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doi:10.2204/iodp.proc.320321.103.2010

Lithostratigraphy

Drilling at Site U1331 recovered a 189 m thick section of pelagic sediments overlying seafloor basalt. The uppermost ~6 m of the section is a late Neogene (~400 ka to 6 Ma based on radiolarian biostratigraphy and magnetostratigraphy) clay with radiolarians and micronodules (Fig. F4). These sediments are, in turn, underlain by ~94 m of Oligocene to late middle Eocene nannofossil and radiolarian ooze and ~87 m of early middle Eocene to early Eocene radiolarian ooze with alternations of nannofossil ooze, clay, and porcellanite. A thin (1.9 m thick) unit of early Eocene calcareous ooze and zeolite clay was recovered at the base of the sedimentary sequence above basalt. Approximately 15 m of the lithologic section was not recovered.

The sedimentary sequence at Site U1331 is divided into four major lithologic units, and two of these units are further divided into three subunits (Table T2; Fig. F4). Unit and subunit boundaries are defined by differences in lithology and measured physical properties. Lithologic differences, based on visual core descriptions and smear slide and thin section analysis (Figs. F5, F6; see "Site U1331 thin sections" and "Site U1331 smear slides" in "Core descriptions"), are primarily attributable to varying distributions of biogenic components (e.g., nannofossils and radiolarians) and clay-sized lithogenic material with minor variations in manganese and iron oxide content. Lithologic descriptions are primarily based on sediments recovered in Hole U1331A, supplemented with observations from Holes U1331B and U1331C.

Unit I

Intervals: 320-U1331A-1H-1, 0 cm, through 2H-1, 85 cm; 320-U1331B-1H-1, 0 cm, through 1H-5, 52 cm; 320-U1331C-1H-1, 0 cm, through 1H-5, 107 cm

Depths: Hole U1331A = 0.0–6.05 m CSF; Hole U1331B = 0.0–6.52 m CSF; Hole U1331C = 0.0–7.07 m CSF

Age: late Neogene

Lithology: clay

The major lithology in Unit I is pale brown (10YR 4/3) to very dark gray (10YR 3/1) to dark brown (10YR 3/3) clay. Pale brown clay occurs in a distinct interval ~70 cm thick (in the upper meter of Hole U1331A). The clay in Unit I occurs with radiolarians and with radiolarians and micronodules as minor lithologic components. In Hole U1331A a ferromanganese crust with associated micronodules and encrusted pebbles occurs between 74 and 85 cm from the top of the sedimentary section. Bioturbation is generally minor in these sediments. Unit I is further characterized by high magnetic susceptibility, low GRA bulk densities, low L* reflectance (lightness; see "Physical properties" for additional reflectance information), and near-zero CaCO₃ content. The downhole transition into the underlying Unit II takes place over an interval that is ~5 cm thick.

Unit II

Intervals: 320-U1331A-2H-1, 85 cm, through 11H-CC, 22 cm; 320-U1331B-1H-5, 52 cm, through 11H-1, 150 cm; 320-U1331C-1H-5, 107 cm, through 8H-5, 16 cm

Depths: Hole U1331A = 6.05–99.52 m CSF; Hole U1331B = 6.52–97.10 m CSF; Hole U1331C = 7.07–98.66 m CSF

Age: early Oligocene to late middle Eocene

Lithology: alternations of nannofossil ooze and radiolarian ooze

The dominant lithologies in Unit II are pale brown (10YR 6/3) to very pale brown (10YR 7/3) nannofossil ooze, yellowish brown (10YR 5/4) to dark brown (10YR 3/3) radiolarian nannofossil ooze, and brown (10YR 5/3) to very dark brown (10YR 2/2) radiolarian ooze. Bioturbation is generally moderate to nonvisible in these sediments, and both gradual and sharp contacts are observed between lithologies. Sharp contacts are often characterized by an inclined and irregular basal surface overlain by noticeably coarser grained, more carbonate-rich (including planktonic and benthic foraminifers), and opaque-coated sediments. The overlying materials grade from coarse to fine grained uphole.

Subunit IIa

Intervals: 320-U1331A-2H-1, 85 cm, through 4H-1, 140 cm; 320-U1331B-1H-5, 52 cm, through 3H-5, 35 cm; 320-U1331C-1H-5, 107 cm, through 3H-6, 2 cm

Depths: Hole U1331A = 6.05–25.60 m CSF; Hole U1331B = 6.52–25.95 m CSF; Hole U1331C 7.07–26.52 m

Age: early Oligocene

Lithology: alternations of nannofossil ooze, radiolarian nannofossil ooze, and radiolarian ooze

The major lithologies in Subunit IIa are very pale brown (10YR 8/2) to brown (10YR 4/3) nannofossil ooze and pale brown (10YR 6/3) to yellowish brown (10YR 5/4) radiolarian nannofossil ooze. Pale brown (10YR 6/4) to light yellowish brown (10YR 6/3) radiolarian ooze with nannofossils are minor lithologies. Within the major lithologies, nannofossil ooze sometimes occurs with radiolarians or with diatoms and radiolarians as minor components. Radiolarian nannofossil ooze occurs with clay as a minor lithologic component, whereas radiolarian ooze occurs with clay or nannofossils as minor lithologic components. Alternations of nannofossil ooze with radiolarian nannofossil ooze and of radiolarian ooze with nannofossils occur on 5 cm to 1 m scales. Bioturbation is generally minor to nonvisible in these sediments, and both gradual and sharp contacts occur between lithologies. Sharp contacts are typically inclined and irregular surfaces with noticeably coarser grained, relatively carbonate-rich (including planktonic and benthic foraminifers), and opaque-coated overlying sediments that fine uphole. GRA bulk density, L*, and CaCO₃ contents all increase downhole at the Unit I/Subunit IIa boundary and maintain relatively high values until they decrease just above the Subunit IIa/IIb boundary (Fig. F4). Magnetic susceptibility is generally elevated but variable in Subunit IIa. The contact with underlying Subunit IIb takes place over a 5 cm interval.

Subunit IIb

Intervals: 320-U1331A-4H-1, 140 cm, through 8H-5, 119 cm; 320-U1331B-3H-5, 35 cm, through 8H-2, 76 cm; 320-U1331C-3H-6, 2 cm, to at least 6H-CC, 25 cm (boundary not cored in Hole U1331C)

Depths: Hole U1331A = 25.60–69.39 m CSF; Hole U1331B = 25.95–69.36 m CSF; Hole U1331C 26.52 to below 68.5 m CSF (boundary not cored in Hole U1331C)

Age: late to late middle Eocene

Lithology: alternations of clayey radiolarian ooze, radiolarian ooze, radiolarian nannofossil ooze, and nannofossil ooze

The absence of nannofossil ooze as a major lithology distinguishes Subunit IIb from Subunit IIa (and Subunit IIc; see below). The major lithologies in Subunit IIb are dark brown (10YR 3/3) clayey radiolarian ooze and yellowish brown (10YR 5/4) to dark brown (10YR 3/3) radiolarian ooze. Yellowish brown (10YR 5/4) to dark brown (10YR 3/3) radiolarian nannofossil ooze and radiolarian ooze with clay as well as yellowish brown (10YR 5/4) nannofossil ooze with radiolarians are minor lithologies. Bioturbation is generally minor to nonvisible in these sediments, and both gradual and sharp contacts are observed between lithologies. Sharp contacts are marked by inclined and irregular basal surfaces with noticeably coarser grained, more carbonate-rich (including planktonic and benthic foraminifers), and opaque-coated overlying coarse-grained sediments. Sediments grade toward finer grain sizes uphole. The transition from Subunit IIa to Subunit IIb is characterized by high-frequency, high-amplitude peaks in magnetic susceptibility, GRA bulk density, and L*. The underlying section shows generally low values for each of these series and for CaCO₃ content. Contact with the underlying Subunit IIc takes place over a 5 cm interval.

