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s
PASSIVE MARGINS: GROUP 2
RIFT AND PASSIVE MARGIN BASINS—-
THE SEDIMENTARY RECORI)
.
PREAMBLE
coastal sea
uroductivitv .
The massive sediment accumulations at rifted margins are
mai or repositories for the records of past terrestrial climate,
level, ocean circulation, geochemical cycl es, organic
and sediment supply. Also inscripted is the record
of interaction between the earth ' s interior and its crust, which
led to the breakup of the continents and the creation of oceans.
One new and fruitful way to read the historical record is by
analysis of the internal geometry, composition, and distribution
of primary sedimentary packages called "depositional sequences."
Over the past 200 million years, environmental change has
occurred not only gradually' but sometimes remarkably suddenly as
the oceans, atmosphere, and solid earth responded-to internal
.
physical and chemical processes as well as external forcing
Consequently, the single most important scenic
~ _
events.
objective is to understand the relations between the stratigraphy
of sedimentary sequences, the processes that form stratification
at all scales, and-the geologic events that triggered the
processes.
The specific processes that~build the sedimentary
essentially those that make space (subsidence, compaction,
erosion, etc.) and those that occupy space (sedimentation).
record are
As with and task, in extracting the creative process from
observation of the finished product, the critical problems are to
derive: (1) a clear picture of the product (its three-dimensions
and time), (2) its variance, (3) the linkages between cause and
effect, and (4)
preservation.
, .
the fidelity of the recording and its
The studies that
dimensional
development
formulation
building of
are needed are a collection of multi-
data with an improved measurement capability,
of quantitative techniques for handling the data, and
of a process-oriented model that can simulate the
stratified sediment bodies beginning at the scale of
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a grain of sand and extending to the scale of an entire delta or
reef complex, including the emerged and submerged extremities.
The strategy that must be developed to achieve the stated
goals is to stack the sedimentary records from a variety of
divergent margins, each with relatively simple boundary
conditions, so as to separate the signals that are common to all
margins (i.e., the-global forcing functions and events) from the
noise that is peculiar to individual margins (e.g., local
tectonics and geography, proximity to sources, etc.~.
Fundamental to success is the necessity to combine the expertise
of terrestrial-based sedimentologists and stratigraphers with
the i r marine-based counterparts and to extend the methodo l og i e s
and] measurement techniques into both realms. The boundaries used
for separately evaluating and funding land and sea efforts must
be recognized ant] made more f flexible .
-- BACKGROUND
Current analysis of depositional sequences demonstrates that
multiple processes are responsible for their origin e The
interplay of ~ 1 ) tectonics, expressed through basin-margin
subsidence Thor uplifts anc} variable-patterns of sentiment supply
over time and in space, (2) eustatic~sea-level, and (3) processes
of sediment transport and deposition that combine to produce
complex but-ordered stratigraphies. A fundamental problem facing
our science is the task of defining and interpreting genetically
significant stratigraphic sequences and differentiating and
quanitifying the interplay of local, regional, and global
processes responsib1-e-for ~ eir formation and preservation in the
geologic record.
. .
New concepts are-challenging traditional paradigms of
classical stratigraphy. On a small scale, realistic mathematical
models appear capable of reproducing deposition of sedimentary
packages laid down during simulated storms and tidal surges.
Within the next decade,---it is--reasonable to anticipate the
computational ability to model entire stratigraphic sequences
formed by a variety of shifting depositional and erosional
environments. Modeling also makes possible the prediction of
unconformities and the roles played by the interaction of
sediment-flux and subsidence. Studies of systems tracts, a
concept introduced during the past decade, are proving to have
considerable integrative power. -The system tract concept is
playing a major role in the recognition of global signals
embedded within stratigraphic sequences, which are then
enormously powerful in making worId-wide geologic correlation.
The benefits of the system tract concept are multiple.
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Scientif ic
- Those: of us del iterating in Incline, Cal if ornia found
ourselves in the midst of a repros ution in thinking about the
stratigraphic interpretation of the sedimentary record. In
particular, there are fundamental questions about how the
development of sequences relates to specif ic sedimentary
processes in space and time, and how these interrel ations can be
used to determine the nature, rate, timing, and scale of
geological events. Although sea-level change commonly is
cons idered to be the primary driving mechanism operating on a
global scale, other processes such as tectonics, c' imate change,
and variations in sediment supply can be significant, not only at
regional ant} local scales, but on global scales as well. We need
to understand these relations to explain observed unique
stratigraphy and to reconstruct the behavior and interactions of
earth systems.
