Site 1222 recovered a 97.8-m-thick section dominated by clay-rich lithologies and chert (Fig. F3). Fifty-five meters of lower Oligocene and younger clay with zeolites and radiolarian clay overlies 3.8 m of basal Oligocene nannofossil ooze. Eocene sediments are represented by clay and chert and are poorly recovered. The sedimentary section was divided into four units based on compositional differences. A fifth unit, seafloor basalt, was also recovered. Hole 1222A had better recovery and less core disturbance than Hole 1222B. For this reason, much of the following description is primarily based on sediments from Hole 1222A.
Unit I is divided into two subunits based upon lithologic components. A gradational contact exists between the subunits and with Unit II below.
Subunit IA consists of light yellowish brown to very dark grayish brown (10YR 6/4 to 10YR 3/2) clay with zeolites. Zeolites (authigenic phillipsite) are common to abundant, especially in the lighter-colored (10YR 6/4) intervals. A minor amount of iron oxides and opaque minerals are also present, and a few small (2- to 3-mm diameter) manganese nodules are disseminated throughout this subunit. A thin interval of diatomaceous clay is present near 17 mbsf (intervals 199-1222A-3H-2, 21 cm, through 3H-4, 82 cm, and 199-1222B-2H-7, 35 cm, through 3H-2, 87 cm). Within the diatom ooze are several unusual layers of laminated diatom ooze that contain a nearly pure diatom component (Fig. F4). Laminations are mainly white to olive (5Y 8/1 to 5Y 5/3). Diatom frustules within some laminae contain significant amounts of pyrite, making them appear very dark gray to black (10YR 2/2 to 10YR 2/1). A sharp contact exists between the base of the diatom ooze interval and the underlying clay of Subunit IA. In Hole 1222A, this contact also contains a large (5 cm diameter) botryoidal manganese nodule (interval 199-1222A-3H-4, 80-85 cm).
Subunit IB consists of very dark grayish brown to dark brown (10YR 3/2 to 10YR 3/3) radiolarian clay. Bioturbation is slight to moderate throughout this interval causing some yellowish brown to brown (10YR 5/6 to 10YR 5/3) mottling. Radiolarian content varies only slightly, comprising ~40% of the lithology. Minor components include oxide minerals, zeolites, volcanic glass, and sponge spicules.
This unit consists of dark brown to brown (10YR 3/3 to 10YR 4/3) nannofossil clay grading downcore to yellowish brown (10YR 5/4) nannofossil ooze. Very pale brown (10YR 8/2) mottles are common. Nannofossil content (estimated from smear slides) gradually increases from 30% in the upper portion to 80% near the base of the unit. Minor amounts of iron oxides, zeolites, volcanic glass, and radiolarians are also present. The transition from Unit II to Unit III is gradual.
Unit III is dominated by clay with iron oxides and zeolites. Sediments are dark brown (10YR 3/3) and slightly mottled with yellowish brown (10YR 5/6). Iron oxide minerals comprise a significant proportion of the sediment, up to 35% in some samples. Minor constituents include zeolites, opaque minerals, and trace amounts of rhombohedral dolomite crystals. The basal contact of Unit III is obscured by sediment flow-in. At Hole 1222B, drilling was terminated at 64.4 mbsf and only the upper portion of Unit III was recovered.
Sediments comprising Unit IV were poorly recovered by drilling and are represented by a few small angular chert fragments recovered in the core catchers of the XCB. The uppermost 5.4 m of this unit (interval 199-1222A-8H-3, 155 cm, through 8H-CC) are highly disturbed by flow-in consisting of small chert fragments mixed with sediments from Unit III. The Unit III-IV transition was placed at the top of the flow-in disturbance because this is probably the depth at which chert was first encountered. Chert fragments are black (N1) and exhibit thin, very dark brown (10YR 2/2) laminations. Just above basement, a large chert nodule rimmed with very pale brown (10YR 8/2) nannofossil chalk provides a datum that indicates early Eocene age (Subzone CP9a [NP10]) (see "Calcareous Nannofossils" in "Biostratigraphy"). A few small fragments of claystone are also mixed with chert breccia near the base of Section 199-1222A-12X-CC.
Core 199-1222A-12X contains a fine-grained, weathered ocean-floor basalt with aphanitic texture and a glassy rim. Fragments are highly altered and show light greenish gray color (5YG 7/1).
The sedimentary section at Site 1222 is dominated by chert and clay-rich lithologies. Nannofossils scraped from the rim of one chert fragment recovered directly above basement basalt suggest sedimentation began during the early Eocene at ~53.5 Ma. Basal sediments at this site are poorly represented but probably include carbonate chalk and claystone lithologies, based on the few fragments recovered with chert drilling breccia in Section 199-1222A-12X-CC and at other Leg 199 drill sites. Sedimentation during the middle Eocene was dominated by clay, probably eolian transported. However, hydrothermal sources may have contributed a substantial amount of material to this site as suggested by the increased amount of iron oxide minerals (35%) compared to other sites. Sediments gradually shift in composition from clay to nannofossil ooze across the Eocene/Oligocene (E/O) boundary interval, a lithologic change that is likely related to the global deepening of the calcite compensation depth in the early Oligocene (van Andel et al., 1975).
Miocene and younger sedimentation is dominantly zeolitic clay. During the Pliocene, however, diatoms become an important sedimentary component and sometimes are present as laminated intervals of diatom ooze, presumably associated with the rapid deposition of large "algal mats." Such events have been observed in the modern Pacific (Barber, 1992) and in Neogene sedimentary records from Ocean Drilling Program Leg 138 (Kemp et al., 1995). The depositional model proposed for laminated diatom intervals suggests algal mats may represent regions near strong oceanic frontal boundaries. This would suggest that a water-mass boundary may have been present near 13°N above Site 1222 during the Pliocene.
Clay once again dominates the sequence in sediments younger than the Pliocene. More than 16 m of clay was deposited since the late Pliocene. Typically, pelagic clays accumulate at much slower rates, thus, sediment reworking in this portion of the sequence seems likely (see "Radiolarians" in "Biostratigraphy").