At Site 1231, we recovered a 121.9-m-thick Holocene through late Eocene age sequence of mostly biogenic and clay sediments from the Peru Basin. The proposed lithostratigraphy is based on three holes that penetrated the entire sedimentary section and reached the top of the oceanic basement (Holes 1231B, 1231D, and 1231E). Based on observations of lithology, sediment color, textures (visual core description), smear slide analysis, and color reflectance, sediments from Site 1231 were subdivided into three lithostratigraphic units (Fig. F1). X-ray diffraction (XRD) and measurements of magnetic susceptibility, density, and natural gamma radiation (see "Physical Properties") were also used to detect lithologic and sedimentologic changes. As Site 1231 is located in close proximity (within 100 m of the reported coordinates) of Leg 34 Site 321, the age framework presented in this chapter follows the chronostratigraphic observations of the Leg 34 Shipboard Scientific Party (Shipboard Scientific Party, 1976) and the calcareous nannofossil biostratigraphy for Site 321 proposed by Blechschmidt (1976) and modified after Berggren et al. (1995).
Unit I consists of gray-brown to pale yellow clay-rich diatom ooze and diatom-rich clay with minor amounts of radiolarians, sponge spicules, and trace amounts of quartz, plagioclase, glauconite, and pyrite (e.g., XRD Sample 201-1231B-2H-6, 45-46 cm) (see "Mineralogy"). The uppermost 60-70 cm is red to brown and is characterized by slightly darker laminae (Fig. F2A). Large black sulfide accumulations are concentrated around 1.0 mbsf (Fig. F2B). Below 0.6 mbsf, sediments of Unit I are generally gray to pale gray and show weak banding either due to the presence of green to gray layers or dark gray to black pyrite-rich laminae. Burrows are common from 0 to 12 mbsf; many of them are open and some are outlined by dark alteration rims. Between 11 and 12 mbsf (Sections 201-1231B-2H-6 and 201-1231D-2H-3), the color of the sediment changes from gray to greenish gray and the sediments are richer in siliciclastic material. The average composition is diatom- and silt-rich clay. This change is also highlighted by negative shifts in both chromaticity (Fig. F1) and magnetic susceptibility (see "Physical Properties"). Below this transition, dark pyrite-rich zones are more abundant and are present concentrated in distinct layers and around burrows (Fig. F2C). The bottom of Unit I is marked by a sharp and prominent change in sediment color from orange-yellow to brown. This change in sediment color coincides with a shift in color reflectance (Fig. F1) and a positive excursion toward higher natural gamma radiation values (see "Physical Properties"). It is worth noting that at about the same depth the iron content of interstitial waters drops very to close to zero (see "Biogeochemistry").
Sediments of Unit II are characterized by relatively higher amounts of clay and small to moderate amounts of volcanic glass. Biogenic components such as diatoms, radiolarians, and foraminifers are rare or absent. The dominant color of the upper 13 m of this unit is brown-orange to dark yellow, whereas the lower 11 m of the unit is brown to dark brown. This color change coincides with compositional variations of the sediment as well as with shifts in some of the physical properties (magnetic susceptibility, natural gamma radiation, and grain density; see "Physical Properties"). These variations caused us to subdivide Unit II into two subunits.
Subunit IIA consists of pale brown to pale pink-orange quartz-, diatom-, and radiolarian-bearing volcanic glass-rich clay. This main lithology alternates with 10- to 40-cm-thick darker layers with more abundant sulfide minerals. The abundance of trace fossils varies from low to moderate and is generally higher in the darker intervals (Fig. F2D). The upper boundary of this subunit is marked by a color change from green to pale brown. This color change coincides with a positive shift in chromaticity values and natural gamma radiation. The transition to Subunit IIB includes a drastic shift of sediment color and a minor compositional change toward zeolite- and volcanic glass-rich clay.
