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doi:10.2204/iodp.proc.329.106.2011 LithostratigraphyThe sediment at Site U1368 is 15–16 m thick and consists of calcareous ooze, pelagic clay, and lithic sand. An additional 1 m of volcaniclastic breccia was recovered from an interval between basalt flows, 80 m below the upper sediment/basalt interface. The principal components of the ooze are calcareous nannofossils, accompanied by red-brown to yellow-brown semiopaque oxide (RSO) and foraminifers (see Site U1368 smear slides in “Core descriptions”; Fig. F7). Clay minerals are in relatively low abundance throughout the sediment. Clay-rich and sandy intervals contain a wide variety of minerals, including albite-anorthite, ankerite, augite, calcite, chlorite, hematite, and titanomagnetite. The volcaniclastic breccia contains altered basaltic, lithic and vitric grains. Based on compositional and textural attributes, the sediment at Site U1368 is divided into three lithologic units (Fig. F7). Unit I is nannofossil ooze, clay-bearing nannofossil ooze, and nannofossil marl. Unit I is separated into Subunits IA and IB based on the vertical distribution of nannofossils and RSO. Subunit IA contains abundant nannofossils and thus qualifies as nannofossil ooze and clay-bearing nannofossil ooze. Nannofossil abundance decreases in Subunit IB as RSO increases proportionately. Unit II is nannofossil-bearing clay that can be distinguished easily from the other units by its uniform, very dark brown color. Unit III includes three sand intervals separated by a thick (20 cm) bed of hematitic nannofossil-bearing clay. The sandy intervals contain a distinctive mineral suite that includes indicators of basaltic origins (plagioclase and augite) and hydrothermal alteration (ankerite, chlorite, and titanomagnetite). Lithostratigraphic correlation among Site U1368 holes shows that sediment composition remains fairly uniform; however, unit thickness varies over the 20–40 m distances that separate holes (Fig. F8). The thickness of Unit I, for example, thins by ~2 m across the 40 m lateral distance that separates Holes U1368B and U1368E. Nonetheless, each hole contains similar ooze, marl, and sand intervals. Description of unitsUnit ISubunit IA
The overall color of Subunit IA is dark yellowish brown (10YR 4/6, 3/6) (Fig. F9A, F9B). Specific intervals of nannofossil ooze with lower RSO abundance are yellowish brown (10YR 5/6). The top 25 cm of this interval is brown (10YR 5/3) and overlies dark grayish brown (10YR 4/2) clay-bearing intervals (Fig. F10A). Several widely dispersed silty layers (as thick as 10 cm) are very pale brown (10YR 7/4). Subunit IA contains too many laminations and beds to allow examination of each layer petrographically. Therefore, we prepared 5–7 samples from each of the light-, intermediate-, and dark-colored intervals and used the results to form a characterization of average lithology and compositional trends. Smear slide analyses indicate that Subunit IA consists primarily of calcareous nannofossil ooze (Fig. F7). The uppermost 25 cm of the subunit contains a higher abundance of RSO (10%–15%) and clay (5%–8%) than the lower interval (5%–10% and 2%–3%, respectively) and is the only portion of the sediment that contains phillipsite. X-ray diffraction (XRD) analysis of the ooze (Sample 329-U1368B-1H-1, 146–148 cm) indicates that the clay mineral in Subunit IA is smectite (Fig. F11A). The mass concentration of foraminifers >63 µm in the ooze varies from <1% in the upper clay-rich interval (Sample 329-U1368B-1H-3, 0–2 cm; ~3 mbsf) to >7% near the base of Subunit IA (Sample 329-U1368-2H-1, 25–27 cm; 5.75 mbsf). In addition to low abundance, most of the foraminifers in the upper clay-rich interval are broken and partially dissolved (see “Paleontology and biostratigraphy”). The lower subunit contact consists of pebbles of indurated pelagic clay overlying a clay-rich matrix that contains abundant RSO (Fig. F12A). Although the sediment is poorly consolidated, it is highly cohesive. The sediment also clings persistently to physical properties and sampling instruments inserted into the split core sections. The dark color of the upper 25 cm of sediment is distinct from the underlying pale brown sediment and contains traces of horizontal burrowers of transition-layer (5–8 cm) organisms (Ekdale et al., 1984). Although the sediment is mottled, individual Planolites and Zoophycos burrows are evident (see core photographs from Site U1368 in “Core descriptions”). Several meters of the middle of Subunit IA contain very faint, short (2–3 cm), vertical burrows (Trichichnus and Teichichnus). Very pale brown, very thin to thick (1–10 cm) beds of foraminiferal ooze occur occasionally. Laminations within the beds exhibit two distinct sedimentary structures: (1) undeformed horizontal bedding and (2) soft-sediment deformation. The lower contact of Subunit IA observed in Hole U1368B is inclined 30° from horizontal. In Hole U1368E, the contact is irregular, showing 8 mm of vertical relief. Subunit IB
