks below the crater may also
be strongly affected by subsequent large-scale movements
during the crater modification stage. Such movements may
produce equally striking but different breccias. In large structures,
where modification involves the development of a central
uplift, deep-seated parautochthonous rocks may be
suddenly uplifted for distances of hundreds of meters to several
kilometers. This uplift may bring distinctively shocked
rocks (e.g., containing shatter cones) to the surface, where
they may provide definite evidence for the impact origin of a
large structure. However, these rapid movements may also
generate additional varieties of breccias and destroy the original
spatial relations of the parautochthonous rocks to each
other, making the geology and history of the structure more
difficult to decipher.
Understanding the variety of breccias in subcrater rocks
is complicated by several factors (e.g., Lambert, 1981; Bischoff
and Oskierski, 1987; Dressler and Sharpton, 1997). Breccias
may form at various stages in the cratering process: (1) during
the initial shock-wave expansion and transient crater
formation; (2) during the subsequent modification of the
transient crater, including (in large structures) movements
associated with the rise of the central uplift and peripheral
collapse around the rim. Even within the brief formation
time of an impact crater, it is possible for multiple generations
of breccia to develop and to produce distinctive crosscutting
relations, even though the time between one breccia
generation and the next may be measured in seconds or minutes
(Lambert, 1981; Bischoff and Oskierski, 1987; Dressler
and Sharpton, 1997). Another problem is melt formation;
rocks can be shock-melted by the initial impact and then
distributed as melts or melt-bearing breccias throughout the
crater basement, but rocks can also be melted subsequently
by friction generated during the rapid movements of large
volumes of rock involved in crater modification and central
uplift formation.
5.3.3. Lithic Breccias (Parautochthonous)
Impactite breccias that form by the shattering and pulverizing
of target rock essentially in place (autoclastic) typically
form irregular bodies tens to hundreds of meters in size,
which show gradational contacts against areas of similar
but more coherent target rocks. These lithic breccias are
composed entirely of rock and mineral fragments in a clastic
matrix of smaller, but similar, fragments. Fragments tend
to be angular to sharp, although fragments of softer rocks
like carbonates and shales may be well rounded. The breccias
themselves tend to be poorly sorted. The fragments are
derived from local target rocks, and the breccias may be
monomict or polymict, depending on the lithologic variety
present in the nearby target rocks. Distinctive shock-metamorphic
effects (e.g., PDFs in quartz) are generally absent
in the fragments. The breccias show no evidence of significant
transport, and they contain no exotic fragments or
glassy material.
These rocks often resemble breccias formed by more normal
geological mechanisms such as volcanic explosions or
tectonic movements, and their identification as impact products
is often difficult and uncertain. In general, the subcrater
regions of impact structures display highly localized and variable
deformation over short distances, a close association of
different kinds of breccias developed from basement rocks,
and the presence of allochthonous dike-like bodies of breccia
and melt. This variability in deformation and rock types
contrasts with the more uniform or gradational effects produced
by endogenic mechanisms. Even so, identification of
these rocks as impact breccias can generally not be done
directly, but depends on demonstrating their association with
more highly shocked rocks whose impact origin is clear (e.g.,
French et al., 1997).
5.3.4. Cross-Cutting (Allogenic) Breccias
Other bodies of breccia in the subcrater rocks contain
significant amounts of material that have clearly been introduced
into them from elsewhere, and they are therefore considered
here as allogenic breccias. These bodies tend to have
more regular shapes and to show sharp contacts and clear
cross-cutting relations against the subcrater rocks. Such breccias
often occur as distinctive breccia dikes, which typically
range from less than a meter to tens of meters in width and
may be as much as a kilometer long (Lambert, 1981; Bischoff
and Oskierski, 1987; Dressler and Sharpton, 1997). These bodies
contain fragments of target rock that are angular to
rounded and range in size from <1 mm to several meters.
These breccias tend to be polymict, with lithologically diverse
fragments, indicating mixing over distances of at least
several hundred meters. In addition, they frequently contain
Shock-Metamorphosed Rocks (Impactites) in Impact Structures 65
significant amounts of allogenic material, such as fragments
from even more distant rock units. This allogenic material is
frequently derived from more central regions of the crater,
often from above the present location of the dike, and it often
consists of distinctive highly shocked rock fragments
or melt.
A wide variety of such cross-cutting breccias has been
reported from several impact structures (Lambert, 1981;
Bischoff and Oskierski, 1987; Dressler and Sharpton, 1997):
(1) melt-free, typically polymict, lithic breccias with a clastic
matrix; (2) melt-fragment breccias containing fragments
of heterogeneous glass, rocks, and minerals in a clastic matrix;
(3) melt-matrix breccias (impact melt breccias), composed
of rock and mineral fragments in a matrix of glassy or
crystalline melt; (4) impact melt rocks, composed of glassy
or crystalline melt with few or no inclusions (e.g., Dence,
1971). Many of these dikes are similar to units of breccia or
melt in the crater-fill units above the crater floor, and they
may in fact be continuous with them (e.g., Lambert, 1981).
Subcrater breccia dikes often contain materials (e.g., rock
fragments or melt) that were originally located at higher
stratigraphic levels closer to the impact point, indicating that
the materials in the dikes have been emplaced downward
and/or outward into fractures that opened in the crater floor
during formation and modification of the crater. In many
structures, more than one generation of dikes occurs, with
later ones cutting earlier ones (Lambert, 1981; Dressler and
Sharpton, 1997). These relations indicate that, even during
the brief duration (seconds to minutes) of crater formation
and modification, a variety of distinct breccia types can be
generated and emplaced. However, in the crater environment,
cross-cutting relations between breccia bodies do not
imply the passage of significant amounts of time between
emplacements, a conclusion supported by the fact that the
cross-cutting relations between different types of breccia may
not be consistent from place to place within the whole structure
(Dressler and Sharpton, 1997).
