第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
11 巻 , 4 号
選択された号の論文の17件中1~17を表示しています
  • 木村 方一, 藤山 広武, 近堂 祐弘, 近藤 錬三, 佐瀬 隆, 大槻 日出男
    1972 年 11 巻 4 号 p. 161-170
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    A number of ancient sand dunes in a periglacial environment of the late Pleistocene are widely developed in the Tokachi plain, east Hokkaido. A detailed stratigraphical and tephrochronologic studies on these sand dunes, which were constructed by reworked Eniwa-a (E-a) pumice-fall deposits, showed that geologic succession of the dune is divided into five stages (Fig. 2 and Table 1).
    The E-a dune deposit is characterized by a) development of laminations associated with windblown ripples, b) presence of a thin loam in the deposit, c) abundance of well sorted (So=1.2-1.7) and fine-graind pumiceous sands (Md=0.3-0.8mm), and d) covering by the paleosols and younger volcanic ash soils. Dating on the charcoal piece discovered from a thin loam in the deposit procured an age of 13, 100±1, 200yrs.B.P. (Fig. 2).
    A dune field on the late Pleistocene terraces (Kamisatsunai I Plain), about 35 square km in area, has blown from the northwest area to the southeast and has formed more than 180 sand dunes. As shown in Fig. 7, the volume of the E-a dune sands are calculated the maximum value 2.6×106m3 par square km.
    Lee-slope beds dipping 5-20 degrees predominate within these dunes, and the orientation of several lee-slopes is reflected modestly the influence of prevailing northwesterly winds (Fig. 8).
    It is probable that the agency and condition that caused the formation and fixing of ancient sand dune in a periglacial area are closely related to the distinctive deposits, such as the pumiceous sands in this district, and the fluctuation of climate during the late Pleistocene.
  • 鈴木 秀夫
    1972 年 11 巻 4 号 p. 171-180
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    Locations of the main frontal zones in the Würm Glacial Age are reconstructed as in Fig. 1. Abbreviations in the figure are as follows: A; Arctic or Antarctic Front, P; Polar Front, NI; Northern Intertropical Convergence Zone, SI; Southern Intertropical Convergence Zone, s; northern summer location and w; northern winter location. An English version of this article with a slight difference in explanation has already appeared in the Bulletin of the Department of Geography University of Tokyo No. 3, 1971, under the title of “Climatic Zones of the Würm Glacial Age.”
  • 下野 洋, 藤 則雄
    1972 年 11 巻 4 号 p. 181-192
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    There are some small tablelands at 800-1, 000 meters above the present sea level of the surround ing areas of Mt. Hakusan, Central Japan. Hirugano tableland which is one of them occupies at about 1, 000 meters of the northwestern part of Gifu Prefecture, and at the tableland some small moores are distributed sporadically. The present writers investigated on the peat deposits of these moores from the viewpoints of palaeoclimate and the cause of formation of the tableland.
    The conclusions of the study are summarized as follows:
    (1) The stratigraphy of Hirugano tableland is shown as the following table.
    Hirugano Member {Holocene Peat Deposits Middle-Late Pleistocene {Loam Layer Conglomerate Layer
    unconformity
    Early Pleistocene Dainichidake-Washigatake Andesite Group
    unconformity
    Pliocene Shirotori Lake Deposits
    unconformity
    Late Cretaceous Nôhi Rhyolite Group
    unconformity
    Late Jurassic Tetori Group
    The lowermost part of the peat deposits was formed in about 7, 000 years before the present, 6, 680±130 years B. P. (GaK-3628), and the ratio of sedimentation of the peat deposits may be estimated as about 0.31mm/year.
    (2) The cause of formation of Hirugano tableland is concluded as follows:
    In the end of Early Pleistocene epoch the lava flows of andesite from Dainichidake and Washigatake damed up the south entrance of the basin which was formed by eroding the Late Jurassic Tetori Group, Late Cretaceous Nôhi Rhyolite Group and Pliocene Shirotori Lake Deposits. For the reason this lake named Lake Hirugano was formed here. The conglomerate and pyroclastics belonging to the Hirugano Member were deposited in the lake throughout the Middle and Late Pleistocene epoch. Many moores which are distributed sporadically in Hirugano tableland show the last stage of Lake Hirugano filled up from the viewpoint of the histroy of lake. That is, the moores which we can see in Hirugano tableland are a relic of Lake Hirugano into our day.
