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doi:10.2204/iodp.proc.320321.104.2010

Lithostratigraphy

Drilling at Site U1332 recovered a 150.4 m thick section of pelagic sediments overlying seafloor basalt. The uppermost 17.7 m of the section is a late Miocene to Pliocene–Pleistocene clay with varying amounts of radiolarians and zeolite minerals (~6 to 22 Ma based on radiolarians and magnetostratigraphy). These sediments are underlain by ~130 m of Oligocene to middle Eocene nannofossil and radiolarian ooze with porcellanite deep in the section. A thin (~3 m thick) unit of middle Eocene zeolite clay bearing small chert nodules was recovered at the base of the sedimentary sequence above basement basalt.

The sedimentary sequence at Site U1332 is divided into five major lithologic units, with one of these units further divided into three subunits (Fig. F3; Table T2). Unit and subunit boundaries are defined by differences in lithology, measured physical properties, and calcium carbonate (CaCO₃) content. Lithologic differences, based on both visual core descriptions and smear slide and thin section analysis, are primarily attributable to varying distributions of biogenic components (e.g., nannofossils and radiolarians) and clay-sized lithogenic material, as well as the presence of porcellanite (Table T2; Figs. F3, F5, F6, F7; see "Site U1332 thin sections" and "Site U1332 smear slides" in "Core descriptions"). Lithologic descriptions are primarily based on sediments recovered in Hole U1332A, supplemented with observations from Holes U1332B and U1332C.

Unit I

Intervals: 320-1332A-1H-1, 0 cm, through 3H-3, 130 cm; 320-U1332B-1H-1, 0 cm, through 3H-4, 150 cm; 320-U1332C-1H-1, 0 cm, through 3H-1, 70 cm

Depths: Hole U1332A = 0–17.7 m CSF; Hole U1332B = 0–17.6 m CSF; Hole U1332C = 0–17.7 m CSF

Age: Miocene–Pliocene–Pleistocene

Lithology: clay and radiolarian clay

The major lithology in Unit I is light yellowish brown (10YR 6/4) to very dark brown (10YR 3/2) to dark gray (10YR 4/1) clay. The light yellowish brown clay with radiolarians occurs in the uppermost ~8 m of the sedimentary section (in Hole U1332A), overlying the darker zeolite clay. The downhole transition from radiolarian clay to zeolite clay is characterized by a change to darker color and shifts to higher magnetic susceptibility but lower gamma ray attenuation (GRA) bulk densities and L* (lightness) (Fig. F3; see "Physical properties" for discussion of additional reflectance parameters a* and b*). CaCO₃ contents are near zero throughout Unit I. The contact with underlying Unit II takes place over a ~5 cm thickinterval.

Unit II

Intervals: 320-U1332A-3H-3, 130 cm, through 5H-1, 150 cm; 320-U1332B-3H-4, 150 cm, through 5H-1, 150 cm; 320-U1332C-3H-1, 70 cm, through 4H-7, 30 cm

Depths: Hole U1332A = 17.7–33.9 m CSF; Hole U1332B = 17.6–30.6 m CSF; Hole U1332C = 17.7–35.8 m CSF

Age: early Miocene to late Oligocene

Lithology: alternations of clayey radiolarian ooze and nannofossil ooze

The dominant lithologies in Unit II are dark brown (10YR 3/2) to very dark grayish brown (10YR 3/2) clayey radiolarian ooze, yellowish brown (10YR 5/4) to pale brown (10YR 6/3) nannofossil ooze, and dark brown (10YR 3/3) radiolarian ooze. Bioturbation is generally minor to moderate in these sediments. Within the major lithologies, nannofossil ooze sometimes occurs with radiolarians and sometimes occurs with radiolarians and clay as minor lithologic components, whereas radiolarian ooze occurs with clay as a minor lithologic component. Alternating sequences of nannofossil ooze and radiolarian ooze occur at decimeter to meter scales. The contact with underlying Unit III takes place over a 5 cm interval. Unit II sediments have CaCO₃ contents (typically ≤40%) that are lower than those of the underlying Unit III, whereas magnetic susceptibility, GRA bulk densities, and L* all show systematically lower values in Unit II than in Unit I (Fig. F3; see "Geochemistry").

