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doi:10.2204/iodp.proc.320321.109.2010

Lithostratigraphy

Drilling at Site U1337 recovered a ~450 m thick section of pelagic sediments overlying seafloor basalt (Fig. F5). The sedimentary sequence is divided into three major lithologic units (Fig. F5; Table T2). The upper part of the sedimentary sequence (Unit I; ~90 CSF) is characterized by an alternating sequence of multicolored (including various hues of brown, gray, and green) nannofossil, diatom, and radiolarian oozes of latest Miocene to Pleistocene age. Unit II (~125 m CSF) consists of gray and green biosiliceous (diatom and radiolarian) ooze with varying amounts of nannofossils. Unit III (~230 m CSF) predominantly consists of white, yellow, green, brown, and gray nannofossil ooze and chalk of late Oligocene to early late Miocene age, with generally low but sometimes common abundances of siliceous microfossils. Unit IV is composed of aphanitic basalt of late Oligocene age.

Lithologic units and boundaries are defined by changes in lithology, physical properties, color reflectance, and calcium carbonate (CaCO₃) content. Lithologic differences, based on both visual core description and smear slide observations, are primarily attributable to varying distributions of biogenic components (nannofossils, diatoms, radiolarians, and foraminifers) (Fig. F6; see "Site U1337 smear slides" in "Core descriptions"). Lithologic descriptions are based primarily on sediments recovered in Hole U1337A and supplemented with observations from Holes U1337B, U1337C, and U1337D.

See "Corrected core composite depth scale" in the "Methods" chapter for an explanation of the CCSF-B depth scale.

Unit I

Intervals: 321-U1337A-1H-1, 0 cm, through 11H-2, 85 cm; 321-U1337B-1H-1, 0 cm, through 10H-5, 56 cm; 321-U1337C-1H-1, 0 cm, through at least 2H-CC, 19 cm; 321-U1337D-1H-1, 0 cm, through 10H-4, 106 cm

Depths: Hole U1337A = 0–93.35 m CSF (0–100.67 m CCSF-A; 0–89.89 m CCSF-B); Hole U1337B = 0–93.06 m CSF (0–100.12 m CCSF-A; 0–89.40 m CCSF-B); Hole U1337C = 0 to at least 11.59 m CSF (0 to at least 12.64 m CCSF-A; 0 to at least 11.29 m CCSF-B); Hole U1337D = 0–89.56 m CSF (0–100.49 m CCSF-A; 0–89.73 m CCSF-B)

Age: Pleistocene to latest Miocene

Lithology: nannofossil ooze, diatom nannofossil ooze, radiolarian nannofossil ooze, calcareous ooze, diatom calcareous ooze, diatom ooze, calcareous diatom ooze, radiolarian diatom ooze, nannofossil diatom ooze, and diatom radiolarian ooze

The major lithologies in Unit I are nannofossil ooze, diatom nannofossil ooze, radiolarian nannofossil ooze, calcareous ooze, diatom calcareous ooze, diatom ooze, calcareous diatom ooze, radiolarian diatom ooze, nannofossil diatom ooze, and diatom radiolarian ooze (Figs. F7, F8). Calcite grains are generally <5 µm and are most abundant in the top 20 m. In Unit I they represent fragments of potentially reworked calcareous fossils, but below 300 m, calcite grains are dominantly needlelike in shape, suggesting a diagenetic origin. Downsection the abundance of siliceous microfossils increases relative to nannofossils, which is consistent with a decreasing trend in CaCO₃ weight percent.

Visual core descriptions and smear slide observations indicate that Unit I generally exhibits meter-scale cyclic alternations in color and lithology. Lighter colored sediments, such as white (2.5Y 8/1), very pale hues of brown (e.g., 10YR 7/3), yellow (e.g., 5Y 8/3), and light greenish gray (e.g., 10Y 8/1), are richer in nannofossils. Darker colored sediments, such as slightly darker hues of brown (e.g., 2.5Y 3/3 and 10YR 6/4 to 10YR 3/3) and gray (e.g., 5Y 6/1 and 5Y 5/2), contain more siliceous microfossils, notably radiolarians (Fig. F7). Alternations in lithology covary with physical properties, including magnetic susceptibility and b* reflectance, as well as CaCO₃ weight percent (Fig. F5). Magnetic susceptibility and b* reflectance show higher amplitude variability than in underlying units. Sediments are intensely bioturbated and mottled.

