以海床迴聲特徵探討台灣海峽及台灣西南海域之淺層沈積作用

Abstract

本研究利用海底迴聲剖面資料探討台灣海峽以及台灣西南海域的迴聲型態與沈積構造，彙整、分析了海研一、二號共十二個航次在研究地區蒐集之地層剖面資料，剖面總長分別超過兩千公里（台灣海峽）及一萬一千公里（台灣西南海域）。此外，亦結合水深資料及前人之反射震測剖面研究與沈積物的地質、地化分析結果，建構台灣海峽及台灣西南海域淺層沈積物可能的傳輸方式，並比較西南海域被動性與活動性大陸邊緣不同沈積環境的特徵。 依據海床不同之迴聲特徵，本研究將台灣海峽的迴聲型態分成三大類（平坦型、土堆型、不規則型）及七小類，各類迴聲型態多呈東北－西南走向之狹長型分佈，與潮流橢圓的長軸方向一致，可以看出台灣海峽的沈積物傳輸受到潮流影響很大。台灣西南海域的迴聲型態則可分為四大類（清晰型、濃密型、雙曲線型、不規則型）及八小類，其迴聲型態分布圖清楚顯示被動性與活動性大陸邊緣主要的迴聲型態不一樣：南中國海大陸坡（被動性大陸邊緣）的上部陸坡多為雙曲線型的迴聲型態，代表該區以海底崩移造成的侵蝕作用為主；下部陸坡海床多呈現清晰型迴聲型態，代表穩定的沈積作用；而活動性大陸邊緣（高屏陸坡）的上部斜坡因為有高屏溪等河流帶來大量沈積物在該地區堆積，海床以清晰型迴聲型態為主；下部斜坡之海床則多呈現雙曲線型迴聲型態，推測沈積物供應少，有小規模海底崩移。本研究顯示海床迴聲型態可以用來探討活動性與被動性大陸邊緣不同之沈積環境與沈積作用。 根據地層迴聲剖面資料，在西彰雲砂脊西側有一條長約一百公里的泥質帶，沿著澎湖水道北延的低凹帶發展。因為泥質帶迴聲剖面顯示其地層面向西傾斜，推測其沈積物來源是台灣西部之河川。台灣海峽北部之觀音凹陷區域有崎嶇的海床，而其東北方有厚度達20公尺的砂質沈積物堆積，參考台灣海峽潮流橢圓資料，本研究認為台灣海峽北部存在一個「觀音凹陷—北台砂脊」潮流沈積體系，為搬運細顆粒沈積物離開台灣海峽的主要營力。 在台灣西南海域，靠近台灣島一側的南中國海陸坡上可以觀察到牽引式海底崩移現象，在發生海底崩移的海床迴聲剖面上清楚顯示一系列剪力面，而且剪力面附近沈積層有被拉張變薄的情況。本研究認為在此地區發生的牽引式海底崩移主要是受到台灣島荷重的影響，且此作用有從東往西逐漸發展的趨勢。屬於活動性大陸邊緣的上部高屏陸坡，在摺皺及逆衝斷層等構造作用下呈現一系列西北－東南走向之海脊與斜坡間盆地。由於高屏溪提供了大量的沈積物，在上部高屏陸坡地區即呈現出沈積物「填滿及溢出」的沈積與傳輸方式，此作用在海床迴聲剖面資料中亦可清楚顯示。 綜言之，本研究顯示海床迴聲特徵可以用來辨識海床沈積物分布並協助探討研究地區之沈積作用。在台灣海峽之海床迴聲特徵分析結果認為台灣海峽北部存在一個「觀音凹陷—北台砂脊」潮流沈積體系，此為在短時間內搬運細顆粒沈積物離開台灣海峽的主要營力。而在台灣西南海域的研究結果則顯示被動性大陸邊緣與活動性大陸邊緣沈積作用有不同之趨勢，在南中國海大陸邊緣是上部陸坡侵蝕、下部陸坡沈積，在高屏陸坡則相反，為上部陸坡沈積、下部陸坡侵蝕。

