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doi:10.2204/iodp.proc.344.102.2013 Lithostratigraphy and petrologyThis section outlines the procedures used to document the sedimentary and igneous composition, texture, and fabric of geologic material recovered during Expedition 344. These procedures included visual core description, smear slide and petrographic thin section description, digital color imaging, color spectrophotometry, and X-ray diffraction (XRD). Because many of the geologic techniques and observations used to analyze sedimentary cores are similar to those used to analyze igneous basement cores, both methods are presented together. However, as conditions warranted, different procedures were used to characterize sedimentary and crystalline basement rocks; these distinctions are also provided. All instrument data from Expedition 344 were uploaded into the IODP-USIO Laboratory Information Management System (LIMS), and core description observations were entered using the DESClogik application (iodp.tamu.edu/tasapps/). DESClogik is a visual core description program used to store a visual (macroscopic and/or microscopic) description of core structures at a core (sediment) or section (igneous basement) scale. Core description data are available through the “Descriptive Information” LIMS Report (web.iodp.tamu.edu/DESCReport/). We used the archive halves of core sections to make sedimentary and petrographic observations and interpretations. Sections dominated by soft sediment were split using a thin wire held in high tension. Recovered hard rock was split with a diamond-impregnated saw. Cuts were oriented so that important compositional and structural features were preserved in both the archive and working halves. The split surface of the archive half of sedimentary rocks was then assessed for quality (e.g., smearing or surface unevenness) and, if necessary, gently scraped with a glass slide or spatula. After splitting, the archive half was imaged using the SHIL and then analyzed for color reflectance and magnetic susceptibility using the SHMSL (see “Physical properties”). The archive-half section was occasionally reimaged when visibility of sedimentary structures or fabrics improved following treatment of the split core surface. Following imaging, the archive-half sections of the sediment cores were macroscopically described for lithologic and sedimentary features. Lithostratigraphic units were characterized by visual inspection, and smear slide observations were used to determine microfossil and sedimentary constituents and abundances to aid in lithologic classification. Cores from the basement were also described visually and subsequently with the aid of thin sections for primary igneous features and secondary alteration features. The majority of thin sections were described on shore following the expedition due to time constraints. Based on preliminary visual descriptions and physical properties data, thin section samples and samples for XRD were extracted from the working-half sections. All descriptions and sample locations were recorded using curated depths (CSF-A) and documented on VCD graphic reports (Fig. F3). Visual core descriptions for sedimentColor and compositionColor was determined qualitatively for core intervals using Munsell Color Charts (Munsell Color Company, Inc., 2000). Visual inspections of archive-half sections were used to identify compositional and textural elements of the sediment, including sediment structures and components such as concretions, nodules, chert, rock fragments, and tephra. To emphasize the different compositions of volcanic sandstones, rocks were classified using a scheme after Fisher and Schmincke (1984). In general, coarser grained sedimentary rocks (63 µm to 2 mm average grain size) are designated as “sand(stone)” where volcanoclastic components are <25% of the total clasts. Volcanoclastics can be (1) reworked and commonly altered heterogeneous assemblages of volcanic material like lava and tuff fragments, as well as compositionally different ash lenses/particles, or (2) fresh or less altered compositionally homogeneous loose pyroclastics resulting directly from explosive eruptions on land or effusive/explosive vents on the seafloor. Pyroclasts are made out of volcanogenic material (Greek “pyro” = fire or magma) that is fragmented (Greek “clast” = broken in pieces) during explosive eruptions. In a given sediment/sedimentary rock, if ≥25% volcanoclasts but <25% pyroclasts are observed, it is designated as a “volcanoclastic sand(stone).” If the clast composition is 25%–75% pyroclasts, the sediment/sedimentary rock is classified as “tuffaceous sand(stone),” but if clast composition is ≥75% pyroclasts, the rock/sediment is classified as a “tuff” or “tephra,” respectively. Depending on grain size and degree of compaction, the nomenclature is adjusted accordingly, as shown in Table T1. Textures, structures, and sedimentary fabricWhen visible at low magnifications, sediment grain size was determined using the Wentworth scale (Wentworth, 1922). Grain size, particle shape, and sorting were also noted; however, these textural attributes required inspection at high magnification and were only performed on smear slides and thin sections (see below). Sedimentary structures observed in recovered cores included bedding, soft-sediment deformation, bioturbation, and early diagenetic mineral authigenesis. Bed thickness was defined according to Boggs (2005):
