Boggs S. Principles of Sedimentology and Stratigraphy
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Ty pe of rock
Volcanic rock
(lava flows and
ash falls)
Plutoc igneous
rocks
Metamorphosed
sedimentary
rocks
Sedimentary rocks
containing
contemporary
organic remains
(fossils, wood)
Sedimentary rocks
containing
authigenic
minerals such
as glauconite
Stratigraphic relationship
Interbedded with
"contemporaneous"
sedimentary rocks
Intrude (cut across)
sedimentary roc
Lie unconformably beneath
sedimentary rocks
15.3 e Geologic Time Scale 527
Reliability of age data
Give actual ages of
sedimentary rocks in close
stratigraphic proximity above
and below volcanic layers
Give minimum ages for e rocks
they intrude
Give maximum ages for
overlying sedimentary rocks
Constitute the rocks whose Give minimum ages for
ages are being determined metamorphosed sedimentary
rocks
Lie unconformably beneath
non-metamorphosed
sedimentary rocks
Give maximum ages for the
overlying non-metamorphosed
sedimentary rocks
Give actual ages of sedimentary
rocks
Give minimum ages for
sedimentary rocks
in age. Thus, estimates of the ages of such associated volcanic rocks also estab
lish the ages of contemporaneous sedimentary rocks.
Ages of whole volcanic rock can be estimated relatively easily by the
potassium-argon method, and ages of minerals in these rocks can be deter
mined by e potassium-argon or other methods. Volcanic rocks that occur in
association with nearly contemporaneous sedimentary rocks whose ages can
also be determined by fossils provide extremely useful reference points for cal
ibration. In fact, establishing the absolute ages of fossiliferous sedimentary
rocks by association with contemporaneous volcanic flows whose ages can be
radiometrically estimated has probably been the single most important
method of calibrating the geologic time scale.
For this method to work, the contemporaneity of the terbedded vol
canic and sedimentary rocks must first be established. If a pyroclastic flow
such as an ash fall or a lava ow erupts over an older, exposed sedimentary
rock surface where erosion is taking place or sedimentation is inactive, the
flow is not contemporeous with the underlying sedimentary rock. The age
calculated for such a flow indicates only that the rock below the flow is older
and the rock above younger than the flow. A geologist can establish contempo
reity by determining if fossils in sedimentary layers above and below the
flow belong to the same biostratigraphic zone or by looking, along the basal
contact of the flow unit, for physical evidence that may show that the underly
ing sediment was still soft at the time of the volcanic eruption. For example,
ash fall material may be mixed by bioturbation into underlying sediment, soft
528
Chapter 15 I Chronostratigraphy and Geologic Time
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Globotruncanita elevata Zone-_-
Figure 15.1.3
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Diagram illustrating how the contemporaneity of sedimentary rocks
to an associated, datable volcanic layer can be established. The
shale beds below and above the volcanic ash bed belong to the
same Foraminiferal biozone and the base of the ash bed has been
bioturbated, indicating that the underlying sediment was still soft
at the time of the ash fall. Therefore, the shale beds are approxi
mately the same age as the ash bed (80 Ma).
sediment may be mixed into e base of a submarine lava flow
,
or oer such
relationships may exist (Fig. 15.1.3).
Bracketed Ages from Associated Igneous or Metamorphic Roc. e ra
diometric ages of igneous rock at are not contemporaneous with associated
sedimentary rocks can be used to estimate the ages of associated sedentary
rocks if two or more igneous bodies "bracket" the sedimentary unit. this case,
the age of e sedimentary unit can be established only as lying between those
of the bracketing igneous bodies. The sedimentary unit will be older an
igneous body that intrudes it, but yoger than an igneous body upon which
it rests unconformably (Fig. 15.1.4A). For example, a sedimentary succession
deposited on the eroded, weathered surface of a granite batholi may subse
quently be intruded by a dike or a sill. The sedimentary unit is obviously
younger than the batholith but older than e dike or e sill. Unfortunately,
there is no way to determine how much younger or older unless other evi
dence is available. Because erosional and depositional processes are relatively
slow, the time represented by a bracketed age may be so long as to be of rela
vely little use in calibrating the geologic time scale. Only a few points on e
time scale have been calibrated by this method.
