Abstract
In the northern part of Nagano Prefecture, the Pliocene strata are well exposed. They consist of the Arakurayama, Ogikubo and Sarumaru Formations in ascending order. The Ogikubo Formation, which age is assigned to 43Ma, yields abundant molluscan fossils. Four molluscan associations, showing different environments, are discriminated in the Ogikubo Formation. They are Mya-Macoma Association (intertidal zone), Mizuhopecten-Chlamys Association (upper sublittoral zone), Conchocele Association and Chlamys-Tridonta Association (lower sublittoral to upper bathyal zone). The first shallow association distributes only in the southeast flank of the Orihashi Syncline, whereas the distribution of the last two deeper ones are limited to the northwest. In this area, the shallower associations are recognized at the upper stratigraphic horizon than the deeper ones. The molluscan fauna of the Ogikubo Formation includes many cold-water species, whereas a few warm-water species are found in the Mizuhopecten-Chlamys Association of the southeast flank of the Orihashi Syncline.
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