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7 Seismic Stratigraphic Record of Sea-Level Change
7 Seismic Stratigraphic Record of Sea-Level Change
Pages 116-140
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From page 116... ...
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.
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From page 117... ...
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 subsidence, 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.
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From page 118... ...
Seismic Stratigraphic Record of Sea-Level Change Seismic stratigraphy is an approach to the investigation of sea-level fluctuations that is less sensitive than conventional 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)
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From page 119... ...
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 quantification of onlap (especially to derive amplitudes of eustatic fluctuations) , and regional bias in the "global" curve.
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From page 120... ...
Arrivals at greater time delays alternately add and subtract, but their contribution 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~.
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From page 121... ...
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.
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From page 122... ...
In seismic stratigraphic work, unconformities are delineated primarily by a downward (basinward) shift in the position of coastal onlap.
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From page 123... ...
, is generally excluded both for observational reasons and because sequence boundaries are regarded by Vail et al. as primarily a response to eustatic fluctuations, with rates comparable to or greater than the rate of tectonic subsidence.
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From page 124... ...
Some unconformities of the continental slopes may therefore be di achronous, and this possibility needs to be investigated by further seismic stratigraphic studies of sediments deposited 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)
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From page 125... ...
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)
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From page 126... ...
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.
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From page 127... ...
discordance, because the surface is initially concordant with the underlying strata, but erosion does result in an apparent shift in the position of A to the erosional edge, A', reducing the coastal encroachment (the horizontal component of coastal onlap; Vail et al., 1977) apparent in the underlying sequence.
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From page 128... ...
There may or may not be a problem in the case of continental margins at which the depositional coastal break and shelf break coincide, depending on the rate of tectonic subsidence, and on whether observed onlap below the edge of the shelf is truly coastal or is marine onlap. Pronounced shelf bypassing and submarine-fan sedimentation, explicitly associated with type 1 unconformities, do not require complete exposure of the shelf (May et al., 1983~.
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From page 129... ...
For faster rates of sea-level fall, the line of critical bypassing may intersect the depositional coastal break before the rate of sea-level fall reaches its maximum value. We therefore expect type 1 sequence boundaries to be slightly diachronous, although the level of diachroneity (<< onequater cycle)
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From page 131... ...
. 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.~.
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From page 132... ...
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.
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From page 133... ...
It may be difficult to demonstrate a eustatic origin for many thirdorder 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)
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From page 134... ...
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 infe~Ted from seismic stratigraphic data alone because coastal aggradation (the vertical component of onlap)
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From page 135... ...
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 sequence A consists largely of differential subsidence during the deposition of A, but the rapid fall of 450 m between cycles A and B includes the differential subsidence during cycles B to D (broken line in Figure 7.10a)
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From page 136... ...
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 boundaries of global extent from those of more restricted distribution. Basins should be selected for study on the basis of stratigraphic completeness; simple tectonic history (e.g., passive continental margins lacking diapirism)
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From page 137... ...
Seismic stratigraphic interpretation of depositional systems: Examples from Brazilian rift and pull-apart basins, in Seismic Stratigraphy-Applications to Hydrocarbon Exploration, C
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From page 138... ...
Low (19879. Unconformable sequence boundaries at Deep Sea Drilling Project Site 612, New Jersey transect: Their characteristics and stratigraphic significance, in Initial Reports of Deep Sea Drilling Project 95, C
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From page 139... ...
Todd (19811. Northern North Sea Jurassic unconformities, chronostratigraphy and sea-level changes from seismic stratigraphy, in Petroleum Geology of the Continental Shelf of North-West Europe, L
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From page 140... ...
G MILLER the Late Cenozoic stratigraphy in Cajon Pass, southern California, in San Andreas Fault~ajon Pass to Wrightwood, R
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