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doi:10.2204/iodp.proc.314315316.203.2011

Results

Description of gravel samples

Site C0007 gravels are clast-supported, granule to fine pebble grade, poorly to moderately sorted, polymictic, and have subrounded to angular clasts. Samples from Core 316-C0007C-17H lack mud matrix, and mud matrix is only locally developed in Core 316-C0007D-12R. In Core 316-C0007C-17H, the gravel is part of a 1.7 m thick layer that is normally graded in the topmost 1.5 m. In Core 316-C0007D-12R, the gravel is from a 4.55 m thick layer that is normally graded in the topmost 1.5 m. Gravel samples from Core 316-C0007C-17H were notably coarser than samples from Core 316-C0007D-12R. Gravels are of granule to fine pebble size in Core 316-C0007C-17H (Tables T1, T2). In Core 316-C0007D-12R the gravel is granule grade with abundant sand-sized grains.

Clast types are similar in both cores. The most common clast types are fine-grained sedimentary rock fragments (Tables T3, T4, T5; Figs. F3, F4, F5). Many of these clasts contain quartz veins. In Core 316-C0007D-12R, many clasts are categorized as combined vein quartz and fine-grained sedimentary rock fragments because quartz veins are so abundant (>50% of the clast). Chert is common and mostly dark gray to black with minor quantities of light-colored yellow-brown and clear chert. In Core 316-C0007D-12R, rare clasts are red to purple chert (jasper). Sandstone clasts are also common in samples from both cores. Vein quartz clasts (5%–26%) are present in all samples. Igneous clasts are mainly intermediate volcanic rock fragments, with additional minor silicic rock fragments (including pumice), scarce basaltic rock, and minor plutonic rock fragments. Metamorphic rock fragments are also scarce. Light brown intraformational soft mud fragments occur in most samples.

Petrography of clast types

It is clear from thin sections that all clast types display considerable variation in characteristics. Counts of clast types in the grain mounts from granule-sized gravel are given in Table T5.

Chert ranges from impure varieties to chert consisting almost wholly of silica (Fig. F4A–F4F). Many chert clasts have quartz veins. Radiolarians occur in some fragments (Fig. F4C–F4F). Some chert clasts are strongly recrystallized.

Siltstone is highly variable and includes silty claystone and impure chert. Most fragments are dark colored with fine to abundant veins of quartz and, less commonly, calcite and chlorite. Several of these clast types were chemically analyzed by Expedition 316 Scientists (see Table T7 in Expedition 316 Scientists, 2009b) and were shown to consist mainly of quartz with lesser plagioclase and clay.

Sandstone is also highly varied (Fig. F5). Grain size ranges from very fine to fine sandstone with scarce medium-grained sandstone. Most sandstone clasts consist of quartz, minor plagioclase, and lithic grains (sedimentary, volcanic rock fragments, vein quartz, and plutonic rock fragments). Some lithic sandstone clasts contain radiolarian chert fragments. Pure quartz sandstone and sandstone with calcite cement are rare. Some sandstone clasts contain quartz veins. Rare lithic sandstone clasts show evidence of low-grade metamorphism.

As noted above for vein quartz, all gradations occur between pure vein quartz and vein quartz containing mostly slivers of sedimentary wall rocks.

Volcanic rock fragments are mainly intermediate in composition with less common silicic and mafic clasts. Intermediate clasts texturally are porphyritic with altered glassy groundmass and abundant phenocrysts of plagioclase and fewer pyroxene phenocrysts (Fig. F6A–F6D). Silicic volcanic rock fragments are porphyritic with quartz and feldspar phenocrysts in a recrystallized, very fine grained groundmass. Mafic clasts are also porphyritic with pyroxene and plagioclase phenocrysts in an altered glassy groundmass. Some volcanic rock fragments are partially altered to epidote and chlorite.

Plutonic rock fragments are mainly coarse to medium-grained plagioclase-rich clasts of intermediate composition and granitic clasts with quartz and feldspar crystals (Fig. F6E–F6H). Medium-grained to fine-grained holocrystalline clasts of mainly intermediate composition also occur, probably derived from shallow and/or small intrusions.

Metamorphic rock fragments include fine-grained metasedimentary rock fragments containing common fine-grained biotite (Fig. F7) probably derived from contact aureoles adjacent to intrusions.

Chemical analyses

The chert and vein quartz clasts, as expected, have very high silica values (>94%). The sandstone and siltstone fragments are also relatively high in silica (77% and 83%, respectively). A volcanic rock fragment, thought to be of intermediate composition, was higher in silica (72%) than expected and probably reflects alteration of the sample. The metamorphic rock was thought to be a metamorphosed mafic volcanic rock but it also is significantly higher in silica (71%) than expected. See Table T6 for X-ray fluorescence (XRF) results.

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