At Site 1230 we recovered a 278.3-m-thick sequence of biogenic and siliciclastic Holocene, Pleistocene, and Miocene sediments from the lower slope of the Peru Trench. The proposed lithostratigraphy is mainly based on Hole 1230A, as it was the only hole at Site 1230 that was continuously cored from the sediment surface to a depth of 278.3 mbsf. The lithostratigraphic description of the sedimentary sequence is based on observation of sediment color, sedimentary structures, and structural features (visual core description), smear slide analysis, and color reflectance. X-ray diffraction (XRD) and measurements of magnetic susceptibility, density, and natural gamma radiation (NGR) were also used to detect lithologic and sedimentologic changes (see "Physical Properties" and "Downhole Logging").
Based on textural descriptions and wireline log data (see "Downhole Logging"), sediments from Site 1230 were subdivided into two lithostratigraphic units (Fig. F1). As Site 1230 is located in close proximity (within 100 m) of Leg 112 Site 685, the age framework presented in this chapter follows the chronostratigraphic observations of the Leg 112 Shipboard Scientific Party (1988).
Unit I consists mostly of biogenic sediments mixed with varying amounts of siliciclastic components. Even though diatom ooze is the dominant component of Unit I, differences in color and in the relative amount of the terrigenous components (mostly quartz) justified further subdivision into four subunits.
Throughout Unit I, several high-angle normal faults with offsets of a few centimeters were observed (Fig. F2A). The first faults are present at shallow depth in Subunit IA (e.g., Section 201-1230B-5H-5) and are common throughout the rest of Unit I. Immediately after splitting, some of the sediments from Subunits IA and IB (e.g., Section 201-1230B-7H-6) were black to dark gray in color. However, these colors changed to lighter olive-green hues in <30 min. The color change is presumably caused by the oxidation of iron monosulfide on the cut surface of the split core. Pyrite was commonly found at lithologic boundaries, in faults (Fig. F2A, F2B), in veins, and in spots and streaks, which often outlined Chondrites-type trace fossils.
The main lithology of Subunit IA is olive-green spicule- and quartz-bearing clay-rich diatom ooze with variable amounts of quartz, feldspar (both usually <10%), nannofossils (usually <5%), and rare foraminifers. Scattered white sponge spicule aggregates, with diameters ranging from a few millimeters to 1 cm, as well as pyrite specks and streaks (e.g., Cores 201-1230A-2H and 5H), are common features of this subunit. In many parts of Subunit IA, the main lithology alternates with layers of different color and composition. The most common alternations occur with dark olive to brown, several-centimeter-thick layers, which usually are composed of diatom-rich nannofossil ooze. The mineralogic composition of these nannofossil layers includes mostly calcite, with minor amounts of quartz, plagioclase, and pyrite (e.g., XRD Sample 201-1230B-1H-4, 15-16 cm) (see also "Mineralogy"). These layers are commonly bioturbated, have sharp basal contacts, and are commonly outlined by dark sulfide accumulations (see "Mineralogy"). A relatively coarse-grained, ~20-cm-thick horizon composed of several yellow layers of foraminifer-bearing clay- and diatom-rich nannofossil ooze was observed in Sections 201-1230B-1H-4 and 201-1230C-1H-3 (Fig. F2C). The layers in this horizon show an increase in foraminifer content toward the base and are associated with dark gray pyrite-rich horizons. The layers have sharp basal contacts and normal grading indicative of redeposition (Fig. F2C). Pale yellow layers composed of quartz-bearing diatom-, foraminifer-, and clay-rich nannofossil ooze are also present in Core 201-1230A-5H. Brown layers composed of diatom-rich clay were observed at several depths in this subunit. These layers are particularly frequent in Core 201-1230A-4H as well as in Section 201-1230A-6H-6. Bioturbation is weak to moderate throughout the subunit and is mostly of Chondrites type. High-angle lamination, most likely due to soft sediment deformation, is present in Sections 201-1230A-3H-5 through 3H-7 (Fig. F2E).
After splitting, Section 201-1230D-2H-5, which contains the base of the sulfate reduction zone at 10.6 mbsf (see "Biogeochemistry"), showed evidence of degassing. Bubbles continued to form on the split surface of the soft sediment several hours after the core had been retrieved. Below this depth, sulfide-rich specks are common in sediments from Site 1230 (e.g., XRD Sample 201-1230B-7H-1, 144 cm).
The lowermost part of Subunit IA (Sections 201-1230A-7P-1 through 9H-3) is characterized by alternations of olive to dark gray nannofossil- and spicule-bearing clay-rich diatom ooze and darker blackish clay-rich diatom ooze. The lower boundary of Subunit IA corresponds to a sharp decrease in chromaticity values (b*) and matches a positive shift in magnetic susceptibility (Fig. F1) (see also "Physical Properties").
