The sedimentary sequence at Site 1226 consists of alternating intervals of nannofossil and diatom ooze, with varying numbers of radiolarians and foraminifers. Accessory minerals such as plagioclase and pyrite are present in minor amounts throughout the cored interval. Two lithostratigraphic units were established at Site 1226 (Fig. F1). Unit I is characterized by decimeter- to meter-scale variations in biogenic silica and carbonate, whereas Unit II is dominated by red, green, and brown metalliferous sediments directly overlying the oceanic basement. Only Hole 1226B was used for lithostratigraphic purposes, as it was the only hole at Site 1226 that was continuously cored from the sediment surface to the oceanic basement. As Site 846, which had previously been drilled during Leg 138, is located in close proximity (within 100 m) of Site 1226, the age framework presented in this chapter follows the chronostratigraphy of Site 846 (Shipboard Scientific Party, 1992) revised according to the timescale of Berggren et al. (1995a, 1995b).
The lithostratigraphic description of the sequence is based on visual observation of sediment color and sedimentary structures, smear slide analysis, and color reflectance. X-ray diffraction (XRD) analyses and laboratory measurements of magnetic susceptibility, density, and water content (see "Physical Properties") were also used to detect lithologic changes.
Unit I contains alternating intervals of diatom-bearing to diatom-rich nannofossil ooze and nannofossil-rich diatom ooze with varying amounts radiolarians, foraminifers, and siliciclastic components. In the deeper part of Unit I below ~370 mbsf, the sediments become progressively consolidated and were classified as chalk or diatomite, depending on their mineralogical composition. The sediments of this unit range in age from Pleistocene to early Miocene (Shipboard Scientific Party, 1992). We divided Unit I into four subunits (IA-ID). Subunit IA consists of Pleistocene pale gray to green nannofossil-rich diatom ooze in the upper part and alternating intervals of diatom-bearing and diatom-rich nannofossil ooze in the lower part of the subunit. The mostly late Pliocene age sediments of Subunit IB are dominated by alternating meter-scale intervals of dark olive-green diatom ooze and pale green diatom-rich nannofossil ooze. Pale greenish gray nannofossil ooze and pale brown to pale olive diatom-rich nannofossil ooze of late Pliocene to late Miocene age are the dominant lithologies of Subunit IC. Subunit ID is characterized by alternating intervals of olive green-gray clay- and radiolarian-bearing nannofossil diatom ooze and white clay- and radiolarian-bearing diatom-rich nannofossil ooze of early to late Miocene age. Incipient lithification in the lower part of this subunit transforms the sediments into diatomite and chalk. The presence of dolomite, porcelanite, and chert provides further evidence for the diagenetic sequence in the carbonate- and silica-rich sediments of Site 1226.
The uppermost part of Subunit IA (Core 201-1226B-1H through Section 3H-1; 0-15.4 mbsf) consists of intensively bioturbated pale gray to green nannofossil-rich diatom ooze. Trace fossils (mostly Zoophycos, Skolithos, and Planolites) as well as open burrows are common (Fig. F2A). The mineralogic assemblage is dominated by varying amounts of biogenic opal-A (siliceous microfossils, mostly diatoms) and calcite (nannofossils and foraminifers) (see "Mineralogy").
Below 15.4 mbsf, the subunit is dominated by alternating intervals of light gray diatom-bearing nannofossil ooze and darker pale green diatom-rich nannofossil ooze. The foraminifer content increases with depth, and a faint gray banding, crosscut by burrows with purple reaction halos, is common. Trace amounts of pyrite were also found (see "Mineralogy"). Color reflectance values are generally high but variable, ranging from ~40% to 65% in the visible bands.
Subunit IB is dominated by alternating meter-scale intervals of dark olive-green diatom ooze and pale green diatom-rich nannofossil ooze. The top of the subunit is marked by a sharp color change from light green to dark olive green, which reflects an overall increase of the diatom component. This increase of biogenic silica (opal-A) in the sediment is also confirmed by XRD analyses (see "Mineralogy"). Color reflectance data show a clear negative excursion toward lower values (35%-45%) caused by the generally lower reflectivity of diatom-rich sediments at Site 1226 (see "Color Reflectance Spectrophotometry"). Color reflectivity returns to higher and more variable values in the lower part of the subunit.
