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The litho‐ and biostratigraphy of the mid‐ to upper‐Cretaceous System around the Yagumaike Pond in the Aridagawa area, Wakayama, Southwest Japan, were investigated. Many Middle to Late Albian megafossils were found in the strata of a block bounded by faults. It was also revealed that the Upper Cretaceous System of other blocks ranges from the Middle Turonian to Santonian. The Albian megafossil assemblage contains few benthic organisms, in contrast with the abundance of nektons found (e.g. cephalopods). Sedimentological observations of the mudstone profiles also indicate that scarcely or weakly bioturbated, well‐laminated mudstone is dominant among the Albian deposits. These results suggest deposition of the Albian mudstone under a dysaerobic to anoxic environment. It is comparable to the extended oceanic anoxia (OAEs) in mid‐Cretaceous time. Albian deposits with similar characteristics are also known to exist in Shikoku, Southwest Japan. A wide sedimentary basin that was directly affected by global environmental events, such as OAEs, seemed to be formed on the Chichibu Belt in the Albian. The Upper Cretaceous strata in the study area are extremely thin, similar to the coeval deposits on the Chichibu Belt in Shikoku. It is suggested that the sedimentation rate in the sedimentary basin on the Chichibu Belt was extremely low during early Late Cretaceous time. 相似文献
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The Upper Cretaceous Toyajo Formation is distributed around the Mt. Toyajo in the Aridagawa area, Wakayama, southwestern Japan. The formation is subdivided into three newly defined members, the Nakaibara Siltstone Member, Hasegawa Muddy Sandstone Member, and Buyo Sandstone Member, in ascending order. Close field observation elucidated the detailed biostratigraphy of the Toyajo Formation, and high-precision biostratigraphic correlation was made with the Yezo Group in Hokkaido (northern Japan) and Sakhalin and the Izumi Group in southwestern Japan.The Toyajo Formation contains diversified lower Campanian to upper Campanian heteromorph ammonoid assemblages, including Eubostrychoceras and Scaphites. Discovery of the heteromorph fauna demonstrates that scaphitid ammonoids survived until Campanian time in the northwestern Pacific region. Although Eubostrychoceras elongatum has been known in the northeastern Pacific region, the occurrence of this species in the northwestern Pacific region has been uncertain before. The rich occurrence of E. elongatum in the Aridagawa area indicates that this species was distributed widely in the northern Pacific realm.The Toyajo Formation is similar to the Izumi Group in various geologic features, and may indicate that the Toyajo Formation was deposited in a strike-slip basin along the Chichibu Belt formed by the movement along the Kurosegawa Tectonic Zone in the latest Cretaceous, like the Izumi Group, along the Median Tectonic Line. 相似文献
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