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OCR for page 116
7
Seismic Stratigraphic Record of
Sea-Level Change
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, and K. G. MILLER
Lamont-Doherty Geological Observatory of Columbia University
ABSTRACT
Seismic stratigraphy is a technique for stratigraphic correlation, which involves the identification of
regional unconformities (sequence boundaries) in seismic reflection profiles. These surfaces form
during times of relative sea-level fall as a result of abrupt basinward shifts in sites of sediment
accumulation, and many have been interpreted to have a eustatic origin. Patterns of progressive
onlap punctuated by downward shifts in onlap have been quantified in numerous petroliferous basins
and form the basis for a eustatic sea-level curve. The interpretation of sea-level change from seismic
stratigraphic data is controversial, however, and the purpose of this chapter is to evaluate critically
the method and assumptions by which the sea-level curve has been obtained.
The geometry of stratal surfaces can be determined from seismic sections because these surfaces
are among the most abundant reflectors of seismic energy in sedimentary basins. Interference
between reflections from different surfaces limits vertical resolution to about one-quarter of the
acoustic wavelength (typically on the order of tens of meters), and because the seismic pulse travels
as a spherical wave front, there are also limits to horizontal resolution, that is, to the accurate
determination of the spatial location and size of features that generate acoustic reflections. Sequence
boundaries are recognized by the oblique termination of seismic events (onlap, downlap, toplap, and
erosional truncation), and if appropriate velocity and density logs or vertical seismic profiles (VSPs)
are available from boreholes, it may be possible to locate a given unconformity in the rock stratigra-
phy to well within the level of seismic resolution (on the order of meters). Nevertheless, owing to
the limits of resolution, many more unconformities are present in a sedimentary basin than can be
confidently traced with seismic data. Not all seismic events have primary stratigraphic significance,
but nonstratigraphic events generally do not interfere seriously with stratigraphic interpretation. The
assumption that unconformities have chronostratigraphic significance is generally a good approxi-
mation at an intrabasinal scale for coastal areas and shallow continental shelves. In this chapter, two
examples of diachronous unconformities are presented that raise questions about the universal
application of seismic sequence analysis to the study of eustatic changes.
Unconformities associated with downward shifts in coastal onlap result primarily from an in-
crease in the rate of sea-level fall or a decrease in the rate of subsidence. A prominent shift in onlap
can occur when the rate of sea-level fall is comparable to the rate of tectonic subsidence (~1 cm/1000
~6
OCR for page 116
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
117
yr). Such an onlap shift is not necessarily accompanied by rapid regression of the shoreline. The
controversial issue of whether sea level was oscillatory or monotonically decreasing from the Late
Cretaceous to the onset of glaciation in the mid-Cenozoic is unresolved by published modeling
studies, which did not account for the magnitude of short-term changes in paleobathymetry or
sediment supply. The formation of sequence boundaries of probable eustatic origin at times of long-
term sea-level rise (such as the Early Cretaceous) suggests that sea-level changes may generally be
oscillatory, though of small amplitude. The approximate correspondence of several second-order
sequence boundaries with times of major plate-boundary reorganization may reflect regional bias in
the data, but in part is probably the result of tectonoeustasy. While the idea of global tectonic events
cannot be entirely discounted, no viable mechanism has yet been established for producing correla-
tive unconformities on a global scale. Most tectonic mechanisms for generating sequence bounda-
ries predict dissimilar ages in different basins.
The key to distinguishing boundaries of eustatic and local origin is geochronological resolution.
Although there are inherent uncertainties in the time scale, and in the correlation of seismic and rock
sections with each other and to the time scale, many second-order boundaries appear to be very
persistent from one basin to another. The occurrence of globally synchronous sea-level change is
also corroborated by abundant conventional stratigraphic evidence for much of the Phanerozoic.
The spacing of third-order boundaries is close to or finer than biostratigraphic resolution, and it may
never be possible to resolve the ages of many of these boundaries with sufficient precision for
objective correlation between basins.
Amplitudes of eustatic fluctuations cannot be inferred from seismic stratigraphic data alone
because coastal aggradation (the vertical component of onlap) is primarily a result of basin subsid-
ence, not sea-level rise; and downward shifts in onlap reflect only the rate of sea-level fall in relation
to the rate of basin subsidence. Variations of coastal encroachment (the horizontal component of
onlap) are sensitive to lateral gradients in the rate of subsidence, which are in general time-dependent
and not necessarily linear. Whichever method is used to gauge changes in coastal onlap, the large
component of subsidence cannot be easily or objectively removed to derive the smaller eustatic
signal, and similarities in the patterns of coastal onlap for different basins for the most part indicate
a similar overall subsidence history. For the purpose of deriving a eustatic sea-level curve, the global
onlap chart provides information only about the times of sea-level fall and rise. We conclude that
the main limitations of the eustatic curve derived from seismic stratigraphy are (1) the uncritical
interpretation of all second- and third-order boundaries as eustatic, (2) the uncertainties about the
calibration of many boundaries to the geological time scale, and (3) the largely conjectured inference
of amplitudes.
INTRODUCTION
Conventional Stratigraphic Record of Sea-Level
Change
Oscillatory changes in sea level relative to the conti-
nents, on time scales of <10 m.y., have long been inferred
from paleobathymetric variations in facies successions,
and from stratigraphic evidence for transgressions and
regressions, that is, the alternating landward and seaward
migration of the shoreline (e.g., Suess, 1906; Hallam,1984~.
If consistent changes in water depth or shoreline location
are determined on a regional to intercontinental scale, in a
range of siliciclastic to carbonate facies, a eustatic (i.e.,
global) control is suggested, with the degree of confidence
depending on the reliability and resolution of the facies
interpretation, the precision of the biostratigraphic corre-
lation, and the extent of such correlation. Examples of this
procedure are described by McKerrow (1979; Ordovician
and Silurian), Lenz (1982; Ordovician to Devonian), M. E.
Johnson et al. (1985; Silurian), J. G. Johnson et al. (1985;
Devonian), Ramsbottom (1979; Carboniferous), Ross and
Ross (1985; Carboniferous), Hallam (1981, 1988; Juras-
sic), and Hancock and Kauffman (1979; Cretaceous).
Longer term eustasy and differential vertical movements
of the continents (>10 m.y.), as indicated by the degree of
continental flooding and the elevation of ancient shore-
lines, have been discussed by Bond (1978a,b, 1979),
Harrison et al. (1981, 1983), and Sahagian (1987) and are
summarized in Chapter 8 of this volume by C. G. A.
Harrison.
Water depth and shoreline location are sensitive over a
wide range of time scales to the rates of tectonic subsid-
ence and clastic sediment supply (or carbonate sediment
production) as well as to eustasy. Even if depositional
base level is modulated by eustasy, a given eustatic event
may not be evident in the facies that are preserved, and
times of maximum or minimum water depth or transgres-
sion/regression may not be precisely synchronous at dif-
ferent localities (e.g., Parkinson and Summerhayes,1985~.
OCR for page 116
118
For example, the time of maximum water depth in a sub-
siding basin starved of sediment is clearly not the time of
maximum transgression because of the time lag between
the onset of subsequent regression and the arrival of sig-
nificant amounts of sediment in the deep basin. Where
sedimentation generally keeps pace with subsidence, and
sea-level changes are oscillatory, maximum transgression
corresponds with times at which the rate of eustatic rise is
fastest; where the sediment supply is low, transgression
may occur even during a eustatic fall, providing that the
rate of net subsidence exceeds the rate of eustatic fall
(Pitman and Golovchenko, 1983~. Analyzing the effects
of sea-level change on shoreline position in differentially
subsiding passive continental margins, Pitman (1978) and
Pitman and Golovchenko (1983) argued that under condi-
tions of falling sea level and constant shelf gradient, a
phase lag exists between the time of an instantaneous
change in the rate of sea-level fall and the time at which
the shoreline reaches a new equilibrium position. Accord-
ing to their analysis, lags are on the order of millions of
years for a typical shelf, with longest phase lags corre-
sponding to broad shelves of steep gradient and to ones
characterized by slow subsidence.
In view of these considerations, the degree of syn-
chroneity evident in the conventional stratigraphic record
of short-term sea-level change is remarkable. Such syn-
chroneity reflects a bias toward data from shallow-water
continental platforms of low gradient. It also suggests that
SW
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
sea level varies continuously at a broad range of frequen-
cies rather than episodically and that the equilibrium shore-
line positions inherent in the model of Pitman (1978) and
Pitman and Golovchenko (1983) are rarely attained.
Moreover, the assumption that the point of zero sedimen-
tation rate either coincides with the shoreline (Pitman and
Golovchenko, 1983) or lies at a fixed distance landward
from it (Pitman, 1978) is unrealistic. The estimates of lag
times are therefore hard to evaluate.
Seismic Stratigraphic Record of Sea-Level Change
Seismic stratigraphy is an approach to the investigation
of sea-level fluctuations that is less sensitive than conven-
tional stratigraphy to variations in sediment supply (Vail
et al., 1977, 1980, 1984; Vail and Hardenbol, 1979; Vail
and Todd, 1981; Vail, 1987~. Developed for interpreting
seismic reflection profiles (Figure 7.1), seismic stratigra-
phy makes use of regional surfaces of erosion or nonde-
position known as unconformities or sequence boundaries.
