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[SCG55-P06] Characteristics of sediments distributed in the submarine canyon off Miho Peninsula
Keywords:Suruga Bay , submarine canyon, clay minerals
The Miho Peninsula, located on the western coast of Suruga Bay, is a composite sand spit formed by northward littoral drift, with sediment primarily supplied from the Abe River. Some of these littoral drift flows into offshore submarine canyons, as indicated by strong backscatter intensities observed in seabed mapping, suggesting the presence of coarse-grained sediments extending continuously from offshore Miho Peninsula to the bottom of the Suruga Trough. However, the detailed sediment transport processes from the Miho Peninsula to the Suruga Trough floor remain unclear. This study aims to elucidate these mechanisms.
Field surveys were conducted between August 2022 and May 2024, targeting the Hagoromo Submarine Canyon, transected in an east-west direction (from the coast to offshore, at depths of 100–1350 m) based on backscatter intensity maps. The study area was categorized into morphological units based on slope gradients from bathymetric profiles. Sediment sampling was performed using a Smith-McIntyre grab sampler equipped with a deep-sea video camera, an orientation inclinometer, and a memory-based CTD, covering depths from 100 to 1200 m. The collected sediments were subsampled into core samples (4–11 cm thick) using acrylic tubes (5×6×20 cm). Lithological observations were conducted through visual inspection, soft X-ray imaging, and grain size analysis via laser diffraction scattering. Additionally, qualitative clay mineral analysis was performed on selected surface samples using X-ray diffraction (XRD), and semi-quantitative analysis was conducted using the Reference Intensity Ratio (RIR) method for peaks below 35°.
Geomorphological analysis classified the study area into five slope units: Slope I (100–250 m depth, ~13.4° slope), Slope II (250–350 m, ~8.4°), Slope III (350–750 m, ~5.8°), Slope IV (750–1000 m, ~8.7°), and Slope V (1000–1350 m, ~11.6°).
The sediment characteristics are as follows: Slope I consists of three layers: an oxidized brownish silt layer (2–3 cm), a silt to very fine sand layer (2–5 cm), and a fine to coarse sand layer with gravel. Slope II consists of an oxidized brownish silt layer (2 cm) and a silt to coarse sand layer with gravel. Slope II lithology varies seasonally, with patterns such as a single gravelly silt layer or a silt layer interbedded with thin (1 cm) sand layers. Slope III consists of a silt to very fine sand layer (4–8 cm) and a well-sorted fine sand layer. Slope IV consists of three layers: an oxidized silt layer (0.5–3 cm), a bioturbated silt layer (2–4 cm), and a fine to coarse sand layer with gravel, with a distinct boundary between the lower two layers. Slope V consists of three layers: a bioturbated silt layer (2–3 cm, sometimes exceeding 6 cm), a laminated silt layer (2–4 cm), and a very fine to medium sand layer (0.2–2 cm), with a distinct boundary between the lower two layers. Variations such as silty deposits without sand layers and silt-rich sediments containing gravel were also observed.
Overall, sediment grain size decreases offshore, and sand layers beyond Slope III become better sorted. Seasonal variations were observed in sediment composition, particularly in Slope II and Slope IV, indicating the need for further investigation.
Clay mineral analysis identified illite, chlorite, and montmorillonite. Illite and montmorillonite proportions increased with depth, while chlorite decreased.
These results indicate that sediments off the Miho Peninsula are classified into surface silt layers and underlying coarse-grained deposits. Lithological changes correlate with topographic variations, and grain size generally decreases with depth, with improved sorting of matrix particles. These characteristics suggest lateral variations in turbidity currents, indicating their role in sediment transport within the Hagoromo Submarine Canyon. Additionally, the identification of illite, chlorite, and montmorillonite provides a basis for further research into the provenance of fine-grained sediments in this region.
Field surveys were conducted between August 2022 and May 2024, targeting the Hagoromo Submarine Canyon, transected in an east-west direction (from the coast to offshore, at depths of 100–1350 m) based on backscatter intensity maps. The study area was categorized into morphological units based on slope gradients from bathymetric profiles. Sediment sampling was performed using a Smith-McIntyre grab sampler equipped with a deep-sea video camera, an orientation inclinometer, and a memory-based CTD, covering depths from 100 to 1200 m. The collected sediments were subsampled into core samples (4–11 cm thick) using acrylic tubes (5×6×20 cm). Lithological observations were conducted through visual inspection, soft X-ray imaging, and grain size analysis via laser diffraction scattering. Additionally, qualitative clay mineral analysis was performed on selected surface samples using X-ray diffraction (XRD), and semi-quantitative analysis was conducted using the Reference Intensity Ratio (RIR) method for peaks below 35°.
Geomorphological analysis classified the study area into five slope units: Slope I (100–250 m depth, ~13.4° slope), Slope II (250–350 m, ~8.4°), Slope III (350–750 m, ~5.8°), Slope IV (750–1000 m, ~8.7°), and Slope V (1000–1350 m, ~11.6°).
The sediment characteristics are as follows: Slope I consists of three layers: an oxidized brownish silt layer (2–3 cm), a silt to very fine sand layer (2–5 cm), and a fine to coarse sand layer with gravel. Slope II consists of an oxidized brownish silt layer (2 cm) and a silt to coarse sand layer with gravel. Slope II lithology varies seasonally, with patterns such as a single gravelly silt layer or a silt layer interbedded with thin (1 cm) sand layers. Slope III consists of a silt to very fine sand layer (4–8 cm) and a well-sorted fine sand layer. Slope IV consists of three layers: an oxidized silt layer (0.5–3 cm), a bioturbated silt layer (2–4 cm), and a fine to coarse sand layer with gravel, with a distinct boundary between the lower two layers. Slope V consists of three layers: a bioturbated silt layer (2–3 cm, sometimes exceeding 6 cm), a laminated silt layer (2–4 cm), and a very fine to medium sand layer (0.2–2 cm), with a distinct boundary between the lower two layers. Variations such as silty deposits without sand layers and silt-rich sediments containing gravel were also observed.
Overall, sediment grain size decreases offshore, and sand layers beyond Slope III become better sorted. Seasonal variations were observed in sediment composition, particularly in Slope II and Slope IV, indicating the need for further investigation.
Clay mineral analysis identified illite, chlorite, and montmorillonite. Illite and montmorillonite proportions increased with depth, while chlorite decreased.
These results indicate that sediments off the Miho Peninsula are classified into surface silt layers and underlying coarse-grained deposits. Lithological changes correlate with topographic variations, and grain size generally decreases with depth, with improved sorting of matrix particles. These characteristics suggest lateral variations in turbidity currents, indicating their role in sediment transport within the Hagoromo Submarine Canyon. Additionally, the identification of illite, chlorite, and montmorillonite provides a basis for further research into the provenance of fine-grained sediments in this region.