FUNDAY

4.1. FORMATION CONDITIONS AND
GENERAL CHARACTERISTICS
The growing recognition since the 1960s of the geological
importance of meteorite impact events, and the large
number of impact structures still preserved on Earth, is largely
the result of two related discoveries: (1) The extreme physical
conditions that are imposed by intense shock waves on
the rocks through which they pass produce unique, recognizable,
and durable shock-metamorphic effects; (2) such
shock waves are produced naturally only by the hypervelocity
impacts of extraterrestrial objects (French, 1968a, 1990b;
French and Short, 1968). Shock-metamorphic effects (also
called “shock effects” or “shock features”) have been critical
to the identification of terrestrial impact structures because
of their uniqueness, wide distribution, ease of identification,
and especially their ability to survive over long periods of
geologic time.
With the acceptance of shock effects as a criterion for
impact, the record of terrestrial impact events is no longer
limited to small young structures that still preserve definite
meteorite fragments. Equally convincing evidence for impact
can now be provided by a wide variety of distinctive
deformation effects in the rocks themselves, and it has become
possible to identify numerous old impact structures
from which weathering and erosion have removed all physical
traces of the projectiles that formed them. The recognition
of preserved shock effects has been the main factor
behind the steady increase in the number of recognized impact
structures since the 1960s (Grieve, 1991; Grieve et al.,
1995; Grieve and Pesonen, 1992, 1996; for historical reviews,
see Hoyt, 1987; Mark, 1987).
The approximate physical conditions that produce shockdeformation
effects in natural rocks have been established
by a combination of theoretical studies, artificial explosions
(both chemical and nuclear), and experiments with laboratory
shock-wave devices (for details, see papers in French
and Short, 1968 and Roddy et al., 1977; also Stöffler, 1972;
Kieffer and Simonds, 1980; Melosh, 1989; Stöffler and
Langenhorst, 1994). Peak shock pressures produced in an
impact event range from >2 GPa ( >20 kbar) near the final
crater rim to >100 GPa (>1000 kbar) near the impact point.
These pressures, and the resulting shock-deformation effects,
reflect conditions that are far outside the range of normal
geological processes (Fig. 4.1, Table 4.1). In ordinary
geological environments, pressures equivalent to those of
typical shock waves are attained only under static conditions
at depths of 75–1000 km within Earth, well below the shallow-
crustal regions in which impact structures are formed.
Shock-wave pressures differ in other important ways from
pressures produced by more normal geological processes. The
application of shock-wave pressures is both sudden and brief.
A shock wave traveling at several kilometers per second will
traverse the volume of a mineral grain or a rock sample in
microseconds, and both the onset and release of pressure are
highly transient. Shock-deformation effects therefore reflect
transient stress conditions, high strain rates, and rapid
quenching that are inconsistent with the rates of normal
geological processes (Table 4.1). In addition, shock waves
deposit energy in the materials through which they pass. A
particular shock pressure will produce a specific postshock
temperature, which depends chiefly on the nature of the target
material. These postshock temperatures increase with increasing
shock pressure (see the P-T curve labeled “Shock
metamorphism” in Fig. 4.1). For large shock pressures, the
resulting temperatures are high enough to produce melting
and even vaporization within the target.
The unique conditions of shock-wave environments produce
unique effects in the affected rocks. The nature and
intensity of the changes depend on the shock pressures
Shock-Metamorphic Effects in
Rocks and Minerals
32 Traces of Catastrophe
Fig. 4.1. Conditions of shock-metamorphism. Pressure-temperature plot showing comparative conditions for shock metamorphism
and normal crustal metamorphism. [Note that the pressure axis (X-axis, in GPa) is logarithmic.] Shaded region at lower left (P < 5 GPa,
T < 1000°C) encloses the conventional facies (labeled) for crustal metamorphism. Shock-metamorphic conditions (at right) extend from
~7 to >100 GPa and are clearly distinct from normal metamorphic conditions. Approximate formation conditions for specific shock
effects (labeled) are indicated by vertical dashed lines, and the exponential curve (“Shock metamorphism”) indicates the approximate
postshock temperatures produced by specific shock pressures in granitic crystalline rocks. Relatively high shock pressures (>50 GPa)
produce extreme temperatures, accompanied by unique mineral decomposition reactions (at left, near temperature axis). Stability curves
for high-pressure minerals (coesite, diamond, stishovite) are shown for static equilibrium conditions; formation ranges under shock
conditions may vary widely. (Adapted from Stöffler, 1971, Fig. 1; Grieve, 1990, p. 72; Grieve and Pesonen, 1992, Fig. 9.)
TABLE 4.1. Shock metamorphism: Distinction from other geological processes.
Regional and Contact Metamorphism;
Characteristic Igneous Petrogenesis Shock Metamorphism
Geological setting Widespread horizontal and vertical regions Surface or near-surface regions of Earth’s crust
of Earth’s crust, typically to depths of 10–50 km
Pressures Typically <1–3 GPa 100–400 GPa near impact point; 10–60 GPa in large
volumes of surrounding rock
Temperatures Generally >1000°C Up to 10,000°C near impact point (vaporization);
typically from 500° to 3000°C in much of
surrounding rock
Strain rates 10–3/s to 10–6/s 104/s to 106/s
Time for completion From 105–107 yr “Instantaneous”: Shock-wave passage through 10-cm
of process distance, <10–5 s; formation of large (100-kmdiameter)
structure <1 hr
Reaction times Slow; minerals closely approach equilibrium Rapid; abundant quenching and preservation of
metastable minerals and glasses
Shock-Metamorphic Effects in Rocks and Minerals 33
(Table 4.2). Lower shock pressures (~2–10 GPa) produce
distinctive megascopic shatter cones in the target rocks
(Milton, 1977; Roddy and Davis, 1977). Higher pressures
(>10–45 GPa) produce distinctive high-pressure mineral
polymorphs as well as unusual microscopic deformation features
in such minerals as quartz and feldspar (Stöffler, 1972).
Even higher pressures (>50 GPa) produce partial to complete
melting and even vaporization (>100 GPa) of large
volumes of the target rocks.
An especially distinctive and convincing form of evidence
for meteorite impact is the suite of unique microscopic deformation
features produced within individual minerals by
higher-pressure (~10–45 GPa) shock waves. During the
impact event, such pressures develop in target rocks near the
center of the crater, and most of these rocks are immediately
broken up and incorporated into the excavation flow that is
being initiated by the expanding release wave (Figs. 3.4 and
3.5). As a result, these shock effects are found chiefly in individual
target rock fragments in the breccias that fill the
crater or in the ejecta deposited beyond the rim.
A wide variety of shock-produced microscopic deformation
features has been identified in the minerals of shockmetamorphosed
rocks (for reviews, see Chao, 1967; papers
in French and Short, 1968; Stöffler, 1972, 1974; Stöffler
and Langenhorst, 1994; Grieve et al., 1996). These include
(1) kink bands in micas and (more rarely) in olivine and
pyroxene; (2) several types of distinctive planar microstructures
and related deformation effects in quartz, feldspar, and
other minerals; (3) isotropic mineral glasses (diaplectic or
thetomorphic glasses) produced selectively, most commonly
from quartz and feldspar, without actual melting; (4) selective
melting of individual minerals. Kink bands, although
common in impact environments (Fig. 4.2), can also be produced
by normal tectonic deformation; they are not a unique
criterion for shock metamorphism, and they will not be discussed
further. The other effects, particularly the distinctive
planar microstructures in quartz and the diaplectic glasses,
are now generally accepted as unique criteria for shock waves
and meteorite impact events.
These shock-produced microscopic deformation features
have several distinctive general characteristics. They are
pervasive, and usually occur throughout a centimeter-sized
rock sample, although they may be more erratically developed
over larger distances (meters or tens of meters). They
TABLE 4.2. Shock pressures and effects.
Approximate Estimated
Shock Pressure Postshock Effects
(GPa) Temperature (°C)*
2–6 <100 Rock fracturing; breccia formation
Shatter cones
5–7 100 Mineral fracturing: (0001) and {1011}
in quartz
8–10 100 Basal Brazil twins (0001)
10 100*
Quartz with PDFs {1013}
12–15 150 Quartz ® stishovite
13 150 Graphite ® cubic diamond
20 170*
Quartz with PDFs {1012}, etc.
Quartz, feldspar with reduced refractive
indexes, lowered birefringence
>30 275 Quartz ® coesite
35 300
Diaplectic quartz, feldspar glasses
45 900 Normal (melted) feldspar glass (vesiculated)
60 >1500 Rock glasses, crystallized melt rocks (quenched
from liquids)
80–100 >2500 Rock glasses (condensed from vapor)
* For dense nonporous rocks. For porous rocks (e.g., sandstones), postshock temperatures = 700°C
(P = 10 GPa) and 1560°C (P = 20 GPa). Data from Stöffler (1984), Table 3; Melosh (1989),
Table 3.2; Stöffler and Langenhorst (1994), Table 8, p. 175.
