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doi:10.2204/iodp.proc.338.103.2014

Structural geology

Structural studies at Site C0002 consist of (1) analyses of cuttings from 865.5 to 2004.5 mbsf in Hole C0002F and (2) analyses of cores from 200–280.5, 277–502.8, 902–933.8, and 1100.5–1112.8 mbsf in Holes C0002K, C0002L, C0002J, and C0002H, respectively.

Structures in cuttings from Hole C0002F

In Hole C0002F, deformation structures in cuttings from the 1–4 and >4 mm size fractions were investigated from 865.5 to 2004.5 mbsf (see “Structural geology” in the “Methods” chapter [Strasser et al., 2014a]). In addition to natural deformation structures such as vein structures, carbonate veins, slickenlined surfaces, and minor faults, a high number of drilling-induced deformation structures were observed. Orientations of structures could not be measured because all information on orientation is lost during recovery of cuttings through the riser. All observed deformation structures that are not drilling induced are summarized in CUTTINGS STRUCTURE.XLSX in STRUCTURE in “Supplementary material.” Figure F54 shows the percentage of deformed grains obtained from dividing the number of grains that show deformation structures by the total number of investigated grains.

Vein structures

Vein structures in cuttings are thin clay- or silty clay–filled extensional cracks or veins (Fig. F55) (Ogawa, 1980; Cowan, 1982; Brothers et al., 1996). The occurrence of vein structures is limited to between 860 and 1050 mbsf (Fig. F54). Maximum concentrations form a sharp peak of 5% at 900 mbsf. Considering cuttings from the drill bit and the underreamer, the depth range of these vein structures corresponds to lithologic Unit III observed in Hole C0002B, which is a clay-rich hemipelagic mud sandwiched between accreted sediments below and silty-clay rich hemipelagic sediments above (Expedition 315 Scientists, 2009b).

Mineral veins

Narrow mineral veins that exclusively consist of carbonate (most probably calcite) occur throughout the entire section below 1050 mbsf (Fig. F54). The veins have widths of less than a few millimeters and are present in mudstone, sandstone, and rare limestone cuttings. Carbonate veins are often exposed at the surfaces of clastic rock cuttings, which are, in most cases, planar and lineated (Fig. F56A, F56B). This suggests shear deformation during vein formation. The lineated surface is also sometimes associated with steps. Fiber growth of carbonate veins (Fig. F56C), where the growth direction is perpendicular to the vein wall, is locally observed, indicating repeated extensional fracturing and vein growth from solution (also see Fig. F56D, where calcite grains in veins grew from very fine calcite grains of the limestone wall rock). Observation of thin sections under optical microscope shows that carbonate veins consist of abundant, very fine calcite grains; a small fraction of larger grains (up to 100 µm) show mechanical twins (Fig. F56E). Also, the wall rocks were fractured during vein formation and incorporated into veins (i.e., selvages; Fig. F56F). Maximum concentrations of cuttings with carbonate veins of up to 2.5% occur between 1050 and 1150 mbsf (Fig. F54). It may be noted that from 1800 to 2000 mbsf the frequency of cuttings with carbonate veins is higher in the 1–4 mm size fraction compared to the >4 mm size fraction. This may indicate that cuttings with carbonate veins can be easily broken into smaller pieces with a diameter of <4 mm.