    (3) The climatic change found in the peat deposits is summarized as follows.
    a: Warm stage: This stage is between about 7, 000 years and about 4, 000 years ago. This stage corresponds to the Postglacial climatic optimum age and the Atlantic and early part of the Subboreal in the pollen chronology of Europe.
    b: Cool stage: This stage is between about 4, 000 years and about 1300 years ago. An annual mean temperature of this stage is 0.5° to 1.0°C lower than that of the present time.
    c: Present climatic stage: This stage is the time after about A. D. 700. The present annual mean temperature of Hirugano tableland is 7.6°C.
  • 森山 昭雄, 小沢 恵
    1972 年 11 巻 4 号 p. 193-207
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    No all-inclusive reports have been made on the morphology of the alluvial plain and the alluvial fills of the lower Yahagi River. The authors investigated them through aerial photographic surveys and analysis of many boring data.
    This alluvial plain morphology is characterized by the presence of numerous small microuplands all over the plain (Fig. 1). Although the area corresponds with the natural levee-back swamp type of the ordinary rivers, the channel pattern at present shows the braided one of bed morphology. The authors recognized it has a connection with the geology of the drainage area. A larger part of the drainage basin of the Yahagi consists of granitic rocks, so in spite of the natural levee-back swamp type, the braided pattern characteristic of the river transporting gravels is predominant near as far as the mouth of the River (Fig. 14). Numberless small micro-uplands are due to the micro-uplands in the braided channels, moreover they are supposed to have been made into “shima-batake” (micro butte-like uplands) by artificial changes of prolonged paddy field cultivation.
    Whereas the sedimentary structure of this alluvial fills at the coastal area represents a well stratified slaty one (Fig. 5, 6, 7), the structure at the inland area north of the New Tokaido Line has a disordered and confusive one (Fig. 8, 9, 10). The authors concluded that these depend upon the expanse of the transported bed loads by tidal or off-shore currents in the open sea and the lens-like deposition of coarse materials in the channel belts on the alluvial upland or in the closed sea.
    Using measured 14C age of these layers at the coastal area, the lower sands (LS) accumulated from about 10, 000 years B. P., and the middle muds (MM) about 7, 000 years B. P. and the upper sands (US) about 4, 500 years B. P. (Fig. 15). The coast of the maximum “Jomon” transgression may have lain a little north of the New Tokaido Line. By rapid and succesive alluviations, the coast at the age of late “Yayoi” may have lain near at the line which links Isshiki with Kira.
    Wider buried terrace exists beneeth the Okazaki district (Fig. 7, 8, 9, 11), and the authors named it the Okazaki buried terrace (Fig. 4). This corresponds with the Koshido terrace on the upper Yahagi River. But they are discontinuous on the projected longitudinal profile (Fig. 13). The authors explained it by the presence of the conspicious gorge between Okazaki and Toyota, as the longitudinal profiles on the alluvial plain and buried valley reveal two graided profiles at the gorge as well as those of the Koshido-Okazaki buried terraces.
  • 小林 国夫
    1972 年 11 巻 4 号 p. 209-210
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
  • 小林 国夫
    1972 年 11 巻 4 号 p. 211-218
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    So much tephra deposits which are popularly called “Loam” or “Brown ash”, obscure discrimination of particular tephra layer from many others within the Quaternary section in Japan. Detailed description in petrography and in lithology is therefore needed for characterizing any particular tephra layers. Our attentions should first be called to petrographic feature of such essential vesiculated materials as pumice and scoria beds which usually prove to be the marker bed in the field. The marker bed may be an early product ejected during a series of many eruptions, of what is called “an eruption cycle”, and individual marker bed represents a lower part of a “tephra member” (Table 1). Recognition of each tephra member becomes more difficult with increasing distance from the source vent, and only pumice or scoria bed unmistakably marks the existence of a member within a tephra formation or merely within a tephra section in far distance. Buried soil or palaeosol can be used for detection of appreciably long interval of ash showering to establish a “member” or a “formation” in the field, but the strict application may not always be easy, as in Japan the accumulation of humic soil through Holocene time is said to have got a much greater rate than that during Pleistocene times.