34 Traces of Catastrophe
Fig. 4.2. Kink-banding; in biotite. Large biotite grain in basement granitic gneisses, northeast side of Sudbury structure (Canada),
showing two sets of kink-banding at high angles to original cleavage (horizontal). Associated quartz (upper and lower left) and feldspar
show no shock-deformation effects. Sample CSF-68-67 (cross-polarized light).
Fig. 4.3. Progressive shock metamorphism in sandstone (I). Unshocked Coconino Sandstone from the Barringer Meteor Crater
(Arizona) is composed of well-sorted quartz grains with minor carbonate cement and pore space. The quartz grains are rounded to
angular, clear, and undeformed; some grains display secondary overgrowths. (Black dots are bubbles in thin section mounting medium.)
Ejecta sample from rim of crater. Sample MCF-64-4 (plane-polarized light).
0.5 mm
0.5 mm
Shock-Metamorphic Effects in Rocks and Minerals 35
Fig. 4.4. Progressive shock metamorphism in sandstone (II). Moderately shocked Coconino Sandstone from the Barringer Meteor
Crater (Arizona). The quartz grains are highly fractured and show numerous sets of subparallel fractures along cleavage planes. The
original interstitial pore space has been collapsed and heated during passing of the shock wave, producing a filling of dark glass that
frequently contains coesite. Ejecta sample from ground surface outside crater. Sample MCF-65-15-4 (plane-polarized light).
Fig. 4.5. Progressive shock metamorphism in sandstone (III). Highly shocked, melted, and vesiculated Coconino Sandstone from the
Barringer Meteor Crater (Arizona). The original sandstone has been converted to a light, frothy, highly vesicular pumice-like material
composed dominantly of nearly pure silica glass (lechatelierite). The vesicular glass contains a few remnant quartz grains (e.g., upper
center, arrow) that are highly fractured and show development of distinctive PDFs in addition to the open cleavage cracks. Ejecta sample
from ground surface outside crater. Sample MCF-65-11-2 (plane-polarized light).
0.5 mm
0.5 mm
36 Traces of Catastrophe
are mineralogically selective; a given effect (e.g., isotropization)
will occur in grains of a single mineral (e.g., quartz or
feldspar), but not in grains of other minerals, even adjacent
ones. Shock metamorphism is also characterized by a progressive
destruction of original textures with increasing shock
pressure, a process that eventually leads to complete melting
or vaporization of the target rock (Figs. 4.3, 4.4, and 4.5).
4.2. STAGES OF SHOCK METAMORPHISM
The fact that different shock pressures produce a variety
of distinctive shock features (Table 4.2) has made it possible
to recognize different levels or stages of shock metamorphism
(Chao, 1967; Stöffler, 1966, 1971, 1984; von Engelhardt
and Stöffler, 1968; Stöffler and Langenhorst, 1994). These stages
are not equivalent to the different facies recognized in normal
metamorphism, because shock metamorphism is a rapid
and nonequilibrium process and many of the most distinctive
features produced by shock waves (e.g., high-pressure
minerals and diaplectic glasses) are metastable under normal
geological conditions. Nevertheless, key shock features
occur frequently and consistently in natural impact structures,
and the production of the same features in experimental
studies has made approximate pressure and temperature
calibrations possible. As a result, the stages of shock metamorphism
have become an important concept for field studies
of impact structures and for using certain features as approximate
shock-wave barometers.
Current classifications of shock-metamorphic stages are
based almost entirely on features developed in nonporous,
quartz-bearing, crystalline igneous and metamorphic rocks.
These lithologies are abundant in many of the impact structures
studied so far, and they develop a varied suite of shock
features over a wide range of shock pressures. Individual classifications
of shock-metamorphic stages in these rocks differ
in details, but the following summary of distinctive shock
features and their approximate shock pressures (based largely
on Stöffler, 1966, 1971, 1984; Stöffler and Langenhorst, 1994)
provides a useful classification based on field and petrographic
characteristics. [Other effects observed with increasing shock
pressure include decreases in refractive index and increasing
structural disorder (shock mosaicism) in mineral grains; for
details, see Stöffler, 1972, 1974; Stöffler and Langenhorst,
1994).] It should be remembered that estimated pressures
are only approximate, and that the formation of a given shock
effect will also reflect such individual factors as rock type,
grain size, and other structural features. The shock effects
observed, and the inferred stages of shock metamorphism,
will be different for other rock types, especially for carbonates,
basaltic rocks, and porous rocks of any type.
For nonporous crystalline rocks, the following stages have
been distinguished (see Table 4.2):
<2 GPa
Fracturing and brecciation, without development of
unique shock features (see Chapter 5).
>2 GPa to <30? GPa
Shatter cones. At lower pressures (2 to <10 GPa), occurring
without distinctive microscopic deformation features.
At higher pressures (10 to >30 GPa), shatter cones may also
contain distinctive microdeformation features.
~8 GPa to 25 GPa
Microscopic planar deformation features in individual
minerals, especially quartz and feldspar. It has been possible
to subdivide this zone on the basis of different fabrics of
deformation features in quartz (Robertson et al., 1968; Stöffler
and Langenhorst, 1994).
>25 GPa to 40 GPa
Transformation of individual minerals to amorphous
phases (diaplectic glasses) without melting. These glasses
are often accompanied by the formation of high-pressure
mineral polymorphs.
>35 GPa to 60 GPa
Selective partial melting of individual minerals, typically
feldspars. Increasing destruction of original textures.
>60 GPa to 100 GPa
Complete melting of all minerals to form a superheated
rock melt (see Chapter 6).
>100 GPa
Complete rock vaporization. No preserved materials
formed at this stage (e.g., by vaporization and subsequent
condensation to glassy materials) have been definitely identified
so far.
4.3. MEGASCOPIC SHOCK-DEFORMATION
FEATURES: SHATTER CONES
Shatter cones are the only distinctive and unique shockdeformation
feature that develops on a megascopic (hand
specimen to outcrop) scale. Most accepted shock-metamorphic
features are microscopic deformations produced at relatively
high shock pressures (>10 GPa). Lower shock pressures
(1–5 GPa) produce a variety of unusual fractured and brecciated
rocks, but such rocks are so similar to rocks formed by
normal tectonic or volcanic processes that their presence cannot
be used as definite evidence for an impact event. However,
such low shock pressures also generate distinctive conical
fracturing patterns in the target rocks, and the resulting shatter
cones have proven to be a reliable field criterion for identifying
and studying impact structures (Dietz, 1947, 1959,
1963, 1968; Milton et al., 1972, 1996a; Roddy and Davis,
1977; Sharpton et al., 1996a; Dressler and Sharpton, 1997).
Shatter cones are distinctive curved, striated fractures that
typically form partial to complete cones (Figs. 4.6 and 4.7).
They are generally found in place in the rocks below the
crater floor, usually in the central uplifts of complex impact
structures, but they are also rarely observed in isolated rock
Shock-Metamorphic Effects in Rocks and Minerals 37
Fig. 4.6. Shatter cones; small, well-developed. Small, finely sculptured shatter cones, developed in fine-grained limestone from the
Haughton structure (Canada). The cone surfaces show the typical divergence of striae away from the cone apex (“horsetailing”). Photograph
courtesy of R. A. F. Grieve.
fragments in breccia units. Shatter cones occur as individuals
or composite groups, and individual cones may range from
millimeters to meters in length (Figs. 4.7, 4.8, and 4.9) (Dietz,
1968; Sharpton et al., 1996a). Far more common, however,
are partial cones or slightly curved surfaces with distinctive
radiating striations (“horsetailing”) on them (Fig. 4.10).
The details of shatter cone morphology are also distinctive.
Smaller secondary (“parasitic”) cones commonly occur
on the surfaces of both complete and partial shatter cones,
forming a unique composite or “nested” texture. The surfaces
of shatter cones, and the striations on them, are definite
positive/negative features. The striations are also
directional; they appear to branch and radiate along the surface
of the cone, forming a distinctive pattern in which the
acute angle of the intersection points toward the apex of the
cone (Figs. 4.6, 4.8, and 4.10).
Shatter cones form in all kinds of target rocks: sandstones,
shales, carbonates, and crystalline igneous and metamorphic
rocks. The most delicate and well-formed cones form in finegrained
rocks, especially carbonates (Fig. 4.6). In coarser
rocks, shatter cones are cruder, and their striations are larger,
making the cones more difficult to recognize and distinguish
from nonshock deformational features such as slickensides
(Figs. 4.8 and 4.10).
