Lithologic description of Site 1143 was made by visual inspection of the cores, smear-slide analysis, and carbonate content determined by coulometer analysis (for methods, see "Organic Geochemistry" in the "Explanatory Notes" chapter).
Only one lithologic unit has been recognized at Site 1143. Unit I can be divided into two Subunits, IA and IB, based on the carbonate content (Fig. F5). The sediments at Site 1143 represent continuous hemipelagic sedimentation of fine-grained terrigenous material and calcareous nannofossils from the late Miocene to present. The occurrence of minor lithologies in the core (ash layers, turbidite sediments, and green clay layers) varies with depth and also differentiates between Subunits IA and IB (Fig. F6).
Subunit IA comprises the upper part of the sedimentary sequence in Holes 1143A to 1143C and consists of olive-gray, light grayish green, and greenish gray clayey nannofossil mixed sediment, clay with nannofossils, and clay. Bedding is not evident, and compositional changes are gradual. Sediment color changes related to varying carbonate content are observed throughout, with lighter layers having higher carbonate concentration. Carbonate content (nannofossils, foraminifers, and other forms of calcite, organic and/or inorganic), estimated by smear-slide analysis, is ~25%.
An oxidized red-brown clay layer is present in the top 3 cm of Holes 1143A and 1143B, whereas coring in Hole 1143C did not recover the mudline. Continuous to discontinuous layers and mottles of green clay are present throughout Subunit IA (Figs. F6, F7). Mottling is generally diffuse, and bioturbation intensity ranges from rare to common. Homogeneous patches of light yellowish to brownish gray nannofossil clay, sometimes with high concentrations of foraminifers and pyrite, are likely burrow fills.
In addition to the dominant hemipelagic sediment, foraminifer ooze turbidites are noted within this subunit. These sediments are normally graded, often with a scoured basal contact on the underlying clayey sediment.
Several volcanic ash layers, ranging from 1 to 9 cm thick, are observed in Subunit IA (Table T5). These ashes have sharp bases and normal size grading, are unaltered, and are composed of angular glass shards and volcanic minerals. The thicker ashes are often dark colored, suggesting mafic composition, whereas the two youngest ash layers are thin, light-colored, and intensely bioturbated. Irregular spots of volcanic material are also detected in the upper 90 m of Subunit IA. The ash layers in Subunit IA tend to be thinner bedded (1 to 9 cm) than the Subunit IB ashes (1 to >20 cm).
A possible unconformity, related to a sharp change in sediment color and marked by a steeply dipping horizon of black material (Fig. F8), is observed in Section 184-1143A-15H-5, 93-96 cm (133.0 mbsf; 140.0 mcd), and is also identified in FMS imaging (see "Wireline Logging"). A similar horizon, suggesting an erosive boundary and likely the same unconformity, is in Section 184-1143B-15H-3, 107 cm (133.7 mbsf; 142.02 mcd), and in Section 184-1143C-16H-5, 50 cm (146 mbsf; 155.58 mcd). The boundary is horizontal in Holes 1143B and 1143C; thus the dip observed in Hole 1143A may be a drilling artifact.
Subunit IB is characterized by a higher carbonate content and an increase in the frequency of turbidites compared with Subunit IA. Carbonate content increases rapidly below 150.0 mbsf, from ~25 wt% in Subunit IA to 30-45 wt% in Subunit IB. This change in composition is not clearly observed during visual inspection of the core, but smear-slide identification does indicate higher carbonate content compared to Subunit IA.
The dominant lithologies of Subunit IB are clayey nannofossil mixed sediment, nannofossil clay, and nannofossil ooze with clay. The green clay layers are not as frequent as in Subunit IA. Colored patches (presumably biogenic), bioturbation, and mottling are present in Subunit IB, without any clear change in proportion and significance from Subunit IA. Recognized trace fossils include Zoophycos and Chondrites, which are best observed between 240.0 and 512.4 mcd (~231.0-500.0 mbsf). Dark gray volcanic ash layers and volcanic breccias occur throughout this subunit (Fig. F9).
Major sedimentary structures observed in Subunit IB are slumps and turbidites (Table T6; Fig. F10). A 5-m-thick interval containing several slumps is observed in Core 184-1143A-36X from the top of the core to Section 36X-4, 50 cm (322.5 mbsf; 333.1 mcd). A 6.5-m-thick foraminifer sand turbidite is observed in intervals 184-1143A-37X-3, 25 cm, to 38X-1, 38 cm (336.8 mbsf; 348.4 mcd), and 184-1143C-36X-5 to 37X-2, 12 cm (336.0 mbsf; 348.4 mcd), but is only 4 m thick in this core. The upper part of this interval is composed of extremely homogeneous and structureless nannofossil clay and corresponds to the hemipelagic fine-grained E interval of the Bouma sequence (Bouma, 1962). The sediments at the base of the turbidite layer are more or less homogeneous foraminifer silt and sand. Flaser, oblique, and low-angle cross-lamination and normal grading are all recognized and testify to the current-dominated environment of deposition.
At the bottom of Holes 1143A and 1143C, where the sediment is more consolidated, offsets of laminations and burrows by microfaults are noted. The microfaults show an inclination of 40°-45° to the horizontal of the core. Motion is dominantly dip slip. A calcite-filled slickenside, observed in Section 184-1143A-44X-2, 76 cm (396.16 mbsf; 409.22 mcd), plunging at 10°-15°, indicates a sinistral component of displacement.
Dark brown to brownish black clay layers, characterized by high concentrations of siliceous material such as spicules, are observed in the lower part of Hole 1143C, below 430.0 mbsf (442.4 mcd). These layers often have parallel laminations and Chondrites traces at their top.
The green layers are concentrated in the top 180 m of sediment, although they persist throughout the core. Smear-slide analysis indicates a nearly complete lack of calcareous and siliceous microfossils within these intervals, and a green mineral that resembles glauconite is observed under the microscope. However, X-ray diffraction analysis of bulk sediment from the green clay failed to confirm the presence of glauconite. The green layers are often discontinuous because of bioturbation and frequently occur as burrow fill and mottled intervals.
The origin of these green layers is enigmatic. Three formation schemes are possible: direct, rapid deposition of clay; alteration of volcanic ash layers; and dissolution and modification of the original nannofossil clay. One mechanism for the rapid deposition of clay is distal turbidites. However, this mechanism would likely also transport calcareous and siliceous microfossils to the site. The second alternative, alteration of volcanic ash layers, seems unlikely because unaltered ash layers are preserved within the nannofossil clay sequence throughout the entire section. Also, the frequency of the green intervals (the maximum approaches 100 green clay layers per core) would require extensive and sustained volcanism. Previous studies of "green layers" that are attributed to the alteration of volcanic ash (Lind et al., 1993; Gardner et al., 1986) describe a very different sedimentary feature: millimeter-scale color bands that overprint the surrounding sedimentary matrix where the color bands are diffuse in nature, that crosscut burrows and sedimentary layering, and that contain the same sediment components, including microfossils, as the surrounding matrix. In contrast, the green clay layers observed at Site 1143 occur as centimeter-scale deposits, are not observed to crosscut or overprint surrounding sedimentary features, are present as burrow fill and discontinuous layers (implying deposition or formation in the surface or near surface of the sediment column), and lack the calcareous microfossils of the surrounding nannofossil clay. The third hypothesis, selective dissolution of the original sedimentary matrix, could be possible under reducing conditions, perhaps caused by periods of intensive primary production in the region. However, no other sedimentary evidence indicates more intensive primary productivity. The origin of these green clay layers can only be determined by further analytical work.
Turbidites are common features at Site 1143. They can easily be correlated between the three holes on the basis of depth, thickness, and composition. Only four Bouma intervals, A (34 occurrences), B (30 occurrences), C (10 occurrences), and D/E (15 occurrences) (Bouma, 1962) were recognized. The A and B intervals are typically composed of foraminifer sands, with an occasional volcanic fraction. Since foraminifers have a low settling velocity because of their shape and low density, the turbidity currents did not have major erosional effect on the underlying sediment, as suggested by the lack of rip-up clasts. The basal turbidite sediments are often structureless or may display graded bedding or upper flow regime parallel laminations. The C intervals display different kinds of rippled beds, including flaser and contorted bedding. They can also be associated with syndepositional structures such as small-scale slumps, contorted bedding, water-escape structures, or soft-sediment faulting. Their composition mainly consists of silt-sized foraminifers with minor detrital input. The D/E intervals are difficult to differentiate here because parallel bedding could not be recognized, but the lack of bioturbation suggests episodic deposition.
The highest carbonate content noted at Site 1143 (84 wt%) is present in the coarse intervals of the calcareous turbidites (see "Organic Geochemistry"). The planktonic/benthic foraminifer ratio is lower in these layers than in the hemipelagic sediment (see "Biostratigraphy"), suggesting that they may have been transported from shallower depths.
Between 226 and 500 mbsf in Hole 1143C, the combined thickness of turbidites reaches 27.96 m, ~10% of the total section. This is a minimum estimate as some turbidites may have been obliterated by burrowing. This episodic sedimentation contributes to the increase in sedimentation rate noted in the lowest part of Site 1143 (see "Sedimentation and Accumulation Rates"). The decrease in calcareous turbidites and slumping upsection may suggest a decreasing paleoslope at Site 1143 over the period represented by the cored interval.