relationship between eustacy and stratigraphic sequence of passive margins walter c. pitman iii
TRANSCRIPT
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Relationship between eustacy and stratigraphic sequence of
passive marginsWalter C. Pitman III
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Outline
• Abstract
• Introduction
• Causes of sea-level change
• Effect of continuing differentiation of lithosphere
• Sediment build-up and removal
• Crustal shortening and thermal welts
• Volume changes of mid-oceanic ridge system
• Sea-level changes, late Mesozoic to Present
• Sedimentary history of Atlantic-type margins
• Computations of position of shoreline as a function of rate of sea-level change
• Computation of a theoretical stratigraphic cross section
• General results and discussion
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Abstract
• It’s is shown here that these events may be caused by changes in the rates of sea-level rise or fall.
• It is concluded also that the presence of the shoreline seaward of the shelf edge of an Atlantic margin should be symptomatic of events that may cause rapid sea-level fall, such as glacial build-up of the sudden flooding of large deep basins.
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Introduction
• The shoreline tends to stabilize at that point on a margin where the rate (or fall) of sea-level is equal to the difference between the rate of subsidence of the platform and the rate of sediment infill.
• It is shown that volume changes of the mid-oceanic ridge system give potentially the greatest rates and greatest magnitude of sea-level change.
• The model developed here quantitatively relates the position of the shoreline to the rates of subsidence, rates of sea-level change, and rates of sedimentation.
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Cause of sea-level change
• Slow long-period changes and shorter duration.
• Volumetric changes in the mid-oceanic ridge system are potentially the fastest way to change sea level.
• h-d= 0.7h (=0.7R)
h: the change in water depth
d: consequent adjustment of this ocean basin relative to the continent
density of upper mantle = 3.4 g/cm3
R= rate of change of water depth
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Effect of continuing differentiation of lithosphere• This process is regarded as the most important mechanism for causing
differentiation between heavier (oceanic) and lighter (continental and volatile) materials.
• The overturn has probably been occurring persistently for more than 2 b.y., a near-equilibrium condition should have been reached .
• Production of juvenile water at active ridge axes and at island arcs; removed by the process of hydrothermal alteration of oceanic crust and by subduction of the oceanic lithosphere and part of its sedimentary overburden.
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Sediment build-up and removal
• Seuss (1906) suggested sediment infill of the ocean basins as a means of raising sea level. Sea level might also be lowered by sediment removal at subduction zones.
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Crustal shortening and thermal welts
• The Himalayas were uplifted in Tertiary time as a result of collision of the Indian subcontinent with Eurasia which caused extensive crustal shortening.
• Occasionally ocean basins traverse hotspots, creating volcanic welts such as the Hawaiian seamount chain.
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Volume changes of mid-oceanic ridge system
• Menard (1969), Hallam (1963), Russell (1968), and Valentine and Moores (1972) have all suggested that major fluctuation in sea level may be caused by changes in volume of the mid-ocean ridge systems.
• Age versus depth relationships for all mid-oceanic ridges are the same regardless of the spreading history.
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Volume changes of mid-oceanic ridge system
• If the depth of the oceans increases by a thickness h, the ocean basins will subside a distance d, and as shown above the change in freeboard will be 0.7h (= h-d)
• We assume that as sea-level rises the area covered by the sea increases linearly. (0.17 x 106 km2 for each 0.001-km rise in the sea-level).
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Volume changes of mid-oceanic ridge system
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Sea-level changes, Late Mesozoic to Present
• The various segments of the ridge system do not necessarily act synchronously, but the net effect since Cretaceous time has been contraction in volume and hence sea-level lowering.
• Another source of significant error is in the computed spreading rate, which depends on the reliability of the magnetic polarity time scale.
• The major modification is the inclusion of the Indian Ocean ridge system and a ridge system in the Tethy.
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Sea-level changes, Late Mesozoic to Present
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Sedimentary history of Atlantic-type margins
• The general structure of Atlantic-type margins is that of a seaward-thickening mass of systematically stratified sediment overlying a deeply subsided, faulted basement platform.
• Driving subsidence that is attributed to increased density of the rifted basement caused by thermal contraction and/or phase changes. (decrease exponentially with time)
• The sediment that fills the basin because of the driving subsidence causes further subsidence.
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Sedimentary history of Atlantic-type margins
• The rate of subsidence at the shelf edge of all passive margins (young or old) where such measurements have been made is usually greater than 2 cm/1000yr and is always greater than the rate at which sea-level may normally fall (or rise).
• Although the rate of driving subsidence decreases with time, the rate of decrease is quite slow for older margins.
• For the sake of simplicity, we will assume that the subsidence of a mature Atlantic-type margin may be modeled as a marginal platform subsiding about a fixed landward hingeline.
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Computations of position of shoreline as a function of rate of sea-level change
D = distance from hinge line to shelf edgeXL = distance from hinge line to shorelineX = distance from hinge line to any point on shelf or coastal plainSL = shelf and coastal plain slopeRSS = rate of subsidence at shelf edge of basement platform relative to a horizontal plane that extends through hinge lineRSL = rate of sea-level change relative to same horizontal planedyss/dt = rate of vertical movement of shelf surface relative to same horizontal planedyws/dt = rate of vertical movement of sea = level surface relative to shelf surfacedx/dt = rate of movement of shoreline relative to hinge lineS = uniform sedimentation
-dsed/dt = erosion
dsed/dt = deposition
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Computations of position of shoreline as a function of rate of sea-level change
The example above illustrates the point that a regression may occur when sea-level is risingsimply because of a decrease in the rate of rise.
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Computation of a theoretical stratigraphic cross section
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Computation of a theoretical stratigraphic cross section
Upper Cretaceous Paleocene
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Computation of a theoretical stratigraphic cross section
Paleocene - Eocene Eocene - Oligocene
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Computation of a theoretical stratigraphic cross section
Oligocene - Miocene
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Computation of a theoretical stratigraphic cross section• Minor oscillations in sedimentation rates can produce interfinfering
transgressive and regressive sequences
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General results and discussion
• Transgressive or regressive events may be caused by changes in the rate of sea-level change.
• Rates of sea-level change are always less than 1cm/1000yr and are usually less than the rate of subsidence at the shelf edge of Atlantic-type margins.
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General results and discussion
(Deep Sea Drilling Project)
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General results and discussion
• The model as presented is in essence predictive and is in fact crudely successful.
• Subsidence may be attributed entirely to the combined effects of thermal cooling and sediment loading.
• Sea-level rise of the past 10,000yr has been so rapid that sedimentation has not been able to keep up.