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doi:10.2204/iodp.proc.324.105.2010

Structural geology

Cores recovered from Hole U1348A between 199.5 mbsf in Core 324-U1348A-14R and 324.1 mbsf in Core 26R can be structurally divided into two parts, an upper part consisting of volcaniclastic sediment with well-developed bedding (Cores 14R through 22R) and a lower part made up of coarse brecciated basalt characterized by massive structure (Cores 23R through 26R). The boundary between the parts is at interval 324-U1348A-23R-1, 0 cm. Primary structures and veins described here are solely within the volcaniclastic rocks. Some alteration veins, however, are developed in both sections.

In the following sections, the characteristics of the two major types of structures are described, followed by a discussion of their distribution and orientations, relationships between structures, and a short summary. Important observations and interpretations include (1) preferred orientations of primary structures such as bedding in sedimentary rocks and (2) variation in structural morphology and orientation of veins in both laminated and more massive volcaniclastic rocks.

Primary structures

Bedding, a major primary structure, can be divided into three types in Hole U1348A. Bedding ranges from massive layering in the lower part to delicate lamination in the upper part. The latter is especially clearly observed in fine-grained rocks such as granular hyaloclastite and fine hyaloclastite interbedded with thick layers of hyaloclastic breccias (Fig. F45). Most bedding dips are gentle, with slopes of ~0°–5°. Occasionally, steeper bedding is observed but never exceeding 30° dip (Fig. F46). These intercalated gentle and steeper bedding in the hole comprise meso-scale cross-bedding, which is characterized by bedding or lamination oriented at an angle to the true bedding surface. Observed true bedding surfaces include the sharply truncated top surface, the middle cross-bedding, and the bottom horizon that is tangential to the middle cross-bedding. Graded bedding in Hole U1348A is dominated by inverse grading sequences, which can coarsen upward through fine hyaloclastite and/or coarse granular hyaloclastite to hyaloclastic breccia. The clasts in the sedimentary rocks are vitric basalts and poorly sorted, semirounded volcaniclastic rocks. At interval 324-U1348A-26R-1, 12–15 cm, some laminated clasts are offset 1 cm by a microfault with a dip of 50°. However, this microfault cannot be traced beyond the drill core, so it is considered to be an intraformational fault (Fig. F47), which could indicate minor submarine slumping after diagenesis.

The third kind of bedding is chaotic bedding and weak bedding with many volcanic clasts. This structure is often developed between well-stratified hyaloclastic sedimentary rocks and massive volcaniclastics, for example, at interval 324-U1348A-26R-1, 10–20 cm (Fig. F47).

The massive volcaniclastics in the lower part of Cores 324-U1348A-23R through 26R consist of hyaloclastite breccias cemented by calcites and zeolites. The vesicularity of individual hyaloclasts is varied, and some observed high vesicularity could be a result of alteration (see "Igneous petrology").

Microfaults and veins

Microfaults can be found in intervals 324-U1348A-24R-2, 99–102 cm (297.87–297.90 mbsf), and 117–121 cm (298.04–298.08 mbsf). Some striations are displayed in the fault surfaces of these microfaults. Most microfaults with steeply dipping striations are normal faults based on smoothening downward–directed striations of the fault plane. However, some microfaults are indentified as strike-slipping faults because of horizontal striations in the fault plane.

Veins in Hole U1348A are few, but vein types are complex. Most veins dip at steep angles of 70°–80° and can be divided into three types by color: white, gray, and yellow, which correspond to calcite-filling veins, gray clay–filling veins, and brown clay–filling veins, respectively (Fig. F48). Figure F48A shows a yellowish vein that is curved, ~1 cm wide, and filled with yellow clay and oriented dark green slivers or laths at interval 324-U1348A-18R-2, 70–118 cm. The yellowish vein cuts a white vein and the primary bedding; therefore, it formed later than the white vein. A grayish vein can be observed at interval 324-U1348A-26R-2, 33–43 cm, where the volcaniclastics are characterized by banding with a symmetrical display of different textures or colors on both sidewalls of the vein (lower inset diagram, Fig. F47). Most white veins have an en echelon geometry and are related to postdepositional deformation because they cut the primary bedding, for example, at intervals 324-U1348A-21R-5, 27–60 cm (Fig. F48B), and 15R-1, 58–62 cm (Fig. F48C). We also observe one quartz vein cutting through a piece of chert with obvious bedding at interval 324-U1348A-5R-1, 8–13 cm (Fig. F48D). This shows that many veins developed late relative to volcanism and sedimentation at this site. No direct intercrossing relationship between the gray vein and the other two types of veins are observed in the hole.

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