探討大漢溪剖面更新世卓蘭層至楊梅層之沉積層序及前陸盆地演化

Abstract

晚中新世以來，弧陸碰撞使得臺灣形成一系列南北向造山帶，在其西側前緣發育一系列前陸盆地。前陸盆地具有高沉降速率及高沉積速率的特性，使得其沉積層序可反映小尺度的海水面變動。更新世的海水面升降受米蘭科維奇行星軌道影響，具有週期性變化。其中，臺灣西北部大漢溪剖面的卓蘭層與楊梅層，為更新世前陸盆地地層，沉積環境呈現海相轉為陸相，也導致卓蘭層上部至楊梅層，缺乏超微化石的資料而無法得知年代。因此，本研究透過岩相分析、岩相組合分析及層序地層分析，調查卓蘭層至楊梅層第五級的沉積層序，並對比至米蘭科維奇週期，建立卓蘭層至楊梅層的年代模式，再透過構造沉降速率及沉積速率的計算，探討沉積層序及北部前陸盆地的演化歷史。 卓蘭層至楊梅層共可分出45個沉積層序，具有4種型態的沉積層序，分別為遠濱型、河口型、分流河道型及陸相河道型。整個沉積層序及沉積環境呈現向上變淺，海相轉變為陸相。其中，沉積層序演化有兩種看法，關鍵在於層序編號42之後的沉積層序有明顯增厚的現象，第一種看法是認為受到構造沉降速率及沉積速率加速的影響，第二種看法除了構造沉降速率及沉積速率加速外，也受到100 ky週期的影響。兩種看法皆透過沉積速率及沉積速率的計算，顯示北部前陸盆地分別在1.86 Ma及0.98-0.87 Ma有增加的趨勢。在1.86 Ma，前陸盆地沉降速率及沉積速率增加，是受到雪山山脈及內麓山帶的抬升造成前陸盆地的沉降速率增加，同時造成雪山山脈及外麓山帶的沉積物大量剝蝕至盆地堆積，沉積速率因此而增加及沉積環境快速變淺。在0.98-0.87 Ma，沉降速率及沉積速率增幅大於1.86 Ma，顯示前陸盆地已經相當靠近斷層前緣，造成沉積速率及沉降速率大幅增加。 此外，將北部前陸盆地（大漢溪剖面）的演化史與西南部前陸盆地（曾文溪剖面）的演化史進行比較，發現西南部前陸盆地整體沉積速率都大於構造沉降速率，而北部前陸盆地整體沉積速率皆接近沉降速率。此差異來自於前陸盆地形成於不同的沉積環境。

In late Miocene, the Taiwan orogeny belt is formed by the arc-continental collision. A series of foreland basin developed in western side of the orogeny belt and have characteristics of high subsidence rate and high sediment accumulation rate, so the depositional sequences in foreland basin strata reflect high-frequency sea level change. Especially, sea level change since Pleistocence is controlled by the Milankovitch cycle. In northwestern Taiwan, Cholan Formation and Yangmei Formation in the Dahan river section are Pleistocence foreland basin strata. The evolution of depositional environment from Cholan Formation to Yangmei Formation show marine to nonmarine setting. Thus, the age from the upper part Cholan Formation to Yengmei Formation are unclear, so this study use lithosfacies analysis, facies association and sequence stratigraphy to establish the 5th order depositional sequences from Cholan Formaton to Yengmei Formation and correlated to Milankovitch cycle to resolve age problem. Then, through the calculation of tectonic subsidence rate and sediment accumulation rate, this study would discuss the evolution of depositional sequence and the northern foreland basin in Taiwan. 45 depositional sequences can be recognized from Cholan Formation to Yangmei Formation and that contain 4 types of the depositional sequences which are offshore type, river mouth type, distributary channel type and fluvial channel type. Two models of evolution of the depositional sequences could be considered because above No. 42 depositional sequence in Yangmei Formation is thicker than below. The first model indicates the increase of tectonic subsidence rate and sediment accumulation rate. The second model indicates the increase of tectonic subsidence rate and sediment accumulation rate and the affect of 100 ky of Milankovitch cycle. The tectonic subsidence rate and sediment accumulation rate of two models show acceleration in 1.86 Ma and 0.98-0.87 Ma. In 1.86 Ma, the uplife of the Hsuehshan range and the Western Foothill cause the increase of tectonic subsidence rate and sediment accumulation rate and depositional environment shallow more quickly. In 0.98-0.87 Ma, tectonic subsidence rate and sediment accumulation rate increase again that is influenced by closing the fold-thrust belt of orogeny belt. Furthermore, comparison the foreland basin history of Chengwenchi section in southwestern Taiwan and Dahan river section in northern Taiwan. In the northern foreland basin, the increase of sediment accumulation rate is closed to the increase of tectonic subsidence rate, but in the southwestern foreland basin, the increase of sediment accumulation rate is higher than the increase of tectonic subsidence rate. The reason is that foreland basin is formed in different environment condition.