宜蘭陸棚上的淺層沈積構造及沈積物傳輸模式
Other Title
Shallow Sedimentary Structures and Sediment Transport Processes on the Ilan Shelf
Date Issued
2007
Abstract
The Lanyang watershed, where annual precipitation is greater than 5 m/yr, presently discharges about 10 million tons of sediment annually from mountains over 3,000 m high to the Ilan Shelf at the southwestern end of the Okinawa Trough. In this study, we use high-resolution chirp sonar and seismic reflection profile data, together with high-resolution bathymetry data, to investigate the characteristics of shallow sedimentary structures and to develop a model for sediment transport on the Ilan Shelf.
Based on different high-resolution subbottom echo images, 8 echo types are identified. Flat echoes are parallel to the coast line within 8 km from shore. The distribution of mounded or irregular echoes corresponds to different topography and the seafloor gradient could be a major factor. Blank zones beneath simple strong seafloor echoes correspond to the basement highs which may be formed by volcanic activities. According to previous studies about sea level changes and coastal evolution in the Lanyang Plan, the major unconformity identified on most of the seismic profiles infers that the Ilan Shelf was exposed above sea level about 20,000 years ago. Seismic profiles also show that sediment thickens in the middle of the Ilan Shelf and toward the outer sea. Furthermore, the depocenter has migrated to the north in the past 7,000 years. The average sedimentation rate in the past 20,000 years on the inner Ilan Shelf is 0.6-0.7 cm/yr. We have also mapped several east-west normal faults and found evidences of volcanic activities on the Ilan Shelf. Broadly, the seafloor slope on the Ilan Shelf does not change sharply in most areas except near the Gueishan Island and in the Okinawa Trough where the shallow sedimentary structure is unstable and many slumps and landslides occurred. Mass wasting processes, seafloor morphology and grain sizes play important roles in transporting sediment from near shore to deep water. Frequent earthquakes, volcanic activities, typhoon-induced floods and ocean currents add to the complexities of sedimentary processes on the Ilan Shelf.
Subjects
宜蘭陸棚
連續變頻聲納
反射震測
沈積物傳輸
Ilan Shelf
chirp sonar
seismic
sediment transport
SDGs
Type
thesis
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