Petrography and Geochemistry of Volcanic Ash Layers from Leg 87A, Nankai Trough (South Japan)

Pouclet, André; Fujioka, Kantaro; Jarvet, J.; Cadet, Jean-Paul

doi:10.2973/dsdp.proc.87.118.1986

Cited by 8 publications

(15 citation statements)

References 10 publications

(9 reference statements)

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“…The geochemistry of the ashes is consistent with known explosive andesitic to rhyolitic volcanism in the Japanese Honshu and Izu-Bonin arcs (Pouclet et al, 1986;Masuda et al, this volume). The geochemistry of the circa-14-Ma rhyolitic pyroclastics at the base of Site 808 is different than for the overlying, younger ashes and tuffs, which suggests a different provenance.…”

Section: General Sedimentological Trends At Site 808supporting

confidence: 74%

Stratigraphic Synthesis of the DSDP-ODP Sites in the Shikoku Basin, Nankai Trough, and Accretionary Prism

Pickering¹,

Underwood²,

Taira³

1993

Proceedings of the Ocean Drilling Program

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The stratigraphy of the Deep Sea Drilling Project/Ocean Drilling Program drill sites in the northern Shikoku Basin and Nankai Trough area are reviewed in the context of data from ODP Site 808, and interpreted with generalized depositional models for the Nankai Trough and Shikoku backarc basin. Backarc rifting between about 27 and 14 Ma caused the partitioning of the Shikoku Basin early in its history into a western (e.g., DSDP Sites 442 and 297) and eastern Shikoku Basin (e.g., DSDP Sites 443 and 444), separated by the linear and topographically high spreading ridge and the Kinan seamount chain. Pliocene terrigenous turbidites recovered at DSDP Site 297 appear to have been restricted to southwest of the spreading center; therefore, terrigenous turbidity currents coming from the Suruga Trough, to the east-northeast, do not appear to have been capable of overspilling the ridge crest, and if they overspilled the confines of the trench would have been confined to the eastern Shikoku Basin. We therefore believe that the terrigenous turbidites at Site 297 probably were delivered to the Shikoku Basin via Ashizuri Canyon at the western end of Shikoku, which lacks volcanoes of this time interval and is therefore a better candidate source for the more quartzofeldspathic sands rather than the Izu collision zone. Alternatively, another viable sediment source for these terrigenous turbidite sands is via a canyon from Kyushu. The ODP Site 808 stratigraphy shows that at about 300 k.y., major terrigenous sediment overspill occurred across the subducting, extinct, spreading ridge crest, marked by a prominent debris-flow event, associated with the abrupt southwestward progradation of the sandy trench-wedge turbidite system. As a consequence, the western and eastern parts of the Nankai Trough became linked depositional basins for the accumulation of terrigenous sands derived mainly from the Izu collision zone.At ODP Site 808, the geochemistry suggests that the source of the approximately 14-Ma rhyolitic pyroclastic unit immediately overlying the basaltic oceanic basement appears to have been different from that for the younger ash units higher up in the succession. A probable source for the rhyolitic pyroclastic unit may be related to the onland geology in Japan where circa-14-Ma unroofed granites occur on Kii Peninsula and in Kyushu. On Kii Peninsula, considerable volumes of middle Miocene pyroclastics remain, and therefore provide good temporal and spatial correlatives for the pyroclastics encountered at the base of ODP Site 808. Some of the subaqueous pyroclastics at Site 808 may also have come from submerged explosive volcanic centers to give reworked deposits. The Miocene volcaniclastics recovered from DSDP Site 297 also may be related to the rhyolites at ODP Site 808.

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Section: General Sedimentological Trends At Site 808supporting

confidence: 74%

Stratigraphic Synthesis of the DSDP-ODP Sites in the Shikoku Basin, Nankai Trough, and Accretionary Prism

Pickering¹,

Underwood²,

Taira³

1993

Proceedings of the Ocean Drilling Program

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“…The chemical heterogeneity of the pyroclastic material is consistent with the documented range of andesitic to rhyolitic sources of explosive volcanism on the Japanese Islands. We agree with the assessment of Pouclet et al (1986) who identified the active volcanic arc of southwest Japan, including the Izu-Honshu collision zone, as the principal source of Quaternary ash ( Figure 1). Other possible sources would be the explosive volcanic centers on Kyushu and the Izu-Bonin arc.…”

Section: Pyroclastic Contributions To Shikoku Basinsupporting

confidence: 68%

Clay-Mineral Provenance, Sediment Dispersal Patterns, and Mudrock Diagenesis in the Nankai Accretionary Prism, Southwest Japan

Underwood

Pickering

1996

Clays and clay miner.

