Previous   |    Next

doi:10.2204/iodp.proc.322.202.2013

Results

Almost all of the samples analyzed in this study were selected from co-located 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, which provided estimates of the relative abundance of total clay minerals (see the “Site C0011” and “Site C0012” chapters [Expedition 322 Scientists, 2010a, 2010b]). All of the values of XRD peak-area (total counts) for minerals in the clay-size fraction are tabulated in Table T2. Table T3 lists the calculated values of mineral abundance (weight percent) using SVD normalization factors, as well as area percent using the Biscaye (1965) peak-area weighting factors. Specimens with 80% or more smectite in the clay-size fraction are classified in this report as bentonites. For smectite, we calculated its abundance in the bulk mudstone by multiplying the percentage of smectite within each clay mineral assemblage (where smectite + illite + chlorite + kaolinite = 100%) by the percentage of total clay minerals in the same bulk mudstone (where total clay minerals + quartz + feldspar + calcite = 100%) (for bulk powder data, see the “Site C0011” and “Site C0012” chapters [Expedition 322 Scientists, 2010a, 2010b]). Indicators of clay diagenesis (I/S expandability, percent illite in I/S, and illite crystallinity index) are tabulated in Table T4. The descriptions below are meant to highlight temporal variations in clay composition organized by lithostratigraphic unit, and Table T5 provides a statistical comparison among the units.

Site C0011

Shipboard scientists during Expedition 322 divided the stratigraphic column at Site C0011 into five lithologic units (see the “Site C0011” chapter [Expedition 322 Scientists, 2010a]). Because of time limitations, sampling began at 340 m core depth below seafloor (CSF) rather than the mudline, and the hole was abandoned ~170 m above the basement total depth target because of premature destruction of the drill bit. The provisional lithologic characterization of Unit I (hemipelagic/pyroclastic facies) was based on logging-while-drilling results, but the upper part of the sedimentary section was eventually verified by coring during IODP Expedition 333 (Expedition 333 Scientists, 2011).

The first cores recovered during Expedition 322 fall within the upper Miocene epoch (~7.6 Ma) and define the top of Unit II (Fig. F4). This volcanic turbidite facies consists of hemipelagic mudstone, sandstone/siltstone turbidites with abundant pyroclastic debris, and mass transport deposits. Smectite is the most abundant clay-size mineral in Unit II (average = 65%) followed by illite (average = 24%). Both of these clay minerals show considerable amounts of scatter. The chlorite + kaolinite content averages 8% within this unit, and clay-size quartz averages 2%. We sampled 11 bentonites within Unit II, and our estimates for the proportion of smectite in the bulk mudstones range from 29% to 87% (average = 42%) (Fig. F5; Table T5).

The dominant lithology of Unit III (hemipelagic facies) is bioturbated mudstone (silty clay to clayey silt); these strata range in age from 9.1 to 12.2 Ma (Fig. F4). Smectite is the dominant clay-size mineral within this unit, making up an average of 59% of the clay-size fraction. Illite content ranges from 10% to 33% (average = 27%). Values of chlorite + kaolinite average 10%, and clay-size quartz content averages 4%. We found two identifiable bentonites within Unit III. Figure F5 shows that smectite abundance in bulk mudstone ranges from 31% to 73% (average = 42%).

Unit IV (silty turbidite facies) is composed of silty claystone, fine-grained siliciclastic sandstone, and siltstone ranging in age from 12.2 to 14.0 Ma (Fig. F4). Core recovery within this unit was incomplete. Smectite (average = 68%) is the most abundant mineral in the clay-size fraction, followed on average by illite (20%), chlorite + kaolinite (7%), and quartz (5%). Unit IV contains five bentonite samples, and smectite abundance in bulk mudstone ranges from 25% to 85% (Fig. F5).

Recovery of Unit V (volcaniclastic facies) was very poor because of technical difficulties, and drilling was terminated at 881 m drilling depth below seafloor (DSF). The estimated depth to the top of basaltic basement is 1050 m seismic depth below seafloor (SSF). Based on limited recovery, the lithology of Unit V includes silicic tuff and tuffaceous sandstone (Fig. F4). Mudstone interbeds contain between 45% and 100% smectite in the clay-size fraction (average = 76%). Average values for illite, chlorite + kaolinite, and quartz are 13%, 4%, and 6%, respectively. The volcaniclastic facies includes seven identifiable bentonites, and smectite abundance in bulk mudstone ranges from 31% to 91% (average = 57%) (Fig. F5).

Figure F6 shows that the expandability of I/S mixed-layer clay minerals does not change significantly from the top of Unit II to base of the cored interval. The average value of I/S expandability is 77%. The proportion of illite in the I/S averages 11% and ranges from 0% to 27%. These plots do not reveal any meaningful progression of smectite-to-illite diagenesis as a function of depth, although it is important to remember that coring terminated ~170 m above the top of igneous basement. Heat flow data collected during Expedition 333 indicate the temperature is equal to ~80°C at the sediment/basalt interface (Expedition 333 Scientists, 2011), so diagenesis could advance to higher proportions of illite in I/S below the deepest core recovery. Values of illite crystallinity index typically range between 0.42 and 0.25 Δ°2θ. Such values are consistent with erosion of the illite as a detrital constituent from sedimentary source terrains that had been exposed to anchizone conditions of incipient metamorphism (e.g., Blenkinsop, 1988; Kisch, 1990).

