On the Taipei Dammed Lake

Abstract

To test the hypothesis that the Taipei Basin was a lake dammed from a coastal bay by the volcanic rocks at Kuandu, we examined the boreholes of the Taipei Basin and found in the shallow subsurface a widespread estuarine deposit indicative of the alleged coastal bay. This deposit, however, extends into the Tanshui estuary west of Kuandu and is neither overlain by any lacustrine deposit nor by volcanics. Instead, it overlies the volcanic rocks at Kuandu, indicating that the Tanshui river channel postdates the volcanics. These features demonstrate that the Basin has not been dammed by volcanics since the bay was formed, and thus the early hypothesis is invalid. The genuine dammed-lake event is recorded in the deep subsurface of the Taipei Basin as shown by a widely distributed laminated mud and associated lahar. The laminated mud is mostly embedded in the fluvial deposits, but in the northwestern part of the Basin, it is underlain by a north-thickening lahar that connects with the Tatun volcanoes. This stratigraphic relationship strongly suggests that the lahar derived from the Tatun volcanoes was responsible for damming the Taipei Basin. Because the areas west of the shangjiao Fault and east of Beitou show no signs of channel blocking, Fushinggang remains the only potential site for lahar damming. The damming took place at 180 ka and the lake lasted till 160 ka based on the integrated stratigaphic and radiochronol logical data. Before the damming, the Tanshui River Flowed north from the Taipei Basin through Fushinggang into the Tatun volcanic area. At around 180 ka, fierce volcanic eruption in the Tatun area spewed voluminous lahars into the Tanshui River valley and blocked the effluent channel. Soon the Tanshui River flooded the Taipei Basin and turned it into a freshwater lake. As the lake expanded and deepened, the lake water eventually spilled over the basement high at Kuandu and cut a new channel between Kuandu and Tanshui. The channel progressively eroded headward and undermined the basement dike at Kuandu. At 160 ka, the dike collapsed, the lake drained, and the Basin returned to a floodplain environment. If the Tanshui River had indeed passed through the Tatun volcanic area before the Taipei Basin was dammed, most of the Tatun volcanoes that presently stand more than 500 m high should not have existed before 180 ka. This age estimate is significantly younger than those exhibited by radiometric data of the Tatun volcanics, which calls for reassessment of the eruption history of the Tatun volcanoes. The dammed-lake event demonstrates that Tatun volcanism is capable of causing catastrophic flooding in the Taipei metropolitan area if the Tanshui River is blocked.This potential hazard, which has been overlooked in the past, needs to be carefully evaluated in the future.

臺北盆地曾為堰塞湖的假說，至今已流傳八十餘年。早期學者認為臺北盆地原本是一海灣，後因關渡出口被火山岩堵塞而形成湖泊。根據新獲得的探井資料，臺北盆地的地下確實有一海灣堆積層; 不過這海灣層可向西越過關渡，延伸到淡水河口。海灣層之上既無火山岩層，也無湖泊堆積層，沒有任何堰塞湖的證據。因此，「海灣堰塞成湖」的說法與事實不符。真正的堰塞湖記錄展現在盆地的深部，是一層分布廣闊的湖相紋泥層。這層紋泥的上下大多是河相堆積物，不過在盆地的西北角，紋泥層覆蓋在一層凝灰質角礫岩之上。角礫岩層的岩性與大屯火山群相似，是一道來自大屯山區的火山泥流，並且是造成盆地堰塞成湖的原因。依據區域地質和地層資料綜合研判：堰塞前臺北盆地應為一河川平原，古淡水河當時由臺北盆地向北流經復興崗後，穿越大屯山區入海。在大約十八萬年前，大屯火山噴發，阻塞了淡水河的河道，使臺北盆地氾濫成湖。這個湖維持到十六萬年前潰決，盆地再度乾涸成河川平原。由於淡水河在臺北盆地堰塞前很可能穿越大屯山區，因此今日所見之大屯火山群當時應尚未成形。這項推論比以往所認知的大屯火山群年代要晚，顯示我們需重新檢討大屯火山群的噴發史及噴發潛能，並審慎評估它們對臺北都會區可能造成的災害。