The origin and internal structures of submarine-slide deposits in a lower Pleistocene outer-fan succession in the Kazusa forearc basin on the Boso Peninsula of Japan

•Variations in deformation styles within submarine-slide deposits were clarified.•Timing of the submarine-slide formation was confirmed.•Seafloor conditions leading to the submarine-slide formation was clarified.•Features of slide planes and the associated sheared deposits were clarified.

This study investigates the internal geometry and formation processes of submarine-slide deposits in a lower Pleistocene outer-fan succession in the Kazusa forearc basin on the Boso Peninsula of Japan. The submarine-slide deposits are ~ 40 m thick, with a minimum length of ~ 900 m and a width of ~ 700 m. Both the submarine-slide deposits and host deposits comprise siltstones intercalated with very thin- to medium-bedded, sheet-like turbidites and volcanic ash beds. Based on the sequence-stratigraphic framework of the submarine-fan succession, we conclude that the submarine-slide deposits formed during a glacioeustatic sea-level lowstand at about 1.16 Ma.The submarine-slide deposits are characterized by thrust sequences with a ramp anticline in the frontal part. A basal slide plane in the lower part of the deposits is developed at a horizon located 2–4 cm below the base of a coarse volcanic ash bed and is associated with sheared deposits. Slide planes are sealed in the upper part of the submarine-slide deposits in association with drag folds and chaotic deposits. Finally, the submarine-slide deposits are transitionally overlain by ~ 3-m-thick chaotic muddy deposits, and are finally overlain by siltstones intercalated with very thin- to medium-bedded, sheet-like turbidites and volcanic ash beds, which show lithofacies features similar to those of the submarine-slide deposits. The variations in the deformation styles indicate that sliding occurred as a synsedimentary process in the outer-fan environment, and the basal slide plane formed when the porosity of the muddy deposits was reduced to ~ 55% or less.Based on the empirical relationship between the submarine-fan length and lower-fan slopes from modern examples, the gradient of the outer-fan is estimated at 0.31°–0.46°, which is lower than the threshold gradient of 1.2° for a 40-m-thick submarine slide with the estimated basal porosity. Based on the distribution of deformed deposits within the lower-fan host deposits, the minimum volume and runout distance of the submarine-slide deposits are estimated to be ~ 3 × 107 m3 and ~ 5 km, respectively. Furthermore, based on data from modern submarine slides, the elevation difference between the failed mass in both the starting and depositional areas is estimated to be 80–170 m. Thus, the initiation of a submarine slide in the outer-fan environment was likely influenced by a combination of external forces, such as seismic shaking and increased pore-water pressure in association with the seepage of gas and water from underlying deposits in the Kazusa forearc basin. This study suggests that submarine slides occur in distal and low-gradient deep-water environments in active-margin basins.