秋田県南部の地質構造発達史
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概要
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The geology of the mentioned area which occupies the central part of Northeast Japan represents the Cenozoic tectonic history of the Northeast Honshu Arc. Through the detail field survey, chronostratigraphic and microbiostratigraphic studies, the stratigraphy of this area was established as shown in Table 1. The tectonic history of this area are divided into eight Stages. The characteristics of each stage are summarized as follows. Island arc tectonics started off with onland and subaqueous stratovolcanos of pyroxene andesite (Stage I; isotopic age:26-33 Ma) along NW-trending fracture zones. It was followed by the eruption of dacitic welded tuff on the NW-trending uplifting zones and pyroxene andesite of Stage II (isotopic age:19-22 Ma). Stage I and II are characterized by the Anial type flora. Marine transgression started definitely at Stage III. The NW-trending subsiding and uplifting zones with 30 km wavelength became distinct. Volcanism changed into bi-modal one; huge amount of lava and pyroclastics of basalt and rhyodacite. Mudstone with foraminifera of N8 zone and welded tuff of around 15 Ma isotopic ages are characteristic. The marine transgression progressed through Stage IV, and mudstone with foraminifera of N9-11 zones deposited under the bathyal condition. The bi-modal volcanism has continued. Sedimentation rate was high. NNE-trending sedimentary basins developed embryonically. The formations of Stage III and IV yield the Daijima type plant fossils. During Stage V, the marine transgression reached the acme and then regression started. Hard mudstone deposited in the backarc side and coarser clastic sediments accumulated on the west of the Ou Backbone Range which started uplifting. Sedimentation rate was very slow. The structures trending in NNE-SSW became distinct in the later half of this stage. The age of this stage is well defined by diatom fossils of Denticulopsis praedimorpha zone to Thalassionema schraderi zone. Through Stage VI, the sedimentary basins have been differentiated. The Ou Backbone Range has continued uplifting, and huge amount of rhyo-dacitic tuffs and lacustrine sediments unconformably covered the older rocks there. These are treated gradually toward the backarc region where mudstone and sandstone deposited. Hyaline augite-hypersthene andesite (isotopic age:around 8 Ma) was erupted there. At the Stage VII, uplifting of the Ou Backbone Range completely separated the sedimentary basin into the backarc and forearc regions. The differentiated basins in the backarc region have been diminished. This stage is characterized by the Miyata and Sanzuguwa floras. The marine of the backarc side retreated to the Japan Sea coastal region in Stage VIII. The shallow marine invated on the east of the Ou Backbone Range. The andesite and dacite volcanism (isotopic ages:around 3. 7 Ma) was occurred.
- 1989-03-15