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doi:10.2204/iodp.proc.320321.108.2010

Lithostratigraphy

Drilling at Site U1336 recovered a ~300 m thick section of pelagic sediments (Fig. F4). The sedimentary sequence at Site U1336 is divided into three lithologic units (Fig. F4; Table T2). Unit I (0–74 m CSF) is composed of nannofossil ooze of Miocene age that displays submeter-scale alternations of white (10YR 8/1) to brown (10YR 5/3) nannofossil oozes with varying amounts of radiolarians, foraminifers, diatoms, and clay as minor constituents. Two white (10YR 8/1) diatom nannofossil oozes (Samples 320-U1336A-5H, 22–36 cm, and 6H-2, 87–92 cm) are present within Unit I. These intervals were not observed in Hole U1336B. Unit II (74–189 m CSF) consists of ~115 m thick nannofossil ooze of early Miocene to late Oligocene age with very weak magnetic susceptibility and vivid color changes. Sediment color changes downhole from pale yellow (2.5Y 8/2) to light greenish gray (10Y 8/1) at 92 m CSF. Below this boundary, the coloration of the Unit II nannofossil oozes alternates between light greenish gray (10Y 8/1) and white (N 8/) to 184.80 m CSF. Unit III is composed of light greenish gray (10GY 7/1) nannofossil chalks with interbedded greenish gray (10GY 6/1) to black (10YR 2/1), olive-yellow (2.5Y 6/6), and pink (7.5YR 7/3) chert layers (189.50–299.6 m CSF) of Oligocene age. Units II and III have similar color alternations with occasional sharply defined packets of light greenish gray (5G 8/1) color bands below 120.86 m CSF.

Lithologic units and boundaries (Table T2) are defined by differences in lithology, physical property data, and calcium carbonate (CaCO₃) content as measured by coulometry (see "Geochemistry"). Lithologic differences, based on both visual core description and smear slide analysis, reflect varying distributions of biogenic components such as nannofossils, radiolarians, diatoms, and clay-sized lithogenic material (Fig. F4; see "Site U1336 smear slides" in "Core descriptions"). Lithologic descriptions are based on sediments recovered in Holes U1336A and U1336B. Sediments from Hole U1336A were described by the scientific party during Expedition 320, whereas those from Hole U1336B were examined and described by the Expedition 321 scientific party.

Unit I

Intervals: 320-U1336A-1H-1, 0 cm, through 8H-7, 5 cm; 320-U1336B-1H-1, 0 cm, through 9H-5, 84 cm

Depths: Hole U1336A = 0–73.05 m CSF; Hole U1336B = 0–75.14 m CSF

Age: middle to lower Miocene

Lithology: nannofossil ooze and diatom nannofossil ooze

Major lithologies in Unit I are white (10YR 8/1) to brown (10YR 5/3) nannofossil ooze. The unit is characterized by alternating submeter-scale light and dark bands of nannofossil ooze. Based on smear slides, the darker bands contain greater amounts of radiolarians and/or clay and some of the white bands are nannofossil ooze with foraminifers (Figs. F5, F6). The alternating white to brown sequence is punctuated by two white (10YR 8/1) layers of diatom nannofossil ooze (Samples 320-U1336A-5H-2, 22–36 cm, and 6H-2, 87–92 cm); however, these sediments were not recovered in Hole U1336B and were likely missed by coring gaps or sampled for whole-round studies before cores were described. Pumice fragments, sometimes as large as 2.0 cm in diameter, occur in Unit I. Most are well rounded and light gray to gray (N7/ to N6/) in color. Throughout Unit I, physical properties, including magnetic susceptibility, b* and L* reflectance, and GRA bulk density, all show higher amplitude variability than underlying units. CaCO₃ content is also more variable in Unit I relative to Unit II, ranging between 48 and 90 wt% (Fig. F4). The transition to Unit II is indicated by an overall decrease in the amplitude of variation in the physical properties, a downhole increase in density and L*, a decrease in magnetic susceptibility, and a lesser abundance of the minor constituent radiolarians, clay, and foraminifers (Fig. F4).

