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[MIS16-P03] Formation processes of hybrid sediment gravity flow deposits based on grain fabric analysis
Keywords:Hybrid event beds, Grain fabric analysis, Machine learning
Hybrid event beds, which are the couplets of debris flows deposits sandwiched by turbidites, have recently attracted much attention as a clue to understanding the behavior of deep-water gravity flows. Previous studies proposed a model for subdividing units in hybrid event beds based on sedimentary structures. However, the universality of this model has not been sufficiently examined in various regions. In addition, the formation mechanism of each division is still controversial and has not been sufficiently verified. In this study, we created a model using machine learning that automatically recognizes grains from sandstone images of cross-sections and reveals each divisions’ characteristics of hybrid event beds based on grain fabric analysis by the model. Furthermore, by comparing the characteristics of their grain fabric and the results of flume experiments, we aim to estimate the formation processes of hybrid event beds. This presentation reports characteristics of hybrid event beds in the Otadai Formation and predictive results.
The Otadai Formation is distributed in the central part of the Boso Peninsula, Chiba Prefecture. This formation has been interpreted as submarine-fan deposits based on facies characteristics. This study surveyed two hybrid event beds observed in the interval 10 m lower from the volcanic ash key layer O7.
We recognized the following four sedimentary structural divisions in two hybrid event beds. (1) Division I is found within about 30 cm from the base of the sandstone bed and consists mainly of coarse to very coarse sandstone. Its structure is massive or weakly graded, and it contains shell fragments. In the base of this division, the lower bed is eroded, and mud clasts are observed. (2) Division II is found within 30-50 cm from the bottom of the bed, and the boundary with division I is clearly distinguished by the grain size break. The matrix in this division is poorly sorted and is mainly composed of fine to medium sandstone, but large mud clasts are contained. (3) Division III is found in the interval 50-65 cm from the base of the bed, has a clear boundary with Division II, and is distinguished by the proportion of mud clasts. The thickness of this division is about 15 cm, and it consists mainly of very fine to fine massive sandstone. (4) Division IV overlies division III with a clear boundary characterized by lamination. This division forms the top of beds and is about 10 cm thick, and is composed mainly of very fine to fine sandstone.
It was revealed that the characteristics of division III are very different from other divisions by grain fabric analysis of division I-III of the hybrid event beds. In this study, we collected sandstone samples from two hybrid event beds. Also, samples were fixed with epoxy resin and polished to scan cross-sections images with a desktop scanner. The analysis of the obtained images indicated that grain fabrics of divisions I and II showed low-angle (10-20°) upcurrent imbrication, while the lower part of division III showed a high-angle (40-50°) imbrication. In the upper part of Division III, the imbrication angle becomes even higher (60-70°).
At present, the causes of these divisions are unknown. However, judging from the characteristics of the sedimentary structure and grain fabrics, division I is a high-concentration turbidity current, while division II and III may be debris flow deposits. A previous study about experimental debris flow deposits showed grain fabric characteristics that the low-angle imbrication at the base shifts to a high-angle imbrication angle (40-80°) at the top. The characteristics of grain fabric from division II to division III indicate the possibility that these are a series of debris flows deposits. However, there is a significant change in the proportion of mud clasts and grain size sorting at the boundary between Divisions II and III. The cause of the boundary in debris flows deposits needs to be investigated in the future.
The Otadai Formation is distributed in the central part of the Boso Peninsula, Chiba Prefecture. This formation has been interpreted as submarine-fan deposits based on facies characteristics. This study surveyed two hybrid event beds observed in the interval 10 m lower from the volcanic ash key layer O7.
We recognized the following four sedimentary structural divisions in two hybrid event beds. (1) Division I is found within about 30 cm from the base of the sandstone bed and consists mainly of coarse to very coarse sandstone. Its structure is massive or weakly graded, and it contains shell fragments. In the base of this division, the lower bed is eroded, and mud clasts are observed. (2) Division II is found within 30-50 cm from the bottom of the bed, and the boundary with division I is clearly distinguished by the grain size break. The matrix in this division is poorly sorted and is mainly composed of fine to medium sandstone, but large mud clasts are contained. (3) Division III is found in the interval 50-65 cm from the base of the bed, has a clear boundary with Division II, and is distinguished by the proportion of mud clasts. The thickness of this division is about 15 cm, and it consists mainly of very fine to fine massive sandstone. (4) Division IV overlies division III with a clear boundary characterized by lamination. This division forms the top of beds and is about 10 cm thick, and is composed mainly of very fine to fine sandstone.
It was revealed that the characteristics of division III are very different from other divisions by grain fabric analysis of division I-III of the hybrid event beds. In this study, we collected sandstone samples from two hybrid event beds. Also, samples were fixed with epoxy resin and polished to scan cross-sections images with a desktop scanner. The analysis of the obtained images indicated that grain fabrics of divisions I and II showed low-angle (10-20°) upcurrent imbrication, while the lower part of division III showed a high-angle (40-50°) imbrication. In the upper part of Division III, the imbrication angle becomes even higher (60-70°).
At present, the causes of these divisions are unknown. However, judging from the characteristics of the sedimentary structure and grain fabrics, division I is a high-concentration turbidity current, while division II and III may be debris flow deposits. A previous study about experimental debris flow deposits showed grain fabric characteristics that the low-angle imbrication at the base shifts to a high-angle imbrication angle (40-80°) at the top. The characteristics of grain fabric from division II to division III indicate the possibility that these are a series of debris flows deposits. However, there is a significant change in the proportion of mud clasts and grain size sorting at the boundary between Divisions II and III. The cause of the boundary in debris flows deposits needs to be investigated in the future.