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doi:10.2204/iodp.proc.314315316.202.2012

Results

Nearly all of the samples analyzed in this study were selected from colocated “clusters” immediately adjacent to the whole-round samples used for shipboard analyses of interstitial water chemistry and shore-based tests of frictional, geotechnical, and hydrogeological properties. Each “cluster” includes a specimen for shipboard bulk-powder XRD analysis. All of the values of XRD peak area for the clay-size fractions are tabulated in Table T2. Table T3 lists the calculated values of mineral abundance (wt%) using both SVD normalization factors and Biscaye (1965) peak-area weighting factors. For smectite and illite, we also illustrate stratigraphic trends in the calculated values of mineral abundance in the bulk mud(stone). Brief descriptions of spatial and temporal variations in clay composition are organized below by tectono-stratigraphic domain (Kumano forearc basin, shallow megasplay fault, and frontal thrust).

Kumano Basin, Site C0002

Shipboard scientists divided the stratigraphic column at Site C0002 (Kumano Basin) into four lithologic units (see the “Expedition 315 Site C0002” chapter [Expedition 315 Scientists, 2009b]). Compositional variations among lithologic units are subtle (Fig. F4). Unit I (upper forearc basin) consists of silty clay to clayey silt, sand and silt turbidites, and volcanic ash beds; the ages of these strata are younger than ~1 Ma. The most abundant clay-size mineral is illite (38% average), followed by chlorite (22% average), smectite (22% average), quartz (13% average), and kaolinite (5% average) (Table T4; Fig. F5). The dominant lithology of Unit II (lower forearc basin) is silty clay to clayey silt with comparatively fewer beds of sand, silty sand, silt, and volcanic ash; these strata range in age from ~1.6 to ~1 Ma. Illite (35% average) is still the dominant mineral in this unit, followed by smectite (25% average), chlorite (22% average), quartz (13% average), and kaolinite (5% average) (Table T4; Fig. F5). Unit III (basal starved basin) is composed of silty claystone ranging in age from ~3.8 to ~1.6 Ma. Smectite (36% average) is the most abundant mineral, followed on average by illite (35%), chlorite (16%), quartz (9%), and kaolinite (4%) (Table T4; Fig. F5). The expandability of I/S mixed-layer clay minerals does not change significantly from the top of Unit I to base of Unit III (60% average), although we note an increase in the scatter of values within Unit I (standard deviation = 5%).

The boundary between Units III and IV at Site C0002 is an angular unconformity separating the accretionary prism below from overlying forearc-basin deposits (Fig. F4). The corresponding hiatus lasted from ~5.0 to ~3.8 Ma. The primary lithology of Unit IV consists of silty claystone to clayey siltstone with sporadic interbeds of siltstone and sandstone. The maximum nannofossil age for this unit is 5.90 Ma. The dominant mineral in this unit is smectite (41% average; 60% maximum), followed in average relative abundance by illite (31%), chlorite (15%), quartz (6%), and kaolinite (5%) (Table T4; Fig. F5). Standard deviations for these abundances increase below the unconformity, particularly for values of smectite (10%). This shift toward higher contents of smectite is consistent with the late Miocene age of the accretionary prism, as documented more thoroughly elsewhere (e.g., Underwood and Steurer, 2003). The expandability of smectite also increases abruptly across the unconformity to an average value of 69% (Tables T2, T4; Fig. F5).

Shallow megasplay fault, Sites C0001, C0004, and C0008

Site C0001 is positioned in the hanging wall of the megasplay fault (Fig. F2), and the stratigraphy there consists of two units: a slope apron facies (Unit I) separated from the upper accretionary prism (Unit II) by a major angular unconformity (see the “Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009a]). The unconformity’s hiatus lasted from ~3.79 to ~2.06 Ma. The principal lithology of Unit I is silty clay to clayey silt with sparse interbeds of volcanic ash, silt, and fine sand (Fig. F6). Illite (36% average) dominates the clay-size fraction in this unit, followed by smectite (31% average), chlorite (19% average), quartz (9% average), and kaolinite (5% average) (Table T4; Fig. F7). Percentages of smectite gradually increase from the top to the base of Unit I, with a range of 17% to 47% of the clay-size fraction (Fig. F6). These increases are balanced by steady decreases in quartz. Below the unconformity, accreted Pliocene and uppermost Miocene strata of Unit II consist of deformed silty claystone to clayey siltstone, rare volcanic ash, siltstone, and silty sandstone, with a maximum age of 5.32 Ma (see the “Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009a]). Illite remains the most abundant clay-size mineral below the unconformity (37% average), but there is a clear shift toward higher values (Fig. F6). Average percentages for the other minerals within Unit II are smectite (36%), chlorite (15%), kaolinite (8%), and quartz (4%) (Table T4; Fig. F7). Compared to Site C0002, this part of the accretionary prism is younger in age and contains lower percentages of smectite. There are no major differences in the expandability of I/S (63% average) at Site C0001, although expandability values are consistently lower in the top 50 m of the section (Tables T2, T4; Fig. F7).

