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doi:10.2204/iodp.pr.338.2013

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 occasional 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 homogeneous, high gamma ray intervals and is interpreted as a homogeneous, clay-rich unit (Fig. F7). Sonic velocity is nearly constant in this interval. 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 the 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 Holes 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 the 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 the 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 the hemipelagic Unit III drilled at the 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, C0002K, and 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. F10C) were observed in cores and cuttings exclusively from Unit III in Holes C0002F and C0002J (Fig. F8). Deformation bands (e.g., Fig. F10D) 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 the 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 identified by LWD resistivity images.

Geochemistry

Geochemical data of interstitial water and gas sampled from cores in Holes C0002H, C0002J, C0002L, and C0002K obtained during Expedition 338 complement the 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 at ~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 918 mbsf boundary.

The ratio of methane to ethane and propane (C₁/[C₂ + C₃]) and δ¹³C concentration in methane (δ¹³C-CH₄) suggest that the 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 both cores and cuttings sampled from Holes C0002F, C0002H, C0002J, C0002K, and 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 previous 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 cuttings 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).

A 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

Remnant magnetization measurements were conducted on discrete samples from Holes C0002K and C0002L. The results show that magnetic inclinations at 200–505 mbsf (Unit II) are mostly negative, except for a positive interval at 240.72–299.37 mbsf. The data and results from Holes C0002B and C0002D during Expedition 315 (Expedition 315 Scientists, 2009) revealed that the 160–490 mbsf interval at Site C0002 ranges from 1.078 to 1.24 Ma and that the entire sampled intervals of Holes C0002K and C0002L should correspond to the middle part of the Matuyama reversed polarity interval. The normal polarity interval observed at 240.72–299.37 mbsf is therefore interpreted as the Cobb Mountain Subchron (1.173–1.185 Ma).

Anisotropy of magnetic susceptibility (AMS) measurements were conducted on discrete samples collected from Holes C0002J, C0002K, and 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 a trench-normal compression in addition to subvertical compaction.

Site C0012

LWD in Hole C0012H was conducted from the seafloor to 709.0 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, seven primary logging units were identified: five within the sediment and two within the basement. The logging units were further divided into subunits based on more subtle variations in the resistivity and sonic velocity (Fig. F16).

Logging Unit I (0–188.1 mbsf) is characterized by a gradually increasing trend in gamma ray values from ~65 to ~95 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 (188.1–372.1 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. An increase in abundance of thin gamma ray and resistivity spikes below 339.6 mbsf suggests an increase in the number of ash and sand beds.

Logging Unit III (372.1–403.3 mbsf) is characterized by overall high gamma ray values (~115 gAPI) and a 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 IV (403.3–463.5 mbsf) has more variability in log character than logging Units I, II, and III, 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 V (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 V 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 VI (530.3–626.6 mbsf) represents the uppermost part of the oceanic basement. Through logging Unit VI, 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 VII (626.6–709 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–V), 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 VI–VII), 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 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 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 the 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). The upper section of logging Unit I in Hole C0012H (0–140 mbsf) shows a slight increase in gamma ray values with minor fluctuations and likely correlates with these seismic units.

The lower section of logging Unit I (140–188 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 (215–415 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 the base of logging Unit II (340–380 mbsf), where low spikes in gamma ray values (~30 gAPI) suggest an increasing number of siltstone turbidites.

Logging Units IV and V (403–530 mbsf) show a similar pattern of increasing gamma ray values with depth (Fig. F18). Lithologic Unit V (415–530 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 VI; Fig. F18). Finally, the oceanic basement is consistently observed at depths greater than ~530 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 four 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.

Log units in Hole C0021A

Subunit IA (0–144 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 the 95–100 mbsf interval, where bedding dips >50° in variable directions. These variable dip directions reflect the expected chaotic nature of an MTD in this subunit.

Within Subunit IB (144–176.8 mbsf), the resistivity log exhibits two sharp peaks (154 and 163 mbsf) but no corresponding change is observed in the gamma ray log (Fig. F20). Bedding dips >50° in variable directions throughout this subunit, which implies that the whole of this subunit represents a thick MTD.

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

Lithology

In Hole C0018A, six MTDs were identified in cores between 0 and 190.0 mbsf (Expedition 333 Scientists, 2012a). The cored MTD section in Hole C0021B is different from that in Hole C0018A, but detailed description and sampling for Hole C0021B will be completed during a postexpedition sampling party. Strikingly, MTDs are unidentifiable from the gamma ray and resistivity logs alone (Figs. F21, F22), suggesting an intrabasin MTD source. Evidence seen in the resistivity images is mainly based on steep and chaotic bedding.

Core-Log-Seismic integration

We base our overall correlation between Sites C0018 and C0021 on the Kumano 3-D seismic reflection data (Fig. F21). A prominent regional seismic reflection identified by Kimura et al. (2011) and Strasser et al. (2011) marks the top of the thickest MTD (MTD 6) (Fig. F5), which was drilled both at Sites C0018 and C0021. The base of MTD 6 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, where the base of MTD 6 cuts deeper into the underlying subunit (Fig. F5), MTD 6 was assigned as a separate logging subunit (IB). Following successive ordering of subunits, therefore, logging Subunit IC at Site C0021 should be equivalent to Subunit IB at Site C0018 and correlates to the regional seismic Subunit IB defined by Kimura et al. (2011) and Strasser et al. (2011).

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. F23).

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. F23).

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. F23). The resistivity fluctuates at ~1.1 Ωm with low (≤0.7 Ωm) and high (≥.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 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. F23).

Lithology

Two lithologic subunits are recognized in Hole C0022B within the slope basin sedimentary section (Fig. F24). 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. F24).

Subunit IB (383.5–415.9 mbsf) consists of a series of interbedded mud clast gravels (Figs. F24, F25) 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 <15° throughout the entire section, except in the vicinity of the possible splay fault (shaded interval in Fig. F24). These higher-than-average bedding dip angles are likely affected by the splay faulting.

Minor faults are clustered in two intervals: 50–83 mbsf and 386–405 mbsf (Fig. F24). 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.

The interval of 100–101 mbsf is a plausible candidate for the location of the splay fault at Site C0022. Observations that support this hypothesis are (1) an increase in bedding dip in the vicinity of this interval; (2) the higher density of minor faults 20 m above this interval; (3) poor core recovery in this interval, suggesting highly fractured or disturbed material; and (4) the presence of three 2 cm thick intervals of claystone showing planar fabrics not encountered elsewhere in Hole C0022B.

Geochemistry

Although overall geochemical trends of interstitial water in Hole C0022B are similar to those in Holes C0004D, C0008A, and C0008D, changes in geochemical trends at ~100 mbsf are noticed (Fig. F26). 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 concentrations increase from the seafloor to ~30 mbsf, then decrease to ~100 mbsf, and gradually increase again with depth (Fig. F27). In addition, methane concentrations exhibit 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. F24). Interestingly, 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 have experienced shear-enhanced compaction.

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. F23) 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.

Several shear zones and minor faults were observed in cores from 80 to 140 mbsf. The depth of this deformation zone correlates with offsets in the seismic reflections at this depth (Fig. F23), 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.

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