臺灣西北部河階之地形學研究

A Geomorphological Study on River Terraces in Northwestern Taiwan

劉明錡(Ming-Chi Liu)

指導教授 ： 張瑞津

國立臺灣師範大學/文學院/地理學系/博士(2004年)

摘要

本島最具系統性的地形面是河階，是研究古氣候變遷、古水文變化、構造運動及侵蝕基準面升降等課題的重要對象。本研究以台灣西北部五條主要河流階地為對象，利用航照判讀、地形計測、野外實察與室內分析等方法探討階地分布、地形特徵與階序對比，分析河階形成與氣候變化、海準面升降及構造活動的情形，進而推論台灣西北部河流地形演育。研究結果顯示，台灣西北部各河流劃分高位階地六階(LH、LT1~LT5)、低位階地三階(FT1~FT3)，高位階地以LT4、LT5規模較大，低位階地以FT2、FT3較廣，高位階地階崖、礫石平均粒徑皆大於低位階地，高低位各段階崖呈現上游大、下游小的趨勢，且上游河階多為岩石階地，谷口以下則為礫石階地。
綜合以上階地形特徵，本區階地之階段數目、階地規模、階崖變化與階地類型等具有相同性，顯示台灣西北部階地有相同的發育歷程，而且在更新世晚期之古氣候、古水文與古海準面變化影響高位階地的演育，全新世以來則以地殼運動主導低位階地演育。整體區域性而言，台灣西北部階地演育模式分為二類型，第一類型為河流谷口上、下游地區均為地盤隆升區，如鳳山溪、頭前溪、中港溪與後龍溪，形成各段階地連續；第二類型為谷口上游段為地盤隆升區，下游段為地盤沉降區，如淡水河，造成低位階地FT1、FT2尖滅(覆蓋)於FT3階地之下。
依據階地地形特徵、階地定年與日本喜界島階地定年推論，本區高位階地(LT1~LT5)分別為海準面高峰期形成(距今13萬、10萬、8萬、6萬、4萬年前)，低位階地(FT1~FT3)分別為距今6千、3~3.9千、2.7千年前形成。

關鍵字

河階 ； 地形特徵 ； 階地對比 ； 地形演育

並列摘要

It is the most important to geomorphological surface system of river terrace in Taiwan, and the proposes of the project are studied on the change of the paleo-climate , paleo-hydrology, tectonic movement and erosion base-level . The study aim at five catchments in the northwestern Taiwan island, and the method based on aerial photgraphic interpreation, field excursion and data analysis. Some conclusions are reached as follow.
1)The result of river terrace includes six lateritic surfaces (LH~LT5) and three non-lateritic surfaces (FT1~FT3) have been recognized in the northwestern Taiwan. 2)To sum up , LT5, FT2 and FT3 distributed widely when the land kept a long stable period with sea level in late Pleistocene and Holocene. 3)The formation and evolution of river terraces are controlled by the sea-level fluctuation and multiple effects of neotectonic. 4)There are two models in this area, the first one is Tan-sui river catchment are controlled by subsidence of Taipei basin and eustatic fluctuation ,according to the distribution, geomorphic characteristics and correlation of river terraces. The morphology of river terraces are marked by meander terraces in Keelung river and Hsin-tien river and paired straight terraces in Ta-han river have been influenced by geologic structure as well as rock control. The second model is dominately controlled by the highest sea-level in Fengshen, Touchein, Chungkung and Houlung catchments. 5)It depended on geomorphic characteristics , 14C dating and Kikai Island in the northern Ryukyu chains that coral reefs terraces are dating which are similar to river terraces in northwestern Taiwan. LT1~LT5 developed late-Pleistocene are correlated about 130ka、100ka、80ka、60ka and 40ka BP. and FT1~FT3 were occurred on about 6ka, 3.9~3ka and 2.7ka BP. in Holocene.

並列關鍵字

river terrace ； geomorphic characteristics ； terrace correlation ； geomorphic evolution