    Through sequence of several tephra members, some of which being marked respectively by a pumice or scoria bed, are often recognized systematic variations in the type of eruption and in chemical composition of their products (Momose et al., 1968; Kobayashi et al., 1971).
    The problem on the homogeneity of vesiculating magma was discussed according to informations from samples at every horizon closely spaced in a single marker bed. As to a single marker bed maybe consisting of single shower beds or fall-units (Nakamura, 1960, 1964), sometimes orthopyroxene in the upper part somewhat contains more amount of Mg than those in the lower part, seemi ngly indicating that the magma chamber or column had a zonation due to crystallization differentiation (Arai, 1970, 1971, 1972, this number). Additionally, slight decrease in Curie temperature and increase in TiO2-content of ferromagnetic minerals also correspond, from lower to upper, well with the coexisting mineral assemblage mentioned above (Kobayashi et al., 1971). In most cases, however, the inhomogeneity of petrographic feature of the marker beds falls within a small range of variation, so that the marker bed can usually be represented by samples from comparatively a few horizons of a single bed.
  • 十勝団体研究会
    1972 年 11 巻 4 号 p. 219-227
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    In the Tokachi Plain of the south-eastern Hokkaido, a number of the pumice-fall and ash-fall deposits ranging from the late Pleistocene to Holocene are widely developed. As shown in Fig. 1, most of these deposits were accumulated on the southeastern or eastern side of the volcanoes.
    Some of these pumice-fall deposits of the late Pleistocene, that is, Orange-colored pumice-fall 3, 2 and 1 (Op-3, Op-2 and Op-1), Shikaribetsu pumice-fall 2 and 1 (Sipfa-2 and Sipfa-1), and Eniwa-a pumice-fall deposits (E-a) were found in this plain by the studies of the Tokachi Research Group, since 1962. This paper is a summary of these studies (Figs. 2, 3, 4, 5, 6, 7 and Table 1).
    In the last glacial period (W I-II), the orange pumice-fall 3, 2 and 1, Shikotsu pumice-fall 2 (Spfa-2) and pumice-fall 1 (Spfa-1) erupted from the Shikotsu volcano are mainly extending to the southern part of Tokachi Plain and across the Hidaka Mountains (Figs. 1 and 3). The Op-1 pumice-falls serupted as the earlier stage ejecta of the Shikotsu volcano, was dated as 35, 750±1, 350yrs. B.P. (GaK-3669) by 14C dating.
    On the other hand, two pumice-fall deposits are distributed in a narrow area of the northeastern part of this plain. These deposits are named as Shikaribetsu pumice-fall 2 and 1 (Sipfa-2 and Sipfa-1), and considered to have been derived from the Shikaribetsu volcano. 14C dating investigation revealed the date of the Sipfa-2 ejection to be older than the Spfa-1 deposits dated as 32, 200±2, 000yrs. B.P.(GaK-714). Then, archaeologic investigation revealed the date of the Sipfa-1 ejection younger than the Late Paleolithic Shimaki remains dated as 19, 000yrs. B.P. using the hydration layer of obsidian artifacts.
    In the latest glacial period (W IV), andesitic pumice-fall (E-a) erupted from the Eniwa volcano near the Shikotsu caldera, is chiefly distributed in the central area of this plain. Tephrochronologic investigation in this plain and Ishikari low land revealed the date of eruption of the E-a deposit to be older than about 13, 000yrs. B.P. and younger than about 15, 000yrs. B.P. On the basis of the correlation of terrace stratigraphy, periglacial phenomena and tephrochronology of these pumice-fall deposits in this plain, Table 1 is established.
