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The sediment at Site U1366 is primarily clay. The principal components of the clay are smectite and mica-group members, phillipsite (a zeolite), and red-brown to yellow-brown semiopaque oxide (RSO) (see Site U1366 smear slides in “Core descriptions”; Fig. F7). Although clay and zeolite abundance rises and falls within the sediment column, they both exhibit overall trends of decreasing abundance with increasing depth. RSO, an iron manganese hydrated material found frequently in the southwestern Pacific (Graham et al., 1997), is the most common constituent of the clay. RSO gives the sediment a range of colors, including dark brown (7.5 Y 3/3, 3/4) to very dark brown (7.5YR 2.5/2, 2.5/3) to black (N2.5), that vary with changes in its abundance.

Subdivisions of the sediment at Site U1366 are based on RSO and zeolite distributions. Unit I includes metalliferous to zeolitic metalliferous pelagic clay. Subunit IA contains equal portions of RSO, zeolite, and clay minerals. The modal RSO concentration increases in Subunit IB as zeolite abundance decreases proportionately. The upper boundary of Subunit IC is defined by an increase in zeolite, whereas its base coincides with the absence of zeolite. RSO dominates the composition of Unit II and gives this metalliferous clay a distinctly black color.

Lithostratigraphic correlation shows that thickness and composition of units are variable among the five holes cored at Site U1366 (Fig. F8). The total thickness of Unit I, for example, is 2.6 m thinner in Hole U1366F than it is in the other holes at this site. Correlative sediment has variable composition. For example, manganese nodules found in Subunit IB in Holes U1366C and U1366D are not found in the equivalent sediment of Holes U1366B and U1366F. Similarly, lithic silt beds recovered in Hole U1366F are altogether absent from the other holes. Given these irregularities, the description of units that follows represents a composite stratigraphic section based on observations of cores from all holes at Site U1366.

Description of units

Unit I

Subunit IA
  • Intervals: 329-U1366B-1H-1, 0 cm, to 1H-CC, 23 cm; 329-U1366C-1H-1, 0 cm, to 2H-1, 150 cm; 329-U1366D-1H-1, 0 cm, to 2H-1, 130 cm; 329-U1366E-1H-1, 0 cm, to 1H-CC, 12 cm; 329-U1366F-1H-1, 0 cm, to 2H-3, 150 cm

  • Depths: Hole U1366B = 0–7.7 mbsf, Hole U1366C = 0–7.5 mbsf, Hole U1366D = 0–10.7 m, Hole U1366E = 0–4.7 mbsf, Hole U1366F = 0–10.0 mbsf

  • Lithology: zeolitic metalliferous pelagic clay

Subunit IA is a uniform dark brown (7.5YR 3/3) (Fig. F9A) that gradually changes to very dark brown. The lower interval is unevenly colored with irregularly shaped areas of interwoven shades of very dark brown (7.5YR 2.5/2 and 7.5YR 2.5/3). The transition to Subunit IB is clearly observed in Holes U1366B and U1366C, partially expressed in Holes U1366D and U1366F, and is not observed in Hole U1366E because that hole was not deep enough to reach the Subunit IA–IB transition.

RSO, zeolite, and clay minerals (Fig. F7) are found in Subunit IA in abundances of 30%, 30%, and 20%, respectively. The color change noted in the lower part of the subunit corresponds to slightly higher abundance of RSO and a corresponding reduction in zeolite. The RSO forms 30%–40% of the sediment and consists of grains that are rounded to irregularly shaped and range in size from 1 to 80 µm. X-ray diffraction (XRD) results of representative pelagic clay taken from Subunit IA at interval 329-U1366C-1H-1, 30–32 cm, contain diffraction peaks that correspond to smectite, chlorite, quartz, and phillipsite. The zeolite crystals observed in smear slides are euhedral and prismatic. A majority of the crystals have long axes that are <20 µm (Fig. F10A). Cobble-sized manganese nodules are present at the top of Subunit IA in Holes U1366C, U1366E, and U1366F (Fig. F9A). Smaller nodules were observed at varying depths within Subunit IA in Holes U1366B and U1366C.

Consolidation of Subunit IA sediment transitions from very poorly consolidated near the mudline to poorly consolidated at the base of the subunit. Most samples of this unit are sticky.

