彰化平原區晚第四紀沉積物高解析度層序地層學研究

High-resolution Sequence Stratigraphic Analysis of Late Quaternary Deposits of the Changhua Coastal Plain in the Frontal Arc-Continent Collision Belt of Central Taiwan

陳華玟(Hua-Wen Chen)

指導教授 ： 李通藝

國立臺灣師範大學/理學院/地球科學系/博士(2010年)

摘要

由於沉積盆地的相對海水面變化(relative sea level change)是由大地構造(tectonic)，全球海水面(eustatic)變遷及沉積物供應(sediment supply) 等的交互作用所產生。而沉積的地層紀錄則可反應此一相對海水面變化的結果。因彰化平原及八卦台地位處褶皺逆衝斷層帶最前緣，為一構造活動活躍的區域，若以一系列依序向西遷移的逆衝斷層而言，彰化斷層應為中臺灣最年輕的逆衝斷層，且彰化平原又緊鄰臺灣海峽，實為研究大地構造及全球海水面變遷對沉積盆地相對海水面變化影響的最佳區域。故本研究主要目的為藉由彰化平原地區晚第四紀鑽井岩心紀錄的研究，來了解全球海水面變遷及構造活動對本區作用影響的相對重要性，並重建本區的沉積演變史。
本研究主要利用中央地質調查所地下水觀測網及區域地質調查於彰化平原地區鑽探的30口岩心（多數岩心深度約200公尺）進行研究，研究結果可辨識出25種岩相，由相岩相組合可區分出九種主要沉積環境。包括遠濱過渡帶屬於大陸棚（shelf）沉積體系，濱面、河口灣、潟湖、潮道、潮坪屬於邊緣海（marginal marine）沉積體系，河流相（包括河道及泛濫平原）及沖積扇則併屬於陸地（terrestrial）沉積體系。
研究結果顯示，井深200公尺左右之岩心依層序地層分析可定出三個層序界限(SB)，由上而下分別為SB1, SB2及SB3，即可分出層序1(SQ1)、2 (SQ2)及3 (SQ3)三個層序，反應了三次主要的相對海水面上升、下降的循環。根據碳十四定年資料指示SQ1之沉積年代在海側約為16 ka至現代，陸側約10 ka至現代，SQ2約為20至40 ka之間。兩層序之界限為不整合面，時間間隔約4 ka至10 ka。
本區自晚更新世至全新世(40-0 ka)的沉積演化史可綜述如下：4萬年前，彰化平原全區為陸相沉積物所覆蓋，之後可能由於彰化斷層不同區段活動性的差異造成盆地在花壇以北較以南為高的形態。在此時期，平原中部的海側有一下切谷地(incised valley)的生成。40 ka以後，相對海水面上升，下切谷地有河口沉積物的充填，南端海側為遠濱過渡帶環境，其它海側則為邊緣海環境。之後相對海水面持續上升，約至35 ka左右河口灣填滿，濱面砂及大陸棚沉積物覆於其上並向陸側退積(retrogradation)，海水面於30 ka左右達到最大海漫面(maximum flooding surface, MFS)，此時期之沉積物係屬海進體系域。約30至20 ka期間，海水面持續下降，大陸棚、邊緣海及河流–沖積扇陸相沉積物向海側加積(progradation)，屬高水位體系域沉積。而古濁水溪河道主要位於平原中央。大約在20 至18 ka之間為全球末次冰期的最大冰期(LGM)，本區除八卦台地邊緣外並無沉積紀錄，顯示SB1的不整合面是與末次最大冰期的不整合一致，此時期有4個下切谷地生成，而在18至16 ka期間，本區仍為一侵蝕區。之後開始海進，約16至10 ka之間下切谷地為河口灣沉積物所充填，是屬於SQ1的下部海進體系域。而在約10至6 ka間持續的海進，於河口填滿後再上覆屬於上部海進體系域的大陸棚及邊緣海沉積物，並持續向陸側退積，約在7至6 ka海水位上升達到MFS。高水位體系域的沉積年代約為6 ka至現代，海退的大陸棚及邊緣海以及河相沉積體系向海側加積，但1千年以來的河相沉積物並未持續往海岸加積，而被侷限於平原中央，海側則為邊緣海沉積物的向上加積（aggradation）所填滿。且在1 千年以後，平原中央的河道沉積物轉為沖積平原沉積物，濁水溪主要河道乃向南遷移。
在全新世時期本區為相對的構造活動較安靜期，但沉積物堆積速率則明顯的較之前快很多，顯示末次間冰期彰化平原區（除了八卦台地邊緣外）沉積體系的變化及分佈主要受控於全球海水面的變化因素。但此時期彰化平原的東南側即使於海水面達最大海漫面時期，全區幾乎覆於海水面的狀況下仍保持陸相的沉積形態，則可能是受控於該區濁水溪沉積物供應速率較快的因素。而在2萬年至4萬年 SQ2 沉積期間，沉積物的堆積空間主要由盆地的下沉所提供而非全球海水面的變動所提供，則指示在末次冰期晚期時，構造活動為影響本區沉積形態的主要控因。

