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A simple relation exists between depth and age for ocean floor created at a spreading center. The depth for about 60 percent of the deep oceans can be characterized by this relationship; areas whose depth differs significantly are called residual depth anomalies. They can be separated into two types: aseismic ridges, which are excess crustal loads created at or near a spreading center; and mid-ocean swells, which are thought to be surface manifestations of convection cells in the upper mantle beneath the lithospheric plate. The past position and depth of both types of feature are discussed.

A simple method is introduced for computing the past depth of sediment recovered in individual Deep Sea Drilling Project holes. The method can be used for sites on both normal ocean floor and aseismic ridges. It is accurate from the beginning of the Neogene (25 Ma) to better than 100 m for almost all sites. Four specific examples are discussed in the text.

Charts showing the ages of the ocean floor and the positions of the major plate boundaries and continents are constructed by combining a digitized version of the sea-floor magnetic isochrons with published rotation poles and angles. This report includes constructions for the present, the time of anomaly 5 (9 Ma), and the time of anomaly 6 (20 Ma). Additional charts which include predicted ocean depths are constructed for the present and for three selected ages in the Neogene (22, 16, and 8 Ma). The predicted bathymetries are derived by combining the depth-age relation for normal ocean floor, simple models for aseismic ridges, and individual analyses for mid-ocean swells. A comparison between the predicted and observed bathymetry for the present is presented as evidence that the predicted contours lie within ± 400 m of the observed for about 80 percent of the deep oceans. The marginal basins are excluded.

The paleogeographic position of the continents and the paleodepth contours are evidence that:

1) There was a shallow water passage path around Antarctica well prior to the Neogene.

2) Just before or just after the onset of the Neogene a deep water passage may have developed around Antarctica.

3) The closure of the equatorial circulation system in the late Neogene was probably complex, starting with the closure of northern Australia and South East Asia, then Africa and Eurasia and finally the formation of the isthmus of Panama. The opening and closing of these seaways are in the correct direction to produce effects which may be responsible for the major climatic events in the Cenozoic. These include the cooling of the high latitudes in the Late Eocene, the formation of an Antarctic ice sheet in the Middle Miocene and, finally, formation of Northern Hemisphere ice sheets in the Pliocene.

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