Previous   |    Next

doi:10.2204/iodp.proc.338.101.2014

Site summaries

Site C0002

Logging

Three logging units (III, IV, and V) (Fig. F7) were defined in Hole C0002F, with logging Units IV and V further divided into five and two subunits, respectively.

Logging Unit III (875.5–918.5 mbsf) is characterized by relatively consistent responses in the gamma ray, resistivity, and sonic logs (Fig. F7). The general logging character suggests this unit is mainly composed of clay- to silt-rich sediment, with some thin interbedded sand layers. Logging Unit III identified in Hole C0002F correlates with Hole C0002A Unit III defined during Expedition 314 (Expedition 314 Scientists, 2009a).

Within logging Unit IV (918.5–1638 mbsf), the gamma ray and resistivity logs have generally higher variability than in the other logging units (Fig. F7). The general log character is interpreted to represent interbedding and alternating layers of thick sand-rich and clay-rich packages (lower to higher gamma ray values). In logging Unit IV, prominent highs in resistivity coincide with gamma ray lows, which are interpreted as sandier lithologies. This unit also shows two low-velocity zones. The fractures in logging Unit IV are mostly resistive and concentrated in an interval from 1500 to 1638 mbsf.

Logging Unit V (1638–2005.5 mbsf) is characterized by high gamma ray values and is interpreted as a homogeneous clay-rich unit (Fig. F7). Sonic velocity is nearly constant in this unit. Resistivity data show some spikes and downhole variation. Logging Unit V contains the largest concentration of conductive fractures in the borehole.

Compressional borehole breakouts and DITFs observed in Hole C0002F suggest a northeast–southwest orientation of the maximum horizontal stress (σ_Hmax) throughout this hole, which is consistent with breakout data obtained in Hole C0002A during Expedition 314 (Expedition 314 Scientists, 2009a).

Lithology

Four lithologic units were identified at Site C0002 based on geological and geochemical characteristics of core and cuttings samples: Unit II (200–505 mbsf in Holes C0002K and C0002L), Unit III (875.5–1025.5 mbsf in Hole C0002F and 902–926.7 mbsf in Hole C0002J), Unit IV (1025.5–1740.5 mbsf in Hole C0002F and 1100.5–1120 mbsf in Hole C0002H), and Unit V (1740.5–2004.5 mbsf in Hole C0002F) (Fig. F8). The difference of the lithologic Unit III/IV boundary between Holes C0002F and C0002J arises from RWD in Hole C0002F, which resulted in mixing of cuttings over an interval of as much as ~100 m. As a result of RWD, the logging Unit III/IV and IV/V boundaries in Hole C0002F are placed at 918.5 mbsf and 1638 mbsf, respectively (Fig. F7), which are ~100 m shallower than the corresponding lithologic unit boundaries (Fig. F9).

Lithologic Unit II in Holes C0002K and C0002L is dominated by silty claystone with subordinate sandstone, sandy siltstone, calcareous silty claystone, and fine ash. Similar to Unit II in Hole C0002B, this unit is interpreted to be lower Kumano forearc basin sediment dominated by the hemipelagic mud of distal turbidites (Expedition 315 Scientists, 2009).

Lithologic Unit III in Hole C0002J is dominated by heavily bioturbated, calcareous silty claystone containing abundant glauconite. Cuttings from lithologic Unit III in Hole C0002F have similar lithologic features. These lithologic features are consistent with those observed in Unit III in Hole C0002B, which was interpreted as basal Kumano forearc basin sediment that accumulated above the carbonate compensation depth (CCD) at sediment-starved conditions (Expedition 315 Scientists, 2009).

The lithologic Unit III/IV boundary was sampled at 926.7 mbsf in Hole C0002J and is characterized by (1) a relatively sharp contact immediately overlain by a 15 cm thick interval with mixed fragments from both calcareous glauconitic silty claystone above (Unit III) and less calcareous, nonglauconitic silty claystone below (Unit IV) (Fig. F10A); (2) an abrupt and substantial increase in the abundance of sand below this boundary; and (3) a substantial decrease in the amount of carbonate in silty claystone below this boundary. The nature of the Unit III/IV boundary suggests that this is an erosional unconformity.

