晚第四紀以來台北盆地樹林地區岩芯的沉積物組成特性: 探討大漢溪的襲奪年代

Abstract

大漢溪河谷在桃園縣石門附近呈90 度轉彎，之前學者即已提出 在三萬年前，或兩萬五千年前河川襲奪的看法。本研究以岩芯沉積物 進行組成分析，探討大漢溪被襲奪，流入台北盆地的年代。 本研究選取中央地質調查所於2002 年鑽探，位於樹林地區的三 口鑽井(SCF14、SCF16、SCF17)。統計礫石的組成分析，並採集岩芯的 中- 粗砂，進行砂質沉積物組成研究。 依據砂質沉積物岩象分析，將其分成兩大類型：第一型砂質沉積 物分布在松山層和景美層，以硬頁岩和變質砂岩岩屑為主，兩種岩屑 含量總和為42-87%；其來源區應是今日大漢溪流域，範圍包含上游流 經雪山山脈的變質岩區，以及中下游的西部麓山帶。第二型砂質沉積 物分布在五股層及板橋層，以單晶質石英含量（41-66%）最多，以及 部分硬頁岩岩屑（10-22%）；其來源區應是現今三峽溪的流域，包括 的地層為中新世為主的沉積岩與部分漸新世硬頁岩的岩層。 樹林SCF17、SCF16、 SCF14、板橋PC2、新莊HC、五股WK1 六口 井岩芯沈積物，都顯示五股層和景美層沉積物組成有明顯的不同，組 成變化是位在五股層與景美層的交界。因此，大漢溪襲奪的年代，應 該是景美層開始沈積時。由岩芯漂木碳14 年代，及河階沉積物光螢光 定年，推斷大漢溪襲奪年代應介於61.2+5.6ka 至50ka 之間，或接近 61.2+5.6ka

In order to understand the timing of river capture of the Dahan River in the north Taiwan, this study collects three boreholes data, using petrographic analysis, radiocarbon and luminescence dating in addition to stratigraphic correlation that suggests the river capture may occurs at a range of 50 ka ~ 61.2 +5.6 ka. Three boreholes of SCF14, SCF16 and SCF17;wells are located around the Shulin village where the Dahan River turns at right angle and flowing northward into the Taipei Basin. This location is believed recording the evolution of the Dahan River. Two types of sediments composition can be distinguished by petrographic analysis. Type I mainly comprises argillite and meta-sandstone fragments (42-87%), this type belongs to the Jinmei and Shunshan Formations. Type II is composed of mono-quartz (41-66%) and argillite fragments (10-22%) belongs to the Banciao and Wuku Formations. A significant composition change is in the bottom of the Jinmei Formation. It is reasonable to interpret that the timing of river capture of the Dahan River occur at the initial deposition of the Jinmei Formation. In addition, laterally, to correlate with the other boreholes (PC2, HC and WK1) and radiocarbon dating that suggests the bottom of the Jinmei Formation is older than 50 ka. On the other hand, the age of the fan terraces that exposed in the western side of the Dahan River is 61.2 +5.6 ka by luminescence dating. Hence, the time range between 50 ka ~ 61.2 +5.6 ka is the timing of river capture of the Dahan River.