金山至萬里岬灣海岸帶地質特徵

Abstract

目前海岸帶為全球人口最密集之生活地帶，其為海陸交界之帶狀區域，其地質地形特徵與人類活動有密切關係。本研究範圍內岬灣相間，依據地形特徵由西至東依序分為金山灣、金山岬、國聖灣、野柳岬、萬里灣。本文探討金山至萬里地區海岸帶之地質、地貌與沈積物特徵，及相關的地質營力。 區內海岸帶陸域上界為平均高潮線向內陸延伸到第一條稜線，或是植被明顯改變之處，海域下界參考碎浪帶區域暫訂為水深30公尺左右。三個海灣的海岸帶長度介於1.3至3.75公里，其寬度約介於1680至2320公尺，其中陸域寬度約介於100至250公尺，近海灣之寬度較寬廣，近海岬之處較為狹窄，此反映陸域丘陵腹地狹窄之地貌特徵，平均坡度約介於0.80至1.17度，地形呈現平緩之丘陵形貌，岩石海岸之海岬地形相當陡峻，愈遠離岬體則地形愈平緩，其海岸帶為海灣地形受斷層構造差異侵蝕與海岬岩體間距限制之小型岬灣海岸帶。 三個海灣海灘沈積物的礦物組成皆以石英、長石與角閃石為主，角閃石的出現可推論沈積物源自於大屯火山群之火山岩屑物質，並經由瑪鍊溪、員潭溪、磺溪搬運堆積於河口，再經波浪作用搬運堆積至海灣灘面上。至於蒙脫石的分佈主要為瑪鍊溪、萬里灣與國聖灣中，但幾乎不存在於員潭溪與金山灣中。而方解石之分佈稀少，僅零星存於海灣灘面上，顯示生物遺骸之分佈不廣。 區內海岸線與東北季風之間幾乎呈垂直交角，平均潮差較小約介於0.6至1.7公尺，相對而言，海岸經常受年平均1.0至2.0公尺波高之波浪影響，可推論本海岸主要受到中高波能的影響。海岬受波浪侵蝕岩石節理形成的海蝕溝與離水海蝕洞的存在顯示過去受海浪侵蝕之海崖地形因陸地抬升運動而達到目前之位置，本海岸帶可視為陸地作用大於海洋作用之原生海岸帶。 海灣海灘沈積物淘選度與歪斜度的相關性指出沈積物之組織特徵與河床砂相近，顯示陸源物質是控制海岸沈積物組成之主要因素。沈積物顆粒的圓度以呈現次角狀之數量居多，而且球度普遍不佳，顯示本流域內砂質沈積物之搬運距離不遠。沈積物粒徑分析與文獻報導之近岸水下之沈積物粒徑相近，顯示三個海灣海岸帶為中砂質海岸帶。

The coastal zone is the region of the highest density of global population presently. It is the narrow transition zone from land to sea. There is a close relation between the geological characteristics of landform and human activity. The coastal zone of this study is characterized by alternate headland and bay. This coastal zone consists of Jingshan bay, Jingshan headland, Kuosheng bay, Yeliu headland, and Wanli bay from west to east, respectively. This thesis discusses characteristcs of geology, landform and sediment of the coastal zone from Jingshan to Wanli and related geological process. The upper boundary of this coastal zone is the first crest line from the average high-tide to inland, or the land region of obvious changes of vegetation. The lower boundary refers to the breaker zone at the shallow sea where is about 30m in depth. The length of coastal zones of these three bays ranges from 1.3 to 3.75km. The width ranges from 1680 to 2320m, of which land region is about between 100 and 250m in width. This characteristic landform reflects hilly and narrow hinterland. The average slope is between 0.80 and 1.17 degrees. The headlands are quite steep in rocky this coast. It becomes gentler far toward land. This is the small coastal zone characterized by alternating headland and bay. This landform was formed by differential erosion due to local structures. The mineral composition of beach sediments of these three bays consists mainly of quartz, feldspar, and hornblende. Appearance of hornblende infers that the sediment comes from volcanic debris of Datun volcanos in the source area. These sediments are transported by the Malian stream, Yuantan stream, and Huang stream to the estuaries, and then redistributed by waves. The Montmorillonite distributes mainly in Malian stream, Wanli bay, and Kuosheng bay. Calcite is rarely found. This hints that biological remains are not widely distributed. The direction of the northeast monsoon is perpendicular to the regional coastal line in northern Taiwan. The average tide range is between 0.6 and 1.7m. This coastal zone has been influenced by the wave of average height between 1.0 and 2.0m, characteristic of medium-high wave energy. The landform due to wave erosion in the marine cliffs at the present location was formed by the uplift movement of the crust there exist wave erosion grooves in rocky joint in headland and wave erosion caves at sea. This coastal zone can be considered as a primary coast because the influences of terrestrial processes are greater than that of marine processes. The relationship between sorting and skewness of beach sediment in bays indicates that the nature of beach sediment is similar to that of river sands. This implies that source areas in land control the composition of the coastal sediment. The roundness of sediment is mainly subangular. The sphericity is generally not good. The transport distance of this coastal sandy sediment is not far. This coastal zone, including three bays, can be considered as the medium sandy coastal zone.