The first part of this section summarizes the methods used to describe sediment cores and the manner in which data collected manually for visual core description forms (VCDs) are summarized and condensed into computer-generated summaries for each core. The second part reviews the sedimentological classifications and terms used in the descriptions.
Shipboard sedimentologists were responsible for visual core logging, smear-slide analyses, and thin-section descriptions of sedimentary and volcaniclastic material. During Leg 149, information recorded section-by-section on VCD sheets was condensed onto computer-generated summaries that give graphic and textual summaries for each core (see "Cores" section, this volume).
Cores were designated using leg number, site number, hole letter,core number, and core type, as discussed in Shipboard Scientific Party (1993c). The cored interval was specified in terms of meters below sea level (mbsl) and meters below seafloor (mbsf). On the basis of drill-pipe measurements (dpm), reported by the SEDCO Coring Technician and the ODP Operations Superintendent, depths were corrected for the height of the rigfloor dual elevator stool above sea level to give true water depth and correct depth below sea level.
The lithology of the recovered material is represented on the computer-generated core description forms by symbols representing as many as three components in the column titled "Graphic Lithology" (see bottom right of Fig. 1). Constituents accounting for < 10% of the sediment in a given lithology (or others remaining after the representation of the three most abundant lithologies) are not shown in the "Graphic Lithology" column, but are listed in the "Lithologic Description" section of the core description form. Because of the limitations of the software used for generating the core summaries, the "Graphic Lithology" column shows only the composition of layers or intervals exceeding 20 cm in thickness. This meant that the VCDs for Leg 149 often do not show the nature of vertical changes in the cores, as many of the repetitive sequences present are less than 20 cm thick.
The chronostratigraphic unit, as recognized on the basis of paleontological and paleomagnetic criteria, is shown in the "Age" column on the core summaries. Boundaries between assigned ages are indicated as follows:
Natural structures, such as cross stratification, grading, and bioturbation, are indicated in the "Structure" column of the core summaries.
The core summary software also enables color banding to be indicated, but does not include symbols to enable one to record bed thickness. During Leg 149, the shipboard sedimentologists decided to use the color banding symbols to indicate both color banding and interbedded lithologies, as the latter were invariably accompanied by color changes. The following definitions were adopted (from Blatt, Middleton, and Murray, 1980, p. 128):
- Thick bedding/color banding >30 cm
- Medium bedding/color banding 10-30 cm
- Thin bedding/color banding <10cm
The core summary software enables sharp and gradational boundaries between color banding to be depicted. Banding showing one sharp boundary and one gradational boundary was arbitrarily defined as having gradational boundaries.
Sediment disturbance resulting from the coring process may be difficult to distinguish from natural structures. Nonetheless, it is important to estimate the degree and nature of core disturbance in the "Disturbance" column on the core summaries (using symbols in Fig. 2). Blank regions indicate an absence of drilling disturbance. The degree of core disturbance is described for soft and firm sediments using the following categories:
The degree of fracturing in indurated sediments and igneous rocks is described using the following categories:
The hue and chroma attributes of color were determined using Munsell Soil Color Charts (1971) and recorded in the "Color" column on the core description form.
The positions and types of samples taken from each core for shipboard analysis are indicated in the "Samples" column on the core description form, as follows:
- C: organic-geochemistry sample,
- D: XRD sample,
- F: XRF sample,
- I: interstitial-water sample,
- M: micropaleontology sample,
- P: physical-properties sample,
- S: smear slide,
- T: thin section,
- W: whole-round sample, and
- X: paleomagnetic sample.
A table summarizing data from smear slide and thin section descriptions appears at the end of each site chapter. The table includes information about the sample location, whether the sample represents a dominant ("D") or a minor ("M") lithology in the core, and the estimated percentages of sand-, silt-, and clay-size material, together with all identified components.
Some samples for carbonate analysis were taken at the same positions from which smear-slide material was taken; additional samples for carbonate analysis were taken by the shipboard geochemists, but the positions of these are not shown on the summaries.
The lithologic description that appears on each core description form consists of three parts: (1) a heading that lists all the major sedimentary lithologies observed in the core; (2) a heading for minor lithologies, and (3) a more detailed description of the sediments, including features such as color, composition (determined from the smear slides), sedimentary structures, or other notable characteristics. Descriptions and locations of thin, interbedded, or minor lithologies that cannot be depicted in the graphic lithology column, are included in the text.
For each hole, a "Master" column was prepared (see "Cores" section, this volume); this indicates core numbers and recovery, graphically summarizes the lithology, and charts a variety of data vs. subbottom depth (biostratigraphic zones, magnetochrons, chemistry, physical properties, and downhole tool measurements). The "Master" columns supplement the information contained on the core description forms, but at a more condensed vertical scale.