Mottling and bioturbation as well as pale purple alteration halos around burrows dominate the lighter-colored diatom-bearing nannofossil ooze intervals. Some of the trace fossils were recognized as burrows of Zoophycos type. The relative thicknesses of the alternating darker diatom ooze and lighter nannofossil ooze intervals are variable, with the pale brown diatom ooze layers becoming thicker toward the center of the subunit. Minor amounts (<5%) of radiolarians and foraminifers were observed in Cores 201-1226B-12H and 13H, as well as a dark spot containing both pyrite and plagioclase (XRD Sample 201-1226B-12H-7, 53-54 cm) (see "Mineralogy").
As already noted by the Leg 138 Shipboard Scientific Party, at Site 846 the color banding that characterizes the subunit is arranged in cycles ranging from 1 to 10 m in thickness. Based on an average sedimentation rate of ~40 m/m.y. (Shipboard Scientific Party, 1992), the duration of these cycles would range between ~25 and ~250 k.y. and may result from Milankovitch-type orbital forcing of the sedimentary regime during deposition of Subunit 1B.
Within the upper 28 m of Subunit IC (Cores 201-1226B-14H through 17H; 118.4-146.9 mbsf), pale greenish gray nannofossil ooze and pale brown to pale olive diatom-rich nannofossil ooze are the dominating lithologies. Mottling and bioturbation range from moderate to intense and are typically associated with pale purple alteration rims. Pale purple and gray banding is either dominant (e.g., Core 201-1226B-15H) or faint and overprinted by bioturbation (e.g., Core 201-1226B-16H). This banding disappears in darker greenish zones, where diatoms are more abundant.
In the lower 125 m (Cores 201-1226B-18H through 29H; 146.9-271.9 mbsf), the subunit is characterized by pale green to pale yellow nannofossil ooze with scattered olive to gray diatom-rich nannofossil ooze layers showing high opal-A content (e.g., XRD Sample 201-1226B-29H-6, 104-105 cm). Intense to moderate bioturbation and burrows with pale purple, green, and gray reaction halos are common. Bioturbated intervals are often characterized by a mottled texture and sometimes by very long (up to 90 cm) subvertical burrows, probably of Skolithos type (e.g., Core 201-1226B-24H). Pale purple to white horizontal alteration bands are scattered throughout the lower part of Subunit IC and are usually either weakly developed or partially obliterated by bioturbation. Scattered millimeter-sized pyrite nodules were also observed and identified in XRD spectra (Sample 201-1226B-29H-7, 56-57 cm).
The main lithology in the uppermost part of Subunit ID (Cores 201-1226B-30X through 34X; 271.9-314.7 mbsf) is characterized by alternating intervals of olive green-gray clay- and radiolarian-bearing nannofossil diatom ooze (of mixed opal-A and calcite composition) and white clay- and radiolarian-bearing diatom-rich nannofossil ooze. Low color reflectance values are common (Fig. F1) because of a marked increase of the diatom component. Yellow clasts of dolomite (Fig. F2D) and dark specks of pyrite, one of which contained trace amounts of barite (Sample 201-1226B-30X-7, 36-38 cm), were also observed.
In the central part of Subunit ID (Cores 201-1226B-34X through 40X; 314.7-371.1 mbsf), the main lithology consists of pale greenish gray and yellow diatom-bearing nannofossil ooze. Variable amounts of clay, plagioclase, and volcanic glass are also present. The sediments are finely laminated and slightly to moderately bioturbated.
