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doi:10.2204/iodp.proc.333.105.2012

Structural geology

Most of the Site C0012 cores had well-measured structure even though the HPCS produced disturbance in Hole C0012D. Structural measurement data are given in C0012.XLS in STRUCTUR in “Supplementary material.” Planar structures such as bedding, faults, shear zones, and veins were reoriented into true geographic coordinates using shipboard paleomagnetic data (see “Structural geology” in the “Methods” chapter [Expedition 333 Scientists, 2012a] for reference).

Holes C0012C and C0012D

Figure F12 presents the overall distribution of planar structures in Holes C0012C and C0012D. In Holes C0012C and C0012D, bedding shows a wide range of dip angles from 3° to 70°. Most of the faults and shear zones have high dip angles. The depth profile of structural features in Holes C0012C and C0012D is divided into four zones based on the distribution of bedding dip angles: Zone I (0–14 mbsf), Zone II (14–86.5 mbsf), Zone III (86.5–146.5 mbsf), and Zone IV (146.5–180 mbsf).

Bedding

Planar structures such as green layers, dark brown sand layers, and volcanic ash layers are recognized as bedding. Bedding was measured both on the split core surfaces and using X-ray computed tomography (CT) images. Beds in Zone I are characterized by gentle dip (generally <30°) (Fig. F13), whereas bedding dip angles within Zone II range from 19° to 70°. In particular, dip angles >60° are commonly observed in Zone II. With few exceptions, beds in Zone II strike northeast–southwest and dip southeast (Fig. F14). There are several changes in bed orientation patterns within Zone II. Abrupt changes of bedding orientation are observed at intervals 333-C0012C-6H-6, 41 cm, and 9H-7, 80–110 cm. The lowest portion of Zone II (Core 333-C0012C-10H) displays a chaotic occurrence that looks like shallow soft-sediment deformation. In contrast to Zone II, bedding orientations in Zone III follow the same lower dip (<30°) pattern as those of Zone I. In Zone IV, bedding strike and dip are scattered.

Deformation structures

Faults

Faults in Holes C0012C and C0012D generally have high dip angles with normal offsets (Figs. F12, F15). They are mostly observed in Zones I and III. Fault planes generally strike northwest–southeast and dip to the northeast or southwest, which suggests conjugate sets formed by northeast–southwest extension (Fig. F16). Although faults are scarce within Zone II, several faults are found slightly above the bottom of Zone II. These faults show a diverse range of strikes and dips. A normal fault in Core 333-C0012C-10H is truncated by bioturbation (Fig. F17), suggesting deformation occurred at very shallow burial depths. At ~145 mbsf, which is the transition zone between low-angle bedding (Zone III) and scattered bedding of chaotic deposits (Zone IV), several sets of faults were observed. A normal fault crosscuts two low-angle faults with 4 mm offsets in interval 333-C0012D-5H-5, 91–96 cm (Fig. F18). Another fault with a moderate dip angle was observed ~1 cm below those fault sets having northeast–southwest strike and southeast dip direction. Only one reverse fault was observed in Holes C0012C and C0012D (interval 333-C0012D-2H-5, 76–82 cm) (Fig. F19).

Chaotic deposits

In this chapter, the term “chaotic deposit” is used to express intervals where bedding and original sedimentary structures were highly disturbed and mixed. Chaotic deposits are developed mainly in three intervals in Holes C0012C and C0012D. All of these intervals are characterized by disrupted beds, folds, and injections of sand or mud.

The first interval occurs at the bottom of Zone I (interval 333-C0012C-2H-5, 53 cm, to 2H-CC, 26 cm; 10.33–14.00 mbsf). The second interval occurs at the bottom of Zone II (interval 333-C0012C-10H-3, 0 cm, to 10H-8, 121.5 cm; 81.63–86.11 mbsf). Within this zone, directions of strikes and dips of bedding are scattered and beds are irregularly truncated by planar structures (Fig. F20). A diapir-shaped dewatering structure (Fig. F21) is also observed here.

