(1) Considering the stratigraphic sequence and geologic structure of the Middle Miocene formations, the Sidara structural basin is divided into four units: the central, the northern, the southern and the eastern Parts., (2) The basement complex is the Ryoke Metamorphic Rocks and unconformably overlain by the Miocene formations., (3) The Tertiary is classified into two groups, the Hokusetu and the Nansetu., The Hokusetu group, composed mainly of clastic sediments, is further divided into four formations, the Taguti, the Kawakado, the Simoda and the Tubogawa in ascending order., The Nansetu group overlies unconformably on the Hokusetu group and comprises pyroclastic and lava flows., (4) The sequence of the volcanic activities in the area is as follows: (a) the eruption of rhyolite and dacite contained in the Nansetu group, (b) the intrusion of andesite sills which have been silicified after the intrusion, and (c) the eruption of andesite and basalt after the formation of the basin structure of Sidara., (5) In the eastern part of the basin the Nansetu group is absent and the Hokusetu group shows a small and rather flat basin structure separated from the major basin of the central part., This minor basin is formed by three fault systems, contemporaneous with each other: N 30°W, N 30°∼50°E and N 10°E in directions., The minor basin, moreover, is abundant in silicified andesite sills., (6) In contrast with the major basin, which has relation with the accumulation of the Nansetu group, the minor basin is caused by the faults which occurred in the Tobe Phase of the Median Line.,
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