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Volume 106, Issue 2
Displaying 1-8 of 8 articles from this issue
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Takeshi Nakajima, Tohru Danhara, Kiyotaka Chinzei2000 Volume 106 Issue 2 Pages 93-111
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Yuda Basin is a narrow lowland situated along the axial part of the Ou Backbone Range in northeast Japan. Stratigraphic study and fission-track datings reveal that the Oishi, Kotsunagizawa, Kurosawa and Hanayama Formations distributed in the Yuda Basin are almost continuous deposits ranging from Middle Miocene to Pliocene (16-3 Ma) with a significant age gap between 12 and 9 Ma.The Hanayama Formation is composed of high frequency sequences formed under shallow marine, deltaic and fluvial environments, suggesting that the basin had been connected with the Japan Sea until Early Pliocene.The subsidence curve of the Yuba Basin suggests that there were three phases of uplifting in the surrounding Ou Backbone Range after the rapid subsidence in early Middle Miocene. 1)Temporary and sudden uplift of the eastern sector of the Backbone Range at 12 Ma, followed by slow deposition within the basin between 12 and 9 Ma, 2) Differential uplifting of the Backbone Range between 6.5 and 3 Ma. 3) Uplifting of the whole Backbone Range by thrusting after 3 Ma, together with formation of angular unconformity within the basin. The eastern sector of the Backbone Range is, therefore, considered to have been uplifted earlier than the western sector. The resultant differential denudation histories of the two sectors may be attributed to difference in topography as well as in geologic structures between the two sectors.View full abstractDownload PDF (2780K) -
Noritaka Endo2000 Volume 106 Issue 2 Pages 112-119
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe mode of grain movement is a significant factor in sedimentology because it influences the grain fabric of sediments. Laboratory studies regarding the relation between bedform and grain fabric have been carried out. However, grain movement and grain fabric of small bed waves formed immediately after the flow velocity decrease from the upper-plane-bed regime to the dune regime ("upper ripples") have not been investigated so far. This study investigated grain movement near the bed using high-speed video system in the ripple and dune regimes. Grains avalanched on lee slopes in the ripple regime. In the dune regime, however, grains moved as sheet flow everywhere on the surface of "upper ripples", which had wave heights comparable to that of ordinary ripples. These grain movements suggest that the dominant direction of long axes of grains in ripple deposits is similar to cross laminae, while the grain fabric of "upper ripples" shows imbrication with laminae as bases. Grain fabric can be a clue to judge whether small-scale cross-stratification was formed in the ripple regime or in early stage of the dune regime following the upper-plane-bed regime.View full abstractDownload PDF (1872K) -
Wataru Hirose, Miyuki Iwasaki, Mitsuhiro Nakagawa2000 Volume 106 Issue 2 Pages 120-135
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSSpatial and temporal variations in late Cenozoic volcanism of southwestern Hokkaido at the northern end of NE-Japan arc have been clarified by 261 K-Ar and 76 FT ages including 49 newly determined K-Ar ages, volcanic stratigraphy, physical volcanology and whole-rock geochemistry. Arc volcanism characterized by rocks with low-Ti and Nb, and by across-arc increase in K2O content in these rocks has continued at least since 12 Ma. Based on volcanic stratigraphy, physical volcanology and whole-rock geochemistry, volcanism after 12 Ma can be subdivided into 4 stages, 12-5, 5-1.7, and 1.7-0 Ma. The volcanism from 12 Ma to 5 Ma extended northward widely compared with distribution of Quaternary arc volcanism (1.7-0 Ma). This suggests that the arc trench junction between Kuril and NE-Japan arc's trenches was located about 100 km northward from the present position. Since around 5 Ma until 1.7 Ma, different type of volcanism under local extension field, characterized by a group of monogenetic volcanoes of alkali basalt and shield volcanoes of calc-alkaline andesite, had occurred at northern end of the volcanic region (Takikawa-Mashike region). During and after this volcanism, the northern edge of arc volcanism in the area has migrated southward. This suggests that the trench junction has migrated about 100 km southward since ∼5 Ma. The Quaternary arc volcanism (1.7-0 Ma) has been restricted at the southern part of the region. The volcanism since 12 Ma might be influenced by oblique subduction of Pacific plate beneath Kuril arc, resulting in the formation of local back arc basin at the junction and to southward migration of the trench junction.View full abstractDownload PDF (2246K) -
