FAN-DELTA SEDIMENTATION ON THE BASIN MARGIN SLOPE OF THE CRETACEOUS, STRIKE-SLIP IZUMI BASIN, SOUTHWESTERN JAPAN

The Late Cretaceous (Campanian-Maastrichtian) Izumi Basin is an E-W-trending, strike-slip basin, some 300 km long and 15 km wide, filled with turbidites and associated coarse elastic deposits. In the northeastern margin of the basin, steep-face fan deltas were built on a steep slope and prograded directly into deep water, forming conglomeratic slope aprons. The fan-delta deposits comprise debris-flow-dominate alluvial-fan conglomerates, delta-slope conglomerates originated from various subaqueous sediment-gravity flows, and prodelta mudstones with intercalations of thin sandstone and channel-fill pebbly sandstone. The most significant feature of the Izumi fan-delta system is a contrast between the gravel-dominated delta slope and the mud-dominated prodelta. The development of this fan-delta system was due to tectonic activity along the northern margin of the basin. Sedimentation took place due to vigorous uplift of the adjacent source terrane, which caused a high rate of supply of coarse detritus, and rapid basin subsidence which resulted in deep water depth.