Abstract
Samples of ash layers and associated background sediments from Site 808 of ODP Leg 131 in the Nankai Trough accretionary prism were analyzed for changes in mineralogy, porosity and micro-fabric associated with alteration of volcanic ash. Ash layers range from incipient stages of alteration and dissolution to complete alteration to clay minerals and clinoptilolite. Ash layers contain greater abundances of total clay minerals and lower percentages of quartz than do surrounding background hemipelagic sediments. The clay-sized fraction of ash layers is dominated by pure dioctahedral smectite, whereas the background sediments contain primarily illite and chlorite with minor amounts of smectite. Analysis of microfabric revealed dramatic changes in the distributions and abundances of grains and pores during ash alteration. The relative abundances of large pores, grains, and matrix material were quantified on digital back-scattered electron images (BSEI) of ash layer and background sediment samples. During burial, the abundant glass shards of shallow ash layers are initially altered, presumably to smectite. Subsequent dissolution of the glass leaves open, shard-shaped pores, resulting in increased porosities. With greater burial, these pores are filled with clinoptilolite. Although the presence of ash and its alteration products clearly influences sediment physical properties, there is no apparent correlation of the abundance of ash or its alteration products with the formation of thrust faults or other structures within the Nankai Trough accretionary prism.
Similar content being viewed by others
References
Bird, P. (1984) Hydration-phase diagrams and friction of montmorillonjte under laboratory and geologic conditions, with implications for shale compaction, slope stability, and strength of fault gauge: Tectonophys. 107, 235–260.
Bruce, C. H. (1984) Smectite dehydration—Its relation to structural development and hydrocarbon accumulation in Northern Gulf of Mexico basin: Bull. Am. Assoc. Petr. Geol. 68, 673–683.
Burst, J. R., Jr. (1969) Diagenesis of Gulf Coast clay sediments and its possible relationships to petroleum migration: Bull. Am. Assoc. Petr. Geol. 53, 73–93.
Gieskes, J. M. (1983) The chemistry of interstitial waters of deep sea sediments: Interpretation of Deep Sea Drilling data: in Chemical Oceanography 8, J. P. Riley and R. Chester, eds., Academic Press, New York, 221–269.
Hanshaw, B. B. and Bredehoeft, J. D. (1968) On the maintenance of anomalous fluid pressures II; source layer at depth: Geol. Soc. Am. Bull. 79, 1107–1122.
Hein, J. R., and Scholl, D. W. (1978) Diagenesis and distribution of late Cenozoic volcanic sediment in the southern Bering Sea: Geol. Soc. Am. Bull. 89, 197–210.
Hodder, A. P. W., Naish, T. R., and Nelson, C. S. (1993) A two-stage model for the formation of smectite from detrital volcanic glass under shallow-marine conditions: Mar. Geol. 109, 279–285.
Karlak, R. F. and Burnett, D. S. (1966) Quantitative phase analysis by X-ray diffraction: Anal. Chem. 38, 1741–1745.
Kastner, M. and Stonecipher, S. A. (1978) Zeolites in pelagic sediments of the Atlantic, Pacific, and Indian Oceans: in Natural Zeolites, Occurrence, Properties, Use, L. B. Sand and F. A. Mumpton, eds., Pergamon Press, New York, 199–220.
Magara, K. (1975) Reevaluation of montmorillonite dehydration as cause of abnormal pressure and hydrocarbon migration: Bull. Am. Assoc. Petr. Geol. 59, 292–302.
Moore, D. and Reynolds, R. (1989) X-Ray Diffraction and the Identification and Analysis of Clay Minerals: Oxford University Press, New York, 332 pp.
Olafsson, G. (1993) Calcareous nannofossil biostratigraphy of the Nankai Trough: in Proc. ODP, Sci. Results, 131, I. A. Hill, A. Taira, J. V. Firth et al., eds., Ocean Drilling Program, College Station, Texas, 3–13.
Russ, J. C. (1990) Computer-Assisted Microscopy: Plenum Press, New York, 453 pp.
Taira, A., Hill, I., and Firth, J. V. (1991) Proc. ODP, Init. Repts., 131, Ocean Drilling Program, College Station, Texas 434 pp.
Tribble, J. S. (1990) Clay diagenesis in the Barbados accre-tionary complex: Potential impact on hydrology and subduction dynamics: in Proc. ODP, Sci. Res., 110, J. C. Moore, A. Mascle et al., eds., Ocean Drilling Program, College Station, Texas, 97–110.
Tribble, J. S., Wilkens, R. H., and Sasaki, S. (1993) Changes in microfabric associated with alteration of volcanic ash: A comparison of the Nankai Trough, Hawaiian Arch, and Barbados accetionary complex: EOS 74, 226.
Underwood, M., Orr, R., Pickering, K., and Taira, A. (1993) Provenance and dispersal patterns for sediments in the turbidite wedge of Nankai Trough: in Proc. ODP, Sci. Results, 131, I. A. Hill, A. Taira, J. V. Firth et al., eds., Ocean Drilling Program, College Station, Texas, 15–33.
Wilkens, R. H., McClellan, P., Moran, K., Tribble, J. S., Taylor, E., and Verduzco, E. (1990) Diagenesis and dewatering of clay-rich sediments, Barbados accretionary prism: in Proc. ODP, Sci. Res., 110, J. C. Moore, A. Mascle et al., eds., Ocean Drilling Program, College Station, Texas, 309–320.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Tribble, J.S., Wilkens, R.H. Microfabric of Altered Ash Layers, ODP Leg 131, Nankai Trough. Clays Clay Miner. 42, 428–436 (1994). https://doi.org/10.1346/CCMN.1994.0420408
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1346/CCMN.1994.0420408