Unit II

Intervals: 320-U1336A-8H-7, 5 cm, through 22X-4, 20 cm; 320-U1336B-9H-5, 84 cm, through 20H-7, 58 cm (end of Hole U1336B)

Depths: Hole U1336A = 73.05–189.50 m CSF; Hole U1336B = 75.14–173.98 m CSF (end of hole)

Age: early Miocene to late Oligocene

Lithology: nannofossil ooze

The dominant lithology in Unit II is nannofossil ooze with vivid coloring. The uppermost 9.6 m of Unit II is pale yellow (2.5Y 8/2 to 2.5Y 8/3) nannofossil ooze that sharply overlies a sequence of alternating white (2.5Y 8/1 and N 8/) and light greenish gray (10GY 8/1) nannofossil ooze at 92.14 m CSF in Hole U1336A (Figs. F7, F8). Below 120 m CSF, light greenish gray (5G 8/1) millimeter-scale color bands are observed in centimeter-scale packets in nearly every core. Chert was encountered in the lower portion of this unit hampering coring recovery, but no layers were recovered directly. Numerous small angular fragments of chert were found in Section 320-U1336B-16H-CC and soupy intervals near tops of Cores 320-U1336B-17H through 19H, representing fall-in sediment. A small fragment of in situ chert was recovered in interval 320-U1336B-18H-4, 106 cm. Bioturbation is generally minor or nonvisible throughout the unit. CaCO₃ content in Unit II is relatively constant, typically >85 wt% (Fig. F4; see "Geochemistry"). Color reflectance parameter b* decreases abruptly at 92.14 m CSF in Hole U1336A and 91.85 m CSF in Hole U1336B and remains low throughout the unit. L* increases across the Unit I/II boundary, reaching a maximum near the top of Unit II (~90) and gradually decreases with depth to ~65–70 at ~180–190 m CSF (Fig. F7). Working downhole through Unit II, GRA bulk density increases from ~1.6 to 1.9 g/cm³. Magnetic susceptibility is very low (typically <5 × 10^–5 SI), sometimes below zero, throughout the unit (Fig. F4; see "Physical properties").

Unit III

Intervals: 320-U1336A-22X-4, 20 cm, through 35X-CC, 38 cm

Depths: Hole U1336A = 189.5–299.6 m CSF

Age: Oligocene

Lithology: nannofossil chalk, and chert

The Unit II–III transition is identified by the transition to chalk. Dominant lithologies of Unit III are light greenish gray (10GY 8/1 and 10GY 7/1) and white (7.5YR 8/1 and N 8/) nannofossil chalk with light greenish gray (5G 8/1) millimeter-scale color banding and chert layers. In the upper 100 m of Unit III cherts are black (10YR 2/1) to dark greenish gray (10GY 4/1). Below 289 m CSF nannofossil chalk contains increasing amounts of micrite and cherts occur in different colors: olive yellow (2.5Y 6/6), dark brown (7.5Y 3/2), and pink (7.5YR 7/3) (see "Site U1336 thin sections" in "Core descriptions"). Thin section analyses show increasing abundance with depth of foraminifers in the cherts (see "Site U1336 thin sections" in "Core descriptions"). Foraminifer tests in the cherts have been filled with microcrystalline quartz or occasionally filled or replaced by opaques, likely pyrite (Fig. F9). GRA density continues to increase with depth, from ~1.9 g/cm³ at the top of the unit and reaching a maximum of ~2.0–2.1 g/cm³ from 282 m CSF and below (Fig. F4; see "Physical properties"). Magnetic susceptibility in Unit III is generally low (<5 × 10^–5 SI) with thin instances of much higher values that correspond to the chert layers. L* is overall more variable and generally lower in Unit III, whereas b* shows more variability than overlying units. CaCO₃ contents remain >88 wt% in the chalk layers. Igneous rocks were not recovered at Site U1336.

