本邦太平洋沿岸地方における海水準静的変化と第四紀編年

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The most significant evidence for the Quaternary chronology in the non-glaciated region may be the glacial eustasy, whose effectes are world-wide in extent. Evidence for changing sea-levels during the late Pleistocene and Holocene is in part topographic and in part biologic in nature. Terraces result from the changing base-levels which are sea-levels in the coastal region. Three types of the change sea-level are dynamic, eustatic and static in nature. Eustatic change of sea-level can be separated from dynamic and static ones by its world-wide extent and independency of local tectonic features. The glacio-eustatism is accompanied by the climatic changes, though it could not be determined independently from the epeirogenic eustatism which is thought to be more gentile in rate than glacio-eustatism. The depositional plains distributed along the Pacific coast of Japan are divided into a coastal plain and four terraced plains, which are termed, from younger to older, I, II, III, IV and V plains in the text. The Plain I, coastal plain, emerged with subrecent lowering of the sea-level of 5-6 meters preceded by the climax of the postglacial marine transgression. The inner parts of the coastal plain and their extension along the river courses were dissected and transformed into the low terraces. The valleys buried beneath the coastal plain formation, ranging between 40 and 60 meters in depth of their bottoms in the coastal areas, may have been graded to the lowest sea-level during the last glacial stage. It is considered that the continental shelf at 130±20 meters below the sea-level. The Plain II is distributed along the drainage systems in the background of the coastal plain. The plain is terraced and subdivided into two berms in many areas. The gradient of the plain is steeper than those of the I and III plains, so that the plain overlies the III plain in some upper parts of the drainage systems, and is approached by or buried beneath the I plain in the lower part. The Plain II formations mainly consisting of fluviatile gravels and sands intercalated with conifer-dominant floras. The sea-levels to which the Plain II has been graded may be expected between near and below the coastal plain. The plain may be dated to the cool and low sea-level time during the last glacial stage, though the lowest sea-level was after the formation of the Plain II, because the latter is dissected by the valleys buried beneath the coastal plain. The Plain III, being well preserved behind the coastal plain or coast line between 20 and 50 meters in average height, is the ancient coastal plain grading backwards into the fluviatile plains. In the marginal parts, the plain is characterized by the gentle to sub-horizontal gradient and its marine formations, which consist of, from the bottom upwards, gravels, sands and gravels varying in their total thickness according to the relief of the surface of the basement. In the thicker parts of the marine formation, silts or clays are added in the middle part but are lacking in the thinner parts. The marine formation has yielded a fauna indicating inner bay environments and a climatic condition similar to the present. Both the marine and fluviatile formations of the Plain III are intercalated with a flora characterized by the dominant Cryptomeria and broad leaf species, which indicates a climate rather warmer than that of the Plain II formations. Topographic, geologic and biologic features of Plain III suggest that it has been graded to the high sea-level during the last interglacial stage. The Plain IV with gravels is distributed along the mountain foot and on the hilltops in the coastal areas. The plain is hardly dissected and is subdivided into two plains in many areas. The gradient of the plains is steeper than that of the Plain III. A feature of the piedmont alluvial plain is restored along the foot of some mountains. The Plain IV formations have yielded the conifer-dominant floras resembling to those of Plain II. In some areas, the Plain IV includes a marine phase which is thought to be tectonically slightly above Plain III. The features of the Plain IV indicate the sea-levels near or slightly lower than that of Plain III and a cool temperature which suggest that the plain has been formed during the penultimate glacial stage. The distribution of Plain V is within the area of resistant rocks. The gradient of the plain with its deposits of sands and gravels is gentle or sub-horizontal. In the northeastern parts of the Kitakami Massif, the Plain V ranges between 160 and 200 meters in altitude. The features of the plain resemble to those of Plain III, though no fossil is found to date. Probably, the Plain V indicates the high sea-level during the penultimate interglacial stage. The distributions and mutual relations of the five depositional plains along the Pacific coast of Japan are shown in the text-figures. With respect to the sea-levels and climatic conditions, indicated by the topographic and biologic features, two types of depositional plains are alternatively developed along the Pacific coast of Japan. This is probably the fundamental succession of the terraces in the coastal region of the world, considering the extent of the glacial eustasy. Tectonic movements are well shown by the dislocation of the marine Plain III which indicates that block tilting prevailed along the Pacific coastal region during the latest Pleistocene and Holocene times. In the uplifted parts of the tilted blocks, the abrasion plains which grade towards the down-tilted parts into the depositional plains are successively developed. Some boundaries between the tilted blocks coincide with the tectonic lines recognized through the Neogene Tertiary history of the areas. It is considered that the geomorphic provinces of the Pacific coastal region of Japan originated during the Miocene or pre-Miocene times. The interglacial terraces with broad gentle plains which are underlain by marine sediments yielding the warm temperature fauna and flora can be correlated to the broad gentle abrasion plains and the glacial terraces with steep plains which are underlain by the fluviatile sediments containing the cool temperature flora to the narrow, locally diverged, steep abrasion plains between the depositional and abrasion areas. In the northern part of the Boso Peninsula, Kanto Region, a series of marine Pliocene and Pleistocene formations are developed without sedimentary break. The depositional surface of the uppermost formation of the series is the Plain III. Two horizons containing the cold water faunas intercalating the warm water horizon between them are recognized in the upper parts of the series. The climatic fluctuation corresponds with the sedimentary cycles. A cold water horizon preceding these horizons is maintained from the studies of the planktonic foraminifers. A diversity of opinion still exists among the paleontologists as to the delimitation of the Pliocene-Plsistocene boundary in the continuous sequence in the northern part of the Boso Peninsula. Volcanism is one of the conspicuous phenomena in the Japanese Quaternary. Around the Pleistocene volcanoes, especially on the eastside of them, the Plain III and older ones are covered by the volcanic ash layers with their characteristic sequences, by which the tephrochronology can be made. The Plain II is also covered with the ash in some areas. One of the climax of the volcanic activities may be expected during the last interglacial stage. The altitudes of the Pleistocene sea-levels along the Pacific coast of Japan, estimated from the present study, are at 200 meters during the penultimate interglacial stage, between -20 and +30 meters during the penultimate glacial stage, between 20 and 50 meters during the last interglacial stage, near the present level, about -50±20 meters and about -130±20 meters during the last glacial stage, and between five and ten meters during the climax of the post-glacial transgression. The glacial chronology along the Pacific coastal region of Japan is shown in Table 3.
東北大学の論文
1961-08-20

本邦太平洋沿岸地方における海水準静的変化と第四紀編年

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東北大学 | 論文

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