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doi:10.2204/iodp.proc.320321.106.2010

Lithostratigraphy

Drilling at Site U1334 recovered an ~285 m thick section of pelagic sediments overlying seafloor basalt (Fig. F5). The sedimentary sequence at Site U1334 is divided into four major lithologic units (Fig. F5; Table T2). The top of the section (0–47 m CSF) is an early to middle Miocene clay intercalated with nannofossil and radiolarian oozes (Unit I). The topmost 15 m interval of Unit I consists predominantly of radiolarian clay with varying amounts (<15%) of micronodules and zeolite. These sediments overlie alternations of radiolarian clay and nannofossil ooze. Unit II consists of ~200 m of early Miocene to Oligocene nannofossil ooze and chalk overlying a ~35 m thick alternating sequence of late Eocene nannofossil chalk, radiolarite, and claystone (Unit III). A thin layer (~1 m thick) of middle Eocene micritic chalk and limestone was recovered at the base of the sedimentary sequence (Unit IV) above basement basalt.

Lithologic units and boundaries are defined by differences in lithology, physical property data series, and calcium carbonate (CaCO₃) content as measured by coulometry. Lithologic differences, based on both visual core description and smear slide analysis, are primarily attributable to varying distributions of biogenic components such as nannofossils, radiolarians, and diatoms and clay-sized lithogenic material (Fig. F5; see "Site U1334 smear slides" in "Core descriptions"). Lithologic descriptions are based primarily on sediments recovered in Hole U1334A, supplemented with observations from Holes U1334B and U1334C.

Unit I

Intervals: 320-U1334A-1H-1, 0 cm, through 5H-CC, 25 cm; 320-U1334B-1H-1, 0 cm, through 5H-3, 55 cm; 320-U1334C-1H-1, 0 cm, through 5H-1, 65 cm

Depths: Hole U1334A = 0.0–46.76 m CSF; Hole U1334B = 0.0–45.25 m CSF; Hole U1334C = 0.0–38.7 m CSF

Age: early to middle Miocene

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

The major lithologies in Unit I are light yellowish brown (10YR 6/4) to very dark brown (10YR 3/2) to very dark gray (10YR 3/1) clay, dark grayish brown (10YR 4/2) to very dark grayish brown (10YR 3/2) radiolarian clay, brown (10YR 4/3) radiolarian ooze, and very pale brown to brown (10YR 8/2 to 10YR 5/3) nannofossil ooze. Clay occurs with minor amounts of zeolites and micronodules, as well as radiolarians and nannofossils. Radiolarian ooze occurs with minor amounts of clay or nannofossils. Nannofossil ooze sometimes occurs with radiolarians or clay. The uppermost 2 to 5 m of the unit is composed of clay with significant amounts of zeolites and micronodules (up to 15%). Radiolarian clay dominates to 15 m CSF, below which radiolarian clay and clayey radiolarian ooze alternate with nannofossil ooze. The transition from the topmost interval of clayey sediments to the underlying clay/nannofossil ooze alternations is characterized by an increase in the amplitude of variations and absolute values in magnetic susceptibility, gamma ray attenuation (GRA) bulk density and color reflectance (b* and L* shown) (Fig. F5; see "Physical properties" for discussion of reflectance, including a*). CaCO₃ contents are near zero at the top of Unit I and highly variable where clays and nannofossil oozes alternate. The transition to Unit II is indicated by the increased importance of biogenic oozes relative to clay and a marked shift in physical properties, including a decrease in magnetic susceptibility, an increase in GRA bulk density, and an increase in L* (Fig. F5).

Unit II

Intervals: 320-U1334A-6H-1, 0 cm, through 27X-2, 26 cm; 320-U1334B-5H-3, 55 cm, through 26X-4, 124 cm; 320-U1334C-5H-1, 65 cm, through 27X-6, 96 cm

Depths: Hole U1334A = 46.76–244.96 m CSF; Hole U1334B = 45.25–243.94 m CSF; Hole U1334C = 38.7–247.66 m CSF