Economic
The thick sedimentary sequences of continental margins and
other subsiding basins are repositories of the worId's fossil
fuel reserves. The land/sea interface, which usually lies atop
this sedimentary prism, is a locus of intense human activity. In
addition, submerged margins have been used as disposal sites for
sol id -and f laid wastes . Understanding how sedimentary processes
operate across these margins in space and time is essential for
sensible decisions in optimizing resource use, in wise management
and husbandry, and for damage control.
Biospheric
The sedimentary records of divergent margins-provide a
uniquely rich and complete archive of information on past
fluctuations in global climate, oceanic circulation, nutrient
cycl ing, organic productivity, and biological evolution. The
sedimentary records complement but differ signif icantly in
process and fidelity from the records of the deep sea and
continents. The reading of this record is fundamental to
understanding the historical behavior and - interact ions o f these
global systems and their implications for future planetary
habitability. Whereas scientists in the past have considered
processes recorded in the record to have operated over time
scales too long to be directly applicable to problems on human
time scales, a number of authors have recently debated whether
the fossil record can offer a number of insights into geographic
patterns and selectivity of present-day extinctions, and into the
nature and timing of post-extinction recoveries.
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THE SINGLE MOST IMPORTANT SCIENTIFIC OBJECTIVE
The major problem of the sedimentary record is that it forms
by a complex interplay and overprinting of processes ranging from
simple subsidence to the practical obliteration of any signature
of an original beach during transgression, by erasure due to
erosion and mass wasting, and by deterioration from burial
diagenesis. Therefore, the single most important scientific
objective is one of a rigorous conceptual reconstruction which
allows one to understand the relations between the existing
stratigraphy of sedimentary sequences, the past processes that
formed them, and the geologic events that forced change and
triggered the processes. There is an overriding need to inject
studies of fundamental processes of sedimentation into broad-
scale stratigraphic analysis, along with numerical modeling,
using better calibrated parameters and integrating the efforts of
both land and marine researchers.
The Specific Processes Involved
Specific processes of interest are those that require
quantification so that their magnitudes and rates of change can
be put into numerical equations for the simulation of sedimentary
sequence formation. The key processes are essentially those that
make space and those that fill space (Figure I). The former
include thermally driven subsidence, down warp from the weight of
the sentiment accumulation, flexure of the crust, compaction of
buried strata, and erosion from the surface of the sediment pile.
The dominant space filling process is sedimentation in its many
forms. Sedimentation is in fact a composite of supply from
external sources, primary organic production, chemical
precipitation, residues from evaporation, and the growth of reefs
and bioherms. In calculating the instantaneous conf iguration of
a sedimentary sequence, particular attention must be placed upon
the resolution to which the rate of subsidence and accumulation
can be determined. Better sampling, the application of isotopic
chemistry, and quantitative techniques for handling data are
contributing to more reliable estimates of rates of supply,
subsidence, compaction, and erosion.
We identified-a number of scientific inquiries that could
lead to great improvement in analyses of divergent margins.
Origin of Stratification
One of the most obvious features of the sedimentary record
is its stratification, that is, the layering imparted by the
alternation of periods of sediment erosion' deposition, and non-
deposition (transport). We nonetheless have an inadequate
analytical understanding of the mechanics of stratification at
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all scales, ranging from the centimeter-sized/second-duration
scale of laminae, to the lOs or loos of meters and hundreds to
millions of years that encompass unconformity-bounded sequences.
We need to inject studies of basic processes of sedimentation
into our description and interpretation of these stratigraphic
bodies, and the thick sedimentary record of divergent continental
margins ~ ant] the ~noclern shel f seas ~ represents a natural
laboratory in which to advance this understanding of physical,
biological, and geochemica~ effects.
Origin of Stratigraphic Sequences
Three topics require particular attention.
1. Patterns of sedimentary fill in rift versus drift
basins. The tectonophysics of faulted rift basin formation are
radically different from those of mature, slowly subsiding
margins; moreover, the nature of base level changes and the
fidelity with which the depositional interface tracks base level
are radically different in closed, predominantly non-marine rift
basins than in drift "basins" open to sea level e In other words,
the key processes that make space and f ill space in these two
phases of divergent margin evolution are fundamentally different.
we need a clearer and more detailed picture of depositional
sequences (and stratal surfaces) in rift basins both to constrain
models for rifting mechanisms and sediment accumulation, and to
evaluate the relative quality of these records as repositories of
information on earth history.
2. The role of siliciclastic sediment influx. Although
relative sea level changes (the combined ef fects of eustatic sea
level change and subsidence to create space for sediment
accumulation) are invoked to explain most stratigraphic
sequences, changes in the provenance of siliciclastics between
successive sequences and other evidence indicate that such
changes alone cannot explain the formation of all such packages.
Instead, changes in the composition and volume of sediment influx
must be invoked, whether driven by tectonic, climatic, or other
factors. The relative importance of the sediment influx factor
needs to be determined: and' most critically, this parameter must
be cal iterated so that it can be incorporated into future
numerical models. Such mociels to date have assumed constant
sediment inf lux over time, or sediment influx as varying as a
(typically monotonic) function of water depth changes. Neither
of these assumptions is geologically satisfying. If we are to
advance our understanding of the formation of sequences in
siliciclastic margins, we must explore the sediment influx factor
as vigorously and realistically as we have the relative sea level
factor. This theme cries out for cooperative efforts by process
sedimentologists, stratigraphic observationists, and numerical
modelers e
3. Distinctive expressions of sequences in siliciclastic
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versus carbonate margins. The geologically stylized models
derived from seismic analysis of siliciclastic-dominated
divergent margins bear little geometric resemblance to sequences
on carbonate shelves e The biogenic buildups that characterize
the edges of most carbonate margins create geomorphic bowls that
are filled with bioclastic or chemical sediments, rather than
ramps across which strata prograde. Furthermore r from what is
qualitatively known from studies of carbonate margins to date,
the formation and destruction/abandonment of carbonate sequences
will be out of phase with the formation and destruction/abandon-
ment of siliciclastic sequences (see Table 1~. Improved
information on the anatomy and distribution of carbonate
sequences will help direct the numerical modeling efforts, and
information on the differential response of carbonate and
siliciclastic margins to relative sea level changes is essential
to reconstructing accurate histories of eustatic sea level
variation and linkages among global systems. Carbonate margins
provide a complementary record of earth history, and provide
historical data themselves on the cycling of CO2 and organics.
Stratigraphic Documentation of Global Synergisms
1. Sea level. Advocate the Ocean Drilling Program
recommendations as outlined in the COSOD II report (pages 3-7 of
the report).
2. Interactions of a large number of integrated systems,
including global climate r oceanic circulation, and oceanic
chemistry which are not independent of sea level, and which have
cascading ef f eats for organic productivity and biological
evolution. Seek evidence and explanations for events and for
secular variation in the states of these systems and their
balance points/thresholds. It will require a cooperative effort
by many specialists and on all scales, ranging from short-term
oceanographic to historic geological.
Improved Temporal Resolution
The "technological" or methodological breakthrough that
would have the greatest and broadest effect on the study of
divergent margins--in fact, on the entire field of physical and
historical geology--would be the improvement of temporal
resolution to the level of O. 5 million years or better. Because
it represents a quantum leap, highest priority breakthrough,
improved temporal resolution must be identified as a specific
experiment rather than a toolbox item.