The upper two-thirds of Subunit IA are homogeneous and without bedding features. Burrows are evident throughout the lower 40 cm of the subunit in Holes U1366B and U1366D. Several individual burrows were noted for their large diameter (1 cm) and length (10 cm).

Subunit IB
  • Intervals: 329-U1366B-2H-1, 0 cm, to 2H-2, 30–70 cm; 329-U1366C-2H-2, 0 cm, to 2H-4, 68–94 cm; 329-U1366D-2H-1, 130 cm, to 2H-3, 130 cm; 329-U1366F-2H-4, 0 cm, to 2H-5, 54–90 cm

  • Depths: Hole U1366B = 7.7 to 9.5–9.9 mbsf, Hole U1366C = 7.5 to 11.2–11.4 mbsf, Hole U1366D = 10.7–13.7 mbsf, Hole U1366F = 10.0 to 12.0–12.4 mbsf

  • Lithology: metalliferous pelagic clay

Subunit IB is exclusively dark brown (7.5YR 3/3) in Holes U1366B, U1366C, and U1366F. In Hole U1366D, the dark brown sediment includes an intervening very thick (1.5 m) bed of very dark brown (7.5YR 2.5/2) clay. Within each interval, the color is nearly uniform with only subtle variations that are detectable on the Section Half Multisensor Logger (SHMSL) spectrometric imaging of core section halves (see “Physical properties”).

Zeolite is present in very low abundance or altogether absent in Subunit IB (Fig. F7). Although the interval is thin (2–4 m), proportions of clays and RSO in the interval fluctuate between ratios of 20%/80% and 35%/62%. In Hole U1366D, RSO concentration is sustained at 75%–80% and forms a very thick (1 m) bed of very dark brown clay within Subunit IB. Manganese nodules are present in Holes U1366C and U1366D.

Consolidation in the subunit is uniformly moderately indurated (firm).

The majority of Subunit IB is homogeneous and lacks any discernible structure (Figs. F9B, F10B). Occasional burrows are visible in the lowermost meter of Subunit IB in Holes U1366C and U1366F. Faint mottling occurs in the stratigraphically equivalent interval in Hole U1366D.

Subunit IC
  • Intervals: 329-U1366B-2H-2, 30–70 cm, to 2H-CC, 23 cm; 329-U1366C-2H-4, 68–94 cm, to 3H-1, 25 cm; 329-U1365D-2H-3, 130 cm, to 2H-CC, 20 cm; 329-U1365F-2H-5, 54–90 cm, to 2H-CC, 10 cm

  • Depths: Hole U1366B = 9.5–9.9 to 17.3 mbsf, Hole U1366C = 11.2–11.4 to 15.7 mbsf, Hole U1366D = 13.7–18.8 mbsf, Hole U1366F = 12.0–12.4 to 14.0 mbsf

  • Lithology: zeolitic metalliferous pelagic clay

Subunit IC has two distinctly colored intervals. The sediment in the uppermost 1 m (Hole U1366F) to 6 m (Hole U1366B) of the subunit is uniform very dark brown (7.5YR 2.5/2) with occasional flecks of very pale brown (10YR 8/4). Below this interval, the very dark brown sediment is punctuated by very pale brown and red-brown laminations, diffuse beds, and rounded to subangular lenses. Where the pale brown sediment blends with the dark brown sediment, the resulting color is brown (7.5YR 5/4) (Fig. F9C; see core photograph of Section 329-U1366B-2H-5 in “Core descriptions”). Other brown beds appear in granular, thin beds of varying thickness in Holes U1366B–U1366D.

The very dark brown intervals of Subunit IC consist of RSO mixed with lesser amounts of smectite-group clay and very small amounts of zeolite (Fig. F7). The abundance of RSO varies without an obvious trend between 25% and 80% of the grains observed in smear slides. XRD analysis of a sample from Section 329-U1366C-2H-6 suggests that (1) the smectite minerals include montmorillonite, (2) the zeolite crystals are likely phillipsite, and (3) fluorapatite could be present. Zeolite is visible in smear slides and consists of small bladed crystals that are present in small amounts (<5%) in the very dark brown clay. The crystals are slightly more abundant (15%) in the very pale brown laminations. Among Holes U1366B–U1366D, 2–5 thin beds (1–2 cm) of porcellaneous clay were observed. A single, very thin (1 cm) bed of chert was encountered in the bottom of Hole U1366D. One manganese nodule was found in the middle of Subunit IC in Core 329-U1366B-3H. A 1.5 cm long ichthyolith was found in Sample 329-U1366C-2H-CC (Fig. F11).

