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SITE SUMMARIES (continued)

Site 1216

Site 1216 is located on abyssal hills just south of the Molokai Fracture Zone at a water depth of 5163 m. The crustal age based on magnetic lineations is ~57 Ma (C25r). The site was chosen for drilling because it is near the thickest section of lower Eocene sediments along the 56-Ma transect. Based on previous coring and drilling ~1° in latitude to the south, we expected to find a moderately thin red clay section overlying middle Eocene radiolarian oozes and lower Eocene carbonates. Instead we drilled a 50-m section of red clay overlying thin cherts in sediment. We recovered only chert in the chert-sediment section. Microfossils are absent until ~40 mbsf, where small numbers of middle Eocene radiolarians appear. The cherts are early middle Eocene–early Eocene in age. We abandoned the site after drilling to 62 mbsf and before drilling to basement because of the likelihood of large amounts of chert in the section with little sediment recovery and because we could use the saved time to ensure more complete programs at the remaining sites.

The red clay unit is similar to the red clay section of Site 1215 but expanded (40 m vs. 25 m thick). The upper part of the Site 1216 red clays are illite rich (based upon LAS), grading to smectite rich at the base. The transition begins at ~10 mbsf. Fe-Mn oxyhydroxides are also abundant in the lower part of the red clays, reaching a maximum ~29 mbsf as shown by bulk sediment analyses and by grain density. A transition from relatively high to low natural gamma ray activity occurred at ~25 mbsf. A similar transition in natural gamma ray activity was observed at Site 1215.

The sediments at Site 1216 were surprisingly barren of microfossils. Upper middle Eocene radiolarian oozes are absent at the site, and lower middle Eocene radiolarians are not abundant. Calcareous microfossils are absent in the drilled section. Only agglutinated benthic foraminifers were found, but none are age diagnostic. The uppermost radiolarians (from RP 13 Zone; ~44 Ma) occur at ~40 mbsf. The base of the drilled section (62.2 mbsf) is in radiolarian Zones RP9 and RP10, which straddle the middle/early Eocene boundary (~49 Ma). The first cherts encountered downhole occurred at ~50 mbsf. These upper cherts thus appear in an interval of slow sedimentation, at most 4–5 m/m.y., presumably at the top of more rapidly deposited lower Eocene sediments with larger amounts of biogenic components. We estimated from the seismic reflection profile that ~90 m of sediments remained to be drilled within the cherty section until basement was reached.

It was possible to detect magnetic reversals in the red clay section, but none were detected in sections with microfossils because of coring disturbance. Based on the microfossil dates, the lowest magnetic chron detected is probably C20n. Magnetic intensity of the red clays is strong, and drilling-induced magnetic overprints appear to be mostly removable by standard procedures.

Red Clay Section

The red clay section at Site 1216 has several similarities to the red clay section cored at Site 1215, although it is 60% thicker. Both red clay units have a transition from illite to smectite with depth. Both show a significant decrease in natural gamma ray activity downcore. The lower parts of the red clay unit are enriched in Fe-Mn oxyhydroxides. When MST records are compared, smaller events appear to correlate between these two sites. Provided that some age control can be developed, it may prove possible to develop a much more detailed red clay stratigraphy for the North Pacific.

Missing Middle Eocene Radiolarian Oozes

One of the major surprises found from dilling Site 1216 is the remarkable lack of late or middle Eocene radiolarian oozes. These are biogenic sediments that have no modern analog but are prominent sedimentary features from piston cores and drill sites only 1° to the south. DSDP Site 40 recovered 140 m of upper–middle Eocene radiolarian ooze beneath ~10 m of red clay. DSDP Site 41 with a thinner sediment column (34 m of sediment above basalt) contains 16 m of radiolarian ooze below 18 m of red clay. Piston core EW9709-3PC, taken on the site survey for the potential Leg 199 drill site PAT-13 (19°46'N, 138°55'W) also cored 5 m of middle Eocene radiolarian ooze beneath 10 m of red clay. On the basis of this apparent sharp zonation in tropical biotic communities, we hypothesize that a major oceanographic boundary existed between the position of Site 1216 and the position of sites presently situated at ~20°N in the middle and late Eocene.

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