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Data report: clay mineral assemblages in the upper Shikoku Basin, results from IODP Expedition 333, Sites C0011 and C0012

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Proceedings of the IODP

Integrated Ocean Drilling Program

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      Abstract

      This report summarizes the results of X-ray diffraction analyses of core samples from Integrated Ocean Drilling Program (IODP) Sites C0011 and C0012, offshore southwest Japan. We analyzed 211 specimens (<2 µm size fraction) recovered during IODP Expedition 333. Most of those samples come from the uppermost hemipelagic/pyroclastic facies (Unit I) of the Shikoku Basin, with a limited number from the volcanic turbidite facies (Unit II) and the pelagic clay facies (Unit V). Minerals of the smectite group are generally the most abundant. At Site C0011, the amount of smectite in the clay-size fraction of Unit I averages 40.8 wt% (standard deviation = 11.5) and increases downsection. Proportions of illite and chlorite average 36.4 and 15.5 wt%, respectively. Average contents of kaolinite and quartz are 3.6 and 3.7 wt%, respectively. At Site C0012 (Unit I), the amount of smectite averages 50.0 wt% (standard deviation = 13.5) and increases downsection. Averages for the other minerals in the clay-size fraction are illite = 31.8 wt%, chlorite = 10.1 wt%, kaolinite = 5.5 wt%, and quartz = 2.6 wt%. Most values of illite/smectite expandability fall between 60% and 70%, and there are no systematic changes downsection. Values of the illite crystallinity index are generally consistent with detrital sources that were exposed to anchizone metamorphic conditions.

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      Mineralogy and Sedimentation of Recent Deep-Sea Clay in the Atlantic Ocean and Adjacent Seas and Oceans

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        NanTroSEIZE Stage 1 expeditions: introduction and synthesis of key results

        Integrated Ocean Drilling Program Expeditions 314, 315, and 316 were carried out as a unified program of drilling collectively known as Stage 1 of the Nankai Trough Seismogenic Zone Experiment, a multistage complex drilling project. A transect of eight sites was selected for riserless drilling to target the frontal thrust region, midslope megasplay fault region, and Kumano forearc basin region. Two of these sites are preparatory pilot holes for planned deep riser drilling operations, whereas the others targeted fault zone material in the shallow, presumed aseismic zone. Expedition 314 was dedicated to in situ measurement of physical properties and borehole imaging through logging while drilling in holes dedicated to that purpose. Expedition 315 was devoted to core sampling and downhole temperature measurements at one site in the megasplay region and one site in the forearc basin. Expedition 316 targeted the frontal and out-of-sequence megasplay fault region in the mid-slope environment. Results on accretionary complex structure, lithology and age, physical properties, and state of stress, which are documented in full in the site chapters of this volume, are here synthesized across the expeditions.
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          Expedition 322 summary

          Integrated Ocean Drilling Program (IODP) Expedition 322 is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and was designed to document characteristics of incoming sedimentary strata and upper igneous basement prior to their arrival at the subduction front. To accomplish these objectives, coring was conducted at two sites in the Shikoku Basin on the subducting Philippine Sea plate. Site C0011 is located on the northwest flank of a prominent bathymetric high (the Kashinosaki Knoll), whereas Site C0012 is located near the crest of the knoll. The resulting data, which include logging while drilling during IODP Expedition 319, provide a wealth of new information on presubduction equivalents of the seismogenic zone. Unfortunately, coring at Site C0011 began at 340 m core depth below seafloor (CSF) and failed to reach the total depth target because of premature destruction of the drill bit at 876 m CSF. Coring at Site C0012, however, penetrated almost 38 m into igneous basement and recovered the sediment/basalt interface intact at 537.81 m CSF. The age of basal sediment (reddish brown pelagic claystone) is >18.9 Ma. This recovery of basement was a major achievement, as was the comprehensive integration of core-log-seismic data at Site C0011. The correlation of lithofacies and age-depth models from the two sites within the Shikoku Basin shows changes from an expanded section (Site C0011) to a condensed section (Site C0012) and captures all of the important ingredients of basin evolution, including a previously unrecognized interval of late Miocene tuffaceous and volcaniclastic sandstone designated the middle Shikoku Basin facies. An older (early to middle Miocene) turbidite sandstone/siltstone facies with mixed detrital provenance occurs in the lower Shikoku Basin; this unit may be broadly correlative with superficially similar Miocene turbidites on the western side of the basin. When viewed together, the two sites around the Kashinosaki Knoll not only demonstrate how basement relief influenced rates of hemipelagic and turbidite sedimentation in the Shikoku Basin, but also build the complete lithostratigraphic template on which all of the postexpedition laboratory results can be placed. Those forthcoming details will include mineral and volcanic ash composition, geotechnical properties, frictional properties, and hydrological properties. Another triumph came from geochemical analyses of interstitial water and hydrocarbons at Site C0012. Unlike other so-called reference sites in the Nankai Trough, interstitial water on top of the basement high is largely unchanged by the effects of focused bedding-parallel flow and/or in situ reactions associated with rapid burial beneath the trench wedge and frontal accretionary prism. Thus, Site C0012 finally provides a reliable geochemical reference site for the subduction zone. In addition, geochemical evidence points to the presence of a seawater-like fluid within the upper basaltic crust, which is actively exchanging with the ocean and altering the interstitial water composition of the deep sediments by diffusional exchange.
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            Author and article information

            Journal
            10.2204/iodp.proc.333.2012
            Proceedings of the IODP
            Integrated Ocean Drilling Program
            1930-1014
            16 August 2017
            10.2204/iodp.proc.333.205.2017

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