Because of age offsets along the major eastern Pacific fracture zones, the site survey cruise (Fig. F1) tracked over ocean crust of different age. We recombined segments of crust that are roughly the same age to make two major transects through the region: a 56-Ma transect (~53-60 Ma) and a 40-Ma transect (~38-43 Ma). Most drill sites for Leg 199 fall on the 56-Ma transect. A segment of Late Cretaceous crust was also surveyed, but we have not compiled this seismic information.
We have identified seismic reflections within the sediment column that appear regionally correlatable, and we have numbered them from P2 to P5, where P represents Paleogene. We have reserved P0 and P1 for further work in the Paleogene interval between 65 and 56 Ma (Table T1).
We cored the upper sediments with a piston core on the site survey and found that the Leg 199 drill sites north of the Clipperton Fracture Zone are typically blanketed by 5-15 m of red clay above the first biogenic sediments. Biostratigraphic analysis of the core bases shows the age progression of near-surface sediment ages with latitude (Fig. F2) (Riedel, 1971). Because the uppermost biogenic sediments are older to the north, the younger horizons are not present at the northernmost sites.
We hypothesize that the seismic horizons identified are chronostratigraphic (Moore et al., 2002), and we used age constraints from the site survey piston cores and initial correlations to Leg 199 drilling to place approximate ages on them. Postcruise analyses will better constrain the ties between the seismic horizons and drilled sediments. The ages for the seismic horizons are listed in Table T1, with updated ages for Mayer et al. (1985) Neogene seismic horizons. Mayer et al. (1985) used the Berggren et al. (1985) timescale. We have updated ages based upon newer age models (see the "Explanatory Notes" chapter). The hypothesis of chronostratigraphy for the Paleogene seismic horizons is being tested by Leg 199 drilling.
This is second lowest Neogene, or Y-reflector, from Mayer et al. (1985). Mayer et al. (1985) noted that this horizon is in part diagenetic in character, since it is present in the transition from ooze to chalk at Site 574. Nevertheless, it is marked by significant change in carbonate content as well, so it should be a reflector in unconsolidated sediments of similar age drilled during Leg 199. We can identify this reflector in the uppermost sediments of the southern drill sites (Sites 1218 and 1219, Fig. F3; proposed Site PAT-23 reflectors, Table T1), but biogenic sediments of this age are not preserved to the north. It is a prominent seismic reflection that marks the top of a series of relatively high-amplitude, high-frequency reflections in the region north of the Clipperton Fracture Zone (Table T2). South of the Clipperton Fracture Zone, these reflections are much lower amplitude, perhaps because diagenesis of the sediments has cemented them. The age of the Y-horizon is poorly defined but lies within the range of 19-23 Ma based on biostratigraphy (Mayer et al., 1985) and revised age assignments used for Leg 199 biostratigraphy (see the "Explanatory Notes" chapter). The seismic layer may mark a drop in carbonates in the early Miocene that can be found throughout the Pacific basin (see Lyle, 1998).
Initial correlations from drilling at Site 1218 and Site 1219 associate the Y-seismic horizon with a major change in CaCO3 in the lower Miocene at 61.5 meters composite depth (mcd) at Site 1218 and at 31 mcd at Site 1219, both in the middle of magnetic Chron C6n (~19.5 Ma).
Seismic horizon P5 lies at the base of a series of high-frequency reflections in the unconsolidated sediments of proposed Site PAT-23 (Fig. F3). It can also be found at all site surveys on 40-Ma crust (e.g., Site 1218 and proposed Site PAT-21) but only is conformable with the underlying sediments at Site 1219 on the 56-Ma line (Fig. F4). We believe we can identify a P5 horizon at Site 1220, but it drapes over faulted earlier sediments (Fig. F5). Sites farther north (Sites 1215, 1216, 1217, 1221, and 1222) do not have this horizon.
We can also identify P5 at Site 574, which provides one way to attach an age to this seismic horizon. P5 is not very strongly developed at Site 574, perhaps because of the diagenesis that has occurred in this interval (Mayer et al., 1985). Nevertheless, it appears at ~450 ms two-way travel time (TWTT) below the seafloor (~370 meters below seafloor [mbsf]). Biostratigraphically, the reflector lies in late Oligocene sediments in the foraminifer Zone P22, CP19 nannofossil zone, and the Dorcadospyris ateuchus (RP21) radiolarian zone. The age for the reflector based on this correlation is ~25-27 Ma.