Lithologic Unit IV in Holes C0002F, C0002H, and C0002J is dominated by noncalcareous silty claystone with subordinate sandstone and sandy siltstone (Figs. F8, F11). Lithologic Unit IV is divided into five subunits based on sand content (Fig. F8). Based on the sandstone-rich deposits recovered in Hole C0002F, lithologic Unit IV is interpreted as upper accretionary prism sediment that accumulated either in a paleotrench or in the Shikoku Basin. Low carbonate concentration in silty claystone (Fig. F8) suggests deposition below the CCD.

Lithologic Unit V in Hole C0002F is composed almost entirely of silty claystone (Fig. F8). Its thickness of several hundred meters suggests that it may be correlatable with hemipelagic Unit III drilled at subduction input Sites C0011 and C0012 (Expedition 322 Scientists, 2010a, 2010b).

Structural geology

In the Kumano Basin sediment (lithologic Units II and III) in Holes C0002J–C0002L, bedding is subhorizontal to gently dipping (Fig. F11). At the base of lithologic Unit III, however, bedding is intensely disrupted and boudinaged (e.g., Fig. F10B). In Unit III in Hole C0002J, east-west–striking and north-dipping low- to moderate-angle faults are dominant. Vein structures (e.g., Fig. F10D) were observed in cores and cuttings exclusively from Unit III in Holes C0002F and C0002J (Fig. F8). Deformation bands (e.g., Fig. F10C) were also observed mostly in cores from Unit III in Hole C0002J (Fig. F11).

In the upper accretionary prism sediment (lithologic Unit IV) in Holes C0002H and C0002J, bedding dips at variable angles of 7°–64° (Fig. F11). Reoriented bedding in Unit IV is subhorizontal to steeply dipping toward the south or north. Four sets of faults were observed in Unit IV in Hole C0002H: north–south-striking and east-dipping high-angle faults, northwest-striking and northeast-dipping high-angle faults, east–west-striking high-angle faults, and north–south-striking and west-dipping low-angle faults.

Cuttings containing carbonate veins (Fig. F10E) occur throughout accretionary prism Units IV and V (Fig. F8). Cuttings with slickenlined surfaces also occur throughout Units IV and V. Cuttings with slickenlined surfaces up to 10% occur at 1550.5–1675.5 mbsf. This interval is correlatable with the high fracture concentration interval of 1500–1550 mbsf and the fault zone at ~1640 mbsf, identified by LWD resistivity images.

Geochemistry

Geochemical data of interstitial water and gas sampled from cores in Holes C0002H and C0002J–C0002K obtained during Expedition 338 complement previous data in Holes C0002B and C0002D obtained during Expedition 315 (Expedition 315 Scientists, 2009), so continuous profiles of geochemical data to ~1100 mbsf are now available for Site C0002 (Figs. F12, F13).

Salinity, chlorinity, and Na⁺ show similar changes with depth, reaching minimum concentrations at 300–500 mbsf (Fig. F12). These minimum concentrations of dissolved salts are attributable to freshwater derived from the dissociation of methane hydrate.

Methane shows a relatively high concentration near 300 mbsf, and propane shows high concentration at 200–400 mbsf (Fig. F13). The methane- and propane-rich interval at 200–400 mbsf corresponds to the gas hydrate zone inferred from resistivity and sonic log data (Expedition 314 Scientists, 2009a). However, no massive gas hydrates were found in this interval, although gas-rich sands were common. This suggests that the methane hydrates are disseminated in porous sand layers. A prominent methane peak was observed in drilling mud-gas data when passing the logging Unit III/IV boundary at 918 mbsf (Fig. F14).

The ratio of methane to ethane and propane (C₁/[C₂ + C₃]) and δ¹³C concentration in methane (δ¹³C-CH₄) suggest that methane in the gas hydrate zone is mostly of microbial origin. Together with C₁/(C₂ + C₃) and δ¹³C-CH₄ data of mud gas sampled during riser drilling in Hole C0002F, thermogenic methane is shown to gradually increase with depth up to ~50% at ~2000 mbsf (Fig. F14).

Physical properties

Discrete moisture and density (MAD) measurements were conducted on cores and cuttings sampled from Holes C0002F, C0002H, and C0002J–C0002L. Combined with MAD data obtained during Expedition 315 (Expedition 315 Scientists, 2009), a continuous profile of density and porosity data to 2005.5 mbsf is now available for Site C0002 (Fig. F15).