5.3.5. Pseudotachylite
Pseudotachylite is an unusual, much-studied, and longdebated
type of impactite breccia that occurs in the parautochthonous
rocks of large impact structures (for recent
reviews, see Reimold, 1991, 1995; Spray, 1995). Pseudotachylite
is most strikingly developed at two large, ancient
impact structures: Vredefort (South Africa) (Shand, 1916;
Reimold, 1991; Reimold and Colliston, 1994) and Sudbury
(Canada) (Fairbairn and Robson, 1941; Speers, 1957; Dressler,
1984; Thompson and Spray, 1994; Spray and Thompson, 1995),
where it forms striking and extensive exposures (Figs. 5.1
and 5.2). The Vredefort pseudotachylite, first described more
than 80 years ago (Shand, 1916), typically occurs as abundant
irregular, anastomosing, and dike-like bodies that
contain numerous large and small rounded inclusions of
target rock set in a dense, aphanitic or crystalline matrix that
is generally black to blackish-green in color. Similar breccias,
although developed on a much smaller scale, have been
observed in other impact structures, e.g., Rochechouart
(France) (Reimold et al., 1987), Manicouagan (Canada)
(Dressler, 1990), and Slate Islands (Canada) (Dressler and
Sharpton, 1997).
At Sudbury and Vredefort, pseudotachylite is extensive.
Pseudotachylite exposures at Sudbury cover as much as 100–
200 km2, or a few percent of the total area of the structure.
Individual pseudotachylite bodies can also be large; the largest
body so far recognized at Sudbury is more than 11 km
long, more than 400 m wide, and contains discrete fragments
that are hundreds of meters in size (Dressler, 1984). In smaller
impact structures, pseudotachylite bodies are smaller and less
abundant; the material typically occurs as irregular dike-like
bodies less than a meter across.
The individual pseudotachylite bodies in impact structures
are not uniform over long distances and may change
size and shape radically within meters or tens of meters.
The more elongate dike-like bodies show little or no preferred
orientation in direction. The fragment/matrix ratio in
Fig. 5.1. Pseudotachylite in granitic gneisses. Pseudotachylite
exposure, showing rounded gneiss inclusions from a few centimeters
up to a few meters in size in a dense black matrix. The inclusions
show a significant amount of rotation relative to each other. Southwest
sector of the Vredefort structure (South Africa) (farm Samaria
484). Black pen on large inclusion in center (arrow) is 15 cm long;
inclusion itself is about 50 cm long. From Reimold and Colliston
(1994); photograph courtesy of W. U. Reimold.
66 Traces of Catastrophe
pseudotachylite bodies also varies significantly over short
distances, and some pseudotachylite breccias consist only of
fractured target rocks cut by thin veins of black matrix less
than a few millimeters wide. (The descriptive term “cobweb
breccias” has been used as a convenient field label for such
occurrences.)
Contacts between pseudotachylite bodies and the enclosing
target rock are irregular and generally not parallel on
opposite sides. Offsets of wallrock along pseudotachylite
bodies are uncommon, and observed displacements are minor
(e.g., <100 m). In very large pseudotachylite bodies with
large inclusions, the boundary between the breccia body and
the unbrecciated wallrock may not be clear. In such occurrences,
e.g., at Sudbury, the exact boundaries between breccia
and undisturbed wallrock may be difficult to establish
(Dressler, 1984).
Inclusions in pseudotachylite range from submicroscopic
to hundreds of meters in size. They invariably consist of local
bedrock, and there is generally no evidence for significant
long-distance (>100 m) transport of fragments during
formation. The inclusions are irregularly oriented, and outcrops
of the breccia give the strong impression of an overall
tensional or explosive environment (Figs. 5.1 and 5.2), rather
than the narrower compressional/shear environment that is
characteristic of zones of major thrust faulting (Philpotts,
1964; Sibson, 1975; Spray, 1995). Larger inclusions (>1 cm)
are generally rounded, while smaller ones tend to be angular
or sharp. Contacts between both large and small inclusions
and the surrounding matrix are generally sharp. However,
some inclusions may be deformed at the rims, forming a
flow structure that can be observed, both megascopically
and microscopically, to grade into the surrounding matrix
(Fig. 5.3).
The matrix between larger rock fragments is dense and
coherent. In hand specimen, the matrix often shows a conchoidal
or hackly texture on broken surfaces. The color is
commonly black to blackish green on fresh surfaces, although
the color may vary slightly with the host rock involved. The
matrix occurs in a wide variety of forms. It may cover large
(meter-sized) areas of inclusion-poor material, or it may form
tiny submillimeter filaments that penetrate bedrock and inclusions
and often terminate within them. In hand specimen
and thin section, the matrix is commonly structureless
(Fig. 5.4), but flow-banding is often observed, especially in
thin section (Fig. 5.3). This flow-banding may involve inclusions
that have been plastically deformed and possibly
melted (Fig. 5.5).
The matrix, generally aphanitic in hand specimen, is extremely
fine-grained and difficult to characterize, even in
thin section. In some samples, the matrix shows definite mi-
Fig. 5.2. Pseudotachylite; metamorphosed, in quartzite. Dark pseudotachylite (“Sudbury Breccia”) in Mississagi Quartzite on South
Range of Sudbury structure (Canada). Exposure shows large rounded blocks of quartzite in a pervasive black matrix (note penetration of
matrix into large quartzite block at lower right). Hammer (upper right) gives scale. Photograph courtesy of W. Peredery.