Initial correlations from drilling at Sites 1218 and 1219 associate this seismic horizon with the base of the large carbonate cycles in the upper Oligocene (~130 mcd at Site 1218 and ~80 mbsf at Site 1219). This places the P5 seismic horizons at the top of magnetic Chron C8r (26.6 Ma; Site 1218) to the top of C9n (27 Ma; Site 1219).
Seismic horizon P4 is situated at the top of the first packet of high-amplitude low-frequency reflectors in the sediment column. It is the lowest of the identified seismic horizons in surveys above the 40-Ma basement but is within the upper sediment column in the southern sites along 56-Ma crust. At Site 1219 on 56-Ma crust (Fig. F4), the seismic unit is identifiable but not prominent, whereas at Site 1218, it forms the top of the low-frequency sequence near basement.
We identify this horizon near the base of the Site 574 seismic reflection profile, at 550 ms TWTT, where high-amplitude, low-frequency reflectors appear. Using the Mayer et al. (1985) time-depth conversion, this is equivalent to ~460-470 mbsf in the sediment column. The seismic horizon falls within nannofossil Subzone CP16C, foraminifer Zone P19, and the Theocyrtis tuberosa (RP20) radiolarian zone (~32 Ma). Initial correlations to the drilled section at Sites 1218 and 1219 identify this horizon with large lower Oligocene carbonate cycles beginning at ~180 mcd at Site 1218 and ~120 mcd at Site 1219 (Table T1). If the correlations are correct, the seismic horizon marks the top of C11n.1n at Site 1218 (29.4 Ma). P4 lies within the only interval of poor magnetic signal at Site 1219. However, it lies in the lower part of the nannofossil Zone CP18, which has an age of 29-30 Ma.
P4 is best developed at the sites on 40-Ma crust, where it marks the upper surface of the high-amplitude, low-frequency basal sediment package. It is present at the southern sites (Site 1219 and Site 1220) on 56-Ma crust, but sites north of Site 1220 (10°N) do not preserve sufficient biogenic sediments of this age. The P4 horizon has been faulted at Site 1220, suggesting that faulting associated with abyssal hill formation continued throughout the Eocene at this site. This is significantly longer than our estimate of cessation of faulting at Site 1221, now at 12°N (see below).
Seismic horizon P3 is readily identifiable only at drill sites along the 56-Ma transect. Along the 40-Ma transect, we have not specifically picked this horizon. It is difficult to identify it based on preliminary data because it is associated with other, older seismic horizons not enumerated here. These older seismic horizons are associated with rapid middle Eocene changes in calcite compensation depth (CCD), which affect sediments above a depth of ~3500 meters below sea level. On the deeper crust of the 56-Ma transect that remains below the late Eocene CCD, only one seismic horizon exists. P3 marks the abrupt changes in CaCO3 associated with the Eocene-Oligocene transition. It is most prominent in sites between 10°N and ~20°N (Sites 1217, 1220, 1221, and 1222 and proposed Site PAT-13) (Fig. F7; proposed Site PAT-10 profile) but can also be found at Site 1219.
The age on P3 is assigned based on its association with the prominent drop in carbonate downcore in the southern sites at the Eocene/Oligocene (E/O) boundary. At the sites north of the Clarion Fracture Zone (Sites 1217 and proposed Site PAT-13) (Fig. F6), the horizon represents a hiatus between Eocene radiolarian oozes and younger clays overlying them rather than the E/O boundary. P3 is not always conformable to the older sediments. At Site 1221 for example (Fig. F7), the sediment packet above the P3 horizon drapes over faulted older sediments and begins to fill in older topography.
Seismic horizon P2 identifies the top of a lower packet of high-amplitude, low-frequency reflections typical of the basal sediments along the 56-Ma line. Based on drilling during Leg 199, we correlate this horizon to the depth of the first consistently chertified zone in the Eocene. The age of the first chert varies from site to site (Table T1) but seems to define a horizon at ~45 Ma.