日本早白垩世非海相软体动物群组合

THE EARLY CRETACEOUS NON-MARINE MOLLUSK FAUNAL ASSOCIATIONS OF JAPAN

日本具有4个标志早白垩世的半咸水和淡水软体动物群组合:欧特里沃期—早巴列姆期的立川和菖蒲动物群组合,晚巴列姆期的濑林动物群组合和早阿普特期动物群组合。纯非海相下白垩统大量分布于日本东北部(本州岛北部)的部分地区,以及日本中部、西南部的内带。在日本东北部的部分地区以及日本西南部的外带(四国岛与本州岛中部),从巴列姆阶下部往上有一些海相地层夹在非海相层中,这为研究非海相动物群提供了准确的年代依据。生物组合之间在种一级水平上的区别,主要由环境的差异而引起。1)立川动物群组合(欧特里沃期—早巴列姆期),含有半咸水Hayamina naumanni组合和淡水Megasphaerioides okurodaniensis组合;手取群的Okurodani、Kuwajima和Izuki三个组内的淡水软体动物可与本动物组合相对比;2)菖蒲动物群组合(欧特里沃期—早巴列姆期),含有Eomiodon nipponicus组合;3)濑林动物群组合(晚巴列姆期),含有Costocyrena radiatostriata组合,可与手取群的Kitadani组和日本东北部关门群的Sengoku组以及Monomiyama组的淡水软体动物组合相对比;4)日比原动物群组合(早阿普特期),含有Costocyrena minor组合,关门群Wakamiya组的淡水软体动物可与本动物组合相对比。

Four faunal associations of brackish and freshwater mollusks characterize the Early Cretaceous in Japan： Hauterivian-Early Barremian Tatsukawa and Sh0bu-faunal associations, Late Barremian Sebayashi-faunal Association and Early Aptian Hibihara-faunal Association. An exclusively nonmarine Lower Cretaceous prevails in parts of North- east Japan （North Honshu） and over the Inner Zone of Central and Southwest Japan. From the Late Barremian upwards, several marine tongues are interspersed with the non-marine sequences, in part of Northeast Japan and in the Outer Zone of Southwest Japan （Shikoku and Central Honshu）, providing accurate age constraints on the well-studied nonmarine faunas. Differences between assemblages, at the specific level, are attributable to environmental variabilities. 1） Tatsu- kawa-faunal Association （Hauterivian-Early Barremian）, yielding the Hayamina naumanni brackish-water and the Megasphaerioides okurodaniensis freshwater assemblages; the fresh-water mollusks of the Okurodani, Kuwajima and Izuki formations of the Tetori Group being related to the Tatsukawa-faunal association; 2） Shobu-faunal Association （Hauterivian-Early Barremian）： yielding the Eomiodon nipponicus Assemblage; 3） Sebayashi-faunal Association （Late Barremian）： yielding the Costocyrena radiatostriata Assemblage, showing a relationship with fresh-water mollusks in the Kitadani Formation of the Tetori Group, Sengoku Formation of the Kanmon Group and Monomiyama Formation of Northeast Japan; 4） Hibihara-faunal Association （Early Aptian）： yielding the Costocyrena minor Assemblage, the fresh water mollusks from the Wakamiya Formation of the Kanmon Group being related to the Hibihara-faunal association.