關鍵字

彰化平原 ； 晚第四紀 ； 沉積環境 ； 層序地層 ； 海水面變化 ； 構造活動

並列摘要

For the aim to reconstruct the depositional evolution and solve the impacts of eustacy and tectonic on relative sea-level change of the Late Quaternary foreland basin- Changhua Coastal Plain, detailed sequence stratigraphic analysis of 30 cores from the Late Quaternary deposits of the Changhua Coastal Plain that provides a 40 ka high resolution record of the depositional history of a region situated in the frontal arc-continent collision belt of Taiwan. Twenty-five lithofacies and nine facies associations are recognized in the cores. Three depositional systems are from terrestrial system (alluvial fan and fluvial) to marginal marine (estuarine, shoreface, lagoon, tidal flat, marsh) and shallow shelf (offshore-transition) systems. Sedimentology and sequence stratigraphic analysis shows that the sedimentation of study area reflects two main cycles (sequence 1 and Sequence 2) of sea-level falls and rises. Transgressive (TST) and highstand (HST) deposits can be differentiated in sequence 1 (SQ1, 0-16 ka) and sequence 2 (SQ2, 20-40 ka). Sequence boundary 1 (SB 1) is an unconformity.
The Late Pleistocene to Holocene (0-40 ka) depositional evolution of the study area is summarized as follows. Fluvial sedimentation prevailed before 40 ka, and then a differential tectonic activity was recorded in regions north and south of the HT well with a wide and shallow incised valley formed in the middle area of Changhua Coastal Plain in this period. Since 40 ka, estuary (middle area) and shelf (southernmost area) deposits formed the lower TST during 40-35 ka. Between 35–30 ka was characterized by retrogradation of shelf and marginal marine systems (upper TST), and then sea-level reached maximum (MFS2) at about 30 ka. HST (30-20 ka) was characterized by shelf, marginal marine, and fluvial–alluvial fan progradation. HST phase was followed by non-deposition and erosion during the Last Glacial Maximum unconformity (SB1), except the western margin of Pakua tableland. Between 16 and 6 ka, the Changhua Coastal Plain experienced rising sea-level with retrogradational deposition. From 16 to 10 ka, estuarine deposits (lower TST) were deposited. From 10 to 6 ka, shelf and marginal marine (upper TST) deposited. By about 7–6 ka, sea level reached its peak (MFS1), and shelf and marginal marine sediments covered most of the study area. After 6 ka, the highstand was dominated by progradation of shelf, marginal marine and terrestrial deposits. About 1 ka, fluvial deposits were preserved in the central part of the Changhua Coastal Plain. Since then, fluvial channels shifted southward to the present-day Chuoshuei River.
The depositional patterns of SQ1 and SQ2 in this tectonically active area reflected the complex interplay between high-frequency sea-level fluctuations, tectonics (subsidence and uplift), and autocyclic processes. The study area was relatively speaking in tectonic quiescence during Holocene but sediments accumulation rate in this period was about 3 times of before which may indicate the depositional pattern of Changhua Coastal Plain in the last interglacial was majorly controlled by eustacy, and fast sediment supply made that southeastern area only recorded the terrestrial sediments in this period. But the accommodation for the SQ2 deposits was largely created by tectonic subsidence since regional tectonism was very active during late last glacial.

並列關鍵字

Changhua Coastal Plain ； Late Quaternary ； sedimentary environments ； sequence stratigraphy ； sea-level change ； tectonic

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