The lowest part of Subunit ID (Cores 201-1226B-41X through 45X; 371.1-400.2 mbsf), is marked by a few brown brittle quartz layers (Sample 201-1226B-41X-1, 2-3 cm) (Figs. F2C, F3D) that are present between 371 and 398 mbsf and correlate with those found at Site 846 between 372 and 398 mbsf (Shipboard Scientific Party, 1992). About 10 m deeper in the section, green porcelanite composed of opal-CT was found (Sample 201-1226B-44X-CC, 9-10 cm). The lowest part of Subunit ID also coincides with the first appearance of semilithified sediments of both siliceous (diatomite) and calcareous composition (chalk).
Unit II consists of pale green (upper part) and light reddish brown to brown and green (lower part) foraminifer- and diatom-bearing clay-rich nannofossil chalk that directly overlies the oceanic basement (Fig. F2E). Black spots and layers are common. The transition between the green upper part and the brown lower part is present at ~405 mbsf (Sections 201-1226B-46X-4 and 201-1226E-24X-4). The reddish color is caused by the presence of hematite (Sample 201-1226B-47X-2, 53-54 cm) (Fig. F3F), whereas the black color represents either chert or manganese oxide minerals. Bioturbation is generally moderate. In Sections 201-1226B-46X-4 and 201-1226E-24X-4, a green band is present within the light reddish brown sediments at a depth of ~413 mbsf. The top of the band is truncated by a sharp contact, and the layer above is a ~5-cm-thick graded bed that contains reworked sediments from below the contact (Fig. F2D, F2E). Near the basalt/sediment interface, a few normal graded beds as well as reversed graded beds were observed. These beds are a few centimeters thick and are composed of angular to subangular granule-, sand-, and silt-sized clasts. The larger clasts in these beds are either fragments of black chert or dolomite, whereas silt is mostly volcanic ash. Below 420.6 mbsf, 80 cm of feldspar-rich basaltic breccia was recovered (Fig. F2G).
Spectrophotometric analyses of Hole 1126B samples provided a detailed and mostly continuous record of color variations in the sediments of Site 1226. Color reflectance data greatly improved lithostratigraphic reconnaissance and allowed correlations and comparisons between lithologic changes and variations in other parameters of the sediment, such as magnetic susceptibility, physical properties, and gamma radiation.
In particular, variations in lightness (L*) match both primary and secondary lithostratigraphic divisions, as shown in Figure F1. Lightness is usually proportional to the relative amount of nannoplankton ooze (usually pale green) and is inversely proportional to the amount of diatom ooze (usually pale brown/olive to brown/olive).
Three negative excursions can be recognized in the lightness curve of Figure F1. The first two excursions correspond to stratigraphic intervals dominated by diatom oozes. The third low-reflectivity interval is present below 400 mbsf in a nannofossil ooze interval that is partially red, brown, and black because of the concentration of hematite and possibly magnetite in the sediment (see "Unit II" in "Description of Lithostratigraphic Units" and "Mineralogy").
The two low-reflectivity intervals located between ~50 and 70 mbsf and between ~270 and 320 mbsf can be also correlated with variations in other parameters of the sedimentary record. For instance, they both match the two main peaks of total organic carbon reported for Site 846 (Shipboard Scientific Party, 1992). Also, lightness variations match well the variations in some of the physical properties of the sediment column (see "Physical Properties"); they are proportional to variations in density and inversely proportional to variations of porosity and water content. Therefore, darker diatom ooze layers at Site 1226 are generally more porous and less dense than the light-colored nannofossil ooze sediments. It is also worth noting that the first negative excursion of the L* curve (located between ~50 and 70 mbsf) matches the only main positive peak of the natural gamma radiation curve (see "Physical Properties").
XRD was performed on 25 samples from Hole 1226B and 11 samples from Hole 1226E. Overall, the two main sediment components detected are calcite and opal-A (Fig. F3A). Their relative proportion reflects the abundance of nannofossils and diatoms, the two main biogenic components of the sediment. Biogenic opal-A silica is particularly abundant in Subunit IB, at the bottom of Subunit IC, and at the top of Subunit ID.