Subunit IIc

Intervals: 320-U1331A-8H-5, 119 cm, through 11H-CC, 22 cm; 320-U1331B-8H-2, 76 cm, through 11H-1A, 150 cm; top not cored in Hole U1331C but extends through 320-U1331C-8H-5, 16 cm

Depths: Hole U1331A = 69.39–99.52 m CSF; Hole U1331B = 69.36–97.10 m CSF; top of Hole U1331C not cored but extends through 98.66 m CSF

Age: middle Eocene

Lithology: alternations of nannofossil radiolarian ooze, radiolarian nannofossil ooze, and nannofossil ooze

The major lithologies in Subunit IIc are dark brown (10YR 3/3) to dark yellowish brown (10YR 5/3) nannofossil radiolarian ooze, dark brown (10YR 3/3) radiolarian nannofossil ooze, and yellowish brown (10YR 5/4) to dark brown (10YR 3/3) nannofossil ooze. Dark brown (10YR 3/3) radiolarian ooze with clay and very dark brown (10YR 2/2) radiolarian clay are minor lithologies. Each of the major lithologies sometimes occurs with clay as a minor component. In addition, nannofossil ooze can be seen to occur with radiolarians as a minor component. Bioturbation is generally minor to nonvisible in these sediments, and both gradual and sharp contacts are observed between lithologies. Sharp contacts are common, with generally inclined and irregular basal surfaces and noticeably coarser grained, more carbonate rich (including planktonic and benthic foraminifers) overlying sediments that grade to a finer grain size uphole. Subunit IIc is characterized by relatively high L* and CaCO₃ content and slightly elevated GRA bulk densities. The contact between Subunit IIc and the underlying Unit III takes place over several centimeters.

Unit III

Intervals: 320-U1331A-11H-CC, 22 cm, through 22X-1, 11 cm; 320-U1331B-11H-150 cm, through at least 17H-CC, 40 cm (bottom not cored in Hole U1331B); 320-U1331C-8H-5, 16 cm, through 17H-3, 62 cm

Depths: Hole U1331A = 99.52–186.51 m CSF; Hole U1331B = 97.10 to at least 156.8 m CSF; Hole U1331C = 98.66–187.62

Age: early middle Eocene to early Eocene

Lithology: radiolarian ooze and radiolarian ooze with clay and with porcellanite

The dominant lithologies in Unit III are dark yellowish brown (10YR 4/4) to dark brown (10YR 3/3) radiolarian ooze and black (10YR 2/1) porcellanite. Bioturbation is generally minor to moderate in Subunit IIIa, minor in Subunit IIIb, and minor to absent (i.e., laminated) in Subunit IIIc. Both gradual and sharp contacts are observed between lithologies. Sharp irregular contacts with pronounced color change are particularly common in Subunit IIIa.

Subunit IIIa

Intervals: 320-U1331A-11H-CC, 11 cm, through 17X-CC, 37 cm; 320-U1331B-11H-1, 150 cm, through 17H-CC, 40 cm (bottom in Hole U1331B not cored); 320-U1331C-8H-5, 16 cm, through 14H-CC, 29 cm

Depths: Hole U1331A = 99.52–157.06 m CSF; Hole U1331B = 97.10–156.88 m CSF; Hole U1331C = 98.66–156.94 m CSF