These form during times of relative sea-level fall, when
patterns of sedimentation shift abruptly, generally toward
the basin (near-horizontal bold lines in Figure 7.1). Vail et
al. (1977) suggested that many sequence boundaries are of
the same age in different parts of the world and are there-
fore due primarily to a global process, eustasy. They also
developed a technique for quantifying the amplitude of
relative sea-level change from the sawtooth patterns of
NE
44 KILOMETERS
FIGURE 7.1 Seismic section northeast of Beatrice Field, Inner
Moray Firth, North Sea, United Kingdom, showing interpreta-
tion of seismic sequences defined by the termination of seismic
events (full arrows). Numbers with TR, J. and K prefixes iden-
tify Triassic, Jurassic, and Cretaceous sequences. Numbers on
either side of the section are estimated ages of sequence bounda-
ries in millions of years. Depth is given in kilometers and two-
way travel time for seismic waves. Cross-cutting bold lines are
inferred faults, with half arrows indicating apparent sense of
displacement. From Vail et al. (1984~.
OCR for page 116
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
onlap observed on seismic sections (see Figure 7.10~. By
comparing results from different basins, they obtained an
estimate of global sea-level changes popularly known as
the Vail sea-level curve. This led to considerable discus-
sion as to why sea level should rise slowly and fall quickly.
In subsequent publications, the original curve is termed a
chart of "relative change of coastal onlap" (e.g., Vail and
Todd, 1981; Vail et al., 1984) and is the basis for a smoothly
varying "eustatic curve," which takes into account the fact
that discontinuous changes in onlap are accompanied by
gradual changes in shoreline position. The most recent
version, synthesizing for the first time the entire Mesozoic
and Cenozoic, and incorporating the results of detailed
well-log and outcrop studies, is that of Haq et al. (1987~.
The so-called sea-level curve has been widely quoted in
the geological literature and in recent textbooks (e.g.,
Kennett, 1982; Miall, 1984; Boggs, 1987), but apart from
the obvious criticism that much of the supporting data has
not yet been published, the possible limitations of the
curve are not generally appreciated. In particular, the
original coastal onlap chart is still commonly but incor-
rectly reproduced as the "sea-level curve." Recent articles
by Brown and Fisher (1980), Watts (1982), Hallam (1984,
1988), Steckler (1984), Thorne and Watts (1984), Watts
and Thorne (1984), Parkinson and Summerhayes (1985),
Miall (1986), Summerhayes (1986), Burton et al. (1987),
and Hubbard (1988) have raised several important issues
for seismic stratigraphic interpretation, such as the origin
and chronostratigraphic significance of seismic reflections,
the precision with which unconformities can be identified
and calibrated, the influence of tectonics in the formation
of sequence boundaries, the significance and quantifica-
tion of onlap (especially to derive amplitudes of eustatic
fluctuations), and regional bias in the "global" curve. In
the light of these criticisms, the purpose of this chapter is
to evaluate the seismic stratigraphic method as a tool for
investigating sea-level change. The main conclusion of
this chapter is that seismic stratigraphy provides important
information about the timing of sea-level fluctuations, on
a time scale of millions of years, but little about magni-
tudes.
SEISMIC IMAGING OF STRATAL GEOMETRY
The gross geometry of unconformities and other stratal
surfaces can be determined in seismic sections because
these surfaces are commonly an important source of acous-
tic impedance contrasts. Acoustic impedance, or the product
of rock density and the velocity of seismic waves traveling
through the rock, determines to what extent seismic energy
is reflected from a given surface. The range of velocities
in common sedimentary rocks is greater than the range of
densities, and velocity is consequently more important in
119
controlling the strength of reflections. For example, lithified
sandstone and shale are characterized by densities of
approximately 2.32 and 2.42 g/cm3, respectively, and by
corresponding velocities of 3.6 and 4.2 km/s Nafe and
Drake, 1963; Dobrin, 1976~. The amplitude of a reflection
from a planar contact between these two rock types is
about 10 percent of that of the incident wave. Nonlithified
deep-sea sediments commonly possess velocity and density
contrasts more subtle than these values, and the amplitudes
of reflections from the more prominent stratal boundaries
in such settings are consequently much weaker, perhaps 1
to 2 percent that of the incident wave. For weak reflections,
the detection of stratal boundaries is thus critically depen-
dent on the distinction of reflected seismic energy from
background noise. This is a function of geologic setting,
the quality of the recording and processing system, and the
experience of the interpreter.
Most reflection events seen on a seismic section are
composites of reflections from individual interfaces, but a
comparison between seismic sections and corresponding
well-log cross sections suggests that in many cases the
configuration of reflections mimics the configuration of
stratal surfaces at the level of resolution permitted by the
seismic data (Vail et al., 1977~. A common but generally
incorrect expectation is that reflections correspond to the
boundaries of major lithological units, even where these
cut across stratal surfaces (e.g., Hallam, 19841. Most
lateral changes in sedimentary facies involve such gradual
var~at~ons ~n acoust~c ~mpedance that they are not a s~g-
nificant source of reflections.
SEISMIC RESOLUTION
Despite major advances in the recording and processing
of seismic reflection profiles over the last two decades,
basic laws of physics limit the precision with which acous-
tic images portray the geometry of subsurface stratal
boundaries. To gauge the limits of reflection profiling in
practical terms, it is helpful to understand the processes
that govern vertical and horizontal resolution. Vertical
resolution concerns the ability to distinguish two closely
spaced reflecting surfaces, regardless of whether these are
boundaries of different beds or the upper and lower sur-
faces of the same bed. Horizontal resolution concerns the
ability to detect narrow features and then to image them in
their proper location.
Vertical Resolution
The limits to vertical resolution, thoroughly discussed
by Widess (1973), Neidell and Poggiagliolmi (1977),
Sheriff (1977, 1985), and Mahradi (1983), can be illus-
trated by considering reflections from a thin tabular layer
OCR for page 116
120
of shale enclosed in a thick sandstone layer of lower
impedance. Assuming the density and velocity of shale
given above, a wave of frequency 20 Hz passing through
the shale has a wavelength of 210 m. Experiments and
synthetic models have shown that for layers as thin as one
quarter wavelength (about 50 m for the shale), local peaks
corresponding to reflections from the upper and lower
surfaces are discernible, and the time separation between
the peaks is an accurate measure of layer thickness (Widess,
1973~. However, for layers thinner than 1 wavelength,
addition of the two reflections results in a composite wave
whose peak amplitude is sensitive to bed thickness (Nei
dell and Poggiagliolmi, 19774. As bed thickness is de
creased, the amplitude attains a minimum value at a thick
ness of one-half wavelength, and it increases again to a
maximum at one-quarter wavelength. For thicknesses less
than one-quarter wavelength, the distinct contributions from
the upper and lower surfaces cannot be identified, and the
shape of the composite wave continues to change until the
layer thickness is one-eighth wavelength. For still thinner
layers, the wave shape stabilizes but peak amplitude de
creases uniformly with decreasing layer thickness. It does
not matter whether a thin layer is a single unit or a com
posite of numerous thin beds. As lone as the aggregate
thickness is less than one-eighth wavelength, the reflected
energy is characterized by a nonunique composite wave
form. For example, if the shale layer were 1 m thick, it
would produce a reflection whose amplitude was roughly
0.5 percent that of the incident wave. This bed would As a result of this phenomenon, the seismic pulse can
produce the same reflected waveform if its thickness were be regarded as having a "footprint" whose size depends on
5 m or 50 cm. Only the amplitude would vary. Unfortu- (1) the fundamental frequency of the source, which deter
mines the critical one-half wavelength distance, and (2)
the depth to the reflector, which determines the radius of
the wave front, and hence the radius of the first Fresnel
zone. For a seismic source centered at 20 Hz, and a
stratigraphic section with an average acoustic velocity of
2.25 km/s, a reflection from a depth of 4 s (two-way travel
time) has a diameter of about 500 m (Sheriff, 1985~. This
simple example shows how objects not directly beneath
the seismic streamer can reflect considerable energy.
Because the width of the footprint depends on the pulse
frequency, sources of low frequency tend to detect reflec
tors out of the plane of the seismic section more readily
than those of high frequency.
The radius of the first Fresnel zone also governs the
width of subbottom reflectors that can be accurately de
tected. Synthetic models have shown that strong returns
can be expected from features less than one Fresnel zone
wide, but little information is preserved about the size or
shape of such bodies (Sheriff, 1985~. All objects apprecia
bly narrower than one Fresnel zone have nearly identical
reflections.
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
Horizontal Resolution
--O ~-=G- -~-
A convenient way of understanding the concept of
horizontal resolution is to think of acoustic pulses travel-
ing in the Earth as spherical wave fronts. Each impedance
change encountered by a downgoing pulse acts as a point-
source reflector that returns energy as a similar three-
dimensional wave front. It is therefore possible to receive
an echo off a buried feature at a considerable horizontal
distance from the feature. In marine seismology, stream-
ers consisting of hydrophores summed together in a linear
array minimize engine noise and reflections returned from
features directly ahead of or behind the streamer, but they
do not eliminate side echoes.