Unit III

Intervals: 320-U1332A-5H-2, 0 cm, through 9H-4, 124 cm; 320-U1332B-5H-2, 0 cm, through 9H-6, 50 cm; 320-U1331B-4H-7, 30 cm, through 10H-1, 41 cm

Depths: Hole U1332A = 33.9–76.14 m CSF; Hole U1332B = 30.6–75.1 m; Hole U1332C 35.8–75.91 m CSF

Age: early Oligocene

Lithology: nannofossil ooze, nannofossil ooze with radiolarians, and radiolarian nannofossil ooze

The dominant lithology in Unit III is white (10YR 8/1) to brown (10YR 5/3) nannofossil ooze, but brown radiolarian nannofossil ooze is also a major lithology in this unit. Within the major lithologies, nannofossil ooze occurs with diatoms as a minor lithologic component. Bioturbation intensity is minor to intense in these sediments. Baseline values of magnetic susceptibility are low with large-amplitude variability in comparison to the overlying units. Data series for GRA bulk density, L*, and CaCO₃ all show high baseline values with large-amplitude variability in comparison to the overlying units (see Fig. F3). The contact with underlying Unit IV is marked by a light to dark color change over a 20 cm bioturbated boundary.

Unit IV

Intervals: 320-U1332A-9H-4, 124 cm, through at least 16X-CC, 42 cm; 320-U1332B-9H-6, 50 cm, through at least 17X-CC, 40 cm; 320-U1332C-10H-1, 41 cm, through at least 17X-2, 7 cm

Depths: Hole U1332A = 76.14–138.29 m CSF; Hole U1332B = 75.1–135.08 m CSF; Hole U1332C = 75.91–138.77 m CSF

Age: middle to late Eocene

Lithology: clayey radiolarian ooze, radiolarian ooze, radiolarian nannofossil ooze, nannofossil radiolarian ooze, nannofossil ooze, and porcellanite

Unit IV is distinguished from Unit III by the dominance of radiolarian ooze. The major lithologies in Unit IV are dark brown (10YR 3/3) to brown (10YR 5/3) radiolarian ooze, very dark grayish brown (10YR 3/2) to brown (10YR 4/3) clayey radiolarian ooze, dark yellowish brown (10YR 3/4) to light gray (10YR 7/2) nannofossil radiolarian ooze, brown (10YR 5/3) to light gray (10YR 7/2) radiolarian nannofossil ooze, and brown (10YR 5/3) nannofossil ooze. Downhole lithologic changes within Unit IV allow division into three subunits based on the significance of nannofossil ooze, radiolarian nannofossil ooze, and porcellanite as secondary major lithologies and the downhole profile of CaCO₃ (Fig. F3; see "Geochemistry").

Subunit IVa

Intervals: 320-U1332A-9H-4, 124 cm, through 13H-1, 20 cm; 320-U1332B-9H-7, 0 cm, through 13H-5, 120 cm; 320-U1332C-10H-1, 41 cm, through 13H-2, 150 cm

Depths: Hole U1332A = 76.14–108.6 m CSF; Hole U1332B = 75.1–107.8 m CSF; Hole U1332C = 75.91–107.0 m CSF

Age: middle to late Eocene

Lithology: alternations of nannofossil ooze, radiolarian nannofossil ooze, radiolarian ooze, and clayey radiolarian ooze

Subunit IVa is distinguished from Subunits IVb and IVc based on the dominance of radiolarian ooze and absence of porcellanite. The major lithologies in Subunit IVa are dark brown (10YR 3/3) to brown (10YR 5/3) radiolarian ooze, dark brown (10YR 3/3) clayey radiolarian ooze, and brown (10YR 4/3) to pale brown (10YR 6/3) radiolarian nannofossil ooze. Within the major lithologies, radiolarian ooze occurs with either clay or nannofossils. Bioturbation is generally minor to moderate in these sediments. GRA bulk density, L*, and CaCO₃ content all decrease downhole across the Unit III/Subunit IVa boundary and maintain relatively low values throughout (Fig. F3). Magnetic susceptibility is generally higher in the upper portion of Subunit IVa than in Unit III and decreases toward the boundary with Subunit IVb. The contact with underlying Subunit IVb takes place over a 5 cm interval.

Subunit IVb

Intervals: 320-U1332A-13H-1, 20 cm, through 14H-1, 150 cm; 320-U1332B-13H-5, 120 cm, through 15H-4, 40 cm; 320-U1332C-13H-3, 0 cm, through 15H-2, 50 cm

Depths: Hole U1332A = 108.6–119.4 m CSF; Hole U1332B = 107.8–121.0 m CSF; Hole U1332C = 107.0–120 m CSF

Age: middle Eocene

Lithology: alternations of nannofossil ooze, radiolarian nannofossil ooze, and nannofossil radiolarian ooze

Subunit IVb is distinguished from Subunits IVa and IVc based on the dominance of radiolarian nannofossil and nannofossil oozes and absence of porcellenite. The major lithologies in Subunit IVb are brown (10YR 5/3) nannofossil ooze, dark yellowish brown (10YR 4/4) radiolarian nannofossil ooze, and dark brown (10YR 3/3) nannofossil radiolarian ooze. Within the major lithologies, nannofossil ooze occurs with radiolarians as a minor component and nannofossil radiolarian ooze occurs with clay as a minor component. Alternations of nannofossil ooze and radiolarian nannofossil ooze in Subunit IVb occur at decimeter to meter scales. Bioturbation is generally minor to moderate in these sediments.