Metamorphic minerals at develop in sedimentary rocks owing to re
gional or contact metamorphism can be studied also to provide a method of
bracketing the ages of sedimentary rocks (Fig. 15.1.4B). The radiometric age of
metamorphic minerals gives a minimum age for the metamohosed sedi
ment; that is, the metamorphosed sedimentary rocks are older than the time of
metamohism. If a succession of metamorphic rocks is overlain uncon
formably by nonmetamorphosed sedimentary rocks, the nonmetamorphosed
rocks are obviously yoger than the age of metamorphism.
15.3 The Geologic Time Scale
529
<135 to >125 m.y.
Figure 15.1.4
Determining the ages of sedimentary rocks indirectly by (A) bracketing
between two igneous bodies and (B) bracketing between regionally
metamorphosed sedimentary rocks and an intrusive igneous body.
Direct Radiochronolo of Sedimentary Roc
The calibration methods discussed above allow the estimation of ages of sedi
mentary rocks only through their association in some manner wi igneous or
metamorphic rocks whose ages can be determined by radiometric meods.
Clearly, the uncertainties involved in finding ages of sedimentary rocks by
these indirect methods could be avoided if ages could be estimated directly. As
mentioned, terrigenous minerals in sedimentary rocks are not useful for ra
diochronology because they yield ages for the parent rocks, not the time of de
position of the sediment. The only materials in sedimentary rocks that can be
used for direct radiochronology are organic remains (wood, calcium carbonate
fossils, and other such remains) that were deposited with the sediment and au
igenic minerals that formed in the sediment while still on the seafloor or
shortly after burial. The principal methods that have been used for dect ra
diochronology of sedimentary rocks are (1) the carbon-14 technique for organic
materials, (2) the potassium-argon and rubidium-strontium techniques for
glauconites, (3) the thorium-230 technique for ocean floor sediments, and (4)
the thorium-230/protactinium-231 tecique for fossils and sediment.
A short discussion of the advantages and disadvantages of each of these
methods follows. For a description of other possible direct datg methods, such
as amino-acid racemization and other methods based on radioactive disequilib
rium of uranium, thorium, and protactium, see Geyh and Schleicher (1990).
530
Chapter 15 I Chronostratigraphy and Geologic Time
Carbon-14 Method. The carbon-14 method c be applied to the radiochnol
ogy of materials such as wood, peat, charcoal, bone, leaves, and the CaC03
shells of marine organisms. The method has been used extensively for estimat
ing ages of archaeological materials, but its application in geology is limited to
Quaternary geology because of the very short useful age range of the method.
Carbon-14 is produced in the atmosphere owing to the impact of cosmic-ray
neutrons on ordinary nitrogen-14 atoms. The nitrogen atoms lose a proton and
are thus converted to carbon-14, which, in turn, decays backs to nitrogen-14
with a half-life of 5730 years. Carbon-14 is incorporated into carbon dioxide
(C02), which is assimilated by plants and animals during their life cycles.
When organisms die, their tissue no longer assimilates new radioactive car
bon; thus, the amount of radiocarbon in the organisms diminishes with time.
The age of a sample is determined by measuring the amount of radiocarbon
per gram of total carbon in a sample and comparing this amount with the ini
tial amount at the time the organism died. The age equation is
t 19.035 X 10
3
log( i )years
(15.1.3)
where A is the measured activity of the sample at the present moment in dis
integrations per minute per gram of carbon (dpm/g) and Ao is the initial ac
tivity (e.g., Bowen, 1998). Burning of trees and fossil fuels in the past few
centuries has produced a relative decrease in radioactive carbon in the atmo
sphere whereas detonation of thermonuclear bombs has caused a slight in
crease. Corrections must be made for these changes to obtain correct
radiocarbon ages.
