摘 要

　　恆春半島墾丁混同岩體位於恆春斷層東側，以含強烈剪切鱗片狀葉理構造之青灰色或灰黑色頁岩為基質，內含各種大小不等、成分迥異的火成岩及沉積岩岩塊。

　　有關墾丁混同岩體的成因、年代、分佈與產狀，過去一直是地質學界爭論的焦點，不同學者所提出的成因模式解釋，彼此間也相互衝突和矛盾：支持沉積作用成因的學者認為墾丁混同岩體發生在被動式亞洲大陸邊緣（詹新甫，1974；Pelletier and Stephan, 1986）或是增積岩體的斜坡上（Page and Lan, 1983）；支持構造運動成因的學者則普遍認為墾丁混同岩體與馬尼拉海溝隱沒作用有關（Biq, 1972、1977；Huang et al., 1997；Chang et al., 2003）。

　　本研究分析出露在恆春半島增積岩體內三區域的墾丁混同岩體中所含有孔蟲化石，其結果顯示：

1. 三條剖面的墾丁混同岩體，絕大部分標本只含晚中新
世（N14-N17帶）浮游性有孔蟲與深海相底棲性有孔
蟲化石，或大約有30％標本不含有孔蟲化石。深海相
底棲性有孔蟲化石如： Cibicidoides sp.、
Cyclammina orbicularis、Bulimina pupoides、
Fontbotia wuellerstorfi、Uvigerina soendaensis
等。

2. 少量靠近上新-更新世斜坡層序的標本，除含晚中新
世（N14-N17帶）浮游性有孔蟲化石外，尚含有早期-
中期中新世浮游性有孔蟲及深海相底棲性有孔蟲化石
（組合如第一類者），並且有上新－更新世浮游性有
孔蟲（N19-N22帶）及淺海相底棲性有孔蟲化石，
如：Amphistegina raidata、Anomalinoides
colligerus、Elphidium crispum、Operculina
ammonoides、Rectobolivina raphan、
Rotalinoides compressiusculus等。
　　
　　上述微體古生物現象，輔以恆春半島地質（包括三區域出現墾丁混同岩體），及橫過臺灣南部20oN海域地質，顯示恆春半島前身為晚中新世時馬尼拉海溝隱沒系統之恆春增積岩體，當南中國海洋地殼隱沒時， 於恆春增積岩體前緣將晚中新世濁流層經拖卸斷層剪裂，並混入南中國海洋地殼，形成隱沒雜岩；當隱沒作用停止後（6 Ma），因隨後的弧－陸碰撞使得不整合於增積岩體上的上新-更新世淺海斜坡盆地層序亦受剪切，因而於恆春斷層附近的墾丁混同岩體中亦混入上新-更新世淺海相底棲性有孔蟲化石。墾丁混同岩體含有上述化石群集特性，無法由簡單的晚中新世沉積作用成因可以解釋。

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