Chaotic deposits in the third interval are widely distributed in Zone IV. In this zone, it is difficult to define whether such structures formed naturally or as a result of coring. However, a chaotic deposit in Core 333-C0012D-6H appears to be a natural example rather than coring-induced disturbance because vertically exaggerated structures are not observed here, whereas other disturbed layers are mainly characterized by near-vertical flow structure of mud, indicative of flow-in. Instead of flow structure of mud, poorly sorted muddy sand containing clasts of silt is recognized as fluidized matrix (Fig. F22), suggesting that chaotic deposits resulted from fluidization of the sand layer and disaggregation of silt. The muddy sand matrix shows fining-upward texture at a ~4 m scale and injects into relatively coherent silt layers. Silt layers are tilted and folded so that they show overall chaotic occurrence. The fluidized muddy sand is also truncated by a later fault (Fig. F22). It is possible that the later fault is related to coring, but the muddy sand injection and layer disruption appear to be natural. The occurrences observed in this core are quite similar to those of slump bodies observed in the Miura and Boso Peninsulas in Japan (Yamamoto et al., 2007). The origin of other chaotic deposits in Zone IV (e.g., Cores 333-C0012D-12H and 13H), whether they are natural or formed due to coring, is still ambiguous.

Chaotic deposits were also described within lithologic Units II and III of Hole C0012A but obscured by severe drilling disturbance (Expedition 322 Scientists, 2010).

Shear zones

As observed at Site C0011, shear zones with high dip angles that result in large offsets can also be observed at Site C0012 (Fig. F23). Typically, shear zones occur in cores where small faults are also observed but the shear zones show more displacement than the small faults.

Mineral veins

Mineral-filled veins were observed between Sections 333-C0012D-11H-4 and 12H-5 (172.6–175.4 mbsf) (Fig. F24). Vein minerals comprise fine-grained calcite with traces of barite. These veins penetrate at a high angle into the silty mud beds of interval 333-C0012D-11H-4, 72–99 cm, and presumably extend from a hard sandstone layer immediately below. Except for one southeastward-dipping example, veins strike northeast–southwest and dip steeply northwestward.

Slumping at Site C0012

The existence of steeply tilted beds developed throughout Zone II and the chaotic interval at the bottom of Zone II suggest that Zone II is a large slump body that was tilted by block rotation, and the chaotic interval corresponds to the basal slip plane of the slump. Hiatuses at the bottom of Zones I and II (see “Paleomagnetism” and “Biostratigraphy”), observed porosity changes (see “Physical properties”), and changes in total sulfur and carbonate concentrations (see “Organic geochemistry”) are consistent with the assumed slump. A possible hiatus at the bottom of Zone I (1.07–3.60 Ma) (Fig. F25) implies that the slide event occurred some time between 1.07 and 3.60 Ma. On the other hand, the hiatus at the bottom of Zone II (4.49–5.24 Ma) (Fig. F25) also suggests that sediments deposited during this time period were removed by displacement of a basal normal fault. Thickness of the lost sediments is estimated as 14.6–24.5 m by assuming a sediment deposition rate of 1.94–3.26 cm/k.y. (Fig. F25), and removal of this thickness of sediments suggests large displacement along the basal slip zone. The age of the slump base (4.49–5.24 Ma) roughly corresponds to the lithologic Subunit IA/IB boundary at Site C0011, where the number of ash layers and porosity decrease (see “Lithology” and “Physical properties” in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). This is indicative that the lithologic contrast may play an important role for slumping.

Chaotic intervals in Zone IV and deeper portions (i.e., within lithologic Unit II and at the top of Unit III in Hole C0012A [Expedition 322 Scientists, 2010]) are also regarded as evidence of slumping. Widespread occurrences of slumping may suggest that the Kashinosaki Knoll was continuously uplifted at least from ~9.5 Ma (the age of the chaotic deposit in Unit III) to ~1.1 Ma.

Holes C0012E, C0012F, and C0012G

In Holes C0012E, C0012F, and C0012G, greenish silty claystone with thin sandstone layers and red calcareous claystone overlie basaltic basement (see “Lithology”). Bedding of silty claystone and calcareous claystone are subhorizontal or gently (<30°) dip to northeast (Fig. F26A). Cleavages parallel to bedding are occasionally found within calcareous claystone.

Two sand dikes occur at intervals 333-C0012E-1X-1, 35–38 cm, and 1X-2, 59–72 cm. The former is relatively thin (~7 mm thick) and zigzag shaped (Fig. F27), whereas the latter, branching from an underlying sandstone layer, is thick (~5 cm thick) and intruded the claystone in a straight manner.

Mineral veins with various dipping angles (subhorizontal to subvertical) develop within red calcareous claystone. Although paleomagnetic correction was limited by coring disturbance, several corrected measurements indicate that some of the veins dip southeastward (Fig. F26B). Veins are mainly composed of calcite with accessory barite.

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