Yoshihisa Hiroki

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 Jmil 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 delta(18)O 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 Jmi I - 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. (C) 2002 Elsevier Science B.V. All rights reserved.

Sedimentary facies and total organic carbon and total sulfur concentrations of mudstones of the lower Miocene Nojima Group confirm that the Nojima Group has been deposited in a fresh-water environment before a major marine incursion into the Japan Sea. We identified nine sedimentary facies, which were grouped into seven facies associations, and recognized three depositional environments : lacustrine, deltaic, and fluvial. Total sulfur concentrations are much lower than 0.3wt%. Total organic carbon concentrations are up to 5.61wt%. The carbon/sulfur ratios show typical values in fresh-water sediments. The sedimentary facies and geochemical analysis indicate that the most of Nojima sedimentary basin was filled with lacustrine, deltaic and fluvial fresh-water deposits. Although no sample was obtained from the upper most part of the Nojima Group (Minamitabira Formation) because of lack of exposure, marine fauna in the uppermost part of the group reported by previous researchers possibly indicate a transgression caused by a combination of a global sea-level rise and an active tectonic activity during the opening of the Japan Sea.

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well-sorted, pebble- to cobble-size gravel beds with minor sand beds. The gravel includes large-scale foreset beds (5-10 m high) and overlying subhorizontal beds (0.5-3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert-type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low-energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large-scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea-level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross-sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large-scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf-margin systems tracts.

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. © 1994.