The problem of the resolution of size and geometry can
be explained by reference to Fresnel zones. The pulse
from a seismic source is typically several cycles long, and
the reflections originating from early and late portions of
the pulse therefore interfere with each other. Early arri-
vals add constructively, but later arrivals are increasingly
out of phase and approach total cancellation when the time
lag corresponds to one-half wavelength. Arrivals at greater
time delays alternately add and subtract, but their contri-
bution is relatively small. Because the seismic pulse is a
three-dimensional spherical wave front, the initial part of
a pulse that reflects constructively from a planar surface
has a measurable cross-sectional area called the first Fresnel
zone (Sheriff, 1977~.
nately, to use these amplitude variations as measures of
layer thickness, a nearby contact with a layer thicker than
1 wavelength is needed for calibration. This is rarely
possible.
The limit to vertical resolution generally increases with
acoustic velocity and burial depth and decreases with
increasing acoustic frequency. Acoustic velocity tends to
increase during burial as a result of progressive compaction
and cementation. Seismic energy is also attenuated by
reflection from successive interfaces, so that deep reflectors
are less easily distinguished from noise than shallow ones.
Assuming a monotonic seismic source of frequency 20
Hz, the practical one-quarter wavelength limit to resolution
cited above for shale is 50 m. For rocks with high acoustic
velocity, such as limestone (about 5.0 km/s), the one-
quarter wavelength limit is higher (about 65 m). Fortu-
nately, tuned arrays of marine seismic sources in common
use are of relatively broad band and generate components
of many tens of hertz. To the extent that sufficient high-
frequency signal is generated and recorded, the practical
vertical resolution is on the order of 10 to 50 m.
OCR for page 116
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
RECOGNITION OF UNCONFORMITIES IN
SEISMIC SECTIONS
Unconformities are recognized in seismic sections by
the oblique termination of seismic events (see Figure 7.1~.
Events that terminate against an underlying boundary
indicate stratal onlap or downlap (Figure 7.2~. Onlap
involves updip terminations, unless the stratigraphic sec-
tion has been subsequently tilted; downlap is generally
downdip. Events terminating against an overlying bound-
ary indicate toplap or erosional truncation. Toplap arises
from sediment bypassing across the top of a prograding
sedimentary wedge, whereas erosional truncation involves
the removal of previously deposited sediment. These stra-
tigraphic relations commonly change laterally along a
boundary, and seismic events locally parallel unconfo~mi-
ties, especially at correlative conformities, where there is
effectively no depositional hiatus. A reflection is gener-
ated by the unconformity itself only if a significant imped-
ance contrast is present. Such reflections may be discon-
tinuous, especially where discordance of overlying and/or
underlying reflectors leads to lateral phase changes (Vail
et al., 1977, 1980; Vail and Todd, 198 13.
Unconformities possessing the fundamental geometri
cat properties of onlap, downlap, toplap, and erosional
truncation vary considerably in lateral extent, from hun-
dreds of meters to hundreds or thousands of kilometers,
and they separate depositional sequences ranging in thick-
ness from meters to thousands of meters. Observations in
ONLAP
............... ~ A
|OLDER STRATA..
TOPLAP
.. . .. .. - .... . ; ..............
I ~- ~A ~4 L:: (OUNCER RIRA~:
DOWNLAP
CHRONOSTRATIGRAPH IC RELATION
121
outcrop indicate that large-scale sequences commonly
contain other sequences of smaller scale or "higher order,"
although there is probably a continuum of possible scales
(Ryer, 1983; Busch and Rollins, 1984~. Owing to the
restrictions of resolution described above, not all instances
of stratal termination against a given unconformity are
imaged in seismic data, so that reflections may locally
appear concordant even where the corresponding strata are
slightly discordant. In addition, apparent seismic toplap
and downlap can arise where clinoforrns merge into shelf
or basin deposits that are too thin to be resolved acousti-
cally (see Figure 8 of Tucholke, 19813. The number of
unconformities identified in a seismic section is therefore
limited by vertical seismic resolution, and in general many
more unconformities are present in a sedimentary basin
than can be confidently traced with seismic data. It is
usually not possible with seismic data to resolve a given
unconformity over the entire area in which it is present.
These limitations are overcome in practice by selection
of unconformities on the basis of their lateral persistence
and by calibration against the rock record. Although ver-
tical seismic resolution is typically on the order of tens of
meters, seismic sections acquired during exploration for
petroleum are routinely and reliably tied to borehole or
well data by means of synthetic seismograms derived from
velocity and density logs, or by using vertical seismic
profiles (Sheriff, 1977; Badley, 19851. Where biostrati-
graphically resolvable or associated with a distinct change
in facies or stratal dip, it may be possible to locate a given
EROSIONAL TRUNCATION
.. , , ; ....
:. YOUNGER STRATA ....
I=
.. , ... ,: ., , :. .: - .; f : . : .;: :.:: . .;.;; . : -.:' :1
~ I'-'' '' '"' ~ A r ~
~ :.-: ~: - a- :1 it-: . ~.. .: :: 1
~ ~ 1 11~
O
o
UJ
FIGURE 7.2 Stratigraphic and chronostratigraphic relations of
onlap, downlap, toplap, and erosional truncation. Vertical ruling
indicates the duration of the hiatus represented by the uncon-
formity. After Ramsayer (1979).
OCR for page 116
122
unconformity in the rock stratigraphy to at least an order
of magnitude better than seismic resolution (see Vail and
Todd, 19819. The most common error in seismic stratigra-
phy arises where such well control is lacking, and an
unconformity reflection is traced laterally into a promi-
nent reflection that actually onlaps the unconformity (Vail
et al., 19803.
In seismic stratigraphic work, unconformities are de-
lineated primarily by a downward (basinward) shift in the
position of coastal onlap. This is not because boundaries
with such geometry are necessarily more important than
boundaries lacking such a shift in onlap, but for the most
part because such boundaries can be traced with the great-
est confidence. Indeed, for seismic interpretation, Vail et
al. (1984) restricted the term unconformity to those sur-
faces involving local erosional truncation or subaerial
exposure, phenomena commonly associated with a down-
ward shift in onlap. According to this usage, marine
surfaces resulting from deep-water sediment starvation and/
or dissolution, and representing significant hiatuses but
lacking evidence for erosion, are not unconformities. The
delineation of unconformities on the basis of onlap and
downlap is especially appropriate where "follow cycles"
are present. A follow cycle is commonly associated with
a strong reflection along an erosional surface, and consists
of a second peak on the waveform beneath the principal
reflection (Vail and Todd, 1981~. Where the follow cycle
masks underlying reflections, the unconformity may ap-
pear to be stratigraphically lower than its true position.
A downward shift in onlap would seem to be most
easily resolved where (1) the gradient of the depositional
surface is large (e.g., the continental slope); (2) there is
significant differential subsidence; (3) the hiatus repre-
sented at a given locality is long; and (4) overall sedimen-
tation rates are high, as in young, rapidly subsiding mar-
gins. However, as discussed below, the situation is com-
plicated by the fact that an unconformity produced by a
given eustatic event is of greatest regional extent in old,
slowly subsiding margins. By considering rates of tec-
tonic subsidence and seismic resolution, Thorne and Watts
(1984) concluded that for the passive margin of the eastern
United States (an "old" margin), it is unlikely that seismic
stratigraphy can resolve unconformities representing a
hiatus of less than 4 m.y. This may be overly pessimistic
for seismic stratigraphy in general if surfaces as close as
one-quarter acoustic wavelength can be resolved, and if
unconformities can be traced toward the continent from
areas of high subsidence rate, where the reflection termi-
nations are most obvious. Assuming a practical limit to
vertical resolution of 50 m and a relatively slow sediment
accumulation rate of 2 cm/1000 yr, it should be possible to
resolve a hiatus of 2.5 m.y. For broadband seismic sources,
the resolution may be considerably better.
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
SEISMIC REFLECTIONS LACKING PRIMARY
STRATIGRAPHIC SIGNIFICANCE
Although seismic sections resemble geologic cross
sections, not all seismic events have primary stratigraphic
significance. Examples of nonstratigraphic events are those
produced by low-angle faults and diagenetic boundaries,
together with such features as multiples, coherent noise,
diffractions not migrated during data processing to their
proper position, and sideswipe (energy returned from out-
side the plane of the section; see Tucker and Yorston,
19731. Low-angle normal faults and thrust faults in places
geometrically resemble some stratigraphic boundaries, but
they are not a significant source of confusion in most of
the basins used to derive information about sea-level
change. Diagenetic boundaries appear to be best devel-
oped in fine-grained marl-limestone successions, in which
much of the "bedding" may of diagenetic origin, or at least
significantly modified by diagenesis (Hallam, 1986; Ricken,
1986~. Fortunately, such diagenetic layering mainly paral-
lels depositional layering, and usually does not generate
artificial reflection terminations that could be confused
with sequence boundaries. Multiples, noise, diffractions,
and sideswipe can generally be recognized by an experi-
enced interpreter because they tend to cut across events of
stratigraphic origin. Of course, the interpretation of seis-
mic data is not always straightforward, and an example
from USGS seismic line 25 of the passive margin of the
eastern United States has been discussed by Thorne and
Watts (1984~.