Because of the short half-life of radioactive carbon, the carbon-14 meod
is commonly used only for materials less than about 40,000 years old; older
materials contain too little carbon-14 to be determined by standard analytical
methods. Special techniques that make use of mass spectrometers that allow
analysis of smaller amounts of carbon-14, or special proportional counters
with high counting efficiencies (e.g., Bowen, 1988), make it possible to extend
the usable ages in some cases to as much as 60,000-80,000 years. These special
methods are very expensive and have not been widely used in the past. Also,
they are exceptionally subject to systematic error because of contamination of
samples with young carbon.
The carbon-14 method has been used successfully for such applications
as estimating ages of very young sediment in cores of deep-sea sediment and
unraveling recent glacial history by analysis of wood in glacial deposits. Its ex
tremely short range renders the method of little value in calibrating the geo
logic time scale except for very recent Quaternary events.
Radiochronology of Glauconites by Use of Potassium-40/ Argon-40 and
Rubidium-87/Strontium-87. Radioactive potassium-40 (
4°
K) is incorporated
into glauconite grains (green clay minerals composed of complex potassium
aluminum-iron silicates) as they evolve by alteration processes on the seafloor.
When the glauconite grains are fully formed, they theoretically become closed
systems with respect to gain or loss of potassium or argon; that is, no addi
tional radioactive potassium is taken into the grains and the
40
Ar that forms by
gradual decay of potassium remains trapped within the glauconite grains
(e.g., Odin and Dodson, 1982). Measurement of the
40
K/
40
Ar ratio in the glau
conite grains thus allows the age of the grains to be estimated. The half-life of
potassium-40 is 1250 million years; therefore, it is theoretically possible to
15.3 The Geologic Time Scale
531
apply the K-Ar method to radiochronology of rocks ranging in age from about
one million years (less in some cases) to the age of Earth.
Several workers have reported that glauconite ages tend to be 10-20
percent too young owing to some argon loss. On the other hand, calculated
glauconite
ages may be too old in some cases owing to the presence of inherit
ed radiogenic argon that was already in sediment at the me the glauconite
grains formed. Also, the formation of glauconite grains and their closure to
loss of argon do not occur simultaneously with deposition of the enclosing
sediment. Glauconite grains, therefore, must yield a slightly younger age than
the sediment in which ey occur, even if uncertainties about inherited or lost
argon are not a problem. Odin and Dodson (1982) suggest that the time re
quired for glauconites to evolve and become closed systems may range up to
25,000 years or more. Thus, in relation to biostratigraphic zonation, the glau
conite K-Ar ages are closer to those of fossils in e horizon immediately above
the glauconites than to the fossils deposited with the glauconites.
The ages of glauconites can also be esmated by the rubidium-strontium
method (Table 15.1.1). Radioactive rubidium (
87
Rb ) is incorporated into glau
conites as they form, along with potassium-40. The long half-life of rubidium-
87 limits the use of the rubidium-strontium method to radiochronology of
rocks older than about 10 million years. Details of the Rb-Sr method as applied
to the radiochronology of sedimentary rocks are given by Clauer (1982).
Eimating es of Sedimentary Roc by Use of Other Authigenic Minerals.
In addition to glauconite, several other authigenic minerals have been used in
direct radiochronology of sedimentary rocks by the K-Ar and Rb-Sr methods.
ese minerals include clay minerals such as illite, montmorillonite, and chlo
rite; zeolites; carbonate minerals; and siliceous minerals such as chert and opal.
Because of uncertainties about their orig-that is, authigenic or detrital-and
time of closure to seawater interactions, none of the clay minerals except glau
conite have so far proven to yield reliable ages. Zeolites, carbonate minerals,
and siliceous minerals have been used for direct radiochronology of sedimen
tary rocks with some success, but the overall usefulness and reliability of meth
ods based on these minerals have not yet been adequately investigated.
