| Summary: | 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.
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