濃尾平野第四系の層序と微化石分析

NDLデジタルコレクション 被引用文献4件

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  • Stratigraphy and microfossil analysis of Quaternary sediments in the Nobi plain, Central Japan

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During the Pliocene and early Pleistocene there existed a large freshwater lake named the Tokai Lake in the Nobi Plain, the Ise Bay and their environs in central Japan. In the Nobi Plain area the tilting movements of the Nobi Plain Tectonic Block have started to develop at the beginning of middle Pleistocene, which resulted in the formation of a sedimentary sequence attaining over 350 m in thickness that covered unconformably the sediments of the Tokai Lake of more than 1,000 m thickness. The present paper describes the subsurface stratigraphy of the middle Pleistocene and later sediments in the Nobi Plain on the basis of the borehole materials obtained from several thousands of water wells and other holes and discusses the changes in sedimentary environments and climatic fluctuations as estimated from the results of microfossil analyses of numerous boring-core samples. Pleistocene and later sediments, and three major periods of low sealevel stand are proved from lithlogical and microfossil analyses. These low sealevel periods are each represented by a buried valley topography and a gravel bed that fills it, and their stratigraphic levels can be located at the following gravel-bed horizons: 1. The Last Glacial gravel bed lying just subjacent to the sediments resulted from a latest Pleistocene minor transgression and the Holocene transgression. 2. The gravel bed that underlies an interglacial marine clay, which is covered by deltaic to lagoonal sediments of a relative high sealevel stand in the early half of the Last Glacial. 3. The gravel bed preceding the marine to lagoonal clay beds which intercaiate two or three gravel beds of probable minor regression period. The studied sediments are represented in their lower horizons by an alternation of freshwater lacustrine clay bed and fluvial sand and gravel bed, all of which are poor in microfossils, whereas in their upper part they are composed of an alternation of marine clay bed that ha s been deposited under innerbay conditions and fluvial gravel bed. Each of these marine clay beds represents a sedimentation cycle spanning from a transgression through a regression, being assignable to a period of relative climatic ameriolation. In contrast the gravel beds have been deposited in the valleys formed during the periods of low sealevel stand in colder spell as either terrace gravels or river-bed gravels, the latter of which has each formed a talweg plane during a maximum of sealevel falling. At present a detailed stratigraphy is established for the upper part of the above-mentioned middle.

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