Site 1216 (21°27.16´N, 139°28.79´W) 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 (magnetic Anomaly An25r). 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 and abandoned the site before reaching basement. We recovered only chert in the chert-sediment section. Microfossils are absent until ~40 meters below seafloor (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 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 Site 1216 red clays are illite rich (based upon light absorption spectroscopy [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 of ~29 mbsf, as shown by bulk-sediment analyses and grain density. A transition from relatively high to low natural gamma ray (NGR) activity occurred at ~25 mbsf. A similar transition in NGR activity was observed at Site 1215.
The sediments at Site 1216 are 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 Zone RP13; ~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. Thus, these upper cherts appear in an interval of slow sedimentation rates, 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 ~60 m of sediments remained to be drilled within the cherty section until basement was reached.
It was possible to identify magnetic polarity chrons in the red-clay section but none in sections with microfossils because of coring disturbance. Based on the microfossil dates, the oldest chron detected is probably C20n. Magnetic intensity of the red clays is strong, and drilling-induced magnetic overprints are mostly removable by standard procedures.
The red-clay section at Site 1216 has several similarities to the red-clay section cored at Site 1215, although it is 165% thicker. Both red-clay units have a transition from illite to smectite with depth. Both show a significant decrease in NGR activity downcore. The lower parts of each red-clay unit are enriched in Fe-Mn oxyhydroxides. When multisensor track (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 than is now available.
One of the major surprises found from drilling 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, a major oceanographic boundary must have existed between the paleoposition of Site 1216 and Sites 40 and 41.