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doi:10.2204/iodp.proc.314315316.221.2012

Results

Using the sandstone classification of Folk (1980), most of the samples are feldspathic litharenite, with the remainder being lithic arkose (Table T2; Fig. F3). All samples contain between 18% and 40% detrital quartz within the total quartz, feldspar, lithic (QtFL; normalized percentages of total quartz, feldspar, and lithic grains) grain population. The feldspar assemblage is strongly dominated by Ca-plagioclase (Fig. F4). Lithic fragments include a mix of sedimentary, metamorphic, and volcanic fragments (Fig. F5). Sedimentary lithics dominate over the other types in most samples. Sedimentary clasts include chert and siliciclastic lithologies with sand/silt/detrital clay ratios ranging from sandstone to mudstone (Fig. F6). Metamorphic clasts are dominantly pelitic in composition and include grains that display a range of crystal size, ranging from slate to schist (Fig. F7). Volcanic lithic clasts are intermediate to mafic in composition and include grains with felsitic, microlitic, and lathwork fabrics and also vitric grains (Fig. F8). Although not included within the point-count categories because of their generally coarse crystal size (>62 µm), grains of plutonic origin are present (Fig. F9). Biogenic grains include fragments of foraminifers, radiolarians, diatoms, sponge spicules, and wood. Minor authigenic components observed during point counting include pyrite framboids and a few instances of grain aggregates cemented by gypsum. The gypsum engulfs sand grains but is highly displasive and in many instances almost devoid of included grains (Fig. F10); it is possibly a product of postcoring oxidation of the sulfide.

Despite use of the Gazzi-Dickinson point-count method, which is intended to normalize grain compositions across variations in grain size, our data display a correlation between grain size and detrital feldspar, which is more abundant in the finer sand samples (Fig. F11). This contrasts with the results of Marsaglia et al. (1992), who also used the Gazzi-Dickinson point-count method but found no such relationship. Furthermore, Expedition 316 sand compositions are somewhat more feldspathic than those reported by Marsaglia et al. (1992) for Deep Sea Drilling Project Sites 298, 582, and 583.

The polymict assemblages observed in the sand grain populations described here are similar those reported for gravels at Site C0007 (Fergusson, 2011). Pliocene samples at Site C0008 are slightly more lithic rich than Pleistocene samples at that site; however, the Pliocene samples at Site C0004 are slightly less lithic rich than the one Pleistocene sample. Thus, no clear trend with sample age can be established. Overall, compositions are more feldspathic than those reported by Marsaglia et al. (1992) for Sites 298, 582, and 583. Samples are less lithic rich than the accreted Quaternary trench-wedge samples at Ocean Drilling Program Site 1174 and less quartz rich than the accreted Pleistocene and Pliocene deposits beneath the slope-apron facies at Site 1175 as reported by Fergusson (2003).

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