5:15 PM - 7:15 PM
[MIS11-P08] Estimation of paleoenvironments and seismic crustal deformation at northern part of Beppu Bay, Oita Prefecture
Keywords:paleo-environmental reconstruction, tsunami deposit, diatom analysis, Radiocarbon dating, Beppu Bay
To investigate the tsunami history associated with submarine faults earthquakes in Beppu Bay, sedimentological surveys of tsunami deposits have been conducted at some coastal lowlands. We studied Ohga marsh of ~4 m high, ~85 m wide, and ~150 m long on the northern coast of the bay. In 2020, a drilling survey was carried out using a Geo-Slicer and obtained seven sediment cores with a maximum depth of 1.8 m (Yamada et al., 2022, JpGU). The sediment cores mainly consist of an organic-rich mud layer, with underlying a basal muddy sand layer identified in the four seaward cores. A distinct sand layer, dated to 6440–6620 cal. yr BP, was interbedded within the organic-rich mud layer at a depth of approximately 1 m in all cores. This sand layer becomes thinner and finer inland, suggesting deposition from a seaward current. Since there are no historical records of storm surges in the surrounding area, and the study site is located far from rivers capable of generating large-scale floods, this layer is interpreted as a tsunami deposit. Above this sand layer, a 15 cm-thick sandy mud layer is consistently present across the site. However, its origin remains uncertain—it may be related to the same event as the sand layer or may result from later environmental changes. When a large earthquake accompanied by a tsunami occurs, significant crustal deformation can induce drastic changes in coastal environments. Detecting such changes is crucial for reconstructing the activity history of the fault system. This study aims to clarify the origins of these deposits through diatom assemblage analysis and additional radiocarbon dating to assess possible seismic crustal deformation.
A sediment core for fossil diatom analysis was collected from a location approximately 70 m inland from the shoreline. The core consists of three lithological units: a basal muddy sand layer (140–160 cm depth), an overlying organic-rich mud layer (60–140 cm), and a surface cultivated soil layer (0–60 cm). The basal muddy sand layer contains Pseudopodosira kosugii, a species considered as an indicator of past sea level associated with the Jomon transgression (Sato, 2014). The overlying organic-rich mud layer is dominated by diatom taxa such as Pseudostaurosira spp. (freshwater–brackish) and Stauroforma spp. (freshwater), indicating a transition from a tidal flat environment to a freshwater or brackish marsh following the deposition of the basal muddy sand layer. Radiocarbon dating of organic materials just above the basal muddy sand layer yielded an age of 6800–7170 cal. yr BP.
The sand layer (89–111 cm depth) contained both freshwater diatoms (Eunotia spp.) and marine diatoms (e.g., Paralia sulcata). The mixed assemblages of freshwater and marine taxa indicates a seawater inundation to the marsh. In contrast, the overlying sandy mud layer (84–89 cm) has diatom assemblages similar to those of the organic mud layer, suggesting it did not result from a seawater inundation. A shift of dominant taxa from freshwater–brackish (Pseudostaurosira spp.) to freshwater (Stauroforma spp.) occurs in the organic-rich mud layers above and below the sand and overlying sandy mud layers, indicating a sudden increase in freshwater influence following their deposition. According to HERP (2005), the rupture of submarine active faults in the bay induces relative uplift along the northern coast. The observed abrupt environmental changes in the diatom assemblages may provide geological evidence supporting this interpretation. Thus, the deposits of Ohga marsh may record not only the tsunami history but also an evidence of rapid environmental changes driven by a seismic crustal deformation.
A sediment core for fossil diatom analysis was collected from a location approximately 70 m inland from the shoreline. The core consists of three lithological units: a basal muddy sand layer (140–160 cm depth), an overlying organic-rich mud layer (60–140 cm), and a surface cultivated soil layer (0–60 cm). The basal muddy sand layer contains Pseudopodosira kosugii, a species considered as an indicator of past sea level associated with the Jomon transgression (Sato, 2014). The overlying organic-rich mud layer is dominated by diatom taxa such as Pseudostaurosira spp. (freshwater–brackish) and Stauroforma spp. (freshwater), indicating a transition from a tidal flat environment to a freshwater or brackish marsh following the deposition of the basal muddy sand layer. Radiocarbon dating of organic materials just above the basal muddy sand layer yielded an age of 6800–7170 cal. yr BP.
The sand layer (89–111 cm depth) contained both freshwater diatoms (Eunotia spp.) and marine diatoms (e.g., Paralia sulcata). The mixed assemblages of freshwater and marine taxa indicates a seawater inundation to the marsh. In contrast, the overlying sandy mud layer (84–89 cm) has diatom assemblages similar to those of the organic mud layer, suggesting it did not result from a seawater inundation. A shift of dominant taxa from freshwater–brackish (Pseudostaurosira spp.) to freshwater (Stauroforma spp.) occurs in the organic-rich mud layers above and below the sand and overlying sandy mud layers, indicating a sudden increase in freshwater influence following their deposition. According to HERP (2005), the rupture of submarine active faults in the bay induces relative uplift along the northern coast. The observed abrupt environmental changes in the diatom assemblages may provide geological evidence supporting this interpretation. Thus, the deposits of Ohga marsh may record not only the tsunami history but also an evidence of rapid environmental changes driven by a seismic crustal deformation.