5:15 PM - 7:15 PM
[HCG21-P08] Bottom sediments distribution around Miyako Island
Keywords:grain size, surface sediments, ripple mark
In order to clarify the relationship between the distribution of shallow- and deep-sea sediments and their depositional environment in the area around Miyako Island, Okinawa, bottom surface sediments were collected using a K-grab sampler at 68 sites in between 63 m and 1906 m water depth around Miyako Island, and the seabed surface was photographed during collection. The relationship between the sedimentary facies and the depositional environment was investigated by observing the collected sediments with the naked eye and under a microscope, grain size analysis, sand grain composition analysis and observing photographs of the seabed surface.
The sediments around Miyako Island contain mainly carbonate skeletons of organisms or their debris. Tsuji (1993) investigated the distribution of surface sediments on the island shelf to the island shelf slope around Miyako Island and classified them into seven sedimentary facies: reef facies, near reef sand facies, muddy sand-sandy mud facies, rhodolith and large foraminifera gravelly sand facies, bryozoan sand facies, planktonic foraminifera sand facies, planktonic foraminifera muddy sand facies.
Rhodolith and large foraminifera gravelly sand(<-1φ) distribute in the water depths of shallower than 300 m around Miyako, Irabu and Tarama islands, while planktonic foraminiferal sand-mud sand ( -1 - 4φ) distribute on the continental slope. Grain size analysis showed the gravel content was more than 40% on the island shelf shallower than 100 m around Tarama Island, while sand content was 40-50% in the north-northeast of Miyako Island and 50-70% in the south-southwestern offshore Miyako Island. The silt sediment content was low at less than 10% on the island shelf to island shelf slope and 60-80% at water depths of 1000m-1300m.
The radiocarbon accelerator mass spectrometry (AMS) dating of rhodoliths at water depths between 60 and 150m in western offshore Miyako Islands suggested those were formed since 6.5 ka(Tsuji, 1993), while AMS ages of rhodoliths from the eastern offshore of Miyako Islands are 2.6 ka (142 m water depth), 10.2 ka (177 m water depth) and 18.3 ka (340 m water depth). The formation age tended to become more recent as the water depth became shallower.
Seafloor photographs show the development of numerous ripples on the island shelf-island shelf slope around Miyako and Irabu islands. Most of the direction of ripple crests extension on the island-shelf slope was parallel to the slope direction of the seafloor topography, suggesting that they were formed by the high-speed tidal currents on the deep-water shelf and slope as reported by Tsuji (1990).
The sediments around Miyako Island contain mainly carbonate skeletons of organisms or their debris. Tsuji (1993) investigated the distribution of surface sediments on the island shelf to the island shelf slope around Miyako Island and classified them into seven sedimentary facies: reef facies, near reef sand facies, muddy sand-sandy mud facies, rhodolith and large foraminifera gravelly sand facies, bryozoan sand facies, planktonic foraminifera sand facies, planktonic foraminifera muddy sand facies.
Rhodolith and large foraminifera gravelly sand(<-1φ) distribute in the water depths of shallower than 300 m around Miyako, Irabu and Tarama islands, while planktonic foraminiferal sand-mud sand ( -1 - 4φ) distribute on the continental slope. Grain size analysis showed the gravel content was more than 40% on the island shelf shallower than 100 m around Tarama Island, while sand content was 40-50% in the north-northeast of Miyako Island and 50-70% in the south-southwestern offshore Miyako Island. The silt sediment content was low at less than 10% on the island shelf to island shelf slope and 60-80% at water depths of 1000m-1300m.
The radiocarbon accelerator mass spectrometry (AMS) dating of rhodoliths at water depths between 60 and 150m in western offshore Miyako Islands suggested those were formed since 6.5 ka(Tsuji, 1993), while AMS ages of rhodoliths from the eastern offshore of Miyako Islands are 2.6 ka (142 m water depth), 10.2 ka (177 m water depth) and 18.3 ka (340 m water depth). The formation age tended to become more recent as the water depth became shallower.
Seafloor photographs show the development of numerous ripples on the island shelf-island shelf slope around Miyako and Irabu islands. Most of the direction of ripple crests extension on the island-shelf slope was parallel to the slope direction of the seafloor topography, suggesting that they were formed by the high-speed tidal currents on the deep-water shelf and slope as reported by Tsuji (1990).