上総層群の堆積盆の形成機構

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The Kazusa Group composed of Plio-Pleistocene marine sediments with a total thickness of about 5000 m, occupies an area of 200 km×150 km in the Kanto Plain, Japan. The sedimentary basin of the Group, which is one of forearc basins along the Japan Trench, has a flat floor at a depth of less than 2000 m below the surface. The Group of the Boso Peninsula is able to be separated into the the lower, middle and upper units based on two local unconfomities, and each depocenter of the units is different one another. Moreover, the lower unit thickness off the east coast of the Peninsula and is subdivided into four subunits by difference of depocenters. On the whole of the sedimentary basin of the Group, the depocenter migrates toward more northwest as time goes on. A local unconfomity between the lower and the middle units is observable at the base of the Higashi-higasa Formation filling a submarine channel. The channel is large scale and develops in an axial part of a syncline formed during sedimentation. While the lower unit onlaps the basement in a similar form to offlapping, the middle and upper units lie in parallel with the surface of unconformity. The sediments of the lowermost to the upper units are distributed in the southwestern part of the basin, but lower units are lacking in the central part, and only the upper unit occupies the northern part: the basin spread itself northward and westward about the same time as depocenters migrate. As a result of analisis by Virtual Basement Displacement method which is a system of the basin analisis by tectonic simulation (KODAMA et al., 1985), it be concluded that the migration of depocentar of the Kazusa Group is caused by various movements of basement blocks along nearly vertical fractures. The tectonic evolution of the sedimentary basin of the Kazusa Group can be interpreted by the tectonic model by KODAMA et al. (1981) as follows. During Plio-Pleistocene time tension stress origineted in a borderland between uplifting Japanese Islands and subsiding Pacifie Oeean and concentrated in an area to form the sedimentary basin. Subsequently the concentrating area of tension stress migrates toward the uplifting area, resulting in the migration of depocenter.
日本地質学会の論文
1988-04-25

上総層群の堆積盆の形成機構

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