Site C0012

Six sedimentary units rest above igneous basement at Site C0012 (Fig. F7), which is positioned at the summit of Kashinosaki Knoll (Fig. F2). The overall trend displayed by clay mineral assemblages shows progressive downsection decreases in illite content coupled with consistent increases in smectite (Fig. F7). Values of smectite also display considerable scatter, particularly within the lower half of the section. The contents of chlorite, kaolinite, and quartz are subsidiary to smectite throughout.

The principal lithology of Unit I (hemipelagic/pyroclastic facies) is hemipelagic mud (silty clay to clayey silt) with numerous interbeds of volcanic ash. Recovery of this unit was poor, however, and drilling disturbance was severe. Consequently, the upper part of the sedimentary section was cored again during Expedition 333 (Expedition 333 Scientists, 2011). Smectite is the most abundant clay-size mineral in Unit I (average = 53%), followed by illite (average = 32%), chlorite + kaolinite (average = 14%), and quartz (average = 1%). We found only one bentonite at the top of this unit (cored during jet-in). Estimates for the proportion of smectite in the bulk mud range from 25% to 56% (average = 36%) (Fig. F8; Table T5).

The age of Unit II (volcanic turbidite facies) ranges from 7.8 to 9.4 Ma, and the sediment consists of hemipelagic mudstone, sandstone/siltstone turbidites with abundant pyroclastic material, and mass transport deposits (Fig. F7). The most abundant clay-size mineral in Unit II is smectite (average = 63%), followed by illite (average = 26%), chlorite + kaolinite (average = 9%), and quartz (average = 2%). We found only one bentonite within this unit. Estimates for the proportion of smectite in the bulk mudstones range from 26% to 62% (average = 43%) (Fig. F8; Table T5).

The dominant lithology of Unit III (hemipelagic facies) is bioturbated mudstone (silty clay to clayey silt); these homogeneous strata range in age from 9.4 to 12.7 Ma (Fig. F7). Smectite is the dominant clay-size mineral within this unit, making up an average of 64% of the clay-size fraction. Illite content ranges from 13% to 35% (average = 25%). Values of chlorite + kaolinite average 8%, and clay-size quartz content averages 5%. We found four identifiable bentonites in the lower portion of Unit III. Smectite abundance in bulk mudstone ranges from 38% to 62% (average = 49%).

Unit IV (silty turbidite facies) is composed of silty claystone, fine-grained siliciclastic sandstone, and siltstone ranging in age from 12.7 to 13.5 Ma (Fig. F7). Smectite (average = 66%) is the most abundant mineral in the clay-size fraction, followed on average by illite (21%), chlorite + kaolinite (8%), and quartz (5%). Unit IV contains two bentonites. Smectite abundance in bulk mudstone ranges from 39% to 87% (Fig. F8).

Recovery of Unit V (siliciclastic/volcaniclastic turbidite facies) was relatively good at Site C0012. The lithology consists of silicic tuff, volcaniclastic sandstone, siliciclastic sandstone, and hemipelagic mudstone (Fig. F7). There is a significant unconformity near the base of the unit, and the maximum age is >18.9 Ma. Mudstone interbeds within Unit V contain between 38% and 97% smectite in the clay-size fraction (average = 74%). Illite content averages 18%, chlorite + kaolinite averages 3%, and quartz averages 5%. This facies includes 11 identifiable bentonites, and Figure F8 shows that smectite abundance in bulk mudstone ranges from 24% to 82% (average = 50%).

Unit IV is ~10 m thick and consists of reddish brown pelagic claystone with high concentrations of calcium carbonate. Samples from that unit were not analyzed for their clay-size mineral assemblages. Igneous basement (basalt) was also omitted from our XRD study, but information on its composition and alteration can be found in Kameda et al. (2011).

Figure F9 demonstrates that the expandability of I/S mixed-layer clay minerals does not change significantly from the top of Unit I to the base of Unit V. The proportion of illite in the I/S averages 12% and ranges from 3% to 48%. The highest values of illite occur near the base of the sedimentary section, but there is no consistent pattern of progressive smectite-to-illite diagenesis as a function of depth. The estimated temperature at the top of basement is ~65°C (Expedition 333 Scientists, 2011). Values of illite crystallinity index typically range between 0.42 and 0.25 Δ°2θ, which is similar to what was detected at Site C0011. When compared to Site C0011, however, the results from Site C0012 display more scatter in illite crystallinity and a larger number of values fall within the diagenetic zone. This is likewise consistent with erosion of the illite as a detrital constituent from sedimentary source terrains that had been exposed to anchizone conditions of incipient metamorphism (e.g., Blenkinsop, 1988; Kisch, 1990).

Top of page   |    Previous   |    Next