orium-230 and orium-230/Protadinium-231 Methods for Estimating
Ages of Recent Sediments. Uranium-238 decays through several intermedi
ate daughter products, including uranium-234, to thorium-230. Uraum-238 is
fairly soluble in seawater and is present in detectable amounts in seawater. By
contrast, the thorium-230 daughter product precipitates quickly from seawa
ter by adsorption onto sediment or inclusion in certain authigenic minerals
and becomes incorporated into accumulating sediment on the seaoor. Thori
um-230 is an unstable isotope and itself decays with a half-life of 75,000 years
to still another unstable daughter product, radium-226. Owing to this fairly
rapid decay of
2
30
Th, cores of sediment taken from the ocean t1oor exhibit a
measurable decrease in
2
content with increasing depth in the cores. If we
assume that sedimentation rates and the rates of precipitaon of
230
Th have re
mained fairly constant through time, the concentration of
230
Th should de
crease exponentially with depth. The ages of the sediments at various depths
in a core can be calculated by comparing the amount of remaining
23D
rh at any
dep to the amount in the top layer of the core (surface sediment). This
method can be applied to the dating of sediments younger than about 250,000
years old, which makes it useful for bridging the gap between maximum car
bon-14 ages and minimum K-Ar ages.
532
Chapter 15 I Chronostratigraphy and Geologic Time
Protactinium-231 is the unstable daughter product of uranium-235 and
itself decays with a half-life of about 34,000 years to actinium-227. Protacni
um-231, like thorium-230, precipitates quickly from seawater and becomes
incorporated into sediment along with thorium-230. Because protactinium-231
decays about twice as rapidly as thorium-230, e
231
Pa/
23
yh ratio in the sed
iments changes with time. Thus, in a sediment core, this ratio is largest in the
surface layer of the core and decreases progressively with depth in the core.
The ae of the sediment at any depth in e core is determined by comparing
e
23
Pa/
23
�
ratio at that depth to the ratio in the surface sediment. The re
liability of e ages determined by ts method rests on the assumption that
protactinium-231 and thorium-230 are produced everywhere the ocean at a
constant rate and that the starting ratio of these two isotopes in surface sedi
ment is constant throughout the ocean. (See Faure, 1986, and Bowen, 1998, for
details.)
alteative method for calculating ages of sediment based on protac
tinium-231 and thorium-230 involves measuring the ratio of these daughter
products to their parent isotopes in the skeletons of marine invertebrates such
as corals. Dissolved uranium-238 and uranium-235 in seawater are incorporat
ed into corals as they grow, whereas seawater contains no appreciable protac
tinium-231 and thorium-231, because of the rapid precipitaon of these
daughter products. erefore, any protactium-231 or thorium-230 present in
corals results from decay of the parent uranium isotopes within the corals. e
ratio of parent isotope to daughter product decreases systematically wi time,
providing a method for dating the corals. These ratios approach an equilibrium
value
with increasing passage of time, owing to the fact that the daughter
products emselves continue to decay. Thorium-230 reaches a steady state
after about 250,000 years and protactium-231 after about 150,000 years.
Thus, these meods can be used only for radiochronology of rocks younger
than these ages. Because corals and other skeletal materials tend to recrystal
lize with burial and diagenesis, the
231
Pa/
23
�
method has severe limitations.
Recrystallization may open the initially closed system and allow escape of e
daughter or parent isotopes. Therefore, this method cannot be applied to esti
mating ages of skeletal materials that have undergone recrystallization.
SUMMARY
Radiochronology of sedimentary rocks whose relative positions in the sati
graphic column are already established can be accomplished by several meth
ods. The choice of method depends upon the age of the rocks and the types of
materials present in them. In general, calibration of the time scale by estimat
ing ages of volcanic rocks associated with essentially contemporaneous sedi
mentary rocks that can be easily correlated by marine fossils is the most useful
and reliable approach. Radiochronology of sedimentary glauconites or brack
eting the ages of sedimentary rocks from associated plutonic intrusive rocks
may also yield usable ages-the oy ages available in some cases. Therefore,
different methods may have to be applied to estimating ages of rocks in each
geologic system. Details of e methods used for estimating ages of bound
aries between and within e different systems are given in Odin (1982),
Snelling (1985a), and Harland et al. (1990).
