台灣西南海域澎湖與高屏海底扇系統之沉積震測特徵

Depositional elements and seismic characteristics of the Penghu and Gaoping submarine fan systems off SW Taiwan

林亮甫(Liang-Fu Lin)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/博士(2020年)

https://doi.org/10.6342/NTU202000637

摘要

台灣造山帶劇烈地抬升，供應了大量的沉積物到周圍的海域，是海底沉積系統發育的良好條件，因此在台灣島周圍可觀察到許多海底峽谷與海底扇形成。在台灣西南海域，西側是張裂後的南海被動大陸邊緣，東側則是因板塊聚合作用形成的擠壓構造帶，除了在兩側能形成不同類型的沉積系統之外，海盆中更記錄了造山帶的歷史、被動邊緣轉換成活動邊緣過程對沉積作用的影響。然而，在褶皺帶的地層受到變形、侵蝕、變質作用，幾乎無法探究過去發育的沉積系統的全貌，使得許多在造山過程中留下的地質資訊不易解讀，甚至被忽略。本研究利用多頻道反射震測資料配合層面拉平技術探討在下部高屏斜坡褶皺帶變形的地層特徵，並在此辨識出了兩個大範圍分布的海底扇－由澎湖峽谷生成的澎湖扇與由高屏峽谷生成的高屏扇。本研究進一步透過震測相分析，探討此兩海底扇的基本性質與演化，也嘗試將這兩個海底扇的特性與演化對應到它們上游的地層紀錄，探討當時造就澎湖扇與高屏扇的沉積物散布系統，以及影響這兩個系統的沉積物特性、構造與海水面變化等因素。
研究結果顯示澎湖扇是由單點供應的多期自然堤水道組成的長型海底扇，具有細顆粒海底扇的特徵。由當時的濁水溪供給造山帶沉積物到前陸盆地的海底峽谷中向南傳輸，最終在下部高屏斜坡堆出澎湖扇，是一個在被動邊緣與活動邊緣間的沉積物散布系統，稱為澎湖系統。高屏扇則是由單點供應的多期砂體堆覆而成的舌狀海底扇，具有偏粗顆粒海底扇的特徵。由當時的高屏溪供給造山帶沉積物進到旗山斷層上盤的背負盆地中，供應至在活動邊緣下切的高屏峽谷上段，再經過平行構造發育的高屏峽谷中段向南輸送，最終在下部高屏斜坡堆出高屏扇，是一個在活動邊緣上的沉積物散布系統，稱為高屏系統。這兩個系統發育的地點與特性都受到構造演化的控制，而在下部高屏斜坡的褶皺構造形成以後，兩條海底峽谷都向西南切過構造帶，在構造帶上的沉積作用改變，沿著峽谷流徑輸送的沉積物在構造高區以掠過為主、在背負盆地中常從峽谷溢出而滯留在褶皺帶上。澎湖扇具有反映海水面循環的特徵，其中辨識出的五個層序邊界似乎與台南一帶辨識出的層序邊界具有類似的變化趨勢，具對比的可能性，而澎湖扇的震測相組合可能與本地的海水面變化有關。
澎湖扇與高屏扇相較於世界各地發育的海底扇規模並不算大，但是若考量台灣造山帶的規模，兩扇體總和面積超過現代台灣島面積的六分之一，體積更是大約現代台灣島體積的三分之一，是一個弧陸碰撞造成的殘餘海盆中顯著的沉積物聚集。

關鍵字

海底扇 ； 沉積物散布系統 ； 多頻道反射震測 ； 台灣西南海域 ； 高屏扇 ； 澎湖扇

並列摘要

The Taiwan orogen, known for its highest erosion rate in the world, discharges voluminous sediments into the nearby ocean basins. During the processes, submarine canyons and fans were developed around Taiwan. In the area off southwest Taiwan, the rift margin of northeastern South China Sea lies in the west and convergent complex of Taiwan orogen develops in the east, the depositional systems have been developed under different tectonic controls on both sides. This area should preserve the sedimentary records of both the passive margin and active margin settings, and also the depositional transformation between the two tectonic settings as the orogeny propagates westward. However, as the sedimentary layers are deformed, eroded, or even metamorphosed in the convergent zone, the stratigraphic characteristics are difficult to be recognized and often being ignored. In this study, the marine seismic reflection data were used for exploring the stratigraphic characteristics of the lower Gaoping Slope where the folded strata are finely imaged. To better understand the deformed depositional features, the “horizon flattening” technique was applied to present the depositional elements. In the study area, two depositional systems with different characteristics have been identified, namely the Penghu Fan and Gaoping Fan. Based on seismic facies analyses, their respective properties and temporal and spatial variations are discussed. These observations are correlated to the stratigraphic characteristics in their upstream area for discussing their respective sediment dispersal systems and the controlling factors of the sediment properties, structural activities and sea-level fluctuations.
The Penghu Fan is a point-source elongate submarine fan characterized by lobe-channel-levee complexes presenting its fine-grained nature. Formed between the passive and active margins, its sediment dispersal system, the Penghu System, comprised by ancient Choshui River and submarine canyons proximally in the foreland basin and distally on the slope area. On the other hand, the Gaoping fan is a point-source lobate submarine fan characterized by stacked sandy deposits presenting its coarser-grain nature. Formed on the active margin, its sediment dispersal system, the Gaoping System, comprised by the ancient Gaoping river on the hanging-wall piggyback basin of the Chishan fault and the submarine canyon on the uplifted slope (the Upper Gaoping Slope ). The courses and properties of both systems are controlled by tectonic settings. Since fold belt is developed in the Lower Gaoping Slope, the Penghu and Gaoping Fans are deformed and uplifted, and both Penghu and Gaoping Canyons tend to cut across the fold belt and terminally connect to the trough along the deformation front, rather than developing along the piggyback basins on the active slope. During this period, the sediments transporting along the canyons either bypass the structural highs or being spilled into the piggyback basins formed on the slope.
Cyclic depositional pattern in Penghu Fan is observed, suggesting the development of Penghu Fan could be related to local sea-level fluctuations. Five identified sequence boundaries are present in both the Penghu Fan and its upstream area. The depositional trends are similar, so these sequence boundaries are suggested to be correlated. The development of Gaoping Fan is probably also related to the sea-level fluctuations, but it does not show a distinct cyclic pattern. This may reflect the system on active margin is more controlled by structural events.
Comparing to other submarine fans reported, the sizes of Penghu and Gaoping Fans are moderate. However, if the size of the sediment source provenance (the Taiwan Orogen) is taken into consideration, the area of Penghu and Gaoping Fans is larger than 1/6 of the modern Taiwan Island, and the total volume is about 1/3 of the modern Taiwan Island. These two submarine fans present significant sediment accumulation in a remnant ocean basin; thus the Penghu and Gaoping Fans could be a representative case of the remnant ocean submarine fans in an arc-continental collision setting.

並列關鍵字

submarine fan ； sediment dispersal system ； multi-channel seismic reflection data ； off southwestern Taiwan ； Gaoping Fan ； Penghu Fan

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