Discovery of the Early Spathian (Late Olenekian, Early Triassic) Ammonoid Tirolites in the Hiraiso Formation, South Kitakami Belt, Northeast Japan
Discovery of the Early Spathian (Late Olenekian, Early Triassic) Ammonoid Tirolites in the Hiraiso Formation, South Kitakami Belt, Northeast Japan
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DOI：
10.2517/2016pr017
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发表时间：
2017-01
影响因子：
0.9
通讯作者：
Y. Shigeta;Yasuhisa Nakajima
中科院分区：
文献类型：
--
作者：
Y. Shigeta;Yasuhisa Nakajima
抽象的。到目前为止，平所组被认为是完全史密斯（早奥伦尼克，早三叠世）的年龄，但菊石Tirolites cf.在日本东北部南北上带的气仙沼市元吉镇的前河原和赤志港口之间的岩石海岸上的地层的下部中的ussuriensis，确定了该部分实际上等同于下部Spathian Tirolites-Amphistephanus带（上部Olenekian）的上部。Hiraso组的动物群，其中包括大型双壳类Eumorphotis iwanowi和海百合小骨，非常类似于俄罗斯远东南滨海边疆区的Tirolites ussuriensis床，表明在早期Spathian期间，在Panthalassa西侧的低到北方地区广泛分布着这种浅海动物群。由于前斯帕蒂安沉积物中只含有小型的Eumorphotis物种和非常罕见的海百合小骨，大型Eumorphotis物种的出现和早期斯帕蒂安浅海环境中海百合的繁荣表明史密斯-斯帕蒂安边界标志着海洋生态系统的重大变化。
Abstract. Until now, the Hiraiso Formation was considered to be entirely of Smithian (early Olenekian, Early Triassic) age, but the discovery of the ammonoid Tirolites cf. ussuriensis in the lower part of the formation on the rocky coast between the Maekawara and Akaushi ports, Motoyoshi Town, Kesennuma City, in the South Kitakami Belt, Northeast Japan, establishes that this portion is actually equivalent to the upper part of the lower Spathian Tirolites-Amphistephanus Zone (upper Olenekian). The fauna of the Hiraiso Formation, which includes the large bivalve Eumorphotis iwanowi and crinoid ossicles, very much resembles the Tirolites ussuriensis beds in South Primorye, Russian Far East, suggesting a wide distribution of this shallow marine fauna in the low to middle northern latitudes on the western side of the Panthalassa during the early Spathian. Because pre-Spathian deposits contain only small-sized species of Eumorphotis and very rare crinoid ossicles, the appearance of large species of Eumorphotis and the flourishing of crinoids in the shallow marine environment during the early Spathian suggest that the Smithian-Spathian boundary marks a major change in the marine ecosystem.