This section outlines the procedures followed to document the sedimentology of the cores recovered during ODP Leg 175, including core description, X-ray diffraction, color spectrophotometry, and smear-slide preparation. General procedures are outlined, and any significant departures from ODP conventions are noted, below.
The visual description of each core was entered directly into the AppleCORE (v 0.7.5g) software, which generates a one-page graphical description ("barrel sheet") of each core. Barrel sheets are presented with whole-core photographs in Section 4 (this volume).
The lithology of the recovered material is represented on barrel sheets by a column entitled "Graphic Lithology" (Fig. 3), and overall summaries of each site are provided in the "Lithostratigraphy" section of each site report (this volume). The sediments recovered from the Benguela Current system generally contain microscopic biogenic particles (e.g., nannofossils, foraminifers, diatoms, radiolarians, silicoflagellates, and sponge spicules) dispersed in clays and carbonate oozes of variable texture. Grain-size divisions for sand, silt, and clay are those of Wentworth (1922). Sediment with a biogenic fraction >10% is normally plotted as a vertical strip within the "Graphic Lithology" column, with the implication that biogenic grains are dispersed in the coexisting sediment.
The nonbiogenic fraction is represented by a single "Clastic Sediment" symbol if it is homogeneous in texture or by two of these symbols if lithologically distinct clastic sediments are interbedded (e.g., interbeds of sand and silty clay). The relative width of the columns indicates the relative proportion of each type of clastic sediment in the interbedded section. Constituents accounting for <10% of a given lithology or stratigraphic interval (or others remaining after the representation of the three most abundant lithologies and components) are not shown in the "Graphic Lithology" column, but they are indicated in the "Description" section of the barrel sheet.
A wide variety of features that characterize the sediment, such as bed thicknesses, primary sedimentary structures, bioturbation parameters, soft-sediment deformation, ichnofossils, and fossils, are indicated in columns to the right of the graphic log. The symbols are schematic, but are placed as close as possible to their proper stratigraphic position. A legend of the symbols used on the graphic sedimentologic columns is shown in Fig. 4 and Fig. 5.
Bed thickness is characterized by the following terms: very thick bedded (>100 cm thick), thick bedded (30-100 cm thick), medium bedded (10-30 cm thick), thin bedded (3-10 cm thick), and very thin bedded (1-3 cm thick) (McKee and Weir, 1953). The hue and chroma attributes of color, as determined visually using Munsell Soil Color Charts (1975), were recorded in the "Description" column.
Deformation and disturbance of sediment that clearly resulted from the coring process are illustrated in the "Drilling Disturbance" column, using symbols shown in Figure 5. Blank regions indicate the absence of coring disturbance. Detailed accounts of drilling disturbance appear in many previous ODP reports (e.g., Shipboard Scientific Party, 1995b).
A summary lithologic description with sedimentologic highlights is given in the "Description" column of the barrel sheet. This generally consists of two parts: (1) a section that lists the major lithologies and (2) an extended summary description of the sediments, including color, composition, sedimentary structures, ichnofossils identified, and other notable characteristics. Descriptions and locations of thin, interbedded, or minor lithologies that could not be depicted in the "Graphic Lithology" column are presented as remarks in the "Description" column.
The sediment classification scheme used during Leg 175 is descriptive and is largely the same as that used during previous ODP legs (Fig. 6). Composition and texture are the only criteria used to define lithology. Genetic terms (e.g., pelagic, hemipelagic, turbidite, debris flow, etc.) do not appear in this classification. The term "clay" is used for both clay minerals and other siliciclastic material <4 µm in size. Biogenic components are not described in textural terms. A sediment with 55% sand-sized foraminifers and 45% siliciclastic clay is thus called a foraminifer clay, not a foraminifer clayey sand.
The principal name is determined by the component or group of components (e.g., total biogenic carbonate) that comprises at least 60% of the sediment—except for equal mixtures of biogenic and nonbiogenic material. The main principal names are as follows:
Example:
Example:
Ichnologic analysis included evaluation of the extent of bioturbation, as well as identification of trace fossil types. The degree of bioturbation was semiquantitatively assessed using a simple modified version of the Droser and Bottjer (1991) ichnofabric index (e.g., barren or no bioturbation, rare, moderate, common, and abundant; see Fig. 4). These indices are illustrated using relative shading in the "Relative Bioturbation" column of the barrel sheets.
