本研究碎屑鋯石的鈾-鉛定年結果顯示，於晚中新世晚期關刀山砂岩段含有大量
100-150 Ma 和 220-250 Ma 的鋯石，推論可能是因為弧陸碰撞的發生而改變了沉積物
的供應。根據 Lee et al. (2006)提出的模式(圖 4-9)，晚中新世晚期時，弧陸碰撞影響
到前陸盆地之發育，大陸邊緣因此快速下陷，其坡度很可能從原來大陸棚遠小於 1
度之坡度，快速變化至大於 1 度；沉積物出河口後，快速下滑而形成海底扇，而後
再逐漸堆積充填(Lee et al., 2006)。由於大陸棚的坡度快速的變陡，造成相對海水面上
升，使得沉積物大都沉積在河口以上而無法大量供應，造成沉積物組成較為單調，
閩浙沿海出露之燕山期岩盤因而成為主要供應沉積物物源，故於關刀山砂岩中出現
了大量燕山期的鋯石。
到了上新世早期桂竹林層-十六份頁岩段沉積時鋯石鈾-鉛年齡峰值出現明顯的
改變，其中呂梁期鋯石年齡大量的出現，可能為臺灣古第三紀沉積層再次堆積所造
成。根據碎屑鋯石鈾-鉛定年資料推論中新世時，華南閩浙地區的岩盤出露受到風化
作用，經由閩江搬運至沿海形成大陸邊緣堆積(Chen et al., 2006)；到了晚中新世晚期
關刀山砂岩段沉積時，弧陸碰撞開始發生，造成大陸邊緣下陷，使得沉積物物源發
生改變，以閩浙沿海出露之燕山期岩盤為主；而到了上新世早期桂竹林層-十六份頁
岩段沉積時，隨著呂宋島弧和歐亞板塊不斷地碰撞，造成前陸盆地東側隆起，古第
三紀沉積層露出水面。Chi et al. (1981)亦提出於上新世時(NN15-NN16)，有一明顯之
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沉積速率迅速增加時期，故與本研究推論相符：上新世時大量的臺灣古第三紀沉積
物堆積到西部前陸盆地中。
周瑞燉 (1980)利用震測剖面，將中新世的地層進行劃分，其厚度為自西北向東
南逐漸增厚，砂岩百分比逐漸降低、顆粒漸細，根據沉積物結構的方向性推測搬運
沉積物的水流方向是西北-東南向。故綜合本研究臺灣苗栗地區中新世地層碎屑鋯石
鈾-鉛定年結果及古水流方向分析，可推論臺灣西部中新世盆地沉積物可能由盆地之
西北方搬運沉積而來，源區可能為大陸閩浙一帶沿海地區。到了上新世時，古水流
方向為西北-東南向及東-西向，本研究的碎屑鋯石鈾-鉛定年結果亦顯示此時鋯石來
源與中新世早期有著相似的峰值，據此推論上新世時，除了繼續沉積來自大陸邊緣
的沉積物外，也加入了部分來自臺灣古第三紀地層的沉積物（圖 4-4；圖 4-10)。
臺灣的造山歷史相當年輕，由於菲律賓海板塊相對於歐亞板塊每年以七公分的
速率向西北斜向聚合，臺灣碰撞運動的發生與地質現象是由北向南遷移(Suppe,
1984)，這代表著臺灣島地體構造的發育是由北向南逐漸被擠出海水面。因此本研究
所揭示之前陸盆地沉積物供應變化，也可能於臺灣北部地層發現類似變化，故利用
臺灣北部的碎屑鋯石鈾-鉛定年資料(邵文佑，未發表資料)來進行分析。此筆資料是
針對自始新世西村層到上新世二鬮層，共 10 個連續地層中的碎屑鋯石來進行鈾-鉛
定年研究(圖 4-12)。其定年結果顯示始新世西村層之碎屑鋯石鈾-鉛年齡以 200-100
Ma 及 540-250 Ma 的峰值為主，而漸新世到晚中新世早期則以 2.0-1.8 Ga 及 2.6-2.4 Ga
的峰值較為顯著，而到了上新世早期時則以 200-100 Ma 峰值為主。所以，可推論臺
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灣北部地層的沉積物供應於始新世到漸新世早期之間，以及晚中新世早期到上新世
早期之間，共發生了兩次改變。