Age: early Miocene to late Eocene

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

The dominant lithology in Unit II is white (10YR 8/1) to very pale brown (10YR 8/2, 10YR 8/3, 10YR 7/3, and 10YR 7/4) nannofossil ooze with an exceptional 65 m thick interval of greenish and yellowish nannofossil ooze (Figs. F5, F7). Between 141 and 206 m CSF (Hole U1334A) sediment color undergoes a downhole transition from pale yellow (5Y 8/2) to several shades of light greenish gray (10Y 8/1, 10G 8/1, and 5 BG 8/1), light gray (N7), and back again through pale yellow (2.5Y 8/2) before returning to very pale brown (10YR 8/3). Bioturbation is generally minor throughout the unit. Nannofossil ooze sometimes includes radiolarians and clay as minor lithologic components. The alteration of relatively pure nannofossil oozes and nannofossil ooze with radiolarians or with clay creates subtle color banding on scales 20 to 80 cm thick (Fig. F8). Unit II sediments have CaCO₃ contents that are typically near 90 wt% (Fig. F5; see "Geochemistry"). L* and GRA bulk densities are elevated throughout Unit II (~90 and <1.6 g/cm³, respectively), whereas magnetic susceptibility remains low (typically <15 × 10^–6 SI). The interval of light greenish gray nannofossil ooze between 143 and 191 m CSF (Hole U1334A) is associated with a pronounced decrease in b* but little change in L* (Figs. F5, F7). Magnetic susceptibility is exceptionally low, approaching zero and sometimes below zero, throughout the greenish and yellowish intervals between 141 and 206 m CSF (Figs. F5, F7; see "Physical properties" and "Geochemistry").

Unit III

Intervals: 320-U1334A-27X-2, 26 cm, through 31X-2, 25 cm; 320-U1334B-26X-4, 124 cm, through 31X-5, 30 cm; 320-U1334C-27X-6, 96 cm, through 31X-2, 40 cm

Depths: Hole U1334A = 244.96–283.55 m CSF; Hole U1334B = 243.94–282.90 m CSF; Hole U1334C = 247.66–279.41 m

Age: middle to late Eocene

Lithology: nannofossil chalk, clayey nannofossil chalk, and clayey radiolarite

The dominant lithologies in Unit III are very pale brown (10YR 8/1 and 10YR 7/3) to light gray (10YR 7/2) to light yellowish brown (10YR 6/4) nannofossil chalk with lesser amounts of very dark brown (10YR 2/2) and very dark grayish brown (10YR 3/2) clayey nannofossil chalk and brown (10YR 5/3 and 10YR 4/3) nannofossil chalk with clay and very dark grayish brown (10YR 3/2) clayey radiolarite. Within the major lithologies, nannofossil chalk occurs with radiolarians and clay as minor lithologic components. The Unit II–III transition is identified by an increase in clay and radiolarian contents relative to nannofossils, an increase in magnetic susceptibility, and a decrease in GRA bulk density, L* and b* reflectance, and CaCO₃ contents (Fig. F5).

Unit IV

Intervals: 320-U1334A-31X-2, 25 cm, through at least 32X-CC, 40 cm; 320-U1334B-31X-5, 30 cm, through at least 31X-CC, 9 cm; 320-U1334C-31X-2, 40 cm, through at least 32X-CC, 43 cm

Depths: Hole U1334A = 283.55–285.40 m CSF; Hole U1334B = 282.90–283.99 m CSF; Hole U1334C = 279.41–280.13 m CSF

Age: middle Eocene

Lithology: micritic nannofossil chalk and limestone

The major lithologies in Unit IV are very pale brown (10YR 8/2) micritic nannofossil chalk and very pale brown (10YR 7/4) to brown (10YR 5/4) nannofossil chalk and white limestone. Unit IV is distinguished from Unit III by the presence of micrite as a major component in nannofossil chalk and of limestone.

Unit V

Intervals: 320-U1334A-32X-CC, 40 cm, to at least 32X-CC, 43 cm; 320-U1334B-31X-CC, 9 cm, to at least 31X-CC, 11 cm; 320-U1334C-32X-CC, 43 cm, to at least 32X-CC, 49 cm

Depths: Hole U1334A = 285.40 to at least 285.43 m CSF; Hole U1334B = 283.99 to at least 284.01 m CSF; Hole U1334C = 280.13 to at least 280.19 m CSF

Several broken pieces of basalt up to 10 cm in length were recovered at the base of each hole at Site U1334. Basalt is either intercalated with or overlain by the micritic chalks and limestone of Unit IV.