Shock-Metamorphosed Rocks (Impactites) in Impact Structures 67
Fig. 5.3. Pseudotachylite; flow-banded texture. Pseudotachylite (“Levack breccia”) in granitic gneisses from the North Range of the
Sudbury structure (Canada). In thin section, the black pseudotachylite matrix material consists of small irregular rock and mineral
inclusions in a dark microcrystalline to aphanitic groundmass. Numerous inclusions (white) show plastic deformation and alignment to
form a flow structure; note concentric deformation of the flow structure around larger inclusions (e.g., top right). Thin vertical white lines
are filled hairline fractures in the specimen. Sample CSF-67-53 (plane-polarized light).
Fig. 5.4. Pseudotachylite; structureless matrix. Pseudotachylite from Vredefort (South Africa), showing typical irregular to rounded
inclusions, ranging in size from <100 μm to several millimeters, in a dark aphanitic groundmass. Inclusions, which are rock and mineral
fragments from granitic gneisses, show sharp contacts with the matrix. In this pseudotachylite sample, the matrix is structureless, and the
inclusions show no deformation, preferred orientation, or other flow structures. Sample AV-81-53 (plane-polarized light).
1 mm
1 mm
68 Traces of Catastrophe
crocrystalline melt textures at SEM or microscopic scales
(Fig. 5.6). This characteristic, i.e., a matrix of igneous melt,
has been proposed (but not unanimously accepted) as a distinguishing
feature of pseudotachylite breccias (Spray, 1995).
In other samples, the matrix appears to consist of small fragments
in a cataclastic texture, and distinguishing between
the two types is a difficult process with important implications
for both classification and origin (Reimold, 1995).
Chemical studies of pseudotachylites (e.g., Dressler, 1984;
Reimold, 1991) have shown that they correspond closely to
Fig. 5.5. Pseudotachylite; extensive melting and flow. Pseudotachylite (“Levack Breccia”) from granitic gneisses in the North Range
of the Sudbury structure (Canada). The pseudotachylite consists of a heterogeneous mixture of plastically deformed and possibly melted
wallrock fragments (light-colored), mixed with discontinuous areas of more typical pseudotachylite material (dark) consisting of small
rock and mineral fragments in a fine black matrix. Sample CSF-88-2A (plane-polarized light).
Fig. 5.6. Pseudotachylite; igneous matrix with microlites. Black pseudotachylite developed in central granitic gneisses at Vredefort
structure (South Africa), consisting of small, irregular, generally rounded rock and mineral fragments in a black, finely crystalline matrix.
Matrix shows igneous flow-banding, expressed by alignment of small feldspar microlites typically 50–100 μm long. The microlites are
often concentrically aligned around larger inclusions. Sample AV81-52A (plane-polarized light).
1 mm
0.5 mm
Shock-Metamorphosed Rocks (Impactites) in Impact Structures 69
the adjacent host rocks, indicating that they have formed
essentially in place by locally generated cataclastic milling
and/or frictional melting processes.
Controversy and debate over the characteristics, terminology,
and origin of pseudotachylite has existed ever since
the term was first used (Shand, 1916) and continues actively
today (e.g., Spray, 1995; Reimold, 1995). Shand (1916,
pp. 188–189) deliberately coined the word “pseudotachylite”
to distinguish the Vredefort material from tachylite (basaltic
glass) and also from highly crushed and melted materials
formed tectonically along major faults (“flinty crush-rock,”
ultramylonite, hyalomylonite, etc.). Unfortunately, Shand’s
term has since been widely applied to the latter type of
material, so that it now designates similar glassy breccias
that are clearly tectonic in origin (Philpotts, 1964; Sibson,
1975; Reimold, 1995). Such breccias form in entirely different
environments and are the results of intense deformation
(including frictional melting) of rocks along the linear
trends of faults. They form in a compressional /shear regime,
but they can resemble impact-produced pseudotachylite,
including the presence of melted material in the matrix
(Philpotts, 1964).
Recently, some workers have suggested that impactproduced
pseudotachylites are formed in the same way as
tectonic ones, i.e., by frictional heating during the rapid
movements of late-stage crater development and modification
(e.g., Thompson and Spray, 1994; Spray, 1995, 1997; Spray
and Thompson, 1995). In this view, impact-produced
pseudotachylites have essentially the same frictional-melt
origin as tectonic ones. One possible way to distinguish between
them may be size. Bodies of tectonic pseudotachylite
tend to be linear and less than a few meters wide (Sibson,
1975; Spray, 1995). Impact-produced pseudotachylites, at
least at Sudbury and Vredefort, form more irregular bodies,
some of which may reach tens to hundreds of meters in size
(Thompson and Spray, 1994; Spray and Thompson, 1995).
Another problem, even within the study of impact-produced
breccias, is that the term “pseudotachylite” has been
used to designate different types of impact-produced breccias
formed at different stages (and possibly by different
mechanisms) during crater formation (Martini, 1991;
Reimold, 1995; Dressler and Sharpton, 1997). One suggestion
(Martini, 1991) is to use the term “type A pseudotachylite”
to designate relatively rare, small, glassy veins,
typically less than a centimeter wide, that contain fragments
in a matrix of melted material, often accompanied by shockproduced
high-pressure mineral polymorphs such as coesite
and stishovite (Martini, 1991). Such veins are believed to
form during the early, higher-pressure, compressive stages
of shock-wave expansion. In contrast, the more abundant,
widespread, and more intensely studied material (called “type
B pseudotachylite”) is thought (Martini, 1991) to form later,
during crater modification and central uplift formation, probably
by friction generated by the rapid movement of large
volumes of target rock below the crater.
