“…On the other hand, there were changes in sedimentation rate caused by the lower accumulation rate of the prodelta sediment (5.4 mm/yr) compared to the rates of the estuary (13.4 mm/yr) and delta front (7.2 mm/yr). Similar changes in accumulation curves in transgressive sediments were also reported on the western margin of the Niigata Plain (Tanabe et al 2010), central Japan, around an active thrust with a 3 mm/yr vertical slip rate (Nakanishi et al 2010). Additional analyses of 14 C dates, diatom assemblages, and grain size will be needed to evaluate the abrupt changes in the trasgressive environments in the Oita area. Figure 4 (A) Geologic and geographic cross-section along the Oita River.…”
Beppu Bay is located on east-central Kyushu, southwest Japan, and is characterized by hydrothermal activity, tectonic deformation, and recent volcanism under the influence of convergence of the Philippine Sea plate. This area, occupying the western portion of an arc-bisecting dextral fault system, is a tectonic depression that has existed since ca. 5 Ma. Sedimentary facies, mollusk assemblages, and radiocarbon (14C) ages of 25 terrestrial plant fragments and 16 marine carbonate shells from a 70-m drill core were determined to estimate the activity of the Funai Fault, which consists of normal faults along the southern margin of the tectonic basin. Based on the analysis, six sedimentary facies, namely braided river channel, estuary, prodelta, delta front, delta plain, and artificial soil, were identified. The vertical slip rate was calculated as 2.6–2.7 mm/yr based on displacements of the braided river channel sediments of the last glacial period and the base of Kikai-Akayoya tephra in the Holocene highstand sediments of this area. Reservoir ages during 6180–10,410 cal BP were determined from marine shell and terrestrial plant pairs from the sediments of the estuary, prodelta and delta front facies, and were correlated with values from a northern coast of Kyushu and the Korean Peninsula.
“…On the other hand, there were changes in sedimentation rate caused by the lower accumulation rate of the prodelta sediment (5.4 mm/yr) compared to the rates of the estuary (13.4 mm/yr) and delta front (7.2 mm/yr). Similar changes in accumulation curves in transgressive sediments were also reported on the western margin of the Niigata Plain (Tanabe et al 2010), central Japan, around an active thrust with a 3 mm/yr vertical slip rate (Nakanishi et al 2010). Additional analyses of 14 C dates, diatom assemblages, and grain size will be needed to evaluate the abrupt changes in the trasgressive environments in the Oita area. Figure 4 (A) Geologic and geographic cross-section along the Oita River.…”
Beppu Bay is located on east-central Kyushu, southwest Japan, and is characterized by hydrothermal activity, tectonic deformation, and recent volcanism under the influence of convergence of the Philippine Sea plate. This area, occupying the western portion of an arc-bisecting dextral fault system, is a tectonic depression that has existed since ca. 5 Ma. Sedimentary facies, mollusk assemblages, and radiocarbon (14C) ages of 25 terrestrial plant fragments and 16 marine carbonate shells from a 70-m drill core were determined to estimate the activity of the Funai Fault, which consists of normal faults along the southern margin of the tectonic basin. Based on the analysis, six sedimentary facies, namely braided river channel, estuary, prodelta, delta front, delta plain, and artificial soil, were identified. The vertical slip rate was calculated as 2.6–2.7 mm/yr based on displacements of the braided river channel sediments of the last glacial period and the base of Kikai-Akayoya tephra in the Holocene highstand sediments of this area. Reservoir ages during 6180–10,410 cal BP were determined from marine shell and terrestrial plant pairs from the sediments of the estuary, prodelta and delta front facies, and were correlated with values from a northern coast of Kyushu and the Korean Peninsula.
“…The earliest evidence for the Holocene sea invasion into Osaka Bay is at 11 000 cal a BP (Masuda et al ., ). The eustatic sea‐level at 11 000 cal a BP has been reported from many places to range from about −49 to −52 m (Edwards et al ., ; Bard et al ., ; Tanabe et al ., ), and about −44 to −45 m (Peltier and Fairbanks, ; Tanigawa et al ., ). Therefore, an elevation of the sill for Osaka Bay may be about −44 to −52 m (−48 ± 4 m) during the earliest Holocene.…”
Section: Geological and Hydrographical Setting And Core Sitementioning
Detailed sea‐level variation was investigated for Marine Isotope Stage (MIS) 19, based on diatom and grain size analyses of a marine sequence in a core of the Osaka Group. Diatom sea‐level proxies represent precession‐related signals correlated with highstands MIS 19.3 and 19.1, and lowstand MIS 19.2. Astronomical tuning shows the marine sequence has a uniform accumulation rate of about 60 cm ka−1. A rapid sea‐level fall event was found in the earliest MIS 19, demonstrated by several independent sea‐level proxies of diatom and grain size. This event began with a rapid sea‐level drop, followed by a gradual recovery, at about 783–782 ka. A maximum abundance of pelagic diatom taxa at a core depth of 402.20 m evidently shows the highest sea‐level peak in MIS 19, supported by many other proxies. Based on the diatom data, sea‐level change across MIS 19.1 is characterized by centennial to millennial fluctuations. The sea‐level fall event began just after the onset of a cooling event previously reported from the same core. Observations of a comparable sea‐level fall signal in many deep‐sea core records suggest the event is global.
“…For these reasons, in the present study area, it is considered that deglacial sea-level changes were the major factor controlling the changes of river gradient. Before and after the YD, sea-level rose at ca 20 mm yr À1 and the sediment accumulation rate in cores GS-KM-1 and GS-KSM-1 was ca 10 mm yr À1 , whereas during the YD, sea-level rose at a rate of ca 4 mm yr À1 and sediment accumulated at 3 mm yr À1 (Tanabe et al, 2010b). Thus, before and after the YD, the rate of sea-level rise exceeded the rate of sediment accumulation, resulting in a gentle river gradient.…”
Spatial and quantitative analysis of infilling processes of the tide-dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post-Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea-level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet-like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea-level rise of 4 to 7 mm yr À1 . Before migration of the Tone River at 5 ka, the tide-dominated bay in the Nakagawa Valley was filled by upward-fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide-dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward-coarsening deltaic sediments.
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