Dark gray to olive, more or less bioturbated clay-rich diatom ooze characterizes the lithology of Subunit IB. Siliciclastic components such as quartz and feldspar generally comprise <10% of the bulk sediment. Diatom content ranges between 50% and 60%. Calcareous nannoplankton is present in trace amounts in the uppermost part of the subunit and disappears below ~79 mbsf (Fig. F1). White spicule aggregates are present but seem to be less frequent compared to Subunit IA. Chondrites-type trace fossils are present in moderate density throughout the subunit. Pyrite specks and streaks are common. High-angle normal faults showing offsets of a few centimeters are also common.
In Hole 1230A, the color of sediments from Subunit IB changes gradually from dark gray at the top (Section 201-1230A-9H-4) to dark olive (Core 13H) and brown at the bottom (Core 15H). This change is also recorded by variations in the chromaticity profile (Fig. F1). These gradual color changes coincide with a steady increase in compaction and "stiffness" of the sediment, which also shows pervasive foliation parallel or subparallel to bedding.
Consolidated, fissile, dark gray to black pyrite-bearing diatom-rich silty clay is the dominant lithology of Subunit IC. A shift toward relatively low values of the chromaticity variable (b*) occurs at the top of this subunit (Fig. F1). The shift is inferred to be due to a relative increase in the siliciclastic fraction, as quartz content is higher than in both the over- and underlying subunits. Although diatom abundance is lower than in Subunit IB, it seems to increase toward the bottom of the subunit (e.g., Section 201-1230A-18H-3, 123 cm). Some portions are characterized by faint banding resulting from decimeter-scale alternation between darker and lighter layers. Bioturbation is moderate to high throughout and is mostly of Chondrites type.
The boundary between Subunits IC and ID is defined by a change in sediment color from dark gray to greenish brown and dark brown. This color change is reflected by a moderate shift of the chromaticity variable b* (Fig. F1). The sediment of this subunit is well consolidated and fissile clay-rich diatom ooze. Fissility is usually pervasive, and in some cases (e.g., Core 201-1230A-19H) is oblique to bedding (Fig. F2F). In the uppermost part of the subunit (Core 201-1230A-19H), dark brown clay-rich intervals alternate with lighter-colored olive diatom ooze. In general, the former appear to be less fractured and more ductile than the latter.
Poor core recovery and strong drilling disturbance characterized a large portion of Hole 1230A, including most of Subunit ID (recovery = 23%) and the underlying Unit II. According to biostratigraphic data presented in the Leg 112 Initial Reports volume (Shipboard Scientific Party, 1988), a major hiatus in the biostratigraphic record is present between ~200 and 203 mbsf at Site 685. This hiatus (with a minimum duration of 4.3 m.y.) separates lower Pleistocene slope deposits (Unit I) from accreted upper Miocene sediments (Unit II). Wireline logging data, in particular P-wave velocity, resistivity, and bulk density, show a prominent shift between 215 and 216 mbsf for Site 1230 (see "Downhole Logging"). These data indicate the presence of a fractured zone, which may represent an unconformable tectonized contact between Units I and II. Accordingly, we established the boundary between Units I and II at the core break between Cores 201-1230A-27H and 28H at 215.8 mbsf (Fig. F1) (see also "Unit II").
Most of Unit II is characterized by both poor recovery and core disturbance. The few sections recovered from the uppermost three cores of this unit (Sections 201-1230A-28X-1 and 30X-1) are characterized by a pervasive scaly fabric (Fig. F2G), which offers further support to the presence of a tectonic boundary between Units I and II. It is also worth noting that several layers of dolomite breccia are present in both Section 201-1230A-30X-1 and the underlying Section 31X-1 (Fig. F2H) (see also "Mineralogy"). Sandy fabric is less pervasive between Sections 201-1230A-31X-1 (Fig. F2I) and Section 35X-2, where the sediment includes a few scattered dolostone nodules and is a consolidated and fissile, commonly bioturbated clay- and quartz-rich diatom ooze (XRD Sample 201-1230A-31X-1, 93-94 cm). Scaly fabric, possibly related to tectonic fracturing, was also observed in the upper part of Section 201-1230A-35X-4. Below this fractured layer, several carbonate concretions are present within relatively undisturbed sediment composed of quartz-bearing dolomite-rich clay.
In general, lighter layers with higher nannofossil concentrations and disseminated carbonate rhombohedra are more common below ~235 mbsf (Fig. F1). Usually, diatom- and silt-rich intervals have a dry, semiconsolidated appearance, whereas clay-rich layers are smoother, with a more homogeneous texture.
