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doi:10.2204/iodp.proc.341.201.2016

Results

Consolidation and grain size data are summarized in Table T2. Complete experiment data for consolidation (341_CONSOL) and grain size (341_GRAIN_SIZE) are provided in “Supplementary material.”

Our laboratory-determined porosity values are within the range of the values determined by shipboard MAD measurements (Fig. F5). It should be noted that both the MAD values and our values include movable water and water bound to clay mineral surfaces and in clay interlayers; therefore, the porosities obtained represent the total porosity (Daigle, 2014).

Laboratory measurements are plotted against depth in Figures F6 and F7. The depth reference is meters core depth below seafloor (CSF-A), which uses the core section length for depth determination and does not include a core expansion correction (see IODP Depth Scales Terminology, v.2, at http://www.iodp.org/program-documents/). Permeabilities (k₀) determined from CRS consolidation experiments range from 1.2 × 10^–17 m² to 2.0 × 10^–13 m² and exhibit little trend with depth. Compression indexes (C_c) range from 0.13 to 0.25, and swelling indexes (C_s) range from 0.023 to 0.036. The C_s/C_c ratio can be used to quantify the fraction of virgin consolidation that is recoverable during unloading. For our data set, C_s can be fit as 16.8% of C_c with a coefficient of determination (R²) of 0.40 (Fig. F8). Neither C_c nor C_s exhibit any depth dependence. Median grain diameters (D₅₀) range from 0.00232 to 0.0969 mm. The grains are generally very poorly sorted in the classification scheme of Folk and Ward (1957), consisting of roughly equal portions of sand-, silt-, and clay-sized grains (Fig. F9) and exhibiting σ values ranging from 2.3ϕ to 4.1ϕ.

Global studies of permeability of marine sediments (Gamage et al., 2011; Daigle and Screaton, 2015) have shown that grain size exerts a first-order control on permeability. Specifically, the relative mass fractions of clay-sized versus larger particles are important determiners of permeability and porosity changes during burial, because a greater abundance of larger particles will shield the clay from consolidation during burial (Schneider et al., 2011; Reece et al., 2013). We did not observe any correlation between k₀ and the clay-sized fraction for the samples we measured (Fig. F10A). However, we did observe a weak correlation between C_c and the clay-sized fraction, with higher clay content corresponding to larger C_c (Fig. F10B). This is consistent with the experimental data of Reece et al. (2013) for samples from the Nankai Trough. Although k₀ and C_c should exhibit dependence on porosity (Scheidegger, 1963; Long et al., 2011), we did not observe any such trends. This is probably due to the small range of porosities among these samples (0.33–0.41).

Because all of our measurements were performed in the laboratory, the results may have been affected by sample disturbance introduced during the coring and preparation processes. We selected the specific intervals for testing from each ~30 cm long whole-round based on visual inspection, and all of the tested intervals appeared free of cracks and voids. However, none of the CRS consolidation experiments showed evidence of a preconsolidation stress, which would have been apparent as an inflection point in the e-log(σ_v′) curve (Casagrande, 1936). Because all samples were from at least 492.63 m CSF-A, a nonzero preconsolidation stress should be expected. The lack of apparent preconsolidation stress is likely due to sample disturbance during coring. Because of the abundance of large clasts in the sediment at Sites U1420 and U1421, core recovery was problematic, with frequent jamming of the core catcher (see the“Site U1420” and “Site U1421” chapters [Jaeger et al., 2014b, 2014c]). Coring-induced disturbance typically alters the fabric of the clay fraction of the sediment, resulting in apparent lower preconsolidation stresses or complete remolding and stress history erasure in severe cases (Silva and Hollister, 1973; La Rochelle et al., 1981; Saffer et al., 2011; Daigle and Dugan, 2014). We conclude that our samples were strongly influenced by coring disturbance. This highlights the importance of the use of consolidation experiments to estimate in situ permeability in remolded samples.

It is important to note that the value of σ_a′ that separates elastic reconsolidation from virgin consolidation is affected by in situ stresses the sediment has experienced and stresses imparted during core recovery. The in situ stresses experienced by sediments at Sites U1420 and U1421 are further complicated by a history of glaciation extending over these locations at various times in the past (Manley and Kaufman, 2002). If the in situ fabric of the sediments had been preserved, we would expect that the glaciation history at these locations would cause the sediments to have large maximum preconsolidation stresses and that all of the stress-strain data recorded in the laboratory would correspond to elastic reconsolidation. However, because we observed distinctly different values of C_c and C_s in all samples, and as mentioned previously, there were no inflection points observed in the stress-strain curves.

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