台灣西南外海高屏峽谷沉積物及沉積機制研究

A study on sedimentary processes and sediments of the Gaoping Canyon offshore SW Taiwan

曾世霖(Shih-Lin Tseng)

指導教授 ： 林殿順

國立中央大學/地球科學學院/地球物理研究所/碩士(2011年)

摘要

水下沉積作用長期監測困難，尤其對快速沉積事件之直接觀測記錄更為稀少，一般都是先藉由間接資料判識，其後再對目標區域做觀測或採樣研究。本研究藉由研究兩次地質事件後所採集之岩心沉積物，包括（1）2009年8月8日莫拉克颱風洪氾事件，其洪水自高屏溪出海，高濃度流體形成異重流沿高屏海底峽谷及馬尼拉海溝流動依序沖毀海底電纜；（2）2010年3月4日六龜地震（也有文獻稱甲仙地震），地震發生其間亦陸續觀測到海底電纜沿高屏海底峽谷下游及馬尼拉海溝受破壞，推測此地震造成海底山崩並伴隨著海底濁流產生。瞭解高屏峽谷流域的近代沉積作用，以期辨別颱風洪水造成之異重流及地震造成之濁流的沉積物特徵。
由海底電纜斷裂之資訊，計算異重流之流速及濁流流速，並分別對兩次不同事件之重力流作進一步分析。異重流事件，對比莫拉克颱風的降雨量與洪氾時期高屏溪的水流量，試圖去找出雨量、水流量及異重流之關係；六龜地震引發之海底濁流事件，藉由電纜斷裂資料分析其流速，並推估其山崩區域。我們也分析所採集之沈積物，沉積物分析包含沈積特徵描述、X光攝影、粒徑分析、礦物組成、磁感率及密度量測及碳13穩定同位素量測。
由海纜斷裂時間可得知莫拉克颱風所引起之重力流事件至少有三次：第一次（2009/8/9）於峽谷中游及下游，水深約1200-2702 m造成3處海底電纜斷裂，由斷裂點推得最大流速超過11 m/s，此次事件與陸域高屏溪觀測之最大逕流量時間相符，應為高沉積物濃度河水入海所形成之異重流事件；第二次（2009/8/12），沿峽谷下游及馬尼拉海溝處（水深約2900-3500 m），造成8處海底電纜斷裂，最大流速超過23 m/s，但此次事件發生時，高屏溪逕流量已明顯降低，推測應為洪水時期堆積在峽谷上游或斜坡不穩固之鬆軟沉積物，因重力作用產生崩移，引發濁流沿峽谷流動造成此次事件；第三次（2009/8/13），於峽谷中游、水深約1600 m處，造成1處海底電纜斷裂，由於高屏峽谷沿線之電纜大多已斷裂，故此次事件資訊不足無法詳加探討。（2）六龜地震事件：時間比對應為地震誘發海底山崩，並形成海底濁流沿地形低區及海溝流動。海纜斷點位於上部及下部增積岩體交界處以及馬尼拉海溝，水深約2700-3700公尺，由相對時間及位置計算出此濁流最大流速超過9 m/s，並推測海底山崩位置應為接近上部及下部增積岩體交界處的大陸斜坡。
經2009年莫拉克颱風異重流及2010年六龜地震濁流沉積事件分析及沉積物之研究後，依沉積物及沉積機制解釋高屏海底峽谷沉積系統。此峽谷沉積物受到異重流、濁流、洋流、河水及潮汐等沉積作用，並綜合出三種不同模式解釋：（1）異重流事件：若河水含大量沉積物時，沉積物隨洪水入海，常形成沉積物羽流漂浮於海水表層。若沉積物濃度過高，一般高於40g/l（Mulder, 2001a），則形成高濃度之異重流下沉入海床並沿著海床低處流動。沉積物羽流將隨河水推擠及表層海流作用下，擴散到高屏陸棚與峽谷上游兩岸沉積；異重流沿海床底部流動並向下切入高屏海底峽谷，並將沉積物往深海搬運，此流體能量大、侵蝕強，故可向下切斷海底電纜；另外，可能因側向侵蝕造成谷壁沉積物鬆動引起濁流事件（如莫拉克事件），使沉積物再次懸浮搬運、堆積。（2）濁流事件：由於台灣南部陸海域地震頻仍，易引發海底山崩形成濁流。濁流沿海床底部流動，可能進入峽谷，傳輸到馬尼拉海溝或直接進入斜坡盆地中堆積。上述兩重力流隨峽谷深度溢流物顆粒變化，上、中游因離源頭近、水道深及坡度陡，重力流能量、下切力雖強，但溢流沉積物卻以粉砂為主。其中，中游東側為斷層控制之峭壁，溢流沉積物無法越過；下游水道較淺、坡度緩，沉積物溢堤堆積砂質顆粒。（3）相對靜水期時，以半遠洋泥質沉積物為主，此時期沉積物難以保存於峽谷中，但普遍存在河口、陸棚、斜坡及峽谷兩岸，因峽谷中主要受控於侵蝕力強的重力流。

