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doi:10.2204/iodp.proc.320321.110.2010

Lithostratigraphy

Drilling at Site U1338 recovered a ~415 m thick section of pelagic sediments overlying seafloor basalt (Fig. F5; Table T2). The sedimentary sequence is divided into three major lithologic units (Fig. F5; Table T3). Unit I (~50 m CSF; middle Pliocene to Pleistocene) consists of an alternating sequence of multicolored (including various hues of white, brown, green, and gray) nannofossil ooze, diatom nannofossil ooze, and radiolarian nannofossil ooze. Unit II (~194 m thick; upper Miocene to middle Pliocene) is mainly composed of light green and light gray nannofossil ooze with varying amounts of diatoms and radiolarians. Unit III (~171 m thick; lower to upper Miocene) predominantly consists of white, pale yellow, light greenish gray, and very pale brown nannofossil ooze and chalk, with generally low but sometimes common abundances of siliceous microfossils. Unit IV is composed of lower Miocene aphanitic basalt.

Lithologic units and boundaries are defined by changes in lithology, physical properties, color reflectance, and CaCO₃ content. Lithologic differences, based on both visual core description and smear slide observations, are primarily attributable to varying amounts of biogenic components (nannofossils, diatoms, and radiolarians) (Fig. F6; see "Site U1338 smear slides" in "Core descriptions"). Lithologic descriptions are based on sediments recovered in Hole U1338B and supplemented with observations from Holes U1338A and U1338C. Three cores were recovered from Hole U1338D and were saved for instructional purposes during upcoming expeditions. No measurements were made on these cores during Expedition 321.

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

Unit I

Intervals: 321-U1338A-1H-1, 0 cm, through 6H-7, 65 cm; 321-U1338B-1H-1, 0 cm, through 6H-4, 51 cm; 321-U1338C-1H-1, 0 cm, through 6H-5, 55 cm

Depths: Hole U1338A = 0–50.35 m CSF (0–55.91 m CCSF-A; 0–50.37 m CCSF-B); Hole U1338B = 0–50.61 m CSF (0–55.76 m CCSF-A; 0–50.23 m CCSF-B); Hole U1338C = 0–48.35 m CSF (0–55.96 m CCSF-A; 0–50.42 m CCSF-B)

Age: middle Pliocene to Pleistocene

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

The major lithologies in Unit I are nannofossil ooze, calcareous nannofossil ooze, nannofossil calcareous ooze, diatom nannofossil ooze, and radiolarian nannofossil ooze (Figs. F6, F7A). Calcite grains, which are generally <5 µm in size, are relatively abundant in this unit.

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

The transition to Unit II (Fig. F8) is defined by the last downcore occurrence of gray (e.g., 10Y 5/1 and N 4/) sediments and the base of the interval of highly variable magnetic susceptibility and coincides with a significant downhole decrease in b* and an increase in L* reflectance.

Unit II

Intervals: 321-U1338A-6H-7, 65 cm, through 27X-3, 74 cm; 321-U1338B-6H-4, 51 cm, through 26H-5, 44 cm; 321-U1338C-6H-5, 55 cm, through 27H-6, 12 cm

Depths: Hole U1338A = 50.35–243.94 m CSF (55.91–271.72 m CCSF-A; 50.37–244.79 m CCSF-B); Hole U1338B = 50.61–244.54 m CSF (55.76–271.47 m CCSF-A; 50.23–244.57 m CCSF-B); Hole U1338C = 48.35–244.92 m CSF (55.96–271.73 m CCSF-A; 50.42–244.80 m CCSF-B)

Age: late Miocene to middle Pliocene

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

The major lithologies in Unit II are alternating nannofossil ooze, diatom nannofossil ooze, nannofossil diatom ooze, and radiolarian diatom ooze (Figs. F6, F7B, F7C). Although sediments in Unit II generally consist of nannofossils, in some intervals diatoms and radiolarians are more abundant sedimentary components (Figs. F5, F6).

Unit II exhibits meter- to tens of meters–scale alternations in color and lithology. Lighter colored sediment, such as white (e.g., 2.5Y 8/1 and N 8/) and lighter hues of gray (e.g., 10Y 8/1, 5PB 8/1, 5BG 8/1, and 5Y 7/2), contains a high abundance of nannofossils. Sediment in darker hues of gray and green (e.g., 10Y 5/1, 5PB 6/1, 5Y 5/2, 2.5Y 5/1, and N 4/) is richer in siliceous microfossils. This unit is also characterized by occasional diatom nannofossil ooze layers (generally several centimeters thick and in some intervals 1–2 m thick; e.g., intervals 321-U1338A-15H-1, 0–95 cm, and 26H-1, 109 cm, to 26H-3, 84 cm), which contain abundant specimens of the diatom Thalassiothrix spp. interbedded with nannofossil ooze. However, laminated diatom ooze intervals (diatom mats), such as those observed in equivalent time intervals at Site U1337, are absent. Pyrite nodules, mainly found as centimeter-sized burrow casts, occasionally occur in Unit II. Color banding (e.g., light greenish gray, light gray, and light bluish gray) of millimeter to centimeter scale is common in all lithologies in Unit II.