Line 25 passes through DSDP Site 612 and within 11
km of the COST B-3 well (Figure 7.3~. Between these two
holes, well-defined reflection terminations on line 25 are
present at depths of between 2.3 and 2.0 s (two-way travel
time), with at least four instances of apparent onlap against
a prominent reflection (event 6 in Figure 7.3) in a lateral
distance of only 8 km. In spite of this geometry, no hiatus
has yet been detected with biostratigraphic data in COST
B-3 at the level of these terminations (Poag, 1980), and
only a minor hiatus has been observed at Site 612 (upper
lower to lower middle Eocene; Miller and Hart, 1987;
Poag and Low, 1987~. Moreover, the high amplitude of
event 6 at Site 612 can be related to a postdepositional
diagenetic front, which is slightly oblique to stratal sur-
faces, and which separates siliceous nannofossil chalk above
from porcellanite-bearing nannofossil chalk below. Two
interpretations are possible. (1) The boundary is not a
sequence boundary, but a diagenetic front, and the appar-
ent termination of reflections is an artifact of poor seismic
resolution (Thorpe and Watts, 1984~. (2) The boundary is
indeed a sequence boundary, but one on which a diagen-
etic front has been superimposed and for which the bio-
stratigraphic evidence is inconclusive. The existence of a
OCR for page 116
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
prominent reflection immediately beneath and parallel to
event 6 favors the second interpretation. If the boundary
were diagenetic, onlapping reflections should continue
across it. In addition, most of the biozonation in the COST
B-3 well (for which the greatest hiatus should be ob-
served) was based on rotary cuttings rather than cores, and
a marked decrease in sediment accumulation rate in the
lower and middle Eocene strata encountered in this well is
consistent with a hiatus (Poag and Schlee, 1984~.
CHRONOSTRATIGRAPHIC SIGNIFICANCE OF
UNCONFORMITIES
An unconformity is a buried surface of erosion or
nondeposition, whose principal significance for seismic
stratigraphy is that it separates younger sediments or sedi-
mentary rocks from older sediments or rocks below (Fig-
ure 7.4~. The term unconformity thus refers to surfaces
not only at a great range of scales but also involving
hiatuses of markedly different duration, so that an uncon-
formity at a small scale might be regarded as a conformity
at a larger scale. An assumption of the technique of
seismic interpretation promoted by Vail et al. (1977, 1984)
is that a given surface has the same chronostratigraphic
significance throughout a given basin; that is, although the
duration of the hiatus represented may vary laterally, the
sediments above are everywhere younger than the sedi-
ments below (Figure 7.4a). The possibility that an uncon-
formity might be diachronous, in the sense that the sedi
COST B-3
2900 3000
1 1 1 1 1 1 1
DSDP 612
10
3
4.0J
in_
1 ~,,l,
2
3 ~ _~.::
m 4
E20- 6
A,
3 10
30
. ~
~ ~. 'it ~
_ At. t. _~ At 'I_.
, , . . . . . . .
_''`l 'at
.'"""..
~L ~_
,....
. = 7_~;;;rr~;;;;rltrt~,-""'~; ~"'~;; ' ;~~ '~-it-
~~~"~
1~
1; V~;~
~ 't-At ~'l)Htt ' It t i, the ~~2 t ~ ;t.;,il; alto t t It At ~
0 1 2 3 4 5
I 1 1 1 1 1
Km
123
meets above the surface at one locality (e.g., Do in Figure
7.4b) might be older than those below it at a different
locality (e.g., D2 in Figure 7.4b), is generally excluded
both for observational reasons and because sequence
boundaries are regarded by Vail et al. as primarily a re-
sponse to eustatic fluctuations, with rates comparable to or
greater than the rate of tectonic subsidence. It is doubtful
that high rates of sea-level change a necessary, but the
chronostratigraphic assumption is probably a good ap-
proximation in coastal areas and on the continental shelves
of passive margins. In such settings, the formation of
unconformities is largely controlled by variations in the
rates of subsidence and sea-level change, and during time
intervals equivalent to a depositional sequence (1 to 10
m.y.), subsidence occurs at a relatively uniform rate. The
assumption of chronostratigraphic significance may not be
valid in the deep oceans, where unconformities are not
necessarily related to variations in depositional base level
(Tucholke, 1981; Tucholke and Embley, 1984), ailed in
some tectonically active areas, where the rate of tectonic
subsidence is not only spatially and temporally variable
but changes diachronously within the basin. We do not
believe that diachronous unconformities constitute a sig-
nificant difficulty for seismic stratigraphic interpretation
in most of the basins used by Vail et al. (1977, 1984) to
derive the sea-level curve, but here we briefly discuss two
examples of diachronous stratigraphic boundaries because
the existence of such boundaries is not recognized by most
. . .
seismic strat~graphers.
6
8
10
1.0
.
30
4.0
FIGURE 7.3 U.S. Geological Survey seis-
m'ic line 25 between shot points 2847 and
3100. DSDP Site 612 was drilled at 3558
on line 25. COST B-3 has been projected
from 11 km to the northeast to 2885. The
numbered bold lines indicate tentative
interpretations by A. B. Watts and N.
Christie-Thick of sequence boundaries.
These are based entirely on reflection ter-
minations (arrows) observed in this single
very short segment of the profile, and are
subject to revision. After Thorne and Watts
(1 984~.
OCR for page 116
124
Diachronous Unconformity on the Blake-Bahama
Outer Ridge
Deposition and erosion in the deep ocean are controlled
by many processes, including bottom-water current speed,
bottom-water chemistry, sediment composition, and sedi-
ment cohesion (Tucholke and Embley, 19841. Of these,
changes in bottom-water currents appear to be primarily
responsible for a diachronous unconformity on the Blake-
Bahama outer ridge (BBOR).
Deep-ocean currents are driven by subtle density differ-
ences related to temperature and salinity distributions, and
they are capable of eroding the sea floor and transporting
particles as large as silt and fine sand in suspension
(Richardson et al., 19811. A particularly well studied
current, the western boundary undercurrent (WBUC), flows
south and west along the continental rise of the eastern
United States (Heezen et al., 19661. The Coriolis force,
which tends to turn the flow to the right in the Northern
Hemisphere, is balanced by an opposing pressure gradient,
and this results in a quasi-steady geostrophic current that
does not necessarily flow in a straight line.
The pressure gradient in a geostrophically balanced
system can be supplied by several factors, including seafloor
topography. The BBOR on the continental rise off Geor-
gia is a topographic obstruction that exerts a substantial
pressure gradient on the WBUC. As the water is diverted
around the ridge toward the east (to the left when looking
A J
o
J
UJ
(a
B
UNCONFORMITY WITH
CHRONOSTRATIGRAPHIC SIGNIFICANCE
~ CORRELATIVE CONFORMITY
- R~_i ~
LATERAL Dl STANCE
-
o
OTT
D'
DIACH RON OUS
UNCONFORMITY
LATERAL DISTANCE
FIGURE 7.4 Chronostratigraphic cross sections of (A), an un-
conformity with chronostratigraphic significance; and (B), a
diachronous unconformity.
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
downcurrent), the current speed increases to maintain a
constant volume rate of transport, and this leads to an
increase in the Coriolis force. The increased Coriolis
force in turn increases the tendency for the water mass to
turn to the right against the BBOR. This self-regulating
system has operated in dynamic balance over the past 25
m.y., and little sedimentation has taken place beneath the
core of the flow. In contrast, the region of more gentle
seafloor gradient along the crest of the BBOR has not
experienced a history of topographically intensified bot-
tom currents. Depositional conditions have been main-
tained, and so much sediment has fallen out of suspension
that the BBOR is now over 2 km high.
The basic elements of the inferred history of the BBOR
are shown schematically in Figure 7.5a, which represents
a view downcurrent. The BBOR is on the right, and cur-
rent strength is portrayed by contours of equal speed.
Sediment carried in suspension by the WBUC accumu-
lates most rapidly in areas away from the core of the flow,
while nondeposition or even erosion takes place beneath
the region of greatest current strength. As sediments lap
onto the base of the BBOR, the zone of nondeposition
migrates upslope (Figure 7.5b). The long-term effect of
this process is to produce a diachronous unconformity
(Figure 7.5c). Without borehole control, incorrect age
relations might be inferred because all of the strata lapping
onto the base of the BBOR appear to be younger than any
of the strata exposed along the erosional upper flank. In
fact, they are of the same age. The error lies in assuming
that the erosional surface was created at the same time
everywhere.
Past contour-following geostrophic currents are likely
to have been important along lower continental slopes and
upper rises. In these regions, unconformities may be re-
lated both to changes in depositional base level, as in the
case of the continental shelves, and to margin-parallel
oceanic currents such as those of the BBOR. Some uncon-
formities of the continental slopes may therefore be di-
achronous, and this possibility needs to be investigated by
further seismic stratigraphic studies of sediments depos-
ited in this setting.
Diachronous Unconformity in Alluvial-Fan Sediments
Along the San Andreas Fault
An example of a diachronous unconformity in an active
tectonic setting has been documented by Weldon (1984) in
the vicinity of the strike-slip San Andreas Fault in south-
ern California (Figure 7.61. The Harold Formation, Shoe-
maker Gravel, and Older Alluvium are Pleistocene allu-
vial and alluvial-fan sediments, derived from the San
Gabriel Mountains southwest of the San Andreas Fault,
and deposited northeast of the fault within the Mojave
OCR for page 116
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
A TOPOGRAPH I CA LLY
INTENSIfIED CURRY\\\
B
EROSIONAL T R UNCAT:ON
~1
~ \ I'\\\'\\\
-
(: DIACHRONOUS UNCONFORMITY
FIGURE 7.5 Development of a diachronous unconformity along
the east side of the Blake-Bahama outer ridge (BBOR). (A)
View downcurrent, with current strength portrayed by contours
of equal speed. (B) Sediment carried in suspension by the west-
ern boundary undercurrent (WBUC) accumulates most rapidly in
areas away from the core of the flow, while the region of greatest
current strength is characterized by nondeposition or erosion.
