臺北盆地之相分析及沉積歷史

Abstract

臺北盆地位於台灣北部，為一發育在山脈上的拉張盆地，地下堆積層沿山腳斷層呈一半地塹，厚達670公尺。為瞭解盆地形成至今的演化過程及控制因素，本研究彙整37口探井，進行地層對比、岩相分析，並結合放射性定年，藉以探究盆地堆積層的堆疊型態及沉積歷史。 因盆地堆積層相變劇烈，本研究以事件地層作為分層及對比的依據，將盆地堆積層由下至上分為板橋層下段(400-180ka)、板橋層上段(180-160ka)、五股層(160-30ka)、景美層(30-23ka)、松山層(23-0ka)。整套堆積層的沉積特徵可以3種礫相、4種砂相、1種砂泥互層相及3種泥相代表，反映沖積扇、火山外圍平原、辮狀河、氾濫平原、內河口灣、中央河口灣及湖泊之沉積特徵。 透過沉積剖面、各地層單元之厚度及沉積相分布圖，可發現臺北盆地各時期的堆積中心及沉積系統不盡相同，整體的沉積相具有逐漸加深的趨勢。大體而言，板橋層下段為沿山腳斷層之條帶狀沖積扇相礫層，其在五股─泰山一帶轉向新店溪上游，且向盆地西北相變為火山外圍平原相。板橋層上段呈平板狀分布，以湖泊相紋泥層為主。五股層為沿山腳斷層產生之堆積楔，可延伸至盆地西南，其中堆積物以辮狀河、氾濫平原、內河口灣相為主，但亦包含零星的沖積扇相。景美層為一廣布盆地之沖積扇相礫層，堆積中心在盆地之西南角。松山層則為向出海口增厚的堆積楔，河口灣為主要的沉積系統。地層中的沉積相變化相當明顯，特別是五股層及松山層，其演變序列可與全球海水面變化相互呼應。透過地層的堆積特徵，可從中洞悉古地形、山腳斷層及火山刻劃了盆地早期之堆積，而全球海水面的影響則在山腳斷層擴張至盆地西南後才逐漸明顯。 整合區域地質及地層後，本研究推斷臺北盆地在40萬年前開始堆積。當時古新店溪在泰山一帶被山腳斷層截切後進入盆地，大屯火山群亦開始提供大量沉積物，導致盆地最初的沖積扇淤塞在山腳斷層崖及古新店溪河谷內。18-16萬年前，大屯火山群的火山泥流將盆地堰塞為一深水湖泊。16-3萬年前，堰塞湖潰堤後，盆地回復為河川平原，且開始受全球海水面影響，感潮河段曾在MIS5及3進入盆地西半部。約3-2萬年前，全球海水面下降導致盆地東半部之沉積物被劇烈侵蝕，而後大漢溪被襲奪入盆地內，與新店溪形成一聯合沖積扇。在2萬年至今，全球海水面逐漸上升，河口灣亦首度深入盆地內部，範圍近乎涵蓋整個臺北盆地，但最後因沉積物向外堆疊而成為現今的樣貌。

In the northern tip of the active Taiwan Mountain Belt, a Quaternary extensional basin, namely Taipei Basin, has developed atop the deformed orogenic basement as a result of post-orogenic collapse, and comprises 670m siliciclastic sediments infill. This study provides preliminary stratigraphic architecture and facies analysis on 37 cores to display the nature of basin filling and its evolution. The basin deposits could be divided into five stratigraphical units, which, from the bottom to the top, are lower member of Banchiao Formation (400-180ka), upper member of Banchiao Formation (180-160ka), Wuku Formation (160-30ka), Jingmei Formation (30-23ka), and Songshan Formation (23-0ka). These deposits could also be classified into 11 lithofacies, which represent alluvial fan, braided river, floodplain, lake, inner estuary, central estuary and volcanic ring plain facies. The lower member of Banchiao Formation exhibits a ribbon-like alluvial-fan gravels intercalating with volcanic deposits in the northwest. The upper member of Banchiao Formation is a planar lacustrine mud layer extending to whole basin. The upper 3 units each appears like a sedimentary wedge that thickens toward the bounding fault and outlet of the basin, except the Jingmei Formation is composed of alluvial-fan gravels whose depocenter is on the southwestern basin margin. Drastic facies change and their stacking pattern imply the fluvial system in this basin was modulated by glacio-eustatic fluctuations since MIS (Marine Isotopic Stage) 5. However, stratigraphic geometry and facies distribution demonstrate the complex interplay among bounded fault, volcanos and inherited paleo-topography, which are predominate controlling factors in this collapse basin. By integrating Taipei Basin stratigraphy with regional geology, it can be inferred that the basin started accumulating sediment along a paleo-valley inherited from the mountain building stage. As the bounding normal fault formed in consequence of orogenic collapse, its hanging wall slid down and captured paleo- Xindian River into a depression that began to pond gravels as the result of Tatun Volcanoes Group growing in the north at 400ka. Meanwhile, deposits from surrounding volcanoes jammed up the paleo-valley, and eventually dammed up the Taipei Basin, forming a deep tropical lake at 180ka. When the lake levee was broken down at 160ka, basin subsidence and bounded fault expansion enlarged the accommodation space, the basin was occupied by widespread fluvio-estuarine deposits, and its tidal river reached the basin at sea-level highstand of MIS5 and 3 as well. Later, a large amount of sediments was fed from a large captured river in the Last Glacial Age to deposit alluvial fan gravels. When sea level rose since Last Glacial Maximum, the basin was first inundated as a large estuary and then filled up by inner estuarine and fluvial sediments.