Redox related color changes

The relatively homogeneous lithology of Unit II is marked by vivid color changes that are made obvious against the backdrop of relatively white nannofossil oozes. Sediment color shifts downhole from white (10YR 8/1) to very pale brown (10YR 8/2) to pale yellow (2.5Y 8/1) to light greenish gray (10GY 8/1) over a 65 m thick interval and persists as light greenish gray for ~40 m before returning back through pale yellow to very pale brown and white (Fig. F7). Reflectance parameters a* and b*, which measure green–red and blue–yellow portions of the spectrum, respectively, shift in a steplike manner to lower values or toward green (a*) and blue (b*) with these observed color changes (Fig. F7). Magnetic susceptibility drops to near zero throughout the light greenish gray interval. Dissolved Fe and Mn concentrations in pore fluids increase 5 to 6 µM/L within these sediments from background concentration of close to zero (see "Geochemistry"). Together with the loss of magnetic signal (see "Paleomagnetism"), the increase in dissolved Fe concentrations and changes in sediment color indicate intensified microbial Fe reduction, perhaps fueled by higher organic carbon accumulation rates across this interval relative to the under- and overlying nannofossil ooze at Site U1334.

Light–dark color cycles within Oligocene nannofossil oozes

Readily observable but subtle color variations are common within the very pale brown nannofossil oozes of Unit II. These meter-scale light–dark cycles are associated with minor variations in the relative amounts of minor lithologic components, including clay, radiolarians, and diatoms (Fig. F8), and also in physical properties including L*, b*, magnetic susceptibility, and GRA bulk density (Fig. F5; see "Physical properties"). Nannofossils dominate the sediments, making up >95% of the fine fraction observed in smear slides. The remaining 5% is dominated by clay, radiolarians, and diatoms. The lithology name remains nannofossil ooze across these slight but apparent variations. The seemingly small shifts in nannofossil contributions, however, often between 95% and 99%, are necessarily associated with an approximately two-fold increase in clay or radiolarian content.

Sediments across the Oligocene–Miocene transition

The Oligocene/Miocene boundary was recovered in all three holes drilled at Site U1334 (Fig. F9). The Oligocene/Miocene boundary is defined by the appearance of the planktonic foraminifer P. kugleri (23.0 Ma) and approximated well by the short-lived (~100 k.y.) calcareous nannofossil Sphenolithus delphix (23.1–23.2 Ma), just below C6Cn.2n. P. kugleri is present in Sample 320-U1334A-10H-2, 38–40 cm, but not in Sample 10H-5, 38–40 cm, placing the midpoint of this datum in Core 320-U1334A-10H-3 (Fig. F9; see "Biostratigraphy"). The base of Chron C6Cn.2n, the magnetostratigraphic Oligocene/Miocene marker, however, locates the boundary 2 to 6 m lower, around interval 320-U1334A-10H-6, 100 cm. S. delphix is identified between intervals 320-U1334A-10H-7, 30 cm, and 11H-2, 20 cm, consistent with the magnetostratigraphic data.

The Oligocene–Miocene transition in Hole U1334A occurs in very pale brown nannofossil ooze with subtle light (10YR 8/2) and dark (10YR 7/4) color alternations. Four darker (10YR 7/4) layers alternating with lighter ones are evident upsection of the Oligocene/Miocene boundary in Hole U1334A (Fig. F9). The same alternating sequence is also observed above the Oligocene–Miocene transition in Cores 320-U1334B-9H and 320-U1334C-9H (Fig. F9). Smear slide observations do not show significant differences in constituents between dark and light layers, but compositional variations are indicated by variations of GRA bulk density, L*, and magnetic susceptibility that accompany the color changes (Fig. F9). A similar pattern in magnetic susceptibility is identified in Holes U1334B and U1334C (Fig. F9) and at Site 1218 (Shipboard Scientific Party, 2002b; Pälike et al., 2005).

