Drilling at Site 1215 recovered a 70-m section dominated by pelagic clay and clayey nannofossil ooze (Fig. F3). Twenty-six meters of pelagic clay of early Eocene and younger age overlies 42 m of clayey calcareous ooze and 2 m of basal hydrothermal sediment. The sedimentary section is divided into three lithologic units on the basis of sedimentary facies, and a fourth unit, basalt, lies below. Sediments in Hole 1215B are more disturbed by piston coring through the cherty horizons than those in Hole 1215A, so this description is largely based on results from Hole 1215A.
Unit I is dominated by clay. The upper few meters are dark yellowish brown (10YR 4/4 and 10YR 3/4) and grade down over an interval of ~100 cm at 5 mbsf to very dark-brown (10YR 2/2) clay. The rich chocolate color of most of Unit I is the dominant characteristic of the unit. Zeolites are common in the interval from ~9 to 24 mbsf, with lithologies of zeolitic clay and clay with zeolites. Zeolites are abundant in thin layers that presumably represent altered volcanic ash. Ash, in the form of clear glass and less commonly brown glass, is present throughout. The most prominent ash/zeolite layer is a strong yellow-brown color (10YR 5/6) and occurs at ~8 mbsf in Hole 1215A and 8.2 mbsf in Hole 1215B. Black chert is present in the bottom 1.5 m of Unit I (24.3-25.8 mbsf) as nodules and angular fragments broken by the piston coring process. In Unit I, in contrast to nearly all prior attempts, cherts were recovered together with their host sediments. The dark clays of Unit I are sometimes mottled with slightly lighter colors, denoting the burrows, and a faint centimeter-scale layering is often evident on the digitally enhanced color scans but not on the analog core photographs. A faint, 10-cm-thick horizon of nannofossil clay is present 25 cm above the base of the unit. The lower contact of Unit I is sharp and is present at the top of a much lighter-colored ooze.
Unit II consists of alternating intervals of brown (10YR 3/4 to 10YR 5/3) clayey nannofossil ooze and darker nannofossil clay (dark brown, 10YR 2/2, to dark grayish brown, 10YR 4/2). The cyclic variations are on length scales of ~20-50 cm and extend throughout the unit. An interval of dark-brown (10YR 2/2) nannofossil clay at ~54.7 mbsf in Hole 1215A (interval 199-1215A-8H-3, 128-148 cm) may represent the P/E boundary event that is characterized elsewhere by a negative carbon isotopic excursion (Dickens et al., 1995). Immediately above this dark layer, sediment flow-in occurs and disturbs the material in Section 199-1215A-8H-3 from 128 cm up to ~40 cm. The lower boundary of the dark layer is sharp; below it the sediments are disturbed by the coring process but retain a coherent stratigraphy.
Chert nodules or horizons are scattered throughout Unit II. The chert is black and commonly broken into pebble-sized fragments by the drilling process. Sediment hosting the cherts was recovered by the APC coring, but often the chert blocked the core barrel and occasioned intervals of flow-in. Portions of the lower Eocene ooze were disturbed by this process, which complicates short intervals of Cores 199-1215A-4H and 6H, about half of Core 5H, and two-thirds of Core 8H. Disturbance in Hole 1215B was generally worse than in Hole 1215A.
Unit III is a very dark-brown (10YR 2/2) metalliferous sediment. It is a nearly pure oxide ooze (see "Solid-Phase Geochemistry" in "Geochemistry") (Table T12; Fig. F15) containing little clay and no ash or fossils. This thin unit immediately overlies the basalt of Unit IV.
Three pebbles were recovered from the bottom of Hole 1215A and two from the base of Hole 1215B. The pebbles are brownish gray aphanitic basalt, the largest of which was ~3 cm in diameter. Two pebbles had portions of a glassy margin.
The sedimentary section at Site 1215 records ~5 m.y. of early Paleogene biogenic deposition, followed by 51 m.y. of pelagic clay accumulation. The light to dark color change in the upper portion of Unit I has been observed in cores of pelagic clay throughout the north central Pacific Ocean and may represent a change in the source of the dust that makes up the clays from China above to North America below (Kyte et al., 1993). The lower Eocene oozes display a cyclicity, probably driven by Milankovitch processes. The P/E boundary is quite distinct, is associated with a clay-rich layer, and is nicely fossiliferous at Site 1215. At the base of the sediments, the hydrothermal oozes record the final phase of hydrothermal activity at the nearby ridge axis.