|Title:||On the Taipei Dammed Lake
|Keywords:||堰塞湖;臺北盆地;大屯火山;自然災害;第四紀;Dammed lake;Taipei basin;Tatun volcanoes;Natural hazard;Quaternary||Issue Date:||Jun-2004||Journal Issue:||36||Start page/Pages:||077-100||Source:||地理學報||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.
|Appears in Collections:||地理環境資源學系|
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