  • 大池 昭二
    1972 年 11 巻 4 号 p. 228-235
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    The Quaternary tephra ejected from the Towada Volcano is widely distributed in its eastern foot-hills. The tephra layers can be classified into four major stratigraphic units; they are, in ascending order, Tengutai Ash, Takadate Ash, Hachinohe Ash, and the Holocene ash alyers. The present paper dealt with the stratigraphy and chronology of the Holocene ash layers. Special reference was made to the archaeological evidences in the absolute chronology of the tephra layers based on the radiocarbon dates. The results are summarized as follows:
  • 豊野層団体研究グループ
    1972 年 11 巻 4 号 p. 236-246
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    The volcanic ash deposits distributed in the vicinity of Lake Nojiri can be classified into three units; the Older, the Middle and the Younger volcanic ash. The nearly 10 meters thick Older volcanic ash consists of brown to reddish scoria beds. Characteristically the about 0.6-1.5 meters thick beds of hornblende andesite pumice occur in the lower and upper parts of the Older volcanic ash. These beds are good guides.
    The Older volcanic ash interfingers with the main part of the Toyono formation which was deposited in the middle Pleistosene lacustrine basin to which the writers adapted the name “the Toyono Lake”. The surface of the Hanami plane I, from 900 to 1000 meters above sea-level, may be accepted as the deposition surface of the Older volcanic ash. This was subjected to up-warping followed with subaerial denudation and the formation of the Hanami plane II, from 700-800 meters above sea-level, around Lake Nojiri. Although the Hanami plane I' about 600-800 meters above sea-level, is separated by the flexure scarp from the Hanami plane I, it is thought that it is a correlative of the latter.
    The nearly 2 meters thick Middle volcanic ash is distributed only on the Hanami planes and is superposed upon the Older volcanic ash with little unconformity. A bed of yellowish, pebble size hornblende pumice occurs at its basal part. It attains about 1 meter in thickness and an excellent stratigraphic marker.
    The Younger volcanic ash covers several planes in different hights is classified into three parts, each of which is separated by an unconformity, and designated as I, II and III in upwards sequence. From the horizon II of the Younger volcanic ash, during the excavation of the bottom of Lake Nojiri, the remains of Paleoloxodon namadicus naumanni and Megaceros (Shimomegaceroides) yabei were collected, and the 14C dates from 16.150±520 to 21.600±900 B.P. years are obtained from two drift woods found together with elephant teeth and bones remains.
  • 横山 卓雄
    1972 年 11 巻 4 号 p. 247-253
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    The correctly identified pyroclastic layers are the best keys for the geologic chronology. The Plio-Pleistocene Osaka and Kobiwako groups are good sediments for the tephrochronology because of the high frequency of intercalations with many thin volcanic ash layers.
    The foundation of the tephrochronology is correct identification of tephra. Especially, the features of the volcanic glass are very important for identification, since main volcanic ash of the Osaka and Kobiwako groups consists chiefly of volcanic glass. The writer tried to determine the titanium content of volcanic glass by polarogram, in order to identify some tephra.
    In this paper, the method of polarographic determination of titanium used in cast iron with good accuracy, reported by Fukui & Arita (1962), was applied to the volcanic glass in a little modification. The sample was dissolved by heating in sulfuric acid-fluoric acid mixture. The fluoric acid was driven off by heating. Then, Fe3+ was reduced to Fe2+ by addition of metallic aluminum. The solution was treated with oxalic acid and the determination of titanium was carried out by recording a polarogram obtained in the range of 0--0.8V.
    The results are shown in Table 4. The individual volcanic ash layer has one's own titanium content. It may be sure that the determination of various metalic elements in the volcanic glass is a good method for identification of tephra.
  • 新井 房夫
    1972 年 11 巻 4 号 p. 254-269
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    This paper deals with my basic study on tephrochronology-an investigation of the availability of refractive indices of orthopyroxenes and hornblendes, contained commonly in the Japanese Quaternary tephras, for exact identification of particular tephras (pumices) which here mean widely spreading tephras so as to be used as stratigraphical key beds.