利用本研究與臺灣北部地層的碎屑鋯石鈾-鉛定年資料(邵文佑，未發表資料)進
行鋯石鈾-鉛年齡的分佈及頻率比對(圖 4-13)，結果顯示本研究之晚中新世晚期關刀
山砂岩段與臺灣北部上新世二鬮層皆記錄下燕山期鋯石年齡大量出現之物源改變現
象。根據 Suppe (1984)提出之模式，則臺灣地體構造的發育應由北向南逐漸被抬升出
海水面，故臺灣北部應較早記錄下轉變為前陸盆地的過程。但根據定年資料之比對，
卻顯示臺灣西部於晚中新世晚期時即記錄下沉積物物源改變的過程，但臺灣北部地
區卻到上新世早期二鬮層沉積時才出現改變，沉積物的紀錄明顯不符合臺灣地體構
造的發育過程。推論有可能是因為臺灣西側的沿海地殼隆起形成觀音高區(圖 4-14)，
將臺灣盆地隔成了北部的南彭佳嶼盆地系統及西部地區的臺西-臺中盆地系統，故造
成其沉積物供應系統不同(Sun, 1985)。
54
圖 4-10 臺灣前陸盆地形成及沉積形貌演化示意圖，引用自 Lee et al. (2006)。
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圖 4-11 臺灣前陸盆地演化圖，修改自 Teng (1990)。紅色箭頭為沉積物移動方向。
(A)早中新世沉積物主要來自於華夏陸塊；(B)晚中新世晚期，因弧陸碰撞臺灣隆起 造山，前陸盆地開始發育。(C)上新世早期古臺灣島因弧陸碰撞而抬升露出水面，前 陸盆地沉積中心西移，古臺灣島開始供應沉積物至前陸盆地中。由此可推論前陸盆 地開始發育的時間較前人所推論的更早，上新世早期臺灣造山活動已非常可觀。
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圖 4-12 臺灣北部地區碎屑鋯石頻譜分析圖(邵文佑，未發表資料)。
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圖 4-13 臺灣北部與臺灣苗栗地區碎屑鋯石頻譜分析圖。(A)臺灣北部地區碎屑鋯石定年資料(邵文佑，未發表資料)；(B) 臺灣地層對比 表(謝凱旋 et al., 2001)；(C)本研究資料，臺灣中部中新世-上新世地層碎屑鋯石定年資料。
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圖 4-14 臺灣及其鄰近區域新生代臺西盆地、臺南盆地及南彭佳嶼盆地剖面圖，引用自 Sun (1985)。
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第5章 結論
本研究嘗試利用碎屑鋯石鈾-鉛定年(LA-ICP-MS) 來探討苗栗出磺坑背斜附近
桂竹林層沉積環境的演變，得到以下結論：
1. 根據碎屑鋯石鈾鉛年齡頻譜分析結果，顯示中新世以來地層記錄下沉積物物
源發生兩次的改變，一為晚中新世早期的東坑層至晚中新世晚期上福基層，而另一
次則為晚中新世晚期桂竹林層-關刀山砂岩段到上新世早期桂竹林層-十六份頁岩段
沉積時。
2. 早中新世至晚中新世早期，前陸盆地主要接受來自閩浙地區的沉積物。晚中
新世晚期關刀山砂岩段沉積時，由於弧陸碰撞開始發生，沉積了大量中國大陸閩浙
沿海地區沉積物，故以燕山期與印支期鋯石所佔的比例大為增加。到了上新世早期，
則因碰撞造山使古臺灣露出水面之上，古第三紀沉積層開始供應沉積物至前陸盆地
中。
3.基於臺灣北部與臺灣西部沉積物物源區鋯石鈾-鉛年齡的分佈及頻率比對，推
論臺灣北部與西部分別為兩種不同之盆地系統，沉積物供應來源也不相同。
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