Based on newly obtained K-Ar whole rock ages of intrusive rocks and previous geological data, the spatial and temporal transition of Tertiary magmatism in Central Hokkaido is summarized as follows: Hidaka magmatism (HM) from 43 (or 53) Ma to approximately 17 Ma; 'Urakawa-Samani'magmatism (USM) at approximately 16-18 Ma; Pipairo-Toyokoro magmatism (PTM) at approximately 15 Ma or from 15 to 12 Ma; andesitic magmatism in northern Central Hokkaido (AMNCH) from 14 to 10 Ma; and Kamishiyubetsu magmatism (KM) at approximately 10 Ma. The following four points are discussed, in relation to the magmatic history mentioned above. (1) Plutonic bodies, emplaced at various levels in the Hidaka Magmatic Zone (HMZ) during the HM, are observed in the Pankenushi-Memuro area with the emplacement level of plutons becoming deeper towards the west. The Nisshoutouge granitic pluton, the shallowest in the area, shows concordant Rb-Sr and K-Ar ages of approximately 17 Ma, it has therefore been concluded that this age represents the time of emplacement. The Pankenushi intrusion, the deepest gabbroic body in the area, exhibits a similar K-Ar age. We note the possibility that deeper level plutonic rocks were rejuvenated by the emplacement of younger granitic rocks at approximately 17 Ma. (2) It is established that the age of the USM in the west of the HMZ partly overlaps the HM, although the tectonic relationship between these magmatisms remains unsolved. (3) The PTM, alkalic and anorogenic magmatism in southern Central Hokkaido, temporaly overlaps the AMNCH. However, the analyzed material indicating an age of 12 Ma is partly altered, therefore it is likely that the PTM is restricted in age to approximately 15 Ma, indicating no overlap between magmatisms. (4) The AMNCH and the KM are gradational in space and time, and the KM probably occurred as the AMNCH migrated southward.

抄録全体を表示

The Tertiary deposits are studied, which are widely distributed in the eastern marginal part of the Backbone range, from western part of Morioka to Hanamaki City. And the new stratigraphic division is proposed, that is, Lower Miocene Mizuwake, Azumaneyama and Yuzawamori Formations, Middle Miocene lioka and Isarizawa Formations and Upper Miocene Masuzawa Formation, in ascending order. The former three formations (pre-Nishikurosawa stage) are named and defined from the discovery of surface distribution, hitherto which were known only in bore hole samples (Okami et al., 1988). They have been regarded as parts of the lioka and/or Osuke Formations by Hayakawa et al. (1954) and Murai (1960). The Mizuwake and Yuzawamori Formations consist of pyroclastic deposits which are strongly welded in part. From the lithologic characters and stratigraphic relationship, they can be correlated to the Lower Miocene welded tuffs distributed in many areas of the Tohoku district. So the two newly discovered welded tuffs in this area can be good key beds of the Lower Miocene sequences which are barren for leading fossils, in central to northern Tohoku district. The acidic pyroclastics are prevailed in the sequence above the lioka Formation (Nishikurosawa stage). They have been divided in western Morioka area, into the Shimosaruta. Osuke and Yabitsu Formations in ascending order (Murai, 1960), and they have been also divided into Osuke and Yuguchi Formation (Hayakawa et al., 1954) in western Hanamaki area. However, the present study clarifies on the basis of the field survey that they are in interfinger in the central part of the studied area. By it the acidic pyroclastics are combined to one unit, namely the Isarizawa Formation. And the Osuke Formation is treated as a member of the Isarizawa Formation. On the basis of the zonal distribution of the zeolites, it is explained that the alteration of the acidic pyroclastics of the Isarizawa Formation have been formed by the diagenesis.

抄録全体を表示

The Ishikari Lowland is situated between Western and Central Hokkaido. In Kuriyama and Kurisawa towns in the eastern marginal area of this lowland, thick marine Upper Cenozoic strata are distributed. They are a part of the thick sedimentary rocks widely distributed in an N-S direction in Central Kokkaido. These Middle Miocene Sedimentary rocks and deduced to have been deposited in a basin formed due to the collision and obduction of the Central Hokkaido Block to the Eurasian plate, which also resulted in the uplifted northern part of Central Hokkaido. The Hidaka Mountains, situated in the southern part of Central Hokkaido, were considered to have formed due to the collision between the Eastern Hokkaido Block and the southern part of the Central Hokkaido Block in Late Miocene time. Many previous sedimentological studies have been made in the northern part (for example, the Habaro district) and the southern part (for example, the Hidaka Coastal district) of the distribution of the thick sedimentary rocks. The study area is situated in the central part, which is in an important tectonic position between the Western and Central Hokkaido Blocks. In this study, I clarified Upper Cenozoic stratigraphy and gave consideration to the Late Cenozoic history of the basin through paleoenvironmental analysis and sedimentological study. The Neogene in this area is divided into the Takinoue, Kawabata, Iwamizawa, Oiwake and Kiyomappu Formations, in ascending order. The Takinoue Formation (Lower-Middle Miocene) is composed mostly of mudstone of deep sea origin, which overlies shallow marine sandstone. The Kawabata Formation (Middle Miocene) is composed of sediments of gravity flow origin, deposited in an N-S trending deep trough. The detrital material was derived mainly from the northern part of Central Hokkaido, as in the cases of the Kotanbetsu Formation in the Haboro district and the Ukekoi Formation in the Hidaka Coastal district. The Iwamizawa Formation (Middle Miocene) consists of bedded siliceous rocks, deposited in an N-S trending deep basin. The Oiwake Formation (Middle-Upper Miocene) consists mostly of sandy mudstone containing lenses of conglomerate, deposited in an open-deep basin, The clastic sediments were supplied mainly from the northern part of Central Hokkaido, as during deposition of the Kawabata Formation. Such provenance is different from that in the Hidaka Coastal district, where clastic sediments were supplied from the Hidaka Metamorphic Belt. This may be ascribed to submarine topography. These strata of Miocene age were deposited in a deep geosynclinal basin. The Kiyomappu Formation (Pliocene) consists of shallow marine tuffaceous sandstone. The Quaternry, consisting of terrace deposits including the Moseushi Formation, and alluvial deposits, rests on the Neogene with an unconformity.

抄録全体を表示