SAND COMPOSITION

Sand and sandstone samples from Leg 190 are split into three major petrofacies related to different stratigraphic units in the Nankai Trough region (Fig. F2). These petrofacies consist of the following:

  1. Volcaniclastic sands at Sites 1173 and 1174,
  2. Quartzose sands at Sites 1175 and 1176, and
  3. Sedimenticlastic sandstones at Site 1178.

Sands and sandstones from Site 1177 include both quartz-rich and sedimenticlastic types. Apart from Site 1177 there were minimal vertical variations in mineral and clast types in sand and sandstone at Leg 190 sites.

Sites 1173-1174: Volcaniclastic Sands

These samples are all from the upper Pleistocene axial trench-wedge facies of the Nankai Trough (Subunit IIA at Site 1174 and Unit I at Site 1173). Site 1173 sands are fine to very fine grained, but petrographic analysis shows that they are compositionally similar to those at Site 1174. Sands at both sites are poorly sorted and consist mainly of angular to subangular grains. Modes were only determined for Site 1174 samples (Table T1). Modally analyzed sands at Site 1174 are medium to very coarse. The sands are all lithic rich and plot near the L (lithic fragments) corner on a QFL (quartz-feldspar-lithic) diagram (Fig. F3A) and near the Lv (volcanic lithic fragments) corner on a LmLvLs (metamorphic lithic-volcanic lithic-sedimentary lithic) plot (Fig. F3B). Mafic to intermediate volcanic rock fragments are dominant with common sedimentary lithic fragments, plagioclase, and quartz (Plate P1).

Quartz

Quartz (7%-21%) is the most abundant mineral component and has both straight and undulose extinction. Fine-grained quartzite and vein quartz are relatively uncommon. Grains of chert, some with recrystallized radiolarians, are relatively rare.

Feldspar

Feldspar (17%-22%) is common and consists mainly of relatively fresh plagioclase. Plagioclase is present as a common phenocryst mineral in the volcanic rock fragments. Many plagioclase crystals contain small glassy blebs and mineral inclusions (Plate P1). K-feldspar is present in all samples (1%-4%).

Mineral Fragments

Additional mineral fragments are minor (<8%) and include pyroxene, olivine, hornblende, biotite, and opaque minerals. These minerals are present as phenocryst phases in volcanic rock fragments. Rare metamorphic and igneous minerals such as chlorite, epidote, and, less commonly, tourmaline are present.

Volcanic Rock Fragments

Volcanic rock fragments are the most common component (25%-46%) and are dominated by mafic to intermediate varieties. These fragments typically consist of phenocrysts and microphenocrysts of olivine, pyroxene, plagioclase, and, more rarely, hornblende contained in an altered black and/or brown glassy groundmass (Plate P1). Many of the porphyritic volcanic grains may well be of basaltic andesitic affinity. Felsic volcanic fragments are more easily distinguished by the presence of quartz (1%-8%), although some of these grains are difficult to distinguish from chert and some sedimentary lithic fragments. The presence of phenocrysts and devitrification textures distinguishes some felsic fragments. Fresh colorless glass is relatively rare.

Sedimentary Lithic Fragments

Sedimentary lithic fragments are common (18%-32%). They are mainly mudstone fragments, some with thin quartz veins, and fine-grained siliceous rock fragments. Rare fine-grained quartzose sandstone, lithic sandstone, and quartz siltstone fragments are present.

Metamorphic Rock Fragments

Metamorphic rock fragments are minor (<8%) and are mainly metamorphosed sedimentary and volcanic rock fragments. The sedimentary rock fragments are typically low-grade slates and in Sample 190-1174B-1R-1, 6-10 cm, include some rare biotite-bearing hornfels. Metamorphosed volcanic rock fragments contain chlorite and epidote. Rare plutonic rock fragments distinguished by graphic intergrowths are present in Sample 190-1174A-3H-5, 69-73 cm. Most plutonic detritus is counted under monomineral component fragments, owing to the use of the Gazzi-Dickinson point-counting method.

