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doi:10.2204/iodp.proc.307.102.2006

Lithostratigraphy

Sediment classifications

Sediments were classified based on their grain-size, composition, macrofossil content, and texture.

Mineralogy

Three sediment types were defined based on the following mineralogy: (1) carbonate-dominated sediments, (2) siliciclastic-dominated sediments, and (3) marine noncarbonate minerals. Their definitions are as follows.

Carbonate-dominated sediments are coarse- to fine-grained particles that consist of calcareous skeletal and nonskeletal grains and fragments (e.g., bioclasts and micrite). The term “micrite” is used to define very fine calcareous particles (<20 µm) of indeterminant origin. Carbonate sediments were classified after the modified Dunham classification from Embry and Klovan (1971). Dominating components were added as a prefix to the name (e.g., coral floatstone).

Siliciclastic grains were defined as (1) grains comprising minerals and rock fragments eroded from plutonic, sedimentary, and metamorphic strata and (2) grains comprising glass shards, rock fragments, and minerals produced by volcanic processes.

Marine noncarbonate minerals were (1) glauconite, a black to greenish, iron-rich sheet silicate that occurs as infill and rounded sand-sized grains; (2) phosphorides, commonly black sand- to gravel-sized grains; (3) framboidal pyrite; (4) barite; and (5) dolomite, which occurs as scattered rhombs.

Carbonate sediments

Carbonates were defined as sediments containing >50% carbonate, whereas siliciclastic sediments were defined as containing >50% siliciclastic grains.

Texture

The texture of calcareous sediments was classified after Dunham (1962) in conjunction with depositional textures described by Embry and Klovan (1971) (Fig. F1). Accordingly, carbonate crystals or fragments that are <20 µm are referred to as micrite. Within the coarse-grained carbonates (i.e., from floatstone to boundstone), the matrix between bioclasts ranges from mudstone to grainstone.

Siliciclastic sediments

Texture, structure, and composition are the main criteria for the selection of a principal name for siliciclastic sediments. The Udden-Wentworth grain-size scale (Wentworth, 1922) (Fig. F2) defines the grain-size ranges and names of the textural groups (gravel, sand, silt, and clay) and subgroups (fine sand, coarse silt, etc.). When two or more textural groups or subgroups are present, the principal names appear in order of increasing abundance. Eight major textural categories can be defined on the basis of the relative proportions of sand, silt, and clay (Fig. F3). The terms “conglomerate” and “breccia” are the principal names of gravels with well rounded and angular clasts, respectively.

Major and minor modifiers

The lithologic classification of granular sediments was refined by adding major and minor modifiers (Table T1). Minor modifiers are preceded by the term “with.” The most common uses of major and minor modifiers are to describe the composition and textures of grain types that are present in major (25%–40%) and minor (10%–25%) proportions. In addition, major modifiers can be used to describe grain fabric, grain shape, and sediment color.

The composition of grains can be described in greater detail with the major and minor modifiers “coral,” “foraminifer,” “calcareous,” and “siliceous.” The terms calcareous and siliceous are used to describe sediments that are composed of calcareous or siliceous pelagic grains of uncertain origin.

The compositional terms for calcareous grains include the following major and minor modifiers as skeletal and nonskeletal grains:

• Bioclast = fragments of skeletal remains (specific names such as “molluscan” or “gastropodan” are used where appropriate).
• Pellet = fecal particles from deposit-feeding organisms.
• Peloid = micritized carbonate particles of unknown origin.
• Intraclast = a reworked carbonate sediment/rock fragments or rip-up clasts consisting of the same lithology as the host sediment.
• Lithoclast = a reworked carbonate rock fragment consisting of a different lithology than the host sediment.
• Calcareous, dolomitic, aragonitic, and sideritic = the mineral composition of carbonate muds or mudstones (micrite) of neritic origin.

The textural designations for siliciclastic grains used standard major and minor modifiers such as gravel(ly), sand(y), silt(y), and clay(ey). The character of siliciclastic grains can be described further by mineralogy using modifiers such as “quartz,” “feldspar,” “glauconite,” “mica,” “lithic,” or “calcareous.” The fabric of a sediment can be described using major modifiers such as “grain-supported,” “matrix-supported,” and “imbricated.” Fabric terms are generally useful only when describing gravels, conglomerates, and breccias.

In lithologies where the dominant grain size was 20–63 µm and the sediments were well sorted with grains in contact with each other, we placed the major modifier “silt-sized” before the principal name (e.g., silt-sized grainstone), according to Dunham’s (1962) classification. Modifiers such as “fine sand-sized” are also used to refine the description of sand-sized grainstones in the Lithologic Description section on the barrel sheets.

Whenever dolomite or dolomitic texture was recognized (>25%) in carbonate sediments, the term “dolomitic” was used as a major modifier (e.g., dolomitic mudstone with clay). When a lithology appeared to contain more than ~75% dolomite, it was called “dolostone” or, if skeletal components can be recognized, “skeletal dolostone.”

Sediments were named on the basis of their texture and major and minor modifiers (Table T1). Principal names define the granular sediment class (Fig. F3). Composition is the most important classifier to distinguish calcareous and siliciclastic sediments. The texture is significant for the further classification of calcareous sediments (Fig. F1). Composition and texture of cored sediments were determined aboard ship by visual observation as well as smear slides, thin sections, and coarse fractions. Calcium carbonate content was qualitatively estimated in smear slides and quantitatively estimated by coulometric analysis (see “Geochemistry and microbiology”).

Lithification

Lithification of recovered materials was defined according to Gealy et al. (1971). Three classes of lithification were used to describe calcareous sediments and rocks.