Magnetic susceptibility is low throughout Unit II, apart from the lowermost 35 m. GRA bulk density, L* reflectance, CaCO₃ content, and TOC content are highly variable throughout the section (Figs. F5, F9), reflecting meter- to tens of meters–scale alternations between nannofossil ooze and diatom nannofossil ooze. Intervals of relatively high TOC concentrations reflect diatom-rich layers and contain abundant pyrite (Fig. F9). Bioturbation is intense throughout.

The transition to Unit III is defined as the base of the interval in which siliceous microfossils are a major sedimentary component and is associated with a significant downhole increase in GRA density and CaCO₃ content (Figs. F5, F10).

Unit III

Intervals: 321-U1338A-27X-3, 74 cm, through 44X-CC, 40 cm; 321-U1338B-26H-5, 44 cm, through 45H-CC, 48 cm; 321-U1338C-27H-6, 12 cm, through 47H-CC, 35 cm (base of hole)

Depths: Hole U1338A = 243.94–408.37 m CSF (271.72–453.43 m CCSF-A; 244.79–408.50 m CCSF-B); Hole U1338B = 244.54–414.94 m CSF (271.47–459.78 m CCSF-A; 244.57–414.22 m CCSF-B); Hole U1338C = 244.92–414.41 m CSF (271.73–455.28 m CCSF-A; 244.80–410.16 m CCSF-B, base of hole)

Age: early to late Miocene

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

The major lithologies in Unit III are nannofossil ooze, nannofossil chalk, calcareous nannofossil chalk, calcareous radiolarian nannofossil chalk, radiolarian nannofossil chalk, nannofossil calcareous chalk, and calcareous chalk (Figs. F6, F7D, F7E). Meter-scale lithologic alternations covary with color changes throughout the section: nannofossil oozes are white (N 8/) or light greenish gray (e.g., 10G 7/1 and 10GY 8/1), whereas increased abundances of siliceous microfossils are slightly darker greenish gray (5G 8/1 to 5G 6/1) and bluish gray (5PB 8/1 to 5PB 4/1) (Fig. F6). A minor lithology of light gray (2.5Y 7/2) nannofossil diatom ooze with opaques, containing abundant pyrite-filled siliceous microfossils, occurs in interval 321-U1338B-33H-4, 58–66 cm, at 297.54–297.62 m CSF, and in interval 35H-5, 76–82 cm, at 318.32–318.38 m CSF (Fig. F11). Accessories include regular millimeter-sized pyrite and manganese oxide grains; the latter are particularly abundant in the lowermost part of Unit III. Interval 321-U1338B-45X-6, 1–13 cm, contains a ground drilling slurry of black (10YR 2/1) manganese oxides. The transition from ooze to chalk occurs between Cores 321-U1338B-41H and 42H at ~377.9 m CSF and at roughly equivalent depths in Hole U1338C between Cores 321-U1338C-43H and 44H at ~386.4 m CSF. This transition occurs at a shallower depth in the cores recovered from Hole U1338A (between Cores 321-U1338A-33X and 34X at ~307.3 m CSF), probably related to sediment disturbance (biscuits) associated with XCB drilling.

Drilling breccia of chert was recovered in Core 321-U1338B-31H (282.1–282.91 m CSF) and interval 321-U1338C-32H-1, 0–6 cm (282.90–282.96 m CSF). The thickness of the original chert interval is undeterminable from the cores because of poor recovery, but downhole logging indicates it is ~16 cm thick (see "Downhole measurements"). The chert interval hampered core recovery across this interval in all holes, but from composite section analysis the unrecovered sediment section is estimated to be <1 m thick (see "Stratigraphic correlation and composite section").

A downhole sharp transition from light greenish gray (5G 7/1) to pale yellow (2.5Y 8/3) nannofossil ooze occurs at 386.72 m CSF in Hole U1338B (interval 321-U1338B-42H-6, 127 cm), at 382.72 m CSF in Hole U1338A (interval 321-U1338A-41X-CC, 25 cm), and at 391.73 m CSF in Hole U1338C (interval 321-U1338C-44H-3, 130 cm) (Fig. F12). In the lowermost part of Unit III, another downhole sharp transition from pale yellow to very pale brown sediments occurs at 405.94 m CSF in Hole U1338A (interval 321-U1338A-44X-3, 20 cm) and at 412.68 m CSF in Hole U1338C (interval 321-U1338C-47H-2, 67 cm), below which nannofossil chalks are very pale brown (10YR 6/4 and 10YR 8/3) (Fig. F12). Bioturbation is intense throughout Unit III. The transition to Unit IV is defined at the base of the sediment column, overlying basement.