The entire interval is moderately well indurated. The porcellaneous clay is marked by granular disturbances in the clay that are attributable to its brittleness. The chert is very well indurated and breaks along flattened blades with conchoidal fractures.

Subunit IC comprises two intervals with distinctive structure. The uppermost 1–6 m of clay is homogeneous and faintly mottled. Burrowed, very pale brown to red-brown laminations are present in the lower interval. The frequency of the laminations and beds is ~3–5 per 10 cm of core (e.g., see photographs of Section 329-U1366B-2H-7 in “Core descriptions”). Porcellaneous clay forms as many as five thin beds (3–5 cm) in Hole U1366B and as few as two in Hole U1366C.

Unit II

  • Interval: 329-U1366F-3H-1, 0 cm, to 4H-5, 116 cm

  • Depth: Hole U1366F = 14.0–30.0 mbsf

  • Lithology: metalliferous clay

Unit II consists of very dark sediment intercalated with laminations and thin beds of a wide variety of colors. The dominant colors of the unit are brown-black (7.5YR 2.5/1, 2.5/2, and 2.5/3) and black (5YR 2.5/1 and GLEY1 2.5/N). Individual beds and laminations have various colors that were interpreted as shades of white (7.5YR 8/1), pale yellow (7Y 8/4), olive-gray (5Y 4/2), brown (7.5YR 4/6), very pale brown (10YR 8/4), and reddish brown (2.5YR 3/4).

Smear slide analyses indicate Unit II is predominantly composed of RSO (Figs. F7, F10C, F10D). Most intervals contain 80%–100% RSO. The unit also contains at least one thick (10 cm) bed of ~60% smectite and numerous other thinner beds of porcellaneous clay, lithic grains, and volcanic ash/altered ash. Zeolite minerals are absent. Seven thin (<5 cm) beds of porcellaneous clay were recovered from the upper half of Unit II. Three distinctive ash/altered ash layers are found in Sections 329-U1366F-3H-4 and 3H-5. The upper layer was partially removed by whole-round sampling prior to sedimentary description. The upper layer is >10 cm thick and consists of brown and black clay. No distinguishing crystals were observed during smear slide analysis. The second ash/altered ash layer contains a core of very soft white clay surrounded by olive-green clay (Fig. F9D). The third bed is 4 cm thick and similar to the overlying white and olive-green ash layer. Olivine and feldspar crystals were identified in lithic silt and sand found in Sections 329-U1366F-3H-1, 3H-7, and 4H-5. Both silt beds also contain numerous radiolarians (Fig. F10D).

The brown- and red-black metalliferous clays that make up most of Unit II are moderately well consolidated (i.e., stiff clay). Porcellaneous clay beds are similarly consolidated and break into granules when cores are sectioned and split. Ash/altered ash layers are soft but consolidated. Lithic silt and sand layers are made of loose grains.

In contrast to Unit I, Unit II contains numerous bedding features. Very pale brown layers are laminated to very thinly bedded and commonly show gaps, offsets, and other irregularities associated with burrows. Porcellaneous clay forms beds that are 3–10 cm thick. Lower in the section, several ash/altered ash layers are thickly (10+ cm) bedded. The lithic silt and sand are thin, 1 and 3 cm, respectively, with distinct upper and lower contacts.

Sediment/Basalt contact

Because of coring difficulties in Holes U1366B–U1366D, components of the sediment/basalt contact were only recovered in the lowermost core of Hole U1366F. The basalt and obsidian that were recovered consist of numerous small pieces (2 cm maximum diameter) that were packed tightly into the upper 7 cm of the core catcher of Core 329-U1366F-4H. Sediment was recovered in the lowermost part of Core 5H. Although probably representational of the basalt and sediment composition, this form of core recovery precludes our ability to observe structural relationships between the sediment and basalt. A description of the volcanic rocks is provided in “Igneous lithostratigraphy, petrology, alteration, and structural geology.” A description of the interface interval is provided below.

