Seismic profiles in the central equatorial Pacific have a remarkable acoustic stratigraphy, with seismic reflections traceable for >1000 km (Mayer et al., 1985; Bloomer et al., 1995). Mayer et al. (1985) identified reflectors in the sediment column associated with changes in carbonate content in equatorial sediments drilled during Deep Sea Drilling Project Leg 85 at Site 574 (4°13´N, 133°20´W). Furthermore, they hypothesized that major paleoceanographic events caused the impedance contrasts and specifically suggested that the seismic horizons are chronostratigraphic. At least two of these horizons were identified in the eastern equatorial Pacific in sediments recovered during Leg 138 at 110°W (Bloomer et al., 1995), which provides support for their chronostratigraphic nature and illustrates their widespread occurrence. The Leg 138 sites are >2500 km east of the "type" locality in the central Pacific.
Because very little sediment accumulates in the central Pacific Ocean outside of the equatorial region and the net drift of the Pacific plate is west-northwest, during Leg 199 only the lowest part of the Neogene section was drilled. The Neogene reflector set of Mayer et al. (1985) is only of marginal direct use as a guiding stratigraphy for sediments north of the Clipperton Fracture Zone. Nevertheless, because of the usefulness of the Neogene seismic stratigraphy of Mayer et al. (1985) for understanding regional Neogene sedimentation, we specifically designed the site survey for the Leg 199 cruise (Maurice Ewing 97-09) (Fig. F1) to collect data pertinent to constructing a Paleogene seismic stratigraphy for the east central tropical Pacific. Our study of the seismic reflection data collected on the site survey cruise has convinced us that such a seismic stratigraphy exists for the Paleogene (Moore et al., 2002). Here, we will identify a set of seismic reflections of probable paleoceanographic significance. Postcruise studies will test whether these seismic horizons are associated with the hypothesized specific sedimentary horizons.
Unlike the seismic stratigraphy of Vail et al. (1977), which is based on the effect of sea level change on continental margin sedimentary sequences, the stratigraphy described by Mayer et al. (1985) is based more on paleoceanographic events. Intervals of pronounced carbonate dissolution, rather than erosional episodes associated with sea level change, produce the seismic reflectors. To the extent that these paleoceanographic events represent synchronous and pervasive changes in deepwater chemistry and surface water production, the seismic stratigraphy of the deep sea is a chronostratigraphy. However, it does not rely solely on lapouts and truncations to define seismic sequence boundaries. Rather, the seismic stratigraphy of the deep sea relies more on the internal patterns of reflection packages to define groups of seismic reflections that can be traced over great distances and can be shown to have time, as well as facies, significance.