The top of Subunit IIB is marked by a change in sediment color from pale orange (Subunit IIA) to dark brown. This change corresponds to a major color reflectance shift (Figs. F1, F2D). This subunit also includes a portion of the section characterized by increased grain density, magnetic susceptibility, and natural gamma radiation values (see also "Physical Properties"). The main lithology of Subunit IIB is dark brown volcanic glass-rich clay with variable amounts of zeolite (e.g., XRD Sample 201-1231B-7H-1, 60-61 cm) (see "Mineralogy") and trace amounts of nannofossils. Signs of bioturbation are visible in the form of several burrows composed of nannofossil-rich clay. This nannofossil-rich clay was most likely transported from the overlying subunit by biological activity. The bottom of Subunit IIB has a sharp contact with the white nannofossil ooze of Unit III and corresponds to a major shift in color reflectance (Fig. F1). This boundary also coincides with decreases in magnetic susceptibility, natural gamma radiation, and porosity, as well as increases in P-wave velocity, thermal conductivity, and bulk density (see also "Physical Properties"). The depth of this boundary is also in close proximity to a small shift of interstitial water sulfate concentrations (see "Biogeochemistry").
Unit III consists of white to dark brown nannofossil ooze with varying amounts of foraminifers, manganese and iron oxides, volcanic glass, calcite grains, and zeolites. Sediment color changes with increasing depth. In general, darker hues are more common in proximity to the basement. As clay is only present in minor amounts, color changes from whitish to brownish hues are attributed to varying amounts of oxide minerals. These color changes, most of which can be correlated to changes in physical properties, justified subdivision into four subunits (Fig. F1).
Sediments in Subunit IIIA are cream to white and are composed of almost pure nannofossil ooze (upper section of Fig. F2E). The nannofossil assemblage is strongly dominated by Discoaster sp. No primary sedimentary textures, such as laminations or burrows, were observed. However, the homogeneous aspect of the sediment might be the effect of pervasive bioturbation. The upper and lower boundaries of the subunit coincide with increases in grain density, bulk density, and P-wave velocity. They can also be correlated with negative excursions in porosity and natural gamma radiation (see "Physical Properties"). Unlike the sediments from the units above, X-ray diffractograms from Subunit IIIA show high intensities, which indicate high concentrations of mineral phases (calcite) and a low abundance of amorphous material (e.g., XRD Sample 201-1231B-8H-4, 60-61 cm) (see also "Mineralogy"). The lower boundary of Subunit IIIA corresponds to an abrupt change in color (Fig. F2E), and it also coincides with the depth interval at which the interstitial water manganese approaches concentrations near zero (see "Biogeochemistry").
Subunit IIIB is ~5 m thick and consists of dark brown diatom- and clay-bearing nannofossil ooze that contains variable amounts of opaque minerals and zeolites (e.g., XRD Sample 201-1231B-8H-5, 60-61 cm) (Fig. F2E). Both the top and the bottom of this subunit correspond to prominent shifts in magnetic susceptibility and natural gamma radiation (see "Physical Properties"). A dark sulfide layer is present a few centimeters below the upper boundary (Fig. F2E). The sediment is characterized by alternating brown and dark brown layers that show signs of moderate to strong bioturbation (mostly of Planolites type). Both the observed color change and the presence of a sulfide layer near the top of this subunit may be related to a drop in interstitial water manganese concentrations (see "Biogeochemistry").
The dominant lithology of Subunit IIIC is orange to pale yellow nannofossil ooze with variable amounts of foraminifers and opaque or semiopaque minerals. Abundance of trace fossils is low to moderate. The only notable textural features of these otherwise homogeneous sediments are scattered lighter- and darker-colored bands present throughout the subunit. However, as already noted in Subunit IIIA, the homogeneous appearance of the sediments may be the result of intensive bioturbation. In all holes, the lower boundary of this subunit is not a sharp contact but rather a gradual transition toward darker red and brown sediments.
The last 5 to 7 m of sediment directly above oceanic basement is characterized by red to very dark brown nannofossil ooze of mostly calcitic composition (e.g., XRD Sample 201-1231B-13H-3, 132-133 cm). Sediments contain up to 5% of dark brown semiopaque mineral grains of unidentified composition (presumably oxides of manganese and iron). Alternation between darker and lighter layers is the only common sedimentary feature for this subunit. The basalt/sediment contact was encountered at 114.4 mbsf in Holes 1231B and 1231D and at 119.1 mbsf in Hole 1231E. Fragments of basalt were recovered in Holes 1231B (Section 201-1231B-14X-CC), 1231D (Section 201-1231D-13X-CC), and 1231E (Section 201-1231E-14H-3) and were completely consumed by microbiological sampling.