Pseudotachylite breccias (especially the more familiar “type
B” variety) are distinctive and recognizable at Vredefort and
Sudbury, but their wider use as unique indicators of impact
is complicated by several factors. First, since they form below
the original crater floor, they are found only in impact
structures that have been deeply enough eroded to expose
target rocks originally located beneath the crater, and
pseudotachylites are usually restricted to the central-uplift
regions of larger structures. Second, pseudotachylites resemble
rocks formed by nonimpact processes, and the
distinction is difficult unless definite preserved shockmetamorphic
effects can be found. The current confusion in
terminology and formation mechanisms, combined with the
scarcity of distinctive shock effects in many impact-produced
pseudotachylites, makes it difficult to use pseudotachylites
by themselves as unique indicators of impact structures.
Despite these problems, well-developed pseudotachylites
may still be a useful field tool for identifying possible
impact structures for more detailed study. Pseudotachylites
can be widespread in impact structures, and their distinctive
appearance can survive even high-grade metamorphism
(Fig. 5.2). The striking irregular and anastomosing character
of pseudotachylite bodies, their rounded inclusions (often
altered at the rims), their development over large areas,
and the frequent absence of a regular shape or of compressional
effects typical of similar fault-related breccias make
them a valuable field indicator of a possible impact structure,
and their discovery should be followed up with an intensive
search for more definite shock effects. In addition, melt-rich
pseudotachylite breccias in established impact structures
have proven valuable for determining the formation ages
of the structures themselves (Spray et al., 1995; Kelley and
Spray, 1997).
5.4. CRATER INTERIOR: CRATER-FILL
DEPOSITS (BRECCIAS AND
MELT ROCKS)
5.4.1. Formation Conditions
During the modification stage, material excavated from
various locations in the growing transient crater is deposited
within the final crater to form crater-fill deposits of breccia
and melt rock. These allogenic units consist of four main
components: (1) material ejected ballistically on steep or
near-vertical trajectories that impacts within the final crater;
(2) large and small bodies of impact melt that do not travel
beyond the rim of the final crater; (3) large and small fragments
of unshocked target rock that collapse from the
oversteepened walls and rim of the original transient crater;
(4) ejecta originally deposited near the transient crater rim
and caught up in the subsequent collapse.
As a result of these processes, the final crater is partially
filled with a complex mixture of rock fragments (shocked
and unshocked) together with bodies of impact melt. These
deposits consist mostly of crater-fill breccias, often accompanied
by discrete units of impact melt rocks. In small, bowlshaped,
simple craters, the various components tend to be
mixed together, and the final deposit may fill the crater to
70 Traces of Catastrophe
about half its depth. [This crater-fill unit is also called the
breccia lens because of its shape (Fig. 3.7).] In larger complex
structures, particularly those formed in crystalline target
rocks, the crater-fill rocks typically contain discrete units
of breccias and impact melts that form a large annular deposit
around the central uplift (Fig. 3.13).
Subsequent to formation of the crater and the deposition
of impact-produced crater-fill breccias, the structure may
be filled, and the breccias buried, by younger crater-fill sediments
deposited more slowly by the conventional processes
of erosion, transport, and deposition. These sediments not
only preserve the underlying impact-produced breccias, but,
because of their circular outcrop pattern and often anomalous
character, they may call attention to previously unsuspected
impact structures. In this section, the discussion and
the term “crater-fill deposits” are limited only to the impactproduced
breccias that fill the crater during and immediately
after formation and do not include any ordinary
sediments that may also be present.
Many of the individual fragments in the crater-fill deposits
have been derived from within the zone of crater excavation
(Fig. 3.4) and may be highly shocked. Much of the
target rock within the excavation zone is subjected to relatively
high shock pressures of about 5 GPa to >100 GPa.
The lowest pressures in this range are sufficient to shatter
and brecciate the target rocks extensively; at higher pressures,
the rocks are deformed and melted as well. Shocked
Fig. 5.7. Crater-fill breccias. Recent drill coring along the southern
flank of the Chicxulub structure (Mexico), has recovered impact
breccias and melt rocks only shallowly buried beneath the younger
carbonate sediments. This mosaic shows the sequence of diverse
crater-fill breccias retrieved from the UNAM-5 drill core located
near the village of Santa Elena in southern Yucatán, ~112 km from
the center of the basin. The core pieces are arranged so that each
represents 10 m of core. The top of the impact sequence (top of
picture) occurs at a depth of ~330 m below the surface and is
characterized by a 30-m interval of highly vesicular and pulverized
impact melt rock (M). The melt rock horizon is almost completely
altered to clay but contains abundant clasts of the target rock
assemblage. Below this horizon is a varicolored continuous unit of
suevite breccia (SB). As is typical of suevites, this unit has a clastic
matrix containing a substantial proportion of highly shocked and
melted clasts derived from lithologies that were originally deep
within the target assemblage. The upper 50 m of the UNAM-5
suevite (SB1) is characterized by abundant, centimeter-scale clasts
of vesicular melt rock, similar to that of the overlying melt horizon
but less altered. The middle 50 m of the suevite (SB2) is dominated
by larger clasts of shocked to partially melted silicate basement
rock showing abundant evidence of shock deformation. The matrix
of the lower section of suevite (SB3) is more melt-rich and contains
a greater proportion of centimeter-scale silicate clasts. Total depth
was reached at the UNAM-5 well while still in the suevite. Coin is
~3 cm in diameter. Photograph courtesy of V. L. Sharpton.
