11:00 〜 11:15
[HGM02-07] The landscape evolution of the buried valley in the lower reaches of the Dawu River, southeastern Taiwan
キーワード：buried valley, fluvial landscape evolution, X-ray fluorescence, resistivity image profile, incised valley
The Dawu River (DR) and the Zhaoyong River (ZR) are rivers that flow eastward into the Pacific Ocean in southeastern Taiwan. A wide and flat valley 2 km in length and 0.35 km in width traverses between the lower reaches of both rivers, and the valley is similar to an abandoned valley after being captured. Therefore, this study reconstructed the landscape evolutionary history based on geomorphic analysis, resistivity analysis, and sedimentary facies analysis, X-ray fluorescence and radiocarbon dating analysis of a 60-meter-long core.
The landscape evolution can be divided into four stages. Stage 1: During the Last Glacial Maximum. The resistivity image profile shows that the valley floor at that time was more than 100 meters lower than the current valley floor, and it may have been affected by global sea level reduction, forming an incised valley. Stage 2: Late Glacial to approximately 9 ka. Due to the rising sea level, the river valley at this time mainly aggraded fluvial gravels and sandy sediments similar to the ZR, as well as debris flow deposits from the surrounding slopes. We consider that this valley is likely to have been a tributary of the DR and originated from the modern ZR catchment. The original incised valley was buried by sediments more than 40 meters thick, which conforms with the resistivity analysis result. Stage 3: Approximately 9 ka to 6.5 ka. The valley mainly aggraded sandy or muddy sediments with mottled or parallel-laminated structures. It may be that the average channel-gradient ratio decreased due to the rapid sea level rise in the early Holocene, resulting in the weakening of river energy. Moreover, there are several layers of coarse sand that may have come from the DR or coast, some of which have obviously low log (Ti/Ca) values. We speculate that the valley at this stage was often flooded by the DR and may even have been affected by the sea, but the current data cannot confirm this speculation. Stage 4: Approximately 6.5 ka to modern. The 4.5-meter-thick muddy sediment at the top of the core with a strongly mottled structure may be related to closing to the surface for a long time or agricultural activities. The relatively thin sediment layer may also indicate that this tributary lacked sediment supply from upstream. Therefore, it is proposed that the alluvial fans began to develop on the left side of this tributary at approximately 6.5 ka, which blocked the channel and caused the river to divert into the ZR while gradually stopping aggradation in the valley and becoming an abandoned channel.
In summary, we propose an incised valley that was buried under the effects of sea level rise. The cause of the buried valley being abandoned is river diversion due to the development of alluvial fans upstream, rather than river capture. Finally, we estimate that the modern altitude of the continental sediments during the highest sea level period at 7.1 ka is approximately 5.8 m a.s.l., and it can be deduced that the uplift rate here may be quite low or even in a subsidence state.
The landscape evolution can be divided into four stages. Stage 1: During the Last Glacial Maximum. The resistivity image profile shows that the valley floor at that time was more than 100 meters lower than the current valley floor, and it may have been affected by global sea level reduction, forming an incised valley. Stage 2: Late Glacial to approximately 9 ka. Due to the rising sea level, the river valley at this time mainly aggraded fluvial gravels and sandy sediments similar to the ZR, as well as debris flow deposits from the surrounding slopes. We consider that this valley is likely to have been a tributary of the DR and originated from the modern ZR catchment. The original incised valley was buried by sediments more than 40 meters thick, which conforms with the resistivity analysis result. Stage 3: Approximately 9 ka to 6.5 ka. The valley mainly aggraded sandy or muddy sediments with mottled or parallel-laminated structures. It may be that the average channel-gradient ratio decreased due to the rapid sea level rise in the early Holocene, resulting in the weakening of river energy. Moreover, there are several layers of coarse sand that may have come from the DR or coast, some of which have obviously low log (Ti/Ca) values. We speculate that the valley at this stage was often flooded by the DR and may even have been affected by the sea, but the current data cannot confirm this speculation. Stage 4: Approximately 6.5 ka to modern. The 4.5-meter-thick muddy sediment at the top of the core with a strongly mottled structure may be related to closing to the surface for a long time or agricultural activities. The relatively thin sediment layer may also indicate that this tributary lacked sediment supply from upstream. Therefore, it is proposed that the alluvial fans began to develop on the left side of this tributary at approximately 6.5 ka, which blocked the channel and caused the river to divert into the ZR while gradually stopping aggradation in the valley and becoming an abandoned channel.
In summary, we propose an incised valley that was buried under the effects of sea level rise. The cause of the buried valley being abandoned is river diversion due to the development of alluvial fans upstream, rather than river capture. Finally, we estimate that the modern altitude of the continental sediments during the highest sea level period at 7.1 ka is approximately 5.8 m a.s.l., and it can be deduced that the uplift rate here may be quite low or even in a subsidence state.