Shatter cones, especially well-formed examples, are
easy to distinguish from similar nonimpact features (see
Table 4.3). Some shatter cone occurrences may superficially
resemble the “cone-in-cone” structures produced during
38 Traces of Catastrophe
lithification of carbonate-bearing clastic sediments. However,
the cones in cone-in-cone features have their axes normal
to the bedding of the host rocks and their apexes pointing
down. Shatter cones generally point upward, and their axes
may lie at any angle to the original bedding, depending on
the preimpact orientation of the target rock and its location
relative to the impact point. Furthermore, the occurrence of
shatter cones in a variety of rock types, especially nonsedimentary
ones, is a good indication of an impact origin.
The horsetailing striations on shatter cone surfaces sometimes
resemble slickensides formed on faults, especially when
the surfaces are approximately flat (Figs. 4.8 and 4.10). However,
unlike slickensides, shatter cone striations are nonparallel
and often show strong radiation and directionality, so
that it is easy to determine the direction of the cone apex.
Shatter cones are now generally accepted as unique indicators
of shock pressures and meteorite impact. They are
especially valuable in this role because they form at relatively
low shock pressures (typically 2–10 GPa, but perhaps as
high as 30 GPa) and therefore develop throughout a large
volume of target rock below the crater floor. They are typically
widely and intensely developed in exposed central uplifts
of large structures. Shatter cones form in a wide range
of rock types, they are resistant to subsequent metamorphism,
and (when well developed) they can be easily and immediately
recognized in the field. Frequently, an initial discovery
of shatter cones has spurred the search for, and discovery of,
a range of equally definite shock effects produced at even
higher pressures.
For well-developed shatter cones, it is possible to measure
the orientation of the cone axes and to statistically determine
the varying orientations of shatter cones throughout
an impact structure. Such measurements (e.g., Manton,
1965; Guy-Bray et al., 1966; Milton et al., 1972, 1996a)
have provided strong support for the use of shatter cones
Fig 4.7. Shatter cones; large. Large shatter cone and crudely
conical striated surfaces in Mississagi Quartzite from the South
Range (Kelley Lake) of the Sudbury structure (Canada). Cone
axes point upward and into the Sudbury Basin (toward viewer) at
a high angle. Cone axes are nearly parallel to the original bedding
in the quartzite, which dips steeply back and to the right.
Fig. 4.8. Shatter cone; huge, wellstriated.
A large shatter cone, 2–
3 m long, in quartzite in the central
uplift of the Gosses Bluff structure
(Australia). The cone axis plunges
gently to the left, nearly normal to
the original bedding in the quartzite,
which appears as parallel joints dipping
steeply to the right. Despite the
crudeness of the large cone, the direction
of the apex (right), parasitic
cones, and distinctive horsetailing are
all visible. Scale rule (at top) is 15 cm
long.
Shock-Metamorphic Effects in Rocks and Minerals 39
Fig. 4.9. Shatter cone; huge. Unusually large shatter cone (megacone) (light-colored area, center) exposed in a cliff along a wave-cut
shoreline on Patterson Island, one of the islands in the Slate Islands impact structure, Lake Superior (Canada). The huge cone, developed
in Archean felsic metavolcanic rocks, points nearly straight up and is at least 10 m in length. At the exposed base, the exposed surface of
the cone is at least 7 m wide. Only ~25° of the cone’s basal perimeter is exposed, indicating that the true width of the feature may exceed
20 m at its base. Horsetail striations and parasitic cones cover all exposed surfaces. Several other large, conical features are obvious on the
near-vertical cliff, but because of the steep scree-covered slopes these features have not yet been examined in detail. Photograph courtesy
of V. L. Sharpton.
Fig. 4.10. Shatter cones; crude,
striated surfaces. Poorly developed
shatter cones in Serpent Quartzite,
Sudbury (Canada). The cones are only
partially developed, appearing as
curved and striated surfaces. Divergence
of the striae indicates that the
cone apexes are to the right. Pen (at
center) is 12 cm long.
40 Traces of Catastrophe
as a criterion for impact. In several impact structures that
formed in originally flat-lying sediments, the apexes of shatter
cones in the rocks point inward and upward when the rocks
are graphically restored to their original horizontal preimpact
position, indicating that the source of the shock wave
that produced the shatter cones was located above the original
ground surface (Guy-Bray et al., 1966; Dietz, 1968;
Manton, 1965; Howard and Offield, 1968; Wilshire et al., 1972;
Milton et al., 1972, 1996a). More recently, shatter cones in
the Beaverhead (Idaho) structure (Hargraves et al., 1990)
have been used to reconstruct the original shape and size
of a large, ancient impact structure that was subsequently
dissected and redistributed by major faulting during the
Laramide Orogeny.
The use of shatter cones to identify impact structures requires
caution, especially in cases where no other shock effects
can be identified. Poorly developed shatter cones
(Figs. 4.8 and 4.10) can be easily confused with normal fractures
and slickensides, and the latter may be misidentified
as shatter cones. Even in well-established impact structures,
shatter cones may be entirely absent or poorly developed, or
their orientations may be locally diverse and ambiguous
(Fig. 4.11). Detailed studies of shatter cone orientations need
to be done at more impact structures where they are well
developed, but such studies need to be done with care (see,
e.g., Manton, 1965; Milton et al., 1972, 1996a).
It is a paradox that, even though shatter cones are a proven
and valuable indicator of shock metamorphism and impact
structures, the exact mechanisms by which the radiating
TABLE 4.3. Shatter cones: Distinction from other geological features.
Cone-in-Cone Shatter Cones
Conical secondary growth features formed during Conical fracture features formed by transient shock waves (P ~2 to
diagenesis; found in undisturbed sedimentary rocks. >10 GPa) and found in meteorite impact structures, typically in uplifted
central rocks.
Restricted to carbonate-bearing rocks (limestones, Found in all rock types (sedimentary, igneous, metamorphic). Best
limy shales); associated with secondary carbonate. developed in fine-grained rocks, especially limestones.
Cone axes normal to bedding planes. Cone axes oriented at any angle to bedding, depending on orientation of
rock at time of impact and on postimpact movements.
Cones oriented point-down. Cones originally form pointing in direction of source of shock wave, i.e.,
inward and upward. Orientation varies over structure. Orientation further
modified by development of central uplift or later postcrater deformation.
When beds restored to original horizontal position, cones point toward a
focus above original surface, indicating external source of shock wave.
Striations along cone surface generally continuous, Striations along cone surface typically show development of divergent
uniform. radiations (“horsetailing”) along surface. Development of secondary
(parasitic) cones on main cone is typical.
Cone surfaces are growth surfaces against other cones Cone surfaces are actual fracture surfaces; rock splits into new shatteror
fine matrix in rock. coned surfaces along cone boundaries. Unlike slickensides, striated cone
surfaces show no relative motion, fit together without displacement.
Rocks typically show no deformation, metamorphism. Frequently contain kink-banded micas or quartz (coarser grains) with
shock-produced planar deformation features (PDFs).
shock wave interacts with the target rock to generate shatter
cones have not been studied in great detail and are still not
understood (e.g., Dietz, 1968; Gash, 1971; Milton, 1977;
Sharpton et al., 1996a). A further complication in shatter
cone formation is the evidence that, although the cones themselves
form at relatively low shock pressures, localized melting
and glass formation can occur along the cone surfaces,
probably as the result of a complex combination of shock
and frictional mechanisms (Gay, 1976; Gay et al., 1978;
Gibson and Spray, 1998). Combined theoretical, experimental,
and field studies to understand the exact conditions of
shatter cone formation are a major challenge for the future.
4.4. HIGH-PRESSURE MINERAL
POLYMORPHS
When subjected to impact-produced shock waves, some
minerals in target rocks (e.g., quartz, graphite) may transform
to high-pressure minerals, just as they do under high
static pressures produced in laboratory experiments or deep
in Earth’s crust. Graphite (C) can be converted to diamond.
Quartz can be converted to stishovite at shock pressures
of >12–15 GPa and to coesite at >30 GPa (Stöffler and
Langenhorst, 1994). [These numbers illustrate one of the
many differences between shock processes and normal geological
deformation. Under conditions of static equilibrium,
where reaction rates are slower and kinetic factors less imShock-
Metamorphic Effects in Rocks and Minerals 41
Fig. 4.11. Shatter cones; small, diversely oriented. This specimen shows a group of small, well-developed shatter cones, formed in a
sample of Precambrian crystalline target rock at the Slate Islands structure (Canada). The cones show two distinct orientations, and cone
axes appear to diverge above and below the coin. This type of diverse orientation may reflect small-scale nonuniformities in the shock
waves, produced by local heterogeneities (bedding planes, joints, etc.) in the rock sample. Coin is about 2 cm in diameter. Photograph
courtesy of V. L. Sharpton.
Fig. 4.12. Diaplectic quartz glass; with coesite. Diaplectic quartz glass (clear), with strings of small, high-relief crystals of coesite (“C”).
From biotite granite inclusion in suevite breccia, Aufhausen, Ries Crater (Germany). Photograph courtesy of W. von Engelhardt (planepolarized
light).
