Subsurface control on seafloor erosional processes offshore of the Chandeleur Islands,Louisiana |
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Authors: | David Twichell Elizabeth Pendleton Wayne Baldwin James Flocks |
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Institution: | 1.U.S. Geological Survey,Woods Hole,USA;2.U.S. Geological Survey,Florida Integrated Science Center,St. Petersburg,USA |
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Abstract: | The Chandeleur Islands lie on the eastern side of the modern Mississippi River delta plain, near the edge of the St. Bernard
Delta complex. Since abandonment approximately 2,000 years b.p., this delta complex has undergone subsidence and ravinement as the shoreline has transgressed across it. High-resolution
seismic-reflection, sidescan-sonar, and bathymetry data show that seafloor erosion is influenced by locally variable shallow
stratigraphy. The data reveal two general populations of shallow erosional depressions, either linear or subcircular in shape.
Linear depressions occur primarily where sandy distributary-channel deposits are exposed on the seafloor. The subcircular
pits are concentrated in areas where delta-front deposits crop out, and occasional seismic blanking indicates that gas is
present. The difference in erosional patterns suggests that delta-front and distributary-channel deposits respond uniquely
to wave and current energy expended on the inner shelf, particularly during stormy periods. Linear depressions may be the
result of the sandy distributary-channel deposits eroding more readily by waves and coastal currents than the surrounding
delta-front deposits. Pits may develop as gas discharge or liquefaction occurs within fine-grained delta-front deposits, causing
seafloor collapse. These detailed observations suggest that ravinement of this inner shelf surface may be ongoing, is controlled
by the underlying stratigraphy, and has varied morphologic expression. |
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