臺灣海岸山脈東翼河階地形發育之研究

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

孫稜翔(Leng-Hsiang Sun)

指導教授 ： 沈淑敏

國立臺灣師範大學/文學院/地理學系/博士(2013年)

摘要

臺灣位於弧陸碰撞帶，東部海岸山脈為抬升速率最高地區之一，該區河階地形發達，前人研究常以高抬升率為造就其階地發達的主因。然而該區內河流普遍短小，還需哪些環境因子的配合，才能在河流持續下切的過程中拓展谷床，成就未來的河階面？因此，本文以海岸山脈東翼為研究區，藉由河階、山麓沖積扇及海階崖的對比，階地露頭的特徵、河床礫石的調查，流域的地層配置與地形特性，輔以前人文獻於階地沉積物年代以及古氣候的研究成果，達成三項目的：(1)了解本區河階特性與分布；(2)分析河階發達的環境因素，尤其是流域內地層配置的重要性；(3)根據河階特性，提出不同分區之河階發育模式。本文研究對象為海岸山脈(長約150公里) 東翼207個中小型流域，不包含豐濱溪、秀姑巒溪、馬武窟溪等較高等級河流和最南端部分地區。
根據本研究判釋結果，研究區中103條河流判釋得河階地形，其中流域面積最小的僅為6公頃 (0.06 km2)。整體而言，流域面積愈大，河階愈發達，階序數愈多；多數河流階序可達5-6階，三仙溪階序最多，可達10階，只有1-2階的均為最小河流。根據河階特徵，本研究區河流可大致分為3區：中段的靜浦至新蘭間約76 km區域，河階最為發達；北段河階主要分布在水璉溪和蕃薯寮溪兩條較大溪流；最南端河階也主要只分布在兩條溪流。此外，中段那些只發育在海階範圍內的小溪流，也有少數發育零星河階。絕大多數的河階分布於砂頁岩互層和泥岩為主的八里灣層、蕃薯寮層和利吉層等弱岩區，屬火成岩硬岩區的都鑾山層區只有少數堆積型階地分布。
本研究野外調查共記錄124個露頭，根據其中46個露頭調查所組合成的21個剖面顯示，本區河階多屬侵蝕型階地(岩石河階)，可反映本區河流在全新世快速抬升背景下，河流持續下切的歷程。雖然本區河階都發育於弱岩區，但根據統計分析，在流域面積相似狀況下，具備上游硬岩、下游軟岩之地層配置流域，河階總面積最大。由此推斷，當上游有都鑾山層供給硬岩巨礫時，可發揮工具效應(tool effect) 拓寬河道，而有利於階面形成。此外，本區中、南段海階發達，隨著海階崖後退，河長變短，會造成最下游段額外下切的效果。
基於河階特徵和影響因素，本研究推論中段河階最發達地區的河階發育過程。靜浦至新蘭間的全新世的抬升率為海岸山脈東翼的高區，且上游硬岩、下游軟岩的地層配置最為典型，在此狀況下，位於上游的都鑾山層受地震或暴雨驅動岩石碎屑進入河道，易在硬軟岩層交界處因土石流堆積，形成堆積型河階。且本區中、下游階地多屬侵蝕型河階，堆積層厚度多小於10公尺，且河口多有沖積扇或扇階分布，顯示上游沈積物大多通過中下游段至河口堆積，當都鑾山層的巨礫通過軟岩區河道時，軟硬岩層岩性的顯著差異，巨礫工具效應更顯著，有利於谷床加寬，而隨著陸地抬升，河流持續下切，河階陸續生成。其他兩區則因隆升率或地層配置的狀況不同，而與中段溪流發育出不同的河階特徵。簡言之，海岸山脈東翼這些中小型流域河階很發達的原因，除了高抬升率和有弱岩出露之外，地層配置的效應顯然也很重要。

關鍵字

河階 ； 地層配置 ； 發育模式 ； 海岸山脈

並列摘要

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.

並列關鍵字

fluvial terrace ； lithological arrangement ； evolution model ； Coastal Range

參考文獻

林名璟（2008）：《利用河階發育年代、堆積特徵與其分佈來探討荖濃溪之河流演育過程》，國立臺灣大學理學院地質科學所碩士論文。

齊士崢、黃美璇（2001）：〈荖濃溪的河階地和地形演育〉，《環境與世界》，5： 123-140。

龔琪嵐、齊士崢（2004）：〈楠梓仙溪流域的河階地與地形演育〉，《地理學報》，38：47-62。

齊士崢、宋國城（2000）：〈台灣界限斷層帶上沖積扇階地地形發育的複雜性〉，《環境與世界》，4：77-91。

齊士崢、宋國城、陳邦禮、謝孟龍、蔡衡、傅炯貴（1998）：〈蘭陽溪上游沖積扇的地形演育〉，《環境與世界》，2：137-150。