- 學位論文
從震測資料探討臺灣西南及東北海域沈積物散布系統
Seismic Imaging of Sediment Dispersal Systems: Southwest and Northeast Taiwan Perspectives
摘要
臺灣是一個構造活動活躍、高降雨量，且為高侵蝕率、並具有山區小河川特性的環境，因而也輸出大量的陸源沈積物至周遭海域。本研究藉由多尺度的資料整合與分析工作，透過震測相分析、層序地層學分析、地形空間分析以及底質剖面回聲特徵的分類等方法，針對臺灣兩處地體架構背景不同的海域–西南海域的高屏棚、坡及東北海域的宜蘭陸棚和南沖繩海槽區域，探討在兩種不同構造的大陸邊緣環境下，沈積物的散布形式。本研究同時討論陸棚、陸坡在沈積物傳輸過程中所扮演的角色及可能的相關影響因子，並提出陸源沈積物輸出至深海的傳輸模式。 臺灣西南海域的高屏棚坡屬於聚合式的大陸邊緣，構造上為褶皺逆衝斷層帶並伴有泥貫入體發育。因為構造活動，該區域發育出數個斜坡盆地，這些在陸坡上所形成的構造性納積空間不僅會影響沈積物的散布方式，也會成為部分沈積物的最終儲存處。透過震測相分析以及盆地中震測反射層接觸關係的辨識，本研究探討斜坡盆地在構造前、同構造與構造後等不同盆地發育階段中，沈積物散布形式的變化，並藉由和陸上泥岩層的對比，認為高屏陸坡斜坡盆地之基底可能為古亭坑層。因為盆地內沈積物的沈積速率與盆地周遭構造高區的抬昇速率間的相對關係，會造成不同的基準面(base level)變化，進而影響陸坡形貌與斜坡盆地內的沈積物散布形式。本研究認為「充填與溢出」模式為高屏棚坡區基本的陸源沈積物散布方式，根據不同的陸坡納積空間型態與發育，沈積物會由近岸到遠洋方向，依序充填各個斜坡盆地。但在局部區域，因為受到海底峽谷發育以及盆地周遭的塊體崩落影響，個別盆地也會出現海底峽谷沖積填充，以及塊體運動所造成之沈積物散布特徵。 臺灣東北海域的宜蘭陸棚和沖繩海槽則為張裂式的大陸邊緣，研究區域中發育了許多正斷層構造並伴隨火山活動。海床底質回聲剖面的分析結果顯示宜蘭陸棚的淺層沈積構造狀況，同時也從中觀察到許多塊體運動的沈積特徵。透過陸上岩心資料與海域震測資料的層序分析，本研究探討宜蘭陸棚約兩萬年以來的沈積物散布形式，並估計沈積物收支。自末次冰盛期以來，宜蘭陸棚每年約沈積了160萬噸的沈積物，對比蘭陽溪的年輸出量與南沖繩海槽年沈積量，宜蘭陸棚大約承接10-20%自臺灣東北部輸出的沈積物。而其餘源自臺灣東北部的沈積物，則通過宜蘭陸棚進入南沖繩海槽，並至少貢獻約45%之南沖繩海槽年平均沈積量。震測相分析顯示，塊體運動沈積物廣泛地分布在外陸棚及其鄰近的海槽陸坡區域，因此推論塊體運動應是陸棚的沈積物再進一步傳輸至海槽的重要方式。活躍構造、陡峻陸坡，加上棚坡間的水道侵蝕以及潮流作用，則可能是導致塊體運動發生的原因。在南沖繩海槽深水區並未觀察到明顯、廣泛的塊體運動沈積物分布，其沈積特徵主要反應了區域性的構造活動。依照地層的震測相形貌與速度特性，本研究將地層分成四層層序，並分別將其對應至南沖繩海槽不同階段的弧後張裂活動。 本研究運用沈積物散布系統的觀點，對臺灣週邊大陸邊緣下陸棚與陸坡兩個不同的沈積物散布系統單元進行討論，並與世界上其他區域進行比較。海底峽谷是將陸源沈積物傳輸至深海的重要管道，但在陸坡區域，若因斜坡盆地的發育，產生出適當的沈積物納積空間，便會影響陸源沈積物的散布形貌，同時陸坡上的斜坡盆地也可成為部分陸源沈積物的最終儲存場所。而在坡度較陡的陸坡環境中，若無適當的納積空間發育，陸坡便不具有長久留置陸源沈積物的能力，特別是當沒有明顯的海底峽谷發育時，塊體運動成為陸棚與陸坡間重要的陸源沈積物傳輸方式。
並列摘要
The Taiwan Island is an active mountain belt formed by the collision of the China continent and Luzon Arcs. Because of active tectonics, high topographic reliefs with steep gradients, frequent typhoons and heavy precipitations, Taiwan is recognized as having particularly high sediment production rates within the small mountainous river systems. In this study, we integrate marine seismic, sub-bottom profiler, and high-resolution bathymetry data together with land well core and log data to investigate the sediment dispersal systems in the areas off southwest and northeast Taiwan. Through seismic facies analyses, sequence stratigraphy studies, physiography analyses and sub-bottom echo image researches, sediment disposal patterns are proposed which help to better understand how the large sedimentary loads were deposited in tectonically active continental margins. Off Southwest Taiwan, the Gaoping Shelf and Slope are in active convergent tectonic environment, where slope basins are formed by the developments of folds, faults and diapiric intrusions. The complex system of folds, faults, diapirs, slope basins, and submarine canyons could affect sediment dispersal and readjust the morphology of the Gaoping Slope. In this study, we have performed seismic facies analyses to investigate sedimentary processes and depositional environments in the accommodations. Four major seismic facies have been recognized in the Gaoping Slope basins. By comparing different deposition patterns in the slope basins, different stages of basin development during the post-, syn-, and pre- structural phases could be recognized. The thick mud