10:45 AM - 11:00 AM
[SGL28-07] Discovery of faulted basin margin from the middle Miocene Tottori Grope, Tottori Prefecture, Southwest Japan and its significance
Keywords:Tottori Group, Middle Miocene, Sedimentary basin, Japan Sea opening event
Some Miocene sedimentary basins in the San’in region extend N-S or NE-SW and, have been called the indentation of sedimentary basins. Several studies speculated that these basins were aulacogenes formed in association with the Japan Sea opening event, or extensional basins opened parallel to the rifting axis before the rotation of Southwest Japan. The little sedimentary geological and chronological studies for this type of sedimentary basins remain their evolution to have been still unknown. The precise reconstruction of evolution of these sedimentary basins leads to understanding of the deformation of Southwest Japan crust associated with the Japan Sea opening event and thus to know the concrete Japan Sea opening process. The geological information in the marginal part of such type of basin fills is indispensable in order to elucidate their evolution.
A sedimentary basin, extending almost N-S and called the Tottori Indentation, is spread in the eastern part of Tottori Prefecture, Southwest Japan. Because the sediments of this basin have not been suffered from extensive intrusion of the volcanic rocks in its southeastern part, unlike the other basin fills in the Shimane Prefecture, this is an ideal sediment succession as the target of the sedimentary basin analysis in order to know the evolution of this type of sedimentary basins. The present study conducted geological survey of the Tottori Group using a conventional sedimentary geological method (facies analysis) for reconstruction of the sedimentary environment of the southeastern part of the Tottori Group (Iwami Formation).
The Iwami Formation of the Tottori Group is made up from the Entsuji Conglomerate and Sandstone Member (EM) and Fuganji Mudstone Member (FM). The FM contains both freshwater and marine mudstones. New faults with large displacement on and near the boundary with the underlying rocks are discovered. One represents the boundary between the granitic rock and the EM sandstone with thin vertical veneer of the deformed Kawabara Volcanic Rock Member of the underlying Yazu Formation along the fault. The other is running within the EM sandstone. The presence of gravels of debris fall origin in the FM mudstone near the fault and the talus deposits together with the presence of the deformed Yazu Formation along the fault imply that the fault started to be activated after the Kawabara Member phase and may have been active during the Fuganji period to have formed the slope along the fault. As the result, the block to the west of the fault subsided to have formed a lacustrine basin. Another faults are also expected along the boundary between the basement rock or the Yazu Formation and the Iwami Formation based on the presence of small slump structures indicating a slope that dipped away from the boundary in the southeastern part of the group. Additional study is necessary in order to elucidate the basin type (e.g. strike slip basin or half graben) and to determine the specific timing of basin generation. The present study is financially supported by the Grant-in-aid for scientific research from the San’in Kaigan UNESCO Global Geopark Promotion Council.
A sedimentary basin, extending almost N-S and called the Tottori Indentation, is spread in the eastern part of Tottori Prefecture, Southwest Japan. Because the sediments of this basin have not been suffered from extensive intrusion of the volcanic rocks in its southeastern part, unlike the other basin fills in the Shimane Prefecture, this is an ideal sediment succession as the target of the sedimentary basin analysis in order to know the evolution of this type of sedimentary basins. The present study conducted geological survey of the Tottori Group using a conventional sedimentary geological method (facies analysis) for reconstruction of the sedimentary environment of the southeastern part of the Tottori Group (Iwami Formation).
The Iwami Formation of the Tottori Group is made up from the Entsuji Conglomerate and Sandstone Member (EM) and Fuganji Mudstone Member (FM). The FM contains both freshwater and marine mudstones. New faults with large displacement on and near the boundary with the underlying rocks are discovered. One represents the boundary between the granitic rock and the EM sandstone with thin vertical veneer of the deformed Kawabara Volcanic Rock Member of the underlying Yazu Formation along the fault. The other is running within the EM sandstone. The presence of gravels of debris fall origin in the FM mudstone near the fault and the talus deposits together with the presence of the deformed Yazu Formation along the fault imply that the fault started to be activated after the Kawabara Member phase and may have been active during the Fuganji period to have formed the slope along the fault. As the result, the block to the west of the fault subsided to have formed a lacustrine basin. Another faults are also expected along the boundary between the basement rock or the Yazu Formation and the Iwami Formation based on the presence of small slump structures indicating a slope that dipped away from the boundary in the southeastern part of the group. Additional study is necessary in order to elucidate the basin type (e.g. strike slip basin or half graben) and to determine the specific timing of basin generation. The present study is financially supported by the Grant-in-aid for scientific research from the San’in Kaigan UNESCO Global Geopark Promotion Council.