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doi:10.2204/iodp.proc.342.105.2014 LithostratigraphyThe sedimentary progression recovered from Holes U1404A–U1404C reveals a diversity of deep-sea pelagic sediments of Pleistocene to middle Eocene age comprising four lithostratigraphic units (Figs. F4, F5, F6, F7, F8; Table T2). Unit I is ~2 m thick and composed of Pliocene–Pleistocene brown foraminiferal sandy clay and nannofossil ooze, with notable manganese nodules, transitioning downhole to Pliocene brown clay with silt. Unit II is green and greenish gray carbonate-poor Oligocene–Miocene clay that occurs in a nearly 200 m thick sequence and contains abundant diatoms, radiolarians, and sponge spicules. A dolomite cobble is present at the top of Unit II. Unit II is divided into an upper Subunit IIa (20 m thick) that is barren of microfossils and lower Subunit IIb (180 m thick) that contains abundant siliceous microfossils (diatoms, radiolarians, and sponge spicules) and comparably minor abundances of calcareous nannofossils. Unit III is a ~26 m interval of carbonate-rich nannofossil ooze alternating with clay-rich nannofossil ooze and clay that spans the lowermost Oligocene and uppermost Eocene. Unit IV is an ~75 m thick succession of clay and claystone with some intervals containing abundant radiolarians and/or calcareous nannofossils. Sand-sized lithoclasts are found in the >63 μm size fraction in Site U1404 sediment, particularly in the Miocene and Oligocene sequences. 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. F4). The lithologic differences observed between units are primarily attributable to varying abundances of nannofossils, diatoms, radiolarians, and foraminifers (Figs. F6, F7, F8). Lithologic descriptions are based on sediments recovered from Hole U1404A and supplemented with observations from the two shorter Holes U1404B and U1404C. Unit I
Unit I is a 2.33 m thick succession of sediment encountered only in Hole U1404B (Fig. F4). This thin lithostratigraphic unit was not recovered in Hole U1404A as a result of poor mudline core recovery (see “Stratigraphic correlation”). The sediment in Core 342-U1404B-1H is very pale brown (10YR 8/2) moderately bioturbated nannofossil ooze (Fig. F5B) with abundant sand-sized foraminifers in the uppermost 57 cm of Section 342-U1404B-1H-1 (Fig. F5A) and Mn nodules as large as 10 cm. Unit II
Unit II and its two subunits, IIa and IIb, comprise a 195–200 m thick succession of predominantly clay and biosiliceous clay with some intervals of biosiliceous clay with nannofossils (Figs. F4, F5). The top of the unit is defined by pale brown (10YR 6/3) clay and silty clay underlying the nannofossil-rich sediment of Unit I. The boundary between Subunits IIa and IIb is defined by the first downhole occurrence of biogenic sediment within Unit II (top of Section 342-U1404A-5H-1 [33.20 mbsf] and top of Section 342-U1404B-4H-3 [24.40 mbsf]). The base of Unit II is defined by the occurrence of nannofossil ooze at 200.60 mbsf in Hole U1404A and 203.56 mbsf in Hole U1404B. A large (7 cm long) dolomite clast interpreted as a glacial dropstone is present in the top of Core 342-U1404A-1H. Subunit IIa
Subunit IIa is a 17–20 m thick succession of pale brown (10YR 6/3), greenish gray (10GY 5/1), and pinkish gray (7.5YR 6/2) to dark greenish gray (10Y 4/1) clay to silty clay with slight to moderate bioturbation (Figs. F4, F5C). Clayey silt is present as a minor lithology, typically as thin (<3 cm) beds alternating with silty clay. Shipboard X-ray diffraction (XRD) analysis indicates distinctive peaks for quartz, illite, plagioclases, montmorillonite, and some chlorite. The presence of disseminated Mn oxides and discrete Mn nodules is variable throughout and, where highly concentrated, impart a dark greenish gray color to the sediment (10Y 4/1). Nodule-shaped patches of possibly authigenic glauconite are present in Sections 342-U1404A-3H-1 through 3H-3 (Fig. F9). Subunit IIa is barren of microfossils. Subunit IIb