XRD mineralogic analyses were performed on 27 samples collected from the three units recognized at Site 1231. Seven main mineral species were found: calcite, plagioclase, quartz, glauconite, clay minerals, pyrite, opal-A, and zeolite (philipsite). In general, their distribution and relative amounts in the mineralogic assemblage reflects the lithologic variations in Site 1231 sediments.
XRD diffractograms from Unit I, and in particular from Unit II, are characterized by very low intensities, which are caused by the presence of large percentages of nondetectable amorphous components in the sediment (e.g., volcanic glass). Low to trace amounts of quartz, opal-A, pyrite, glauconite, plagioclase, and clay minerals compose the mineralogic assemblage of most of the samples from these two units. In Unit I, quartz and opal-A are slightly more abundant. Most of the samples collected from Unit II contain zeolites, which appear to be more abundant in Subunit IIB, where they can comprise up to 15% of the sediment. Zeolite mineral grains commonly show twinning and have a cloudy appearance, which causes almost isotropic behavior under cross-polarized light. In the same subunit, volcanic glass, most likely the precursor for the zeolites, is present in concentrations as high as 5%.
Calcite was the main and, in most samples, the only detectable mineral in Unit III. XRD intensities from this unit are much stronger than in Units I and II, thus suggesting a higher abundance of crystalline phases and a much smaller percentage of amorphous components. Calcite, which is clearly of biogenic origin, shows higher XRD intensities in the white and orange nannofossil oozes of Subunits IIIA and IIIC. In contrast, calcite is less abundant in the brown sediments of Subunit IIIB and is present mixed with quartz, plagioclase, and zeolites.
Reddish brown basalt was recovered in Section 201-1231B-14X-1, and a small piece was archived for postcruise description (see "Site 1231 Thin Sections") and geochemical analysis. In thin section, this sample is much different than the basalt recovered from Site 1226 in that there is no olivine and only rare plagioclase phenocrysts. In addition, iron-titanium oxide in the groundmass is much more abundant in the sample from Site 1231. Clinopyroxene microphenocrysts are rare but are present as glomerocrysts with small plagioclase laths. Smectite is the most common alteration phase filling vesicles and microfractures, with rare calcite and some palagonite. Compositionally, this sample is a iron-titanium basalt (Table T2) with a similar composition to the average reported composition for samples from DSDP Site 321 (Bunch and LaBorde, 1976).
At Peru Basin Site 1231, a ~120-m-thick section of Pleistocene through late Eocene age mostly biogenic and clayey sediments was divided into three lithostratigraphic units. Unit I consists of Pleistocene to Miocene gray and brown to pale yellow clay-rich diatom ooze and diatom-rich clay with minor amounts of radiolarians, sponge spicules, and trace amounts of quartz, plagioclase, glauconite, and pyrite. Unit II (late Oligocene to Miocene) is characterized by clay-rich sediments poor in biogenic components. Subunit IIA is composed of pale brown to pale pink-orange variably bioturbated, quartz-, diatom-, and radiolarian-bearing volcanic glass-rich clays. Subunit IIB consists of dark brown volcanic glass-rich clay with variable amounts of zeolite and trace amounts of nannofossils. Unit III (late Oligocene to late Eocene), which directly overlies the oceanic basement, is composed of nannofossil ooze. Within this unit, four subunits were recognized. Subunit IIIA is composed of cream to white nannofossil ooze. Subunit IIIB is a brown to dark brown nannofossil ooze. Subunit IIIC is orange to pale yellow nannofossil ooze with variable amounts of foraminifers. Subunit IIID is composed of red to dark brown nannofossil ooze rich in foraminifers and opaque minerals, which directly overlies the oceanic basement.
Seven mineral groups characterize the mineralogic assemblage of Site 1231 sediments: calcite, plagioclase, quartz, glauconite, clay minerals, pyrite, opal-A, and zeolite (phillipsite). XRD diffractograms from Unit I, and in particular from Unit II, are characterized by very low intensities caused by the presence of large percentages of nondetectable amorphous components. Most samples collected from Unit II contain zeolite. Calcite was the dominant mineral in all samples collected from Unit III.