Clay-mineral provenance, sediment dispersal patterns, and mudrock diagenesis in the Nankai accretionary prism, Southwest Japan

Author(s): Underwood, Michael B.; Pickering, Kevin T.
Author Affiliation(s): Primary:
University of Missouri, Department of Geological Sciences, Columbia, MO, United States
University College London, United Kingdom
Volume Title: Clays and Clay Minerals
Source: Clays and Clay Minerals, 44(3), p.339-356. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604 CODEN: CLCMAB
Note: In English. 102 refs.; illus., incl. sketch map
Summary: 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 µm and 2-6 µm) 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 (I/S) mixed-layer clay at ∼555 mbsf and an estimated temperature of ∼65 °C. Ordered (R = 1) I/S interlayering first appears at ∼1220 mbsf (<2 µm size fraction) and ∼1100 mbsf (<0.2 µm size fraction). The illitization gradient coincides with a reduction in pore-water chlorinity, but depth-related changes in bulk mudstone geochemistry (K2O, Rb) are subtle. The absolute abundances of discrete smectite and I/S 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.
Year of Publication: 1996
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Accretionary wedges; Areal studies; Clastic rocks; Clay mineralogy; Clay minerals; Deep-sea environment; Detrital sedimentation; Diagenesis; Hemipelagic environment; Illite; Illitization; Leg 131; Leg 132; Lithofacies; Marine environment; Mineral composition; Mineral-water interface; Mixed-layer minerals; Mudstone; Nankai Trough; North Pacific; Northwest Pacific; ODP Site 808; Ocean Drilling Program; Pacific Ocean; Patterns; Provenance; Sedimentary rocks; Sedimentation; Sediments; Sheet silicates; Shikoku Basin; Silicates; Smectite; Trenches; Volcaniclastics; West Pacific
Coordinates: N322105 N322111 E1345646 E1345634
Record ID: 1996063786
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland