radiation at 40 kV and 35 mA with a focusing graphite monochromator and the
following slit settings:The description of sedimentary units recovered during Leg 185 included estimates of sediment composition based on smear slides, thin sections, carbonate measurements, and XRD, documentation of sedimentary and deformational structures, drilling disturbance, presence and type of fossils, bioturbation intensity, induration, diagenetic alteration, and color. These data were recorded manually for each core section on standard visual core description (VCD) paper forms that are archived by ODP.
Information on the VCDs was summarized and entered into AppleCORE (version 0.7.5g) software, which generated a one-page graphical log of each core ("barrel sheet"). Barrel sheets are presented with core photographs (see the "Core Descriptions" contents list). A wide variety of features, such as sediment lithology, bed thickness, primary sedimentary structures, bioturbation parameters, soft-sediment deformation, and structural and diagenetic features are indicated by patterns and symbols in the graphic logs. A key to the full set of patterns and symbols used on the barrel sheets is shown in Figure F3. The symbols are schematic, but they are placed as close as possible to their proper stratigraphic position, or arrows indicate the interval for which the symbol applies. For exact positions of sedimentary features, copies of the detailed section-by-section VCD forms can be obtained from ODP. The columns on the barrel sheets are as follows.
Sediment lithologies are represented by patterns in the "Lithology" column (see the "Core Descriptions" contents list). This column may consist of up to three vertical strips, depending on the number of the major end-member constituents (see "Sediment Classification"), thus reflecting intermixing of different components. Sediments with only one major component group (i.e., all other component groups are <10% each) are represented by one strip. Because of the limitations of the AppleCORE software, thin intervals of interbedded lithologies cannot be adequately displayed at the scale used for the barrel sheets, but they are described in the "Description" columns of the barrel sheets where appropriate.
Symbols in these columns are explained in Figure F3. The bulk of the clayey sediments recovered during Leg 185 was relatively homogeneous. Stratification, bioturbation, or other sedimentary structures were usually discernible only where textural or compositional differences were present (e.g., close to ash layers). In the homogeneous background sediment, however, it was difficult to distinguish the destruction of primary structures due to bioturbation from the actual absence of primary structures. Given the light gray and reddish colors of the clayey sediments, the absence of lamination, and the very low organic matter contents (see "Interstitial Water Chemistry and Headspace Gas"), it is reasonable to assume that the remaining sediment has been pervasively bioturbated as well because benthic burrowing activity was not limited by oxygen deficiency. To convey the maximum amount of information without confusing interpretation with observation, we used the "Bioturbation" column to display only visible bioturbation or sediment mottling. The bioturbation column of the barrel sheets shows four levels of intensity:
Stratification thickness was characterized by a combination of the terms given by McKee and Weir (1953) and Ingram (1954): very thick bedded (>1 m thick), thick bedded (30-100 cm thick), medium bedded (10-30 cm thick), thin bedded (3-10 cm thick), very thin bedded (1-3 cm thick), thickly laminated (3-10 mm thick), thinly laminated (1-3 mm thick), and very thinly laminated (<1 mm thick).
Natural structures (physical or biological) can be difficult to distinguish from disturbance created by the coring process. Deformation and disturbance of sediment that resulted from the coring process are illustrated in the "Drilling disturbance" column with the symbols shown in Figure F3. Blank regions indicate the absence of drilling disturbance. The degree of drilling disturbance for soft sediments was described using the following categories:
The stratigraphic position of samples taken for shipboard analysis and the location of close-up photographs is indicated in the "Samples" column of the barrel sheet according to the following codes:
CAR = carbonate content,PAL = biostratigraphy,
PHO = close-up photograph,
SS = smear slide,
THS = thin section,
WR = whole-round sample,
XRD = X-ray diffraction analysis, and
XRF = X-ray fluorescence analysis.
Sediment color was determined visually by comparison with standard color charts (Munsell Color Company, Inc., 1975; Rock Color Chart Committee, 1991) and is reported in the "Description" column of the barrel sheets. In addition to determining color visually, all cores were scanned at 2- to 4-cm intervals using a Minolta CM-2002 spectrophotometer mounted on the AMST. The spectrophotometer measures reflectance in thirty-one 10-nm-wide bands of the visible spectrum (400-700 nm) on the archive half of each core section. Spectrophotometer readings were taken after cleaning the surface of each core section and covering it with the clear plastic film (Glad brand Cling Wrap, a brand of polyethylene food wrap). Calibration of the color scanner did not include a correction for the plastic film because we found that the effect is very minor even with very bright colored lithologies. The measurements were taken automatically and recorded by the AMST at evenly spaced intervals along each section. There was no way to program the AMST software to avoid taking measurements in intervals with a depressed core surface or in disturbed areas of core containing drilling slurry or biscuits. The data are part of the Janus database and can be obtained from ODP. Additional detailed information about measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Balsam et al. (1997, 1998, 2000).
