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題 名 | 林口臺地北緣的火山碎屑岩層=Volcaniclastic Deposits of the Northern Linkou Tableland |
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作 者 | 鄧屬予; 宋聖榮; 劉志學; 蕭丁槐; 李錫堤; 莊釗鳴; | 書刊名 | Western Pacific Earth Sciences |
卷 期 | 8 2008.12[民97.12] |
頁 次 | 頁43-75 |
分類號 | 356.232 |
關鍵詞 | 地層; 火山作用; 觀音山; 林口臺地; 大屯火山; Statigraphy; Volcanism; Kuanyinshan; Linkou Tableland; Tatun Volcanoes; |
語 文 | 中文(Chinese) |
中文摘要 | 林口台地和觀音火山雖然緊密相鄰,但兩者的先後關係至今仍有爭議。最近我們在台地和火山接壤處的探井中,發現一段火山碎屑岩層和沈積岩層的互層,為台地和火山的地層關係和年代提供了重要的新線索。 這些井下火山碎屑岩層位於荖阡坑山南麓,岩體呈舌狀分布,東西寬約 1公里,南北長 3公里,總厚度最高達 70公尺。火山岩層以火山角礫為主,內含許多沈積岩礫和少量火山灰及沈積岩屑,單層厚度約 2-50公尺。火山岩層間穿插有沈積岩層,包括礫岩、砂岩及泥岩,其岩性與林口台地週緣所出露的林口層相當。岩層中的火山岩礫以斑狀安山岩為主,地球化學性質屬於島弧型的鹼長基玄武岩系列,岩性特徵類似觀音火山岩。 綜合探井和鄰近地區的地層資料,可知觀音火山岩層和林口層有互層的關係,而觀音火山在林口層堆積時候就已經開始噴發。由於火山碎屑岩層的年代最老可達 100萬年,這表示觀音火山 100萬年前已開始噴發,比前人推測的 63萬年要早了許多。而火山碎屑岩層中含有觀音火山中層熔岩流的岩塊,但無上層熔岩流碎屑,表示林口層頂部的年代,也就是林口台地面開始形成的年代,應介於兩層熔岩流之間,大約是 44萬年前。這也比以往所推測的 80萬年前要年輕。 |
英文摘要 | The Linkou Tableland and the Kuanyin Volcano in northern Taiwan are contiguous geomorphic entities with ambiguous stratigraphical relationships and debatable ages. Recent discovery of intercalated volcanic and sedimentary strata in the northern Linkou Tableland sheds new lights on its stratigraphic relationship with the Kuanyin Volcano, which helps us to delineate the ages for both Tableland and Volcano. The volcaniclastic strata were unveiled by a series of boreholes drilled in the northern tip of the Linkou Tableland near the southern slope of the Kuanyin Volcano. They consist of flat-lying, up to 50-m thick, layers of volcanic breccias and lapilli tuffs, in which angular igneous rock fragments mixed with various amount of sedimentary clasts, including pebbles, sand, and mud. These volcaniclastic deposits are interlayered with conglomerates, sandstones, and mudstones of the Linkou Formation that crop out widely in the neighboring river valleys. The whole volcanic deposits form a wedge-like body, 2 km wide and up to 150 m thick, that rapidly pinches out southward in a distance of 3 km. The igneous rock fragments of these volcanic layers are dominated by porphyritic andesites similar to those exposed in the Kuanyin Volcano, but some are previously unknown in this area. Petrographic observations and geochemical analyses show that the andesitic fragments of the volcaniclastic strata pertain to the shoshonitic rock series with prominent volcanic arc signature, similar to the lava flows of the Kuanyin Volcano. These features demonstrate that the newfound volcanic deposits were derived from the Kuanyin Volcano, most likely through debris avalanches or lahars, and were laid down contemporaneously with the alluvial fan deposits of the Linkou Formation. From the stratigraphical relationship described, it is obvious that the Kuanyin volcanism commenced well before the formation of the Linkou Tableland. The first volcanic eruption, as indicated by the chronometric data of the lowermost volcaniclastic strata, took place around 1.08 Ma, which is 0.4 m.y. earlier than what was previously thought. The age of the Linkou Tableland can also be re-evaluated by its stratigraphical relationship with the Kuanyin Volcano. Since the volcaniclastic deposits underlying the Tableland include rock fragments of the middle lava flow but lack those of the upper lava flow of the Kuanyin Volcano, the age of the Tableland can be bracketed by the ages of these two lava flow episods (0.43 Ma and 0.45 Ma). This gives an age around 0.44 Ma, which is considerably younger than previous estimates. |
本系統中英文摘要資訊取自各篇刊載內容。