0.1 mm
42 Traces of Catastrophe
portant, coesite forms from quartz at lower pressures
(>2 GPa) than does stishovite (10–15 GPa).]
The identification of coesite and stishovite at several sites
in the early 1960s provided one of the earliest criteria for
establishing the impact origin of several structures, most
notably the Ries Crater (Germany) (Chao et al., 1960;
Shoemaker and Chao, 1961) (Fig. 4.12). Most subsequent
identifications of impact structures have been based on
shock-produced planar deformation features (PDFs) in
quartz, which are more widely distributed and simpler to
identify. However, the discovery of both coesite and stishovite
in the ancient Vredefort structure (South Africa) (Martini,
1991) was an important step in the growing acceptance of
this structure as an impact site. Diamond and other highpressure
carbon compounds [e.g., lonsdaleite (hexagonal diamond)]
produced from graphite in the shocked target rocks
have also been identified at an increasing number of impact
structures (Masaitis, 1998; Masaitis et al., 1972; Hough et al.,
1995; Koeberl et al., 1997c).
Coesite, stishovite, and diamond, when they are found in
near-surface rocks, are unique and reliable indicators of meteorite
impact. None of these minerals has been identified,
for example, as the result of explosive volcanic eruptions. The
use of coesite and diamond as impact criteria does require
some care, however, because both minerals also occur naturally
in deep-seated (depth >60 km) terrestrial rocks, where
they have formed in stable equilibrium at the high static pressures
(>2 GPa) present at these depths. Both minerals may
then be transported to Earth’s surface: coesite by tectonic
processes and diamond in fragments carried up by unusual
mafic (kimberlite) volcanic eruptions. However, stishovite,
formed only at pressures >10 GPa, has never been identified
in a nonimpact setting. Such static pressures could be produced
only at depths of 300–400 km within Earth. Furthermore,
the occurrence of such high-pressure minerals as
coesite, stishovite, or diamond in near-surface crustal rocks
[e.g., coesite and stishovite in sandstone at Barringer Meteor
Crater (Arizona)], particularly when they occur as a disequilibrium
assemblage with other chemically equivalent
minerals (e.g., coesite + stishovite + silica glass + quartz), is
definite evidence for meteorite impact.
4.5. PLANAR MICROSTRUCTURES
IN QUARTZ
Shock waves produce a variety of unusual microscopic
planar features in quartz, feldspar, and other minerals. These
features typically occur as sets of parallel deformation planes
within individual crystals. The recognition and interpretation
of these features, particularly those in quartz, as unique
products of meteorite impact has been a critical factor in
identifying most new impact structures, in recognizing the
impact origin of large, ancient, or deeply eroded structures,
and in demonstrating the role of meteorite impact in the
K/T extinction event.
Distinctive planar features in quartz (SiO2) have been one
of the most widely applied criteria for recognizing impact
structures (for reviews, details, and literature references, see
papers in French and Short, 1968; also von Engelhardt and
Bertsch, 1969; Stöffler and Langenhorst, 1994; Grieve et al.,
1996). Quartz is an ideal mineral for this purpose. It is abundant
in a wide range of sedimentary and crystalline rocks. It
is stable over long periods of geologic time, and it resists
change by alteration and metamorphism. It is an optically
simple (uniaxial) mineral to study and to analyze on the Universal
Stage (U-stage). In particular, it displays a variety of
different planar features whose development can be correlated
with shock pressure (Table 4.2) (Hörz, 1968; Robertson
et al., 1968; Stöffler and Langenhorst, 1994), and can thus be
used as a shock barometer to reconstruct the shock-pressure
distribution that existed within an impact structure during
the impact event (Robertson, 1975; Grieve and Robertson,
1976; Robertson and Grieve, 1977; Grieve et al., 1996; Dressler
et al., 1998).
The production and properties of planar microstructures
in quartz have been studied intensely since the early 1960s
by geological investigations, shock-wave experiments, and
both optical and electron microscopy (papers in French and
Short, 1968; also Stöffler and Langenhorst, 1994). It is now
recognized that shock waves produce several kinds of planar
microstructures in quartz, and their detailed characterization
and interpretation has been — and still is — an active
and much-debated problem (e.g., Alexopoulos et al., 1988;
Sharpton and Grieve, 1990). At present, two basic types of
planar features can be recognized, planar fractures and planar
deformation features (PDFs) (Table 4.2).
4.5.1. Planar Fractures
Planar fractures are parallel sets of multiple planar cracks
or cleavages in the quartz grain; they develop at the lowest
pressures characteristic of shock waves (~5–8 GPa)
(Figs. 4.13 and 4.14). The fractures are typically 5–10 μm
wide and spaced 15–20 μm or more apart in individual quartz
grains. Similar cleavage also occurs rarely in quartz from nonimpact
settings, and therefore planar fractures cannot be used
independently as a unique criterion for meteorite impact.
However, the development of intense, widespread, and closely
spaced planar fractures (Fig. 4.15) is strongly suggestive of
shock, and such fractures are frequently accompanied in
impact structures by other features clearly formed at higher
shock pressures (Robertson et al., 1968; Stöffler and
Langenhorst, 1994; Grieve et al., 1996; French et al., 1997).
4.5.2. Planar Deformation Features (PDFs)
Planar deformation features (PDFs) is the designation
currently used for the distinctive and long-studied shockproduced
microstructures that were formerly given a variety
of names (e.g., “planar features,” “shock lamellae”). In contrast
to planar fractures, with which they may occur, PDFs
are not open cracks. Instead, they occur as multiple sets of
closed, extremely narrow, parallel planar regions (Fig. 4.16).
Individual PDFs are both narrow (typically <2–3 μm) and
more closely spaced (typically 2–10 μm) than planar fractures
(Figs. 4.17 and 4.18). Detailed optical and TEM studies
have shown that, within individual PDFs, the atomic
Shock-Metamorphic Effects in Rocks and Minerals 43
Fig. 4.13. Quartz; cleavage and PDFs. High-magnification view of relict deformed quartz grain in highly shocked and vesiculated
Coconino Sandstone [Barringer Meteor Crater (Arizona)]. The quartz grain shows irregular, subparallel fractures (dark, near-vertical),
combined with shorter cross-cutting light-and-dark planar features, possibly PDFs (upper right/lower left). Note the irregular extinction
in the grain. Sample MCF-65-15-3 (cross-polarized light).
0.05 mm
Fig. 4.14. Quartz; cleavage. Quartz grain in moderately shocked Coconino Sandstone from Barringer Meteor Crater (Arizona), showing
irregular extinction and multiple sets of cleavage fractures parallel to c(0001), m{1010}, r{1011}, and r'. c-axis direction (arrow) and
directions of cleavage traces indicated in inset. Photograph courtesy of T. E. Bunch (cross-polarized light).
0.1 mm
44 Traces of Catastrophe
Fig. 4.15. Quartz; fractured, in quartzite. Intense fracturing of quartz in a coarse-grained metamorphosed orthoquartzite target rock
from the Gardnos structure (Norway). The large quartz grain (right) grades into a finer-grained recrystallized shear zone (left). The
quartz grain is cut by numerous subparallel planar fractures (longer, dark, subhorizontal lines) and by much shorter planar features (short,
dark, near-vertical lines) that originate along the fracture planes. These latter features may be relicts of actual PDFs or of Brazil twins
parallel to the base (0001). Within the Gardnos structure, the originally white quartzite is dark gray to black and highly fractured, and the
fractures within the quartz grains contain carbonaceous material. Sample NG-94-17B (cross-polarized light).
and the Ries Crater (Germany) (age 15 Ma) (Fig. 4.16).
However, preservation of fresh, continuous PDFs depends
on geological circumstances, including cooling rate and
postimpact temperatures. Fresh, well-preserved PDFs are
also present in older structures, e.g., Sierra Madera (Texas)
(age <100 Ma) (Fig. 4.19) and Gardnos (Norway) (age
>400 Ma) (Fig. 4.20). The occurrence of striking fresh PDFs
in quartz exactly at the K/T boundary, a worldwide layer of
ejecta from the Chicxulub structure (Mexico) (age 65 Ma)
(Figs. 4.17 and 4.18), provided some of the most important
initial evidence that a large meteorite impact event had
occurred at that time.
In altered, geologically old, or metamorphosed samples,
PDFs have an equally distinctive but discontinuous character.
The original amorphous material in the PDF planes is
recrystallized back to quartz, and in the process, arrays of
small (typically 1–2 μm) fluid inclusions (“decorations”)
develop along the original planes (Figs. 4.21 and 4.22). The
resulting features, called decorated PDFs (Robertson et al.,
1968; Stöffler and Langenhorst, 1994) preserve the orientation
of the original PDFs, and the distinctive shock-produced
fabric can still be recognized in old rocks that have
even undergone metamorphism [e.g., greenschist facies at
Sudbury (Canada); Fig. 4.23]. More intense recrystallization
produces arrays of small mosaic quartz crystals
(subgrains), especially along PDFs originally parallel to the
base c(0001) of the quartz grain (Leroux et al., 1994).
