Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
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 45
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. 46
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. ~7
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 48
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 49
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. 50
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
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- 52
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. 53
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 54
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 55
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). 56
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 57
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 58
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
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 cvu 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
