The present otariid fossil was discovered by high school teachers in 1986, at the river side in the Horonitachibetu. Those bones were yielded at the base of Horokaoshirarika Formation, which was determined to be formation ranges Early Pliocene by the fission trach method (KIMURA et al., 1987). The specimen has several primitive characters with respect to the right mandibular cheektooth row and the alveolus of the cheektooth, distinct from those of modern otariid. Observing the alveolusli, it become clear that P1 shows record of single rootendness and both of P2 and P3 are duble rootendness.

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The Koto Rhyolites in Shiga Prefecture, Southwest Japan, have undergone hydrothermal alteration, whose character is propylitic and deuteric. Alkali feldspar phenocryst, which is replaced more or less by secondary minerals, shows various textures, most of which are perthitic. Microscopically homogeneous clear feldspar of about Or_<68> survives within altered perthitic feldpar. Submicroscopic perthitic texture is observed in the microscopically homogeneous clear part. The compositions of Or-rich and Ab-rich feldspars in perthitic texture are almost of end members, which are comparable to those in granites. The somewhat Ca-loss during the change of primary alkali feldspar to perthitic one is recognized. Or-rich halo with a compositional gradient develops along cracks and surrounding Ab-rich parts. Microscopic perthitic textures are irregularly developed where turbidity and secondary alteration minerals are present. The degree of coarseness of perthitic textures corresponds to the grade of hydrothermal alteration of alkali feldspar. The proportion of the constituent two feldspars of coarse perthitic textures is variable in each crystal. From these facts, it is reasonable to consider that the microscopic perthitic textures of the alkali feldspar in the Koto Rhyolites were formed mainly by metasomatism (replacement) rather than only by the simple coarsening of primary exsolved cryptoperthites during hydrothermal alteration.

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The type I Suite, one of the doubly piled tectonostratigraphic units consisting of the Tamba Group, is extensively distributed in the investigated area. The suite is composed of coarsening-upward succession including bedded chert (50 m+), greenish-gray siliceous shale (60 m), sandstone-shale alternation (410 m), black shale (550 m) and melange (muddy mixed rock; 1100 m+) in ascending order. Lithologic change of the succession, from the base upward, shows a shift of sedimentary environments from a deep-sea basin to a subduction zone. Lithologic nature of the succession is described in detail. Especially, sedimentological and petrographical features of the sandstone-shale alternation show that these strata settled as the typical turbidite sequence on the middle to outer part of the deep-sea fan. Mineralogical composition of the sandstone in this suite is discriminated from that of the Type II Suite, another tectonostratigraphic unit of the Tamba Group, by the relative abundance of quartz grains and the slight amounts of garnet grains. The age of clastic sediments in the Type I Suite is late Late Jurassic on the basis of radiolarian biostratigraphy. This suite can be compared to the Kanayama unit defined by Wakita (1988b) in the Mino Belt.

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