The lowermost sediment interval at Site U1366 consists of lithic sand, nodular clay, and massive red clay. The basal interval (329-U1366F-4H-5, 113–116 cm) is black (5YR 2.5/1) clayey lithic sand that includes a firm, large (2 cm × 3 cm) nodule of dark gray (GLEY 1 3/N) clay (Fig. F9E). The composition of the sand is a mix of lithic fragments (olivine and plagioclase feldspar) and clay. Shipboard scientists concluded that the large clay nodule imbedded in the sand is made of altered basalt. The interval overlying the lithic sand contains 10.5–14.5 cm of friable, very dusky red and reddish black clay nodules. The uncertainty in thickness of this interval is due to whole-round sampling of the core sections prior to sedimentary description. Above the sampled zone, the interface is reddish brown (2.5YR 4/4) metalliferous clay. XRD analysis of the clay indicates the presence of smectite and hematite (Fig. F12). The upper limit of the red clay is an abrupt transition to black metalliferous clay at interval 329-U1366F-4H-5, 53 cm, ~63 cm above the basal lithic sand.


Sediment composition and texture

Smear slide and XRD analyses identify smectite, mica, zeolite, and RSO as the principal components of the zeolitic, metalliferous, and pelagic clay at Site U1366. Overall, the proportion of zeolite in the sediment decreases with depth. Conversely, the portions of RSO increase with depth. In the 10 m of sediment that overlie the basement, RSO becomes the dominant (occasionally exclusive) lithologic component. These trends define the two lithologic units found at Site U1366: Unit I contains RSO, clay, and zeolite and Unit II contains RSO and clay.

Within the broad trends of changing principal components outlined above, several of the less abundant lithologies help refine the description of sedimentary units and provide insight into the depositional history of Site U1366. These features are described below.

Porcellaneous clay

The porcellaneous intervals at Site U1366 are very poorly developed. Although radiolarians occur throughout the upper half of Unit II, their volume is relatively small compared to the RSO and clay fractions of the sediment. The porcellaneous beds retain the dark color of surrounding metalliferous clay and can be easily disaggregated into granules. Thus, we identify this sediment as “porcellaneous clay” rather than by the name “porcellanite,” which carries connotations of being a well-indurated microcrystalline rock with dull luster (Keene, 1975). Sediment at the site does include chert; however, the only chert recovered during coring was one thin (<5 cm) bed in Hole U1366C. The high concentration of clay likely precluded the formation of porcellanite and chert at this site despite the numerous occurrences of radiolarians in the sediment (Kastner et al., 1977).

Volcanic ash

The ash/altered ash layers at Site U1366 show variable thicknesses and compositions. Most of these ash layers are in the lower half of Unit I and upper half of Unit II. They consist of laminated to very thinly bedded, uniformly very pale brown zeolitic clay layers. Using sediment samples from DSDP Site 597, Kastner (1986) identified these layers as altered volcanic ash layers. Although her description of the altered volcanic ash at Site 597 is similar to our observations at Site U1366, we prefer to call the altered volcanic ash by the name ash/altered ash because the thorough studies needed to assess alteration of the ash were beyond the scope of our shipboard lithologic observations.

In the lower half of Unit II, three ash/altered ash layers differ from those found in the overlying sediment. The first of these, found at interval 329-U1366F-3H-4, 40–50 cm, is brown and may have received ash from the same source as the numerous overlying ash/altered ash layers. However, this bed is at least five times thicker than any of the other very pale brown layers found at Site U1366. The lower two ash/altered ash layers (located at intervals 329-U1366F-3H-4, 96–103 cm, and 3H-5, 15–19 cm) are also thick (7 and 4 cm, respectively) and possess unique compositions as indicated by their white, pale yellow, and olive-green colors. The small size of the crystals in these ash layers makes them petrographically indeterminate. However, the unique colors of the ash layers imply they (currently) possess chemical constituencies that differ from all other ash/altered ash recovered at Site U1366. Because these layers in Unit II are the oldest and reside in sediment that is similar to numerous overlying very pale brown altered ashes, it is possible that the older ashes originated from a different volcanic source than the younger ash or that ash composition from a single source evolved over time.

Lithic silt and sand

The lithic silt beds and sand recovered in Cores 329-U1366F-3H and 4H suggest that weathering, erosion, and lateral sediment transport impacted Site U1366. Because the sand contains numerous olivine and plagioclase grains and directly overlies basement rocks, it is likely a product of basaltic seafloor weathering and short-distance sediment transport.