Age: middle Eocene

Lithology: radiolarian ooze

The major lithology in Subunit IIIa is dark yellowish brown (10YR 4/4) to dark brown (10YR 3/3) radiolarian ooze. Dark yellowish brown (10YR 4/4) nannofossil radiolarian ooze, very dark grayish brown (10YR 3/2) radiolarian ooze, and black (10YR 2/1) porcellanite are minor lithologies. Radiolarian ooze occurs with clay and nannofossils in varying amounts as minor components. Bioturbation is generally minor to moderate in these sediments. Sharp irregular contacts between lithologies of different color are common in the upper portion of this subunit. The sediments immediately overlying these contacts are noticeably coarser grained and more carbonate rich (including planktonic and benthic foraminifers) than those immediately below the contacts and grade into finer grained material uphole. Subunit IIIa is characterized by overall low magnetic susceptibility, GRA bulk density, L*, and CaCO₃ content, as well as logging magnetic susceptibility (Fig. F4). The contact with underlying Subunit IIIb was not sufficiently well recovered to be identified with confidence based on core observation, but it occurs near the top of Core 320-U1331A-18X based on drilling operations reports (see "Operations") and downhole logs (see "Downhole measurements").

Subunit IIIb

Intervals: 320-U1331A-17X-CC, 37 cm, through at least 18X-1, 25 cm (bottom not recovered); not recovered in Hole U1331B; 320-U1331C-14H-CC, 29 cm, through 16H-1, 0 cm

Depths: Hole U1331A = 157.06–157.55 m CSF (bottom not cored in Hole U1331A); Hole U1331C = 156.94–177.00 m CSF

Age: early Eocene

Lithology: radiolarian ooze and porcellanite

Subunit IIIb is distinguished from Subunit IIIa (and IIIc; see below) based on the presence of porcellanite as a major lithology. The major lithologies in Subunit IIIb are dark brown (10YR 3/3) radiolarian ooze and black (10YR 2/1) porcellanite. Very dark brown (10YR 2/2) radiolarian clay and very dark brown (10YR 2/2) clayey radiolarian ooze are less important. Clay and opaques sometimes occur as minor components within radiolarian oozes. Oozes show minor bioturbation, and, where recovered without drilling disturbance, porcellanites are associated with intervals rich in clay.

Subunit IIIc

Intervals: top in Hole U1331A not cored, present in 320-U1331A-22X-1, 0 cm, through 22X-1, 11 cm; Subunit IIIc not cored in Hole U1331B; 320-U1331C-16H-1, 0 cm, through 17H-3, 62 cm

Depths: Top of Hole U1331A not cored to 186.51 m CSF; Hole U1331C = 177.00–187.62 m CSF

Age: early Eocene

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

The major lithologies in Subunit IIIc are dark brown (10YR 3/3), brown (10YR 4/3), and light yellowish brown (10YR 6/4) radiolarian ooze and dark yellowish brown clayey radiolarian ooze (10YR 3/4). Dark brown (10YR 3/3) radiolarian clay, dark yellowish brown (10YR 3/4) clayey radiolarian ooze, and dark brown (10YR 3/3) zeolite clay are minor lithologies. Bioturbation is minor to nonvisible in these sediments, and some intervals are distinctly laminated (e.g., Samples 320-U1331C-16H-2, 80–145 cm, and 16H-3, 80–120 cm). Only logging magnetic susceptibility data are available for comparison to the lithologic descriptions in Subunit IIIb. Formation magnetic susceptibility is highly variable, with high-amplitude short peaks likely associated with partially lithified porcellanite and clay-rich layers that characterize this subunit. The contact with underlying Unit IV occurs over a 10–15 cm interval containing centimeter-scale laminae.

Unit IV

Intervals: 320-U1331A-22X-1, 11 cm, through 22X-CC, 25 cm; Unit IV not cored in Hole U1331B; 320-U1331C-17X-3, 62 cm, through 17H-CC, 8 cm

Depths: Hole U1331A = 186.51–187.02 m CSF; Hole U1331C = 187.62–189.48 m CSF

Age: early Eocene

Lithology: calcareous ooze and zeolite clay

The major lithologies in Unit IV are light yellowish brown (10YR 6/4) calcareous ooze (composed of calcareous nannofossils and micrite) and very dark brown (10YR 2/2) zeolite clay. A basal 1.2 m thick interval of Unit IV was recovered in Hole U1331C and is characterized by calcareous ooze with concretions and reddish color streaks. Physical property data series show high variability within Unit IV consistent with the heterogeneous lithologies recovered.