Sediment in Subunit IB is dark yellowish brown (10YR 3/4, 3/6) to dark brown (10YR 3/3) (Fig. F9C). The uniformity of Subunit IB lends itself to a simple description of its lithology. Smear slide analyses indicate an overall abundance of nannofossils and RSO (Fig. F10B). RSO increases from ~10% in the upper 10–50 cm of the subunit to 60%–80% in the dark brown clay at the bottom of the subunit (Fig. F7). Sample 329-U1368B-2H-4, 60–61 cm, was selected to represent an average Subunit IB lithology and analyzed by X-ray diffraction (XRD). An analysis of the sample’s XRD pattern resolves calcite and smectite but fails to contain any diagnostic indicators of the RSO. Foraminifers >63 µm constitute slightly more than 6% (by mass) of the sediment in the upper 10–50 cm of the interval but <1% (by visual estimation) in the lower 30–100 cm. Overall preservation of planktonic foraminifers in Subunit IB is moderate to good (see “Paleontology and biostratigraphy”). Firmness of the sediment increases with depth and varies with color, not bulk density. Dark, RSO-rich sediment in the lower half of Subunit IB is firm. Layers of very pale brown nannofossil marl are poorly consolidated. The upper half of Subunit IB is massive. The lower half exhibits light and dark layering suggestive of bedding, although all sediment is affected by bioturbation. In Section 329-U1368B-2H-4, Zoophycos burrows as wide as 5 mm stand out as evidence of the bioturbation. Contrasting very dark brown and dusky red sediment form a steeply (~45°) inclined interval of metalliferous clay in interval 329-U1368E-2H-3, 48–55 cm. The texture of the clay on either side of the line separating these colors is similar and thus the origin of this structure is unclear. Unit II
Unit II sediment is black to very dark brown (7.5YR 2.5/1, 2.5/3, 3/4) (Fig. F9D). Color differences define three zones in Unit II that range in thickness from 15 to 50 cm. Smear slide analyses indicate that all three intervals are predominantly RSO, accompanied by nannofossils (Fig. F10C). RSO abundance increases slightly with depth from 60% to 80% in the uppermost interval and from 70% to 90% in the lowermost interval (Fig. F7). XRD analysis of Sample 329-U1368B-2H-6, 60–61 cm, from the middle interval clearly confirms the presence of calcite and suggests the presence of hematite and montmorillonite (Fig. F11B). In Sample 329-U1368B-2H-7, 10–11 cm, XRD confirms the presence of calcite and hematite in the lowermost interval and contains several peaks that cannot be easily explained. Specifically, the very strong peak at 7.12°2θ (5.23Å) defies easy recognition. The automated search/match function available through the R/V JOIDES Resolution’s X-ray pattern evaluation software couples this peak with other unmatched peaks at 24.33°2θ (1.82Å) and 15.84°2θ (2.82Å) and indicates that the mineral calciohelairite is the most likely match; however, the common association of this mineral with perialkalic igneous intrusions casts suspicion on the likelihood of this mineral’s presence in clay obtained at Site U1368. The uniform dark brown clay is firm. During description, the clay shrunk quickly and developed transverse and longitudinal desiccation cracks. This relatively thin unit (120 cm maximum thickness in Hole U1368E) is massive. The boundary between the uppermost and intermediate intervals is sharp but contains small-scale irregularities indicative of horizontal burrowing organisms. The boundary between the intermediate and lower layer contains short (<21 cm) Chrondrites burrows. The lower contact of Unit II is highly irregular and exhibits 5 cm of vertical relief (Fig. F12B). Similar to the overlying sediment, the dusky red clay immediately overlying the irregular contact is massive. Unit III
