Fluvial Terrace Landscape in Tai-Yuan Basin and Its Implications

• Main Authors: Ting-Yi Chang, 張庭漪
• Other Authors: Jiun-Yee Yen
• Format: Others
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/9t7t4w

碩士 === 國立東華大學 === 自然資源與環境學系 === 105 === Fluvial terraces are the residual expression showing how a river responds to the allogenic (climate change, tectonic movement and sea level eustacy) and autogenic controls. As a river reacts to the environmental changes, the channel adjusts its characteristic such as channel width, channel slope, regional topography and so forth. Terraces are shaped accordingly by river incision, lateral erosion, and abrasion under the processes of channel migration or meander cut off. The formation of the terraces, therefore, archives the evolution history of the river network. This study aims to investigate the timing and mechanisms regarding the formation of fluvial terrace landforms in the Tai-Yuan Basin. Tai-Yuan Basin is located in the southern section of the Coastal Range, eastern Taiwan, where actively uplifting has been continuously monitored. The geological framework of the basin is a structural syncline resulted from the ongoing arc-continent collision between the Eurasian plate and Philippine Sea plate. The most prominent landforms in Tai-Yuan Basin are fluvial terraces and meanders. Mawuku River, the major river within the basin, consists of the north-south trending North River, South River and several other east-west trending tributaries. These rivers almost all display similar features of terraces and meander developments. Investigation areas in this study include the Mawuku River, North River, South River and Madajida River, one of the tributaries of South River. Field observations were further analyzed and validated with the assistance of digital elevation models(DEM), radiocarbon dating and Chi analysis using LSDTopoTools. Fluvial terraces in Tai-Yuan Basin are mainly built of bedrock strath and covered with relatively thin fluvial sediments. The relative height of the terraces range from 3 to 80 meters, and thereby can be identified as sequence T1 to T8, from low to high. The thickness of alluvium above bedrock is about 6~8 m in general, with a maximum of VI 14~20 m. Based on these data, the forming process of terraces in North River, Mawuku River and Madajida River can be reconstructed as a consecutively intermittent downcutting, while in the South River it reveals a period of sedimentation followed by the normal river incision near 7~6 ka. Radiocarbon dates from the acquired terrace sediments suggest that all of these terraces had formed in Holocene (the oldest one dates about 9050~8990 cal. y BP and the youngest dates about 925~785 cal. y BP). The incision rate of Mawuku River is then estimated about 4.1 mm/yr. The χ plot of the North River shows more distinct signatures of graded river profile than that of the South River. This is consistent with our observation of the characteristic behavior for the intermittent downcutting terraces. The χ plot of South River shows transient profile at the elevation of 200~250 m, which could be a knickpoint. The changes of Mχ the gradient of χ plot, indicates the segment break in χ plot. These segment breaks are found in specific elevation both commonly in the North River, the South River and other tributaries. This indicates the increases of uplift rate for several stages and suggests that the Mawuku River is highly influenced by tectonic uplifting. In conjunction with the climatic variations, sea level eustacy and tectonic uplifting, the uplift rate of southern Coastal Range can be estimated around 5~7 mm/yr since the late Pleistocene. The glacial-interglacial transition at late Pleistocene- Holocene(12.8~11.5 ka), fits in the window and provides a possible cause that has triggered these Holocene terraces to form. To sum up, the Mawuku River has developed prominent terraces and meanders for the past 10 ka. The reconstruction of terrace forming process of the major tributaries suggest that these strath terraces were strongly influenced by the constant tectonic force related to the syncline structure and the migration of meanders.