Unit V

Intervals: 320-U1331A-22X-1-CC, 25 cm, through 22X-1-CC, 44 cm; not cored in Holes U1331B and U1331C

Depth: Hole U1331A = 187.02–187.21 m CSF

Age: early Eocene

Lithology: basalt

Fine-grained basaltic fragments from 1 to 5 cm in size were recovered at the base of Hole U1331A. Some of the recovered basalt shows a fresh glassy chilled margin (Fig. F7C, F7D). Thin section analysis indicates a sparsely phyric basalt mainly composed of phenocryst plagioclase (Sample 320-U1331A-22X-CC [Piece 1, 26–29 cm]) overlying a microphyric basalt mainly made of phenocryst of clinopyroxene and plagioclase (Sample 320-U1331A-22X-CC [Piece 4, 37–44 cm]) (see "Site U1331 thin sections" in "Core descriptions"). Minor minerals are olivine, opaque minerals, and glass.

Sediments across the Eocene–Oligocene transition

A downhole Eocene–Oligocene transition was recovered in all three holes drilled at Site U1331 (Fig. F8). The Eocene/Oligocene boundary sensu strictu is defined on the extinction of the planktonic foraminifer genus Hantkenina but cannot be identified at Site U1331 because although calcareous benthic foraminifers show good to very good preservation, the planktonic foraminifer assemblage is highly dissolved (see "Biostratigraphy"). Based on magnetostratigraphic and both radiolarian and nannofossil biostratigraphic results, however, the boundary falls somewhere between Section 320-U1331B-3H-5, 45 cm, and the upper part of Core 320-U1331B-4H, probably in Section 320-U1331B-3H-5 or 3H-6 (Fig. F6). The lithostratigraphy of the Eocene–Oligocene transition at Site U1331 defines the Subunit IIa/IIb boundary and is marked by a sharp (~5 cm interval) downhole transition from very pale brown nannofossil ooze to homogeneous dark brown clayey radiolarian ooze. An associated pronounced downhole increase occurs in magnetic susceptibility together with pronounced downhole decreases in GRA bulk density, L*, and CaCO₃ content (Figs. F4, F8) (see "Physical properties"). These lithostratigraphic results for the Eocene–Oligocene transition at Site U1331 are consistent with those obtained from multiple sites drilled during Leg 199—in particular, with those of Site 1220 (Shipboard Scientific Party, 2002).

Gravity flow deposits

Throughout the sedimentary section drilled at Site U1331, sharp irregular contacts occur between lithologies (Table T3; Fig. F4). Many are instantly recognizable by distinct changes in color (Table T3; Figs. F9, F10). Many of these sharp contacts are directly overlain by coarser grained, more carbonate rich (including planktonic and benthic foraminifers), and/or opaque-coated sediments than those below the contact. The overlying sediments fine uphole (Figs. F9, F10). In some cases, the coarse-grained sediments directly overlying the sharp contacts are multicolored and these contacts are often clearly of erosional origin (Figs. F9, F10). Planktonic foraminifers are rare in sediments from Site U1331 because of seafloor dissolution below the shallow CCD during the Eocene. The occurrence of planktonic foraminifers above many of the sharp contacts is not consistent with a winnowing origin for the coarse-grained intervals because this would require comparatively prolonged exposure on the seafloor. Instead, their presence in the coarse-grained intervals is indicative of improved preservation through rapid burial. For these reasons, the erosional contacts and overlying graded sediments are interpreted as the product of mass flow events, likely turbidity currents. The provenance of the inferred turbidites observed at Site U1331 is unknown, but their typically calcareous composition points to a source that lay above the CCD at the time that the reworked sediments were originally deposited, possibly the seamount lying a few kilometers to the south (present water depth at summit >500 m shallower than Site U1331) (Fig. F3).