Figure 15.3 shows the calibration of the Geological Society of America
1999 Geologic Time Scale on the basis of absolute ages obtained from a num
ber of different sources. Readers should be aware, however, that other pub
lished geologic time scales have slightly different values for some of these
15.4 Chronocorrelation
533
boundaries (e.g., Odin, 1982; Curry et al., 1982; Snelling, 1985b; Harland et aL,
1990), indicating differences in opinion about the ages of e boundaries. Cali
bration of the geologic time table has changed steadily through the years as ra
diochronologic methods have improved and more absolute ages have become
available. Although the ages now used to calibrate the major boundaries of the
geologic time scale are unlikely to undergo major revision in the future, it is
safe to assume that refinements in these ages will continue for some time.
15.4 CHRONOCORRELAT ION
Chronostratigraphic units are extremely important in stratigraphy because they
form the basis for provincial to global correlation of strata on the basis of age
equivalence. We have already established at chronostragraphic correlaon is
correlation that expresses correspondence in age and chronostratigraphic posion
of stratigraphic uni. To many geologists, correlation on the basis of age equiva
lence is by far the most important type of correlation and, in fact, it is commonly
the only e of correlation possible on a truly global basis. Methods of establish
ing the age equivalence of strata by magnetosatigraphic and biologic techniques
have already been discussed (Chapters 13, 14). Several other methods of time
stragraphic correlation are also in common use, including correlation by short
term depositional events, correlation based on transgressive-regressive events,
correlation by stable isotope events, and correlation by absolute ages. These
meth
ods are discussed below.
Event Correlation and Event Stratigraphy
Event correlation constitutes part of what has come to be known as event stratig
raphy. Event stratigraphy focuses on the specific events that generate a strati
graphic unit or succession rather than on the physical or biological characteristics
of the unit. For example, a eustatic rise in sea level can affect sedimentation pat
terns worldwide. As a result of this event, sedimentary facies are generated in a
variety
of environments in various parts of the world. These facies may not be
equivalent in terms of their physical characteristics; however, they are equivalent
in e sense that they were produced as a result of the same event. Thus, they are
chronological equivalents.
Events can be considered to have different scales depending upon their du
ration (Fig. 15.4), intensity, and geologic effect. Some convulsive events are extra
ordinarily energetic, occur quickly, and have regional influence (e.g., explosive
volcanic eruptions, impact of large exaterrestrial bodies (bolides), great earth
quakes, catastrophic floods, large violent storms, large tsunamis). These events
may produce widespread effects, including mass extinctions. Because of their
magnitude, the deposits of such events may form important parts of the geologic
record; in fact, the stratigraphic record tends to overemphasize extraordinary per
turbations (Schleicher, 1992). On the other hand, the products of a particular event
may not be well enough preserved in the geologic record to be recognized as an
event marker (Clifton, 1988), and synchroneity of event deposits from one region
to another may not be easily recognized. Other events occur more slowly and
produce important stratigraphic successions that may be well preserved and rec
ognized over large areas, such as the rise and fall of sea level that generates a
transgressive-regressive stratigraphic succession.
534
Chapter 15 I Chronostratigraphy and Geologic Time
10-
3
10"
2
10-
1
l
10
c
�
10
2
c
0
�
10
3
l
10
4
E
10
5
10
6
10
7
10
8
Age before present (years)
Approximate duration of
_
E
xperimental
Holene
Historical
I
Pleistocene
I
Te rt
I
Mesoz
ce
r
tain classes of events
of significance to the
10·
3
10·
2
1 o-1 1
1 0 1 0
2
1 0
3
1 0
4
10
5
1 0
6
1 0
7
10
8
geological record
Figure 15.4
I
1 Accuracy of
14
C
1
..............
1 hour: Ts unami, gravity flow
1 10 days: Ashfall, lava flow
70 years: Human lifespan
100 1000 years: Continent-wide
expansion of successful immigrant;
iohrcmoloaiical-l
deposition 1-cm pelagic ooze
0.1 my: Average paleomagnetic event
0.5
m
y: Average glaciation cycle
1 my: Cyclothem, paleomnetic epoch
1 -1 0 my: Species lifespan
5
50
my: Orogenic cycle
Resolving power of geochronologic systems in the Cenozoic on the basis of
1
4
C and K-Ar
absolute age discrimination and biochronological discrimination. The vertical axis shows
the duration of events ranging from hours to hundreds of millions of years, and the hori
zontal axis shows age before the present ranging from hours to hundreds of millions of
years. Note that
1
4
C dating can resolve events that range in age from tens of years to less
than 100,000 years and that are years to tens of thousands of years apart. K-Ar dating can
resolve events that are older than 1 00,000 years and that are separated by at least 1 0,000
years. Biochronology is most effective in resolving events that are older than about one
million years and that are spaced at least one million years apart. [After Berggren, W. A.,
and J. A. Van Couvering, 1978, Biochronology, in G. V. Cohee, M. F. Glaessner, and H. D.