Some samples were inspected with a 10× hand lens for micrograded bedding (i.e., graded bedding occurring within laminations) and indications of preferred particle orientations, including lineation and imbrication of elongated detrital and biogenic material. Estimations in abundances of components were made semiquantitatively using the following scheme:
The abundance of bioturbation was measured using the semiquantitative ichnofabric index as described by Droser and Bottjer (1986, 1991) and with the help of visual comparative charts (Heard and Pickering, 2008). The indexes refer to the degree of biogenic disruption of primary fabric such as lamination and range from 1 for nonbioturbated sediment to 6 for total homogenization:
Additionally, the inventory of ash layers, pods, and dispersed ash layers was documented separately to describe their textural and structural occurrence within the sedimentary column. Detailed results are given in the tephra log (see “Core descriptions”). Smear slides and thin sectionsSmear slides and thin sections are useful for identifying and reporting basic textural and compositional attributes, even if the results are only semiquantitative. We estimated the texture of the sediment with the help of a visual comparison chart (Rothwell, 1989). However, errors can be large, especially for fine silt and clay-sized fractions. Smear slide analysis also tends to underestimate the amount of sand-sized grains because they are difficult to incorporate evenly onto the slide. Nevertheless, in order to define unit boundaries and subunits, smear slides are the most efficient way to evaluate differences in lithology, textures, and composition on board semiquantitatively using a point counting method. When point-counts were conducted, we used four different predefined rectangles, randomly distributed over the slide, and counted every component in this area at 100× to 200× magnification until 200 points were reached (e.g., Galehouse 1969, 1971). This is a relatively imprecise method compared to the normal net-based point counting method; however, it is quick and works well for sandstone and sandy siltstone. The theoretical 2σ error for a total of 200 counted particles is between 3% and 7%, depending on the portion of the total inventory (van der Plas and Tobi, 1965). The various components were then binned into several categories (e.g., feldspar, pyroxene, metamorphic lithics, sedimentary lithics, etc.) to facilitate the optimum reproducibility among different scientists (see smear slides in “Core descriptions”). For fine sediment (silt and silty claystone), rough estimations were made regarding the matrix using the visual comparison chart from Rothwell (1989). These estimates were supplemented with point counting of the coarser fraction. Coarse and fine fractions were subsequently combined and normalized to 100% of the total component inventory. Because most of the point counting is based on estimates, the 2σ error for the finer sediment is much higher and must be considered when interpreting the results; overall trends, especially for coarser grains, are considered more reliable. Detailed results are summarized in the smear slides (see “Core descriptions”). The relative abundance of major components was also validated by XRD (see “X-ray diffraction”), and the absolute weight percent of carbonate was verified by chemical analysis (see “Sediment geochemistry”). The sample location of each smear slide and thin section is shown in the Shipboard samples column of the VCDs. X-ray diffractionWe completed routine XRD analyses of bulk powder using a Bruker D-4 Endeavor diffractometer mounted with a Vantec-1 detector using nickel-filtered Cu-Kα radiation. Our principal goal was to identify the different minerals that are present in the sediments of the different lithologic units. We also estimate relative percentages of major components—total phyllosilicate minerals, quartz, plagioclase, and calcite—using peak areas. Most samples were selected from intervals adjacent to whole-round samples, and most are part of clusters with physical properties and carbonate samples. A few additional samples were collected periodically when there were lithology changes. Samples were freeze-dried, crushed using a mortar and pestle (along with powder for X-ray fluorescence and carbonate), and mounted as random bulk powder. Standard locked coupled scan conditions were as follows:
Diffractograms of single samples were evaluated with the Bruker DiffracPlus Evaluation software package (EVA). The upper and lower limits of each peak on the diffractogram (Fig. F4) were adjusted following the guidelines shown in Table T2, where the peak areas used for calculations of relative mineral abundances are given. As calibration was not performed, mineral proportions should only be used to assess the variation of the relative proportion of a four-component system where phyllosilicate minerals + quartz + plagioclase + calcite = 100%. The quality of these estimates relative to the absolute percentages within the mass of total solids depends on the abundance of amorphous solids (e.g., biogenic opal and volcanic glass), as well as the total of all other minerals that occur in minor or trace quantities. Thus, the primary value of bulk powder XRD data should only be to identify spatial and temporal trends in sediment composition and to assist with core-log integration. Hard rock descriptionMineralogy and unit classificationIn order to preserve important features and structures, all cores were visually examined before splitting. Large pieces of hard rock core were marked on the bottom with a red wax pencil to preserve orientation when