Drilling of the three holes at Site 1218 recovered a 274-m section of pelagic sediment (Fig. F3). The four sedimentary units are differentiated by sedimentary composition. Fifty-two meters of early Miocene and younger pelagic and radiolarian clay overlies 165 m of nannofossil ooze and chalk, 33 m of radiolarite, and 24 m of nannofossil chalk. A fifth unit consists of seafloor basalt and was sampled only in Hole 1218A. The basement was not reached in Hole 1218B. Hole 1218C was washed down to 55.0 mbsf, and drilling was terminated at 256.8 mbsf.
The sediment of Unit I alternates between light yellowish brown (10YR 6/4) radiolarian clay and dark yellowish brown (10YR 4/4) clay with radiolarians. At 15.8 mbsf (Hole 1218A), there is a 56-cm-thick interval of dark grayish brown (10YR 4/2) to very dark grayish brown (10YR 3/2) clay with zeolites. Clay with zeolites is found at 17.7 mbsf and gradually changes to clay with radiolarians downcore. In Hole 1218B, smear slide estimates show diatoms comprise the main siliceous component from 0 to 10.8 mbsf. Distinct 40- to 60-cm-thick yellowish brown to brown (10YR 4/3 to 10YR 5/4) nannofossil ooze layers with various amounts of clay and radiolarians are present through the interval from 35.7 to 46.3 mbsf in Hole 1218A and from 34.3 to 44.1 mbsf in Hole 1218B.
All lithologies exhibit slight to moderate mottling. Lithologic contacts are obscured by mottles. In interval 199-1218B-3H-2, 94-97 cm, a 3-cm-diameter manganese nodule was recovered (Fig. F4).
The upper part of Unit II consists of nannofossil ooze, with clay content varying from 5% to 20% (Hole 1218A: 52.1-84.7 mbsf; Hole 1218B: 51.0-80.0 mbsf; and Hole 1218C: 55.0-84.2 mbsf). The sediment color alternates from pale brown (10YR 7/3) to yellowish brown (10YR 5/3) and brown (10YR 4/3). The lowermost part of this unit is a pale brown (10YR 8/2) to white (10YR 8/1) nannofossil ooze grading downcore to a nannofossil chalk with minor amounts of radiolarians (Hole 1218A: 84.2-216.9 mbsf; Hole 1218B: 80.0-215.55 mbsf; and Hole 1218C: 84.2-218.5 mbsf). In Hole 1218B, several centimeter-scale nannofossil chalk layers are present within the light nannofossil ooze at 122.9 and 124.8 mbsf and at 147.5 mbsf in Hole 1218C.
Diatom content increases in the lower 15 m of Unit II, with intervals containing up to 50% diatoms (e.g., intervals 199-1218A-23X-1, 0 cm, to 24X-CC, 35 cm; 199-1218B-22X-4, 0 cm, to 22X-CC, 6 cm; and 199-1218C-17X-5, 2 cm, to 17X-6, 90 cm). A stepwise shift to the darker radiolarite of Unit III is seen in Sections 199-1218C-17X-5 and 17X-6. Interval 199-1218C-17X-5, 2 cm, through 17X-6, 90 cm, contains brown (10YR 5/3) nannofossil chalk with clay, diatoms, and radiolarians. Mottles are common to abundant throughout Unit II, often obscuring lithologic contacts. Rice-shaped carbonate crystals without square-tipped terminations, 15-20 µm long, are occasionally observed in the dominant lithology (~5%) and burrow infills. Although similar in appearance to aragonite needles (Rothwell, 1989), X-ray diffraction analysis shows only calcite mineralogy. Manganese oxides are common in black (10YR 2/1) burrows. All lithologic contacts are gradational.
Small millimeter- to centimeter-sized medium gray (N5) pumice fragments are sporadically present throughout Holes 1218A, 1218B, and 1218C (e.g., intervals 199-1218A-17H-7, 84 cm; 199-1218B-22X-3, 70-71 cm; and 199-1218C-1H-3, 5-7 cm). Black (N1) centimeter-sized chert fragments and nodules are occasionally found throughout the unit. A 50-µm spherical, glassy green mineral was found in a sieved core catcher sample from Core 199-1218A-23X and was inferred to be a microtectite.
The nannofossil chalk and underlying units are biscuited (commonly 5-50 cm in size) by XCB coring. The biscuits are surrounded by a slurry matrix.