MAD data on cuttings (below 875.5 mbsf in Hole C0002F) show lower bulk density and higher porosity compared to measurements on discrete samples from cores (Fig. F15). This was also observed during riser drilling Expeditions 319 and 337 (Expedition 319 Scientists, 2010; Inagaki et al., 2012). Analyses on Expedition 338 cuttings revealed that these differences resulted from mixing of aggregates produced during the drilling process, termed drilling-induced cohesive aggregates. MAD data from discrete core samples and a few selected intact formation cuttings from lithologic Unit V suggest that bulk density increases continuously from 1.6 g/cm³ at the seafloor to ~2.3 g/cm³ at 2005.5 mbsf. Porosity decreases from ~65% at the seafloor to ~23% at the base of lithologic Unit V (2005.5 mbsf) (Figs. F11, F15).

An LOT to estimate the least principal stress was performed at 875.5 mbsf, which is 12.3 m below the 20 inch casing shoe. Two cycles of pressurization were conducted by injecting drilling mud at 31.8 and 47.7 L/min, respectively. The first cycle was not successful because of loss of a large volume of mud. The second cycle yielded an estimate of 32 MPa as the least horizontal principal stress (σ_hmin) at 875.5 mbsf.

Paleomagnetism

Remanent magnetization measurements were conducted on discrete samples from Holes C0002K and C0002L. Results show that magnetic inclinations at 200–505 mbsf (lithologic Unit II) are mostly negative, except for a positive interval at 240.72–299.37 mbsf. Because the nannofossil event of 1.04 Ma is found at ~250 mbsf in Hole C0002K, this normal polarity interval observed at 240.72–299.37 mbsf is correlated to the Jaramillo Subchron (0.988–1.072 Ma). However, the top of the Jaramillo Subchron was also found at 119.58 mbsf in Hole C0002D (Expedition 315 Scientists, 2009). This duplicate occurrence of the Jaramillo Subchron suggests the presence of a fault between Holes C0002D and C0002K, as discussed in “Biostratigraphy” below.

Anisotropy of magnetic susceptibility (AMS) measurements were conducted on discrete samples collected from Holes C0002J–C0002L. AMS data show that the samples from Holes C0002K and C0002L (200–505 mbsf) exhibit a subhorizontal, uniaxially oblate magnetic fabric, likely formed by subvertical compaction. In contrast, AMS data show that the samples from Hole C0002J (902–940 mbsf) exhibit more prolate magnetic fabric elongated in the northeast–southwest direction, suggesting trench-normal compression in addition to subvertical compaction.

Biostratigraphy

Calcareous nannofossils indicated middle Pleistocene ages (0.903–1.34 Ma) of the Kumano Basin sediment recovered from Holes C0002K and C0002L. A nannofossil event of 1.04 Ma was found at ~250 mbsf in Hole C0002K (Fig. F11). However, this event was also encountered at 137.46 mbsf in Hole C0002D (Expedition 315 Scientists, 2009). Thus, the Jaramillo Subchron (0.988–1.072 Ma) and the nannofossil event of 1.04 Ma occur in both Holes C0002D and C0002K at different depth intervals. Such duplicate occurrence of the Jaramillo Subchron and the nannofossil event may be due to the presence of a normal fault between Holes C0002D and C0002K, where the former hole penetrated the footwall and the latter hole penetrated the hanging wall. The lithologic Unit III/IV boundary is interpreted as unconformity between sediments with early to middle Pliocene calcareous nannofossil and radiolarian species above and those with only few and poorly preserved late Miocene species below.

Site C0012

LWD in Hole C0012H was conducted from the seafloor to 710.0 mbsf and provided a suite of LWD data that can be combined with core analyses from previous expeditions (Expedition 322 Scientists, 2010b; Expedition 333 Scientists, 2012b) and seismic data (Park et al., 2008) to characterize the subduction zone inputs.

Log data and lithologic characterization

Based on variations of the gamma ray data from baselines and changes in trend and log character, eight primary logging units were identified: six within the sediment and two within the basement. The logging units were further divided into subunits based on more subtle variations in resistivity and sonic velocity (Fig. F16).