Slickenlined surfaces

Similar to mineral veins, cuttings with slickenlined surfaces occur throughout the entire section below 1050.5 mbsf. A slickenlined surface is the polished surface of a cutting that shows striations (Fig. F57A, F57B). Under the optical microscope, clay minerals are observed along incipient slickenlined surfaces, where they build a clay mineral–rich zone up to 100 µm in width on both sides of the incipient surface (Fig. F57B, F57C). Slickenlines are commonly associated with steps (Fig. F57D) from which the sense of shear can be inferred (e.g., Petit, 1987; Angelier, 1994; also see Expedition 319 Scientists, 2010, for detailed explanation of steps on faults). The degree of the preferred alignment of clay minerals seems to increase with depth, but this requires more detailed investigation. Depths or depth intervals for which the proportion of cuttings showing slickenlined surfaces exceeds 3% are found at 1060.5, 1215.5–1285.5, 1345.5–1375.5, 1550.5–1675.5, and 1895.5–1985.5 mbsf (Fig. F54). Among these depths, the 1550.5–1675.5 mbsf interval shows anomalously high concentrations of slickenlined surface–bearing cuttings of up to 10%. Also at these depths, a high number of lens-shaped cuttings, which are completely surrounded by slickenlined surfaces, are observed. In the shallower intervals of Hole C0002F (1010.5–1635.5 mbsf depth, mostly 1010.5–1235.5 mbsf), grains with a shiny surface but without slickenlines are commonly observed. These grains could be related to fracture surfaces coated by clay minerals, but the relationships with shear deformation are unclear. The abundance of grains with a shiny surface is listed in CUTTINGS STRUCTURE.XLSX in STRUCTURE in “Supplementary material” but not included in Figures F54 and F58.

Minor faults

Only two minor faults were observed within the cuttings. One is in a calcareous siltstone chip from cuttings Sample 338-C0002F-169-SMW, >4 mm (1565.5 mbsf bit depth) (Fig. F59A–F59D), and the other is in a laminated sandstone from cuttings Sample 238-SMW, >4 mm (1835.5 mbsf bit depth) (Fig. F59E, F59F). The first fault is characterized by two thin, black-colored parallel zones with thicknesses of up to 100 µm (Fig. F59A–F59C). Although the displacement along the faults is unclear, they are distinguished from other structures (e.g., stylolites) because of their planar shape and stepovers (Fig. F59C). Under the optical microscope, the fault slip zones are composed of dark-colored clay minerals with no preferred orientation (Fig. F59C, F59D). Inspection of thin sections shows that detrital quartz grains and foraminifer fossils adjacent to the fault do not show any deformation (Fig. F59D). In the case of the second fault (Sample 238-SMW, >4 mm; 1835.5 mbsf), laminations in the sandstone are displaced ~0.6 mm along the observed plane (Fig. F59E). The fault plane is accompanied by a very thin zone (<100 µm) in which neither comminuted material nor concentration of clay minerals is observed (Fig. F59F). The nature of both faults suggests that cataclastic flow, characterized by grain comminution or crushing, was not dominant during faulting.

Diagenesis and lithification processes of sediment

In the shallow part of Hole C0002F (above 1100 mbsf), sandstone is not observed in 1–4 mm and >4 mm cuttings. Between 1100 and 1500 mbsf, sandstone commonly occurs as rounded clasts that easily disaggregate. Under the optical microscope, such clasts appear to be composed of loosely packed sand grains surrounded by clay minerals (Fig. F60A). Because of the low degree of induration, large amounts of unconsolidated sandstone may have been dispersed during riser drilling. Below ~1500 mbsf, the sand becomes indurated enough to produce sandstone cuttings that remain intact during drilling, recovery, and sieving. At these depths, sedimentary structures such as graded bedding and laminations are commonly observed in cuttings (Fig. F60B). Quartz cement fills the gaps between the closely packed detrital grains (Fig. F60C, F60D). Compaction and cementation seem to have played important roles in the lithification process of sandstone.

Angular-shaped silty claystone cuttings gradually appear near 1600 mbsf. On a microscopic scale, the degree of parallel alignment of clay minerals increases with depth (compare Fig. F61A–F61F, retrieved from 1215.5, 1475.5, 1565.5, 1625.5, 1875.5, and 2004.5 mbsf). This increase could be caused by growth of clay minerals that became more significant with increasing depth, corresponding to increases in temperature, time, or tectonic compaction (Milliken and Reed, 2011; Day-Stirrat et al., 2011).