Unit III is characterized by a sequence of interlayered radiolarite and nannofossil chalk, each containing various amounts of clay (up to 30%). The color alternates between very dark brown (10YR 2/2) and brown (10YR 4/3). In Holes 1218A, 1218B, and 1218C, several 7- to 12-cm-thick, 6- to 27-cm-thick, and 6- to 10-cm-thick, respectively, very dark brown to black (10YR 2/2 to 10YR 2/1) intact chert layers were recovered in the intervals 199-1218A-25X-1, 49 cm, to 27X-1, 41 cm (225.2-244.3 mbsf), 199-1218B-25X-1, 12 cm, to 25X-1, 141 cm (223.4-224.7 mbsf), and 199-1218C-18X-3, 90 cm, to 20X-2, 120 cm (227-244.9 mbsf). For details, see Table T2 and Figure F5. These layers are present in the very dark brown (10YR 2/2) radiolarian intervals. Mottling in the lithified chert layers is common.
Gray (10YR 6/1) siliceous horizons, probably porcellanite, are present in interval 199-1218B-26X-7, 27-82 cm (237.1-237.6 mbsf). In interval 199-1218A-24X-4, 0 cm, to 24X-CC, 10- to 40-cm-thick layers of dark grayish brown (10YR 4/2) diatomite are present within dark brown (10YR 3/3) radiolarite (Sections 199-1218B-24X-2 through 24X-CC).
The entire unit contains abundant mottles. Zoophycos and Chondrites burrows are common through this unit. Small fractures with centimeter-scale offset are present in Core 199-1218B-24X.
Sediments from this unit are strongly biscuited (10-20 cm in size) by XCB coring. A slurry matrix surrounds the biscuits.
Unit IV is differentiated into two subunits on the basis of mineralogical and sedimentary composition. Subunit IVB was not recovered in Holes 1218B and 1218C.
Subunit IVA consists of a nannofossil chalk containing 10%-30% clay and 5%-15% diatoms. Color varies from very pale brown (10YR 8/2) to white (10YR 8/1). Layers of fragmented, very dark brown (10YR 2/2) chert are present within intervals 199-1218A-29X-2, 117 cm, to 29X-CC, 20 cm (265.8-268.7 mbsf) and 199-1218B-29X-1, 96 cm, to 29X-3, 102 cm (258.2-261.2 mbsf). Strong mottling is present throughout Subunit IVA.
Subunit IVB is composed of nannofossil chalk with dolomite and various amounts of clay (up to 15%) and radiolarians (up to 20%). Sediment varies between dark yellowish brown and light yellowish brown (10YR 4/6 and 10YR 6/4). The color change between Subunits IVA and IVB is gradational. Chert layers are present in interval 199-1218A-29X-4, 20 cm, to 29X-CC, 20 cm (267.8-268.7 mbsf). Bedding shows inclinations up to 45°. Centimeter-scale faults are observed throughout this unit (Fig. F6). The dolomite content increases to 20% in the coarser sediment near the base of Section 199-1218A-30X-CC. Dolomite crystals, commonly as large as 100 µm, are authigenic and euhedral. Smear slide analysis indicates that dolomite is principally replacing discoasters and coccoliths. Sediments of Subunit IVB contain abundant mottles.
At the bottom of Hole 1218A, a centimeter-sized piece of weathered basalt was recovered. The fragment is fine-grained phaneritic and has a glassy rind. We terminated coring before reaching basement in Holes 1218B and 1218C.
The middle Eocene basalt is overlain by 274 m of pelagic sediments at Site 1218. The middle Eocene pelagic unit is represented by carbonate-rich sediments with radiolarians. The middle-late Eocene is dominated by a dark radiolarite, reflecting the paleoequatorial productivity zone of the Pacific Ocean. The E/O boundary interval is characterized by a major lithologic change from dark radiolarite to pale nannofossil chalk. The first interval with remarkable radiolarian content in the nannofossil-dominated lithology above the radiolarite emphasizes a stepwise shift to the pale-colored lower Oligocene sediments. DSDP Sites 161 and 162 show similar compositional changes near the E/O boundary interval (van Andel, Heath, et al., 1973). The MST data (see "Composite Depths") indicate a stepwise shift in the lithologies at the E/O boundary. Tektites were not found in the upper Eocene sediments in the interval proposed for the late Eocene impact. The abrupt change in the lithologies at the E/O boundary interval may reflect a rapid deepening of the CCD (van Andel et al., 1975). The lower Oligocene-lower Miocene interval is characterized by an alternating sequence of dark and pale nannofossil ooze with various amounts of clay (5%-20%). Holocene-lower Miocene sediments are dominated by pelagic clay.