11:45 AM - 12:00 PM
[HQR04-10] The landscape evolution of the buried valley driven by sea level and tectonic controls in the lower reaches of the Dawu River and the Zhaoyong River, southeastern Taiwan
Keywords: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 0.36 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 research reconstructs the landscape evolutionary history based on electrical resistivity analysis, quantitative geomorphological 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 I: Last Glacial Maximum to approximately 10 ka. The resistivity image profile shows that the buried valley floor at that time was approximately 100 meters lower than the current valley floor, and it may have been affected by global sea level fall, forming an incised valley. Due to the sea level rising in the Late Glacial, fluvial gravels and sandy sediments similar to the ZR aggraded in the river valley. This valley was 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 35 meters thick, which conforms with the resistivity analysis result.
Stage II: Approximately 10 ka to 9 ka. A dammed lake with approximately 3.5 m mottled mud sediments formed due to no tectonic uplift in this area, and as well as debris flow deposits from the surrounding tributaries aggraded in the river valley.
Stage III: Approximately 9 ka to 6.5 ka. Sandy or muddy sediments with mottled or parallel-laminated structures mainly aggraded in the buried valley. It may be that the average relief ratio decreased due to the rapid sea level rise in the early Holocene, resulting in the weakening of river energy, starting forming alluvial fans at the west side of the valley and creating the floodplains, wetlands or dammed lakes environment. Moreover, there are three layers of coarse sand that may have come from the DR or coast, some of which have obviously low log (Ti/Ca) values. The valley at this stage was often flooded by the DR and may even have been affected by the sea water.
Stage IV: 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 also indicates that this tributary (the buried valley) lacked sediment supply from the upstream (the ZR), which resulted from the development of the alluvial fans at the west side of the valley blocked the channel, causing the river to divert into the ZR, and let the buried valley become an abandoned river at approximately 6.5 ka at the latest. After the diversion of the ZR, the tributaries of the valley slowly and gradually supplied fine-grain sediments into the valley, finally flattening the buried valley.
In summary, the incised valley was buried under the effects of sea level rise since the Late Glacial. The cause of the buried valley being abandoned is river diversion due to the development of alluvial fans upstream, rather than river capture. Finally, this research found that the uplift rate is lower than 0.24 millimeters per year in the lower reaches of the DR and the ZR.
The landscape evolution can be divided into four stages.
Stage I: Last Glacial Maximum to approximately 10 ka. The resistivity image profile shows that the buried valley floor at that time was approximately 100 meters lower than the current valley floor, and it may have been affected by global sea level fall, forming an incised valley. Due to the sea level rising in the Late Glacial, fluvial gravels and sandy sediments similar to the ZR aggraded in the river valley. This valley was 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 35 meters thick, which conforms with the resistivity analysis result.
Stage II: Approximately 10 ka to 9 ka. A dammed lake with approximately 3.5 m mottled mud sediments formed due to no tectonic uplift in this area, and as well as debris flow deposits from the surrounding tributaries aggraded in the river valley.
Stage III: Approximately 9 ka to 6.5 ka. Sandy or muddy sediments with mottled or parallel-laminated structures mainly aggraded in the buried valley. It may be that the average relief ratio decreased due to the rapid sea level rise in the early Holocene, resulting in the weakening of river energy, starting forming alluvial fans at the west side of the valley and creating the floodplains, wetlands or dammed lakes environment. Moreover, there are three layers of coarse sand that may have come from the DR or coast, some of which have obviously low log (Ti/Ca) values. The valley at this stage was often flooded by the DR and may even have been affected by the sea water.
Stage IV: 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 also indicates that this tributary (the buried valley) lacked sediment supply from the upstream (the ZR), which resulted from the development of the alluvial fans at the west side of the valley blocked the channel, causing the river to divert into the ZR, and let the buried valley become an abandoned river at approximately 6.5 ka at the latest. After the diversion of the ZR, the tributaries of the valley slowly and gradually supplied fine-grain sediments into the valley, finally flattening the buried valley.
In summary, the incised valley was buried under the effects of sea level rise since the Late Glacial. The cause of the buried valley being abandoned is river diversion due to the development of alluvial fans upstream, rather than river capture. Finally, this research found that the uplift rate is lower than 0.24 millimeters per year in the lower reaches of the DR and the ZR.