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doi:10.2204/iodp.proc.314315316.207.2011

Results

The grain size distribution from specimens recovered at Sites C0001, C0002, C0004, and C0006–C0008 is presented as volumetric fractions of clay, silt, and sand next to the lithostratigraphic columns (Figs. F2, F3, F4, F5, F6, F7) and as variation of mean grain size versus depth (Fig. F8). Note that we do not present the data in consecutive order with respect to site numbering (i.e., the chronological order of drilling) but as a function of their position across the accretionary margin. Sites C0001 and C0002 represent the upper slope and seaward forearc basin, Sites C0004 and C0008 are in the mid-slope, whereas Sites C0006 and C0007 are at the lowermost slope near the frontal portion of the accretionary wedge (Fig. F1B).

At Site C0001, the grain size distribution of lithologic Units I and II is fairly similar, with the former being somewhat coarser than the latter (Fig. F2). On average, sand content in the slope apron deposits (Unit I) reaches ~5%, with maximum values reaching 12% in the silty turbidite sequence near the unconformity. Below the unconformity, the accreted, sometimes bioturbated, muds of Unit II are generally <5% sand (Fig. F2). Silt is the dominant grain size class in both units, whereas clay is ~40% (Unit I) and 45% (Unit II).

Further landward (northwest) at Site C0002, grain size data were collected only for three of the four lithologic units drilled (Fig. F3). No data exist for the Quaternary sediments of Unit I in the upper Kumano Basin sequence. In the lower forearc basin sediments, silt is the dominant grain size, whereas sand approaches and occasionally exceeds 10% and clay may be as low as 20% in places (largely in the coarser turbidite layers that are interbedded within the hemipelagic mud). Unit III represents the basal forearc basin facies of predominantly Pliocene age, where a gradual increase in clay fraction is observed (from~30% to ~50%; see Fig. F3). Sand content from either turbidites or ash ranges between 5% and 15%. In Unit IV, representing the uppermost accreted sediment, the sand fraction drops to a nearly constant 5% (exceptions are occasional thin turbidite beds). Silt accounts for ~50% of the particles, whereas clay content is lower than in Unit III (Fig. F3).

When moving to the southeast and down the slope, the shallow sedimentary succession changes drastically. At Site C0004 (Fig. F4), the 78 m thick slope apron sediments are still clayey (~30%–40%) silt (~50%–60%) with occasional fine sand or ash layers (usually <5% sand fraction). Just below (see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009a]), a 40 m thick MTC (Subunit IIA) comprising sedimentary breccia and hemipelagic muds was recovered. If the individual clasts are disregarded for grain size analysis, the overall pattern in this unit is fairly homogeneous: ~45% clay, ~55% silt, and ~5% sand (Fig. F4). With the exception of a prominent ash layer at~130 mbsf, Subunit IIB has an extremely similar grain size distribution as Subunit IIA. Unit III, the fault-bounded package separating the upper accretionary prism from the underthrust slope apron of Quaternary age, shows some scatter in the clay-silt range but has otherwise a similar grain size distribution as Unit II (Fig. F4). Finally, Unit IV shows a coarsening in overall grain size, with sand content up to 10%, silt content at 50% or slightly higher, and clay decreasing from ~40% to 30%–35%. Not unexpectedly, the underthrust slope apron (Unit IV) mimics the uppermost deposits (slope apron in Unit I) regarding both the grain size distribution and scatter (Fig. F4). The fault zone in Unit III, which represents the updip end of one branch of the megasplay fault, is somewhat finer grained than the other units (Fig. F4). However, it is coarser grained than what may be expected from potential particle disaggregation, cataclasis, and similar processes that are usually associated with large strains.

At Site C0008, adjacent to Site C0004 but a little further downslope, the entire grain size spectrum with depth shifted to coarser sizes. The slope apron (Subunit IA) largely ranges between 20% and 50% clay, ~30%–60% silt, and up to 30% sand (Fig. F5). Interestingly, the mass transport complex at this location (termed Subunit IB because, in contrast to Site C0004, the MTC deposits are within the slope basin unit; see the “Expedition 316 Site C0008” chapter [Expedition 316 Scientists, 2009d]) is coarser grained and further shows an increase in sand toward its base. Just below, Unit II represents the accreted trench wedge facies, where sand dominates over mud and recovery was poor, which may be taken as indirect evidence for the occurrence of sand. Since we tried to exclude individual clasts and other deposits of secondary (i.e., mass wasting) origin, we assume that the coarsening may be explained by amalgamation of trench wedge facies during emplacement.

