Abstract
Fluidization structures are clearly observed at base of the upper unit of the Lower Miocene Himaka Formation on Saku Island, Aichi Prefecture, central Japan. Characteristic ones are 1) the truncation-like structure of lamina at the top of a convolute sandstone layer, 2) the injection of small sandstone dikes into the siltstone layer above, and 3) the "top-to-NNE" shear deformation indicated by the attitudes of the axial surfaces of minor folds and the wedge-shaped small dikes. These fluidization structures suggest that dehydration occurred in response to the bedding slip to NNE, thereby forming a water film beneath the impermeable silt layer.
Such a sequential fluidization seems to be understood as the phenomenon that excess interstitial water upwelling through the sand/mud interlayers has been trapped under an impermeable layer, then dewatered through the lateral sliding and fracturing of the overlying layers. Water films are likely to cause the bedding slip in un- and semi-consolidated interlayers. They temporarily form parallel with bedding planes, leaving only subtlest traces. Such traces will become important clues to understand the deformational mechanism of soft sediments.