The transition to Unit II (Fig. F9) is defined by the last downcore occurrence of brown (e.g., 10YR 7/4, 10YR 8/2, and 2.5Y 6/3) sediments and the base of the interval of highly variable magnetic susceptibility and b* reflectance values.

Unit II

Intervals: 321-U1337A-11H-2, 85 cm, through 24X-4, 27 cm; 321-U1337B-10H-5, 56 cm, through 23H-4, 4 cm; 321-U1337C-4H-1, 0 cm, through 8H-7, 55 cm (top of unit not recovered in Hole U1337C); 321-U1337D-10H-4, 106 cm, through 24H (shattered liner)

Depth: Hole U1337A = 93.35–218.27 m CSF (100.67–238.81 m CCSF-A; 89.89–213.23 m CCSF-B); Hole U1337B = 93.06–214.54 m CSF (100.12–238.83 m CCSF-A; 89.40–213.24 m CCSF-B); Hole U1337C = at least 192.65–211.85 m CSF (at least 192.96–237.65 m CCSF-A; at least 160.80–212.19 m CCSF-B); Hole U1337D = 89.56 to ~216 m CSF (100.49 to ~243.25 m CCSF-A; 89.73 to ~217.19 m CCSF-B)

Age: latest Miocene to middle Miocene

Lithology: diatom ooze, radiolarian diatom ooze, nannofossil diatom ooze, radiolarian nannofossil diatom ooze, diatom radiolarian ooze, diatom nannofossil radiolarian ooze, nannofossil radiolarian diatom ooze, nannofossil ooze, diatom nannofossil ooze, and radiolarian nannofossil ooze

The major lithologies in Unit II are nannofossil ooze, diatom nannofossil ooze, radiolarian nannofossil ooze, diatom ooze, radiolarian diatom ooze, nannofossil diatom ooze, radiolarian nannofossil diatom ooze, diatom radiolarian ooze, diatom nannofossil radiolarian ooze, and nannofossil radiolarian diatom ooze (Figs. F7, F8). Even though nannofossils sometimes comprise the major sedimentary components, sediments in Unit II are generally composed of siliceous microfossils (i.e., diatoms and radiolarians with common silicoflagellates) (Figs. F5, F6). Unit II exhibits meter-scale cyclic alternations in color and lithology. Darker hues of gray and green (N 6/ and 5GY 5/1 to 7.5Y 5/1) contain high abundances of siliceous microfossils (Fig. F7). Lighter colored intervals, such as white (N 8/), pale yellow (2.5Y 8/2 to 2.5Y 7/3) and light greenish gray (5G 4/1 to 10Y 7/1), are richer in nannofossils. In fact, the entire unit is characterized by the occasional occurrence of 30–150 cm laminated diatom ooze that likely reflects diatom mat deposition, interbedded with nannofossil diatom ooze (Fig. F10). Both lithologies are overprinted with millimeter- and centimeter-scale color banding with gray (N 5/ to N 6/) and greenish gray (5G 4/1). Dolomite concretions (centimeter sized) commonly occur throughout Unit II. Laminated diatom ooze intervals contain abundant specimens of the diatom Thalassiothrix spp. and intercalate with very thin (millimeter scale) porcellanite layers (Fig. F10). Layers (millimeter and centimeter scale) and concretions of dolomite occur regularly throughout Unit II.

Magnetic susceptibility and b* color reflectance are extremely low in Unit II, except for an interval between ~170 and 190 m CSF (Hole U1337A). L* reflectance and CaCO₃ content are highly variable throughout the unit (Fig. F5), reflecting decimeter- to meter-scale alternations between nannofossil diatom ooze and diatom ooze. Note that the frequency of these alternations is higher than the shipboard CaCO₃ sampling resolution in most of Unit II. Bioturbation is intense except in the laminated diatom mats.

The transition to Unit III is defined as the base of the interval in which siliceous microfossils are the dominant sedimentary component and coincides with a significant downhole increase in GRA density (Fig. F11) and a general increase in CaCO₃ content (Fig. F5).

