白亜系有田層の化石層序

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The name Arida Series has been applied to the formations in Japan which are assigned approximately to the Upper Neocomian age. MATSUMOTO (1954) classified the Cretaceous System of the Japanese Islands into six major time-stratigraphic units, K1 to K6. K2 is denominated the Aridan (previously proposed as Aritan). The definition of the Aridan was not biostratigraphically accurate. The name was derived from the Lower Cretaceous formation of the Arida district along the Aridagawa valley, Kii Peninsula, in the Outer zone of Southwest Japan. However, as our research work progressed, it has become evident that the Cretaceous Arida Formation is only a fraction of the whole range of the Upper Neocomian. Meanwhile, the Cretaceous deposits of the Choshi area, about 100 km east of Tokyo, which were assigned roughly to the Miyakoan (Aptian plus Albian), have been intensively studied, revealing that they are not contemporaneous with those of the Miyako area on the Pacific side of the Kitakami Mountains, northeastern Honshu and that they include Barremian beds in the lowermost part (OBATA et al., 1975). The present paper reports the results of the ammonites biostratigraphic investigations of the Lower Cretaceous Arida Formation exposed in the northern part of Yuasa-machi, Wakayama Prefecture. The Arida Formation is subdivided into four members, i. e., lower conglomerate (Al), middle sandstone (Am), upper siltstone (Au) and uppermost alternation (At), in ascending order. The geological map and the geological sections surveyed by us are presented in Figure 1. The synthetized stratigraphic section is shown in Figure 2. The individual columnar sections are illustrated in Figure 3. in which horizons of the fossiliferous beds are indicated, with their index map in Figure 4. The cephalopod species and the fossil localities of the middle sandstone (Am) and upper siltstone (Au) members are listed in Table 1,and those of the uppermost alternation (At) in Table 2. Figure 5 shows the localities of the specimens obtained. In the area surveyed we recognized at least four horizons where the fossil cephalopods predominate. From the fossil evidence, the subdivided units, Am, Au and At, are clearly correlated with the reference scales abroad. The upper siltstone (Au) yeilds Paracrioceras aff. elegans and Anahamulina aff. subcylindrica, so the member is concluded as a representative of the upper zone of the Lower Barremian. The uppermost alternation (At) yields Heteroceras aff. astieri, Barremites (Barremites) cf. strettostoma and Barremites (Cassidoiceras) aff. cassidoides, so the member is concluded as a representative of the lower zone of the Upper Barremian. Thus the Cretaceous sequence of the fossiliferous part in the Arida Formation ranges probably from upper Lower Barrmian to lower Upper Barremian. The knowledge newly obtained from the ammonites biostratigraphy of the Arida Formation would prove useful for future correlation of the Arida Series in Japan. The Isejigaura Formation, an Aridan deposit, the lowermost member of the Cretaceous deposits of the Choshi area, is mostly assigned to lower Lower Barremian, because the formation yields Crioceratites (Emericiceras) cf. emerici and Holcodiscus sp., the zonal indices. Another interesting fact is that the Upper Cretaceous indices, Inoceramus spp., Gaudryceras sp. and Polyptychoceras sp., were newly discovered from Kibi-machi, south of Yuasa-machi. The deposits of this area were formerly regarded as a representative of the southern facies of the Arida Formation. The Upper Cretaceous fossils were obtained abundantly from the massive siltstone intercalated in a thinbeded alternation of siltstones and shales. From the evidence of lithofacies and fossils contained, it is strongly suggested that the deposits belong to the Santonian to lower Campanian Futagawa Formation. Figure 8 shows the localities of the Upper Cretaceous fossils obtained from Kibi-machi. KANIE (1972) reported a Coniacian deposit near Minami-d
国立科学博物館の論文
1976-06-22

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小畠郁生
国立科学博物館

白亜系有田層の化石層序

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