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doi:10.2204/iodp.proc.342.107.2014 LithostratigraphyCoring in Holes U1406A–U1406C recovered a sedimentary succession of deep-sea pelagic sediment of Pleistocene to middle Eocene age, comprising four lithostratigraphic units (Figs. F5, F6, F7, F8, F9, F10; Table T2). Unit I is ~2 m thick and composed of Pleistocene brown foraminiferal sand and nannofossil ooze with manganese nodules. Unit II is brown, transitioning downhole to a green Oligocene to Miocene nannofossil ooze that occurs in a ~180 m thick sequence and contains abundant diatoms and/or radiolarians in some intervals. Unit III is a ~40 m interval of alternating carbonate-rich nannofossil ooze, clay-rich nannofossil ooze, and clay of earliest Oligocene to middle Eocene age. Unit IV is also largely composed of nannofossil ooze and marks the downhole transition from reduced to oxidized sediment that is middle Eocene to Paleocene in age. Sand-sized lithoclasts are found in the >63 µm size fraction in Site U1406 sediment, particularly in sediment of Miocene and Oligocene age. Lithostratigraphic units and boundaries are defined by changes in lithology (as identified by visual core description and smear slide observations), physical properties, color reflectance (L*, a*, and b*), and biogenic content (calcium carbonate and silica) (Fig. F5). The lithologic differences observed between units are primarily attributable to varying abundances of nannofossils, diatoms, radiolarians, and foraminifers (Figs. F8, F9, F10). Lithologic descriptions are based on sediment recovered from Hole U1406A and supplemented with observations from the two shorter holes, U1406B and U1406C. The Unit II/III boundary is defined by a transition from ooze to chalk. As the ooze–chalk transition is a broad zone, we have tied the lithostratigraphic Unit II/III boundary to a magnetic susceptibility peak in the middle of this lithologic transformation. This unit boundary falls in nannofossil Zone NP22 and planktonic foraminifer Zone O1 (see “Biostratigraphy”). The Unit III/IV boundary is marked by a distinct color change that reflects redox conditions in the sediment and may be related to lower productivity in Unit IV when compared to younger parts of the sedimentary record. Unit I
Unit I is a 2.61 m thick succession of sediment encountered in all three holes at Site U1406 (Fig. F6A). This thin unit in Core 342-U1406A-1H is composed of pale brown (10YR 6/3) and light brown (7.5YR 6/4) nannofossil foraminiferal ooze with clay and “foraminiferal sand” that occurs downhole to interval 342-U1406A-1H-2, 111 cm. Bioturbation is extensive to complete with no discrete burrows. Unit I contains abundant sand-sized foraminifers throughout as well as manganese nodules and ice-rafted clastic material that includes sand- to pebble-sized clasts. The lithologies range from high-grade metamorphic rocks and granodiorites to dolomites and limestones. Unit II
Unit II is a ~180 m thick succession of dominantly nannofossil ooze (Figs. F5, F6B, F6C). The top of the unit is defined by homogeneous, light yellow (2.5Y 8/2) and light greenish gray (10GY 7/1) clayey nannofossil ooze with foraminifers. At Section 342-U1406A-3H-1, 98 cm, sediment color distinctly changes from light yellow to light greenish gray. This sediment color change likely reflects sedimentary redox conditions and is not associated with a change in lithology (Fig. F5). The vast majority of Unit II is composed of light greenish gray (10GY 7/1) nannofossil ooze with common 1 cm thick green layers of disseminated glauconite and chlorite. Bioturbation traces are slightly darker light greenish gray (10Y 6/1) and result in subtle mottling of the core surface. Sulfide blebs (1–2 cm in diameter) are present throughout Unit II. Unit III
The top of Unit III is defined as the transition from ooze to chalk, which occurs within nannofossil Zone NP22 in the lower Oligocene in Holes U1406B and U1406C. Hole U1406A undergoes the transition to chalk deeper in the core than Holes U1406B and U1406C, so a tie was picked based on the magnetic susceptibility peak in Hole U1406A (184.86 mbsf). Unit III is 50–60 m thick and composed of light grayish (10Y 7/1) nannofossil chalk with foraminifers; the latter are visible macroscopically. The chalk is slightly bioturbated and mottled. Section 342-U1406A-27X-3 contains centimeter-scale laminated beds that display indications of low-angle cross lamination. Native copper metallogenic phases were observed in thin veins and as single crystals in interval 342-U1406A-27X-1, 80–120 cm. Biscuiting is common throughout Unit III. Unit IV
Lithostratigraphic Unit IV is distinguished by a color change from light grayish nannofossil chalk (10Y 7/1) in Unit III to pale brown (10YR 6/3) nannofossil chalk with foraminifers and nannofossil chalk with radiolarians (Figs. F5, F6, F7, F8, F9, F10). The unit is ~38 m thick. Unit IV also contains variable abundances of radiolarians and/or calcareous nannofossils. Centimeter- to decimeter-scale color variation in various shades of light grayish brown (10YR 8/1 to 10YR 8/2) occurs through most of the unit and is the result of differential bioturbation intensity and, in some cases, of diagenesis. The diagenesis results in two important features, gray sulfide precipitation and green chlorite with glauconite bands. These bands, commonly characterized by diffuse margins, are recognized as the result of an early diagenetic phenomenon because of their crosscutting