Previous   |   Next

doi:10.2204/iodp.proc.320321.107.2010

Lithostratigraphy

Drilling at Site U1335 recovered a ~420 m thick section of pelagic sediments overlying seafloor basalt (Fig. F4). The sedimentary sequence at Site U1335 is divided into two major lithologic units (Fig. F4; Table T2). The upper part of the sedimentary sequence (0–64 m CSF; Unit I) is characterized by an alternating sequence of multicolored nannofossil, diatom, and radiolarian oozes of early late Miocene to Pleistocene age. Unit II is a ~350 m thick sequence of nannofossil ooze and chalk of late Oligocene to early late Miocene age. This unit is characterized by pronounced changes in color, both at the thick stratigraphic scale (20–190 m) (Fig. F6) and superimposed centimeter- to millimeter-scale color banding. Unit II contains many thin beds of nannofossil foraminifer ooze overlying sharp basal contacts (Fig. F4).

Lithologic units and boundaries are defined by changes in lithology, physical properties, and calcium carbonate (CaCO₃) content as measured by coulometry. 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). Lithologic descriptions are based primarily on sediments recovered in Hole U1335A, supplemented with observations from Hole U1335B.

Unit I

Intervals: 320-U1335A-1H-1, 0 cm, through 7H-6, 44 cm; 320-U1335B-1H-1, 0 cm, through 8H-5, 75 cm

Depths: Hole U1335A = 0–64.34 m CSF; Hole U1335B = 0–67.05 m

Age: Pleistocene to early late Miocene

Lithology: alternation of nannofossil ooze, diatom nannofossil ooze, clayey nannofossil ooze, foraminifer nannofossil ooze, nannofossil foraminifer ooze, radiolarian diatom ooze, nannofossil diatom ooze, clayey diatom ooze, clayey radiolarian ooze, and clay

The major lithologies in Unit I are white (10YR 8/1) through dark grayish brown (2.5Y 4/2) nannofossil ooze, very pale brown (10YR 8/2) foraminifer nannofossil ooze, very pale brown (10YR 7/3) through olive-brown (2.5Y 4/3) diatom nannofossil ooze, light gray (10YR 7/2) to very pale brown (10YR 7/4) radiolarian diatom ooze, very pale brown (10YR 8/4) and olive-brown (2.5Y 4/3) nannofossil diatom ooze, dark gray (10YR 4/1) clayey nannofossil ooze, very dark grayish brown (10YR 3/2) clayey radiolarian ooze, brown (10YR 4/3) clayey diatom ooze, and brown (10YR 4/3) clay.

Sediments in Unit I show cyclic alternations in lithology of 30–80 cm thickness. An exception to this is a ~9 m thick white (10YR 8/1) to very pale brown (10YR 8/2) nannofossil ooze observed between 45.5 and 54.5 m CSF (from interval 320-U1335A-5H-6, 80 cm, to 6H-6, 9 cm). Beneath this interval, the lowermost part of Unit I consists of clay-rich sediments of very dark grayish brown (10YR 3/2) clayey radiolarian ooze and brown (10YR 4/3) clay that alternate with grayish brown (2.5Y 5/2) to light brownish gray (2.5Y 6/2) nannofossil ooze with diatoms (e.g., Cores 320-U1335A-7H and 320-U1335B-7H). Clay-rich intervals are characterized by high magnetic susceptibility (up to 22 x 10^–5 SI), low L* (25–30), elevated b* and L* (10–15), and low GRA bulk density (<1.2 g/cm³) values (Fig. F4). The nannofossil ooze layer shows low magnetic susceptibility (<6.0 x 10^–5 SI), high L* (>80), low b* (<10), and high bulk density (up to 1.7 g/cm³) values. Bioturbation is generally intense in Unit I, particularly in the alternating multicolored biogenic oozes.

