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doi:10.2204/iodp.proc.317.204.2015

Results and observations

Lithologies

Imaging data for each sample including core photo, thin section scan, optical microscopy (plane and polarized light), SEM-BSE, and X-ray elemental maps were combined into one document to provide a visual reference that highlights sample features at different magnification scales (see COREIMAGES in “Supplementary material”).

The results of general observations and more detailed point counting of grains for each of the samples is presented in Tables T2 and T3. The lithology naming scheme for the samples follows the method adopted by Expedition 317 scientists, which was modified from the “Expedition 317 summary” chapter (Expedition 317 Scientists, 2011). The samples were separated into three categories: limestone, marlstone, and sandy marlstone. Four samples (from Cores 317-U1351B-12H, 19X, and 96X and 317-U1354C-20X) were found to contain primarily bioclastic material. In contrast, samples from Cores 317-U1352B-2H, 317-U1352C-10R, 317-U1353B-88X and 92X, and 371-U1352B-13H were found to contain primarily siliciclastic sand. The remaining samples showed mixed content, with variable bioclastic/sand ratios.

Grain sizes are relatively uniform across all samples, on the order of <100 µm for the siliciclastic particles. These particles are generally poorly to moderately sorted. Grain-to-grain contacts range from point to floating, and grain shapes range from angular to subangular. Dominant terrigenous sand constituents are quartz, feldspar, and mica, and these total up to 87% of the sample. Bioclasts range up to ~40% of the sample and are mainly foraminifers and mollusk fragments. Other carbonate components include calcareous micritic matrix (calcareous biogenic silt to authigenic cement?) and fine pore-filling cement ranging up to ~60% of the sample in fine micritic concretions.

The samples exhibit three main types of porosity: interparticle, intraparticle, and moldic. Porosity values were first estimated using area percentage charts ranging from <1% to 30% for the sample set (Table T2). Point count results of porosity values were generally lower than those obtained using the estimation charts and range from <1% to 22%. Samples with the highest porosity contain the highest percentage of bioclasts. No significant correlation exists between the remaining point count data and the elemental, carbonate XRD, or isotope data.

XRD and SEM

The elemental composition of the cement and aggregate components were examined in thin section using low-vacuum SEM-EDS mapping. The elemental maps are located in COREIMAGES in “Supplementary material,” whereas tabulated values in counts per second are provided for the cement in each sample and selected rhombohedral crystals in Table T5.

Results of XRD analyses are summarized in Table T4 and are presented as counts per second for quartz and the carbonate phases that were detected. This was done to simplify the data set, as the main focus of this research is on cement identification.

All of the elemental maps contain calcium as a significant element, and bulk XRD analysis confirmed the presence of calcite at some level in all of the samples. Multiple samples (from Cores 317-U1352B-2H, 317-U1352C-10R, 317-U1353B-88X and 92X, 317-U1354A-15H, and 317-U1354B-13H) were found to contain magnesium as a major component of the cement correlated with calcium suggestive of magnesium-enriched phases such as dolomite or magnesian calcite. Evaluating the ratio of Ca/Mg signal within the elemental maps in counts per second from reconstructed spectra of the cemented zones separates these samples into two groups. Samples from Cores 317-U1352B-2H and 10R, U1354A-15H, and U1354B-13H have ratios ranging from 1.4 to 2.0. XRD analysis identified these samples as containing ferroan dolomite. Samples from Cores 317-U1353B-88X and 92X, which have ratios of 7.3 and 10.1, respectively, were shown to contain magnesian calcite via XRD. This link between Ca/Mg ratio and verified mineralogic phase provides a way to identify the location of these phases within samples where they occur in minor amounts. Samples such as those from Cores 317-U1352B-52X and 317-U1352C-15R and 21R, which show small crystals containing magnesium and calcium, can be evaluated. In all three cases, reconstructed spectra of the crystals show that the Ca/Mg ratio falls in the same range as ferroan dolomite. A closer look at the XRD data table reveals that there are counts related to the ferroan dolomite peak for each of these samples.

Exceptions to these nice-fitting correlations exist. Sample 317-U1351B-46X-CC, 1–5 cm, shows 41 counts per second for ferroan dolomite, yet looking at the elemental maps, no correlation exists between the magnesium and calcium maps. This might be due to the fact that sample locations for thin section and XRD samples differ slightly.

Very minor (<1%) pyrite crystals were found in all 30 samples, noted by their characteristic high contrast in the BSE images and their morphological structure and confirmed through the correlation of Fe and S intensities in EDS maps; a group of pyrite crystals can be seen in Figure F3. Other minerals observed by optical microscopy, such as micas, feldspars, and quartz, were also easily observed using the SEM-EDS maps and their characteristic structural features. For example, a particle of mica interleaved with pyrite can be observed in the lower left corner (see Fig. AF9 in COREIMAGES in “Supplementary material”), where magnesium, aluminum, and iron are all present in a single grain and the characteristic lamellae of the mica can be observed in the BSE image (part A).

Isotopes

The isotopic data are presented as a bivariate plot of δ¹⁸O PDB (‰) vs. δ¹³C PDB (‰) in Figure F4 with various grouping overlays including (A) arbitrary grouping by clustering; (B) categories of depositional environments as modified from Nelson and Smith (1996) following the work of Hudson (1977), Bathurst (1981), Choquette and James (1987), Moore (1989), Morse and Mackenzie (1990), and Marsaglia and Carozzi (1990); (C) mineral classification from Mozley and Burns (1993); and (D) color coded with XRD identification. The results cluster into four distinct groupings (labeled A–D). Groups A and B have relatively similar δ¹³C values and could potentially be grouped together; however, there is a large enough gap between the δ¹⁸O values that two separate groups were assigned. Group A consists of three samples with δ¹³C values ranging from −2.3‰ to –3.5‰ and δ₁₈O values ranging from −2.0‰ to –2.4‰ and falls into a warm-water skeleton classification. Group B consists of six samples with δ¹³C values ranging from –1.2‰ to –3.9‰ and δ¹⁸O values ranging from –0.1‰ to 1.4‰. Using the Nelson classification scheme, these materials are derived from oozes (note that the relative percentage of calcareous components in these ooze samples is not specified).

The remaining two groups diverge significantly from the δ¹³C values that are observed in Groups A and B. Group C contains three samples (317-U1352B-2H, 317-U1354A-15H, and 317-U1354B-13H) that have δ¹³C values between –12‰ and –24‰ and δ¹⁸O values between 1.9‰ and 5.6‰. All three samples were identified as containing ferroan dolomite by XRD. The closest corresponding category in the Nelson and Smith identification scheme on the bivariate plot would be for mixing zone dolomites.

Group D contains two samples (317-U1353B-88X and 92X) that have δ¹³C values below –40‰; these two samples stem from the same drill site and were identified by XRD as containing magnesian calcite (see above). The extremely low δ¹³C values comfortably place these samples as being derived from a methanogenic origin. Having such low δ¹³C values is unusual but not unheard of for the region. Lawrence (1991) reported δ¹³C values below –30‰ PDB in concretionary dolomite deposits in Eastern Marlborough, New Zealand, on the South Island. Sample 317-U1354C-17X-CC, 3 cm, was originally assigned to Group C; however, it is known to contain mainly calcite from XRD and SEM analysis and could potentially also be assigned to Group B. It appears that the isotopic values could be transitional between the two groups or this may represent sampling of a dolomite-rich zone of the sample billet.

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