SB3 SB2 SB1 M
330 m
Shock-Metamorphosed Rocks (Impactites) in Impact Structures 71
rock fragments, derived from this zone and deposited in the
crater-fill breccias, have provided the best evidence for the
impact origin of numerous structures.
The crater-filling process is both rapid and chaotic, and
mixing of the different components is not complete. The
crater-fill deposits therefore contain a variety of distinctive
allogenic breccias and melt rocks (Fig. 5.7). The simple classification
used below is based on (1) fragment lithologies
(lithic vs. melt-fragment breccias; (2) nature of the matrix
(clastic vs. melt-matrix). (For more detailed discussions and
classifications, see, e.g., Stöffler et al., 1979; Taylor et al., 1991;
Stöffler and Grieve, 1994, 1996.)
5.4.2. Lithic Breccias (Allogenic)
Melt-free breccias (lithic breccias) form a common and
distinct lithology in both large and small impact structures
(Figs. 3.7 and 3.13). In small impact structures, e.g., Brent
(Canada) (Dence, 1968; Grieve and Cintala, 1981), lithic breccias
may form units hundreds of meters thick that extend
over much of the final crater. At the larger Ries Crater
(Germany), a distinctive allogenic polymict lithic breccia
[the Bunte (“colored”) Breccia] occurs beneath the overlying
melt-bearing suevite breccias both inside and outside
the crater (Hörz, 1982; Hörz et al., 1983), with a sharp contact
between the two units. In some impact structures, especially
those formed in carbonate target rocks, lithic breccias
may be the only type of crater-fill material present (Roddy,
1968; Reiff, 1977).
Lithic breccias consist of rock and mineral fragments in a
clastic matrix of finer-grained similar material (Fig. 5.. The
breccias are poorly sorted; fragment sizes generally range from
<1 mm to tens of meters. Fragments are typically sharp to
angular in appearance. Unlike the lithic breccias found in
parautochthonous rocks, crater-fill lithic breccias are more
apt to be polymict because their fragments have been derived
from a wider region of the original target rocks. Because
most of the material in lithic breccias is derived from
less-shocked regions around the walls and rim of the transient
crater, distinctive shock effects are only rarely observed
in the fragments.
Within the crater-fill deposits, lithic breccias are often
associated, both horizontally and vertically, with units that
contain a melt component as discrete fragments or as a matrix
for lithic fragments. Breccias with a few percent or more
of a melt component are regarded as melt-bearing breccias,
but the transition between these breccia types appears continuous,
and no formal boundary has been established. Such
melt-bearing breccias typically form a smaller proportion of
the crater fill, perhaps 10–25 vol%, and the amount of melt
component they contain varies from a few percent to
>90 vol% (e.g., Hörz, 1982; Masaitis, 1983; von Engelhardt,
1990, 1997).
Two basically different types of melt-bearing breccias can
be distinguished. In melt-fragment breccias (suevites), the
melt component occurs as large (centimeter-sized) discrete
bodies; in melt-matrix breccias (impact melt breccias), the
melt forms a matrix for rock and mineral fragments (Stöffler
and Grieve, 1994, 1996).
5.4.3. Melt-Fragment Breccias (Allogenic) (Suevites)
Melt-fragment breccias (suevites, pronounced “SWAYvites”)
are composed of discrete fragments of rocks and minerals,
together with bodies of melt, in a clastic matrix of similar
but finer-grained materials. Many of the rock and mineral
Fig. 5.8. Crater-fill breccia; lithic breccia. Poorly sorted crater-fill lithic breccia composed of angular to sharp fragments of granitic
rocks and constituent minerals (quartz, feldspar, etc.) in a finer clastic matrix. Drill core sample from the Brent Crater (Canada). Photograph
courtesy of R. A. F. Grieve (cross-polarized light).
0.1 mm
72 Traces of Catastrophe
Fig. 5.9. Crater-fill breccia; suevite. Large hand specimen, about 45 cm long, of typical fresh suevite from the Ries Crater (Germany)
(Otting quarry). The specimen consists of irregular and contorted individual fragments of glass (dark), which show a roughly parallel
elongation, and crystalline rock fragments (light) in a fine clastic matrix. The glass fragments, which range up to 5 cm in size, are
composed of a mixture of rock and mineral fragments in heterogeneous, flow-banded glass. Photograph courtesy of D. Stöffler.
Fig. 5.10. Crater-fill breccia; suevite. Suevite breccia from Nicholson Lake (Canada), containing glass fragments (dark) with rock and
mineral clasts in a finer fragmental matrix. The glass fragments are heterogeneous mixtures of mineral clasts (light) in dark, flow-banded
glass. Photograph courtesy of M. R. Dence (plane-polarized light).
5 mm
Shock-Metamorphosed Rocks (Impactites) in Impact Structures 73
fragments are highly shocked, and these breccias often provide
the most distinctive evidence for a meteorite impact
origin of the structures in which they are found.
The term suevite was originally applied to melt-fragment
breccias from the type occurrence at the Ries Crater (Germany),
a relatively young (15 Ma) and well-preserved structure
24 km across, in which well-exposed suevites and other
impactites have been extensively studied and drilled (for reviews,
see von Engelhardt et al., 1969; von Engelhardt and
Graup, 1984; von Engelhardt, 1990, 1997). Suevite breccias
are found both inside the structure (crater suevite or fallback
suevite) and as preserved ejecta deposits (ejecta or fallout
suevite) as far as 40 km from the center of the Ries structure.