Drilling-induced deformation

Cuttings generally show severe drilling-induced disturbance. The most common drilling-induced structure is a characteristic sawtooth shape that is observed in many cuttings samples (Fig. F62A). This shape is likely formed by the drill bit or the underreamer. Because of their characteristic shape, those drilling-induced structures could be easily distinguished from natural deformation structures.

At shallow depths (above 1400 mbsf), drilling mud invasion is commonly observed in cuttings. Figure F62B shows a typical microscopic example of such an invasion. Under an optical microscope, drilling mud is characterized by a low birefringence matrix that contains angular grains of minerals with a wide range of grain sizes. Also, some of the original silty claystone shows embayed surfaces, suggesting that drilling mud with high fluid pressure invaded less cohesive formations.

In addition to those cuttings that were deformed by drilling mud injection, some cuttings are likely to be artificially formed during drilling and recovery operations. Such drilling-induced cohesive aggregates (DICAs), which occur in the 1–4 mm and >4 mm size fractions, contain less sorted angular mineral grains and fragments of small cuttings from the formation in a low-birefringence drilling mud matrix (Fig. F62C). Matrix-supported texture, scattered grain-size distribution, and low birefringence of matrix suggest that the DICAs are in fact aggregates of dispersed sand and small fragments of the formation that formed when mixed with drilling mud and remained intact during subsequent recovery, washing, and sieving.

In the deeper part of this hole (especially below 1800 mbsf), rounded DICAs predominantly consisting of silty clay start to appear. After vacuum drying, these aggregates are visually similar to formation silty claystone cuttings. However, when exposed to water, they easily disaggregate, and they do not show the angular shape of “real” silty claystone cuttings (Fig. F62D). It may also be noted that cuttings from Samples 338-C0002F-311-SMW, >4 mm, and 322-SMW, >4 mm (1975 and 1982.5 mbsf), which are produced only by the underreamer, do not contain DICAs and do not show a sawtooth shape.

Drilling-induced disturbance not only destroys preexisting rock textures but also creates DICAs. Careful mesoscopic and microscopic observations of cuttings are therefore necessary in order to exclude DICAs from any subsequent analysis.

Relationship between structural observations and lithology

During the investigation of deformation structures in cuttings, we also estimated the amount of sandstone versus that of silty claystone in the >4 mm and 1–4 mm size fractions (Figs. F58, F63). The derived concentrations of silty claystone are in good agreement for both size fractions. Down to ~1150 mbsf, only silty claystone cuttings are observed. From 1150 to 1650 mbsf, silty claystone concentrations fluctuate between 60% and 90%. Below 1650 mbsf, silty claystone concentrations increase to >90%. These results can be qualitatively compared to the silty claystone to sandstone ratio determined by lithologic observations of the cuttings mix, sieved at >63 µm (Fig. F63; see also “Lithology”).

The overall trends derived from structural and lithologic analyses are in good agreement. Low concentrations of sandstone are observed above 1150 mbsf and below 1700 mbsf, whereas the interval between shows higher concentrations. Although the overall trends agree rather well, the absolute values as well as the locations of local maxima and minima do not always match exactly.

Over most of the interval from 1150 to 1750 mbsf, the overall concentration of sandstone inferred from lithologic observations on bulk cuttings is slightly higher than the concentration inferred from the appearance of sieved cuttings for 1–4 mm and >4 mm size fractions. One reason for this discrepancy possibly originates in the different methods applied. Lithologic observations were done on easy-sieved cuttings with a 63 µm mesh (see “Lithology”). Structural observations of cuttings were carried out after standard sieving and drying (see “Structural geology” in the “Methods” chapter [Strasser et al., 2014a]). Many of the sandstones were less consolidated and therefore could have disaggregated during processing. Therefore, some of the sandstones investigated directly after easy sieving may have been disaggregated during the standard cuttings workflow and were not preserved in the 1–4 mm and >4 mm size fractions. As structural observations of cuttings only counted intact cuttings, this may explain the observed lower sandstone percentages.

