Alternative Title手取層群から産出する前期白亜紀植物化石群集とその古気候学的意義
Note (General)The Early Cretaceous floras of East Asia have been divided into three types, the Tetori-, Ryoseki- and Mixed-type. The Tetori-type Flora, distributed in the Inner Zone of southwest Japan, Siberia and northeast China, flourished under warm-temperate and humid climate conditions. The Ryoseki-type Flora, distributed in the Outer Zone of southwest Japan and southern China, flourished under tropical to subtropical climate conditions with a seasonal aridity. The composition of the Tetori- and Ryoseki-type floras was completely different with no species in common. The Mixed-type Flora was distributed in between the Tetori- and Ryoseki-type floristic provinces. This study discusses the Early Cretaceous climate on the eastern margin of Asia, target at the Barremian to Albian non-marine deposits of the Tetori Group. First, the stratigraphy of the Tetori Group in the Kuzuryu area of Fukui Prefecture, which is the type area of the Tetori Group, was reexamined. The group in the area is divided into Yambara, Ashidani, Obuchi, Itsuki and Nochino formations, in stratigraphic ascending order. The plant assemblage from the Ashidani Formation has no Ryoseki-type floral element, whereas the assemblages from the Itsuki and Nochino formations have some Ryoseki-type floral elements such as microphyllous conifers. Second, the stratigraphy of the Tetori Group in the Shiramine area of Ishikawa Prefecture and the Takinamigawa area of Fukui Prefecture was reexamined. The group in the area is divided into Gomijima, Kuwajima, Akaiwa and Kitadani formations, in stratigraphic ascending order. Plant assemblage from the Akaiwa Formation around Mt. Oarashiyama in the Shiramine area includes 18 genera and 23 species typified by the abundant of the Tetori-type floral elements including ferns, ginkgoaleans and conifers, associated with macrophyllous bennettitalean (or cycadalean) and microphyllous coniferous foliages represented by Brachyphyllum which favored climate with drying season(s). It had been presumed that the occurrence of the Ryoseki-type floral elements is consistent with floral change and a warming and drying climate trend during the deposition time of the Kitadani Formation. However, this study suggests that such a trend started from the deposition time of the Akaiwa Formation earlier than previously thought. Finally, the stratigraphy of the Oguchi area of the Ishikawa Prefecture was reexamined. The group in the area is divided into the Gomijima and Kuwajima formations, in stratigraphic ascending order. The plant assemblage of the Kuwajima Formation collected from the Onabara section includes 18 genera and 26 species. This study describes the co-occurrence of plant fossils in 10 plant fossil-bearing beds. It is considered that these fossil occurrences are allochthonous. This study described the co-occurrence of plant fossils in these plant fossil-bearing beds. It is likely that the occurrence of Podozamites is not influenced by depositional environment. In contrast, it is obvious that the occurrence of ferns is varied by depositional environment. The difference of taphonomic process and sedimentary environment probably become a factor for this trend. This study provides clues for understanding the local floral compositional change and reveals the details of the regional vegetation difference and floral succession in the Tetori Group. The Ryoseki-type floral elements occur at the Nochino, Akaiwa and Kitadani formations. The floral element analysis infers a warming and drying climate trend between the Itsuki Formation and the Nochino Formation and between the Kuwajima Formation and the Akaiwa Formation. The trend suggests that the floral composition of the Tetori Group changed from the Tetori-type to the Mixed-type. Therefore, the drying on the Tetori basin has started much earlier than previously thought. The floral composition of the Tetori Group changed from the Tetori-type to the Mixed-type because the climate condition around the Tetori basin became warmer and more arid in the Aptian. The warming and drying trend in the Tetori Group probably expresses the potential links with the Lower Cretaceous climate transition in the mid-latitudes in the Northern Hemisphere. Thermophilic reptiles such as crocodilians also have been discovered in the uppermost part of the Tetori Group, and they unequivocally indicate warmer environments around the Tetori basin. East Asia on Early Cretaceous was divided into some basins by mountain range. The coastal area was under humid climate, and the inland area was under dry climate at that time. It remains even now doubt on the differences of fauna in the basins of the Tetori and Jehol groups, which were located at the same latitude in the same age. The cmparison of floral succession between inland (Jehol) and the coastral (Tetori) basins revealed that successive climate change recorded in the Tetori Group is representing the moving northward process of the Ryoseki-type Flora along the coastal area on East Asia. Thus, the Tetori basin, which was located in the eastern margin of Asia, is an important area that leads to the elucidation of the establishing process of the terrestrial ecosystem in East Asia.
新大院博(理)第476号
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2022-08-08T06:02:54+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション