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doi:10.2204/iodp.proc.329.109.2011 LithostratigraphyThe sediment at Site U1371 consists of ~130 m of diatom ooze and pelagic clay (Fig. F7). The strata of the site’s seven APC-cored holes are divided into two lithologic units based on their markedly different mineralogy. Here we will describe the sediment that was recovered from Holes U1371D–U1371F. Because they represent full penetrations of the sediment, shipboard sedimentologists, geochemists, and microbiologists relied on these holes primarily to characterize the subseafloor environment at Site U1371. Consequently, detailed descriptions of sediment recovered in Holes U1371B, U1371C, U1371G, and U1371H will not be included. See “Core descriptions” for images, basic visual descriptions, and data pertaining to the sediment in those holes. The two lithologic units at Site U1371 have sharply contrasting mineralogies. Unit I is ooze with average diatom and clay content of 56% and 17%, respectively. It is 104–107 m thick and contains numerous ash layers and multiple thin hardgrounds. Unit II is a blend of clay (32%), zeolite (30%), and red-brown to yellow-brown semiopaque iron manganese oxides (RSO; 15%). It contains an average modal abundance of up to 26% diatoms, but only in the upper 5 m of the unit where the lithology transitions from ooze to clay. Other minor constituents of the sediment include quartz, pyrite, manganese oxide/hydroxide, and biogenic particles including radiolarians, spicules, and silicoflagellates. The clay-bearing diatom ooze and pelagic clay at Site U1371 form interbedded intervals of highly fossiliferous and clay-rich layers. Bioturbation is a prominent feature of the sediment and causes diffuse boundaries on most beds. Numerous coring-related disturbances and whole-round core sampling affect our ability to correlate specific intervals among the holes of Site U1371. In a broad sense, however, the overall sediment thickness and composition appear to be very uniform (Fig. F8). Description of unitsUnit I
Unit I consists of three end-member colors: gray, olive, and brown (Fig. F9A, F9B). Pale olive-gray and greenish gray colors (5Y 6/2 and 5G 5/1 through 10GY 6/1 to 5GY 5/1) constitute the majority of Unit I’s color. The two colors frequently blend together and change gradationally. The longest interval between color changes is ~5.5 m and occurs in the middle of the unit (e.g., Sections 329-U1371D-6H-3 through 6H-7). All olive and gray intervals are interrupted by numerous black (5GY 2.5) laminations, beds, and irregular lenses that are between ~1–2 mm and 6 cm thick. Brown sediment (10YR 5/3 through 2.5Y 6/3) is most common in the uppermost 6 m, the middle 7.5 m, (e.g., Sections 329-U1371D-4H-5 through 5H-3), and the lowermost 6 m of Unit I. Approximately 15 short brown intervals of 0.1–1 m fall between thicker layers of gray and olive ooze throughout the remainder of Unit I. Similar to the gray and olive intervals, the brown layers are interbedded with dark laminations, beds, and irregular patches. The dark sediment lying within the brown layers is dark gray (5Y 4/1). Thin (<5 cm) white, pink, dark green, and very dark brown layers are associated with ash beds and hardgrounds that are described below. Smear slide analyses identify diatoms as the major component of Unit I (Fig. F10A); they constitute 31%–85% of the modal abundance of particles and are present in all layers except those that are >90% volcanic glass. Other biogenic particles in Unit I include siliceous spicules, radiolarian tests, and silicoflagellates. Like diatoms, spicules are present in all but the most volcanic of layers. Where present, their abundance ranges between 2% and 21% in roughly inverse proportions to diatom values. Average radiolarian abundance is 4.5% of the particles identified in smear slides. Radiolarians are present throughout the unit but are slightly more common (by 6%–10% above their average abundance) in the uppermost 6 m and in two very thin beds at 87 and 92 mbsf in Hole U1371D. Silicoflagellates are found above 8.5 mbsf and below 30.5 mbsf in Hole U1371D. Clay, RSO, and quartz are the majority of accessory minerals at Site U1371. X-ray diffraction (XRD) analyses of samples