Suevite breccias from the Ries Crater and other impact
structures typically consist of large (centimeter-sized)
and smaller glassy bodies (typically 5–15 vol%), together
with rock and mineral clasts in a matrix of finer fragments
(Figs. 5.9 and 5.10). Glass-rich suevites are also known, in
which the glass fragments may make up >50 vol% of the
rock (Masaitis, 1994). Individual rock and glass fragments
typically range from a maximum size of 10–20 cm down to
submillimeter dimensions.
The glassy bodies in the fallout suevite beyond the Ries
Crater rim typically show irregular to contorted shapes and
textures (Hörz, 1965). These bodies are typically heterogeneous,
consisting of a polymict mixture of rock and mineral
clasts (frequently highly shocked or partially melted) in a
matrix of glass that may be compositionally heterogeneous
and often shows well-developed flow structure (Fig. 5.11).
At the Ries Crater, the larger (5–20 cm) glassy fragments
in the ejecta deposits outside the structure, called Fladen,
show a grooved and lobate flow structure that is evidence of
aerodynamic sculpturing during their flight through the
atmosphere (Hörz, 1965). These bodies also show brittle fractures
developed on landing, implying that they were solid
when they struck the ground. In contrast, glass bodies in the
crater suevite are smaller (normally <5 cm) and lack distinctive
sculpturing, implying that they did not travel through
the atmosphere for any significant length of time (Fig. 5.12)
(von Engelhardt and Graup, 1984; von Engelhardt, 1990).
Although the Ries suevites are the best-known and most
intensely studied examples of this rock type, impressive
suevite breccias have been recognized in many other impact
structures. However, in many of these structures, erosion has
largely removed the ejecta deposits outside the crater, and
the suevites occur only as crater-fill units, where they are
associated with, and often interbedded with, lithic breccias
and impact-melt rocks. Examples include Brent (Canada)
(Dence, 1965, 1968; Grieve, 1978); Rochechouart (France)
(Kraut and French, 1971); Popigai (Russia) (Masaitis et al.,
1980; Masaitis, 1994); Manson (Iowa) (Koeberl and Anderson,
1996a; Koeberl et al., 1996b); Gardnos (Norway) (French
et al., 1997); Slate Islands (Canada) (Dressler and Sharpton,
1997); and Roter Kamm (Namibia) (Reimold et al., 1997a).
The Onaping Formation, a complex and metamorphosed
Fig. 5.11. Crater-fill breccia; suevite; glassy inclusion. Heterogeneous, fragment-rich glassy fragment (Fladen) in suevite breccia from
Lake Mien (Sweden), showing complex, multiple layering with varying amounts of rock and mineral inclusions. The mineral inclusions
are typically sharp to angular and do not show the phenocryst shapes that are typically observed in glassy volcanic rocks. The generally
laminar flow-banding is emphasized by a sharp difference in clast content and by dark streaks that may represent decomposed and melted
opaque minerals. Note that flow-banding in the clast-rich layers (e.g., top) is more highly contorted. Sample NBS-61-0487 (planepolarized
light).
1 mm
74 Traces of Catastrophe
Fig. 5.12. Crater-fill breccia; suevite. Typical poorly sorted suevite
breccia in a core sample from the Nördlingen deep drill hole
(369.9 m depth), Ries Crater (Germany). The unit contains crystalline
rock fragments (light-colored) and glassy fragments (Fladen)
(dark) in a fine clastic matrix. Inclusion at upper left contains a
rock fragment (core) surrounded by a rim of flow-banded glass.
Specimen is 10 cm wide. Photograph courtesy of H. Newsom.
crater-fill unit at the 1.85-Ga Sudbury (Canada) impact
structure, contains the oldest suevite unit identified so far
(Fig. 5.13) (French, 1968b; Muir and Peredery, 1984;
Avermann, 1994).
Because of their high melt content and the occurrence of
individual glassy bodies, suevite breccias resemble conventional
volcanic breccias, and the suevite from the Ries Crater
was considered to be a volcanic tuff for nearly two
centuries. However, suevites differ from volcanic breccias in
several ways, both in hand specimen and microscopically.
Fragments in suevites show no volcanic textures; such typical
volcanic features as feldspar phenocrysts or corroded
quartz phenocrysts are absent (Figs. 5.10, 5.11, 5.14, and
5.15). Rock fragments in suevites are not deep-seated volcanic
xenoliths but are derived entirely from the underlying
shallow target rocks. Suevites often contain cored inclusions,
composite fragments in which a rim of glass is wrapped
around a fragment of basement rock, indicating that both
rock and melt were ejected into the air at the same time
(Figs. 5.16, 5.17, and 5.18). Most convincing is the presence
of unique high-pressure shock-metamorphic effects
(such as PDFs in quartz or the high-pressure minerals coesite
and stishovite), in rock and mineral inclusions in the suevite.
High-temperature melting effects, e.g., the formation of silica
glass (lechatelierite) from quartz, may also be present in the
glass fragments in suevite.
Despite their widespread distribution, suevite breccias are
not found in all meteorite impact structures. In some cases,
their absence is probably due to erosion, which has removed
these near-surface deposits from the structure. However, the
nature of the target rocks also seems important in determining
whether suevites are formed (Kieffer and Simonds, 1980;
Grieve and Cintala, 1992). Suevites have so far been observed
only in impact structures formed largely or entirely in crystalline
silicate rocks, possibly because these rocks melt to
produce cohererent and durable bodies of glass. No suevite
deposits have yet been found in impact structures formed in
carbonate rocks, in which decarbonation and volatile loss,
rather than melting, would be important.
