Drill cores recovered from the Shikoku Basin (seaward of the Nankai accretionary prism) during Deep Sea Drilling Project (DSDP) Leg 87 and Ocean Drilling Program (ODP) Leg 190 reveal unusual downhole trends in key physical properties, most notably porosity and density. Unlike typical normally consolidated marine sediments, which show decreasing porosity with depth (e.g., Athy, 1930; Hamilton, 1976), strata in the upper Shikoku Basin (USB) unit exhibit anomalously high porosities, up to 10%–15% higher than predicted for their depths (Bray and Karig, 1986; Shipboard Scientific Party, 2001a). Porosities in the surrounding stratigraphic units follow more typical depth trends. In addition, porosities within the correlative USB unit in the accretionary prism, at Sites 1174 and 808, lie much closer to typical porosity depth trends (Shipboard Scientific Party, 1991; Shipboard Scientific Party, 2001b), denoting changes that accompany sediment accretion (e.g., Bray and Karig, 1985). The origin of these anomalous physical properties in front of the prism is a subject of debate. They may result from localized overpressures in the USB unit (e.g., Bray and Karig, 1986, 1988) or, alternatively, from intergranular cementation that supports the sediment matrix against consolidation (e.g., Karig, 1993; Moore, Taira, Klaus, et al., 2001). The downhole and downdip transitions in physical properties reflect major changes in mechanical behavior that may govern the types and distributions of deformation structures that develop in the accretionary prism.
Here, we examine additional evidence to determine the origin of the anomalous physical properties in the nontectonically deformed USB sediments. Following a review of new shipboard data collected during Leg 190 that bear on the interpretation of the physical property trends, we present scanning electron microscopy (SEM) analyses of sediment microstructure and microfabrics in and adjacent to the USB. These observations confirm the anomalous character of sediments in the USB at Site 1173 and demonstrate systematic downhole and downdip changes in microfabric, lithology, and corresponding physical properties. Comparison of these results with grain size, mineralogy, and pore fluid chemistry data point to the combined effects of diagenetic and mechanical changes as the sediments are buried and tectonically loaded. These spatial and temporal variations in properties and microfabric may play an important role in localization of deformation in the prism, including the positioning of the basal décollement fault.