Abstract The distribution and characteristics of marine surface sediments are a basic marine geological information. Large river floods are a frequent natural hazard that transport substantial terrigenous sediments into the marine environment. In August 2003, TY ETAU (0310) caused heavy rainfall in the southern coast of Hokkaido, north Japan, where some mountainous rivers in the Hidaka region flooded. Two deposition modes for the 2003 flood sediments can be identified by comparing the pre- and post-flood surface sediment distribution. Shore-normal shallow depressions off the mouth of the Saru and Atsubetsu rivers served as channels for the discharged floodwater preventing dispersion and maintaining the necessary water density to transport the materials as density bottom currents. This action also promoted long-distance transport of flood materials across the continental shelf. Absence of depression on the inner shelf off the mouth of the Niikappu and Shizunai rivers may have dispersed floodwaters near the river mouth and deposited the flood materials close to the shore. Marine geological mapping suggests that the differences in submarine topography (the presence or absence of shallow depressions) are closely related to the regional geological structure. Thus, submarine geology is a controlling factor of the seafloor environments influenced by the river flood.

The Hidaka Metamorphic Belt (HMB) in Hokkaido, northern Japan, consists of tilted metamorphic layers of an island-arc type crust from lower (granulite facies) to upper (very low-grade metasedimentary) horizons. Abundant granitic rocks, mainly S-type tonalites of crustal origin, intrude various metamorphic layers and are classified into four depth types, namely upper, middle, lower and basal. The basal orthopyroxene-garnet (S-type) tonalities were intruded into granulite facies country rocks. Textural and compositional evidence from minerals in the basal tonalite indicates that the crystallisation sequence is Grt–Pl–Opx–Bt–Qtz–Crd–Kfs, and that crystallisation took place at about 600 MPa and 900°C–700°C. Some crystallisation experiments were carried out in an internally heated pressure vessel, using the basal tonalite, under the conditions of 300 and 600 MPa, 700–900°C, and with 0–20 wt% H 2 O, respectively. The results show that the primary S-type tonalite magma was at a temperature above 900°C and contained 3–4 wt% H 2 O at the beginning of crystallisation. In order to study the influence of normative orthoclase content on orthopyroxene crystallisation, some starting materials also included 15, 20 and 25% normative orthoclase, by adding KAlSi 3 O 8 gel to the rock powder. Normative orthoclase content has an influence on the subliquidus crystallisation limit of orthopyroxene. The changes in P–T conditions and chemical composition of the magma during ascent would generate the sequence from the basal to upper S-type granite. Opx-free S-type granitic magma can be generated from lower crustal Grt–Opx S-type granitic magma, by differentiation with falling magmatic temperature.