We have identified four Paleogene seismic horizons in the central tropical Pacific. Their distribution is bounded by younger basement to the east and by preservation of biogenic sediments to the north. These seismic horizons apparently mark important paleoceanographic events, and postcruise studies of Leg 199 physical properties will be important to better define the paleoceanographic links. Development of this seismic stratigraphy has been critical to defining differences in tectonics at sites along the 56-Ma transect vs. the 40-Ma transect. The 56-Ma transect is much more highly faulted than the younger crust, perhaps because of an Eocene plate reorganization.