A second type of PDF, oriented parallel to the base
c(0001), has recently been identified, chiefly by studies of
1 mm
structure of the original crystalline quartz is severely deformed,
so that the quartz has been transformed into a distinct
amorphous phase (Müller, 1969; Kieffer et al., 1976a;
Goltrant et al., 1991, 1992).
The importance of PDFs arises from the fact that they
are clearly distinct from deformation features produced in
quartz by nonimpact processes, e.g., cleavage or tectonic
(metamorphic) deformation lamellae (Böhm lamellae)
(Carter, 1965, 1968; Alexopoulos et al., 1988; Stöffler and
Langenhorst, 1994). Cleavages are open fractures; they tend
to be relatively thick (~10 μm) and widely spaced (>20 μm).
Deformation lamellae consist of bands of quartz typically
10–20 μm thick and >10 μm apart that are optically distinct
and slightly misoriented relative to the host grain. In
contrast to these features, shock-produced PDFs are narrow
(<2–3 μm) straight planes consisting of highly deformed or
amorphous quartz, and they are generally oriented parallel
to specific rational crystallographic planes in the host quartz
crystal, especially to the base c(0001) or to low-index rhombohedral
planes such as w{1013}, p{1012}, and r{1011}
(Table 4.4).
The presence of well-developed PDFs produces a striking
and distinctive appearance in thin section. Unaltered
PDFs form multiple sets of continuous planes that extend
across most or all of the host grain (Figs. 4.16, 4.17, and
4.18). These fresh, continuous PDFs tend to be observed
only in unaltered material from shock-wave experiments
and from younger, well-preserved impact structures, e.g.,
Barringer Meteor Crater (Arizona) (age 50 ka) (Fig. 4.13)
Shock-Metamorphic Effects in Rocks and Minerals 45
Fig. 4.17. Quartz; multiple PDFs, fresh. Small quartz grain
(0.20 mm long) from K/T boundary ejecta layer, showing two
prominent sets of fresh (undecorated) PDFs. (Small dots with
halos are artifacts.) Specimen from Starkville South, a few kilometers
south of Trinidad, Colorado. Photograph courtesy of
G. A. Izett. Spindle stage mount (plane-polarized light).
Fig. 4.18. Quartz; multiple PDFs, fresh. Small quartz grain
(0.36 mm long) from K/T boundary ejecta layer, containing one
opaque inclusion and multiple (3–5?) prominent sets of fresh
(undecorated) PDFs. Specimen from Clear Creek North, a few
kilometers south of Trinidad, Colorado. Photograph courtesy of
G. A. Izett. Spindle stage mount (plane-polarized light).
0.1 mm
Fig. 4.16. Quartz; multiple PDFs, fresh. Striking multiple sets of PDFs developed in a quartz grain from a shocked granite inclusion
in suevite from the Ries Crater (Germany). “A” indicates PDFs parallel to {1013} or {0113}; “B” indicates PDFs parallel to {1011} or
{0111}. Note the irregular mottled extinction within the quartz grain. From von Engelhardt and Stöffler (1965), Fig. 1. Photograph
courtesy of W. von Engelhardt (cross-polarized light).
46 Traces of Catastrophe
Fig. 4.19. Quartz; multiple PDFs, fresh. Shocked quartz grain containing multiple sets of fresh PDFs. The grain is included with rare
sandstone fragments in a carbonate breccia dike that cuts the deformed basement rocks at Sierra Madera (Texas), an impact structure
developed in a target composed dominantly of carbonate rocks. The closely spaced PDFs give a distinctive darkened, yellowish appearance
to the quartz grain. Sample SMF-65-2-2 (plane-polarized light).
0.1 mm
TABLE 4.4. Typical crystallographic orientations of planar
microstructures in shocked quartz (modified from
Stöffler and Langenhorst, 1994, Table 3, p. 164).
Polar Angle
(Angle Between Pole to Plane
Symbol Miller Indexes and Quartz c-axis)
c * (0001) 0°
w, w' * {1013},{0113} 23°
p, p' * {1012},{0112} 32°
r, z * {1011},{0111} 52°
m {1010} 90°
x {1122},{2112} 48°
s {1121},{2111} 66°
a {1120},{2110} 90°
* {2241},{4221} 77°
t {4041},{0441} 79°
k {5160},{6150} 90°
x {5161},{6511} 82°
{6151},{1561}
— {3141},{4311} 78°
{4131},{1341}
— {2131},{3211} 74°
{3121},{1231}
*Prominent planes in typical shock fabrics.
Shock-Metamorphic Effects in Rocks and Minerals 47
Fig. 4.21. Quartz; multiple PDFs, decorated. Large compound quartz grain from shocked basement rock inclusion in suevite breccia
from Rochechouart (France), showing two prominent sets of partially decorated PDFs (north-northeast/south-southwest; northeast/
southwest). Original, partly continuous PDF traces are still recognizable from the location of small fluid inclusions (black dots) along the
original PDF planes. Sample FRF-69-16 (cross-polarized light).
0.1 mm
0.1 mm
Fig. 4.20. Quartz; multiple PDFs, slightly decorated. Quartz grain in a carbon-bearing crater-fill breccia from Gardnos (Norway),
showing two well-developed sets of {1013} PDFs. In places, the normally continuous PDFs break down into a string of small fluid
inclusions (small black dots) that follow the original trace of the PDFs. This process, by which the originally glassy material in the PDFs
is recrystallized and replaced by fluid inclusions, has produced decorated PDFs, in which the original PDFs are visible only by the arrays
of fluid inclusions that reproduce their original orientations. Sample NG-94-31 (plane-polarized light).
48 Traces of Catastrophe
Fig. 4.22. Quartz; multiple PDFs, decorated. Compound quartz grain showing two prominent sets of decorated PDFs (north/south;
northwest/southeast). The original PDF planes are now largely replaced by arrays of small fluid inclusions that preserve the original PDF
orientations. Sample from Precambrian basement gneiss in the central uplift of the Carswell Lake structure (Canada). Photograph
courtesy of M. R. Dence. Sample DCR-11-63B (cross-polarized light).
Fig. 4.23. Quartz; multiple PDFs, decorated. High-magnification view of shocked quartz from ejecta block in metamorphosed suevite,
showing multiple sets of recrystallized PDFs (northwest/southeast; east/west) now expressed by arrays of small fluid inclusions
(black dots). Quartz grain also contains numerous random larger fluid inclusions scattered through the grain. Sample from a small
granitic gneiss inclusion in the Onaping Formation “Black Member,” from the type locality, Onaping Falls (Highway 144, Dowling
Township), northwestern corner of the Sudbury structure (Canada). Photograph courtesy of N. M. Short. Sample CSF-66-39 (crosspolarized
light).
0.1 mm
0.1 mm
Shock-Metamorphic Effects in Rocks and Minerals 49
Fig. 4.24. Quartz; basal PDFs. Large irregular quartz grain associated with sericitized feldspar (dark) in footwall granitic rocks on
North Range of Sudbury structure (Canada), together with shatter cones and pseudotachylite. Grain shows one well-developed set of
PDFs (upper left/lower right), which appear as linear arrays of small fluid inclusions parallel to the base (0001) of the quartz grain.
Sample CSF-67-55-2 (cross-polarized light).
shocked quartz with transmission election microscopy
(TEM), as Brazil twins (Fig. 4.24) (Leroux et al., 1994; Joreau
et al., 1996). This form of twinning also occurs in natural
unshocked quartz, but it has never been observed parallel to
the base in such samples. Experimental formation of basaloriented
Brazil twins in quartz requires high stresses (about
8 GPa) and high strain rates, and it seems probable that such
features in natural quartz can also be regarded as unique
impact indicators (Stöffler and Langenhorst, 1994).
4.5.3. PDF Orientations
Despite the distinctive appearance of PDFs in thin section,
appearance alone is not adequate to distinguish them
from nonshock features or to argue that they are impact
produced. An additional and definitive characteristic of PDFs
is their tendency to form along specific planes in the quartz
crystal lattice. Measurements of PDF orientations within the
host quartz grain therefore provide a simple and reliable
method to distinguish them from planar structures produced
by nonshock processes. PDF orientations can be measured
using standard petrofabric procedures on a U-stage (for details,
measurement techniques, and specific studies, see Carter,
1965, 1968; Robertson et al., 1968; von Engelhardt and Bertsch,
1969; Alexopoulos et al., 1988; Stöffler and Langenhorst, 1994)
or on the related spindle stage (Bloss, 1981; Medenbach, 1985;
Bohor et al., 1984, 1987; Izett, 1990).
The procedures involve measuring, in a single quartz grain,
both the orientation of the pole (normal) to each set of PDFs
0.2 mm
and the orientation of the c-axis (= optic axis) of the grain.
