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doi:10.2204/iodp.proc.302.103.2006

Lithostratigraphy

Visual core descriptions

Expedition 302 sedimentologists were responsible for visual core descriptions and smear slide analysis of cored sediment and rock. Lithology of recovered material is recorded on the barrel sheets (found in “Core Descriptions”) using symbols to represent up to three components in the Graphic Lithology column (Fig. F2). Where an interval of sediment is a mixture of lithologic components, the constituent categories are separated by a solid vertical line, with each category represented by its own symbol. In contrast, constituent categories separated by a dashed vertical line indicate intervals of thinly interbedded sediments comprising two or more lithologies of different compositions. Constituents accounting for <10% of the sediment in a given lithology are not shown in the Graphic Lithology column but are listed in the Lithologic Description section of the core description form. Because of the limited scale of the core summaries, the Graphic Lithology column usually shows only the composition of layers or intervals exceeding 20 cm in thickness.

The Structure column indicates the presence of primary sedimentary structures, soft-sediment modification features, structural features, and diagenetic features observed visually (Fig. F3). The following definitions were adopted from Blatt et al. (1980, p. 128):

• Thick bedding/color banding = beds/bands > 30 cm thick.

• Medium bedding/color banding = beds/bands 10–30 cm thick.

• Thin bedding/color banding = beds/bands < 10 cm thick.

The following scale is used to describe bioturbation as measured by the percentage of burrow features:

• Slightly bioturbated = 1%–30% bioturbation.

• Moderately bioturbated = 30%–60% bioturbation.

• Heavily bioturbated = 60%–90% bioturbation.

Total mixing of sediment by bioturbating organisms produces homogeneous sediment with an appearance similar to nonbioturbated sediments that result from the deposition of material of homogeneous color and grain size. Therefore, a bioturbation scale can not be applied to homogeneous sediment with confidence.

Deformation and disturbances of sediment that clearly resulted from the coring process are illustrated in the Drilling Disturbances column of the barrel sheets (see “Core Descriptions”). The degree of drilling disturbance is described for soft and firm sediments using the categories listed below (blank regions indicate the absence of drilling disturbance):

• Slightly disturbed = bedding contacts that are slightly bent or drilling biscuits that are unrotated and in stratigraphic order.

• Moderately disturbed = bedding contacts that are extremely bowed or sediment biscuits that are rotated but likely still in stratigraphic order.

• Very disturbed = bedding that is completely disturbed or sediment biscuits that are likely rotated and no longer in stratigraphic order.

• Drilling slurry or flow-in = intervals that are water-saturated or have otherwise lost all aspects of original bedding resulting from flow-in or the presence of drilling slurry.

The degree of fracturing in indurated sediments is described in the Drilling Disturbance column using the following categories:

• Slightly fractured = core pieces that are in place and contain little drilling slurry or breccia.

• Moderately fragmented = core pieces that are in place or partly displaced, but the original orientation is preserved or recognizable (drilling slurry may surround fragments).

• Highly fragmented = pieces that are from the cored interval and probably in the correct stratigraphic sequence (although they may not represent the entire section), but the original orientation is completely lost.

• Drilling breccia = core pieces that are no longer in their original orientation or stratigraphic position and may have been mixed with drilling slurry.

The hue and chroma attributes of color are recorded in the Color column of the barrel sheet and were determined using Munsell Soil Color Charts (1971).

Figures summarizing key data from smear slides and XRD analyses appear in the “Sites M0001–M0004” chapter.

The lithologic description that appears in the Description column of each barrel sheet lists all the major and minor sediment lithologies observed in the core, as well as a more detailed description of these sediments including features such as color, composition (determined from the analysis of smear slides), or other notable characteristics. Descriptions and locations of thin, interbedded, or minor lithologies or thin color banding that could not be depicted in the Graphic Lithology column are included in the text. Terms to describe sediment induration (e.g., firm) follow those used during ODP Leg 105 (Shipboard Scientific Party, 1987).

X-ray diffraction

Equipment parameter

X-ray diffraction (XRD) measurements were performed at the Crystallography Department of Geosciences, University of Bremen, on a Philips X’Pert Pro X-ray diffractometer equipped with a Cu tube (K_α λ 1.541), a fixed divergence slit (¼°2θ), a 15-sample changer, a secondary monochromator, and the X’Celerator detector system. Measurements were made from 3° to 85°2θ with a calculated step size of 0.016°2θ. The calculated time per step was 100 s.

Peak identification was done graphically through the MacIntosh program MacDiff (version 4.5) (available at servermac.geologie.uni-frankfurt.de/​Staff/​Homepages/​Petschick/​RainerE.html) (Petschick et al., 1996).

Mineral identification

Integrated intensities for the investigated mineral peaks were calculated by MacDiff. Based on this intensity, ratios were calculated. To provide an easy comparison to published data on surface samples of the potential source regions (Andersen et al., 1996; Vogt, 1997; Vogt et al., 2001), the fixed divergence was changed to automatic divergence using an algorithm integrated in MacDiff.

Sediment classification

Expedition 302 sediment classification is based primarily on visual core descriptions and smear slide analyses. A modified version of the sediment classification format established during ODP Leg 151 (Shipboard Scientific Party, 1995) is used here to allow for easier comparison of the lithostratigraphy of cores retrieved during both cruises. As during Leg 151, the principal lithologic name (e.g., diatom ooze, silty clay) is based on the major (>50%) sediment component. Secondary components, composing 25% to 50% of the sediment, are included as major modifiers preceding the principal name (e.g., diatom ooze, silty clay). Minor constituents, composing 10% to 25% of the sediment, are included using the term “-bearing” (e.g., mud-bearing diatom ooze). The sediment modifiers are ordered so that minor modifier(s) precedes major modifier(s). Specific nomenclature for the two compositional groups, appropriate for sediments recovered during this expedition, is given below.

Siliciclastic sediments

The siliciclastic category includes sediments having >50% siliciclastic sediments. These sediments are classified on the basis of grain size (Fig. F4). Siliciclastic sediment having >75% of a single component are given the name of that major component. For example, siliciclastic sediment containing 80% clay and 20% silt would be classified simply as “clay.” When siliciclastic sediment consists of a two-component mixture, the major component is preceded by a modifier describing the secondary grain size. For example, siliciclastic sediment containing 70% clay and 30% silt would be classified as “silty clay.” In situations where the siliciclastic sediment contains a mixture of sand, silt, and clay and the least abundant of these components comprises 10% of the sediment, the term “mud” is given as the major component and is modified by the name given to the most abundant grain size. For example, a mixture of 10% sand, 60% silt, and 30% clay would be classified as “silty mud.”

Biogenic sediments

The biogenic category includes fine-grained sediments containing >50% biogenic sediments. Designation as siliceous or carbonate depends upon the relative abundance of these two biogenic categories (Fig. F5). When the biogenic component is >90%, the sediment is classified as ooze. When biogenic sediments are mixed with 10% to 25% siliciclastic components, the siliciclastic component name followed by the word “-bearing” is used as a major modifier. For example, if the sediment contains 80% biogenic silica of mixed types (e.g., diatoms, ebridians, and silicoflagellates) and 20% siliciclastic clay, it would be classified as “clay-bearing biosiliceous ooze.” When the siliciclastic component is a mixture of sand, silt, and clay, the modifier “mud-bearing” is used (e.g., mud-bearing siliceous ooze).

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