(C) Migration of the zone of nondeposition upslope produces a
diachronous unconformity. Conventional seismic sequence analy-
sis would incorrectly predict that the onlapping strata on the left
are entirely younger than the truncated strata on the right.
Desert. Sedimentation was accompanied by uplift and
tilting, with most of the deformation concentrated in the
time interval represented by the angular unconformity
between the Shoemaker Gravel and Older Alluvium
(Miesling, 1984~. Magnetostratigraphic results show that
the prominent unconformity between the Shoemaker Gravel
and the Older Alluvium is markedly diachronous, occur-
ring within the Matuyama reversed interval in the vicinity
of Crowder Canyon, but within the Brunhes normal inter-
val at Phelan Peak and near Puzzle Creek, approximately
23 km northwest of Crowder Canyon. We should expect
similar diachronous unconformities on a variety of scales
wherever blocks are uplifted or folded, or basins subside
in a diachronous fashion, as is common in strike-slip ba-
sins such as those of southern California (Christie-Brick
and Biddle, 1985), and in basins where sedimentation is
accompanied by the propagation of thrust faults (J. Suppe,
Princeton University, personal communication, 19863.
ORIGIN OF UNCONFORMITIES
Even the most widespread unconformities are of finite
areal extent because they tend to pass laterally into cor
i25
relative conformities (Figure 7.4a). The formation of an
unconformity can therefore be considered in terms of the
expansion and subsequent burial of zones of nondeposi-
tion or erosion. At the scale of a seismic section, uncon-
formities in shelf and coastal environments are controlled
largely by two factors: (1) changes in depositional base
level, an imaginary surface asymptotic to sea level, and
above which significant sediment accumulation is not
possible; and (2) sediment supply, or production in the
case of carbonates. On a smaller scale, of course, factors
such as the grain size and cohesion of available sediment,
the direction and strength of currents, water depth, depth
to wave base, and the geometry of the depositional surface
influence the development of unconformities. These are
subordinate in comparison with depositional base level
and sediment supply and are ignored in the following
discussion. We also focus primarily on those shelf and
coastal environments in which paleoclimatic and paleo-
oceanographic changes are relatively unimportant in the
~ . ,` , . .
formation of unconformities.
The elevation of base level at a particular locality is a
function of the rates of change of tectonic subsidence and
sea level. By definition, points at base level are subject to
sediment bypassing, and those above base sea level, to
erosion. Expansion of the zone of bypassing is therefore
promoted by a decrease in the rate of subsidence and by an
increase in the rate of sea-level fall. Unconformities tend
to become buried when the rate of subsidence increases or
the rate of sea-level fall decreases (or sea level is rising).
Nondeposition and the development of condensed inter-
vals in deep water tend to occur at times of sea-level rise
because at these times available terrigenous sediment is
trapped preferentially in nearshore and coastal areas. A
decrease in sediment supply or production also promotes
the development of unconformities in coastal regions, as
in the familiar example of the switching of delta lobes; and
a minor downward shift in the position of coastal onlap
can be produced by a decrease in regional sediment supply
(see below). However, the lowering of depositional base
level is a more effective mechanism for the depression of
coastal onlap on a regional scale.
A different interpretation of marine unconformities along
continental margins has been suggested by Brown and
Fisher (1977, 19801. According to them, marine onlap
against the continental slope is commonly initiated not by
a relative sea-level fall but during times of diminished
sediment supply. At these times, sediment is thought to be
eroded from the shelf and redeposited in deeper water.
This mechanism was invoked to explain instances of marine
onlap where no evidence exists for a downward shift in
coastal onlap. However, a decrease in sediment supply
and corresponding increase in water depth would seem to
be unfavorable for the reworking of shelf sediments. This
OCR for page 116
126
FIGURE 7.6 Magnetic stratigraphy of the
Victorville Fan sediments on the northern
flank of the San Gabriel Mountains, south-
ern California. Black stripes represent
chrons and subchrons of normal polarity;
white stripes represent intervals of reversed
polarity. All of the units and the angular
unconformity between the Shoemaker
Gravel and Older Alluvium are younger to
the northwest (Puzzle Creek) than to the
southeast (Crowder Canyon). After Wel-
don (1984~.
- o
is because storms and tides are less effective in transport-
ing sediment as the shelf becomes deeper. Moreover, as
discussed above, not all stratal terminations are acousti-
cally resolved, especially in shelf deposits. Thus the
mechanism proposed by Brown and Fisher (1977, 1980)
may not be necessary.
Sea-Level Change and Sediment Supply
Conditions for the formation of sequence boundaries in
passive continental margin settings have been considered
quantitatively by Thorne and Watts (1984~. The following
is an elaboration of their analysis. To simplify the discus-
sion, we first consider instantaneous changes in the rates
of sediment supply and sea-level fall, and time intervals
that are sufficiently long for topographic profiles to be-
come dynamically graded. The analysis is then extended
to oscillatory sea-level changes and to profiles that are not
. . . . .
In dynamic equal ~ strum.
The rate of tectonic subsidence (Y) of a basin can be
written (modified from Steckler and Watts, 19823:
Y = BUS* (Pm Ps )+ W
VPm Pw J
/\ (Pm Pw (1 qj') )
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
PUZZLE CREEK
rho _
OLDER o o ° °.
A LLUVIUM
SHOEMAKER
G RAV E L
HARO L D
FORMATION
m
r lOO
- 50 ~I
., ,.,.. ,.
I': ::. ::':
Mo
1
_ _
-L1
-1-,
'-1.7
-n 0
1.1
l.1
- 0~5
-0-7
BR U N H E S
MATUYAMA
) A RA M I L L O
where Wd is the rate of increase in water depth; ~SL iS the
rate of eustatic sea-level fall (using the sign convention of
Thorne and Watts, 1984~; S* is the rate of increase in the
sedimentary thickness, corrected for the effects of
compaction; Pm' Ps, and Pw are the densities of mantle,
sediment, and water; and ~ is the basement response
function relating sediment and water loads to tectonic
subsidence. Note that p and p are constants but that p
m w s
actually changes with time as the sediment becomes
lithified. If we assume Airy isostasy, ~ = 1, Eq. (7.1)
. . a.
sump aloes to
· · * Pm W s . (7.2)
(Pm Pw ) (Pm Pw )
For a point above sea level, Eq. (7.2) must be modified to
correct for the sediment load above the datum (-h):
Y = S* ( Pm Ps )+ (h + /\ ~ ( m ), (7.3)
Pm Pw SLY Pm Pw
where h is the rate of decrease of elevation with respect to
sea level. If the lithosphere has strength, and loads are
compensated regionally rather than locally, the basement
response function (~) in Eq. (7.1) assumes a time-depend-
ent value that varies from less than to greater than unity,
but this does not change the overall conclusions of the
following discussion.
(7.1) Equations (7.2) and (7.3) can be rearranged as follows:
OCR for page 116
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30
r
1 1
1 1
1 1
I i
l l .
I Pl Fl r T ~ .
_
=: 1
I SW 31545 _
30~dclnS
dUlNhAOC
A'd~t0N nol3
33N3n'33s
·~ l
~ O
.~' ~ ~z
cn ~
~c
Z
LU _ ,~ lo~o awl,
C, 0 S313A3d3dn'
o
~ 313A3d3dnS
. 5313A3~03W
o '35
~31:)A3~)3W
_
Sd~d3A W N' 3WI] ·<
~ '~z
~ 5°
I ~Go~''~c
~ =o,
m0 OOz
,[ 0 ~ 0
1 O ~ AS J O ' ~ ~ . X 7
o`7 ~1 11- ~1, =! '
~CE ~ -~ZW
53NOZON0~14D _
O . AlidUlOd
\ --~ S~30d3 Alid~O
z ~ ~ A-LIH
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
and Golovchenko, 1983), the rate of change of sea level is
affected immediately (Heller and Angevine, 19851. An
alternative interpretation of apparently synchronous se-
quence boundaries on different continents is that they are
a result of global tectonic events (Sloes, 1979; Bally, 1982~.
While this possibility cannot be entirely discounted, no
viable mechanism has yet been established. Even if there
is communication of stresses between plates, there seems
to be little reason for changes in the rate of basin subsi-
dence to be synchronous on a global scale.
Most "tectonic" mechanisms for generating sequence
boundaries predict dissimilar ages from one continent to
another, or even on different parts of the same contir~-nt.
