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doi:10.2204/iodp.proc.331.107.2011

Lithostratigraphy

Site C0017 consists mainly of hemipelagic homogeneous mud, pumiceous sediment, and volcaniclastic-pumiceous mixed sand, without the obvious hydrothermal alteration seen at previous sites. Based on sediment character and structure, major mineral composition, and the occurrence of microfossils, sediments recovered from the four holes drilled (Holes C0017A, C0017B, C0017C, and C0017D) consist of three main sediment types: hemipelagic mud, coarse angular pumiceous gravel, and volcanic sands with erosional bases. These sediments and lithologies are similar to those of Unit I encountered at Sites C0014 and C0015. However, the unaltered sedimentary sequence drilled at Site C0017 is much more vertically extensive, and subdivision may be drawn on the basis of the relative predominance of hemipelagic mud or volcaniclastic sediments. This divides the stratigraphy at Site C0017 into four units (Fig. F2; Table T2). An upper unit (Unit I), dominantly composed of hemipelagic mud from 0 to 18.5 mbsf, is underlain by two units dominated by pumiceous gravel from 19.1 to 36.2 mbsf (Unit II) and 61.1 to 78.8 mbsf (Unit III). The interval from 36.1 to 61.1 mbsf was not cored, so it was not possible to group Unit II and Unit III as a single unit, despite the strong sedimentological similarity between them. Additionally, no core was recovered in the interval from 78.8 to 94.3 mbsf. The deepest portion of the cored interval, from 94.3 to 144.7 mbsf (Unit IV) is dominated by hemipelagic mud. The thicknesses of individual beds of mud and gravel at Site C0017 are significantly greater than those at Site C0014, consistent with Site C0017’s location on the downthrown side of a normal fault.

Sediment types at Site C0017

There is little apparent systematic vertical variation in sediment types at Site C0017. Unless stated otherwise, the following description of sediment types applies to all four lithostratigraphic units at the site.

Hemipelagic mud

Hemipelagic mud is the dominant sediment type in Unit I (0 to ~18.5 mbsf) and Unit IV (~94 to ~145 mbsf) but also occurs in Units II and III (Fig. F2). This mud is primarily composed of hemipelagic homogeneous silty clay or clayey silt, with widely distributed foraminifers and occasional lenticular pumiceous sand patches, gravels, and mud clasts. The grayish olive to dark olive-gray color and uniformity of the mud make it easy to recognize. Foraminifers are mostly fresh and intact and are only rarely reworked, reflecting an autochthonous source. They should be useful for postcruise research that aims to establish the late Quaternary chronostratigraphy in this area and to reconstruct paleoenvironments. The relatively uniform, homogeneous, and structureless mud in the upper 18.5 m of sediments in Holes C0017A and C0017B (Unit I) reflects a typical hemipelagic depositional environment.

In contrast, Unit IV (Cores 331-C0017D-6X, 7H, 9X, 10X, and 11X; ~94.3 to ~144.7 mbsf) comprises laminated and interbedded clay and silt and very fine sand, with homogeneous clay predominant. Layers of silt and very fine sand (1–5 cm thickness) commonly form upward-fining sequences and possess erosional bases that create sharp contacts with the underlying homogeneous dark gray clay (Fig. F3). From X-ray diffraction (XRD) analyses (Table T3), the mineralogy of these silty and fine sandy layers is dominated by probably locally derived volcanic glass, quartz, and feldspar. Along with calcite, muscovite, chlorite, and dolomite, they are also the main components of the overlying and underlying homogeneous clay. These coarser grained layers are probably formed primarily by volcanic eruption and episodic gravity flows related to volcanic and seismic activity.

Pumiceous gravel and lithoclast-rich horizons

The pumiceous gravel and lithoclast-rich horizons sediment type consists of poorly sorted pumiceous sediments dominated by coarse-grained angular woody pumiceous gravels (Fig. F4). It generally has a gradational contact with finer overlying pelagic or volcaniclastic sediments but mostly sharp contacts with the underlying sediments. Pumiceous gravels predominate in Unit II (19.1–36.2 mbsf) and Unit III (61.1–78.8 mbsf) but are also found within Units I and IV. This sediment type shares similar characteristics and major mineral compositions with pumiceous sediments encountered at Site C0014, and especially Site C0015.

The pumice encountered at Site C0017 can be divided into two types: one fresh and white-gray to greenish and the other oxidized and stained yellow to brown. It has similar appearance and bulk mineralogy to that found at Site C0014. Oxidized pumice is abundant between ~26 and 35 mbsf in Hole C0017C. Occasionally, the greenish pumice gravels and clasts (Fig. F3), which are inferred to contain glauconite (Weigland et al., 2003), are mixed with oxidized fragments, which are softer and more fragile than the fresh pumice and easily broken during visual examination. The significance of the oxidized pumice is discussed in more detail in “Petrology.”

Volcaniclastic and pumiceous mixed sand

Gray to black, fine to coarse sand and gravels with a mixture of volcaniclastic and pumiceous components are irregularly distributed throughout all four units at C0017 (Fig. F2). A number of individual beds show clear normal and, in one case, inverse graded bedding, generally varying from coarse sand to coarse silt. The thickness of graded beds varies from <10 cm to 0.95 m.

Volcaniclastic and pumiceous sand beds generally have sharp contacts with underlying layers but gradational contacts with overlying units, indicating a waning sediment supply. This observation is consistent with deposition from a gravity flow, probably resulting from episodic volcanic and seismic activity within and around the area, coupled with the instability of volcanic flanks in this region. The bulk mineralogy of these sediments is volcanic glass, anorthite, quartz, albite, muscovite, and trace amounts of microline and augite, based on microscopic and XRD examinations (Table T3). Shards of volcanic glass are usually brown and yellow-brown and are poorly sorted, ranging in size from coarse sand to pebbles as large as 1.2 cm in diameter. Vesicular structure is well developed on the surface of the volcanic glass shards, and pores are somewhat elongate and filled with diagenetic white minerals.

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