Chirp sonar profile data have been analyzed and compiled in this study to map the echo character distribution of the sea floor sediments in the Taiwan Strait and in the area offshore Southwestern Taiwan. These chirp sonar profile data were collected from 12 cruises of the Ocean Researcher 1 and Ocean Researcher 2 from 2002 to 2007, with total lengths of over 2,000 km in the Taiwan Strait and over 11,000 km in offshore Southwestern Taiwan area. By integrating the echo character studies with sea floor topography and results of previously published seismic and coring studies, sea floor sedimentary processes and possible sediment transport mechanisms in the study area can be established. Based on the different echo characters, three types of echo character patterns (flat, mound and irregular) and seven sub-types have been recognized in the Taiwan Strait. In general, those patterns are distributed in several NE-SW trending long and narrow zones, parallel to the M2 axis of the local tidal current ellipses, suggesting that sediment transport in the Taiwan Strait is strongly influenced by the tidal current. Offshore southwestern Taiwan, four types of echo character patterns (distinct, indistinct, hyperbolic and irregular) and eight sub-types have been identified. The echo character distribution patterns reveal that there are district sedimentary processes in the upper and lower continental slopes of the passive South China Sea continental margin, and in the active orogenic margin of the Kaoping slope area. The upper continental slope of the South China Sea continental margin presents hyperbolic echo characters which suggests that the submarine landslides is the dominative processes here, while most chirp profiles show distinct echo characters in the lower South China Sea continental slope, implies a more stable depositional environment. On the contrary, the distinct echo character patterns are prevailing in the upper Kaoping Slope, reflecting a depositional environment here due to the huge influx of the orogenic sediments from Taiwan. In the lower Kaoping Slope, sea floor echo characters appear to be most hyperbolic, suggesting the sedimentary environment is less depositional and many submarine landslides occurred here. In the Taiwan Strait, the chirp sonar data show that a mud belt extending northward from the north end of the Penghu Channel along the western flank of the Yun-Chang Ridge for over 100 km. The westward dipping reflectors and acoustic blanking caused by shallow gas in the chirp sonar profiles suggest that the mud should come from Taiwan. In the northern Taiwan Strait, the rugged sea-floor in the Kuayin Depression and the presence of a 20-m thick sand ridge suggests that there is a tidal deposition system here which plays a key role in transporting fine grain sediments from the Taiwan Strait eastward to the Okinawa Trough. In the passive South China Sea continental margin near Taiwan, retrogressive failures have been observed. There are many shear planes on the sub-bottom profiles where the submarine slides occurred and the sediment layers near the shear plane were stretched, bended and thinned. We suggest that the retrogressive failure processes are mainly caused by the loading of the Taiwan mountain belt as this process is well developed on the South China Sea continental slope near Taiwan while is less developed westward along the continental margin. In the active margin of the Kaoping Slope, folds and thrusts are the dominative structure features which form a series of submarine ridges and intra-slope basins. In the upper Kaoping Slope region, the “fill and spill” processes are observed, and echo character patterns also support this process here. In summary, this study has demonstrated that sea floor echo characters can be used to identify the sediment distribution patterns and to reveal the sedimentary processes involved. In the northern Taiwan Strait, the existence of a tidal deposition system, named the “Kuayin Depression and North-Taiwan Sand Ridge system”, has been suggested which is the driving force for transporting fine grain sediments from Taiwan Strait to deep sea in a short period. In the passive continental margin offshore Southwestern Taiwan, sea floor erosion caused by submarine slides is widespread on the upper South China Sea continental slope, while the lower South China Sea continental slope is a stable depositional environment. In contrast, huge amount of sediments are deposited on the upper Kaoping Slope, while rugged sea floor topography and submarine landslides are the main features in the lower Kaoping Slope.