Sediments across the Eocene–Oligocene transition

An Eocene–Oligocene transition section was recovered in all three holes drilled at Site U1334 (Holes U1334A–U1334C) (Figs. F5, F10). The Eocene/Oligocene boundary marker, Hantkenina species, has not been found at Site U1334 (see "Biostratigraphy"). However, radiolarian and nannofossil biostratigraphy provide excellent age control, indicating that the Eocene/Oligocene boundary falls near the middle of Biozone NP21 and just above the Zone RP20/RP19 boundary (near the base of Core 320-U1334A-27X, in the upper part of Core 320-U1334B-26X, and toward the lower part of Core 320-U1334C-27X) (Fig. F10).

The lithostratigraphy of the Eocene–Oligocene transition is well captured in all three holes drilled at Site U1334. In Hole U1334B, starting in Core 320-U1334B-26X, a downhole transition takes place from light gray (10YR 7/2) nannofossil ooze to dark brown (10YR 3/3) clayey nannofossil chalk (Figs. F10, F11), and these sediments overlie an alternating sequence of grayish brown (10YR 5/2) nannofossil chalk and very dark grayish brown (10YR 3/2) clayey nannofossil chalk with an underlying prominent (~60 cm thick) bed of very dark brown (10YR 2/2) radiolarian clay in Section 320-U1334B-27X-5 (Figs. F10, F11). Similar downhole lithologic transitions are seen in Holes U1334A and U1334C (Fig. F10). Thus, the Eocene–Oligocene transition at Site U1334 is marked by a distinct stepwise downhole color change from pale nannofossil ooze through dark clayey nannofossil chalk to alternations of dark nannofossil chalk and even darker clayey nannofossil chalk. Magnetic susceptibility, a*, and b* display pronounced downhole stepwise increases with pronounced downhole deceases in GRA bulk density, L*, and CaCO₃ content (Figs. F5, F10; see "Physical properties").

The lithostratigraphic results for the Eocene–Oligocene transition at Site U1334 are broadly consistent with those at Sites U1331–U1333 and multiple sites drilled during ODP Leg 199, in particular, Site 1218 (see the "Site U1331," "Site U1332," and "Site U1333" chapters) (Shipboard Scientific Party, 2002a). The most obvious lithostratigraphic difference between the Eocene–Oligocene transition at Sites U1334 and U1333 is that the transition at Site U1334 takes place within sediments that are significantly darker in color (L* values up to 60% at Site U1334 and up to 80% at Site U1333) (Fig. F12). The darker sediments at Site U1334 correspond to a more calcareous lithologic sequence and higher CaCO₃ contents than the darker Eocene sediments at Site U1333 (compare Fig. F5 in this chapter with Fig. F4 in the "Site U1333" chapter) (Fig. F12). The darker Eocene–Oligocene transition at Site U1334 is therefore attributed to higher clay content than at Site U1333, as indicated by magnetic susceptibility records for the two sites (Site U1334 = up to 60 × 10^–5 SI units; Site U1333 = up to 40 × 10^–5 units) (Fig. F12).

Summary

At Site U1334, Eocene seafloor basalt is overlain by about 285 m of pelagic sediments that are divided into four major lithologic units. Sediments are dominated by nannofossil oozes and nannofossil chalks. The early Miocene sedimentary sequence contains more clay and radiolarians relative to the Oligocene and Eocene sediments. The near-white Oligocene nannofossil oozes are characterized by both subtle meter-scale dark–light color cycles and vivid color variations that take place over 60 m. The subtle color cycles are the manifestation of small changes in the relative proportions of lithologic components, namely radiolarians, clay, and diatoms. The more vivid color cycles are related to changes in the oxidation state of Fe in the sediments and sedimentary pore waters. The oxidation-reduction reactions responsible for the observed color and pore water chemistry changes are likely fueled by enhanced availability of organic carbon relative to overlying and underlying sediments. The Oligocene–Miocene transition at Site U1334 is characterized by four subtle light–dark color alternations in very pale brown nannofossil ooze. The Eocene–Oligocene transition at Site U1334 is marked by a distinct stepwise color change from pale nannofossil ooze to dark clayey nannofossil chalk to alternations of dark nannofossil chalk and even darker clayey nannofossil chalk. Compared to Site U1333, the Eocene–Oligocene transition at Site U1334 takes place in significantly darker sediments. This observation is attributed to higher clay content, indicated by the higher values in the magnetic susceptibility record for Site U1334.

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