5.4.4. Melt-Matrix Breccias (Impact-Melt Breccias)
Suevites inside the crater are closely associated with a different
type of melt-bearing breccia: melt-matrix breccias
or impact-melt breccias. In these units, the melt occurs, not
as individual fragments, but as a matrix that typically makes
up 25–75 vol% of the rock and may range from glassy material
to completely crystalline igneous rock. The fragments,
which consist of target rocks and minerals, are frequently
shocked or melted.
Impact-melt breccias form distinct bodies of widely varying
size, from small glassy inclusions in suevite breccias to
distinct dike-like and sill-like units tens to hundreds of meters
thick. As the melt component increases, impact-melt breccias
grade into impact melt rocks (see Chapter 6), in which
the melt component is dominant and the included fragments
are minor or entirely absent. These rocks often have the appearance
of conventional igneous rocks.
5.5. CRATER RIM ZONE AND PROXIMAL
EJECTA DEPOSITS
The region near the rim of the transient crater is subjected
to relatively low shock pressures (typically <1–2 GPa
in smaller structures; Fig. 3.4) (Kieffer and Simonds, 1980).
These pressures are high enough to fracture and brecciate
target rocks but are too low to produce unique shock-deformation
features in them. The dominant effects in this region
are related to the excavation of the crater and the ejection
of material from it. In simple craters, which are only slightly
larger than the original transient crater, the rim is characterized
by structural uplift (and even overturning) of the target
rocks that occurs during development of the original transient
crater (Fig. 3.3). Even though much of this original
transient crater rim may collapse into the final crater during
modification, significant uplift may be preserved, especially
in smaller and younger craters (e.g., Shoemaker, 1963; Roddy
et al., 1975; Roddy, 1978). Such rim uplift and overturnShock-
Metamorphosed Rocks (Impactites) in Impact Structures 75
Fig. 5.13. Crater-fill breccia; suevite, metamorphosed. Typical exposure of Onaping Formation “Black Member,” showing centimetersized
fragments of rock fragments and contorted recrystallized glassy inclusions in a black fragmental matrix. Despite color differences,
the unit has a strong resemblance to fresh suevite from the Ries Crater (Germany) (see Fig. 5.9). Exposure located at “Black Member”
type locality at Onaping Falls (Highway 144, Dowling Township) in the northwestern part of the Sudbury structure (Canada). Diameter
of coin near large glassy inclusion is about 2 cm. Photograph courtesy of J. Guy-Bray.
Fig. 5.14. Crater-fill breccia; suevite, heterogeneous glasses. Complex heterogeneous glassy breccia from West Clearwater Lake
(Canada), composed of distinct areas of light- and dark-colored mixed glasses, which show short-range turbulent flow and mixing.
The glassy areas contain abundant small rock and mineral fragments. Photograph courtesy of M. R. Dence (plane-polarized light).
0.1 mm
76 Traces of Catastrophe
Fig. 5.15. Crater-fill breccia; suevite, metamorphosed. Heterogeneous glassy breccia consisting of fragments of recrystallized glass,
together with rock and mineral fragments, in a fine opaque carbon-bearing matrix. Despite greenschist-level metamorphism, the glassy
fragments still preserve original melt textures such as flow banding and vesicles (now filled with chlorite; gray). Many of the fragments
display sharp crosscutting fractures, indicating that they were cool and brittle when deposited. The rock and mineral clasts represent
broken basement (target) rocks; no typical volcanic textures (phenocrysts, etc.) are observed. Discrete fragments as small as 5 μm across
can be distinguished in the opaque matrix. Onaping Formation “Black Member,” from type locality at Onaping Falls (Highway 144,
Dowling Township), northwestern corner of Sudbury structure (Canada). Sample CSF-66-36-1 (plane-polarized light).
Fig. 5.16. Crater-fill breccia; suevite, “cored” inclusion. Large flow-banded fragment (about 15 cm long) from a larger glassy inclusion
in the suevite unit of the Ries Crater (Germany) (Bollstadt quarry). The specimen is a composite or “cored” inclusion containing a large
block of shocked and fractured crystalline rock (light) surrounded by dark, flow-banded glass. Photograph courtesy of F. Hörz.
0.5 mm
Shock-Metamorphosed Rocks (Impactites) in Impact Structures 77
Fig. 5.17. Crater-fill breccia; suevite, “cored” inclusion. Composite (cored) inclusion in Onaping Formation “Black Member” in
northwestern corner of Sudbury structure (Canada). Inclusion consists of a core fragment of crystalline granitic rock (light-colored)
surrounded by flow-banded glassy material, now recrystallized. Similar inclusions are observed in fresher suevite deposits, e.g., at the Ries
Crater (Germany) (see Figs. 5.12 and 5.16). A separate angular granitic fragment appears at lower right. Coin at left of inclusion is about
2 cm in diameter. Exposure located at “Black Member” type locality at Onaping Falls (Highway 144, Dowling Township). Photograph
courtesy of J. Guy-Bray.
Fig. 5.18. Crater-fill breccia; suevite, “cored” inclusion. Composite rock fragment in metamorphosed suevite unit. The fragment contains
a core of fine-grained granitic basement rock surrounded by a rim of microcrystalline recrystallized glass. The fragment is associated
with smaller individual clasts of glassy material and rock and mineral fragments in a black, opaque, carbon-bearing matrix. Onaping
Formation “Black Member,” from type locality at Onaping Falls (Highway 144, Dowling Township), northwestern corner of Sudbury
structure (Canada). Sample CSF-66-36-2 (cross-polarized light).
