Controlling factors of fluvial terrace evolution in the small drainage basins, eastern Coastal Range, eastern Taiwan

• Main Authors: Sun, Leng-Hsiang, 孫稜翔
• Other Authors: Shen, Su-Min
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/23860944274485178741

博士 === 國立臺灣師範大學 === 地理學系 === 101 === Located at the Taiwan–Luzon arc-continent collision boundary, Coastal Range is one of the areas with the highest Holocene uplifted rate in Taiwan. Widespread fluvial terraces on the eastern flank of Coastal Range is usually believed due to the high uplift rate and the consequent channel downcutting. The picture of how well-developed the terraces is and what environmental factors combined to contribute to the development of the terraces is not clear. Thus, this research aims (1) to understand the morphological characteristics and spatial distribution of the fluvial terraces, (2) to analyze the major controlling factors of the terraces, and (3) to suggest various evolution models (scenarios) of formation of the terraces in the drainage basins on the eastern flank of Coastal Range. The study area consists of 207 drainage basins and the three river with the highest drainage order are not included. Based on the aerial photo interpretation and field investigation, fluvial terraces have been found in the 103 drainage basins on the eastern flank of Coastal Range, among them the smallest drainage basin is only 6 ha (0.06 km2) in size. As a whole, the greater the area of the drainage basin is, the larger the total terrace surface is and the number of terrace flight is. The number of terrace flight in most drainage basins is 5 to 6, and is 12 in the Sansien Basin. The terraces of the rivers from Chinpu to Hsinlan (76 km in length along the coast) are the most well-developed in the study area. In terms of geology, terraces are almost exclusively distributed in the less resistant formations (soft rocks) which are characterized by interbedded sandstone and shale, and mudstone, i.e., Paliwan Formation, Fanshuiliao Formation and Lichi Formation; in the more resistant volcanic bedrocks (hard rocks), i.e., Tuluanshan Formation, only a few depositional terraces developed. 46 out of 124 outcrops on the terrace scarps, which were investigated in the filed, consisting of 21 cross sections of the fluvial terraces show the most terraces are erosional terraces and it also reflects the rapid Holocene uplift rate and the continuous channel downcutting. Statistical analysis also shows those drainage basins with hard rocks (Tuluanshan Formation) in the upper stream and soft rocks (mainly Paliwan Formation) in the lower stream have the larger total terrace area. It is believed that such lithological arrangement strengthens so-called tool effect as the resistant boulders from Tuluanshan Formation transported downstream through much weaker Paliwan Formation the channel widened. In terms of the regional difference of the fluvial terraces in the study area, the area between the Chinpu and Hsinlan has the highest uplift rate and the most typical lithological arrangement of hard rocks upstream and soft rocks downstream. Under such circumstance, triggered by frequent earthquakes or typhoon rainfalls, abundant coarse sediments move into channels and are subject to form depositional terraces at the boundary of hard/soft rock boundary. The thickness of the depositional layer on the straths usually less than 10 m and the widespread debris-flow fan or fan terrace at the river mouth shows that most sediments from upstream reaches are quickly transported downstream along the very steep channel. On the way, resistant boulders, probably in the form of debris flows, increase the width of the channels effectively. In short, apart from the rapid uplift rate and the existence of weak rocks, lithological arrangement is also a crucial factor in contributing to the very well-developed fluvial terraces in such small drainage basins in the eastern flank of Coastal Range.