6. Clay Mineralogy of Mudstones from the Nankai Trough Reference Sites 1173 and 1177 and Frontal Accretionary Prism Site 11741

Joan F. Steurer2 and Michael B. Underwood2


X-ray diffraction analyses of the clay-sized fraction of sediments from the Nankai Trough and Shikoku Basin (Sites 1173, 1174, and 1177 of the Ocean Drilling Program) reveal spatial and temporal trends in clay minerals and diagenesis. More detrital smectite was transported into the Shikoku Basin during the early-middle Miocene than what we observe today, and smectite input decreased progressively through the late Miocene and Pliocene. Volcanic ash has been altered to dioctahedral smectite in the upper Shikoku Basin facies at Site 1173; the ash alteration front shifts upsection to the outer trench-wedge facies at Site 1174. At greater depths (lower Shikoku Basin facies), smectite alters to illite/smectite mixed-layer clay, but reaction progress is incomplete. Using ambient geothermal conditions, a kinetic model overpredicts the amount of illite in illite/smectite clays by 15%-20% at Site 1174. Numerical simulations come closer to observations if the concentration of potassium in pore water is reduced or the time of burial is shortened. Model results match X-ray diffraction results fairly well at Site 1173. The geothermal gradient at Site 1177 is substantially lower than at Sites 1173 and 1174; consequently, volcanic ash alters to smectite in lower Shikoku Basin deposits but smectite-illite diagenesis has not started. The absolute abundance of smectite in mudstones from Site 1177 is sufficient (30-60 wt%) to influence the strata's shear strength and hydrogeology as they subduct along the Ashizuri Transect.

1Steurer, J.F., and Underwood, M.B., 2003. Clay mineralogy of mudstones from the Nankai Trough reference Sites 1173 and 1177 and frontal accretionary prism Site 1174. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/190196SR/211/211.htm>. [Cited YYYY-MM-DD]

2Department of Geological Sciences, University of Missouri, Columbia, 101 Geology Building, Columbia MO 65211, USA. Correspondence author: underwoodm@missouri.edu

Initial receipt: 18 November 2002
Acceptance: 5 August 2003
Web publication: 26 November 2003
Ms 190SR-21