Abstract

We suggest that the magnitude of the early-late Oligocene eustatic fall, as predicted from sequence stratigraphy, has been overestimated because of flexural rebound along margins that was induced by late Eocene to early Oligocene erosional unloading. The unloading of sediments from a continental margin during periods of deep-sea erosion and slope retreat will be accompanied by some form of flexural rebound. The magnitude and form of this rebound are controlled by (1) the amount of sediment removed from the margin and (2) the strength of the lithosphere at the time of sediment removal. Along margins with narrow continental shelves, this rebound will induce a relative sea-level fall resulting in a basinward shift in the shoreline, and the predicted systems tracts will be similar to those expected for a eustatic fall. In contrast, rebound along wide continental shelves will not alter the position of the shoreline, even though a relative sea-level fall may be recorded across the outer shelf. Deep-sea drilling and seismic reflection data from many margins support the interpretation of a late Eocene to early Oligocene (∼38-34 Ma) deep-sea erosional event best developed along the continental slope and rise. Seismic reflection and drilling data also document a prominent canyon-cutting event across the shelf during the early-late Oligocene (∼32-29 Ma). This event has been interpreted in terms of a large eustatic fall (>150 m). Nevertheless, there appears to be a correlation between late Eocene to early Oligocene deep-sea erosion and the development, or at least enhancement, of unconformities across the shelf during the early-late Oligocene.

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