Relation of structural observations and logging data

A fundamental difference between the structural observations on cuttings and the LWD data is the vertical resolution. Cuttings were sampled every 5 m but were generally analyzed every 10 m and were mixed at least over the 43.8 m depth interval spanning from the drill bit to the underreamer (see “Structural geology” in the “Methods” chapter [Strasser et al., 2014a]); the LWD data have a sampling interval (vertical resolution) of 0.152 m. These differences in vertical resolutions make correlations between log features and structural observations difficult.

However, there are some ways to qualitatively compare the results obtained by the different methods. Figure F54 shows the downhole distribution of deformation structures in cuttings. These can be correlated to the distribution of faults and fractures documented in the logging data (Fig. F8). In the structural data, the type of deformation structures changes at ~1020 mbsf. Here, the last occurrence of vein structures coincides with the first occurrence of slickenlines and carbonate veins. This structural change likely reflects the Unit III/IV boundary (see “Logging while drilling” and “Lithology”) and may be caused by different styles of deformation in the Kumano Basin sediment and the accretionary prism.

Maximum concentrations of carbonate veins (2.5%) between 1050 and 1150 mbsf may correlate to logging Subunit IVB. However, no increased concentration of slickenlined surfaces is found at this depth.

Intervals with a high abundance of slickenlined surfaces are observed at 1060.5, 1215.5–1285.5, 1345.5–1375.5, 1550.5–1675.5, and 1895.5–1985.5 mbsf. The 1345.5–1375.5 mbsf interval correlates to an interval where the LWD data show a prominent change in the dominant dip direction (Fig. F8). The 1550.5–1675.5 mbsf interval, which hosts the highest concentrations of slickenlined surfaces, correlates to the basal part of Unit IV, including the boundary to Unit V, which is situated at ~1638 mbsf based on LWD data and 1740.5 mbsf based on lithology data. For a comparison between the above discussed variations in the lithology of the cuttings and the LWD data, refer to “Lithology.”

Structures in core from Holes C0002H, C0002J, C0002K, and C0002L

Cores retrieved from Holes C0002H and C0002J–C0002L during Expedition 338 show a large variety of structures (e.g., Fig. F64). Bedding, faults, and deformation bands are well represented and locally abundant, whereas shear zones, carbonate-cemented breccias, fractures without noticeable displacement, vein structures, disrupted bedding, fissility, and incipient scaly cleavage are rare.

Deformation observed in core or X-ray CT images is localized in specific core intervals. Deformation structures are rarely observed in cores from the upper part of the Kumano Basin deposits (Unit II), whereas they are numerous in cores from the lowermost part of the Kumano Basin sediment (Unit III) and from the accretionary prism sediment (Unit IV). A total of 27 bedding orientations, 49 faults, 13 striations, and 24 deformation bands measured on core from Holes C0002H and C0002J were reoriented into true geographic coordinates using paleomagnetic data measured on board the ship.

Bedding

Bedding from Holes C0002K and C0002L (lower Kumano Basin sediment; Unit II) is subhorizontal to gently dipping and dips at angles <30° (Fig. F65). In cores from Hole C0002J, which were retrieved from the interval including the Unit III (basal Kumano Basin)/IV (accretionary prism) boundary, bedding dips gently at angles <12° at the interval 900–922.77 mbsf, whereas bedding angle gradually increases with depth from 923.0 mbsf to 61° at 932.2 mbsf. In Hole C0002H (Unit IV), bedding shows a tendency to increase in dip angle with depth (7°–50° in Core 338-C0002H-1R and 17°–64° in Core 2R). However, the limited data set does not allow us to determine if this tendency is significant at the scale of Hole C0002H.

Reoriented bedding in Unit III from Hole C0002J is subhorizontal (Fig. F66A). On the other hand, reoriented bedding in Unit IV from Holes C0002H and C0002J is subhorizontal to steeply dipping toward south or north (Fig. F66B). Poles to bedding roughly lie on a girdle, suggesting the presence of an east-west–trending fold. However, the scarcity of orientation data and the lack of layer polarity indicators (see “Lithology”) do not clarify this hypothesis. Bedding dipping north or south at 900–1100 mbsf is consistent with bedding orientations derived from resistivity images obtained during Expedition 314 (Expedition 314 Scientists, 2009).

