海岸山脈南段泰源盆地之地層架構及沉積歷史

Abstract

泰源盆地位於海岸山脈南段，為原呂宋火山島弧的弧前盆地之一，盆地內有厚數千公尺的蕃薯寮層與八里灣層，沉積於火成岩基盤上，記錄了台灣弧陸碰撞過程中的山脈剝蝕歷史。雖前人對這套地層已進行了相當豐富的研究，但對泰源盆地的地層與構造分布仍存在許多爭議，其關鍵是缺乏區域地層對比。為解決此問題，本研究進行泰源盆地詳盡的地質調查，重新繪製泰源盆地的地質圖與構造剖面，再透過沉積學與岩石學的分析，探討沉積物的來源與成因，最後整合年代地層資料，重建泰源盆地的沉積歷史。 調查發現，蕃薯寮層與八里灣層以砂泥質濁流岩為主，其中穿插多層水平延展性良好的礫質泥岩與凝灰層可作指準層。本研究利用這些指準層，將八里灣層分為下、中、上三段。砂泥質濁流岩的砂泥比例側向變化劇烈，常見崩滑作用產生的軟質變形褶皺；古水流資料指出其沉積物主要來自北方，並隨時間逐漸受到東北方的物源影響。礫質泥岩為海底岩屑流的陸坡堆積物，在盆地中總共有五層，本文由上至下分別命名為Pm1至Pm5，沉積物主要源自北方；從礫石的岩性組成可知Pm1、Pm4與Pm5沉積物主要來自呂宋火山島弧，Pm2及Pm3則主要來自碰撞增生楔。凝灰層僅出現於八里灣層中，以濁流岩或礫質砂岩組成，共可辨別出T0至T11等十二層；從岩層厚度變化及古水流資料可見，火成碎屑源自盆地東南方，且因碎屑礦物組成均含有黑雲母，可能與綠島的火山活動有關。此外，古坡向的分析指出，泰源盆地的古地貌大體以西側碰撞增生楔與東側呂宋火山島弧的陸坡包圍而成，沉積物常沿著這些陸坡崩滑產生崩移褶皺。 綜合上述地層架構、沉積物來源與盆地古地貌的解釋，及前人的年代地層資料，本研究推演古泰源弧前盆地的演化史如下述：中新世晚期至上新世早期，弧陸碰撞活動正要開始，隱沒作用與呂宋火山島弧的火山活動趨緩，古泰源弧前盆地東側的安山岩基盤已大體形成，西側的碰撞增生楔亦逐漸長高，構成盆地的初始樣貌。到上新世早期至更新世(約4.0至1.4百萬年前)，因島弧及增生楔的抬升與剝蝕，大量沉積物以濁流與岩屑流機制由北方進入此盆地，以超覆與疊覆形式堆積於盆地基盤上，形成蕃薯寮層與八里灣層之砂泥質濁流岩與礫質泥岩Pm1至Pm5；在八里灣層沉積期間，盆地東南方的綠島火山活動產生許多含黑雲母之火山碎屑，亦以濁流與岩屑流方式搬運至盆地內，形成凝灰層T0至T11。此弧前盆地隨後因碰撞活動的持續進行而逐漸關閉停止沉積，地層受到後期的抬升、變形與侵蝕作用，最終演變成今日海岸山脈南段之形貌。

Located in southern Coastal Range of eastern Taiwan, the Tai-Yuan Basin is one of the arc-continent collisional forearc basins in the Luzon Arc between the Eurasian and the Philippine Sea plates. These basins are composed of Miocene volcanic basement (Tuluanshan Formation) sharply overlain by over 5 km thick Plio-Pleistocene siliciclastic sedimentary rocks (Fanshuliao Formation and Paliwan Formation) which recorded the geotectonical evolution of the Taiwan Orogeny. Despite the stratigraphic system has been established, the regional lithostratigraphic correlation and the depositional evolution of the basin are not well understood. For this purpose, this study first reexamines the stratigraphic subdivision, then discusses the sources and sedimentary processes of marker beds by deposits composition, paleocurrent and paleoslope analysis. Finally, an explanation of depositional history of Tai-Yuan Basin is proposed. The Fanshuliao Formation and Paliwan Formation contain sandy to muddy turbidites and several widespread marker beds, including 5 pebbly mudstones (Pm1 to Pm5) and 12 tuffaceous layers (T0 to T11). Based on these marker beds, this study subdivides Paliwan Formation into the lower, middle, and upper sections. In the turbidites facies, considerable lateral change of the sand to mud ratio could be recognized. According to the paleocurrent analysis, these turbidite are mainly derived from the north, partly from north-eastern source. The pebbly mudstones facies are originated by slumping and related debris flow processes in submarine canyon/slope or near slope environments. The results of paleocurrent and clast composition analysis imply that all pebbly mudstones were derived from northern instable slopes, where is at volcanic arc side for Pm1, Pm4, and Pm5, and at Taiwan orogenic wedge side for Pm2 and Pm3. The tuffaceous layers, which are mainly turbidites or debrites, are only discovered within the Paliwan Formation. Based on petrographic data, paleocurrent analysis and thickness variations, the volcanic clasts are considered to be derived from south-eastern side source, which could be Lutao volcano. Furthermore, the paleoslope analysis result shows that this basin was shaped by western orogenic wedge slope and eastern volcanic arc slope, and several slumped sturctures were formed by frequently slumping events. By integrating the above-mentioned, this study provides a new evolution model of Tai-Yuan Basin. Prior to the arc-continent collision, the subduction and arc volcanism subsided, and the arc andesite basement (Tuluanshan Formation) was established. The collisional orogeny started to grow around early Pliocene. During this period (4.0-1.4 Ma), lots of clasts eroded form the nortern mountain belts and submarine slopes were transported into the basin and formed Fanshuliao Formation and Paliwan Formation turbidites and pebbly mudstones Pm1 to Pm5. At around 2.13 to 1.4 Ma, several biotite-containing tuffaceous layers produced by eruptive events of the south-eastern side volcano are recorded in the sequence. As the collosion propogated toward the south, the Tai-Yuan forearc basin was progressively closed and accreted to the orogen, which eventually grew up to its present appearance.