北陸地域における新第三系の層序対比と新第三紀古地理  [in Japanese] Neogene paleogeography in the Hokuriku region, Central Japan, based on the revised stratigraphic correlation  [in Japanese]

微古生物層序学と放射年代の進歩により,北陸地域の新第三系の精度の良い対比表ができた。新しい対比表を基に,最初の2時期は岩相分布図,後の7時期は古地理図をつくった。1)楡原期とそれ以前と2)岩稲期と医玉山期(20-17Ma):この2時期には火山活動が盛んで安山岩や流紋岩の火砕岩が広く分布した。3)黒瀬谷期(16.5-16Ma):海進により堆積盆が形成され"八尾一門ノ沢"動物群が支配した。4)東別所期(16-15Ma):海は拡大・深化し黒色泥岩が厚く堆積した。5)下部音川期(15-14Ma):堆積盆は再び浅くなり,暖流が流れていた。6)上部音川期(13.5-8Ma):堆積盆は浅く寒流が流れ,"音川"動物群で特徴づけられる。7)阿尾期(6.5-3.5Ma):能登半島は陸域となり,堆積盆は狭くなった。8)薮田期(3.5-1.5Ma):前期と同様寒流が流れ,石灰質砂岩が堆積した。9)大桑期(1.5-0.8Ma)浅海で寒流系の"大桑・万願寺"動物群で特徴づけられる。

Recent progress of biostratigraphic investigation and radiometric age dating has established a revised correlation of the Neogene formations in the Hokuriku region, Central Japan. Two rock distribution maps and seven paleogeographic maps are presented, corresponding to seven stages from Miocene to early Pleistocene. Succesive changes of paleogeography and paleoenvironmental conditions are outlined as follows: 1) Nirehara and older stages (older than 20 Ma): Distribution map of the formations older than 20 Ma is shown with radiometric ages. 2) Iwaine and Iozen stages (20-17 Ma): Andestic and rhyolitic pyroclastic materials of Early Miocene time are indicated on a map. 3) Kurosedani stage (16.5-16 Ma): After an intensive volcanic episode during the Early Miocene, marine transgression occurred in this region. It is characterized by the tropical/subtropical molluscan Yatuo-Kadonosawa fauna including mangrove swamp plants and Potamid-Arcid molluscs. This sedimentary basin was filled by a great amount of coarse clastic sediments transported by rivers from the south and north. 4) Higashibessho stage (16-15 Ma): The sedimentary basin of the former stage became rapidly wider and deeper, and subbathyal homogenous black muds were deposited. 5) Lower Otogawa stage (15-14 Ma): The deep marine basin of the Higashibessho stage became shallower because coarse clastic sediments and calcareous sandstones were deposited. A warm current flowed into the basin as in the preceding stage. 6) Upper Otogawa stage (13.5-8 Ma): The warm marine environment abruptly cooled at about 13.5 Ma, Silts and sands were deposited in the central and northen parts of the present Noto Peninsula. The sea became narrower, partly changing to land area. This stage is characterized by the shallow and cold Otogawa fauna. 7) Ao stage (6.5-3.5 Ma): The whole area of the Noto Peninsula became land, and the sedimentary basin became narrower than during the former stage. Sands and silts were deposited under a cold current environment, 8) Yabuta stage (3.5-1.5 Ma): The cold current still flowed into the basin and an open sea condition prevailed. Siltstone and sandstones in the basin contain deep sea molluscs. 9) Omma stage (1.5-0.8 Ma): The basin became narrower than in the preceding stage and was under a cold current with an occasional warm current invasion from the south. This stage is characterized by the cold shallow "Omma-Manganji fauna".