Logging Unit I (0–144.3 mbsf) is characterized by a gradually increasing trend in gamma ray values from ~65 to ~75 gAPI and roughly constant, low resistivity of ~0.7 Ωm (Fig. F16). The gamma ray values and resistivity through Unit I are interpreted to reflect sandy mud lithologies.

Logging Unit II (144.3–188.1 mbsf) is characterized by a gamma ray increase from ~75 to ~95 gAPI and roughly constant, low resistivity of ~0.7 Ωm (Fig. F16). The gamma ray values and resistivity are interpreted to reflect sandy mud lithologies.

Logging Unit III (188.1–339.6 mbsf) is characterized by gamma ray values near ~95 gAPI with minor fluctuations (Fig. F16) that are interpreted to reflect changes in the relative proportions of silt and clay in the hemipelagic mud. Resistivity is roughly constant at ~0.9 Ωm.

Logging Unit IV (339.6–403.3 mbsf) is characterized by an increase to higher gamma ray values (~115 gAPI) and fairly constant resistivity of ~1 Ωm (Fig. F16). This is interpreted as a clay-dominated unit. The base of logging Unit III is placed at 403.3 mbsf, where a step down in gamma ray, resistivity, and sonic velocity values occurs.

Logging Unit V (403.3–463.5 mbsf) has more variability in log character than logging Units I–IV, especially in P-wave velocity, which fluctuates between 1800 and 2000 m/s (Fig. F16). This is interpreted to be a heterogeneous mixture of hemipelagic mudstone, sand, and ash.

Logging Unit VI (463.5–530.3 mbsf) exhibits a gradual decrease in gamma ray from ~95 to ~60 gAPI, whereas resistivity is fairly constant at ~0.85 Ωm (Fig. F16). The base of Unit VI is placed where a significant change in the overall log character occurs, with a sharp decrease in gamma ray and corresponding sharp increase in resistivity and sonic velocity. This boundary is interpreted as the sediment/basement contact.

Logging Unit VII (530.3–626.6 mbsf) represents the uppermost part of the oceanic basement. Through logging Unit VII, the gamma ray log exhibits significant variation with depth (15–45 gAPI) (Fig. F16). These fluctuations in gamma ray value possibly represent changing sediment volume within the basement or alterations to the basalt.

Logging Unit VIII (626.6–710 mbsf) is characterized by low gamma ray values of ~15 gAPI with only minor fluctuations (±5 gAPI) (Fig. F16), suggesting the presence of uniform or fresh basalt. Resistivity exhibits some variability with depth but remains high relative to all the other logging units.

Resistivity image analysis

In the Shikoku Basin sediment (logging Units I–VI), bedding dips are 10°–30°. The dominant dip direction is bipolar with strong clustering in west–northwest and east–northeast directions (Fig. F16).

In the basement (logging Units VII–VIII), there are a wide variety of textures and fracture patterns that can broadly be summarized as (1) mottled texture, distinct from other regions of clear fracturing (Fig. F17D); (2) “turtleshell” texture as approximately circular regions of high-resistivity clasts within a lower resistivity network (Fig. F17B, F17D, F17E); and (3) zones of homogeneous background resistivity, often with subvertical fractures. The mottled texture can be interpreted to represent a multitude of small fractures, alteration of basaltic basement, or, alternatively, volcaniclastic sediment gravity flow deposits. The turtleshell texture possibly represents pillow basalts. The homogeneous zones containing subvertical fractures may represent sheet flows (Fig. F17C, F17D).

Four compressional borehole breakouts were observed in the Shikoku Basin sediment. The uppermost and lowermost breakouts (424.8 and 517.7 mbsf) agree with a north–south orientation of the maximum horizontal stress (σ_Hmax), whereas the central two breakouts (442.6 and 446.6 mbsf) demonstrate a northeast–southwest σ_Hmax orientation.

Core-log-seismic integration

Cores recovered from Holes C0012A–C0012E provided detailed lithologic information for the Shikoku Basin sediment; those intermittently recovered from Holes C0012A and C0012E–C0012G provided some lithologic information about the underlying oceanic basement of the Philippine Sea plate (Expedition 322 Scientists, 2010b; Expedition 333 Scientists, 2012b). Hole C0012H provided detailed LWD data. Seismic Units A–G were identified on the Institute for Research on Earth Evolution (IFREE) 3-D prestack depth migration (PSDM) (Park et al., 2008) In-line 95 by the Expedition 322 Scientists (2010a, 2010b). Seismic Unit G represents the oceanic basement (Fig. F18).

