彰雲潮流沙脊之形貌、沈積作用及演化模式

Abstract

彰雲沙脊位於台灣彰化雲林外海，以砂質沈積物為主組成的地形高區。鳥瞰整個彰雲沙脊，呈現東邊寬往西逐漸尖滅的三角形，在西側又產生與台灣西岸平行的北向延伸帶。依形貌的特徵，彰雲沙脊可分成四區：東彰雲沙脊、槽溝、西彰雲沙脊南端淺灘、西彰雲沙脊北延伸沙席。各沙體表層沈積物組成依序是中粗砂、中砂、中細砂、細砂與粉砂，顯示沈積物顆粒由東向西變細的趨勢。 東彰雲沙脊的走向與台灣西岸平行，長軸長為65公里，短軸為17公里，長軸/短軸的比值為4，東西向橫剖面顯示兩側對稱，呈現典型沙脊形貌特徵。西彰雲沙脊淺灘的走向與台灣西岸近乎垂直，長軸長約53公里，短軸約26公里，長軸/短軸的比值約為2，橫剖面顯示南側較陡、北側較平緩。北延伸沙席走向和長寬比與東彰雲沙脊相似，但其高度與坡度太小，非為脊狀，因此以沙席名之。 變頻聲納回聲剖面顯示在西彰雲沙脊下方，水深75公尺處有一明顯的不整合面，往東逐漸上揚，在東彰雲沙脊下方消失。此不整合面可能是上次冰期暴露在大氣的陸棚侵蝕面，未被古濁水溪帶來的沈積物所覆蓋。 由沈積物粒徑分佈、沙波的陡、緩坡面方向（沈積物由緩坡往陡坡方向移動）、粒徑趨勢分析結果，推測沈積物可能由澎湖水道往北帶至東沙脊，再向西依序搬運到西沙脊南端淺灘、往北搬運至西彰雲沙脊北延伸沙席。 台灣海峽東側潮流的強度、流向、潮流橢圓，均與沈積物的搬運方向和沙脊形貌相符合，推測潮流為形成彰雲沙脊的主要動力。由淺層回聲剖面顯示澎湖水道為受潮流侵蝕成沖刷槽，並提供沈積物，往北搬運堆積形成彰雲沙脊，因此彰雲沙脊與澎湖水道共同組成潮流沈積體系。 彰雲沙脊的形成與末次冰期以來海平面上升有關，其發育可分為三時期。第一期（低水位期，17ka-15ka）：澎湖水道為陸棚，南端開口為海岸帶，當時南海潮流流向可能垂直海岸，沖刷澎湖水道。第二期（海進期，15ka-14ka）：海水面快速上升至75公尺，南海水經澎湖水道進入將沈積物往北搬運，受北邊濁水溪口淺灘阻擋，沈積物在此或往西堆積。第三期（高水位期，14ka至今）：海水淹沒台灣海峽陸棚，潮流流向近乎平行台灣西岸，且近岸為往復型強潮流、離岸為旋轉型弱潮流，潮流將彰雲沙脊修飾成目前的形貌。

The bathymetric Changyun Ridge, located in the central-east of the Taiwan Strait, consists of three sand bodies: the eastern Changyun sand ridge, a ridge-like sand shoal and a sand sheet. Off the western coast of Taiwan about 30 km the eastern Changyun sand ridge is about 65 km long and 15 km wide, showing typical morphology of linear sand ridges. Its long axis is oriented parallel to the coastline and narrow reciprocal tidal ellipses. Farther west the ridge-like sand body has a length of 53 km and a maximum width of 26 km, trending northwest-southeast normal to the coast and oblique to the broad rotary tidal ellipses. Distal to the western Taiwan shoreline the sand shoal is 20 to 40 km wide, 22 m high and 60 km long, oriented parallel to the coastline and broad rotary tidal ellipses. Surface sediments of the Changyun Ridge are composed of very fine to coarse sands in a progressive seaward decrease in grain size. The configuration and spatial distribution of these three sand bodies with corresponding decrease in tidal speed and in grain size together suggest that the sand bodies developed sequentially from near-shore to offshore, presumably in response to westward shifting of the north-flowing tidal currents. Chirp sonar profiles indicate that sand bodies of the Changyun Ridge rest upon a relative flat basal reflector at a water depth of 75m, suggesting a mainly hydrodynamically controlled origin. The variations of the hydrodynamic conditions (mainly tidal currents) on the central-eastern Taiwan Strait shelf seem to be the primary control on sedimentary facies shift, morphological changes and sand ridge growth for the Changyun Ridge. The Changyun sand ridge is ongoing to receive sands from the south in a transitional development stage. It is proposed that there are three stages in the sand ridge evolution. The first stage (low stand ):17ka-15ka, the sea waters from the South China Sea began to flow over the sea floor between the Penghu Islands and Taiwan and the paleo-tidal currents finally excavated the sea bed into a channel now known as the Penghu Channel. The second stage (transgression stage): 15ka-14ka, the northward flowing currents from the Penghu Channel are blocked by paleo-Choshui River delta and slowed down. The sediments are deposited on the delta and carried out westward. The third stage (high stand): 14ka-the present, the early formed Changyun Ridge is continued to be modified by the tidal currents in the Taiwan Strait and forms the present sand ridge.