| 摘要: | 摘要
潮汐沙脊是現代陸棚中常見的地形特徵,是為海岸表層沉積,主要透過潮汐、潮
流影響,使沉積物堆積形成現今的沙脊。台灣地處於新生代弧陸碰撞造山帶,是高侵
蝕速率與高沉積速率的環境,地層受到造山帶的壓力擠壓以及沉積物的重力堆積影響
下形成前陸盆地,後期造山運動趨緩穩定後,沉積物往大陸邊緣推進,將前陸盆地填
滿變成淺水環境。根據前人研究古環境與古地形的重建,台灣的構造活動非常的劇
烈,不管是沉積或是侵蝕速率都非常快速且複雜,其中最近代的一次大事件是末次冰
期,全球在末次冰期間海平面比現今低約 120 公尺,使台灣海峽出露地表呈現侵蝕環
境。末次冰期以後海水位開始上升,台灣西部山脈前緣低海拔的盆地大多都受到海水
的侵入而成為淺水環境並開始沉積。
本研究利用電火花反射震測剖面以及底質剖面來更好的了解當時彰雲沙脊地區淺
水環境的變化。本研究中垂直海岸線的測線可以發現前陸基底不整合面,沉積物的堆
積重力促成前陸凸起形成,使得彰雲沙脊成為沉積盆地。剖面中的不整合面為末次冰
期間所形成的,震測剖面的特徵有被侵蝕過的強反射,隨著海平面的上升,持續堆積
會看到末次冰期、海進時期、以及高水位時期的層序。在底質剖面的部分可以清楚看
到沉積物有不同的沉積方向,由此探討沉積物運輸的過程。
根據震測剖面中的層序地層、震測相可以辨認出前陸基底不整合面,末次冰期不
整合面、快速海進時期、最大海漫面以及高水位時期。本研究利用各層面之等時面去
做網格化分析得知,從末次冰期到海進以及最大海漫面時期的海進過程並非只有濁水
溪的來源也有來自研究區域的北側開始進入,由沉積物的厚度來看可以發現當時最厚
的區域在濁水溪出海口附近,隨著海進開始有潮汐流發生,沉積物被潮流運輸開始往
東北向移動。;Abstract
Tidal sand ridges are common geomorphic features in modern continental shelves, formed by
coastal surface sedimentation primarily influenced by tides and currents, resulting in the
accumulation of sediment to form present-day sand ridges. Taiwan is situated in a collisional
orogeny zone of the Neogene, characterized by high erosion and sedimentation rates. The
geological formations are affected by the compressional forces from the orogenic belt and the
gravitational stacking of sediments, leading to the formation of foreland basins. Subsequently,
as the orogenic activity wanes and stabilizes, sediments prograde towards the continental
margin, infilling the foreland basins to transform them into shallow water environments.
Previous reconstructions of paleoenvironments and paleotopography indicate intense tectonic
activities in Taiwan, with rapid and complex rates of both sedimentation and erosion. The
most recent significant event was the Last Glacial Maximum (LGM), during which global sea
levels dropped approximately 120 meters, resulting in erosional environments. Following the
LGM, sea levels began to rise, inundating low-lying basins along the western foothills of
Taiwan, transitioning them into shallow water environments conducive to sediment
deposition.
This study utilizes high-resolution seismic reflection profiles and sediment cores to better
understand the variations in shallow water environments during that time. The results reveal
clear unconformities along the vertical shoreline transects, indicating discontinuities in the
foreland basement. Sediment thickness decreases seaward as the sediment is sourced from
river mouths, and the continuity of the unconformities stops at the defined foreland bulge. The
unconformities in the shallower sections formed during the LGM exhibit erosional features in
seismic profiles. With rising sea levels, continued sedimentation reflects products of
lowstand, rapid transgression, and highstand periods. In sediment core profiles, different
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sediment transport directions are clearly observed, elucidating the processes of sediment
transportation.
The study results identify foreland unconformities, LGM unconformities, rapid
transgression periods, maximum marine inundation, and highstand periods based on sequence
stratigraphy and seismic facies from seismic profiles. Utilizing isochronal surfaces for grid
mapping, it is inferred that during the LGM, transgression initiated from the north, with the
final deposition observed near the mouth of the Choshui River. As transgression commenced,
tidal currents began transporting sediment in a northeastward direction. |