    So far as I am concerned, the phase-contrast technique is the most desirable and efficient in the determination of accurate and semi-statistic range of the highest refractive indices of cleavage flakes of phenocrysts. Some examples of refractive indices measured by this method are shown in Figs. 2-9.
    The highest principal refractive indices (γ) of orthopyroxenes contained in the great majority of the Quaternary pumices in Japan are almost invariably within a range of 1.700-1.720 (Fs30-Fs45), except for the special case of Shikotsu tephras in Hokkaido (Fig. 5), while the refractive indices n2 on hornblende cleavage flakes fall within a rather wide range of 1.665-1.695 (Fig. 9).
    In most cases refractive indices of orthopyroxenes and hornblendes in peculiar tephras, however, are confined respectively within a rather definite range of their own. For instance, the notable “Pm-I”(pumice layer), which originated from Ontake volcano in Shinshu, shows a good agreement in the refractive index n2 of hornblende cleavage flakes in samples collected from the wide area from Shinshu to South Kanto, regardless of their degrees of argillization of glass component of pumices at each locality (Figs. 10-12). Therefore, refractive indices of characteristic minerals can be a great help to identification of some kinds of particular tephra layers.
    In some exceptional cases, the top unit of pumice layers such as “Tokyo pumice” in South Kanto and “Imaichi pumice” in North Kanto, characteristically contains En-richer hypersthenes than the lower units (Figs. 6, and 2). It suggests that for some reason somewhat layered magma in chemical composition may have existed in the magma column or chamber prior to the eruption.
    On the other hand, in a series of pumice falls which deposited intermittently at intervals of 10-103 years as an approximate estimation throughout an active stage (103-105y.) of volcanism, the pumices in later stages abound in Fs-richer orthopyroxenes than the pumices in earlier stages. This case is shown most typically by the tephras of Shikotsu volcano in Hokkaido (Figs. 5, and 8). The same tendency seems to be generally evident through the activity of other volcanoes (Fig. 7). This tendency not only helps the distinction of the pumices with similar mineral composition by making use of the properties of orthopyroxenes, but also provides an interesting subject for volcanology.
  • 前中 一晃, 横山 卓雄
    1972 年 11 巻 4 号 p. 270-280
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    大阪層群の火山灰層から磁気分離法によって抽出された強磁性鉱物について, その磁気的性質が熱磁気分析, X線分析および化学分析によって調べられた. その結果は大阪層群の火山灰層中の含有強磁性鉱物はアズキ・ピンクIおよびピンクIIの三つのタイプに分けられる事を示している.
    アズキタイプは500℃近いキュリー温度をもち, 等軸晶系に属し, その化学組成は磁鉄鉱に近い.
    ピンクIは100℃近いキュリー温度をもち三方晶系に属し, その化学組成は, 0.6 FeTiO3, 0.4 Fe2O3に近い.
    ピンクIIは100℃および500℃の二相のキュリー温度をもち, ノレルコのチャートもまた等軸晶系および三方晶系の両相の存在を示し, 化学成分もまた両相の混合物たる組成を持つことを示している.
  • 鈴木 正男
    1972 年 11 巻 4 号 p. 281-288
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    南関東武蔵野・相模野台地立川ローム層出土の先土器文化30層準に由来する約1,500点の黒曜石について, 原産地推定, 水和層厚の測定を行なった.
    原産地推定には, フィッション・トラック法 (SUZUKI, 1970) と晶子形態法 (SUZUKI, 1972) を併用した. 水和層厚の測定は, FRIEDMAN and SMITH (1960) およびKATSUI and KONDO (1965) とほぼ同様の手法によった.
    測定の結果, 以下の知見を得た.
    i) 水和速度は, 原産地ごとの黒曜石によって, 著しい差が認められる.
    ii) 各産地ごとに分離された水和層厚は, 各層準でそれぞれほぼ一定の比をとる.
    iii) 4個のフィッション・トラック法による鍵年代と水和層厚の2乗とは, 一次関係y=ax2(y: 年代, x: 水和層厚, a: 定数) が認められる.
    iv) ii, iiiの事実は, KATSUI and KONDO (1965) が指摘した, 過去数万年の気候変動による水和層形成速度への影響を考慮しなければ, FRIEDMAN et al. (1967) が実験的に証明した時間-(水和層厚)2の方程式を支持する.
    v) 過去の気候変動による水和層形成速度の変動は, 14C年代自身が気候変動の影響を受けている可能性があること (RALPH, 1971), その補正が現状では, 必ずしも正しく行えないかあるいは, 不可能であり, さらに, それに代りうる可能性をもつフィッション・トラック法による年代測定値も, その数が限られている等の理由で, 補正困難である.
    vi) この問題の解決は, 将来, 年代測定値が多数得られてはじめて検討されるべき問題である.
    vii) 不十分なデータに基づく, 気候変動による黒曜石水和層年代の誤差の試算によれば, 15,000年につき高々1,500年程度である (SUZUKI, 1972).
    viii) iv, v, vi の理由で, y=ax2で算出された各文化層の黒曜石水和層年代を, 立川ローム層の編年に応用した. 作業仮説は以下のとおりである.
    a) 経過時間 (T) と水和層厚 (L) の2乗とは一次関係にある.
    b) 文化層の示す時間と, それを包含するテフラの堆積年代とは同時性がある.
    ix) viiiで述べた前提により得られた立川ロームの編年は下記のとうりである.
    ソフトローム (III) 9,000~12,500年B. P.
    ハードローム (IV) 12,500~18,500年B. P.
    ブラック・バンドI (V) 18,500~21,000年B. P.
    ブラック・バンドII下半部 約25,000年B. P. (立川ローム最下部の推定年代)
    約30,000~35,000年B. P.
    x) 月見野, 野川および平代坂遺跡におけるテフラの堆積速度は, 町田 (1971) の指摘のようにほぼ一定であると判断される.
    xi) 町田・鈴木 (1971) に報告されたTP (東京パミス) のフィッション・トラック年代, 49,000±5,000年B. P. は, 黒曜石水和層法による年代46,000±3,000年B. P. と誤差内でほぼ一致する.
    xii) ここで得られた年代を, 他の方法で得られた年代と比較するにあたっては, それぞれの方法のもつ問題を十分吟味して行われるべきである. このことは, 14C年代と黒曜石水和層年代とで, 不一致がかなり見られることからもいえる.
    xiii) 黒曜石水和層年代測定法は, 立川ロームの下部から, 弥生時代まで長期にわたって, 試料が入手できること, それぞれの層準で多数個入手できて得られた結果を統計処理できること等の利点があるので, このシステムの内部での地層の対比や, 文化層の編年には極めて有用な方法である.
  • 近藤 精造, 高井 憲治
    1972 年 11 巻 4 号 p. 289-295
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    Studies of the grain-size analyses, mineral analyses, chemical analyses and X-ray analyses were carried out on the weathered volcanic ash beds at 13 locations of the Boso peninsula (Fig. 1).
    Their volcanic ash beds are divided into three types, according to the composition of mafic minerals, clay minerals and the chemical composition. The characteristics of the types are summarized as follows:
    (1) The volcanic ash beds of the first type are fine (Md: 4.7-5.0) and 30-60% of the mafic minerals in these beds is olivine (Fig. 3). Allophane is dominant clay mineral.
    (2) The second type of ash beds have much iron minerals but less of olivine among mafic minerals (Fig. 3). The clay mineral of these beds is mostly halloysite (Figs. 4 and 5).
    (3) The volcanic ash beds of the third type have much of volcanic glass and the contents of SiO2, Na2O and K2O are more than those of the other ash beds (Table 2).
  • 渡辺 晃二
    1972 年 11 巻 4 号 p. 296-304
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    The Ontake pyroclastic fall deposits, which have been accumulated in Pleistocene time, show widespread occurrence in the eastern areas of the Ontake volcano in Honshû. In this study, the weathering sequence of these deposits was given, and it was concluded that the alteration followed the sequence of volcanic ash→allophane→halloysite in the order of progressive alteration.