Coring at Site C0004 successfully penetrated the megasplay fault near its updip intersection with the seafloor (Fig. F2), and shipboard scientists subdivided that section into four lithologic units (Fig. F8). The main lithology of Unit I (slope apron facies) is silty clay with a substantial component of calcareous nannofossils (as much as ~25%). Illite is the most abundant mineral within this unit (36% average), followed by average values of smectite (32%), chlorite (17%), kaolinite (6%), and quartz (7%) (Table T4; Fig. F7). The boundary between Units I and II is an unconformity, with a hiatus that lasted from ~2.0 to ~1.6 Ma (see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009a]). Below that contact, Subunit IIA (mass transport complex) consists of synsedimentary breccia with rounded to subangular clasts of mudstone and subsidiary beds of silty clay. The average content of illite in Subunit IIA is 37%. Other relative percentages are, on average, smectite (26%), chlorite (23%), quartz (10%), and kaolinite (4%) (Table T4; Fig. F7). Thus, mineralogy changes significantly across the unconformity, particularly with respect to percent smectite. The dominant lithology of Subunit IIB (accretionary prism) is silty clay. In this unit as well, the dominant mineral is illite (38% average) followed by smectite (28% average), chlorite (21% average), quartz (7% average), and kaolinite (6% average) (Table T4; Fig. F7). Unit III is the fault-bounded package of uncertain stratigraphic origin. Its boundary with Unit II is marked by a time reversal from ~4.13 Ma above to ~2 Ma below (see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009a]). Mudstones from below the fault contain more calcite, and there are ubiquitous beds of volcanic ash. Illite (36% average) and smectite (36% average) are the two most abundant clay minerals (Fig. F8). The average content of chlorite is 18%; averages for kaolinite and quartz are 6% and 5%, respectively (Table T4; Fig. F7). This assemblage of clay minerals, with generally higher contents of smectite, is consistent with what has been documented elsewhere in the upper Shikoku Basin facies (Steurer and Underwood, 2003). The main trace of the megasplay fault forms the lower boundary of Unit III, and the footwall (Unit IV) is interpreted to be a slope-apron facies with an age of ~1.6 Ma. The dominant clay minerals in Unit IV are smectite (34% average) and illite (33% average), followed on average by chlorite (19%), quartz (9%), and kaolinite (4%) (Table T4; Fig. F7). These relative percentages are similar to what we report for Unit I, although marginally higher in percent smectite.

Site C0008 is located just seaward of the shallow tip of the megasplay fault (Fig. F2), and shipboard scientists divided that section into three units/subunits (see the “Expedition 316 Site C0008” chapter [Expedition 316 Scientists, 2009d]). Subunit IA (slope-apron facies) consists of silty clay with a substantial concentrations of calcareous nannofossils, thin sand/silt turbidites, and volcanic ash. Illite (35% average) is the most abundant clay mineral. Other mineral contents are smectite (29% average), chlorite (20% average), quartz (10% average), and kaolinite (6% average) (Table T4; Fig. F7). Subunit IB (mass transport complex) includes a series of interbedded mud-clast gravels and silty clay beds with an age range of ~2.9 to ~1.6 Ma. Illite (35% average) is the most abundant clay mineral in the silty clay beds, followed on average by smectite (30%), chlorite (20%), quartz (10%), and kaolinite (5%) (Table T4; Fig. F7). We analyzed carefully separated specimens of silty clay matrix and mudstone clasts from the conglomerates to test for differences in composition. Those tests show a significant increase in the abundance of smectite in the mudstone clasts relative to clay matrix and the background of bedded hemipelagic mudstone (Fig. F9). The clasts contain approximately 43% to 52% smectite. Beneath the mass transport deposits, Unit II is interpreted as accreted trench-wedge facies (Fig. F9), but poor recovery within the sand-rich turbidites precluded our sampling and analysis of clay minerals.