Unit III

Intervals: 321-U1337A-24X-4, 27 cm, through 48X-CC, 21 cm; 321-U1337B-23H-4, 4 cm, through at least 27H-CC, 25 cm (base of hole); 321-U1337C-8H-7, 55 cm, through 33X-4, 1 cm; 321-U1337D-24H (shattered liner) through 49X-3, 0 cm

Depths: Hole U1337A = 218.27–448.04 m CSF (238.81–493.40 m CCSF-A; 213.23–440.53 m CCSF-B); Hole U1337B = 214.54–245.19 m CSF (238.83–268.52 m CCSF-A; 213.24–239.75 m CCSF-B); Hole U1337C = 211.85–439.77 m CSF (237.65–494.47 m CCSF-A; 212.19– 441.49 m CCSF-B); Hole U1337D = ~216–441.32 m CSF (~243–494.51 CCSF-A; ~217–441.53 m CCSF-B)

Age: middle Miocene to latest Oligocene

Lithology: nannofossil ooze, nannofossil chalk, diatom nannofossil ooze, radiolarian nannofossil chalk, diatom ooze, nannofossil diatom ooze, and nannofossil radiolarian ooze

The major lithologies in Unit III are nannofossil ooze, nannofossil chalk, diatom nannofossil ooze, radiolarian nannofossil chalk, diatom ooze, nannofossil diatom ooze, and nannofossil radiolarian ooze (Figs. F7, F8). Meter-scale lithologic alternations co-occur with color variations throughout the section: nannofossil oozes are white (N 8/) or light greenish gray (10G 7/1 and 10GY 8/1), whereas increased abundances of siliceous microfossils are generally slightly darker, with light greenish gray (10Y 8/1 to 5G 6/1) and sometimes light bluish gray (5PB 8/1) ooze (Fig. F6). Drilling breccia of chert and porcellanite was recovered in the lower part of Core 321-U1337C-11X (238.79–238.91 m CSF; 267.65–267.77 m CCSF-A) and in Sections 321-U1337D-28H-1 and 28H-2 (242.63–243.37 m CSF; 270.14–270.88 m CCSF-A). This represents a chert layer, which may act as a barrier to flow of pore waters (see "Geochemistry"). The thickness of the chert layer is unclear from the cores because of poor recovery, but downhole logging indicates it is ~40 cm (see "Downhole measurements"). The chert layer hampered core recovery across this interval in all holes, but correlation between density data from downhole logging and the cores indicates a gap in the composite section <1 m.

The transition from ooze to chalk occurs between Cores 321-U1337A-35X and 36X at ~318 m CSF (and at equivalent depths at the other Site U1337 holes). Color banding of millimeter- to centimeter-scale is common in Unit III with light greenish gray (10Y 8/1), greenish gray (5G 6/1), grayish green (5G 4/2), and darker hues of gray (N 4/ and N 5/). A ~10 m thick interval around 340 m CSF (late early Miocene) is characterized by increased magnetic susceptibility; decreased L*, GRA density, and CaCO₃ content; and the absence of planktonic foraminifers, suggesting carbonate dissolution. A sharp downcore transition from pale green to pale yellow sediments occurs at 412.36 m CSF in Hole U1337A (interval 321-U1337A-44X-5, 146 cm), at 396.56 m CSF in Hole U1337C (interval 321-U1337C-28X-2, 146 cm), and at 396.25 m CSF in Hole U1337D (interval 321-U1337D-44X-3, 106 cm), below which nannofossil chalks are predominantly pale yellow (5Y 8/2) (Fig. F12). Lithologic accessories include regular millimeter-scale pumice, pyrite and manganese oxide grains (the latter are particularly abundant in the lower part of Unit III), porcellanite, and one vitric volcanic ash in Core 321-U1337A-34X. Bioturbation is intense throughout Unit III. The transition to Unit IV is defined at the base of the sediment column, overlying basement basalt.

Unit IV

Intervals: 321-U1337A-48X-CC, 21 cm, through 48X-CC, 36 cm; 321-U1337C-33X-4, 1 cm, through 33X-4, 46 cm; 321-U1337D-49X-3, 0 cm, through 49X-3, 32 cm

Depths: Hole U1337A = 448.04–448.42 m CSF (493.40–493.78 m CCSF-A; 440.53–440.88 m CCSF-B); Hole U1337C = 439.77–440.23 m CSF (494.47–494.93 m CCSF-A; 441.49–441.90 m CCSF-B); Hole U1337D = 441.32–441.64 m CSF (494.51–494.83 m CCSF-A; 441.53–441.81 m CCSF-B)

Age: latest Oligocene

Lithology: aphanitic basalt

Aphanitic basalt with calcite veins was recovered at the base of Holes U1337A, U1337C, and U1337D, indicating penetration into basement. The early-stage veins are cemented with microcrystalline white (N 8/) calcite. Along the surface of partially open fractures are 2–3 mm euhedral crystals of bluish gray calcite. Unit III nannofossil chalks overlie the basalt.