The Unit I/II boundary is located at interval 320-U1335A-7H-6, 44 cm (64.34 m CSF), and is defined at the base of an alternating sequence of clayey radiolarian ooze with clay and nannofossils and nannofossil ooze with diatoms (Table T2). Below the unit boundary to interval 320-U1335A-8H-2, 50 cm (67.90 m CSF), physical property data show a transition to lower magnetic susceptibility, higher L*, lower b*, and higher GRA bulk density values (Fig. F4).

Unit II

Intervals: 320-U1335A-7H-6, 44 cm, to at least 45X-CC, 15 cm; 320-U1335B-8H-5, 75 cm, to 46X-CC, 8 cm

Depths: Hole U1335A = 64.34 to at least 420.08 m CSF; Hole U1335B 67.05–418.04 m CSF

Age: early late Miocene to late Oligocene

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

Major lithologies in Unit II are white (10YR 8/1) to very pale brown (10YR 8/4) and light greenish gray (10Y 8/1 to 5GY 7/1) nannofossil ooze and white (2.5Y 8/1) and pale yellow (2.5Y 7/3) nannofossil chalk. Several 10–50 cm thick interbedded layers of yellow (10YR 7/6) to light greenish gray (5GY 7/1) nannofossil radiolarian ooze occur in the middle part of Unit II (157–176 m CSF in Hole U1335A). In addition, two distinct thin beds (18–23 cm thick) of nannofossil diatom ooze are interbedded within the nannofossil ooze (Samples 320-U1335B-10H-3, 62–80 cm, and 24H-6, 40–63 cm) (Figs. F8, F9). In the middle part of Unit II (157–176 m CSF in Hole U1335A), some intercalations of white (N 8) to pale yellow (2.5Y 7/4) and light greenish gray (5GY 7/1) nannofossil ooze contain radiolarians or diatoms with abundances reaching between 10% and 25% (see "Site U1335 smear slides" in "Core descriptions"). These intervals of nannofossil radiolarian ooze, nannofossil diatom ooze, and nannofossil ooze with radiolarians or diatoms show darker colors relative to the immediately overlying and underlying nannofossil ooze intervals. Bioturbation is generally minor throughout Unit II.

One of the most prominent features of Unit II is the intercalation of at least 49 beds (2–162 cm thick) of white (N 8 and 10YR 8/1), light greenish gray (10Y 8/1, 10GY 8/1, and 5GY 8/1), and light gray (N 7) nannofossil foraminifer ooze (Table T3). These beds have sharp basal boundaries, many of which are irregular, some of which are inclined. Grain size fines upward from medium sand to silt within these beds. Basal sediments are often darker colored than the immediately overlying deposits. Basal sediments are dominated by nannofossil foraminifer ooze and are instantly recognizable in the split core by their coarse texture during preparation (surface scraping) for line scan imaging. Angular basalt fragments (<1 mm), fish teeth, and pyritized foraminifers and radiolarians are also found concentrated in these basal deposits (Fig. F10). At least three of these beds show parallel or cross-laminations in their middle or upper part.

In the basal part of Unit II, at least five thin (4–13 cm thick) intervals of nannofossil chalk contain sand- to pebble-sized fresh basalt clasts with a glassy rim in nannofossil chalk (see "Site U1335 smear slides" in "Core descriptions"). Boundaries between basalt-containing nannofossil ooze and adjacent nannofossil oozes are not clear because of bioturbation and XCB drilling disturbance (biscuits).