Subunit IVc

Intervals: 320-U1332A-14H-2, 0 cm, through at least 16X-CC, 42 cm (base not recovered); 320-U1332B-15H-4, 40 cm, through at least 17X-CC, 40 cm (base not recovered); 320-U1332C-15H-2, 50 cm, through 17X-2, 7 cm

Depths: Hole U1332A = 119.4 to at least 138.39 m CSF; Hole U1332B = 121.0 to at least 135.08 m CSF; Hole U1332C = 121.0 to least 138.77 m CSF

Age: middle Eocene

Lithology: alternations of radiolarian nannofossil, radiolarian ooze, and nannofossil radiolarian ooze with porcellanite layers or nodules

Subunit IVc is distinguished from Subunits IVa and IVb based on the presence of porcellanite. The major lithologies in Subunit IVc are dark brown (10YR 3/3) to dark yellowish brown (10YR 4/4) radiolarian ooze, brown (10YR 5/3) radiolarian nannofossil ooze, very dark grayish brown (10YR 3/2) clayey radiolarian ooze, and porcellanite. Within the major lithologies, radiolarian ooze occurs with clay as well as with clay and nannofossils as minor components. Nannofossil radiolarian ooze is a minor lithology in Hole U1332A. Alternations of nannofossil radiolarian ooze with radiolarian nannofossil ooze and of radiolarian ooze with clay and radiolarian ooze with clay and nannofossils occur on decimeter to meter scales in Subunit IVc. Bioturbation is generally minor to moderate in these sediments. Magnetic susceptibility, GRA bulk density, and L* are comparatively low in Subunit IVc at Site U1332. In thin section, porcellanite layers and nodules contain flat flakes of clay minerals, radiolarians, nannofossils, and foraminifers. Radiolarian and foraminifer tests are partially replaced with microcrystalline quartz.

Unit V

Intervals: At least 320-U1332A-17X-1, 0 cm (top not recovered), through 17X-CC, 3 cm; 320-U1332B-18X-1, 0 cm, through at least 18X-CC, 16 cm (base not recovered); 320-U1332C-17X-2, 7 cm, through at least 18X-CC, 46 cm (base not recovered)

Depths: Hole U1332A = 144.50 to at least 148.15 m CSF; Hole U1332B = 143.90 to at least 146.09 m CSF; Hole U1332C = 138.77 to at least 147.36 m CSF

Age: middle Eocene

Lithology: clay, zeolite clay, and chert

The major lithologies in Unit V are very dark grayish brown (10YR 3/2) to black (10YR 2/1) clay, very dark grayish brown (10YR 3/2) to black (10YR 2/1) zeolite clay, and chert. Sediments at the very base of the sedimentary section directly overlying basalt are partially lithified with nonvisible bioturbation.

Unit VI

Intervals: 320-U1332A-17X-CC, 3 cm, to at least 18X-CC, 52 cm; 320-U1332B-18X-CC, 16 cm, to at least 18X-CC, 34 cm; 320-U1332C-18X-CC, 46 cm, to at least 18X-CC, 62 cm

Depths: Hole U1332A = 148.15–150.56 m CSF; Hole U1332B = 146.09–146.27 m CSF; Hole U1332C = 147.36–147.52 m

Small broken basalt pieces were recovered at the base of each hole at Site U1332. Thin section analysis indicates a highly altered phyric basalt with sparse plagioclase (Sample 320-U1332A-18X-CC (Piece 3A, 16–19 cm) (see "Site U1332 thin sections" in "Core descriptions"). Fragments of glass in the groundmass are highly altered and show spherulitic texture, and ferromagnesian minerals (mainly clinopyroxene) are replaced with chlorite. Sample 320-U1332A-18X-CC (Piece 2A, 12–16 cm) is a partly altered phyric basalt with sparse plagioclase. Glass and clinopyroxene in the groundmass are preserved in a chilled margin. Calcite veins are observed in both pieces and show a distinct radiaxial fabric.