Discussion

Color changes, lithology, and redox state

Smear slide analyses and visual core descriptions show that many of the meter-scale color variations in Units I and II to some extent relate to changes in lithology (Fig. F7). We suspect, however, that many of the color variations on centimeter to meter scale, notably the transitions between pale yellow and green lithologies, are controlled by sediment redox state, similar to those recorded at Sites U1331–U1336 and earlier work in the equatorial Pacific (e.g., cf. Lyle, 1983).

Magnetic susceptibility is relatively low in the light greenish gray intervals in Unit I and for all of Unit II. A significant decrease in the intensity of the magnetic signal in Unit II suggests dissolution of magnetite (see "Paleomagnetism") resulting from intensified microbial iron reduction. In the lower part of Unit III (~410 m CSF) a sharp downcore transition from green to yellow is not associated with any lithologic change (Fig. F12). Although pore water iron concentrations reach 5–6 µM in the green interval, it is absent below the transition to yellow. Although some of this signal may be affected by seawater contamination during XCB drilling, all available information suggests that the lowermost color change represents a redox front.

Diatom mat deposition

Unit II at Site U1337 is mostly composed of biosiliceous lithologies, notably diatoms (Figs. F5, F6, F7). The abundance of diatoms in the middle and upper Miocene section at Site U1337 is much higher than encountered in any interval at Sites U1331–U1336. Several decimeter- to meter-scale intervals of diatom ooze are laminated, and smear slide analyses indicate that the diatom assemblage is composed primarily of pennate taxa with abundant needlelike Thalassiothrix spp., indicating diatom mat deposition (Fig. F10). The lowermost laminated diatom mat is in the upper portion of Unit III at ~15 Ma. Much larger intervals are present in Unit II at ~10 Ma and shorter intervals at ~4.5 Ma. The ages of laminated diatom mats at this site are similar to those found at Leg 138 sites farther to the east (Mayer, Pisias, Janecek, et al., 1992), which have been interpreted to reflect regional bursts of export silica production in the eastern equatorial Pacific (Kemp and Baldauf, 1993). No laminated diatom oozes were observed at Expedition 320 drill sites farther to the west; however, near 10 Ma at Site U1335, drilling recovered clayey diatom ooze and clayey radiolarian ooze containing no carbonate at all (Pälike et al., 2009) suggesting that dissolution may also play an important role in the deposition of laminated diatom mats.

Oligocene–Miocene transition

The Oligocene/Miocene boundary was recovered in Holes U1337A, U1337C, and U1337D (Fig. F5). In Hole U1337A, the Oligocene/Miocene boundary is estimated to fall between intervals 321-U1337A-48X-2, 85–87 cm, and 48X-3, 55 cm (445.56–446.75 m CSF; 490.92–492.11 m CCSF-A) (see "Biostratigraphy"). It occurs in white (2.5Y 8/1) nannofossil chalk with foraminifers, interbedded and heavily mottled with pale yellow (2.5Y 7/4) to very pale brown (10YR 7/4) nannofossil chalk. Abundant millimeter-scale dendritic manganese grains composed of manganese oxide occur throughout this interval. The lower 15 cm of the core catcher of this core represents basement. No prominent change in lithology, GRA bulk density, reflectance, or magnetic susceptibility is seen through the Oligocene–Miocene transition.

Summary

At Site U1337, latest Oligocene seafloor basalt is overlain by ~450 m of nannofossil and biosiliceous oozes and nannofossil chalks that are divided into four lithologic units. Pleistocene through uppermost Miocene sediments of Unit I are characterized by multicolored (various hues of white, brown, green, and gray) nannofossil oozes, diatom oozes, and radiolarian oozes that alternate on a meter scale with a general downsection increase in siliceous microfossils relative to nannofossils. Green and gray biosiliceous lithologies, interbedded on a meter scale with white and light greenish gray nannofossil ooze comprise the dominant sedimentary constituents in the uppermost Miocene to middle Miocene Unit II, which includes regular diatom mat deposits. Meter-scale color alternations in Units I and II are associated with variations in lithology and physical properties. However, similar to the common millimeter- and centimeter-scale color banding that does not mark compositional changes, they are likely associated with sediment redox conditions. White, pale yellow, and pale green nannofossil oozes and chalks dominate the sediments of middle Miocene to latest Oligocene age, although diatoms and radiolarians remain present in low abundances. Latest Oligocene seafloor basalt (Unit IV) was recovered at the base of the sedimentary section.

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