Subunit IIb is a ~170–180 m thick succession of moderately bioturbated, greenish gray (5GY 5/1) to dark greenish gray (5GY 4/1 and 10Y 4/1) biosiliceous clay or biosiliceous ooze with clay with intervals of biosiliceous clay with calcareous nannofossils (Figs. F4, F5C). Subunit IIb is differentiated from overlying Subunit IIa based on the presence of siliceous microfossils. Siliceous biogenic components include diatoms, radiolarians, sponge spicules, and silicoflagellates (Figs. F6, F7, F8). Shipboard XRD analyses for Subunit IIb yield similar results to those for Subunit IIa. Only kaolinite and calcite seem to be more common in Subunit IIb than Subunit IIa, although kaolinite is still relatively rare. In contrast, calcite is more common in Subunit IIb as a result of more common nannofossils. The clay also contains variable abundances of quartz, sulfides, oxides, biotite mica, feldspar, and heavy minerals in the clay to fine silt size range that were observed in smear slides. Color variations in Subunit IIb are largely centimeter- to decimeter-scale banding resulting from the bioturbation of Mn oxide layers. Color bands have minor differences in nannofossil abundance (Fig. F5C). Core 342-U1404A-11H contains small (millimeter sized) white blebs composed of highly angular silt-sized quartz grains with occasional mica or feldspar grains. Similar sandy patches, possibly ice-rafted debris (IRD), were also observed in Section 342-U1404B-21H-1 (Fig. F10). Coarse sand-sized clasts of quartz siltstone are present at Site U1411, where they produce blebs of quartz silt similar to those at Site U1404 upon dissolution of their calcite cement. Unit III
Unit III is 22–26 m thick and composed of brownish gray (2.5Y 6/2), gray (5Y 6/1), to light yellowish brown (2.5Y 6/3) moderately bioturbated nannofossil ooze and clayey nannofossil ooze alternating with clay (Figs. F4, F5, F6, F7, F8, F11). The color variation observed in Unit II corresponds well with qualitative smear slide analysis of carbonate content. Moreover, a distinguishing characteristic of this lithostratigraphic unit is the highest calcium carbonate content values (several data points >40 wt%) observed within any unit at Site U1404 (Figs. F4, F12; see also “Geochemistry”). These intervals of high calcium carbonate are associated with the EOT (Figs. F11, F12; see also “Eocene–Oligocene transition”). Section 342-U1404A-26H-1 contains a conspicuous ~40 cm thick greenish gray (5BG 5/1) silty layer within the background grayish brown (2.5Y 5/2) nannofossil ooze contained within Chron C17 (see “Paleomagnetism”). Unit IV
Unit IV is a 73 m thick succession of clay/claystone and radiolarian clay recovered primarily in Hole U1404A (Figs. F4, F5, F6, F7, F8). Only 3.64 m of this lithostratigraphic unit, the deepest and oldest lithostratigraphic unit drilled at Site U1404, was encountered in Hole U1404B at the base of Core 342-U1404B-27H. Unit IV contains variable abundances of radiolarians and/or calcareous nannofossils in some intervals (Fig. F7). Centimeter- to decimeter-scale color variation, from greenish gray (5GY 6/1 and 5GY 5/1) to dark greenish gray (5GY 4/1), occurs through most of the unit and is the result of differential bioturbation intensity and, in some cases, diagenesis. Cores 342-U1404A-33X through 36X were drilled with the XCB and, as a result, are significantly disturbed by biscuiting and fracturing. Notable eventsCarbonate content in Site U1404 sediment is generally very low, suggesting the site was positioned close to the CCD for much of its Cenozoic history. Increases in carbonate abundance may well reflect CCD deepening events in the Eocene and the EOT. For much of the Eocene–Miocene interval, the site is below the CCD, with discrete horizons indicating CCD deepening events in the lowermost Oligocene. At 195 mbsf in Hole U1404A, we observed a peak in the carbonate concentration, presumably reflecting deepening and overshoot of the CCD in the early Oligocene. Surprisingly, the record in the Eocene records a number of possible carbonate “events.” Eocene–Oligocene transitionThe EOT was recovered in a rare carbonate-rich interval at Site U1404. The EOT was identified on the basis of biostratigraphic and magnetostratigraphic observations (see “Biostratigraphy” and “Paleomagnetism”) and is characterized by a series of striking lithologic changes (Fig. F11). Oligocene and late Eocene sediment over- and underlying the EOT are moderately bioturbated, greenish gray (5GY 5/1) biosiliceous clay to radiolarian claystone typically containing <5 wt% calcium carbonate. In Hole U1404A, the EOT is overlain by 2.67 m of relatively unmottled (i.e., more completely bioturbated) greenish gray sediment overlain by the typical mottled greenish gray sediment of the early Oligocene. We are unable to comment on the presence or absence of the Eocene/Oligocene boundary in Hole U1404B because of extensive core disturbance (flow-in) in the lowermost four sections of Core 342-U1404B-23H. The general interval associated with the