A summary of the sedimentologic data is given in the "Description" column of the barrel sheet. It consists of four parts: (1) a heading in capital letters that lists only the dominant sediment lithologies observed in the core; (2) a general description of the sediments in the core, including color, composition, sedimentary structures, bed thicknesses, drilling disturbance, as well as any other general features in the core; and (3) descriptions and locations of thin, interbedded, or minor lithologies.
Sediments were analyzed petrographically using smear slides and thin sections. Tables summarizing these data (see the "Core Descriptions" and "ASCII Tables" contents lists) include information about the sample location, whether the sample represents a dominant (D) or a minor (M) lithology in the core, and the percentages of sand, silt, and clay size fractions, along with all identified components. We emphasize here that smear-slide and thin-section analyses provide only estimates of the relative abundances of the constituents. The comparison charts of Baccelle and Bosellini (1965) were used to refine abundance estimates in thin sections. However, these charts cannot be used for smear slides because they are designed to simulate a field of view that is completely and evenly covered with particles. Quantification of data from smear slides is further aggravated by the difficulty in identifying fine-grained particles using only a microscope and by the tendency to underestimate sand-sized grains because they cannot be incorporated evenly into the smear. Biogenic opal and its diagenetic modifications are particularly difficult to determine from smear slides (van Andel, 1983). Previous experience has shown that the largest variations in smear-slide determination correlate with the change from one observer to another, or with shift changes. The accuracy problem is indicated in the "Explanatory Notes" chapters of several recent ODP volumes in which sedimentologic numerical data in general, and those of smear-slide determination in particular, are consistently de-emphasized and replaced by semiquantitative categories (e.g., Shipboard Scientific Party, 1998a, 1998b, 1998c). A limitation to semiquantitative categories, such as the ones proposed during previous legs would have seemed all the more appropriate during Leg 185. Current ODP policies, however, require the input of numerical data; therefore, the reader is warned that the tabulated smear-slide results (see the "Core Descriptions" and "ASCII Tables" contents lists) largely reflect the need to comply with these regulations rather than actual accuracy. Smear-slide and thin-section data were reviewed for internal consistency and correct sedimentologic nomenclature, and the qualitative composition was confirmed by XRD. Accuracy of the carbonate content estimated from smear slides and thin sections was confirmed by chemical analyses (see "Interstitial Water Chemistry and Headspace Gas").
Selected samples were
taken for qualitative mineral analysis by XRD using a Philips diffractometer
with Cuk
radiation at 40 kV and 35 mA with a focusing graphite monochromator and the
following slit settings:
,
/min,
Bulk samples were
freeze-dried, ground with an agate mortar and pestle, and packed in sample
holders, which, together with the ship's movement, probably imparted some
orientation to the mineral powder. These samples were scanned from 2° to 70° 2.
MacDiff software (v. 4.0.4 PPC, by Rainer Petschick) was used to display
diffractograms and to identify the minerals. Most diffractograms were corrected
to match the main peaks of quartz, calcite, or clinoptilolite at 3.343, 3.035,
and 8.95 Å, respectively. Identifications are based on multiple peak matches
with the mineral data base provided with MacDiff. Each site chapter includes
selected diffractograms to illustrate which peaks were associated with various
minerals. Relative abundances reported in this volume are useful for general
characterization of the sediments, but they are not quantitative concentration
data.
One major goal of Leg 185 is to quantitatively assess the composition of the input material to the Izu-Bonin Trench, which includes the sedimentary cover of the oceanic crust at the drill site. Achieving this goal requires extrapolating petrographical and geochemical measurements on discrete samples over long core intervals, which, in turn relies on an accurate core description. We evaluated the methods and the classification used for sediment description and found that there is a need for a less ambiguous and more flexible classification than the proposed ODP standard classification by Mazzullo et al. (1988). Our classification is neither comprehensive nor entirely descriptive, but it is simple to use for the purpose of Leg 185, and it circumvents some of the disadvantages of the Mazzullo classification. Notably, it avoids the impression of a level of accuracy that is not achievable under the conditions of most ODP cruises. Also, we tried to use common and relatively simple names, which led us to abandon a number of ambiguous or meaningless terms like "Radiolarite" and "Mixed sediment" (see "Sediment Nomenclature"). We used three end-members (i.e., calcareous, siliceous, and silicate) (Fig. F4), similar to the classification by Dean et al. (1984). In response to the absence of reliable quantitative petrographic data mentioned above, the Leg 185 sediment classification provides only a limited number of principal sediment names supplemented by a preliminary modifier that relates to the carbonate content (e.g., calcareous, clayey) (Fig. F4). The preliminary modifiers were eventually adjusted to the corresponding measured carbonate content when those measurements were available.
