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SITE SUMMARIES (continued)

Site 1221

Site 1221 (12°01.999'N, 143°41.572'W) forms an equatorial component of the 56-Ma transect drilled during Leg 199. It is situated about three quarters of the way between the Clipperton and Clarion Fracture Zones in typical abyssal hill topography at a water depth of 5175 mbsl. On the basis of regional magnetic anomalies, we anticipated basement age at Site 1221 to be equivalent to C25n (~56 Ma; Cande et al., 1989), about the same age as Site 1220. At the outset of drilling at Site 1221, our estimate for total sediment thickness was ~150 m.

Based upon a fixed hotspot model (Gripp and Gordon, 1990, for 0- to 5-Ma Pacific hotspot rotation pole and Engebretson et al., 1985, for older poles) Site 1221 should have been located ~1° south of the equator at 56 Ma, in an equatorial position at ~50 Ma and ~2° north at 40 Ma. Thus, Site 1221 should have been situated underneath the South Equatorial Current in the early Eocene. A nearby piston core (EW9709-14PC) taken during the site survey cruise recovered more than 11 m of red clay, with the base of the core containing a calcareous radiolarian ooze dated as early Oligocene in age on the basis of radiolarian biostratigraphy (Lyle, 2000).

Four holes were drilled at Site 1221. Hole 1221A was abandoned because of a stuck XCB barrel that resulted in a pipe trip to clear the BHA. Hole 1221B recored the Oligocene interval at the top of the sediment column, whereas Hole 1221C cored to cover the major gaps in the upper sediment column and then cored continuously to basement. Hole 1221D was drilled to obtain a second copy of the lower Eocene and Paleocene sediments.

The heave of the vessel frequently exceeded 5 m while we drilled at Site 1221. The ship heave compromised the quality and recovery of the core. The uppermost ~20 mbsf encountered severe flow-in problems, whereas in the remaining intervals, the often soupy nature of the sediment (radiolarian ooze) resulted in MST data that were not suitable for correlation purposes, with the exception of the P/E boundary interval. A continuous sediment column could not be correlated below 19.5 mcd.

The sediment column is a variation of lithologies seen before at other Leg 199 sites. A very thin clay and radiolarian ooze lithologic unit overlies lower Oligocene nannofossil ooze, terminated by an abrupt transition to clay rapidly grading to radiolarian ooze. The abrupt transition is typical of the
E/O boundary. At Site 1221, however, latest Eocene and earliest Oligocene time is represented by a hiatus of perhaps 1 m.y. in extent. The middle and upper Eocene radiolarian ooze section is the thickest of any site of Leg 199. Deposition of the radiolarian ooze was highest in the period between 45 and 43 Ma, when sedimentation rates reached 18 m/m.y.

A chert-rich section covering the time interval from ~48 Ma (radiolarian zone RP11) to 53.8 Ma (nannofossil Subzone CP9a) lies beneath the radiolarian ooze. Despite the fact that coring times were relatively fast, only chert was recovered, suggesting a significant amount of radiolarian ooze or clay is also present. Hole 1221C drilled somewhat faster than Hole 1221D, indicating some variability in the thickness and areal extent of the chert. In the recovered sediments below the chert unit, clay layers in Hole 1221C can be correlated to clay layers in Hole 1221D. Beneath the chert-rich interval are nannofossil chalks of early Eocene–Paleocene age that comprise the basal sedimentary unit. Within this basal chalk we recovered two copies of the P/E boundary interval. The P/E boundary interval is recognizable by a multicolored clay-rich unit remarkably similar in structure to the P/E boundary interval at Site 1220, 206 km to the south. The lowermost chalks above basement were dated at 56.5–57 Ma with foraminifers and nannofossils.

Physical properties of the sediments follow lithology. The densest unit is the lower Eocene chalk (~1.6–1.8 g/cm3) followed by the upper clays and nannofossil ooze. The radiolarian ooze has the lowest wet bulk densities (~1.14 g/cm3) and the highest porosities (~84%), as well as the highest velocities of the unconsolidated sediments (1530–1550 m/s).

Biostratigraphic analyses indicate that Site 1221 contains a continuous sequence of radiolarian-bearing sediments from the early Oligocene to the early Eocene. The uppermost sequence contains considerable quantities of reworked middle Eocene radiolarians, while the interval from ~7 to 111 mbsf is almost exclusively brown radiolarian ooze with a sugary texture. No radiolarians were recovered in the chert-rich interval except in the core catcher of Core 199-1221C-10X at a depth of 141 mbsf. These radiolarians are from the early Eocene–late Paleocene Zone RP7. No radiolarians are found in the basal chalk.

The sediments of Site 1221 contain calcareous microfossils in only the uppermost and lowermost units. Calcareous microfossils are completely absent in the siliceous sediments between ~7 and ~111 mbsf, and biostratigraphic control through this interval was provided entirely by radiolarians. The carbonate content of sediments increased in the basal reaches of Holes 1221C and 1221D, and nannofossil and planktonic foraminiferal biostratigraphy allow basic zonation of a condensed sequence of lower Eocene nannofossil ooze and chert. Section 199-1221C-11X-3 contains an interval of dramatic and colorful banding, the base of which, as at Site 1220, corresponds to the extinction of Paleocene benthic foraminifers and therefore the P/E boundary. Preservation of calcareous microfossils improves below the P/E boundary, and planktonic foraminifers and nannofossils allow a detailed zonation of upper Paleocene nannofossil ooze and clay. Calcareous benthic foraminiferal assemblages occur only in the basal sediments below 140 mbsf. These assemblages are moderately well preserved, but the presence of small calcite microcrystals on the surfaces of the tests indicate some diagenetic alteration.

Interstitial pore water profiles from Site 1220 are very similar to the profiles of all other Leg 199 sites except Site 1219 and primarily reflect minor organic matter degradation, the dissolution of biogenic silica, and minor alteration of underlying basalt. The bulk geochemistry of the sediments from Site 1221 reflect the shifts in lithology between sediments dominated by silica and carbonate.

Paleocene/Eocene Boundary

The P/E boundary was recovered twice (interval 199-1221C-11X-3; 50–90 cm and interval 199-1221D-4X-2, 70–125 cm). The two intervals are very similar, with the interval in Hole 1221D appearing slightly expanded relative to that in Hole 1221C and exhibiting more disturbance.

The benthic foraminiferal fauna were examined in small surface scrapes prior to more complete shore-based analysis. The first occurrence of Eocene benthic foraminifers is at 50 cm in Section 199-1221C-11X-3, whereas the last occurrence of Paleocene forms is at ~91 cm in the same section.

The Site 1221 P/E boundary interval appears remarkably similar to that recovered at Site 1220. Distinctive brown, pink, black, and dark brown layering can be correlated between the two sites 206 km apart.

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