When regarding the frontal portion of the accretionary wedge, two nearby sites were aimed at intersecting the frontal thrust (Sites C0006 and C0007; Fig. F1B).

At Site C0006, the uppermost ~600 m of sediment material was penetrated (Fig. F6). The uppermost ~30 m corresponds to lithologic Unit I, where hemipelagic mud with minor sand (10% ± 5% on average, with one interval containing up to 30%) were interpreted as trench–slope transitional deposits. The clay volume is 35% on average, and silt dominates (~55%–60%). Unit II (~30–450 mbsf) consists of accreted trench wedge sediment (Fig. F6), with the uppermost part to ~320 mbsf characterized by substantial scatter. Clay is 40% ± 15%, silt ranges from ~55% to 60% ± 10%, and sand is absent to >40%. In the lowermost portion of Unit II, the variability in grain size is less pronounced, the sand fraction is almost negligible, and clay steadily increases from 40% at ~320 mbsf to 50% at 450 mbsf (Fig. F6). The lowermost Unit III, the upper Shikoku Basin facies, is characterized by fairly homogeneous mudstones with generally >50% clay, 45% silt, and very little sand-sized material (often tephra). Although several structural features (e.g., shear bands) were penetrated by drilling in this unit, there is no systematic textural relationship between these thrust zones at this sampling scale.

The seawardmost drilling location cored during NanTroSEIZE Stage 1 was Site C0007 (Fig. F7) (see the “Expedition 316 Site C0007” chapter [Expedition 316 Scientists, 2009c]). Given the proximity to Site C0006, it is not surprising to find many similarities in the grain size patterns. Lithologic Unit I, comprising the trench–slope transition, shows a wide scatter with 20%–35% ± 8% clay, a more or less steady 55% silt, and 5%–35% sand (Fig. F7). In the accreted trench wedge facies below this interval, we observe again a larger scatter in texture in the upper portion and more uniform patterns in the lower portion. Unit II spans from ~30 mbsf to an unconformity at 362 mbsf, with substantial variability in texture to ~200 mbsf. Similar to Site C0006, the lower portion has increasing clay (up to 50%), a constant 50%–60% silt, and only occasional sand beds (Fig. F7). In Unit III, the upper Shikoku Basin facies, mudstones with clay fractions slightly exceeding 50% and little sand (0%–10%) are found. The scatter seen in the data does not correlate with any structural observations but merely reflects somewhat increased sand or ash contents in some intervals. In the accreted trench wedge facies underneath (Unit IV), recovery was insufficient to regularly sample the core for a continuous grain size record with depth (Fig. F7).

In Figure F8, we show the variation in mean grain size (φ) (sensu Folk and Ward, 1957) and grouped the six drill sites with respect to their position in the accretionary complex (Fig. F1B).

Sites C0001 and C0002 show fairly homogeneous curves with φ ranging predominantly between 4–7 µm (Site C0001) and 4–12 µm (Site C0002) (Fig. F8). Mean grain size is larger in the slope apron and forearc basin when compared to the upper accretionary mud and mudstones.

When moving downslope, φ at Site C0004 ranges mostly from 4 to 6 µm, with the exception of the coarser, lowermost part of the underthrust slope apron, whereas Site C0008 is coarser (φ = 5–12 µm). Interestingly, the MTC at Site C0008 shows a much larger mean value (up to >20 µm), whereas the MTC at Site C0004 exhibits the opposite trend with φ close to 4 µm in this interval (Fig. F8).

Near the toe of the accretionary complex, Sites C0006 and C0007 show the largest mean values of this study. In either location, Unit II (accreted trench wedge) φ is >70 µm (Site C0006) and 90 µm (Site C0007) in the upper portion (Fig. F8). In the lower portion of Unit II, the mean values drop drastically and φ approaches 4 µm, a value which is also typical for the mudstones of Unit III (upper Shikoku Basin facies).

All data of this study will be part of an electronic appendix that will be uploaded to the IODP Web site as well as the World Data Centre WDC-MARE (www.pangaea.de) and can be used by other NanTroSEIZE researchers. See PARTSIZE in “Supplementary material.”

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