Shimozaisho Group

The Shimozaisho Group is Carboniferous in age (Konishi, 1954, Kawai, 1959), and consists of the Limestone Formation and the Limestone and Slate Formation, in ascending order (Konishi, 1954; Fig. 2). The Shimozaisho Group is unconformably overlain by the Cretaceous Tetori Group and Tertiary volcanic rocks (Maeda, 1957).

Figure 2. Geological map of Paleo-Mesozoic systems in the Shimozaisho area (after Konishi, 1954; Maeda, 1957)

The base of the Shimozaisho Group is not exposed. The Shimozaisho Limestone Formation yields Late Carboniferous fusulinid fossils (Konishi, 1954). Low-grade metamorphism of unknown age has affected these rocks, changing limestones into marbles and Fe-/Al-rich deposits into chloritoid-bearing metapelites.

The Limestone and Slate Formation is composed of metaclastic rocks with intercalated meta-mafic tuffs, and marbles (Fig. 3).

Chloritoid occurs as porphyroblasts in some metaclastic layers. Relict sedimentary structures such as cross lamination and normal grading are observed in metaclastic layers (Fig. 4A).

Marbles are composed of carbonate clasts including bioclasts and partly show normal grading. Crinoids are observable in recrystallized limestones. Meta-mafic tuffs are composed of chlorite, clay minerals and opaque minerals, partly including hematite. Metamorphic minerals in meta-mafic tuffs include chlorite and hornblende.