北部石灰アルプスのジュラ系とその国際境界模式層序・位置

This paper summarises the recent progress and current status of research undertaken on the Jurassic strata of the Northern Calcareous Alps. The Jurassic GSSP (Global Boundary Stratotype Section and Point) at the Kuhjoch section of the Northern Calcareous Alps is also explained in detail. The base of the Jurassic strata is defined at this location by the occurrence of the oldest known Jurassic ammonite, Psiloceras spelae tirolicum Hillebrandt & Krystyn. The Upper Triassic to Lower Jurassic successions of the Northern Calcareous Alps were developed along the passive continental margin of the Neotethys Ocean. In Middle Jurassic time, a change in the plate tectonic setting influenced the sedimentary facies, which suggest northwest-verging nappe stacking in association with the partial closure of the Neotethys Ocean. Deep-water radiolarite basins developed in the area in front of the advancing nappes and were the sites of mass flow deposits that produced olistoliths of various sizes. These olistoliths were included in the radiolarite matrices. Therefore, the use of radiolarian fossils as a dating method plays an important role in understanding the formations of the Northern Calcareous Alps.

ジュラ系の国際境界模式層序・位置がオーストリア国チロル州のクーヨッホ層序断面に置かれることが2010年に正式に決まった．それはジュラ紀最古のアンモナイトPsiloceras spelae tirolicumが初産出する層準で，ケンドゥルバッハ層の基底から5.8m上位に位置する．筆者らは2012年に行われた国際堆積学会の巡検でクーヨッホ層序断面を訪れたので，その概要を紹介する．
ジュラ系の国際模式境界が置かれた北部石灰アルプスでは，近年の研究でオリストリスの基質をなす珪質堆積岩からジュラ紀の放散虫化石が数多く報告され，年代決定に有効なことが示された．本稿では特に研究が進んでいるザルツカンマーグート地方の代表的なジュラ紀層を紹介し，北部石灰アルプスの地質構造発達史を概観する．