    In view of the sedimentary environments, the pyroclastic fall deposits may be divided into water-laid, sea-laid and air-laid sediments.
    In the case of “Pm I” pumice fall bed which served as a key bed in these pyroclastic fall deposits, the degree of alteration is reflected by the delicate shade of different environmental conditions. That is to say, in air-laid sediment argillization is remakable, where as, in water-laid and sea-laid sediments, the volcanic glasses are usually prevented from conspicuous alteration, and only small amount of allophane is recognized. These facts suggest that the sufficient water supply and the retaining of dead water in the sediments might be essential factors in progressive alteration.
    In general, during the course of weathering, alkalies and alkaline-earth metals might be removed by slightly acid water. The fact that iron and manganese are concentrated as hydroxide at the base of pumice bed may be explained by more alkaline conditions at this site. Under these physicochemical environments, silica and alumina might be leached out as colloidal forms and precipitate as gels. The usual occurrence of allophane, α-cristobalite and gibbsite in the weathering profile will support this view.
    The features of alteration mentioned above, may indicate that the hydrogen ion concentration and the redox potential as a function of it, are the dominant factor to controll the mineral species which were formed in the course of weathering of these pyroclastic fall deposits.
  • 中谷 進
    1972 年 11 巻 4 号 p. 305-317
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
    The tephras supplied from Late Quaternary volcanoes in Central Japan have been distinguished into some tephra regions. In this paper, the author has made the characterization, classification and tephrochronologic consideration of the Omachi Tephra formations, which are developed in the Omachi Tephra region. The Omachi Tephra formations consist of pumices, scoriae and fine volcanic ash, of which lithologic and petrographic characterization have been made.
    (1) Studies were especially made on pumices and scoriae by means of (a) field observations, (b) analysis of heavy mineral assemblage, (c) thermomagnetic and X-ray analysis of Fe-Ti oxides, (d) chemical analysis of pyroxene by electron micro-probe technique. Throughout the whole sequence of petrographic features of the Omachi Tephra formations two cycles were found. If the biotite and/or hornblende phenocrysts are typical of acid liquids, the followings may be concluded; (i) From the Lower to Middle Tephra units, the composition of magma changed from salic one (biotite-hornblende-dacite) to mafic (two pyroxene andesite). (ii) After an interval of ash shower, the Upper Tephra unit shows a change from slightly salic (hornblende-two pyroxene-dacitic andesite) to mafic activity (hornblende-two pyroxene andesite). And the ash fall activity of the source volcano finished.
    (2) Based on the distribution of the Omachi Tephra, it is inferred that the source vent should be at Mt. Tateyama.
    (3) If the assumption mentioned above could be correct, two cycles recognized from the section of the Omachi Tephra formations seem to correspond with the already known two cycles in chemical composition of volcanic products from Tateyama volcano (after Yamasaki et al., 1966).
    (4) The Omachi Tephras have been traced to the East Shinshu district and show the relationships with the tephras of other tephra regions. The pumice layer DPm of the Omachi tephra stratigraphically lies above the pumice layer Kwp from Mt. Yatsugatake, and also a little lower horizon of the Pm-I from Mt. Ontake. On the one hand, as the latter is embedded within the middle part of the Shimosueyoshi “Loam” tephra unit in the South Kanto districts, the Upper unit of the Omachi Tephra formation should naturally be correlated with the Shimosueyoshi stage.
    (5) As the DPm layer is thought to be correlated with the pumice flow activity of the third period in volcanic history, so the activity of Mt. Tateyama must at least have continued down to the Shimosueyoshi “Loam” age.
    (6) Many pumice grains which are identical with those of the IIIrd period pumice flow are contained within the gravel bed of the highest terrace on the right side of the Joganji river. So this terrace could be correlated to the Narimasu or Musashino terraces.
  • 三村 弘二
    1972 年 11 巻 4 号 p. 318-319
    発行日: 1972/12/28
    公開日: 2009/08/21
    ジャーナル フリー
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