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doi:10.2204/iodp.proc.338.207.2017

Results

Tables T2 and T3 show the peak-area values (total counts) for common minerals in the clay-size fraction, segregated by lithologic unit. Tables T2 and T3 also include the values of mineral abundance calculated with SVD normalization factors and weighted peak area (%) using Biscaye (1965) factors. Values are rounded to the nearest 1 wt% in acknowledgment of the statistical disadvantages of having to use three sets of SVD normalization factors. In addition, some of these results suffer from unusually low peak intensities caused by relatively high concentrations of biogenic calcite in the clay-size fraction (Table T2).

Site C0018

Detrital illite is the most abundant clay mineral in the slope basin deposits, followed by detrital smectite. The relative abundance of illite within Subunit Ia ranges from 32 to 46 wt% (Fig. F3) with a mean value (µ) of 37.7 wt% and a standard deviation (σ) of 4.2. The amount of smectite in the clay-size fraction ranges from 14 to 38 wt% (µ = 26.8; σ = 5.1). Percentages of chlorite range from 12 to 36 wt% (µ = 20.2; σ = 3.8). Percentages of clay-size kaolinite and quartz are subordinate, averaging 4.1 wt% (σ = 3.4) and 11.3 wt% (σ = 4.7), respectively. There are no obvious differences in composition between intact hemipelagic deposits and MTDs.

The clay minerals in samples from Subunit Ib are similar to those in overlying deposits (Fig. F3). The range of relative percentages for illite is from 30 to 47 wt% (µ = 37.2; σ = 5.3), whereas smectite varies from 8 to 39 wt% (µ = 23.6; σ = 7.2). For 6 specimens, it was impossible to resolve the chlorite/kaolinite ratio because of interference by a strong barite peak (Table T2). The average value for undifferentiated chlorite + kaolinite is 30.7 wt%, and the average for clay-size quartz is 8.5 wt%.

Figure F4 illustrates how abundances of smectite and illite within the bulk sediment change as a function of depth. Smectite is probably the most influential for modulating frictional and geotechnical properties of the sediment. Within Subunit Ia, bulk sediment smectite ranges from 8 to 18 wt% (µ = 13.8; σ = 2.6). Comparable values within Subunit Ib range from 3 to 22 wt% (µ = 12.6; σ = 4.6).

Illite crystallinity (Kübler) index values for strata within Subunit Ia range from 0.26 to 0.42Δ°2θ (µ = 0.33Δ°2θ). The results from Subunit Ib are basically the same (µ = 0.33Δ°2θ), with no systematic changes as a function of depth (Fig. F4). 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) coincides with 0.32Δ°2θ (Warr and Mählmann, 2015). Deposits from Subunit Ia yield I/S expandability values that range from 48 to 68 wt% (µ = 61%), with no systematic changes as a function of depth (Fig. F4). The average value of I/S expandability for Subunit Ib is 64%.

Site C0021

The 21 specimens analyzed from Site C0021 (1 replicate) are limited to Subunit Ia and reveal clay mineral assemblages similar to those from Subunit Ia at Site C0018 (Fig. F3). Some of these results also suffer from unusually low peak intensities caused by relatively high concentrations of biogenic calcite (Table T3). The relative abundance of illite in the clay-size fraction ranges from 39 to 52 wt% (µ = 43.7; σ = 4.5). Smectite abundance ranges from 15 to 38 wt% (µ = 23.6; σ = 5.2). Chlorite remains consistently subordinate, ranging from 9 to 27 wt% (µ = 19.5; σ = 4.5), whereas clay-size kaolinite (µ = 4.8; σ = 3.8) and quartz (µ = 8.4; σ = 5.3) remain consistently low. Calculated values of smectite in bulk sediment (Fig. F4) range from 7 to 22 wt% (µ = 11.7, σ = 3.2). Illite crystallinity (Kübler) index values range from 0.21 to 0.42Δ°2θ (Fig. F4), with an average value of 0.35Δ°2θ (i.e., mostly anchizone source rocks). The expandability of I/S clays ranges from 49% to 66%, with an average value of 58%.

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