Sites 1175 and 1176: Quartzose Sands

These samples come from the lower part of Units II and III (Pliocene-Pleistocene) at Sites 1175 and 1176 (Fig. F2). The modally analyzed samples are medium to very coarse, poorly sorted sands and pebbly sands with dominantly angular to subangular grains. They contain abundant quartz, sedimentary rock fragments, and minor feldspar. Site 1175 and 1176 samples overlap near the Q corner on the QFL plot (Fig. F3A). On the LmLvLs plot they all occur near the Ls corner (Fig. F3B). Volcanic detritus is relatively minor in contrast to the axial trench sands at Sites 808, 1173, and 1174.

Quartz

Quartz (55%-72%) includes monocrystalline quartz including some grains exhibiting undulose extinction, incipient polygonal grains, and rare deformation bands. Polycrystalline quartz is also common and with common fragments of veins with comb structure. Chert is common (3%-23%) with black and red chert containing recrystallized radiolarian tests that are circular in thin section (Plate P2).

Feldspar

Feldspar is a persistent component (5%-24%) and consists mainly of plagioclase and altered fragments of probable untwinned albite. K-feldspar (3%-14%) includes rare grains of microcline with its distinctive cross-hatched twinning (Plate P2).

Mineral Fragments

Additional mineral fragments are relatively minor (<2%) and include low-grade metamorphic minerals such as muscovite, biotite, chlorite, and epidote. Others are rare and include pyroxene and hornblende, probably of igneous derivation, and rare tourmaline, garnet, and zircon.

Sedimentary Lithic Fragments

Sedimentary lithic fragments are common (10%-24%). They include fine-grained mudstone that is clearly lithified as shown by the presence of thin quartz veins (Plate P2) and easily distinguished from unconsolidated mud that was inadvertently included during the sampling process. Many fine-grained siliceous fragments are also present that are transitional to chert and polycrystalline quartz but contain more abundant impurities than fragments identified as chert. Rare fine-grained quartzose sandstone, lithic sandstone, and quartz siltstone fragments are present. Much of the vein quartz in these samples is most likely derived from sedimentary rocks including chert (e.g., Plate P2).

Volcanic Rock Fragments

Volcanic rock fragments are minor (<8%) and are dominated by felsic volcanic fragments. The presence of phenocrysts, flow textures, and spherulites distinguishes some felsic fragments (Plate P2). Rare mafic to intermediate volcanic fragments are present in some samples. Fresh and altered glass also is present but relatively rare (e.g., Sample 190-1175A-43X-3, 75-78 cm). Tube pumice only occurs in Sample 190-1175A-39X-CC, 10-13 cm.

Metamorphic Rock Fragments

Metamorphic rock fragments are also relatively minor (<5%) and are mainly metamorphosed sedimentary rock fragments. These consist of two main types: foliated quartz-chlorite fragments (impure quartzite) and unfoliated low-grade metamorphic rock fragments containing minerals such as epidote and chlorite. The presence of relict volcanic textures and the abundance of chlorite and epidote distinguish metamorphosed volcanic rock fragments. Some metamorphic rock fragments are too altered to distinguish their parent material. Fragments of slate are scarce.

Plutonic Rock Fragments

Plutonic rock fragments are distinguished by intergrowths of quartz and feldspar (e.g., graphic intergrowth) that is too fine to be recorded as separate mineral components. Most plutonic detritus is counted as crystal fragments.