A total of 21 XRD analyses were performed on samples collected from both Holes 1230A and 1230B. Overall, quartz was the primary mineral detected; it was detected together with plagioclase in all analyzed samples. The highest concentrations of quartz were recorded in the highly consolidated and fractured dark silt- and clay-rich diatom oozes of Unit II (Samples 201-1230A-31X-1, 93-94 cm; 33X-2, 14 cm; and 33X-CC, 14 cm). Plagioclase seems to be more concentrated in the brown nannofossil ooze layers of Subunit IA (e.g., Sample 201-1230B-1H-4, 15-16 cm). Glauconite, pyrite, and clay are also common minerals at this site; however, they are only present in small or trace amounts. In Subunits IA and IB, dark pyrite concentrations commonly outline boundaries between alternating olive diatom ooze and brown nannofossil ooze layers (Fig. F2C). Pyrite is also a common mineral in the consolidated, fissile, and fractured sediments of Unit II (e.g., Sample 201-1230A-33X-CC, 16 cm). Glauconite is present in most of the analyzed samples, and the highest concentrations of this mineral were detected in the quartz-rich samples of Unit II (e.g., Sample 201-1230A-31X-1, 93-94 cm). The amount of calcite is generally related to the presence of nannofossils in the biogenic assemblage. In Unit I, calcite was detected only in nannofossil-rich layers (e.g., Sample 201-1230B-1H-4, 15-16 cm) (Fig. F2C; interval between 15 and 16 cm). In the nannofossil-rich sediments of Unit II, calcite was found in most of the analyzed samples (e.g., Sample 201-1230A-35X-5, 68-68 cm). A dolomite layer was found in Unit II (Sample 201-1230A-31X-1, 0-18 cm). The latter is probably a cemented breccia, which is present below the tectonized interval that marks the boundary between Units I and II. Even though most of the sediments from Site 1230 are composed of variable amounts of diatom ooze, opal-A could only be detected in the samples from Unit II that had the lowest concentrations of quartz (Samples 201-1230A-30X-1, 93-94 cm; 35X-4, 40 cm; 35X-4, 80 cm; and 35X-5, 68 cm).
The mineralogy of Site 1230 sediments offers some insights on the depositional setting of both Units I and II. The common presence of quartz and plagioclase in most of the analyzed samples indicates that there was a relatively constant input of fine-grained terrigenous components during the deposition of both units. The origin of both calcite and opal-A is clearly biogenic, as the presence of these minerals is linked to nannofossil- and diatom-rich sediments, respectively. The presence of authigenic dolomite at or near the boundary between the Pleistocene to Holocene sediments of Unit I and the Miocene sediments of the accretionary prism (Unit II) might indicate fluid flow and carbonate precipitation along a tectonic surface.
Drilling on the lower slope of the Peru Trench produced a 278.3-m-thick sequence of both biogenic and siliciclastic sediments. The majority of the sedimentary section at Site 1230 consists of predominantly biogenic Holocene and Pleistocene sediments with varying amounts of siliciclastic components (Unit I; 0-215.8 mbsf). Unit I overlies Miocene sediments that probably belong to the accretionary prism of the Peru Trench (Shipboard Scientific Party, 1988). Although diatom ooze is the main component in most of the Unit I sediments, differences in color and in the relative amount of the terrigenous component suggested a further lithostratigraphic subdivision into four subunits. Subunit IA is a green spicule- and quartz-bearing clay-rich diatom ooze interbedded with brown bioturbated diatom and nannofossil ooze layers. The mineralogy of the nannofossil ooze layers is characterized by the presence of calcite, with smaller amounts of quartz, plagioclase, and pyrite. Small offsets resulting from high-angle normal faults were observed at the very top of Subunit IA and are commonly present throughout Unit I. Subunit IB is a dark gray to dark olive clay-rich diatom ooze that is characterized by a gradual increase in both consolidation and foliation of the sediment from top to bottom. Subunit IC consists of a mixed siliciclastic and diatomaceous sediment. Generally, its sediment is consolidated, fissile, and dark gray to black. Subunit ID is characterized by greenish brown to dark brown well-consolidated and very fissile clay-rich diatom ooze. Poor core recovery in the lower part of Site 1230 made it difficult to establish the boundary between Units I and II. However, the boundary was placed at 215.8 mbsf because downhole logging data indicate a distinct break in resistivity and bulk density at that depth, and very fractured layers are present just below that depth. This boundary, which probably represents a tectonized unconformable contact between Pleistocene and Miocene sediments, is also characterized by a hard layer of authigenic dolomite that may be the product of fluid flow along the tectonic surface. Sediments of Unit II are very consolidated and fissile. They often show a pervasive slaty fabric, which accounts for tectonic deformation in an accretionary prism. Their main composition is clay- and quartz-rich diatom ooze.