Unit IV

Intervals: 321-U1338A-44X-CC, 40 cm, through 44X-CC, 48 cm; 321-U1338B-45X-CC, 48 cm, through 45X-CC, 53 cm

Depths: Hole U1338A = 408.37–408.45 m CSF (453.43–453.51 m CCSF-A; 408.50–408.58 m CCSF-B); Hole U1338B = 414.94–414.99 m CSF (459.78–459.83 m CCSF-A; 414.22–414.27 m CCSF-B)

Age: early Miocene

Lithology: aphanitic basalt

Aphanitic basalt fragments were recovered at the base of Holes U1338A and U1338B. Unit III nannofossil chalk overlies the basalt. Coring in Hole U1338C ceased before reaching basement.

Discussion

Color changes, lithology, and redox state

Smear slide analyses and visual core descriptions show that many of the decimeter-, meter-, and tens of meters–scale color variations in Units I and II to some extent relate to changes in lithology (e.g., Fig. F9). We suspect, however, that some of these color variations, notably the transitions between pale green and pale yellow lithologies, are controlled by sediment redox state similar to those recorded at Sites U1331–U1337 and earlier work in the equatorial Pacific (e.g., Lyle, 1983).

Magnetic susceptibility is relatively low in the light gray and light brown intervals in Unit I and for most 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 Fe reduction. In the lower part of Unit III, a sharp downhole transition from green to yellow is not associated with any lithologic change and does not occur at the same stratigraphic level between holes (see "Stratigraphic correlation and composite section"), and thus should not be considered as an equivalent time horizon (Fig. F12). Whereas pore water Fe concentrations reach 6–7 µM/L in the green interval, it is absent below the transition to yellow and brown (see "Geochemistry"). Although some of this signal may be affected by seawater contamination during XCB drilling in Hole U1338A, all available information suggests that the lowermost color change represents a redox front.

Occurrence of diatom-rich layers

Unit II at Site U1338 is mainly composed of nannofossil ooze with relatively high abundances of biosiliceous components, notably diatoms (Figs. F5, F6). The relative abundance of diatoms is lower than that at Site U1337, and the record lacks laminated diatom ooze intervals (diatom mats) such as those observed at Site U1337. However, 1 cm to sometimes 1–2 m thick intervals of diatom nannofossil ooze containing abundant specimens of the diatom Thalassiothrix spp. are occasionally interbedded with nannofossil ooze (e.g., ~126.2–127.1 and ~231.8–234.3 m CSF in Hole U1338A and ~127.3–128.0 and ~233.8–234.8 m CSF in Hole U1338C) (Fig. F9). Units II and III also contain significant amounts of pyrite, particularly in diatom-rich intervals in Unit II (e.g., Cores 321-U1338B-14H, 19H–21H, 26H, 28H, 29H, and 32H–41H). In addition, the middle part of Unit III contains thin intervals of abundant pyrite-filled siliceous microfossils (e.g., intervals 321-U1338B-33H-4, 58–66 cm, and 35H-5, 76–82 cm) (Fig. F11). These diatom-rich layers, pyrite nodule occurrences, and pyrite-rich siliceous microfossil layers in Units II and III are associated with higher TOC content (Figs. F9, F11), suggesting a relation between the abundance of diatoms in the sediments, sediment redox state, and the production or preservation of organic carbon.

Summary

At Site U1338, ~415 m of nannofossil ooze and chalk with varying concentrations of diatoms and radiolarians overlie early Miocene seafloor basalt and are divided into three lithologic units. Unit I Pleistocene through middle Pliocene sediments are characterized by multicolored (various hues of white, brown, green, and gray) nannofossil ooze, diatom nannofossil ooze, and radiolarian nannofossil ooze that alternate on decimeter to meter scales. Light green and light gray nannofossil ooze with occasional darker intervals with abundant siliceous microfossils, notably diatoms, comprise the upper Miocene to middle Pliocene Unit II. Decimeter-, meter-, and tens of meters–scale color alternations in Units I and II are associated with variations in lithology and physical properties. Some of these color changes, as well as common millimeter- and centimeter-scale color banding, are not associated with compositional changes and likely reflect variations in sediment redox state. White, pale yellow, light greenish gray, and very pale brown nannofossil oozes and chalks dominate Unit III of the lower to upper Miocene, although slightly darker green and gray intervals with larger amounts of siliceous microfossils remain present. Lower Miocene seafloor basalt (Unit IV) was recovered at the base of the sedimentary section.

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