Sediments across the Eocene–Oligocene transition

An Eocene–Oligocene transition was recovered in two of the three holes at Site U1332 (Holes U1332A and U1332B) (Fig. F8). The transition was not recovered in Hole U1332C because of core disturbance associated with a shipboard power outage (see "Operations"). The Eocene/Oligocene boundary is formally defined by the extinction of the planktonic foraminifer genus Hantkenina but cannot be identified at Site U1332 because of poor preservation of planktonic foraminifers (see "Biostratigraphy"). Radiolarian and nannofossil bio- and magnetostratigraphy provide excellent age control, indicating that the Eocene/Oligocene boundary falls somewhere between the base of Chron 13n and the Biozone RP20/RP19 boundary (within Cores 320-U1332A-9H, 320-U1332B-9H, and 10H). The lithostratigraphy of the Eocene–Oligocene transition at Site U1332 is well captured in both of these holes and consists of a downhole change from light gray (10YR 7/2) and very pale brown (10YR 7/3) nannofossil ooze with diatoms to very pale brown (10YR 8/2) nannofossil ooze to brown (10YR 4/3) radiolarian nannofossil ooze and dark yellow brown (10YR 3/4) radiolarian ooze with clay (Fig. F8). The transition from pale nannofossil ooze to radiolarian ooze is comparatively abrupt (~1 m interval) and defines the Unit III/Unit IV boundary. An associated pronounced downhole increase occurs in magnetic susceptibility, together with pronounced downhole decease in GRA bulk density, L*, and CaCO₃ content (Figs. F3, F8). These lithostratigraphic results for the Eocene–Oligocene transition at Site U1332 are consistent with those obtained from Site U1331 and multiple sites drilled during ODP Leg 199, in particular Site 1220 (Shipboard Scientific Party, 2002b).

Approximately 8 m above the Eocene–Oligocene transition in Hole U1332C, a prominent sharp contact (interval 320-U1332C-9H-2, 95 cm) occurs between very pale brown (10YR 8/2) radiolarian nannofossil ooze to overlying brown (10YR 5/3) radiolarian nannofossil ooze (Fig. F9). In turn, this radiolarian nannofossil ooze transitions uphole into light gray (10YR 7/2) nannofossil ooze with radiolarians over an ~3.5 m thick interval. Magnetostratigraphy and radiolarian and nannofossil biostratigraphy, together with physical property series from the WRMSL and Section Half Multisensor Logger (SHMSL), demonstrate that this 3.5 m thick sequence is a duplication of latest Eocene through earliest Oligocene age interval cored below (see "Biostratigraphy," "Paleomagnetism," "Physical properties," and "Stratigraphic correlation and composite section"). A similar duplicated (or replicated) sequence is also documented in Hole U1332B, but in this hole the sharp contact between pale earliest Oligocene sediments and darker overlying latest Eocene sediments occurs in the core catcher (Sample 320-U1332B-8H-CC, 3 cm) and is consequently disturbed (Fig. F9). This duplicated sequence, with its sharp basal contact, is interpreted to result from a mass movement, probably a slump or slide that reworked older sediments (that happened to be of Eocene–Oligocene transition age) into sediments of early Oligocene (C12r) age (Fig. F9). The lithostratigraphic integrity of the Eocene–Oligocene transition that lies ~8 m deeper in the section in all holes cored at Site U1332 is not affected (Fig. F8) and is very well correlated with that at Site 1220 using WRMSL physical property data (see "Stratigraphic correlation and composite section").

Discussion

Eocene intervals with nannofossil ooze

The dominant lithology of Unit IV at Site U1332 is radiolarian ooze, but this unit also contains four discrete intervals where nannofossil ooze and radiolarian nannofossil ooze is a second major lithology (Fig. F3; see "Site U1332 smear slides" in "Core descriptions"). Two of these carbonate-rich intervals are comparatively thin (≤5 m thick each), consist entirely of radiolarian nannofossil ooze, and occur in Subunit IVa. The third carbonate-rich interval is thicker (~25 m), consists of an alternating sequence of nannofossil ooze and radiolarian nannofossil ooze, and spans almost all of Subunit IVb. The fourth interval occurs in the upper half of Subunit IVc (Fig. F3) and consists of alternating radiolarian ooze with clay and radiolarian nannofossil ooze. All four intervals show CaCO₃ contents (as measured by coulometry) that are above background for Unit IV with up to 60 wt% obtained in Subunit IVb (Fig. F3) and are separated by intervals dominated by radiolarian ooze. According to shipboard magnetostratigraphic results, the uppermost of these carbonate-rich intervals at Site U1332 occurs in sediments of late Eocene age (C16n.2n to C16r; ~35.5–36.5 Ma) (Fig. F3). The other three carbonate-rich intervals fall in the middle Eocene (estimated ages: Interval 2 = C18n.1n to C18n.1r, ~38.5–39.5 Ma; Interval 3 = within C18r middle RP15 to upper RP14 radiolarian zone, ~40–41.5 Ma; Interval 4 = middle to lower part of RP14, 42.5–43.8 Ma). These lithostratigraphic results are broadly consistent with those of Leg 199, especially ODP Sites 1218 and 1219 (Shipboard Scientific Party, 2002a) and the carbonate accumulation events (CAE [2?], 3, 4, and 6/7) of Lyle et al. (2005) that have been used to refine the Paleogene record of the CCD for the equatorial Pacific (Shipboard Scientific Party, 2002a; Van Andel, 1975).

