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doi:10.2204/iodp.proc.323.110.2011

Summary of core description

This paper describes the cored volcanic section recovered during IODP Expedition 323 from Holes U1342A and U1342D. In the description, we used the nongenetic terms of volcanic conglomerate and volcanic sandstone (Suthren, 1985) instead of lapilli tuff, tuff breccia, and volcanic breccia. Mineral abundances in the igneous rocks were determined by point counting using 1000 points per thin section.

The volcanic sequence in Cores 323-U1342D-7X through 19X was divided into six major lithologic units as described below. Figure F1 schematically shows the lithology of the material recovered in Cores 323-U1342D-7X through 19X.

Unit I is a vesiculated andesitic lava flow that was recovered in Sections 323-U1342D-7X-1 through 8X-1 (Figs. F2, F3). A comparable lithology appears in Core 323-U1342D-9X in Hole U1342A (Fig. F4). The andesite contains phenocrysts (~20–30 vol%) of magnetite (Mt), orthopyroxene (Opx), clinopyroxene (Cpx), and plagioclase (Pl) and has a groundmass showing intersertal texture. It also contains ellipsoidal vesicles <7 mm long. This unit has a minimum thickness of 0.9 m. The Unit I/II boundary was not recovered.

Unit II consists of interbedded volcanic sandstones and polymict volcanic conglomerates. These were recovered in Cores 323-U1342D-8X through 10X (Figs. F3, F5, F6). Volcanic clasts in the conglomerates and gravel sandstones are mainly angular to subangular fragments of basalts and andesites. The clasts have various phenocryst assemblages, phenocryst proportions, extent of vesiculation, and groundmass textures. For example, the following clasts are observed: olivine (OL)-Cpx-Pl porphyritic basalt, Cpx-Pl porphyritic basalt, Pl-sparsely phyric basaltic andesite, and aphyric basaltic andesite. Sandstones and the matrix of conglomerates consist of basaltic and andesitic fragments, together with mineral grains of pyroxene, plagioclase, and magnetite. Biogenic material is absent. The bedding planes of this unit have dips of 0° to 10°. The unit has a minimum thickness of 8 m. The base of this unit gradually changes to Unit III, which is monomict volcanic conglomerate.

Unit III is formed by monomict volcanic conglomerates, which are observed in Cores 323-U1342D-10X and 11X (Figs. F6, F7). Clasts in the conglomerates are made up of aphyric basalt, in which there are phenocrysts (<3 vol%) of clinopyroxene and plagioclase and glassy to intersertal groundmass. Clasts are an average of ~1.5 cm in size, with a maximum size of >6 cm. The matrix is basically composed of fine- to medium-sized altered glass of the same basalt as that making up the clasts. The minimum thickness of this unit is 6.5 m. The presence of radial cooling joints in large basaltic clasts and the cognate origin of the clasts and the glassy matrix suggest that this unit is probably hydroclastic volcaniclastics (e.g., Suthren, 1985).

Unit IV consists of interbedded volcanic sandstones and polymict volcanic conglomerates. This unit appears in Cores 323-U1342D-11X through 14X (Figs. F7, F8, F9, F10). The sandstones and the matrix of the polymict conglomerates are composed of various types of volcanic fragments. Because large amounts of reddish sand are included in this unit compared to Unit II, the sandstone and matrix of the conglomerate are dark red to reddish gray in color. Clasts in the conglomerate are angular to subangular basaltic and andesitic pebbles. This unit is characterized by the presence of fault and slump structures. The fault planes have dips of ~85° and displacements of ~7 cm. This unit is 9.5 m thick. There is a sharp contact between the base of this unit and the top of Unit V.

Unit V is comprised of monomict volcanic conglomerates recovered from Core 323-U1342D-14X (Fig. F10). Clasts are angular to subangular vesiculated basalt. The clasts reach 7 cm in size, with an average size of 1 cm. The basalt clasts contain phenocrysts (less than ~10 vol%) of completely altered olivine, clinopyroxene, and plagioclase, and exhibit variable vesiculation up to ~15 vol%. The matrix consists of very fine fragments of glassy basaltic material, which is the same as the clasts. The minimum thickness of this unit is 1 m. This unit is probably hydroclastic volcaniclastics. The Unit V/VI boundary was not recovered.

Unit VI consists of polymict volcanic conglomerates recovered from Cores 323-U1342D-15X through 19X (Figs. F11, F12, F13, F14, F15). Clasts are angular to subangular and consist of various types of basalts to andesites. The most abundant type of clasts is a highly porphyritic basalt (consisting of ~30–40 vol% phenocrysts) with phenocrysts of altered olivine, clinopyroxene, plagioclase, and minor orthopyroxene. The basaltic clasts have variable vesicularities up to ~15 vol%. The matrix of the clasts is made up of medium-grained to very coarse grained basaltic and andesitic fragments together with plagioclase and pyroxene grains. Depending on the fraction and size of the clasts, four subunits are recognized. The boundaries between the subunits are gradual. The lower three subunits are characterized by normal grading of clasts. The minimum thickness of this unit is 18 m.

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