Abstract The Boso Peninsula is located in central Japan near the junction of the subduction boundary of three tectonic plates. A forearc basin has been developing there since 3 Ma and has been uplifting since 1 Ma. The basal surface of the Holocene deposits in the offshore area was investigated based on a seismic survey and is very similar to the adjacent land areas (the Iioka Plateau, the Kujukuri Plain and the Kazusa Hills). The basal surface in the Kujukuri Plain and its corresponding offshore area contains many incised valleys. Most of them extend southeastward, parallel to the direction from the hinterland to the ocean, but one incised valley (Kujukuri-oki Buried Valley) lies perpendicular to the others. A buried terrace is located SE of the valley and along the area where mudstone (of the Kiwada Formation) is distributed. The present observations indicate that differential erosion formed the terrace, after which the valley bent to follow the terrace. The rivers tend to be perpendicular to the strike of the sediment in the forearc basin owing to tectonic movement. Thus, the valley must have been incised into the underlying strata with a perpendicular strike and may have become bent in uplifting forearc basins.

Abstract Examination of the detailed facies architecture of Holocene depositional systems contributes to the understanding of the rock record when coupled with high-resolution radiocarbon dating. Case studies of the depositional succession of Holocene to modern prograding shorefaces in the Kujukuri and Sendai areas along the Pacific coast of eastern Japan are summarized and compared to determine the relationships between the depositional architecture and wave conditions, and the shoreline trajectory. These areas differ from each other in history of relative sea level and characteristics of sediment source, but their wave climates are similar. The 20-m-thick Holocene depositional succession of the Kujukuri coast overlies the Plio-Pleistocene basement upon a ravinement surface. The succession consists of basal, lower-shoreface and upper-shoreface deposits, and foreshore and backshore deposits, in ascending order. The shoreface receives sediment from the coastal cliffs, and it has prograded seaward since 6 ka, when relative sea level stopped rising and then started to fall. The shelf and shoreface were sediment-starved during transgression, when sediment was deposited near the cliffs and were not transported to the central coast. Along the Sendai coast, the Holocene marine deposits lie upon a ravinement surface cutting the latest Pleistocene to Holocene nonmarine deposits. The succession is less than 30 m thick and is composed of transgressive ravinement deposits, sand-mud alternations of stacked inner-shelf storm deposits, lower-shoreface and upper-shoreface deposits, and foreshore and backshore deposits, in ascending order. The onset of shoreface progradation was at 7-8 ka, during a period of relative-sea-level rise, reflecting a high and continuous sediment supply from adjacent rivers. The Kujukuri shelf is sandy with a small amount of mud, but in the Sendai succession the base of the sandy shoreface is marked by the deposition of inner-shelf mud, and its water depth is 17 m, much shallower than half the length of an average deepwater wave. Thus, the depth of the base of the sandy shoreface is determined not solely by wave conditions but also by other factors, such as supply of mud and formation of sediment plumes. On the Sendai shelf, flood discharge of the relatively large rivers probably led to a well-developed sediment plume, resulting in shallow deposition of mud. The erosional boundary between upper-shoreface and lower-shoreface facies, defined as the surf diastem, shows abrupt increases in grain size and the angle of cross lamination, and there is a gap in the depositional age record in some places. These features indicate that formation of longshore bars dominated over the formation of storm-induced low-angle bedforms at this boundary. The water depth of the surf diastem is typically 5-7 m, which is approximately equivalent to the depth at which storm waves break, and to the depth limit of longshore bar migration observed along the modern coast. The spatial distribution of the surf diastem reflects the shoreline trajectory, although it is influenced partly by changes in the wave climate during progradation.

Abstract The Kanto Basin developed, starting c. 3 Ma, influenced by the subduction of the Philippine Sea plate and the Pacific plate. Sediments in this basin have influenced the geomorphology of the Kanto region. In the Boso Peninsula, located at the eastern edge of the Kanto Basin, uplift continues in what is called the Kashima–Boso uplift zone. Although the development of this uplift after the Late Pleistocene is well understood, there are few data from the Early to Middle Pleistocene. In this study, we investigated the offshore shelf area east of the Boso Peninsula using a high-resolution seismic reflection survey, and report new information on the geological structure and uplift processes in the area from the Early to Middle Pleistocene. We identified the Kujukuri-oki anticline and the Kujukuri-oki normal fault zone. The Kujukuri-oki anticline, more than 47 km long, is north–south striking and deforms the Kujukuri-oki Group. There are numerous normal faults with displacements of less than tens of metres spread widely in the survey area (Kujukuri-oki normal fault zone). These findings reveal that the Kujukuri-oki anticline uplifted during the end of the Early Pleistocene and attenuated during the Middle Pleistocene. This anticline comprised the axis of the Kashima–Boso uplift zone at the Boso Peninsula from the Early to Middle Pleistocene and the Boso Peninsula is located at the western limb of this anticline.

Abstract The Japanese islands comprise 59%% mountains, 6% volcanoes and 35% hills and (mostly coastal) lowlands (Yonekura 2001). Quaternary strata excluding volcanoes occupy around 25% of the geological map of Japan, and constitute hills and lowland plains. Holocene strata are distributed mainly in the lowlands and occupy 13-15% of Japan (Murata Kano 1995; D. Kawabata 2013, pers. comm.). These extensive lowlands comprise four depositional environments, namely delta, fan delta, strand plain and barrier/estuary systems, all of which have been impacted by Holocene sea-level changes. Most of Holocene Japanese coastal plains record a hydro-isostatically controlled relatively stable to falling sea level over the last 6-7ka (Ota et al. 1981, 1987 a , 1990; Nakada et al. 1991; Okuno et al. 2014). This hydro-isostatic effect varies spatially, being more strongly developed in the central rather than marginal parts of the islands (Yokoyama et al. 1996; Nakada et al. 1998; Nakada Okuno 2011; Okuno et al. 2014). Local tectonics have also strongly impacted relative sea levels since middle Holocene times, ranging from +30 m in the southern tip of the uplifting Boso Peninsula, SE of Tokyo, to-20 m in the subsiding Echigo Plain (Ota et al. 1987 a, b ; Shishikura 2001; Urabe et al. 2004; Tanabe et al. 2009).