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Structural analysis of Tashiro Formation of Hyuga Group based on planktonic foraminiferal zonation
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| Abstract | Two markedly contrasting views, one based on the geosynclinal concept and the other on the accretion tectonics, have been presented to explain sedimentary and tectonic processes of the Shimanto Superg...roup. Discrepancies between these two views seem to have arised from differrences in geological data base on stratigraphic sequences and geolgoic structures. Rocks of the Shimanto Terrane are so lithologically monotonous and structurally extremely complex that it is necessary to analyse them in more detailed fashion to obtain warrantable configuration of the geologic development of the terrane. This paper describes the results of analysis of the stratigraphy and geologic structure of the Tashiro Formation of the Hyuga Group in the Mimi-River area, Miyazaki Prefecture, Kyushu, using planktonic foraminiferal fossils, and discusses sedimentary and tectonic environments of the formation. The Paleogene Hyuga Group is lithologically divided into four formations, the Aradani, Tashiro, Unama and Ouchibaru. These formations are distributed repeatedly by thrusts dipping north and shows an imbricate structure. Of these formations, the Lower Member of the Tashiro Formation is composed mainly of massive mudstone with thin sandstone interbeds and sandy laminae, and occur in seven belts, of which the Shiitani, Nagano and Unama belts have a larger width than others. As this formation yields abundant planktonic foraminiferal fossils, systematic sampling was made along a representative section of these three belts and biostratigraphic sequence and geologic structure of these belts were analysed. Five planktonic foraminiferal zones were recognized in the strata of these three sections. They are Morozovella spinulosa Zone (I), Globigerinatheka index Zone (II), Globorotalia cerroazulensis cocoaensis Zone (ill), Pseudohastigerina barbadoensis Zone (N) and Chiloguembelina cubensis Zone (V) in ascending order. They are correlated with the planktonic foraminiferal zones known from the tropical to subtropical regions and are referable to as latest Middle Eocene, earliest Late Eocene, Latest Eocene, Earliest Oligocene and Early Oligocene age, respectively. On the basis of the distribution of these fossil zones of the three sections, the stratigraphy and geologic structure of the Lower Member of the Tashiro Formation are summarized as follows: 1) The strata of each section are cut up into many sheets mainly by thrusts dipping north and form an imbricate structure. One and the same zone appears repeatedly many times with a disordered array within one section; 2) Many of the sheets dip north and show a normal sequence, but a few sheets show an overturned sequence; 3) The thickness of individual sheets is mostly less than 250m; 4) The thickness of the overall sequence of the five zones is approximately 700 to 800m; 5) The zones I to V are distributed in the southernmost Shiitani section, I to IV in the central Nagano section, and m and IV in the northernmost Unama section, respectively. 6) In each section, the lower zones occur widely and many times in the southern part and the upper ones in the northernpart. Therefore, the strata of each section show a tendency of northward (continentward) younging; 7) Viewed collectively the distribution of the foraminiferal zones of the three sections, the upper zones (IV to V) are more widely distributed in the Shiitani section than in the Nagano section; 8) The youngest zone (V) is found only in the Shiitani section. Therefore, it is desiphered that each section shows a roughly northward (continentward) younging, but that the arrangement of the overall foraminiferal zones among three sections shows a southward (oceanward) younging. The above-mentioned features of the stratigraphy and geologic structure of three sections of the Lower Member of the Tashiro Formation are consistent with those of accretionary prisms of a Recent trench lower slope to trench system in many respects, and suggest that the Lower Member of the Tashiro Formation is referable to as an accretionary prism.show more |
| Table of Contents | I. まえがき II. 九州の古第三系四万十累層群の研究史 Ⅲ. 岩相と地質構造 IV. 化石帯の分布 V. 考察 VI. まとめ |
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| Created Date | 2021.10.25 |
| Modified Date | 2022.06.20 |