Hedberg (eds.), The geologic time scale: Am. Assoc. Petroleum Geologists Studies in Geol
ogy
6, Fig. 1, p. 43, reprinted by permission of AAPG, Tulsa, Okla.]
To be useful in chronocorrelation, events should be relavely sudden, thus
producing abrupt changes in liology, chemistry, biology, and/ or paleomagnetism
that can be recognized. Physical events that meet this criterion include tsunamis,
storms, oods, sediment gravity ows, volcanic eruptions, meteorite and comet
impacts, rapid sea-level changes, and abrupt reversals of Earth's magnetic field
(e.g., Einsele, 1998; Shiki, Chough, and Einsele, 1996). Chemical events, which may
be related to physical events, include sudden changes in stable isotope (e.g., oxy
gen, carbon) content of the ocean and development of anoxic (low-oxygen) condi
tions in e ocean. Biologic changes such as sudden appearance of new species or
sudden extinction of species are also useful events (e.g., Walliser, 1996). Biolocal
events may be related to relatively sudden environmental changes such as major
changes in current patterns or to other physical events (e.g., meteorite impacts).
These various types of events are summarized in Figure 15.5. As mentioned,
physical, chemicat and biological events generate corresponding event deposi
(e.g., a volcanic eruption produces an ash bed). Combining several kinds of event
15.4 Chronocorrelation
535
Figure 15.5
Events
R =regional
+
=global
Transgression
Black
shale
s
{
Anoxic
event
Nodular lime-
.
} Mass extinction
stone of special
Time-specific
pattern
+
facies
Black shales
Monomorph
fossil layer
Te phra
Tu rbidites
Storm layer
Black shales
Rythmic
sedimentary
succession
Fossiliferous
marker bed
Disconformity
with hardground
------------
}
R
Anoxic event
R
Ts unami
R
_rVolcanic activity
Mass moality
R
R
Suspension
currents
Storms
J Anoxic event
• '-Extinctions
Milankovitch
cycles
R
R
Transgression
above gap
B1
?
A6
� L
Schematic illustration of events and event deposits that are useful in chronostratigraphic
correlation. Note that most events are restricted to regions; however, a few, such as anox
ic events that cause mass extinctions or mass mortality, may be of global scope. Combin
ing various kinds of events leads to identification of high-resolution stratigraphic units
(holostratigraphic units) and biostratigraphic units that have chronostratigraphic signifi
cance. Column sea level: F = fall, Ri rise. Column biostratigraphic units: Et earliest,
E = early, M middle, L = late. Column evolution: thick lines of species ranges = index
species. [After Barnes et al., 1995, Global event stratigra phy, in Walliser, 0. H. (ed.), Glob
al events and event stratigraphy: Springer-Verlag, Fig. 1, p. 320.]
markers to identify correlatable horizons has come to be known as high-resolution
event stratigraphy (Kauffman, 1988). Many events are of local or provincial scope;
however, some produce event deposits that are globally traceable, holding out
the possibility of global event stratigraphy (Barnes et al., 1996; Wallister, 1996).
Other than paleomagnetic correlation, most worldwide correlation is based on
biostratigraphy.
