Abstract
The Maizuru Terrane in the Tatsuno-Aioi area, Hyogo Prefecture is composed of the Upper Permian Tatsuno Formation and the Middle Triassic Hiraki Formation, besides the Yakuno Complex. Both formations are redefined based on fossil evidence from the limestone. They are correlatable to the Middle and Upper formations of the Maizuru Group and the Waruishi Formation of the Yakuno Group in the type area, respectively. The Hiraki Formation consists mostly of mudstone and sandstone and contains limestone blocks less than 20 m in longer diameter in three horizons. Limestones consist of bioclastic oolid grainstone/packstone with many detrital quartz grains, and arrange irregularly regardless the general stratification of the formation. Mode of occurrence and lithologic characters of limestone blocks of the Hiraki Formation are closely similar to those of the Kaizawa Formation and Tanoura Formation. Eight species of foraminifers including Meandrospira dinarica and M. pusilla are identified from limestone blocks of the Hiraki Formation. Taxonomic diversity of the Hiraki foraminiferal fauna is exceedingly low in comparison with the Kaizawa and the Tanoura faunas. Along with low taxonomic diversity, absence of Pilammina densa and involutinid foraminifers, which are characteristic in Kaizawa, Tanura and other late Anisian (Pelsonian and Illyrian) faunas, suggests early Anisian age (Aegean) of the Hiraki fauna.
