10:00 AM - 10:15 AM
[HQR04-05] Stratigraphic division, age and distribution pattern of the Middle to Upper Pleistocene Shimosa Group in central Tokyo area
Keywords:Middle Pleistocene, Upper Pleistocene, Stratigraphy, 3D geological map, Tokyo Formation, Shimosa Group
The Middle to Upper Pleistocene deposits, mainly composed of shallow marine and fluvial sediments, are distributed in the upper subsurface of the central Tokyo area. This Pleistocene deposit has been called “Tokyo Formation” for a long time, and especially “Tokyo Gravel Bed” at the basal part of “Tokyo Formation” has been paid attention from the viewpoint of applied geology because of its importance such as a bearing layer for a mid–high rise buildings, an important aquifer for groundwater utilization, and a key bed for active tectonics analysis. However, the detailed age of “Tokyo Formation” has remained undetermined for many decades.
We are preparing a 3D geological map of the central Tokyo area (Urban Geological Map: Central Tokyo Area), which is scheduled to be released in April 2021, as part of GSJ project to develop geological information on urban area. In order to develop the Urban Geological Map, we have re-examined the stratigraphy and age of “Tokyo Formation” based on reconstruction of depositional environments according to facies analysis, tephra analysis, and pollen assemblages of borehole cores (Nakazawa et al. 2019, 2020: Naya et al. 2020). The results indicate that the strata previously considered to be “Tokyo Formation” are a mixture of several depositional cycles formed under the influence of eustatic sea-level changes in different periods. In the Middle to Upper Pleistocene Shimosa Group of Boso Peninsula, a sedimentary cycle formed by a single transgression–regression event is defined as a single formation. In this study, the criteria of the Shimosa Group in Boso Peninsula are applied to the stratigraphic division of the central Tokyo area, and the previous “Tokyo Formation” is subdivided into three formations; the Yabu, Kamiizumi, and Tokyo formations. In addition, based on this stratigraphic division, a 3D geological model was created using a large amount of borehole data over a wide area, and the distribution pattern of each formation was also clarified. In this presentation, we introduce an overview of the newly applied stratigraphic division, age and distribution pattern of the Shimosa Group in the central Tokyo area.
Nakazawa et al. (2019) The Journal of the Geological Society of Japan, 125, 367–385.
Nakazawa et al. (2020) Bulletin of the Geological Survey of Japan, 71, 19–32.
Naya et al. (2020) The Journal of the Geological Society of Japan, 126, 575–587.
We are preparing a 3D geological map of the central Tokyo area (Urban Geological Map: Central Tokyo Area), which is scheduled to be released in April 2021, as part of GSJ project to develop geological information on urban area. In order to develop the Urban Geological Map, we have re-examined the stratigraphy and age of “Tokyo Formation” based on reconstruction of depositional environments according to facies analysis, tephra analysis, and pollen assemblages of borehole cores (Nakazawa et al. 2019, 2020: Naya et al. 2020). The results indicate that the strata previously considered to be “Tokyo Formation” are a mixture of several depositional cycles formed under the influence of eustatic sea-level changes in different periods. In the Middle to Upper Pleistocene Shimosa Group of Boso Peninsula, a sedimentary cycle formed by a single transgression–regression event is defined as a single formation. In this study, the criteria of the Shimosa Group in Boso Peninsula are applied to the stratigraphic division of the central Tokyo area, and the previous “Tokyo Formation” is subdivided into three formations; the Yabu, Kamiizumi, and Tokyo formations. In addition, based on this stratigraphic division, a 3D geological model was created using a large amount of borehole data over a wide area, and the distribution pattern of each formation was also clarified. In this presentation, we introduce an overview of the newly applied stratigraphic division, age and distribution pattern of the Shimosa Group in the central Tokyo area.
Nakazawa et al. (2019) The Journal of the Geological Society of Japan, 125, 367–385.
Nakazawa et al. (2020) Bulletin of the Geological Survey of Japan, 71, 19–32.
Naya et al. (2020) The Journal of the Geological Society of Japan, 126, 575–587.