| 摘要: | 海底峽谷為陸上沉積物搬運至深海的主要通道。高屏峽谷為第一種類型峽谷，其與陸上的高屏溪直接相連，高屏溪的沉積物可透過高屏峽谷直接輸送至南海海盆。前人研究結果顯示，高屏峽谷歷史上有多次濁流事件發生，峽谷內的濁流亦可能透過河道轉彎處溢流而出。高屏峽谷的極上游區域為陸源濁流源頭，過去研究認為峽谷東岸主要受侵蝕作用影響，但該侵蝕作用的三維構造與受侵蝕的地層層序無法僅透過二維測線得知。因此，本研究於小琉球北方區域進行仿三維震測，蒐集78 條密集式電火花反射震測測線，資料先經過二維處理後進行三維幾何定義並輸出製成三維立方體，進行後續震測解釋與分析。本研究劃分出八種震測地層特徵，分別為末次冰盛期前沉積 (Pre – LGM Deposition)、海進體系 (Transgressive Systems Tract；T.S.T.)、高位體系 (Highstand Systems Tract；H.S.T.)、泥貫入體 (Mud Diapir)、邊坡滑移 (Slop failure)、溢流沉積 (Overflow deposition)、峽谷底沉積 (Canyon bottom deposition)與階地 (Terrace)。綜合分析結果顯
示，本研究的研究區域近海床地層遭受強烈的侵蝕作用，主要侵蝕動力可以分為西北往東南向表面海流與峽谷溢流，由兩者導致小琉球北方Ｕ型西北往東南走向侵蝕通道，該侵蝕通道最深處相較於高屏峽谷西側地形落差將近40 公尺，且侵蝕通道中央處的高位體系沉積物被侵蝕殆盡，而海進體系沉積物亦遭受侵蝕僅剩約十公尺，遭受侵蝕或受底流所搬運的沉積層體積估算約為0.544 立方公里。另一顯著地質作用為南北走向活動泥
貫入體，該泥貫入體最北側緊鄰峽谷東岸谷壁，往南連接至小琉球基盤。受泥貫入體推擠影響，峽谷東側谷壁地層產生大規模海底崩塌並可延伸至小琉球西側。此外，峽谷溢流所堆積的砂質沉積物廣泛分佈在小琉球西方海床，侵蝕量較少。但小琉球北方侵蝕通道幾無砂質堆積物，此可能為表層海流長期侵蝕下將溢流沉積物搬運至高屏陸坡附近堆積所致。;The submarine canyon is the main pathway that transports terrestrial sediments down to the deep sea. As the type I canyon, the Gaoping Submarine Canyon (GPC) connects to the Gaoping River (GPR) directly which is capable for moving gravity driven flow and sediments to the northern South China Sea. The turbidity current originated from the Gaoping River mouth could destroy telecommunication cables along and across the canyon. The along canyon
sediment flows may also overflow through canyon breakage and meandering segments from the uppermost of the canyon to downstream of the canyon. As the origin of the turbidity current, the near seafloor fine sedimentary structures and dynamics are rarely studied in the uppermost GPC. Although a previous sparker seismic study shows an eastern bank of the uppermost GPC is erosional, three-dimensional sediment structures and erosional volume are still unknown. In this study, we applied a 2.5-dimensional sparker seismic exploration method that collected 75 sixty meters interval in-line across uppermost GPC and 3 cross-line parallel to the eastern
Gaoping Slope. The data processing was gone through 2D and 3D processings. The 2D processing steps are as followed: trace editing, 2D geometry settings, band-pass filters, stack, swell correction, shaping filter, predictive deconvolution. After 2D processing, the data was performed in 3D geometry settings, 3D stack, and 3D F-X decon then output a final 3D cube.
The seismic characteristics analysis shows that there are eight significant sedimentary sequences and features amongst the study area: Pre-Last Glacial Maximum Sequence (Pre- LGM), Transgressive Systems Tract (T.S.T.), Highstand Systems Tract (H.S.T.), Mud Diapirism, Slope failures, Overflow depositions, Canyon bottom depositions, and Terrace. There is a U-shape channel in the north of Xiaoliuchiu islet that is caused by a strong NW-SE current erosion. In the middle of the channel, the H.S.T. section was gone and only ~10 meters thick T.S.T. was left with respect to the west of the uppermost GPC. There are about 0.544 km3 volume sediments were removed. The east bank of the uppermost GPC is suffered by N-S active mud diapir and triggered severe slope failures in the east bank of the uppermost GPC. In III addition, there are about 5-10 meters thick sandy overbank sediments deposited widely in the west of Xiaoliuchiu islet but only very thin one can be found to the U-shaped channel. This indicates most overbank sediments in the U-shaped channel was removed by NW-SE current. Those eroded sediments are probably deposited along east Gaoping Slope. |
