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doi:10.2204/iodp.proc.331.105.2011

Lithostratigraphy

Despite the relatively short interval cored (0–9.5 mbsf), a broad diversity of sediment types was drilled at Site C0015, with coarse pumiceous gravel and grit, siliciclastic sand, hemipelagic mud, bioclastic gravel, and foraminiferal sediment all recovered. In order to be consistent with nomenclature applied to other sites drilled during this expedition, these interlayered sediments are grouped as a single lithostratigraphic unit (Unit I), but 14 subunits (Ia–In) are defined based on sediment type (Fig. F3; Table T2).

Sediment types cored at Site C0015

Table T2 describes the lithologic subunits cored at Site C0015. This information is presented as a sedimentary log in Figure F3. Subunits are described below and grouped by lithotype.

Coarse pumiceous gravel and grit

Woody pumice gravel

Woody pumice exhibits tubular structure, with tubes ~2 mm in cross section. These tubes give the pumice unique sedimentological properties, as they rapidly imbibe water and reduce the buoyancy of clasts, causing them to sink near the locale of effusion (Kato, 1987). Woody pumice is a local term for the tube pumice found widely in the Okinawa Trough region, and the term has been used in the region for some time. Woody pumice breccia near the seafloor comprises angular clasts of woody pumice that are generally black at this site (lithologic Subunit Ia). Near the top of Hole C0015B, a 2.8 m thick layer of pumice breccia displays inverse to normal grading (Subunit Ib). The base and top of the deposit comprise a coarse sand matrix of pumice fragments with a minor but significant mud fraction that was observed to make the unit more cohesive. Without this matrix, pumice gravels are effectively unconsolidated submarine talus.

Woody pumice gravel also occurs deeper in the sequence as Subunits If and Ik. In these intervals, pumice beds grade upward into matrix-supported bioclastic gravel and siliceous sand, respectively (Fig. F3). Pumice breccia with a poorly sorted brown mud-sand matrix and a significant (20%–30%) bioclast component was cored in Hole C0015B (Subunit Ie). This bed passes up into hemipelagic mud without any sharp depositional contact (Subunit Id). Coarse-grained sand was cored in Hole C0015C, at the base of which is a pumice layer (Subunit Ii). In this instance, the unconsolidated nature of the coarse-grained sediment, combined with disruption during core recovery, prevented observation of the depositional contact.

Pumice gravel and grit

Other pumice breccias in which the significant clastic component is not woody pumice were encountered in both holes (Subunits Ih, Ii, Im, and In; Fig. F3). In Subunits Ih and Ii, clasts are vesicular and have a frothy texture, but not to the extent of tube pumices. For Subunits Im and In in Hole C0015C, disruption due to drilling is high, and lack of a supporting matrix (which may have been flushed from the core) makes it difficult to discern sedimentary contacts.

Siliciclastic sand

Two siliciclastic sand intervals are present in Hole C0015C (Subunits Ij and Il; Fig. F3). Although the core is highly disturbed by drilling, the sands are texturally mature, comprising well-rounded, spherical, and well-sorted medium-grained quartz.

Hemipelagic mud

Hemipelagic mud is a major matrix component of most of the coarser sediments at Site C0015 and forms a distinct subunit (Subunit Ig) between 5.5 and 5.89 mbsf in Hole C0015B (Fig. F3). This interval displays visible laminae of silt and fine sand, indicating a possible mixed hemipelagic-siliciclastic origin. Foraminifers are abundant in these muds and are also common where mud forms a component of the matrix in volcanic breccias, although in this instance the foraminifers have holes and other evidence of damage, suggesting that they have been reworked.

Bioclastic gravel

The interval from 3.2 to 5.07 mbsf in Hole C0015B (Subunit Ie; Fig. F3) comprises bioclastic gravel that contains ~20% 1–2 cm fragments of branching coral and mollusk shells supported in a matrix of dark olive-green plastic reduced silty hemipelagic mud. The silt fraction in the interval consists of biogenic material (foraminifers, fish teeth, and shell and coral fragments), with minor pumice and trace fine-grained framboidal pyrite. Rare subrounded pumice fragments, ranging in size up to 4 cm, are also present in the subunit. Subunit Ie has gradational contacts into both the overlying foraminiferal sand and underlying woody pumice gravel subunits.

Foraminiferal sediment

Subunit Ic (2.95–3.05 mbsf) and Subunit Id (3.05–3.2 mbsf) are baby poo yellow foraminifer-rich sedimentary units separated by a sharp depositional contact (Fig. F3). The upper subunit is well sorted, well rounded, and medium grained and contains abundant foraminifers, minor scaphopods, fish teeth, and other biogenic material. Siliciclastic components comprise ~10% of the interval. Subunit Id is finer grained and less well sorted and grades into the underlying bioclastic gravel subunit.

Discussion and implications

The tendency for woody pumice to be deposited close to its point of eruption/effusion in a subaqueous volcanic setting (Kato, 1987) must be considered in any interpretation of the pumice horizons encountered at Site C0015. The inverse to normal grading of the thickest pumice breccia (Subunit Ib) in Hole C0015B (0.15–2.95 mbsf) is significant, as it is diagnostic for a proximal debris flow or mass wastage deposit. Inverse to normal grading is observed in breccia associated with mass wasting (Walker, 1992). The poorly sorted mud matrix and the large variety of texturally immature clasts in the bioclast-rich breccia (3.2–5.07 mbsf; Hole C0015B, Subunit Ie) is also consistent with a debris flow or mass waste; in this instance a greater variety of clast types has been resedimented. Thus, volcanic and other clast types at this locality have been redeposited from their initial point of deposition, albeit in close vicinity to their source.

Sharp sedimentary contacts with overlying pelagic mud are absent for most breccia horizons, probably because of the high porosity of the talus or scree beneath. Under these conditions, sediment of clay-size fraction infiltrates the top of the underlying formation.

In many oceanic volcanoes, debris flows triggered by flank collapse outnumber those triggered by caldera collapse or other volcanic activity (Schmincke, 2004). In such a depositional regime, lithology varies over short distances and thick units do not necessarily have a wide lateral extent. Stratigraphic correlation in an actively mass wasting environment, particularly the flank of a volcano, is thus complicated, although bounding surfaces will form during the long hiatuses between depositional events, when hemipelagic sedimentation dominates. These surfaces, however, may still be disrupted by further debris flows. Figure F4 illustrates an analogous setting to Iheya North Knoll, illustrating the difficulty inherent in understanding subsurface fluid flow at the site. Laterally extensive hemipelagic mud would be expected to act as a barrier or baffle to fluid flow. Coarser, more permeable and porous layers will often possess better hydraulic conductivity, although matrix-supported intervals would be significantly less permeable.

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