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TABLE ~ Carbonate Versus Clastic Responses to Changing Sea Level
Carbonate
Environment
Classic
Environment
1. Sea level
rise
accumulate on flat tops
(aggrade)
carbonate is "fixed"
(sink)
carbonate is exported
to basins
carbonate tracks sea
level by:
(I) keeping up to it
(shallow sequence)
(2) catching up to it
(shallowing upward)
(3) giving up (deepening
upward)
periplatform sequence gets
mud, high water content, low
velocity, rapid deposition
rate
trapped on upper
shelf (coastal
plain ~ estuary)
supply to basin
is shut off
deep slope
collects carbon-
ate/veneer in
absence of
Plastics (hard-
grounds form)
2. Sea level
drop
-
shut off supply (arose
"factory")
expose bank tops (basin
"starved") -
expose fixed carbonate by
cement
karst surface forms on top
of bank
laterite soil forms
the.-al convection over
banks
concentrates rain--scrubs out
dust and deposits bauxite
precursors
fresh water caps basin, and
basin gets stratified
black shales form in sink holes
with laterite on top
oxygenated basins collect only
pelagic sediment (calcitic,
low rates of accumulation, high
seismic velocities result from
cementation)
5
erode-remobilize
toward basin
basin gets sediment
(gets "fed")
Ford shallow unac~n-
formities
valleys are scoured
prograde shelf
build out shelf edge
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Critical Problems
Stratigraphic Problems
1. Facies anatomy of sequences developed during the rift
phase and their spatial variability. Present sequence concepts
have been developed in marine basin margins. However, deposition
during initial rifting is initially within non-marine, commonly
lac:ustrine basins, where relative base-level change is untenable
as a basis for sequence def inition. Instead, tectonic phases or
events are ~ ikely to def ine primary depositional style and
stratigraphy. The sequence paradigm must be expanded to
encompass such a tectono-stratigraphic setting.
2 ~ racier anatomy of sequences formed curing drift ~ thermal
subsidence phase) are better known but highly variable. The
processes that control this variabi~ ity are complex and
~- controvers ial .
3. Unconformities and other hiatuses are known from outcrop
analysis to be highly variable in physical expression, lateral
extent, origin, and chronostratigraphic utility, but this is not
reflected in current subsurface-based sequence stratigraphic
models.
4. The sedimentary and stratigraphic significance of
seismic facies and geometries needs to be tested and refined
using independently derived interpretations of facies and stratal
patterns.
Processes
1. Interpretation of sequences requires that we be able to
extrapolate the small-scale, space/time physics of sediment
erosion, transport, and deposition to stratigraphic scale
problems, including the formation of beds, facies, depositional
systems and surfaces.
2. For biogenic and chemical sediments, we need similarly
detailed, complementary information on mass budgets and bio-
geochemical cycles. Accumulation of such sediments reflects an
interplay of physical, biological, and chemical processes that
are not well constrained.
3. Recognition of sequences places a priority on the timing
and interpretation of stratal surfaces, which define all scales
o f s ec3 imentary packages used in hi storica l reconstruct ion . The
sophi st i cation o f process model s f or eras ion and non-depos it i on
lag far behind] models for deposition.
Events
1. The stratigraphic expressions of local, regional, and
global events, such as autocyclic shifts of c3epocenters, intra-
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plate deformation, eustatic sea-level changes, and reorganization
of ocean circulation patterns, must be established so that these
very different mechanisms can be differentiated. To do this, it
is essential that we ~ ~ ~ def ine the three-dimensional
distribution of sequences, and (2) improve the precision of
temporal resolution in reading and correlating the stratigraphic
record.
2. Linkages between external forcing events and sedimentary
features need to be established. Factors that complicate simple
one-to-one correlation between the event and its record include
thresholds, nonlinear and/or chaotic behavior, and diversity of
response.
3. An essential key to accurate histories, and thus to
well-constrained dynamical models at any scale, will be improved
understanding ( and estimates and analytic methodology) of the
preservation potential of recorded events. Implicit in most
histories and models is an assumption that the record is complete
or, at the least, unbiased in any systematic way, yet this
remains an untested hypothesis.
NEEDED STUDIES
Kinds of Information
The fundamental requirement is the 3-D large-scale surface
and subsurface anatomy of the sedimentary record in rift and
post-rift settings from divergent continental margins. Such an
anatomy is provided (Table 2) by indirect geophysical
prospecting, direct sampling by coring, monitoring via well-
logging tools, measurement in field outcrop, and decipherment by
various age-dating methods.
Data Collection Methods and Tools
The scale of investigation of extensional terrains is
important. Of primary concern is the nature of rifted regimes,
which because of extension must be examined at large geophysical
scale. The stratigraphic response to extensional tectonism is
the unconformity-bounded sequence, which records major episodes
of basin evolution, measured in tens of millions of years.
