Earth Science (Chikyu Kagaku)

Late Pleistocene tephras in the southern Ishikari Low-land, Hokkaido

A comparative study of the relation between refractive indices and Mg-Fe ratios of orthopyroxene

Akira KASUGAI, Akira FUJITA, Koshiro HOSOKAWA, Satoshi OKAMURA, Hiroyu ...

Article type: Article
1980 Volume 34 Issue 1 Pages 1-15
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_1

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In this paper, a score of stratigraphic units of tephras erupted mainly from Shikotsu Caldera are studied on the distribution of the tephras, the assemblage of color minerals, the Mg-Fe ratios of orthopyroxenes by EPMA analysis and the refractive indices of minerals and glass in the tephras. These tephras are composed in ascending order of 1) four units of the Azuma Ash-fall Deposits (Aafa 4-1), 2) four units of the Mukawa Pumice-fall Deposits (Mpfa 3, 2 b, 2 a, 1), 3) half a score of units of the Shikotsu Pumice-fall Deposits (Spfa 10-1) intercalating a unit of the Hidaka Pumice-fall Deposits (Hpfa) and 4) a unit of the Eniwa-a Pumice-fall Deposits (En-a). Each of the upper three tephras, namely En-a, Spfa 1 and Spfa 2 has been traced to the Tokachi Plain about 150 km east-ward. In the Southern Tokachi Plain, downward sequence of Orange Pumice-fall Deposits-1 (Op1), Orange Pumice-fall Deposits-3 (Op-3), Taiki Pumice-fall Deposits (Tpfa) and Kamisatsunai Pumice-fall Deposits (Kpfa) is found in peaty deposits of the Biraotori Formation. Kpfa is found in the upper part of the peaty Horokayantoh Formation, too. In the Southern Ishikari Low-land, Aafa 1-4 are bedded in peaty deposits of the Shiomi Formation. Spfa 3-5, Mpfa 3, Aafa 1 and Aafa 2 can be correlated to Op-1, Op-3, Tpfa and Kpfa, respectively, based on the investigation of the stratigraphic sequence, the mineral assemblage, the chemical composition of orthopyroxene and the refractive indices of orthopyroxene, hornblende and glass in the tephras. Such a correlation of the marker tephras makes it possible to compare the above two areas from the viewpoint of paleoclimatology and paleogeography. Table 3 shows the chronological sequence between the tephras and their ¹⁴C ages. It is also confirmed that the frequency distribution of the refractive indices of orthopyroxenes is similar to that of Mg-Fe ratios. It has been clarified that the refractive index (γ) of the orthopyroxene which contains less than 0.07 atoms of Al (per 6 atoms of oxygen) decreases by approximately 0.00125 for each atomic per cent of Mg/(Mg+Fe⁺²+Fe⁺³+Mn), but in the Al-rich orthopyroxene which contains 0.07〜0.14 atoms of Al, the index (γ) is higher by 0.005 for the composition En₁₀₀, and the effect of Al weakens with decrease of En component. Therefore, in order to estimate the refractive index (γ) of orthopyroxene from the analysed Mg-Fe ratio, it is indispensable to analyse not only Mg, Fe and Ca but Al and Mn.

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Stratigraphy and geologic structure of the Sanbagawa metamorphic belt in the Besshi district, Shikoku, Japan

Reexamination of the recumbent fold structures

Akira TAKASU, Kuniaki MAKINO

Article type: Article
1980 Volume 34 Issue 1 Pages 16-26c
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_16

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In the Besshi district of the Sanbagawa metamorphic belt, the highest temperature part of the metamorphism occurs in the middle stratigraphic horizon. So, the existence of largescale recumbent anticlines has been proposed by HIDE et al. (1956) and other geologists. Then we report on the results of our reexamination of those recumbent folds. We restore metamorphic formations to the original states. Most of the original rocks are pelitic, psammitic, siliceous and basic rocks, and gabbroic and ultrabasic intrusive rocks. The discrimination between basic schists, which are originated in basic tuff or lava, and amphibolites derived from gabbroic intrusive rocks is of much importance to precise mapping. The criteria of the discrimination between them are made of following points; banded structure, schistosity, gabbroic texture, grain size, albite porphyroblast and zonal structure of clinozoisite. Then we construct the stratigraphy and clarify the geologic structure, based upon the tracing of marker beds. It becomes clear that the large-scale recumbent anticlines never exist in this district. Not the recumbent fold strucrure but the monoclinic structure of northward dipping controls the Sanbagawa belt of the Besshi district. On the basis of the established stratigraphy and geologic structure, we point out the possibility that the thermal structure of the Besshi district was formed owing to the thermal supply of the original rocks of the amphibolite, namely, gabbroic intrusive rocks.

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Size-frequency distributions of dead shells of Batillaria zonalis(BRUGUIERE), and the mode of their formation

Shoichi SHIMOYAMA

Article type: Article
1980 Volume 34 Issue 1 Pages 27-39
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_27

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The mode of formation of the primary size-frequency distribution and its post-mortem modification in Balillana zonalis (BRUGUiERE) have been studied with population ecologic tequnuque for the living and death populations in the bottom sediments in Irie Bay, Tsuyazaki, Kyushu. The size-frequency distribution of the primary death population, directly derived from the living population of the species, shows a bimodal pattern with a large variance, because of recruitment at long intervals of a few years, slow growth-rate and longevity. The size-frequency distribution patterns of dead shells within or near the habitat of the living population are, on one hand, similar to that of the primary death population. This fact implies that the degree of the post mortem modification of the size-frequency distribution might have been scare. On the other hand, those of dead shells far from the habitat of the living population have been strongly modified from that of the primary death population. The main factor controlling the postmortem modification of size-frequency distribution pattern of primary death population in this bay is attributed to the size selective use and transportation by intertidal hermit crabs, and the effect of tidal currents is considered to be much smaller and slower than the former.

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Editorial comment on the discussion about the origin of deep-sea terraces less than 2,000 m deep

Editors

Article type: Article
1980 Volume 34 Issue 1 Pages 40-
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_40

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Discussion of paper by H. AOKI, Y. MISAWA and M. ISHIKAWA, "The results of Deep Sea Driling Project (DSDP), Kaiyo-kagaku (an extra), vol.1, no.1, 193-203, 1978"

Michihei HOSHINO

Article type: Article
1980 Volume 34 Issue 1 Pages 41-46
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_41

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Authors' reply to the discussion by M. HOSHINO of <<Discussion of paper by H. AOKI, Y. MISAWA and M. ISHIKAWA, "The results of Deep Sea Drilling Project (DSDP), Kaiyo-kagaku (an extra) vol.1, no.1, 193-203, 1978>>

Hitoshi AOKI, Yoshifumi MISAWA, Masanori ISHIKAWA

Article type: Article
1980 Volume 34 Issue 1 Pages 47-52
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_47

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A new note on the Tertiary strata, Ichishi Group and Suzuka Group in the north-western environs of Tsu City, Mie Prefecture

Yukio WADA

Article type: Article
1980 Volume 34 Issue 1 Pages 53-56
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_53

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¹⁴C age of the Satsunai pumice bed, Noboribetsu City, SW Hokkaido, Japan

¹⁴C age of the Quaternary deposits in Japan (133)

Shinpei KAWACHI, Shogo MIYASAKA, Morio AKAMATSU, Akira KASUGAI, Masaru ...

Article type: Article
1980 Volume 34 Issue 1 Pages 57-59
Published: January 25, 1980
Released on J-STAGE: July 26, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.34.1_57

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