末次冰期以來彰雲沙脊地層構造演化過程

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Title:	末次冰期以來彰雲沙脊地層構造演化過程
Authors:	郭馥瑄;Kuo, Fu-Hsuan
Contributors:	地球科學學系
Keywords:	沙脊;末次冰期;彰雲地區;台灣海峽;電火花反射剖面
Date:	2025-01-23
Issue Date:	2025-04-09 16:53:41 (UTC+8)
Publisher:	國立中央大學
Abstract:	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 vii 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.
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