Nannofossil ooze in Unit II displays pronounced changes in color, both at the thick stratigraphic scale (20–190 m) (Fig. F6) and superimposed centimeter- to millimeter-scale color banding (Fig. F11). At the top of Unit II, sediment color shifts downhole from pale yellow (2.5Y 8/2) toward white (N 8) and light greenish gray (10Y 8/1) around 67–72 m CSF (Hole U1335A). The white and light greenish gray color persists for ~100 m downhole and then reverts back to pale yellow and very pale brown for ~20 m (Fig. F6). Below 192 m, sediments are consistently white (N 8) and light greenish gray over a 180 m thick interval to 372 m CSF (Hole U1335A). Around 372 m CSF, color shifts back from white (N 8) to pale yellow (2.5Y 7/3) toward pale brown (10YR 7/3) to the base of the sediment column (Fig. F6). Superimposed on these gross changes in sediment color within the light greenish gray intervals millimeter- to centimeter-scale light greenish gray (5GY 8/1, 10GY 8/1, 5G 8/1, 5GY 7/1, 10GY 7/1, and 5G 7/1), light gray (N 7), and gray (N 6) color bands are common (Figs. F4, F11). The color bands occur both with sharply defined boundaries and with more diffusive boundaries. Sometimes the sharply defined color bands occur in discrete beds that are distinctly more consolidated than immediately over- and underlying sediments (Fig. F11). Based on smear slide observations, the lithology of the color-banded sediments (nannofossil ooze) is identical to the immediately over- and underlying sediments (see "Site U1335 smear slides" in "Core descriptions"). Submillimeter-sized opaques, presumably sulfide minerals such as pyrite, form streaks and fill burrows.

Magnetic susceptibility is extremely low in Unit II between 68 and 118 m CSF and between 200 and 400 m CSF (Fig. F4). These two intervals broadly correspond to the light greenish gray intervals. Reflectance parameters a* and b* shift in a stepwise manner to lower values or toward green (a*) and blue (b*) with the observed color shift toward light greenish gray (Fig. F6). Within the stratigraphic interval between 118 and 199 m CSF (Hole U1335A), magnetic susceptibility data show sporadic peaks that correspond to the slightly darker, siliceous fossil-rich intervals (i.e., nannofossil radiolarian ooze, nannofossil ooze with radiolarians, and nannofossil ooze with diatoms). Higher magnetic susceptibility values in the radiolarian- and diatom-rich intervals indicate higher clay contents than the adjacent pure nannofossil oozes.

Unit III

Intervals: 320-U1335B-46X-CC, 8 cm, through at least 46X-CC, 46 cm; no basalt drilled in Hole U1335A

Depth: Hole U1335B = 418.04 to at least 418.42 m CSF

Lithology: basalt

Pieces of basalt up to 10 cm in length were recovered at the base of Hole U1335B. These basalt pieces have glassy rims and are overlain by the nannofossil chalks of Unit II.

Redox-related color changes

The homogeneous lithology of Unit II is overprinted by vivid color changes similar to those observed at Site U1334. Light greenish gray nannofossil ooze occurs in two stratigraphic intervals, between 70 and 170 m and 192 and 323 m. Magnetic susceptibility drops to near zero, and reflectance parameters a* and b* shift to lower values with the observed color shifts toward light greenish gray (Fig. F6). Pore water profiles for dissolved Fe and Mn concentrations show downhole changes that are consistent with the changes in sediment color and magnetic susceptibility data (see "Geochemistry"). A significant peak in dissolved Fe in the pore fluids occurs in the light greenish gray intervals between 80 and 165 m and 203 and 316 m CSF (Hole U1335A), with distinct minima that correspond to the return to pale brown color and higher magnetic susceptibility between the greenish layers and at the base of the section (see "Geochemistry"). The pattern of changing sediment color, physical and magnetic properties, and pore water chemistry in the two intervals suggests loss of Fe-bearing minerals during diagenesis by microbial Fe reduction. A diagenetic origin for the color bands is suggested by (1) their occurrence in the interval of the sequence that has undergone Fe reduction, (2) their lack of lithological specificity, and (3) the consolidation sometimes associated with discrete beds that show particularly prominent banding. Similar gray and light greenish gray banding is reported from sediments recovered from Ongtong Java Plateau (ODP Leg 130), where they are enriched in iron sulfides in gray layers and in iron and magnesium silicates in green layers (Lind et al., 1993).