Seismic Units A and B (0–120 mbsf) exhibit low-amplitude reflectivity with some irregular and discontinuous reflections (Fig. F18). In cores, the same interval shows a dominant lithology of hemipelagic mud(stone) with a few volcanic ash/tuff beds (Expedition 322 Scientists, 2010b; Expedition 333 Scientists, 2012b). Logging Unit I in Hole C0012H (0–144.3 mbsf) shows a slight increase in gamma ray values with minor fluctuations and likely correlates with these seismic units.

Logging Unit II (144.3–188.1 mbsf) has a greater variability in gamma ray values (Fig. F18). This interval corresponds to lithologic Unit II, which contains several volcanic sandstone beds (each ~5 m thick), and seismic Unit C, which contains a series of high-amplitude, continuous reflections.

Lithologic Units III and IV (188.1–403.3 mbsf) comprise hemipelagic mudstone and hemipelagic mudstone with interbedded siltstone sediment gravity flow deposits, respectively. Seismic Unit D, at the corresponding depth interval of 200–405 mbsf, is seismically transparent (Fig. F18). Lithologic Unit IV is correlatable with logging Unit IV (339.6–400.3 mbsf), where low spikes in gamma ray values (~30 gAPI) suggest an increasing number of siltstone turbidites.

Logging Units V and VI (403.3–530.3 mbsf) show a similar pattern of increasing gamma ray values with depth (Fig. F18). Lithologic Unit V (403.3–463.50 mbsf) contains a series of sandstone sediment gravity flow deposits and volcanic sandstone layers interbedded in hemipelagic mudstone. An increase in the ratio of mudstone to sandstone with depth may explain the observed increases in gamma ray values with depth. Seismic Unit E contains a series of strong reflections between 405 and 530 mbsf, corroborating the observation of sandstone layers interbedded in hemipelagic mudstone.

Lithologic Unit VI (~530–537 mbsf) was identified as thermally altered hemipelagic mudstone, resulting from contact with the basement rocks of lithologic Unit VII (seismic Unit G, logging Unit VII; Fig. F18). Finally, the oceanic basement is consistently observed at depths below 530.3 mbsf.

Sites C0018 and C0021

LWD logs were collected in Holes C0018B and C0021A as well as cores in Hole C0021B for the purpose of characterizing the MTDs in the slope basin seaward of the megasplay fault zone.

Logging

During Expedition 333, which collected cores in Hole C0018A (Expedition 333 Scientists, 2012c), slope basin sediment was defined as Unit I. To maintain consistency, only one unit is defined encompassing the entire sections logged and cored at Sites C0018 and C0021. The gamma ray log supports classification as one unit, as its character does not change significantly through the entire drilled section. However, based on changes in the character of resistivity logs, two subunits were identified in the Hole C0018B logs (Fig. F19) and three subunits were identified in the Hole C0021A logs (Fig. F20).

Logging units in Hole C0018B

Subunit IA (0.0–179.8 mbsf) is characterized by variable gamma ray and resistivity values (75 ± 30 gAPI and 1.0 ± 0.5 Ωm, respectively) (Fig. F19). Bedding is mostly subhorizontal, except for two intervals of high-angle dips (40°–80°) at 81.0–83.0 and 127.0–168.0 mbsf (Fig. F18), which correlate with MTDs observed in Hole C0018A cores (Expedition 333 Scientists, 2012a) (Fig. F21).

In contrast, Subunit IB (179.8–350.0 mbsf) is characterized by highly variable gamma ray and resistivity values (80 ± 40 gAPI and 1.5 ± 1.5 Ωm, respectively) (Fig. F19). From the gamma ray signature, coarsening- and fining-upward packages on a scale of tens of meters are interpreted throughout this subunit. Resistivity exhibits fluctuations over narrow (<2 m) horizons, possibly indicative of thin interbedded sand/ash and muddy sediment.