The measurement data are then plotted on a standard
stereonet, and the results are expressed as the location of the
pole to the PDFs relative to the c-axis. If a large number of
PDF measurements can be made on a sample, a convenient,
although not entirely rigorous, method to present comparative
results is to plot a frequency diagram (histogram) of the
angles between the c-axis and the pole to each set of PDFs.
Because shock-produced PDFs in a given quartz grain
are parallel to only a few specific crystallographic planes, the
angles measured between the quartz c-axis and the poles to
the PDFs tend to concentrate at a few specific values. In a
histogram plot, the poles appear as sharp concentrations at
specific angles, each of which corresponds to a particular
plane (Figs. 4.25 and 4.26).
This sharply peaked pattern of PDF orientations, typically
characterized by peaks at c(0001) (0°), w{1013} (23°),
and p{1012} (32°), is one of the most useful and most-used
indicators of meteorite impact. Such plots clearly demonstrate
the great difference between PDF distributions
(Figs. 4.25a–c) and the more widely distributed, bell-shaped
distribution characteristic of metamorphic deformation
lamellae (Fig. 4.25e). Such plots are also used to distinguish
different shock-produced fabrics that reflect different shock
pressures (Fig. 4.26).
Experimental and geological studies have demonstrated
that PDFs form in quartz at pressures of ~7–35 GPa, or at
the lower end of the range of shock-metamorphic pressures
50 Traces of Catastrophe
Fig. 4.25. Quartz; PDF orientations. Comparative histograms showing orientations of shock-produced PDFs and other planar
deformation features in quartz (from Carter, 1965). In each diagram, the angle between the quartz c-axis and the pole to the planar
feature is plotted on the x-axis; y-axis indicates frequency for each given angle. Shock-produced fabrics are characterized by strong
orientations parallel to a few specific crystallographic planes. (a) and (b) Basal-oriented sets of deformation lamellae in shocked sandstones
from the Vredefort (South Africa) and Barringer Meteor Crater (Arizona) structures; (c) distinctive PDFs showing the distinctive
concentration parallel to w{1013} [shocked crystalline rocks; Clearwater Lakes (Canada)]; (d) low-angle, near-basal fabric of deformation
lamellae generated under high-strain experimental conditions; (e) broad distribution of metamorphic deformation lamellae (Böhm lamellae)
produced by normal metamorphic conditions. The distinctive differences between shock-produced fabrics (a), (b), and (c) and those of
normal metamorphism (e) have been one of the strongest arguments for the meteorite impact origin of suspected impact structures.
Shock-Metamorphic Effects in Rocks and Minerals 51
Fig. 4.26. Quartz; PDF orientations. Comparative histograms showing different fabrics displayed by PDFs in quartz produced at
different shock pressures, based on measurements of shocked crystalline rocks from several Canadian impact structures (from Robertson
et al., 1968). With increasing shock pressures, both the total number of PDFs and the number of different orientations increase. The
following fabrics, and the minimum shock pressures estimated to form them (Grieve and Robertson, 1976, pp. 39–40), can be recognized:
type A (P > 7.5 GPa): basal PDFs only; type B (P > 10 GPa), appearance of w{1013} planes, typically with basal planes; type C
(P > 14 GPa), appearance of {2241} planes with others; type D (P > 16 GPa), appearance of p{1012} planes with others. These fabrics
have been used as shock barometers to measure the intensity and distribution of shock pressures in several structures (Grieve and Robertson,
1976; Robertson and Grieve, 1977; Dressler and Sharpton, 1997). From Carter (1965).
52 Traces of Catastrophe
been observed in sedimentary rocks from several impact
structures (Kieffer, 1971, Kieffer et al., 1976a; Grieve et al.,
1996).
Despite these similarities, a growing amount of data now
indicates that sedimentary rocks, especially porous ones, respond
differently to shock waves than do nonporous crystalline
rocks. One indication of significant differences is that
PDF fabrics measured in sediments show a large proportion
of PDFs whose poles are oriented at high angles (>45°) to
the quartz c-axis (Grieve et al., 1996; Gostin and Therriault,
1997). Other possible differences are that PDFs may first
appear, or a particular PDF fabric may develop, at different
shock pressures in sedimentary rocks than in crystalline rocks.
A more important difference between porous and nonporous
rocks is that a shock wave passing through porous
sediments will generate more heat than in passing through
crystalline rocks, chiefly because more of the shock-wave
energy is absorbed by the numerous grain interfaces and pore
spaces in the sediment (Kieffer, 1971; Kieffer et al., 1976a;
Kieffer and Simonds, 1980; Stöffler, 1984). As a result, extensive
melting will occur at lower shock pressures in sediments
than in crystalline rocks, i.e., at about 15–20 GPa in sandstone
vs. 50–60 GPa in crystalline rocks (Stöffler, 1972, 1984).
Therefore, the higher-pressure fabrics of quartz PDFs, which
form at 20–30 GPa in crystalline rocks, may not be found in
sediments, either because they did not form or because they
Fig. 4.27. Quartz; multiple PDFs, fresh. Photomicrograph showing at least four sets of fresh PDFs in a shocked quartz grain from
crystalline target rocks at the Lake St. Martin impact structure, Manitoba (Canada). Two prominent PDF sets (northwest/southeast and
west-northwest/east-southeast) are accompanied by less obvious sets oriented approximately north/south and east/west. Petrofabric
measurements with a U-stage show that the PDFs are oriented parallel to both w{1013} and p{1012}, indicating moderately high shock
pressures (>15 GPa). Patches of diaplectic glass, associated with the shocked quartz, appear as dark zones (e.g., upper right). Width of
field is ~100 μm. Photograph courtesy of V. L. Sharpton (cross-polarized light).
(e.g., Hörz, 1968; Stöffler and Langenhorst, 1994). However,
the relative abundance of different PDF orientations varies
significantly with shock pressure. Basal Brazil twins, although
little studied so far, appear restricted to shock pressures below
10 GPa. PDFs parallel to w{1013} develop at about >7–
10 GPa, and PDFs parallel to p{1012} at about >20 GPa.
At higher pressures, e.g., 20–35 GPa, the total number of
PDF sets increases, and additional orientations appear
(Fig. 4.26). The PDFs formed at these higher levels tend to
be intensely developed and very closely spaced within the
quartz grains (Figs. 4.16, 4.18, and 4.27).
4.5.4. PDFs in Sedimentary Rocks
Although PDFs and their orientations can be reliably used
as indicators of shock and impact events, it is becoming clear
that our current knowledge about such features is incomplete
and unrepresentative. Nearly all our information to date
has come from impact structures formed in dense, coherent,
quartz-bearing crystalline rocks. There is relatively little information
about the effects of shock deformation in other
kinds of quartz-bearing rocks, e.g., porous sandstones or finegrained
shales.
Several studies have demonstrated that shocked sandstones
and shales also develop PDFs in quartz, and even
diaplectic quartz and feldspar glasses, similar to those observed
in shocked crystalline rocks, and these features have
Shock-Metamorphic Effects in Rocks and Minerals 53
were immediately destroyed by postshock melting. The
unique shock effects observed in sedimentary rocks can still
provide conclusive evidence for an impact origin [e.g., at
Barringer Meteor Crater (Arizona) (Kieffer, 1971)], but the
details of such occurrences cannot be accurately interpreted
on the basis of results from shocked nonporous crystalline
rocks (Grieve et al., 1996).
4.6. PLANAR MICROSTRUCTURES IN
FELDSPAR AND OTHER MINERALS
Similar planar microstructures are produced by shock in
many other minerals (e.g., Stöffler, 1972, 1974), but such
features have been less used as indicators of meteorite impact.
Feldspars of all kinds (both alkali varieties and plagioclase)
display various shock-produced planar microstructures:
fractures, deformation bands, kink bands, and actual PDFs.
Frequently, short and closely spaced PDFs may be combined
with longer and more widely spaced features (deformation
bands or albite twinning) to produce a distinctive ladder texture
(Figs. 4.28, 4.29, and 4.30).
Although several studies have been made of shock-produced
planar features in feldspars (e.g., Stöffler, 1967, 1972;
papers in French and Short, 1968), these features have been
less studied and less well characterized than those in quartz.
There are several reasons for this: the greater diversity and
complexity of such features, the greater optical complexity
(biaxial) of feldspars, and the common secondary alteration
of the feldspar and its planar features to clays, iron
oxides, etc. (Figs. 4.29 and 4.30). Another factor in studies
focused on identifying new impact structures is the fact
that shocked feldspar in crystalline rocks is generally associated
with shocked quartz, whose features (especially PDFs)
provide a quicker and simpler method for establishing an
impact origin.