Disrupted bedding

Intensely disrupted bedding is observed in Sections 338-C0002J-5R-3 through 5R-8 between 922.76 and 927.7 mbsf. An example of such disrupted bedding is depicted in Figure F64A. Where they are still recognizable, disrupted beds have variable thicknesses and a boudinaged appearance. Sets of Riedel shears and preferred orientations (P-foliations) within those intervals suggest bedding-parallel shearing to form disrupted bedding. Bedding orientation measurement cannot be done with accuracy in the disrupted interval. In particular, among the five bedding orientations measured in this interval, the two ~30° values, which depart from the low (<20°) dip values measured elsewhere in Unit III, likely result from disrupted bedding.

Faults

Most faults were observed in cores from the bottom of Kumano Basin Unit III (Hole C0002J) and from Unit IV (Hole C0002H). Of 48 observed faults, only 4 faults were observed in Kumano Basin Unit II (Holes C0002K and C0002L, Fig. F67). The lowermost part (Unit III) of the Kumano Basin sedimentary pile appears more intensely faulted than the shallower layers of Unit II. Fault dips range between 11° and 82°.

In Kumano Basin Unit III (Hole C0002J), fault orientations are variable and no preferred orientation is clearly expressed (Fig. F68A). However, contouring of poles to faults suggests a predominance of east-west–striking and north-dipping low-angle to moderate-angle faults. The scarcity of striations and sense of slip data as well as the lack of relative chronology constraints prevent any paleostress analysis for Unit III faults.

In accretionary prism Unit IV (Hole C0002H), four fault sets can be distinguished (Fig. F68B): 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. Only four faults bear striations with clear slip sense. The trend of these striations suggests extension in the east–west to northwest–southeast directions, which is consistent with normal fault data from Hole C0002B (Byrne et al., 2009; Lewis et al., 2013).

A series of faults occur in interval 338-C0002H-1R-1, 99–121 cm (Fig. F69). Their dip angles are between 57° and 76° for faults with normal displacement components and between 75° and 82° for faults with reverse displacement components. Faults with normal displacement components strike north–south to northwest–southeast, whereas those with reverse displacement components strike around east–west. This contrast in strike suggests that the two fault types pertain to two diachronous episodes of deformation. As observed on split core surfaces, most of these faults have apparent displacements of no more than a few centimeters (Fig. F69A).

In summary, faulting at Site C0002 increases in intensity with depth, but the lack of information regarding slip sense along most faults and the lack of relative chronology criteria between faults prevent any reliable paleostress analysis.

Deformation bands

Most deformation bands (26 out of 27 occurrences) were observed in Kumano Basin sediment Unit III (Fig. F67). On cores, deformation bands appear as dark bands with thicknesses between <1 and 5 mm (Fig. F64B). The boundary between a deformation band and the host sediment is sharp, at least to the naked eye. Thickness commonly changes along strike over a few centimeters. Most deformation bands are oblique to bedding. No clear offset could be observed along these structures.

Deformation bands dip variably between 0° and 90° but predominantly between 20° and 60° (Fig. F67). Deformation bands, reoriented based on paleomagnetic data, do not show any preferred orientation (Fig. F70).

Shear zones

Shear zones are found only in Section 338-C0002J-1R-3. Unlike faults, for which displacement is accommodated along discrete planar surfaces, shear zones are several millimeter thick zones consisting of an anastomosing network of undulating fault surfaces (Fig. F64C). The boundary between shear zones and host sediment is usually not clear and can look progressive. Displacement along shear zones is on the order of a few centimeters. The absence of crosscutting relationships in cores precludes any tentative relative chronology among deformation bands, faults, and shear zones.

