Previous   |    Next

doi:10.2204/iodp.proc.348.204.2017

Results

Table T2 shows the peak-area values (total counts) for common minerals in the clay-size fraction, segregated by lithologic unit. The table also includes the values of mineral abundance (wt%) calculated via SVD normalization factors and weighted peak-area percentages using Biscaye (1965) factors. Relative abundances of smectite within lithologic Subunit Va range from 39 to 21 wt%, with a mean value (µ) of 27.3 wt% and a standard deviation (σ) of 3.2. A gradual depletion of smectite with increasing depth begins at approximately 2355 mbsf (Fig. F4). Values for illite in the clay-size fraction range from 48 to 23 wt% (µ = 34.3; σ = 5.4), and those values begin to increase with depth at approximately 2355 mbsf. Percentages of kaolinite + chlorite in Subunit Va range from 32 to 20 wt% (µ = 26.4; σ = 2.9); in all cases, chlorite is the dominant mineral over kaolinite. The average percentage of clay-size quartz is 12.0 wt% (σ = 5.1). Values for clay-size smectite continue to decrease within Subunit Vb, with a range of 27–6 wt% (µ = 12.0; σ = 5.0). Abundances of illite increase steadily with depth, with a range of 45–27 wt% (µ = 43.6; σ = 3.3), whereas the content of kaolinite + chlorite ranges from 27 to 17 wt% (µ = 35.0; σ = 3.3). The average percentage of clay-size quartz is 9.4 wt% (σ = 2.2).

Figure F5 illustrates how relative mineral abundances change within the bulk sediment as a function of depth. Within lithologic Subunit Va, values of bulk sediment smectite range from 26 to 14 wt% (µ = 18.0, σ = 2.8). Those percentages are significantly lower than what Underwood and Guo (2013) documented at IODP Sites C0011 and C0012 in the Shikoku Basin (i.e., Nankai subduction inputs) (Fig. F1), where coeval (9–11 Ma) Miocene strata contain averages of 42–49 wt% smectite. Illite in the bulk sediment of Subunit Va ranges from 31 to 17 wt% (µ = 22.4, σ = 3.2), and the amount of undifferentiated kaolinite + chlorite ranges from 22 to 12 wt% (µ = 17.3, σ = 1.8). Within the bulk sediment of lithologic Subunit Vb, the abundance of smectite decreases even more to a range of 19–4 wt% (µ = 7.9, σ = 3.4). Illite in the bulk sediment ranges from 33 to 25 wt% (µ = 28.4; σ = 1.9), and kaolinite + chlorite ranges from 27 to 18 wt% (µ = 22.8; σ = 2.2).

Indicators of clay diagenesis are tabulated in Table T3 and plotted as a function of depth in Figure F6. The expandability of I/S mixed-layer clays within Subunit Va ranges from 77% to 59%, with an average value of 62% (σ = 4.0). A steady decrease in expandability is evident with increasing depth, beginning at approximately 2350 mbsf (Fig. F6). Values within Subunit Vb range from 67% to 40% (µ = 51.0, σ = 5.8). The lowest possible value recorded by this method is 40% (i.e., saddle:peak ratio = 1.0). Within Subunit Va, the proportion of illite in I/S mixed-layer clays ranges from 16% to 53%, with an average value of 39% (σ = 10.0). Those values are scattered, with a slight increase in the proportion of illite as depth increases. Illite crystallinity (Kübler) indexes range from 0.56 to 0.23Δ°2θ, with an average value of 0.32Δ°2θ. As a frame of reference, the boundary between advanced diagenesis and anchizone metamorphism is set at 0.52Δ°2θ, and the achizone/epizone boundary (incipient greenschist facies) is 0.32Δ°2θ (Warr and Mählmann, 2015). The scatter of expandability values decreases markedly within Subunit Vb, illitization increases steadily with depth from 47% to 68% illite in the mixed-layer clay (µ = 59.0, σ = 5.5), and the average Kübler index increases to 0.42Δ°2θ. Broadening of the illite peak (i.e., lower crystallinity) seems to coincide with illitization of the I/S mixed-layer clay, as described above.

Top of page   |    Previous   |    Next