Depths: Hole U1368B = 13.5–15.1 mbsf, Hole U1368C= 15.50–15.13 to 16.1 mbsf (end of core), Hole U1368D = 13.0–13.25 to 14.9 mbsf (end of core), Hole U1368E = 10.16–10.50 m (end of core) Unit III contains between one and three sandy intervals and one clay-rich interval (Fig. F8). All three sandy intervals are found in the lowermost core section of Hole U1368B. The upper sand is very dark gray (N3) with white (2.5Y 8/1) and dark reddish brown (5YR 3/3) highlights on the edges of numerous pebble-sized nodules. The middle sand is multicolored, including dark greenish gray (5GY 3/1), bluish black (10B 2.5/1), and very dark brown (7.5YR 2.5/1) (Fig. F9E). Part of the well-indurated dark greenish gray sand contains reddish yellow (5YR 6/6) grains. The lower sand is mostly black (2.5Y 2.5/1) with dark gray and white grains. The clay interval, between the upper and medial sands, is very dark brown (7.5YR 2.5/3) at its core and grades into gray/dark gray (7.5R N5/N3) and white (GLEY 1 N8) in the few centimeters adjacent to its lower contact. The apparent continuity of the three sandy intervals from hole to hole varies from one layer to another. The upper layer was observed only in material recovered from Hole U1368B. The middle layer was observed in cores from Holes U1368B–U1368D. The lower layer was observed in cores from Holes U1368B and U1368C. The sandy layer found in Hole U1368E has textural similarity to the upper sand in Hole U1368B, but has magnetic susceptibility similar to the lower sand of Hole U1368B. Consequently, this sandy layer’s relationship to the three sand layers of Hole U1368B is not clear. The absence of the upper layer from Hole U1368C and U1368D cores may be due to either whole-round sampling for biogeochemistry and microbiology or to limited spatial continuity of the layer. The absence of the lower layer in Hole U1368D (and possibly Hole U1368E) may be due to poor recovery of the lowermost few centimeters of sediment. All three sands are lithic, but each possesses a distinct assemblage of minerals. The upper (dark gray) sand contains numerous plagioclase minerals confirmed by thin section observation and XRD patterns of albite-anorthite (Fig. F11C). The upper sand’s other abundant primary mineral component is more difficult to identify because cross-sectional views of its crystal habit were not diagnostic. However, XRD intensity peaks suggest that it is augite. A thin section created from the sand’s lower contact shows that most minerals are weathered to clay (Fig. F13A). The middle (green) sand also contains albite-anorthite and augite, as well as petrographic and very strong XRD indications of abundant chlorite (Fig. F11C). The lowermost (black) sand contains abundant calcareous nannofossils and is strongly magnetic. Sieved samples of the black sand yield granular carbonate pieces with bulbous outer surfaces. Planktonic foraminifers with recrystallized tests are also evident in the sieved samples (Fig. F14). Confirmed visually and by XRD analyses, the magnetic mineral is titanomagnetite (Fig. F11C). A major uncertainty about the composition of this sand involves our visual observation of numerous acicular crystals (Fig. F10D). Visual observations and XRD analyses could not positively identify this mineral, although our studies did rule out several suspected minerals including antigorite, aragonite, and natrolite. The very dark brown clay is calcareous. Smear slide analyses of sediment drawn from the middle of the clay showed numerous nannofossils and RSO (Fig. F7). In Hole U1368B, the upper part of the clay contains four small nodules with composition similar to the overlying lithic sand. A thin section prepared from the sediment at the upper (clay/sand) contact contains minute titanomagnetite (octahedral) and apatite (hexagonal and botryoidal) crystals and many microfossil (foraminifer and radiolarian) molds, some containing secondary calcite crystals (Fig. F13B, F13C). The upper sand of this unit is well consolidated. Nodules within the sand are either well indurated (rock) or brittle and crumbly. The green sand contains two thin (2–3 cm), firm layers but is otherwise easily disaggregated into loose sand-sized particles. The lower black sand is very poorly consolidated (soupy). The clay interval between the upper (dark gray) sand and middle (green) sand is mostly firm but tends to become crumbly in the short (1–5 cm) intervals adjacent to the lithic sands. The lower two sands (green and black) are merged into one contiguous interval in Holes U1368B and U1368C. Contacts between neighboring lithotypes are abrupt. In Hole U1368B, the uppermost sand’s contact with overlying clay is very irregular (see “Unit II”). The lower contact of the upper sand is very sharp and concave against an underlying clay interval. The lower contact of the clay, against the middle (green) sand, is concave and gradational at the top. Based on observations of core from Hole U1368C, the contact between the middle (green) sand and underlying (black) sand is sharp and slightly convex. The black sand is highly disturbed (soupy) and devoid of internal structures. The lower contact of the sand was not observed. Sediment texture is variable within the