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doi:10.2204/iodp.proc.347.102.2015

Lithostratigraphy

This section provides a summary of the procedures for the description and documentation of the sedimentology of cores recovered during Expedition 347. It outlines the methodology of the visual core description and our system for sediment classification, as well as the data entry procedure upon manual completion of visual core description (VCD) sheets. Information presented here is a result of observations made by members of the OSP sedimentology team.

Shipboard descriptions from core catcher samples and preliminary observations were incorporated in these descriptions.

Onshore methodology

Visual core description

Visual description of the recovered cores was conducted by members of the sedimentology team. This description was performed on the archive halves of the split cores at the BCR.

Individual core sections measuring a maximum of 1.5 m in length were placed into core trays for visual inspection. Descriptions were initially made by hand on VCD template sheets. These featured a high-resolution line-scan image of each core section, with a series of columns for manual entry of graphics to denote lithology and illustrate aspects such as grain size variations and sedimentological features (Figs. F13, F14, F15; see handwritten VCD scans in HANDVCD in “Supplementary material”). Grain size divisions for clay, silt, sand (very fine, fine, medium, coarse, and very coarse), granules, and pebbles were defined in accordance with Wentworth (1922) (Fig. F16), and we employed subdivisions of this to allow for a more robust classification with reference to Shepard (1954) (i.e., “silty sand”) (Fig. F17). Divisions were assessed through visual comparison using a hand lens and grain size card.

Color was noted with reference to the Munsell color chart (Munsell Color Company, Inc., 1988) to assign an estimated value; however, the core was also fed through an automated color reflectance procedure (see “Physical properties”), which provides a quantification of color based on the Munsell index.

Composition was estimated using a combination of hand lenses and microscopy observations of smear slides. This allowed for semiquantified estimates of mineral and bioclastic composition to produce a sediment classification.

Bioturbation intensity (1–6) was determined using the ichnofabric index from Bann et al. (2008) (e.g., 1 = bioturbation absent, 3 = moderate bioturbation, and 6 = total biogenic homogenization of sediment).

The number of gravel clasts (>2 mm) was counted for each 10 cm section of core in gravel-bearing sediments. Lithology, shape, and surface features were recorded on the VCD sheets for clasts with diameter >2 cm.

Core disturbance

Evidence of drilling-related disturbance was captured on the VCD sheets (Fig. F13). Drilling disturbance of relatively soft or firm sediments (i.e., where intergrain motion was possible) was classified into four categories:

1. Slightly disturbed: bedding contacts are slightly bent.
2. Moderately disturbed: bedding contacts are extremely bowed.
3. Extremely disturbed: bedding is completely deformed and may show diapiric or minor flow structures.
4. Soupy: sediments are water saturated and show no traces of original bedding or structure.

In addition to these main categories for soft and lithified sediments, several other terms were used to characterize drilling disturbance:

• Washed gravel: fine material is suspected to be lost during drilling, with only washed coarse material remaining. This may have resulted from problems in the drilling and recovery of coarse-grained lithologies.
• Flow-in: soupy, displaced sediment pulled into the core liner during retrieval.
• Fall-in: downhole contamination resulting from loose materials falling from the drill hole walls.
• Sand/gravel contamination along core liner: isolated pieces of coarse contamination occurring alongside the core liner away from the core top.

Smear slides

Smear slide observations were utilized in the identification of fine-grained sediments (clay, silt, and very fine sand) and sediment matrix. Smear slides were prepared by mixing sediment and distilled water on a glass slide. Distilled water was evaporated on a hot plate, and the dried sample was mounted in Norland optical adhesive 61 using ultraviolet light. Relative abundances of silt and clay, accessory minerals, and biogenic components were estimated for each smear slide using a polarizing microscope (Marsaglia et al., 2013). Data were entered into the DIS using a Smear Slide Input Form template previously created during the offshore phase of the expedition.

Percentage compositions of sand, silt, and clay were estimated semiquantitatively using a standard visual composition chart by Terry and Chilingar (1955); however, finer grain sizes may be underestimated using this method (Bahn et al., 2008).

Preliminary petrography of the sediments was also performed on the smear slides. Microfossil and mineral identification in smear slides during the description process were noted to help direct sampling strategies.

Data entry and the Digital Information System

Visual core descriptions were entered into the DIS using a template for Visual Section Unit Description. Results of smear slide analyses were entered using the Smear Slide Input Form. VCD forms were scanned and linked to the Section Unit Description in DIS.

Offshore methodology

During the offshore phase, the lithologic characterization of sediments was based on visual analysis of core catcher material. For visual grain size classification, we used the approach of Wentworth (1922) (see “Visual core description”). The Munsell color chart (Munsell Color Company, Inc., 1988) was used to define in situ/wet sediment color. Because of possible core catcher disturbance, a classification of bioturbation intensity was not always possible and was only noted where the sediment was clearly bioturbated. Smear slides from the core catcher material were prepared in the same manner as described in “Smear slides” (see SMEARSLIDSCANS in “Supplementary material”). Onboard analyses of smear slides were carried out using a polarization microscope, with special focus on determining grain size, mineral composition, texture, and microfossils. Results were documented within the DIS. It should be noted that the possibility of artificial mixing of core catcher sediment is quite high, and the results should therefore be treated with some caution. All offshore core catcher descriptions were directly entered into the DIS without using any handwritten VCDs.

In addition to the description of core catcher material, entire liner sections were also studied to detect sedimentological features not always visible in the core catcher material, such as major hiatuses and glacial varves.

Sediment classification

The sediment classification scheme used during Expedition 347 is descriptive and defines lithology on the basis of composition and texture. The scheme has been used previously in the Ocean Drilling Program.

Nomenclature criteria for fine-grained sediments that do not contain gravel are defined by the relative proportions of sand, silt, and clay-sized material, based on the ternary classification scheme of Shepard (1954) (Fig. F17). Generic or interpretive terms such as pelagic and turbidite do not appear in this classification and were avoided because the aim was to provide a purely descriptive account of the sediments. Based on the classification scheme employed, the term “clay,” for example, is used for both clay minerals and other siliciclastic material <4 μm in size. The term “mud” describes a subequal mixture of silt and clay. For sediments with a gravel component, a modified version of the classification scheme of Moncrieff (1989) was used (Fig. F18).

Examples of principal names of fine-grained sediments are clay, silty clay, silt, sandy silt, or sand. For lithified sediments, the suffix “-stone” is added to the principal names of sand, silt, or mud. Where quartz is not the dominant mineral, a modifier is used to denote the dominant mineral phase (e.g., glauconite sand). Biogenic components are not described in textural terms at this stage; instead, they are given more detailed observations using microscopy of smear sections. In terms of visual core description, sediments with visible bioclastic components as the dominant phase are classified with a similar modifier as mentioned above (i.e., sediment with 55% sand-size foraminifers and 45% siliciclastic clay is called foraminifer clay).

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