drawn from clay-rich intervals in the uppermost part of Unit I reveal well-defined illite and chlorite patterns (Fig. F11A). Smectite is also present but only above 12 mbsf. The average modal abundance of clay in Unit I is ~18%. Although clay abundance varies from 5% to 30%, no clear trends are apparent. On the other hand, RSO demonstrates a clear pattern of decreasing abundance with increasing core depth. Maximum RSO abundance (47%) occurs in the uppermost 2 m below mudline, decreases sharply to ~10% at 3–4 mbsf, and is absent below 12 mbsf. All RSO grains are small (<50 µm) and subangular. Quartz grains have similar sizes and shapes but occur throughout Unit I. They occur in 85% of all smear slides with low modal abundances (<1%–7%). Other accessory minerals are rare, restricted vertically, or exist in minute proportions. For example, pyrite is found only in the uppermost 10 m of Unit I and typically constitutes <2% of the sediment. Pyrite crystals are very small (<10–20 µm) and form euhedral pyritohedrons. The manganese oxyhydroxide nodules commonly associated with surface and shallowly buried sediments in the South Pacific were not observed in Unit I. However, black laminations (1–3 mm thick) occurring regularly throughout Unit I are composed of clay-sized particles that are probably manganese oxides or oxyhydroxides. The common zeolite, phillipsite, was not observed in smear slides; however, XRD analyses of clay-rich sediment in the uppermost 10 m of core contain peaks at 26.8° and 8.9°2θ that are strongly suggestive of phillipsite. Unit I contains a minimum of 20 pumice and ash layers and 6 hardgrounds (Tables T2, T3). Pumice layers have low clay content and exhibit grainy textures and distinctive white to light gray color. Volcanic glass shards in the pumice are large (as large as 200 µm), highly angular, and possess smooth surfaces and edges that are free of discoloration, pits, or other obvious signs of alteration. Ash layers have smooth, claylike texture and variable colors that include white, green, and red-brown. Dark greenish gray hardgrounds consist of clay and biogenic debris. The debris consists of diatom girdles (very few valves were observed), fragmented radiolarian tests, and spicules. Sediment consolidation increases with depth. Clay-bearing diatom ooze in the uppermost 10 mbsf is highly cohesive but easily deformed. Below 75 mbsf, sediment is very firm and brittle. Hardgrounds are particularly compact and difficult to divide for sampling purposes. Regardless of the vertical position of the ash and pumice layers, they are easily disaggregated into individual clay particles and glass shards. Bedding and bioturbation structures are the dominant features of Unit I. Bedding exists across a broad spectrum of thicknesses from ~1 mm ashy laminations to 1.5 m sections of uninterrupted diatom ooze. Bedding contacts range from sharp, horizontal transitions that are common in clay-rich portions of Unit I to diffuse, barely perceptible transitions among adjacent layers of diatom ooze. The sediment lacks other primary sedimentary structures (e.g., cross-laminations, graded bedding, and so on). Burrowing organisms impacted most sedimentary layers. With a frequency of at least one per core, extensive bioturbation created thick beds (10 cm to 1.5 m) of homogenized clay-bearing diatom ooze. On a much finer scale (1–3 cm), burrowing organisms blended and broadened bedding contacts among diatom, clay, ash, and other mineral components of individual layers. Although few burrows are distinct, large (~2 cm), subhorizontal, elliptical Planolites are evident in the uppermost 10 m, and small (~2–4 mm), subvertical Chrondrites burrows are faintly visible in the lower diatom ooze. The Chondrites burrows are especially well defined in the 2–10 cm of sediment that lies beneath thick ash layers in Section 329-U1371D-3H-6. The further redistribution of volcanogenic sediment is illustrated by a pair of glass-filled burrows in the lower part of Unit I (Section 329-U1371D-11H-3). These large (1.5 cm diameter × 8 cm length) horizontal and vertical burrows are part of a Thalassinoides network and are filled with light gray volcanic glass. The volcanogenic fill in these burrows is particularly interesting given that the nearest overlying ash layer is ~6 m upcore. Sediment structures related to the volcanogenic sediment in Unit I are generally the most well preserved. The basal contacts of many ash layers are sharp despite disturbances by small burrowing organisms. The upper contacts of the ash/pumice layers typically show more significant bioturbation. The absence of detectable ash layers in the middle of Unit I (Table T2) is likely related to bioturbation given the following three reasons:
Unit II
Unit II includes a variety of brown colors that become increasingly darker with depth (Fig. F9C–F9E). At its uppermost contact, the unit consists of a mixture of medium brown (2.5Y 6/2) clay with thin layers and irregular lenses and ellipses of very pale brown (10YR 8/4). The clay that forms the majority of the sediment darkens to dark grayish brown (10YR 4/2) and eventually very dark gray (10YR 3/1) in the 2 m that overlies the lowest recovered sediment. The very pale brown lenses and ellipses become less common and darken to light yellowish brown (10YR 6/3) in the lower part of Unit II. Black sediment occurs as small (~1–20 mm), spherical, granular masses that occur irregularly throughout the entire unit and in five large (4–7 cm) nodules found in Holes U1371E and U1371F (Fig. F9D). Light red (2.5YR 7/6) clay fills 7 burrows found in Section 329-U1371D-12H-1 and >20 burrows found in Sections 329-U1371F-12H-1 and 12H-2. Smear slide analyses show that Unit II is clay with varying amounts of phillipsite, RSO, volcanic glass, and biogenic particles and minor amounts of quartz and manganese oxide (Fig. F10B). On average, clay comprises the majority component of Unit II. Modal concentrations range from 0%–11% to 68%–77%. Where lower concentrations exist, RSO, phillipsite, or volcanic glass are the dominant particles. Clay mineralogy changes slightly through Unit II. In the uppermost 6 m, XRD results show well-defined illite peaks (Fig. F11B). In the lowermost 12 m, diffraction peak intensities corresponding to illite decrease by 50% and peak areas broaden by ~2°2θ, changes that are somewhat suggestive of smectite. Phillipsite is a major component of Unit II that is unevenly distributed. Although it is visibly absent from the uppermost 6 m, distinctive diffraction peaks from Sample 329-U1371D-12H-5 indicate phillipsite is probably a minor constituent of this interval (Fig. F11B). A similar set of conflicting results exists in the lower half of Unit II, where smear slide analyses indicate phillipsite is the majority (~60%) component of the clay, whereas XRD diffractograms show decreased (~30%) phillipsite peak intensities. Where visible, the phillipsite crystals are single laths and twinned crosses that are typically <10 µm × 40–60 µm and subhedral. The phillipsite crystals are heavily pitted (Fig. F10B). Unit II clay is moderately metalliferous, containing both RSO and manganese oxide-oxyhydroxide. RSO is not present in the uppermost 8 m of Unit II. Through the middle 10 m of Unit II, RSO abundance fluctuates erratically between <1% and 100%. In the lowermost 4 m, RSO maintains a constant presence with a range of modal abundance between 30% and 42%. Although RSO is amorphous and thus difficult to quantify using XRD (Kastner, 1986), diffractograms from five samples spanning Unit II support smear slide results. Specifically, the characteristic broad hump (between 20.5° and 22°2θ) that is associated with RSO is not present in the samples drawn from the uppermost 10 m of Unit II but is present in the XRD patterns from the samples drawn from the lowermost 4 m of Unit II (Fig. F11B). Manganese oxide-oxyhydroxide grains and nodules occur between ~110 and 120 mbsf. In the upper part of this interval, small (1–2 mm) spherical to medium (2 cm) elliptical black granular masses lie along burrow walls, bedding surfaces, and randomly in the intervening sediment. The masses are aggregated clay-sized grains. XRD analyses included one of the masses as part of the bulk sediment sample. Diffraction patterns for typical manganese oxides and oxyhydroxides (e.g., ramsdellite, manganite, romanèchite, etc.) could not be resolved in the resulting diffractograms. Five manganese nodules were recovered in Holes U1371E and