5:15 PM - 6:45 PM
[SGL18-P06] Depositional age and origin of chaotic beds, Paleogene Muroto Formation, Shimanto Belt
Keywords:Shimanto Belt, Muroto Formation, Paleogene, chaotic beds
Gravitationally unstable sediments on the seafloor can migrate offshore in masses. The chaotic beds formed by this phenomenon are collectively called mass–transport deposits (MTDs). MTDs reflect the topography, change, depositional environment, etc., at the time of deposition of the strata (e.g., Chanier and Ferriére, 1991; Festa et al., 2022). Those are important information for estimating the depositional environment of the strata. In addition, the formation process is a key to understanding the movement that caused the deformation (e.g., Strachan, 2008).
In the southern part of the Shimanto Belt, several strata that contain many large–scale chaotic beds originating from MTD have been widely recognized (e.g., the Muro Group in Wakayama Prefecture, the Muroto Formation in Kochi Prefecture, and the Nichinan Group in Miyazaki Prefecture). The study of these formations will lead to the estimation of the factors that caused the MTD and the plate convergence area at that time. In this study, we focus on the Muroto Formation to obtain more detailed information on the deformation process and depositional age of the chaotic beds.
The field survey revealed that the Muroto Formation consists of several chaotic beds, which account for about 40% of the main layer exposed on the beach. The thickness of individual chaos beds ranges from 1–25 m at Cape Gyoto to more than 100 m at the large scale observed at Kuromi Coast. The Muroto Formation has the following characteristics as MTD: (1) They show a certain succession, although some parts are lacking, sandy mudstone and sandy pebbly mudstone formations are present in the lower part, and sandstone–shale alternation with clastic intrusion is accumulated in the upper part. (2) soft–sediment deformation representing layer–parallel extension and shortening, (3) The clastic intrusion observed in the entire area indicate that they were formed at the same time as the deformation of the turbidite beds, based on the relationship between the intrusions and cuttings. The direction of movement inferred from the deformation structure within the MTD at Cape Gyoto is parallel to the direction of paleo–current indicated by the surrounding strata. The lithological and deformation characteristics indicate that these MTDs were formed by migration by slides and slumps.
U–Pb dating of zircons extracted from the siliciclastic tuff and sandstone of the Muroto Formation show that the youngest peaks correspond to the Early Oligocene (31–33 Ma). These ages are within the range of the late Eocene to Oligocene age inferred from fossil ages, which further constrains the depositional age of the rocks.
These results indicate that MTDs in the Muroto Formation occurred after the Early Oligocene. These occurrences may have been related to increased sediment supply and changes in slope angle .
In the southern part of the Shimanto Belt, several strata that contain many large–scale chaotic beds originating from MTD have been widely recognized (e.g., the Muro Group in Wakayama Prefecture, the Muroto Formation in Kochi Prefecture, and the Nichinan Group in Miyazaki Prefecture). The study of these formations will lead to the estimation of the factors that caused the MTD and the plate convergence area at that time. In this study, we focus on the Muroto Formation to obtain more detailed information on the deformation process and depositional age of the chaotic beds.
The field survey revealed that the Muroto Formation consists of several chaotic beds, which account for about 40% of the main layer exposed on the beach. The thickness of individual chaos beds ranges from 1–25 m at Cape Gyoto to more than 100 m at the large scale observed at Kuromi Coast. The Muroto Formation has the following characteristics as MTD: (1) They show a certain succession, although some parts are lacking, sandy mudstone and sandy pebbly mudstone formations are present in the lower part, and sandstone–shale alternation with clastic intrusion is accumulated in the upper part. (2) soft–sediment deformation representing layer–parallel extension and shortening, (3) The clastic intrusion observed in the entire area indicate that they were formed at the same time as the deformation of the turbidite beds, based on the relationship between the intrusions and cuttings. The direction of movement inferred from the deformation structure within the MTD at Cape Gyoto is parallel to the direction of paleo–current indicated by the surrounding strata. The lithological and deformation characteristics indicate that these MTDs were formed by migration by slides and slumps.
U–Pb dating of zircons extracted from the siliciclastic tuff and sandstone of the Muroto Formation show that the youngest peaks correspond to the Early Oligocene (31–33 Ma). These ages are within the range of the late Eocene to Oligocene age inferred from fossil ages, which further constrains the depositional age of the rocks.
These results indicate that MTDs in the Muroto Formation occurred after the Early Oligocene. These occurrences may have been related to increased sediment supply and changes in slope angle .