layers beneath the slope basin which are regarded as the source of diapiric intrusions could be related to the Gutingkeng Formation on land Southwest Taiwan. The sedimentary patterns are controlled by the sedimentation rate and structure uplifting (which also means the basin subsidence) rate caused by the convergent and diapiric tectonics in the Gaoping Slope. A“filling-and-spilling” model is suggested to be a basic sedimentary process in this area; however, canyon feeding and mass wasting could be other important processes of sediment dispersal and causing rapid deposition of sediments in the basins. Off Northeast Taiwan, the Ilan Shelf and Southern Okinawa Trough are in extensional tectonic setting where several normal faults and volcanic activities have also developed. Eight echo types have been recognized from high-resolution sub-bottom profiler images that represent different deposition patterns and interpret shallow sedimentary structures in the Ilan Shelf. Mass transport deposits are widely observed in the outer shelf and the adjacent slope areas of the Southern Okinawa Trough on sub-bottom and seismic profiles. However, mass transport deposits are not observed in the deep sea area of the Southern Okinawa Trough, where sediment dispersal features are mainly controlled by tectonics. Four major sequences have been recognized, and they could be related to different stages of the back-arc extension of the Southern Okinawa Trough. Through sequence stratigraphy analyses of well data onshore and seismic data offshore, a regional unconformity was formed during the last glacial maximum about 20,000 years ago. The estimated amount of deposited sediments since the late Pleistocene is about 1.6 million tons per year on the average. Comparing to the sedimentary budgets of the Lanyang River drainage and the Southern Okinawa Trough, the shelf has trapped 10-20% sediments discharged from the Lanyang River, and the sediments output from the Northeast Taiwan contribute at least 45% sediment depositions in the trough. Mass movement process is suggested to be an important factor for sediments dispersal from the Ilan Shelf to the Southern Okinawa Trough. Submarine mass movement is a rapid sediment transport process which results in extremely high sedimentation rate in the southern end of the Southern Okinawa Trough. Active structures, channel axial incisions, and tidal currents could be the triggers for frequent mass wasting processes in the study area. Comparing two studied continental margin areas and sediment dispersal systems in other parts of the world, regional structural activities are important factors besides submarine canyons for sediments dispersal. The slope usually represents a bypass zone for most sediment in a sediment dispersal system. However, if slope basins can be developed in the slope area, not only trench and deep sea basins but this kind of structural accommodation spaces in the slope basins could also be another ultimate sink for sediments. Mass movement is another important process for sediments transport from shelf to deep basin, especially when there are no obvious submarine canyons developing in the continental margin areas.