末次最大冰期以來台灣西部平原的環境變遷

Environment Evolution of the Western Coastal Plain in Taiwan Since the Last Glacial Maximum Period

楊詠然(Yung-Jan Yang)

指導教授 ： 陳文山

國立臺灣大學/理學院/地質科學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201602157

摘要

臺灣地處新生代弧陸碰撞造山帶，高侵蝕速率與高沉積速率的影響下，第四紀沉積層廣布於海岸平原前陸盆地。受到構造活動、海水面變遷與沉積物供應等因素影響，晚更新世以來沉積層中記錄豐富的環境變化史。為了解西部平原的環境變化，本研究分析了雲林至嘉義一帶尚未詳細研究的鑽井岩心的沉積相，並蒐集了台灣西部以及西南部平原地區鑽井資料與研究結果，期望能了解晚第四紀末次最大冰期以來海水面上升對西南部海岸環境所造成的影響。另外針對全球海水面、構造活動與沉積物供應等沉積環境變化的控制因素，以解析這三項因素如何操控西部平原沉積環境與海岸線的變遷。
經由層序與環境分析，得知末次最大冰期時西部平原出露水面，受到風化侵蝕形成不整合面（層序界限）。而此時期旺盛的河流下蝕作用，在海側形成數個下蝕凹谷，其中以高屏溪與曾文溪所形成的規模最大。由剖面對比建立的古環境變化史如下：約18,000年前因大規模暴露地表而形成土壤層及侵蝕面。約16,000~14,000年前，海水入侵到下蝕凹谷形成河口灣。約14,000~12,000年前，海水面逐漸上升，下蝕河谷被淹沒形成溺谷，且海岸線已推移至現今海岸平原中部。約9,500~9,000年前屏東平原以及雲嘉南平原南段先達到最大海漫面時期，而其他區域仍持續海進。約8,500~8,000年前雲嘉南平原北段至中段、台南與高雄北部地區以及高雄地區才陸續達到最大海漫面時期，此時屏東平原以及雲嘉南平原南段已呈現海退樣貌。彰化平原則必須到7,500~7,000年前才達到最大海漫面時期。6,000~4,000年前環境逐漸變淺，海岸線向海側移動至平原中部，此時台南台地已出露水面，而大灣低地仍覆蓋著海水形成大灣海峽。4,000年前至今環境快速變淺，海岸線快速向海側移動至今日樣貌。
研究結果顯示西部平原的環境變遷受到全球海水面、構造活動與沉積物三者交互作用，使不同平原具有不盡相同的古環境演變，而古地形起伏與坡度同樣會影響海岸線與環境的變遷。如各個平原達到最大海漫面時期的時間並不相同，但大致可以由形成時間分為三種區域，分別代表受到不同程度海水面變化、構造作用與沉積物供應的影響。另外經由自8,000年來平原地殼垂直變動速率的計算，得知6,000~4,000年之間嘉南平原海岸線快速朝海側遷移15~10公里，可歸因於受到平原東側傾斜台地的抬升，造成海岸線快速移動到平原西側的沉降盆地內。

關鍵字

海岸平原 ； 沉積環境 ； 海岸線 ； 末次最大冰期

並列摘要

In order to understand the depositional environment evolution and solve the impacts of eustacy, tectonic and sediment yield on western coastal plain of Taiwan since the last glacial maximum period, we use several borehores to reconstruct it. Based on the depositional environment, sequence stratigraphy and radiocarbon dating, significant environment and shoreline changes are recognized.
(1) The western Taiwan is characterized by a subaerial environment and formed several incised valleys during the sea-level fall in the last glacial maximum period.
(2) The environment experienced the sea-level rising with retrogradational deposition of the trangression system tracts and the shoreline was migrated into the incised valley and formed a drowned valley during 16,000~14,000 years ago.
(3) The ongoing sea-level rise causes the valley filled and environment changed deepen into intertidal zone during 14,000~12,000 years ago.
(4) Most areas occurred the deepest environment from shoreface to inner offshore during 8,500~8,000 years ago, and maximum flooding surface formed. But Tainan and Pingdong plain was experience the deepest environment earlier than 8,500~8,000 years ago.
(5) After 8,500~8,000 years ago, the system occurred progradational deposition of the highstand system tracts. The sediments progradated to the seaward, hence, the environment was shallow from intertidal zone to fluvial environment and the shoreline was retreated and migrated to the westward to the recent appearance.
The period of maximum flooding surface in different parts of the western plains are not the same. Based on the occurrence time of maximum flooding surface, it can be roughly divided into three types. Each type represents the complex interplay between eustacy, tectonics and sediment supply. In addition to the above three factors will control the depositional environment and shoreline changes, the ancient topography and slope will also affect the shoreline and environment evolution.
Based on the vertical deformation rate rechecked in order to solve the impact on shoreline evolution during 6,000~4,000 years ago. The result shows the eastern part of coastal plain is uplift environment, causing shoreline move quickly into the western side of the subsidence coastal plain.

並列關鍵字

coastal plain ； sedimentary environment ； shoreline ； last glacial maximum period

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