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Abstract--Offscraped strata within the toe of Nankai accretionary prism display an overall facies pattern of thickening and coarsening upward. Detrital clay minerals within the Quaternary trench-wedge facies are dominated by illite; chlorite is the second-most abundant clay mineral, followed by smectite. Relative mineral percentages change only modestly with depth. The hemipelagic clay-mineral population is virtually identical to clays washed from turbidite matrix, and different size fractions (<2 ixm and 2-6 ixm) show nominal amounts of mineral partitioning. Smectite content increases beneath the trench-wedge deposits, where the upper subunit of the Shikoku Basin stratigraphy (late Pliocene and early Pleistocene) includes abundance of volcanic ash. Syneruptive, subaerial chemical weathering of volcanic source rocks, together with in situ alteration of disseminated glass shards, caused the increase in smectite. Smectite begins a monotonic transformation to illite/smectite (US) mixed-layer clay at --555 mbsf and an estimated temperature of -65 ~ Ordered (R = 1) I/S interlayering first appears at --1220 mbsf (<2 ~m size fraction) and --1100 mbsf (<0.2 ~m size fraction). The illitization gradient coincides with a reduction in pore-water chlorinity, but depth-related changes in bulk mudstone geochemistry (K20, Rb) are subtle. The absolute abundances of discrete smectite and US appear to be insufficient to account for the magnitude of pore-water dilution via in situ dehydration reactions. Instead, pore water probably was transported to Site 808, either from sources located deeper in the accretionary prism, where bulk mudstone porosities are lower, or from strike-parallel sources where mudstones originally deposited in the Shikoku Basin might contain higher percentages of smectite.

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“…We agree with the assessment of Pouclet et al (1986), who identified the principal pyroclastic source as the Quaternary volcanic arc of southwest Japan, including the Izu-Honshu collision zone (Fig. 1).…”

Section: Volcanic Ash Geochemistrysupporting

confidence: 66%

“…Chemical data are sparse for Site 297, so it is not clear how much of the disparity is due to ash alteration with depth, as opposed to chemical differences in the volcanic sources that supplied Sites 297 and 808, respectively. Chemical analyses of volcanic glass from Sites 582 and 583 (Pouclet et al, 1986) add to the complexity. These ash layers also yield consistently higher silica contents than the background mudstones (Fig.…”

Section: Volcanic Ash Geochemistrymentioning

confidence: 99%

Sediment Geochemistry, Clay Mineralogy, and Diagenesis: A Synthesis of Data from Leg 131, Nankai Trough

Underwood¹,

Pickering²,

Gieskes³

et al. 1993

Proceedings of the Ocean Drilling Program

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This paper presents a synthesis of data from X-ray diffraction analyses of clay minerals and X-ray fluorescence analyses of bulk mudstones from Ocean Drilling Program Site 808. The samples come from three closely spaced holes drilled through the toe of the Nankai accretionary prism offshore Shikoku, Japan. Detrital assemblages of clay minerals are unusually uniform throughout the Nankai trench-wedge facies. Illite is the most abundant detrital clay mineral, followed by chlorite, smectite, and traces of kaolinite. Relative percentages of smectite increase within the upper subunit of the Shikoku Basin stratigraphy. This subunit contains abundant layers of volcanic ash, and the corresponding change in clay mineralogy probably was caused by synvolcanic weathering in source areas during the Pliocene, together with in-situ alteration of disseminated glass shards within the Shikoku Basin.At a depth of ~555 mbsf, the detrital and/or authigenic smectite component begins its transformation to illite/smectite mixed-layer clay. With increasing depth below this horizon there is a monotonic increase in illite interlayers. The onset of illitization occurs at an estimated temperature of ~65°C. Ordered (R = 1) illite/smectite interlayering first appears at depths of -1220 mbsf (<2-µm size fraction) and -1100 mbsf (<0.2-µm size fraction). The depth interval of the smectite-to-illite transformation coincides with a reduction in pore-water chlorinity; however, the absolute abundance of smectite appears to be insufficient to account for the changes in aqueous chemistry via in-situ dehydration reactions. Instead, significant volumes of diluted pore water probably were transported to Site 808, either from sources located deeper in the accretionary prism (where bulk mudstone porosities are lower) or from lateral sources where mudstones originally deposited in the Shikoku Basin may contain higher percentages of smectite. Significantly, we did not detect any anomalies in clay mineralogy or clay-mineral diagenesis within or near the décollement zone (945-964 mbsf).X-ray fluorescence analyses show that hemipelagic muds and mudstones at Site 808 are chemically uniform throughout most of the section. There are no geochemical perturbations, for example, within the décollement zone. Data from interbeds of volcanic ash demonstrate that the chemical effects of mud/ash dissemination and/or in-situ alteration of pyroclastic material are limited. In addition, ash layers are chemically heterogeneous within Unit III and Subunit IVa, which indicates that tephra was transported from a variety of andesitic to rhyolitic sources on the Japanese Islands during the Pliocene. In contrast, Miocene rocks of Unit V display a clear chemical divergence (little mixing) between rhyolitic tuffs and interbeds of multicolored mudstone.The most significant geochemical anomaly at Site 808 occurs well below the décollement zone between 1087 and 1111 mbsf. Variegated mudstones in this interval contain unusually high ratios of MgO/Al 2 O 3 , Fe total /Al 2 O 3 , Mn...

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Petrography and Geochemistry of Volcanic Ash Layers from Leg 87A, Nankai Trough (South Japan)

Cited by 8 publications

References 10 publications

Stratigraphic Synthesis of the DSDP-ODP Sites in the Shikoku Basin, Nankai Trough, and Accretionary Prism

Stratigraphic Synthesis of the DSDP-ODP Sites in the Shikoku Basin, Nankai Trough, and Accretionary Prism

Clay-Mineral Provenance, Sediment Dispersal Patterns, and Mudrock Diagenesis in the Nankai Accretionary Prism, Southwest Japan

Sediment Geochemistry, Clay Mineralogy, and Diagenesis: A Synthesis of Data from Leg 131, Nankai Trough

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