they were removed from the split core liner. Each piece was numbered sequentially from the top of the core section and labeled on the outside surface. Broken core pieces that could be fitted together along fractures were assigned the same number and lettered consecutively from the top down (e.g., 1A, 1B, 1C, etc.). Plastic spacers were placed between pieces with different numbers; however, the presence of a spacer may represent a substantial interval without recovery. Fitted core pieces and fragile pieces were wrapped together with shrink wrap prior to splitting the core into archive and working halves. To determine the most appropriate split line, the morphology of magmatic contacts, veins, lava, and porphyroclastic flows were initially identified on the whole-round cores. Additionally, all pieces that could be confidently joined were aligned. All subsequent shipboard characterization of hard rocks was based on visual core description and thin section analysis, and hard rock core descriptions and associated shipboard analyses were archived electronically in DESClogik. First-order identification of rock type, primary igneous textures, and alteration (veins, halos, breccia, and vesicle fill) was carried out by visual observation using a hand lens (10×) and a binocular microscope. Observations including petrographic texture, grain size, groundmass mineralogy, phenocryst mineralogy, and alteration mineralogy were determined by thin section analysis. Thin section descriptions were utilized as a point of reference from which more precise hand specimen descriptions could be made. Where possible, point counting using an automated stepping stage was carried out to estimate groundmass compositions. Phenocryst crystal habit and size (minimum, maximum, and mode) were estimated for each thin section. Mineral morphologies and textural features were also recorded. Whole-section digital photomicrographs in cross-polarized light and plane-polarized light were taken, and these, along with the thin section descriptions, are available in this volume and in the IODP database. Classification of igneous units and subunits was based on variations in occurrence and abundance of primary mineralogy (e.g., phenocryst abundance), color, grain size, textural changes, the occurrence of chilled margins, or tectonic contacts. Where possible, a geological indicator is used to define a unit boundary (e.g., volcanic cooling margins); however, limited recovery lead to the emplacement of artificial boundaries where the units above and below are deemed sufficiently different. Subunit classifications were used to define zones of significant textural change, where appropriate. For phenocryst descriptions, the most abundant type is named last during any written description. For example the most abundant mineral in a clinopyroxene-plagioclase phyric basalt is plagioclase. Definition limits are defined as follows:
Color was described visually on a wet cut rock surface. Wetting, using tap water and sponge, was kept to a minimum to minimize uptake of water by expanding clay minerals (smectite). Vesicle descriptions are described as follows:
Breccia was described as basalt-clastic (i.e., clasts are formed in situ with a matrix composed of secondary minerals). This form of in situ brecciation may be considered an end-member that stems from vein-net, incipient brecciation (basaltic groundmass is nearly detached and surrounded by secondary minerals), followed by full brecciation (full detachment of basaltic fragments where they have been totally surrounded by secondary minerals). Groundmass grain size characterization includes the following:
Groundmass textures used during Expedition 344 include
AlterationAlteration from basement recovered during Expedition 344 was defined as (1) background alteration, (2) alteration halos, (3) veins and vein nets (where veining was numerous), (4) secondary minerals filling vesicles, and (5) basaltic breccia. Alteration features were recorded into the DESClogik program as separate logs. Levels of background alteration were defined following the convention in Figure F5. Vein mineralogy, orientation, width, shape, and location were recorded. Where possible, the order of infill for vesicles and veins was determined; in addition, halos and vesicle size, color, shape, and secondary mineralogy was described. Overprinting or crosscutting of veins, minerals within veins, and halos were noted. Thin sections were used to determine secondary mineralogy and describe the timing and method of secondary mineral emplacement through crosscutting relationships observed in veins, vesicles, and alteration halos. Glass was recorded in terms of percent fresh and percent altered. Visual Core Descriptions for hard rockVCD forms were used to describe each hard rock core. A key to the symbols used on the hard rock VCDs is given in Figure F5. On the VCDs, the following information is displayed from left to right:
The unit summary, located on the right-hand side of the VCD includes the following information
In instances where an individual rock fragment could not have rotated about a horizontal axis during drilling, an arrow pointing to the top of the section is shown in the Orientation column. The term “vein” describes filled crosscutting fractures and includes breccia, cataclastic injections, epigenetic mineralized veins, shear veins, and vein networks. Vein geometry and mineralogy is given in detail in the additional comments. Comments also include accessory mineral occurrences. |
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