During Leg 149, sedimentologists used a modified version of the ODP sediment classification scheme (Shipboard Scientific Party, 1990; Mazzullo et al., 1987) for granular sediment types (Fig. 3). Variations in the relative proportions of siliciclastic, volcaniclastic, pelagic, and neritic components define five major classes of granular sediments (Fig. 3). The neritic component consists of skeletal and nonskeletal shallow-water bioclasts, including shallow-water benthic foraminifers. Pelagic grains are the skeletal remains of open-marine siliceous and calcareous microfauna and microflora (e.g., radiolarians, diatoms, planktonic foraminifers, nannofossils) and associated organisms. Note that the term micrite is used to define very fine-grained calcareous particles (~<10 m), observed in smear slides with no clear identification of origin. Calcium carbonate content was qualitatively estimated using smear slides and quantitatively estimated by using coulometric analyses (see "Organic Geochemistry" section, this chapter). Siliciclastic grains are mineral and rock fragments derived from igneous, sedimentary, and metamorphic rocks. Volcaniclastic grains include those of pyroclastic (direct products of magma degassing) and epiclastic (detritus derived from erosion of volcanic rocks) origins.
A granular sediment is classified by designating a principal name and major and minor modifiers. The principal name of a granular sediment (e.g., ooze, fine sand or sandstone, volcanic breccia) defines its granular-sediment class as depicted in Figure 3; the major and minor modifiers may include a description of the texture, composition, and fabric.
For siliciclastic sediments, the principal name describes the texture and is assigned according to the following guidelines:
Conglomerate and breccia are used as principal names of lithified gravels with well-rounded and angular clasts, respectively.
For pelagic sediments, the principal name describes the composition and degree of lithification using the following terms:
The principal name of a granular-sediment class is preceded by major modifiers and followed by minor modifiers (the latter preceded by "with") that describe the lithology of the granular sediment in greater detail (e.g., a siliciclastic rock composed of 50% clay, 35% quartz-silt, and 15% foraminifers is classified as a silty claystone with foraminifers). Major and minor modifiers, as applied in the above example, are used most commonly to describe composition and textures of grain types present in major (>25%) and minor (10%-25%) proportions. Major and minor modifiers also may be used to describe grain fabric (e.g., matrix-supported) in siliciclastic and volcaniclastic sediment types.
The composition of pelagic grains can be described with the major and minor modifiers diatom(-aceous), radiolarian, spicule(-ar), siliceous, nannofossil, foraminifer(-al), and calcareous. The terms siliceous and calcareous are used generally to describe sediments composed of siliceous or calcareous pelagic grains of mixed origins.
Grain shape, mineralogy, rock fragment types, fabric, degree of induration, and color can be used as major modifiers. Grain shapes are described by the major modifiers rounded, subrounded, subangular, and angular. The character of siliciclastic grains can be described by mineralogy (using modifiers such as quartzose, feldspathic, glauconitic, micaceous, zeolitic, lithic, or calcareous). Modifiers such as volcanic, sed-lithic, meta-lithic, gneissic, and plutonic, which describe the provenance of rock fragments, also can be used in the classification of sediments (particularly in gravels, conglomerates, and breccias). The fabric of a sediment can be described as well using major modifiers such as grain-supported, matrix-supported, and imbricated. Generally, fabric terms are useful only when describing gravels, conglomerates, and breccias. The degree of lithification is described using the following major modifiers: "unlithified" designates soft sediment that is readily deformable by finger pressure, "partially lithified" designates firm sediment that is incompletely lithified, and "lithified" designates hard, cemented sediment that must be cut with a saw. Finally, sediment color, as determined visually with the Munsell Soil Color Chart (1971), also can be employed as a major modifier.
Mixed sediments are described using major and minor modifiers indicating composition and texture.
The fine fraction of selected samples was analyzed on board the ship using X-ray diffraction techniques. Sediments were put into suspension by ultrasonic disaggregation, and the fine fraction (<1 m) was separated by centrifuging and then used to prepare air-dried specimens on glass slides. X-ray diffraction patterns of these oriented specimens were produced using the shipboard Philips AD 3420 X-ray diffractometer (CuK alpha emission source). Selected samples were treated with ethyleneglycol and re-analyzed. Other selected samples were heated at 550°C for 1 to 1.5 hr, and then reanalyzed. Peaks were visually inspected and matched to standard reference peaks for various minerals (quartz, feldspar, hornblende, calcite, pyrite, and clay minerals, etc.).