Sedimentological Analysis of the Neogene Basins in the Central Part of the Northern Fossa Magna Region, Central Japan

Abstract

The Development of the Neogene sedimentary basins in the Northern Fossa Magna region was examined mainly on the basis of stratigraphic and sedimentologic viewpoints. The Fossa Magna region, an important geologic province which divides the Japanese Islands into Northeast Japan and Southwest Japan, is characterized by strong volcanism and subsidence which resulted in very thick Neogene volcaniclastic and terrigenous accumulation. The western boundary of the region is marked by a remarkable fault called Itoigawa-Shizuoka Tectonic Line, and the eastern one is called Kashiwazaki-Choshi Tectonic Line. Further, this region is divided into two parts, north and south. The basin analysis indicates that the Northern Fossa Magna region is included in the inner arc of Northeast Japan, being separated by the central massif from the Southern Fossa Magna region which is believed to be a continuation of the Izu-Bonin arc. Thick Neogene sequence in the Northern Fossa Magna region is composed mainly of flysch-type alternations of sandstone and mudstone, mudstones, thick-bedded sandstones, large-scale cross-lami-nated sandstones and conglomerates with intercalations of volcanic rocks, and altered pyroclastics. These are divided into seven formations, the Moriya, Uchimura, Bessho, Aoki, Ogawa, Shigarami and Sarumaru Formations in ascending order. The former five belong to Miocene and the latter two to Pliocene. Based on the lithofacies characters, three stages of geologic development are discriminated, that is, initial volcanism stage (Moriya and Uchimura Formations), flysch stage (Bessho and Aoki Formations), and molasse stage (Ogawa, Shigarami and Sarumaru Formations). Lower Neogene formations of gently folded are distributed in the Central (southern) and Otari (northern) Upheaval Zones, whereas thick and moderately folded formations of middle to upper Neogene are distributed in the Subsidence Zone situated between the above two Upheaval Zones. Three coarsening-upward mega-sequences can be recognized in the Neogene strata. Each sedimentary sequence is composed of several formations which begins with mudstone or muddy alternations, and terminates with massive and/or thick-bedded sandstone and conglomerate through sandy alternation. The first mega-sequence indicates the extinction of the initial volcanic basin, and second one corresponds to the transition from flysch to molasse stage, reflecting gradual filling-up of the southern half of the molasse basin. The third one corresponds to the extinction of the molasse basin. Paleocurrents shown by sole markings in the flysch sequence and clastic intercalations of initial volcanism stage are generally northeastward. In the molasse stage the paleocurrent directions deduced from the cross-laminations are variable such as from south, east and west. The Neogene geoyncline in the Northern Fossa Magna region is not a simple basin but much complicated one constituted by many small basins, therefore, thickness variation of each formation is conspicuous. The first-order migration is northwestwardly from initial volcanism site to flysch and molasse sedimentation site, and such migration can be recognized throughout the Inner arc province of Japan and is assigned to be general in the course of Neogene sedimentary basins. Second- and third-order migration in the molasse basin is a movement of rather small-scale and the trend of migration is a reflection of local upheaval within the Shin'etsu mega-sedimentary basin.