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doi:10.2204/iodp.proc.330.102.2012

Sedimentology

The lithology of the sediment recovered during Expedition 330 was primarily determined using observations based on visual (macroscopic) core description, thin sections, and smear slides. In some cases digital core imaging, color reflectance spectrophotometry, and magnetic susceptibility analysis provided complementary discriminative information. The methods employed during this expedition were similar to those used during IODP Expedition 324 (Expedition 324 Scientists, 2010) to Shatsky Rise. Expedition 330 used the DESClogik application to record and upload descriptive data into the LIMS database (see DESClogik user guide), which was first implemented during IODP Expedition 320T (Expedition 320T Scientists, 2009). Three spreadsheet templates were set up in DESClogik and customized for Expedition 330 before the first core arrived on deck. The spreadsheet templates were used to record macroscopic sedimentologic core descriptions and data from smear slides and thin sections, which were used to quantify the texture and relative abundance of biogenic and nonbiogenic components. The locations of all smear slide and thin section samples taken from each core were recorded in the Sample Master application. Descriptive data uploaded to the LIMS database were also used to produce visual core description (VCD) standard graphic reports.

Visual core descriptions

After descriptions of the cores were uploaded into the central LIMS database, the data were used to produce VCDs, which include a simplified graphical representation of the core (per section) with accompanying descriptions of the features observed (Figs. F1, F2). Depending on the type of material drilled, two VCDs were sometimes produced for the same section: one to describe sediment or sedimentary rocks and the other to describe igneous features.

Site, hole, and depth in mbsf, calculated according to the CSF-A depth scale, are given at the top of each VCD, with depth of core sections indicated along the left margin. Observations of the physical description of the core correspond to entries in DESClogik, including grain size, bioturbation intensity, fossils, ichnofossils, lithologic accessories, sedimentary structures, and drilling disturbance. Symbols used in the VCDs are given in Figure F2. Additionally, sedimentary VCDs display magnetic susceptibility, color reflectance, paleontological observations, and the locations of samples taken for shipboard measurements. Section summary text provides a generalized overview of the core section’s lithology and features. This summary text and individual columns shown on the VCDs are described below in greater detail, followed by an outline of the lithostratigraphic classification used during Expedition 330.

Section summary

A brief overview of major and minor lithologies present in the section, as well as notable features (e.g., sedimentary structures) and the composition of basalt clasts (if present), is presented in the section summary text field at the top of the VCDs. The summary includes sediment color determined qualitatively using Munsell soil color charts (Munsell Color Company, Inc., 2000). Because sediment color may evolve during drying and subsequent oxidization, color was described shortly after the cores were split and imaged or measured by the SHIL and SHMSL.

Section-half image

The high-resolution scans of each core section (at 20 pixels/mm) made by the SHIL are included to provide a continuous image of the whole section.

Graphic lithology

The lithology of the recovered core is represented on the VCDs by graphic patterns. See Figure F2 for an explanation of patterns used during Expedition 330.

Magnetic susceptibility

When recovery and core length permitted, the magnetic susceptibility of both whole rounds and split sections of the core was measured, which roughly indicates the concentration of magnetic minerals. A filter was applied to remove spurious data related to gaps between broken pieces of the hard rock cores (see “Physical properties”). Both filtered and raw data are shown on the VCDs.

Color reflectance spectrophotometry

Color reflectance spectrophotometry of visible light (“Reflectance” in the VCDs) was routinely measured on archive halves of sediment (and hard rock) cores using the SHMSL, which was equipped with OceanOptics software for analysis of color reflectance data (see details in “Physical properties”). Cores consisting of soft sediment were covered with clear plastic wrap and then placed on the SHMSL. Hard rock sections were run without the protective plastic cover. Measurements were taken at 1 cm spacing. The SHMSL is set to skip empty intervals in the core liner, but it cannot recognize relatively small cracks, disturbed areas of cores, or plastic section dividers. Thus, raw SHMSL data may contain spurious measurements. Therefore, during postprocessing a filter was applied to the reflective data set (see “Physical properties”). Both filtered and unfiltered data are shown on the VCDs.

Grain size

The Grain size column displays sediment grain size as a block chart. The dominant grain size is represented numerically for each lithologic interval using a range of 1–6:

  • 1 = clay size (<3.9 µm).