To the extent that major boundaries in passive continental
margins are globally synchronous, these mechanisms are
of secondary importance, so that the precision with which
unconformities can be correlated has become a critical
issue. Cloetingh et al. (1985), Cloetingh (1986), and Karner
(1986) suggested that variations in horizontal stresses in
the lithosphere of a few hundred bars can induce vertical
motions of tens of meters, at rates comparable to typical
rates of tectonic subsidence (1 cm/1000 yr). According to
these authors, such motions may be responsible for the
third-order boundaries on the coastal onlap chart (those
representing base-level changes on a time scale of 1 to 3
m.y.~. There appears to be good evidence that appropriate
reorganization of in-plane stress takes place, but it has not
yet been established that stresses vary frequently enough
or that stress changes are of sufficient magnitude to pro-
duce most or even many of the observed third-order bounda-
ries. According to the model, compressive stress acceler-
ates basin subsidence and produces uplift of the basin
margins; tensile stress retards basin subsidence and en-
hances subsidence of the margins. The rates of uplift and
subsidence are sensitive to the magnitudes of stress vari-
ations, which vary from place to place within a given
lithospheric plate, to flexural rigidity (a function of age),
and to basin geometry. In particular, stratigraphically
recorded vertical motions are a composite of motions at
each of the density interfaces within the existing sedimen-
tary section and beneath the basin (Karner, 1986~. For
these reasons, no two basins should behave in the same
way, and depending on flexural rigidity, the response of a
given basin may vary according to the orientation of its
margins in map view. For example, in a large basin an
individual unconformity might pass to a correlative un-
conformity not only in the direction of increasing tectonic
subsidence but also parallel to the margin of the basin
where its orientation changes through 90°. Indeed, several
unconformities in one part of a basin might be exactly half
a cycle out of phase with unconformities in an adjacent
part of the same basin. Such unusual stratal geometry has
131
never been described in published seismic stratigraphic or
outcrop studies.
Cloetingh (1986) and Karner (1986) both imply that
downward shifts in coastal onlap should be associated
with compression, and Cloetingh (1986) even attempts to
calibrate the coastal onlap chart of Vail et al. (1977) in
terms of paleostress fluctuation. It is assumed for this
purpose that the chart is weighted in favor of the North
Atlantic region, which is regarded as being sufficiently
small to have experienced the same stress history. Apart
from the obvious hazards of such a correlation, compres-
sion does not necessarily produce a downward shift in
onlap unless eustatic sea level is near stationary or rising.
Under compression, the transition from increased basin
subsidence to margin uplift occurs near the fiexural node.
We have shown above that if sea level is falling, the line of
critical bypassing assumes a position within the basin. An
increase in the rate of tectonic subsidence under this circum-
stance would lead not to a downward shift in onlap, but to
renewed (or continued) onlap toward the basin margin.
Morner (1976, 1980, 1981) argued that sea-level changes
are markedly diachronous in different parts of the Earth
and that they result in part from variations in the configu-
ration of the geoid through geological time. The geoid, or
equipotential surface of the Earth's gravity field, contains
irregularities as great as 180 m, and undoubtedly has var-
ied in the past. We question Morner's interpretation of
available biostratigraphic data, but more important, we
think that his reasoning is flawed because he assumes that
geoidal changes affect only the oceans, whereas on geo-
logical time scales they result in concomitant adjustments
of the solid earth (Steckler, 1984~. Thus while geoidal
changes affect sea level during intervals of thousands of
years, they are not relevant to longer-term eustasy.
A fundamental feature of eustatic unconformities is
their global persistence in marine basins. In attempting to
distinguish eustatic unconformities from those of local
significance (related to tectonics, sediment supply, or paleo-
oceanographic conditions, for example), the apparent ab-
sence of a "global" sequence boundary in a given basin is
generally regarded as evidence against a eustatic origin
(e.g., Thorne and Watts, 1984; Hubbard, 1988~. Such a
criterion should be applied with caution. Different seis-
mic stratigraphers may subjectively select different un-
conformities for correlation in the same basin; many un-
conformities pass laterally into correlative conformities;
and a given unconformity may not be seismically resolved
even though known to be present (from biostratigraphic
data, for example). On the other hand, unconformities of
local origin in different basins may fortuitously be of
approximately the same age, and thus incorrectly corre-
lated and assumed to represent a eustatic signal. In order
132
to resolve these problems objectively, it is essential that
the stratigraphy of every basin used for comparison is
based on an internally consistent detailed interpretation of
a grid of seismic sections. Few such interpretations have
ever been attempted outside the petroleum industry. It is
also imperative to consider the geochronological precision
that may be achieved for each sequence boundary.
GEO CHRO NO LOG Y
Any approach to global seismic stratigraphy requires
calibration to geological time through rock stratigraphy,
but there are inherent uncertainties in the time scale, and
in the correlation of seismic and rock sections with one
another and with the time scale. The choice of an appro-
priate geological time scale is controversial, although every
time scale involves similar components. Stratotypes of
standard chronostratigraphic units (stages) are correlated
with one another in order to establish a chronostratigraphic
framework, and these units are then calibrated against a
numerical scale. One approach is to use all available
radiometric age measurements, including those obtained
from "low-temperature" minerals such as glauconite, which
commonly differ significantly from ages derived from
"high-temperature" minerals (e.g., Odin, 1982; Haq et al.,
1987~. Another is to correlate stratotypes with changes in
geomagnetic polarity, and to calibrate this magnetochronol-
ogy with the few reliable high-temperature age measure-
ments (e.g., Heirtzler et al., 1968; Berggren et al., 1985;
Kent and Gradstein, 1985~. Once an age calibration for the
chronostratigraphic framework has been chosen, other
geological data such as multiple biostratigraphic donations
and geochemical fluctuations can be calibrated to the time
scale.
Uncertainties in time scales are caused by errors in
isotopic age measurements, and especially by problems in
correlation between stratotypes and age measurements.
Numerical uncertainties are partly a function of the age of
the strata and the techniques employed. For example, the
K-Ar technique widely used in Mesozoic and Cenozoic
geochronology involves potential errors on the decay
constant of less than 2 percent, and a typical accuracy of
better than 5 percent (Dalrymple and Lanphere, 19653.
Errors in astronomical (Milankovitch) estimates of the
ages of boundaries for the latest Quaternary may be less
than 5000 years (Imbrie et al., 1984~. Comparisons of
different time scales suggest that errors are typically about
0.5 to 2 m.y. for the Neogene (cf. Odin, 1982; Berggren et
al., 1985', 1 to 7 m.y. for the Paleogene (cf. Odin, 1982;
Berggren et al., 1985), 2 to 8 m.y. for the Cretaceous, and
as much as 10 m.y. for the Jurassic (Kent and Gradstein,
19851. Numerical calibration of biostratigraphic zones
within these intervals is commonly less precise than bio
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
stratigraphic correlation. For example, Paleogene fora-
miniferal zones are typically 1 to 2 m.y. in duration, whereas
radiometric precision is about 2 to 3 m.y.
The boundary between the middle and late Miocene
provides a good example of correlation and time scale
problems. Depending on the author, the age of this bound-
ary varies from 9.5 to 11.5 Ma (Figure 7.9), a range of
about 20 percent of the age. Direct calibration of plank-
tonic foraminifera, nannofossils, and magnetostratigraphy
has removed some of the ambiguity, and the boundary is
now thought to be about 10.4 Ma (Miller et al., 1985a).
Differences among earlier estimates were due to miscorre-
lations. (1) Biostratigraphic correlation of the stratotype
Tortonian (basal upper Miocene) was incorrect. Zone
NN8 is found in the basal upper Miocene stratotype (Miller
et al., 1985a), and this requires Zone NN9 to be well
within the upper Miocene (Figure 7.9, column BKV85),
and not straddling the boundary as shown in columns
VB74, BKD85a, and BKD85b. (2) Correlations of nanno-
fossils and foraminifera with the geomagnetic polarity
record were also incorrect. A long normal magnetozone
associated with Zone NN9 (Epoch 11, Figure 7.9) was
improperly correlated with the Geomagnetic Polarity Time
Scale. Instead of being about 11 Ma (columns VB74,
BKD85a in Figure 7.9), Zone NN9 must be younger than
about 10 Ma (column BKV85~.
Virtually all interregional seismic stratigraphic com-
parisons rely on biostratigraphic correlations. Where
synthetic seismograms can be obtained from geophysical
logs, or VSPs are available, the major limitations to achiev-
able age resolution have to do with the lack of appropri-
ately positioned boreholes, the use of cuttings rather than
cores, and errors or lack of biostratigraphic resolution.
Sequence boundaries are most precisely dated at correla-
tive conformities (Figure 7.4), but such conformities may
not be penetrated in drilling, or in the absence of sufficient
seismic data, may even be misinterpreted as indicating
that no unconformity is present. Where two or more
unconformities are superimposed, a considerable hiatus
may be present, and zonations in such circumstances are
usually equivocal. In the case of commercial wells, most
biostratigraphic work is based on cuttings, and the strati-
graphic significance of such samples is reduced by
downhole caving. Problems associated with biostrati-
graphic errors can be reduced by restricting interregional
stratigraphic comparisons to sections that have been stud-
ied by the same author, but errors in identification, taxon-
omy, or calibration of taxa with those studied by others
may be significant. Another limitation to biostratigraphic
resolution involves the diachrony of taxa (between low
and high latitudes, for example). First appearances of taxa
are commonly diachronous (Johnson and Nigrini, 1985),
and although last appearances are more likely to be syn
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
VB74 BKD85a
in. A ~ AAB
1
YW
(D
z
z
~ A ,.
chronous, even these may be diachronous between sec-
tions at different latitudes (Aubry, 1983~. In order to use
biostratigraphy confidently, ranges of taxa must be cali-
brated against an independent chronology. Direct calibra-
tion to magnetostratigraphy has potential for greatly im-
proved correlations (e.g., Berggren et al., 1985; Miller et
al., 1985a). The precision and reproducibility of bio-
stratigraphic picks are not readily quantifiable. Recent
Neogene studies have claimed biostratigraphic resolution
as good as 100,000 yr (e.g., Keller and Barron, 1983~.