Gravity flow deposits

Throughout the sedimentary section drilled at Site U1335, sharp irregular contacts occur between lithologies (Table T3; Figs. F10, F12). Many of these sharp contacts are directly overlain by coarser sediment (i.e., nannofossil foraminifer ooze), generally showing a normal grading. The coarse-grained sediments directly overlying the sharp contacts are clearly of erosional origin and sometimes contain angular basalt fragments (Fig. F10), pyritized foraminifers and/or radiolarians, and fish teeth at the base. There is a concentration of grayish layers in the basal intervals of the nannofossil foraminifer ooze that may reflect increased porosity in coarse sediments and intensified iron sulfide precipitation. In some cases, coarse intervals show cross or parallel laminations in the middle of the bed. These features indicate that the erosional contacts and their overlying coarse sediments are the product of mass flow events, likely turbidity currents. The thickness of the inferred turbidites is typically between 2 and 25 cm, with a maximum thickness of 176 cm in Sections 320-U1335B-36H-2 and 36H-3 (Table T3). Gravity flow deposits occur with an approximate frequency of one or two beds per core, and their total thickness occupies a minimum of 2% of the recovered sediment. The abundance and thickness of turbidites is highest within Cores 320-U1335A-21H through 37X (189.4–350.1 m CSF). In contrast, no gravity flow deposits are observed between Cores 320-U1335A-3H and 8H (18.4–75.9 m CSF). The presence of basalt fragments at the base of some turbidites suggests that the provenance of the inferred turbidites observed at Site U1335 is likely one or both of the two seamounts located 15 to 20 km northeast and southeast (present summit water depth is ~400–600 m shallower than Site U1335) (Fig. F1).

Sediments across the Oligocene–Miocene transition

The Oligocene/Miocene boundary was recovered in both holes at Site U1335 (Fig. F4). The Oligocene/Miocene boundary (23.0 Ma) is approximated by the first appearance of the planktonic foraminifer P. kugleri, whereas the last occurrence of nannofossil S. delphix falls 100 k.y. before the Oligocene/Miocene boundary (23.1 Ma). P. kugleri is present in Sample 320-U1335A-37X-4, 136–138 cm, and the top of S. delphix falls between Samples 320-U1335A-37X-6, 50 cm, and 37X-CC (see "Biostratigraphy"). Therefore, the Oligocene/Miocene boundary is estimated to fall between Samples 320-U1335A-37X-4, 136–138 cm, and 37X-CC. Magnetostratigraphic data are not available for these intervals because of XCB coring in Hole U1335A (below Core 320-U1335A-37X) and loss of signal in Hole U1335B (see "Magnetostratigraphy"). No shipboard splice is available for this interval (see "Stratigraphic correlation and composite section"). The Oligocene–Miocene transition in Holes U1335A and U1335B occurs in white to light greenish gray nannofossil ooze with frequent millimeter- to centimeter-scale color bands and a sharp boundary (interval 320-U1335A-37X-6, 44 cm) overlain by an 18 cm thick bed of nannofossil foraminifer ooze that fines upward. No prominent change in lithology, GRA bulk density, reflectance, or magnetic susceptibility is seen through the Oligocene–Miocene transition.

Summary

At Site U1335, Oligocene seafloor basalt is overlain by ~420 m of calcareous sediments that are divided into two lithologic units. The sediments are primarily composed of nannofossil oozes and nannofossil chalks. The Pleistocene through late middle Miocene sedimentary sequence (Unit I) contains more radiolarians, diatoms, and clay relative to the Oligocene through late middle Miocene sediments (Unit II). Unit II is characterized by homogeneous nannofossil ooze and nannofossil chalk with interbedded nannofossil radiolarian ooze that corresponds to the transition from early to middle Miocene. Unit II is marked by two intervals of major color change from very pale brown to pale yellow to light greenish gray and back to pale brown nannofossil ooze. Intervals of light greenish gray nannofossil ooze are overprinted by vivid color banding ranging from millimeter- to centimeter-scale similar to that observed at Site U1334. Throughout Unit II, sharp irregular contacts are frequent and often overlain by coarser sediment (i.e., nannofossil foraminifer ooze) interpreted as mass flow events and appear to be turbidites. Fresh seafloor basalt was recovered at the base of the sedimentary section.

Top of page   |   Previous   |   Next