Porcellanite and chert

In Subunit IVc, layers and pebbles of very dark brown (10YR 2/2) partially to well-lithified claystones, often layered or even laminated, are observed within alternating sequences of radiolarian nannofossil ooze, radiolarian ooze, and nannofossil radiolarian ooze of middle Eocene age in Cores 320-U1332A-15X through 17X. Within this sequence (Core 320-U1332A-16X = ~130 wireline log matched depth below seafloor [WMSF], ~135 CSF) (see "Downhole measurements"), a small peak above a stepwise change in NGR is observed in the downhole logging data but no equivalent feature is seen in the core NGR data. In hand specimen, the partially lithified claystones cleave along bedding planes that are particularly rich in clay and the well-lithified specimens exhibit some concoidal fracture indicative of partial secondary silicification. In a single sample (320-U1332-17X-1, 0–4 cm), a black very hard and vitreous pebble with distinct concoidal fracture was recovered. During thin section preparation this single sample proved significantly more resistant to cut by rock saw than the other samples taken from Subunit IVc for this purpose.

In thin section, all of the samples taken from Subunit IVc at Site U1332 show evidence of partial secondary silicification (see "Site U1332 smear slides" in "Core descriptions"). All but one of these samples are porcellanites (Fig. F6A, F6B, F6C). The single black very hard and vitreous pebble (Sample 320-U1332-17X-1, 0–4 cm), is termed "chert" (Fig. F6D; see "Site U1332 smear slides" in "Core descriptions"). In thin section, major components of the porcellanites are clay minerals, microcrystalline quartz, opaques (Fe oxides), and calcite, as well as biogenic shells and fragments from radiolarians and foraminifers. Foraminifer tests are predominantly filled with microcrystalline quartz. In many cases the original calcite mineralogy of the foraminifer test wall is preserved, but some are partially or entirely replaced by diagenetic silica. All of the porcellanite samples retain a distinct sedimentary fabric, with layers rich in clay, radiolarians and foraminifers, and microcrystalline quartz. Chert is mainly composed of microcrystalline quartz, clay minerals, and opaques. Areas show a breccia-like fabric of angular material with infill of chalcedonic quartz (Fig. F6D). No biogenic components were observed within the chert.

Summary

At Site U1332, Eocene seafloor basalt is overlain by ~150.4 m of pelagic sediments that are divided into five major lithologic units and subunits. Sediments are dominated by radiolarian and nannofossil ooze with varying amounts of clay and can be correlated with Sites 1219 and 1220 using biostratigraphic, magnetostratigraphic, and cyclostratigraphic (magnetic susceptibility and GRA density) results. The early Miocene sedimentary sequence is dominated by clay with radiolarians followed downhole by a late Oligocene alternation of radiolarian ooze with clay, nannofossil ooze with radiolarians, and nannofossil ooze. The early Oligocene is predominantly characterized by white nannofossil ooze with minor intercalations of radiolarian nannofossil ooze in the middle early Oligocene. The early middle Eocene sequence is very low in carbonate followed by a porcellanite interval of ~5 m. The middle through late Eocene section (Hole U1332A; ~95–140 CSF) is dominated by radiolarian ooze with varying amounts of clay, whereas nannofossil ooze is a secondary major lithology and occurs in four distinct intervals that broadly correlate with the lithostratigraphic results of Leg 199 and the CAEs of Lyle et al. (2005). The Eocene/Oliocene boundary is marked by a transition from dark brown radiolarian ooze to pale brown nannofossil ooze with radiolarians. A transition from Eocene siliceous sedimentation to Oligocene carbonate deposition is also observed in sediments from several other sites in the equatorial Pacific Ocean (e.g., Sites 1218 and 1219 and DSDP Sites 161 and 162) and probably reflects a deepening of the CCD associated with Antarctic glaciation (van Andel et al., 1975; Coxall et al., 2005).

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