Smaller scale adjustments to tectonics are expressed in local
stratigraphic and structural packages, which are measured in
millions of years. Facies and seismic variants are also critical
in reconstructing depositional and tectonic events on the scale
of the outcrop, which reflects events generally shorter than
millions of years because they impose geometric constraints on
the major fault systems.
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TABLE 2 Hierarchy of Information
Obtained by geophysical exploration
Surf icial geomorphology
Seabed texture
Seismic reflection
Seismic refraction
Gravity
Magnetics
Obtained by well-l ogging
Heat flow
Elastic properties
In situ density
Poros ity
Permeabil ity
Some chemistry
Obtained by drilling and wireline coring
Composition
Texture
Bedding
Pore Fat nicts
Consol idation,
Lithofacies
Biofacies
Chemofacies '-
strength
(e.g., organic matter, C, P. Si, 02)
Obtained by analysis of samples
Biostratigraphy
Magnetostratigraphy
Chemostratigraphy
Isotope stratigraphy
Radiometric dating
Event stratigraphy (e.g., tephra-chronology, tektites,
planktonic blooms, rare catastrophic)
Obtained by outcrop measurements
Planar geometry of strata along bedding planes
Stacking patterns of sequences
Internal geometry at a range of scales
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To assess the scientif ic merits of a continental margin, it
is necessary to use a broad variety of techniques. Information
gathering would proceed from an evaluation of the large-scale
structures of different margins to detailed analyses of
sedimentary sequences, and to the establishment of their time-
space framework. To def ine its large-scale anatomy, an array of
geophysical and geological methods is employed.
Seismic reflection provides the gross geometry of the margin
including its rift and drift stages. Seismic refraction yields
information about seismic velocities, which constrain time-depth
section and serve to define the nature of the basement. Further
constraints of basement structure and composition are given by
magnetic and gravity data. Magnetic data of the adjacent crust
also yields important information on the kinematics of early
opening, particularly about ridge jumps and asymmetric spreading
To increase our understanding of the continental margins
origin, construction, and internal fabric and to fulfill the
scientific objectives given previously, a broad spectrum of
techniques has to be employed. In such a task, we can see a
natural hierarchy of techniques. Some will provide a general
overview to address our understanding of mega-scale phenomena,
such as the margin makeup and its relation to its principal
carriers (such as the continental and ocean crust). Others will
address micro-scale phenomena such as transport of a single
clastic or carbonate grain, or geochemical species residing at
the margin.
To define the mega-scale phenomena, a whole suite of
geophysical data, including multichannel reflection seismic
corrected with a broad variety of techniques (high resolution,
deep penetration, low angle, multi-ship, undershooting, 3-D
grid), refraction, gravity, and magnetics will be needed . Such a
data base will provide the spatial information about margins,
architecture , sedimentary basins formation, and characteristics
of the underlying basemen" e It-provides valuable information
about the tectonic history of-the margin from its birth as a rift
basin to its maturity expressed as a divergent. Such data for
offshore regimes need to be augmented by sub-surface geological
information, provided by drilling (ODP) and recovery of
conventional cores. Employment of modern logging techniques will
enable constraint in the interpretation of seismic reflection
data and testing of seismic concepts. Geological studies, such
as mapping, sampling in outcrops or by continental drilling are
the principal gathering methods for the sub-aerially exposed
parts of the continental margins or their ancient equivalents.
Data collecting on a more detailed scale is a determinant
for reconstructions of strata geometries, stacking patterns of
sequences, and studies of internal geometries at all scales. For
successful accomplishment of such a task, collecting data in a
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3-D grid is essential. Correlation of geophysical and
drilling/crop data will provide the means of testing the seismic
sequence concept in relation to sea level changes; the data
gathering for this purpose has to be done in both carbonate and
clastic regimes.
Undisturbed samples are required for lithofacies, biofacies,
and chemofacies studies. This step can be accomplished only if
rock samples for laboratory studies can be obtained e To achieve
this, a variety of techniques such as drilling, dredging, bottom
sampling, submersible or ROW sampling needs to be employed. The
samples will provide data to describe texture, structure,
mineralogical, chemical anti physical properties (porosity,
permeability, stress/strain, thermal conductivity) of the rock.
