亀尾 浩司

北大西洋で設定された最上部新生界の石灰質ナンノ化石対比基準面を古地磁気層序および酸素同位体層序と比較し,少なくとも第四系は10万年のオーダーで年代決定が可能であることを改めて指摘した.その結果から房総半島および本邦に分布する海成第四系に発達するテフラの地質年代を石灰質ナンノ化石層序から総括し,近年報告された広域テフラ対比が支持できることを示した.また,石灰質ナンノ化石層序と酸素同位体ステージとの関係に基づいて,上総層群に認められるテフラ鍵層と酸素同位体ステージとの関係もあわせて明らかにした.一方,鮮新世の石灰質ナンノ化石群集は,日本海側が北極海域の群集と類似し,太平洋側が低緯度海域の群集に対比される.これは鮮新世の古海洋が日本列島を挟んだ東西でまったく異なった生物地理区を形成していたことを示唆する.本論ではその群集変化が,パナマ地峡の成立と関連した鮮新世古海流の変遷によることを述べた.

北大西洋の第四系深海底堆積物の中に,13の石灰質ナンノ化石基準面が認定され,その地質年代も地磁気層序との対応から算出された(Takayama and Sato, 1987).この基準面は,赤道太平洋やインド洋の深海底堆積物,およびわが国の上部新生界の中にも確認される.その結果,第四系の認定と相互の対比,および10万年オーダーでの地質年代の決定が可能になった.本論では,その成果の概略を紹介し,あわせて基準面の有効性について,化石の同定に関する個人差,基準面の同時性,研究者によって異なる基準面の年代値といったさまざまな問題点からこれを検証した.一方,鮮新統/更新統境界の模式地である南イタリア,ヴリカ(Vrica)のセクションにおける石灰質ナンノ化石層序を検討したところ,基準面のうちの4つが確認された.鮮新統/更新統境界は,筆者らの基準面(12)(Gephyrocapsa caribbeanicaの出現:図7参照)に近く,その年代を基準面の位置とCande and Kent (1992)の古地磁気年代尺度を基礎に求めたところ,1.74Maとなった.

Three nannofossil datum planes of the Early Pleistocen e are identified in the cores from subsurface at Irabu Island, South Ryukyus. The result allows for the first time to date the initial sedimentation of the reef-related Riukiu Limestone there. It represents the age (1.2M a) turning from siliciclastics to shallow-water carbonate regime. Implication of this transition time is discussed briefly in relation with both local neotectonis m resulting in the birth of a backarc basin (Okinawa Trough) and regional to global reor ganization of paleoceanographic environments.

On the basis of calcareous nannofossil assemblages in samples recovered at six sites in the North Atlantic Ocean during DSDP-IPOD Leg 94, eleven calcareous nannofossil biohorizons are recognized in the Quaternary sequences deposited at middle and high latitudes. Correlation of these biohorizons with the magnetostratigraphy is established, and ages of all datums are estimated by interpolations between magnetic reversals (CLEMENT and ROBINSON, 1987; TAKAYAMA and SATO, 1987). The calcareous nannofossil and the foraminiferal assemblages of the Yabuta and the Junicho formations in Toyama Prefecture and the Omma formation at its type locality in Kanazawa City are described in detail. Based on the above mentioned results of DSDP-IPOD Leg 94, the Yabuta, the Junicho and the Omma formations are late Pliocene, Plio-Pleistocene and Pleistocene in age, respectively.<br>In 1984, small mammalian fossil bone of Stegodon aurorae (elephant) was found from the upper Omma formation. This level of S. aurorae is probably referred to the stratigraphic interval between the calcareous nannofossil biohorizon (5) (0.83Ma) and the Brunhes/Matuyama boundary of the magnetostratigraphy (0.73Ma).<br>The Omma formation overlies unconformably the Saikawa formation. In contrast to the Omma formation, the Saikawa formation contains middle to lower Miocene calcareous nannofossils. The upper Miocene and the Pliocene sediments are, therefore, completely missing between these two formations.

We describe in detail the calcareous nannofossil assemblages present in the uppermost Cenozoic sequences in the southern Kwanto region, Japan and in the Vrica section (the Pliocene/Pleistocene boundary stratotype), southern Italy. Ten of twelve calcareous nannofossil biohorizons previously recognized in the Quaternary sequences in the Northeast Atlantic Ocean (TAKAYAMA and SATO, 1987) also occur in these areas. These datums allow us to correlate the uppermost Cenozoic sequences in the southern Kwanto region with these sequences along the Japan Sea margins. We clarify the relationships between the calcareous nannofossil biohorizons and the benthonic foraminiferal zones recognized in the sedimentary basins along the coastal regions of the Japan Sea. We also clarify the relation between these biohorizons and the Pliocene/Pleistocene boundary at its stratotype. The first occurrence of Gephyrocapsa caribbeanica, which occurs seventeen meters above the Pliocene /Pleistocene boundary and thirty meters above the Olduvai event at the Vrica section, is usable as the excellent indicator to detect the Pliocene/Pleistocene boundary.