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＜紀要論文＞
山口県新第三紀油谷湾層群の堆積地質学的研究
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| 概要 | The Early to Middle Miocene Yuya-wan Group is well exposed in the Yuya-wan area in the westernmost part of Honshu. This group is regarded to represent Miocene marine sequences developed in close relat...ion to the spreading of the Sea of Japan. This paper describes and analyzes sedimentary facies, sedimentary structures, paleocurrent patterns and petrological features of the Yuya-wan Group in order to clarify the depositional environment. The main results are summarized as follows (1) The Yuya-wan Group is divided into the Igami Formation and its equivalent Kadoyama Formation in the lower and the Kawashiri Formation in the upper on the basis of the characteristics of litho-and bio-facies. (2) The Igami (more than 250m thick) and Kadoyama Formations (150-300m thick) are composed mainly of well-sorted, thin-to medium-bedded sandstones with pebble conglomerate in the lower and upper parts. Tabular and trough-like cross-stratifications are the most common sedimentary structures. Channel structures, hummocky structures and herringbone cross-stratification are also found at some horizons. Glauconitic sandstones are intercalated in the lower part of the Igami and Kodayama Formations. The sediments contain molluscan fossils and foraminifers abundantly, which show shallow sea environments. In addition, trace fossils Cylindrichnus sp. occur exclusively in these formations, which corresponds to the Skolithos assemblage of the SEILACHER Model proposed as the depth-indicator. All these features suggest strongly that the Igami and Kadoyama Formations were deposited in the tidal flat to very shallow sea environments including the prodeltaic environment (3) The Kawashiri Formation (more than 350m thick) conformably overlies the Kadoyama Formation. It is wholly characterized by gravity-flow sediments showing turbidite facies with intercalations of slump deposits. Most of sandstones show the graded bedding with the Tb-Tc intervals. Lithothamnium limestones occur as exotic blocks of various sizes in slump beds in the upper part of the formation. Zoophycos-like trace fossils and foraminifera assemblages so far reported from the Kawashiri Formation indicate that the sediments were deposited in the upper bathyal enviroments. (4) The sandstone composition of the Yuya-wan Group indicates that sandstones are mostly lithic to feldspathic arenite and are characterized by stable heavy mineral assemblages of high ZTR index (5) The petrographic and paleocurrent data suggest that elastic materials may have been derived mainly westwards and/or northwards from the Cretaceous Kwanmon and Abu Groups and Paleogene Hioki Group as source rocks.続きを見る |
詳細
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| 登録日 | 2021.10.20 |
| 更新日 | 2022.06.20 |