台灣西南海域陸棚及峽谷內沈積物傳輸方式

Abstract

本文研究區域位於台灣西南海域陸棚、高屏海底峽谷及枋寮海底峽谷。西南海域具有陸棚短窄及海底峽谷地形等特徵，其中高屏海底峽谷頭部接連著陸上的高屏溪；枋寮海底峽谷從陸棚開始、頭部沒有河川連接。本研究希望能夠藉由陸棚之表層沈積物及峽谷上、中、下部的岩芯分析中找出沈積物傳輸方式，且對不同時間同一採樣點所採集之岩芯做時間尺度上的對比。 從X光攝影之結果來看可知此區的岩芯中幾乎均含濁流沉積物。粒徑分析的結果顯示從峽谷上部至峽谷下部的沈積物顆粒含砂比越來越大，其峽谷上部沈積物主要為粉砂及泥而峽谷深部沉積物之含砂量較大，說明此區的沈積物主要因濁流或重力流攜帶輸出。210Pb分析中發現有一近期快速堆積之沈積物分佈於整個高屏海底峽谷和枋寮海底峽谷西側。在相隔一年多後，於相對高屏海底峽谷之OR1-785 2A同一站位採集OR-851 GCC岩芯中發現此一快速沈積事件之沈積物厚度應超過1.5公尺，比對當時地震及水文資料推測乃因洪氾所引起。另外，也間接的証實海研一號舊型的重力岩芯採集器會造成表層沈積物損失。峽谷中大量堆積沈積物經過長時間底流淘選後僅剩數公分薄層，說明了峽谷為沈積物的通道，長時間後沈積物會被帶離。枋寮海底峽谷之岩芯中亦有如高屏海底峽谷新一期快速堆積之沈積物，但從其X光攝影可看出部分岩芯層理並非相當完整，甚至其淘選度相對於岩芯本身早期之沈積物較好，比對採樣時間點，推測其為2006年地震所產生之濁流沈積物。高屏溪河流攜帶沈積物大量輸出，主要堆積於高屏海底峽谷中，溢流部分受海流及潮流作用飄送至鄰近區域沉積。

The southwestern Taiwan offshore is mainly composed by narrow continental shelf with several submarine canyons, in which the Gaoping canyon is connected with the Gaoping River, and in contrary, the Fangliau canyon is without river connected. This study wishes to combine the grain size and 210Pb profile data in conjunction with X-radiograph images to understand the transportation of sediments on the shelf off southwestern Taiwan, especially in the Gaoping and Fangliau canyons. X-ray radiography images show the turbidites are widespread in the study area, and grain size results show the sand contents are increased form upper canyon to lower canyon. It implies the sediments were transported by turbidity or gravity flow in this area. 210Pb analysis shows new turbidite layer in all the Gaoping canyon and western Fangliau canyon core samples. In the March of 2006, we collected a core (OR1-785-2A) from Gaoping canyon and found a newest forming thick turbidite in the box core. A year later, at the same location, a gravity core (OR1-851 GCC) was taken and finds the thickness of the turbidite layer is over 150 cm, and this turbidite layer is related to the flooding event in 2005. In the lower part of the gravity core OR1-785-2A, the thin layered turbidites are appeared repeatedly. We suggest the huge and thick sediments were deposited in the canyon in temporary, followed by long-term re-suspension, re-transportation and erosion processes, the thick turbidite layer transferred into thin layer, and the whole transformation illustrates why the submarine canyon is a conduit of sediments transportation. Compare to the Gaoping canyon, the new turbidites deposited in the Fangliau canyon was probably caused by the 2006 Hengchun earthquake.