Frontal thrust, Sites C0006 and C0007

The main goal of coring at Site C0006 was to cross the frontal thrust of the accretionary prism (Fig. F2), but recovery was hampered by high concentrations of unlithified sand, particularly near the bottom of the hole. The primary lithology of Unit I (trench-slope transition) is silty clay (Fig. F10) with a maximum age of ~0.44 Ma (see the “Expedition 316 Site C0006” chapter [Expedition 316 Scientists, 2009b]). The most abundant clay mineral in Unit I is illite (38% average), followed on average by smectite (25%), chlorite (24%), quartz (10%), and kaolinite (3%) (Table T4; Fig. F11). Unit II (trench-wedge facies) displays an overall upward coarsening trend accentuated by progressive increases in silt and sand turbidites; the unit ranges from ~1.5 to ~0.44 Ma in age. Illite (35% average) is the dominant clay mineral within the accreted trench-wedge deposits, followed on average by smectite (27%), chlorite (26%), quartz (8%), and kaolinite (4%) (Table T4; Fig. F11). A gradual increase in smectite content is evident downsection within this unit, from a minimum of 15% to a maximum of 43% of the clay-size fraction (Fig. F10). The boundary between Unit II and Unit III (upper Shikoku Basin facies) is an unconformity with a hiatus from ~2.87 to ~1.46 Ma (see the “Expedition 316 Site C0006” chapter [Expedition 316 Scientists, 2009b]). Below the unconformity, the age of Unit III extends into the uppermost Miocene (5.32 Ma), and the lithology consists of silty claystone with abundant interbeds of volcanic tuff (Fig. F10). The dominant clay mineral in this facies is smectite (37% average; 49% maximum), followed on average by illite (36%), chlorite (17%), kaolinite (6%), and quartz (5%) (Table T4; Fig. F11). In addition to the unit’s significantly higher abundance of smectite, the expandability of I/S shifts to modestly lower values (by ~3%) across the Unit II/III boundary.

After attempts failed to core across the frontal fault at Site C0006, Site C0007 was positioned closer to the trench (Fig. F2). Unit I (trench–slope transition) is dominated by silty clay and thin, fine-grained turbidites (Fig. F12), similar in age and composition to what was recovered at Site C0006 (see the “Expedition 316 Site C0007” chapter [Expedition 316 Scientists, 2009c]). Consistent with our results from Site C0006, the most abundant clay mineral within this facies is illite (39% average), and the other average percentages are chlorite (25%), smectite (23%), quartz (10%), and kaolinite (2%) (Table T4; Fig. F11). One anomalous specimen within this unit contains unusually high amounts of smectite (66%). Unit II (accreted trench-wedge facies) displays an overall upward coarsening trend marked by progressive increases in silt, sand, and gravel turbidites (Fig. F12). That unit’s age ranges from ~1.46 to ~0.4 Ma. Similar to correlative deposits at Site C0006, illite is the most abundant clay-size mineral (35% average), followed on average by smectite (25%), chlorite (25%), quartz (9%), and kaolinite (3%) (Table T4; Fig. F11). The boundary between Units II and III is an unconformity, with a hiatus that lasted from 3.65 to 2.06 or 1.46 Ma (see the “Expedition 316 Site C0007” chapter [Expedition 316 Scientists, 2009c]). Unit III (upper Shikoku Basin facies) consists of silty claystone with abundant interbeds of volcanic ash layers, including one calcite-cemented tuff. Below the boundary, Pliocene to uppermost Miocene mudstone reaches a maximum age of 5.32 Ma and possesses a total-clay-mineral content substantially higher than overlying Unit II, averaging 65% of the bulk sediment (see the “Expedition 316 Site C0007” chapter [Expedition 316 Scientists, 2009c]). Smectite increases to the most abundant clay mineral (39% average). Average percentages for the other minerals are illite (36%), chlorite (16%), quartz (6%), and kaolinite (3%) (Table T4; Fig. F11). The expandability of I/S decreases by ~3% across the boundary between accreted trench-wedge facies and upper Shikoku Basin facies (Tables T2, T4; Fig. F12). Once again, this mineral assemblage is consistent with what has been documented for the upper Shikoku Basin facies elsewhere in the Nankai Trough (Steurer and Underwood, 2003). Coring demonstrated that the age reverses to 3.65 Ma below the frontal thrust, but recovery of sandy material from Unit IV was not high enough to permit sampling for clay mineralogy.

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