Site 1091 was drilled to a total depth of 311 mbsf (325 meters composite depth [mcd]). Sediments from this site constitute one lithologic unit, are of Pleistocene and late Pliocene age, and consist of light to olive green diatom-rich ooze, with minor and varying amounts of nannofossils, foraminifers, and mud (Fig. F3). Smear-slide analysis (see the "Core Descriptions" contents list) shows that diatom abundance typically varies from 30% to 90%, total carbonate components (nannofossils + foraminifers) typically range from 0% to 40%, whereas mud exceeds 20% of the total composition only near the base of the section in Cores 177-1091A-30H through 33H. X-ray diffraction (XRD) analysis reveals similar trends in major lithologic components (Fig. F4; Table T1, also in ASCII format in the TABLES directory). Diatom-rich intervals are pronounced between 91 and 262 mcd. About 10 very thin (5-25 cm) and often calcareous-rich beds provide stratigraphic correlation among the three deeper holes (1091A, 1091B, and 1091D) drilled at this site.
Diatoms are the dominant lithologic component at this site, with lesser amounts of nannofossils, foraminifers, and mud (Fig. F4). A distinctive characteristic of the diatom-rich sediments is the presence of diatom mats, marked by a rough spongy surface texture that was observed after scraping the split cores (Fig. F5). The diatom assemblages observed are often dominated by a few species or contain near-monospecific assemblages of Fragilariopsis kerguelensis, Thalassiothrix sp., or Actinocyclus ingens. Although the sediments are classified mainly as diatom oozes and nannofossil-, foraminifer-, and/or mud-bearing diatom oozes, diatom nannofossil oozes occur in several sections, with nannofossil abundance as much as 90%. Total carbonate percentages obtained from smear-slide analysis (typically 0%-40%) are higher than percent CaCO3 determined by coulometry, which is typically less than 20 wt% (see "Geochemistry"). In previous sites drilled during this leg, the carbonate concentrations determined by these two techniques were comparable. The discrepancy at this site may be caused by overestimation of the minor carbonate fraction in smear slides against the background of a dominant opal fraction. Foraminifer abundance obtained from smear-slide analysis in some cases exceeded 50%, and was especially high in the upper intervals (typically 10 m) of all holes. Mud content was uniformly low except in the basal sediments from this site, where mud exceeded 40% of the total composition and sedimentation rates were lower (~30 m/m.y.) than rates in the sediments above (~145 m/m.y.).
A variety of lithostratigraphic features, including paler carbonate and distinctive diatom horizons, were used to develop a sedimentological interhole correlation (Fig. F6). This information complemented multisensor track (MST) data (see "Chronostratigraphy", "Physical Properties") and aided in the preparation of a composite section that, in turn, allowed placing cores within their proper stratigraphic context in terms of the mcd scale (Fig. F6). One lithologic unit was recognized at this site.
Intervals: 177-1091A-1H through 33H (0-310.9 mbsf; 324.61 mcd); 177-1091B-1H through 29H (0-273.8 mbsf; 281.21 mcd); 177-1091D-1H through 22H (0-203.1 mbsf; 218.67 mcd); 177-1091E-1H through 6H (0-51.7 mbsf; 56.30 mcd)
Age: Pleistocene to Pliocene
The unit consists of Pleistocene and upper Pliocene medium green to olive diatom ooze, nannofossil-, foraminifer-, and mud-bearing diatom ooze, and diatom nannofossil ooze. Several intervals are essentially carbonate free, with the exception of the discrete carbonate-ooze horizons indicated on Figure F3 and several of the thin marker beds (Fig. F6). Carbonate is a minor but significant component of the sediment in intervals 0-9, 67-78, 111-140, 165-175, and 206-215 mbsf (all depths correspond to Hole 1091A; Fig. F3). Intervals of intermittently laminated diatom mats occur between 85 and 210 mbsf (Fig. F3), with minor layers above and below this interval. Color mottling is present throughout the unit and is manifested as brighter green and tan irregular layers, but specific ichnofossils are rare. Millimeter-scale laminations are intermittent, are purple and white in color with various textures (Fig. F7), and in many cases are present below diatom-mat layers. Small dropstones (1-2 cm) are scattered throughout the cores, and one large dropstone is present in Core 177-1091A-13H (Fig. F8). The bottom three cores of Hole 1091A are uniformly diatom rich and display a darker green color and more extensive bioturbation than the cores above. These cores are also quite mud rich (Fig. F4) and indicate lower sedimentation rates for the late Pliocene section (see "Chronostratigraphy").
A number of distinctive 7- to 34-cm-thick marker beds occur at Site 1091 (Table T2; Fig. F6) and were used for interhole correlation among Holes 1091A, 1091B, 1091D, and 1091E. The beds are of two types: autochthonous carbonate-rich deposits likely related to peak interglacials (see "Chronostratigraphy"), and allochthonous deposits. The allochthonous beds have a number of common features, including marked color and textural differences with the surrounding sediment (including lack of bioturbation and absence of diatom mats), sharp lower contacts, and grading with 3- to 10-mm-thick basal layers of foraminifers and up to ~5% well-sorted, very fine-grained sand. Some beds appear to display cross-lamination at the base (Fig. F9), whereas others display little internal structure (Fig. F10). Several of these calcareous layers are enclosed within carbonate-free sediment and some contain reworked older nannofossils (Table T2; see "Chronostratigraphy"). Evidence of sedimentary structures indicating current activity combined with their carbonate contents suggests that these beds represent turbidity-current deposits, perhaps derived from the Meteor Rise to the east of Site 1091. The relatively high carbonate values associated with some of these beds may favor the interpretation of downslope transport of this material by turbidity currents (from above the carbonate compensation depth) rather than lateral transport by contour currents. However, any genetic interpretation of these deposits is ambiguous until more detailed studies of grain-size distribution and geochemistry are performed.