Trace-fossil identification was restricted to intervals where biogenic structures were discrete (e.g., where burrows exhibited sharp walls or had fills that contrasted well in texture, composition, or color with surrounding sediments). Discrete biogenic structures (burrows, burrow systems, borings, etc.), were identified based on morphologic attributes as manifested on two-dimensional core surfaces. Recognizable biogenic structures are illustrated in the "Ichnofossils" column of the barrel sheets, using symbols depicted in Figure 5.
Graphic sedimentologic columns are presented in the "Litho-stratigraphy" section of each site chapter (this volume) and are based on the information compiled from the barrel sheets (Fig. 7). The columns show the sequence and recovery of the drilled cores and the coring device used (H = APC and X= XCB coring). The "Lithology" columns illustrate the major lithologic units and subunits. Biostratigraphic zones are indicated for the "Foraminifer," Nannofossil," "Diatom," and "Radiolarian" Zones. The "Paleomagnetics" column indicates paleomagnetic time periods and stratigraphic positions of magnetic reversals. The last three columns on the right contain the spliced total reflectance and magnetic susceptibility data and the concentration of calcium carbonate (by weight percent) analyzed for sediments from Hole A at each site.
Relative abundances of the main silicate and carbonate minerals were determined using a Philips model PW-1729 X-ray diffractometer with CuKα radiation (Ni filter). Bulk-sediment samples were freeze-dried, ground, and mounted with a random orientation into an aluminum sample holder. The instrument conditions were as follows: 40 kV, 35 mA, goniometer scan from 2° to 70° 2θ for bulk samples, step size of 0.01° 2θ, scan speed at 1.2° 2θ/min, and count time of 0.5 s. Peak intensities were converted to values appropriate for a fixed slit width (Table 1).
An interactive software package (MacDiff 3.2b5 PPC) was used on a Macintosh computer to identify the main minerals. The relative abundances of the minerals were established based on the peak heights. The locations of the peaks used for mineral recognition are presented in Table 1. Although the ratios and relative abundances reported in this volume are useful for general characterization of the sediments (Moore and Reynolds, 1989), they should not be viewed as precise quantitative data.
Clay and microfossil analyses of the sediment were complemented by smear-slide description. Tables summarizing data from smear slides appear in Section 5 on CD-ROM (back pocket, this volume). These tables include information about the sample location; whether the sample represents a dominant or a minor lithology in the core; the estimated abundances of sand, silt, and clay; and the major biogenic components. We emphasize here that smear-slide analysis provides only estimates of the relative abundances of detrital constituents. The mineral identification of finer grained particles is difficult using only a binocular microscope, and sand-sized grains tend to be underestimated because they cannot be incorporated into the smear evenly. The mineralogy of smear-slide components was complemented by X-ray diffraction.
Relative abundances are defined as follows:
The position of samples taken from each core for shipboard analysis is indicated in the "Samples" column on the core description form as follows: IW = interstitial water sample; PAL = micropaleontology sample; and SS = smear slide.
Reflectance of visible light from cores was routinely measured downhole using a Minolta spectrophotometer CM-2002. Color reflectance was used to establish semiquantitative relationships between lithology and spectral reflectance for visible (VIS) wavelengths to provide a continuous stratigraphic record of color variations downhole and to attempt to recognize climatic signals in Miocene- to Pleistocene-aged sediments.
Spectrophotometer readings were taken before the working halves of the cores were sampled. Strips of very thin, transparent plastic film (Glad Cling Wrap, a brand of polyethylene food wrap) were used to cover the cores to prevent the spectrophotometer from becoming dirty. Routine measurements were made at evenly spaced intervals (generally 2 or 4 cm) of each core section but were modified according to void space and regions of coring disturbance.
Before obtaining measurements from each core, the spectrophotometer was calibrated by attaching a white calibration cap (Balsam et al., 1997). The spectrophotometer measurements were then recorded using the program Spectrolog (v 3.0). Each measurement consists of 31 separate determinations of reflectance in 10-nm-wide spectral bands from 400 to 700 nm, which covers the VIS spectrum. Selected reflectance curves are shown in the site chapters (this volume). Additional detailed information about measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Schneider et al. (1995) and Balsam et al. (1997). Color-reflectance values were smoothed using a nine-point running average for total reflectance and a five-point running average when calculating the ratio for the 650/450 nm red-to-blue ratios.