Planar microstructures, both planar fractures and true
PDFs, have also been observed in other minerals, including
pyroxene, amphiboles, and several accessory phases (apatite,
sillimanite, cordierite, garnet, scapolite, and zircon) (Stöffler,
1972). Less is known about PDF formation and orientations
in these minerals, because appropriate rocks are less
abundant in most impact structures, and because the specific
minerals have not been studied in detail. However, recognition
of shock-produced PDFs in zircon has been
especially important in applying U-Th-Pb dating methods
to individual zircons in shocked target rocks to determine
the ages of impact structures (e.g., Krogh et al., 1984, 1993;
Kamo and Krogh, 1995).
The development of distinctive shock-metamorphic features
such as PDFs in denser mafic minerals like amphibole,
pyroxene, and olivine apparently occurs at higher pressures
and over a more limited pressure range than in quartz and
feldspar. At pressures <30 GPa, sufficient to form PDFs in
both quartz and feldspar, the most common shock effects
observed in mafic minerals are planar fractures, mechanical
twins, and general comminution (Stöffler, 1972); features
Fig. 4.28. Feldspar; multiple PDFs and diaplectic glass
(maskelynite). Shocked plagioclase feldspar grain from the Ries
Crater (Germany), showing development of multiple sets of PDFs
(lower right) and gradational conversion of the same crystal to
diaplectic glass (maskelynite) (upper left). Original polysynthetic
albite twin lamellae (northwest/southeast) are still preserved in
part of the crystal (lower right), but alternate twin lamellae have
either been converted to maskelynite (clear) or are crosscut by short,
closely spaced PDFs to form a distinctive “ladder” structure.
Elsewhere in the crystal (upper left), both the original twins and
the subsequent shock-produced PDFs disappear, and the whole
crystal consists of maskelynite. Sample from a moderately shocked
amphibolite fragment in suevite breccia. From Stöffler (1966), Fig. 4
(plane-polarized light).
resembling true PDFs are only rarely observed. At higher
pressures, mafic minerals in naturally and experimentally
shocked basalts generally show only extreme comminution,
accompanied by the melting and flow of associated feldspar
(Kieffer et al., 1976b; Schaal and Hörz, 1977). PDFs are therefore
unlikely to be observed in mafic minerals in impact structures.
The higher pressures apparently required for their
formation imply that they will form in a correspondingly
smaller volume of shocked rock in the structure. Furthermore,
the higher shock pressures required are closer to pressures
that produce partial to complete melting of the rock,
so that PDFs, even if formed, would not survive any subsequent
melting episode.
0.1 mm
54 Traces of Catastrophe
Fig. 4.29. Feldspar; multiple PDFs, “ladder” texture. Shocked K-feldspar, showing multiple sets of altered PDFs. Two types of planar
deformation features are present: (1) long, thicker, widely spaced planes (clear areas, approximately east/west) that may be deformation
bands or kink bands; (2) short, narrower, closely spaced features (northeast/southwest and north-northwest/south-southeast) that combine
with the first type to form a distinctive “ladder” texture. The planar features have a brownish-red color, possibly caused by alteration of the
feldspar to clay minerals and iron oxides. Sample from a small granitic gneiss inclusion in the Onaping Formation “Black Member” from
the type locality, Onaping Falls (Highway 144, Dowling Township), northwestern corner of the Sudbury structure (Canada). Photograph
courtesy of N. M. Short. Sample CSF-66-39 (cross-polarized light).
Fig. 4.30. Feldspar; twinning and PDFs. Large deformed feldspar crystal (microcline?) in granitic fragment in suevite breccia. Original
twinning in the feldspar (light/dark pattern, northwest/southeast) is deformed and faulted along multiple parallel fractures (east-northeast/
west-southwest). Elsewhere, the feldspar is cut by a single set of short, narrow, closely spaced planar features (northeast/southwest) that
may be actual PDFs. Sample from a small block of granitic gneiss from the Onaping Formation “Black Member,” Sudbury (Canada).
Sample CSF-67-73 (cross-polarized light).
0.1 mm
0.1 mm
Shock-Metamorphic Effects in Rocks and Minerals 55
4.7. SHOCK ISOTROPIZATION AND
DIAPLECTIC GLASSES
Planar microstructures form at relatively low shock pressures
(>7–35 GPa) (Table 4.2) (Stöffler and Langenhorst,
1994) and involve only partial and localized deformation of
the host crystal. PDFs, which develop in the upper part of
this range (10–35 GPa), involve actual conversion of the
quartz crystal structure to an amorphous phase within the
individual planes. Higher shock pressures (35–45 GPa),
which transmit more energy into the crystal, do not form
PDFs. Instead, the shock waves convert the entire crystal to
an amorphous (glassy) phase.
This shock-produced diaplectic glass (also called thetomorphic
glass) (Stöffler, 1966, 1967, 1972, 1984; Chao, 1967;
papers in French and Short, 1968) is completely different from
conventional glasses produced by melting a mineral to a liquid
at temperatures above its melting point. Diaplectic glasses
do not melt or flow; they preserve the original textures of the
crystal and the original fabric of the mineral in the rock. In
addition, although diaplectic glasses are optically isotropic
(i.e., they show no birefringence when examined petrographically
under crossed polarizers), studies of quartz and feldspar
diaplectic glasses by X-ray diffraction and infrared
spectrometry have shown that they retain much of the ordered
atomic structure of the original crystal (e.g., Bunch et
al., 1967, 1968; Stöffler, 1974, 1984; Arndt et al., 1982).
Samples of diaplectic feldspar glasses have also been experimentally
annealed by heating at ambient pressure to produce
original single crystals (Bunch et al., 1967, 1968; Arndt
et al., 1982) or microcrystalline aggregates that preserve the
shapes of the original feldspar crystals (Arndt et al., 1982;
Ostertag and Stöffler, 1982).
Quartz and feldspar are the most common examples of
minerals converted to diaplectic glasses by shock waves.
Diaplectic plagioclase feldspar glass, called maskelynite, was
in fact observed in meteorites more than a century before it
was discovered in shocked terrestrial rocks. The same material,
often well preserved, is also observed at several impact
structures where highly shocked rocks are preserved, e.g.,
the Ries Crater (Germany) (Figs. 4.28, 4.32, and 4.33) and
Manicouagan (Canada) (Fig. 4.31).
In these occurrences, the unique textures of the diaplectic
glasses clearly indicate formation without melting to the liquid
state. The overall grain fabric of the rock is unchanged,
and the diaplectic glasses preserve the shapes of the original
quartz and feldspar grains. In some grains, the transformation
to diaplectic glass is incomplete, and areas of relict birefringence
remain in the otherwise isotropic material
(Figs. 4.28 and 4.31). In some shocked plagioclase grains,
one set of alternating albite twins is converted to maskelynite,
while the twins of the other set remain birefringent. Other
minerals (e.g., amphibole, garnet, micas), associated with (or
even in contact with) grains of diaplectic glass, show little
Fig. 4.31. Feldspar; diaplectic glass (maskelynite). Shocked plagioclase feldspar, partially converted to isotropic diaplectic feldspar
glass (maskelynite). Parts of the original coarse feldspar grains remain crystalline and birefringent (light areas); these regions grade into
adjoining areas of maskelynite (dark). Drill-core sample from coarse-grained basement anorthosite, exposed in the central uplift of the
Manicouagan structure (Canada). Photograph courtesy of M. R. Dence. Sample DMM-73-63B (cross-polarized light).
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56 Traces of Catastrophe
Fig. 4.32. Feldspar and quartz; diaplectic glasses. Biotite gneiss containing diaplectic feldspar glass (maskelynite) (clear, low relief;
e.g., upper right) and diaplectic quartz glass (clear, higher relief, e.g., lower right). The associated biotite crystals (dark) have retained
their original shape and have remained crystalline and birefringent, despite the complete transformation of adjacent quartz and plagioclase
into glassy phases (compare with Fig. 4.33). Biotite gneiss inclusion in suevite breccia, Otting, Ries Crater (Germany). From Stöffler
(1967), Fig. 12a. Photograph courtesy of D. Stöffler (plane-polarized light).
Fig. 4.33. Feldspar and quartz; diaplectic glasses. Biotite gneiss containing diaplectic feldspar glass (maskelynite) and diaplectic quartz
glass (compare with Fig. 4.32). Both phases are isotropic (dark) under crossed polarizers. The associated biotite crystals have retained
their original shape and have remained crystalline and birefringent, despite the complete transformation of adjacent quartz and plagioclase
into glassy phases. Biotite gneiss inclusion in suevite breccia, Otting, Ries Crater (Germany). From Stöffler (1967), Fig. 12b. Photograph
courtesy of D. Stöffler (cross-polarized light).
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Shock-Metamorphic Effects in Rocks and Minerals 57
deformation and retain their original form (Figs. 4.32 and
4.33), although they may show reduced birefringence and
reddening produced by the formation of hematite (e.g.,
Feldman, 1994) and cordierite (Stähle, 1973).