Site 1177: Quartzose and Sedimenticlastic Sands and Sandstones

Sand and sandstone samples at Site 1177 are from Unit III, the lower-upper Miocene lower Shikoku Basin turbidite facies (Fig. F2). Many of the samples consist of very fine to fine sand and sandstone, but only fine to medium sand samples were modally analyzed. These sands are poorly sorted with angular to subangular grains and less common subrounded grains. Their finer grain sizes means that they are less suited for modal analysis than sands from Sites 1175 and 1176. Mineral and clast types in Site 1177 sands are similar to those from Sites 1175, 1176, and 1178 (Plate P3). Sand in the unit occurs in four sandy intervals, with interval D at the base and intervals C, B, and A in ascending order to the top of the unit (Shipboard Scientific Party, 2001b). The four sand samples modally analyzed from sand interval A and the top of sand interval B contain subequal amounts of quartz (36%-47%), feldspar (20%-30%), and lithic fragments (22%-43%). Lithic fragments are dominantly of sedimentary type with a smaller quantity of silicic volcanic rock fragments (2%-5%). On the QFL diagram the sands from intervals A and B plot closer to sands from Site 1178 than those from Sites 1175 and 1176, whereas on the LmLvLs diagram they overlap with sands from Sites 1175 and 1176 (Fig. F3). The three samples analyzed from sand interval D have higher quartz content (54%-67%), similar feldspar content (21%-34%), and lower lithic content (11%-15%) than the samples from intervals A and B. The content of silicic volcanic fragments in interval D (2%-7%) is comparable to the upper intervals. On the QFL diagram the lower sands compositionally overlap those from Sites 1175 and 1176, whereas on the LmLvLs plot two samples plot with the sands from Site 1174 and one plots with sands from Sites 1175 and 1176 (Fig. F3).

Site 1178: Sedimenticlastic Sandstones

The Site 1178 samples are all from Unit II, which consists of upper Miocene accreted trench-wedge facies (Fig. F2). The sandstones are medium to very coarse and poorly sorted with subrounded to angular grains. The larger grains are usually more rounded than the smaller grains. Compared to samples from most other sites they are dominated by sedimentary lithic fragments (mudstone, siltstone, fine sandstone, and siliceous claystone) (Plate P4) with minimal chert. On the QFL plot they lie near Site 1174 sands, but they have only lower quartz content in common with these sediments (Fig. F3A). On the LmLvLs plot they plot on or very close to the Ls corner (Fig. F3B). Clast types are similar to those for Sites 1175, 1176, and 1177 but the overall proportions differ with lower total quartz and higher lithic fragments compared with these other sites:

Quartz (23%-36%) includes monocrystalline quartz, polycrystalline quartz, and chert (<5%).
Feldspar is common with subequal plagioclase and K-feldspar (9%-21%).
Additional mineral fragments are rare (<1%) and include muscovite, chlorite, epidote, garnet, and zircon.
Sedimentary lithic fragments are abundant (49%-59%) and are mainly mudstone and less common quartz sandstone, lithic sandstone, and quartz siltstone fragments.
Volcanic rock fragments are minor (<6%) and are mainly felsic volcanic fragments.
Metamorphic rock fragments are minor (<5%) and are mainly low-grade metamorphosed sedimentary rock fragments.

Provenance

Petrographic discrimination diagrams, such as those created by Dickinson and Suczek (1979) and Dickinson et al. (1983), are useful indicators of distinctive source regions with implications for the tectonic setting of the Japan region (e.g., Marsaglia et al., 1992) and Leg 190 sand provenance. Late Pleistocene sands from Sites 1173 and 1174 have similar compositions to other Nankai Trough trench wedge Sites 808, 582, and 583 (Fig. F1); these plot in the undissected to transitional arc field on a QFL diagram (Fig. F3A, F3D) (De Rosa et al., 1986; Taira and Niitsuma, 1986; Marsaglia et al., 1992; Underwood et al., 1993). Five of the samples analyzed from Site 1174 fall in the undissected arc field (Fig. F3A). Miocene and Pliocene-early Pleistocene sands from Sites 1175, 1176, and 1177 (sand interval D) plot in the recycled orogen field (Fig. F3A). Site 1178 late Miocene sandstones plot on the QFL diagram in the transitional arc and dissected arc fields, and Site 1177 (sand intervals A and B) sandstones plot on the recycled orogen dissected arc boundary (Fig. F3A). On the LmLvLs diagram Site 1174 sands are easily distinguished from all other sites apart from two samples (Site 1177, sand interval D) that contain small quantities of lithic fragments (Fig. F3B). The distinction is clearer on the QmKP diagram where Site 1174 sands plot close to the plagioclase corner in a separate group from the other Leg 190 sites (Fig. F3C).

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