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At Sites U1325–U1329 of Expedition 311, the clay mineral assemblages are composed of smectite, illite, chlorite, kaolinite, and illite-vermiculite (I/V) mixed-layers. I/V layers have not been observed in the Cascadia margin area (e.g., Karlin, 1980; Underwood and Torres, 2006), whereas they are present in Pleistocene northwestern Atlantic sediments (e.g., Vanderaveroet et al., 2000) and are related to interglacial periods. However, in this study, the sampling resolution and the age control are not good enough to define or recognize any climate control in the clay mineral record.

Clay mineral variations are slight and probably partly due to the natural heterogeneity of the clay-sized sediment budget. Nevertheless, it is possible to see some differences between sites and between units in the same site. Lithostratigraphic Unit I at Site U1329 does not contain kaolinite. The lowest smectite contents are observed in lithostratigraphic Unit IV at Site U1325 and Unit III at Site U1326, whereas higher values are recorded in the Miocene of lithostratigraphic Unit III at Site U1329. In all sites, a global anticorrelation trend between illite and smectite fractions is observed. This trend could be correlated to sediment grain size, hemipelagic sediments being slightly richer in smectite and turbiditic samples slightly enriched in illite. In the samples studied, there is no evidence of diagenetic impact of gas hydrate and/or carbonate diagenesis occurrence on the clay mineral fraction.