Logging units in Hole C0021A

Subunit IA (0–176.8 mbsf) is characterized by variable gamma ray and resistivity values (75 ± 15 gAPI and 0.9 ± 0.3 Ωm, respectively) (Fig. F20). The majority of bedding dips moderately at 15°–40°, except for two intervals (95–100 mbsf and 148–178 mbsf), where bedding dips >50° in variable directions. These variable dip directions reflect the expected chaotic nature of MTDs in this subunit.

Subunit IB (176.8–276.5 mbsf) exhibits several increasing and decreasing cycles of gamma ray and resistivity values with some low spikes (Fig. F20). These cycles likely reflect lithologic coarsening- and fining-upward cycles.

Subunit IC (276.7–294 mbsf) exhibits two cycles of gamma ray and resistivity value increases with depth (Fig. F20), which can be correlated with lithologic fining-upward cycles.

Lithology

The slope basin sedimentary succession in Hole C0021B was drilled to 194.5 mbsf. Cores were collected in two intervals of 0–5.93 and 80–194.5 mbsf. Two lithologic units are defined: Subunits IA and IB (Fig. F22). Subunit IA is dominated by silty clay and contains two MTDs labeled as MTD A (94.16–116.75 mbsf) and MTD B (133.76–176.16 mbsf). The base of MTD B corresponds to the lithologic subunit boundary. Subunit IB contains thin, frequent sand interbedded with silty clay. Volcaniclastic ash layers are present in both units. This succession is lithologically similar to nearby slope basin sites drilled during previous expeditions (Expedition 333 Scientists, 2011; Expedition 316 Scientists, 2009a, 2009b) and correlated to regional seismic-stratigraphic framework (Strasser et al., 2011; Kimura et al., 2011).

Structural geology

Structural observations of cores from Hole C0021B confirm the conclusions obtained from nearby Hole C0018A (Expedition 333 Scientists, 2012b). Slope sediment is characterized by flat or gently dipping bedding and a lack of shear zones (Fig. F22). In contrast, MTDs are characterized by a wider range of dip angles (0°–50°) and by the occurrence of millimeter to centimeter thick shear zones, reflecting the disruption during their emplacement. The base of both MTDs is defined by the presence of shear zones.

Geochemistry

In addition to generally normal geochemical trends with depth, a low PO₄^3– anomaly is associated with MTD B, as is observed for MTD 6 in Hole C0018A (Fig. F22). Li, Ca²⁺, and Mg²⁺ activity also show remarkable changes across the lithostratigraphic Subunit IA/IB boundary, and thus across the base of MTD B.

Physical properties

Both bulk density and porosity generally increase and decrease with depth, respectively. Undrained shear strength values generally increase with depth. With the two MTDs, the gradient of shear strength increase with depth is higher and the base of these MTDs is characterized by an abrupt decrease in strength (Fig. F22). Similar strength depth profiles were also observed in MTDs in Hole C0018A (Expedition 333 Scientists, 2012b). In addition, magnetic susceptibility, electrical resistivity, and thermal conductivity show distinct changes across the lithologic Subunit IA/IB boundary; magnetic susceptibility and thermal conductivity are higher in lithologic Subunit IB, whereas electrical conductivity is lower in Subunit IB.

Paleomagnetism

Inclination profiles after demagnetization of samples from Hole C0021B at 20 mT generally agree well with the expected inclination value at the site (52°) for the intervals 0–10, 80–100, and 126–136 mbsf. They all show positive values, suggesting that the sections above MTD B belong to the Brunhes Normal Polarity Chron. Inclination and declination values in the MTDs are widely scattered, which likely reflect internal deformation structures of MTDs. In the lower part of the drilled succession and within lithologic Subunit IB, paleomagnetic data also show large scatter (likely due to the EPCS coring system used), which does not allow us to discriminate the magnetic polarity pattern.

Biostratigraphy

Calcareous nannofossil and planktonic foraminifer examination from recovered samples consistently indicate that sediment at the bottom of Hole C0021B (195 mbsf) is ~1.3 Ma in age. Biostratigraphic age datums generally are in stratigraphic order; however, calcareous nannofossils indicate age reversals and thus the presence of older sediment mixed with younger sediment at ~99 mbsf and between 146.48 and 175.17 mbsf, which are the intervals of MTDs A and B.