Correlation by Short-Te rm De
p
ositional Events
Some events produce key beds, or marker beds, that can be traced in outcrop or
subsurface sections for long distances. These marker beds are useful for time
stratigraphic correlation, as well as for lithostratigraphic correlation, if they were
deposited as a result of a geologic event that took place essentially "instanta
neously." The most striking short-term depositional event ash fall from volcanic
eruptions, which can take place in 1 to 10 days (Fig. 15.4). Beds formed from ash
falls are called ash layers, tephra layers, bentonite beds (if the ash alters to ben
tote clays), or tuff layers. The ash fall from a single eruption may produce ash
layers several centimeters thick that can cover thousands to hundreds of thou
sands of square kilometers. For example, ash from the eruption of Mt. Mazama in
li'l�
536
Chapter 15 1 Chronostratigraphy and Geologic Time
southeastern Ogon about 6500-7000 years ago, an eruption that subsequently
led to the formation of the Crater Lake caldera, was carried noreastward by
winds and deposited as far away as Saskatchewan and Manitoba, Canada. Ash
from the May 1980 eruption of Mt. St. Helens also spread over ousands of
square kilometers east and north of Mt. St. Helens in Washington and Idaho.
Other historic examples of widespread ashfalls include the 1932 eruption of
Quizapu in Chile, an eruption that distributed volcanic ash eastward for 1500
across South America and into e Atlantic Ocean, and the eruption of Perbuatan
Vo lcano at Krakatoa Island, Indonesia, in 1883, an eruption that spread volcanic
dust around the world.
Te phra layers make extremely useful reference points in stratigraphic sec
tions. They provide a means for reliable time-stratigraphic correlation if they are
of suicient lateral and vertical extent and if they can be identified as the product
of a particular volcanic eruption. Identification of individual ash layers or ben
tonite beds can often be made on the basis of petrographic characteristics-types
of mineral grains, rock fragments, glass shards, or other components-or trace-el
ement composition. Ages of these layers may be determined also by radiometric
meods, allowing the layers to be identified and correlated by contemporaneous
age. Tephra
are particularly useful in correlating across marine basins, and
it may even be possible to correlate ash layers in marine basins to well-dated lava
flows or ash
on land, thereby extending marine correlations onto land.
Turbidity currents constitute another type of "instantaneous" geologic event
that can produce tn, widespread deposits (e.g., Einsele, 1998). Turbidites may have
chronostratigrapc sificance if a particular turbidite bed, or succession of beds,
can be dferentiated from other turbidite uts and traced laterally. Unfortunately,
most turbidites commonly consist of rhythmic or cyclic successions of uts that have
very similar appearance and are very diicult to differentiate. us, in practice, the
usefuess of turbidites in time-stratigraphic correlation is rather limited.
Other types of "catastrophic" short-term geologic events include dust
storms that produce fine-grained loess deposits on land or silt-sand layers in ma
rine basins. Storms at sea can stir up and transport sediment on the continent shelf
to produce thin "storm layers" of sand or silt, as discussed in a preceding chapter.
Slower, noncatastrophic depositional conditions also may generate thin, dis
tinctive, widespread stratigraphic marker beds under some depositional condi
tions. Deposition of these beds does not necessarily take place "instantaneously."
Nevertheless, they can be used for time-stratigraphic correlation if they formed as
a result of deposition that took place over a large part of a basin during a relavely
short period of time under essentially uniform depositional conditions. For ex
ample, changes in ocean circulation patterns may bring about anoxic conditions
(Fig. 15.5), leading to widespread deposition of organic-rich black shales. A thin,
widespread limestone bed within a dominantly shale or silt succession implies
deposition of the limestone under conditions that were in effect essentially simul
taneously throughout a geologic province. Such a thin limestone bed within a suc
cession of nonmarine clastic units may represent a brief incursion of marine
conditions into a nonmare environment or the temporary ponding of fresh
water to form a large, shallow lake. Thin limestone units in a thick succession of
marine clastic deposits may indicate shelf carbonate deposition during brief peri
ods when clastic detritus was temporarily trapped in estuaries or deltaic environ
ments and thus prevented from escaping onto the shelf. By contrast, thin
interbeds of sand, clay, or silt in a thick carbonate or evaporite succession may rep
resent temporary incursions of clastic detritus into a carbonate or evaporite basin.
Such incursions may be due to a sudden increase the supply of detritus as a re
sult of tectonic events, periodic flooding on land, or deposition by windstorms or
turbidity currents.