U1371F. The nodules range in size from 2 to 4 cm and are situated between horizontal burrows and bedding planes. Volcanogenic components of Unit II include glass shards and ash. Volcanic glass shards are small (<10 µm × <30 µm), angular, and exhibit pitted surfaces and edges. The glass particles demonstrate a nearly symmetrical distribution of abundance. They are absent in the uppermost and lowermost 1 m and increase in abundance toward the midpoint of the unit, where their abundance peaks at 29% to 37% of the sediment. Light red clay, likely the alteration product of volcanic ash, fills 7 Planolites burrows in Core 329-U1371D-12H and >20 burrows in Core 329-U1371F-12H. A second ash layer, very pale brown and moderately indurated, lies ~4 m above the lowest recovered sediment and separates overlying medium brown and underling very dark brown sediment. Quartz is a relatively minor constituent of Unit II. Its abundance is distributed unevenly, with 8%–11% in the uppermost 8 m and 0% through the remainder of Unit II, except for the lowermost recovered in situ sedimentary layer where its abundance jumps to 100% of the particle abundance. Biogenic particles, including diatoms, radiolarians, spicules, and silicoflagellates, are present in the transitional interval that separates Units I and II. In the uppermost 2 m of Unit II, they constitute as much as 47% of the sediment. However, their overall abundance in Unit II is ~5% because they are entirely absent below the upper 3 m that makes up the transition zone. The upper half of Unit II is firm and pliable. The clay in the lower half of the unit is stiff and crumbly. Bioturbation effects are the most pronounced sedimentary structures in Unit II. Throughout the uppermost 6 m, the diatom-bearing clay is mottled. Although their ichnofacies association is indeterminate, the numerous elliptical burrows (1–2 cm diameter) in the uppermost 50 cm of this interval are distinct because they are filled with red ash. Individual burrows become easily distinguishable ~7.5 m below the upper contact. Traces in this interval are transverse and longitudinal cross-sections through Planolites burrows. The increased visibility of the burrows is largely attributable to the development of “halo burrows” caused by reduction and loss of clay-bound iron associated with respiration of the burrowing animals (Ekdale, 1977). Planolites halos continue to be prominent in the lower third of Unit II, although they become far less common. The lowermost 60–80 cm of sediment has a mottled structure and a few small (1–4 mm diameter) burrows exhibiting both horizontal and vertical aspects (Chrondrites?). Bedding in Unit II is rare. The three different types of bedding observed in Unit II and the intervals they cover are as follows:
Sediment/Basalt contactIndividual vitric grains and clusters of vitric grains were recovered in Holes U1371D–U1371F. In all holes, the grains lie in long (1–2 m) intervals of flow-in. The flow-in material is similar to the lowermost sediment of Unit II, very dark brown zeolitic metalliferous clay. The distribution of the grains along vertical streaks of deformed clay suggests the grains were created by the impact of the APC shoe on basement basalt and were subsequently sucked into the core barrel during recovery operations. Consequently, the sedimentary characteristics of the sediment/basalt contact could not be discerned at Site U1371. The igneous grains are glassy and possess alternating bands of red-brown and olive-yellow. They are translucent to semiopaque and are accompanied by thin coatings of yellow clay. These characteristics suggest that the material is altered basalt originating from an exterior position on an eruptive basaltic unit (e.g., pillow lava or sheet flow). Interhole correlationLithologic units were correlated among holes at Site U1371 to facilitate the integration of physical property, geochemical, and microbiological data. The stratigraphic correlation panel for Site U1371 is presented in Figure F8. Correlations shown in this figure are based on principal characteristics of the sediment, including
Correlations show that strata at Site U1371 have uniform unit thickness and composition. |
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