Pyrite was detected in most of the sediments of Subunits IA, IB, and IC. However, the highest concentrations of pyrite were found on top of Subunit ID (Samples 201-1226B-30X-7, 36-37 cm, and 31X-5, 120-121 cm), together with trace amounts of barite (Sample 201-1226B-30X-7, 36-37 cm) (Fig. F3B). Dolomite was detected as yellow nodules in Subunit ID (Samples 201-1226B-33X-3, 125-126 cm, and 36X-1, 105-106 cm) (Fig. F3C).
Both opal-CT and quartz, two diagenetic products of biogenic opal-A, were found in the lowermost part of Hole 1226B. Quartz composes the brittle brown chert layers that are present in the lowermost part of Subunit ID and Unit II below 371 mbsf (Samples 201-1226B-41X-1, 2-3 cm; 44X-CC, 9-10 cm; and 47X-1, 7-8 cm) (Fig. F3D). Opal-CT was found in green porcelanite layers at a depth of ~380 mbsf (Sample 201-1226B-43X-1, 19-20 cm) (Fig. F3E), below the depth of the first occurrence of chert layers (Fig. F1). The presence of opal-CT below chert indicates anomalous conditions during silica diagenesis at Site 1226. The mineralogic assemblage of an orange-red sample collected from the lowermost part of Unit II (Sample 201-1226B-47X-2, 53-54 cm) (Fig. F3F) includes hematite, clay minerals (illite/smectite), and dolomite.
Light gray basalt was recovered from the core catcher of Core 201-1226B-47X. A small rounded fragment was archived, and a thin section (see "Site 1226 Thin Sections") and bulk geochemical analysis was acquired immediately postcruise. In thin section, the sample has abundant fresh plumose to sheaf quench-textured plagioclase. The ratio of quench-textured plagioclase to altered groundmass glass varies from plagioclase much more abundant than groundmass to the opposite across the 2 cm of the section, indicating the sample was taken from near the margin of a flow. Calcite is the most common alteration phase, filling vesicles and fractures with less abundant smectite. The major element oxide bulk chemistry of this sample (Table T2) is average East Pacific Rise (EPR) mid-ocean-ridge basalt (MORB) and is virtually identical to the composition reported for Sample 138-846B-45X from the basement sampled at this location during Leg 138. Trace element abundances are also consistent with normal EPR MORB compositions.
At Site 1226, a nearly complete Pleistocene to early Miocene section of alternating intervals of nannofossil-rich diatom ooze and diatom-rich nannofossil ooze with varying amounts of foraminifers and radiolarians was recovered. The thickness of the sedimentary section at this site is ~420 m. This depth also marks the contact between the sediments and the oceanic basement.
The main sedimentological characteristic of the sediments is variation in abundance of biogenic carbonate (mostly nannofossils) and silica (mostly diatoms) at a scale of decimeters to several meters. Leg 138 studies inferred that this variation might be the result of Milankovitch-type orbital forcing during deposition of the sediments at Site 1226 (Shipboard Scientific Party, 1992). Two sedimentary units were distinguished. Unit I is characterized by alternating intervals of diatom-bearing to diatom-rich nannofossil ooze and nannofossil-rich diatom ooze with varying amounts of radiolarians, foraminifers, and siliciclastic components. In the deeper part of Unit I, the sediments become progressively consolidated and were classified as chalk and diatomite. Within Unit I, four subunits were distinguished (Subunits IA-ID). Boundaries between these subunits coincide with major breaks in color reflectance and changes in most physical properties of the sediments (see "Physical Properties"). The presence of dolomite, porcelanite, and chert layers below 300 mbsf documents the onset of both carbonate and silica diagenesis. The presence of porcelanite below the depth of the first occurrence of chert layers indicates a somewhat anomalous diagenetic sequence for biogenic silica at Site 1226.
Unit II consists of ~20 m of light reddish brown and green metalliferous nannofossil chalk, which directly overlies the oceanic basement. Manganese- and iron-rich minerals, volcanic glass, and reworked horizons are present above plagioclase-rich basaltic basement.