A number of the turbidites identified in cores recovered at Site U1331 are sufficiently distinctive that they are readily correlated among two or all three sites drilled. One example is located ~0.5 m below the Unit I/Subunit IIa boundary (Fig. F10). Nannofossil biostratigraphy indicates that the sediments 7 cm below the basal scour of this turbidite seen in Hole U1331A are Oligocene in age, whereas the overlying clay (Unit I) at all three sites is substantially younger based on radiolarians (<6 Ma, most likely Pliocene–Pleistocene) (see "Biostratigraphy"). Magnetostratigraphic results support the existence of a hiatus somewhere between these two age control points (see "Paleomagnetism"). This hiatus is interpreted to fall at the base of the turbidite based on the erosional nature of its basal contact (Fig. F10).

The turbidites documented in sediments recovered from Site U1331 vary significantly in prominence. Many have barely discernible changes in grain size, whereas others contain sand- and even pebble-sized material. The coarsest grains observed occur within a partially lithified conglomerate that is surrounded by a ~1 cm thick sand layer overlying a clay horizon within the radiolarian oozes of Subunit IIc in Hole U1331B (Fig. F9C). The lithified conglomerate is ~7 cm in length. Thin section analysis indicates that centimeter-scale angular rip-up clasts of mudstone and millimeter- to micrometer-scale biogenic components are contained within a fine-grained, partially silicified clay matrix (microcrystalline quartz). The major biogenic components are benthic and planktonic foraminifers, radiolarian spines, and nannofossils. Foraminifers and nannofossils retain their calcareous composition (they have not been silicified), but foraminifer chambers are typically infilled with chalcedonic quartz.

Clay horizons and porcellanite

In Subunit IIc and Unit III, dark brown clay horizons occur within biogenic oozes at Site U1331 (Table T4; Fig. F11). Some of the observed clay horizons are associated with sharp erosional boundaries and partial silicification of the associated coarser grained sediments (Fig. F11C). Clay horizons contain nannofossils, radiolarians, and, in particular, zeolite minerals in minor (<10%) amounts. Porcellanite pebbles and discrete layers are observed in sediments of middle Eocene age in Cores 320-U1331A-12H, 320-U1331B-11H, and 320-U1331C-10H (see "Site U1331 smear slides" in "Core descriptions"). Porcellanite layers 2 to 4 cm thick were recovered in Core 320-U1331A-19X from sediments of early middle Eocene age (see "Site U1331 smear slides" in "Core descriptions"). Massive or more frequent porcellanite layers are inferred to be present in the underlying 10–15 m interval where recovery was near zero based on the high-frequency, high-amplitude peaks in magnetic susceptibility data from downhole logging (Fig. F3).

Summary

At Site U1331, Eocene seafloor basalt is overlain by 187.2 m of pelagic sediments that are divided into four major lithologic units (Units I–IV); Units II and III are both divided into three subunits. Site U1331 sediments are dominated by radiolarian and nannofossil ooze with varying amounts of clay and can be correlated with Site 1220 using biostratigraphic and magnetostratigraphic results. Basal early middle Eocene sediments (Unit IV) are carbonate rich and overlain by a ~20 m thick radiolarian ooze–porcellanite interval. The middle Eocene sediment section (Hole U1331A; 49–158 m CSF) is dominated by radiolarian ooze with varying amounts of clay and nannofossil ooze and frequent turbidites. The late Eocene is characterized by radiolarian ooze with relatively less nannofossil ooze. The Eocene/Oligocene boundary is marked by a relatively sharp transition from dark brown radiolarian ooze beneath to pale brown nannofossil ooze with radiolarians above. A transition is indicated from siliceous sedimentation during the late Eocene to carbonate deposition during the early Oligocene, as observed previously in sediments from other sites in the region (e.g., ODP Sites 1218, 1219, and 1220 and DSDP Sites 161 and 162). This transition is interpreted to reflect deepening of the CCD associated with Antarctic glaciation (van Andel et al., 1975; Shipboard Scientific Party, 2002; Coxall et al., 2005).

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