臺北盆地五萬年來之沉積體系與盆地下陷演育研究

Abstract

本研究基於54口探井岩芯的岩相分析所解釋沉積環境以及153筆定年數據的分析，重整了臺北盆地松山層與景美層之地層架構，並重建過去5萬年來的盆地時空環境與地層演育過程。此外又根據年代資料、定年樣品之取樣深度和所在環境沉積相，對比全球海水面變動曲線，推算盆地不同時期的盆地下陷速率，並探討海水面變動、盆地下陷速率與沉積物供應速率之互制關係如何影響盆地的地層發育。 本研究發現在全球海水面下降期間 (50－20 ka)，早期氣候潮濕時盆地廣布礫質辮狀河，而後氣候轉乾，約在35－20 ka，乾燥氣候下較低的沉積物供應速率和/或快速盆地下陷主導盆地內的沉積環境變化，粗顆粒的沉積物供應不足使近斷層的相對下游地區沉積環境呈現相對海水面上升的變化特徵，從礫質辮狀河轉變到砂質辮狀再轉為曲流河相，遠離斷層的相對上游側可能因為地形坡度較陡和近沉積物來源，而還能堆積礫質辮狀河相。 在全球海水面上升的階段 (20－6 ka)，使環境變淺的沉積物供應速率和使環境加深的盆地下陷速率達到平衡，因此盆地長時間維持在曲流環境，臺北盆地西側於20－10.2 ka因此堆積了厚達40公尺的曲流相沉積物。而此時期遠離斷層的盆地南側和東南側受盆地下陷影響有限，並無足夠的納積空間，以致無法保存沉積物，也造成景美層與松山層之間長達約1萬年的沉積中斷。約11－8.8 ka的融冰脈衝MWP 1B造成全球海水面快速上升，促使河口灣相於約10.2 ka首次出現於位處內陸的臺北盆地，較世界其他地區的海岸河口灣還要早了2－3千年，可能係受到10.6－10.2 ka盆地快速下陷的影響。海水面持續的快速上升也逐步擴大了河口灣環境的分布，河口灣環境的擴展約略與隨後的兩次盆地快速下陷期 (9.4－9.0 ka和8.9－8.5 ka) 同時發生，可見盆地快速下陷對河口灣環境的出現和擴展有一定程度的幫助。 全球海水面雖然到6 ka後才趨於穩定，但7 ka後以減緩上升速度，此時盆地下陷速率也保持相對穩定，充足的沉積物供應隧逐漸填滿了河口灣，使得臺北盆地逐漸演變到現今的曲流河相環境。 本研究所建立的地層演育特徵，為具有高沉積物供應速率的連海內陸半地塹盆地的地層發育提供了新的見解，尤其在盆地下陷速率與全球海水面變動之間的相互作用上提供新的認識。

This study establishes the stratigraphic architecture of the Taipei Basin and its temporal and spatial paleoenvironmental development during the past 50 kyr based on analyses of 54 borehole cores and 153 age data. We calculate the rates of basin subsidence from the borehole data at depths where radiocarbon age dates are available. Our results indicate that, during the eustatic sea level falling period (50–20 ka), the basin was covered by gravelly braided river deposits in the cold but wet climate. However, during 35-20 ka, the climate changed to cold and dry, low rates of sediment supply and/or rapid basin subsidence controlled the development of sedimentary environments, leading to a change in the depositional environment from gravelly braided rivers, through sandy braided rivers, to meandering rivers with falling eustatic sea level. During the early stage of eustatic sea level rise (~20–10.2 ka), balanced rates of sediment supply and basin subsidence maintained the meandering river environment, and the meandering river deposits accumulated up to 40 m thick. On the other hand, in the relative upstream area, the far-fault area, lack of accommodation space created by the basin subsidence led to the lack of sediment preservation. Thus there is a 10-kyr gap between Sungshan Fm and Jingmei Fm. Rapid sea level rise caused by melt water pulse 1B took place during 11–8.8 ka led to the initial appearance of estuarine facies at ~10.2 ka and widened the distribution of the estuarine environment after 9.4 ka; however, the coeval phases of rapid basin subsidence ~10.6–10.2 ka, 9.4–9.0 ka and 8.9–8.5 ka promoted the first appearance and widening of the estuary, respectively. After 7 ka, when the eustatic sea level and rate of basin subsidence remained relatively stable, sufficient sediment supply gradually infilled the estuary although the eustatic sea level went stable after 6 ka. The established stratigraphy improves our understanding of the factors controlling stratigraphic development in marine-influenced inland half grabens, specifically in the Taipei Basin with high rates of sediment supply. Accommodation space in the Taipei Basin was governed by eustatic fluctuations with modulations by basin subsidence and sediment supply since the last glacial period.