Such claims are extravagant (see Berggren et al., 1983),
but properly calibrated biostratigraphic ranges have poten-
tial for Neogene correlations of better than 0.5 m.y.
(Berggren et al., 1985~.
Our ability to test the scheme of global unconformities
proposed by Vail et al. (1977, 1984) that is, to distin-
guish between global unconformities and those developed
on only a regional or local scale is limited by our ability
to determine the ages of unconformities in continental-
margin successions. Biostratigraphic uncertainties of the
sort outlined above are often so large that later drilling
requires substantial revisions of preliminary findings. A
good example is attempts to date the mid-Oligocene un-
conformity in the continental margin of the eastern United
States. Olsson et al. (1980) used well cuttings to suggest
that the oldest sediments above a prominent unconformity
in that region are about 34 to 31 Ma, and therefore not
consistent with a major erosional event predicted by Vail
et al. (1977) at about 30 Ma. Subsequent examination of
boreholes on the Irish margin, which indicated a hiatus
between 34 Ma and 30 Ma, prompted a reevaluation of the
133
B KD85b H VH 87
B KV85
.
rat :~ ~ ~ ~ ; - -
, ^|. LEJ I~ I ~ ~ ~ ~ ~ L in ~ rm of ep chs ( to 1 ) and c rons
l~ ~ ~A ; ~l ID I ~ LIJ (C4 to C5; black, normal polarity; white,
I1~1 I If I z I z ~'=L: I ~ I z ~ z ~ I rev rse polar ty). iostrat graphy Nl
,, ~o 0 Lo,,~ ~-I to N16 are planktonic foramlniferal zones;
z z ~_z ~ _ NN7 to NNll are nannofossil zones. Note
~ ~.~ oo _ co _ ~ z the variable position of Zone NN9 (coarse
77 _ ~_ z zig _ z z z stipple) relative to the inferred age, Zone
.,. | | z N16 (fine stipple), and magnetic stratigra
- ~z ~phy. Symbols for different time scales:
_ ~ ~_ ~ z ~ ~== l l _ ~VB74, van Couvering and Berggren (1976)
_ z z z z _ z _ ~ _ z ~based on biochronology; BKD85a, Barron
_ _ ~ (D ~ ~et al. (1985), based on second-order mag
~ _ 0 ~_ z z ~ z z z netobiostratigraphic correlations and the
/v: ~ ~ ~ ~ ~ ~ ~ assumption that Epoch 9 is equivalent to
\ ~Chron CSn; BKD85b. Barron et al. (1985)
assuming correlation of Epoch 11 with
Chron CSn; HVH87, Haq et al. (1987) based upon essentially the same geomagnetic time scale as Berggren et al. (1985), and second
order magnetostratigraphic correlations in this time interval; BKV85, Berggren et al. (1985) based on first-order magnetobiostrati
graphic correlations of Miller et al. (1985a), calibrated to the geomagnetic polarity time scale.
FIGURE 7.9 Comparison of time scales
for middle to late Miocene time. Mag
. . . . .
nets polarity chronoloc~c units are given
.
. ha:
.
['I V \AA .
biostratigraphic record from the U.S. margin (Miller et al.,
1985b). These workers determined that the hiatus there
probably extends to at least 30 Ma, and is thus consistent
with the Vail scheme.
In spite of the geochronological problems outlined here
and the potential danger for circularity, in which bounda-
ries are assumed incorrectly to be globally synchronous,
many second-order sequence boundaries (such as those of
the mid-Cenomanian and mid-Oligocene) appear to be
very persistent from one basin to another. Although the
ages of all boundaries are subject to refinement, the idea
that some sequence boundaries record eustatic events
remains an appealing working hypothesis. The seismic
stratigraphic evidence is supported by a good deal of out-
crop evidence, referred to above, for globally synchronous
sea-level change through much of the Phanerozoic. It may
be difficult to demonstrate a eustatic origin for many third-
order sequence boundaries, those derived largely from
higher-resolution well-log and outcrop studies (Figure 7.8;
Haq et al., 1987~. This is because in spite of considerable
recent efforts to calibrate the seismic stratigraphic record
(Haq et al., 1987), it may never be possible to resolve the
ages of many of these boundaries sufficiently well for
objective correlation between basins because the spacing
of third-order boundaries is close to or finer than bio-
stratigraphic resolution.
INTERPRETATION OF SEA-LEVEL CHANGE
For sequence boundaries and condensed intervals of
eustatic origin (perhaps many of the second-order se
134
quences), seismic stratigraphy provides information only
about times of rapid sea-level fall and rise, respectively.
Seismic stratigraphy provides no direct information about
times of eustatic highstands and lowstands; they must be
interpolated (Figure 7.8; Vail et al., 1984; Haq et al.,
1987~. Amplitudes of eustatic fluctuations cannot be in-
fe~Ted from seismic stratigraphic data alone because coastal
aggradation (the vertical component of onlap) is primarily
a result of basin subsidence, not sea-level rise; and down-
ward shifts in onlap reflect only the rate of sea-level fall
relative to the rate of basin subsidence [Eq. (7.6~. Vari-
ations of coastal encroachment (the horizontal component
of onlap) are sensitive to lateral gradients in the rate of
subsidence, which are in general time dependent and not
necessarily linear.
Coastal Aggradation
Basin subsidence is primarily a response to tectonic
subsidence, amplified by sediment loading and modified
to a limited degree by sea-level change and sediment
compaction LEq. (7.1) integrated with respect to time].
Although Vail et al. (1977) used coastal aggradation as a
direct measure of eustatic sea-level rise (Figure 7.10), it is
not an especially good approximation even of a relative
sea-level rise (net subsidence plus eustasy). Four difficul-
ties are as follows:
1. As recognized by Vail et al. (1984), the upper part
of many sequences consists of alluvial as well as coastal
FIGURE 7.10 Procedure for constructing
regional chart of relative changes of coastal
onlap from estimates of coastal aggrada
tion and downward shifts in coastal only
(A) stratigraphic cross section and (B)
regional chart of cycles of relative change
of coastal onlap. The letters A to E are
arbitrary labels for five depositional cycles
shown. A supercycle is a group of cycles
during which there are only minor down
ward shifts in onlap. After Vail et al.
(1977, 1984~. See text for an evaluation of B
this procedure.
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
plain sediments, so that the observed aggradation exceeds
the relative sea-level change.
2. For divergent reflection pattems, to be expected in
differentially subsiding basins, estimates of the magnitude
of aggradation are critically dependent on the path taken
across the seismic section, greater values being obtained
in basinward locations (Miall, 1986~. Incremental mea-
surements of aggradation near the point of onlap (e.g.,
Vail et al., 1977) lead to minimum estimates of aggrada-
tion, but do not eliminate subjectivity from the procedure.
3. Aggradation varies within a basin according to the
local rate of subsidence, basin geometry, and the degree of
subsequent erosion and compaction. It is not clear how
any measurement on a single seismic section can be objec-
tively regarded as the most representative for inclusion in
the coastal onlap chart of the basin.
4. The accurate measurement of aggradation within a
sequence requires reliable estimates of interval velocities
for each of the stratal segments used.
Ideally, coastal aggradation could be corrected for
subsidence, compaction, and water-depth changes (e.g.,
Watts and Steckler, 1979; Hardenbol et al., 1981), but
magnitudes of eustatic fluctuations are difficult to esti-
mate even at a single site. This is due to uncertainties in
estimating paleobathymetry, particularly where sediments
accumulated in water more than 200 m deep; in determin-
ing how sediments compacted; in making corrections for
sediment loading on lithosphere with finite but poorly
known flexural rigidity; and in separating tectonic subsid
A
500 ~
;
; ~ COASTAL DEPOSITS
~1 MARINE DEPOSITS
0J 1
1
0 25 meters
l~m
S00 400 300 200 100 0 -100
v 1
o
~ S
L~
0
~ 15
O 20
o
UJ
(: 25
STILLSTAND 1 LCOASTAL TOPLAP I
1 ~
~,-t---
- __
FALL ~1\
V _ __
_ COASTAL I
CYCLES SUPER
. E
D
5 BCD
_ A
HIGHSTANDLOWSTAND
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
ence from eustasy. Once observed stratigraphic thick-
nesses have been corrected for water-depth changes,
compaction, and sediment loading, the derived subsidence
curve can be compared with a best-fit model curve for
tectonic subsidence. If high-frequency deviations from
smoothly varying long-term tectonic subsidence are largely
of eustatic, not local tectonic, origin, the misfit between
the two curves is a first approximation to the eustatic
signal (e.g., Watts and Steckler, 1979; Hardenbol et al.,
1981~. However, the estimated amplitudes of eustatic
oscillations vary according to assumptions involved in
selecting the best-fit model. The model curve may also be
biased by long-term eustatic effects, and is not necessarily
a true measure of tectonic subsidence. These problems for
a single site are compounded if the stratigraphic input is
coastal aggradation measured incrementally along a sur-
face, and the errors involved are difficult to assess. For all
of the reasons summarized above, the measurement of
coastal aggradation is an inappropriate method for esti-
mating eustasy.
