西南日本白亜系の古地理と古環境

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This paper expresses an idea that during the period from Hauterivian (Early Cretaceous) to Middle Eocene (Early Tertiary), deltaic sediments may have widely been distributed longitudinally from the Korean Peninsula to about one thousand kilometers south of the recent position of southwest Japan, probably in the eastern marginal part of the Asian continent. This idea is based entirely upon the assumption that the movements of lateral faults represented by the so-called Kurosegawa, Median and Maizuru Tectonic Belts or Lines, evidenced from recent studies, and the paleogeography of the Japanese Islands, figured out from the paleomagnetic studies, are all realistic. This supposed great "delta" is schematically illustrated. The main part of the "delta" comprises faunas and lithofacies which are the characteristics of the Lower Cretaceous Nankai Group and the Upper Cretaceous of central Kyushu. In other words, this part is composed of protected-bay or delta area deposits and associated faunas. The "brackish" faunas from this part are usually characterized by the occurrence of Eomiodon spp. On the northern and southern borders of this "delta", red sediments and conglomerate beds, of nonmarine origin, are usually found as though they were at the extremes of the lateral extension of the "delta". The sediments on the northern and southern borders of the "delta" include faunas and lithofacies common to the Lower Cretaceous Monobegawa Group and the Upper Cretaceous Sotoizumi Group. In other words, those areas are underlain by "brackish", nearshore, and offshore sediments which are characterized by open sea faunas and lithofacies along the continental margin. "Brackish water" bivalves, Costocyrena spp. are characteristic species in the "brackish" or intertidal environment of these areas. The outskirts of the "delta" (distal bar areas) are characterized by an intermediate fauna between open sea and protected-bay area ones, which is common to the Doganaro, Uwagumi and Uwajima Formations of the Shimanto Belt. In central Kyushu, the Median Tectonic Line has probably thrust up eastward as early as the Middle Eocene over not only the Sanbagawa Belt, but also the Chichibu Belt. As a result, the Yatsushiro, Tano and Gumizaki Formations (Albian-Cenomanian protected-bay area deposits) are all found as the overthrust sheets on these two belts.
日本古生物学会の論文
1986-11-20

西南日本白亜系の古地理と古環境

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