  • 2 = silt size (3.9–62.5 µm).

  • 3 = very fine to fine sand size (>62.5–250 µm).

  • 4 = medium to very coarse sand size (>250 µm–2 mm).

  • 5 = granule to cobble size (>2–256 mm).

  • 6 = boulder size (>256 mm).

Because the bulk of the sediment observed during Expedition 330 is composed of basalt breccia and volcanic sandstone, the size of detrital grains in coarse-grained sediment was estimated during visual description of the cores by measuring the largest grain size per 10 cm interval. Grains overlapping several intervals were measured only once. Also, the roundness of grains was estimated on the basis of the average roundness of the largest grains in each 10 cm interval for each lithology. Six categories of roundness were defined with the help of a visual comparison chart (Shepard and Young, 1961): very angular, angular, subangular, subrounded, rounded, and well rounded (no measurement of roundness was made for clasts <1 mm). These measurements are not shown on the VCDs but are available in supplementary tables (see SIZE in SEDIMENT in “Supplementary material”) and are displayed in stratigraphic summary figures in each site chapter. The average maximum grain size of coarse-grained sediments is available in LIMS.

Bioturbation intensity

The degree of bioturbation was determined by observing how intensely the sediments were altered by the action of organisms (Droser and Bottjer, 1986). The following categories were used to describe degree of bioturbation:

  • Intense (ichnofabric index 5 and 6): 60%–100% of original bedding disturbed.

  • Moderate (ichnofabric index 3 and 4): 10%–60% of original bedding disturbed.

  • Minor (ichnofabric index 1 and 2): 0%–10% of original bedding disturbed.

The Bioturbation column was left blank for intervals composed of volcaniclastic sediments.

Fossils/Ichnofossils

Identifiable fossils (or fossil fragments) and trace fossils (ichnofossils) are identified in the Fossils/Ichnofossils column.

Sedimentary structures

Structures resulting from physical sedimentary processes are represented in the Sedimentary structures column. Identified features include laminations and cross-bedding, contacts between sediment of differing lithologies, and any soft-sediment deformation structures.

Lithologic accessories

Some postdepositional features (e.g., ferromanganese encrustations) and grains of interest (e.g., pumice and coated grains) are recorded in the Lithologic accessories column.

Drilling disturbance

The Drilling disturbance column indicates the mode and type of disturbance (e.g., flow-in or fall-in) caused by the drilling process. The degree of disturbance within soft sediment is characterized using the nomenclature of Ocean Drilling Program (ODP) Leg 180 (Taylor, Huchon, Klaus, et al., 2000):

  • Slightly disturbed: bedding contacts are slightly deformed.

  • Moderately disturbed: bedding contacts have undergone extreme bowing.

  • Highly disturbed: bedding is completely deformed as flow-in and other soft stretching and/or compressional shearing structures attributed to coring/drilling.

  • Soupy: intervals are water saturated and have lost all aspects of original bedding.

The degree of fracturing within indurated sediments is described using the following categories:

  • Slightly fractured: core pieces are in place and broken.

  • Moderately fractured: core pieces are in place or partly displaced, but original orientation is preserved or recognizable.

  • Highly fractured: core pieces are probably in correct stratigraphic sequence, but original orientation is lost.

  • Drilling breccia: core is crushed and broken into many small and angular pieces, with original orientation and stratigraphic position lost.

Age

The nannofossil or foraminiferal zone that defines the age of the sediments was provided by the shipboard biostratigraphers (see “Paleontology”) and is listed in the Age column.

Samples

The exact positions of samples used for microscopic descriptions (i.e., smear slides and thin sections), biochronological determinations, and shipboard analysis of chemical and physical properties of the sediments are recorded in the Samples column.

Sediment classification

Lithologic names, including sediment composition, degree of lithification, and texture, are based mostly on conventions outlined by Mazzullo et al. (1988) in the ODP sediment classification scheme (Fig. F3). The term “mixed sediment” was not used during Expedition 330 because thin-bedded lithologic variations did not occur. Pelagic sediments are defined as >50% pelagic and neritic components and <50% siliciclastic and volcaniclastic components, with a higher proportion of pelagic than neritic components. Neritic sediments, on the other hand, are defined as >50% pelagic and neritic components and <50% siliciclastic and volcaniclastic components, with a higher proportion of neritic than pelagic components. Siliciclastic sediments are defined as >50% siliciclastic and volcaniclastic components and <50% neritic and pelagic components, with a higher proportion of siliciclastic than volcaniclastic components. Finally, volcaniclastic sediments are defined as >50% siliciclastic and volcaniclastic components and <50% neritic and pelagic components, with a higher proportion of volcaniclastic than siliciclastic components.

The nomenclature includes a principal name based on the composition of the major lithology, preceded by major modifiers (in order of increasing abundance) that refer to components making up at least 25% of the sediments. The principal name is followed by minor modifiers for components forming between 10% and 25% of the sediments, in order of increasing abundance. In clastic deposits, components refer to clasts and do not include the matrix. For example, a well-indurated breccia sample containing 40% basalt, 35% bioclasts, 20% cement, and 5% foraminifers would be described as a “bioclast basalt breccia.” The principal name (breccia, in the preceding example) is displayed in the Graphic lithology column of the VCDs with a general descriptive prefix (e.g., volcanic), whereas major and minor components are available from the LIMS database or section summaries of the VCDs. The term “sandy foraminiferal ooze” is applied to the youngest sediments encountered during Expedition 330, which likely represent a winnowed residue of foraminiferal ooze originally richer in nannofossils.

The subclassification of volcaniclastic sediments followed here differs from the standard ODP classification (Mazzullo et al., 1988) in that we adopted a descriptive (nongenetic) terminology similar to that employed during ODP Leg 197 and Expedition 324 (Shipboard Scientific Party, 2002; Expedition 324 Scientists, 2010). Unless an unequivocally pyroclastic origin for volcanogenic particles could be determined, we simply described deposits composed of these materials as being of volcanic provenance according to the classification scheme for clastic sediments, noting the dominance of volcanic grains (e.g., volcanic sandstone). We followed the clastic textural classification of Wentworth (1922) to separate the various volcanic sediment types and sedimentary rocks (according to grain size) into volcanic gravel (>2 mm), volcanic sand (2 mm–62.5 µm), volcanic silt (62.5–3.9 µm), and volcanic clay (<3.9 µm). For coarse-grained, poorly sorted volcaniclastic sediments rich in basalt clasts, such as those produced by gravity currents, we applied the terms “basalt breccia” (angular clasts) or “basalt conglomerate” (rounded clasts) and used lithologic or structural modifiers for further description.

Where evidence for a pyroclastic origin was compelling, we adopted the classification scheme of Fisher and Schmincke (1984). In these instances we used the grain-size terms “volcanic blocks” (>64 mm), “lapilli/lapillistone” (2–64 mm), and “ash/tuff” (<2 mm). The term “hyaloclastite” was used for vitroclastic (i.e., glassy) materials produced by the interaction of water and hot magma or lava (Fisher and Schmincke, 1984). The term “peperite” was applied to a rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet, sediments. The term also refers to similar mixtures generated by the same processes operating at the contacts of lavas and other hot volcaniclastic deposits with such sediments (Skilling et al., 2002).

Smear slide observation

Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition) in soft sediments, but the results are not quantitative. Similar to the procedure applied during IODP Expedition 315, we estimated the abundances of biogenic, volcaniclastic, and siliciclastic constituents with the help of a visual comparison chart (Rothwell, 1989). Errors can be large, however, especially for fine silt- and clay-size fractions, and reproducibility among different sedimentologists is expected to be poor. Smear slide analysis also tends to underestimate the amount of sand-size grains because they are difficult to incorporate evenly onto the slide. Thus, it would be misleading to report values as absolute percentages. Instead, our descriptive results are tabulated as visual percentage estimates in the LIMS database, with values grouped into the following broad range of categories:

  • D = dominant (>50%).

  • A = abundant (>20%–50%).

  • C = common (>5%–20%).

  • P = present (>1%–5%).

  • R = rare (0.1%–1%).

  • T = trace (<0.1%).

Semiquantitative X-ray fluorescence analysis

Semiquantitative XRF analysis using a portable XRF spectrometer was applied for some sedimentary intervals, such as ferromanganese encrustation and phosphatized limestone (see “Geochemistry”; see also XL3_EVAL.PDF in XRF in “Supplementary material”).

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