Rei Nakashima, Ryuichi Majima2000 Volume 106 Issue 2 Pages 136-150
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSFossil shell beds of the shelf can be used as the good tools in the analysis of paleoenvironments because they include many features clearly associated with sedimentary processes. Thier usefulness, however, has not been evaluated adequately. The upper Miocene to lower Pliocene Horokaoshirarika Formation is distributed in central Hokkaido. The upper part of the Horokaoshirarika Formation is composed mainly of sandy and fossiliferous deposits and is interpreted to be deposited in the inner-shelf. Analysis of the sedimentary facies suggests it as a successive upward-shallowing sequence. Based on the shell arrangement in the deposits, the shell beds of the upper part of the formation can be divided into two types. Type 1 shows regularly arranged shells which are stacked, imbricated, and arranged parallel to the bedding plane and is formed by physical reworking. Type 2 represents randomly arranged shells and is formed by bioturbation. We subdivide the upper part of the formation into even stratigraphic unit I to IV. Number of both types increases gradually from I to III, but abruptly decreases from III to IV. The ratio between Types 1 and 2 does not change from I to IV. The increase of the type 1 from I to III reflects the increase frequency of storm reworking in ascending order, and its abrupt decrease from III to IV reflects severe amalgamation of the shell beds. The constant ratio of Types 1 and 2 indicates the constant extent of bioturbation in each unit. Thus, in the inner-shelf deposits of the Horokaoshirarika Formation, the physical reworking increases upward with shallowing, while extent of biological reworking is almost the same throughout the successtion.View full abstractDownload PDF (4754K) -
Michio Takami, Yujiro Nishimura2000 Volume 106 Issue 2 Pages 151-160
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Jurassic accretionary complex in eastern Yamaguchi Prefecture, Southwest Japan, is intruded by Late Cretaceous granite, generating a contact aureole more than 2 km wide which is divided into the biotite and cordierite mineral zones. X-ray diffraction analyses of carbonaceous material were carried out for pelitic rocks collected systematically not only from the contact aureole but also from its outer side. The apparent interplanar spacing d002 values of carbonaceous material rapidly decrease from 3.590 to 3.371 Å in the upper biotite zone, and reach 3.353 Å in the cordierite zone. Although all d002 data from the lower biotite zone and from the outside are calculated to be around 3.6 Å, the peak shapes can be divided into three types in a figure of peak-height versus peak-width at half height : Types I, II and III with advance of graphitization. Types I and II occur both in the lower biotite zone and in the outside, whereas Type III yields only in the lower biotite zone. These aspects suggest that graphitization caused by contact metamorphism has slightly advanced in the lower biotite zone.Rejuvenation of K-Ar white mica ages in the contact aureole reported by Takami et al. (1993) was examined on the basis of the graphitization. In the lower biotite zone where the d002 values are more than 3.5 Å, K-Ar ages change systematically from 122 to 97 Ma, indicating that rejuvenation of the ages has rapidly advanced. In the upper biotite and cordierite zones where the d002 values are less than 3.5 Å, the ages (83-86 Ma)coincide with those of the granite (82 and 86 Ma), implying that the rejuvenation has completed. These results indicate that a metamorphic temperature represented by d002=3.5 Å mostly coincides with the closure temperature of K-Ar white mica system. The value of d002=3.5 Å is therefore an useful indicator for considering the degree of rejuvenation of K-Ar age.View full abstractDownload PDF (1369K) -
Hajime Naruse, Haruyoshi Maeda, Yasunari Shigeta2000 Volume 106 Issue 2 Pages 161-164
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSPachydiscus flexuosus, Tetragonites popetensis, Spheonoceramus hetonaianus, and other molluscan fossils are newly discovered from the Senpohshi, Hamanaka and middle Akkeshi formations, Nemuro Group, eastern Hokkaido, Japan. They are stage index fossils of Lower Maastrichtian. According to calcareous nannofossil biostratigraphy suggesting that the upper Akkeshi Formation is Paleocene in age, the K/T boundary in the Nemuro Group is possibly intercalated between the middle and upper Akkeshi Formation.View full abstractDownload PDF (1115K) -
Tetsuro Hirono, Satoru Nakashima2000 Volume 106 Issue 2 Pages 165-168
Published: February 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDownload PDF (691K) -
Aiming Lin, Toni Ouchi, Allen Chen, Tadashi Maruyama2000 Volume 106 Issue 2 Pages III-IV
Published: 2000
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESS
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