Core-log-seismic integration

We base our overall correlation between Sites C0018 and C0021 on the Kumano 3-D seismic reflection data. A prominent regional seismic reflection identified by Kimura et al. (2011) and Strasser et al. (2011) marks the top of the thickest MTD (Fig. F5), which represents MTD 6 and MTD B at Sites C0018 and C0021, respectively. Core observation and analyses of MTD B in Hole C0021B and steep and chaotic bedding inferred from resistivity images of MTD B correlative to logging Subunit IB are strikingly similar to MTD 6 of Hole C0018A and correlative to the logging interval in Hole C0018B. The MTD, however, is unidentifiable from the gamma ray and resistivity logs alone (Figs. F21, F23), suggesting an intrabasin MTD source. Using seismic stratigraphy and bio-, magneto- and tephrastratigraphic age constraints from Sites C0018 and C0021, this thick MTD is inferred to have emplaced between 1.05 and 0.78 Ma (Strasser et al., 2012).

The base of this MTD is not marked by one coherent single reflection, and regional correlation is affected by basal erosion of the MTD. At Site C0018, the logging Subunit IA/IB boundary has been defined at the base of MTD 6 (Fig. F21). At Site C0021, the base of the thick MTD B cuts deeper into the underlying subunit (Fig. F5).

Site C0022

LWD data and cores were collected at Site C0022 to characterize the uppermost 400 m of sediment near the tip of the megasplay fault zone where the seaward-most branch of this fault system approaches the surface (Figs. F1, F4) (Moore et al., 2007, 2009).

Logging

As at Sites C0018 and C0021, the main slope sediment section was defined as a single lithologic and logging unit at Site C0022. The gamma ray log supports classification as one unit, as its value maintains a baseline near ~75 gAPI and the character does not change significantly through the entire drilled section. However, three subunits were identified based on changes in the character of resistivity logs (Fig. F24).

Subunit IA (0.0–74.3 mbsf) is characterized by gradual increases in gamma ray and resistivity values with minor fluctuations (75–85 gAPI and 0.9–1.1 Ωm, respectively) (Fig. F24).

Throughout Subunit IB (74.3–212.9 mbsf), the gamma ray log exhibits repeated decimeter-scale decreases and increases, which are interpreted to reflect coarsening- and fining-upward cycles (Fig. F24). Resistivity fluctuates at ~1.1 Ωm with low (≤0.7 Ωm) and high (≥1.5 Ωm) anomalies. A high-resistivity (up to ~1.5 Ωm) interval at 85–88 mbsf, a low-resistivity (0.72 Ωm) peak at 100 mbsf, and a high-resistivity (up to ~1.7 Ωm) interval at 102–106 mbsf correspond to a highly fractured zone, and these intervals are likely related to the megasplay faulting. In particular, the low-resistivity peak at 100 mbsf is likely the location of the megasplay fault because the fault zone is presumed to be more porous because of fracturing and therefore richer in pore fluid than its host rock.

Throughout Subunit IC (212.9–420.5 mbsf) the gamma ray log exhibits small-scale variations around the baseline of ~90 gAPI, whereas resistivity generally maintains minor fluctuations (±0.2 Ωm) around a constant baseline (~1.2 Ωm) (Fig. F24).

Lithology

Two lithologic subunits are recognized in Hole C0022B within the slope basin sedimentary section (Fig. F25). Subunit designations applied here are adopted with minor modification from Site C0008 (Expedition 316 Scientists, 2009b).

Subunit IA (0.0–383.5 mbsf) is dominated by silty clay with a variable component of calcareous nannofossils, foraminifers, siliceous biogenic debris, and volcanic ash. A trend toward diminishing carbonate content with depth is observed (Fig. F25).

Subunit IB (383.5–415.9 mbsf) consists of a series of interbedded mud clast gravels (Figs. F25, F26) with thin sand, clayey silt, and silty clay in the upper part and is dominated by silty clay in the lower part. This mud clast gravel is correlatable with a similar section in lithologic Subunit IB of Hole C0008A.

Structural geology

Bedding is subhorizontal with dip angles <20° throughout the entire section, except for the interval of 73.49–143.82 mbsf, where bedding dips steeper than 20° (up to ~50°) (shaded interval in Fig. F25). This interval of disturbed bedding possibly corresponds to the megasplay fault zone, which is supported by several biostratigraphic age reversals at this interval.

Minor faults are clustered in two intervals: 50–83 and 386–405 mbsf (Fig. F25). The faults of the megasplay hanging wall strike northwest–southeast, whereas those of the footwall preferentially strike north–south to northeast–southwest. Such a difference in fault orientation suggests different stress conditions on either side of the splay fault.

Geochemistry

Although overall geochemical trends of interstitial water in Hole C0022B are similar to those in Holes C0004D, C0008A, and C0008D, there are changes in geochemical trends at ~100 mbsf (Fig. F27). For example, pH decreases from ~20 mbsf (>8.0) to ~100 mbsf (~7.5), below which it increases to the bottom of the hole (>8.0). Chlorinity and bromide concentrations increase to ~100 mbsf and then become roughly constant (~630 and ~1.2 mM, respectively) below that depth. Na⁺ shows a trend similar to chlorinity and bromide. Ca²⁺ concentration increases rapidly to ~100 mbsf and then increases gently downward. Fe²⁺ concentration has a high peak at ~100 mbsf. Li⁺ concentration decreases from the seafloor to ~100 mbsf and then increases to ~200 mbsf. Rb⁺ concentration rapidly decreases from the seafloor to ~100 mbsf and then slightly decreases to ~350 mbsf. All these geochemical changes at ~100 mbsf are considered to be associated with the megasplay fault.

Hydrocarbon gas trends in Hole C0022B are similar to those in Holes C0004D, C0008A, and C0008D. Methane and ethane increase from the seafloor to ~30 mbsf, then decrease to ~100 mbsf, and gradually increase again with depth (Fig. F28). In addition, methane exhibits a high peak at ~100 mbsf, which may be associated with the megasplay fault.

Physical properties

MAD measurements on discrete samples show that porosity decreases quickly from 70% at the seafloor to 45%–50% at ~100 mbsf and then increases to 60% at 150 mbsf (Fig. F25). The minimum porosity occurs at 93.4–94.7 mbsf, which is close to the proposed location of the megasplay fault tip. Thus, this trend suggests that sediment near the splay fault has experienced shear-enhanced compaction.

Paleomagnetism

Paleomagentic data from Hole C0022B are interpreted with caution because demagnetization was only conducted with two demagnetization steps on sections without discrete sample measurement because of time constraints on board the Chikyu and because magnetostratigraphic interpretation in structurally complex environments (i.e., splay fault tip) is not straightforward. The inclination profile shows indistinct inclination changes from normal to negative inclination values in the upper 100 m, for which interpretation of paleomagnetic stratigraphy correlation remains nonunique. In contrast, the interval dominated by a clear positive inclination between 330 and 380 mbsf can be assigned to the Olduvai Subchron, ranging from 1.77 to 1.95 Ma.

Biostratigraphy

Calcareous nannofossils and planktonic foraminifers consistently indicate an age of ~2.0 Ma for sediment at the bottom of Hole C0022B and a stratigraphic reversal or reworking of older sediment at ~130 mbsf. Calcareous nannofossils indicate additional stratigraphic reversals or reworking of older sediment at ~80 and ~140 mbsf. Stratigraphic reversals are likely associated with megasplay faulting, whereas reworking of older sediment could also be explained by mass-movement processes.

Core-log-seismic integration

At Site C0022, a single unit was defined from analysis of both core samples and LWD data. Two subunits were identified based on core lithology, whereas three subunits identified from the LWD data (Fig. F24) correlate to seismic units defined by Kimura et al. (2011). According to this unit definition, lithologic Subunit IA correlates to logging/seismic Subunits IA and IB and the upper part of Subunit IC, whereas lithologic Subunit IB correlates to the lower part of seismic/logging Subunit IC. This is in agreement with the compiled stratigraphy by Kimura et al. (2011) and Strasser et al. (2011) in the slope basin.

An interval of disturbed bedding, where bedding dips >20°, was observed from 80 to 140 mbsf. Within this deformation interval, biostratigraphic age constraints reveal several age reversals. The depth of this deformation zone correlates with offsets in the seismic reflections at this depth (Fig. F24), suggesting that it may represent the updip extension of the megasplay fault. A zone of high deformation and fracturing was also identified in the LWD resistivity images at 85–106 mbsf with a low-resistivity interval at ~100 mbsf, which is likely the location of the splay fault at Site C0022.

Top of page   |    Previous   |    Next