To place the geometrically-defined sequence and its
lithologic content into a time frame, high resolution
stratigraphy is needed. This means integrating biostratigraphic,
magnetostratigraphic and chemostratigraphic data into a
chronostratigraphic framework. Additional data must come from
radiometric dating of authigenic low-temperature minerals, e.g.,
glauconite, and/or high-temperature minerals, e.g., sanidine,
from intercalated volcanic rocks or sediment. Event stratigraphy
provides an additional tool for time correlation on a local,
regional, or global scale.
Laboratory, Theoretical, and Numerical Developments
Dynamical Sediment Flux and Accumulation Models
We need to build our continental margin sediment prisms by
means of forward models, supported by extensive observations of
modern processes and of the record of sedimentary response. The
models must contain enough of the small-scale physics of sediment
accumulation (i.e., they must be dynamical, rather than kinematic
models) so as to constrain the large-scale stratigraphic
geometries that we wish to build (Figure 2~. To be realistic,
such models must have the ability to create stratigraphic
discontinuities without external forcing.
Modeling will occur at two basic scales. On the event
stratigraphic scale, the models should use principles of fluid
and sediment dynamics to create the horizontal grain-size
gradients, and the stratification patterns that characterize
sedimentary lithofacies (Figure 1~. At the sequence
stratification scale, the models must be able to vary the
stratigraphic process variables of eustacy, subsidence, and
sediment input, so that stratal sequences (event stratigraphy
fabrics) are shaped into larger scale units of organization
(facies, depositional systems, and depositional systems tracts).
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my sew few ink
· V~:~-~:~.~::::.~.4 ~~
J ~ By'.-.... .-.- _ ~~
`-.~.~.:r:~-~-~-' ~ -at -~.P~-P-=~=:t-r~
,
scqu~ fommson
foment chime"
s~g~ph ~ ~ -
~u pmducum
biological c~oluiion
occult cimulanon
organic productivity
cam
rift t~tO=C5
FIGURE 1 The sedimentary record is essentially constructed from
a combination of space making and space filling processes acting
in an environment that change" with time.
STORM BED FORMATION
t ~ · ._ ~ .. ~ · ~ _
'~e ~~
FIGURE 2 Dynamic models of process sedimentology incorporate the
physics of particle movements. They describe the sediment
texture and geometry prior to, during and after the sedimentary
event across a section of seafloor that might range from the
proximal nearshore to the distal shelf edge.
Our modeling must be intimately associated with an extensive
program of field verification and calibration. We must determine
critical rates in analogous modern environments, including the
sediment accumulation rate at short and long time scales, the
shear stress cremate in the benthic boundary layer, and the
resulting bedding thickness frequency distributions. We must
determine the relevant spatial dimensions from the depositional
record (strata! continuity and the geometries of facies masses,
depositional systems, and systems tracts).
Backstripping Models
We also need to examine subsidence histories by means of
backstripping models. Contemporary back stripping models are
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mainly one-dimensional, but we need a three-dimensional
understanding of the subsidence process. Subsidence, due to
crustal cool dung, sediment loading or external tectonics, is the
mechanism whereby most of the space is created for the depos ition
of sediments on continental margins . Thus, it is essential that
we have the capabi~ ity to model subsidence. It shouict also be
noted that load-incluced subsidence causes ups ift at the margins
of the depositional axis. Thus, we al so need to model loadl-
induced uplift.
The model ing of subsidence carries with it the requirement
for correcting for compaction during deposition of sediments. We
currently correct for compaction in a crude manner as a result of
a poor understanding of the compaction process e Compaction data
are not directly accessible e without continuous coring, a
condition that will usual ly not prevail over wide areas. We are
therefore constrained to estimating compaction from seismic and
well-Iog data. We would like to have the ability to determine
from wed I-log measurements, cores, and seismic data the pre-
compaction thickness of buried strata of various lithologies,
facies, and sediments with different diagenetic histories.
Seismic Expression of Stratal Geometries
We needs a better understanding of the patterns of reflectors
associated with different facies, depositional systems, and
systems tracts. This is a difficult probe em because of inherent
ambiguities in the equivalency of seismic and rock properties,
and because of the variation from one Repositioned system to
another. Nevertheless, seismology remains the most important
means of reconnaissance of rock properties on the continental
margin. We therefore need to accelerate studies of the ways in
which different aspects of depositional events appear on seismic
sections in order to enhance our interpretational capabilities.