0.1 mm
78 Traces of Catastrophe
ing are only rarely observed in volcanic explosion structures
such as maars and diatremes, and the presence of such rim
deformation provides a strong indication of an impact origin
for a structure.
In a newly formed crater the rim and the surrounding
region are generally covered with allogenic ejecta ejected from
the growing transient crater (Melosh, 1989; Chapter 6). Two
kinds of ejecta deposits can be distinguished: those deposited
near the crater (proximal ejecta) and those distant from
the crater (distal ejecta).
Most of the material ejected beyond the crater rim is deposited
near the crater (Melosh, 1989, p. 90). In terms of
crater radius (Rc, the distance from the center of the crater
to the final rim), approximately half the ejecta is deposited
within 2 Rc from the center (or 1 Rc from the rim) to form a
continuous ejecta blanket that may be tens to hundreds of
meters thick, depending on the size of the crater. At greater
distances, the ejecta unit becomes thinner and increasingly
discontinuous; most of the ejecta (>90%) is deposited within
about 5 Rc. (This value may serve as an arbitrary boundary
between proximal and distal ejecta.) Because many of the
fragments in the ejecta deposits were originally close to the
impact point, they are often distinctively shocked and melted.
Ejecta blankets, where they are preserved, may therefore
provide the best and most accessible evidence for an impact
origin of the structure.
Ejecta deposits around impact craters are not homogeneous,
but are made up of distinct lithologic units derived
from different regions of the transient crater and transported
by different mechanisms to the site of deposition. Mixing
during the ejection and deposition process is not complete,
and the ejecta deposits that surround a crater contain the
same diversity of rock types that are found as crater fill within
the structure: lithic breccias, suevites, and impact melt rocks.
In large impact structures, the ejecta deposits preserved outside
the crater contain a recognizable sequence of different
lithologies. The sequence at the Ries Crater (Germany) (see
von Engelhardt, 1990, 1997, and references therein) contains
a lower unit of polymict lithic melt-free breccia (Bunte Breccia)
overlain by melt-bearing breccia (suevite). Some of the
ejecta at the Ries also occurs as large (tens to hundreds of
meters in size) limestone blocks ejected intact from the crater
and skidded for many kilometers across the surrounding
ground surface (von Engelhardt, 1990, pp. 264–265).
In impact structures formed on land, the near-surface
regions are quickly removed by erosion, and the distinctive
rim uplift and ejecta deposits are observed only at relatively
young structures such as the Barringer Meteor Crater (Arizona)
(age 50 ka) (Shoemaker, 1963) and the Ries Crater
(Germany) (age 15 Ma) (von Engelhardt, 1990). At older
structures (e.g., Dence, 1965, 1968), distinctively shocked
rocks tend to be preserved in only two areas: in the target
rocks immediately beneath the crater floor, and in the breccia
and melt deposits that fill the crater itself.
5.6. DISTAL EJECTA
Although most of the material (about 90 vol%) ejected
from the crater is deposited relatively close (<5 Rc) to the
crater (Melosh, 1989, p. 90), a significant amount (about
10 vol%) may travel to even greater distances (>5 Rc) to form
deposits of distal ejecta. Where an atmosphere is present, as
in terrestrial impact events, a combination of disruption of
the atmosphere by the impact fireball, ballistic ejection from
the crater, and subsequent atmospheric transport can distribute
the smaller ejecta particles (typically >1 mm) to regional
or even global distances (Alvarez et al., 1995). The
resulting deposits, usually less than a few centimeters thick,
may contain distinctive evidence for impact: shocked rock
and mineral fragments, distinctive chemical and isotopic signatures,
and unusual glassy objects. It has thus become possible
to recognize debris from a given impact structure over
a large area of Earth, and even to establish the existence of a
major impact event from a globally distributed ejecta layer
before the structure itself could be located.
Although few layers of distal ejecta have been identified,
they have been critical to recognizing large impact structures
and determining their age. Coarse ejecta (millimeterto
centimeter-sized fragments) from the Acraman structure
(Australia) (D = 90 km) has been recognized as a discrete
layer several centimeters thick at distances of 300–400 km
from the site (Gostin et al., 1986; Williams, 1986). Ejecta
from the Manson structure (Iowa) (D = 36 km) has been
recognized more than 250 km away (Izett et al., 1993). The
most striking and best-known example of distal ejecta is the
thin layer of material ejected from the Chicxulub structure
(Mexico) and distributed worldwide to form the K/T boundary
layer (Alvarez et al., 1980; papers in Sharpton and Ward,
1990, and in Ryder et al., 1996). The occurrence in this layer
of shocked quartz grains and small spherules of melted target
rock, accompanied by an anomalously high content of
the element iridium (derived from the projectile), provided
conclusive evidence that a large meteorite impact had occurred
at the end of the Cretaceous Period, even before the
Chicxulub impact structure itself was identified. The layer
also provided key geochemical and geochronological evidence
to demonstrate that the Chicxulub structure was identical in
age to the K/T boundary and that it was also the source for
the global ejecta layer itself.
Generally, ejecta found at greater distances from the crater
displays a higher level of shock effects, and much distal
ejecta consists of small fragments of melted target rock. One
peculiar and much-studied variety of distal ejecta is tektites
and microtektites, small (centimeter- to millimeter-sized)
bodies of pure glass that have been ejected from a few impact
structures and spread over areas (strewnfields) that may
be thousands of kilometers in extent (see Chapter 6).