台湾西北部海岸阶地的研究

海岸阶地的形成通常是海准面变动、地壳变动或两者共同作用之结果．研究海阶不仅可印证推论古气候、往昔海准面变化及地壳变动状况，更可藉以了解各区域间变动的差异，全盘了解大地构造的意义，而且小规模的海阶变动，时常与地震活动伴生，因此，研究海阶更可作为判读地震周期的依据之一，所以，世界各地位于地壳活动带的国家对于海阶的型态与演育过程均作详细的调查及研究．本研究以淡水河以南至大安溪以北之海阶作为研究范围，发现台湾西北部位于观音山北部沿海、新竹山子顶沿海、客雅溪口南岸、后龙溪口南岸等4个地区，都有零星的海阶分布．经过阶地分布、地形特征与阶序对比，并利用世界海阶对比基图求出该地区的地壳隆升率与海阶可能形成的年代，藉以了解各区域间变动的差异．台湾西北部海岸地区的海阶大致上可划分为高位及低位2群阶：也，高位海阶分布的高度在15～275m之间，阶面覆盖着红壤层，为晚更新世时所造成；低位海阶分布的范围与高度较小，在海滩与高位阶地末端阶崖之间，沿着海岸成带状分布，阶面无红壤掩覆，海拔大多在10m以下．低位海阶构成的物质多以砾石及砂为主，属全新世海阶，即第四纪最后一次冰期结束后，近1万年以来全球高海水位时期所形成．利用海阶对比基图与已有的定年数据，辅以地形地貌及堆积物特征比较，获得各段阶地之平均隆升率，观音山北部沿海、客雅溪口南岸、后龙溪口南岸3个地区，经过比对，分别是2．1mm／a、2．2mm／a、2．15mm／a，数值相近，显示该区之地盘隆升率及海准面变动状况大致相同；仅新竹山子顶沿海地盘隆升率较小，为1．4mm／a．此表示，台湾岛的海阶变化不仅受到海准面变动的影响，尚受到区域性地壳隆升的控制．

The Research of Marine Terraces in Northwestern Taiwan

Influenced by eustatic changes and tectonic movement, marine terraces are considered as indicators of regional tectonic movement, paleo-climate ＆ sea-level variation and crust movement. The study area, from Tamsui river to Ta＇an river, is divided into four groups, and by the use of aerial photographic interpretation, field survey and data analysis, we draw conclusions as following： 1 ） Classified and correlated by continuity of altitude and extension of terrace surface, two type of marineterraces are found in study area, one is Pleistocene terrace type with heights distributed from 15 -275 m and covered with thick laterite, the other belongs to Holocene terrace type whose heights are mostly below 10 m, developed along the coastal lowland and consisted of sand and gravels. 2） Based on dating, elevation, mode of erosion and deposits feature of terraces, the marine terraces uplift rate of Northwestern Taiwan can reflect regional tectonic activities and sea level variation. The uplift rate of marine terraces on the hsinchu coast is 1.4mm/a, and the uplift rates on the north coast of Kuanyin Volcano, on the south coast of Keya river estuary and the south coast of Houlong river estuary are 2. 1 mm/a, 2. 2 mm/a and 2. 15 mm/a, respectively. The uplift rate on the Hsinchu coast is smallest in Northwestern Taiwan, indicating that perhaps the area was influenced by Hsinchu Subsidence Area.