Stratigraphy of the Lower Cretaceous Tetori Group and stratigraphic implication of plant assemblages in the border area between Ishikawa and Fukui Prefectures, central Japan

Abstract

The stratigraphy of the Tetori Group in the Shiramine area of Ishikawa Prefecture and the Takinamigawa area of Fukui Prefecture has been reexamined, and the plant-bearing horizons stratigraphically compared. The Tetori Group in the study areas is divided into the Gomijima, Kuwajima, Akaiwa, and Kitadani Formations, in ascending stratigraphic order. Plant assemblages from the Akaiwa Formation around Mt. Oarashiyama in the Shiramine area comrise 23 species of 18 genera typified by the abundant occurrence of Tetoritype floral elements including ferns, ginkgoaleans, and conifers. The floral elements are associated with minor Zamites-like macrophyllous bennettitalean (or cycadalean) and microphyllous coniferous foliage represented by Brachyphyllum, indicating a climate with a dry season(s). Previously the occurrence of the Ryoseki-type floral elements was presumed to be consistent with floral change and a warming and drying climate during the deposition of the Kitadani Formation. However, this study suggests that such a trend started from the deposition of the Akaiwa Formation, earlier than previously thought.