宜蘭陸棚上的淺層沈積構造及沈積物傳輸模式

Abstract

台灣東北部的蘭陽溪流域年平均降雨量超過5公尺，每年從三千公尺高的山區傳輸大約一千萬噸的沈積物經宜蘭陸棚進入兩千公尺深的沖繩海槽中。這些陸源沈積物在宜蘭陸棚上的分佈狀況究竟為何？宜蘭陸棚又在沈積物傳輸的過程中扮演什麼樣的角色？本研究係針對上述課題，利用連續變頻聲納資料、反射震測資料及每100公尺見方為一網格點的高解析度水深資料來進行探討。 宜蘭陸棚上的淺層沈積構造展現三種不同的連續變頻聲納迴聲型態：層狀（Type 1）、堆狀或不規則狀（Type 2）、以及無底層反射型態（Type 3）。隨著細部形貌的不同，可再細分為8種反射訊號型態。Type 1的分佈狀況與海岸線大致平行，Type 2的分佈受地形影響很大，海床坡度增大為形貌改變的主因，Type 3則可能為火山活動所造成，同時也呼應了海底地形與基盤上的高區。而根據相對海水面的變化週期及蘭陽平原上的岩心資料，我們從震測剖面上推論，宜蘭陸棚大約在2萬年前是出露於海水面之上，而後逐漸沒入海中。陸棚上的沈積中心自上次冰期以來有北遷的跡象，平均沈積速率約為0.6-0.7 cm/yr。由於研究區域受晚更新世以來的火山活動影響，震測剖面中可觀察到一些火山噴發紀錄及部分區域地層抬升的現象。整體而言，宜蘭陸棚上的沈積物厚度在南北兩端較薄而往外海方向逐漸增厚，陸棚東方的陡坡及水道則相當不穩定，有許多海底崩移或地層滑動的現象。就沈積物傳輸的機制來看，除了粒徑大小會影響沈積物傳輸外，崩移和地層滑動則是使原本堆積在陸棚上的沈積物大量且快速進入沖繩海槽的方式。另外龜山島與南、北龜山峽谷造成的地形效應及峽谷內海流的作用，使南、北龜山峽谷成為另一個沈積物傳輸的重要管道，並在其出口處形成沖繩海槽內具最高沉積速率的區域。考慮到台灣東北海域的環境因素，宜蘭陸棚上的淺層沈積構造應受當地間歇性的火山活動、頻繁的地震、颱風所帶來的洪氾以及複雜的海流影響，而海底地形及水流則是影響沈積物傳輸形式的重要關鍵。

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.