Sediment names consist of a principal name relating to the dominant composition of the sediment (e.g., claystone, marl, or chert) and one or two modifiers that precede the principal name (e.g., ash-bearing siliceous clay). Besides composition, principal names vary according to the grain size and the induration of the sediment. Principal sediment names are as follows.
The calcareous end-member of our classification includes sediments made up of all kind of calcareous fossil shells or tests, resedimented and diagenetic carbonate grains, and cements. Sediments that contain more than ~70% calcareous components, the majority of which were secreted by pelagic organisms (planktonic foraminifers and calcareous nannofossils) are called ooze if they are soft, chalk if they are firm, and limestone if they are hard. These names may be preceded by the dominant calcareous microfossil. Mixtures of the calcareous and the silicate or the siliceous end-member that contain between 70% and 30% carbonate are called marl or marlstone, depending on their induration.
The siliceous end-member of our classification includes sediments rich in siliceous microfossils, as well as the diagenetic modifications of these sediments, and silica-rich hydrothermal precipitates. Sediments dominated by siliceous microfossils and indeterminate silica that contain less than ~30% carbonate are called radiolarian, diatom, or siliceous ooze if they are soft, porcelanite if they are firm to hard, and chert if they are hard enough not to be scratched by a stainless steel probe. In addition to this field classification, the terms porcelanite and chert bear a strong compositional notion. Thus, porcelanite is typically composed of opal-CT (christobalite-tridymite), but it may also contain diagenetic quartz, carbonate, and silicates (mostly clay minerals). Chert is usually dominated by quartz and tends to be a purer silica, but may also contain clay minerals and carbonate (Kastner, 1979; Isaacs, 1982; Isaacs et al., 1983).
A number of different lithologic definitions for siliceous rocks were used by the Leg 129 participants, which resulted in considerable confusion (Lancelot, Larson, et al., 1990; Behl and Smith, 1992; Fisher et al., 1992; Karl et al., 1992; Karpoff, 1992; Ogg et al., 1992). This unsatisfactory situation partly reflects the difficulty to quickly determine the composition of siliceous sediments and rocks (van Andel, 1983) but also relates to regional differences in usage of specific rock names. Thus, "radiolarite" was used during Leg 129 to describe a soft, friable sediment that contains abundant radiolarians. Most European geologists, and probably many others, use radiolarite for an extremely hard radiolarian-bearing rock (i.e., as a synonym of radiolarian chert) (e.g., Bernoulli, 1972; Baltuck, 1983, 1986; Jenkyns, 1986). In view of the intended integration of core and logging data, which involves the comparison of physical properties, we avoided the Leg 129 usage of radiolarite. Soft, friable sediments dominated by radiolarians are called radiolarian ooze or radiolarian marl, depending on their carbonate content. The more indurated forms of this sediment are called radiolarian porcelanite and radiolarian chert or radiolarian marlstone, respectively.
Sediments dominated by nonbiogenic, mostly detrital, silicate components are further subdivided based on the relative proportion of siliciclastic and volcaniclastic sediments. If the majority of the detrital components are siliciclastic, the sediment is called sand if the average grain size is between 63 µm and 2 mm, silt (2-63 µm), or clay (<2 µm). Mixtures of sand, silt, and clay are named according to the classification of Shepard (1954). Note that the silt/clay boundary has been placed at 2 µm as suggested by Doeglas (1968). The suffix "-stone" is added if the sediment is indurated. The principal name for sediments dominated by volcaniclastic components in the silt and fine sand size range (2-250 µm) is volcanic ash.
Principal names may be preceded by a major modifier, (e.g., diatomaceous) that relates to a component group that is common or abundant, but not dominant. Alternatively, where this makes for odd names, the modifier may be used in conjunction with the suffix "rich." As an example, diatomaceous claystone or, alternatively, diatom-rich claystone describes a hard sediment that contains more than 50% siliciclastic clay and more than 30% diatoms. Minor modifiers were used to specify a common component group (i.e., less than ~30%, but more than ~10% of the sediment). Minor modifiers are used with the suffix "-bearing" and precede the major modifiers. Thus, a soft sediment with dominant radiolarians, abundant calcareous nannofossils, and common volcanic ash would be called an ash-bearing nannofossil-rich radiolarian marl. Note that the more specific microfossil names take the places of the terms "calcareous" and "siliceous" that are shown in Figure F4. Where appropriate, the names of important, but accessory (<10%) components, or those not covered by the end-member component groups, such as manganese oxyhydroxides, phosphate, zeolite, or barite, were added to the sediment names preceded by the word "with" (e.g., nannofossil marl with Mn micronodules).