EOT is identified as a lithostratigraphic unit (Unit III) on the basis of the unique lithology, including the presence of clayey nannofossil ooze to nannofossil ooze, light brownish gray (2.5Y 6/2) to gray (5Y 6/1) sediment, and variable carbonate content (Fig. F4). The uppermost part of Unit III, a brown mottled and banded 0.14 m thick section in Hole U1404B, is not observed in Hole U1404A (Fig. F11). This mottled and banded section is followed by a 3.77–4.92 m thick carbonate-rich interval that is mottled in Hole U1404A and banded in Hole U1404B. Underlying this carbonate-rich interval is the Chron C13r/C13n boundary, the Eocene/Oligocene boundary, and a ~4 m thick interval that is brown with reddish mottles. This interval is followed in Hole U1404A by a second, short (0.275 m thick) carbonate-rich interval. This second carbonate-rich interval and the underlying light brown interval (2.125 m thick) are not observed in Hole U1404B (Fig. F11). Carbonate concentrations reach their highest value (>50 wt%) in calcareous nannofossil Zone NP21 and Chron C13n and coincide with the lightest colored sediment recovered at Site U1404 (Figs. F11, F12). Color reflectance, magnetic susceptibility, and carbonate content are broadly correlated in Hole U1404A. Color reflectance data, particularly a* and b*, and magnetic susceptibility increase through the late Eocene (~207 to ~227 mbsf in Hole U1404A). Magnetic susceptibility, a*, and b* peak in the earliest Oligocene, followed by a peak in L* that correlates with the highest carbonate content (purple shading in Fig. F12). Calcium carbonate content increases through the late Eocene in a series of discrete intervals. All three data types (magnetic susceptibility, color reflectance, and calcium carbonate) drop to a sustained low in the early Oligocene. For instance, mean magnetic susceptibility values increase from 26 to 45 instrument units (IU) from ∼207 to ∼227 mbsf in Hole U1404A during the late Eocene ramp up and then decrease to 20 IU at ∼198 mbsf. Sand-sized lithic grainsSand-sized (>63 μm) lithics were observed within two intervals at Site U1404. These lithics consist of quartz grains, which are mostly transparent but also pink and smoky varieties, as well as metamorphic rock fragments within the 63–150 μm sieve size fraction (Fig. F10). The quartz grain shape is often angular and occasionally shows conchoidal fractures. The first common occurrence of sand-sized lithic grains takes place in the lowermost Oligocene directly downhole from the change from carbonate-bearing brownish sediment to darker, grayish green clay in Section 342-U1404A-23H-4. An analysis of core catcher and section samples revealed that these grains are common constituents within the fine sand fraction uphole to the base of Core 342-U1404A-19H (172 mbsf) and are interpreted to be IRD. Larger particles, as large as coarse sand– or granule-sized grains, have also been observed but are comparatively rare (Figs. F10, F13). The sand-sized lithic grains occur in intervals 342-U1404A-18H-CC through 23H-3, 117–119 cm (163–199 mbsf), which can be stratigraphically correlated with the EOT. The abundance of sand-sized lithic grains in this deep-sea sediment is suggestive of IRD, supporting the existence of continental ice sheets in the northern hemisphere before 15 Ma, as previously proposed by Tripati et al. (2007) and Eldrett et al. (2008). Another explanation for the occurrence of the observed coarse siliciclastic sediment might be strongly enhanced bottom-current activity transporting fine sand material into the drift deposits. However, the well-developed angular habits of the grains argue for short transport typical for IRD. A second sand-sized lithic grain occurrence was reported from Section 342-U1404A-12H-5, with a composition and size comparable to the occurrence described above. This second interval may correspond to the Miocene Mi-1 glaciation. Deepwater authigenic glauconiteSediment in Subunit IIa includes nodular-shaped green accumulations that we interpreted as Mn nodules altering to glauconite/illite (Fig. F9). XRD data from Sample 342-U1404A-3H-1, 140–141 cm, indicate a concentration of glauconite/illite. Given the shape and distribution of this authigenic material, we speculate that the glauconite/illite lumps derived from the alteration of Mn nodules. To our knowledge, this style of deep-marine glauconite occurrence has not been reported in any detail in the geologic literature. |
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