Downslope Variability in Seal Rock Characteristics in the Kazusa Forearc Basin, Japan

Summary This study investigated the variability of seal rock characteristics in the Pleistocene Kazusa Group, a forearc basin succession in Japan. Regional- to local-scale variations in mudstone characteristics, including mineralogy, seal capacity, and permeability, were examined using closely spaced onshore outcrops with precise chronostratigraphic correlation based on volcanic ash beds. The results show remarkable downslope variations in the seal rock properties, especially in the shelf margin to the slope environments. In contrast, basin floor mudstones exhibit only minor downslope variations. These findings can be used as a geologic analogue for potential carbon storage sites in the offshore part of the Kazusa sedimentary basin, where subsurface data remains limited. Furthermore, it can be utilized as a geologic analogue for areas with similar geologic settings, such as young siliciclastic-dominated successions formed in active margin basins.