• Unlithified = soft sediments that have little strength and are readily deformed under the pressure of a fingernail or the broad blade of a spatula. In the classification used here, the prefix “unlithified” is used (e.g., unlithified packstone).
• Partially lithified = firm but friable sediments that can be scratched with a fingernail or the edge of a spatula blade. Here the prefix “partially lithified” is used, as in “partially lithified grainstone.”
• Lithified = hard, nonfriable, cemented rock that is difficult or impossible to scratch with a fingernail or the edge of a spatula. Here the prefix “lithified” is used, as in “lithified wackestone” or “lithified coral rudstone.”

Two classes of lithification were used to describe siliciclastic sediments and rocks as follows:

• Soft = sediment core can be split with a wire cutter.
• Hard = the core is hard (i.e., consolidated, well indurated, or cemented) if it must be cut with a hand or diamond saw. For these materials, the suffix “-stone” is added to the soft-sediment name, as in “sandstone, siltstone, and claystone.” Note that this varies from terms used to described calcareous sediments, for which the suffix “-stone” has no firmness implications.

Core description

The visual core description (VCD) forms and barrel sheets summarize the data obtained during shipboard core description (see “Core descriptions”). Cores were described according to IODP conventions with modifications for coral-bearing sediment sequences recovered during Expedition 307. Barrel sheets represent the summarized electronic information from the VCD forms using AppleCore software.

Graphic lithology

The lithologic classification is based on a modified version of Mazzullo et al. (1988). Different sediment types are graphically represented on the barrel sheets using the symbols illustrated in Figure F4.

The Graphic Lithology column can comprise a maximum of three different lithologies (for interbedded sediments) or three different components (for mixed sediments) for the same core interval. Lithology and components are quantitatively plotted in 10% intervals. Only interbedded lithologies that constitute at least 10% are displayed and represented by a vertical line dividing the lithologies. The textures of carbonates following Dunham’s 1962 classification (Fig. F1) are shown in the same column.

Bioturbation

The degrees of bioturbation are defined as “abundant,” “common,” “moderate,” “rare,” and “not evident.”

Sedimentary structures

The locations and nature of sedimentary structures are shown in the Structure column of the barrel sheets. The legend of symbols used to designate structures is shown in Figure F4.

Fossils

The occurrence of macro- and microfossils is marked in a separate column. The symbol legend is shown in Figure F4. For coral fragments in the cores, additional characteristics were applied. The following three stages of preservation were differentiated: good, moderate and poor. If possible, the degree of bioerosion and dissolution was estimated and noted in the VCD.

Lithologic accessories

Lithologic accessories comprise various additional sedimentary features (e.g., dropstones and cements) as listed in Figure F4. Positions in the core are marked in the Accessories column by the appropriate symbol.

Core disturbance

Observations of drilling-related disturbance were recorded in the Disturbance column using the symbols shown in Figure F4. The degree of drilling disturbance in soft and firm sediments is as follows:

• Slightly disturbed = bedding contacts are slightly deformed.
• Moderately disturbed = bedding contacts have undergone extreme bowing.
• Very disturbed = bedding is completely deformed as flowing, coring/​drilling slough, and other soft-sediment stretching and/or compressional shearing structures attributed to the coring/​drilling.
• Soupy = intervals are water-saturated and have lost all aspects of original bedding.

The degree of fracturing in indurated sediments and rocks 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, and original orientation is preserved or recognizable (drilling slurry may surround fragments [i.e., drilling/​coring “biscuits” are evident]).
• Highly fragmented = core pieces are probably in correct stratigraphic sequence (although they may not represent the entire sequence), but original orientation is lost.
• Drilling breccia = the core is crushed and broken into many small and angular pieces with original orientation and stratigraphic position lost. Often drilling breccia is completely mixed with drilling slurry.

Smear slides and thin sections

Smear slides were analyzed for matrix compositions of the recovered sediments. The following categories were used: trace (<0.1%), rare (0.1%–1%), present (1%–5%), common (5%–20%), abundant (20%–50%), and dominant (>50%). Visual quantitative estimation of components is based on reference charts in Flügel (1982). When the sediments were too lithified for smear slide analyses, thin sections were analyzed instead. Tables containing data from smear slide and thin section analyses (see “Core descriptions”) include information about the sample location and whether the sample represents a dominant (D) or a minor (M) lithology in the core.

Samples

The positions of subsamples are indicated in the Sample column of VCD forms and barrel sheets and are abbreviated as follows: SS (smear slide), THS (thin section), PAL (micropaleontology), IW (interstitial water), XRD (X-ray diffraction), CARB (total carbon, total organic carbon), and MBIO (microbiology and geochemistry).

Color

After the core was split, color was determined visually using the Munsell Color Company (1994) color chart. In addition, color was measured with a Minolta CM-2002 spectrophotometer mounted on the AMST. These measurements were determined on the damp core surface, and clear plastic film was used to cover the core. The Minolta CM-2002 measures reflected visible light in thirty-one 10 nm wide bands ranging from 400 to 700 nm. Routine measurements were made at evenly spaced intervals within each section, taking into account section length and the position of voids within the section. Measurement spacing was 2 cm for core intervals.

Once daily, the spectrophotometer was calibrated for white color reflectance by attaching its white calibration cap. These white color calibrations were made to avoid variation in color readings due to the laboratory environment (temperature, humidity, and background light) and instrument variations. Spectrophotometer readings were recorded using Labview (version 5.0).

Digital camera imagery

A track-mounted Kodak DCS 460 digital camera was set up in the core laboratory so that complete cores or specific sections of interest could be imaged, cataloged, and stored.

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