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doi:10.2204/iodp.proc.338.206.2016

Results

Table T2 shows the peak-area values (total counts) for common minerals in the clay-size fraction, segregated by lithologic unit. We also tabulated the values of mineral abundance (wt%) calculated by SVD normalization factors (Fig. F3) and by Biscaye (1965) peak-area weighting factors (Table T2). The peak intensities for four additional specimens were too low to generate reliable results, so those values were omitted. Relative abundances of clay-size smectite within Unit IV range from 55 to 30 wt%, with a mean (µ) of 44.3 wt% and a standard deviation (s) of 6.3. Values for illite in the clay-size fraction range from 42 to 29 wt% (µ = 33.4; s = 2.7). Percentages of kaolinite + chlorite in Unit IV range from 26 to 12 wt% (µ = 19.9; s = 3.8); in all cases, chlorite is the dominant mineral over kaolinite. Percentages of clay-size quartz average only 2.5 wt% (s = 2.5). Within Unit V, the clay-size smectite ranges from 44 to 28 wt% (µ = 36.7; s = 6.2). Illite ranges from 55 to 32 wt% (µ = 37.6; s = 4.7), whereas kaolinite + chlorite ranges from 36 to 19 wt% (µ = 23.9; s = 2.6). The average percentage of clay-size quartz is only 1.8 wt%.

Figure F4 illustrates how relative mineral abundances within the bulk sediment change with depth. Bulk sediment smectite values within Unit IV range from 33 to 19 wt% (µ = 25.1; s = 3.6). Those percentages are significantly lower than what Underwood and Guo (2013) documented from coeval (5–6 Ma) Miocene strata at IODP Sites C0011 and C0012 in Shikoku Basin (i.e., the Nankai subduction inputs) (Fig. F1). Illite in the bulk sediment of Unit IV ranges from 25 to 15 wt% (µ = 18.9; s = 2.0), and kaolinite + chlorite ranges from 17 to 7 wt% (µ = 11.3; s = 2.3). Within Unit V, smectite within bulk sediment ranges from 26 to 16 wt% (µ = 21.8; s = 2.8). Illite in the bulk sediment ranges from 27 to 16 wt% (µ = 21.8; s = 3.5), and kaolinite + chlorite ranges from 18 to 11 wt% (µ = 14.1; s = 2.4).

Indicators of clay diagenesis are tabulated in Table T3 and plotted in Figure F5. Illite crystallinity (Kübler) indexes range from 0.67Δ° to 0.41Δ°2θ, with an average value of 0.52Δ°2θ. As a frame of reference, the boundary between advanced diagenesis and anchizone metamorphism is set at 0.52Δ°2θ, and the limit for epizone metamorphism (incipient greenschist facies) is 0.32Δ°2θ (Warr and Mählmann, 2015). The expandability of illite/smectite mixed-layer clays ranges from 89% to 59%, with an average value of 68% (s = 6.9). The proportion of illite in illite/smectite mixed-layer clays ranges from 6% to 49%, with an average value of 25% (s = 8.5). Values are scattered, but we note an increase in the levels of diagenesis with increasing depth (Fig. F5).

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