By formation age, depositional feature, and spatial distribution to deduce the evolutional history of Laonong River, southern Taiwan
Date Issued
2008
Date
2008
Author(s)
Lin, Ming-Ching
Abstract
Lao-Nong River is mostly located in Kaohsiung County, whose drainage basin is bounded by Central Mountains on the east and the fold-and-thrust belt of Western Taiwan on the west. There have been multiple landslides since Mindulle Typhoon, and so many outcrops are present that it is easier to sample substances for radiocarbon dating. Therefore this study aims to re-examine river terraces in this area. By studying aerial photos and undertaking field investigation, this research aims to understand the types of Lao-Nong River terraces and their sedimentary faces, and also to obtain the age of these river terraces by radiocarbon dating. The purpose is to correlate river terraces with an eye to establishing the evolutionary history of Lao-Nong River’s drainage basin and examining the factors that influence the distribution of its terraces. This study has altogether sampled twenty radiocarbon substances. While it is hard to correlate the river terraces between Mei-Shiow and Shi-Sher, it may be identified that rock terraces span south of Shi-Sher while some fill terraces are scattered north of Mei-Shiow. Thus the river terraces in this section can be divided into two groups: northern terrace group (NT) and southern terrace group (ST). NT is located north of Mei-Shiow and ST, south of Shi-Sher. NT consists of five terraces, all of which were generated about three thousand years ago. ST can be subdivided into five terraces as well, the fourth and fifth of which were produced more than ten thousand years ago. By radiocarbon dating method, the downward erosion rate of Lao-Nong River is estimated to be around 0.8 cm/yr on average. This rate is higher upstream and lower downstream. The evolution of river terraces of Lao-Nong River may be grossly divided into five stages: The first stage spanned from 27000 to 28000 years ago, the second 8750 to 11500 years ago, the third 3000 to 5400 years ago, the fourth 2100 to 2800 years ago, and the fifth, 900 to 1800 years ago. Mudslide to the north of Mei-Shiow was larger and younger than that to the south, and it might cause obstruction and local accretion. The mainstream terrace generated by the impact of local accretion constitutes a major difficulty in correlating the river terraces of upstream and downstream Lao-Nong River. Debris fans of the tributaries in the drainage basin are mainly located on the right bank. Probable causes include geological and topographic features. Because it is steep on the northern side but even on the southern side of Jade Mountains, tributaries north of Mei-Shiow tend to form mudslides repeatedly, so debris fans and river terraces in this area are younger.
Subjects
Lao-Nong River
river terrace
debris fan
radiocarbon dating
fluvial evolution
SDGs
Type
thesis
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