Carbonate-cemented breccia

Fragments of calcite-cemented breccia were observed in indurated claystone at interval 338-C0002J-7R-1, 6–11 cm (Fig. F64D). This breccia, which can be described as a mosaic breccia (Mort and Woodcock, 2008), clearly experienced dilatancy in several directions, suggesting that it was formed by hydraulic fracturing (pore pressure in excess of the least principal stress; Cosgrove, 1995). The breccia was retrieved from 932.11 to 932.6 mbsf, which is <5 m below the Unit III/IV boundary. It is, however, difficult to correlate this occurrence to any specific structure (e.g., unconformity or fault zone) crossed by Hole C0002J. Moreover, the breccia fragments were found at the top of Core 338-C0002J-7R, suggesting that they may have fallen from above when drilling resumed after recovery of Core 338-C0002J-6R. The fact that the fragments are rounded and bear RCB tool scars supports this hypothesis.

Fractures without noticeable displacement

Natural fractures in cores from Site C0002 are not readily distinguished from drilling-induced fractures. In some cases, however, features borne by fracture surfaces allow rejecting a drilling-induced origin. One joint striking N89°E to N94°E and dipping 78°N to 81°N has been observed at interval 338-C0002H-1R-1, 65–83 cm (Fig. F64E). Its smooth surface suggests a Mode I opening, similar to the joint described in Section 316-C0006F-18R-1 (Expedition 316 Scientists, 2009). Other natural fractures have shiny surfaces that bear faint striations. Since no displacement across them can be noticed with the naked eye, these fractures are interpreted as shear or hybrid fractures (Hancock, 1985).

Vein structures

Sediment-filled vein structures (Cowan, 1982) were observed in silty claystone in cores from the lowermost part of Unit III in Hole C0002J (e.g., Fig. F64F). They appear as sets of vertical to steeply dipping, parallel, fine veins with either planar or sigmoidal shapes (Ohsumi and Ogawa, 2008). The distribution of vein structures with depth is consistent with core data from Hole C0002B (Expedition 315 Scientists, 2009b) and cuttings data from Hole C0002F (Fig. F54).

Fissility and incipient scaly cleavage

Fissility is locally observed in Holes C0002K and C0002L (e.g., Fig. F64G). It is generally well developed in mudstone layers and absent in coarser siltstones or sand intervals. Fissility is always horizontal and is suspected to result from drilling-induced sediment unloading. Orientation of fissility was not measured at Site C0002.

Incipient scaly cleavage is locally observed in mudstone from interval 338-C0002J-3R-5, 76–79 cm (Fig. F64H). Incipient scaly cleavage is an irregularly spaced cleavage along which the mudstone easily breaks apart. Cleavage surfaces are shiny and bear faint striations. Given the scarcity of incipient scaly cleavage, orientation of this structure was not measured.

Unit III/IV structural boundary

As already reported from Holes C0002A (Expedition 314 Scientists, 2009), C0002B, C0002C, C0002D (Expedition 315 Scientists, 2009b), and C0002F (“Logging while drilling”), Kumano Basin forearc sediment is characterized by subhorizontal to gently dipping bedding with dips <30° (Fig. F65). A total of 87% of the 238 bedding dip angles measured in Kumano sediment are <10°, and 11% are between 11° and 30°. In contrast, bedding in the accretionary prism (20 measurements) dips between 5° and 64° with 11 measurements steeper than 30°. This difference in bedding dip can help locate the boundary between the lowest Kumano Basin sediment (Unit III) and the underlying accretionary prism (Unit IV). The enlarged part of Figure F65 shows that a gap in bedding angles is present at ~923–927 mbsf in Hole C0002J, suggesting that this hole likely intersected the Unit III/IV boundary there. As we mentioned in “Disrupted bedding,” two relatively high dip angles at 923.9–924.09 mbsf seem to be related to bedding disruption. In that case, the structural boundary can be defined between 925.91 and 926.78 mbsf, which is comparable with the boundary defined by lithologic analyses (see “Lithology”).

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