Measurement Capabil ities Needed
Understanding the stratigraphy of sedimentary sequences in
divergent margins requires that we determine as accurately as
possible the physical, chemical, paleontologic, and temporal
features of the rock record. In achieving this goal, we will
face a number of technological challenges including:
· Drilling technology that assures complete core recovery
of unconsolidated sands, shallow water carbonates, inter-
bedded hard/soft lithologies in all depth ranges
including shallow water settings. These settings include
atolls, lagoons, carbonate platforms and the inner
portions of siliciciastic margins. Sub-bottom
depths as great as 10 km are also required and should
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include riser capability augmented by submersibles and
remotely operated vehicles.
· Logging that yields continuous measurements of in s itu
temperature, formation porosity, permeabil ity, acoustic
properties, grain orientation ~ bedding dip, formation pressure,
and formation consolidations
· Maj or element components of the sediment column and pore
water.
· Se ismic pro f il ing that de f ines sequence strat igraphy
to the bottom of the sedimentary column with vertical
resolution that can image the internal stratal geometry
of depositional sequences in a grid that is dense enough
to def ine the geometry of stacked sequences acquired and
processed with state-of-the-art technol ogy.
~ Integrated big-, magneto-, and isotopic stratigraphy
adequate for developing chronostratigraphic resolution
below 1 m.y. throughout the sedimentary column.
· Ability to derive quantitative pa~eobathymetry
approaching Tom resolution.
· High spatial resolution 3-D imaging of sequence tract
geometries using acoustic sources and sensors in boreholes.
STRATEGIES REQUT=D TO A=IE~ THE GOAT
To differentiate local or regional geologic impulses from
global impulses, it will be necessary to study the sedimentary
record of a variety of different divergent margins and then
determine whether these individual records show corre~atable
elements that can be attributed to synchronous impulses (Figure
3~. The margins should have the following characteristics: (1)
simple, predictable subsidence history; (2) different ages (i.e.,
different subsidence rates); and (3) relatively complete but
accessible sedimentary records. Because of the generally
different ways in which divergent margins accumulate sedimentary
records on the eastern and western s ides of ocean teas ins and in
s i ~ ic icl ast ic versus carbonate environments (Table 1), it is
desirable to study and compare examples of each.
For practical reasons, it will be necessary to limit the
intercomparisons to certain time-stratigraphic intervals. For
example, finite resources will not allow a detailed investigation
of a large number of margins-that would be necessary to piece
together and compare the entire Jurassic to Recent sedimentary
record in the foreseeable future. Thus, it is sensible to select
two time-stratigraphic intervals, one (Oligocene to Pliocene)
which records a known glacioeustatic signal, and one (early
Cretaceous) which is thought not to have a glacioeustatic signal.
We currently do not know whether there are enough margins
with both of the above characteristics and appropriate time-
stratigraphic intervals to make valid intercomparisons and test
59
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for synchronism. Therefore, an essential prerequisite is to
conduct a global reconnaissance review of existing data
(acad~ic/industry) to determine (~) whether the comparisons can
be made and (2 ~ the optimum time-stratigraphic in~cer~rals that
should be studied.
Many 0 f these capab i ~ it i es f or measurement and ana ~ ys i s
exist at present, but they have not been aclecTuately applied to
the study of divergent margin sediments. A major obstacle is the
absence of a facility through which data relevant to the studies
described in this document can be acquired, archived' and made
available to the research community. The data should be
accessible through a digital database available via academic
computer-networks.
I_
~~ If_
2 --~=-r——"
delays
c~-e
gLa:~
n ~ Ion"
n
~~ £~
l
not noise · Iec~ Veldt'
corbeled Usual a Slot fetrciag hit Ed cv—u
FIGURE 3 The sedimentary records of numerous margins are
"stackedn. The correlated signal represents the forcing
functions and events of a global nature acting synchronously on
all margins. The uncorrelatec3 noise equates with the local
variability of individual margins.
60
Representative terms from entire chapter:
sedimentary record