upper and middle sand layers. Within the uppermost sand, particles are arranged in nodules that are separated by wide, irregular gaps. The nodules are generally small (2 cm in diameter). When they possess visibly long and short axes, the long axis is usually oriented horizontally. In the green sand, particle size increases from silty to sand in the lower 12 cm. The black sand is soupy; textures and structures were unobservable. The nannofossil-bearing clay is massive and featureless except for several isolated locations that are associated with indurated nodules. The unit possesses burrowed laminations. Burrows are filled with nannofossil and microfossil debris. The edges of the burrows and space between laminations host consecutive layers of botryoidal apatite, secondary calcite, and pore/vein-filling radiating acicular crystals (Fig. F13C). Sediment/Basalt contactWe attempted to sample the sediment/basalt interface in each hole. Mixed sediment and basalt fragments were recovered in Holes U1368B, U1368C, and U1368D. Lithologic types associated with the basalt included green and black lithic sand (see “Unit III”). Textural and structural properties of the sediment were not recorded because the sediment was very highly disturbed. In general, the basalt at the sediment/basalt interface is highly altered. Of the ~10 fragments recovered at Site U1368, most were friable and grainy and contained abundant zeolite. More information regarding the altered and relatively unaltered basalt fragments recovered in APC cores in “Igneous lithostratigraphy, petrology, alteration, and structural geology.” The lowermost core (2H) in Hole U1368E did not contain basalt. However, we believe the sediment/basalt interface was penetrated in the hole because (1) the sediment recovered in Section 329-U1368E-2H-CC contained pebble-sized clasts of friable, highly altered basalt and (2) the core barrel’s cutting shoe was irreparably damaged by impact with a very hard substrate during the Core 329-U1368E-3H coring attempt (the cutting shoe was gouged to a depth of ~2 cm, the stroke was incomplete, and the core barrel was recovered empty). We attempted to recover the sediment/basalt contact in Hole U1368F using RCB coring. The core that spanned the interface (Core 329-U1368F-2R) recovered 4.01 m of the advanced interval (11.6 m). No sediment was recovered in the core and the basalt appeared only mildly altered. See “Igneous lithostratigraphy, petrology, alteration, and structural geology” for more details regarding the basalt recovered in Hole U1368F. Volcaniclastic brecciaVolcaniclastic breccia was recovered during RCB coring of basaltic rocks ~80 m below the upper sediment/basalt contact. Sedimentologists and igneous petrologists worked cooperatively to describe the core, and results are presented in “Igneous lithostratigraphy, petrology, alteration, and structural geology.” DiscussionThe foregoing results support preliminary interpretations of depositional and diagenetic environments. The interpretations that pertain to sediment accumulation and alteration in Units I–III warrant discussion and are listed below. Sediment accumulationBy thickness and duration of sediment accumulation, the majority of sediment at Site U1368 is in Unit I. The lithologically similar calcareous oozes and marls of Unit I comprise >80% of the 12–15 m of sediment thickness and ~80% of the ~14 m.y. depositional history (see “Paleontology and biostratigraphy”). To the level of specificity afforded by our smear slide and X-ray analyses, the minerals that make up Subunits IA and IB are identical. The lithologic differences found in Unit I are caused by variations in the abundances of calcite, clay, and RSO. The RSO is particularly important because it is responsible for the dark reddish brown colors that mark bedding features and because it is associated with slow rates of sediment accumulation (Heath and Dymond, 1977). Although the abundance of RSO generally increases with depth, the trend has several notable deviations. First, RSO abundance does not increase at a constant rate; it experiences highs and lows that exist in layers ranging from 10 to 30 cm thick. Second, several beds (3–10 cm thick) of laminated foraminiferal silt are nearly devoid of RSO and have very low clay content. Third, the unit boundary between Subunits IA and IB is very thin (2–6 cm) but sharp and contains abundant RSO and indurated clay clasts (0.5–1 cm in diameter). Because seafloors are evolving environments responsive to internal and external processes, variations in RSO abundance are expected. However, the very low abundance of clay and RSO in the foraminiferal silt and dramatic abundance at the Subunit IA/IB boundary carries sedimentary and oceanographic implications briefly outlined in the following paragraphs. The foraminiferal silt layers are devoid of clay and RSO. Options for limiting the clay content in mid-Pacific seafloor sediments include
