Yoshihisa Hiroki

Correlation of Miocene (18–12 Ma) sequence boundaries in central Japan to major Antarctic glaciation events

Abstract Using magnetostratigraphy constrained by biostratigraphy and radiometric dating, we have investigated whether early to early-middle Miocene (18–12 Ma) sequence boundaries in central Japan were synchronous with major Antarctic glaciation events. We have correlated Miocene sequence boundaries across three sedimentary basins across central Japan, representing backarc basin (Yatsuo area), intra-arc basin (Mizunami area), and forearc basin (Kakegawa area) environments. These boundaries were formed during four time intervals, designated as Zones Jmi1 to Jmi4: Zone Jmi1 correlates with Chron C5Dn (17.61–17.27 Ma), Zone Jmi2 correlates with Chron C5Br (16.01–15.15 Ma), Zone Jmi3 correlates with Subchron C5Bn1r (15.03–14.88 Ma), and Zone Jmi4 ranges from 14.88 to 12.3±1.9 Ma. The age range of Japanese Miocene (Jmi) zones represents the range of uncertainty (error limits) in the age of individual sequence boundary formation. We have correlated Zones Jmi1–3 to positive shifts of δ18O in the deep-sea cores from ODP Leg 120 Site 747, southern Indian Ocean, where magnetostratigraphic data are available, suggesting that the sequence boundaries of Zones Jmi1–3 were formed by eustatic sea-level falls during major Antarctic ice sheet developments. Zone Jmi4 age constraints are too poor to test correlation with ODP Site 747.

Quaternary crustal movements examined from facies distribution in the Atsumi and Hamana areas, central Japan

Abstract Quaternary sediments are distributed from the east coast of Lake Hamana to the Atsumi Peninsula, on the Pacific side of central Japan. The sediments in the eastern Hamana area include fluvial, estuary and deltaic facies, whereas sediments in the western Atsumi area include fluvial, estuary, shoreface, beach and coastal marsh facies. The sediments in the Hamana area comprise four sedimentary units; those in the Atsumi area comprise seven sedimentary units. Each unit in the Hamana area was formed by the alternation of a transgressive estuary system and a regressive fan-delta system. In contrast, each unit in the Atsumi area was formed by the alternation of a transgressive estuary system and a regressive fan-delta system. In contrast, each unit in the Atsumi area was formed by the alternation of a transgressive estuary system and a regressive strand plain system. Stratal correlation with the ocean oxygen isotope variation indicates that the four younger units in both areas can be correlated and that the five younger units were formed at the sea-level maxima during interglacial stages, the oxygen isotope stages being 1, 5, 7, 9 and 11. The top of each sequence is marked by the beach or fluvial facies, deposited during highstand, the present altitude of which was used as a marker for the palaeo-highstand sea-level. A record of palaeo-highstand sea-level change was obtained from five locations in the Atsumi area and one location in the Hamana area. Comparison of the palaeo-highstand sea-level changes with the glacio-eustatic highstand sea-level change, determined from raised coral reef terraces, shows that the Atsumi area subsided at the mean rate of 27 cm/1000 yr until 331,000 yr B.P. and the whole area was then uplifted between 331,000 and 122,000 yr B.P., with a maximum uplift rate of 29 cm/1000 yr. The differential crustal movements were more active in the latter period. The wavelength of the crustal bending is estimated at 50–100 km and the amplitude of the bending was less than 10 m before 331,000 yr B.P. and was 50 m thereafter. The change in vertical crustal movement is synchronous in Japan, New Zealand and New Guinea. Such a synchronous change in the vertical crustal movement may be explained by a change in the regional horizontal stress field due to rotation of the Pacific plate.