關鍵字

高屏海底峽谷 ； 沉積特徵 ； 異重流 ； 濁流 ； 由源至匯

並列摘要

The Gaoping submarine canyon, connecting to the Gaoping river, is located offshore southwestern Taiwan on an accretionary wedge. Two major sediment transport processes that deliver Taiwan sediments to abyssal South China Sea are operating along the Gaoping canyon. They are flood-induced hyperpycnal flows and turbidity currents caused by submarine landslides. This study examines sediment cores to infer recent depositional processes along the Gaoping submarine canyon. In addition, I use the sequential submarine cable breakages along the Gaoping canyon during 2009 Morakot typhoon to calculate the flow velocity for the flood-induced hyperpycnal flows.
This study collected 17 gravity cores after the 2009 Morakot typhoon and 9 piston cores after the onshore 2010 Liouguei earthquake. Both events are accompanied by two episodes of gravity flows as revealed by series of submarine cable breakages along the canyon.
There are, at least, 3 episodes of submarine cable breakages along the Gaoping canyon during immediately after the 2009 Morakot typhoon. The first hyperpycnal event (2009/8/9) resulted in three locations of cable breakages in the middle and lower reaches (1200-2702 m). The flow velocity exceeded 11 m/s. Timing for this event correlates well with the peak flood of the Gaoping river. It is therefore interpreted as a hyperpycnal-flow event caused by high sediment concentration. For the second event (2009/8/12), there are 8 locations of cable breakages along the lower reach of the Gaoping submarine canyon and the Manila trench with water depth ranging from 2900 m to 3500 m. The flow velocity exceeded 23 m/s. Timing for this event coincides with lower river run-off. I therefore interpret that this event is a turbidity current caused by, perhaps, submarine landslides. For third event (2009/8/13), there is only one location of cable breakage in the middle reach lying at a water depth of 1600 m.
The onshore Liouguei earthquake induces submarine landslides that evolves into turbidity currents and flows the lower slope of accretionary wedge and the Manila trench as evidenced from. Submarine cable breakages lying in a water depth of 2700-3700 m. The velocity for this turbidity current exceeded 9 m/s.
Analyses on sediment cores reveal that the Gaoping submarine canyon is controlled by three main depositional processes: hyperpycnal flows triggered by extreme onshore floods; turbidity currents caused by submarine landslides; hemipelagic deposition.
Sediment by passing and erosive currents are the characteristic features in the upper and middle reaches of the canyon. By contrast, coarser-grained sediment deposition both in channel thalwegs and overbank areas are characteristic in the lower reach of the canyon due to a sudden decrease on canyon gradient. This study reveals that terrigenous materials are transported to deep sea along canyon by hyperpycnal flows during severe floods. Mineral contents and values of δ13C measured from sediments indicate that most of the sediments along the Gaoping canyon are sourced from the Taiwan mountain belt. Turbidity currents triggered by submarine landslides serve as another major sediment transport agent that deliver sediments accumulated in the Gaoping canyon to the Manila trench.

並列關鍵字

source to sink ； turbidity currents ； hyperpycnal flows ； Gaoping submarine canyon

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