Option 1 is unlikely because reducing the influx of terrigenous clay should reduce overall sediment accumulation and thus promote greater RSO abundance in the foraminiferal silt layers. Option 2 follows similar logic and is therefore unsupported by the observed paucity of RSO in the silt layers. Therefore, we speculate that the foraminiferal silts correspond to episodes of increase biogenic flux and thus higher sediment accumulation rates, option 3. The Subunit IA/IB boundary represents an event in which reduced sediment accumulation is inferred. The irregularity of this surface and abundance of indurated clay clasts may have resulted from prolonged bioturbation and not subsequent removal of previously deposited sediment (Stow and Lovell, 1979; Ekdale et al., 1984; Ujiié, 1984; Uchman, 2007). The great abundance of RSO in the sediment is consistent with very slow sediment accumulation rates (~0.001 m/m.y.). Preliminary shipboard biostratigraphy places this event between Miocene and Pliocene foraminiferal Zones M11/M10 (~12 Ma) and PL2/PL1 (5.8–4.3 Ma) (see “Paleontology and biostratigraphy”). Lithologic alteration and diagenesisEach stratigraphic unit at Site U1368 contains compositional and textural evidence of alteration and/or diagenesis. Alteration found in carbonate minerals and basalt is the most evident and is discussed below. The most common biogenic constituents of the sediment at Site U1368 are calcareous nannofossils and microfossils. Spot checks of foraminiferal preservation found dissolution effects in the uppermost part of Unit I and extensive recrystallization in Unit III (see “Paleontology and biostratigraphy”). Alteration of fossils in Unit III included infilling of shell pores and entire shells with secondary calcite, zeolite, RSO, and hematite. Nannofossils in the lowermost sand appear only as outlines in carbonate masses that include the tests of dozens of organisms. The lithic sand intervals in Unit III may have resulted from alteration of basaltic debris. Evidence supporting this interpretation includes (1) mineral constituency, (2) sharp contacts with the overlying and underlying clay layers, and (3) proximity (<2 m) to underlying basaltic basement. Each of these intervals possesses unique compositional and textural attributes. For example, the uppermost sand of Unit III consists of a series of indurated nodules with strong partings that correspond to weak zones of elevated clay content. Zeolite crystals are also common among its veins and pores. In contrast to the uppermost sand, the medial (green) sand is more uniform in texture and possesses a uniquely high abundance of chlorite. The lowermost (black) sand consists almost entirely of titanomagnetite, zeolite, and altered microfossils. Given that these three sand layers lie within approximately one vertical meter of each other, these compositional and textural differences are remarkable. Accurately identifying the origin of these lithologic differences requires detailed petrographic and geochemical analyses beyond the scope of our shipboard studies. Interhole correlationLithologic units are correlated among holes at Site U1368 to facilitate the integration of physical properties, geochemical, and microbiological data. The stratigraphic correlation panel for Site U1368 is presented in Figure F8. Correlations shown in this figure are based on a number of horizons with characteristic features, including
Assuming subsea depths and coring intervals were recorded accurately and/or consistently, interhole correlation shows lithologic units at Site U1368 to generally thin to the south and east (Fig. F8). Comparison of the continuously cored intervals in Holes U1368B and U1368E illustrates this point. The total thickness of Unit I, for example, thins from 12.5 m in Hole U1368B to 10.4 m in Hole U1368E. Despite this change in thickness, the composition of Unit I changes very little; the two sites each possess the surficial NGR response, the phillipsite constituency near the top of the unit, the clay clast and RSO sequence in the middle of the unit, and the distinctive lower contact with Unit II. The difference in thickness is therefore associated with slight changes in sediment accumulation and not deposition of unique sedimentary units (i.e., mass wasting deposit) or postdepositional phenomena such as erosion or faulting. |
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