Stratigraphical Study of the Miocene Series in the Eastern Part of Tottori Prefecture, Southwest Japan

In this paper, the stratigraphy of the Miocene series distributed in the eastern part of Tottori Prefecture is described in detail, with some considerations on correlation and geologic history. The results obtained are summarized as follows.
The eastern part of Tottori Prefecture occupies a part of the so-called Green Tuff region, and is characterized by the development of a thick series of Miocene volcanic and clastic rocks formed by strong volcanism and subsequent subsidence. The Miocene series is called the Tottori Group, and is divided into two formations, i. e., the lower Yazu and the upper Iwami Formation. The Yazu Formation is subdivided into the Koge Conglomer-ate Member below and the Kawabara Volcanic Member above, which are conformable with each other. The Koge Member is the basal conglomerate member of the Tottori Group. The Kawabara Member consists mainly of thick piles of volcanic rocks such as andesite lavas and andesitic to dacitic pyroclastics, with intercalation of layers of clastic sediments. The Iwami Formation has an unconformable relationship with the Yazu For-mation, and is lithologically subdivided into the Entsuji Conglomerate and Sandstone, Moroga Conglomerate, Fuganji Mudstone, Oda Andesite, Aragane Pyroclastic and Shichi-yama Sandstone and Mudstone Members. The first two members are interfingered with the lower part of the Fuganji Member, while the last three with the upper part of the Fuganji. The Entsuji and Moroga Members are composed mainly of alternating beds of conglomerate and sandstone. The Fuganji Member is made up of well-stratified mudstone, accompanied with parallel-laminated, fissile one in the lower part. The Oda and Aragane Members consist largely of thick accumulations of volcanic rocks such as andesite lavas and dacitic pyroclastics. The Shichiyama Member is represented by fine- to medium-grained sandstone, accompanied with mudstone and dacitic pyroclastic rocks.
Two molluscan assemblages of relatively deep sea type, namely, the Acesta assem-blage below and the Propeamussium-Delectopecten assemblage above, are recognized in the main part of the Fuganji Member. Judging from the faunal and floral assemblages, as well as from the lithofacies, it is inferred that the sedimentary environment of the Iwami Formation changed from fresh-water condition to brackish-water or shallow marine one and further to the deeper sea. The main part of the Fuganji Member represents deposits of maximum transgression at Middle Miocene time.
The northward tilting of the basement blocks controlled by the uplifting of the island arc has been revealed through the analysis of stratigraphic relationship, geologic structure and lateral change in lithofacies. The uplifting and tilting may be closely related not only to the migration of depocenters of the Miocene and Pliocene sedimentary basins toward the Sea of Japan but also to the longitudinal gravity faulting in horizontally tensional stress field. The initiative uplifting may have begun at Early Miocene time.