Miall (1986) questioned the method for quantifying
onlap changes for the additional reason that it appears to
ignore the fact that reflections are dipping (Figure 7.10a).
Though contrary to geological intuition, for immigrated
seismic data, stratigraphic thickness is most accurately
measured on a vertical scale (two-way travel time multi-
plied by appropriate velocities, or vertical thickness in a
depth section). Two-way travel time is the time required
for the most direct reflection, which is perpendicular to
dipping strata. As a consequence, no correction for stratal
dip is required. In migrated data, subsurface points are
corrected for spatial mislocation, and aggradation is more
properly measured perpendicular to reflections.
Downward Shifts in Onlap
The sawtooth asymmetry of the coastal onlap chart
reflects the tendency for intervals of progressive onlap to
be punctuated by downward shifts in onlap that appear to
be geologically rapid, but the inferred magnitudes of
downward shifts have no physical meaning. In Figure
7.10, the coastal aggradation of 400 m measured in se-
quence A consists largely of differential subsidence dur-
ing the deposition of A, but the rapid fall of 450 m be-
tween cycles A and B includes the differential subsidence
during cycles B to D (broken line in Figure 7.10a), which
clearly has nothing to do with the formation of the bound-
ary at the top of sequence A. As shown above, a down-
ward shift in onlap is a not a response to a sea-level fall but
to an increase in the rate of sea-level fall. Thus even if the
downward shift were somehow corrected for the effects of
later subsidence and for the various sources of error de-
scribed in the section above, the shift in onlap would still
135
provide no direct information about the magnitude of sea-
level change.
Coastal Encroachment
Recognizing the problems inherent in measuring coastal
aggradation, Vail and colleagues have recently begun to
use variations of coastal encroachment (the horizontal
component of onlap) to construct coastal onlap charts (P.
R. Vail, Rice University, personal communication, 19871.
Horizontal distances can be measured accurately on seis-
mic profiles, and for superposed sedimentary cycles the
degree of coastal encroachment may allow a qualitative
comparison of the eustatic fluctuations associated with
each cycle. However, the use of coastal encroachment
does not remove the ambiguity associated with the fluvial
wedge in the upper part of many sequences, and the sig-
nificance of coastal encroachment in cycles of markedly
different age is uncertain. This is because changes of
coastal encroachment are sensitive to lateral gradients in
the rate of subsidence, which in general are time depend-
ent and not necessarily linear. Identical eustatic fluctua-
tions might produce very different variations of coastal
encroachment at different stratigraphic levels within a basin.
Moreover, it is unclear how variations of coastal encroach-
ment (measured laterally) can be quantitatively "corrected"
for subsidence, compaction, and water-depth changes
(measured vertically), and therefore how resulting coastal
onlap charts for different basins can be usefully compared
to derive a eustatic signal.
Global Onlap Chart
By qualitatively comparing charts of relative change of
coastal onlap for different basins, Vail et al. (1977, 1980,
1984), Vail and Todd (1981), and more recently Haq et al.
(1987) derived a global onlap chart for the Mesozoic and
Cenozoic (see Figure 7.81. Apart from indicating the
timing of global unconformities, assuming that many of
the second-order unconformities are global, the signifi-
cance of such a chart is unclear. It appears to be strongly
biased toward North America and Europe (see Figure 11
of Vail et al., 1984), but in our view the most significant
problem is that similarities in the patterns of coastal onlap
for different basins for the most part indicate similar sub-
sidence history.
Eustatic Curve
In another departure from earlier procedures, attempts
have been made recently to estimate the magnitudes of
eustatic falls from the degree to which coastal onlap shifts
below the depositional coastal break at type 1 unconformi
136
ties, making corrections for compaction, loading, and water-
depth changes (Greenlee et al., 1988~. Some of the prob-
lems inherent in measuring downward shifts in coastal
onlap are thereby avoided. However, it is critically impor-
tant to demonstrate that the onlapping strata below the
depositional coastal break accumulated near sea level rather
than in a deeper marine environment. Only rarely can
paleobathymetry at the point of onlap be established with
confidence owing to the lack of cores and appropriately
positioned wells or boreholes. An additional limitation of
this approach is that only part of a eustatic fall is sampled
because sea level is already falling before the line of
critical bypassing reaches the depositional coastal break,
and nearshore sediments begin to onlap the sequence
boundary before the onset of the next sea-level rise. In
spite of these limitations, we nevertheless expect this
approach to yield improved estimates of the magnitudes of
sea-level falls if procedures are refined and applied to
cored boreholes in transects across continental margins.
The technique of Greenlee et al. (1988) does not appear
to have been used systematically to derive the eustatic
curve published by Haq et al. (1987). Indeed. their eu-
static curve is strikingly similar to a smoothed global
onlap curve, in which the ages of inflection points are
constrained by the ages of sequence boundaries and con-
densed intervals (see Figure 7.8 for the Cenozoic seg-
ment). In view of the many arguments outlined above for
doubting that the onlap curve is a good measure of the
amplitudes of eustatic oscillations, we think that the de-
rived eustatic curve contains information for the most part
about the timing of eustatic rises and falls. Large-ampli-
tude eustatic oscillations indicated by Haq et al. (as much
as 100 m or more for some type 1 unconformities) appear
to be inferred largely by analogy with Pleistocene sea-
level changes (e.g., Vail et al., 1984), and generally not on
the basis of firm evidence for every boundary shown.
Amplitudes of sea-level falls associated with type 2 se-
quence boundaries cannot be determined by Greenlee's
method, and these must have been inferred from the shape
of the global onlap chart.
In summary, the main limitations of the eustatic curve
are (1) that all of the observed sequence boundaries (third
order as well as second order) are uncritically assumed to
be of eustatic origin; (2) that questions persist about the
calibration of many boundaries to the geological time scale;
and (3) that the inferred amplitudes of sea-level fluctua-
tions are for the most part conjectural.
SUMMARY AND RECOMMENDATIONS
The technique of seismic stratigraphy has led to a fun-
damental reevaluation of our approach to stratigraphic and
sedimentological studies. It provides a new way of inter
NICHOLAS CHRISTIE-BLICK, G. S. MOUNTAIN, AND K. G. MILLER
preting subsurface stratigraphy, and of comparing the stra-
tigraphic record in different basins. The identification of
unconformities of apparently global extent has spawned
renewed interest in eustasy and vigorous debate about the
interpretation of seismic stratigraphic data.
The key to seismic stratigraphy is the identification and
calibration of regional unconformities (sequence bounda-
ries), which in most shelf and coastal areas have chrono-
stratigraphic significance. Sequence boundaries associ-
ated with a downward shift in coastal onlap develop in re-
sponse to changes in the rate of subsidence and rate of sea-
level change. The distinction of these phenomena hinges
largely on the precision with which individual boundaries
can be dated and correlated on a regional to global scale.
Amplitudes of eustatic fluctuations cannot be inferred from
seismic stratigraphic data alone because coastal aggrada-
tion is primarily a result of basin subsidence, not sea-level
rise; and downward shifts in onlap reflect only the rate of
sea-level fall relative to the rate of basin subsidence.
Variations of coastal encroachment are sensitive to lateral
gradients in the rate of subsidence. Whichever method is
used to gauge changes in coastal onlap, the large compo-
nent of subsidence cannot be easily removed to derive the
smaller eustatic signal.
For continued improvement of the seismic stratigraphic
record of eustasy, we recommend objective reevaluation
of the ages of sequence boundaries in individual basins,
with the aim of distinguishing more confidently bounda-
ries of global extent from those of more restricted distribu-
tion. Basins should be selected for study on the basis of
stratigraphic completeness; simple tectonic history (e.g.,
passive continental margins lacking diapirism); and the
current or future availability of high-resolution seismic
sections, fully cored boreholes, and state-of-the-art geo-
physical logs and VSPs for calibration of the boundaries.
The basins should also be of a range of ages and widely
separated to avoid regional tectonic bias. Chronostratigra-
phic control should be obtained by integrating multiple
biostratigraphic, isotopic (stable and radiometric), and
magnetostratigraphic criteria. For this purpose, basins at
mid-latitudes offer the most favorable trade-off between
biostratigraphic and magnetostratigraphic techniques. For
improved estimates of the magnitudes of sea-level change,
a transect of boreholes is required for two-dimensional
analysis of tectonic subsidence. The best resolution of
paleobathymetry is obtained from shallow-water carbon-
ate platforms (e.g., Kendall and Schlager, 19811.
ACKNOWLEDGMENTS
This chapter was reviewed by W. A. Berggren, A.
Hallam, J. Ladd, M. Levy, M. A. Kominz, C. G. St. C.
Kendall, W. C. Pitman III, and A. B. Watts. We thank A.
SEISMIC STRATIGRAPHIC RECORD OF SEA-LEVEL CHANGE
Hallam and B. U. Haq for preprints, and T. S. Loutit, H.
W. Posamentier, and P. R. Vail for recent discussions
about seismic stratigraphic principles and interpretation.
The research was supported by NSF grants OCE 86-00249
(Mountain, Miller, and Christie-Thick), OCE 85-00859 and
OCE 87-00005 (Miller), and by the Donors of the Petro
leum Research Fund, administered by the American Chemi
cal Society (PRF 16042-G2 to Christie-Thick). Lamont
Doherty Geological Observatory Contribution No. 4271.
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