Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
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5 Passive Margins: Group 2
5 Passive Margins: Group 2
Pages 45-60
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From page 45... ...
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.
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From page 46... ...
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.
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From page 47... ...
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.
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From page 48... ...
There is an overriding need to inject studies of fundamental processes of sedimentation into broadscale 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.
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From page 49... ...
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)
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From page 50... ...
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/abandonment 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.
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From page 51... ...
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)
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From page 52... ...
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.
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From page 53... ...
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)
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From page 54... ...
Obtained by outcrop measurements Planar geometry of strata along bedding planes Stacking patterns of sequences Internal geometry at a range of scales 54
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From page 55... ...
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.
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From page 56... ...
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)
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From page 57... ...
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!
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From page 58... ...
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.
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From page 59... ...
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.
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From page 60... ...
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.
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