Diaplectic glasses formed from other minerals (e.g., scapolite)
have rarely been observed. Mafic minerals (e.g., pyroxene,
amphibole, and biotite) do not seem to form diaplectic
glasses, probably because the pressures required are higher
than those for quartz and feldspar, high enough so that shockproduced
melting occurs instead.
Diaplectic quartz and feldspar glasses are metastable. They
apparently do not survive if they are exposed to even relatively
mild postimpact thermal effects. Diaplectic glasses are
not observed in impact structures that have been even slightly
metamorphosed, even though decorated PDFs may still be
preserved in associated quartz. In such settings, instead of
diaplectic glasses, one observes quartz and feldspar grains
that are recrystallized to microcrystalline aggregates that replace
the original crystal (Figs. 4.34, 4.35, and 4.36). Textures
in the altered feldspars sometimes suggest intense plastic
deformation and flow within the original grain. These features
are often accompanied by the development of plumose
or spherulitic microcrystalline textures that may reflect significant
thermal effects as well. Such grains of quartz and
Fig. 4.34. Feldspar; possible diaplectic glass, recrystallized. Large, highly deformed and recrystallized feldspar clast in suevite breccia,
surrounded by finer fragments in an opaque carbon-bearing matrix. The feldspar shows deformation and recrystallization throughout, as
indicated by the intensely mosaic extinction. The crystal is subdivided by thin irregular zones of nearly isotropic material, possibly
original melt. Plastic behavior of the fragment is also suggested by indentations of the matrix into the clast (e.g., at top). This clast can be
interpreted as a fragment of diaplectic feldspar glass that has subsequently been recrystallized to form a fine-grained microcrystalline
texture that is still similar to the original crystal. Similar reactions have been produced in experimentally annealed maskelynite. Another
possibility is that the fragment was shock-heated above its melting point, but was rapidly quenched (perhaps during deposition) before
extensive flow could occur. In any case, the unusual texture has been preserved despite subsequent metamorphism of the unit in which it
occurs. Fragment in Onaping Formation “Black Member” from type locality, Onaping Falls (Highway 144, Dowling Township),
northwestern corner of Sudbury structure (Canada). Sample CSF-66-37-2 (cross-polarized light).
feldspar have been tentatively interpreted as original diaplectic
glasses that have been annealed and recrystallized,
either by immediate postshock thermal effects or by subsequent
metamorphism (McIntyre, 1968; French, 1968b,
pp. 401–404).
4.8. SELECTIVE MINERAL MELTING
The high-pressure (35–45 GPa) shock waves that produce
diaplectic glasses also generate significant and sudden
postshock temperature rises of several hundred degrees
Celsius in the rocks and minerals through which they pass
(Fig. 4.1). In the region of diaplectic glass formation,
postshock temperatures are still low enough (300°–900°C)
that virtually no actual melting occurs, and rapidly quenched
samples of diaplectic glasses suffer no further immediate
alteration. However, at slightly higher shock pressures
(~45–50 GPa), the higher postshock temperatures (>1000°C)
begin to exceed the melting points of typical rock-forming
minerals, and distinctive localized melting effects appear in
the affected rocks.
This shock-produced selective mineral melting differs
significantly from normal equilibrium melting. Under nor-
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58 Traces of Catastrophe
Fig. 4.35. Feldspar; possible diaplectic glass, recrystallized.
Shock-deformed and recrystallized feldspar and quartz from a
coarse-grained granitic fragment in suevite breccia. Large original
quartz grains (lower center; gray, higher relief ) are recrystallized
to finely crystalline mosaic quartz. Original feldspar grains (clear,
lower relief ) are generally finely recrystallized and virtually isotropic
in some areas (compare with Fig. 4.36), although some areas of
original feldspar crystals are preserved. From granitic inclusion in
Onaping Formation “Black Member” at type locality, Onaping
Falls (Highway 144, Dowling Township), northwestern corner of
Sudbury structure (Canada). Sample CSF-66-50-13 (planepolarized
light).
Fig. 4.36. Feldspar; possible diaplectic glass, recrystallized.
Shock-deformed and recrystallized feldspar and quartz from a
granitic fragment in suevite breccia. Large original quartz grains
are recrystallized to finely crystalline mosaic quartz. Original
feldspar grains are generally finely recrystallized and virtually
isotropic in some areas, although some areas of original feldspar
crystals are preserved (compare with Fig. 4.35). In one such area
(right center), a plagioclase crystal has been plastically deformed,
bending the original polysynthetic albite twinning (light/dark
bands) through a large angle. Despite the intense deformation of
quartz and feldspar, a single apatite grain (lower right) shows no
deformation. Sample from granitic inclusion in Onaping Formation
“Black Member” at type locality, Onaping Falls (Highway 144,
Dowling Township), northwestern corner of Sudbury structure
(Canada). Sample CSF-66-50-13 (cross-polarized light).
mal conditions of increasing overall temperature, melting
occurs first at the boundaries between different mineral
grains. Two or more different minerals are involved, and the
resulting eutectic melt has a composition intermediate between
that of the adjacent minerals and forms at a temperature
well below that of their individual melting points. In a
shock-wave environment, each mineral grain is instantaneously
raised to a postshock temperature that depends on
the shock-wave pressure and on the density and compressibility
of the mineral itself. If the postshock temperature produced
in a mineral exceeds its normal melting temperature,
each grain of that mineral in the rock will melt, immediately
and independently, after the shock wave has passed. The melt
will have approximately the same composition as the original
mineral before any flow or mixing takes place, and the
melt regions will initially be distributed through the rock in
the same pattern as the original mineral grains.
Selective melting therefore produces unusual textures in
which one or more minerals in a rock show typical melting
features while others — even immediately adjacent ones —
do not. Shocked granitic inclusions from the Ries Crater
(Germany) frequently show a texture in which feldspar has
melted, flowed, and vesiculated, but the adjacent quartz remains
in the form of unmelted diaplectic glass (Chao, 1967;
Stöffler, 1972, 1984). Similar textures can be preserved even
in subsequently metamorphosed rocks, in which flowed and
recrystallized feldspar is accompanied by recrystallized but
undeformed grains of quartz (Fig. 4.37).
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Shock-Metamorphic Effects in Rocks and Minerals 59
Fig. 4.37. High-temperature effects; plastic deformation, grain-boundary melting. Highly shocked and recrystallized quartzofeldspathic
inclusion in metamorphosed suevite breccia, showing extreme deformation of quartz and feldspar. Quartz (gray, higher relief, lower right)
is recrystallized to a fine mosaic of small quartz grains. Feldspar (clear, lower relief, top) shows intense, contorted flow structure, indicating
either incipient melting or extreme plastic flow. Definite incipient melting has occurred at the grain boundaries, forming a brown melt
(dark) with lath-like microlites (white; feldspar?). (Circular feature at center is a bubble in the thin section.) Coarse-grained granitic
inclusion in Onaping Formation “Black Member,” Sudbury structure (Canada). Sample CSF-67-67 (plane-polarized light).
At higher shock pressures, where temperatures are higher
and cooling times may be longer, these selective melting textures
may be complicated by the effects of normal eutectic
melting at grain boundaries (Fig. 4.37). In some shocked
rocks, postshock temperatures may exceed the melting points
of all the minerals present, and the rock will melt to a mixture
of heterogeneous glasses that may preserve (depending
on the amount of subsequent flow and mixing) the original
shapes and mineral compositions. If such rocks are quenched
before flow and mixing can occur, the chemically diverse
glasses can survive and be recognized, even after significant
metamorphism (Fig. 4.38) (Peredery, 1972).
Such distinctive selective melting textures are relatively
uncommon in rocks from impact structures. The region of
shock pressures that produces them is relatively narrow (~45–
55 GPa), and their preservation, once formed, requires rapid
quenching, most commonly as small inclusions in crater-fill
breccias. At progressively higher shock pressures (>55 GPa),
postshock temperatures increase rapidly, melting becomes
complete, flow and mixing processes become dominant in
the melted rock, and more chemically homogeneous bodies
of impact melt are produced (see Chapter 6).
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60 Traces of Catastrophe
Fig. 4.38. High-temperature effects; complete melting. Highly shocked, melted, and recrystallized rock inclusion in metamorphosed
suevite breccia. Postshock temperatures apparently exceeded the melting points of all component minerals, converting the originally
crystalline rock into an initially heterogeneous glass that developed limited flow textures before it was quenched. The inclusion was
subsequently recrystallized to secondary minerals such as quartz, feldspar, amphibole, and chlorite, but the original mineralogy and the
character of the shock-formed heterogeneous glass are still detectable in the distribution and chemical variations in the secondary mineral
assemblage. Inclusion in Onaping Formation “Black Member” at the type locality, Onaping Falls (Highway 144, Dowling Township),
northwestern corner of Sudbury structure (Canada). Sample CSF-66-50-3 (plane-polarized light).
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