Discussion

Unit I light–dark color cycles

Pronounced submeter-scale color variations are common within the nannofossil oozes of Miocene age found in Unit I. These light–dark cycles are associated with variations in the relative amounts of accessory lithologic components within the nannofossil oozes, including clay, radiolarians, and diatoms (Fig. F5). These lithologic cycles are also apparent in a higher amplitude of variation in physical properties, including L*, b*, magnetic susceptibility, and GRA bulk density (Fig. F4; see "Physical properties"). Nannofossils dominate these sediments, making up between 50% and 95% of smear slide samples. The remainder is made up of clay, radiolarians, or diatoms (Fig. F5; see "Site U1336 smear slides" in "Core descriptions"). Two white (10YR 8/1) diatom nannofossil ooze layers (Samples 320-U1336A-5H-2, 22–36 cm, and 6H-2, 87–92 cm) occur at 38.34 and 48.40 m CSF. These layers stand out because they are visually brighter than the surrounding sediments (nearest L* measurements are 88 and 83.5 for the upper and lower layers, respectively) and also have a unique spongy texture that causes pilling during surface scraping in preparation for line scan imaging. Smear slide analysis indicates that the diatom assemblage is composed primarily of pennate taxa, with abundant needlelike Thalassiothrix spp. (Fig. F6). The low abundance of diatoms (<5%) in most dark layers, where abundance of radiolarians increases relative to nannofossils, suggests that the observed lithologic variations within the nannofossil oozes could be the product of dissolution cycles; however, more detailed mass accumulation rate data are needed to test this.

Redox-related color changes

Site U1336 sediments are marked by vivid color changes that are made obvious against the backdrop of otherwise relatively white nannofossil oozes and chalks. Sediment color shows a downhole shift from white (10YR 8/1) to very pale brown (10YR 7/3) through pale yellow (2.5Y 8/2) to light greenish gray (10GY 8/1) over a 30 m thick interval (65–95 m CSF). This greenish gray color starts abruptly (Fig. F8) at 92.14 m CSF in Hole U1336A and at 91.8 m CSF in Hole U1336B and spans a ~198 m thick interval before shifting to white and pink below 290.41 m CSF in Hole U1336A (not recovered in Hole U1336B). 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 with these observed color changes (Fig. F7). Magnetic susceptibility drops to near zero in the transition from very pale brown to pale yellow and remains very weak throughout the light greenish gray interval, the same pattern observed at Sites U1334 and U1335. With elevated Fe and Mn pore water concentrations, discrete millimeter- to centimeter-scale color bands occur within the interval of light greenish gray to white nannofossil ooze where Fe reduction has occurred in Units II and III. Although there is no obvious lithologic specificity, the banded sediments are sometimes more consolidated than the immediately overlying and underlying sediments.

The pattern of changing sediment color, physical and magnetic properties, and pore water chemistry seen at Site U1336 suggests loss of Fe-bearing minerals during diagenesis by microbial Fe reduction. A diagenetic origin for the color bands is suggested by their occurrence in the interval of the sequence that has undergone Fe reduction, their lack of lithologic specificity, and induration sometimes associated with discrete beds with particularly prominent banding. Similar "light gray-green banded chalk" is reported from DSDP Site 79 (2°33.02′N, 121°34.00′W) with high Fe (105 µM/kg) and Mn (1000 µM/kg) interstitial water concentrations (Presley and Kaplan, 1972).

Summary

At Site U1336 ~300 m of pelagic sediments are divided into three major lithologic units. Sediments are composed mainly of nannofossil oozes, nannofossil chalks, and chert. The early to middle Miocene sedimentary sequence of Unit I contains more radiolarians, clay, foraminifers, and diatoms relative to the early Miocene to early Oligocene sediments below ~75 m CSF. Subtle changes in the relative proportions of these minor components produce meter-scale dark–light color cycles and two diatom-rich layers. Numerous rounded fragments of pumice occur throughout this unit. The oxidation-reduction reactions responsible for the observed vivid colors and pore water chemistry changes are likely fueled by enhanced availability of organic carbon relative to overlying and underlying sediments. Thin chert layers were encountered near 124 m CSF in Unit II, but mainly broken fragments were recovered, except for a small in situ chert fragment at 159.6 m CSF in interval 320-U1336B-18H-4, 106 cm. More abundant chert layers are common in Unit III.

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