relationship with sedimentary structures such as burrows (Fig. F12 in the “Site U1408” chapter [Norris et al., 2014e). Foraminiferal tests are macroscopically visible, and radiolarians are also concentrated in certain intervals. The sediment is moderately to slightly mottled, resulting from bioturbation. Cores drilled with the XCB are significantly disturbed in places (biscuiting and fracturing). Lithostratigraphic unit summaryCarbonate content in Site U1406 sediment is generally moderate (typically ~40 wt% CaCO3; Fig. F5), suggesting that the site was positioned above the CCD for much of its Cenozoic history. No distinct lithostratigraphic boundary is associated with the Oligocene/Miocene chronostratigraphic boundary. At ~199 mbsf in Hole U1406A, just above the Eocene/Oligocene boundary, we observed a peak in carbonate concentration, presumably reflecting a deepening and overshoot of the CCD (see "Background and objectives" and a similar signal at Site U1404). The sedimentary succession at Site U1406 also records a number of carbonate content increases or “events” in middle to upper Eocene strata, including an a peak of 90 wt% CaCO3 at 250.45 mbsf, during the middle Eocene (46.29 Ma) nannofossil Subzone NP15a (see “Biostratigraphy”). Oligocene–Miocene transitionThe Oligocene–Miocene transition was recovered in a moderately carbonate rich interval in Holes U1406A, U1406B, and U1406C at 81.16, 82.48, and 75.71 mbsf, respectively (Fig. F11). The Oligocene/Miocene boundary is approximated by the first occurrence and last occurrence datums of the nannofossil taxon Sphenolithus delphix and defined by the base of Chron C6Cn2n. The Oligocene/Miocene boundary zone does not stand out from the underlying and overlying sediment in terms of physical properties such as color or carbonate content (Fig. F11). Color is relatively uniform greenish gray (10GY 6/1 to 10GY 7/1), and carbonate content varies between 30 and 50 wt% at 60–100 mbsf in Hole U1406A. Eocene–Oligocene transitionThe EOT was recovered in a carbonate-rich interval in Holes U1406A, U1406B, and U1406C at ~199, ~198, and ~195 mbsf, respectively. These depths are approximate because the EOT is transitional rather than a boundary and occurs midway through nannofossil Zone NP21 near the top of Chron C13r. Lithologic expressions of this interval relative to underlying and overlying sediments are less pronounced than at the deeper water Site U1404, where background levels of carbonate accumulation are lower (Fig. F12). Upper Eocene sediments below the Eocene/Oligocene boundary are composed of very uniform, light greenish gray (10Y 7/1) nannofossil ooze with a carbonate content close to 50 wt%. Slight mottling as a result of moderate bioturbation occurs throughout these sediments. In Hole U1406A, the EOT (199.33–190.44 mbsf) is composed of greenish gray (5GY 6/1) nannofossil ooze. The slightly darker, light greenish gray regions (typically ~1 interval per section) display more visible bioturbation. Disseminated sulfide blebs (10Y 3/1) occur throughout. Post-EOT Oligocene sediment (above 190.44 mbsf in Hole U1406A) typically has a carbonate content of 40 wt% (Fig. F12). Carbonate content reaches a peak value (~80 wt%) in calcareous nannofossil Zone NP21 and Chron C13n (Fig. F12) and coincides stratigraphically with the lightest colored sediments recovered at Site U1404. Color reflectance and carbonate content are broadly correlated with one another in Hole U1406A. Magnetic susceptibility, a*, and b* reach peaks in the earliest Oligocene (Core 342-U1406A-23H) (Fig. F5). Stratigraphically above this early Oligocene peak in magnetic susceptibility, color reflectance and calcium carbonate drop to a sustained low. Sand-sized lithic grainsSand-sized (>63 μm) grains were observed above the Eocene/Oligocene boundary at Site U1406. These grains consist mostly of angular quartz within the 63–150 μm sieve size fraction. Rock fragments also occur but are rare. In general, the composition of the sand-sized grains resembles those observed at Site U1404 in time-correlative strata (Fig. F13). Deepwater authigenic glauconiteSediment in Units II and III includes green layers that we interpret to contain mostly glauconite and chlorite. We hypothesize that these layers represent diagenetic layers that form on an oxidation/reduction boundary tens of centimeters deep in the sediment when sedimentation rate slows (Figs. F6C, F13). Copper veinsSection 342-U1406A-28X-3 contains an occurrence of native copper associated with a minor fracture (Fig. F14). Copper is found both as grains in the sediment and as vein fill in Cores 342-U1406A-27X and 342-U1406B-28X. Native copper is very rarely seen in pelagic sediments, though it has been reported previously (Dickens et al., 1995). The presence of copper was confirmed by X-ray diffraction. Vein fills of copper are up to centimeters in length, whereas the grains are hundreds of micrometers across. The grains are clearly euhedral, and the crystal shapes are consistent with native copper. Unlike the copper reported by Dickens et al. (1995), no “halos” surround the copper occurrence; however, blue inclusions of azurite [Cu3(CO3)2(OH)2] can be seen in Figure F14 associated with the vein fill. The copper found at Site U1406 on J-Anomaly Ridge was recovered from ~230 mbsf, whereas the copper reported by Dickens et al. (1995) was found between ~500 and 800 mbsf at Ocean Drilling Program (ODP) Leg 145 Site 884. |
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