The origin of the lithic silt is less clear. The composition, including the presence of olivine and plagioclase grains, also implies a basaltic source. However, the silt beds are separated from the basement rocks by 10 m of metalliferous clay. Given the mean rate of sediment deposition at this site (D’Hondt et al., 2009), the silt was deposited millions of years after the basement was formed. Exposed basalt near the ridge could still be the source, however, because spreading rates along the East Pacific Ridge were generally slow between 60 and 70 Ma (Smith and Sandwell, 1997) and Site U1366 was likely within several hundred kilometers of the ridge when the silt beds were deposited. Transport of the silt grains to Site U1366 by low-density gravity flow is suspected but cannot be substantiated because the silt beds lack indicators of depositional processes (e.g., graded bedding, cross bedding, flute marks, and so on). Additional information regarding the areal distribution of the silt or paleogeography of Site U1366 is needed before the origin of the silt can be accurately interpreted.

Sedimentary structures

Sedimentary structures at Site U1366 define a continuum from thoroughly massive to thickly bedded sediment. The majority of the pelagic clay in Subunits IA and IB lacks any discernible structure. We attribute this to the uniformity of sediment composition because small burrows (2–4 mm diameter and 1–2 cm length) are evident at the compositional boundary between Subunits IA and IB. The occurrence of manganese nodules in the upper 15 m of core and RSO throughout the sedimentary succession at Site U1366 implies that sedimentation rates at Site U1366 were very low for a protracted period of time (Hein et al., 1979). Therefore, the lack of sedimentary structures in Subunits IA and IB may have resulted from prolonged exposure of uniform sediment to burrowing organisms.

The presence of massive and bedded sediment in Subunit IC provides further insight into biologic activity at Site U1366. The massive sediment found in Subunits IA and IC transitions to bedding in Subunit IC through a gradual transition zone of 10–30 cm. Through this short interval, uniform very dark brown pelagic clay grades into dark brown clay with occasional fine flecks of very pale brown sediment and then into dark brown clay with thin beds of very pale brown sediment (see core photographs of Cores 329-U1366B-1H and U1366B-2H in “Core descriptions”). The very pale sediment comprising the flecks and thin beds is interpreted to be the ash/altered ash lithology previously described. Burrowing impacted the laminations in the lower part of Subunit IC and in the upper half of Unit II; however, the disturbance of this sediment was incomplete and numerous beds were retained in the developing strata. The loss of bedding features in the upper part of Subunit IC could correspond to an increase in biologic activity or a marked decrease in sediment accumulation rates.

Bedding is absent below the porcellaneous clay layers in Unit II. Although the massive texture of this sediment could result from bioturbation, it could be a primary feature of the sediment; 70%–100% of the sediment is RSO. Consequently, the lack of bedding in Unit II could have resulted from active burrowing through sediment that possesses insufficient contrast to record the biologic activity or it could have resulted from deposition of homogeneous sediment in an environment free of active burrowing.

Interhole correlation

  • Lithostratigraphic correlation among the five APC holes at Site U1366 shows differences between the units and subunits of Site U1366. Several of the differences are artificial, created by coring operations. They include

  • Coring operations inadvertently missed 0.8 m of Subunit IA between Core 329-U1366C-1H and 2H,

  • The lower 9.2 m of Core 329-U1366C-3H is flow-in and not in situ sediment, and

  • Whole-round sampling created a large number of gaps in Holes U1366D and U1366F that masked unit boundaries and some beds.

Despite these challenges, we created a tentative correlation of lithostratigraphic units among the holes of Site U1366 (Fig. F8). Noteworthy observations illustrated by the correlation include

  • Subunit IA thickness varies by as much as 2.6 m;

  • Subunit IB varies in thickness by ~0.5 m and contains alternating composition that is not observed in the other holes;

  • Manganese nodules are not evenly distributed by unit or depth; and

  • Porcellaneous clay beds occur in Subunit IC in Holes U1366B–U1366D but in Unit II in Hole U1366F. This difference is an artifact of our lithostratigraphic classification system. Because the clay that envelopes the porcellaneous beds in Holes U1366B–U1366D is zeolitic, whereas the clay that over and underlies the beds in Hole U1366F is metalliferous, the beds are assigned to different units. However, given the short distances that separate holes at Site U1366, the porcellaneous beds are likely continuous across the site.